GB2496833A

GB2496833A - Mode-selective launching and detecting in an optical waveguide

Info

Publication number: GB2496833A
Application number: GB201113488A
Authority: GB
Inventors: Ian Giles
Original assignee: PHOENIX PHOTONICS Ltd
Current assignee: PHOENIX PHOTONICS Ltd
Priority date: 2011-08-04
Filing date: 2011-08-04
Publication date: 2013-05-29
Also published as: WO2013017822A1; GB201113488D0

Abstract

An optical communications system comprises a multi-modal optical waveguide 3 which comprises a core and a cladding surrounding the core. At least one launching apparatus 5 selectively launches waves of a plurality of different modes m or modal groups in the waveguide, and at least one detecting apparatus 7 detects waves of a plurality of different modes m or modal groups in the waveguide. 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 m or modal groups in the waveguide thereby providing for multiplexed communication over the waveguide. The launching and detecting apparatus may have optical couplers 11 for coupling to the waveguide. The couplers may have an overlay which overlies a coupling face on the waveguide, where the overlay can have a higher refractive index than the cladding and the core.

Description

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

-

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.

Claims

<claim-text>CLAIMS1, An optical communications system, comprising: a multLmodal 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 pluraiity 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 demultiplexer for dc-multiplexing light from a plurality of separate modes m 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.</claim-text> <claim-text>2. The system of claim 1, wherein the launching apparatus comprises an optical coupler, which is coupled to the waveguide for coupling, at least partially, light delivered to the optical coupler to guided modes m in the waveguide.</claim-text> <claim-text>3. The system of claim 1 or 2, wherein the detecting apparatus comprises an optical coupler, which is coupled to the waveguide for coupling, at least partially, guided modes m in the waveguide to light propagated from the optical coupler.</claim-text> <claim-text>4. The system of claim 2 or 3, wherein the optical coupler comprises a support which supports a coupling section of the waveguide, and an overlay which overlies the waveguide at the coupling section of the waveguide, the overlay having a refractive index greater than the -24 -refractive index of the cladding, and preferably greater than the refractive index of the core.</claim-text> <claim-text>5. The system of claim 4, 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.</claim-text> <claim-text>6. The system of daim 5, wherein the coupling section of the waveguide includes a coupling face over which the overlay is disposed.</claim-text> <claim-text>7. The system of claim 6, wherein the coupling face comprises a flat, polished face.</claim-text> <claim-text>8. The system of claim 6 or 7, wherein the coupling face extends substantially parallel to the longitudinal axis of the core of the wa veg u d e.</claim-text> <claim-text>9. The system of claim 6 or 7, wherein the coupling face is inclined relative to the longitudinal axis of the core of the waveguide, such that the overlay acts to tap the evanescent fields for the respective guided modes m at spaced locations along a length of the coupling face, thereby causing increased spatial separation of light components corresponding to the respective guided modes m as propagated from the overlay.</claim-text> <claim-text>10. The system of ciaim 9, wherein the coupling face is inclined at an angle of less than 5 degrees, preferably less than 3 degrees, and more preferably less than 2 degrees.</claim-text> <claim-text>11. The system of claim 4, wherein the core of the waveguide is exposed at the coupling section of the waveguide.</claim-text> <claim-text>-25 - 12. The system of claim 11, wherein the cladding is removed only from one side of the waveguide, so as to retain integrity of the waveguide.</claim-text> <claim-text>13. The system of any of claims 4 to 12, wherein the support has a support surface at which the coupling face of the waveguide is located and to which the overlay is disposed.</claim-text> <claim-text>14. The system of claim 13, wherein the support surface includes a recess in which the waveguide is located, with the coupling face of the waveguide being parallel to or located just below the support surface.</claim-text> <claim-text>15. The system of any of claims 4 to 14, wherein the overlay includes a coupling face which overlies the coupling section of the waveguide and provides for coupling to the evanescent fields of the guided modes m in the waveguide.</claim-text> <claim-text>16. The system of claim 15, wherein a coupling medium is provided between an interface of the coupling section of the waveguide and the overlay.</claim-text> <claim-text>11. The system of claim 16, wherein the coupling medium is a high refractive index oil, preferably having substantially the same refractive index as the overlay.</claim-text> <claim-text>18. The system of claim 16, wherein the coupling medium is a high refractive index adhesive, preferably having substantially the same refractive index as the overlay.</claim-text> <claim-text>19. The system of any of claims 15 to 18, wherein the overlay further includes at least one propagation face located on the optical axis of the overlay, from which light components corresponding to the guided modes m of the waveguide are propagated from or into the overlay.</claim-text> <claim-text>-26 - 20. The system of claim 19, wherein the or each propagation face of the overiay encloses an acute angle with the coupling face of the over!ay.</claim-text> <claim-text>21. The system of claim 20, wherein the or each propagation face of the oveday is angled such that the fundamental mode m of the waveguide radiates substantially perpendicular to the respective propagation face.</claim-text> <claim-text>22. The system of any of claims 19 to 21, wherein the overlay includes first and second propagation faces located to opposite ends of the optical axis of the overlay, from which light components corresponding to the guided modes m of the waveguide are propagated from or into the overlay, thereby allowing for launching or detecting in both directions through the waveguide.