GB2570440A

GB2570440A - Optical source and method of assembling an optical source

Info

Publication number: GB2570440A
Application number: GB1721349.7A
Authority: GB
Inventors: James Poustie Alistair; Ashley Harrison James
Original assignee: Rushmere Tech Ltd
Current assignee: Rushmere Tech Ltd
Priority date: 2017-12-19
Filing date: 2017-12-19
Publication date: 2019-07-31
Also published as: GB201721349D0; WO2019122877A1

Abstract

An optical source 10 comprises: a laser 12 comprising an optical gain section 20 and an optical phase control section 22; a transmission filter 16 configured to receive and filter light output from the laser 12; a partial reflector 18 configured to receive filtered light from the filter 16 and to input filtered light back into the laser 12; at least one base member 32: and a temperature control element 30. At least the transmission filter 16 and the partial reflector 18 are fixedly mounted on a first of the at least one base member 32. The temperature control element 30 is configured to control the temperature of the at least one base member 32.

Description

(57) An optical source 10 comprises: a laser 12 comprising an optical gain section 20 and an optical phase control section 22; a transmission filter 16 configured to receive and filter light output from the laser 12; a partial reflector 18 configured to receive filtered light from the filter 16 and to input filtered light back into the laser 12; at least one base member 32: and a temperature control element 30. At least the transmission filter 16 and the partial reflector 18 are fixedly mounted on a first of the at least one base member 32. The temperature control element 30 is configured to control the temperature of the at least one base member 32.

Figure 1

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Figure 1

Figure 2

OPTICAL SOURCE AND METHOD OF ASSEMBLING AN OPTICAL SOURCE

TECHNICAL FIELD

The present invention relates to an optical source and a method of assembling an optical source. In some configurations, the invention may allow the optical source to operate over a wide temperature range, with the source wavelength determined by an optical filter whose centre frequency may have a small variation with temperature.

BACKGROUND

With increasing demand for data capacities and bandwidth, optical technologies have been successfully developed to facilitate high-capacity, long distance transmission of optical data over optical fibre networks. These networks often use dense wavelength division multiplexing (DWDM) to allow one or more optical sources with different wavelengths to traverse a single optical fibre. More recently, DWDM has also been considered for access applications in passive optical networks (WDM-PON) where each wavelength is routed to a customer via a wavelength selective device such as an arrayed waveguide grating (AWG). These networks require stable control of the optical source wavelength in order to keep the optical signal within the passband of one or more optical filters within the DWDM network. A known solution to maintain the wavelength of a DWDM optical source is to design the optical source to emit a single-mode output, such as a Distributed Feedback (DFB) laser, and then control its wavelength by temperature or electrical current injection in the device. However, it is complex and expensive to fabricate DFB lasers at many wavelengths required for WDM grid operation.

GB 2516679 describes thin film optical interference filters combined with multi-section semiconductor lasers where the temperature insensitive filter determines the wavelength of the laser operation. The filter in GB 2516679 can be angle tuned to vary the absolute wavelength of laser operation. However, in certain packaging geometries that form an external optical cavity to the laser chip, the temperature dependent variation in the length of the distance of the laser chip to the filter and reflector, can also affect the absolute laser operation wavelength.

It is against this background that the present invention has been devised.

SUMMARY OF THE INVENTION

In one aspect, there is presented an optical source comprising: a laser comprising an optical gain section and an optical phase control section; a transmission filter configured to receive and filter light output from the laser; a partial reflector configured to receive filtered light from the filter and to input filtered light back into the laser; at least one base member; and a temperature control element. At least the transmission filter and the partial reflector are fixedly mounted on a first of the at least one base member. The temperature control element is configured to control the temperature of the at least one base member.

In this way, a wavelength stable optical source is provided that uses low cost optical components to achieve a largely single-mode optical output spectrum. Temperature control of the at least one base member assists in providing a source that is wavelength stable over external temperature variations. In some embodiments, wavelength stability of the source with external temperature variation is maintained through thermal compensation of packaging of components of the optical source or through packaging components of the optical source onto a single thermal substrate that is temperature controlled.

The laser may be fixedly mounted on at least one base member. The laser, the transmission filter and the partial reflector may be fixedly mounted on the said first base member.

Alternatively, the laser may be fixedly mounted on a second base member. The first and second base members may be attached to one another.

At least one of said base members may comprise a coefficient of thermal expansion no greater than 1 x 10'⁶ K’¹. In particular, the first base member may comprise a coefficient of thermal expansion no greater than 1 x 10'⁶ K’¹.

