WO2023041171A1

WO2023041171A1 - Distributed feedback laser with integrated mpd

Info

Publication number: WO2023041171A1
Application number: PCT/EP2021/075577
Authority: WO
Inventors: Xin Chen
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2021-09-17
Filing date: 2021-09-17
Publication date: 2023-03-23

Abstract

A compound laser structure comprising: a substrate; an active waveguide structure comprising: a laser emitter (301) on the substrate, the laser emitter (301) comprising a Bragg grating (318) on a first area of the substrate; and a monitoring photodetector (317) integrated in the laser emitter (301) on a second area of the substrate different to the first area, the monitoring photodetector (317) being configured to measure the intensity of the laser emitter (301). In this way, the monitoring photodetector (317) may be integrated in the compound laser structure in such a way that minimizes the effect on the size and performance of the compound laser structure.

Description

DISTRIBUTED FEEDBACK LASER WITH INTEGRATED mPD

FIELD OF THE INVENTION

This invention relates to providing a compound semiconductor device including a monitoring photodetector.

BACKGROUND

Compound semiconductor devices, in particular compound laser structures, are widely used in telecommunications. The compound laser structure may comprise an active waveguide structure with a laser emitter. A monitoring photodetector (mPD) may be used to measure the intensity of the laser emitter. Prior art compound laser structures are known to provide the monitoring photodetector separate to the compound laser structure.

Figure 1 shows a first prior art compound semiconductor structure. In this example, the compound semiconductor structure 100 comprises a laser emitter 101 , such as a distributed feedback (DFB) laser, a monitoring photodetector (mPD) 102 is located external to the compound semiconductor structure 100. In particular, the monitoring photodetector 102 is located on the back of the laser 101. The monitoring photodetector 102 is soldered to a submount, such as a silicon platform. This solution may result in a number of disadvantages. The monitoring photodetector 102 may be large in comparison to the laser emitter 101. The monitoring photodetector 102 may therefore take up a significant amount of space on the submount. Additionally, the soldering of the monitoring photodetector 102 may be an additional process which incurs additional time and cost to the manufacturing.

Figure 2 shows a second prior art compound laser structure. In this example, the compound laser structure 200 comprises a laser emitter 201 . A monitoring photodetector 202 is located inset to the compound laser structure 200. In particular, the monitoring photodetector 202 is located on, and inset to, the back of the laser emitter 201. This integrated solution may result in a number of disadvantages. Normally, to increase the output power of the laser emitter 201 a high reflection (HR) coating is applied to the back facet of the laser emitter 201. By integrating the monitoring photodetector 202 on the back of the laser emitter 201 this may have the effective result of removing the back facet of the laser emitter 201 . This may result in the laser emitter 201 having no light reflected from both the front facet and the back facets. As a result, the output power of the laser emitter 201 may be greatly reduced. Additionally, integrating the monitoring photodetector 202 in this way may increase the overall size of the compound laser structure, which may increase the manufacturing cost.

It is desirable to develop improved compound laser structure which may overcome some of the disadvantages described above.

SUMMARY

According to a first aspect, there is provided a compound laser structure comprising: a substrate; an active waveguide structure comprising: a laser emitter on the substrate, the laser emitter comprising a Bragg grating on a first area of the substrate and a monitoring photodetector integrated in the laser emitter on a second area of the substrate different to the first area, the monitoring photodetector being configured to measure the intensity of the laser emitter. In this way, the monitoring photodetector may be integrated in the compound laser structure in such a way that minimizes the effect on the size and performance of the compound laser structure. In particular, the Bragg grating may be located in the waveguide close to the output facet side. The length of the Bragg grating may be between 40%-60% of the total laser cavity length L. The Bragg grating may comprise a Kappa*L value between 0.6 - 1 .4.

In some implementations, the compound laser structure may be configured wherein the monitoring photodetector is electrically isolated from the laser emitter.

