WO2012095641A1

WO2012095641A1 - Assembly for multiplexing light of different polarisation states and wavelengths

Info

Publication number: WO2012095641A1
Application number: PCT/GB2012/050001
Authority: WO
Inventors: Peter Wigley; Ian Peter Mcclean
Original assignee: Oclaro Technology Limited
Priority date: 2011-01-11
Filing date: 2012-01-03
Publication date: 2012-07-19
Also published as: GB201100413D0; GB2487194A

Abstract

An assembly is provided for multiplexing light having a plurality of wavelengths and polarised states so as to produce an output of generally unpolarised light having at least two components with different wavelengths. The assembly comprises a plurality of polarisation multiplexers and a selective polarisation rotation device configured to rotate the polarisation of light at a first wavelength through a first controlled angle and to rotate light of a second wavelength through a second controlled angle.

Description

ASSEMBLY FOR MULTIPLEXING LIGHT OF DIFFERENT POLARISATION STATES AND WAVELENGTHS

Technical Field The present invention relates to a method and assembly for multiplexing light of different polarisation states and wavelengths. In particular, although not exclusively, the invention relates to a system for multiplexing components for pumping a Raman amplifier, and such a system integrated into a single package. Background

In this specification the term "light" will be used in the sense that it is used in optical systems to mean not just visible light, but also electromagnetic radiation having a wavelength outside that of the visible range.

A Raman amplifier is an optical amplifier based on Raman gain, which results from the effect of stimulated Raman scattering. The Raman-active gain medium is often an optical fibre, although it can also be a bulk crystal, a waveguide in a photonic integrated circuit, or a cell with a gas or liquid medium. In an optical fibre Raman amplifier an input signal is amplified by providing co-propagating and/or or counter-propagating pump light, usually provided by a pump laser or lasers. The wavelength of the pump light is typically a few tens of nanometres shorter than the signal wavelength when using a Silica optical fibre as the gain medium. Raman amplifiers are often used for optical telecommunications to allow optical data signals to travel further along system fibre. In these amplifiers a Raman pump module is necessary that outputs pump light in various forms depending upon the requirement. Raman amplifiers can be operated in many different wavelength regions, provided that a suitable high power pump light source, or sources, are available. It is also very common for multiple pumps to be simultaneously coupled in a Raman pump module for several reasons. To achieve high gains in such amplifiers it is often desirable to combine multiple pump lasers to provide higher pump light power than can be achieved with a single pump at any wavelength. Raman gain is also very dependent on polarisation, and a pump orthogonally polarised to a signal will provide essentially no gain, so a polarisation multiplexer is generally used to combine the outputs of two lasers operating with orthogonal polarisations at the same wavelength, so as to minimise this polarisation-dependent gain as far as possible. This is sometimes called a polarisation based depolariser, but strictly it does not depolarise the light: it merely mixes two polarisation states in roughly equal proportions to give equal contribution to parallel and orthogonal Raman gain contributions in the Raman active gain media. Once the polarisation states have been mixed, the light will lose coherence as it travels through the media. In all these cases it is clear that there is a need to combine more than one pump source to provide the required output from the Raman pump module. Also the gain spectrum of the amplifier is affected by the choice of pump wavelengths, and often pumps of different wavelengths are multiplexed or coupled to provide a wider useable gain bandwidth than can be achieved with a single pump wavelength.

In many Raman systems, both a polarisation based depolariser and a wavelength multiplexer is used to combine lasers including those having different wavelengths. Figure 1 is a schematic illustration of a typical four-laser, two-wavelength multiplexer, which provides separate polarisation and wavelength multiplexing of four laser sources 101 -104 with two wavelengths and two orthogonal polarisations. The outputs of two lasers 101 , 102 operating at a first wavelength λ-ι, and orthogonal polarisations, are combined in one polarisation multiplexer 105. The outputs of the other two lasers 103, 104 operating at a second wavelength λ₂ and orthogonal polarisations are combined in another polarisation multiplexer 106. The outputs of the polarisation multiplexers 105, 106 are combined in a wavelength multiplexer 107 to provide generally unpolarised light of two wavelengths λ-ι, λ₂. Polarisation multiplexing is accomplished using a variety of techniques including fused- fibre couplers with polarisation-maintaining fibre and crystal-based micro-optic polarisation splitters. Wavelength multiplexing is generally achieved using micro-optic thin film multiplexers and fibre-based fused biconic taper or Mach-Zehnder interferometer multiplexers.

One problem with this arrangement is that later optical components have a tendency to "re-polarise" unpolarised light due to their polarisation dependent properties. So the passage of light through the wavelength multiplexer will cause a certain amount of re- polarisation of light. It would therefore be desirable to provide polarisation multiplexers as far downstream in the system as possible. In addition, Raman pump modules also emit light consisting of several longitudinal frequency modes which is favourable in order to reduce stimulated Brillouin Scattering (SBS) effects. The total emitted Raman pump light power is split between the modes, and this results in an effective increase in the pump power threshold for SBS (which is caused by high power pump propagation within the system fibre). It is difficult to achieve multi-longitudinal spectra over the full operating power range of a Raman pump from just above threshold to maximum emission power. It would therefore be desirable to reduce the dynamic range of the pump output power to allow a more optimal pump design.

