GB2499255A

GB2499255A - Integrated Raman Amplification Pump Assembly

Info

Publication number: GB2499255A
Application number: GB1202446.9A
Authority: GB
Inventors: Ian Peter Mcclean; Nadhum Zayer; Peter Wigley
Original assignee: Oclaro Technology Ltd
Current assignee: Lumentum Technology UK Ltd
Priority date: 2012-02-13
Filing date: 2012-02-13
Publication date: 2013-08-14
Also published as: GB201202446D0

Abstract

An optically integrated Raman amplification (RA) pump assembly 722 for use in an optical system includes an optical fibre 714 carrying an optical signal. The optical signal includes a plurality of optical channels. The RA assembly 722 includes a plurality of pump lasers 702a to 702n generating light of a plurality of wavelengths and polarised states. The plurality of wavelengths for use in Raman amplification of the plurality of optical channels. A combining unit 706 is provided for multiplexing the generated light, and a depolarising unit 708 is provided to generate generally unpolarised light from the multiplexed light, the generally unpolarised light having at least two components with different wavelengths. The RA 722 includes an output for injecting the generally unpolarised light into the optical fibre 714. Monitors 724a to 724n may be included in the assembly 722 at various measuring points to measure the power of the generated light and multiplexed light that is output for use in controlling the Raman gain. The plurality of pump lasers 702a to 702n, the combining unit 706, the depolarising unit 708, and output can be optically coupled within the assembly by free space optical paths.

Description

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Integrated Raman Amplification Pump Assembly Field of the Invention

5 The present invention relates to a Raman amplification (RA) pump assembly for dense wavelength division multiplexed optical systems and, more particularly, to an optically integrated RA pump assembly for use in providing suitably depolarised pump light to a DWDM optical system for Raman amplification of the optical signal light therein.

10 Background of the Invention

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.

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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 20 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.

25 Raman amplifiers are often used for optical telecommunications to allow optical data signals to travel further along system span fibre. In these amplifiers a Raman pump module is necessary that outputs pump light in various forms depending upon the requirement. Distributed Raman amplifiers use the fibre span to provide the gain medium, although discrete Raman amplifiers may also be used these have their own 30 fibre dedicated for the amplifier only. However, it is to be understood the techniques described herein are applicable to discrete Raman amplifiers. Raman amplifiers for dense wavelength division multiplexed optical systems may operate in many different wavelength regions provided that a suitable high power pump light source or sources are available. Multiple pumps can be simultaneously coupled in a Raman amplifier 35 (RA) pump unit or module for several reasons. To achieve high gains in such

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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. Also the gain spectrum of the Raman amplifier is affected by the choice of pump wavelengths, and often pumps of different wavelengths are multiplexed or coupled to provide a wider 5 useable gain bandwidth than can be achieved with a single pump 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 often used to combine the outputs of two lasers operating with orthogonal polarisations at the same 10 wavelength, which produces light that is essentially depolarised so as to minimise this polarisation-dependent gain as far as possible. This is sometimes called a polarisation multiplexed 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. .

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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 unit or module.

A conventional RA pump unit is made up of individual optical component packages or 20 assemblies connected by fibre tails. Figure 1 is a schematic illustration showing an example of a multiple-pump RA pump unit 10 as described in figure 2 of US6724524. The multiple-pump Raman pump unit 10 is made up of individual optical component packages. The unit 10 includes four pump lasers 12A, 12A', 12B and 12B', in which pump lasers 12A and 12A' have a wavelength A,1 and pump lasers 12B and 12B' have 25 a wavelength 12, the pump lasers 12A to 12B' are connected to two pump combiners 14A and 14B, which are pump multiplexers (P-MUX). The pump lasers 12A to 12B' and P-MUXes 14A and 14B are separate components, which are coupled together at the points marked X. Conventionally the pump lasers 12A to 12B' and P-MUXes 14A and 14B have fibre tails and are fusion spliced together at point X. The P-MUXes 14A 30 and 14B also act as pump depolarisers, which requires pump lasers 12A and 12A' to be of the same wavelength. When only one pump laser per wavelength is required it is common to use an alternative depolariser such as a fibre Lyot.

Although not shown in figure 1, it is common to include an optical isolator between 35 pump lasers 12A to 12B' and the output components. This prevents back reflections of

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laser light interfering with the stability of the pump lasers 12A to 12B'. The assembly 10 also includes 98/2 taps 18A and 18B for use in monitoring the power of the pump lasers 12A to 12B'. The two pump wavelengths M and 12 are combined using a narrow band wavelength demultiplexer (NBWDM) 16 before being combined with the 5 optical transmission signal using wavelength demultiplexer (WDM) 21. An additional 98/2 tap 22 is used for further monitoring of the optical powers. Each component of the assembly is fusion spliced at the places marked X.

However, using individual fibre tailed components results in a more complicated 10 manufacturing process due to the many fusion splices required. This means more fibre has to be managed within the enclosure of the Raman pump unit and each component needs its own package and fibre tails, resulting in a large final device. In addition, fibre tails can result in losses of the optical pump light and subsequent components may affect the polarisation performance of the RA pump unit 10.

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Although it is common for each function within a RA pump unit to be a separate component package, US6404542 describes combining some of the functions into a single package as illustrated in figure 2 of US6404542, which is also reproduced in figure 2 of the current application. Figure 2 is a schematic illustration of several pump 20 components being combined into a single package. In this case a single semiconductor is used, but with two laser stripes 202 and 204 within the single wafer 200, thus producing two pump lasers on one wafer. The two beams from the laser stripes 202 and 204 are combined via an optical assembly 208 and coupled into a fibre 222. Fibre Bragg grating 224 is placed in the output fibre 222 to lock the two lasers 25 202 and 204 to a fixed wavelength. This figure shows a minor level of integration of a few optical components that may be used in a RA pump unit. Figure 2 simply provides an optical component that can be used in place of two lasers of the same wavelength and a P-MUX in the RA pump unit of figure 1 (e.g. combining 12A/A' and 14A).

30 However, when compared to the full RA pump unit optical train this level of integration only includes a small number of the optical components and removes only two fusion splices per pump pair. The reduction in component count and fibre to be managed is small with only a minor improvement in manufacturing and performance losses.

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In distributed Raman amplification the wavelength profile of the gain and the bandwidth of gain are dependent upon many factors including the fibre type of the system span, the temperature of the system, the loss of other components in the system as well as component changes over a component's lifetime and so requires careful selection to 5 ensure optimum performance of a Raman amplifier in all conditions. To achieve optimum performance, the pump power and pump wavelength need to be adjusted to account for the current operating conditions. Conventionally pump laser powers can be modified by a control unit with the RA pump unit, however, the pump wavelengths are fixed through use of a fibre Bragg grating as illustrated in figure 2 and cannot be 10 modified.

It is desirable to improve system performance of a RA pump unit using tunable pump lasers and to integrate as many of the optical components of a RA pump unit as possible to achieve a higher manufacturing yield, lower performance losses, efficient 15 operation, and portability within a smaller footprint.

Summary of the Invention

According to a first aspect of the invention there is provided an optically integrated 20 Raman amplification (RA) pump assembly for use in an optical system comprising an optical fibre carrying an optical signal, the optical signal including a plurality of optical channels. The assembly includes a plurality of pump lasers generating light at a plurality of wavelengths and polarised states, a beam combining unit for multiplexing the light generated by the plurality of lasers, a depolarising unit to produce an output of 25 generally unpolarised light from the multiplexed light, the generally unpolarised light having at least two components with different wavelengths, and an output for injecting the generally unpolarised light into the optical fibre.

Preferably, at least one optical path between the plurality of pump lasers and the output 30 is a free space optical path. Alternatively or additionally, a plurality of optical paths between the plurality of pump lasers and the output that are free space optical paths. Preferably, the optical components are arranged to be optically coupled by free space optical paths. Free space optical paths provide the advantage of reducing optical path losses and thus allowing the RA assembly to output higher pump powers or with

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reduced electrical power consumption for a specific output power which can reduce total heat dissipation.

Optionally, each of the plurality of pump lasers is a dynamically tunable pump laser. 5 Each tunable pump laser is configured for emits light tuned to a specific wavelength of the plurality of wavelengths. Each tunable pump laser comprises a pump laser source and a tuning unit, where the tuning unit locks the pump laser source to the specific wavelength of the plurality of wavelengths. This provides the advantage that the assembly can be dynamically controlled such that the output power of the unpolarised 10 light is optimised for specific optical applications to provide an optimised gain profile.

As an option, the assembly includes at least one optical isolator located in the optical path between the pump lasers and the output. Additionally, the optical isolator is located in the optical path prior to the depolarising unit. As an option, the assembly 15 may include a plurality of reflective devices located at the output, the reflective devices for use in locking the plurality of pump lasers to the plurality of wavelengths.

Optionally, each of the plurality of pump lasers includes a collimating lenses for collimating the generated light. Additionally, the assembly further includes a focussing 20 lens and the output of the assembly further comprises an output optical fibre, wherein the focussing lens is arranged for coupling the generally unpolarised light to the output fibre.

As an option, the assembly may further include a plurality of pump laser monitors for 25 measuring pump data representative of the power of the generated light from the plurality of pump lasers, the measured pump data for use in Raman gain control. Alternatively or additionally, the assembly may further include an output power monitor located in the optical path after the beam combiner, the output power monitor configured to measure the total output power of the plurality of pump lasers. 30 Alternatively or additionally, the assembly may further include an output power monitor located in the optical path after the beam combiner, the output power monitor configured to measure the total output power of the plurality of pump lasers. Alternatively or additionally, the assembly may further include a signal power monitor for measuring the power of signal light passing into the output of the optically integrated 35 RA pump assembly. The assembly may further comprise a controller unit for receiving

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data representative of power measurements for use in controlling the plurality of pump lasers.

As an option, the optical components of the integrated RA pump assembly are 5 mounted onto a cooling unit for removing heat from the optical assembly. The cooling unit may include a heat sink. Alternatively, the cooling unit may include a thermoelectric cooler and a sub-mount. The optical components of the integrated RA pump assembly may be mounted on the cooling unit. The optical components may be mounted on the TEC or the sub-mount, or both the TEC and the sub-mount. 10 Additionally, the housing of the integrated RA pump assembly is a hermetically sealed package and the output comprises an output port passing through the hermetically sealed package for use in injecting the unpolarised light into the optical fibre of the optical system.

15 According to a second aspect of the invention, there is provided an optically integrated RA pump assembly for use in an optical system comprising an optical fibre carrying an optical signal, the optical signal including a plurality of optical channels. The assembly comprising a plurality of pump lasers generating light at a plurality of wavelengths and polarised states, the pump lasers being of fixed wavelength output or dynamically 20 tunable, a beam combining unit for multiplexing the light generated by the plurality of lasers, a depolarising unit to produce an output of generally unpolarised light from the multiplexed light, the generally unpolarised light having at least two components with different wavelengths, an isolating device for attenuating any reflected light to the plurality of lasers, and an output for injecting the generally unpolarised light into the 25 optical fibre, wherein the optical components are optically coupled based on free space optical paths.

As an option, the assembly may further include a plurality of pump monitor units configured for generating measurement data representative of the power of the light 30 generated by the plurality of pump lasers, the pump lasers being of fixed wavelength output or dynamically tunable. Each of the pump laser monitors may be configured for generating measurement data representative of the power of the generated light from one or more of the plurality of pump lasers. The assembly may further include an output power monitor configured for generating measurement data representative of 35 the total output power of the plurality of pump lasers. Additionally, the assembly may

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include a reflection power monitor configured for generating measurement data representative of the power of any laser light reflected from the output of the assembly. Optionally, the assembly may further include a signal monitor configured for generating measurement data representative of the power of signal light passing into the output of 5 the optically integrated RA pump assembly. As an option, the monitors within the assembly are optically coupled to the corresponding optical components by free space optical paths. Additionally or alternatively, the RA pump assembly may be further configured for receiving control data from a controller unit, the controller unit may be coupled to one or more of the monitors for receiving measurement data for use in 10 controlling the plurality of pump lasers and/or for performing Raman gain control. Optionally, the controller unit is further configured to receive measurement data from an external signal monitor, the external signal monitor configured to generate measurement data representative of the power of signal light at the output of the RA pump assembly.