</claim-text> <claim-text>23. The system of claim 22, wherein the overlay comprises at least one optical element disposed to the respective at least one propagation face thereof, which acts to collimate light components propagated from the overlay corresponding to the respective guided modes m of the waveguide.</claim-text> <claim-text>24. The system of claim 23, wherein the or each optical element is a lens, preferably a cylindrical lens.</claim-text> <claim-text>25. The system of claim 24, wherein the or each optical element is fixed to the at least one propagation face of the overlay.</claim-text> <claim-text>26. The system of claim 24, wherein the or each optical element is integrally formed from the material of the overlay.</claim-text> <claim-text>27. The system of any of claims 4 to 26, wherein the overlay is permanently mounted to the support. -27</claim-text> <claim-text>28. The system of any of claims 4 to 25, wherein the overlay is movably disposed relative to the support, and the optical coupler further comprises a positioner which allows for adjustment of a spacing between the overlay and the coupling section of the waveguide, such as to control an extent of the evanescent coupling.</claim-text> <claim-text>29. The system of any of claims 4 to 28, wherein the overlay is formed of glass.</claim-text> <claim-text>30. The system of any of claims 4 to 30, wherein the overlay comprises a prism.</claim-text> <claim-text>31. The system of any of claims 4 to 30, wherein the launching apparatus comprises a launch unit for selectively providing light waves to the overlay, such as to launch light in selected ones of the guided modes rn of the waveguide.</claim-text> <claim-text>32. The system of claim 31, wherein the launch unit comprises a plurality of optical assemblies, each configured to deUver a light component at a propagation angle corresponding to a respective one of the guided modes m of the waveguide.</claim-text> <claim-text>33. The system of claim 32, wherein the optical assemblies each comprise a beam combiner for combining light from a plurality of light sources, each having a subsequently-resolvable characteristic, in a single beam, which provides for light to be selectively launched in a respective one of the guided modes m of the waveguide.</claim-text> <claim-text>34. The system of claim 33, wherein the light sources provide light with different polarization.</claim-text> <claim-text>35. The system of claim 33, wherein the light sources provide light of different wavelength.</claim-text> <claim-text>-28 - 36. The system of claim 32, wherein the optical assemblies each comprise a light source, preferably a laser light source, which provides for light to be selectively launched in a respective one of the guided modes m of the waveguide.</claim-text> <claim-text>37. The system of ciaim 32, wherein groups of the optical assemblies are commonly coupled to respective light sources, whereby light from separate light sources is selectively launched into respective modal groups, each comprising groups of individual guided modes m of the waveguide.</claim-text> <claim-text>38. The system of any of claims 32 to 37, wherein the optical assemblies each comprise processing optics for processing light to be launched in the waveguide.</claim-text> <claim-text>39. The system of claim 38, wherein the processing optics comprise beam-shaping optics for shaping the light beam with a predetermined profile for the respective mode rn of the waveguide.</claim-text> <claim-text>40. The sysLem of any of claims 32 to 39, wherein the optical assemblies each comprise a beam director for directing the respective light beam at the required propagation angle for coupling into the respective mode m in the waveguide.</claim-text> <claim-text>41. The system of claim 40, wherein the beam director comprises a reflector, preferably a prismatic reflector or a mirror reflector.</claim-text> <claim-text>42. The system of any of claims 4 to 41, wherein the detecting apparatus comprises a detecting unit for detecting light components propagated from the optical coupler, which correspond to the guided modes m of the waveguide.</claim-text> <claim-text>-29 - 43. The system of claim 42, wherein the detecting unit comprises a plurality of optical assemblies, each configured to collect Ught from a respective one of the light beams propagated from the optical coupler, which correspond to the respective guided modes m of the waveguide.</claim-text> <claim-text>44. The system of claim 43, wherein the optical assemblies each include a beam director for separating the light beams propagated from the optical coupler.</claim-text> <claim-text>45. The system of claim 44, wherein the beam director comprises a reflector, preferably a prismatic reflector or a mirror reflector.</claim-text> <claim-text>46. The system of any of claims 43 to 45, wherein the detecting apparatus further comprises a positioner for positioning the optical coupler relative to the beam directors of the optical assemblies, such as to provide for a required orientation of the light beams corresponding to the guided modes n of the waveguide relative to the beam directors of the optical assemblies.</claim-text> <claim-text>47. The system of claim 46, wherein the detecting apparatus further comprises instrumentation, which is operable to calibrate and configure the position of the optical coupler relative to the beam directors of the optical assemblies.</claim-text> <claim-text>48. The system of any of claims 43 to 47, wherein the optical assemblies each further include processing optics for processing the collected light beam.</claim-text> <claim-text>49. The system of claim 48, wherein the processing optics comprise at least one optical element for collimating the light beam corresponding to the respective mode m of the waveguide.-</claim-text> <claim-text>50. The system of claim 49, wherein the at least one optical element comprises a cylindrical lens.</claim-text> <claim-text>51. The system of any of daims 43 to 50, wherein the optical assemblies each comprise a beam splitter for separating at least two resolvable light components from the received light beam associated with each respective mode m of the waveguide.</claim-text> <claim-text>52. The system of claim 51, wherein the received light beam has light components with at least two different polarizations, and these light components are separated by the beam splitter.