In some implementations, the compound laser structure may be configured wherein the monitoring photodetector is electrically isolated from the laser emitter by a further waveguide extending between the monitoring photodetector and the laser emitter. In this way, the monitoring photodetector may be isolated from the laser emitter in a way which still enables a small amount of light to transfer from the laser emitter to the monitoring photodetector.

In some implementations, the compound laser structure may be configured wherein the laser emitter comprises a multi-mode interferometric (MMI) coupler section on a third area of the substrate different to the first area and the second area. In this way, the light passing through the laser emitter may be split by the MMI coupler section.

In some implementations, the compound laser structure may be configured wherein the further waveguide extends from a corner of the MMI coupler section. In some implementations, the compound laser structure may be configured wherein the MMI coupler section comprises a rectangular section on the substrate.

In some implementations, the compound laser structure may be configured wherein the further waveguide extends between 20um to 100um. In this way, the monitoring photodetector may be sufficiently displaced from the main part of the laser emitter such that the chance of electrical conduction is reduced.

In some implementations, the compound laser structure may be configured wherein the laser emitter comprises a p-doped layer on a multi-quantum well layer.

In some implementations, the compound laser structure may be configured wherein the p- doped layer comprises a second p-doped layer on a first p-doped layer.

In some implementations, the compound laser structure may be configured wherein the further waveguide is integrated in the second p-doped layer. In this way, the further waveguide may form part of the p-doped layer which may enable applying bias.

In some implementations, the compound laser structure may be configured wherein the Bragg grating is integrated in the multi-quantum well layer. In this way, the Bragg Grating may enable reflecting particular wavelength and transmit others depending on the arrangement of the Bragg Grating.

In some implementations, the compound laser structure may be configured wherein the active waveguide structure comprises a monitoring photodetector electrode (319) contacting the monitoring photodetector. In this way, a reverse bias may be applied to the monitoring photodetector.

In some implementations, the compound laser structure may be configured wherein the active waveguide structure comprises a laser emitter electrode contacting the laser emitter. In this way, together with the top contact, an electrical current may be applied to the laser emitter.

In some implementations, the compound laser structure may be configured wherein the active waveguide structure comprises a base electrode located between the substrate and the laser emitter. In this way, an electrical current may be applied to the base of the laser emitter. In some implementations, the compound laser structure may be configured wherein the active waveguide structure comprises a n-doped layer located between the base electrode and the laser emitter.

In some implementations, the compound laser structure may be configured wherein the laser emitter is a distributed feedback laser.

In some implementations, the compound laser structure may be configured wherein the compound laser structure further comprises an optical modulator on a fourth area of the substrate different to the first, second and third areas. In this way, the optical modulator may not affect the performance of the different components of the laser emitter.

In some implementations, the compound laser structure may be configured wherein the optical modulator is an electroabsorption modulator. The optical modulator may be a Mach-Zehnder modulator.

In some implementations, the compound laser structure may be configured wherein the laser emitter is at least partially coated in a reflectivity-increasing coating and the optical modulator is at least partially coated in a reflectivity-reducing coating.

According to a second aspect, there is provided a method of forming a compound laser structure comprising forming a laser emitter and a monitoring photodetector on a substrate so as to at least partially provide an active waveguide structure, wherein the laser emitter comprises a Bragg grating on a first area of the substrate and the monitoring photodetector is integrated in the laser emitter on a second area of the substrate different to the first area. In this way, during manufacture, the monitoring photodetector may be integrated in the compound laser structure in such a way that minimizes the effect on the size and performance of the compound laser structure.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will now be described by way of example with reference to the following accompanying drawings.

Figure 1 shows a first compound laser structure of the prior art. Figure 2 shows a second compound laser structure of the prior art.

Figures 3A to 3F shows an exemplary embodiment of a compound laser structure.

DETAILED DESCRIPTION

The compound laser structure and the method of forming a compound laser structure described herein concern integrating a monitoring photodetector into the compound laser structure.

Embodiments of the present system may tackle one or more of the problems previously mentioned by integrating the monitoring photodetector in the laser emitter on a second area of the substrate different to the first area where the Bragg grating is. In this way, the monitoring photodetector may be integrated in the compound laser structure in such a way that minimizes the effect on the size and performance of the compound laser structure.

