US20040100693A1 - Reflection-type polarization-independent optical isolator, optical isolator/amplifier/monitor, and optical system - Google Patents

Abstract

A variety of polarization independent optical isolators, including a single stage polarization independent optical isolator, a single stage broadband polarization independent optical isolator, a double stage polarization independent optical isolator, a double stage broadband independent optical isolator, and an optical isolator/monitor/amplifier, provide improved isolation characteristics and functionality. Optical systems are based upon input light traveling twice through the optical isolator/monitor/amplifier, and upon input light traveling twice through the optical isolator/monitor/amplifier connected in cascade with a double stage broadband polarization independent optical isolator.

Description

BACKGROUND OF TE INVENTION
• 1. Field of the Invention[0001]
• The present invention relates to a polarization independent optical isolator and, more particularly, to a reflection type of a polarization independent isolator.[0002]
• 2. Description of the Related Art[0003]
• Optical fiber communication systems are now in practical use, and efforts are being made to advance research and development in this field. Accordingly, requirements for optical devices with more versatile functions have also increased.[0004]
• An optical isolator is used as a functional component in a light transmission system such that light transmission therethrough is permitted in only one direction. A common use of optical isolators is as constituents of so-called “optical passive components” within optical amplifier systems, which are themselves important components of fiber-optic communication systems. Optical amplifier systems generally include optical isolators residing on both sides of an optical gain element such as an Er-doped fiber. Other optical passive components include Wavelength Division Multiplexers (WDM's) and signal monitors.[0005]
• A polarization independent[0006]optical isolator100 is shown in FIG. 1 as an example of a traditional and typical optical isolator of the prior art. As illustrated in FIG. 1, there is provided a 45-degree Faraday rotation element (which is also referred to as a Faraday rotator)101 which always rotates light input thereto in one direction by virtue of a permanent magnet. Apolarizer102 and ananalyzer103 are respectively placed before and after the Faraday rotation element, with thepolarizer102 andanalyzer103 being maintained at relative positions rotated 45 degrees with respect to one another.
• As shown in FIG. 1, light emitted from an[0007]optical fiber104 is divided or separated into parallel beams by alens105, and of the parallel beams, thepolarizer102 allows only polarized light oriented in a particular direction to pass through it; any other light is absorbed or reflected and eliminated. Polarized light that has passed through thepolarizer102 emanates from the Faradayrotation element101 with its plane of polarization rotated by 45 degrees. Theanalyzer103 is so arranged that polarized light with its plane of polarization rotated by 45 degrees passes through theanalyzer103, is focused by alens106 and enters anoptical fiber107.
• On the other hand, and also as shown in FIG. 1, of light entering the polarization independent[0008]optical isolator100 in the reverse direction (from the optical fiber107), only polarized light that is rotated by 45 degrees relative to thepolarizer102 may pass through theanalyzer103. Polarized light that has passed through theanalyzer103 will have its plane of polarization rotated by 45 degrees by the Faradayrotation element101, and then emanates therefrom. The resulting light is rotated by 90 degrees relative to thepolarizer102 and is eliminated. Because of this, light in the forward direction propagates forwardly while light in the reverse direction is eliminated.
• However, the[0009]isolator100 just described is polarization dependent, even with respect to light propagating in the forward direction. More particularly, only specific, polarized light can pass through theisolator100 in the forward direction, and the remaining propagating light is not effectively utilized because it is eliminated. Typical optical fibers used in light wave communication and data transfer systems do not preserve optical polarization over long distances. Light emanating from such a fiber consists of a randomly mixed state of light polarized in all directions, regardless of the state of polarization of light input to the fiber. Polarization-preserving fiber is well known but is too expensive for general use over long distances. Polarization independent optical isolators have therefore found a wide variety of applications in fiber-optic light wave systems.
• FIG. 2A shows a well-known prior-art polarization independent optical isolator that is disclosed in U.S. Pat. No. 4,548,478. In the prior art polarization independent[0010]optical isolator200 shown in FIG. 2A, tapered birefringent plates (tapered plates)201 and202 are placed on either side of a 45-degree Faradayrotator203. Referring now to FIG. 2A, when light emanates from theoptical fiber204 into the prior art polarization independentoptical isolator200 and enters in the forward direction into the firsttapered plate201, the light is divided or separated into ordinary rays (o-rays) and extraordinary rays (e-rays) because of the differences in the index of refraction of the firsttapered plate201 due to polarization. These rays are refracted to different directions, and enter the 45-degree Faradayrotator203 of FIG. 2A.
• Ordinary and extraordinary rays of which planes of polarization are rotated 45 degrees by the Faraday[0011]rotator203 are caused to enter the secondtapered plate202. The secondtapered plate202 is arranged such that an optical axis of the secondtapered plate202 is rotated 45 degrees around or about the light propagation direction relative to an optical axis of the firsttapered plate201. Therefore, the foregoing ordinary and extraordinary rays correspond to ordinary and extraordinary rays in the secondtapered plate202, respectively. Accordingly, ordinary rays and extraordinary rays that pass through the secondtapered plate202 emanate parallel to each other. These parallel beams of ordinary and extraordinary rays are focused onto theoptical fiber207 by thelens206.
• On the other hand, light traveling in the reverse direction (emanating from[0012]fiber207 and traveling toward the direction offiber204 as shown in FIG. 2B) is divided into ordinary rays and extraordinary rays after entering the secondtapered plate202. The ordinary rays and the extraordinary rays are refracted to different directions by the secondtapered plate202, enter the 45 degree Faradayrotator203, and are emitted therefrom with their plane of polarization rotated by 45 degrees.
• For the light propagating in the reverse direction as shown in FIG. 2B, ordinary rays and extraordinary rays in the[0013]second plate202 are converted to extraordinary rays and ordinary rays, respectively, in thefirst plate201 by the Faradayrotator203, so that the direction of each of these rays after passing through the firsttapered plate201 is different from that of incident light. Accordingly, when these rays are converged by thelens205, focal points are formed outside the face of thefiber end204 so that the light traveling in the reverse direction does not enter theoptical fiber204.
• Since optical isolators typically utilize Faraday rotators and since the angular polarization rotation of Faraday rotators typically depends on wavelength of the light propagating therethrough, the wavelength region that provides the 45-degree rotation is very narrow. Therefore, a high isolation is maintained only in a very limited wavelength region, unless deviation from 45-degree rotation is compensated for.[0014]
• In U.S. Pat. No. 4,712,880, two optical isolators and two polarization rotation compensators which are incorporated into these optical isolators are disclosed. The first polarization rotation compensator described in U.S. Pat. No. 4,712,880 is shown in FIG. 3A as[0015]element300 and is composed of a combination of a half-wave plate301 whose principal axis is inclined at an angle of 0/2 with respect to the plane of polarization of theincident light302 and a quarter-wave plate303 whose principal axis is inclined at an angle of θ with respect to the plane of polarization of theincident light302, with the half-wave plate301 and the quarter-wave plate303 disposed in this order with respect to the forward light propagation direction.
• The second polarization rotation compensator described in U.S. Pat. No. 4,712,880 (not shown) is similar except that the principal axis of the quarter-wave plate is parallel to the plane of polarization of the incident light, the principal axis of the half-wave plate is inclined at an angle of θ/2 with respect to the plane of polarization of the incident light, and the quarter-wave plate and half-wave plate are disposed in this order with respect to the forward light propagation direction.[0016]
• The first optical isolator described in U.S. Pat. No. 4,712,880 utilizes the first polarization compensator, and is shown in FIG. 3B. This first optical isolator[0017]304 comprises a first birefringent wedge plate305, the firstpolarization rotation compensator300 described herein above with reference to FIG. 3A, a Faraday rotator306, and a second birefringent wedge plate307, all arranged in this order with respect to the direction of propagation of the forward light. The forward light emanates fromfiber308, and is collimated bylens309 onto the first birefringent plate305. After passing through the first birefringent plate305, the firstpolarization rotation compensator300, the Faraday rotator306, and the second birefringent plate307, the light is focused bylens310 intofiber311, as shown in FIG. 3B.
• The second optical isolator described in U.S. Pat. No. 4,712,880 (not shown) is similar except that the second embodiment of the polarization rotation compensator is used and the first birefringent wedge plate, the Faraday rotator, the second polarization rotation compensator, and the second birefringent wedge plate are arranged in this order with respect to the propagation direction of the forward light.[0018]
• The prior-art optical isolators discussed above are of the transmission type. Reflection-type optical isolators can reduce the number of optical components, because some components are used twice due to the double pass characteristics of the device. FIG. 4 is a perspective view of a prior art reflection-type polarization independent optical isolator that is disclosed in U.S. Pat. No. 5,033,830. As shown in the prior art reflection-type polarization independent[0019]optical isolator400 of FIG. 4, a pair of stackedreciprocal rotators401 and402, namely half-wave plates, aFaraday rotator403, and reflector404 (including lens404-1 and mirror404-2) are positioned in tandem adjacent to thebirefringent plate405. In the forward (transmitting) direction, a light wave signal exiting anoptical fiber406 is split into a pair of orthogonal rays by thebirefringent plate405. The orthogonal rays then pass through a firstreciprocal rotator401 and theFaraday rotator403 for rotating polarizing light planes. TheFaraday rotator403 rotates polarizing light planes 22.5 degrees. The rotated rays are then redirected by thereflector404 back through theFaraday rotator403. After passing through the secondreciprocal rotator402, the orthogonal rays re-enter the samebirefringent plate405 where they are recombined and launched in anoutput fiber407.
• Since a[0020]Faraday rotator403 is a non-reciprocal device, any signal traveling through the isolator in the reverse (isolation) direction will be split on both passes through thebirefringent plate405 such that neither will intercept theinput fiber406.
• A second prior-art reflection-type polarization independent optical isolator suitable for use as an optical passive component in an optical amplifier is disclosed in U.S. Pat. No. 5,499,132, incorporated herein by reference, and is shown in FIGS. 5A and 5B. The second prior-art reflection-type polarization independent optical isolator[0021]500 shown in FIGS. 5A and 5B includes at least twooptical fibers501 and502, anoptical fiber array503 into which the fibers are secured and whose tip end is polished at the angle of approximately 8 degrees, abirefringent crystal504 for dividing the input light into two linearly polarized lights, ahalf wave plate505 for reversibly rotating the direction of polarization of the input light by approximately 45 degrees, a graded index type rod-lens506 to collimate and focus the light, amagnetooptical crystal507 and associatedmagnet508 for non-reversibly rotating the direction of polarization of light transmitted therethrough by 22.5 degrees counter clockwise on each pass, areflector509, and aglass plate510.
• In the second prior-art reflection-type polarization independent optical isolator[0022]500 shown in FIG. 5A, the signal lights output from the firstoptical fiber501 are divided by thebirefringent crystal504 into two linearly polarized light rays, which are then collimated by thelens506. Thereafter, the planes of polarization of the two linearly polarized light rays are rotated by π/8+nπ/2 (n=0, 1, . . . ), respectively, in, for example, the left-hand direction by themagnetooptical crystal507. After reflection by thereflector plate509, the two linearly polarized light rays each receive a further rotation of π/8+nπ/2 (n=0, 1, . . . ) in the same direction by themagnetooptical crystal507. A further rotation π/4 in the same direction is caused by thehalf wave plate505. Thereafter, a polarized light coupling operation is effected by thebirefringent crystal504 so as to input the light rays into the secondoptical fiber502. Thus, the above described arrangement functions as a polarization-independent optical isolator.
• Also disclosed in U.S. Pat. No. 5,499,132 are additional embodiments illustrating extended function of the optical isolator disclosed therein so as to provide an integrated set of optical passive components for use in an optical amplifier. An example of such an embodiment is shown in FIG. 5 of U.S. Pat. No. 5,499,132 in which provision is made for injection, in a direction counter to the signal propagation direction, of two 1480 nm laser-diode pump beams into the optical path as well as for detection and monitoring of a portion of the amplified signal. FIG. 5 of the '132 patent is reproduced herein as FIG. 6, for the convenience of the reader.[0023]
• The above described prior art optical isolators are all of the single-stage type—that is to say that light inputted thereto passes at most one time through the full set of isolation-providing components. Generally, one stage of isolation structure provides an isolation characteristic of about 35 dB. Therefore, the prior art polarization independent optical isolators described above, although useful for many applications, have an insufficient isolation characteristic for applications to the high quality transmission systems or the optical fiber amplifiers.[0024]
• In U.S. Pat. No. 5,689,360, incorporated herein by reference, a double-stage reflection isolator is disclosed. FIG. 1 of U.S. Pat. No. 5,689,360 discloses a device comprising both first and second optical isolation units and also a reflection unit for coupling the output of the first optical isolation unit to the input of the second optical isolation unit by directing the outputted signal ray from the first optical isolation unit to the second optical isolation unit, with the signal ray received by the first optical isolation unit transmitting in the opposite direction to the outputted signal ray from the second optical isolation unit. FIG. 1 of the '360 patent is reproduced herein as FIG. 7, for the convenience of the reader, using the reference numerals provided in the '360 patent. The operation of each of the single stage isolators disclosed in U.S. Pat. No. 5,689,360 is similar to that disclosed in U.S. Pat. No. 5,499,132 (and is not repeated in detail here) except that the reflector is not incorporated within each isolator. Instead, the reflector element is positioned after the output of the first optical isolator and before the input of the second optical isolator, with respect to the forward light propagation direction, so as to provide optical coupling between the two isolators. The sequential operation of two optical isolators in this fashion provides improved isolator performance with respect to the operation of a single-stage isolator.[0025]
SUMMARY OF THE INVETION
• An object of the present invention is to provide a single-stage polarization independent optical isolator with improved isolation characteristics.[0026]
• Another object of the present invention is to provide a single-stage broadband polarization independent optical isolator with improved performance characteristics.[0027]
• An additional object of the present invention is to provide a double-stage polarization independent optical isolator with improved isolation characteristics.[0028]
