OPTICAL FIBER BUNDLE WITH A SINGLE MODE OPTICAL FIBER SURROUNDED BY MULTI- MODE OPTICAL FIBERS AND METHOD OF MANUFACTURE THEREOF
Field of the Invention
This invention relates to an optical fiber coupler, more particularly, a wavelength division multiplexer coupler forming a fiber bundle
Background ofthe Invention Optical couplers permit optical beams to be combined As such optical couplers have a multitude of uses Optical couplers for example are used to multiplex optical signals in the same optical fiber in a telecommunications network and to combine signal radiation and pump radiation in an optical amplifier
Optical couplers may be constructed as bulk optical couplers or fiber optic couplers A conventional "bulk optic" optical coupler, known to the prior art, for example for combining signal radiation and pump radiation in an optical amplifier, is shown Fig 1 As shown, a dichroic mirror 1 which is transmissive to the signal radiation wavelength and reflective to the pump radiation wavelength, receives an input optical data signal from signal radiation source 2 and an input pump laser beam from pump laser source 3 The data signal is transmitted through the mirror 1 and the pump beam is reflected by the mirror 1 The data signal and the pump beam are thus simultaneously combined in the gain medium 4 which amplifies the signal radiation This amplified data signal is, for example, then transmitted into an optical fiber which is a part of an optical fiber network Although this bulk optic optical coupler effectively combines the data signal and the pump beam, it is difficult to fabricate such a coupler so that it is compact, robust, and available at a low cost
Within the past decade or so, optical couplers employing optical fibers have emerged The use ofthe fiber optic couplers has become widespread due in part to the high degree of energy containment available within optical fibers, the good coupling properties possessed thereby, and their small size In such a coupler as known to the prior art and, shown in Fig 2, the cladding 5 is removed from the region 8 of each ofthe optical fibers 6, 7 to be joined The cores 9 of each optical fiber 6, 7 are fused together in the region 8 Light injected (Ii) in fiber 6 and light injected (I2) in fiber 7 are combined in the region of couphng 8 and the combined hght (Oi) may be obtained from the opposite end of one of the coupled fibers 7 Such a coupler, for example, is used to combine the signal radiation and the pump radiation for an optical amplifier
In addition to such signal amplification in telecommunications networks, high power optical radiation is often required other applications Broadcast cable suppliers require high power signals to reach subscribers High speed networks require high power signals for non¬ linear optical switching Medical instruments for surgery and diagnostics also require high power light beams for certain applications
The present invention relates to a high efficiency optical coupler suitable for applications requiring amplified optical radiation
Summary ofthe Invention It is therefore an object ofthe invention to provide a fiber coupler device, particularly an optical fiber spatial wavelength division multiplexer coupler (hereinafter 'WDM coupler') which efficiently couples optical radiation from a laser signal radiation source and a pump laser radiation source, into an optical fiber It is another object of this invention to provide a method of efficiently combining optical radiation from such radiation sources in a WDM coupler In one embodiment the WDM coupler of the present invention forms a fiber bundle comprising a plurahty of multimode optical fibers in parallel juxtaposition and a single mode optical fiber in substantially parallel juxtaposition with and substantially surrounded by the plurality of multimode optical fibers The single mode optical fiber is coupled to a first optical radiation source transmitting optical radiation at a first wavelength, and the multimode optical fibers are coupled to a plurahty of second optical radiation sources transmitting optical radiation at a second wavelength In one embodiment the first and second wavelengths are the same In another embodiment the first and second wavelengths are different
In yet another other embodiment of the invention, the WDM coupler acts as a preamplifier to an optical amplifier In this embodiment, a cladless single mode fiber ofthe WDM coupler is doped with an optically active material such that when signal radiation is transmitted therethrough it is combined with pump radiation absorbed from the surrounding cladless multimode optical fibers. In this way energy from the pump laser radiation is transferred to the signal radiation in the single mode doped fiber, resulting in the amplification thereof.
The WDM coupler in another embodiment, acts as a coupler to a double clad fiber amplifier. In this embodiment, a double clad fiber is coupled to an output end ofthe WDM coupler through an optical system. The double clad fiber comprises a single mode optical fiber core surrounded by a multimode optical material. The single mode optical fiber core of the double clad optical fiber receives signal radiation from the single mode optical fiber ofthe WDM coupler. The multimode optical material ofthe double clad optical fiber receives pump radiation from the plurality of multimode optical fibers ofthe WDM coupler
In another embodiment ofthe present invention, a method of transmitting optical radiation through an optical coupler comprises providing a plurality of multimode optical fibers in parallel juxtaposition, providing a single mode optical fiber in substantially parallel juxtaposition with and substantially surrounded by said plurality of multimode optical fibers, the single mode optical fiber and the multimode optical fibers forming a fiber bundle; transmitting optical radiation at a first wavelength through said single mode optical fiber; and transmitting optical radiation at a second wavelength through the plurality of multimode optical fibers. In one embodiment, the first wavelength and the second wavelength are the same In another embodiment the first wavelength and the second wavelength are different.