Figures 3Ato 3F shows a laser emitter 301. Figure 3A shows the laser emitter 301 from above. Figure 3B shows a first section through compound laser emitter 301 , the section being shown in Figure 3A. Figure 3C shows the laser emitter 301 from above. Figure 3D shows a second section through compound laser emitter 301 , the section being shown in Figure 3C. Figure 3E shows the laser emitter 301 from above. Figure 3F shows a third section through compound laser emitter 301 , the section being shown in Figure 3E.

A compound laser structure may comprise an active waveguide structure comprising the laser emitter 301. The active waveguide structure may be located on a semiconductor substrate (not shown in Figures 3A to 3F). The term ‘on’ may be defined as a component being located above another component in the orientation shown in the Figure. For example, the active waveguide structure may be located on an area of the substrate. In other words, the active waveguide structure may be located in a column volume above the area of the substrate in the orientation shown in Figures 3A to 3F.

The laser emitter 301 may be located on a laser emitter area of the substrate. In other words, the laser emitter 301 may be located in a column above the laser emitter area of the substrate. The laser emitter 301 may be a distributed feedback (DFB) laser. The active waveguide structure may optionally comprise an optical modulator. The optical modulator may be located on an optical modulator area of the substrate. In other words, the optical modulator may be located in a column above the optical modulator area of the substrate. The optical modulator may be an electro absorption modulator (EAM).

The active waveguide structure may comprise a ridge or buried heterostructure (BH).

The light may be received by the laser emitter 301. The light may be outputted 316 from a front facet of the laser emitter 301 .

The laser emitter 301 may comprise a p-doped layer 307. Additionally, or alternatively, the laser emitter 301 may comprise a multi-quantum well layer 310. The p-doped layer 307 may be located on the multi-quantum well layer 310. In other words, the p-doped layer 307 may be located in the column above the multi-quantum well layer 310.

The active waveguide structure may comprise a base electrode 313. The base electrode 313 may be located between the substrate and the laser emitter 301. In other words, the base electrode 313 may be located on the bottom of the laser emitter 301. In this way, the base electrode 307 may be used for providing an electrical contact for the laser emitter 301 .

The active waveguide structure may further comprise an n-doped layer 312 located between the base electrode 313 and the laser emitter 301. In this way, the n-doped layer 312 may be used for the laser emitter 301 .

The active waveguide structure may comprise a laser emitter electrode 304 on the laser emitter 301. In other words, the laser emitter electrode 304 may be located in the column above the laser emitter 301 . Preferably, the laser emitter electrode 304 may contact the laser emitter 301. In this way, the laser emitter electrode 304 may provide an electrical contact for providing a current to the laser emitter 301 .

The laser emitter 301 may be at least partially coated in a reflectively-increasing coating 314, or high reflectivity (HR) coating. The reflectively-increasing coating 314 may provide increased reflectivity to the surface of the laser emitter 301 . Preferably, the reflectively-increasing coating 314 covers a surface of the back facet of the laser emitter 301 . In this way, the reflectively- increasing coating 314 may face in the general direction from which the light is received. The optical modulator may be at least partially coated in a reflectively-decreasing coating 315. The reflectively-decreasing coating 315 may provide decreased reflectivity to the surface of the optical modulator. Preferably, the reflectively-decreasing coating 315 covers a surface of the front facet of the optical modulator. In this way, the reflectively-decreasing coating 315 may face in the general direction from which the light is outputted 316.

As shown in Figures 3A to 3F, the laser emitter 301 may comprise a Bragg Grating 318. The Bragg Grating 318 may enable the laser emitter 301 to reflect particular wave lengths and transmit others depending on the arrangement of the Bragg Grating 318. The Bragg grating may be located on a first area of the substrate. In other words, the Bragg grating 318 may be located in the column above the first area of the substrate. Preferably, the first area does not cover all of the substrate. In this way, the Bragg grating 318 may not cover all of the laser emitter 301.