Summary

In accordance with one aspect of the present invention there is provided an assembly for multiplexing light having a plurality of wavelengths and polarised states so as to produce an output of generally unpolarised light having at least two components with different wavelengths. The assembly comprises one or more polarisation multiplexers and at least one selective polarisation rotation device configured to rotate the polarisation of light at a first wavelength through a first controlled angle and to rotate the polarisation of light at a second wavelength through a second controlled angle.

The first and second controlled angles may be the same, or may be different from one another. The first controlled angle may be approximately 0° or an integer multiple of 180° and the second controlled angle approximately 90° or an integer multiple of 180° plus 90°. The selective polarisation rotation device will then allow light at the first wavelength to pass therethrough with its polarised state substantially unchanged, but will rotate the polarisation state of light at the second wavelength by approximately 90°.

The assembly may be configured to propagate into an output polarisation multiplexer a first input of light in a first polarised state and having components with the first wavelength and second wavelength, and a second input of light in a second polarised state (optionally substantially orthogonal to the first polarisation state) and having components with the first wavelength and second wavelength. The output polarisation multiplexer may be configured to combine said inputs to produce generally unpolarised light having components with the first and second wavelengths. The first and second inputs may be incoherent and may be of substantially equal power.

The assembly may be configured so that light propagated into the selective polarisation rotation device produces a first output of light in the first polarised state and having components with the first wavelength and second wavelength, and a second output of light in the second polarised state and having components with the first wavelength and second wavelength, the first output from the selective polarisation rotation device being coupled to the first input to the output polarisation multiplexer, and the second output from the selective polarisation rotation device being coupled to the second input to the output polarisation multiplexer.

First and second inputs to the selective polarisation rotation device may be provided, so that light having a component with the first wavelength and the first polarised state and a component with the second wavelength and the second polarised state can be propagated into the selective polarisation rotation device via the first input and acted on by the device to produce the first output, and light having a component with the first wavelength and the second polarised state and a component with the second wavelength and the first polarised state can be propagated into the selective polarisation rotation device via the second input, and is acted on by the device to produce the second output.

The assembly may comprise a first polarisation multiplexer for multiplexing light having a component with the first wavelength and the first polarised state with light having a component with the second wavelength and the second polarised state, where an output of the first polarisation multiplexer is coupled to the first input of the selective polarisation rotation device. A second polarisation multiplexer may be provided for multiplexing light having a component with the first wavelength and the second polarised state and light having a component with the second wavelength and the first polarised state, where an output of the second polarisation multiplexer is coupled to the second input of the selective polarisation rotation device.

A first light source may be coupled to the first polarisation multiplexer and configured to emit light at the first wavelength in the first polarised state. A second light source may be coupled to the first polarisation multiplexer and configured to emit light at the second wavelength in the second polarised state. A third light source may be coupled to the second polarisation multiplexer and configured to emit light at the first wavelength in the second polarised state. A fourth light source may be coupled to the second polarisation multiplexer and configured to emit light at the second wavelength in the first polarised state.

Alternatively, the assembly may comprise first and second light sources, configured to emit polarised light at the first and second wavelengths, respectively. The outputs from each the first and second light sources may be split into two propagation paths which are coupled to the first and second polarisation multiplexers. One propagation path of each output may pass through a 90° splice to rotate its polarised state by 90° before coupling to the respective polarisation multiplexers.

The inputs to the first and second polarisation multiplexers may be provided from outputs of further selective polarisation rotation devices. Additional polarisation multiplexers may be included upstream of the further selective polarisation rotation devices.

The selective polarisation rotation device may be configured to rotate the polarisation state of light incident thereon by an amount varying periodically with respect to the frequency or wavelength of the light. In this context, it will be appreciated that a property of a material which varies periodically with respect to frequency will also effectively vary periodically with respect to wavelength to a first order approximation over a short range of wavelengths. In this specification, although a frequency periodicity is technically correct, references are generally made to wavelength periodicity, since this is the terminology generally used in the art.

The selective polarisation rotation device may comprise a birefringent material, and may comprise one or more birefringent crystals. Alternatively or in addition, the selective polarisation device may be formed from an optically active, electro-optic or magneto-optic element or comprise one or more optically active, electro-optic or magneto-optic elements.

Further selective polarisation rotation devices having a periodicity of rotation with respect to frequency or wavelength double that of the selective polarisation rotation device may also be included. Further selective polarisation rotation devices having a periodicity of rotation with respect to frequency or wavelength which are an integer multiple of that of the selective polarisation rotation device may also be included. The assembly may be configured to multiplex light of four or more substantially equally spaced frequencies or wavelengths. Light having first and third wavelengths may be polarisation multiplexed and selectively rotated by one of the further selective polarisation rotation devices to produce light having components with the first and third wavelengths in the first polarised state and an output having components with the first and third wavelengths in the second polarised state. Light having second and fourth wavelengths may be polarisation multiplexed and selectively rotated by another of the further selective polarisation rotation devices to produce an output having components with the second and fourth wavelengths in the first polarised state and an output having components with the second and fourth wavelengths in the second polarised state. The light having components with the first and third wavelength in the first polarised state may be multiplexed with the light having components with the second and fourth wavelengths in the second polarised state to produce light propagated into the first input of the selective polarisation rotation device. The light having components with the first and third wavelength in the second polarised state may be multiplexed with the output having components with the second and fourth wavelengths in the first polarised state to produce light propagated into the second input of the selective polarisation rotation device.