Forming the or each base member from a material having a very low thermal expansion coefficient provides a degree of passive thermal compensation to the optical source, reducing the variation in optical path length between components of the optical source.

At least one of the said base members may be comprised of Inovco, a glass material or Zerodur (RTM).

The transmission filter and partial reflector may be optically aligned on the first base member to form a pre-aligned module having a first optical axis. The first optical axis of the prealigned module may be optically aligned with an optical axis of the laser.

The temperature control element may be arranged to control the variation in optical path length between the transmission filter and the partial reflector. The temperature control element may be arranged to control the variation in optical path length between an output facet of the laser and the transmission filter and/or the partial reflector. In one example, the temperature control element may be attached to, and act on, a base member on which the optical source is mounted.

In another aspect of the invention there is provided a method of assembling an optical source comprising a laser, a transmission filter, a partial reflector, at least one base member and a temperature control element, the method comprising;

fixedly mounting the laser on at least one base member;

fixedly mounting the transmission filter and partial reflector on to a first of the at least one base members; and coupling the thermal control element to the at least one base member.

The method may comprise fixedly mounting the laser on a second base member, and rigidly coupling the first and second base members to one another.

The method may comprise attaching the first base member and the second base member to one another.

The method may comprise optically aligning the transmission filter and partial reflector on the first base member to form a pre-aligned module having a first optical axis, before coupling the first and second base members to one another.

The method may comprise optically aligning the transmission filter and the partial reflector on the first base member using an alignment laser, wherein the alignment laser is different to the laser that is fixedly mounted on the second base member.

The method may comprise aligning the first optical axis of the pre-aligned module to an optical axis of the laser.

The method may comprise attaching the thermal control element to one of the at least one base member.

The thermal control element may comprise a thermo-electric cooler (TEC).

In another aspect of the invention there is provided an optical source comprising:

a laser comprising an optical gain section and an optical phase control section;

a transmission filter configured to receive and filter light output from the laser;

a partial reflector configured to receive filtered light from the filter and to input filtered light back into the laser; and at least one base member having a coefficient of thermal expansion no greater than 1 x 10’⁶ K’¹;

wherein at least the transmission filter and the partial reflector are fixedly mounted on a first of the at least one base member.

The first base member may have a coefficient of thermal expansion no greater than 1 x 10'⁶K’¹. The laser may be fixedly mounted on at least one base member.

The laser, transmission filter and partial reflector may be fixedly mounted on the said first base member. Alternatively, the laser may be fixedly mounted on a second base member, and the first and second base members may be attached to one another.

The optical source may further comprise a temperature control element coupled to the second base member.

The optical source may be configured such that variation in optical path length between the transmission filter and the partial reflector is controlled.

The optical source may be configured such that variation in optical path length between an output facet of the laser and the transmission filter and/or the partial reflector is controlled.

In another aspect of the invention there is provided a method of assembling an optical source; the optical source comprising a laser, a transmission filter, a partial reflector and at least one base member having a coefficient of thermal expansion no greater than 1 x 10'⁶ K’ ¹, the method comprising;

fixedly mounting the laser on the at least one base member;

fixedly mounting the transmission filter and partial reflector on to a first of the at least one base members.

The method may comprise attaching the first and second base members to one another.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic representation of a wavelength stable optical source in accordance with a first embodiment of the optical source; and.

Figure 2 is a schematic representation of a wavelength stable optical source in accordance with a second embodiment of the optical source..

DETAILED DESCRIPTION

The present invention relates to a wavelength stable optical device, and in particular, but not limited to, one suitable for dense wavelength division multiplexing (DWDM) applications, where the variation in laser wavelength with external environment temperature is substantially controlled or compensated. A method of arranging or packaging an optical source as a thermally compensated or hermetic package is also presented.

There is provided herein an optical source comprising a laser that includes an optical gain section and an optical phase control section. The optical source further comprises a transmission filter configured to receive and filter light output from the laser, a partial reflector configured to receive filtered light from the filter and to input filtered light back into the laser, at least one base member and a temperature control element. At least the transmission filter and the partial reflector are fixedly mounted on a first of the at least one base member. The temperature control element is configured to control the temperature of the at least one base member.

The laser may be a Fabry Perot semiconductor laser operating at a central wavelength of 1550nm. However, the laser may take other forms. For example, the laser may alternatively be any semiconductor laser operating from 350nm to 5000nm in wavelength. The laser may operate in different wavelength regions in dependence on the intended application of the optical source.

