Photonic devices based on anisotropic microring resonators

Abstract

An external cavity laser having a photonic integrated platform including a gain chip and at least one reflector chip coupled to form a resonant cavity is provided. The reflector chip includes a waveguide structure made of an anisotropic material having ordinary and extraordinary refractive indices along respective principal axes. An ordinary microring resonator (MRR) and an extraordinary MRR, each having an oblong shape oriented such that a polarisation direction of polarised light guided therealong extends in majority along the corresponding principal axis, are coupled together in a Vernier configuration to provide wavelength-selective optical feedback. In another aspect, a photonic device including at least one reflector chip comprising an extraordinary MRR oriented such that a guided transverse mode experiences the extraordinary refractive index for more than half of a round trip, and an ordinary MRR oriented at a non-zero angle with respect to the extraordinary MRR is provided.

Description

[0001] PHOTONIC DEVICES BASED ON ANISOTROPIC MICRORING RESONATORS

[0002] TECHNICAL FIELD

[0003] The technical field generally relates to external cavity lasers and other photonic devices using microring resonators in a Vernier configuration.

[0004] BACKGROUND

[0005] Integrated external cavity lasers (ECL) have received considerable interest due to their potential applications in numerous high-performance photonic applications, offering benefits in terms of compactness, integrability, and performance. In particular, integrated ECLs can provide both wide wavelength tunability of over 40 nm and a narrow linewidth below 100 kHz. An integrated ECL typically consists of a laser diode or a gain medium with integrated frequency-selective external feedback circuits to form a Fabry-Perot (FP) cavity or a ring cavity. The term “external cavity” refers to the configuration where at least part of the feedback circuit or cavity, involving frequency-selective elements, mirrors, or reflectors, is placed outside the gain medium or solitary laser diode, either on one or both sides. This contrasts with solitary diode lasers, where the cavity or feedback circuit components are integral parts of the gain medium or main cavity of the laser diode. In addition to reduced size, weight, and cost, the ECLs provide an efficient route towards cointegration of lasers with other highly functional building blocks.

[0006] Various integrated tunable ECL configurations have been demonstrated in the art, differing from each other in aspects of the feedback circuit such as the type of frequency-selective elements, mirrors or reflectors, optical coupling systems, gain media and integration methods. Regardless of the integration techniques, one key part to the ECLs is the design of the external cavity feedback circuit, which is commonly achieved by utilizing the Vernier effect, being realized both with sampled Bragg grating reflectors and more commonly with microring resonators (MRRs), in conjugation with other passive and active components. The design of the frequency selective elements of the feedback circuit directly impacts laser parameters such as the wavelength tuning range, linewidth, side mode suppression ratio (SMSR), wavelength and single-mode lasing stability, wavelength, etc.

[0007] Typically, integrated ECLs use a pair of MRRs made of a same material in the feedback circuit. FIGs. 1A and 1B (PRIOR ART) show the typical configurations of integrated ECLs100 forming Fabry-Perot cavities. Both illustrated ECLs 100 include a gain section 102 provided between a passive cavity reflector 104 and a tunable cavity reflector 106 forming a frequency selective feedback circuit. The tunable cavity reflector 106 includes a pair of MRRs 108 and 110 cascaded in add-drop configuration. The MRRs can be placed in a loop configuration, as in FIG. 1A, or a separate loop mirror 112 can be added to the end of the MRR cascade, as shown in FIG. 1B. Broadband and / or tunable couplers are typically used to form the loop of the structures. The tunable cavity reflector 106 serves as a single wavelength filter for the ECL 100, and a phase shifter 114 positioned between the gain section 102 and tunable cavity reflector 106 is used to tune the laser cavity modes. FIG. 1C (PRIOR ART) shows another example of an ECL configuration in which the Vernier-based frequency feedback circuit is formed using two MRRs 108, 110 in a ring cavity. The MRRs 108, 110 are coupled by two bus waveguides 116a, 116b with a gain region 102 in between.

[0008] In all topologies, the cascaded MRRs 108 and 110 are designed in a Vernier configuration, whereby they have slightly different radii, giving slightly different free spectral ranges (FSRs), to obtain a Vernier effect and therefore shape the Vernier spectrum needed for wavelength tuning. The optical output of the laser, whether as the main output or a monitoring output, can be extracted from various points within the laser structure. These points include the high-reflectivity mirror 104, the ports of the MRRs, and the coupler. For the latter, this is applicable if the coupler is non-symmetric (other than 50% / 50%) or if a tunable coupler is employed.