• A further object of the present invention is to provide a double-stage broadband polarization independent optical isolator with improved isolation characteristics.[0029]
• Still another object of the present invention is to provide an isolator/monitor/amplifier.[0030]
• Another object of the present invention is to provide an optical system based upon the isolator/monitor/amplifier of the present invention.[0031]
• Yet a further object of the present invention is to provide an optical system based upon the double-stage broadband polarization independent optical isolator and the isolator/monitor/amplifier.[0032]
• Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.[0033]
• The present invention is a polarization independent isolator and an optical system based thereon. The polarization independent isolator of the present invention includes a single stage polarization independent isolator, a single stage broadband polarization independent isolator, a double stage polarization independent isolator, and a double stage broadband independent isolator. The present invention also includes an isolator/monitor/amplifier, and respective optical systems based upon the isolator/monitor/amplifier of the present invention and on the isolator/monitor/amplifier in cascade with the double-stage broadband polarization independent isolator of the present invention.[0034]
• The single stage polarization independent isolator of the present invention comprises an input fiber; an output fiber; optical elements including a birefringent walk-off plate, a counterclockwise rotating λ/2 plate provided adjacent thereto, and a Faraday rotator and associated magnets provided adjacent to the birefringent walk-off plate and the counterclockwise rotating λ/2 plate; a lens; and a mirror. Input light traveling in the forward direction from the input fiber passes through each of the above-mentioned optical elements and enters the output fiber. However, the above-mentioned optical elements prevent input light from the output fiber traveling in the reverse direction from entering the input fiber.[0035]
• The single stage broadband polarization independent isolator of the present invention comprises an input fiber; an output fiber; optical elements including a birefringent walk-off plate, a counterclockwise rotating λ/2 plate and a broadband polarization rotation compensator provided adjacent thereto, and a Faraday rotator and associated magnets provided adjacent to the birefringent walk-off plate and the counterclockwise rotating λ/2 plate and broadband polarization rotation compensator; a lens; and a mirror. Input light of many wavelengths traveling in the forward direction from the input fiber passes through each of the above-mentioned optical elements and enters the output fiber. However, the above-mentioned optical elements prevent input light from the output fiber traveling in the reverse direction from entering the input fiber.[0036]
• The double stage polarization independent isolator of the present invention comprises an input fiber; an output fiber; optical elements including a birefringent walk-off plate, a counterclockwise rotating λ/2 plate provided adjacent thereto, a clockwise rotating λ/2 plate provided adjacent to the birefringent walk-off plate and to the clockwise rotating λ/2 plate, a Faraday rotator and associated magnets provided adjacent to the birefringent walk-off plate and the counterclockwise rotating λ/2 plate and the clockwise rotating λ/2 plate, and a second birefringent walk-off plate; a lens; and a mirror. Input light traveling in the forward direction from the input fiber passes through each of the above-mentioned optical elements and enters the output fiber. However, the above-mentioned optical elements prevent input light from the output fiber traveling in the reverse direction from entering the input fiber.[0037]
• The double stage broadband polarization independent isolator of the present invention comprises an input fiber; an output fiber; optical elements including a birefringent walk-off plate, a counterclockwise rotating λ/2 plate and broadband polarization compensator provided adjacent thereto, a clockwise rotating λ/2 plate and second broadband polarization compensator provided adjacent to the birefringent walk-off plate and to the clockwise rotating λ/2 plate and broadband polarization compensator, a Faraday rotator and associated magnets provided adjacent to the birefringent walk-off plate and to the broadband polarization compensator and the clockwise rotating λ/2 plate, and a second birefringent walk-off plate; a lens; and a mirror. Input light traveling in the forward direction from the input fiber passes through each of the above-mentioned optical elements and enters the output fiber. However, the above-mentioned optical elements prevent input light from the output fiber traveling in the reverse direction from entering the input fiber.[0038]
• The isolator/monitor/amplifier of the present invention is based upon the single stage broadband polarization independent isolator of the present invention, and further includes a laser input and monitor output.[0039]
• An optical system of the present invention includes two of the isolator/monitor/amplifiers of the present invention coupled to each other in series through an Er-doped fiber or other suitable optical gain element. The two isolator/monitor/amplifiers of the present invention coupled in series replace the optical passive components of a prior art optical system.[0040]
• In addition, the present invention is a cascaded optical system including a double sided broadband polarization independent optical isolator of the present invention coupled in series to an isolator/monitor/amplifier of the present invention. Input signal light makes two passes through the optical system, being output from the isolator/monitor/amplifier along Er-doped fiber after the first pass therethrough. The Er-doped fiber then carries the input light back to the isolator/monitor/amplifier and the double sided broadband polarization independent isolator for a second pass through the cascaded optical system.[0041]
• Because of its function described herein, a λ/2 plate is also referred to as a reciprocally rotating optical element.[0042]
BRIEF DESCRIPTION OF THE DRAWINGS
• These and other objects and advantages of the invention will become apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which:[0043]
• FIG. 1 is a schematic view of a prior-art polarization dependent optical isolator.[0044]
• FIGS. 2A and 2B are side views of the operation of a prior-art polarization independent optical isolator in both the forward (FIG. 2A) and reverse (FIG. 2B) light propagation directions.[0045]
• FIG. 3A is a perspective view of a prior-art polarization rotation compensator and FIG. 3B is a side view of the operation of a prior-art polarization independent optical isolator which utilizes the same polarization rotation compensator.[0046]
• FIG. 4 is a perspective view of a prior-art polarization independent optical isolator of the reflection type.[0047]
• FIGS. 5A and 5B are side views of a second prior-art reflection-type polarization independent optical isolator with delineated light paths in both the forward (FIG. 5A) and reverse (FIG. 5B) propagation directions.[0048]
• FIG. 6 is a sectional view illustrating the construction of a prior-art optical passive component.[0049]
• FIG. 7 is a side view of a prior-art double-stage polarization independent optical isolator showing the loci of forward propagating light rays.[0050]
• FIGS. 8A and 8B are side views showing the structure and operation of a first embodiment of a single-stage polarization independent optical isolator of the present invention showing, respectively, loci of forward and backward propagating central light rays of two principal polarization states.[0051]
• FIG. 9 is a side view showing the structure and operation of a first embodiment of a single-stage polarization independent optical isolator of the present invention showing loci of the full sheath of forward propagating light rays of one of the two principal polarization states.[0052]
• FIG. 10 is a graph showing an approximation to the expected wavelength variation of the performance of a typical single stage optical isolator.[0053]
• FIG. 11 is a perspective view of a polarization rotation compensator for use in conjunction with an embodiment of a polarization independent optical isolator of the present invention.[0054]
• FIG. 12 is a side view showing the structure and operation of a second embodiment of a single-stage polarization independent optical isolator of the present invention having polarization rotation compensation with the loci of forward propagating central light rays of two principal polarization states denoted.[0055]
• FIGS. 13A and 13B are side views showing the structure and operation of a first embodiment of a double-stage polarization independent optical isolator of the present invention showing, respectively, loci of forward and backward propagating central light rays of two principal polarization states.[0056]
• FIG. 14 is a side view showing the structure and operation of a second embodiment of a double-stage polarization independent optical isolator of the present invention having polarization rotation compensation with the loci of forward propagating central light rays denoted.[0057]
• FIG. 15 is a schematic view of a third embodiment of a double-stage polarization independent optical isolator having an additional signal-monitoring function.[0058]
• FIG. 16 is a basic block diagram of an optical fiber amplifier showing the assembly of conventional optical passive components and their functional correspondence to the port locations of an integrated optical passive component of the present invention.[0059]
• FIG. 17 is a side view of an embodiment of an integrated optical passive component of the present invention for use with an optical fiber amplifier and which encompasses the combined functions of pre-amplification and post-amplification single-stage signal isolation, pre-amplification and post-amplification signal monitoring, co-propagating and counter-propagating pump beam injection (multiplexing), and mutual co-propagating and counter-propagating pump beam isolation.[0060]
• FIG. 18 is graph of the preferred variation with wavelength of the reflectivity of the center partial reflector element of the integrated optical passive component of FIG. 17.[0061]
• FIG. 19 is a pair of end views of the front and rear fiber (port) configurations and associated optical elements of the integrated optical passive component of FIG. 17.[0062]
• FIGS. 20A and 20B are a pair of schematic side views of the loci of light ray paths during first and second passes through the integrated optical passive component of FIG. 17.[0063]
• FIG. 21 is a detailed view of the loci and polarization states of central light rays of two principal polarizations propagating in the forward direction during the first pass (pre-amplification) through the integrated optical passive component of FIG. 17.[0064]
• FIG. 22 is a detailed view of the loci and polarization states of central light rays of two principal polarizations propagating in the reverse direction within the specific optical pathways defining the first pass through the integrated optical passive component of FIG. 17.[0065]
• FIG. 23 is a detailed view of the loci and polarization states of central light rays of two principal polarizations propagating in the forward direction during the second pass (post-amplification) through the integrated optical passive component of FIG. 17.[0066]
• FIG. 24 is a detailed view of the loci and polarization states of central light rays propagating in the reverse direction within the specific optical pathways defining the second pass through the integrated optical passive component of FIG. 17.[0067]
• FIG. 25 is a schematic side view of the loci of light ray paths during first and second passes through a double-stage set of integrated optical passive components comprising a twin single-stage isolator in series arrangement with the integrated optical passive component of FIG. 17.[0068]
DESCRIPTION OF THE PREFERRED EMBODIMENTS
• Reference will now made in detail to the present preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures.[0069]
• Beginning with FIGS. 8A and 8B, and in all other subsequent figures included herein, solid and/or dashed lines with directional arrows affixed represent signal (light ray) trajectories and circles containing one or two double-headed arrows represent light beam polarization directions of the signals to which they are adjacent. Neither these trajectory indicators nor polarization direction indicators represent actual physical components of the embodiments to which they apply, and are provided as visual aids for the reader. Furthermore, the polarization direction indicators are all drawn and are always drawn as if the respective device were viewed end-on from a fixed reference point at the left side of the respective figure.[0070]
• The first embodiment of the polarization independent optical isolator of the present invention is illustrated in a side view in FIG. 8A and FIG. 8B. FIG. 8A and FIG. 8B illustrate operation of this first embodiment of the polarization independent[0071]optical isolator800 of the present invention during, respectively, forward and reverse propagation of light therethrough.
• Forward signal light propagation through the first embodiment of the present invention is now explained with reference to FIG. 8A. As shown in FIG. 8A, input fiber(s)[0072]801 and output fiber(s)810 are all contained within and secured to aferrule815 capable of containing at least two and up to four parallel optical fibers. The end face of theferrule815 together with the fibers contained therein is polished flat and cut at a tapered angle of approximately 8°.
• Unpolarized light entering the polarization independent[0073]optical isolator800 of the present invention via theinput fiber801 is first split intosub-signals802 and803 by the birefringent walk-off plate804. The principal optical axes of birefringent walk-off plate804 are aligned such thatsub-signal802 propagates through as an ordinary ray (o-ray) and is not deflected while sub-signal803 propagates through as an extraordinary ray (e-ray) and is deflected by the well-known birefringence walk-off effect, as shown in FIG. 8A. In FIG. 8A and in all subsequent figures, o-rays and e-rays are drawn as horizontally and vertically polarized, respectively.
• The amount of walk-off of a signal entering the birefringent walk-[0074]off plate804 which is introduced by the birefringent walk-off plate804 is dependent upon the thickness of theplate804 in the direction of the signal pathway. More particularly, the birefringent walk-off plate804 introduces approximately 20 μm of walk-off for each 200 μm of thickness of the birefringent walk-off plate804.
• After passing through birefringent walk-[0075]off plate804, each of the sub-signals802 and803 enters the λ/2 (half-wave)plate805 which reciprocally rotates the polarization of each of the sub-signals802 and803 by 45° in a counterclockwise direction. Bothsub-signals802 and803 are collimated and directed bylens806 ontomirror807, which reflects thesubsignals802 and803 back to and through thelens806.Lens806 then directs the reflected light through theFaraday rotator808 which, in response to a magnetic field applied bymagnets809, non-reciprocally rotates the polarization of bothsub-signals802 and803 by 45° in a counterclockwise direction. After passing through theFaraday rotator808, bothsub-signals802 and803 reenter the birefringent walk-off plate804 such thatsub-signal802 is vertically polarized and sub-signal803 is horizontally polarized. Sub-signal802 therefore re-enters the birefringent walk-off plate804 as an e-ray and is deflected by an amount equal to and opposite from the original deflection ofsub-signal803. Furthermore, sub-signal803re-enters element804 as an o-ray and is not deflected. Because of these switches in character with respect to the two passes through birefringent walk-off plate804, from o-ray to e-ray forsub-signal802 and from e-ray to o-ray forsub-signal803, their respective deflections in the birefringent walk-offelement804 are canceled and these two sub-signals thus recombine (after passage through birefringent walk-off plate804) and enter theoutput fiber810 as a single combined signal.
• [0076]Elements804,805, and808, in combination as shown in FIGS. 8A and 8B are referred to collectively as a single stage polarization independent optical element.