The partial reflector may have a reflectivity in the range of 5 to 95%, and may be formed of thin film coated transparent substrate such as glass. It should be noted that the chosen material of the partial reflector may depend on the required reflectivity of the partial reflector and the laser wavelength. In an embodiment, the partial reflector is positioned at a distance of 3mm from an output facet of the laser. It will be understood that in other embodiments of the optical source, the partial reflector may be positioned closer to, or further from, the output facet of the laser.

Referring now to Figure 1, which illustrates a first embodiment of the optical source 10, a laser 12, lens 14, filter 16 and external cavity partial reflector 18 are fixedly mounted on a first base member 32. A temperature control element 30, which in this case takes the form of a TEC, is coupled to the first base member 32 and is configured to control the temperature of the first base member 32. In other words, the packaging of the optical filter 16 and external cavity partial reflector 18 is thermally connected to the same thermal mount of the TEC cooled laser chip. In this way, variation in optical path length between components of the optical source 10, which is affected by temperature variation of the first base member 32, may be reduced through use of a single TEC 30 which may also be operable to control the laser 12 temperature. It should be noted that whilst a TEC 30 forms the temperature control element 30 in this embodiment of the optical source, this is not essential. The temperature control element 30 could be formed of another suitable element, for example a heater.

Note that whilst the laser 12, filter 16, and partial reflector 18 are mounted on the same base member in this embodiment, this should not be considered limiting. As will be described below with reference to a second embodiment of the optical source, it is possible for more than one base member to be used in the optical source arrangement.

The first base member 32 comprises a glass substrate, the glass substrate having a thermal expansion coefficient of close to zero. As an example, the substrate may be formed of the extremely low expansion glass ceramic Zerodur (RTM), which may have a coefficient of linear thermal expansion (CTE) of 0 ± 0.007 x 10'⁶/°K in the temperature range 0°C to 50°C. It is possible for other materials to be used for the first base member 32. For example, the first base member 32 may comprise a substrate having a different thermal expansion coefficient, e.g. Inovco (Super Invar (RTM)) which is a material best known in the area of mechanical engineering. Typically, Inovco is known to have a CTE of 0.55 x 10'⁶/°C in the temperature range 20°C to 100°C.

It is preferable that the thermal expansion coefficient of the chosen substrate does not exceed 1 x 10'⁶K, as this provides a degree of passive thermal compensation. That is to say, use of a material having a lower thermal expansion coefficient for the first base member 32 (or at least a part of the first base member 32) results in less expansion of said material for an identical temperature variation, and therefore less variation in optical path length of the optical source 10. Thus, the passive thermal compensation provided by the low thermal expansion coefficient material of the first base member 32 may work in conjunction with the active thermal control element 30 to control the variation in optical path length between components of the optical source 8.

To assemble or construct the optical source 10 of the first embodiment of the invention, the laser 12, lens 14, filter 16 and partial reflector 18 are positioned on the first base member 32. The laser 12 is fixedly mounted in its final position on the first base member 32, and the lens 14, filter 16 and partial reflector 18 are arranged on the first base member 32 in optical alignment with the optical axis of the laser 10. The angular position of the filter element 16 with respect to the collimated beam from the laser 12 is adjusted until the required angle is achieved. Once the filter 16 and partial reflector 18 are arranged as required with respect to the laser 12, the filter 16 and partial reflector 18 are fixedly mounted on the first base member 32.

To attach the TEC 30 to the first base member 32, a planar face 34 of the TEC 30 is placed in abutment with a planar face 36 of the first base member 32, and the TEC 30 is mounted to the first base member 32 via solder (not shown), or another appropriate form of attachment means, for example mechanical fixings such as screws, bolts or an adhesive.

When attached to the first base member 32 as described above, the TEC 30 is arranged to control the temperature of the first base member 32, and thereby to control variation of the optical path length between the optical components of the optical source 10. For example, the variation in optical path length between the transmission filter 16 and the partial reflector 18 may be controlled in this way, in addition to the variation in optical path length between an output facet 38 of the laser and the transmission filter 16 and/or the partial reflector 18.

In the first embodiment of the invention, the optical filter 16 and external cavity partial reflector 18 are co-packaged on the same base member, i.e. the same thermal substrate, as the laser chip within a single hermetic package, so that the overall optical path length is controlled by a single TEC 30 that is in the hermetic package to control the laser chip temperature.