The selective polarisation rotation device may be adjustable so as to adjust a power output of the assembly. Adjustment of the selective polarisation rotation device may include rotation of the device so as to control the first and/or second controlled angles by which the polarisation of light is rotated. In addition or alternatively, adjustment of the selective polarisation device may include tilting the device so as to control a free spectral range of the device. The adjustment of the device may result in selective adjustment of transmission of light through the device. Additional adjustable selective polarisation rotation devices may be included for controlling the relative power at different wavelengths. Adjustment of the selective polarisation rotation device may include use of an electro-optic material in combination with a variable applied voltage. The assembly may further comprise a plurality of lasers for providing light to be multiplexed. Optical isolators may be provided between the lasers and an output of the assembly. The assembly may be provided as an integrated unit. The invention also provides a Raman pump unit comprising the assembly as described above. The invention also provides a Raman amplifier comprising the assembly described above and a Raman pumped gain block.

In accordance with another aspect of the present invention there is provided a method of multiplexing light having a plurality of wavelengths and polarised states so as to produce an output of generally unpolarised light having at least two components with different wavelengths. The method comprises passing the light through a plurality of polarisation multiplexers and a selective polarisation rotation device configured to rotate the polarisation of light at a first wavelength through a first controlled angle and to rotate light of a second wavelength through a second controlled angle.

Brief Description of the Drawings

Some preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a schematic illustration of a four-laser, two-wavelength multiplexer;

Figure 2 is an exemplary schematic drawing of a birefringent crystal placed between two crossed polarisers;

Figure 3 illustrates the transmission function of an exemplary set of components arranged as shown in Figure 2; Figure 4 is a schematic drawing of a four-laser, two-wavelength multiplexer;

Figure 5 is a schematic drawing of a two-laser, two-wavelength multiplexer;

Figure 6 is a schematic drawing of an eight-laser, four-wavelength multiplexer; and Figure 7 illustrates the transmission function of two of the crystals shown in Figure 6.

Figure 8 is a schematic drawing of a four-laser, two-wavelength multiplexer with optical isolators;

Detailed Description

As discussed in the introduction, wavelength multiplexing can be achieved using micro- Mach-Zehnder interferometer multiplexers. As will be discussed below, Mach-Zehnder functionality can also be obtained using a crystal-based micro-optic approach. In order to understand the principles of the invention, some aspects of the behaviour of birefringent crystals will be discussed. This discussion is provided to assist in understanding the operation of the invention, and should not be considered to be limiting.

Figure 2 is an exemplary schematic drawing of a birefringent crystal 203 placed between two crossed polarisers 201 , 202 that will produce a transmission function that varies sinusoidally with respect to wavelength in a manner similar to a Mach-Zehnder type wavelength multiplexer. This concept has been previously applied to interleaver filter designs, and is commonly referred to as a Lyot or Sole filter. Figure 2 shows one possible combination of input and output polarisers 201 , 202 having axes 204, 205 which are orthogonal to each other and each orientated at 45° relative to the c-axis 206 of the crystal 203.

In this crystal orientation, if unpolarised light 208 is incident on the assembly, half of the incident light experiences a group refractive index n_e and the other half experiences a group refractive index n₀. The difference between the two group refractive indices is the birefringence of the crystal, Δη₉=η_β9-η₀₉. The transmission coefficient is given by:

where φ_σ is a phase constant, f is the optical frequency, and τ is a time delay defined by where L is the thickness of the crystal and c is the speed of light.

The transmission spectrum has a period (in the frequency domain) of 1/τ. This corresponds to a period in the wavelength domain (in a first order approximation) of λ²

, which is more commonly referred to as the free spectral range (FSR). Rotating

LAn

the crystal slightly tunes φ₀ and modifies the precise location of the wavelength peaks. As an example, the birefringence of crystalline quartz at 1500 nm is Δη=0.0085. Figure 3 illustrates the transmission function of the arrangement of Figure 2 using such quartz with a crystal length of 26.47 mm. The FSR is approximately 10 nm for light at 1500 nm. The solid line 301 is the spectrum for the system exactly as shown in Figure 2, where the polarisers 201 , 202 are orthogonal to each other. It will be appreciated that a crystal material with larger birefringence can be used to shorten the material length required to produce the same FSR. If the input polariser 201 in Figure 2 is rotated by 90° so that its polarisation axis 204 is parallel to the axis 205 of the output polariser 202, the transmission spectrum shifts to the dashed line 302 of Figure 3. From these plots, it is clear that the crystal has the effect of rotating the polarisation of the light at 1481 nm by 90° to allow it to pass through the second polariser 202 of Fig. 2. Light at 1486 nm emerges from the crystal 203 with the same polarisation as at the input and so cannot pass through the second polariser. Both wavelengths experience a time delay between the ordinary and extraordinary refractive indices n₀, n_e. However, for light at 1481 nm, the relative delay between the two has changed by 90° at the output. The crystal 203 operates as a half- wave plate for this case. For light at 1486 nm, the relative delay at the output is the same as at the input. It will be appreciated that a crystal thickness which produces delays which are multiples of 90° will also produce output polarizations which are orthogonal to the input and that crystal thickness which produces delays which are multiples of 180° will also produce output polarizations which are parallel to the input. .This property can be used to assist with the construction of a device having wavelength combining functionality, as is described below.