• Polarization independent optical isolators generally contain one reciprocal (or reversible) polarization rotator (or “rotator”, for short) and one non-reciprocal (or non-reversible) polarization rotator. This and subsequent embodiments of the present invention described herein each include at least one of each of these two types of rotators. In the polarization independent[0077]optical isolator800 of the present invention shown in FIGS. 8A and 8B, the reciprocal rotator is the λ/2 (half-wave)plate805 and the non-reciprocal rotator is theFaraday rotator808 together with the associatedmagnets809. Both such optical elements (the reciprocal and the non-reciprocal rotators) are used such that the direction of the plane of linearly polarized light that passes through them is rotated after such passage. When so used, reciprocal rotators have the property such that, given the polarization direction of a traversing light beam both to one side and to the other side of the reciprocal rotator, it is impossible to determine the propagation direction of the light beam traveling therethrough. Equivalently stated, for polarization plane rotation by a reciprocal rotator, the direction of rotation about the axis of light propagation, either clockwise (CW) or counter-clockwise (CCW), is always the same when viewed facing the reciprocal rotator towards the side at which the linearly polarized light beam enters the element.
• Conversely, non-reciprocal (non-reversible) rotators have the property such that the direction of polarization plane rotation about the axis of light propagation, either clockwise (CW) or counter-clockwise (CCW), is always the same when viewed facing the non-reciprocal rotator from a fixed reference point in a fixed direction, regardless of the propagation direction of the light ray through the element.[0078]
• Keeping these points in mind, FIG. 8B illustrates the behavior of light rays propagating in the reverse direction through the polarization independent[0079]optical isolator800 of the present invention. Light entering the polarization independentoptical isolator800 of the present invention viaoutput fiber810 is first split by the birefringent walk-off plate804 into reverse-propagatingsub-signals812 and813. In this case, sub-signal812 passes through birefringent walk-off plate804 as a vertically polarized e-ray and is deflected, whereas sub-signal813 passes through as an o-ray, which is not deflected. Bothsub-signals812 and813 then pass through theFaraday rotator808 which non-reciprocally rotates the polarization planes of both by 45° in the counterclockwise direction.Sub-signals812 and813 are then collimated and directed bylens806 ontomirror807, which reflects them back to and throughlens806.Lens806 then directs the reflectedsignals812 and813 to the λ/2plate805 which rotates the plane of polarization of both of them by 45° counterclockwise.
• Because λ/2[0080]plate805 is a reciprocal rotator, the counterclockwise rotation of each sub-signal812 and813 is as viewed facing toward the side at which the sub-signal entry to the λ/2plate805 occurs. However, as viewed end-on from a fixed reference point at the left side of FIG. 8B, the rotation of the sub-signal is in a clockwise direction. After the rotation by λ/2plate805, bothsub-signals812 and813 re-enter the birefringent walk-off plate804 such thatsub-signal812 is vertically polarized and sub-signal813 is horizontally polarized. The horizontally polarized sub-signal813 passes throughelement804 as an o-ray and is not deflected; conversely the vertically polarized sub-signal812 passes throughelement804 as a vertically polarized e-ray and is deflected for a second time by an amount equal and opposite to its original deflection after exitingfiber810. Because of these signal trajectories, the sub-signals812 and813 fail to re-combine and both fail to enter theinput fiber801 in the reverse direction. Thus, the function of the single stageoptical isolator800 of the present invention as a “one-way gate” is realized.
• Unless otherwise stated, in FIG. 8A and FIG. 8B as well as in all subsequent drawings herein, only the paths of representative centrally located rays of each polarization state are depicted. For instance,[0081]ray paths802 and803 (FIG. 8A) and812 and813 (FIG. 8B) correspond to such representative central rays. Rays such as those depicted byreference numerals802,803,812, and813 are each one of a plurality of rays of each sub-signal, as shown in FIG. 9 for the sub-signal corresponding toray path802. In FIG. 9, in addition toray path802, are also shown the representative boundinglight ray paths814A and814B. The boundingray paths814A and814B represent the loci of two rays that fall on the boundary of the full assemblage or plurality of rays in one of two mutually orthogonal polarization states which emanate from fiber801 (or from fiber810). Such boundary exists because, in three dimensions, the light emanating from fiber801 (or810) comprises a diverging (or converging) cone between the fiber end andlens806 and comprises a cylinder between thelens806 andmirror807.Reference numeral802 represents the unique central light ray at the center of the full assemblage of rays in one polarization state whose boundary is represented byreference numerals814A and814B. For simplicity, rays of the complementary polarization state, corresponding to ray path803 (FIG. 8A) are not shown in FIG. 9.
• Also as shown in FIG. 9, the focal length f from the[0082]lens806 to the birefringent walk-off plate804 is the same as the focal length f from thelens806 to themirror807. By this means,lens806 collimates light inputted from either of thefibers801 or810 and focuses output light onto either of these fibers. One of ordinary skill in the art will recognize that modifications may be made by which such collimating and/or focusing is performed by one or more lenses which are not necessarily disposed between these fibers and thenurror807. Any and all such modifications are within the scope of the present invention.
• Since optical isolators typically utilize Faraday rotators and since the angular polarization rotation of Faraday rotators typically depends on wavelength, the wavelength region that gives the 45-degree rotation of the input light signal is very narrow. This makes it possible to maintain a high isolation of the signal only in a very limited wavelength region, unless deviation from 45-degree rotation is compensated for. More particularly, optical isolators of the prior art generally work well within one narrow band of wavelength due to the above-mentioned 45° rotation mentioned herein above, and other wavelengths above and below may provide or receive leakage.[0083]
• For instance, FIG. 10 shows a graph of the approximate isolation performance, plotted against wavelength, which can be typically expected for a single-stage optical isolator which is not compensated for wavelength. The use of a polarization rotation compensator, similar to that disclosed in U.S. Pat. No. 4,712,880, can significantly broaden the wavelength region of maximum isolation.[0084]
• FIG. 11 illustrates a[0085]polarization rotation compensator1100 suitable for use in a second embodiment of the present invention, explained herein below with reference to FIG. 12. Thepolarization rotation compensator1100 includes a λ/2 (half-wave)plate1101 whose principal axis is inclined at an angle of θ/2′ with respect to the plane of polarization of theincident light1102 and a quarter-wave plate1103 whose principal axis is inclined at an angle of θ° with respect to the plane of polarization of theincident light1102, with the half-wave plate and the quarter-wave plate disposed in this order with respect to the forward light propagation direction.
• The[0086]polarization rotation compensator1100 introduces a range of polarization angles which vary based on wavelength based upon the adjustment ofplates1101 and1103. The range of polarization angles versus wavelength of therotation compensator1100 can be adjusted to be opposite to that of another element within an optical system to counter balance or compensate for the aberrations introduced by the other element, as is well-known in the art and disclosed in U.S. Pat. No. 4,712,880.
• If the[0087]polarization rotation compensator1100 is included in the polarizationindependent isolator800 of the first embodiment of the present invention, in addition to and adjacent to, the half-wave plate805, then the resulting apparatus is a second embodiment of the present invention, referred to as a single stage broadband polarizationindependent isolator1200 and disclosed herein below beginning with reference to FIG. 12. Thus, polarization rotation compensation functionality is added toisolator800 through the insertion, immediately after λ/2plate805 with respect to forward light propagation, of a new polarization rotation compensator of the type shown in FIG. 11.
• A second embodiment of the polarization independent optical isolator of the present invention, referred to as a single stage broadband polarization independent optical isolator, is shown in FIG. 12.[0088]
• The single stage broadband polarization independent[0089]optical isolator1200 shown in FIG. 12 includes a quarter-wave element (λ/4plate1103 shown in FIG. 11), along with a half-wave element (λ/2plate1101 shown in FIG. 11), collectively referred to as thepolarization rotation compensator1100 shown in FIG. 11 and denoted byreference numeral1201 in FIG. 12, along with a λ/2-wave plate805 shown in FIGS. 8A and 8B. Theisolator1200 shown in FIG. 12 is otherwise identical in construction to theisolator800 shown in FIGS. 8A and 8B except for the addition ofpolarization rotation compensator1100. The addition of thepolarization rotation compensator1100 to theisolator800 of the present invention shown in FIGS. 8A and 8B provides an isolator1200 with improved performance.
• The[0090]isolator1200 provides acceptable isolation performance over a broader wavelength range than is realized forisolator800. In an other aspects, the operation ofisolator1200 is similar to that already described forisolator800 and is not repeated in detail here. The angle which the λ/2 plate ofpolarization rotation compensator1201 makes with a vertical axis of theinput fiber801 must be tuned through a compensator for one specific device, to provide specific rotation of input light, such as 45° rotation.
• The broadband polarization[0091]optical compensator1201 of the single stage broadband polarization independentoptical isolator1200 of the present invention shown in FIG. 12 provides even greater effective isolation of input light, allowing more channels to be included in an optical fiber of a transmission band, with reduced leakage between channels, and improved performance over the entire bandwidth of input light.
• [0092]Elements804,805,808, and1201, in combination as shown in FIG. 12 are referred to collectively as a single stage broadband polarization independent optical element.
• The two already-described embodiments,[0093]isolator800 andisolator1200, of the polarization independent optical isolator of the present invention are both isolators of the single-stage type. A third embodiment of the present invention, comprising a double-stage polarization independent optical isolator, is shown in FIG. 13A and FIG. 13B. The double-stage polarization independentoptical isolator1300 of the present invention shown in FIGS. 13A and 13B provides even greater isolation of signals than does theisolator800 of the first embodiment of the present invention because the number of optical isolation elements that a signal travels through from input to output is effectively doubled over that of theisolator800 of the first embodiment. Also, as discussed further below, the double stage polarization independentoptical isolator1300 has the advantage of being free from polarization mode dispersion (PMD) that, in some cases, may be problematic forisolator800 orisolator1200.
• Optical isolators of the prior art generally include leakage of light backward through the system. The leaked light, which can be damaging to the transmitted optical light, is generally produced by reflections off of components within the optical transmission system and is compensated for in the prior art by placing two isolators of the prior art in series with each other.[0094]
• In FIG. 13A and FIG. 13B, the double-stage polarization independent[0095]optical isolator1300 of the present invention comprises a birefringent walk-off plate1301, a first λ/2 (half-wave)plate1302 with its principal optical axes oriented such that linearly polarized light propagating therethrough has the orientation of its polarization plane reversibly rotated by 45° counterclockwise about the propagation axis, a second λ/2 (half-wave)plate1303 with its principal optical axes oriented such that linearly polarized light propagating therethrough has the orientation of its polarization plane reversibly rotated by 45° clockwise about the propagation axis, aFaraday rotation crystal1304 and associatedmagnets1305 which provides a non-reversible polarization plane rotation of 45° counterclockwise to light propagating therethrough, a secondbirefringent plate1306 having properties, orientation, and dimensions identical to those ofelement1301, a focusing/recollimation lens orlens assembly1307 and amirror1308. Also provided in double-stage isolator1300 is a fiber holder orferrule1309 within which is provided at least one inputoptical fiber1310 and at least one outputoptical fiber1311.
• The operation of the[0096]isolator1300 with signal light propagating in its normal forward direction is illustrated in FIG. 13A whereas the operation ofisolator1300 with light propagating in the undesired reverse direction is illustrated in FIG. 13B. Also indicated in both FIG. 13A and FIG. 13B are the locations and polarization states of representative signal and sub-signal light rays propagating in both the forward (FIG. 13A) and reverse (FIG. 13B) directions.
• The forward propagation of signal light through double-[0097]stage isolator1300 will now be described with reference to FIG. 13A. Unpolarized light entering the polarization independentoptical isolator1300 via theinput fiber1310 is first split into polarized sub-signals,1312 and1313, by the birefringent walk-off plate1301. The principal optical axes of birefringent walk-off plate1301 are aligned such that sub-signal1312 propagates through as an ordinary ray (o-ray) and is not deflected while sub-signal1313 propagates through as an extraordinary ray (e-ray) and is deflected by the well-known birefringence walk-off effect.
• In FIG. 13A and FIG. 13B, o-rays and e-rays are drawn as horizontally and vertically polarized, respectively, although this specific orientation is not required. After passing through[0098]element1301, each of the sub-signals1312 and1313 enters the first λ/2 (half-wave)plate1302 that reciprocally rotates the polarization of each of the sub-signals1312 and1313 by 45° in a counterclockwise direction. Both sub-signals1312 and1313 then pass through theFaraday rotator1304 which, in response to a magnetic field applied bymagnets1305, non-reciprocally rotates the polarization of bothsub-signals1312 and1313 by 45° in a counterclockwise direction. After passing through theFaraday rotator1304, bothsub-signals1312 and1313 enter the second birefringent walk-off plate1306 such that sub-signal1312 is vertically polarized and sub-signal1313 is horizontally polarized. Since the thickness, composition, and orientation of birefringent walk-off plate1306 are identical to the respective properties of birefringent walk-off plate1301, the sub-signal1312 propagates through birefringent walk-off plate1306 as an e-ray and is deflected by an amount equal to the original deflection of sub-signal1313 inelement1301. Furthermore, sub-signal1313 propagates through birefringent walk-off plate1306 as an o-ray and is not deflected.
• Because of these switches in character with respect to the travel through the two[0099]birefringent plates1301 and1306, from o-ray to e-ray for sub-signal1312 and from e-ray to o-ray for sub-signal1313, their relative deflections in the birefringent walk-offelements1301 and1306 are canceled and these two sub-signals thus recombine so as to follow identical paths towardslens1307. The passage of sub-signals1312 and1313 throughbirefringent plate1301, half-wave plate1302,Faraday rotator1304 andbirefringent plate1306 in this order comprises a first stage of optical isolation.
• The[0100]lens1307 intercepts both co-propagating sub-signals1312 and1313 after their emergence from secondbirefringent plate1306 and collimates and directs both of these sub-signals ontomirror1308. Themirror1308 reflects bothsub-signals1312 back towards and throughlens1307 which then directs both sub-signals back towards the second birefringent walk-off plate1306. Upon entering second birefringent walk-off plate1306 for a second time, the paths of sub-signals1312 and1313 are re-separated. The vertically polarized sub-signal1312 once again propagates throughelement1306 as an e-ray and is deflected by an amount equal and opposite to its deflection during its first pass throughelement1306. The horizontally polarized sub-signal1313 once again propagates throughelement1306 as an o-ray and is not deflected. Thus, after passing throughbirefringent plate1306 for a second time, the relative physical separation of sub-signals1312 and1313 is identical to what it was just prior to enteringelement1306 for the first time (FIG. 13A). After passing throughbirefringent plate1306 for the second time, sub-signals1312 and1313 make a second pass throughFaraday rotator1304 which non-reversibly rotates the polarization planes of both of these sub-signals by450 CCW (counterclockwise).
