Integrated semiconductor laser with transversely coupled fabry-perot cavity

Abstract

An integrated semiconductor laser includes a lasing cavity, a passive waveguide, and a transversely coupled Fabry-Perot (TCFP). The passive waveguide is coupled to receive an optical input from the lasing cavity. The laterally coupled pseudo-linear external cavity includes a cavity waveguide laterally coupled to the passive bus waveguide, wherein the cavity waveguide is terminated by at least one linear waveguide reflector.

Description

Client Docket No.2024-865 B&A Docket No.4059.068PCT1 INTEGRATED SEMICONDUCTOR LASER WITH TRANSVERSELY COUPLED FABRY-PEROT CAVITY STATEMENT OF GOVERNMENT INTERESTS

[0001] This invention was made with government support under HR001-20-2-0044 awarded by DARPA. The government has certain rights in the invention. CROSS-REFERENCE TO RELATED APPLICATION(S)

[0002] This application claims the benefit of and priority to U.S. provisional application 63 / 666,578, titled “Integrated Semiconductor Laser with Transversely Coupled Fabry- Perot Cavity”, filed July 1, 2024, the contents of which are incorporated by reference herein. BACKGROUND

[0003] A laser cavity is defined – in part – by the mirrors located at either end of the laser cavity. Traditional integrated lasers may utilize cyclic resonators (rings and / or racetrack structures) to generate the desired reflections (i.e., to act as a passive mirror). To reduce bending losses, traditional cyclic resonators must be relatively large. It would therefore be desirable to provide a passive structure that generates the desired reflections required by an integrated semiconductor laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0004] FIG. 1A is a diagrammatic view of an integrated semiconductor laser with a transversely coupled Fabry-Perot cavity according to some embodiments.

[0005] FIG. 1B is a graph illustrating resonance power transfer associated with the transversely coupled Fabry-Perot cavity shown in FIG. 1A.

[0006] FIG. 1C is a graph illustrating bus reflections Rbusgenerated in response to termination of the transversely coupled Fabry-Perot cavity with metal coated etched facet (MCEF) mirrors.Client Docket No.2024-865 B&A Docket No.4059.068PCT1

[0007] FIG. 2A is a diagrammatic view of an integrated semiconductor laser with a laterally coupled pseudo-linear external cavity utilizing distributed Bragg reflectors (DBRs) according to some embodiments.

[0008] FIG. 2B illustrates bus reflections Rbusprovided by the laterally coupled pseudo- linear external cavity 204 utilizing distributed Bragg reflectors (DBRs) according to some embodiments.

[0009] FIG. 3A is a diagrammatic view of an integrated semiconductor laser with a laterally coupled pseudo-linear external cavity utilizing sampled grating distributed Bragg reflectors (SGDBRs) according to some embodiments.

[0010] FIG. 3B illustrates bus reflections Rbusprovided by the laterally coupled pseudo- linear external cavity 204 utilizing sampled grating distributed Bragg reflectors (DBRs) according to some embodiments.

[0011] FIG. 4 is a diagrammatic view of an integrated semiconductor laser with a transversely coupled Fabry-Perot external cavity utilizing a combination of DBRs and SGDBRs according to some embodiments.

[0012] FIG. 5 is a diagrammatic view of a transversely coupled Fabry-Perot external cavity capable of higher harmonic generation (HHG) according to some embodiments.

[0013] FIG. 6 is a diagrammatic view of a cascaded transversely coupled Fabry-Perot external cavity according to some embodiments. DETAILED DESCRIPTION