[0009] As understood by those skilled in the art, the so-called Vernier Effect uses two (or more) resonators at two (or more) different resonant frequencies having FSRs that are slightly different, such that they overlap perfectly only at multiples of their individual FSRs. The FSR of a given MRR for light at a wavelength is given by FSRj = n R-’w^ere FSRi

[0010]

[0011] is the FSR of the ithMRR, Rj is the MRR radius and nggroup index of the MRR waveguide. By convention, the Vernier FSR of a pair of MRRs such as shown in FIGs. 1A, 1B and 1C is determined by:

[0012] ~ FSRTx FSR2A2(1)

[0013]

[0014] v~ |FSR1- FSR2| ~ ng2n|R2- R1|R2~ mRi

[0015] where m and m-1 are coprime integers for any m in a two MRR Vernier filter, m quantifies the increased tuning efficiency of the Vernier MRRs in comparison to a single MRR, i.e.,

[0016]

[0017] FIGs. 1D and 1E (PRIOR ART) show an example of the spectral responses of a dual MRR mirror 106 such as shown in either one of FIGs. 1 A and 1 B, with cross-coupling coefficients of K-L = K2= 0.23 and MRR radii of RT= 32.5 |im and R2= 25 |im. FIG. 1D shows the individual spectra of the MRRs with several-nm FSRs. The synthesized spectral response of the dual-MRR mirror, FIG. 1 E, shows a much broader FSR (FSRV« (m- 1)FSR2« mFSR . This Vernier FSR sets the range for the wavelength tuning of an ECL. ECLs can also utilize the thermo-optic effect for tuning and switching between adjacent and non-adjacent wavelength channels. Typically, metallic heaters are placed above MRR waveguides to tune the individual resonances of the MRRs. The heaters can either be controlled together for continuous wavelength tuning within a single MRR’s FSR, or controlled individually for discrete wavelength tuning, stepping by a single MRR’s FSR.

[0018] Referring to international patent application WO2024254713A1 (MARAM, et al), it has been suggested to use multiple materials to form the MRRs in the feedback circuit of an ECL, to enhance the performance of single-material ECLs. This design leverages the strengths of different material platforms, each with distinct refractive indices and thermooptic coefficients. For example, FIG. 2 (PRIOR ART) illustrates a Si-Si N ECL 200 including a gain section 202 provided between a passive cavity reflector 204 and a tunable cavity reflector 206 forming a frequency selective feedback circuit. The tunable cavity reflector 206 includes a pair of MRRs 208 and 210 cascaded in add-drop configuration, similar to that of FIG. 1A. In this instance, however, each MRR 208 and 210 is positioned on a different material platform. SiN and Si can be fabricated monolithically on the same platform due to their compatibility in deposition processes, thermal stability, and optical properties, allowing seamless integration in standard silicon photonics foundries for scalable and high-performance PICs. In this case, the perimeter of the MRRs can be approximately related as:LSi - -LSiN

[0019] m ngsi

[0020] This leads to the Vernier FSR of:

[0021] A2(4)

[0022]

[0023] ngSiLsingSiNLsiN

[0024] where Lsi, LsiNare the MRR perimeters and ng si,ng siNare the group indices of the Si and SiN waveguides, respectively.

[0025] This multi-material ECL approach, for example using Si and SiN as the materials of the two MRRs, may advantageously integrate the compactness and compatibility of the Si platform with the low propagation loss and high nonlinearity threshold of lower index silicon material (e.g. SiN) platforms. By combining these attributes, the resulting ECL can offer enhanced static performance compared to Si-based ECLs, such as narrower linewidth, wider tunability range, and higher output power. In some instances, the monolithic fabrication of a two-material structure may pose a challenge. As a result, heterogeneous integration techniques, such as wafer bonding or micro-transfer printing, and hybrid approaches are typically employed. While these methods facilitate integration, they may also increase fabrication complexity and cost.

[0026] There remains a need for MRR-based reflector configurations for ECLs that mitigate at least some of the drawbacks of the prior art.

[0027] SUMMARY

[0028] In accordance with one aspect, there is provided an external cavity laser, comprising: a photonic integrated platform comprising a gain chip providing a gain medium and at least one reflector chip coupled to the gain medium to provide a resonant cavity, the at least one reflector chip comprising a waveguide structure made of an anisotropic material, the anisotropic material having an ordinary refractive index along an ordinary axis and an extraordinary refractive index along an extraordinary axis, the waveguide structure comprising;- an ordinary MRR having an oblong shape oriented such that a polarisation direction of polarised light guided therealong extends in majority along the ordinary axis of the anisotropic material; and

[0029] - an extraordinary MRR having an oblong shape oriented such that the polarisation direction of the polarised light guided therealong extends in majority along the extraordinary axis of the anisotropic material, the ordinary MRR and extraordinary MRR coupled together in a Vernier configuration.

[0030] In some implementations, the anisotropic material comprises X-cutthin-film lithium niobate (TFLN).

[0031] In some implementations, the anisotropic material comprises Lithium Tantalate (LiTaO3), Barium Titanate (BTO), or Electro-Optic Polymers.

[0032] In some implementations, the oblong shape of the extraordinary MRR is oriented such that the polarisation direction of the guided TE mode is predominantly aligned with the extraordinary axis of the anisotropic material.

[0033] In some implementations, the oblong shape of the ordinary MRR is oriented such that the polarisation direction of the guided TE mode is predominantly aligned with the ordinary axis of the anisotropic material.

[0034] In some implementations, the oblong shapes of the ordinary and extraordinary MRRs are each defined by a pair of parallel elongated waveguide segments joined by a pair of bent waveguide segments at opposite ends thereof. The elongated waveguide segments of the extraordinary MRR may extend along the ordinary axis, and the elongated waveguide segments of the ordinary MRR may extend along the extraordinary axis. In some variants, the elongated waveguide segments of the ordinary MRR may extend at a non-zero angle with respect to the elongated waveguide segments of the extraordinary MRR.