</claim-text> <claim-text>53, The system of claim 51, wherein the received light beam has light components with at least two different wavelengths, and these light components are separated by the beam splftter.</claim-text> <claim-text>54. The system of any of claims 43 to 53, wherein the opticai assemblies each further comprise at least one detector for detecting the separated light from a respective one of the guided modes m of the waveguide.</claim-text> <claim-text>55. The system of any of claims 43 to 53, wherein the detecting unit comprises a plurality of detectors, each for detecting light from groups of the individual modes m of the waveguide.</claim-text> <claim-text>56. The system of any of claims 1 to 55, wherein the detecting apparatus further comprises instrumentation, which is operable to provide information on performance of the system, enabling analysis of the propagating light in each mode m and/or measurement of fiber mode properties.</claim-text> <claim-text>57. The system of any of claims 1 to 56, wherein the overlay is configured to propagate light from the guided modes rn of the waveguide along the optical axis of the waveguide.</claim-text> <claim-text>-31 - 58. The system of any of claims 1 to 56, wherein the overlay is configured to propagate light from the guided modes in of the waveguide laterally to the optical axis of the waveguide.</claim-text> <claim-text>59. The system of any of claims 1 to 58, wherein the overlay is a prismatic beam splitter which is arranged to separate the light from each guided mode m of the waveguide into at least two components.</claim-text> <claim-text>60. The system of claim 59, wherein the prismatic beam splitter is arranged such as to propagate a first component of the light from each of the guided modes m of the waveguide laterally to the optica' axis of the waveguide and a second component of the light from each of the guided modes in of the waveguide a'ong the optical axis of the waveguide.</claim-text> <claim-text>61. The system of any of claims 1 to 50, wherein the flght coupled to a guided mode in of the waveguide is a DWDM channel, thereby allowing multiple DWDM channeis to be launched in respective modes m of the fiber.</claim-text> <claim-text>62. The system of any of claims 1 to 61, wherein the waveguide comprises an optical fiber.</claim-text> <claim-text>63, The system of any of claims 1 to 62, comprising a plurality of launching apparatus.</claim-text> <claim-text>64. The system of any of daims 1 to 63, comprising a plurality of detecting apparatus, 65. The system of any of claims 1 to 64, wherein the waveguide forms part of an optical network, optionally a telecommunications network. 32 -66. The system of any of claims 1 to 65, wherein at least one of the launching apparatus or the detecting apparatus is implemented as a combined launching and detecting apparatus, which provides at the same location both for launching of waves in a plurality of different guided modes m or modal groups in the waveguide and detecting of waves from a plurality of different guided modes m or modal groups in waveguide.67. The system of any of claims 1 to 66, wherein the light comprises visible light.68. The system of any of claims 1 to 66, wherein the light comprises infra red light, including near infra red (NIR) light.69. 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.70. The apparatus of daini 69, wherein the optical coupler comprises a support which supports a coupling section of the waveguide, and an overlay which overlies the waveguide at the coupling section of the waveguide, the overlay having a refractive index greater than the refractive index of the cladding, and preferably greater than the refractive index of the core.71. The apparatus of claim 70, wherein the cladding has a reduced dimension at the coupling section of the waveguide, such that the -33 -evanescent fields from the guided modes m extend to a surface of the cladding.72. The apparatus of claim 71, wherein the coupling section of the waveguide includes a coupling face over which the overlay is disposed.73. The apparatus of claim 72, wherein the support has a support surface at which the coupling face of the waveguide is located and to which the overlay is disposed.74. The apparatus of any of claims 70 to 73, wherein the overlay includes a coupling face which overlies the coupling section of the waveguide and provides for coupling to the evanescent fields of the guided modes rn in the waveguide.75. The apparatus of claim 74, wherein the overlay further includes at least one propagation face located on the optical axis of the overlay, from which light components corresponding to the guided modes m of the waveguide are propagated from or into the overlay.76. The apparatus of claim 75, wherein the overlay comprises at least one optical element disposed to the respective at least one propagation face thereof, which acts to collimate light components propagated from the overlay corresponding to the respective guided modes m of the waveguide.77. The apparatus of claim 76, wherein the or each optical element is a lens, preferably a cylindrical lens.78. The apparatus of any of claims 70 to 77, wherein the overlay comprises a prism.-34 - 79. The apparatus of any of claims 70 to 78, wherein the Haunch unit comprises a plurality of optical assemblies, each configured to deliver a light component at a propagation angle corresponding to a respective one of the guided modes m of the waveguide.80. The apparatus of claim 79, wherein the optical assemblies each comprise a beam combiner for combining light from a plurality of light sources, each having a subsequently-resolvable characteristic, in a single beam, which provides for light to be selectively launched in a respective one of the guided modes m of the waveguide.81. The apparatus of claim 80, wherein the optical assemblies each comprise a light source, preferably a laser light source, which provides for light to be selectively launched in a respective one of the guided modes rn of the wavequide.82. The apparatus of claim 80, wherein groups of the optical assemblies are commonly coupled to respective light sources, whereby light from separate light sources is selectively launched into respective modal groups, each comprising groups of individual guided modes in of the wave guide.83. The apparatus of any of claims 79 to 82, wherein the optical assemblies each comprise processing optics for processing light to be launched in the waveguide.84. The apparatus of any of claims 79 to 83, wherein the optical assemblies each comprise a beam director for directing the respective light beam at the required propagation angle for coupling into the respective mode rn in the waveguide.