WO2003105294A1

WO2003105294A1 - Wavelength stabilized optical device

Info

Publication number: WO2003105294A1
Application number: PCT/GB2003/002453
Authority: WO
Inventors: Rong Huang; Duncan Walker; Jay Liu
Original assignee: Bookham Technology Plc
Priority date: 2002-06-07
Filing date: 2003-06-05
Publication date: 2003-12-18
Also published as: AU2003244772A1

Abstract

Described is an optical device (10) having a thermally stabilized optical component (32) with temperature dependent optical characteristics. The device (10) includes a temperature dependent optical component (32) attached to a thermally conductive surface (35) maintained at a constant temperature. A thermal shield (40) attached to the thermally conductive surface (35) substantially surrounds the optical component (32) and insulates it from thermal sources located outside the shield (40). The device includes a housing (38) that encloses the temperature dependent optical component (32), thermally conductive surface (35), thermal shield (40) and, optionally, a low thermal conductivity gas. The thermal shield (40) and low thermal conductivity gas significantly reduce the radiant and the conductive heat transfer between the temperature dependent optical component (32) and other device components.

Description

WAVELENGTH STABILIZED OPTICAL DEVICE

FIELD OF THE INVENTION

The invention relates generally to optical sources. More specifically, the invention relates to devices and methods for thermal stabilization of optical components having temperature dependent optical characteristics.

BACKGROUND

Optical communication links continue to be pushed to support increasing data rates using various techniques. One common technique is based on dense wavelength division multiplexing (DWDM) transmission systems that implement multiple communication channels over a single link or optical fiber. Each channel is separated from adjacent channels by a small frequency increment that can be 25 GHz or less. By decreasing the channel spacing, additional channels can be added to the transmission link. As channel spacing decreases, however, laser frequency stability becomes increasingly important.

Optical frequency lockers (or wavelength lockers) are commonly used to achieve precise and stable control over the laser wavelength used in a DWDM channel. Optical frequency lockers typically include multiple optical components and one or more photodetectors. Some of the optical components of the frequency locker can exhibit temperature dependent characteristics. In particular, the optical parameters of the components can change in value as the temperature changes For example, the reflectance and/or transmittance of components can be temperature dependent. Consequently, the performance of frequency lockers having temperature dependent optical components is degraded if the transmitter is operated in an uncontrolled thermal environment. To compensate for temperature variations, conventional optical transmitters often utilize compensating elements. These elements are fabricated from materials having temperature dependent characteristics that are opposite to the characteristics of the fundamental elements. For example, mechanical spacers fabricated from thermally stable materials can be used to maintain critical separations between optical components such as the reflective surfaces of an air- spaced etalon. In another example, compound spacers fabricated from materials having different coefficients of thermal expansion are configured such that the expansion of one spacer compensates for the expansion of the other spacer to effectively maintain the etalon cavity length over a range of temperature. Unfortunately the additional manufacturing complexity results in a significant increase to the overall cost of the laser module. In addition, as channel spacing decreases, such laser devices can be disqualified for certain applications based on their temperature variations. In another technique to reduce temperature sensitivity of the frequency locker, the temperature dependent optical component is wrapped in a miniature heater and maintained at a temperature slightly above that of the local environment. This technique results in increasing both the manufacturing complexity and the power consumption of the optical device.

The trend towards narrower frequency spacing between laser frequencies in DWDM systems is resulting in stricter requirements on the allowable frequency change (i.e., stability) of each laser in the system. In addition, it is desirable to deploy these systems in areas subject to a wide range of operating temperature. Consequently, optical frequency lockers must provide better frequency stability than that currently available from conventional lockers.

SUMMARY

The present invention generally relates to an optical device having a thermally stabilized optical component. The optical device utilizes a low thermal conductivity gas inside a device housing and/or a thermal shield to substantially surround one or more components having temperature dependent optical characteristics. The device reduces the thermal transfer between the housing or the external environment, and the temperature dependent optical component, preferably resulting in an extended operating temperature range and/or improved wavelength stability for the device.