The Bragg grating may be integrated into the multi-quantum well layer 307 of the laser emitter 301 . As shown in Figure 3B, the Bragg grating 318 may comprise a thickness that is less than the thickness of the multi-quantum well layer 307 of the laser emitter 301. Although, it may be appreciated that the relative thicknesses shown in Figures 3A to 3F may only be illustrative. In either case, in may be preferable for the Bragg grating 318 to be thinner than multi-quantum well layer 307 of the laser emitter 301 .

The active waveguide structure may comprise a monitoring photodetector 317. The monitoring photodetector 317 may be configured to measure the intensity of the laser emitter 301. This may be linked, or proportional, to the intensity of the light output 316 of the laser emitter 301. The monitoring photodetector 317 may be used, directly or indirectly, to control the intensity of the light output 316.

Conventionally, the Bragg grating 318 may cover the whole of the laser emitter 301 . However, the waveguide width should be compatible with the grating pitch/period to have stable emission wavelength. For a commercial compound laser structure, it is preferable for manufacture and high yield for the entire laser emitter 301 to comprise a constant grating period and waveguide width. Therefore, it can be difficult to introduce a monitoring photodetector 317 into the laser emitter.

The monitoring photodetector 317 may be integrated in the laser emitter 301. In other words, the monitoring photodetector 317 may form an integral part of the laser emitter 301. The monitoring photodetector 317 may be located on a second area of the substrate. In other words, the monitoring photodetector 317 may be located in the column above the second area of the substrate. Preferably, the first area is different, or independent, to the second area. In other words, the first and second areas do not overlap. In this way, the monitoring photodetector 317 and the Bragg grating 318 do not overlap. This may ensure that the variable light deflection from the Bragg grating 318 does not affect the monitoring of the monitoring photodetector 317.

By locating the monitoring photodetector 317 in the laser emitter 301 , this may provide a compact compound laser structure. In this way, the compound laser structure may not result in the disadvantage of being bulky and having high manufacturing costs that can occur in the first prior art compound laser structure 100.

The monitoring photodetector 317 may be configured to measure the intensity of the laser emitter 301. This may be the intensity of the light output 316 of the laser emitter 301. The monitoring photodetector 317 may be used, directly or indirectly, to control the intensity of the light output 316.

The active waveguide structure may comprise a monitoring photodetector electrode 319. The monitoring photodetector electrode 319 may be located on, and/or contact the monitoring photodetector 317. In this way, the monitoring photodetector electrode 319 may be used for providing an electrical contact for the monitoring photodetector 317.

The monitoring photodetector 317 may only require a pA level photocurrent. Such as a 10^A-6 Amper photocurrent. This may keep the loss due to the monitoring photodetector 317 minimal. In this way, the compound laser structure may not result in the disadvantage of output power loss that can occur in the second prior art compound laser structure 200.

As shown in Figures 3A, 3B and 3C, the monitoring photodetector 317 may be transversely located away from the laser emitter 301. Preferably, the monitoring photodetector 317 is electrically isolated from the laser emitter 301. The monitoring photodetector 317 may be electrically isolated from the laser emitter 301 by using a further waveguide 321 . Additionally, or alternatively, the monitoring photodetector 417 may be electrically isolated from the laser emitter 301 etching away the top p-doped layers or by ion-implantation the area surrounding the monitoring photodetector 317. As shown in Figure 3A, 3B and 3C, the waveguide 321 may extend between the monitoring photodetector 317 and the laser emitter 301 . The laser emitter 301 may further comprise a multi-mode interferometric (MMI) coupler section 320. The MMI coupler section 320 may be located on a third are of the substrate. In other words, the MMI coupler section 320 may be located in a column volume above the third area. Preferably, the third area is different, or independent, to the first area and the second area. In other words, the third area and the first area and second area do not overlap. In this way, the MMI coupler section 320 and the Bragg grating 318 do not overlap. This may ensure that the variable light deflection from the Bragg grating 318 does not affect the monitoring of the monitoring photodetector 317.