[0035] In some implementations, the external cavity laser is configured to guide said polarised light in a Transverse Electric (TE) mode.

[0036] In some implementations, the external cavity laser is configured to guide said polarised light in a Transverse Magnetic (TM) mode.In some implementations, the external cavity laser further comprises a tuning mechanism fortuning a spectral resonance of the extraordinary MRR. The tuning mechanism may be a thermo-optic and electro-optic tuning mechanism, and / or may be further used for tuning a spectral resonance of the ordinary MRR.

[0037] In some implementations, the external cavity laser further comprises an in-phase / quadrature (IQ) detection module configured to generate a cavity-feedback error signal for wavelength stabilization, the IQ detection module including a 90-degree optical hybrid coupler optically coupled to two monitoring outputs respectively associated with the ordinary and extraordinary MRRs. The 90-degree optical hybrid coupler may be configured to mix signals from said monitoring outputs to generate four optical outputs in quadrature, the IQ detection module further comprising two balanced photodiode pairs producing respective in-phase (I) and quadrature (Q) electrical signals by differential detection of said four optical outputs.

[0038] In accordance with another aspect, there is provided a photonic device, comprising: a photonic integrated platform comprising at least one reflector chip comprising a waveguide structure made of an anisotropic material, the anisotropic material having an ordinary refractive index along an ordinary axis and an extraordinary refractive index along an extraordinary axis, the waveguide structure comprising;

[0039] - an extraordinary MRR having an oblong shape oriented such that a transverse mode guided around said extraordinary MRR experiences the extraordinary refractive index for greater than 50% of one round trip around the extraordinary MRR; and

[0040] - an ordinary MRR having an oblong shape oriented at a non-zero angle with respect to the oblong shape of the extraordinary MRR.

[0041] In some implementations, the anisotropic material comprises X-cutthin-film lithium niobate (TFLN), Lithium Tantalate (LiTaO3), Barium Titanate (BTO), and / or Electro-Optic Polymers.In some implementations, the oblong shape of the ordinary MRR is oriented such that the transverse mode when guided around said ordinary MRR experiences the ordinary refractive index for greater than 50% of one round trip around the ordinary MRR.

[0042] In some implementations, the oblong shape of the extraordinary MRR is oriented such that the transverse mode guided around said extraordinary MRR experiences the extraordinary refractive index for greater than about 80% of one round trip around the extraordinary MRR.

[0043] In some implementations, the oblong shape of the ordinary MRR is oriented such that the transverse mode guided around said ordinary MRR experiences the ordinary refractive index for greater than about 80% of one round trip around the ordinary MRR.

[0044] In some implementations, the non-zero angle of the oblong shape of the ordinary MRR with respect to the oblong shape of the extraordinary MRR is about 90 degrees.

[0045] In some embodiments, by leveraging the inherent anisotropic behavior of anisotropic materials such as X-cut thin-film lithium niobate (TFLN) crystals, the X and Y axes correspond to the ordinary refractive index, while the Z axis, the optical axis, displays, there is provided a novel design paradigm for external cavity lasers (ECLs) that exploits these material properties to achieve ultra-stable lasering operation, transforming challenges into opportunities. In some variants, the ECL design may be based on a Vernier feedback circuit that incorporates carefully engineered anisotropic MRRs, in which circulating photons inherit the directional properties of the material. The designed anisotropic MRRs feature extended optical paths and are oriented along specific directions on the wafer surface to take full advantage of the corresponding axis's properties. In particular, some embodiments of the disclosed design benefit from the disparity between the thermo-optic coefficients of the ordinary and extraordinary axes to enable thermally stable and robust lasing performance. This new ECL design may potentially offers exceptional capabilities for diverse applications such as telecommunications, sensing, LI DAE, and as a quantum light source.

[0046] Other features and advantages will be better understood upon reading of detailed embodiments with reference to the appended drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0047] FIGs. 1A, 1B and 1C (PRIOR ART) are schematic representation of ECL configurations according to prior art; FIGs. 1 D and 1 E (PRIOR ART) are spectra of the individual spectra of the MRRs (FIG. 1D) and the Vernier reflection spectrum (FIG. 1E).

[0048] FIG. 2 (PRIOR ART) is a schematic representation of an ECL configuration using two MRRs respectively made of Si and SiN.

[0049] FIG. 3A is a schematic representation of an ECL configuration according to one embodiment; FIG. 3B is a side elevation view of a photonic integrated platform embodying the configuration of FIG. 3A; FIG. 3C is a cross-section view of the reflector chip of FIG.

[0050] 3B; FIG. 3D is a representation of the ordinary and extraordinary principal axes system of the platform of FIG. 3B; and FIG. 3E is a cross-sectional view of a reflector chip according to an alternative embodiment.