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According to a third aspect of the invention there is provided an optically integrated RA pump assembly for use in an optical system comprising an optical fibre carrying an optical signal, the optical signal including a plurality of optical channels. The assembly comprising a pump laser generating light at a wavelength and a polarised state, a 20 depolarising unit to produce an output of generally unpolarised light from the generated light, the generally unpolarised light having at least two components with different wavelengths, and an output for injecting the generally unpolarised light into the optical fibre, wherein the pump laser, the depolarising unit and the output are optically coupled by free space optical paths.

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As an option, the pump laser is a tunable pump laser for emitting light tuned to a specific wavelength or a fixed wavelength pump laser. The tunable pump laser may include a pump laser source and a tuning unit, wherein the tuning unit locks the pump laser source to a required wavelength. In addition, the assembly may include at least 30 one optical isolator located in the optical path between the pump laser and the output. Alternatively, the assembly may further include a reflective device located at the output, the reflective devices for use in locking the fixed wavelength pump laser to the required wavelength.

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As an option, the RA pump assembly may include a pump laser monitor configured for generating measurement data representative of the power of the generated light from the pump laser. In addition, the assembly may further include an output power monitor configured for generating measurement data representative of the total output power of 5 the pump laser. Additionally, the assembly may include a reflection power monitor configured for generating measurement data representative of the power of any laser light reflected from the output of the assembly. Optionally, the assembly may include a signal monitor configured for generating measurement data representative of the power of signal light passing into the output of the optically integrated RA pump assembly. 10 The monitors within the assembly may be optically coupled to the corresponding optical components by free space optical paths. The RA pump assembly may be further configured for receiving control data from a controller unit, the controller unit coupled to one or more of the monitors for receiving measurement data for use in performing Raman gain control of the RA pump assembly. As an option, the controller unit is 15 further configured to receive measurement data from an external signal monitor, the external signal monitor configured to generate measurement data representative of the power of signal light at the output of the RA pump assembly.

As an option, the optically integrated RA pump assembly as described is provided as 20 an integrated circuit. In another aspect of the invention, there is provided a Raman pump unit including the RA assembly as described herein. Other aspects of the invention provide a Raman amplifier including the RA assembly as described herein.

Brief Description of the Drawings

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In order that the invention may be more fully understood, some of the embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

30 Figure 1 is a schematic diagram illustrating a prior art example of a RA pump unit;

Figure 2 is a schematic diagram illustrating a prior art example of a dual pump laser RA pump unit;

Figure 3a is a schematic diagram illustrating an example of an optically integrated RA pump assembly according to the invention;

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Figure 3b is a schematic optical diagram illustrating an example of the optically integrated RA pump assembly of figure 3a;

Figure 4a is a schematic diagram illustrating another example of an optically integrated RA pump assembly according to the invention;

5 Figure 4b is a schematic optical diagram illustrating an example of the optically integrated RA pump assembly of figure 4a;

Figure 5a is a schematic diagram illustrating further example of an optically integrated RA pump assembly according to the invention;

Figure 5b is a schematic optical diagram illustrating an example of the optically 10 integrated RA pump assembly of figure 5a;

Figure 5cis a schematic optical diagram illustrating another example of the optically integrated RA pump assembly of figure 5a with internal chip tuning;

Figure 5d is a cross-sectional schematic optical diagram illustrating a further example of the optically integrated RA pump assembly of figure 5a with integrated external 15 wavelength locking;

Figure 6a is a schematic diagram illustrating yet another example of an optically integrated RA pump assembly according to the invention;

Figure 6b is a schematic optical diagram illustrating an example of the optically integrated RA pump assembly of figure 6a with external tuning optics;

20 Figure 6c is a schematic optical diagram illustrating another example of the optically integrated RA pump assembly of figure 6a with internal chip tuning;

Figure 6d is a cross-sectional schematic optical diagram illustrating a further example of the optically integrated RA pump assembly of figure 6a with internal chip tuning; Figure 7a is a schematic diagram illustrating an example of an optically integrated RA 25 pump assembly according to the invention with integrated pump power monitor;

Figure 7b is a schematic optical diagram illustrating an example of the optically integrated RA pump assembly of figure 7a with integrated pump power monitor and external tuning optics

Figure 7c is a schematic optical diagram illustrating another example of the optically 30 integrated RA pump assembly of figure 7a with integrated rear pump power monitor and internal chip tuning;

Figure 7d is a schematic optical diagram illustrating a further example of the optically integrated RA pump assembly of figure 7a with integrated forward pump power monitor and internal chip tuning;

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Figure 7e is a cross-sectional schematic optical diagram illustrating yet a further example of the optically integrated RA pump assembly of figure 7a with integrated forward pump power monitor and internal chip tuning;

Figure 8a is a schematic diagram illustrating an example of an optically integrated RA 5 pump assembly according to the invention with pump monitor and total power monitor; Figure 8b is a schematic optical diagram illustrating an example of the optically integrated RA pump assembly of figure 8a;

Figure 8c is a cross-sectional schematic optical diagram illustrating another example of the optically integrated RA pump assembly of figure 8a; and 10 Figure 9 is a schematic diagram illustrating an optically integrated RA pump assembly according to the invention with pump monitors, total power monitor, signal monitor and control electronics.

Detailed Description

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Figure 3a is a schematic diagram illustrating an example optically integrated RA pump assembly 322 for use in Raman amplification. The integrated pump assembly 322 includes a pump laser 303 such as a semiconductor stripe, and a depolariser 308. Laser light from laser 303 will be input to the depolariser 308 plane polarised but will be 20 emitted depolarised. The depolariser 308 may include a wave-plate combination, which can be achieved in bulk optic material. The output depolarised laser light from the depolariser 308 is coupled into an optical fibre 314 at the output, the optical fibre 314 includes a reflective device 323 that returns a portion of the depolarised laser light back towards the laser, which creates a cavity and locking the laser at the wavelength 25 defined by the reflector device 323. An example of the reflector device 323 is a fibre Bragg grating. Most of the depolarised laser light is transmitted through reflective device 323 and passes to the output fibre 314.

The integrated pump assembly 322 removes the need to have separate pump and 30 depolariser component packages and also removes the need to have fibres between them with an associated splice. Instead, the optical path between the pump laser 303 and the depolariser 308 is a free space optical path. Additionally, the optical path between the depolariser 308 and the output of the assembly 322 may also be a free space optical path. Free space optical paths are considered to be optical paths that 35 have no waveguides such as optical fibre or fibre tails for propagating light. It is to be

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appreciated that if the optical components abutt each other or are butted up, then the optical components may be considered coupled together by a free space optical path as there is no waveguide provided. In addition, when the integrated pump assembly 322 is provided as a single pump chip, it can be depolarised rather than requiring the 5 two separate same wavelength stripes as shown in figure 2. The integrated pump assembly 322 provides the advantage of reducing the size and thermal dissipation of the integrated package, which means a higher power laser stripe can be used to achieve the same power as a dual stripe design.

10 Figure 3b is a schematic optical diagram illustrating an example optical design for the integrated pump assembly 322. The pump laser 303 is represented by a semiconductor laser chip 351 which outputs light via front facet 360. The semiconductor laser chip 351 is coupled via a collimating lens 352 made to produce collimated beam 353, which passes into depolariser 308. The depolariser 308 is 15 represented by quarter wave-plate 354 that is designed to convert plane polarised light from the semiconductor laser 351 into substantially depolarised light in beam 355. A focussing lens 356 focuses the depolarised light beam 355 into an optical fibre 357. A set of optical paths between the laser chip 351 and the output to optical fibre 357 are selected to be free space optical paths for reducing optical performance losses within 20 the integrated package and to improve thermal dissipation. As an example, the set of optical paths may include the optical paths between the collimating lens 352 and the wave-plate 354, the wave-plate 354 and the focussing lens 356, and the focussing lens 356 and the optical fibre 357 at the output.

25 The reflective device 323 is represented within optical fibre 357 by a fibre Bragg grating

358, which is positioned to reflect a small portion of the focussed depolarised light beam in optical fibre 357 to create a cavity with the end of the semiconductor laser chip

359. The fibre Bragg grating 358 is designed to produce a laser spectrum with multiple longitudinal modes, thus spreading the laser power spectrally and reducing the risk of

30 stimulated Brillouin scattering.

In this example, the semiconductor laser 351 may be Gallium Indium Arsenide Phosphide (GalnAsP) based, which can be designed to emit light having a wavelength substantially in the range 1400nm to 1500nm. It can also be mounted either to a 35 cooling unit (not shown) for removing heat from the optical assembly. The cooling unit

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may include a heat sink including a thermal conducting material or block. Alternatively or additionally, the cooling unit may include a heat pump such as a Peltier cooler or a thermo-electric cooler and a sub-mount. The optical components of the integrated pump assembly 322 may be mounted on the TEC or the sub-mount, or both the TEC 5 and the sub-mount. It is to be appreciated that the depolariser 308 includes a quarter wave-plate 354 by way of example only, and that depolariser 308 may include different wave-plates such as half or three quarter wave-plates or any other type of suitable depolariser or material for depolarising collimated beam 353.

10 The integrated RA pump assembly 322 may further include apparatus for monitoring the output power of the pump laser 303 and the total output power of the depolarised light at the output of the assembly 322. Examples of such apparatus and monitors are described with respect to figures 8a to 9. In particular the assembly 322 may include a pump laser monitor configured for generating measurement data representative of the 15 power of the generated light from the pump laser 303. The pump laser monitor may be configured to measure a portion of the generated light from the pump laser 303 and generate measurement data representative of the measured portion of the generated light. The assembly 322 may further include an output power monitor configured for generating measurement data representative of the total output power of the pump 20 laser 303. For safety, the assembly 322 may include a reflection power monitor configured for generating measurement data representative of the power of any laser light reflected from the output of the assembly 322.

In addition, the integrated RA pump assembly 322 may further include a signal power 25 monitor configured for generating measurement data representative of the power of signal light passing into the output of the optically integrated RA pump assembly 322. The signal power monitor may be configured to measure a portion of the signal light passing into the output of the assembly 322 and generate measurement data representative of the power of the measured portion of the signal light. Including the 30 signal power monitor within assembly 322 minimises the need to deploy separate signal power monitors within the vicinity of the injection point of the depolarised light output from the assembly 322 into the optical fibre of the optical system.

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The pump laser monitor, output power monitor, reflection power monitor, and signal power monitor (if included in the assembly 322) may be optically coupled to the corresponding optical components of the assembly 322 by free space optical paths.

5 The RA pump assembly 322 may be further configured for receiving control data from a controller unit for use in controlling the pump laser and/or for performing Raman gain control of the RA pump assembly. The controller unit may be coupled to one or more of the monitors for receiving measurement data for use in performing Raman gain control. The controller unit may be located externally to the pump assembly 322 or 10 may be incorporated within the pump assembly 322.

Although a signal power monitor may be included within the RA pump assembly 322, a signal power monitor positioned within the optical train of the RA pump assembly 322 can cause a reduction in the output power of the output depolarised laser light from 15 assembly 322. Instead of a signal power monitor within the optical train of assembly 322, an external signal monitor may be configured to generate measurement data representative of the power of the signal light at the output of the assembly 322. The external signal monitor may be positioned in the vicinity of the injection point of the output depolarised laser light at an external point in the optical fibre of the optical 20 system. The controller unit can be further configured to receive the measurement data from the external signal monitor for controlling the assembly 322.

