WO2012046079A1

WO2012046079A1 - Semiconductor laser device and method for stabilising the wavelength of a semiconductor laser device

Info

Publication number: WO2012046079A1
Application number: PCT/GB2011/051932
Authority: WO
Inventors: Nadhum Kadhum Zayer
Original assignee: Oclaro Technology Limited
Priority date: 2010-10-07
Filing date: 2011-10-07
Publication date: 2012-04-12
Also published as: GB201016929D0; GB2484334A

Abstract

A semiconductor laser device is described comprising an optical emitter and a wavelength locking mechanism arranged to control the temperature of the optical emitter in order to maintain a desired optical wavelength. Also described is a wavelength locking mechanism for a semiconductor laser and a method for stabilising the wavelength of a semiconductor laser comprising monitoring the wavelength of the laser and controlling the temperature of the laser in order to maintain a desired optical wavelength.

Description

Semiconductor Laser Device and Method for Stabilising the Wavelength of a

Semiconductor Laser Device

Field of the Invention

This invention relates to a semiconductor laser device and a method for stabilising the wavelength of a laser device.

Background to the Invention

Laser devices are often employed in applications requiring an optical beam of relatively constant wavelength. For example, semiconductor diode lasers are commonly employed as pump lasers for use in doped fibre amplifiers whereby optimal amplification is achieved when the pump laser maintains a desired wavelength.

In the case of erbium-doped fibre amplifiers, semiconductor pump lasers operating at approximately 980nm are traditionally employed. However, it has been found that the wavelength of such pump lasers can shift by approximately 7 nm/A as the drive current is increased and by approximately 0.35nm/°C.

To date this problem has been addressed by locating a thermistor on a semiconductor laser chip carrier, mounting the carrier on a thermoelectric cooler (TEC) and operating the TEC to maintain a carrier temperature of approximately 25°C (as monitored by the thermistor). However, it has been found that simply maintaining a constant carrier temperature is not sufficient for maintaining a constant wavelength and so it is common for such devices to employ an external fibre Bragg grating (FBG) to lock the chip output to a specific wavelength. While this approach can provide an adequate result, such an arrangement can be very expensive and requires careful chip and FBG design in order to make it work.

It is therefore an aim of the present invention to provide an alternative semiconductor laser device and a method for stabilising the wavelength of such a semiconductor laser device, which addresses the afore-mentioned problems.

Summary of the Invention

According to a first aspect of the present invention there is provided a semiconductor laser device comprising: an optical emitter; and

a wavelength locking mechanism arranged to control the temperature of the optical emitter in order to maintain a desired optical wavelength. Embodiments of the present invention therefore provide a semiconductor laser device effectively comprising a self-adjusting wavelength locking mechanism that combines temperature and wavelength control. Thus, the present invention allows product simplicity, cost and part reduction as well as providing an effective and efficient wavelength stabilisation scheme.

It will be understood that the desired optical wavelength may comprise a range of wavelengths. The range of wavelengths may be relatively narrow compared to the output spectrum of the device. The laser may be configured as a pump laser, such as that employed in a doped fibre amplifier. For example, the laser may be configured for emission at a wavelength of approximately 980nm, such as required for an erbium-doped fibre amplifier.

The optical emitter may comprise a gain medium in the form of a semiconductor laser diode disposed within an optical resonator such as a Fabry-Perot cavity.

A temperature controller may be employed to control the temperature of the optical emitter. The temperature controller may comprise a TEC. The Applicants have found that in the case of a semiconductor diode laser the wavelength shift by drive current is mainly due to a change in the junction temperature of the laser chip. They have also discovered that a free-running laser wavelength can be locked to a single wavelength by keeping the chip junction temperature constant. Thus, the wavelength locking mechanism may be configured to control the junction temperature of a semiconductor diode laser in order to maintain a desired optical wavelength.