[0051] FIG. 4A shows the index ellipsoid of X-cut TFLN illustrating the principal refractive indices along the crystal axes; FIG. 4B is a schematic representation of an anisotropic MRR with a maximal optical path (waveguide) tilted at an arbitrary angle 0; FIG. 4C is a schematic representation of a special case of a MRR which is aligned along the Y axis (0 = 0°); and FIG. 4D is a schematic representation of a special case of a MRR which is aligned along the Z axis (9 = 90°).

[0052] FIG. 5A illustrates the structure of the ridge waveguide used for an ECL according to one embodiment; FIG. 5B illustrates the corresponding group index ellipsoid for an X-cut TFLN; and FIG. 5C is a graph of a group indices of the fundamental TE mode in the waveguide with different propagation directions 9.

[0053] FIG. 6A is a graph of the group indices of ordinary and extraordinary waveguides vs. wavelength; FIG. 6B shows the reflection spectra of individual MRRs; and FIG. 6C shows the combined spectrum of the dual-MRR mirror.

[0054] FIG. 7A illustrates the thermo-optic ellipsoid for an X-cut TFLN; and FIG. 7B is a graph of the TOC of a waveguide with different propagation directions 0.FIGs. 8A to 8D compared the wavelength control methodology exploiting the anisotropic nature of an ECL according to prior art (FIGs. 8A and 8B) and according to an embodiment of the present description (FIG. 8C and 8D).

[0055] FIGs. 9A to 9E are schematic representations of ECL configuration according to various embodiments.

[0056] FIG. 10 is a schematic representation of a feedback loop to implement a wavelength control scheme.

[0057] DETAILED DESCRIPTION

[0058] It is to be understood that the phraseology and terminology employed in the present description is not to be construed as limiting and are for descriptive purposes only.

[0059] Furthermore, it is to be understood that the technology can be carried out or practiced in various ways and that it can be implemented in embodiments other than the ones outlined described herein.

[0060] Meanings of technical and scientific terms used herein are to be commonly understood as by one of ordinary skill in the art to which the invention belongs, unless otherwise defined.

[0061] In the following description, similar features in the drawings have been given similar reference numerals. In order not to unduly encumber the figures, some elements may not be indicated on some figures if they were already mentioned in preceding figures. It should also be understood herein that the elements of the drawings are not necessarily drawn to scale and that the emphasis is instead being placed upon clearly illustrating the elements and structures of the present embodiments.

[0062] The terms “a”, “an” and “one” are defined herein to mean “at least one”, that is, these terms do not exclude a plural number of items, unless stated otherwise. Terms such as “substantially”, “generally” and “about”, that modify a value, condition or characteristic of a feature of an exemplary embodiment, should be understood to mean that the value, condition or characteristic is defined within tolerances that are acceptable for the proper operation of this exemplary embodiment for its intended application.Unless stated otherwise, the terms “connected” and “coupled”, and derivatives and variants thereof, refer herein to any structural or functional connection or coupling, either direct or indirect, between two or more elements. For example, the connection or coupling between the elements may be mechanical, optical, electrical, logical, or any combination thereof.

[0063] In the present description, the terms “light” and “optical”, and variants and derivatives thereof, are used to refer to radiation in any appropriate region of the electromagnetic spectrum. The terms “light” and “optical” are therefore not limited to visible light, but can also include, without being limited to, the infrared or ultraviolet regions of the electromagnetic spectrum. Also, the skilled person will appreciate that the definition of the ultraviolet, visible and infrared ranges in terms of spectral ranges, as well as the dividing lines between them, may vary depending on the technical field or the definitions under consideration, and are not meant to limit the scope of applications of the present techniques.

[0064] To provide a more concise description, some of the quantitative expressions given herein may be qualified with the term "about". It is understood that whether the term "about" is used explicitly or not, every quantity given herein is meant to refer to an actual given value, and it is also meant to refer to the approximation to such given value that would reasonably be inferred based on the ordinary skill in the art, including approximations due to the experimental and / or measurement conditions for such given value.

[0065] In the present description, the term “about” means within an acceptable error range for the particular value as determined by one of ordinary skill in the art, which will depend in part on how the value is measured or determined, i.e. the limitations of the measurement system. It is commonly accepted that a 10% precision measure is acceptable and encompasses the term “about”.

[0066] In the present description, when a broad range of numerical values is provided, any possible narrower range within the boundaries of the broader range is also contemplated. For example, if a broad range value of from 0 to 1000 is provided, any narrower range between 0 and 1000 is also contemplated. If a broad range value of from 0 to 1 ismentioned, any narrower range between 0 and 1, i.e. with decimal value, is also contemplated.

[0067] In accordance with one aspect, there are provided photonic devices, such as for example an External Cavity Laser (hereinafter referred to as an ECL), based on microring resonators (MRRs) in a Vernier configuration. Photonic devices according to embodiments described herein may be used in the context of various applications, such as optical sensing, fiber-optic communications, Global Positioning System (GPS), clocks in space applications, fundamental metrology, and the like. In accordance with one aspect, as described further below, photonic devices described herein introduce a new design paradigm that uses a single anisotropic material for the MRRs, exploiting both the ordinary and extraordinary refractive indices to implement anisotropic MRRs within the feedback cavity circuit. In this approach, anisotropic MRRs are designed with extended optical paths and oriented along specific directions on the wafer surface to fully exploit the properties of each axis. For ECL embodiments, this method effectively emulates the behavior of two distinct materials, resulting in a ECL with performance similar to that of a multi-material ECL. In other embodiments, two MRRs may also be configured in non-Vernier arrangements, for example to be used for optical filtering to shape a desired spectral response, such as identical cascaded or parallel rings, directly coupled photonic-molecule pairs, coupled resonator optical waveguides (CROW), ring-loaded MZI-based filters, or the like.