Further integration of the integrated RA pump assembly can be achieved by combining at least two laser pumps together and depolarising them within one optical structure as 25 shown in figure 4a. Figure 4a is a schematic block diagram that illustrates an example optically integrated RA pump assembly 422 in which light emitted by pump lasers 403a and 403b are combined in pump combiner 406, which emits a combined light beam to depolariser 408. A reflective device 423, which is shown external to integrated pump assembly 422, is used such that both pump lasers 403a and 403b can be locked to a 30 single wavelength. By adding a second reflective device (not shown) of a different wavelength, each pump laser 403a and 403b can be locked to a separate wavelength. At least one optical path between the pump lasers 403a and 403b and the depolariser 408 is a free space optical path. Additionally, at least one optical path or a plurality of optical paths between the depolariser 408 and the output of the assembly 422 may 35 also be a free space optical path(s). This provides the advantages of an integrated

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pump assembly with fewer components and reducing the size and thermal dissipation of the integrated package.

Figure 4b is a schematic optical diagram illustrating an example of the integrated pump 5 assembly 422 of figure 4a. In this example, a second reflective device is used to allow each pump laser 403a and 403b to be locked to a separate wavelength. The components are arranged such that the pump lasers 403a and 403b are not polarisation multiplexed, but instead are combined as shown in figure 4b. In this example, the two pump lasers 403a and 403b are represented by two semiconductor 10 laser chips 451a and 451b, respectively. These laser chips 451a and 451b may be GalnAsP based and configured to emit light in the wavelengths substantially in the range 1400nm to 1500nm. The semiconductor laser chips 451a and 451b emit light from front facets 460a and 460b, respectively. If required, two lasers may be used that emit the same wavelength to increase the output power of the laser at the particular 15 wavelength. The laser light beams emitted from front facets 460a and 460b are each collimated by collimating lenses 452a and 452b into beams 453a and 453b, respectively. The pump combiner 406 is represented by a beam combiner 461 such that beams 453a and 453b are combined to produce a collimated and polarised beam 462 comprising the laser light from lasers 451 a and 451 b.

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The depolariser 408 is represented by a quarter wave-plate 454 and receives the collimated and polarised beam 462 comprising beams 453a and 453b. The quarter wave-plate 454 acts to depolarise the both beams of light 453a and 453b to produce a substantially depolarised combined beam 455. It is to be appreciated that the 25 depolariser 408 includes a quarter wave-plate 454 by way of example only, and that the depolariser 408 may include different wave-plates such as half or three quarter wave-plates or any other type of suitable depolariser or material for depolarising both beams of light 453a and 453b.

30 The combined depolarised beam 462 is received at focussing lens 456, which focuses the beams into fibre 457 to output from the integrated pump assembly 422. A set of optical paths between the laser chips 451a and 451b and the output to optical fibre 457 are selected to be free space optical paths for reducing optical performance losses within the integrated package and to improve thermal dissipation. As an example, the 35 set of optical paths may include the optical paths between the collimating lenses 452a

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and 452b and the beam combiner 461, the beam combiner 461 and the wave-plate 454, the wave-plate 454 and the focussing lens 456, and/or the focussing lens 456 and the optical fibre 457 at the output.

5 Within optical fibre 457 there are two reflective devices 458a and 458b, which can be reflective fibre Bragg gratings. The reflective devices 458a and 458b are placed in fibre 457 to reflect back a small portion of each laser light beam creating a cavity with the end of the laser semiconductor chip 459a and 459b, respectively. In this example, since each semiconductor laser 451a and 451b emits light of a different wavelength 10 two reflective devices 458a and 458b are used in which each match the corresponding wavelength emitted by semiconductor laser 451a or 451b. The reflective devices 458a and 458b are designed to produce a laser spectrum with multiple longitudinal modes, thus spreading the laser power spectrally and reducing the risk of stimulated Brillouin scattering. Alternatively, integrated pump assembly 422 may be configured in which 15 the two semiconductor lasers 451 a and 451 b have the same wavelength, which means only one reflective device 423 (e.g. either 458a or 458b) is necessary.

Further integration can be achieved if the pump lasers are locked using an alternative technique such as including a local locking reflector, which can be integrated into the 20 optical train as shown in figure 5a. Figure 5a illustrates a schematic diagram for another example of an optically integrated RA pump assembly 522. The integrated RA pump assembly 522 includes two pump lasers 503a and 503b, which emit light to pump combiner 506 for emitting a combined light beam to depolariser 508 producing a depolarised combined light beam. The depolarised combined light beam is input to an 25 isolator device 510 and output to optical fibre 514. It is not possible to integrate an isolator device into an integrated package when the pump lasers are locked by a reflective device in the output fibre as illustrated in figure 2. The isolator device 510 is incorporated into the optical train of the integrated RA pump assembly 522 and does not require a separate component or fibre tail for fusion splicing to the depolariser 508.

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Figure 5b is a schematic optical diagram illustrating an example of the integrated RA pump assembly 522, which is based on the integrated RA pump assemblies 322 and 422 as described with reference to figures 3b and 4b. In the example of figure 5, the pump lasers 503a and 503b are represented by two semiconductor laser chips 551a 35 and 551b, which may be GalnAsP based and designed to emit light with different

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wavelengths substantially in the range of 1400nm to 1500nm. The semiconductor lasers 551a and 551b emit light from front facets 560a and 560b, respectively. Although the two semiconductor lasers 551a and 551b may be configured to emit different wavelengths of light, it is to be appreciated by a person skilled in the art that 5 the lasers 551a and 551b can be configured to emit the same wavelength light, which is useful when increasing in the output power of the integrated RA pump assembly 522 at a particular wavelength.

The emitted light beams from the semiconductor lasers 551a and 551b are individually collimated in collimating lenses 552a and 552b into beams 553a and 553b. A first beam splitter 565a is located in the path of collimated beam 553a, which reflects a small portion of the laser beam at right angles to the path of collimated beam 553a. A first reflective volume grating 564a is located in the reflected path to create a cavity with the rear of the semiconductor laser chip facet 559a and so locks the semiconductor laser 551a to the wavelength of the volume grating 546a. Similarly, a second beam splitter 565b is located in the path of collimated beam 553b and reflects a small portion of the laser beam at right angles to the path of collimated beam 553b. A second reflective volume grating 564b is also located in the reflected path to create a cavity with the rear of the semiconductor laser chip facet 559b and so locks the semiconductor laser 551b to the wavelength of the volume grating 546b. This provides the advantage of not requiring a reflective device or grating within the output fibre 514. The volume gratings 564a and 564b are positioned far enough from the semiconductor lasers 551a and 551b, respectively, so that the optical paths are longer than the coherence length of the corresponding laser such that the semiconductor lasers 551a and 551b operate in coherence collapse mode. This produces a laser spectrum with multiple longitudinal modes, thus spreading the laser power spectrally and reducing the risk of stimulated Brillouin scattering.

The pump combiner 506 is represented by a beam combiner 561 such that beams 30 566a and 566b are combined to produce a collimated and polarised beam 562 comprising the laser light from lasers 551a and 551b. As the locking cavity represented by semiconductor lasers 551a and 551b and reflective devices 565a, 564a and 565a, 564b, respectively, is not in the fibre 514 an isolator device 510 may be placed in the path of the optical beam after the locking cavity. In figure 5a, the isolator 35 device 510 is placed after the depolariser 508 or after the pump combiner 506. The

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isolator device 510 may be placed anywhere in the optical train after the locking cavity. In the example of figure 5b, the isolator device 510 is represented by isolator 563, which is placed in the optical train after the beam combiner 561. The isolator 563 is designed to provide a loss to any reflected laser light that would otherwise destabilise 5 the laser output power of semiconductor lasers 551 a and 551 b.

The depolariser 508 is represented by a quarter wave-plate 554 and is placed after the isolator device 563. The depolariser 508 receives the collimated and polarised beam 562 that is emitted from the isolator device 563. The collimated and polarised beam 10 562 comprises beams 566a and 566b. The quarter wave-plate 554 acts to depolarise both beams of light 566a and 566b to produce a substantially depolarised combined beam 555. Although depolariser 508 includes a quarter wave-plate 554 by way of example only, it is to be appreciated that depolariser 508 may include different wave-plates such as half or three quarter wave-plates or any other type of suitable 15 depolariser or material for depolarising both beams of light 566a and 566b. A following focussing lens 556 focuses the depolarised beams 555 into fibre 557 for output from the integrated RA pump assembly 522.

Figure 5c is a schematic optical diagram of an alternative design of integrated RA 20 pump assembly 522, in which the pump lasers 503a and 503b are represented by semiconductor laser chips 571a and 571b. The semiconductor laser chips 571a and 571b may be implemented, by way of example only, as distributed feedback (DFB) semiconductor lasers. In this example, the semiconductor laser chips 571a and 571b are DFB semiconductor lasers which have inherent locking within the laser structure. 25 The overall structure of the integrated RA pump assembly 522 of figure 5c is similar to that of figure 5b except that the semiconductor lasers 551a and 551b and the external locking optics 564a, 565a, 564b, and 565b of figure 5b have been replaced with the inherently locked semiconductor laser 571a and 571b. This provides the advantage of reducing the number of optical components while retaining the same functionality as 30 described in figure 5b.

Figure 5d is a cross-sectional schematic optical diagram illustrating a physical realisation of the integrated RA pump assembly 522 of figure 5c. In this example, the optical components of the integrated RA pump assembly 522 of figure 5c are mounted 35 into a hermetically sealed butterfly package 581 (or integrated RA pump assembly

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package 581), within which the optical train is mounted onto a cooling unit comprising a thermo-electric cooler (TEC) 582 and a sub-mount 583. The optical components of the integrated pump assembly 522 may be mounted on cooling unit, the TEC 582 or the sub-mount 583, or both the TEC 582 and the sub-mount 583. The TEC 582 and sub-5 mount 583 maintain the optical components at a fixed, defined temperature. Although the cooling unit uses a TEC 582, by way of example only, to remove heat from the assembly 522, it will be appreciated by the person skilled in the art that the cooling unit may include or use other forms of heat removal such as apparatus for controlling the temperature of the optical components or a heat sink comprising a thermally 10 conductive material to assist in transferring heat generated by the laser chips to outside of the assembly 522. For simplicity, the remaining embodiments of the invention are described with a cooling unit comprising a TEC 582 and sub-mount 583. However, it is to be appreciated by a person skilled in the art that the cooling unit may comprise other mechanisms for providing heat removal.

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Commonly the optical components are maintained at room temperature, such as around 20 to 25°C, but other temperatures are possible to improve aspects of performance. For example, the TEC 582 and sub-mount 583 may be configured to allow the optical components to operate at a higher desired temperature than room 20 temperature, which means the TEC 582 and sub-mount 583 expends less energy (e.g. electrical power consumption) cooling the optical components to the desired temperature. As an example, the TEC 582 and sub-mount 583 may be configured to operate at a temperature of around 40°C, which reduces the overall electrical power consumption of the integrated RA pump assembly when it is running at high 25 temperatures as the TEC 582 and sub-mount 583 will cool the optical components to 40°C rather than room temperature such as 25^.

The locked semiconductor laser chips 571a and 571b are bonded side by side to the sub-mount 583. The laser output is directed into suitably aligned collimating lenses 30 552a and 552b, which may be incorporated into a single collimating lens. The semiconductor laser chips 571a and 571b may be two individual semiconductor stripes or two stripes from the same wafer and cleaved to operate together as one laser stripe pair. The collimated beams 553a and 553b are passed into beam combiner 561, where the optical beams are combined or moved into a combined beam 562 on one 35 physical path. The combined beam 562 is directed to the isolator device 563, which

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may be an optical isolator core and thereafter to the depolariser 510 represented by quarter wave-plate 554 producing combined depolarised beam 555, which is directed to focussing lens 556. The collimating lenses 552a and 552b, beam combiner 561, isolator device 563, quarter wave-plate 554, focussing lens 556are also mounted onto 5 the sub-mount 583 so that the optical path passes through each optical component. The optical fibre 557 is held onto the sub-mount 583via a fixing block 590 which is aligned to the focal point of the focussing lens 556 such that the depolarised beam 555 is coupled to the fibre 557. The alignment of fibre 557 is optimised during a manufacturing alignment process. The fibre 557 is routed to the outside of the 10 integrated RA pump assembly package 581 using an optical fibre ferrule 591, which is hermetically sealed to the package nose cone 592.