An optical source 100 in accordance with a second embodiment will now be described with reference to Figure 2. The optical source 100 of the second embodiment includes many of the same components as in the first embodiment of the invention. As such, like components of the first and second embodiments are given identical reference numerals, and will not be described in detail again here.

Referring now to Figure 2, the optical source 100 comprises a laser 12, a lens 14, an optical filter 16 and an external cavity partial reflector 18. The optical filter 16 and partial reflector 18 are fixedly mounted on a first base member 102. The laser 12 and lens 14 are fixedly mounted on a second base member 104, along with power monitor 28. A thermal control element 30 in the form of a TEC is coupled to the second base member 104 to control, at least, the temperature of the laser 12. As in the first embodiment, the TEC 30 is attached to the second base member 104 by means of solder, but may be attached by other suitable means in other embodiments.

In this embodiment, the first base member 102 is formed of glass having a coefficient of thermal expansion of close to zero (e.g. Zerodur (RTM)). In other embodiments, the first base member 102 may be formed of a different material, so long as the material has a coefficient of thermal expansion no greater than 1 x 10'⁶ K, e.g. Inovco. It should be understood that, whilst it has been described that the entire first base member 102 is formed of the same material, in practice the first base member 102 could be formed by multiple different materials, provided that the low thermal expansion coefficient material is in contact with the filter 16 and partial reflector 18.

When the optical source 100 is assembled, the first and second base members 102, 104 are rigidly coupled to one another, and in this embodiment the first and second base members 102, 104 are physically attached to one another such that they are in thermal contact with one another. In this way, the TEC 30 may control the temperature of both the first and second base members 102, 104, although this is not required. For example, in some embodiments, the first and second base members 102, 104 may be attached in such a way that the TEC 30 only controls the temperature of the second base member 104.

The variation in optical path length between the components of the optical source 100 is controlled both passively and actively, the TEC 30 forming an active control element and the low temperature coefficient first base member 102 providing passive control. Variation in optical path length between components of the optical source 100 is compensated by means of the low temperature coefficient material used to form the first base member 102 and is controlled by means of the TEC 30.

As noted above, it is possible that the first and second members 102, 104 are not in thermal contact with each other. In this case, the temperature of the first base member 102 is not controlled by the TEC 30 and only passive compensation for optical path length variation is provided in the vicinity of the filter 16 and partial reflector 18.

To assemble the optical source 100, the filter 16 and partial reflector 18 are optically aligned to a first optical axis on the first base member 102, and are fixedly mounted on the first base member 102 to form a pre-aligned module 106. Optical alignment of the filter 16 and partial reflector 18 to the first optical axis is achieved using an alignment laser (not shown), the alignment laser being different from the above-described laser 12.

The laser 12 and lens 14 are fixedly mounted on the second base member 104, such that the lens 14 is optically aligned to the optical axis of the laser 10.

To assemble or construct the optical source 100, the pre-aligned module 106 comprising the first base member 102, the filter 16 and the partial reflector 18 is rigidly coupled to the second base member 104 comprising the laser 12 and the lens 14. Once attached in this way, the first and second base members 102, 104 are arranged such that the optical axis of the laser 12 is aligned with the first optical axis of the pre-aligned module 106 to define an optical axis of the optical source 100.

In the second embodiment, the packaging of the optical filter 16 and partial reflector 18 uses a material with a very low temperature expansion coefficient (i.e. no greater than 1 x 10'⁶K), which provides passive thermal compensation such that variation in optical path length is brought within the adjustment range of the laser chip temperature and phase control section 22.

Thus, the present invention may package a filter and partial reflector onto the same thermal mount as the laser chip, or into a thermally compensated mechanical housing, so that the temperature of the overall optical cavity is controlled by a single thermo-electrical cooler (TEC).

The components of the optical source 10, 100 of the first and second embodiments, respectively, will now be described in further detail. As already noted, the laser 12 in the above-described embodiments comprise a Fabry-Perot (FP) semiconductor laser that includes an optical gain section 20 and an electrically isolated phase section 22 disposed between first and second optical reflectors 24, 26. Use of a Fabry-Perot laser in the optical source 10, 100 provides a cost effective option. The electrically isolated phase section 22 enables the absolute frequency of the laser longitudinal modes to be changed by means of current injection or applied voltage to the phase section 22. The second optical reflector 26 is formed of a partial reflector having a reflectivity in the range of 5 to 95%, such that the second optical reflector 26 acts as an output coupler of the laser 12.