Figure 4 is a schematic diagram of a system suitable for multiplexing the outputs of four lasers 401 , 402, 403, 404 at two different wavelengths λ₁₍ λ₂. The lasers at each wavelength emit light at orthogonal polarisations to each other (herein referred to as parallel and perpendicular polarisations), but light from the two lasers at the same wavelength are not multiplexed immediately. In the example shown in figure 4, a first laser 401 emits parallel light at a first wavelength (λ-ι, || ), a second laser 402 emits perpendicular light at a second wavelength (λ₂,±), a third laser 403 emits perpendicular light at the first wavelength (λ-ι,_Ι_) and a fourth laser 404 emits parallel light at the second wavelength (λ₂, || ). The polarisation of each laser source is known and is maintained in the optical path 405, 406, 407, 408. The optical path could be realised in free-space, fibre or other waveguides. It can be seen from Figure 4 that the structure of Figure 2 is effectively recreated where the input PMux (409, 413) acts as the input polariser (201 ) and the output PMux (416) acts as the output polariser (202) and the birefringent crystal (41 1 ) is the same as that in Figure 2 (203). Light from the first two lasers 401 , 402, at different wavelengths and different polarisations, is combined using a polarisation multiplexer 409 so as to produce light with two different wavelengths and different polarisations (λ-ι, || ;λ₂,±), propagating along the same path 410. This light is then transmitted through a birefringent crystal 41 1 similar to the crystal 203 shown in Figure 2 (i.e. the crystal is such that it operates as a half-wave plate for the first wavelength and does not affect the polarisation of the second wavelength as described in the example above). In this example, the crystal 41 1 is configured to rotate the polarisation of the light at the second wavelength λ₂ by 90° but transmit light of the first wavelength λ-ι with its polarisation unchanged. This results in light at two wavelengths but the same polarisation (λ-ι, || ; λ₂, || ) passing along the subsequent propagation path (free space, fibre or waveguide) 412. Similarly, light from the third and fourth lasers 403, 404 is combined using a polarisation multiplexer 413 to produce light with different wavelengths and polarisations (λ-ι,_Ι_; λ₂, || ) propagating along an optical path 414. This light also passes through the crystal 41 1 (or another crystal with similar properties) so that the polarisation of light with the second wavelength λ₂ is rotated by 90° but light with the first wavelength λ-ι is transmitted unchanged. This results in light at both first and second wavelengths but the similar polarisation (orthogonal to the polarisation of light in the propagation path 412) (λι,-L; λ₂,_Ι_) propagating along a subsequent propagation path 415. Light in both propagation paths 412, 415 is subsequently polarisation multiplexed at an output polarisation multiplexer 416 to produce a combined output at both first and second wavelengths and both parallel and orthogonal polarisations coupled into a single optical fibre. The coupling mechanisms between input fibres and polarisation multiplexers, between polarisation multiplexers and polarisation rotation crystals between polarisation rotation crystals and final polarisation multiplexer and from the final polarisation multiplexer into the output fibre is not shown. This may be accomplished through standard micro-optic fibre coupling techniques.

The use of polarisation sensitive optics enables polarisation multiplexing and wavelength multiplexing to be accomplished using a reduced number of components which can be integrated into a single, compact package. Furthermore, the polarised states at the multiplexed wavelengths are maintained and controlled throughout the optical path up to and including the final polarisation multiplexer, allowing unpolarised light to be produced by the output polarisation multiplexer 416.

The maximum output power available from commercially available pump lasers is currently limited. If higher output power pump laser sources become available or the total output power required at the output fibre is achievable with a single pump, it may be possible to use only one laser per wavelength. If so, at a suitable point within the optical path, each single wavelength Raman pump light source is split into two equal parts and one part is rotated in polarisation by 90° and one part, which may be the same, undergoes a delay compared to the other part so that the pump light of the two paths, although of the same wavelength, are no longer coherent and are orthogonally polarised prior to being combined in the final PM Mux.

Figure 5 is a schematic drawing of an example arrangement of this in line with Figure 4, in which the same components are referred to by the same reference numerals. The arrangement of Fig. 4 is effectively recreated within section 514 of Fig. 5, including two initial polarisation multiplexers 409, 413, a birefringent crystal 41 1 and an output polarisation multiplexer 416. In this embodiment two pump light lasers 501 , 502 provide pump light at different wavelengths λι, λ₂ but the same polarisation. The output of each pump laser is transmitted along polarisation Maintaining (PM) fibre and provided as input to respective PM 3dB couplers 503, 504. The pump light is output in equal powers and polarisation states from the 3dB couplers 503, 504 in two arms 510 and 51 1 for λι and 512 and 513 for λ₂. One output arm 51 1 of λι and 513 of λ₂ is then passed through a delay line made up of a length of PM fibre 507 for λι and 508 for λ₂. The delay lines 507 and 508 are made long enough that the difference between path length 510 and 51 1 of λι and 512 and 513 of λ₂ is longer than the coherence length of the respective pump light source lasers so that, at the ends of the delay lines 507, 508, the pump light λι in path 510 and λι of path 51 1 are incoherent. In the same manner pump light λ₂ in path 512 and λ₂ of path 513 are also incoherent. In addition, along paths 51 1 and 513 the pump light polarisation state is rotated using a 90° PM splice 505, 506, whilst the polarisation of light along paths 510 and 512 are not changed. It is possible that power equalisation is also necessary at this point to ensure equal powers, but this is not shown. Thus the configuration of pump light entering PM Mux 409 and 413 at points 515, 516, 517 and 518 is the same as in Figure 4 and all performance within 514 is the same as described for Fig. 4.