A set of optical paths between the laser chips 551a and 551b and the output to optical fibre 557 are free space optical paths for reducing optical performance losses within 15 the integrated package and to improve thermal dissipation. In this example, the set of optical paths that are free space optical paths are shown to include the optical paths between a) the semiconductor lasers 551a and 551b and collimating lenses 552a and 552b, respectively; b) the collimating lenses 552a and 552b and beam combiner 561; c) the beam combiner 561 and the isolator device 563; d) the isolator device 563 and 20 the quarter wave-plate 554, e) the quarter wave-plate 554 and focussing lens 556, and f) the focussing lens 556 and the optical fibre 557 at the output. It is to be appreciated that one or more of the optical components in the optical train may be arranged to abutt or butt up to another optical component to reduce the size of the integrated package further and/or for enhancing optical performance. The butted up optical components 25 are coupled by free space optical paths as no waveguide is provided.

Figure 6a is a schematic diagram illustrating a further example of an optically integrated RA pump assembly 622 showing an alternative example for locking a pump laser source 603a. In this example, the integrated RA pump assembly 622 includes a 30 tunable laser assembly 602a optically coupled to a depolariser 608 and isolator device 610. The tunable laser assembly 602a includes pump laser source 603a and tuning element 604a. The pump laser source 603a of the tunable laser assembly can be dynamically tuned during its operation to anywhere within a defined bandwidth. This provides the advantage that the light output from the tunable laser assembly 602a can 35 be tuned to any combination of wavelengths during system set-up or as required during

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system operation or maintenance, which allows the Raman pump assembly 622 to be optimised for many different operating conditions. The depolarised RA pump laser light is coupled to fibre 614. The wavelength for locking the pump laser source 603a is achieved with tuning element 604a, which allows the pump laser source 603a to be 5 tuned to a specific wavelength and optimisation of the gain profile at any time.

Several locking techniques could be included such as those described in GB Patent Application GB1100225.0 entitled "TUNABLE PUMPING LIGHT SOURCE FOR OPTICAL AMPLIFIERS", filed on 07 January 2011, the contents of which are 10 incorporated herein by reference. GB1100225.0 describes a technique for incorporating an external cavity to a pump laser semiconductor chip. Alternatively, a pump laser source with an inherent tuning capability, such as the pump laser source described in GB Patent Application GB1114823.6 entitled "TUNABLE MULTI-MODE LASER", filed on 26 August 2011, the contents of which are incorporated herein by 15 reference, could be included directly into the optical train of the integrated RA pump assembly 633. This is illustrated in figure 6a in which the tunable laser assembly 602a includes the pump laser source 603a and tuning element 604a that is optically coupled to the depolariser 608 and isolator device 610 in a similar fashion as described in the integrated RA pump assemblies of figures 3a to 5d. In this example, tunable laser 20 assemblies 602a and pump laser sources 603a are described as tunable, however, it will be appreciated by the person skilled in the art that fixed wavelength laser assemblies or sources such as those described in figures 3a to 4b may be used in place of tunable laser assemblies 602a or source 603a to achieve the some of the benefits described.

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The integrated RA pump assembly 622 may further include apparatus for monitoring the output power of the pump laser 603a and the total output power of the depolarised light at the output of the assembly 622. Examples of such apparatus and monitors are described with respect to figures 8a to 9. In particular the assembly 622 may include a 30 pump laser monitor configured for generating measurement data representative of the power of the generated light from the pump laser 603a. The pump laser monitor may be configured to measure a portion of the generated light from the pump laser 603a and generate measurement data representative of the measured portion of the generated light. The assembly 622 may further include an output power monitor 35 configured for generating measurement data representative of the total output power of

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the pump laser 603a. For safety, the assembly 622 may include a reflection power monitor configured for generating measurement data representative of the power of any laser light reflected from the output of the assembly 622.

5 In addition, the integrated RA pump assembly 622 may further include a signal power monitor configured for generating measurement data representative of the power of signal light passing into the output of the optically integrated RA pump assembly 622. The signal power monitor may be configured to measure a portion of the signal light passing into the output of the assembly 622 and generate measurement data 10 representative of the power of the measured portion of the signal light. Including the signal power monitor within assembly 622 minimises the need to deploy separate signal power monitors within the vicinity of the injection point of the depolarised light output from the assembly 622 into the optical fibre of the optical system. The pump laser monitor, output power monitor, reflection power monitor, and signal power 15 monitor (if included in the assembly 622) may be optically coupled to the corresponding optical components of the assembly 622 by free space optical paths.

The RA pump assembly 622 may be further configured for receiving control data from a controller unit for use in controlling the pump laser and/or for performing Raman gain 20 control of the RA pump assembly. The controller unit may be coupled to one or more of the monitors for receiving measurement data for use in performing Raman gain control. The controller unit may be located externally to the pump assembly 622 or may be incorporated within the pump assembly 622.

25 Although a signal power monitor may be included within the optical train of the RA pump assembly 622, a signal power monitor positioned within the optical train of the RA pump assembly 622 can cause a reduction in the output power of the output depolarised laser light from assembly 622. Instead, to minimise losses in the output power of the output depolarised laser light, an external signal monitor may be 30 configured to generate measurement data representative of the power of the signal light at the output of the assembly 622. The external signal monitor may be positioned externally to the assembly 622 within the vicinity of the injection point of the output depolarised laser light into the optical fibre of the optical system. The controller unit can be further configured to receive the measurement data from the external signal 35 monitor for controlling the assembly 622.

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Figure 6b is a schematic optical diagram illustrating an example embodiment of the integrated RA pump assembly 622 of figure 6a. This example includes tuning the wavelength of the tunable laser assembly 602a via an external physical mechanism.

5 The tunable laser assembly 602a includes the pump laser source 603a represented by a semiconductor laser chip 651 and the tuning element 604a represented by a beam splitter 665, a rotatable mirror 667, and a diffraction grating 668. The semiconductor laser chip 651 may be GalnAsP based and may be designed to emit light with different wavelengths substantially in the range of 1400nm to 1500nm.

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The semiconductor laser 651 emits a light beam from front facet 660. The emitted light beam is collimated in collimating lens 652 into collimated beam 653. At this point the beam splitter 665, located in the path of collimated beam 653, reflects a small portion of the collimated beam 653 towards the mirror 667. The reflected light is represented 15 by reflected beam 669, which is reflected at right angles to the path of collimated beam 653. The reflected beam 669 is incident on the rotatable mirror 667, which can be rotatably adjusted to an angle that directs the reflected beam 669 onto a new path towards diffraction grating 668, this is represented by light beam 670. These optical components provide a wavelength locker that is designed to have a spatially varying 20 grating reflectance, such that by varying the angle of the light beam 670 using the rotating mirror means a different dominant reflective wavelength can be achieved, thus the tunable laser assembly 602a can be locked at a particular wavelength depending on the angle of the rotating mirror 667 and the spatially varying grating reflectance. The wavelength locker creates a cavity between the diffraction grating 668 and the 25 back facet 659 of the semiconductor laser chip 651 so that the semiconductor laser chip 651 emits light at a defined wavelength or frequency. The diffraction grating 668 is designed to produce a laser spectrum with multiple longitudinal modes, thus spreading the laser power spectrally and reducing the risk of stimulated Brillouin scattering.

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As the cavity that locks the laser wavelengths is not within the fibre 614, the isolator device 610 represented by isolator 663 can be placed in the path of the optical beam that is transmitted through the beam splitter 665, i.e. the light that is not reflected by the locking cavity. In this example, isolator 663 is located after the beam splitter 665 and 35 before the depolariser 608 represented by quarter wave-plate 654. The isolator 663

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provides attenuation or loss to reflected laser light which would otherwise destabilise the laser output power. Although the isolator 663 is located before the quarter wave-plate 654, it is to be appreciated that the isolator 663 may be placed at other locations such as after the quarter wave-plate 654 or after depolariser 608 as illustrated in figure 5 6a. The depolariser 608 acts to substantially depolarise both the beams of light to produce a depolarised combined beam 655. Alternative depolariser designs could be used such as an alternative wave-plate e.g. half or three quarter wave-plates, or a combination of wave-plates or other suitable material. A focussing lens 656 placed after the quarter wave-plate 654 is used to focus the depolarised beams 655 into fibre 10 614 represented by fibre 657, which outputs the depolarised beams 655 from the integrated RA pump assembly 622.

Figure 6c is a schematic optical diagram illustrating an alternative example of the integrated RA pump assembly 622 in which the tunable laser assembly 602a includes 15 a semiconductor laser chip 671 that is capable of being tuned through control of internal currents within its structure. An example of such a semiconductor laser chip 671 is described in GB Patent Application GB1114823.6 entitled "TUNABLE MULTI-MODE LASER", filed on 26 August 2011, the contents of which are incorporated herein by reference. In this example, the integrated RA pump assembly 622 includes the 20 tunable laser assembly 602a represented by semiconductor laser chip 671, the isolator device 610 represented by bulk isolator 663, and the depolariser 608 represented by quarter wave-plate 654, which are optically coupled together.

The semiconductor laser chip 671 emits laser light from the front facet 660. The 25 semiconductor laser chip 671 may be GalnAsP based and may be designed to emit laser light of a wavelength substantially in the range of 1400nm to 1500nm. The laser light emitted from the front facet 660 is collimated in a collimating lens 652 and the collimated beam 653 is directed into the bulk isolator 663 which is located within the integrated RA pump assembly 622 as described with reference to figure 6b. The 30 function of the bulk isolator 663 is to prevent unwanted reflections that may result in destabilising the pump laser power. The collimated beam 653 is passed through the quarter wave-plate 654 to produce a substantially depolarised beam, which is then focussed by focussing lens 656 into output fibre 657.

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Figure 6d is a cross-sectional schematic diagram illustrating a physical realisation of the integrated RA pump assembly 622 as described with reference to figure 6c. In this example, the optical components of the integrated RA pump assembly 622 of figure 6c are mounted within a hermetically sealed butterfly package 681. The optical train is 5 mounted onto a cooling unit comprising a TEC 682 and a sub-mount 683, which is configured to maintain the optics and optical components at a fixed, defined temperature. The optical components of the integrated RA pump assembly 622 may be mounted on cooling unit, the TEC 682 or the sub-mount 683, or both the TEC 682 and the sub-mount 683. Commonly the optical components can be maintained, by way 10 of example only, at room temperature, such as around 20 to 25*0, but other temperatures are possible to improve aspects of performance. For example, the TEC 682 may be configured to allow the optical components to operate at a higher desired temperature than room temperature, which means the TEC 682 expends less energy (e.g. electrical power consumption) cooling the optical components to the desired 15 temperature. By way of example only, the TEC 682 may be configured to operate at a temperature of around 40°C, which reduces the overall electrical power consumption of the integrated RA pump assembly when it is running at high temperatures as the TEC 682 will cool the optical components to 40°C rather than room temperature such as 25°C.

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The tunable laser semiconductor chip 671 is bonded to the sub-mount 683 and the emitted laser light that is output is directed into a suitably aligned collimating lens 652. The collimated beam is passed through the isolator 663 (e.g. an optical isolator core) then through a depolariser 654 (e.g. a quarter wave-plate), and the depolarised light 25 then passed through focussing lens 656. As all of these optical components are mounted onto the sub-mount 683, the optical path passes through each component. In addition, as can be seen in figure 6d, each component is arranged such that there is a free space optical path between the output of one optical component and the input to the adjacent optical component in the optical path.