[0014] The present disclosure describes an integrated semiconductor laser having a transversely coupled Fabry-Perot resonator (TCFP). In particular, the TCFP acts as a passive structure that provides the desired reflections or mirror-like functionality required by the integrated semiconductor laser. The TCFP (i.e., a cavity that does not include a cyclic ring-type resonator) is transversely (or laterally) coupled to the laser’s passive waveguide and utilizes one or more types of linear waveguide reflectors to generate resonance within the cavity. The resonant light built up within the TCFP is reflected back to the laser’s passive waveguide (in the opposite direction that it was initially provided toClient Docket No.2024-865 B&A Docket No.4059.068PCT1 the TCFP). Because the TCFP does not rely on cyclic structures, the cavity requires much less area than a traditional cyclic structure. In addition, one of the benefits of the TCFP is the ability to design the cavity to reflect specific wavelengths (and / or transmit specific wavelengths). In particular, if some process converts light in the resonator to another wavelength, the reflectors utilized in the cavity may be designed to reflect the new wavelength appropriately to create an optimal output at the new wavelength. In addition, the use of the external cavity allows for a greater build-up of light within the cavity as compared to devices using a linear reflector without a cavity. In some embodiments, the greater build-up of light afforded by the external cavity provides narrower linewidth outputs. Finally, in some embodiments the use of TCFP allows the cavity to pass alongside the active cavity further reducing the overall footprint of the integrated semiconductor laser as compared with traditional integrated lasers relying on cyclic resonators.

[0015] FIG. 1A is a diagrammatic view of an integrated semiconductor laser 100 with a transversely coupled Fabry-Perot 104 according to some embodiments. In some embodiments, the integrated semiconductor laser 100 includes a lasing cavity 102 and the TCFP 104. An active cavity of a laser is typically bookended by reflective mirrors. In the embodiment shown in FIG. 1A, the active cavity 106 includes a coupler 108 located at a first end that couples the lasing cavity 102 to passive bus waveguide 110. In some embodiments, a second end of the active cavity 106 located opposite the first end includes a rear mirror 105, such as a metal coated etched facet (MCEF) mirror. In some embodiments, the MCEF is a lithographically defined metal deposition. By using an etched facet and conformal metal deposition, an in-plane waveguide mode can be reflected by the MCEF. The TCFP 104 acts as the second mirror to the lasing cavity 102, providing desired reflections 120 back to the active cavity 106.

[0016] As shown in FIG. 1A, the passive bus waveguide 110 is laterally coupled to the cavity waveguide 112 of the TCFP 104. In some embodiments, the cavity waveguide 112 is laterally offset in a non-vertical direction from the passive bus wave guide 110. In other embodiments, the lateral coupling of the cavity waveguide 112 and the busClient Docket No.2024-865 B&A Docket No.4059.068PCT1 waveguide 110 includes at least some vertical offset. In some embodiments, a heater 122 is located adjacent the lateral coupling between the bus waveguide 110 and the cavity waveguide 112 and is utilized to adjust the coupling between the respective waveguides. Although not shown in other embodiments, heater 122 could be included in each of the additional configurations taught herein.

[0017] The TCFP 104 includes a first linear waveguide reflector 114 located at a first end of the TCFP 104 and a second linear waveguide reflector 116 located at a second end of the TCFP 104. As described in more detail below, a number of different types of linear waveguide reflectors may be utilized either alone or in combination with one another to provide the desired reflection properties, including distributed bragg reflectors (DBRs), sampled grating distributed bragg reflectors (SGDBR), metal coated etched facet (MCEF) mirrors, and other more complex designs such as retroreflectors and inversely designed structures. In the embodiment shown in FIG. 1A the cavity waveguide 112 is linear (i.e., no bends or curves), but in other embodiments some degree of curvature may be introduced into the cavity waveguide 112. However, the cavity waveguide 112 does not rely on cyclic geometries (e.g., rings, racetracks), but instead relies on first and second linear waveguide reflectors 114, 116 to generate the desired resonance within the cavity waveguide 112. As described in more detail below, the first and second linear waveguide reflectors 114, 116 may be selected to reflect certain wavelengths of light and transmit other wavelengths of light in order to build-up light of a desired wavelength within the cavity waveguide 112. Resonant light within the cavity waveguide 112 is laterally coupled – but in the opposite direction – with the passive bus waveguide 110 to provide reflections 120 back to the active cavity 106. In this way, the TCFP 104 acts as the second mirror bookending the active cavity 106 of the lasing cavity 102. Benefits of TCFP include the ability to provide the desired resonance and reflections required at a reduced footprint.

