MODE-SELECTIVE LAUNCHING AND/OR DETECTING IN AN OPTICAL
WAVEGUIDE
The present invention relates to mode-selective launching and/or detecting in an optical waveguide, typically an optical fiber, which allows for multiplexing of discrete, individual modes or modal groups launched in an optical waveguide and dc-multiplexing of discrete, individual modes or modal groups from guided waves within an optical waveguide.
It is known to launch waves of a selected mode in an optical waveguide using a high refractive index prism, and, similarly, to detect waves of a selected mode from a guided wave in an optical waveguide using such a prism.
GB-A-1527228 and US-A-4125768 disclose examples of apparatus for such launching and detecting in an optical fiber, Tien, P K et al (Journal of the Optical Society of America, Vol 60, No 10, pages 1325 to 1337) discloses the use of a prism for launching waves of a selected mode in a thin-film waveguide.
Mendes et al (Optics Communications, Vol 136, pages 320 to 326) also discloses the use of a prism for launching waves of a selected mode in a thin-film waveguide.
Sorin, W V et al (Optics Letters, Vol 12, No 2, pages 106 to 108) discloses the use of a prism for detecting waves of multiple modes from an optical fiber.
The present inventor has now recognized that selective, independent control of the launching and detecting of a plurality of different modes or modal groups in a multi-modal optical waveguide would provide significant benefits, particularly in enhancing the transmission capacity of an optical waveguide link, such as a telecommunications link.
In one aspect the present invention provides an optical communications system, comprising: a multi-modal optical waveguide through which light waves are propagated, which comprises a core and a cladding surrounding the core; at least one launching apparatus for selectively launching waves of a plurality of different modes m or modal groups in the waveguide; and at least one detecting apparatus for detecting waves of a plurality of different modes m or modal groups in the waveguide; whereby the launching apparatus is operative as a multiplexer for selectively launching waves of a plurality of different modes m or modal groups in the waveguide and the detecting apparatus is operative as a de-multiplexer for de-multiplexing light from a plurality of separate modes rn or modal groups in the waveguide, and the optical system, through operation of the launching apparatus and the detecting apparatus, provides for multiplexed communication over the waveguide.
In another aspect the present invention provides a launching apparatus for selectively launching light in a plurality of different modes m or modal groups of a multi-modal waveguide, the apparatus comprising: an optical coupler, which is in use coupled to the waveguide for coupling, at least partially, light beams delivered to the optical coupler to guided modes m of the waveguide; and a launch unit for selectively providing light beams to the optical coupler, such as to launch light in selected ones of the guided modes m of the waveguide.
In a further aspect the present invention provides a detecting apparatus for detecting light from a plurality of different modes m or modal groups of a multi-modal waveguide, the apparatus comprising: an optical coupler, which is in use coupled to the waveguide for coupling, at least partially, guided modes m in the waveguide to light beams propagated from the optical coupler; and a detecting unit for detecting light beams propagated from the optical coupler, which correspond to the guided modes m of the waveguide.
In a still further aspect the present invention provides a combined launching and detecting apparatus for selectively launching light in a plurality of different modes m or modal groups of a multi-modal waveguide and detecting light from a plurality of different modes m or modal groups of the waveguide, the apparatus comprising: an optical coupler, which is in use coupled to the waveguide for coupUng, at least partially, light beams delivered to the optical coupler to guided modes m of the waveguide and guided modes m in the waveguide to ight beams propagated from the optical coupler; a launch unit for selectively providing light beams to the optical coupler, such as to launch light in selected ones of the guided modes m of the waveguide; and a detecting unit for detecting light beams propagated from the coupler, which correspond to the guided modes in of the waveguide.
In a yet further aspect the present invention provides a multiplexed optical communications method, comprising the steps of: selectively launching light in a plurality of different modes m or modal groups of a multimodal waveguide, such as to provide a multiplexed signal; detecting light beams propagated from a plurality of different modes m or modal groups in the waveguide, such as to de-multiplex light from a plurality of separate modes m in the waveguide into individual signals; and processing the de-multiplexed light components.
In a still yet further aspect the present invention provides an optical coupler for coupling, at east partially, light to and/or from guided modes m of a multi-modal waveguide comprising a core and a cladding which surrounds the core, the optical coupler comprising: a support which in use supports a coupling section of the waveguide; and an overlay which in use overlies the coupling section of the waveguide, the overlay having a refractive index greater than a refractive index of the cladding, and preferably greater than a refractive index of the core.