As will be described below, embodiments of the present invention can also be configured to allow wavelength locking without the need for external feedback (for example, from a FBG). The wavelength locking mechanism may be configured to directly or indirectly monitor the wavelength of the optical emitter. In certain embodiments, the wavelength locking mechanism may comprise a detector configured to monitor an output power of the optical emitter at the desired optical wavelength. The detector may be a photo-detector and may be configured to generate a current in response to the output power detected. The current may be provided to a temperature controller (e.g. a TEC) to control the temperature of the optical emitter. The detector may be constituted by a back facet monitor (BFM) and a filter may be provided to ensure that the BFM is responsive at the desired optical wavelength.

The wavelength locking mechanism may comprise a pass band filter or an edge filter at the desired optical wavelength. The pass band filter may be a narrow band filter and may be disposed between the optical emitter and the detector. Thus, the detector will only receive power from a portion of the beam passing through the filter at the desired wavelength. If the wavelength shifts, less power will be received by the detector. Accordingly, the detector may determine, from the power it receives, whether the emitter is operating at, above or below the desired wavelength and this information can be used in a feedback loop to control the junction temperature and therefore the desired wavelength of a semiconductor diode laser.

The narrow band filter may be provided as a discrete component or may be integrated with the optical emitter or the detector. Thus, in certain embodiments the filter may be deposited directly onto a back facet monitor. In other embodiments, the filter may be deposited onto a back laser facet. In either case, the filter may comprise a dielectric mirror stack having a mirror optical loss associated therewith, wherein the mirror optical loss is increased for wavelengths other than the desired optical wavelength. The mirror optical loss may be increased for wavelengths longer than the desired optical wavelength and/or shorter than the desired optical wavelength. Accordingly, aspects of the present invention may be used in conjunction with aspects of the Applicants earlier invention described in US 6,819,702.

The wavelength locking mechanism may comprise a closed control loop (or feedback loop) comprising a detector and a temperature controller wherein the temperature controller is configured to control the temperature of the optical emitter so as to achieve a maximum signal at the detector, when the optical emitter is operating at the desired wavelength. In other embodiments, the wavelength locking mechanism may comprise a wavelength selection system arranged in a feedback loop with a temperature controller such that the temperature of the optical emitter can be controlled to lock onto the desired optical wavelength. The wavelength selection system may comprise a wavelength detector which may be provided inside the laser device or may be external thereto. The wavelength detector may comprise a spectrometer and may be pre-programmed to adjust a current supplied to the temperature controller in order to control the temperature of the optical emitter and therefore its wavelength to the desired optical wavelength.

According to a second aspect of the present invention there is provided a wavelength locking mechanism, for a semiconductor laser device, arranged to control the temperature of the semiconductor laser device in order to maintain a desired optical wavelength.

According to a third aspect of the present invention there is provided a method for stabilising the wavelength of a semiconductor laser device comprising:

monitoring the wavelength of the semiconductor laser device; and

controlling the temperature of the semiconductor laser device in order to maintain a desired optical wavelength.

The step of monitoring the wavelength of the semiconductor laser device may comprise directly or indirectly monitoring the wavelength. For example, the wavelength may be monitored directly using a spectrometer or indirectly by monitoring an output power from the laser and using the output power to determine whether the laser is operating at, above or below the desired wavelength. Where the wavelength is indirectly monitored, a narrow band filter may be provided at the desired optical wavelength.

The optional features described above in relation to the first aspect of the invention apply equally to the second and third aspects of the invention and vice versa. Brief Description of the Drawings

Some embodiments of the present invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1A shows a plan view of a portion of a semiconductor laser device in accordance with a first embodiment of the invention;

Figure 1 B shows a side elevation view of the portion of the semiconductor laser device shown in Figure 1A;

Figure 2 shows a plan view of the inside of a laser package in accordance with a second embodiment of the invention;

Figure 3 shows the shift in operating wavelength as a result of increasing drive current for a free-running laser;

Figure 4 shows a detector response when a narrow band filter is employed in accordance with an embodiment of the present invention, compared to the theoretical narrow band filter response;

Figure 5A shows the wavelength spectra for three different drive currents when no filter is provided and the chip junction temperature is not controlled;

Figure 5B is similar to Figure 5A but shows the wavelength spectra for the three different drive currents when a filter is provided and the chip junction temperature is actively controlled in accordance with an embodiment of the present invention;

Figure 6 shows plots of output power and monitor current against drive current for each of a fixed thermistor temperature and a fixed chip wavelength;

Figure 7 shows plots of output power and temperature against drive current when the wavelength is locked at 980nm in accordance with an embodiment of the present invention; and

Figure 8 shows a plot of back facet monitor (BFM) current against sub-mount temperature when the chip wavelength is locked.