[0018] For the generic TCFP, the steady state power reflection and transmission in the bus waveguide 110 may be described by equation:Client Docket No.2024-865 B&A Docket No.4059.068PCT1 (1) (2)transmission between the coupler and the reflector further from the input, ^1is the field reflection at the first linear reflector 114 (e.g., the reflector furthest from the input) and a is the intrinsic round trip field transmission of the cavity. The intrinsic round trip transmission may be written explicitly as: (3) Where a2and r2aredescribed above. It is clear from equation (1) that for strongly coupled cavities (i.e., log(1-t2) >> log(1-a2), or a ≈ 1), the reflection approaches unity on resonance. Assuming the linear waveguide reflectors 114, 116 allow some light to transmit into a continuing waveguide, the power transfer function from the bus input to the continuing waveguides are given by: (4) and(5) wherein T1is the fieldT2 is the field transmission through the second linear waveguide reflector 116.

[0019] FIG. 1B is a graph illustrating the bus transmission Tbus, bus reflection Rbus, and mirror transmission T2on resonance as bus cavity coupling varies relative to cavity round trip loss (wherein coupling is illustrated on the x-axis with increasing coupling in theClient Docket No.2024-865 B&A Docket No.4059.068PCT1 direction of the arror). As illustrated, weak coupling, the incident power is split between bus reflection, bus transmission and mirror transmission. For stronger coupling, the reflection increases, and the bus transmission falls off rapidly, while the mirror transmission declines more slowly. The continuing waveguide output (Tbus, T1, or T2, or some combination of all three) acts like an additional drop port, offering isolation away from resonance and improved linewidth performance.

[0020] FIG. 1C is a graph illustrating bus reflections Rbusgenerated in response to termination of the TCFP with metal coated etched facet (MCEF) mirrors. That is, both the first and second linear waveguide reflectors 114, 116 are constructed using MCEF mirrors provide broadband reflection and compact form factor but very low transmission so no continuing waveguide (i.e., T1and T2are close to zero). Assuming strong coupling, this type of termination of the TCFP may be utilized to act as a rear laser mirror.

[0021] FIG. 2A is a diagrammatic view of an integrated semiconductor laser 200 with a TCFP 204 utilizing distributed Bragg reflectors (DBRs) according to some embodiments. In this embodiment, first and second linear waveguide reflectors 214, 216 make use of distributed bragg reflectors (DBRs). In some embodiments, the DBRs are selected to reflect certain wavelengths of light and transmit other wavelengths of light based on the desired reflection 220. As compared with the embodiment described in FIG. 1C in which the laterally coupled pseudo-linear cavity is terminated with MCEF mirrors, termination of the TCFP 204 with DBRs offers a high reflection and a continuing waveguide. In some embodiments, one of the DBRs (either the first linear waveguide reflector 114 or the second linear waveguide reflector 116) is a relatively weak DBR that allows incident power to be efficiently divided between reflection (e.g., reflection r2) and a single output (e.g., transmission T2).

[0022] FIG. 2B illustrates bus reflections Rbus provided by the TCFP 204 utilizing distributed Bragg reflectors (DBRs) according to some embodiments. As illustrated, termination of the external cavity 204 with DBRs at the first and second linear waveguide reflectors 214, 216 provides bus reflections Rbus at only selected wavelengths (in contrastClient Docket No.2024-865 B&A Docket No.4059.068PCT1 with the bus reflections Rbusgenerated by termination of the TCFP with MCEFs as described with respect to FIG. 1C).

[0023] FIG. 3A is a diagrammatic view of an integrated semiconductor laser 300 with a TCFP 304 utilizing sampled grating distributed Bragg reflectors (SGDBRs) 314, 316 according to some embodiments. SGDBRs can be utilized to tune the wavelengths of light reflected / transmitted by the SGDBRs and control the reflections 420 provided to the active cavity 106 of the lasing cavity 102.