-35 - 85. A detecting apparatus for detecting light from a plurahty of different modes m or modal groups of a muitHmodai 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.86. The apparatus of claim 85, wherein the optical coupler comprises a support which supports a coupling section of the waveguide, and an overlay which overlies the waveguide at the coupling section of the waveguide, the overlay having a refractive index greater than the refractive index of the cladding, and preferably greater than the refractive index of the core.87. The apparatus of claim 86, wherein the ciadding has a reduced dimension at the coupling section of the waveguide, such that the evanescent fields from the guided modes rn extend to a surface of the cladding.88. The apparatus of claim 87, wherein the coupling section of the waveguide includes a coupling face over which the overlay is disposed.89. The apparatus of claim 88, wherein the support has a support surface at which the coupling face of the waveguide is located and to which the overlay is disposed.90. The apparatus of any of claims 86 to 89, wherein the overlay includes a coupling face which overlies the coupling section of the waveguide and provides for coupling to the evanescent fields of the guided modes m in the waveguide.-36 - 91. The apparatus of claim 90, wherein the overlay further includes at east one propagation face located on the optical axis of the overlay, from which light components corresponding to the guided modes m of the waveguide are propagated from or into the overlay.92. The apparatus of claim 91, wherein the overlay comprises at east one optical element disposed to the respective at least one propagation face thereof, which acts to collimate light components propagated from the overlay corresponding to the respective guided modes m of the waveguide.93. The apparatus of c!aim 92, wherein the or each optical element is a lens, preferably a cylindrical lens.94. The apparatus of any of claims 86 to 93, wherein the overlay comprises a prism.95. The apparatus of any of claims 85 to 94, wherein the detecting unit comprises a plurality of optical assemblies, each configured to collect light from a respective one of the light beams propagated from the optical coup!er, which correspond to the respective guided modes rn of the waveguide.96. The apparatus of claim 95, wherein the optical assemblies each include a beam director for separating the light beams propagated from the optical coupler.97. The apparatus of claim 96, wherein the detecting unit further comprises a positioner for positioning the optical coupler relative to the beani directors of the optical assemblies, such as to provide for a required orientation of the light beams corresponding to the guided modes rn of the waveguide relative to the beam directors of the optical assemblies.-37 - 98. The apparatus of claim 97, wherein the detecting unit further comprises instrumentation, which is operable to calibrate and configure the position of the optical coupler relative to the beam directors of the optical assemblies.99. The apparatus of any of claims 95 to 98, wherein the optical assemblies each further include processing optics for processing the collected light beam.100. The apparatus of claim 99, wherein the processing optics comprise at least one optical element for collimating the light beam corresponding to the respective mode in of the waveguide.101. The apparatus of claim 100, wherein the at least one optical element comprises a cylindrical lens.102. The apparatus of any of claims 95 to 101, wherein the optical assemblies each comprise a beam splitter for separating at east two resolvable light components from the received light beam associated with each respective mode rn of the waveguide.103. The apparatus of any of claims 95 to 102, wherein the optical assemblies each further comprise at least one detector for rJetecting the separated light from a respective one of the guided modes m of the waveguide.104. The apparatus of any of claims 95 to 102, wherein the detecting unit comprises a plurality of detectors, each for detecting light from groups of the individual modes m of the waveguide.105. A combined launching and detecting apparatus for selectiv&y launching light in a plurality of different modes rn 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 coupling, 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 light 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 rn of the waveguide.106. The apparatus of claim 105, wherein the optical coupier comprises a support which supports a couplinq section of the waveguide, and an overlay which overlies the waveguide at the coupling section of the waveguide, the overlay having a refractive index greater than the refractive index of the cladding, and preferably greater than the refractive index of the core.107. The apparatus of claim 106, wherein the cladding has a reduced dimension at the coupling section of the waveguide, such that the evanescent fields from the guided modes rn extend to a surface of the cladding.108. The apparatus of claim 107, wherein the coupling section of the waveguide includes a coupling face o'ier which the overlay is disposed.109. The apparatus of claim 108, wherein the support has a support surface at which the coupling face of the waveguide is located and to which the overlay is disposed. 39 -110. The apparatus of any of claims 106 to 109, wherein the overlay includes a coupUng face which overlies the coupling section of the waveguide and provides for coupling to and from the evanescent fields of the guided modes rn in the waveguide.111. The apparatus of claim 110, wherein the overlay further includes at least one propagation face located on the optical axis of the overlay, at which light components corresponding to the guided modes m of the waveguide are propagated from or into the overlay.112. The apparatus of claim 110, wherein the overlay includes first and second propagation faces located to opposite ends of the optical axis of the overlay, with the first propagation face being optically coupled to the launch unit such as to allow for launching of light into guided modes m of the waveguide in one direction through the waveguide and the second propagation face being optically coupled to the detecting unit such as to allow for detecting of light components from the guided modes m of the waveguide in the other, opposite direction through the waveguide.113. The apparatus of claim 111 or 112, wherein the overlay comprises at least one optical element disposed to the respective at least one propagation face thereof, which acts to collimate light components propagated from the overlay corresponding to the respective guided modes in of the waveguide.114. The apparatus of claimll3, wherein the or each optical element is a lens, preferably a cylindrical lens.115. The apparatus of any of claims 106 to 114, wherein the overlay comprises a prism.