• After passing through[0101]Faraday rotator1304, sub-signals1312 and1313 then pass through the second half-wave plate1303. The half-wave plate1303 reversibly imposes a 45° CW (clockwise) rotation about the propagation axis upon the polarization planes of bothsub-signals1312 and1313. Because half-wave plate1303 is a reciprocal rotator, this polarization plane rotation is in the CCW direction as viewed end-on from a fixed reference point at the left side of the diagram. The two sub-signals1312 and1313 thus pass throughbirefringent plate1301 for a second time with horizontal and vertical polarization plane orientations, respectively. Because of these polarization plane orientations during their respective second passes throughbirefringent plate1301, sub-signal1312 propagates throughplate1301 as an o-ray and is not deflected whereas sub-signal1313 passes throughplate1301 as an e-ray and is deflected. This deflection of sub-signal1313 during its second pass throughelement1301 is identical to the deflection of sub-signal1312 during its second passage throughelement1306. Because of these switches in character with respect to the second passage through the twobirefringent plates1301 and1306, from e-ray to o-ray for sub-signal1312 and from o-ray to e-ray for sub-signal1313, their relative deflections in the birefringent walk-offelements1301 and1306 are canceled and these two sub-signals thus recombine so as to enter theoutput fiber1311 as a single combined signal. The passage of sub-signals1312 and1313 throughbirefringent plate1306,Faraday rotator1304, half-wave plate1303, andbirefringent plate1301 in this order comprises a second stage of optical isolation.
• The reverse propagation of signal light through double-[0102]stage isolator1300 will now be described with reference to FIG. 13B. Unpolarized light entering the double-stage polarization independentoptical isolator1300 via theoutput fiber1311 is first split into polarized sub-signals,1322 and1323, by the birefringent walk-off plate1301. The principal optical axes of birefringent walk-off plate1301 are aligned such that sub-signal1322 propagates therethrough as a horizontally polarized o-ray and is not deflected while sub-signal1323 propagates therethrough as a vertically polarized e-ray and is deflected by the well-known birefiingence walk-off effect. After passing throughbirefringent plate1301, bothsub-signals1322 and1323 enter and pass through half-wave plate1303 that reversibly imposes a 45° CW rotation on both their polarization plane orientations about the axis of propagation. After passing through half-wave plate1303, bothsub-signals1322 and1323 then enter and pass throughFaraday rotator1304 that non-reversibly imposes a 45° CCW rotation on both of their polarization plane orientations about the axis of propagation. After passing throughFaraday rotator1304, bothsub-signals1322 and1323 then enter and pass through the secondbirefringent plate1306.
• Upon passing through[0103]birefiingent plate1306, sub-signal1322 is horizontally polarized and thus propagates throughelement1306 as an o-ray that is not deflected. Also upon passing throughbirefringent plate1306, sub-signal1323 is vertically polarized and thus propagates throughelement1306 as an e-ray that is deflected for a second time by an amount and in a direction similar to the amount and direction of its deflection upon passage throughelement1301. The two sub-signals1322 and1323 therefore do not recombine after passage throughbirefringent plate1306 and continue on towardslens1307 along separate paths.
• [0104]Lens1307 intercepts both reverse propagating sub-signals1322 and1323 and collimates and directs them ontomirror1308 which reflects them back to and throughlens1307 for a second time.Lens1307 then directs sub-signals1322 and1323 towardsbirefringent plate1306 for a second time. Upon enteringbirefringent plate1306 for the second time, sub-signal1322 and sub-signal1323 maintain the same horizontal polarization plane orientation and vertical polarization plane orientation, respectively, with which they emerged-after their first passage throughplate1306. Therefore, sub-signal1322 once again propagates throughbirefringent plate1306 as an o-ray that is not deflected and sub-signal1323 once again propagates throughbirefringent plate1306 as an e-ray that is deflected. The deflection of sub-signal1323 upon its second passage throughplate1306 is equal and opposite to its deflection during its first passage throughplate1306. As illustrated in FIG. 13B, this causes the physical separation between the two sub-signals1322 and1323 to increase during their second passage throughbirefringent plate1306. After leavingplate1306, sub-signals1322 and1323 then both pass throughFaraday rotator1304 that non-reversibly imposes a 45° CCW rotation on both their polarization plane orientations about the axis of propagation.
• After passing through[0105]Faraday rotator1304, sub-signals1322 and1323 then both pass through half-wave plate1302 that reversibly imposes 45° CCW rotation on both their polarization plane orientations about the axis of propagation. Because half-wave plate1302 is a reciprocal rotator, this polarization plane rotation is in the CW direction as viewed end-on from a fixed reference point at the left side of the diagram. After passing through half-wave plate1302, sub-signals1322 and1323 then enter and pass through birefringent walk-off plate1301 with polarization plane orientations that are horizontal and vertical, respectively. Therefore, sub-signal1322 once again propagates throughbirefringent plate1301 as an o-ray that is not deflected and sub-signal1323 once again propagates throughbirefringent plate1301 as an e-ray that is deflected. The deflection of sub-signal1323 during its second passage throughbirefringent plate1301 is identical to its deflection during its second passage throughbirefringent plate1306. As illustrated in FIG. 13B, this causes the physical separation between the two sub-signals1322 and1323 to increase during their second passage throughbirefringent plate1301 such that both fail to intercept the face ofinput fiber1310 by a wide margin. In this fashion, thedevice1300 operates as a double-stage polarization independent optical isolator.
• [0106]Elements1301,1302,1303,1304, and1306, in combination as shown in FIGS. 13A and 13B are referred to collectively as a double stage polarization independent optical element.
• The polarization rotation compensator[0107]1100 (FIG. 11) may also be incorporated into the double-stage polarization independentoptical isolator1300 of the present invention to give improved broadband isolation performance. Thus, a fourth embodiment of the present invention, which comprises a broadband double-stage polarization independentoptical isolator1400, is provided and is as shown in FIG. 14. The double-stage polarization independent optical isolator1400 (FIG. 14) is identical to the isolator1300 (FIGS. 13A, 13B) except for the addition of polarization rotation compensator1100 (shown aselement1401 in two places in FIG. 14). In all other aspects, the operation ofisolator1400 is similar to that already described for isolator1300 and is not repeated in detail here.
• [0108]Elements1301,1302,1303,1304,1306, and1401, in combination as shown in FIG. 14 are referred to collectively as a double stage broadband polarization independent polarization-mode-dispersion-free optical element.
• The double stage polarization independent optical isolators,[0109]isolator1300 andisolator1400, have the advantage relative to the single stage isolators,isolator800 andisolator1200, of freedom from Polarization Mode Dispersion (PMD). Polarization Mode Dispersion is the phenomenon by which differently polarized components, or sub-signals, comprising an optical signal propagate with different speeds. This duality of speeds can cause unacceptable broadening of the digital pulses comprising a signal. Such pulse broadening may, in turn, cause digital reception errors at the receiver end of an optical communications system. The maximum acceptable level of PMD broadening, in time units, between transmitter and receiver is generally taken as equivalent to one-tenth the width of a digital light pulse. For example, for data transmission rates corresponding to the OC-192 standard, where nominal pulse widths are on the order of 100 pico-seconds, the maximum acceptable level of pulse broadening is on the order of 10 pico-seconds. This translates into a total maximum optical path length difference between sub-signal components of 3 mm between transmitter and receiver, a distance that may encompass many hundreds of kilometers. Different data transmission rates will correspond to different maximum optical path length differences, accordingly.
• The maximum acceptable PMD-induced optical path length difference is the cumulative result of all PMD effects in all the optical elements through which a signal propagates, including fiber and non-fiber optical components. Although the PMD broadening of optical fiber increases as the square root of fiber length, the PMD broadening caused by birefringent components is linearly related to the cumulative optical path difference of all such components. Thus, if any PMD effects are produced by non-fiber optical components, either the number of such components, the PMD effect per component, or the data transmission rate must be limited so as to derive acceptable data transmission performance.[0110]
• The most suitable option is for all components to be PMD-free. However, as may readily be seen by inspection of FIG. 8A or FIG. 12, the two sub-signal components,[0111]802 and803, may traverse different physical and optical path lengths within the first and second single-stage polarization independent optical isolators,isolator800 andisolator1200, respectively. This difference in optical path lengths may cause PMD problems in some situations. However, it is readily seen by inspection of FIG. 13A and FIG. 14 that the two components sub-signals,1312 and1313, in the double stage isolators of the current invention,1300 and1400, traverse identical physical and optical path lengths. Thus PMD effects are eliminated inisolator1300 andisolator1400.
• The reflection-type polarization independent optical isolators as already described in the various embodiments of the present invention all incorporate mirrors, such as[0112]mirror807 of the first embodiment (FIGS. 8A, 8B) ormirror1308 of the third embodiment (FIGS. 13A, 13B). These mirrors all have reflectivity of preferably 100% so as to fold the signal light rays back, without power loss, for a second passage through the various optical elements comprising the respective isolator and back to the fiber-holding ferrile. In the case of integrated optical passive components, it is desirable to remove a small portion of the signal ray energy for monitoring purposes. This may be easily accomplished by using, for instance, a mirror with less than 100% reflectively and non-zero transmissivity. Thus, for instance, if the 100%reflective mirror807 of the first embodiment of the present invention,isolator800, is replaced by a mirror with, for instance, 95% reflectivity and 5% transmissivity, then additional signal monitoring components can be placed on the side of the partially reflective mirror opposite to the isolator. In this way, the isolator is modified so as to become an integrated isolator/passive component set.
• In view of the discussion in the above paragraph, there is provided a fifth embodiment of the present invention that comprises an integrated single-stage polarization independent isolator and monitor as illustrated in FIG. 15. The integrated isolator/[0113]monitor1500 of the present invention comprises all of the components of the double-stage isolator1300 (FIGS. 13A, 13B) except for themirror1308 that is replaced by a new partiallyreflective mirror1501 having approximately 95% reflectivity and approximately 5% transmissivity; lens806 (FIGS. 8A, 8B,9, and12) or lens1307 (FIGS. 13A, 13B and14) is also replaced byfront lens1506 in FIG. 15. In addition to these components, isolator/monitor1500 also comprises a second lens orlens assembly1502 positioned to the side ofmirror1501 opposite the isolator components together with an anti-reflection (AR)coated window1503, a photo-detector1504 and alight absorber1505 disposed to the side oflens1502 opposite the isolator components. Taken together with the partiallyreflective mirror1501, the components of the isolator/monitor1500 that are identical to those ofisolator1300 comprise the isolator portion of isolator/monitor1500. The remaining components of isolator/monitor1500 comprise the monitor portion of the device. The operation of the isolator portion of isolator/monitor1500 is identical to that ofisolator1300 except that a portion of the signal light is lost, upon reflection at the partiallyreflective mirror1501, to the monitor portion ofdevice1500 by transmission of that portion of the signal through the partiallyreflective mirror1501. The operation of the isolator/monitor1500 as an optical isolator will therefore not be re-discussed in detail here.
• [0114]Lenses806,1307,1502 and1506 can be either curved surface lenses or graded index lenses, which are well-known in the art.Lenses806,1307,1502 and/or1506 could be replaced by any type of lens without departing from the scope of the present invention.
• In addition to its polarization independent optical isolator function, the isolator/[0115]monitor1500 also provides a signal monitoring function. This signal monitoring function of isolator/monitor1500 is now discussed with reference to FIG. 15. The monitoring function uses a portion of the energy of the forward propagatinglight signal1512 as shown in FIG. 15. The partiallyreflective mirror1501 separates signal1512 into two sub-signals, a first sub-signal1512A which is reflected off partiallyreflective mirror1501 and back into the second stage of the isolator portion of isolator/monitor1500, and a second sub-signal1512B which is transmitted through partiallyreflective mirror1501 to the signal monitoring stage of isolator/monitor1500. In FIG. 15 and the discussion pertaining thereto, the partiallyreflective mirror1501 is shown as having approximately 95% reflectivity and approximately 5% transmissivity. Thus, in this discussion of the operation of isolator/monitor1500, the sub-signals1512A and1512B contain approximately 95% and approximately 5%, respectively, of the light signal power originally contained insignal1512 prior to encountering partiallyreflective mirror1501. One of ordinary skill in the art will readily recognize, however, that, without departing from the spirit or scope of the present invention, the partially reflective mirror15.01 may have values of reflectivity and transmissivity different from 95% and 5%, respectively, depending upon the needs of the user. The effects of the device upon sub-signal1512A will not be discussed further, having already been discussed with reference to the operation of the double-stage optical isolator1300 (FIGS. 13A, 13B).
• The sub-signal[0116]1512B is intercepted by therear lens1502 and directed towards the anti-reflection-coatedwindow1503. Preferably, the coating on this window is designed such that, for all possible wavelengths of light comprising thesignal1512B, at most 5% of the light impinging uponwindow1503 is reflected back, in the reverse direction, along the paths of sub-signal1512B andsignal1512. With this type of anti-reflection coating and the assumed transmissivity of partiallyreflective mirror1501, then, at most, a proportion of thesignal1512 equivalent to (0.05)³or 0.0125% can be reflected back into the light transmission system in the reverse direction. This level of back-reflection, 0.0125%, is equivalent to 39 dB of isolation, which is adequate performance for most applications. The isolator/monitor.1500 also contains alight absorber1505 which is adjacent to thewindow1503 and which absorbs any remaining stray light so as to prevent that stray light from returning to the light transmission system as a spurious signal. The portion of the sub-signal1512B that is transmitted through anti-reflection-coatedwindow1503 then impinges upon photo-detector1504. The well-known operation of the photo-detector, which may be any one of a number of well-known types, is to convert the light energy of sub-signal1512B into an electronic signal which may be used for monitoring purposes. In this way, the monitoring function of the isolator/monitor1500 is accomplished.
• An optical isolator can be one of a set of optical passive components within an optical amplifier system. FIG. 16 is a basic block diagram of an optical fiber amplifier system showing the assembly of conventional optical passive components within an Er-doped fiber optical amplifier system (EDFA)[0117]1600. In FIG. 16, anoptional input tap1602 optionally directs asmall proportion1601A (i.e., 5%) of theinput signal light1601 alongoptical pathway1603 to first photo-detector1604. Taken together, theinput tap1602,pathway1603 and photo-detector1604 comprise theinput monitor component1605 of the amplifier system. The remainingportion1601B of the input signal light, which comprises the majority of the input signal, is directed to firstoptical isolator1606, which only permits signal light transmission in the forward direction. After passing throughisolator1606, thesignal light1601B is directed to the first Wavelength Division Multiplexer (WDM)1607. First laser light1608 fromco-pump laser1609 is also directed toWDM1607. The function ofWDM1607 is to direct the pathways and directions of three separate lights—signal light1601B,first laser light1608, and residualsecond laser light1611—according to their respective wavelengths. The origin ofsecond laser light1611, which passes throughfiber1610 in the reverse direction, is discussed further below. InWDM1607, signal light1601B is passed together withlaser light1608 into the Er-dopedfiber1610 in the forward direction. A further function ofWDM1607 is to direct residualsecond laser light1611 alongpathway1616 in the reverse direction.