It will be appreciated that other designs providing the same function of Fig. 5 are possible. For example the structure could be made out of bulk optic components or the delay line could be placed in path 414 or 415. However other designs will all have the same effect of providing incoherent, orthogonally polarised two or more pump light sources as input to the final PM Mux 416. It will be appreciated that variations from the arrangement of Figure 5 are possible: the 90° splices and outputs can be arranged so that the lasers do not need to operate with the same polarisation states.

This concept can also be extended to a larger set of wavelengths with the addition of another crystal with a different FSR (in this context, the "FSR" of a crystal is used in the sense of Figure 2: i.e. it refers to the transmission periodicity which would be exhibited by that crystal if placed between two crossed polarisers). In this case, the wavelength spacing of the pump lasers is determined by the crystal with the smaller FSR. Figure 6 is a schematic drawing of a suitable layout for an eight-laser device.

Eight lasers 601 -608 provide light at four wavelengths and two polarisations. The output of pairs of lasers are polarisation multiplexed using four polarisation multiplexers 609-612, into four propagation paths (free space, fibres or waveguides) 613-616, each propagating light at two different wavelengths and different polarised states. The light from two of these propagation paths 613, 614 is transmitted through a first birefringent crystal 617, and the light from the other two paths 615, 616 is transmitted through a second birefringent crystal 618 with the same FSR as the first crystal. The result of this is four propagation paths 619-622, each propagating light at two wavelengths and the same polarised state. Light from pairs of these propagation paths 619,622; 620,621 is multiplexed by two further polarisation multiplexers 623, 624, and the outputs passed through a third birefringent crystal 625 so that there are two propagation paths 626, 627, each passing light of all four wavelengths and a single polarisation. Finally, light from these two polarisation paths is passed through an output multiplexer 628 to produce unpolarised light at all four wavelengths.

If the wavelengths λ-ι, λ₂, λ₃, λ₄ are evenly spaced, the three crystals should be configured so that the first and second crystals 617, 618 have an FSR that is twice as large as the FSR of the third crystal 625. This can be achieved by using crystals with the same birefringence, in which case the first and second crystals 617, 618 will be half the length of the third crystal 625. In order to understand the operation of the device, by way of example it may be considered that the third crystal 625 is the same as the crystal 41 1 used in the four channel device. Consider four wavelengths where two are at maxima and two are at minima of the solid curve 301 shown in Figure 3 (e.g., λ_!=1476 nm, λ₂=1481 nm, λ₃=1486 nm, and λ₄=1491 nm). Those wavelengths at minima in Figure 3 (λ-ι and λ₃) will behave in the same way as each other when light is passed through the third crystal 625, and those wavelengths at maxima in Figure 3 (λ₂ and λ₄) will behave in the same way as each other. Thus if λ-ι and λ₃ with similar polarisations are on the same paths as are λ₂ and λ₄ (with polarisations similar to each other but orthogonal to λ-ι and λ₃), upon entering the third crystal 625 (i.e. the various inputs to the third crystal 625 are || ;λ₃, || ;λ₂,_Ι_;λ₄,_Ι_ and λ₁,±;λ₃,±;λ₂, || ;λ₄, || ) the operation is similar to the four channel device described above, but requires an additional wavelength multiplexing stage. It will be appreciated that a crystal thickness which produces delays which are multiples of 90° will also produce output polarizations which are orthogonal to the input and that crystal thickness which produces delays which are multiples of 180° will also produce output polarizations which are parallel to the input.

In order to achieve these inputs to the third crystal 625, the first and second crystals 617, 618 with larger FSR are required. Figure 7 shows the transmission spectra 701 , 702 of the first and second crystals 617, 618 through crossed polarisers. The first crystal 617 rotates the polarisation of light with λ₃ but passes light with λ-ι unrotated, and similarly the second crystal 618 rotates the polarisation of light with λ₄ but leaves λ₂ unrotated. The first and second crystals 617, 618 each have thickness approximately one half that of the third crystal 625. The second crystal 618 has a slight phase delay shift relative to the first crystal 617 in order to be operative at λ₂ and λ₄ instead of λ-ι and λ₃.

Thus the first, second, third and fourth lasers 601 -604 emitting light with wavelengths λ-ι and λ₃ in two polarised states, and the fifth, sixth, seventh and eighth lasers emitting light with wavelengths λ₂ and λ₄ in two polarised states are initially multiplexed into four outputs comprised of two wavelengths and two orthogonal polarised states each. Following passage through first and second crystals (617, 618) those four outputs (613, 614, 615, 616) each comprise light with two different wavelengths and one common polarised state. Each set of four lasers (e.g. 601 -604 and 606-609), polarisation multiplexers (609, 10, 61 1 , 612) and polarisation rotation crystal (617, 618) thus behaves similarly to the four channel device described previously with an additional polarisation rotation crystal and multiplexing stage appended at the output. This final stage (625, 628) is chosen to ensure that the wavelengths and polarised states from the previous stage are multiplexed correctly by appropriate choice of crystal thicknesses and corresponding phase delays.