[0024] In addition, in the embodiment shown in FIG. 3A, the cavity waveguide 312 associated with the TCFP 304 extends alongside the active cavity 106. In the embodiment shown in FIG. 3A, the first SGDBR 314 is located on a first side of the lasing cavity 102 and the second SGDBR 316 is located on a second side of the lasing cavity 102 opposite the first side. As compared with the embodiments shown in FIGS. 1A and 2A, the embodiment provided in FIG. 3A provides a longer external cavity 304 while still maintaining a relatively small overall footprint. The longer cavity results in a smaller fraction of the average cavity loss being from the mirror loss and provides a higher quality (Q) factor of the cavity. Although the embodiment shown in FIG. 3A utilizes SGDBRs, in other embodiments various other linear waveguide reflectors may be utilized, such as DBRs, either alone or in combination with the SGDBRs. For example, one linear waveguide reflector could utilize SGDBRs and the other linear waveguide reflector may utilize DBRs.

[0025] FIG. 3B illustrates bus reflections Rbusprovided by the TCFP 204 utilizing sampled grating distributed Bragg reflectors (DBRs) according to some embodiments. As illustrated, termination of the external cavity 304 with SGDBRs at the first and second linear waveguide reflectors 314, 316 provides bus reflections Rbusat only selected wavelengths (in contrast with the bus reflections Rbus generated by termination of the pseudo-linear cavity with MCEFs as described with respect to FIG. 1C).

[0026] FIG. 4 is a diagrammatic view of an integrated semiconductor laser 400 with a TCFP 404 utilizing a combination of DBRs 418, 420 and SGDBRs 414 and 416 according to some embodiments. The embodiment shown in FIG. 4 may be utilized forClient Docket No.2024-865 B&A Docket No.4059.068PCT1 second harmonic generation (SHG). In this embodiment, it is assumed that the input provided by the active cavity 106 has a wavelength of λ. The lateral coupling between the bus waveguide 410 and the cavity waveguide 412 is characterized by weak directional coupling at wavelength λ and no coupling at wavelength λ / 2. Located at opposite ends of the cavity waveguide 412 are first and second SGDBRs 414, 416. In some embodiments, the SGDBRs 414, 416 are tuned to provide reflections at the wavelength λ and provide transmissions at the wavelength λ / 2. In this way, the SGDBRs 414, 416 provide confinement of the λ wavelength. In addition, the external cavity 404 includes first and second DBRs 418, 420 located outside of the SGDBRs 414, 416. In some embodiments, the first DBR 418 is selected to provide near 100% reflection of the λ / 2 wavelength while the second DBR 420 allows some transmission of the λ / 2 wavelength. In this way, the embodiment shown in FIG. 4 provides separate confinement for the λ / 2 wavelength without any bending loss.

[0027] FIG. 5 is a diagrammatic view of a TCFP 504 capable of higher harmonic generation (HHG) according to some embodiments. In this embodiment, the active cavity has been omitted and only the bus waveguide 510 and TCFP 504 are shown. In the embodiment shown in FIG. 5, the TCFP 504 utilizes post-grating DBRs 516 and 518 to confine longer wavelengths in the cavity while allowing a shorter wavelengths to pass. The longer wavelength experiences less lateral confinement and interacts with the grating and the lateral coupler, while the shorter wavelength is more tightly confined and experiences no interactions. In this application, the TCFP 504 can simultaneously act as a strong reflector and store a resonantly enhanced field to facilitate nonlinear behavior. In some embodiments, if the nonlinear loss is the dominant cavity loss mechanism, it takes the place of the transmission field ^2 (shown in FIG. 1B, for example). Further, because shorter wavelengths are more strongly guided in any given platform, the ends of the LCPC may be continued and joined together to form a closed loop resonator (e.g., racetrack, ring) that allows the build up of higher harmonic wavelengths. Alternatively, a second grating might be used to form a pseudo-linear cavity for the higher harmonic wavelengths. In some embodiments, this configuration has natural applications to low-Client Docket No.2024-865 B&A Docket No.4059.068PCT1 confinement silicon nitride for second harmonic generation via the photogalvanic effect, lithium niobate, and other nonlinear platforms.