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For example, as shown in figure 6d, the optical paths between the semiconductor laser 671, the collimating lens 652, the isolator 663, depolariser 654, focussing lens 656 and the optical fibre 657 are free space optical paths. It is to be appreciated that some of the optical components may be arranged to abutt each other to reduce the size of the 35 integrated package or for reducing performance losses. The butted up optical

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components are coupled by free space optical paths as no waveguide is provided. The optical fibre 657 is held onto the sub-mount 683 via a fixing block 690 and is configured and aligned to the focal point of the focussing lens 656. This allows the depolarised light output from the depolariser 654 to be coupled into the optical fibre 657. During 5 manufacture of integrated RA pump assembly 622, the alignment of the optical fibre 657 is optimised. The optical fibre 657 is routed to the outside of the integrated RA pump assembly package 681 using an optical fibre ferrule 691, which is hermetically sealed to the package nose cone 692.

10 Figure 7a is a schematic diagram of another example of an optically integrated RA pump assembly 722 in which further integration and functionality is achieved by including at least two tunable pump laser assemblies into the integrated RA pump assembly 722. The integrated RA pump assembly 722 includes a plurality of tunable laser assemblies 702a to 702n for generating light at a plurality of wavelengths and 15 polarised states (e.g. at least two tunable laser assemblies), each of which emits laser light that is optically coupled to and subsequently combined with a beam combining unit such as pump combiner 706, the combined light is then directed into a depolarising unit such as depolariser 708, which outputs depolarised light for use in Raman amplification of a plurality of optical channels. In this example, tunable laser 20 assemblies 702a to 702n and/or pump laser sources 703a to 703n are described as tunable, however, it will be appreciated by the person skilled in the art that fixed wavelength laser assemblies or sources such as those described in figures 3a to 4b may be used in place of tunable laser assemblies 702a to 702n or pump laser sources 703a to 703n to achieve some of the benefits described.

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As laser assemblies 702a to 702n or pump laser sources 703a to 703n are tunable, an isolator device 710 is located after the depolariser 708 to attenuate reflected laser light and thus minimise destabilisation of the tunable laser assemblies 702a to 702n. The integrated RA pump assembly 722 outputs the depolarised combined light on optical 30 fibre 714, which is injected into an optical system for use in Raman amplification.

Each tunable laser assembly 702a includes pump laser source 703a and tuning element 704a for generating laser light of a specific wavelength or set of wavelengths. It can be seen in figure 7a that there are a plurality of pump laser monitors 724a to 35 724n for generating measurement data representative of the power of the emitted laser

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light from the corresponding plurality of laser sources 703a to 703n. For example, the tunable laser assembly 702a includes pump laser monitor 724a, which receives a proportion of the laser light for use in monitoring the optical properties, e.g. output power, of the emitted laser light. Pump laser monitor 724a generates measurement 5 data representative of the power of the generated light from the pump laser source 703a. The pump laser monitor 724a measures a portion of the generated light and outputs data representative of the optical properties of the tunable laser light over line 726a, which may be an electrical connector or data line.

10 The pump laser monitors 724a to 724n provide a valuable measure of the pump power useful for Raman gain control and/or control of the pump laser sources 703a to 703n. The measurements can be fed to electrical connectors or data lines 726a to 726n coupled to a controller unit (not shown). The controller unit may be configured to receive the measurement data and, in response, it may be configured for generating 15 control data for controlling laser assemblies 702a to 702n and/or pump laser sources 703a to 703n and/or for performing Raman gain control. The assembly 722 may include the controller unit. Alternatively, the controller unit may be external to assembly 722, in which case measurements are fed out of the assembly 722 on the electrical connectors or data lines 726a to 726n for coupling to the controller unit.

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The optical components of the integrated RA pump assembly 722 are arranged or configured in an integrated optical package without optical fibres or fibre tails between each other. This means that there are free space optical paths between the above optical components. Free space optical paths are considered to be optical paths 25 between components that have no waveguides such as optical fibre or fibre tails for propagating light. It is to be appreciated that if the optical components abutt each other or are butted up, then the optical components may be considered coupled together by free space optical paths as there are no waveguides provided.

30 As an example, the optical paths between a) the tunable laser assemblies 702a to 702n and the pump combiner 706; b) the pump combiner 706 and the depolariser 708; and b) the depolariser 708 and the isolator device 710 may be free space optical paths. In addition, within the tunable laser assembly 703a, the optical path between the tuning element 704a and the pump laser monitor 724a may be a free space optical path. The 35 integrated RA pump assembly 722 removes the need for fusion fibre splices between

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components, and the need for tap and the fusion fibre splices between the pump monitoring components, which further reduces losses or attenuation of the light between components due to fibre splicing. Free space paths may also reduce the optical losses and result in reduced laser power for a given output power, which further 5 assists cooling of the optical components.

Figure 7b is a schematic optical diagram illustrating an example of the integrated RA pump assembly 722 of figure 7a in which the tuning of the tunable laser assemblies 702a to 702n is achieved via an external physical mechanism. For simplicity, two 10 tunable laser assemblies are illustrated as a first tunable laser assembly 702a and a second tunable laser assembly 702b. The first tunable laser assembly 702a includes a pump laser source 703a, which is represented by the semiconductor laser chip 751a and a tunable element 704a represented by beam splitter 765a, rotatable mirror 767a and diffraction grating 768a. Similarly, the pump laser source 703b of the second 15 tunable laser assembly 702b is represented by semiconductor laser chip 751b and a tunable element 704b represented by beam splitter 765b, rotatable mirror 767b and diffraction grating 768b. The semiconductor laser chips 751a and 751b may be GalnAsP based and each may be designed to emit light in the wavelengths substantially within the range the 1400nm to 1500nm. Each semiconductor laser chip 20 751a or 751b emits light from its front facet 760a or 760b, respectively. Each of the emitted light beams is collimated in collimating lenses 752a or 752b and into a first or second light beam 753a or 753b, respectively.

Referring to the first light beam 753a, the beam splitter 765a is located into the optical 25 path of the first light beam 753a. The beam splitter 765a reflects a small portion of the first light beam 753a producing first reflected light beam 769a at right angles to the beam path of the first light beam 753a. The remaining or majority portion of the first light beam 753a is transmitted through the beam splitter to the pump combiner 706 represented by beam combiner 761. The first reflected beam 769a is incident on the 30 rotatable mirror 767a that can be rotated to an angle reflecting the first reflected beam 769a into a new optical path 770a such that the first reflected beam 769a is directed towards the diffraction grating 768a.

This produces a wavelength locker that has a spatially varying grating reflectance, such 35 that, using rotatable mirror 767a, a first angle between the optical path of the first

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reflected beam 769a and the new optical path 770a of the first reflected beam 769a produces a different dominant reflective wavelength than a second angle between the optical path of the first reflected beam 769a and the new optical path 770a. A cavity is created between the diffraction grating 768a and the back facet 759a of semiconductor 5 laser chip 759a so that the laser emits at a defined frequency and wavelength. The diffraction grating 768a is designed to produce a laser spectrum from the semiconductor laser chip 751a with multiple longitudinal modes, which spreads the laser power spectrally and reduces the risk of stimulated Brillouin scattering.

10 Similarly, the second light beam 753b is directed to beam splitter 765b, which reflects a small portion of the second light beam 753b producing a second reflected light beam 769b at right angles to the beam path of the second light beam 753b. The remaining or a majority portion of the second light beam 753b is transmitted through the beam splitter 765b to the pump combiner 706 represented by beam combiner 761. The 15 second reflected beam 769b is incident on rotatable mirror 767b, which can be rotated to an angle reflecting the second reflected beam 769a into a new optical path that is directed towards a diffraction grating 768b. This also produces a wavelength locker that has a spatially varying grating reflectance, such that, using rotatable mirror 767b, a first angle between the optical path of the second reflected beam 769b and the new 20 optical path 770b produces a different dominant reflective wavelength than a second angle between the optical path of the second reflected beam 769b and the new optical path 770b of the second reflected beam 769a. This creates a cavity between the diffraction grating 768b and the back facet 759b of semiconductor laser chip 759b so that the laser emits at a defined frequency and wavelength. The diffraction grating 25 768b is designed to produce a laser spectrum from the semiconductor laser chip 751a with multiple longitudinal modes, which spreads the laser power spectrally and reduces the risk of stimulated Brillouin scattering.

Each pump laser assembly 702a and 702b includes a pump laser monitor 724a and 30 724b, respectively, which may be located within the tunable laser assemblies 702a and 702b to collect light from each pump laser source 703a and 703b. There may be several locations from which light may be collected within the tunable laser assemblies 702a and 702b. For example, the pump laser monitors 724a and 724b may be represented by monitors 795a and 795b, which may be PIN diodes that are placed to 35 collect light from the back facet of each semiconductor laser 751a and 751b,

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respectively. This will provide a measure of the laser power. Additionally or alternatively, pump laser monitors 724a and 724b represented by monitors 796a and 795b may be placed behind diffraction gratings 768a and 768b, respectively, which can be made to transmit a small amount of light onto the monitors 796a and 796b, and 5 reflecting the majority. The monitors 796a and 796b are placed to capture the small amount of transmitted light to provide an accurate figure for the forward laser power. Although two monitor techniques are shown in figure 7b, it is to be appreciated that one or the other technique may also be used.

10 After the locking of the tunable laser assemblies 702a and 702b, the remaining or majority portions of the first and second beams 753a and 753b that are directed through beam splitters 765a and 765b pass into beam combiner 761 for coupling the two optical beams into a single physical path to produce a combined beam comprising the two optical beams. Since the cavity that locks the laser wavelengths is not in 15 optical fibre 757, an isolator device 710 can be placed in the optical beam after the locking cavity, that is before the depolariser 708 represented by quarter wave-plate 754. In this example, the isolator device 710 is represented by an isolator 763, which is mounted after the beam combiner in the single physical path of the combined beam. The isolator 763 provides a loss or attenuation to any reflected laser light, which would 20 otherwise destabilise the laser output power. After the isolator 763, the combined beam is directed to the quarter wave-plate 754, which acts to substantially depolarise both beams of light within the combined beam to produce a depolarised combined beam 755. Although the depolariser 708 is represented by a quarter wave-plate 754, it is to be appreciated that alternative depolariser designs can be used, for example, 25 alternative wave-plates, half or three quarter wave-plates, or a combination of wave-plates or other suitable depolarising material. The depolarised combined beam 755 is then focussed by focussing lens 756 into the optical fibre 757 and subsequently output from the integrated RA pump assembly 722.

30 Figure 7c is another schematic optical diagram illustrating an alternative example of the integrated RA pump assembly 722 in which each of the tunable laser assemblies 702a to 702n incorporate a tunable laser semiconductor chip that is capable of being tuned through control of internal currents in the laser semiconductor structure as described in GB Patent Application GB1114823.6 entitled "TUNABLE MULTI-MODE LASER", filed 35 on 26 August 2011, and the contents of which are incorporated herein by reference.

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For simplicity, two tunable laser assemblies 702a and 702b will be described, by way of example only, in which each assembly 702a and 702b incorporates a tunable laser semiconductor chip 771a and 771b, respectively. The tunable laser semiconductor chips 771a and 771b may be GalnAsP based and may be designed to emit laser light 5 from the front facets 760a and 760b, respectively, each having a wavelength substantially in the range of 1400nm to 1500nm. The laser light emitted from each tunable semiconductor laser device 771a and 771b is collimated in a lens 752a and 752b, respectively. The resulting collimated beams 753a and 753b are directed into a beam combiner 761 to couple the optical beams into a single physical path resulting in 10 combined beam 762 (this is considered a substantially polarised beam comprising the collimated beams 753a and 753b).