It will be appreciated that the arrangements described make it possible to provide wavelength and polarisation multiplexing together in a single integrated unit. Furthermore, the device can be designed to be adjustable, at installation and/or dynamically reconfigured during operation. This can be achieved by rotating one or more of the elements or by substituting dynamic or adjustable elements, respectively, such as a Liquid Crystal or other electro-optic modulator, or a Piezo-electric electro- optic ceramic material such as PLZT (Lead-Lanthanum-Zirconate-Titanate). The use of dynamic or adjustable elements enables the relative contribution from any input laser wavelength or polarised state to the total output power to be adjusted by adjusting the relative polarisation state at any intermediate stage preceding any of the polarisation multiplexers, effectively causing a phase change in the transmission spectra for the different polarisations. A dynamic device is beneficial to controlling the gain shape or polarization dependent gain properties of a Raman gain module pumped using the component arrangement described here. An adjustable device that is configured during deployment is beneficial to reducing the number of different versions of manufactured products needed to support various Raman pump wavelengths using the component arrangement described here.

Raman amplifiers used for telecommunications often require pumps emitting very high optical powers to provide gain at the required signal wavelength range using the Raman effect in a length of transmission fibre or within a shorter length of highly nonlinear fibre configured as a discrete Raman amplifier. However, such high powers also induce Stimulated Brillouin Scattering (SBS), a detrimental, competing gain process. SBS is particularly detrimental if the optical pumps have power which is concentrated within a very small optical spectral bandwidth. SBS can be alleviated by spectral broadening (via modulation) or by ensuring the optical spectrum of the pump laser is spread over a wide optical spectral bandwidth such as that associated with multi-longitudinal mode pump emission. Multi-longitudinal mode pump sources also exhibit good spectral stability and good RIN (Relative Intensity Noise) performance compared to few-mode sources which results in a correspondingly stable gain spectral shape and overall stable operating performance.

In order to provide controllable and predictable Raman gain there is a need to change the optical power of the pump over a wide dynamic range depending upon the system operating conditions. Maintaining a broad, multimode spectral shape over the full pump operating power dynamic range from threshold to pump roll-over is very challenging because the internal gain of the pump changes significantly as the output power is adjusted. Any technique that reduces the dynamic range of output powers required for the pump will help reduce spectral variation, maintain stable Raman gain and reduce the likelihood of unwanted SBS effects. An example can be illustrated where four pump sources, as in Figure 4, are operated continuously at full output power where stability is optimum and optical spectral bandwidth is broadest (thus minimizing SBS impact). It is desirable to be able to attenuate the total power transmitted through the polarisation and wavelength multiplexing device shown in Figure 4 without adjusting individual pump source powers at the inputs. First birefringent crystal 41 1 could be rotated to shift the transmission spectra of the composite Polarisation and Wavelength Multiplexer (shown in Figure 3) through a phase change, having the effect of adjusting the total power multiplexed at the output. Alternatively, the angle of rotation provided by the birefringent crystal could be modified so that each wavelength is not rotated to align with the maximum transmission axis of polarisation of the second polariser, 416.

To vary the relative amounts of power at λ-ι and λ₂, an additional adjustable polarisation rotation device (not shown in Figure 4) may be provided between each laser 401 -404 and the first polarisation multiplexer 409, 413 so that the polarisation of either one or both wavelengths entering the first input polarisation multiplexer 409, 413 are not aligned perfectly with the axis of maximum transmission, so that the polarized light is attenuated by an amount dependent upon the adjustment angle applied to each wavelength either individually or in any combination.

Modification of the angle of rotation can be realised by modifying any parameter that changes the FSR of the assembly, namely the path length or birefringence of the crystal. For example, by applying a tilt to the birefringent crystal 41 1 along the light propagation direction the effective thickness of the crystal and thus the path length L is varied and the FSR is changed. For an untilted crystal, orthogonal rotation occurs for the two input wavelengths with subsequent maximum transmission through the output polarising element, whereas once a controlled tilt is effected the rotation of the two wavelengths is no longer orthogonal so that the transmission through the output polariser (416) no longer provides maximum transmission, by an amount dependent upon the rotation angle experienced by the two wavelengths. It is understood that other methods for changing the path length are possible and can equally be applied to this disclosure. Alternatively by changing the birefringence (Δη) of the crystal it is also possible to change the FSR and correspondingly the amount of polarisation rotation in the crystal, and in a similar manner as tilt the crystal the amount of transmission through the output polariser is controllable. It will be appreciated from the foregoing discussion that, in many cases, the polarisation states of the light will not always be exactly orthogonal to each other, and the polarisation rotations effected induced by the birefringent crystal will not always be precisely 0° or 90°. It is variation from these exact values that allows the system to be adjusted to control the transmission of light from the various input lasers. The primary benefit from adjusting the transmitted multiplexed pump power contributions from the pump sources using the techniques described above is that it relaxes the requirement for the laser sources to operate with broad, stable spectra and low RIN associated with multiple longitudinal modes over a wide dynamic operating optical power range. This can provide a simpler route to operation over a wide dynamic range.