Detailed Description of Certain Embodiments

With reference to Figures 1A and 1 B, there is illustrated a portion of a semiconductor laser device in the form of a carrier assembly 10 in accordance with a first embodiment of the invention. The laser carrier assembly 10 comprises an optical emitter in the form of a semiconductor diode laser chip 12 having an InGaAs gain medium 14 disposed within a Fabry-Perot cavity having a front laser facet 16 and a back laser facet 18. The laser chip 12 is configured for emission at a wavelength of approximately 980nm (such as required for an erbium-doped fibre amplifier) and is mounted on a chip carrier substrate 20 having a front facet 22 and a back facet 23 such that the front laser facet 16 is disposed substantially in line with the front facet 22 of the substrate 20. A narrow band pass filter 24 is mounted on the substrate 20. The filter 24 is generally planar and is disposed a short distance from the back laser facet 18 (and is orientated approximately parallel thereto). The filter 24 in this embodiment is made from a transparent silica substrate having multi-layer dielectric thin film deposited thereon before the substrate is diced into a suitable size for the present application.

A back facet monitor (BFM) 26 in the form of an InGaAs photo-detector is also mounted on the substrate 20, is located a short distance behind the filter 24 and is orientated parallel thereto, such that the filter 24 is provided between the back laser facet 18 and the BFM 26. In other embodiments other types of photo-detector may be employed, such as a silicon photo-detector. If required, a collimating lens (not shown) may be used to collimate the light emitted from the back laser facet 18 prior to the light being received at the BFM 26. Although not shown in this embodiment, the BFM 26 may be angled relative to the laser chip 12 to suppress any reflections back into the emitting portion of the chip 12.

As illustrated in Figure 2, a portion of a semiconductor laser device in the form of a carrier assembly 10' (which is similar to the laser carrier assembly 10 shown in Figures 1A and 1 B) is incorporated into a laser package 30. The laser carrier assembly 10' in Figure 2 is largely the same as the laser carrier assembly 10 in Figures 1A and 1 B and so like reference numerals will be employed as appropriate. In fact, the only significant differences are that the filter 24' and the BFM 26' shown in Figure 2 are not parallel with the back laser facet 18. More specifically, the filter 24' is angled by approximately 7° from parallel and the BFM 26' is angled by approximately 14° from parallel. It will be understood that the angling or off-setting of each of these components 24', 26' is to minimise reflected rays from being directed back into the laser chip 12 through the back laser facet 18.

The laser carrier assembly 10' is mounted on a temperature controller in the form of a thermoelectric cooler (TEC) 32 such that the back facet 23 of the substrate 20 is disposed substantially in line with a back facet 34 of the TEC 32. It will be noted that the TEC 32 is significantly larger than the substrate 20 in this embodiment and so the front laser facet 16 is set back a distance from a front facet 36 of the TEC 32.

It is also shown in Figure 2 that the TEC 32 is mounted towards the rear 38 of the package 30 with a gap provided between the front facet 36 of the TEC and the front wall 40 of the package 30. A generally cylindrical port 42 is provided in the front wall 40 for connection to a fibre optic cable (not shown). The front laser facet 16 is, of course, aligned with the port 42 so that emitted light from the laser chip 12 can be directed into a fibre optic cable when connected thereto.