[0068] In some implementations, the photonic device is an ECL which includes a photonic integrated platform providing a resonant cavity. As understood by those skilled in the art, integrated photonics refers to substrate-supported structures that integrates, on a common chip or on a set of chips optically coupled together, at least one component configured to perform an optical, electro- optica I, or opto-electronic function. A photonic integrated platform may include passive, active, nonlinear, or electro-optic components fabricated using lithographic, epitaxial, bonding, or hybrid-integration processes. The photonic integrated platform may include a gain chip and at least one reflector chip. Various non-limitative configurations of the photonic integrated platform are described below.Referring to FIGs. 3A, 3B and 30, there is shown an ECL 20 according to one embodiment. As mentioned, the ECL 20 includes a photonic platform 21, in this example including a gain chip 22 and one or more reflector chips 30. Although the example of these figures is used as a basis for describing the components of an ECL, it will be readily understood that the same considerations and options may be applied to other variants, such as for example described further below.

[0069] The gain chip 22 of the ECL 20 provides a gain medium configured to enable amplification of light. The gain chip 22 may have any structure known in the art. For example, the gain chip may be embodied by a Semiconductor Optical Amplifier (SOA) using a direct bandgap lll-V medium such InP or the like, depending on the wavelength of interest. In some implementations, the gain chip includes a gain waveguide 24 embodying the gain medium. The gain medium may be quantum well-based, quantum dash-based or quantum dotbased. It will be readily understood that in other variants the gain medium may be embodied by any material or structure amplifying light via stimulated emission, the choice of gain medium determining the operational wavelength of the laser. SOAs, such as those based on InP, are for example commonly used in lasers that operate in the 1300-1600 nm wavelength range. A Fabry-Perot diode laser can also be used as the gain medium of the ECL. In other examples, gain media without semiconductors may be used, such as EDFAs, etc. The gain medium may have a bandwidth covering the C-band, L-band, O-band or any other wavelength range providing amplification at the required wavelengths for the lasing of the ECL to occur at the desired output wavelength.

[0070] The ECL 20 further includes a resonant cavity 26 optically coupled to the gain medium 22. As used herein, the term “resonant cavity” refers to an optical structure configured to support and sustain electromagnetic radiation at one or more resonant wavelengths by providing feedback through one or more reflective, refractive, or interferometric elements. A resonant cavity confines light such that it undergoes multiple round-trips, thereby enabling constructive interference at discrete longitudinal modes determined by the optical path length, refractive index distribution, and boundary conditions of the cavity. In some embodiments, the resonant cavity may include a pair of reflectors positioned to define a closed or partially closed optical path. Such a configuration may form a Fabry-Perot cavity, a ring cavity, or any hybrid or compound resonant structure. In the illustrated variant, theresonant cavity has a Fabry-Perot configuration, also understood as a linear cavity defined by a pair of light reflectors, or mirrors, provided at opposite ends of the gain waveguide 24. In some implementations, such as shown in the illustrated variant of FIGs. 3A, 3B and 30, the pair of light reflectors may include a fixed cavity reflector 28 optically coupled to one extremity 23 of the gain waveguide 24, and a tunable cavity reflector 32 optically coupled to an extremity 25 of the gain waveguide 24 opposite the fixed cavity reflector 28. The expression “fixed cavity reflector” will be understood by one a skilled in the art as a reflector that is passive, i.e. not tunable. In some implementations, the fixed cavity reflector 28 may be embodied by a layer of reflective material deposited on a surface at the corresponding extremity 23 of the gain chip 22, embodying a Reflective Semiconductor Optical Amplifier (RSOA) design. It will however be readily understood that the fixed cavity reflector may alternatively be external to the gain chip, and for example includes a separate mirror, a Sagnac loop mirror or the like.

[0071] As mentioned above, the ECL 20 of the illustrated variant includes a reflector chip 30, on which is provided the tunable cavity reflector 32. The reflector chip 30 is external to the gain chip 22, as is inherent to an ECL design. In the present context, the term “reflector chip” refers to a photonic integrated structure configured to provide optical feedback, filtering, wavelength selection, or other cavity-defining functions through one or more integrated waveguides, resonators, couplers, or reflective elements. A reflector chip may include passive, active, or electro-optic components. The reflector chip 30 may be embodied by a structure comprising several layers, for example, but not limited to a SOI structure. In some implementations, as best seen in FIG. 3C, the reflector chip 30 may include a substrate 60, for example made of Si, on which is provided a cladding 62, for example made of SiO2. In typical embodiments, the reflector chip 30 includes at least waveguide structure 33 made of an anisotropic material embedded in the cladding 62, the waveguide structure 33 being engineered to form reflective or resonant elements such as the anisotropic microring resonators described further below. In some embodiments, the reflector chip may additionally support heaters, modulators, phase shifters, mode-converters, tunable couplers or the like integrated above or adjacent the waveguides.