It is desirable to prevent any spurious reflections from the elements in the optical polarisation and wavelength multiplexing components from inducing instability which could detrimentally impact Raman gain performance. This is achieved by carefully orienting all of the element surfaces to minimize back reflection, and by incorporating anti-reflection coatings on each surface where possible. The inclusion of an optical isolator into the optical assembly between the pump source and the multiplexing optics is even more beneficial and is commonly achieved with a discrete fibre pigtailed optical isolator. An optically isolating component element can be incorporated within the optical structure of the current disclosure without any significant impact on performance and at minimal cost or increase in size. One embodiment places the isolator downstream of each pump laser source and upstream of the first polarisation mulitplexer as shown in Figure 8. Isolators 817, 818, 819 and 820 are placed in optical paths 805, 806, 807 and 808 to provide the highest level of isolator in front of the pump lasers 801 , 802, 803 and 804. Isolator core elements appropriated from commercially available micro optic isolator technology would enable an integrated optical isolation solution. It is also appreciated that the isolator core element could be placed anywhere downstream between the output of the pump laser and the output of the polarisation and wavelength multiplexing device. Placing the isolator element further downstream from the pump laser requires fewer core isolator elements as the number of individual paths decreases but exposes each pump to a larger number of non-isolated elements. Placing the isolation elements upstream, close to the pump laser is more effective as it exposes the pump sources to fewer non-isolated elements,.

It will be appreciated that variations from the embodiments described above may still fall within the scope of the invention. In particular, very specific combinations of wavelengths and polarised states have been described, but it will be appreciated that any suitable binary or other bifurcating tree arrangement of polarisation multiplexers with interposed wavelength selective or wavelength periodic polarisation rotation elements will produce similar results and can be used to similarly produce wavelength and polarisation multiplexing of multiple pump sources.

Claims

1 . An assembly for multiplexing light having a plurality of wavelengths and polarised states so as to produce an output of generally unpolarised light having at least two components with different wavelengths, the assembly comprising one or more polarisation multiplexers and a selective polarisation rotation device configured to rotate the polarisation of light at a first wavelength through a first controlled angle and to rotate the polarisation of light at a second wavelength through a second controlled angle.

2. The assembly of claim 1 , wherein the first controlled angle is different from the second controlled angle.

3. The assembly of claim 2, wherein the first controlled angle is approximately 0° or an integer multiple of 180° and the second controlled angle is approximately 90° or an integer multiple of 180° plus 90°.

4. The assembly of claim 1 , 2 or 3, wherein:

the assembly is configured to propagate into an output polarisation multiplexer a first input of light in a first polarised state and having components with the first wavelength and second wavelength, and a second input of light in a second polarised state and having components with the first wavelength and second wavelength; and the output polarisation multiplexer is configured to combine said outputs to produce generally unpolarised light having components with the first and second wavelengths.

5. The assembly of claim 4, wherein the first and second inputs of light to the output polarisation multiplexer are incoherent.

6. The assembly of claim 4 or 5, wherein the first and second inputs of light to the output polarisation multiplexer are of substantially equal power.

7. The assembly of claim 4, 5 or 6, wherein the assembly is configured so that light propagated into the selective polarisation rotation device produces a first output of light in the first polarised state and having components with the first wavelength and second wavelength, and a second output of light in the second polarised state and having components with the first wavelength and second wavelength, the first output from the selective polarisation rotation device being coupled to the first input to the output polarisation multiplexer, and the second output from the selective polarisation rotation device being coupled to the second input to the output polarisation multiplexer.

8. The assembly of claim 7, further comprising first and second inputs to the selective polarisation rotation device, so that:

light having a component with the first wavelength and the first polarised state and a component with the second wavelength and the second polarised state can be propagated into the selective polarisation rotation device via the first input and acted on by the device to produce the first output; and

light having a component with the first wavelength and the second polarised state and a component with the second wavelength and the first polarised state can be propagated into the selective polarisation rotation device via the second input, and is acted on by the device to produce the second output.

9. The assembly of claim 8, further comprising:

a first polarisation multiplexer for multiplexing light having a component with the first wavelength and the first polarised state with light having a component with the second wavelength and the second polarised state, an output of the first polarisation multiplexer being coupled to the first input of the selective polarisation rotation device; and

a second polarisation multiplexer for multiplexing light having a component with the first wavelength and the second polarised state and light having a component with the second wavelength and the first polarised state, an output of the second polarisation multiplexer being coupled to the second input of the selective polarisation rotation device.

10. The assembly of any of claims 1 to 4, comprising:

a first polarisation multiplexer for multiplexing light having a component with the first wavelength and a first polarised state with light having a component with the second wavelength and a second polarised state; and a second polarisation multiplexer for multiplexing light having a component with the second wavelength and first polarised state with light having a component with the first wavelength and second polarised state;

wherein the selective polarisation rotation device is coupled to the first and second polarisation multiplexers so as to produce a first output of light in the first polarised state having a component with the first wavelength and a component with the second wavelength and a second output of light in the second polarised state having a component with the first wavelength and a component with the second wavelength.

1 1 . The assembly of claim 9 or 10, further comprising:

a first light source coupled to the first polarisation multiplexer and configured to emit light at the first wavelength in the first polarised state;

a second light source coupled to the first polarisation multiplexer and configured to emit light at the second wavelength in the second polarised state;

a third light source coupled to the second polarisation multiplexer and configured to emit light at the first wavelength in the second polarised state;

a fourth light source coupled to the second polarisation multiplexer and configured to emit light at the second wavelength in the first polarised state.