The thermally stabilized optical device preferably includes a temperature dependent optical component, a housing and a low thermal conductivity gas. The housing preferably encloses a volume that includes the temperature dependent optical component and the low thermal conductivity gas. The gas preferably surrounds the temperature dependent optically component and is preferably sufficient to thermally insulate it from the housing.

According to a first aspect, the present invention provides a thermally stabilized optical device according to independent claim 1 of the claims appended hereto.

According to a second aspect, the invention provides a thermally stabilized optical device according to independent claim 18 of the claims appended hereto.

According to a third aspect, the invention provides a method for fabricating a thermally stabilized device, according to independent claim 19 of the claims appended hereto.

Preferred and optional features of the invention are described herein and in the dependent claims.

In one embodiment the low thermal conductivity gas includes Xenon. In other embodiments the temperature dependent optical component is an etalon or an element of an optical frequency locker. In another embodiment the device also includes a tunable laser module disposed in the device housing and optically coupled with the temperature dependent optical component.

The optical device preferably includes a thermally conductive surface having a substantially constant temperature, a temperature dependent optical component and a thermal shield. The temperature dependent optical component preferably is attached to the thermally conductive surface. The thermal shield preferably is attached to the thermally conductive surface and preferably substantially surrounds the temperature dependent optical component to insulate the optical component from one or more external sources.

In one embodiment the thermal shield has an external surface that is optically polished. In another embodiment the thermal shield is fabricated from a thermally conductive material such as aluminium, copper, copper-tungsten alloy or aluminium nitride.

In a further embodiment the optical device also includes a housing and a low thermal conductivity gas. The housing preferably encloses a volume that includes the thermal shield, the thermally conductive surface, the temperature dependent optical component and the low thermal conductivity gas. The thermal conductivity of the gas preferably is sufficient to thermally insulate the thermal shield from the housing.

As mentioned above, in another aspect the invention features a method for fabricating a thermally stabilized optical device having at least one temperature dependent optical component. The method preferably includes the steps of attaching a thermally conductive surface to a thermo-electric cooler or other cooling device and attaching a temperature dependent optical component to the thermally conductive surface. The cooling device preferably is adapted to maintain the thermally conductive surface at a substantially constant temperature. The method preferably also includes the steps of enclosing the temperature dependent optical component, the thermally conductive surface and the cooling device in a volume defined by a housing and introducing a low thermal conductivity gas into the volume of the housing. The thermal conductivity of the gas preferably is sufficient to thermally insulate the temperature dependent optical component from the housing.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood, by way of example, in consideration of the detailed description of some preferred embodiments of the invention which follows in connection with the accompanying drawings, in which:

FIG. 1 is a block diagram of an optical device having an optical frequency locker as known to the prior art;

FIG. 2 is a block diagram illustrating components of the optical frequency locker of FIG. 1;

FIG. 3 is an illustration of the transfer of thermal energy from a device housing to a temperature dependent optical component;

FIG. 4 is an illustration of a thermally stabilized optical device according to an embodiment of the present invention; and

FIG. 5 is an illustration of a thermally stabilized optical device according to an embodiment of the present invention. DETAILED DESCRIPTION

FIG. 1 illustrates a simplified optical device 10 having a laser modulel2, an optical frequency locker 14 and an optical tap 16. The optical frequency locker is equivalently referred to as a wavelength locker. The laser module 12 includes a temperature controller 28 in communication with a tunable laser 30. The optical tap 14 generally is a partial optical reflector that reflects a small portion (e.g., 2%) of the optical power in an incident beam and transmits the remainder of the optical power in the beam. The tunable laser 30 generates a laser beam 20 that is sampled by the optical tap 16 to create a sample beam 18. The remainder of the laser beam 20 passes through the optical tap 16 to the output port 24 of the optical device 10 as output beam 22. The optical frequency locker 14 receives the sample beam 18 and provides a feedback signal FB over communication channel 26 to a laser controller. In the illustrated embodiment the laser controller is a temperature controller 28. Alternatively, the laser controller can be a controller that is adapted to change some other operating parameter of the tunable laser 30 to cause a change in the operating wavelength.

