US-12627112-B2 - Fabry-Perot based multi resonant cavity tunable laser

Abstract

There is provided a laser, and/or a reflector for a laser cavity comprising: a ring resonator structure; and a Fabry-Perot filter connected in cascade to the ring resonator structure by a coupling waveguide. The coupling waveguide is configured to propagate light having a frequency corresponding to any of the resonant frequencies of the ring resonator structure to the Fabry-Perot filter, and the Fabry-Perot filter is configured to select one or more frequencies and return light having a frequency matching any of the selected frequencies to the ring resonator structure via the coupling waveguide.

Inventors

• Sergio PINNA
• Yi Zhang
• Richard Grote

Assignees

• ROCKLEY PHOTONICS LIMITED

Dates

Publication Date

20260512

Application Date

20221220

Claims (10)

1. 1 . A laser comprising: a ring resonator structure; and a Fabry-Perot filter connected in cascade to the ring resonator structure by a coupling waveguide, wherein the coupling waveguide is configured to propagate light having a frequency corresponding to any of the resonant frequencies of the ring resonator structure to the Fabry-Perot filter, and the Fabry-Perot filter is configured to select one or more frequencies and return light having a frequency matching any of the selected frequencies to the ring resonator structure via the coupling waveguide, wherein the ring resonator structure comprises a plurality of ring resonators, each connected to one another in cascade, wherein each pair of adjacent ring resonators is connected by a respective connecting waveguide, wherein one or more of the connecting waveguides is a straight waveguide, wherein the coupling waveguide is a first coupling waveguide, and a first ring resonator amongst the plurality of ring resonators is connected to a gain medium of the laser by a second coupling waveguide, wherein a last ring resonator amongst the plurality of ring resonators is coupled to the first coupling waveguide, and wherein the first ring resonator is coupled to the second coupling waveguide by evanescent coupling.
2. 2 . The laser according to claim 1 , wherein the ring resonator structure together with the Fabry-Perot filter define a reflective end of a laser cavity of the laser.
3. 3 . The laser according to claim 2 , wherein the ring resonator structure together with the Fabry-Perot filter define a rear mirror of the laser cavity.
4. 4 . The laser according to claim 1 , wherein the Fabry-Perot filter is defined by a straight waveguide configured to operate as a Fabry-Perot cavity; and a coupling region of the coupling waveguide, wherein the Fabry-Perot cavity is coupled to the coupling region of the coupling waveguide.
5. 5 . The laser according to claim 4 , wherein the coupling waveguide comprises a back-end high-loss propagation region, wherein the coupling region of the coupling waveguide is located between the ring resonator structure and the back-end high-loss propagation region.
6. 6 . The laser according to claim 1 , wherein the coupling waveguide comprises a front-end high-loss propagation region, wherein the ring resonator structure is coupled to the coupling waveguide at a position along the coupling waveguide between the Fabry-Perot filter and the front-end high-loss propagation region.
7. 7 . The laser according to claim 1 , wherein the second coupling waveguide comprises a high-loss propagation region, wherein the ring resonator is coupled to the second coupling waveguide at a position along the second coupling waveguide between the gain medium and the high-loss propagation region.
8. 8 . The laser according to claim 1 , wherein one or more of the connecting waveguides comprise: a respective high-loss propagation region, wherein each of the ring resonators is coupled to a corresponding one or more of the connecting waveguides at a position along said connecting waveguide distal from the respective high-loss propagation region.
9. 9 . The laser according to claim 8 , wherein one or more of the connecting waveguides having a high-loss propagation region further comprise: a further high-loss propagation region, wherein each of the ring resonators coupled to the respective connecting waveguide are coupled at positions along the respective connecting waveguide between the high-loss propagation regions.
10. 10 . The laser according to claim 1 , wherein the second coupling waveguide comprises a high-loss propagation region, wherein the first ring resonator is coupled to the second coupling waveguide at a position along the second coupling waveguide between the gain medium and the high-loss propagation region.