[0028] FIG. 6 is a diagrammatic view of a cascaded TCFP 604 according to some embodiments. In this embodiment, the active cavity has once again been omitted and only the bus waveguide 610 and a cascaded TCFP 604 are shown. In this embodiment, the cascaded TCFP 604 includes a first TCFP 606 and a second TCFP 608, wherein the second TCFP 608 is laterally coupled to the first TCFP 606.

[0029] The first TCFP 606 includes a cavity waveguide 612 terminated at a first end by first linear waveguide reflector 614 and at a second end by the second laterally coupled cavity 608. In some embodiments, the second TCFP 608 includes a cavity waveguide 616 terminated by second and third linear waveguide reflectors 618, 620. This cascade configuration shown in FIG. 6 allows for further wavelength selection. In general, this configuration utilizes the second TCFP 608 as one of the terminations for the first TCFP 606. In particular, the cascaded configuration shown in FIG. 6 enables vernier tuning as both cavities must be on resonance in order to deliver a strong reflection. As described above, in some embodiments, the first, second, and third linear waveguide reflectors 614, 618, and 620 may be MCEF mirrors, DBRs, SGDBRs, or other types of well linear waveguide reflectors depending on the application.

Claims

Client Docket No.2024-865 B&A Docket No.4059.068PCT1 CLAIMS:

1. An integrated semiconductor laser comprising: a lasing cavity; a passive bus waveguide coupled to receive an input from the lasing cavity; and a transversely coupled Fabry-Perot (TCFP) having a cavity waveguide terminated by at least one linear waveguide reflector, wherein the TCFP is laterally coupled to the passive bus waveguide.

2. The integrated semiconductor laser of claim 1, wherein the cavity waveguide is pseudo-linear or linear.

3. The integrated semiconductor laser of claim 1, wherein at least one linear waveguide reflector is a metal coated etched facet (MCEF) mirror.

4. The integrated semiconductor laser of claim 1, wherein at least one linear waveguide reflector is a distributed bragg reflector.

5. The integrated semiconductor laser of claim 1, wherein at least one linear waveguide reflector is a sampled grating distributed bragg reflector.

6. The integrated semiconductor laser of claim 1, wherein the at least one linear waveguide reflector is a metal coated etched facet mirror.

7. The integrated semiconductor laser of claim 1, wherein the cavity waveguide extends laterally alongside the lasing cavity and the passive bus waveguide.Client Docket No.2024-865 B&A Docket No.4059.068PCT1 8. The integrated semiconductor laser of claim 1, wherein the cavity waveguide is terminated by a first linear waveguide reflector at a first end and a second linear waveguide reflector at a second end.

9. The integrated semiconductor laser of claim 1, wherein at least one linear waveguide reflector is a retroreflector.

10. The integrated semiconductor laser of claim 1, wherein at least one linear waveguide reflector is an inversely designed photonic structure.

11. The integrated semiconductor laser of claim 1, further including: a heater located adjacent to the lateral coupling of the of the TCFP to the passive bus waveguide and selectively controlled to adjust the coupling between the TCFP and the passive bus waveguide.

12. The integrated semiconductor laser of claim 1, wherein the cavity waveguide is terminated by first and second linear waveguide reflectors at a first end and by third and fourth linear waveguide reflectors at a second end.

13. The integrated semiconductor laser of claim 12, wherein the first and third linear waveguide reflectors are sampled grating distributed bragg reflectors (SGDBRs) and the second and fourth linear waveguide reflectors are distributed bragg reflectors (DBRs), wherein the second and fourth linear waveguide reflectors are located outside of the first and third linear waveguide reflectors.

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