In operation, optical frequency locker 14 receives the low power sample beam 18 and generates the feedback signal FB which is responsive to the difference between the actual wavelength of the output beam 22 and the desired operating wavelength. As the wavelength of the laser beam 20 begins to change, the optical frequency locker 14 responds by providing a feedback signal FB. The feedback signal FB is used to control an operating parameter of the tunable laser 30 to counteract the changing wavelength. In the illustrated example, the feedback signal FB drives the temperature controller 28 to cause a heating or cooling of the tunable laser 30 to maintain the desired operating wavelength of the output beam 22 FIG. 2 is a block diagram of the optical frequency locker 14 of FIG. 1 showing a limited number of its optical and electrical components. The frequency locker 14 includes a spectrally selective optical component 32 in optical communication with a photodetector 34 The photodetector 34 is in electrical communication with conditioning electronics 36. Although the illustrated spectrally selective optical component is an etalon 32, other filters or optical components that provide a significant change in transmission as a function of wavelength about the desired wavelength can be used. For example, an optical Mach-Zehnder interferometer can be used as the spectrally selective optical component 32.

The filtered sample beam 18' exiting the spectrally selective optical component 32 has a varying optical power according to its wavelength. Consequently, as the wavelength changes, the optical power incident on the photodetector 34 also changes. The conditioning electronics module 26 generates the feedback signal FB indicative of the wavelength variations in the output beam 22 on electrical channel 26.

The design of the optical frequency locker 14 is based on the requirements of the specific application for the optical device 10. In a DWDM transmission system the frequency locker 14 is required to stabilize the optical frequency to a fraction of the channel spacing. For example, in a DWDM system having a channel spacing of 100 GHz, the optical frequency of the laser module 12 should be within 2.5 GHz of the nominal value. External environmental conditions around the laser module 12 can have a significant influence on its internal components. For example, the temperature can range from less than 0°C to greater than 80° C in some applications. The feedback loop described above may not be sufficient to stabilize the wavelength in systems having small channel separations. Moreover, using a separate feedback loop to stabilize the temperature in the vicinity of the laser module 12 and/or the frequency locker 14 can be inadequate due to temperature gradients established between the housing of the optical device 10 and components of the laser module 12. These temperature gradients can limit the wavelength stability of the optical device 10.

FIG. 3 illustrates a portion of an optical device 10 having a frequency locker 14 with a temperature dependent optical component 32', The temperature dependent optical component 32' can be, for example an optical Mach-Zehnder interferometer or other spectral filter. In the illustrated example the temperature dependent optical component 32' is an etalon. The etalon 32' is attached to the surface 35 of a thermally conductive platform 37 by thermal conductive epoxy. A device housing 38 encloses a volume that includes the internal components of the device 10', including the laser module 12 and optical frequency locker 14. The housing 38 is shown in a cutaway view and only the etalon 32' and thermally conductive platform 37 are shown for clarity.

The dashed arrowhead lines between the housing 38 and etalon 32' represent the transfer of thermal energy from the housing 38 to the etalon 32' by way of conduction and radiation. As the temperature of the housing 38 increases, the temperature of the etalon 32' also increases. The refractive index and thickness of the etalon 32' are each a function of temperature, therefore, the transmittance characteristic of the etalon 32' changes with temperature. Although the wavelength feedback control of the device 10' continues to operate, the reference wavelength as defined by the etalon 32' changes and the device 10' is locked to a different wavelength.

FIG. 4 shows an embodiment of an optical device 44 constructed in accordance with the principles of the invention. The device 44 exhibits a reduced transfer of thermal energy (as indicated by the shortened dashed arrowhead lines} compared to the device 10' of FIG. 3. The device 44 includes a housing 38 that encloses a volume containing the laser module 12, a modified optical frequency locker and other internal device components. For clarity, the only illustrated internal components of the device 44 are the etalon 32' and the platform 37 having a thermally conductive surface 35, both of which are part of the modified optical frequency locker. The laser module 12 can include a VCSEL laser adapted for wavelength adjustment. In one example, the wavelength adjustment of the tunable laser module 12 is achieved using a micro electro-mechanical structure (MEMS).