In operation, optical output from the laser 12 propagates through lens 14, where it is collimated before passing through the filter element 16 and to the external cavity partial optical reflector 18. In this way, the optical output from the laser 12 is filtered by filter element 16 before reaching the partial reflector 18. The partial reflector 18 reflects a portion of the incident filtered beam back towards the laser 12 to form a filtered reflected beam. The reflected beam is reflected back along the optical axis of the optical source such that the reflected beam is coupled back into the semiconductor laser. It should be noted that the term Optical axis’ is well-known in the art and, as such, would be well understood by the skilled person. The optical axis of the optical source is defined as the axis that passes through the centre of each optical element of the source, and is aligned to the axis along which optical output from the laser 12 propagates, i.e. the optical axis of the laser.

The filter element 16 is a thin-film coated etalon bandpass design, whose centre frequency of transmission changes as a function of the filter angle with respect to the collimated optical beam. The etalon free-spectral range (FSR) is designed such that only a single order of the etalon filter 16 is transmissive over the required laser wavelength operation range. The absolute filter centre frequency, ω1, is determined by the angle of the filter 16 with respect to the laser beam, the filter angular tuning rate and the normal incidence wavelength of the filter 16. The filter 16 comprises a plane parallel substrate such that the collimated optical beam is mostly laterally displaced as the filter angle is changed. The filter element 16 passband is designed to primarily pass one of the longitudinal modes of the laser 12, such that re-injection of the reflected beam results in single longitudinal mode operation of the laser 12.

The phase section 22 of the laser 12 allows a laser longitudinal mode to be fine-tuned in frequency to coincide with the filter frequency, ω1. The phase section 22 also thus allows optical phase tuning between the laser mode and the phase of the reflected signal.

The power monitor 28, which in this embodiment is in the form of a photodiode, is provided to receive optical output from the first optical reflector 24 of the laser 12. The power monitor 28 allows monitoring of the laser power with respect to the wavelength of operation of the laser 12.

The reflectivity of the partial reflector 18 is chosen to inject the optimum power back into the laser 12 to achieve stable single-mode laser operation. In some embodiments of the invention, the partial reflector 18 design incorporates wavelength selective coatings such that the reflection is only effective over a defined wavelength range. In this manner, the laser 12 is wavelength tuned to an absolute frequency by adjusting the filter angle and laser phase section until the required frequency is selected. The laser 12 can be directly modulated, via either the gain or phase section 20, 22, respectively, or both, in order to obtain a digitally modulated output signal from the laser source 12.

The tuning of the laser mode frequency via the phase section 20 can compensate for the changes in laser mode frequency with small device temperature variations, allowing operation of the wavelength source over certain temperature ranges. However, for stable wavelength operation over wide external temperature ranges, the overall optical length variation with temperature between elements of the optical source 10, in particular between the laser 12 and the external partial reflector 18, must be reduced. This may be achieved, at least in part, by means of a temperature control element 30.

In summary, a wavelength stable optical source is disclosed, which may comprise an optically filtered, optically self-injected Fabry-Perot semiconductor laser with a phase control section. The optical filter and feedback elements may be packaged on a thermally compensated structure, or on the same thermal mount as the laser chip, so that a single thermoelectric cooler can be used for the whole source.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding description, in the claims and/or in the drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

Furthermore, features of the invention may include any one or more of the following aspects. Any of these aspects may be modified or adapted with any of the features and configurations described herein.

1. A wavelength stable optical source featuring a multi-section semiconductor laser where the wavelength is determined by a thin-film coated filter and the laser cavity length is thermally controlled.

2. A wavelength stable optical source as in aspect 1 where the laser cavity length is thermally compensated.

3. A wavelength stable optical source as in aspect 1 to 2 where the absolute wavelength of operation is achieved by angle tuning a thin-film coated filter.

4. A wavelength stable optical source as in aspects 1 to 3 where a separate phase section on the laser is used to tune the laser longitudinal mode frequencies.

5. A wavelength stable optical source as in aspects 1 to 4 where the thin-film coated filter is on a thermally matched substrate to reduce the variation in filter wavelength with temperature.

Claims

1. An optical source comprising:

a partial reflector configured to receive filtered light from the filter and to input filtered light back into the laser;

at least one base member; and a temperature control element;

wherein at least the transmission filter and the partial reflector are fixedly mounted on a first of the at least one base member, and wherein the temperature control element is configured to control the temperature of the at least one base member.

2. An optical source as claimed in Claim 1, wherein the laser is fixedly mounted on at least one base member.

3. An optical source as claimed in Claim 2, wherein the laser, the transmission filter and partial reflector are fixedly mounted on the said first base member.