[0072] In some embodiments, the waveguide structure 33 may be provided in the reflector chip 30 to receive, guide and reflect light to form an ECL feedback circuit, hence performingthe reflecting function of the tunable cavity reflector 32. The waveguide structure 33 may typically include one or more waveguide branches 43a, 43b, 43c (...) designed to guide light throughout the tunable cavity reflector 32 and coupled together and to the MRRs (defined below) in a variety of possible configurations, several of which are presented below. Light may be coupled between the gain chip 22 and the reflector chip 30 in any manner known in the art. Other optical components may be integrated in the tunable cavity reflector 32, such as one or more couplers 40, phase shifters 42 or the like.

[0073] The waveguide structure 33 includes two micro-ring resonators (MRRs), referred to herein as the ordinary MRR 34 and the extraordinary MRR 36, made of the anisotropic material. As used herein, the term “microring resonator” or “MRR” refers to a closed-loop optical waveguide structure configured to support circulating optical modes at discrete resonant wavelengths. An MRR confines light through total internal reflection along a closed path such that constructive interference occurs when the optical path length is an integer multiple of the guided wavelength. This resonance condition produces sharp spectral features that may be exploited for filtering, wavelength selection, modulation, or reflection within a photonic device. The MRRs 34 and 36 preferably extend generally parallel to the substrate 60. The MRRs 34 and 36 are positioned within the reflector chip 30 so as to perform the reflecting function of the tunable cavity reflector 32. By way of example, FIGs.

[0074] 3A and 3B shows a configuration where the ordinary MRR 34 and the extraordinary MRR 36 are positioned in a loop mirror configuration. It will be readily understood that the specifications of this configuration is but one manner of implementing the tunable cavity reflector 32 and that in other variants, the different components of the tunable cavity reflector may be arranged differently than illustrated here, depending on design considerations and target specifications. In the illustrated configuration of FIGs. 3A and 3B, the waveguide structure 33 of the tunable cavity reflector 32 includes an input-output waveguide 41 optically coupled to the gain waveguide 24. A phase shifter 42 may be provided to tune the cavity modes. The waveguide structure 33 next includes a coupler 40 optically coupling the input / output waveguide 41 to a first and a second waveguide branches 43a and 43b each optically connected to one of the MRRs 34 and 36. A third waveguide branches 43c connects the ordinary MRR 34 and the extraordinary MRR 36 together. Preferably, the waveguide branches 43 and input / output waveguide 41 are made of the anisotropic material. In other embodiments, however, the waveguide branches,i nput / output waveguide and other waveguiding components other than the MRRs may be made of a different material without departing from the scope of protection.

[0075] In the configuration of FIGs. 3A and 3B, light from the gain chip 22 entering the tunable cavity reflector 32 travels from the input / output waveguide 41 to the coupler 40 at which it is then split into two light portions, respectively travelling clockwise and counterclockwise through the waveguide structure 33. The clockwise light portion travels sequentially in the first waveguide branch 43a and around the ordinary MRR 34, through the third waveguide 43c, around the extraordinary MRR 36 and back to the coupler 40 through the second waveguide branch 43b. The counterclockwise light portion travels from the second waveguide branch 43b though the same components in the reverse order. Both the clockwise and counterclockwise light portions are combined by the coupler 40 and finally carried out of the tunable cavity reflector 32 through input / output waveguide 41. It will be noted that in MRRs, the most common coupling mechanism to have light coupled between the waveguide branches and the MRRs is via codirectional evanescent coupling, where the optical fields of adjacent waveguide branches and MRRs overlap to facilitate efficient energy transfer between them through directional coupling. Other options include multimode interference (MMI) couplers, etc.

[0076] As will be readily understood by one skilled in the art, an optical output 45 of the ECL whether as a main output or a monitoring output, may be provided from various points within the laser structure to extract light therefrom. Referring to FIG. 3A, by way of example, the optical output 45 may be provided at the coupler 40 if the coupler is non-symmetric (other than 50% / 50%) or if a tunable coupler is employed. In other variants, the optical output 45 may be provided at the fixed reflector 28, at any port of the MRRs 34, 36 or at other connectable components of the ECL.

[0077] As will be readily understood by those skilled in the art, in Vernier reflectors made of materials having isotropic properties in the light propagation plane, such as Si, SiN, InP, or Z-cut TFLN Vernier reflectors, the refractive index, and consequently, the group index of the MRR waveguides remains the same in all directions. Therefore, the FSR can be calculated using known formula, and appropriate radii can be determined for specific ECL designs. However, if the MRR waveguides are made of an anisotropic material in the light propagation plane, such as in X-cut Thin-Film Lithium Niobate (TFLN), light experiencesdifferent refractive indices and group indices when propagating in different directions on the wafer, affecting the FSR.