The optical modulator may be located on a fourth area of the substrate. In other words, the optical modulator may be located in a column volume above the fourth area. Preferably, the fourth area is different, or independent, to the first area, second area and third area. In other words, the fourth area and the first area, second area and third area do not overlap.

The MMI coupler section 320 may comprise a rectangular section on the substrate, as shown in Figures 3A, 3B and 3C. The MMI coupler section 320 may comprise a 1x1 square section. Alternatively, the MMI coupler section 320 may comprise another polygonal section on the substrate. In other words, from the above view in Figures 3A, 3B and 3C, the section of the MMI coupler section 320 can be seen.

The further waveguide 321 may comprise a length of between 20um and 100um. In other words, the monitoring photodetector 317 may be located between 20um and 100um from the main body of the laser emitter 301 or, in particular, from the MMI coupler section 320.

The further waveguide 321 may extend from a corner of the MMI coupler section 320. In Figures 3A, 3B and 3C, the further waveguide 321 extends from a corner closest to the Bragg grating 318.

The further waveguide 321 may enable a small amount of light to leak into a passive waveguide linked to the absorbing region under reverse bias via the monitoring photodetector electrode 319.

As shown in Figures 3B, 3D and 3F, the p-doped layer 307 may comprise a first p-doped layer 307 and a second p-doped layer 322. As shown in Figures 3D and 3F, the monitoring photodetector 317 may be provided by the second p-doped layer 322. This p-doped layer of the monitoring photodetector 317 may be the same material as the second p-doped layer 322.

As shown in Figure 3F, the MMI coupler section 320 may be provided by the second p-doped layer 322. This p-doped layer of the MMI coupler section 320 may be the same material as the second p-doped layer 322.

As shown in Figures 3D and 3F, the p-doped layers around the monitoring photodetector 317 may be etched away, so that the monitoring photodetector 317 is electrically isolated from laser emitter. The electrical isolation can also be done via ion-implantation the surround area of the monitoring photodetector 317. Together with top metal contact 319 and bottom contact 313, a reverse bias may be applied to the monitoring photodetector 317, the collected photocurrent from monitoring photodetector 317 can be used by an external circuit to control the electrical current injected to the laser emitter 301 , hence control the output power of the laser emitter 301.

The laser emitter 301 may include the MMI 321 and may have the same top p-doped layers as the monitoring photodetector 317.

As shown in Figure 3F, the laser emitter 301 may comprise a MMI coupler section electrode 323. The MMI coupler section electrode 323 may be located on, and/or contact the MMI section coupler 320. In this way, the MMI coupler section electrode 323 may be used for providing an electrical contact for the MMI section coupler 320.

The further waveguide 321 may be integrated in the second p-doped layer 322. As shown in Figure 3F, the further waveguide 321 may comprise a thin layer extending between the p- doped layer of the MMI coupler section 320 and the p-doped layer of the monitoring photodetector 317. This may provide an optical coupling between the doped layer of the MMI coupler section 320 and the p-doped layer of the monitoring photodetector 317. The further waveguide 321 may comprise a width of 0.6um to 4um.

The monitoring photodetector 317 may comprise a length between 5um-300um. Alternatively, the monitoring photodetector 317 may comprise a diameter between 20um and 100um if a further waveguide 312 is used. The monitoring photodetector 317 may comprise a width between 0.8um to 5.0um. The distance between the monitoring photodetector 317 and the p- doped layer of the MMI coupler section 320 may be between 20um to 100um. The distance between the monitoring photodetector 317 and the second p-doped layer 322 may be between 20um to 100um.

The compound laser structure may be formed, or manufactured, by forming the laser emitter 301 and the monitoring photodetector 317 on the substrate. The laser emitter 301 and the monitoring photodetector 317 may be each deposited on the substrate. In particular, the monitoring photodetector 317 may be deposited with the laser emitter 301. Additionally, a Bragg grating 318 may be deposited with the laser emitter 301. The p-layer 317 of the monitoring photodetector 319 may be grown simultaneously with the p layer 307 of the laser emitter 418301.