-40 - 116. The apparatus of any of claims 105 to 115, wherein the launch unit comprises a plurality of optical assemblies, each configured to deliver a light component at a propagation angle corresponding to a respective one of the guided modes m of the waveguide.117. The apparatus of claim 116, wherein the optical assemblies of the launch unit each comprise a beam combiner for combining light from a plurality of light sources, each having a subsequently-resolvable characteristic, in a single beam, which provides for light to be selectively launched in a respective one of the guided modes m of the waveguide.118. The apparatus of claim 117, wherein the optical assemblies of the launch unit each comprise a light source, preferably a laser light source, which provides for light to be selectively launched in a respective one of the guided modes m of the waveguide.119. The apparatus of claim 117, wherein groups of the optical assemblies of the launch unit are commonly coupled to respective light sources, whereby light from separate light sources is selectively launched into respective modal groups, each comprising groups of individual guided modes m of the waveguide.120. The apparatus of any of claims 116 to 119, wherein the optical assemblies of the launch unit each further comprise processing optics for processing light to be launched in the wavequide.121. The apparatus of any of claims 116 to 120, wherein the optical assemblies of the launch unit each further comprise a beam director for directing the respective light beam at the required propagation angle for coupling into the respective mode rn in the waveguide.-41 - 122. The apparatus of any of claims 105 to 121, wherein the detecting unit comprises a plurality of optical assemblies, each configured to collect light from a respective one of the light beams propagated from the optical coupler, which correspond to the respective guided modes m of the waveguide.123. The apparatus of claim 122, wherein the optical assemblies of the detecting unit each include a beam director for separating the light beams propagated from the optical coupler.124. The apparatus of claim 123, wherein the detecting unit further comprises a positioner for positioning the optical coupler relative to the beam directors of the optical assemblies of the detecting unit, such as to provide for a required oheritation of the Ught beams corresponding to the guided modes rn of the waveguide relative to the beam directors of the optical assemblies of the detecting unit.125. The apparatus of claim 124, wherein the detecting LInt further comprises instrumentation, which is operable W calibrate and configure the position of the optical coupler relative to the beam directors of the optical assembhes of the detecting unit.126. The apparatus of any of claims 122 to 125, wherein the optical assemblies of the detecting unit each further include processing optics for processing the collected light beam.127. The apparatus of claim 126, wherein the processing optics comprise at least one optical element for collimating the light beam corresponding to the respective mode m of the waveguide.128. The apparatus of claim 127, wherein the at least one optical element comprises a cylindrical lens. -42129. The apparatus of any of claims 122 to 128, wherein the optical assemblies of the detecting unit each comprise a beam splitter for separating at least two resolvable light components from the received Ught beam associated with each respective mode in of the waveguide.130. The apparatus of any of ciaims 122 to 129, wherein the optical assemblies of the detecting unit each further comprise at least one detector for detecting the separated light from a respective one of the guided modes rn of the waveguide.131. The apparatus of any of claims 122 to 129, wherein the detecting unit comprises a plurality of detectors, each for detecting light from groups of the individual modes m of the waveguide.132. A multiplexed optical communications method, comprising the steps of: selectively launching light in a plurality of different modes in or modal groups of a multi-modal 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-muitiplex light from a plurality of separate modes ai in the waveguide into individual signals; and processing the de-rnultiplexed light components.133. The method of claim 132, wherein the step of selectively launching light comprises selectively launching light in a plurality of different modes in or modal groups of a multi-modal waveguide through an optical coupler which is coupled to the waveguide, and the step of detecting light beams comprises detecting light beams propagated from a plurality of difierent modes rn or modal groups in the waveguide through. an optical coupler which is coupled to the Wave guide -. 43 134. The method of claim 133, wherein the step of selectively launching light and the step of detecting light beams are performed using separate optical couplers.135. The method of claim 133, wherein the step of selectively launching light and the step of detecting light beams are performed using a cornrnor1 optical coupler.136. The method of any of claims 133 to 135, wherein the or each optical coupler comprises a support which supports a coupling section of the waveguide, and an overlay which overlies the waveguide at the coupling section of the waveguide, the overlay having a refractive index greater than the refractive index of the cladding, and preferably greater than the refractive index of the core.137. The method of claim 136, 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.138. The method of claim 137, wherein the coupling section of the waveguide includes a coupling face over which the overlay is disposed.139. The method of claim 138, wherein the coupling face comprises a flat, polished face.140. The method of claim 138 or 