In yet another aspect the present invention provides an optical coupler for coupling, at least partially, light to and/or from guided modes m of a multi-modal waveguide comprising a core and a cladding which surrounds the core, the optical coupler comprising: a support which in use supports a coupling section of the waveguide, wherein the cladding has a reduced dimension at the coupling section of the waveguide, such that the evanescent fields from the guided modes m extend to a surface of the cladding; and an overlay which in use overlies the coupling section of the waveguide, the overlay having a refractive index greater than a refractive index of the cladding, arid preferably greater than a refractive index of the core; wherein the coupling section of the waveguide includes a coupling face over which the overlay is disposed and the coupling face is inclined relative to the longitudinal, optical axis of the core of the waveguide, such that the overlay acts to tap the evanescent fields for the respective guided modes rn at spaced locations along a length of the coupiing face, thereby providing for increased spatial separation of light components corresponding to the respective guided modes rn as propagated from the overlay.
In yet another aspect the present invention provides an overlay for coupling, at least partially, light to and/or from guided modes m of a multi-modal waveguide, the overlay comprising: a coupling face which in use overlies a coupling section of the waveguide and provides for coupling to the evanescent fields of the guided modes m in the waveguide; and at least one propagation face located on the optical axis of the overlay, from which light corresponding to the guided modes m of the waveguide is propagated from or into the overlay, wherein the or each propagation face encloses an acute angle with the coupling face.
In yet another aspect the present invention provides an overlay for coupling, at least partially, light to and/or from guided modes m of a multi-modal waveguide, the overlay comprising: a coupling face which in use overlies a coupling section of the waveguide and provides for coupling to the evanescent fields of the modes m in the waveguide; and first and second propagation faces located to opposite ends of an optical axis of the overlay, from which light corresponding to the guided modes m of the waveguide is propagated from or into the overlay, thereby allowing for launching or detecting of Ught in both directions through the waveguide.
In yet another aspect the present invention provides an overlay for coupling, at least partially, light to and/or from guided modes m of a mufti-mod& waveguide, the oveday comprising: a coupling face which in use overfles a coupling section of the waveguide and provides for coupling to the evanescent fields of the guided modes in in the waveguide; at least one propagation face located on an optical axis of the overlay, from which light corresponding to the guided modes m of the waveguide is propagated from or into the overlay; and at least one optical element disposed to the respective at least one propagation face, which acts to collimate light beams propagated from the overlay corresponding to the respective guided modes m of the waveguide.
Preferred embodiments of the present invention will now be described hereinbelow by way of example only with reference to the accompanying drawings, in which: Figure 1 illustrates an optical communications system in accordance with one embodiment of the present invention; Figure 2(a) illustrates a side view of an optical coupler in accordance with one embodiment of the present invention; Figure 2(b) illustrates a plan view of the optical coupler of Figure 2(a); Figure 3 fllustrates the launching apparatus of the optical system of Figure 1; Figure 4 illustrates the detecting apparatus of the optical system of Figure 1; Figure 5 schematicaHy represents the evanescent coupling of an optical waveguide and an overlay in accordance with an embodiment of the present invention; Figure 6 illustrates the beam proffles of the light beams as propagated from the optical coupler of the detecting apparatus of Figure 4; Figure 7 illustrates the beam profiles of the light beams as propagated from the overlay of the arrangement of Figure 5; Figure 8 illustrates an optical coupler in accordance with an alternative embodiment of the present invention; Figure 9 illustrates a combined launching and detecting apparatus in accordance with an embodiment of the present invention for use in the optical system of Figure 1; Figure 10 illustrates a launchinq apparatus in accordance with an alternative embodiment of the present invention; Figure 11 illustrates a detecting apparatus in accordance with an alternative embodiment of the present invention; Figure 12 illustrates a fragmentary view of an optical coupler in accordance with an alternative embodiment of the present invention; Figure 13 illustrates a detecting apparatus in accordance with another alternative embodiment of the present invention; FIgure 14 Illustrates a launching apparatus in accordance with another alternative embodiment of the present invention; Figure 15 illustrates a detecting apparatus In accordance with another alternatIve embodiment of the present invention; Figure 16 illustrates a launching apparatus in accordance with still another alternative embodiment of the present Invention; FIgure 17 illustrates a detecting apparatus in accordance with still another alternative embodiment of the present invention; Figure 18 Illustrates a launching apparatus in accordance with yet another alternative embodiment of the present invention; FIgure 19 illustrates a detecting apparatus in accordance with yet another alternative embodiment of the present invention; Figure 20 illustrates an optical coupler In accordance with another alternative embodiment of the present invention; and FIgure 21 illustrates an optical coupler in accordance with still another alternative embodiment of the present invention.