The deposition may be provided by ion-implantation. To form the monitoring photodetector 317, with the gap on each side, the monitoring photodetector 317 may only be deposited in the desired location and the gaps on each side are provided by non-ion-implantation. Alternatively, a single p-doped layer may be deposited, and the gap on each side of the monitoring photodetector 317 may be provided by etching away.

The materials may comprise InP, GaAs, GaN, and/or other semiconductor materials.

The phrase "configured to" or “arranged to” followed by a term defining a condition or function is used herein to indicate that the object of the phrase is in a state in which it has that condition, or is able to perform that function, without that object being modified or further configured.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description, it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

1. A compound laser structure comprising: a substrate; an active waveguide structure comprising: a laser emitter (301) on the substrate, the laser emitter (301) comprising a Bragg grating (318) on a first area of the substrate; and a monitoring photodetector (317) integrated in the laser emitter (301) on a second area of the substrate different to the first area, the monitoring photodetector (317) being configured to measure the intensity of the laser emitter (301).

2. A compound laser structure of claim 1 , wherein the monitoring photodetector (317) is electrically isolated from the laser emitter (301).

3. A compound laser structure of claim 1 or claim 2, wherein the monitoring photodetector (317) is electrically isolated from the laser emitter (301) by a further waveguide (321) extending between the monitoring photodetector (317) and the laser emitter (301).

4. A compound laser structure of any preceding claim, wherein the laser emitter (301) comprises a multi-mode interferometric (MMI) coupler section (320) on a third area of the substrate different to the first area and the second area.

5. A compound laser structure of claim 4 when dependant on claim 3, wherein the further waveguide (321) extends from a corner of the MMI coupler section (320).

6. A compound laser structure of claim 4 or claim 5, wherein the MMI coupler section (320) comprises a rectangular section on the substrate.

7. A compound laser structure according to any of claims 3 to 4, wherein the further waveguide (321) extends between 20um to 100um.

8. The compound laser structure of any preceding claim, wherein the laser emitter (301) comprises a p-doped layer (307) on a multi-quantum well layer (310).

9. The compound laser structure of claim 8, wherein the p-doped layer (307) comprises a second p-doped layer (322) on a first p-doped layer (307).

10. The compound laser structure of claim 9, wherein the further waveguide (321) is integrated in the second p-doped layer (322).

11 . The compound laser structure of any of claims 8 to 10, wherein the Bragg grating (318) is integrated in the multi-quantum well layer (311).

12. The compound laser structure of any preceding claim, wherein the active waveguide structure comprises a monitoring photodetector electrode (319) contacting the monitoring photodetector (317).

13. The compound laser structure of any preceding claim, wherein the active waveguide structure comprises a laser emitter electrode (304) contacting the laser emitter (301).

14. The compound laser structure of any preceding claim, wherein the active waveguide structure comprises a base electrode (313) located between the substrate and the laser emitter (301).

15. The compound laser structure of claim 14, wherein the active waveguide structure comprises a n-doped layer (312) located between the base electrode (313) and the laser emitter (301).

16. The compound laser structure of any preceding claim, wherein the laser emitter (301) is a distributed feedback laser.

17. The compound laser structure of any preceding claim, wherein the compound laser structure further comprises an optical modulator on a fourth area of the substrate different to the first, second and third areas.

18. The compound laser structure of claim 17, wherein the optical modulator is an electroabsorption modulator.

19. The compound laser structure of claim 17 or 18, wherein the laser emitter (301) is at least partially coated in a reflectivity-increasing coating (314) and the optical modulator is at least partially coated in a reflectivity-reducing coating (315).

20. A method of forming a compound laser structure comprising forming a laser emitter (301) and a monitoring photodetector (317) on a substrate so as to at least partially provide an active waveguide structure, wherein the laser emitter (301) comprises a Bragg grating (318) on a first area of the substrate and the monitoring photodetector (317) is integrated in the laser emitter (301) on a second area of the substrate different to the first area.

14