139, wherein the coupling face extends substantially parallel to the longitudinal axis of the core of the waveguide.141. The method of claim 138 or 139, wherein the coupling face is inclined relative to the longitudinal axis of the core of the waveguide, whereby the overlay acts to tap the evanescent fields for the respective guided modes in at spaced locations along a length of the coupling face, thereby providing for increased spatial separation of light components corresponding to the respective guided modes m as propagated from the overlay.142. The method of claim 141, wherein the coupling face is inclined at an angle of less than 5 degrees, preferably less than 3 degrees, and more preferably less than 2 degrees.143. The method of claim 136, wherein the core of the waveguide is exposed at the coupling section of the waveguide.144. The method of claim 143, wherein the cladding is removed only from one side of the waveguide, so as to retain Integrity of the wavegulde.145. The method of any of claims 136 to 144, wherein the overlay includes a coupling face which overlies the coupling section of the wavegulde and provides for coupling to the evanescent fields of the guided modes m in the waveguide.146. The method of claim 145, whereIn the overlay further includes at least one propagation face located on the optical axis of the overlay, from which light components corresponding to the guided modes in of the waveguide are propagated from or into the overlay.147. The method of claim 146, wherein the or each propagation face of the overlay encloses an acute angle with the coupling face of the overlay.148. The method of claim 147, wherein the or each propagation face of the overlay is angled such that the fundamental mode m of the waveguide radIates substantially perpendicular to the respective propagation face. -. 45149. The method of any of claims 146 to 148, wherein the overlay includes first and second propagation faces located to opposite ends of the optical axis of the overlay, from which light components corresponding to the guided modes m of the waveguide are propagated from or into the overlay, thereby allowing for launching or detecting in both directions through the waveguide.150. The method of claim 149, wherein the overlay comprises at least one optical element disposed to the respective at least one propagation face thereof, which acts to collimate light components propagated from the overlay corresponding to the respective guided modes m of the waveguide.151. The method of claim 150, wherein the or each optical element is a lens, preferably a cylindrical lens.152. The method of any of claims 136 to 151, wherein the overlay is permanently mounted to the support.153. The method of any of claims 136 to 151, wherein the overlay is movably disposed relative to the support, and further comprising the step of: adjusting a spacing between the overlay and the coupling section of the waveguide, such as to control an extent of the evanescent coupling.154. The method of any of claims 136 to 151, wherein the overlay comprises a prism.155. The method of any of claims 132 to 154, wherein the step of selectively launching light comprises the step of launching a plurality of light components, each as a single beam at a propagation angle -46 -corresponding to a respective one of the guided modes m of the waveguide.156. The method of daim 155, wher&n one or more of the Ught components comprise light combined from a plurality of light sources, preferably laser light sources, each having a subsequently-resolvable characteristic, in a single beam.157. The method of claim 156, wherein the light sources provide light with different polarization 158. The method of claim 156, wherein the light sources provide light of different wavelength 159. The method of any of claims 155 to 158, wherein one or more of the light components comprise light from separate, respective light sources, preferably a laser light source.160. The method of any of claims 155 to 159, wherein one or more groups of the light components comprise light from separate, respective light sources, preferably laser light sources, whereby light from separate light sources is selectively launched into respective modal groups.161. The method of any of claims 155 to 160, wherein one or more of the launched light beams are shaped to have a predetermined profile for the respective mode m of the waveguide.162. The method of any of claims 151 to 161, wherein the launched light beams are each directed by beam directors at the required propagation angle for coupling into the respective mode m in the wave g u d e.-47 - 163. The method of claim 162, wherein the launching beam directors comprise a reflector, preferably a prismatic reflector or a mirror reflector.164. The method of any of claims 132 to 163, wherein the step of detecting light beams comprises the step of separately collecting light from each of the respective detected flght beams, which correspond to the respective guided modes m of the waveguide.165. The method of claim 164, wherein the detected ight beams are directed by beam directors such as to separate the detected light beams.166. The method of claim 165, wherein the detecting beam directors comprise a reflector, preferably a prismatic reflector or a mirror reflector.167. The method of claim 165 or 166, further comprising the step of: positioning the optical coupier relative to the detecting beam directors, such as to provide for a required orientation of the ight beams corresponding to the guided modes m of the waveguide relative to the detecting beam directors.168. The method of claim 167, further comprising the step of: calibrating the detected light beams; and adjusting a position of the optical coupler relative to the beam directors in response to the step of calibrating the light beams.169. The method of any of claims 164 to 168, further comprising the step of: collimating one or more of the detected light beams. -. 