The combined beam 762 is then passed into isolator device 710, which is represented by bulk isolator 763 that is mounted within an optical package as previously described. The bulk isolator 763 has the function of preventing unwanted reflections that would otherwise destabilise the pump laser power. The combined beam is then passed through depolariser 708 represented by quarter wave-plate 754 to produce a substantially depolarised beam 755. The substantially depolarised beam 755 is then focussed in focussing lens 756 into output optical fibre 714 represented by optical fibre 757. The laser powers can be monitored by placing monitors 795a and 795b, which may be PIN diodes, to capture the power output from the back facets 759a and 759b of the tunable semiconductor lasers 771a and 771b, respectively. Alternatively or in addition, laser light may be captured from the front facets 760a and 760b of tunable semiconductor lasers 771a and 771b, respectively, which is illustrated in figure 7d. Figure 7d illustrates the integrated RA pump assembly 722 of figure 7c, except that the collimated beams 753a and 753b have a small amount of light tapped off using beam splitters 797a and 797b such that the tapped off light is captured in monitors 796a and 796b. The monitors 795a and 795b or 796a or 796b may be connected to electrical connectors or data lines (not shown) for outputting measurement data representative of the power of the lasers 771a and 771b to a controller unit (not shown).

Figure 7e is a cross-sectional schematic diagram illustrating an example physical realisation of the integrated RA pump assembly 722 of figures 7c or 7d. The optical components that comprise the integrated RA pump assembly 722 of figures 7c or 7d 35 are mounted into a hermetically sealed butterfly package 781. As illustrated, the

20

25

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optical train (optical components) is mounted onto a cooling unit comprising a TEC 782 and a sub-mount 783, which can be optional, to maintain the optics at a fixed, defined temperature. The TEC 782 maintains the optical components at a fixed, defined temperature. The optical components of the integrated RA pump assembly 722 may 5 be mounted on the cooling unit, the TEC 782 or the sub-mount 783, or both the TEC 782 and the sub-mount 783. Commonly the optical components are maintained, by way of example only, at room temperature such as around 20 to 25^, but other temperatures are possible to improve aspects of performance. For example, the TEC 782 may be configured to allow the optical components to operate at a higher desired 10 temperature than room temperature, which means the TEC 782 expends less energy (e.g. electrical power consumption) cooling the optical components to the desired temperature. By way of example only, the TEC 782 may be configured to operate at a temperature of around 40°C, which reduces the overall electrical power consumption of the integrated RA pump assembly 722 when it is running at high temperatures as the 15 TEC 782 will cool the optical components to 40^ rather than room temperature such as 25^.

The tunable semiconductor laser chips 771a and 771b are bonded side by side to the sub-mount 783. The semiconductor laser chips 771a and 771b may be two individual 20 semiconductor stripes or two stripes from the same wafer and cleaved to operate together as one laser stripe pair. The laser light emitted from the tunable semiconductor lasers 771a and 771b is directed into suitably aligned collimating lenses 752a and 752b.

25 A portion of the collimated beam from each of the collimating lenses 752a and 752b is tapped off using a suitably aligned beam taps or beam splitters 797a and 797b, respectively, as described with reference to figure 7d. The remaining or the majority of the collimated beams from collimating lenses 752a and 752b pass through the beam taps or beam splitters 797a and 797b, respectively. The portion of light tapped off by 30 the beam taps or beam splitter 797a and 797b is directed substantially orthogonally, for example at 90°, from the main collimated beam into monitors 796a and 796b, respectively. Monitors 796a and 796b may be optical PIN diodes that are mounted on beam taps or beam splitters 797a and 797b or they may also be arranged and mounted on the sub-mount 783. The monitors 796a and 796b can provide an accurate 35 measure of the forward power of the emitted beams from the tunable semiconductor

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lasers 771a and 771b. Additionally or alternatively, further monitors 795a and 795b, which may be PIN diodes, can be mounted on the sub-mount 783 adjacent to the rear facets of the tunable semiconductor laser chips 771a and 771b, respectively. These monitors 795a and 795b can capture the laser light from the rear facet of each laser 5 771a and 771b to give a measure of the amount of light being emitted from the front facet of the lasers 771a and 771b, respectively, as described above with respect to figure 7c. The monitors 795a and 795b or 796a or 796b may be connected to electrical connectors or data lines (not shown) for outputting measurement data representative of the power of the lasers 771a and 771b to a controller unit (not shown).

10

The collimated beams are combined in beam combiner 761 and then passed to an isolator 763 comprising an optical isolator core. The combined collimated beams are then directed to a quarter wave-plate 754, which produces the depolarised combined beam. The depolarised combined beam is focussed by focussing lens 756 into optical 15 fibre 757. The optical components of the integrated RA pump assembly 722 are all arranged and mounted onto the sub-mount 783 such that most of the optical paths between the optical components are free space optical paths.

For example, as shown in figure 7e, the optical paths between the semiconductor 20 lasers 771a, 771b, the collimating lenses 752a, 7562b, the beam splitters 797a, 797b, the beam combiner 761, then isolator 763, the wave-plate 754, the focussing lens 756, and the optical fibre 757 are free space optical paths. It is to be appreciated that some of the optical components may be arranged to abutt each other to reduce the size of the integrated package or for reducing performance losses. The butted up optical 25 components are coupled by free space optical paths as no waveguide is provided. The optical paths comprise a main optical path that passes through each optical component.

The optical fibre 757 is held onto the sub-mount 783 via a fixing block 790, which is 30 aligned to the focal point of the focussing lens 756, this is optimised during the alignment process in manufacture. The optical fibre 757 is routed to the outside of the integrated RA pump assembly package 781 using an optical fibre ferrule 791, which is hermetically sealed to the package nose cone 792.

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Figure 8a is a schematic diagram illustrating an example of an optically integrated RA pump assembly 822, which comprises the integrated RA pump assembly 722 as described with reference to figures 7a to 7e in which further integration has been achieved by including one or more further power monitor(s) 812. For example, the 5 power monitor(s) 812 may be used to generate measurement data representative of the total output power of the depolarised laser light at the output of the assembly 822. For simplicity, the reference numerals of figure 7a have been reused for the same or similar optical components and only the modification of further monitors such as the power monitor(s) 812 will be described.

10

The power monitor(s) 812 include an output power monitor that is configured to generate measurement data representative of the output laser pump power at the output of assembly 822, and/or include a signal power monitor that is configured to generate measurement data representative of signal power entering the output of the 15 assembly 822. The measurement data may be used by a controller unit (not shown) or circuitry for controlling the laser pump powers and/or performing Raman gain control. Monitor(s) 812 generate the measurement data, which is transmitted as electrical signals over electrical connection(s) or line(s) 815 for use by the controller unit.

20 The measurement data generated by the output power monitor may include data representative of the total output power from the plurality of lasers 703a to 703n at the output of assembly 822. The measurement data generated by the signal power monitor may include data representative of the total signal power present in the fibre span. Although power monitor(s) 812 are shown to be placed within assembly 822, it is 25 to be appreciated by those skilled in the art that the output power monitor may be placed in the vicinity of the injection point of the depolarised laser light output from assembly 822 at the fibre output 714 to provide a measure of the total fibre power from the plurality of pump lasers 703a to 703n being emitted onto the output fibre 714. However, in this example the output power monitor is placed within the integrated 30 optical package of the integrated RA pump assembly 822. A total power measurement as well as a measure of the amount of pump power being reflected by the system (indicating a fault condition) is key for RA pump power control as well as safety. This is shown figure 1, in which the total power measurement is implemented using a photodiode that is conventionally spliced with a separate coupler. However, in figure 1, 35 the photodiode and separate coupler are each contained within their own component

34

housing. However, in the example integrated RA pump assembly 822, the optical components are optically coupled together using free space optical paths such that individual fibres, fibre tails between the optical components and consequential fusion splicing are not required.

5

Figure 8b is a schematic optical diagram illustrating an example embodiment of the integrated RA pump assembly 822 of figure 8a. Figure 8b corresponds with the integrated RA pump assembly 722 as described with reference to figures 7c and for simplicity, the reference numerals of figure 7c will be used for the same or similar 10 optical components. For simplicity, the integrated RA pump assembly 822 of figure 8b is described, by way of example only, as having two tunable laser assemblies 702a and 702b incorporating semiconductor laser chips 771a and 771b, respectively. In this example, the polarisation state of the laser light will be described, although this is applicable to any prior or later explanations. The two internally tunable semiconductor 15 laser chips 771a and 771b as previously described, emit light from their front facets 760a and 760b to produce a laser spectrum from each laser with multiple longitudinal modes. Although this example describes that laser assemblies 702a and 702b, or laser assemblies 702a to 702n, include tunable laser sources, it will be appreciated by the person skilled in the art that laser assemblies 702a to 702n may alternatively 20 include fixed laser sources as described with respect to figures 3 and 4, which can also achieve the benefits described.

Laser pump monitors 724a and 724b is represented by monitors 795a and 795b, respectively, which can be PIN diodes placed to collect light from the back facet 759a 25 and 759b of lasers 771a and 771b, respectively. The emitted light beams from lasers 771a and 771b are linearly polarised and are collimated in collimating lenses 752a and 752b into collimated beams 753a and 753b and maintain their linear polarisation state as indicated by the arrows in beams 753a and 753b. Additionally or alternatively, with reference to figure 7c and the corresponding description, the laser pump monitors 724a 30 and 724b may be configured to comprise or include forward pump monitors 796a and 796b at this point as described with reference to figure 7b or 7d for measuring laser power in conjunction to or separately from monitors 795a and 795b.

The collimated beams 753a and 753b pass into a beam combiner 761, which couples 35 the collimated beams 753a and 753b into a single combined beam on a single physical

35

path 762, where the beams are still linearly polarised. Isolator 763 is mounted after the beam combiner 761 to prevent reflections passing back to the tunable semiconductor laser chips 771a and 771b and destabilising the output power. After the isolator 763, the combined beam 880 is directed to power monitors 812, which may include an 5 output power monitor and/or a signal power monitor comprising a suitably aligned beam tap 899, monitors 896 and 833, suitably aligned signal band splitter 894, and a further monitor 892. The output power monitor includes the beam tap 899, monitors 896 and 833. The signal power monitor includes the signal splitter 894 and the further monitor 892.

10

The output power monitor is configured for generating measurement data representative of the total output power of the plurality of pump lasers. In this example, the total output power of pump laser assemblies 702a and 702b. This is achieved by directing the combined beam 880 to the suitably aligned beam tap 899, which taps off a 15 small amount of the laser pump power from the combined beam 880 in the form of tapped off beam 898, which is substantially orthogonal, for example at 90°, to the combined beam 880. The tapped off beam 898 is captured by monitor 896, which may be a PIN diode, to provide a measure of the forward total laser power. The monitor 896 is used to output measurement data representative of the total output power of the 20 pump lasers 703a and 703b.

In addition, a reflection power monitor may be used to measure any laser light that is reflected back from the system from optical fibre 714, which is represented by output fibre 757. In this case, the reflection monitor includes beam tap 899 and monitor 833. 25 The reflection power monitor is configured to generate measurement data that could indicate a fault condition due to the power of the reflected laser light, which can pass back from the output fibre 757 and along combined beam path 834a and pass into the beam tap 899. The beam tap 899 is configured to reflect the reflected laser light substantially orthogonally, for example at 90°, to the beam path 834a and into beam 30 path 834b, but in an opposite direction to the forward laser beam 880 such that the reflected laser light is captured by monitor 833, which may be a PIN monitor or diode. Monitor 833 is used to output measurement data representative of the reflected pump laser light. The output monitor and the reflection monitors provide an integrated solution for measuring the total power and the reflected power of the forward main 35 combined laser beams 880. This provides the advantage of not requiring a separate

36

external fibre coupled tap and external output and/or reflection power monitors as shown in figure 1.

The signal power monitor is configured for generating measurement data 5 representative of the power of signal light passing into the output of the optically integrated RA pump assembly 822. The signal power monitor includes the signal band splitter 894 and monitor 892. The forward main combined laser beams 880 pass through the beam tap 899 and subsequently pass through the signal band splitter 894, which allows the capture of any signal light passing into the integrated RA pump 10 assembly 822 from optical fibre 757. The signal power at this point of the system span can be monitored by allowing a small amount of the signal light 897b to be passed into the integrated RA pump assembly 822 from output fibre 757 in an opposite direction to the forward main combined laser beams of the pump laser assemblies 702a and 702b. The forward main combined laser beams are directed into the signal band splitter 894, 15 which may be made from a thin film filter but which has a low loss to the outgoing forward main combined laser beams. All of the signal light 897a coming into the integrated RA pump assembly 822 may be directed by the signal splitter 894 to an optical path that is substantially orthogonal, for example is 90°, to the forward main combined beam path such that the resulting signal light 897b is directed into monitor 20 892, which may be an optical PIN diode, for providing an accurate measure of the signal span data and/or signal power of the span.