Figures 1 to 4 illustrate an optical communications system in accordance with an embodiment of the present invention.
The optical communications system comprises a multi-modal optical wavegulde, in this embodiment an optical fiber 3, a launching apparatus $ for launching waves of a plurality of different modes m or modal groups In the fiber 3, and a detecting apparatus 7 for detecting waves of a plurality of different modes m or modal groups in the fiber 3.
In this embodiment, as iflustrated in Figures 2(a) and (b), the fiber 3 comprises a core 3a having a refractive index n1, and a cladding 3b which surrounds the core 3a and has a refractive index n2, which is smaller than the refractive index n1 of the core 3a.
In this embodiment the core 3a is of circular cross-section and the cladding 3b is of annular cross-section.
In this embodiment the core 3a is a solid core glass fiber, typically a silica fiber.
The launching apparatus 5 and the detecting apparatus 7 each comprise an optical coupler 11, which is coupled to the fiber 3 for coupling, at least partially, propagated light to guided modes rn in the fiber 3 in the launching apparatus S and guided modes rn in the fiber 3 to propagated light in the detecting apparatus 7, as will be described in more detail hereinbelow.
In this embodiment the optical coupler 11 comprises a support 15, here a substrate, such as silica slide or block, which supports a section 17 of the fiber 3 at which the cladding 3b has, to at least one side, a reduced cross-section, which provides a coupling face 18, here having a flat, polished surface, and an coupler overlay 19, in this embodiment comprising a prism 20, which overlies the coupling face 18 at the reduced section 17 of the cladding 3b and has a refractive index n3 greater than the refractive index n2 of the cladding 3b, and preferably greater than the refractive index n1 of the core 3a.
With this configuration, the evanescent field from each of the guided modes m in the fiber 3 extends beyond the interface between the core 3a and the cladding 3b, and, by reducing the thickness of the cladding 3b or removing the cladding entirely, the guided modes m can be coupled to the prism 20 such as to propagate therefrom, and vice versa, light propagated into the prism 20 can be coupled to guided modes m in the fiber 3. -9.-
In an alternative embodiment the cladding 3b could be configured such that the evanescent field extends to the outer surface of the cladding 3b.
In the fiber 3, guiding occurs as a consequence of the refractive index n7 of the dadding 3b being lower than the refractive index n1 of the core 3a, and the prism 20, having a higher refractive index n3 than the refractive index n2 of the cladding 3b, interacts with the evanescent field to reduce or eliminate guiding in that region, and the light within each mode m is radiated at a specific angle °m dependent on the effective mode refractive index or propagation constant.
Figure 5 schematically represents this evanescent coupling effect between a prism 20 and a fiber 3.
The light from each mode m is radiated at angle °m, where Oni is the propagation angle and m is the mode number, and the propagation angle O is given by the following equation: (n \ 1 2 2\1/2 n3 -nme) It can be seen that the effective refractive index (nrne) of each mode m will determine the propagation angle °m, and each propagating mode m is radiated at a different angie and spatially separated.
In this embodiment the cladding 3b is removed from one side of the fiber 3 by grinding and polishing the side of the fiber 3 to remove the cladding 3b to a specified depth, leaving the flat, polished coupling face 18.
In this embodiment the cladding 3b is removed only from one side of the fiber 3, so as to retain the integrity of the fiber 3
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In preferred embodiments the dadding 3b is removed by suspending the fiber 3 over a rotating wheel and grinding and polishing with appropriate grinding and polishing materials, or by setting the fiber 3 with an arc in a substrate and grinding and polishing the surface of the whole composite substrate.
The length and amount of the cladding 3b removed can be controlled to provide different levels of coupling from the fiber 3 to the prism 20.
in an alternative embodiment the fiber 3 can be etched using an etching solution, which is selective to the material of the cladding 3b, such as to remove the cladding 3b, either entirely or to a predetermined thickness.