• The[0118]signal light1601B and thelaser light1608 propagate together in the forward direction through Er-dopedfiber1610. Also propagating through Er-dopedfiber1610 in the reverse direction is laser light1611 that originates fromcounter-pump laser1612. The wavelength oflaser light1608 is ordinarily 980 nm whereas the wavelength oflaser light1611 is ordinarily 1480 nm. The wavelength of thesignal light1601B is always greater than the wavelength of either of thelaser lights1608 or1611.
• As shown in FIG. 16, signal light[0119]1601B becomes amplified by the optical gain of the Er-doped fiber under the condition of laser light excitation and is therein transformed into amplified signal1601C. Amplified signal light1601C andresidual laser light1608 are passed from the Er-dopedfiber1610 to thesecond WDM1613. Thesecond WDM1613 separates the pathways of the threelights1601C,1608, and1611 according to their respective wavelengths. Amplified signal1601C is directed in the forward direction alongpathway1614 whereasresidual laser light1608 is directed in the forward direction alongpath1615. Moreover, laser light1611 fromcounter-pump laser1612 entersWDM1613 in the reverse direction viapath1615, and therefore a further function ofWDM1613 is to direct light1611 into Er-dopedfiber1610 in the reverse direction.
• After leaving[0120]second WDM1613 and propagating alongpath1614 in the forward direction, amplified signal1601C passes through an optional gain-flatteningfilter1619 and thence to secondoptical isolator1620.Optical isolator1620 only permits signal light transmission in the forward direction away from the Er-dopedfiber1610 so that spurious back-reflected signals do not become amplified. After passing through thesecond isolator1620, amplified signal1601C passes through anoptional output tap1621 that optionally directs asmall proportion1601D (i.e., 1%) of the amplified signal alongpathway1622 to second photo-detector1623. Taken together, theoutput tap1621,pathway1622 and photo-detector1623 comprise theoutput monitor component1624 of the amplifier system. The remainder of the amplified signal1601E exits the amplifier system after passing throughoptional output tap1621.
• Also shown in FIG. 16 are a first bandpass filter or isolator[0121]1617 disposed betweenco-pump laser1609 andpathway1616 and a second bandpass filter orisolator1618 disposed betweencounter-pump laser1612 andpathway1615. The functions ofelements1617 and1618 are to prohibit the entry of reverse propagatingsecond laser light1611 intoco-pump laser1609 and the entry of forward propagatingfirst laser light1608 intocounter-pump laser1612, respectively. The use of bandpass filter or isolator1617 and bandpass filter orisolator1618 is necessitated by the fact that eitherlaser1609 or1612 could be severely damaged by entry of laser light of the other kind.
• The Er-doped fiber optical amplifier system (EDFA)[0122]1600 as described herein above is known in the art.
• Further shown in FIG. 16 are the functional correspondences of[0123]various ports #1 through #8 of the sixth embodiment of the present invention (described herein below) to the various locations within the generalizedoptical amplifier system1600. When the sixth embodiment of the present invention is used,elements1602,1604,1606,1607, and1617 are replaced by a first pass through the sixth embodiment, andelements1613,1618,1620,1621, and1623, are replaced by a second pass through the sixth embodiment, in theEDFA1600 of FIG. 16. During the first pass through the sixth embodiment, 5% ofsignal1601 followspathway1603 throughPort #8. During the second pass through the sixth embodiment, 0.2% ofsignal1601 followspathway1622 throughPort #6. The operation and advantages of the sixth embodiment of the present invention, which comprises a set of integrated optical passive components with single stage isolation, are discussed in greater detail below.
• Finally, also shown in FIG. 16 is an optional comparison and control[0124]logic system1625 which represents a set of electronic or computer systems together with decision-making software or firmware which monitors the electronic outputs of both optional photo-detectors1604 and1623, and controls the outputs of the twolasers1609 and1612 accordingly so as to obtain optimal amplification performance. Thesystem1625 is only shown to illustrate the context of the present invention and is not a component of the present invention or necessarily of optical amplifiers in general.
• The sixth embodiment of the present invention, referred to as a polarization independent integrated single stage isolator, monitor, and amplifier[0125]1700 (referred to as isolator/monitor/amplifier1700) and which comprises a set of integrated optical passive components with single stage isolation, is illustrated in FIG. 17. The integrated set of optical passive components (or broadband single-stage reflection isolator)1700A, which is shown to the left side of FIG. 17, is physically and functionally identical to the broadband single-stage polarization independentoptical isolator1200 except that (a) themirror807 ofisolator1200 is replaced by a partiallyreflective mirror1701 and (b) there are twoinput fibers801A,801B (collectively shown and referred to as801 in FIG. 17) and twooutput fibers810A,810B (collectively shown and referred to as810 in FIG. 17), as explained herein below. A preferred graph of the reflectivity versus wavelength properties of partiallyreflective mirror1701 is shown in FIG. 18. Except for this mirror substitution, the identity and arrangement of components of the isolator half ofintegrated components1700A are identical to those ofisolator1200. These common components include theferrule815, theinput fibers801, theoutput fibers810, thebirefringent plate804, the λ/2plate805 providing reversible 45° CCW polarization plane rotation, theFaraday rotation element808 providing non-reversible 45° CCW polarization plane rotation, themagnets809, thelens806, and thepolarization rotation compensator1201. Together with the partiallyreflective mirror1701, these components comprise a single-stage broadbandoptical isolator1700A whose operation is identical to that ofisolator1200 except: (a) a portion of the input signal light is transferred to the second half ofintegrated components1700B by transmission through thepartial reflector1701, (b) two separate laser lights are transmitted into and out of the isolator/monitor/amplifier1700 via transmission through thepartial reflector1701, (c) the twoinput fibers801 and twooutput fibers810 are used in pairs such that one input fiber and one output fiber correspond to a single signal pass through theisolator1700, and (d) forward propagating signals make two consecutive passes through theisolator1700 first using one and then the other pair of fibers.
• As discussed herein above,[0126]elements804,805,808, and1201, in combination as shown in FIGS. 12 and 17 are referred to collectively as a single stage broadband polarization independent optical element.
• [0127]Additional components1700B, referred to as monitor/amplifier components, ofdevice1700 which are not found inisolator1200 include arear lens1702, a first rear λ/2plate1703 and a second rear λ/2plate1704, arear Faraday rotator1705 and associatedmagnets1706, a rear birefringent walk-off plate1707, a rear fourfiber ferrule1708 and four rear fibers orports1715,1716,1717, and1718.
• Although the second rear λ/2[0128]plate1704 cannot be shown separately from therear Faraday rotator1705 in the side view which is FIG. 17, their respective locations are adequately represented in the cross sectional view which is shown in FIG. 19. Finally, for clarity, the sets ofmagnets809 and1706 are not shown in FIG. 19 or in subsequent figures.
• Collectively,[0129]elements1703,1704,1705, and1707 in the configuration shown in FIGS. 17 and 19 are referred to as a monitor/amplifieroptical element1700B.
• In the isolator/monitor/[0130]amplifier1700, which comprises a set of integrated optical passive components (FIG. 17), the partially reflective mirror preferably has a variation of reflectivity with wavelength as shown in FIG. 18, with preferably 95% reflectivity for signal wavelengths and preferably 0% for the shorter pump laser wavelengths. Themirror1701 must have good reflectivity for telecommunications signals of 1550 nanometers.
• Also, in the isolator/monitor/[0131]amplifier1700, the four front fibers or ports are tightly secured within front four-fiber ferrule815 and the four rear fibers or ports are tightly secured within rear four-fiber ferrule1708 and arranged as shown in FIG. 19. The cross-sectional views of FIG. 19 are both drawn as viewed from the left side of the device shown in FIG. 17. The end face ofrear ferrule1708 is polished flat together with the fibers contained therein. The fibers or ports are so arranged (FIG. 19) such that Fiber (Port) #51715, #61716, #7, 1717, and #81718 in therear ferrule1708 are directly opposite, respectively, to Fiber (Port) #1801A, #2801B, #3810B, and #4810A in thefront ferrule815.
• FIG. 19 illustrates the numbering scheme of the fibers (or ports) of the sixth embodiment of the present invention, shown in FIG. 17 (which is a side view of the sixth embodiment of the present invention), and, further, shows the optical elements mounted in front of or adjacent to each port to rotate the light entering or exiting each port.[0132]
• As shown in FIG. 19,[0133]fibers801A,801B,810A and810B are located in thefront ferrule815 of the isolator/monitor/amplifier1700.Fibers801A and810A (FIG. 19) comprise the input an d output, respectively, for the first pass of a signal through theisolator portion1700A of isolator/monitor/amplifier1700. Likewise,fibers801B and810B (FIG. 19) comprise the input and output, respectively, for the second pass of a signal through theisolator portion1700A of isolator/monitor/amplifier1700. Disposed to the rear of—that is, in the direction ofpartial reflector1701—and immediately adjacent to thefront ferrule815 andfibers801A,801B,810A and810B is the front birefringent walk-off plate804, which, for clarity, is not shown in FIG. 19. As shown in FIG. 19, the front λ/2plate805 and thefront Faraday rotator804 are disposed immediately adjacent to and to the rear ofbirefiingent plate804 such that they intercept the optical pathways of the pair of input fibers,801A and801B and the pair of output fibers,810A and810B, respectively
• As further shown in FIG. 19,[0134]fibers1715,1716,1717 and1718 are located in therear ferrule1708 of the isolator/monitor/amplifier1700. Disposed to the front of—that is, in the direction ofpartial reflector1701—and immediately adjacent to therear ferrule1708 andfibers1715,1716,1717 and1718 is the rear birefringent walk-off plate1707, which, for clarity, is not shown in FIG. 19. Also present, in isolator/monitor/amplifier1700, are first and second rear λ/2plates1703 and1704 which are both oriented so as to provide reversible 45° CW and CCW, respectively, polarization plane rotation of light signals transmitted therethrough. Also present, in isolator/monitor/amplifier1700, is arear Faraday rotator1705 andmagnets1706, which are oriented so as to provide non-reversible 45° CW polarization plane rotation of light signals transmitted therethrough. The first and second rear λ/2plates1703 and1704 and therear Faraday rotator1705 are disposed directly adjacent to and to the front of the rear birefringent walk-off plate1707 such that the first rear λ/2plate1703 intercepts the optical pathway tofibers1715 and1716, the second rear λ/2plate1704 intercepts the optical pathway tofiber1718 andFaraday rotator plate1705 intercepts the optical pathway tofiber1717, as shown in FIG. 19.
• Each[0135]fiber801A,801B,810A,810B,1715,1716,1717, and1718 shown in FIG. 9 is a conventional fiber of uniform size having an approximately 8 micrometer core surrounded by approximately 120 micrometer cladding. When a signal light is directed by the present invention to not enter a fiber by the walk-off effect, the signal light is typically absorbed by the cladding. Therefore, optical isolators are not perfect isolators, but single-stage isolators typically provide 35 db of isolation, which is a critical point for analog communications, as is well known in the art.
• With four fibers in each ferrule, as shown in FIG. 19 and in the prior art, the alignment (both lateral and rotational) of each fiber is easy, controlled, and reproducible.[0136]
• As will be apparent from the collective descriptions of the subsequent figures,[0137]port #1 andport #4 form an input/output pair of ports, andport #2 andport #3 form another input/output pair of ports.
• The isolator/monitor/[0138]amplifier1700 of the second embodiment of the present invention, shown in FIG. 17, is an integrated system of optical passive components. The isolator/monitor/amplifier1700 can replace, with improved function and performance, the following optical passive components in theoptical system1600 shown in FIG. 16:input tap1602,isolator1606,wavelength division multiplexer1607,wavelength division multiplexer1613,isolator1620, andoutput tap1621. In replacing the foregoing optical components, two passes of signal light through the isolator/monitor/amplifier1700 of the present invention must be made to duplicate the function of the replaced optical components. The first pass through isolator/monitor/amplifier1700 corresponds to the function provided byinput tap1602,isolator1606, andwavelength division multiplexer1607 shown in FIG. 16. After the first pass through the isolator/monitor/amplifier1700, the output signal light then travels through Er-dopedfiber1610 back to isolator/monitor/amplifier1700 for a second pass therethrough. The second pass through isolator/monitor/amplifier1700 corresponds to the function provided bywavelength division multiplexer1613,isolator1620, andoutput tap1621 shown in FIG. 16.
• FIGS. 20A and 20B show a schematic overview of the operation of isolator/monitor/[0139]amplifier1700 of the present invention. FIGS.21-24 are more detailed views of the isolator/monitor/amplffier1700 of the present invention.
• FIG. 20A is an overview of the first pass through isolator/monitor/[0140]amplifier1700. FIG. 21 and FIG. 22 illustrate optical pathways comprising the first pass through the isolator/monitor/amplifier1700 in the forward and reverse directions, respectively, shown in FIG. 20A.
• Likewise, FIG. 20B is an overview of the second pass through isolator/monitor/[0141]amplifier1700. FIG. 23 illustrates optical pathways comprising the second pass through the isolator/monitor/amplifier1700 in the allowed direction, shown in FIG. 20B, and FIG. 24 illustrates the pathway of residual pump laser light and amplified input light in a second pass through the isolator/monitor/amplifier1700, which is not directed to any port in the isolator/monitor/amplifier1700.
• The following explanation of the isolator/monitor/[0142]amplifier1700 of the present invention shown in FIG. 17 is made with reference to FIGS. 20A, 20B. For clarity in FIGS. 20A and 20B,lenses806 and1702, andmagnets809 and1706 are not shown, and optical elements adjacent to each port (discussed herein above with reference to FIGS. 17 and 19, and herein below with reference to FIGS.21-24) are shown collectively as follows. In the following explanation with reference to FIGS. 20A and 20B, and in the explanation with reference to FIGS.21-24, as shown in the foregoing figures, theoptical elements850 adjacent toPort #1 include birefringent walk-off plate804, λ/2plate805, andpolarization rotation compensator1201; theoptical elements852 adjacent toPort #2 include birefringent walk offplate804, λ/2plate805, andpolarization rotation compensator1201; theoptical elements854 adjacent toPort #3 include birefringent walk-off plate804 andFaraday rotator808; theoptical elements856 adjacent toPort #4 include birefringent walk-off plate804 andFaraday rotator808; theoptical elements858 adjacent toPort # 5 include λ/2plate1703, and birefringent walk-off plate1707; theoptical elements860 adjacent toPort #6 include λ/2plate1703, and birefringent walk-off plate1707; theoptical elements862 adjacent toPort # 7 includereflector1750,Faraday rotator1705, and birefringent walk-off plate1707; and theoptical elements864 adjacent toPort # 8 include anti-reflective coated λ/2plate1704, and birefringent walk-off plate1707.