[0078] Referring to FIG. 3D, using the typical convention whereby the waveguide structure extends in the Y-Z plane, the anisotropic material by definition has an ordinary refractive index n0along an ordinary axis Y and an extraordinary refractive index nealong an extraordinary axis Z. The ordinary and extraordinary axes of an anisotropic material are typically referred to in the art as “principal” axes. As know in the art, in anisotropic materials, the polarisation direction of a light beam determines the refractive index “seen” by the light. For example, a transverse-electric (TE) or quasi-TE light beam, hereafter referred to simply as TE for simplicity, i.e. light propagating in a TE mode, is defined as an optical mode in which the electric field vector is predominantly oriented perpendicular to the direction of propagation. In other words, if the light propagates along a given axis, the electric field oscillates mainly in a direction transverse to that axis. In planar photonic platforms, TE polarisation is typically understood as an electric field lying within the plane of the wafer. Hence, such a TE light beam would be polarised in the Z direction when propagating along the Y direction in the waveguiding structure of FIGs. 3A and 3B. It would therefore see the extraordinary refractive index ne. Conversely, if the same TE light beam propagates along the Z direction, its polarisation is along the Y axis, and it would see the ordinary index n0.

[0079] In some implementations, the ordinary and extraordinary MRR 34 and 36 both have oblong shapes. As used herein, the term “oblong shape” refers to a closed, elongated geometric contour in which one dimension is longer than the other, such that the resulting shape is stretched along a longitudinal axis. In the context of microring resonators, an oblong shape may comprise two substantially parallel straight or curved or bent waveguide segments joined by two curved segments, such as semicircular, Euler, or Bezier bends, to form a continuous closed path, or a rectangular shape with rounded corners. An oblong microring resonator therefore includes a maximal optical path aligned with the longitudinal direction of the shape. It will be noted that the oblong shape is not limited to a specific curvature profile, aspect ratio, or bend geometry.

[0080] In some implementations, the oblong shape of the extraordinary MRR 36 is oriented such that the polarisation direction of the polarised light guided therealong extends in majorityalong the extraordinary axis Z of the anisotropic material. In some variants, the oblong shape of the ordinary MRR 34 is oriented such that a polarisation direction of polarised light guided therealong extends in majority along the ordinary axis Y of the anisotropic material. As used herein, the expression “in majority” refers to a condition in which more than half of the total optical-path length of a MRR is oriented in such a way that the electric-field vector of a guided TE mode is aligned with a specific principal axis of the anisotropic material. This alignment causes the guided mode to experience the refractive index associated with that principal axis for greater than 50% of one round trip around the resonator. The ordinary MRR 34 is said to guide light “in majority” along the ordinary axis when the TE-mode electric field is aligned with the ordinary axis over more than half of the resonator’s optical path. The extraordinary MRR 36 is said to guide light “in majority” along the extraordinary axis when the TE-mode electric field is aligned with the extraordinary axis over more than half of the resonator’s optical path.

[0081] In some variants, the oblong shape of the extraordinary MRR 36 is oriented such that the polarisation direction of the guided TE mode is predominantly aligned with the extraordinary axis Z of the anisotropic material. As used herein, “predominantly” means that about 80% or more of the total optical-path length of the extraordinary MRR corresponds to propagation directions for which the TE-mode electric-field vector is aligned with the extraordinary axis, such that the guided mode experiences the extraordinary refractive index for at least 80% of one round trip around the resonator. Similarly, in some variants the oblong shape of the ordinary MRR 34 is oriented such that the polarisation direction of the guided TE mode is predominantly aligned with the ordinary axis Y of the anisotropic material.

[0082] The chosen waveguide mode travelling in the MRRs 34 and 36 will experience alternating refractive indices, transitioning between the ordinary (no) and the extraordinary (ne) values, depending on the relative orientation of the light propagation axis with respect to the ordinary principal axis Y and extraordinary principal axis Z. Indeed, the extraordinary axis lies along the Z-direction. Thus, light polarised with its electric field along the Z axis experiences the extraordinary refractive index (ne), while light with polarisation perpendicular to the Z axis, hence along the Y axis, experiences the ordinary refractive index (n0). Therefore, as a result of the different orientations of the ordinary MRR 34 and extraordinary MRR 36, light travelling in the waveguide structure 33 will in majorityexperience the ordinary reflective index n0when travelling around the ordinary MRR 34 and conversely, will in majority experience the extraordinary reflective index newhen travelling around the extraordinary MRR 36.

[0083] In the example of FIGs. 3A and 3B, the oblong shapes of ordinary MRR 34 and extraordinary MRR 36 are oriented at a right angle to each other. In other variants, the oblong shape of the ordinary MRR is oriented at a non-zero angle with respect to the oblong shape of the extraordinary MRR. As will readily be understood by one skilled in the art, when a MRR has a maximal optical path at an arbitrary angle 6 with respect to one of the principal axes of the anisotropic material, light guided in the MRR can experience a superposition of the ordinary and extraordinary indices, leading to unique refractive index, dispersion and thermo-optic responses.