In this particular embodiment, the BFM 26' will be linked to the TEC 32 in order to form a feedback loop constituting a wavelength locking mechanism in which the temperature of the laser chip 12 (and in particular the chip junction temperature) is controlled so as to achieve a maximum signal at the detector, when the optical emitter is operating at the desired wavelength (i.e. as determined by the filter 24'). More specifically, as only the desired wavelengths emitted from the back laser facet 18 will be allowed to pass through the filter 24', the BFM 26' will receive a maximum output power signal when the laser chip 12 is operating at the desired wavelength. As the wavelength drifts (e.g. as a result of with a change in drive current), the output power received at the BFM 26' will drop. Thus, the BFM 26' will be configured so as to generate a current in response to the output power detected, and to provide this current to the TEC 32 circuit in order to adjust the temperature of the laser chip 12 so as to continually maximise the BFM 26' response and thereby lock the laser chip 12 to the desired wavelength. Figure 3 shows the shift in operating wavelength as a result of increasing drive current for a free-running laser similar to that described above in relation to Figures 1A, 1 B and 2 but wherein a wavelength locking mechanism is not employed. Thus, it is clear that the position of maximum amplitude shifts from approximately 973nm to approximately 980nm as the drive current increases from 50mA to 700mA. The Applicants believe that this shift is a result of the chip junction temperature increasing and so embodiments of the present invention are designed with this in mind.

Figure 4 shows a comparison between the theoretical and measured profiles of the narrow band filter 24' employed in the embodiment shown in Figure 2. Thus, it can be seen that the response of the BFM 26' is largely the same as the design intention of the filter 24'. It will be noted that although the measured response is wider than the theoretical model this does not represent any issues in the embodiment described.

Figure 5A shows the wavelength spectra for three different drive currents (100mA, 350mA and 700mA) when no filter 24' is provided and the chip temperature is not controlled. Thus, it can be seen that the amplitude peaks in each case are at significantly different wavelengths. More specifically, the wavelength shifts from approximately 973nm (at 100mA) to approximately 976nm (at 350mA) and to approximately 979nm (at 700mA).

Figure 5B is similar to Figure 5A but shows the wavelength spectra for the three different drive currents when the filter 24' is provided and the chip temperature is controlled from the response from the BFM 26' in accordance with an embodiment of the present invention. This, it can be seen that in each case the amplitude of the signal peaks at approximately 982nm.

Figure 6 shows plots of output power and BFM 26' current against laser drive current for each of a fixed thermistor resistance (i.e. a fixed substrate 20 temperature) and a fixed chip wavelength (in accordance with an embodiment of the present invention). In this case, the output power is substantially similar regardless of whether the temperature of the substrate is being kept constant or the wavelength is being kept substantially constant in accordance with the present invention. However, it will be noted that a greater BFM 26' current is generated using the wavelength locking technique of the present invention, when compared to that generated when a fixed substrate 20 temperature is employed.

Figure 7 shows plots of output power and temperature against drive current when the wavelength is locked at 980nm in accordance with an embodiment of the present invention. Accordingly, while the output power increases with drive current it has been found that it is necessary to reduce the temperature of the substrate 20 from approximately 50°C to approximately 32°C in order to maintain an relatively constant chip junction temperature as the drive current is increased, so that the wavelength of the laser is kept approximately constant. Figure 8 shows a plot of BFM 26' current against substrate 20 temperature when the laser chip 12 is wavelength locked in accordance with an embodiment of the present invention. Once again, this shows that as the signal strength is increased it is necessary to reduce the temperature of the substrate 20 in order to maintain a relatively constant chip junction temperature and thereby a relative constant wavelength.

It will also be appreciated by persons skilled in the art that various modifications may be made to the above embodiments without departing from the scope of the present invention.

Claims

CLAIMS:

1 . A semiconductor laser device comprising:

an optical emitter; and

a wavelength locking mechanism arranged to control the temperature of the optical emitter in order to maintain a desired optical wavelength.

2. The laser according to claim 1 wherein the desired optical wavelength comprises a range of wavelengths.

3. The laser according to claim 1 or claim 2 wherein the optical emitter comprises a gain medium in the form of a semiconductor diode laser disposed within an optical resonator.