48170. The method of any of claims 164 to 169, further comprising the step of: separating at east two resolvable light components from one or more of the detected light beams.171. The method of daim 170, wherein the one or more detected light beams have light components with at east two different polarizations.172. The method of claim 170, wherein the one or more detected light beams have light components with at east two dftferent wavelengths.173. The method of any of claims 132 to 172, further comprising the step of: detecting light from one or more of the detected light beams, each using separate detectors.174. The method of any of claims 132 to 172, further comprising the step of: detecting light from one or more groups of the detected light beams, each using separate detectors.175. The method of any of daims 132 to 174, further comprising the steps of: obtaining information on performance of the communication over the waveguide; and analysing propagating light in each mode in and/or measurement of fiber mode properties.176. The method of any of claims 136 to 175, wherein the overlay propagates light from the guided modes m of the waveguide aong the optical axis of the waveguide. 49 -177. The method of any of claims 136 to 175, wherein the overlay propagates light from the guided modes rn of the wavequide laterally to the optical axis of the waveguide.178. The method of any of claims 136 to 175, wherein the overlay is a prismatic beam splitter which separates the light from each guided mode m of the waveguide into at least two components.179. The method of claim 178, wherein the prismatic beam splitter propagates a first component of the light from each of the guided modes m of the waveguide laterally to the optical axis of the waveguide and a second component of the light from each of the guided modes m of the waveguide along the optical axis of the waveguide.180. The method of any of claims 132 to 179, wherein the light coupled to a guided mode m of the waveguide is a DWDM channel, the step of selectively launching light comprises launching multiple DWDM channels in respective modes m of the fiber, and the step of detecting light beams comprises detecting multiple DWDM channels in respective modes m of the fiber.181. The method of any of claims 132 to 180, wherein the waveguide comprises an optical fiber.182. The method of any of claims 132 to 181, wherein the step of launching Ught comprises launching light at a plurality of locations within the waveguide.183. The method of any of claims 132 to 182, wherein the step of detecting light comprises detecting light at a plurality of locations within the waveguide.-50 - 184. The method of any of claims 132 to 183, wherein the waveguide forms part of an optical network, optionally a telecommunications network, 185. The method of any of claims 132 to 184, wherein the light comprises visible light! 186. The method of any of claims 132 to 184, wherein the light comprises infra red light, including near infra red (NW) light, 187. 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; 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! 188. The optical coupler of claim 187, 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 clad di rig.189. The optical coupler of claim 188, wherein the coupling section of the waveguide includes a coupling face over which the overlay is disposed.190. The optical coupler of claim 189, wherein the coupling face comprises a flat, polished face.191. The optical coupler of claim 189 or 190? wherein the coupling face extends substantially parallel to the longitudinal axis of the core of the waveguide.-51 - 192. The optical coupler of claim 189 or 190, wherein the coupling face is inclined relative to the longitudinal axis of the core of the waveguide, such that the overlay acts to tap the evanescent fields for the respective guided modes m at spaced locations along a length of the coupling face, thereby providing for increased spatial separation of light components corresponding to the respective guided modes m as propagated from the overlay.193. The optical coupler of claim 192, wherein the coupling face is incUned at an angle of less than 5 degrees, preferably less than 3 degrees, and more preferably less than 2 degrees.194. The optical coupler of claim 187, wherein the core of the waveguide is exposed at the coupiing section of the waveguide.195. The optical coupler of claim 194, wherein the cladding is removed only from one side of the waveguide, so as to retain integrity of the wa veg u ide.196. The optical coupler of any of claims 187 to 195, wherein the support has a support surface at which the coupling face of the waveguide is located and to which the overlay is disposed.197. fhe optical coupler of claim 196, wherein the support surface includes a recess in which the waveguide is located, with the coupling face of the waveguide being parallel to or located just below the support surface.198. The optical coupler of any of claims 187 to 197, wherein the overlay includes a coupling face which overlies the coupling section of the waveguide and provides for coupling to the evanescent fields of the guided modes m in the waveguide.-52 - 199. The optical coupler of claim 198, wherein a coupling medium is provided between an interface of the coupling section of the waveguide and the overlay.200. The optical coupler of claim 199, wherein the coupling medium is a high refractive index oil, preferably having substantially the same refractive index as the overlay.201. The optical coupler of claim 199, wherein the coupling medium is a high refractive index adhesive, preferably having substantially the same refractive index as the overlay.202. The optical coupler of any of claims 198 to 201, wherein the overlay further includes at least one propagation face located on the optical axis of the overlay, from which light components corresponding to the guided modes in of the wavequide are propagated from or into the overlay.203. The optical coupler of claim 202, wherein the or each propagation face of the overlay encloses an acute angle with the coupling face of the overlay.204. The optical coupler of claim 202 or 203, wherein the or each propagation face of the overlay is angled such that the fundamental mode m of the waveguide radiates substantially perpendicular to the respective propagation face.205. The optical coupler of any of claims 202 to 204, wherein the overlay includes first and second propagation faces located to opposite ends of the optical axis of the overlay, from which light components corresponding to the guided modes rn of the waveguide are propagated from or into the overlay, thereby allowing for launching or cietecting in both directions through the waveguide.206. The optical coupler of any of claims 202 to 205, wherein the overlay comprises at least one optical element disposed to the respective at least one propagation face thereof, which acts to colflmate light components propagated from the overlay corresponding