This method allows access completely around the liber 3, but has the disadvantage of rendering the fiber 3 very fragile.
In another alternauve embodiment the thickness of the cladding 3b can be reduced by heating the fIber 3 and locally drawing the fiber 3 to reduce the dimension of the fiber 3, which has the effect of reducing the thickness of the cladding 3b. This method also reduces the diameter of the core 3a, which provides a benefit, in aflowing the evanescent field to extend further into the cladding 3b.
It will be understood that the present invention allows for the use of any fiber processing method which provides access to the evanescent field.
In this embodiment the support 15 has a flat mounting surface 23 which includes a recess 25, here a groove, formed therein, typically by cutting, in which the reduced section 17 of the fiber 3 is located, with the flat, polished coupling face 18 of the cladding 3b oriented to face upwards, such that the coupling face 18 is parallel to or located just below the mounting surface 23.
In this embodiment the prism 20 includes a coupling face 26, here a flat, polished surface, which is located on the mounting surface 23 of the support -11 - 15, such as to overhe the coupling face 18 at the reduced section 17 of the fiber 3 and provide for couphng to the evanescent field of the modes in in the fiber 3.
In this embodiment a coupUng medium is provided between the interface or the couphng face 18 at the reduced section 17 of the dadding 3b and the couphng face 26 of the prism 20, in order to provide for efficient coupling therebetween.
In one embodiment the coupling medium can be a high refractive index oil, here selected to have the same refractive index as the prism 20.
In another embodiment the coupling medium can be a high refractive index epoxy, here selected to have the same refractive index as the prism 20.
In one embodiment the prism 20 can be permanently mounted to the support 15.
In an aiternative embodiment the prism 20 can be removable from the support 15.
In one embodinient the prism 20 can be niovabiy disposed to the support 15, and, with a suitable positioning mechanism which allows for control of the spacing between the coupling face 26 of the prism 20 and the mounting surface 23 of the support 15, and hence the coupling face 18 of the fiber 3, which can be manually adjusted or automated under the control of an actuator, the extent of the evanescent coupling can be controlled.
In this embodiment the refractive index n3 of the prism 20 is greater than that of the mode effective index n.
In this embodiment the prism 20 is formed of a glass, such as silica glass, br example, a Schott BK7 glass.
-12 -In this embodiment the prism 20 comprises first and second propagation faces 27, 28 at respective ends along the optical axis ol the prism 20, from which light corresponding to the modes m of the liber 3 is radiated from or into the prism 20. By providing the two propagation faces 27, 28 at the opposfte ends of the optical axis of the prism 20, the optical coupler 11 allows for launching or detecting in both directions through the fiber 3.
It should be understood that the prism 20 could instead be configured to have only a single propagation face 27, 28, which would allow for uni-directional launching or detecting in the fiber 3.
In this embodiment the propagation faces 27, 28 are each angled to the coupling face 26 at a face angle such that the fundamental mode in1 of the fiber 3 radiates perpendicular to the respective propagation face 27, 28. In preferred embodiments the face angle 0 is 90 degrees plus or minus the propagation angle 8 of the fundamental mode m1 of the fiber 3.
In this embodiment the coupler overlay 19 further comprises first and second beam-shaping optics 29, 30, here each comprising a cylindrical lens, which are disposed to the respective ones of the propagation faces 27, 28 and each act to collimate, in one axis, here the horizontal axis, the light beams radiated from the prism 20 corresponding to the respective modes m, resulting in narrowly-defined light beams, as illustrated in Figure 6.
In this embodiment the lenses 29, 30 are fixed, such as by bonding, for example, with an epoxy adhesive, to the prism 20.
Jn an alternative embodiment the lenses 29, 30 could be integrally formed from the material of the prism 20.
Without the incorporation of the optical elements 29, 30, the prism 20 would radiate the modes m in a series of spatiallyseparated divergent bands, in -13 the form of crescents, as schematicaUy represented in Figure 7, each representing a mode m with a different effective refractive index nme.
By coUimating the light beams corresponding to the respective modes m, the light beams can be more readily collected for further processing.