The pump laser monitors 724a to 724n, output power monitor, reflection power monitor, and signal power monitor may be optically coupled to the corresponding optical 25 components by free space optical paths. The output, reflection and signal power monitors are configured to be connected to electrical connectors or data lines (not shown) for outputting the measurement data to a controller unit (not shown) for use in controlling the lasers and/or Raman gain. The controller unit may be configured to a) read the measurement data representing the signal power from the signal power 30 monitor, b) determine the per laser powers from the measurement data received from the plurality of pump laser monitors 724a to 724n, c) determine the total output laser power from the measurement data received from the output power monitor, and d) calculate the correct pump current and or wavelength settings for laser assemblies 702a to 702n and/or pump lasers 703a to 703n to give the correct gain and gain profile

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for the distributed Raman gain. The controller unit may be external to the assembly 822 or it may be included in the internal circuitry of assembly 822.

Meanwhile, the outgoing forward main combined laser beams pass out of the signal 5 band splitter as combined laser beam 895 and into the quarter wave-plate 754, which acts to substantially depolarise the combined beams of light comprising collimated beams 753a and 753b to produce a depolarised combined beam 755. Although the depolariser 708 is represented as a quarter wave-plate 754, it is to be appreciated that alternative depolariser designs may be used, for example, half or three quarter wave-10 plates or a combination of wave-plates, or suitable depolarising material. The depolarised combined beam 755 is passed through a focussing lens 756 such that the depolarised combined beam 755 is focused into fibre 757 and is output from the integrated RA pump assembly.

15 Although the output and reflection power monitors are described as being situated between the pump combiner (e.g. beam combiner) 706 and depolariser 708 of assembly 822, it is to be appreciated by a person skilled in the art that the optical components of these monitors may be situated in any other suitable location in the optical train of the assembly 822. For example, the output power monitor may be 20 situated in the optical train between the depolariser 708 and the output of assembly 822.

Although the signal power monitor has been described, by way of example only, to be situated within the assembly 822, it is to be appreciated by the person skilled in the art 25 that the signal power monitor may be situated externally to the assembly 822, for example, the signal power monitor may be an external signal monitor that is situated in the optical fibre of the optical system in the vicinity of the injection point of the depolarised laser light output from assembly 822. In this case, the external signal monitor is configured to generate measurement data representative of the signal power 30 of signal light at the output of assembly 822. The external signal monitor is configured to be coupled to electrical connectors or data lines (not shown) for outputting the measurement data to a controller unit (not shown) for use in controlling the lasers and/or Raman gain.

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Figure 8c is a cross-sectional schematic diagram illustration a physical example of the integrated RA pump assembly 822 of figure 8b. For simplicity, the reference numerals of figure 7d have been reused for the same or similar optical components. The optical components of the integrated RA pump assembly 822 of figure 8b are mounted into a 5 hermetically sealed butterfly package 881, a so-called integrated RA pump assembly package 881. The optical train (the optical components) is mounted onto a TEC 782 and sub-mount 783to maintain the optics at a fixed, defined temperature as has already been described with reference to figures 5d, 6d, and 7e.

10 The tunable semiconductor laser chips 771a and 771b are bonded side by side to the sub-mount 783. The semiconductor laser chips 771a and 771b may be two individual semiconductor stripes or two stripes from the same wafer and cleaved to operate together as one laser stripe pair. The laser light emitted from the tunable semiconductor lasers 771a and 771b is directed into suitably aligned collimating lenses 15 752a and 752b. The collimated beams are combined in beam combiner 761 and then passed to an isolator 763 comprising an optical isolator core.

Prior to the combined collimated beams being directed to a quarter wave-plate 754, which produces the depolarised combined beam, the combined collimated beams are 20 directed into suitably aligned beam taps or beam splitters 899 mounted onto the sub-mount 783. The light from the beam taps or splitters 899 is directed at 90° or orthogonally from the main beam path of the combined collimated beams into an monitor 896, which may be an optical PIN diode, that is mounted on the TEC 782 or the sub-mount 783 (optional) or on the beam taps or splitters 899 as shown in figure 25 8c. This provides an accurate measure of the total forward power of the lasers emitted beams. In addition, another PIN monitor 793 may be mounted on the sub-mount 783 to capture the laser light from the rear facet of each laser 771a and 771b to give a measure of the amount of light being emitted from the front facet of the laser 771a and 771b as previously described with reference to figures 7a to 7e. The main combined 30 beams are then directed into a suitably aligned signal band splitter 894, which may be a thin film band pass filter that passes the combined beams in the forward direction and reflects any backward travelling signal light substantially orthogonal, for example at 90°, to the combined beams into PIN diode 892, which may be mounted onto the sub-mount 783 or on the signal band splitter 894 as shown in figure 8c.

35

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After passing though the signal band splitter 894, the combined collimated beams are directed to the quarter wave-plate 754, which produces the depolarised combined beam. The depolarised combined beam is focussed by focussing lens 756 into optical fibre 757. The main optical components of the integrated RA pump assembly 822 are 5 all arranged and mounted onto the sub-mount 783 such that the optical paths between the optical components are free space optical paths. As shown in figure 8c, the optical paths between the semiconductor lasers 771a and 771b, the collimating lenses 752a and 752b, the beam combiner 761, the isolator 763, signal beam splitter 899, the signal band splitter 894, the wave-plate 754, the focussing lens 756 and the output optical 10 fibre 757 are free space optical paths. The optical paths comprise a main optical path that passes through each optical component. The optical fibre 757 is held onto the sub-mount 783 via a fixing block 790, which is aligned to the focal point of the focussing lens 756, this is optimised during the alignment process in manufacture. The optical fibre 757 is routed to the outside of the integrated RA pump assembly package 15 881 using an optical fibre ferrule 791, which is hermetically sealed to the package nose cone 792.

Figure 9 is a schematic diagram illustrating a further example optically integrated RA pump assembly 922, which is based on the integrated RA pump assembly 822 of 20 figures 8a to 8c, but with the addition of a controller unit 916 for receiving measurement data from the pump laser monitors 724a to 724n, and monitor(s) 812, and an external signal monitor 920 for use in controlling pump laser assemblies 702a to 702n or sources 703a to 703n. The optical train of the integrated RA pump assembly 922 is similar to that described with reference to figures 7a to 8c, but with the addition of the 25 controller unit 916 and the external signal monitor 920. For simplicity, reference numerals for similar or the same optical components of figures 7a and 8c will be used when describing similar or the same optical components in respect of assembly 922.

The external signal monitor 920 is used to determine the signal power of the optical 30 signal in the vicinity of the injection point of the depolarised laser light output from assembly 922. In this example, the monitors(s) 812 may include at least the output power monitor and optionally a reflection power monitor as described with reference to figures 8a to 8c. The reflection power monitor may be included for safety reasons. However, a signal power monitor within assembly 922 is not necessary due to the

40

external signal monitor 920. The external signal monitor 920 provides the advantage of minimising power losses in the output depolarised light output from assembly 922.

Each of the plurality of pump laser monitors 724a to 724n may be configured for 5 generating measurement data representative of the power of the generated light from a corresponding pump laser of the plurality of pump lasers 703a to 703n. Although figures 7a to 9 illustrate that there may be a one-to-one correspondence between pump laser monitors 724a to 724n and the plurality of pump lasers 703a to 703n, it is to be appreciated by the person skilled in the art that there may instead be a plurality of 10 pump laser monitors 724a to 724i, i.e. fewer than the number of pump lasers 724a to 724n, such that some of the plurality of pump laser monitors 724a to 724i are configured for generating measurement data representative of the power of the generated light from more than one of pump lasers 703a to 703n.

15 The output power monitor is configured for generating measurement data representative of the total output power of the plurality of pump lasers 724a to 724n. The monitors 724a to 724n, output power monitor and/or reflection power monitor may be optically coupled to the corresponding optical components by free space optical paths.

20

The RA pump assembly 922 is configured for receiving control data from the controller unit 922, which is configured for performing Raman gain control and/or for controlling the operation of the laser assemblies 702a to 702n. The controller unit 916 is further configured for receiving measurement data from the one or more monitors (e.g. the 25 pump laser monitors 724a to 724n, the output power monitor and/or the reflection power monitor) for use in controlling the plurality of pump lasers 703a to 703n and/or assemblies 702a to 702n. The controller unit 916 is further configured to receive measurement data from the external signal monitor 920 in which the external signal monitor 920 is configured for generating measurement data representative of the 30 power of signal light at the output of the RA pump assembly.

In particular, the controller unit 916 includes control electronics configured for receiving the measurement data and generating control data for controlling laser assemblies 702a to 702n or pump laser sources 703a to 703n. For example, the control data may 35 be used by the assembly 922 for adjusting the operation of one or more of the pump

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lasers 703a to 703n, which may be required for controlling the Raman gain when the assembly 922 is used for Raman amplification or for operational or safety purposes. The external signal monitor 920 captures a portion of the signal power of signal light on an optical fibre 918 that may part of or coupled to fibre 714 in the vicinity of the injection 5 point of the depolarised output light from the integrated RA pump assembly 922. The external signal monitor 920 is configured to generate measurement data representative of the power of the optical signal light in fibre 918. The external signal monitor 920 converts the measurement of the measured optical signal power in fibre 918 into measurement data, which is received by controller unit 916 over an electrical line or 10 data line.

The controller unit 916 is configured to a) read the measurement data representing the signal power from signal monitor 920, b) determine the per laser powers from the measurement data from the plurality of pump laser monitors 724a to 724n received 15 over electric monitor lines or data lines 726a to 726n, c) determine the total output laser power from the measurement data from output power monitor 812 received over data line 815, and d) calculate the correct pump current and or wavelength settings for lasers 702a to 702n to give the correct gain and gain profile for the distributed Raman gain. Although controller unit 916 is described, by way of example only, as being 20 external to the assembly 922, it is to be appreciated by the person skilled in the art that controller unit 916 may also be included in the assembly 922. In addition, the control electronics of the controller unit 916 may also be formed as part of the integrated RA pump assembly 922 and included on the substrate or within the package of the integrated RA pump assembly 922.

25

In this example, tunable laser assemblies 702a to 702n and/or pump laser sources 703a to 703n are described as tunable. However, it will be appreciated by the person skilled in the art that fixed wavelength laser assemblies or sources such as pump laser sources 303, 403a, 403b, 503a and/or 503b as described with respect to figures 3 to 30 5b may be used in place of tunable laser assemblies 702a to 702n or sources 703a to 703n to achieve some of the benefits described. In such examples, the electronic control circuit 916 may be configured to set the correct laser drive currents to provide the optimum Raman gain for the system.

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The integrated RA pump assembly 922 may be provided as an integrated circuit. The integrated RA pump assembly 922 provides the advantage of reducing losses in the laser pump power that are due to separate fibre coupled optical components, fusion fibre splices and a separate electronic printed circuit boards. In addition, provides 5 improvements in energy efficiency of a RA pump assembly due to all optical components being provided on the same substrate and having free space optical paths between a substantial number of the optical components.

Although pump lasers 303, 403a, 403b, 503a, and 503b may be implemented using 10 laser assemblies or sources designed or manufactured to each emit a specific, fixed wavelength (e.g. fixed wavelength lasers), it will be appreciated by the person skilled in the art that pump lasers 303, 403a, 403b, 503a, and 503b may also be implemented as tunable laser assemblies or sources in which the wavelength of a single tunable laser assembly or source of the output light can be dynamically tuned during its operation to 15 anywhere within a defined bandwidth. Tuneable elements 604a and 704a to 704n as described with respect to figures 6a to 9 may be included, where applicable, in place of the wavelength locking optics as described with respect to figures 3a to 5e.