• As illustrated in FIGS. 20A and 20B and[0143]21-24, a signal light makes two passes through theoptical isolator section1700A of isolator/monitor/amplifier1700. These first and second passes correspond to passes throughisolator1606 and1620 of FIG. 16. Because of these and other exact correspondences, the same reference numerals introduced in FIG. 16 are used for the various signal and laser lights illustrated in FIGS. 20A and 20B and21-24. In addition, when reference is made to the port number, the fiber corresponding to that port number in FIG. 17 is also supplied (i.e.,port #1801A).
• In FIG. 20A,[0144]input signal light1601 enters for a first pass through isolator/monitor/amplifier1700 throughPort #1,fiber801A (FIG. 19). This signal passes through the optical elements adjacent to Port #1 (including birefringent walk-off plate804, λ/2plate805, andpolarization rotation compensator1201, in this order) and thence through thelens806 and thence to thepartial reflector1701. A small proportion of this signal light is transmitted through partial reflector1701 (shown in FIG. 20A assignal1601A) and thence torear lens1702 which directs it through the optical elements adjacent toPort #81718 (including 1/2plate1704, and birefringent walk-off plate1707, in this order) and ultimately intoPort #81718 (FIG. 19). Thelight signal1601A enteringPort #8 is delivered to a photo-detector (not shown) for input monitoring and thus this corresponds to signal1601A (FIG. 16). A much larger proportion ofsignal light1601 is reflected off ofpartial reflector1701 and back tofront lens806 which directs it through the optical elements adjacent toPort #4810A (includingFaraday rotator808, and birefringent walk-off plate804, in this order) and ultimately intoPort #4810A (FIG. 19). The signal light that exits isolator/monitor/amplifier1700 throughPort #4 is directed to an Er-doped fiber1610 (FIG. 16) and thus this corresponds to signal1601B.
• In addition to signal lights traveling through isolator/monitor/[0145]amplifier1700 of the present invention, there are laser lights traveling therethrough, as well. The additional pathways of the signal lights, includingsignal1601B, followed through isolator/monitor/amplifier1700 after entering the Er-dopedfiber1610 are further discussed with reference to FIG. 20B, after a discussion of the laser light entering isolator/monitor/amplifier1700 throughPort #51715 andPort #71717 shown in FIG. 20A and FIG. 20B, respectively.
• As shown in FIG. 20A, a first laser[0146]light pump beam1608 is input to isolator/monitor/amplifier1700 throughPort #51715 (FIG. 19). Isolator/monitor/amplifier1700 is configured such that laser light1608 passes through the optical elements directly adjacent toPort #51715 (including birefringent walk-off plate1707 and λ/2plate1703, in this order) and thence throughrear lens1702 and thence throughpartial reflector1701 without being reflected and thence into the same optical pathway assignal1601B which ultimately leads toPort #4810A (FIG. 19). Thus, bothfirst laser light1608 and signal1601B exit the first-pass pathway of isolator/monitor/amplifier1700 throughPort #4810A to be delivered to Er-doped fiber1610 (FIG. 16).
• Furthermore, residual[0147]second laser light1611, produced bycounter-pump laser1612 as shown in FIG. 16, travels in the Er-fiber1610 (FIG. 16) and may enter the isolator/monitor/amplifier1700 in the reverse direction—opposite to thesignal1601 normal propagation direction—throughPort #4810A. Thissecond laser light1611, which is discussed in further detail herein below with reference to FIG. 20B, follows a path through isolator/monitor/amplifier1700 (FIG. 20B) exactly opposite to that offirst laser light1608, enters isolator/monitor/amplifier1700 throughPort #71717, passes throughreflector1701 without being reflected (since the wavelength of thissecond laser light1611 is below 1490 nanometers) enters the Er-dopedfiber1610 throughPort #2801B, returns to isolator/monitor/amplifier1700 throughPort #4810A, and arrives atPort #51715 (FIG. 19).Second laser light1611 cannot enterPort #5 because the optics of the front birefringent walk-off plate804, theFaraday rotator808, the λ/2plate1703, and the birefringent walk-off plate1707 of the isolator/monitor/amplifier1700, which are along the pathway followed bysignal1611 as shown in FIG. 20B, comprise an optical isolator similar to that shown in FIG. 8B.
• Returning now to the foregoing discussion regarding signal lights, upon exit through[0148]Port #4810A from the isolator/monitor/amplifier1700 of the present invention, signal light1601B is amplified and transmitted through an Er-doped fiber (not shown in FIG. 20A or FIG. 20B) toPort #2801B of isolator/monitor/amplifier1700 as signal light1601C.
• After passage through Er-doped[0149]fiber1610, the amplified signal1601C (FIG. 16), along with residualfirst laser light1608, is re-delivered to isolator/monitor/amplifier1700 for a second pass therethrough (FIG. 20B) via secondinput Port #2801B (FIG. 19). The signal1601C passes through the optical elements adjacent to Port #2 (including birefringent walk offplate804, λ/2plate805, andpolarization rotation compensator1201, in this order) and thence through thelens806 and thence to thepartial reflector1701. Most of the energy of signal1601C is reflected offpartial reflector1701, back throughfront lens806, and thence through the optical elements (includingFaraday rotator808, and birefringent walk-off plate804, in this order) adjacent toPort #3810B (FIG. 19) to finally exit throughPort #3. This exiting signal therefore corresponds to signal1601E of FIG. 16. However, a small proportion of signal light1601C and all of the residualfirst laser light1608 is transmitted throughpartial reflector1701 and thence torear lens1702, which directs it through the optical elements (includingreflector1750,Faraday rotator1705, and birefringent walk-off plate1707, in this order) adjacent toPort #71717 (FIG. 19). This small proportion of the signal light1601C which is transmitted throughpartial reflector1701 is thesignal light1601D shown in FIG. 20B.
• The optical components adjacent to[0150]Port #7 include a partial reflector orreflective coating1750 that permits most of the energy ofsignal light1601D and all of the energy of thefirst laser light1608 to pass through towardsPort #7 and that also reflects a small proportion of the signal light away fromPort #7. Most of thesignal light1601D orfirst laser light1608 that impinges upon thispartial reflector1750 is transmitted theiethrough towardsPort #7. The light transmitted towardsPort #7 cannot actually enterPort #7, however, because the optical components betweenPort #2 and Port #7 (including the above-mentionedelements804,805,1201,1705, and1707) comprise an optical isolator that only permits reverse-direction passage of thesecond laser light1611. The small proportion of signal light that is reflected back from thepartial reflector1750 is directed back throughrear lens1702 for a second time, thence to partial reflector1701 (FIG. 20B), thence torear lens1702 for a third time and finally toPort #61716 (FIG. 19). Signal light enteringPort #6 is directed to a photo-detector (not shown) for output monitoring.
• Finally, the second laser light pump beani[0151]1611 is input (FIG. 20B) to isolator/monitor/amplifier1700 throughPort #71717 (FIG. 19). Isolator/monitor/amplifier1700 is configured such that second laser light1611 passes through the optical elements directly adjacent toPort #71717 in the reverse direction, thence throughrear lens1702 and thence throughpartial reflector1701 and thence into the same optical pathway as signal1601C in the reverse direction. In the reverse direction, this pathway ultimately leads (FIG. 20B) toPort #2, from which thesecond laser light1611 is directed into the Er-dopedfiber1610 in the reverse propagation direction. The optical components in the pathway (FIG. 20B) betweenPort #71717 andPort #2801B comprise an optical isolator that only permits passage of light in the reverse direction.
• Therefore, the amplifier components of the isolator/monitor/[0152]amplifier1700 comprise additional single-stage optical isolators for the counter-pump and theco-pump laser lights1611 and1608, respectively. The additional single-stage optical isolators prevent forward-propagating co-pump laser light1608 from travelling all of the way to thecounter-pump laser1612 and prevent the reverse-propagating counter-pump laser light1611 from travelling all of the way to theco-pump laser1609. The isolator/monitor/amplifier1700 further comprises optical combining and re-separating means (the partially-reflective mirror1701). Thepartial reflector1701 injects the co-pump and counter-pump laser lights into the Er-doped fiber, along with the signal lights, and then removes the co-pump and counter-pump laser lights from the signal lights after travelling through the Er-doped fiber. Thus, referring to FIGS. 20A and 20B, the optics betweenPort #5 andPort #4 make up an optical isolator which only lets light propagate in the forward direction (defined relative to the signal) and the optics betweenPort #7 andPort #2 make up another optical isolator which only lets light propagate in the reverse direction. The construction of these optical isolators for the lasers are similar to those for the signal, except that instead of going through the lens, mirror, back to the lens, the light goes in sequence through a collimating lens, through a mirror, and then through a second focusing lens.
• The passage of both signal and laser light through isolator/monitor/[0153]amplifier1700 of the present invention is presented in further detail, with references to FIGS.21-24. Since the operation of the optical elements adjacent to each port were previously discussed, no further explanation is provided.
• FIG. 21 and FIG. 22 illustrate optical pathways comprising the first pass through the isolator/monitor/[0154]amplifier1700 in the forward and reverse directions, respectively, shown in FIG. 20A.
• More particularly, FIG. 21 shows the operation of the isolator/monitor/[0155]amplifier1700 during a first pass of signal light, in a forward direction.
• As shown in FIG. 21, signal light enters isolator/monitor/[0156]amplifier1700 through Port #1 (input fiber801A), included in front four-fiber ferrule815. The input signal light is then divided into an o-ray and an e-ray by the optical elements adjacent toPort #1. The o-ray and e-ray components of the input signal light are then collimated ontoreflector1701 bylens806. The majority of the power included in the input signal light is reflected byreflector1701. The remaining power from the input signal light not reflected byreflector1701 is re-focused bylens1702 onto the optical elements adjacent toPort #8, which recombine the o-ray and e-ray components thereof, and present the recombined input signal light toPort #8 for monitoring.
• Concurrently, input laser light[0157]1608 from a co-pump laser (not shown in FIG. 21) is input throughPort #5 included in rear four-fiber ferrule1708. The input laser light is divided into an o-ray component and an e-ray component, and is collimated ontoreflector1701 bylens1702.Reflector1701 allows all of the input laser light to pass through thereflector1701, and, hence, the input laser light joins with the input signal light to be re-focused bylens806 onto the optical components adjacent tooutput Port #4. The optical elements adjacent tooutput Port #4 recombine the o-ray component of the input signal light with the e-ray component of the input signal light, and, further, recombine the o-ray component of the input laser light with the e-ray component of the input laser light, and present the combined light tooutput Port #4, which exits the isolator/monitor/amplifier1700 throughfiber810A. The output signal light is then transmitted along, for example, an Er-fiber1610 of FIG. 16, which feeds the signal light back to isolator/monitor/amplifier1700 for a second pass therethrough.
• FIG. 22 shows the operation of the isolator/monitor/[0158]amplifier1700 during the re-entry of residual counter-pump light, in a reverse direction. As shown in FIG. 22, light, most likely residual counter pump laser light traveling in the Er-fiber1610, enters the isolator/monitor/amplifier1700 through Port #4 (fiber810A), is divided into an o-ray component and an e-ray component by the optical elements adjacent toPort #4, and is collimated bylens806 ontoreflector1701, through which the residual counter pump laser light passes.Lens1702 then re-focuses the residual counter pump laser light onto the optical elements adjacent toPort #5, and, by their operation explained previously, the residual counter pump laser light is not recombined to exit the isolator/monitor/amplifier1700 throughPort #5. In addition, at most 0.25% of reflected input monitor light is reflected fromPort #8 by or through the optical elements adjacent to Port #8 (in which the light is divided into o-ray and e-ray components), and toreflector1701, through which at most 0.0125% passes through.
• FIG. 23 shows the operation of the isolator/monitor/[0159]amplifier1700 during a second pass of signal light therethrough, in a forward direction. The signal light enters the isolator/monitor/amplifier1700 as amplified signal light, throughPort #2,fiber801B, for the second pass therethrough. In passing through the optical elements adjacent toPort #2, the input signal light is divided into o-ray and e-ray components, as shown. The divided, amplified input signal light is then collimated bylens806 ontoreflector1701,95% of which is reflected back tolens806, to eventually pass through the optical elements adjacent toPort #3, and out of isolator/monitor/amplifier1700 throughoutput fiber810B.
• A portion (5%) of the amplified input signal light passes through[0160]reflector1701, and is re-focused bylens1702 ontoreflector1750, which reflects a small percentage of the portion of the amplified signal light back throughlens1702 toreflector1701. The reflected, amplified signal light is directed bylens1702 andreflector1701 onto the optical elements adjacent toPdrt #6, which recombines the o-ray and e-ray components thereof for output toPort #6 for output monitoring purposes.
• Also shown in FIG. 23 is counter pump light[0161]1611 which is output fromPort #7 through the optical elements adjacent thereto (which divide the counter pump light into o-ray and e-ray components thereof). The divided counter pump light is collimated bylens1702 ontoreflector1701, which allows passage of the divided counter pump light therethrough along the same path (but in opposite direction to) the amplified input signal light input throughPort #2. The optical elements adjacent toPort #2 recombine the divided counter pump light so as to enterPort #2, and thence the Er-dopedfiber1610, in the reverse direction.
• FIG. 24 shows the operation of the isolator/monitor/[0162]amplifier1700 during a second pass of signal light therethrough, in directing residualco-pump laser light1608 and the portion of amplified input signal light which is not reflected byreflector1750 away from enteringPort #7. The path that the residualco-pump laser light1608 and unreflected amplified input signal follows until it impinges uponreflector1701 is explained with reference to FIG. 23 and is not repeated herein. After residualco-pump laser light1608 and 5% of the amplified input signal light passes throughreflector1701, it is re-focused bylens1702 onto the optical components adjacent toPort #7, which do not allow the o-ray and e-ray components thereof to recombine, and thus prevents the residual co-pump laser light and the amplified input signal light from enteringPort #7.
• The integrated set of optical passive components, isolator/monitor/[0163]amplifier1700, is a single-stage isolation device. For many applications, this may not be adequate. Therefore, FIG. 25 illustrates a polarizationindependent isolator1800 including integrated optical components with double stage isolation. In the polarizationindependent isolator1800, double-stage isolation is achieved by using an instance of the second embodiment of the present invention,isolator1400, in cascade arrangement with an instance of the sixth embodiment, isolator/monitor/amplifier1700, of the present invention. The two isolators of FIG. 25 are disposed in a sequential arrangement such that the signal makes a first pass throughisolator1400 followed by a first pass through isolator/monitor/amplifier1700 and then passes to the Er-doped fiber (EDF)1610. This double pass through the twoisolators1400 and1700 in series comprises double-stage isolation at the input to theEDF1610. The operation of either isolator1400 or isolator/monitor/amplifier1700 is as discussed previously, however. After passing through theEDF1610, the signal makes its second pass through isolator/monitor/amplifier17⁰⁰as described previously. After this second pass through isolator/monitor/amplifier1700, the signal is once again directed toisolator1400 to make a second pass therethrough using the second set of fibers in the four-fiber ferrule. These consecutive passes through isolator/monitor/amplifier1700 followed byisolator1400 comprise double-stage isolation at the output end of theEDF1610.
• Although a few preferred embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.[0164]
• The many features and advantages of the invention are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.[0165]