4. An optical source as claimed in Claim 2, wherein the laser is fixedly mounted on a second base member, and wherein the first and second base members are attached to one another.

5. An optical source as claimed in any preceding claim, wherein at least one of said base members comprises a coefficient of thermal expansion no greater than 1 x 10'6 K’1.

6. An optical source as claimed in any preceding claim, wherein the first base member comprises a coefficient of thermal expansion no greater than 1 x 10'6 K’1.

7. An optical source as claimed in any preceding claim, wherein at least one of the said base members comprises Inovco or a glass material.

8. An optical source as claimed in any preceding claim, wherein at least one of said base members comprises Zerodur (RTM).

9. An optical source as claimed in Claims 4 to 8, wherein the transmission filter and partial reflector are optically aligned on the first base member to form a pre-aligned module having a first optical axis.

10. An optical source as claimed in any preceding claim, wherein the first optical axis of the pre-aligned module is optically aligned with an optical axis of the laser.

11. An optical source as claimed in any preceding claim, wherein the temperature control element is arranged to control the variation in optical path length between the transmission filter and the partial reflector.

12. An optical source as claimed in any preceding claim, wherein the temperature control element is arranged to control the variation in optical path length between an output facet of the laser and the transmission filter and/or the partial reflector.

13. A method of assembling an optical source comprising a laser, a transmission filter, a partial reflector, at least one base member and a temperature control element, the method comprising;

fixedly mounting the laser on at least one base member;

14. A method as claimed in Claim 13, comprising fixedly mounting the laser on a second base member, and rigidly coupling the first and second base members to one another.

15. A method as claimed in Claim 14, comprising attaching the first base member and the second base member to one another.

16. A method as claimed in Claims 14 or 15, comprising optically aligning the transmission filter and partial reflector on the first base member to form a pre-aligned module having a first optical axis, before coupling the first and second base members to one another.

17. A method as claimed in Claim 16, comprising optically aligning the transmission filter and the partial reflector on the first base member using an alignment laser, wherein the alignment laser is different to the laser that is fixedly mounted on the second base member.

18. A method as claimed in Claims 16 or 17, comprising aligning the first optical axis of the pre-aligned module to an optical axis of the laser.

19. A method as claimed in Claims 13 to 18, comprising attaching the thermal control element to one of the at least one base member.

20. A method as claimed in Claims 13 to 19, wherein the thermal control element comprises a thermo-electric cooler (TEC).

21. An optical source comprising:

a partial reflector configured to receive filtered light from the filter and to input filtered light back into the laser; and at least one base member having a coefficient of thermal expansion no greater than

1 x 10’6 K’1;

22. An optical source as claimed in Claim 21, wherein the first base member has a coefficient of thermal expansion no greater than 1 x 10'6 K’1.

23. An optical source as claimed in Claims 21 or 22, wherein the laser is fixedly mounted on at least one base member.

24. An optical source as claimed in Claims 21 to 23, wherein the laser, transmission filter and partial reflector are fixedly mounted on the said first base member.

25. An optical source as claimed in Claim 21 to 23, wherein the laser is fixedly mounted on a second base member, and wherein the first and second base members are attached to one another.

26. An optical source as claimed in Claim 25, further comprising a temperature control element coupled to the second base member.

27. An optical source as claimed in Claims 21 to 26, wherein the optical source is configured such that variation in optical path length between the transmission filter and the partial reflector is controlled.

28. An optical source as claimed in Claims 21 to 27, wherein the optical source is configured such that variation in optical path length between an output facet of the laser and the transmission filter and/or the partial reflector is controlled.

29. A method of assembling an optical source; the optical source comprising a laser, a transmission filter, a partial reflector and at least one base member having a coefficient of thermal expansion no greater than 1 x 10'6 K’1, the method comprising;

fixedly mounting the laser on the at least one base member;

30. A method as claimed in Claim 29, comprising fixedly mounting the laser on a second base member, and rigidly coupling the first and second base members to one another.

31. A method as claimed in Claim 30, comprising attaching the first and second base members to one another.

32. A method as claimed in Claims 29 to 31, comprising optically aligning the transmission filter and partial reflector on the first base member to form a pre-aligned module having a first optical axis, before coupling the first and second base members to one another.

33. A method as claimed in Claim 32, comprising optically aligning the transmission filter and the partial reflector on the first base member using an alignment laser, wherein the alignment laser is different to the laser that is fixedly mounted on the second base member.