Although the tunable laser assemblies 602a and 702a to 702n as described with 20 respect to figures 6a to 9 may be implemented as tunable laser assemblies or sources, it will be appreciated by the person skilled in the art that fixed laser assemblies or sources can also be implemented in the integrated Raman pump units 622 to 722 as described in figures 6a to 9 and still gain the benefits of integration and/or further functionality of monitors and electronic control as described with respect to figures 6a 25 to 9. Where applicable, the tuning elements 604a to 704a to 704n as described with respect to figures 6a to 9 may be replaced with the appropriate wavelength locking optics as described with reference to figures 3a to 5e.

Although the above examples describe semiconductor laser chips based on GalnAsP 30 technologies that may be designed to emit laser light of a wavelength substantially in the range of 1400nm to 1500nm, it is to be appreciated by a person skilled in the art that any suitable semiconductor laser chip emitting laser light of any wavelength or range of wavelengths suitable for use in Raman amplification may be used in the optically integrated Raman pump assembly as described herein. Although the above 35 examples describe, by way of example only, that the optical components can be

43

maintained at room temperature such as around 20 to 25°C, or at other temperatures such as around 40°C, it is to be appreciated by a person skilled in the art that the optical components may be maintained at any desired temperature or within any desired temperature range such that the optically integrated Raman pump assembly 5 remains operational for use in Raman amplification applications.

Although the invention has been described in terms of preferred examples or embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled 10 in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification and in each example or embodiment may be incorporated into the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

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Claims

CLAIMS:

1. An optically integrated Raman amplification (RA) pump assembly for use in an optical system comprising an optical fibre carrying an optical signal, the optical signal including a plurality of optical channels, the assembly comprising:

5 a plurality of pump lasers generating light at a plurality of wavelengths and polarised states;

a beam combining unit for multiplexing the light generated by the plurality of lasers;

a depolarising unit to produce an output of generally unpolarised light from the 10 multiplexed light, the generally unpolarised light having at least two components with different wavelengths; and an output for injecting the generally unpolarised light into the optical fibre.

2. An optically integrated RA pump assembly according to claim 1, wherein at 15 least one of optical paths between the plurality of pump lasers and the output is a free space optical path.

3. An optically integrated RA pump assembly according to claim 1, wherein a plurality of optical paths between the plurality of pump lasers and the output are free

20 space optical paths.

4. An optically integrated RA pump assembly according to any of claims 1 to 3, wherein each of the plurality of pump lasers is a tunable pump laser assembly for emitting light tuned to a specific wavelength of the plurality of wavelengths.

25

5. An optically integrated RA pump assembly according to claim 4, wherein each tunable pump laser assembly comprises a pump laser source and a tuning unit, wherein the tuning unit locks the pump laser source to the specific wavelength.

30 6. An optically integrated RA pump assembly according to any of claims 1 to 5, further comprising at least one optical isolator located in the optical path between the pump lasers and the output.

7. An optically integrated RA pump assembly according to claim 6, wherein the 35 optical isolator is located in the optical path prior to the depolarising unit.

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8. An optically integrated RA pump assembly according to any of claims 1 to 3, further comprising a plurality of reflective devices located at the output in the optical fibre, the reflective devices for use in tuning or locking the plurality of pump lasers to

5 the plurality of wavelengths.

9. An optically integrated RA pump assembly according to any preceding claim, wherein each of the plurality of pump lasers further comprise a collimating lens for collimating the generated light.

10

10. An optically integrated RA pump assembly according to any preceding claim, further comprising a focussing lens and the output further comprising an output optical fibre, wherein the focussing lens is arranged for coupling the generally unpolarised light to the output fibre.

15

11. An optically integrated RA pump assembly according to any preceding claim, further comprising a plurality of pump laser monitors, each pump laser monitor configured for generating measurement data representative of the power of the generated light from one or more of the plurality of pump lasers.

20

12. An optically integrated RA pump assembly according to any preceding claim, further comprising an output power monitor configured for generating measurement data representative of the total output power of the plurality of pump lasers.

25 13. An optically integrated RA pump assembly according to any preceding claim, further comprising a reflection power monitor configured for generating measurement data representative of the power of any laser light reflected from the output of the assembly.

30 14. An optically integrated RA pump assembly according to any preceding claim , further comprising a signal power monitor configured for generating measurement data representative of the power of signal light passing into the output of the optically integrated RA pump assembly.

46

15. An optically integrated RA pump assembly according to any of claims 11 to 14, wherein the RA pump assembly is configured for receiving control data from a controller unit, the controller unit configured for receiving measurement data from the one or more monitors for use in controlling the plurality of pump lasers.

5

16. An optically integrated RA pump assembly according to claim 15, wherein the controller unit is further configured to receive measurement data from an external signal monitor, the external signal monitor configured to generate measurement data representative of the power of signal light at the output of the RA pump assembly.

10

17. An optically integrated RA pump assembly according to any preceding claim, wherein the optical components of the integrated RA pump assembly are mounted onto a cooling unit for removing heat from the optical assembly.

15 18. An optically integrated RA pump assembly according to claim 17, wherein the cooling unit includes a heat sink.

19. An optically integrated RA pump assembly according to claims 17 or 18, wherein the cooling unit includes a heat pump.

20

20. An integrated RA pump assembly according to claim 19, wherein the heat pump is a thermo-electric cooler and a sub-mount, wherein the optical components of the integrated RA pump assembly are arranged and mounted on the sub-mount or the thermo-electric cooler or both.

25

21. An optically integrated RA pump assembly according to any preceding claim, wherein the housing of the integrated RA pump assembly is a hermetically sealed package and the output comprises an output port passing through the hermetically sealed package for use in injecting the unpolarised light into the optical fibre of the

30 optical system.

35

22. An optically integrated RA pump assembly according to any preceding claim, wherein the optical components are arranged to be optically coupled by free space optical paths.

47

23. An optically integrated RA pump assembly for use in an optical system comprising an optical fibre carrying an optical signal, the optical signal including a plurality of optical channels, the assembly comprising:

a plurality of pump lasers generating light at a plurality of wavelengths and 5 polarised states;

a beam combining unit for multiplexing the light generated by the plurality of lasers;

a depolarising unit to produce an output of generally unpolarised light from the multiplexed light, the generally unpolarised light having at least two components with 10 different wavelengths;

an isolating device for attenuating any reflected light generated by the plurality of lasers; and an output for injecting the generally unpolarised light into the optical fibre, wherein a set of the optical components are optically coupled based on free space 15 optical paths.

24. An optically integrated RA pump assembly according to any of claims 23, further comprising:

a plurality of pump laser monitors configured for generating measurement data 20 representative of the power of the generated light from the pump lasers; and an output power monitor configured to generate measurement data representative of the total output power of the pump lasers.

25. An optically integrated RA pump assembly according to claim 24, wherein the 25 monitors are optically coupled to the corresponding optical components by free space optical paths.

26. An optically integrated RA pump assembly according to claims 24 or 25, wherein the integrated RA pump assembly is configured for receiving control data from

30 a controller unit coupled to the monitors, the controller unit configured for receiving measurement data from the monitors for use in controlling the pump lasers.

27. An optically integrated RA pump assembly according to claim 26, wherein the controller unit is further configured to receive measurement data from an external

48

signal monitor, the external signal monitor configured to generate measurement data representative of the power of signal light at the output of the RA pump assembly.

28. An optically integrated RA pump assembly according to any of claims 23 to 27, 5 wherein each of the plurality of pump lasers is a tunable pump laser or a fixed wavelength pump laser.

29. An optically integrated Raman amplification (RA) pump assembly for use in an optical system comprising an optical fibre carrying an optical signal, the optical signal

10 including a plurality of optical channels, the assembly comprising:

a pump laser generating light at a wavelength and a polarised state, the wavelength based on at least one of the plurality of optical channels;

a depolarising unit to produce an output of generally unpolarised light from the generated light, the generally unpolarised light having at least two components with 15 different wavelengths; and an output for injecting the generally unpolarised light into the optical fibre; and wherein the pump laser, the depolarising unit and the output are optically coupled by free space optical paths.

20 30. An optically integrated RA pump assembly according to claim 29, wherein the pump laser is a tunable pump laser assembly for emitting light tuned to the wavelength.

31. An optically integrated RA pump assembly according to claim 30, wherein the tunable pump laser assembly comprises a pump laser source and a tuning unit,

25 wherein the tuning unit locks the pump laser source to the wavelength.

32. An optically integrated RA pump assembly according to claims 30 or 31, further comprising at least one optical isolator located in the optical path between the pump laser and the output.

30

33. An optically integrated RA pump assembly according to claim 29, further comprising a reflective device located at the output, the reflective device for use in tuning or locking the pump laser to the wavelength.

49

34. An optically integrated RA pump assembly according to any of claims 29 to 33, further comprising a pump laser monitor configured for generating measurement data representative of the power of the generated light from the pump laser.

5 35. An optically integrated RA pump assembly according to any of claims 29 to 34, further comprising an output power monitor configured for generating measurement data representative of the total output power of the pump laser.

36. An optically integrated RA pump assembly according to any of claims 29 to 35, 10 further comprising a reflection power monitor configured for generating measurement data representative of the power of any laser light reflected from the output of the assembly.

37. An optically integrated RA pump assembly according to any of claims 29 to 36, 15 further comprising a signal monitor configured for generating measurement data representative of the power of signal light passing into the output of the optically integrated RA pump assembly.

38. An optically integrated RA pump assembly according to any of claims 34 to 37, 20 wherein the RA pump assembly is further configured for receiving control data from a controller unit, the controller unit coupled to one or more of the monitors for receiving measurement data for use in controlling the pump laser.

39. An optically integrated RA pump assembly according to claim 38, wherein the 25 controller unit is further configured to receive measurement data from an external signal monitor, the external signal monitor configured to generate measurement data representative of the power of signal light at the output of the RA pump assembly.

40. An optically integrated RA pump assembly according to any preceding claim, 30 provided as an integrated circuit.

41. A Raman amplifier comprising an optically integrated RA pump assembly according to any preceding claim.

42. An optically integrated RA pump assembly as herein described with reference 35 to the accompanying drawings.

•.'????.• INTELLECTUAL

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50

Application No: GB 1202446.9 Examiner: Dr Claire Williams

Claims searched: all Date of search: 12 July 2012

Patents Act 1977: Search Report under Section 17

Documents considered to be relevant:

Category

Relevant to claims

Identity of document and passage or figure of particular relevance

X

1-41

US2003/133180 A

(WUHAN RES and TELECOM) see FIGURES 1 and 10

X

1-41

US2008/044936 A

(FURUKAWA) see paragraphs 0157 and 0239 and feature 54

X

1-41

US2006/159149 A

(SANMINA SCI CORP) see paragraphs 0031-0040

X

1,4-8, 11-21

EP1229673 A

(FURUKAWA ELECTRIC ) see abstract and paragraphs 0020, 0021 and 0050

X

1,4-8, 11-21

W003/034557 A1

(FURUKAWA) see abstract and Figure 23 and paragraph 0149

Categories:

X

Document indicating lack of novelty or inventive

A

Document indicating technological background and/or state

step

of the art.

Y

Document indicating lack of inventive step if

P

Document published on or after the declared priority date but

combined with one or more other documents of

before the filing date of this invention.

same category.

&

Member of the same patent family

E

Patent document published on or after, but with priority date

earlier than, the filing date of this application.

Field of Search:

Search of GB, EP, WO & US patent documents classified in the following areas of the UKC :

Intellectual Property Office is an operating name of the Patent Office www.ipo.gov.uk

•.'????.• INTELLECTUAL

*.*. .V PROPERTY OFFICE

51

International Classification:

Subclass

Subgroup

Valid From

HOIS

0003/30

01/01/2006

HOIS

0003/067

01/01/2006

HOIS

0003/094

01/01/2006

Intellectual Property Office is an operating name of the Patent Office www.ipo.gov.uk