Claims (33)

What is claimed is:

1. An optical isolator including an input fiber carrying input light and an output fiber and comprising:

a birefringent walk-off plate dividing the input light into an ordinary ray sub-light and an extraordinary ray sub-light thereof, and deflecting the extraordinary ray sub-light thereof;

a reciprocally rotating optical element provided adjacent to the birefringent walk-off plate and reciprocally rotating the polarization of each of the sub-lights by 45° in a first direction;

at least one lens collimating the input light and focusing output light;

a mirror reflecting the collimated, divided, reciprocally rotated sub-lights; and

a Faraday rotator and associated magnets provided adjacent to the birefringent walk-off plate and to the reciprocally rotating optical element, non-reciprocally rotating the polarization of the sub-lights by 45° in the first direction, wherein input light received from the input fiber travels in a forward direction from the input fiber to the birefringent walk-off plate, to the reciprocally rotating optical element, to the mirror, back to the Faraday rotator, and back to the birefringent walk-off plate and the sub-lights of the forward-traveling input light are recombined by the birefringent walk-off plate into output light exiting the isolator through the output fiber, whereas input light received from the output fiber travels in a reverse direction opposite to that of the forward direction and the sub-lights thereof are deflected away from each other during each traversal of the birefringent walk-off plate.

2. The optical isolator as recited inclaim 1, wherein the birefringent walk-off plate divides the reverse direction input light into an ordinary ray sub-light and an extraordinary ray sub-light and deflects the extraordinary ray sub-light, the Faraday rotator non-reciprocally rotates the polarization of the reverse direction sub-lights by 45° in the first direction, the at least one lens collimates and directs the reverse direction sub-lights onto the mirror, which reflects them to the reciprocally rotating optical element which rotates their polarizations by 45° in the first direction, and the birefringent plate directs the reverse direction sub-lights away from entering the input fiber.

3. The optical isolator as recited inclaim 1, wherein the at least one lens is disposed between the mirror and the birefringent plate and the distance from the at least one lens to the mirror is the same as the distance from the at least one lens to the far side relative to the mirror of the birefringent walk-off plate.

4. The optical isolator as recited inclaim 1, wherein the optical isolator is a single stage polarization independent optical isolator.

5. The optical isolator as recited inclaim 1, further comprising a ferrule including the input fiber and the output fiber.

6. The optical isolator as recited inclaim 1, further comprising:

a polarization rotation compensator provided adjacent to the reciprocally rotating optical element and providing a range of polarization angles with respect to wavelength of the sub-lights input thereto.