4. The laser according to claim 3 wherein the wavelength locking mechanism is configured to control the junction temperature of the semiconductor diode laser in order to maintain the desired optical wavelength.

5. The laser according to any preceding claim, configured as a pump laser.

6. The laser according to any preceding claim wherein a temperature controller is employed to control the temperature of the optical emitter.

7. The laser according to any preceding claim wherein the wavelength locking mechanism is configured to indirectly monitor the wavelength of the optical emitter.

8. The laser according to claim 7 wherein the wavelength locking mechanism comprises a detector configured to monitor an output power of the optical emitter at the desired optical wavelength.

9. The laser according to claim 8 wherein the detector is a photo-detector and is configured to generate a current in response to the output power detected.

10. The laser according to claim 9, when dependent upon claim 6, wherein the current is provided to the temperature controller to control the temperature of the optical emitter.

1 1. The laser according to any one of claims 8 to 10 wherein the detector is constituted by a back facet monitor (BFM) and a filter is provided to ensure that the BFM is responsive at the desired optical wavelength.

12. The laser according to any preceding claim wherein the wavelength locking mechanism comprises a pass band filter or an edge filter at the desired optical wavelength.

13. The laser according to claim 12, when dependent upon claim 8, wherein the pass band filter is a narrow band filter and is disposed between the optical emitter and the detector.

14. The laser according to claim 13 wherein the detector is configured to determine, from the power it receives, whether the emitter is operating at, above or below the desired wavelength and this information is used in a feedback loop to control the temperature of the optical emitter.

15. The laser according to any one of claims 12 to 14 wherein the pass band filter pr edge filter is integrated with the optical emitter or the detector.

16. The laser according to claim 15 wherein the filter is deposited directly onto a back facet monitor or a back laser facet.

17. The laser according to claim 16 wherein the filter comprises a dielectric mirror stack having a mirror optical loss associated therewith, wherein the mirror optical loss is increased for wavelengths other than the desired optical wavelength.

18. The laser according to any preceding claim wherein the wavelength locking mechanism comprises a closed control loop comprising a detector and a temperature controller wherein the temperature controller is configured to control the temperature of the optical emitter so as to achieve a maximum signal at the detector, when the optical emitter is operating at the desired wavelength.

19. The laser according to any one of claims 1 to 6 wherein the wavelength locking mechanism comprises a wavelength selection system arranged in a feedback loop with a temperature controller such that the temperature of the optical emitter is controllable in order to lock onto the desired optical wavelength.

20. The laser according to claim 19 wherein the wavelength selection system comprises a wavelength detector provided inside the laser device.

21. The laser according to claim 19 the wavelength detector comprises a spectrometer pre-programmed to adjust a current supplied to the temperature controller in order to control the temperature of the optical emitter and therefore its wavelength to the desired optical wavelength.

22. A wavelength locking mechanism, for a semiconductor laser device, arranged to control the temperature of the semiconductor laser device in order to maintain a desired optical wavelength.

23. A method for stabilising the wavelength of a semiconductor laser device comprising:

monitoring the wavelength of the semiconductor laser device; and controlling the temperature of the semiconductor laser device in order to maintain a desired optical wavelength.

24. The method according to claim 23 wherein the step of monitoring the wavelength of the semiconductor laser device may comprise directly or indirectly monitoring the wavelength.

. The method according to claim 24 wherein the wavelength is monitored directly using a spectrometer.

26. The method according to claim 24 wherein the wavelength is monitored indirectly by monitoring an output power from the semiconductor laser device and using the output power to determine whether the semiconductor laser device is operating at, above or below the desired wavelength.

27. The method according to claim 26 wherein a narrow band filter is provided at the desired optical wavelength.

28. A semiconductor laser device substantially as hereinbefore described with reference to the accompanying drawings.

29. A wavelength locking mechanism substantially as hereinbefore described with reference to the accompanying drawings.

30. A method for stabilising the wavelength of a semiconductor laser device, substantially as hereinbefore described with reference to the accompanying drawings.