It should be understood that, although a preferred embodiment of the prism has been described, evanescent coupling is not dependent upon the exemplified shape of the prism 20. The prism 20 can have any shape, provided that, in the interaction region, the prism 20 is large compared to the propagating wave. Figure 8 illustrates a rectangular prism 20 which provides for evanescent coupling in the same manner as the above-described prism 20, The launching apparatus S comprises a launch unit 31 for launching light in selected ones of the guided modes rn of the fiber 3.
In this embodiment the launch unit 31 comprises a plurality of optical assemblies 33a, 33b, 33c, 33d, each configured to deliver a light beam at a propagation angle em corresponding to a respective one of modes m of the fiber 3.
In this embodiment the fiber 3 has at least four guided modes m, and the launch unit 31 comprises four optical assemblies 33a, 33b, 33c, 33d. ft will be understood, however, that the launch unit 31 could comprise any number of optical assemblies 33a, 33b, 33c, 33d.
In this embodiment the optical assemblies 33a, 33b, 33c, 33d each comprise a hght source 35, here a laser, which provides for light to be selectively launched in a respective one of the modes rn of the fiber 3.
In an alternative embodiment groups of the optical assemblies 33a, 33b, 33c, 33d can be commonly coupled to respective light sources 35, enabling light -14 from separate light sources 35 to be selectively launched into respective modal groups, each comprising groups of individual modes m of the fiber 3.
In one embodiment the optical assemblies 33a, 33b, 33c, 33d each further comprise processing optics 37 for processing light to be launched in the fiber 3.
In this embodiment the processing optics 37 comprise beam-shaping optics for shaping the light source 35 in accordance with the beam profile of the respective mode m. thereby providing for efficient power transfer.
In this embodiment the optical assemblies 33a, 33b, 33c, 33d each further comprise a beam director 39, here a reflector, for directing the respective light beam at the required propagation angle for coupling into the respective mode rn in the fiber 3. In this way, the light sources 35 and the processing optics 37 can be located laterafly of the axes of the radiated light beams.
With this configuration, the launching apparatus 5 is operative as a multiplexer for selectively launching light from a plurality of separate light sources 35 into a multi-modal fiber 3.
The detecting apparatus 7 comprises a detecting unit 41 for detecting light propagated from the prism 20, which correspond to the guided modes m of the fiber 3.
In this embodiment the detecting unit 41 comprises a plurality of optical assemblies 43a, 43b, 43c, 43d, each configured to collect light propagated from the prism corresponding to the respective guiding modes rn of the fiber 3.
In this embodiment the fiber 3 has at least four guided modes m, and the detecting unit 41 comprises four optical assemblies 43a, 43b, 43c, 43d. It -15 wiH be understood, however, that the detecting unit 41 could comprise any number of optical assembfles 43a, 43b, 43c, 43d.
In this embodiment the optical assemblies 43a, 43b, 43c, 43d each include a beam director 45, here a prism reflector, for separating light from the respective propagaUon modes m, such as to allow for detection.
in this embodiment the optical assemblies 43a, 43b, 43c, 43d each further include processing optics 47 for processing the collected light components.
In one embodiment the processing optics 47 comprise a beam collimator for collimating the received light component for a propagation mode m.
In one embodiment the processing optics 47 comprise a power splitter for splitting the received light into a plurality of components.
In one embodiment the processing optics 47 comprise a wavelength filter for filtering the received light in accordance with wavelength.
In this embodiment the optical assemblies 43a, 43b, 43c, 43d each further comprise a detector 49 for detecting the separated light component corresponding to a respective one of the guided modes m in the fiber 3.
In an alternative embodiment the detecting unit 41 could comprise a plurality of detectors 49, each for detecting light from groups of the individual modes m of the fiber 3.
In this embodiment, as a consequence of collimating the light beams corresponding to the respective modes m to be collimated in the horizontal axis, the detectors 49 can be provided by a linear detector array aligned to the vertical axis.
-16 - In an afternative embodiment the detectors 49 can be provided by a two-dimensional array detector.
With this configuration, the detecting apparatus 7 is operative as a de-muftiplexer for selectively de-multiplexing light from a plurality of separate modes m in a multi-modal fiber 3.
In this way, the optical system, through operation of the launching apparatus S and the detecting apparatus 7, provides for multiplexed communication over an optical fiber link.
It should be understood that this embodiment has, for ease of description, been described as comprising only a single launching apparatus 5 and a single detecting apparatus 7. The optical system could comprise a plurality of launching apparatus 5 and a plurality of detecting apparatus 7, which can be coupled to a single optical fiber ink or in an optical fiber network.