7. The optical isolator as recited inclaim 6, wherein the birefringent walk-off plate divides the reverse direction input light into an ordinary ray sub-light and an extraordinary ray sub-light and deflects the extraordinary ray sub-light, the Faraday rotator non-reciprocally rotates the polarizations of the reverse direction sub-lights by 45° in the first direction, the at least one lens collimates and directs the reverse direction sub-lights onto the mirror, which reflects their polarizations to the polarization rotation compensator and, upon exit thereof, to the reciprocally rotating optical element which rotates them by 45° in the first direction, and the birefringent plate directs the reverse direction sub-lights away from entering the input fiber.

8. The optical isolator as recited inclaim 6, wherein the at least one lens is disposed between the mirror and the distance from the at least one lens to the mirror is the same as the distance from the at least one lens to the far side relative to the mirror of the birefringent walk-off plate.

9. The optical isolator as recited inclaim 6, wherein the optical isolator is a single stage broadband polarization independent optical isolator.

10. An optical isolator including an input fiber carrying input light and an output fiber and comprising:

a first birefringent walk-off plate dividing light input thereto into an ordinary ray sub-light and an extraordinary ray sub-light thereof, and deflecting the extraordinary ray sub-light thereof;

a first reciprocally rotating optical element provided adjacent to the birefringent walk-off plate and reciprocally rotating the polarization of each of the sub-lights of light input thereto by 45° in a first direction;

a second reciprocally rotating optical element provided adjacent to the birefringent walk-off plate and reciprocally rotating the polarization of each of the sub-lights of light input thereto by 45° in a second direction opposite to the first direction;

a Faraday rotator and associated magnets provided adjacent to the first reciprocally rotating optical element and to the second reciprocally rotating optical element and non-reciprocally rotating the sub-lights of light input thereto by 45° in a first direction;

a second birefringent walk-off plate deflecting the extraordinary ray of light input thereto;

at least one lens collimating light input thereto a mirror receiving and reflecting collimated light directed thereon by the at least one lens, wherein input light received from the input fiber travels in a forward direction from the input fiber to the first birefringent walk-off plate, to the first reciprocally rotating optical element, to the Faraday rotator, to the second birefringent walk-off plate, to the mirror, back to the second birefringent walk-off plate, back to the Faraday rotator, to the second reciprocally rotating optical element, and back to the first birefringent walk-off plate and the sub-lights of the forward-traveling input light are recombined by the first birefringent walk-off plate into output light exiting the optical isolator through the output fiber, whereas input light received from the output fiber travels in a reverse direction opposite to that of the forward direction and the sub-lights thereof are deflected away from each other during each traversal of each of the first and the second birefringent walk-off plates.

11. The optical isolator as recited inclaim 10, wherein the optical isolator is a double stage polarization independent optical isolator.

12. The optical isolator as recited inclaim 10, further comprising a four-fiber ferrule including the input fiber and the output fiber.

13. The optical isolator as recited inclaim 10, further comprising:

polarization rotation compensators, one of which is located between the first reciprocally rotating optical element and the Faraday rotator and the other of which is located between the second reciprocally rotating optical element and the first birefringent plate, said polarization rotation compensators providing a range of polarization angles with respect to wavelength of the sub-lights input thereto.

14. The optical isolator as recited inclaim 13, further comprising a ferrule including the input fiber and the output fiber.

15. An integrated single-stage polarization independent optical isolator and monitor including an input fiber carrying input light and an output fiber and comprising:

a double-stage reflection isolator transmitting light received from the input fiber in a forward direction therethrough to the output fiber and preventing transmission in a reverse direction therethrough to the output fiber of light received from the output fiber, said double-stage reflection isolator including a reflective mirror passing a portion of the input light travelling in the forward direction to pass therethrough;

a window receiving the passed input light; and

a photo-detector coupled to the window and monitoring the passed input light.

16. The integrated single-stage polarization independent optical isolator and monitor as recited inclaim 15, wherein the double-stage reflection isolator comprises:

a first birefringent walk-off plate dividing light input thereto into an ordinary ray sub-light and an extraordinary ray sub-light thereof, and deflecting the extraordinary ray sub-light thereof,

a first reciprocally rotating optical element provided adjacent to the birefringent walk-off plate and reciprocally rotating the polarization of each of the sub-lights of light input thereto by 45° in a first direction,

a second reciprocally rotating optical element provided adjacent to the birefiingent walk-off plate and reciprocally rotating the polarization of each of the sub-lights of light input thereto by 45° in a second direction opposite to the first direction,

a Faraday rotator and associated magnets provided adjacent to the first reciprocally rotating optical element and to the second reciprocally rotating optical element and non-reciprocally rotating the sub-lights of light input thereto by 45° in a first direction,

a second birefringent walk-off plate deflecting the extraordinary ray of light input thereto,

at least one lens collimating light input thereto, and

a mirror receiving and reflecting collimated light directed thereon by the at least one lens, wherein input light received from the input fiber travels in a forward direction from the input fiber to the first birefringent walk-off plate, to the first reciprocally rotating optical element, to the Faraday rotator, to the second birefringent walk-off plate, to the mirror, back to the second birefringent walk-off plate, back to the Faraday rotator, to the second reciprocally rotating optical element, and back to the first birefringent walk-off plate and the sub-lights of the forward-traveling input light are recombined by the first birefringent walk-off plate into output light exiting the optical isolator through the output fiber, whereas input light received from the output fiber travels in a reverse direction opposite to that of the forward direction and the sub-lights thereof are deflected away from each other during each traversal of each of the first and the second birefringent walk-off plates.

17. An optical isolator/monitor/amplifier receiving from the input fiber input light traveling in a forward propagation direction and input from the output fiber light traveling in a reverse propagation direction, said optical isolator/monitor/amplifier comprising:

a broadband single-stage reflection optical isolator transmitting light received from one of light input fibers in a forward direction therethrough to one of light output fibers and preventing transmission of light into the input fibers; and

monitor/amplifier components monitoring and amplifying the light traveling in the forward direction.

18. The optical isolator/monitor/amplifier as claimed inclaim 17, wherein the broadband single-stage reflection optical isolator comprises:

at least one lens collimating the input light and focusing the output light,

a mirror reflecting the ordinary ray sub-lights and the extraordinary ray sub-lights of the input light, and

a single stage broadband polarization independent optical element dividing, deflecting, and rotating the input light such that input light entering the optical isolator/monitor/amplifier from the input fiber passes through the single stage broadband polarization independent optical element onto the mirror, and is reflected by the mirror to the single stage broadband polarization independent optical element and passes therethrough to the output fiber, whereas input light traveling in the reverse propagation direction from the output fiber is prevented from entering the input fiber by the single stage broadband polarization independent optical element.

19. The optical isolator/monitor/amplifier as recited inclaim 17, further comprising a ferrule including the input fiber and the output fiber.

20. The optical isolator/monitor/amplifier as recited inclaim 17, wherein the monitor/amplifier components input laser light from a co-pump laser along a first input port and from a counter-pump laser along a second input port, and monitor the light, wherein the light is prevented from entering the first input port and the second input port.

21. The optical isolator/monitor/amplifier as recited inclaim 18, wherein the mirror is a partially-reflective mirror and said monitor/amplifier components receive input counter-pump laser light from a counter-pump laser and input co-pump laser light from a co-pump laser, said monitor/amplifier components comprising a birefringent walk-off plate, a first reciprocally rotating optical element, a Faraday rotator, and a second reciprocally rotating optical element which, when placed in combination with the single stage broadband polarization independent optical element, form single-stage optical isolators preventing the input co-pump laser light from travelling to the counter-pump laser and the input counter-pump laser light from travelling to the co-pump laser.

22. An optical system comprising:

an Er-doped fiber; and

an optical isolator/monitor/amplifier coupled to the Er-doped fiber, said optical isolator/monitor/amplifier comprising:

a broadband single-stage reflection optical isolator; and

a front four-fiber ferrule including a first and a second light input fiber and a first and a second light output fiber, said Er-doped fiber being coupled between the first output light fiber and the second input light fiber, said broadband single-stage reflection optical isolator transmitting light received from one of the light input fibers in a forward direction therethrough to a corresponding one of the light output fibers and preventing transmission of light in a reverse direction to the input fibers; and

monitor/amplifier components monitoring and amplifying the light traveling in the forward direction, wherein a light entering the first input light fiber travels in a forward propagation direction through the optical isolator/monitor/amplifier and is output by the first output light fiber into the Er-doped fiber, which transmits the light to the second input light fiber of the optical isolator/monitor/amplifier, which outputs the light through the second output light fiber.

23. The optical isolator/monitor/amplifier as recited inclaim 22, wherein the broadband single-stage reflection optical isolator comprises:

at least one lens collimating the input light and focusing the output light,

a mirror reflecting the ordinary ray sub-lights and the extraordinary ray sub-lights of the input light, and

a single stage broadband polarization independent optical element dividing, deflecting, and rotating the input light such that input light entering the optical isolator/monitor/amplifier from the input fiber passes through the single stage broadband polarization independent optical element onto the mirror, and is reflected by the mirror to the single stage broadband polarization independent optical element and passes therethrough to the output fiber, whereas input light traveling in the reverse propagation direction from the output fiber is prevented from entering the input fiber by the single stage broadband polarization independent optical element, wherein the mirror is a partially-reflective mirror and said monitor/amplifier components receive input counter-pump laser light from a counter-pump laser and input co-pump laser light from a co-pump laser, said monitor/amplifier components comprising a birefringent walk-off plate, a first reciprocally rotating optical element, a Faraday rotator, and a second reciprocally rotating optical element which, when placed in combination with the single stage broadband polarization independent optical element, form single-stage optical isolators preventing the input co-pump laser light from travelling to the counter-pump laser and the input counter-pump laser light from travelling to the co-pump laser.

24. An optical system comprising:

an Er-doped fiber;

a broadband double-stage polarization independent polarization-mode-dispersion-free optical isolator coupled to the Er-doped fiber and comprising a first input fiber, a second input fiber, a first output fiber, and a second output fiber; and

an optical isolator/monitor/amplifier coupled to the broadband double-stage polarization independent polarization-mode-dispersion-free optical isolator and comprising a third input fiber, a fourth input fiber, a third output fiber, and a fourth output fiber, wherein the broadband double-stage polarization independent polarization-mode-dispersion-free optical isolator and the optical isolator/monitor/amplifier are coupled sequentially to each other such that the first output fiber is coupled to the third input fiber, the third output fiber is coupled to one end of the Er-doped fiber and the fourth input fiber is coupled to the other end of the Er-doped fiber, the fourth output fiber is coupled to the second input fiber, and a light entering the first input fiber travels in a forward propagation direction from the first input fiber to the second output fiber, making two passes through each of the broadband double-stage polarization independent polarization-mode-dispersion-free optical isolator and the optical isolator/monitor/amplifier, and a light entering the optical system from any of the output fibers is prevented from entering into any of the input fibers.

25. An integrated single-stage polarization independent optical isolator/monitor transmitting light received from an input fiber in a forward direction therethrough to an output fiber and preventing transmission of light received from the output fiber in a reverse direction therethrough to the input fiber and detecting the power of a light transmitted in the forward propagation through the optical isolator/monitor.

26. An apparatus coupled to an input fiber and to an output fiber and receiving from the input fiber input light traveling in a forward propagation direction and input from the output fiber light traveling in a reverse propagation direction, said apparatus comprising:

at least one lens collimating the input light and focusing output light;

a mirror reflecting the ordinary ray sub-lights and the extraordinary ray sub-lights of the input light; and

optical isolator means for transmitting the input light traveling in the forward direction and preventing transmission of the input light traveling in the reverse direction, said optical isolator means dividing, deflecting, and rotating the input light such that input light entering the apparatus from the input fiber passes through the optical isolator means to the mirror, and is reflected by the mirror to the optical isolator means and passes therethrough to the output fiber, whereas input light traveling in the reverse propagation direction from the output fiber is prevented from entering the input fiber by the optical isolator means.

27. The optical isolator as recited inclaim 1, wherein the reciprocally rotating optical element includes a λ/2 plate.

28. The optical isolator as recited inclaim 10, wherein the first reciprocally rotating optical element includes a λ/2 plate and the second reciprocally rotating optical element includes a λ/2 plate.

29. The integrated single-stage polarization independent optical isolator and monitor as recited inclaim 16, wherein the first reciprocally rotating optical element includes a λ/2 plate and the second reciprocally rotating optical element includes a λ/2 plate.

30. The optical isolator/monitor/amplifier as recited inclaim 21 wherein the first reciprocally rotating optical element includes a λ/2 plate and the second reciprocally rotating optical element includes a λ/2 plate.

31. The optical isolator/monitor/amplifier as recited inclaim 23, wherein the first reciprocally rotating optical element includes a λ/2 plate and the second reciprocally rotating optical element includes a λ/2 plate.

32. The integrated single-stage polarization independent optical isolator as recited inclaim 15, further comprising a rear lens collimating the portion of the passed input light.

33. The optical system as recited inclaim 22, wherein the optical isolator/monitor/amplifier further comprises rear ports including two input ports and two output ports.

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