12. The assembly of claim 9 or 10, further comprising first and second light sources, configured to emit polarised light at the first and second wavelengths, respectively, wherein:

the outputs from each the first and second light sources are split into two propagation paths which are coupled to the first and second polarisation multiplexers; and

one propagation path of each output passes through a 90° splice to rotate its polarised state by 90° before coupling to the respective polarisation multiplexers.

13. The assembly of claim 9 or 10, wherein the inputs to the first and second polarisation multiplexers are provided from outputs of further selective polarisation rotation devices.

14. The assembly of claim 13, further comprising additional polarisation multiplexers upstream of the further selective polarisation rotation devices.

15. The assembly of any preceding claim, wherein the first and second polarisation states are substantially orthogonal to each other.

16. The assembly of any preceding claim, wherein the selective polarisation rotation device is configured to rotate the polarisation state of light incident thereon by an amount varying periodically with respect to the frequency or wavelength of the light.

17. The assembly of claim 16, wherein the selective polarisation rotation device comprises a birefringent material.

18. The assembly of claim 16 or 17, wherein the selective polarisation rotation device comprises one or more birefringent crystals.

19. The assembly of claim 16, wherein the selective polarisation rotation device is formed from an optically active, electro-optic or magneto-optic element.

20. The assembly of claim 16, 17 or 15, wherein the selective polarisation rotation device comprises one or more optically active, electro-optic or magneto-optic elements.

21 . The assembly of any of claims 16 to 20, further comprising further selective polarisation rotation devices having a periodicity of rotation with respect to frequency or wavelength double that of the selective polarisation rotation device.

22. The assembly of claim 21 , configured to multiplex light of four or more substantially equally spaced frequencies or wavelengths, and further configured so that:

light having first and third wavelengths is polarisation multiplexed and selectively rotated by one of the further selective polarisation rotation devices to produce light having components with the first and third wavelengths in the first polarised state and an output having components with the first and third wavelengths in the second polarised state;

light having second and fourth wavelengths is polarisation multiplexed and selectively rotated by another of the further selective polarisation rotation devices to produce an output having components with the second and fourth wavelengths in the first polarised state and an output having components with the second and fourth wavelengths in the second polarised state;

the light having components with the first and third wavelength in the first polarised state is multiplexed with the light having components with the second and fourth wavelengths in the second polarised state to produce light propagated into the first input of the selective polarisation rotation device; and

the light having components with the first and third wavelength in the second polarised state is multiplexed with the output having components with the second and fourth wavelengths in the first polarised state to produce light propagated into the second input of the selective polarisation rotation device.

23. The assembly of any preceding claim, wherein the selective polarisation rotation device is adjustable so as to adjust a power output of the assembly.

24. The assembly of claim 23, wherein adjustment of the selective polarisation rotation device includes rotation of the device so as to control the first and/or second controlled angles by which the polarisation of light is rotated.

25. The assembly of claim 23 or 24, wherein adjustment of the selective polarisation device includes tilting the device so as to control a free spectral range of the device.

26. The assembly of claim 23, 24 or 25, wherein adjustment of the device results in selective adjustment of transmission of light through any branch of the device.

27. The assembly of any preceding claim, further comprising additional adjustable selective polarisation rotation devices for controlling the relative power at different wavelengths.

28. The assembly of any preceding claim, further comprising a plurality of lasers for providing light to be multiplexed.

29. The assembly of claim 28, further comprising optical isolators between the lasers and an output of the assembly.

30. The assembly of any preceding claim, provided as an integrated unit.

31 . A Raman pump unit comprising the assembly of any preceding

claim 32. A Raman amplifier comprising the assembly of any preceding claim and a Raman pumped gain block.

33. A method of multiplexing light having a plurality of wavelengths and polarised states so as to produce an output of generally unpolarised light having at least two components with different wavelengths, the method comprising passing the light through a plurality of polarisation multiplexers and a selective polarisation rotation device configured to rotate the polarisation of light at a first wavelength through a first controlled angle and to rotate light of a second wavelength through a second controlled angle.

34. The method of claim 33, wherein the first controlled angle is approximately 0° or an integral multiple of 180° and the second controlled angle is approximately 90° or an integral multiple of 180°plus 90°, so that the polarisation state of light having the first wavelength is substantially unchanged when such light passes through the polarisation rotation device but the polarisation state of light having the second wavelength is rotated through approximately 90° when such light passes through the polarisation rotation device.

35. The method of claim 33 or 34, further comprising passing the light through the assembly of any of claims 1 to 30.

36. The method of claim 33 or 34, further comprising:

propagating light into the selective polarisation device so as to produce a first output of light in a first polarised state and having components with the first wavelength and second wavelength, and a second output of light in a second polarised state, substantially orthogonal to the first polarised state, and having components with the first wavelength and second wavelength; and

combining light from the outputs in an output polarisation multiplexer so as to produce generally unpolarised light having components with the first and second wavelengths.

37. The method of claim 33 or 34, further comprising:

propagating light having a component with the first wavelength and the first polarised state and a component with the second wavelength and the second polarised state into the selective polarisation rotation device;

rotating the polarised state of the component with the second wavelength and transmitting the component with the first wavelength with its polarised state unchanged to produce the first output; and

propagating light having a component with the first wavelength and the second polarised state and a component with the second wavelength and the first polarised state into the selective polarisation rotation device; and

rotating the polarised state of the component with the second wavelength and transmitting the component with the first wavelength with its polarised state unchanged to produce the first output to produce the second output.