In yet another embodiment ofthe invention, the method can include providing a doped cladless single mode optical fiber, transferring high power optical radiation from cladless multimode optical fibers to the cladless doped single mode optical fiber, and amplifying the signal radiation in the cladless doped single mode optical fiber. In still another embodiment ofthe invention, the method can include coupling an optical fiber amplifier to the fiber bundle.
These and other objects, aspects, features and advantages ofthe invention will become more apparent from the following drawings, detailed description and claims
Brief Description of the Drawings This invention is pointed out with particularity in the appended claims The above and further advantages of this invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings, in which. Fig 1 shows a prior art bulk optic wave division multiplexer
Fig 2 shows a prior art fiber coupler
Fig 3 shows an embodiment of the WDM coupler device ofthe present invention including a plurality of multimode fibers encircling a single mode fiber
Fig 4 shows another embodiment ofthe fiber WDM coupler device ofthe present invention coupled to a double clad fiber through an optical system
Fig 4a shows yet another embodiment ofthe fiber WDM coupler device ofthe present invention coupled to a single mode optical amplifier through an optical system
Fig 4b shows an embodiment of a communication system using the WDM couplers ofthe invention with optical amplifiers
Fig 5 shows another embodiment ofthe present invention, particularly the WDM coupler device ofthe present invention used as a preamplifier
Detailed Description ofthe Invention Referring to Fig 3, an embodiment ofthe WDM ofthe present invention is shown in which a cladded single mode optical fiber 20 is substantially surrounded by a plurality of cladded multimode optical fibers 22 The fibers 20, 22 are close packed such that the centers ofthe fibers 20, 22 in alternating layers are alligned
Referring to Fig 4, shown is an embodiment ofthe WDM ofthe present invention in which an optical system 30 is used to optically couple the WDM 10 to a double clad optical fiber 40 The double clad optical fiber 40 includes a single mode optical fiber 42 surrounded by a multimode optical material 44 The multimode optical material is in turn surrounded by cladding 45 As shown in this figure, the single mode optical fiber 20 ofthe WDM coupler 10 ofthe present invention receives laser radiation from a signal radiation source 12 through an intermediate optical fiber 13 Similarly each of the plurality of multimode optical fibers 22 receives pump radiation from a respective one of a plurality of pump laser radiation sources 14 (only one shown for clarity) through an intermediate fiber 15 In one embodiment the signal radiation source 12 emits laser radiation at a different wavelength and power than the pump laser radiation sources 14 In another embodiment, the signal radiation source 12 emits laser radiation at the same wavelength as the pump radiation sources 14
In one embodiment the signal radiation source 12 is a diode laser with an attached intermediate optical fiber 13 The signal radiation source emits signal radiation of a narrow spectral width Each pump laser source 14 is preferably a semiconductor pump laser with an attached intermediate optical fiber 15 As opposed to the signal radiation source 12, the pump radiation source 13 emιts_optιcal radiation of a broad spectral width The signal radiation in one embodiment has a wavelength of about 1550 nm, and low power, typically within the range of 1 mW-JO mW The signal radiation source 12 typically transmits optical radiation which has been modulated to carry data The pump laser radiation, in one embodiment has a wavelength within the range of 800 nm to 1480 nm, and high output power, for example 0 1 W - 5 W In another embodiment, the signal radiation source 12 and the pump laser radiation sources 13 emit the same wavelength
In one embodiment, Fig 4 A, the double clad fiber 40 to which the coupler ofthe invention is optically joined is an optical amplifier 16 comprising a single-mode rare-earth doped optical fiber 42' The rare earth doped single mode optical fiber 42' may be doped with any appropriate rare earth including but not limited to Erbium, Ytterbium, Thulium, and Praseodymium Such single mode optical fibers are inherently diffraction-limited As a result, such an optical amplifier is frequently used to amplify the signals injected into it by the coupler 10 The output end 46 of the single mode fiber amplifier 16 is typically connected to an optical commumcation system such as a telecommunications network (not shown)
Referring to Fig 4C, a series of optical amplifiers 16 and couplers 10 may be connected in tandem in a communications network to provide sequential amplification of a data signal
Whether the double clad optical fiber is an optical amplifier or simply a transmission optical fiber, the optical system 30 is constructed to image the single mode optical fiber 20 ofthe WDM coupler 10 to the single mode optical fiber 42 or 42' ofthe double clad optical fiber 40 or 16, respectively Similarly, the multimode optical fibers 22 ofthe optical coupler 10 are imaged to simultaneously the multimode portion 44 or 44' ofthe dual clad optical fiber 40 or 16, respectively In this way, the single and multimode light from the signal 12 and pump 14 lasers, respectively, are channeled to the correct portions ofthe double clad optical fiber 40 In the embodiment shown, the single mode optical fiber 20 ofthe WDM coupler has a diameter within the range of 3 um - 10 um and a numerical aperture (hereinafter 'NA') of about 0.1. Similarly, each of the multimode optical fibers 22 has a diameter preferably within the range of 100 um - 1 10 um and a NA of about 0.12. The single mode optical fiber 20 is coupled to the signal radiation source 12, and each ofthe multimode optical fibers 22 is coupled to a pump laser optical source 14. The purpose ofthe optical system 30 is generally to match the exit windows of the single mode 20 and multimode fibers 22 in the coupler 10 to the entrance windows of the single mode 42 and multimode 44 portions of the double clad optical fiber 40. With this arrangement, the WDM coupler 10 of the present invention reduces losses attendant with the coupling ofthe pump laser source 14 to a single mode fiber amplifier 16.