Description

CROSS-REFERENCE TO RELATED APPLICATION(S) The present application claims priority to and the benefit of U.S. Provisional Application No. 63/292,341, filed Dec. 21, 2021, entitled “FABRY-PEROT BASED MULTI RESONANT CAVITY TUNABLE LASER”, the entire content of which is incorporated herein by reference. FIELD One or more aspects of embodiments according to the present invention relate to widely tunable, ultra-low linewidth lasers, and more particularly to such lasers implementing a Fabry-Perot structure cascaded with a ring resonator. BACKGROUND Widely tunable, ultra-low linewidth lasers (i.e., lasers with less than 10 kHz linewidth) with a high side-mode suppression ratio (for example 30 dB or more) are core components in a multitude of applications including coherent communications, and sensing applications such as LiDAR and spectroscopy amongst others. In the context of coherent communications, low linewidth and high side-mode suppression ratio (SMSR) are valuable because using lasers with such parameters allow the phase locking circuitry in coherent communication systems to be simplified. Meanwhile, in the context of LiDAR, low linewidth and high side-mode suppression ratio are particularly important because these properties facilitate an improved accuracy for LiDAR sensors, thereby enabling coherent architectures even for long range applications. Further, in the context of spectroscopy, low linewidth and high side mode suppression ratio enable a spectrometer to achieve a higher spectral resolution. On the other hand, having a wide tuning range yields improvements in coherent communication systems because it allows a single part to be used as a source for any of the International Telecommunication Union (ITU) channels of a communication system. In the context of spectroscopy, using a laser with a wide tuning range is beneficial because it allows the spectrometer to scan a broad spectral range, for example to analyze multiple gas species in the field of gas spectroscopy. Given the clear importance of both low-linewidth/high-side-mode-suppression-ratio lasers and widely tunable lasers, it is unsurprising that may solutions have been, and are currently being, studied and proposed. To achieve low linewidth, an ultra-high quality factor cavity is required, this corresponds to a laser with a long effective cavity length. In contrast, achieving a large single longitudinal mode tuning range requires elements with a wide free spectral range (FSR), which corresponds to a small optical path length. Many architectures have been proposed, some based on diffraction gratings—e.g., sampled grating distributed Bragg reflectors (SG-DBR), micro-ring lasers, etc. However, these solutions typically aim to improve either linewidth or tunability, often at the expense of the other. In other words, current solutions typically trade off between linewidth and tunability depending on the context in which the laser is deployed. As the skilled person will appreciate, in order to achieve a laser source that simultaneously has a narrow linewidth, a broad single-mode tuning range and high side-mode suppression ratio, the common solutions such as distributed feedback (DFB), distributed Bragg reflection DBR) or SG-DBR architectures are insufficient. Those architectures, such as SG-DBR that are able to provide a broad tuning range also suffer from a broad linewidth—typically on the order of a few MegaHertz. Meanwhile, those architectures, such as DFB lasers, that can provide narrow linewidth usually have a very limited tuning range—on the order of a few nanometres. One approach that attempts to simultaneously achieve low linewidth and wide-range tunability is a high performance laser architecture that creates an external cavity laser configuration based on mechanically tuned diffraction gratings. This architecture, which may be employed in bench-top test equipment can be designed to provide an extremely wide tuning range—in some cases up to over 100 nm around the designated communication band, a narrow linewidth—in the range of tens to hundreds of kilohertz, and an extremely high side-mode suppression ratio—sometimes exceeding 50 dB. However, this solution is physically bulky and different to integrate in some contexts, such as commercial settings. Alternative solutions that may be easier to integrate into existing infrastructure may exploit the Vernier effect between multiple long high quality factor cavity resonator elements. A common resonator element used in the art is a large-radius (e.g., 20 microns or more) micro-ring structure. If a single resonator structure were used, a reduction in the laser linewidth would typically lead to a corresponding reduction of the resonators free spectral range, and consequently to the laser single-mode tuning range. However, if multiple ring resonators with slightly different free spectral ranges are cascaded, the free spectral range of the overall cascaded structure can be drastically increased. An exam