The thermally conductive platform 37 is fabricated from a material having a high thermal conductivity, such as aluminium nitride. A thermo-electric cooler is thermally coupled by thermal conductive epoxy to the thermally conductive platform 37 to maintain it at a substantially constant operating temperature (e.g., 25° C).

The volume inside the housing 38 is filled with a low thermal conductivity gas; In one embodiment the low thermal conductivity gas includes Xenon. In a further embodiment the composition of the gas is between about 70% Xenon to 100% Xenon. Other low conductivity gases such as Argon can also be used.

The low thermal conductivity gas is introduced into the housing 38 during the seam sealing process for the housing 38. A small opening (not shown) in the housing 38 is coupled to a vacuum line to generate a vacuum within the housing 38. The vacuum line is replaced by a line coupling the opening to a tank or container supplying the low thermal conductivity gas. The device 44 is then hermetically sealed to protect the internal components and maintain the low thermal conductivity gas environment

In an alternative method, the gas is introduced into the housings 38 of multiple devices 44 at the same time. In this method, the devices 44 are loaded in a single vacuum chamber and a vacuum is generated in each device 44 as the chamber is evacuated. The vacuum chamber is then filled with the low thermal conductivity gas, thereby also filling the housings 38 of the devices 44 with the gas. Each device 44 is then hermetically sealed to protect its internal components and maintain the low thermal conductivity gas environment.

The use of a low thermal conductivity gas has significant advantages. Less thermal energy is conducted from the housing 38 to the internal components resulting in a substantial reduction in the change of the operating wavelength with temperature. Consequently, a significant improvement in the wavelength stability is achieved using the modified optical frequency locker. As a result, the operational temperature range of the device 44 is increased. Other benefits include the reduction in power consumed by the thermo-electric cooler and the simplicity of the additional steps in the manufacturing process.

FIG. 5 shows an embodiment of an optical device 44' constructed in accordance with the principles of the invention. The device 44' exhibits a further reduction in the transfer of thermal energy (as indicated by the dashed arrowhead lines) compared to the device 44 of FIG. 4.

In addition to the components for the embodiment illustrated in FIG. 4, the optical device 44' also includes a thermal shield 40. The thermal shield 40 is constructed from a material having a high thermal conductivity and is attached to the surface 35 of the thermally conductive platform 37 using, for example, a thermally conductive epoxy to allow efficient heat transfer. The shield material can be a metal such as aluminium, copper or copper-tungsten alloy. Alternatively, the shield material can be ceramic such as aluminium nitride.

The thermal shield 40 is positioned to substantially surround the etalon 32' but not obstruct the optical path. The thermal shield 40 can also surround other thermally sensitive components of the device 44'. Alternatively, multiple thermal shields 40 can be included in the device 44' so that each shield 40 surrounds one or more temperature dependent components.

Due to its high thermal conductivity, the thermal shield 40 remains at approximately the same temperature as the thermally conductive surface 35 and forms an isothermal barrier around the etalon 32'. Thermal energy that is emitted or conducted from any source external to the thermal shield 40, such as the housing 38, is effectively prevented from reaching the temperature dependent optical component 32. In one embodiment, the thermal shield 40 has its outer surfaces 42a, 42b and 42c (opposite 42b) treated to reflect a substantial portion of the radiant thermal energy. Surface treatment can include, for example, polishing the surfaces 42 or applying a reflective coating to the surfaces 42, thereby reducing the heat absorbed by the thermal shield 40 and reducing the power consumption of the thermo-electric cooler.

The use of the thermal shield 40 yields several benefits. The temperature stability of the modified optical frequency locker is improved and the operational temperature range is increased. The thermal shield 40 also can reduce the background light incident on the temperature sensitive optical component 32' and the photodetectors (not shown). Integration of the thermal shield 40 into the manufacturing process is relatively simple and less costly than other alternatives such as inclusion of an air-spaced etalon.