According to the "Brightness Theorem" in classical optics, a lossless optical system preserves the space-angle bandwidth product, known as the etendue ofthe source. The etendue is one measure ofthe number of spatial modes in an optical source. For any source, the product ofthe area ofthe emission aperture, A, and the solid angle subtended by the beam Ω, is equal to the square ofthe product of the number of modes, M, and the wavelength, λ. This is mathematically represented by:
AΩ = (M λ)2
For many sources the solid angle is well approximated by the square ofthe numerical aperture (NA) ofthe source, which is the trigonometric sine of the half angle ofthe beam emitted from the aperture. For signal radiation source 12, of the embodiment disclosed above, M is approximately 1. For the pump laser radiation sources 14, M is typically larger, in one embodiment 5-10. The WDM coupler 10 and optical system 30 of the present invention accomplishes the requisite etendue matching ofthe laser sources 12, 14 to the respective portions 42, 44 single mode fiber amplifier 16.
It should also be noted that in the embodiment shown, the cross sectional areas of the
WDM coupler 10 and the double clad fiber 40 are of substantially the same shape and aspect ratio. This occurs because the number of multimode optical fibers 22 is preferably of an amount and arrangement such that the etendue of the double clad fiber 40 is completely filled within the limits imposed by the close packed geometrical stacking of the individual multimode fibers 22. That is, the stacking geometry is set to match that ofthe double clad fiber 40 to within an overall magnification factor
Given such similarities, in one embodiment, the optical system 30 may comprise a simple two lens system for magnifying or demagnifying the image ofthe fibers 20, 22 ofthe WDM coupler 10 to overlap the relevant portions 42, 44 ofthe double clad fiber 40 Because the shape and aspect ratio is the same for both the WDM coupler 10 and the double clad fiber 40, in this embodiment, the lens system need not be astigmatic
Furthermore, the coupling ofthe WDM coupler 10 with the optical amplifier 16 is such that the match ofthe single mode optical fibers 20 ofthe WDM coupler 40 and the single mode fiber 42 ofthe optical amplifier 16 need not be exact In fact, the single mode optical fibers 20 and 42 may be mismatched as long as a portion ofthe signal radiation is transmitted from the single mode optical fiber 20 ofthe coupler 10 to the single mode optical fiber 42 However, the multimode optical fibers 22 should be matched with the multimode portion 44 ofthe optical amplifier 16, as closely as possible In this way, whatever amount of signal radiation is injected into in the single mode fiber 42 portion ofthe optical amplifier 16 can be amplified by the pump radiation injected in the multimode portion 44 ofthe optical amplifier 16
If, in addition to the shape and aspect ratio, the size of WDM coupler 10 and the dual clad optical fiber 40 match, another embodiment ofthe invention permits butt-coupling the WDM coupler 10 with the double clad fiber 40 (not shown) directly without an mtervemng optical system 30
Fig 5 shows an alternative embodiment of the fiber WDM coupler 1 10 ofthe present invention In this embodiment, the WDM optical coupler 1 10 acts as a preamplifier to the optical amplifier 16 As similarly described in the embodiment of Fig 4, the WDM coupler 1 10 comprises a plurality of_cladless multimode 22 optical fibers encircling a cladless single mode fiber 120 which is rare earth doped to act as an amplifier Pump and signal radiation are transmitted from the signal radiation source 12 and the plurality of pump laser sources 14, (only one shown for clanty) respectively, by the cladless multimode optical fibers 22 and the cladless single mode optical fiber 120, respectively Some ofthe pump radiation propagating through each ofthe multimode fibers 22, passes through the doped single mode fiber core 120 where energy conversion to the signal radiation takes place, leading to an increase in the output signal power in the single mode optical fiber 120
This WDM coupler 1 10 is optically coupled to the optical amplifier 16, such that the signal radiation transmitted through the single mode optical fiber 120 ofthe fiber bundle is optically coupled to the single mode fiber ofthe optical amplifier Similarly, all the multimode optical fibers 22 ofthe coupler are optically coupled to the multimode portion ofthe optical amplifier 16 The signal radiation is thus transmitted into the optical amplifier 16 having a greater amplitude than the original signal due to the preamplification ofthe original signal by the WDM coupler 1 10
The foregoing description has been limited to specific embodiments of this invention It will be apparent however, that variations and modifications may be made to the invention, with the attainment of some or all ofthe advantages ofthe invention Therefore, it is the object ofthe appended claims to cover all such variations and modifications as come within the true spirit and scope ofthe present invention