In an alternative embodiment, as illustrated in Figure 9, the optical system could utilize a combined launching and detecting apparatus 51, which provdes at the same location both for launching waves of a plurality of different modes m or modal groups in the fiber 3 and detecting waves of a plurality of different modes m or modal groups in the fiber 3.
This combined apparatus 51, in the same manner as the launching apparatus and the detecting apparatus 7 as described hereinabove, comprises an optical coupler 11 which is coupled to the fiber 3, a launch unit 31 for launching light into selected ones of the guided modes m of the fiber 3, and a detecting unit 41 for detecting light propagated from the prism 20, correspond to the respective guided modes m of the fiber 3.
Figures 10 and 11 illustrate launching apparatus 105 and detecting apparatus 107 for an optical communications system in accordance with an alternative embodiment of the present invention.
-17 -The launching apparatus 105 is very similar to the launching apparatus 5 of the above-described embodiment, and thus, in order to avoid unnecessary duplication of description, on the differences will be described in detail, with like parts being designated by like reference signs.
in this embodiment each of the optical assemblies 33a, 33b, 33c, 33d comprises a plurality of light sources, here two light sources 35a. 35b, which each have a discernable or filterable characteristic, and a beam combiner 111 for combining the light from the light sources 35a, 35b in a single beam.
In this embodiment the light sources 35a, 35b provide signals which have a different polarization, and can be subsequently split, as will be described in more detail hereinbelow.
In an alternative embodiment the light sources 35a, 35b provide signals having a different wavelength, and can be subsequently split, as will be described in more detail hereinbelow.
With this configuration, the light launched in each mode m of the fiber 3 comprises two resolvable components, which in effect doubles the bandwidth of the communication.
The detecting apparatus 107 is very similar to the detecting apparatus 7 of the above-described embodiment, and thus, in order to avoid unnecessary duplication of description, on the differences will be described in detail, with like parts being designated by like reference signs.
in this embodiment each of the optical assemblies 43a, 43b, 43c, 43d comprises a plurality of detectors, here two detectors 49a, 49b, and a beam splitter 113 for separating the two resolvable light components from the light associated with each respective mode m, which light components are delivered to the respective detectors 49a, 49b.
-18 -In thk embodiment the received light has light components with two different polarizations, and these light components are separated by the beam splitter 113, which is a polarizing beam splitter.
In an alternative embodiment the received light has light components with two different wavelengths, and these light components are separated by the beam splitter 113, which is a wavelength beam splitter.
Figure 12 illustrates a fragmentary view of an optical coupler 211 in accordance with an alternative embodiment of the present invention.
The optical coupler 211 of this embodiment is very similar to the optical coupler 11 of the above-described embodiment, and thus, in order to avoid unnecessary dupUcation of description, only the differences will be described in detail, with like parts being designated by like reference signs.
The optical coupler 211 of this embodiment differs from that of the above-described embodiment in that the coupling face 18 of the reduced section 17 of the cladding 3h is inclined relative to the longitudinal axis of the core 3a.
In the optical coupler 211 of the above-described embodiment the coupling face 18 of the reduced section 17 of the cladding 3b is parallel to the longitudinal axis of the core 3a.
This inclination can be conveniently controlled for a side-polished fiber 3 by changing the size of the polishing wheel, and for an embedded, polished fiber 3 by changing the radius of the slot arc.
In one embodiment the coupling face 18 of the reduced section 17 of the cladding 3b is inclined at an angle of less than 5 degrees, preferably less than 3 degrees, and more preferably less than 2 degrees.
-19 -This arrangement is advantageous, in that the evanescent fields from the different modes m each extend to a different extent into the dadding 3b, with the depth of penetration increasing with the increasing order of the mode m, and thus, by providing for the cladding 3b to have a progressively decreasing or increasing thickness along the reduced section 17, the prism acts to tap the evanescent field for the respective modes m at spaced locations along the length of the reduced section 17, which acts to cause increased spatial separation of the propagating modes m. This is particularly advantageous because the prism 20 tends to disperse light, which can make separation of the respective modes m difficult, in that dispersion can creates an overlap between short wavelength Ught in one mode rn and long wavelength hght in the adjacent mode m.
Figure 13 illustrates a detecting apparatus 307 in accordance with an alternative embodiment of the present invention.