The heat transfer from the device housing 38 and other thermal sources outside the thermal shield 40 to the temperature dependent optical component 32' is further reduced by filling the volume enclosed by the housing 38 with a low thermal conductivity gas. In one embodiment the gas includes Xenon. In a further embodiment the gas comprises between about 70% Xenon to 100% Xenon. Other gases having a low thermal conductivity, such as Argon, can also be used. As a result of the additional insulation provided by the gas, the electrical power consumed by the thermo-electric cooler is further reduced and the operating temperature range is increased.

While the invention has been shown and described with reference to specific preferred embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the following claims.

Claims

CLAIMS:

1. A thermally stabilized optical device, comprising: a temperature dependent optical component; a housing defining an enclosed volume having the temperature dependent optical component therein; and a low thermal conductivity gas enclosed in the volume by the housing and substantially surrounding the temperature dependent optical component, the low thermal conductivity gas sufficient to thermally insulate the temperature dependent optical component from the housing.

2. An optical device according to claim 1, wherein the low thermal conductivity gas comprises Xenon.

3. An optical device according to claim 1 or claim 2, wherein the temperature dependent optical component comprises an etalon.

4. An optical device according to any preceding claim, wherein the temperature dependent optical component comprises an element of an optical frequency locker.

5. An optical device according to any preceding claim, further comprising a thermally conductive surface coupled to the temperature dependent optical component.

6. An optical device according to claim 5, wherein the thermally conductive surface is at a substantially constant temperature.

7. An optical device according to claim 5 or claim 6, wherein the thermally conductive surface is enclosed by the housing in the enclosed volume.

8. An optical device according to any one of claims 5 to 7 further comprising a cooling device coupled to the thermally conductive surface.

9. An optical device according to claim 8, wherein the cooling device comprises a thermo-electric cooler.

10. An optical device according to any preceding claim, further comprising a tunable laser module disposed in the housing and being in optical communication with the temperature dependent optical component.

11. An optical device according to any preceding claim, further comprising a thermal shield substantially surrounding the temperature dependent optical component.

12. An optical device according to claim 11, wherein the thermal shield is enclosed by the housing in the enclosed volume.

13. An optical device according to claim 12, wherein the low thermal conductivity gas thermally insulates the thermal shield from the housing.

14. An optical device according to any one of claims 11 to 13, wherein the thermal shield thermally insulates the temperature dependent optical component from the housing.

15. An optical device according to any one of claims 11 to 14, wherein an outer surface of the thermal shield is reflective.

16. An optical device according to any one of claims 11 to 15, wherein the thermal shield is fabricated from a thermally conductive material.

17. An optical device according to claim 16, wherein the thermally conductive material comprises one of aluminium, copper, copper-tungsten alloy and aluminium nitride.

18. A thermally stabilized optical device, comprising: a thermally conductive surface having a substantially constant temperature; a temperature dependent optical component attached to the thermally conductive surface; and a thermal shield attached to the thermally conductive surface and substantially surrounding the temperature dependent optical component, the thermal shield thermally insulating the temperature dependent optical component from one or more external thermal sources.

19. A method for fabricating a thermally stabilized optical device having at least one temperature dependent optical component, the method comprising the steps of:

attaching a thermally conductive surface to a cooling device, the cooling device adapted to maintain the thermally conductive surface at a substantially constant temperature;

attaching a temperature dependent optical component to the thermally conductive surface;

enclosing the temperature dependent optical component, the thermally conductive surface and the cooling device in a volume defined by a housing; and: (a) introducing a low thermal conductivity gas into the volume of the housing, the low thermal conductivity gas sufficient to thermally insulate the temperature dependent optical component from the housing and/or

(b) attaching a thermal shield to the thermally conductive surface such that the thermal shield substantially surrounds the temperature dependent optical component, the thermal shield being enclosed by the housing in the enclosed volume.

20. A method according to claim 19, wherein the cooling device comprising a thermo-electric cooler.