to the respective guided modes m of the waveguide.207. The optical coupler of claim 206, wherein the or each optical element is a lens, preferably a cylindrical lens.208. The optical coupler of daim 206 or 207, wherein the or each optical element is fixed to the at least one propagation face of the overlay.209. The optical coupler of claim 206 or 207, wherein the or each optical element is integrally formed from the material of the overlay.210. The optical coupler of any of claims 187 to 209, wherein the overlay is permanently mounted to the support.211. The optical coupler of any of claims 187 to 209, wherein the overlay is movably disposed relative to the support, and the optical coupler further comprises a positioner which allows for adjustment of a spacing between the overlay and the coupling section of the waveguide, such as to control an extent of the evanescent coupling.212. The optical coupler of any of claims 187 to 211, wherein the overlay is formed of glass.213. The optical coupler of any of claims 187 to 212, wherein the overlay comprises a prism. 54 -214. 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 in 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, and 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 longitudinai, optical axis of the core of the waveguide, such that the overlay acts to tap the evanescent fields for the respective guided modes m at spaced locations along a length of the coupling face, thereby providing for increased spatial separation of light components corresponding to the respective guided modes m as propagated from the overlay.215. The optical coupler of claim 214, wherein the coupling face is inclined at an angle of less than 5 degrees.216. The optical coupler of claim 214, wherein the coupling face is inclined at an angle of less than 3 degrees.217. The optical coupler of claim 214, wherein the coupling face is inclined at an angle of less than 2 degrees.218. An overlay for coupling, at least partially, light to and/or from guided modes rn of a multi-modal waveguide, the overlay comprising: -55 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 east 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.219. The optical coupler of claim 218, wherein the or each propagation face of the overlay is angled such that the fundamental mode m of the waveguide radiates substantially perpendicular to the respective propagation face.220. The optical coupler of claim 218 or 219, wherein the overlay includes first and second propagation faces located to opposite ends of the optical axis of the overlay, from which light components corresponding to the guided modes m of the waveguide are propagated from or into the overlay, thereby allowing for launching or detecting in both directions through the waveguide.221. The optical coupler of any of claims 218 to 220, wherein the overlay comprises at least one optical element disposed to the respective at least one propagation face thereof, which acts to collimate light components propagated from the overlay corresponding to the respective guided modes m of the waveguide.222. The optical coupler of claim 221, wherein the or each optical element is a lens! preferably a cylindrical lens.223. The optical coupler of claim 221 or 222, wherein the or each optical element is fixed to the at least one propagation face of the overlay.-56 - 224. The optical coupler of claim 221 or 222, wherein the or each optical element is integrally formed from the material of the overlay.225. 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 light in both directions through the waveguide.226. The optical coupler of claim 225, wherein the propagation faces of the overlay are angled such that the fundamental mode m of the waveguide radiates substantially perpendicular to the respective propagation face.227. The optical coupler of claim 225 or 226, wherein the overiay comprises first and second optical elements disposed to the respective propagation faces thereof, which act to collimate light components propagated from the overlay corresponding to the respective guided modes m of the waveguide.228. The optical coupler of claim 227, wherein one or each of the optical elements is a lens, preferably a cylindrical lens.229. The optical coupler of claim 227 or 228, wherein one or each of the optical elements is fixed to the respective propagation face.230. The optical coupler of claim 227 or 228, wherein one or each of the optical elements is integrally formed from the material of the overlay. 57 -231. An overlay for coupling, at least partiaUy, light to and/or from guided modes n-i of a multi-modal waveguide, the overlay comprising: a coupling face which in use overiles a coupling section of the waveguide and provides for coupling to the evanescent fields of the guided modes m 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 east one propagation face, which acts to collimate light beams propagated from the overlay corresponding to the respective guided modes m of the wave guide.232. The optical coupler of claim 231, wherein the or each optical element is a lens! preferably a cylindrical lens.233. The optical coupler of claim 231 or 232, wherein the or each optical element is fixed to the at least one propagation face of the overlay.234. The optical coupler of claim 231 or 232, wherein the or each optical element is integrally formed from the material of the overlay.235. The optical coupler of claim 231, wherein the overlay includes first and second propagation faces located to opposite ends of the optical axis of the overlay, from which light components corresponding to the guided modes m of the waveguide are propagated from or into the overiay, thereby aliowing for launching or detecting in both directions through the waveguide.236. The optical coupler oF claim 235, wherein the overlay comprises first and second optical elements disposed to the respective propagation faces thereof, which act to collimate light components propagated -58 -from the overlay corresponding to the respective guided modes m of the waveguide.237, The optical coupler of claim 236, wherein one or each of the optical elements is a lens, preferably a cylindrical lens.238. The optical coupler of claim 236 or 237, wherein one or each of the optical elements is fixed to the respective propagation face.239. The optical coupler of claim 236 or 237, wherein one or each of the optical elements is integrally formed from the material of the overlay.</claim-text>