The detecting apparatus 307 of this embodiment is similar to the detecting apparatus 7 of the first-described embodiment, and thus, in order to avoid unnecessary duplication of description, only the differences will be described in detail, with like parts being designated by like reference signs.
In this embodiment the detecting apparatus 307 further comprises a positioner 311 for positioning the optical coupler 11 relative the beam directors 45 of the optical assemblies 43a, 43b, 43c, 43d.
In this embodiment the positioner 311 is configured to move the optical coupler 11 in relation to the beam directors 45 of the optical assemblies 43a, 43b, 43c, 43d.
In an alternative embodiment the positioner 311 could be configured to move the beam directors 45 of the optical assemblies 43a, 43b, 43c, 43d in relation to the optical coupler 11.
-20 - In this embodiment the positioner 311 comprises adjusters for providing 2-axis, and optionally 3-axis translational adjustment, and/or rotational adjustment.
In this embodiment the positioner 311 is under automated control.
In another embodiment the positioner 311 could be manually operable.
With this configuration, the positioner 311 allows for adjustment of the position of the optical coupler 11 relative to the beam directors 45 of the optical assemblies 43a, 43b, 43c, 43d, such as to provide for an optimal or required orientation of the radiated light corresponding to the modes rn of the fiber 3 relative to the beam directors 45 of the optical assembfles 43a, 43b, 43c, 43d. In particular, this allows for adjustment to ensure optimized power transfer.
In this embodiment the detecting apparatus 307 further comprises instrumentation 317, which is operable both to calibrate and configure the position of the optical coupler 11 relative to the beam directors 45 of the optical assemblies 43a, 43b 43c, 43d, through characterization of the detected light, and to provide information on performance of the optical Unk, enabling analysis of the propagating light in each mode m and the measurement of fiber mode properties, including phase, frequency, polarization, amplitude and modulation.
In one embodiment the instrumentation 317 can operate, through feedback control to calibrate and configure the position of the optical coupler 11 relative to the beam directors 45 of the optical assemblies 43a, 43b, 43c, 43d, by automated controi of the positioner 317.
Finally, it will be understood that the present invention has been described in its preferred embodiments and can be modified in many different ways without departing from the scope of the invention as defined by the appended claims.
-21 -For example, the present invention has application to any fiber type.
Aithough the described embodiments utilize a solid core glass fiber, the present invention has application to fibers of different materials and structures, such as photonic bandgap fibers and also planar waveguides.
In the above-described embodiments the reflectors 39, 45 are prism reflectors which are utilized as right-angle surface reflectors, but in other embodiments the prism reflectors 39, 45 could be utilized as corner reflectors, as illustrated in the embodiments of Figures 14 and 15.
In addition, the prism reflectors 39, 45 are exemplified each as separate components, but the prism reflectors 39, 45 acting on adjacent modes m could be provided a single, common reflector or in an embodiment where the fiber 3 is bi-modal, as illustrated in the embodiment of Figures 16 and 17.
Furthermore, the reflectors 39, 45 could be mirror reflectors, as illustrated in the embodiments of Figures 18 and 19, In another modification, as illustrated in Figure 20, the prism 20, here a right-angled prism, could be configured to reflect the light from the modes m of the fiber 3 laterally to the longitudinal axis of the fiber 3, here orthogonally to the longitudinal axis of the fiber 3.
In a further modification, as illustrated in Figure 21, the prism 20 could be a prismatic beam splitter which is arranged to separate the light from each mode rn of the fiber 3 into at least two components. In this embodiment the prismatic beam splitter 20 is arranged such as to direct a first component of the light from each of the modes m of the fiber 3 laterally to the longitudinal axis of the fiber 3 and a second component of the light from each of the modes m of the fiber 3 aligned to the longitudinal axis of the fiber 3. 22 -
in one embodiment the light coupled to a mode m of the fiber 3 can be a DWDM channel, thus allowing multiple DWDM channels to be moduiated and put in respective modes m of the fiber 3. These DWDM chann&s in the respective modes in of the fiber 3 can then be de-multiplexed at the detecting apparatus 7, 107, 207, and in one embodiment applied to single-mode fibers 3. in this way, the multiplexer can then provide polarization multiplexing and spatial multiplexing of DWDM channels, enhancing significantly the transmission capacity of the fiber link.