[0084] In some implementations, the oblong shapes of the ordinary and extraordinary MRRs are defined by a pair of elongated waveguide segments 37a, 37b, preferably straight and parallel to each other, joined by a pair waveguide bent segments 38a, 38b at their opposite ends. By way of example, the bend segments 38a, 38b may be half circles. In other variants, different bend shapes may be used, such as Euler, Bezier, etc. In the illustrated embodiment of FIGs. 3A and 3B, the elongated waveguide segments 37a, 37b of the ordinary MRR 34 extend along the Z axis, such that the TE polarised light propagating in these elongated waveguide segments is polarised along the Y axis, and therefore experiences the ordinary refractive index n0. The elongated waveguide segments 37a, 37b of the extraordinary MRR 36 extend along the Y axis, such that the TE polarised light propagating in these elongated waveguide segments is polarised along the Z axis, and therefore experiences the extraordinary refractive index ne.

[0085] The oblong shape illustrated in the accompanying figures for the MRRs 34 and 36 is sometimes referred to in the art as a racetrack shape and the MRRs having such as shape as racetrack MRRs. One skilled in the art will readily understand that other configurations defining an oblong shape as defined above may be used. In some examples, the extraordinary MRR 36 may have a different oblong shape than the ordinary MRR 34 without departing from the scope of protection.By way of example, in the configuration of FIGs. 3A and 3B, for a chosen fundamental TE mode, one MRR 34 is oriented such that its optical path is maximized along the ordinary refractive index direction (ordinary MRR), while the other MRR 36 is oriented to take full advantage of the extraordinary refractive index direction (extraordinary MRR). FIG. 3B provides a 3D perspective of the setup shown in FIG. 3A, featuring an X-cut TFLN wafer that is hybrid-integrated with an RSOA as the gain chip. In such a configuration, the two MRRs not only exhibit different effective index characteristics but also all the other attributes. For example, the thermo-optical coefficient (TOC) and electro-optic coefficient (EOC) of waveguides differ between the ordinary and extraordinary directions, potentially providing additional degrees of freedom for the ECL design. The predominant EOC corresponds to the largest nonlinear tensor component lies along the extraordinary optical axis Z within the plane of the anisotropic material. In addition, the TOC for the ordinary index is nearly one-sixteenth that of the extraordinary index (approximately 4 x 10-5K-1for neand 2.5 x 10-6K'1for n0). Consequently, the ordinary MRR 34 (i.e. , the one with the longest optical path along which light experiences the ordinary refractive index) benefits from reduced sensitivity to temperature fluctuations, which can otherwise cause drift in the laser wavelength. However, this thermal stability advantage may come at the expense of increased electrical power consumption for tuning, in which case the extraordinary MRR 36 would perform better. By combining these attributes, the resulting ECL can offer enhanced performance compared to isotropic MRR-based ECLs, the same advantages promised also by multi-material ECLs.

[0086] In some implementations, the orientation of the oblong shape of the extraordinary MRR 36 is selected to achieve a large target TOC consistent with the desired ECL parameters. The TOC increases as a larger fraction of the round-trip optical path places the TE-mode electric field along the extraordinary axis Z, which occurs when the local propagation direction is closer to Y (since the TE field is transverse to propagation). Accordingly, the maximum TOC is obtained when the longitudinal axis of the oblong MRR is perpendicular to the extraordinary axis (i.e., parallel to Y).

[0087] In other embodiments, the extraordinary MRR may be tilted by an in-plane angle 0 with respect to the Y axis, 0° < 0 < 90°, so that the longitudinal direction includes components along both Y and Z. For smaller 0 (closer to 0°, i.e., longitudinal direction closer to Y), a larger TOC is achieved; for larger 0 (closer to 90°, i.e., longitudinal direction closer to Z),the TOC is reduced. In all such cases, the extraordinary MRR remains extraordinary so long as the fraction of the optical path that yields a TE-field alignment along Z is greater than that along Y.

[0088] In some implementations, the oblong shape of the ordinary MRR 34 is oriented to minimize its thermo-optic coefficient (TOC) in view of the desired ECL parameters. The lowest TOC is obtained when the longitudinal axis of the resonator is parallel to Z (i.e., perpendicular to the ordinary axis Y), because TE-mode propagation along Z places the electric-field vector along Y, where the ordinary index exhibits a much smaller TOC. In other embodiments, the ordinary MRR may be tilted by an in-plane angle (p with respect to the Z axis, 0° < cp < 90°, (where cp=90- 0) so that its longitudinal direction includes components along both Z and Y. For larger cp (closer to 90°, i.e., longitudinal direction closer to Y), a larger fraction of the round-trip optical path places the TE field along Z, and the effective TOC increases; for smaller cp (closer to 0°, i.e., longitudinal direction closer to Z), the effective TOC decreases. The ordinary MRR remains “ ordinary”, so long as more than half of its optical-path length yields a TE-field alignment along Y.

[0089] As will be readily understood by one skilled in the art, the orientation of the ordinary and extraordinary MRRs has a threshold to which their ordinary and extraordinary component is still dominates (are in majority); if that threshold is passed, an ordinary MRR is converted to an extraordinary MRR and vice versa.

[0090] The ordinary MRR 34 and extraordinary MRR 36 are coupled together in a Vernier configuration. Since an anisotropic material wafer with different refractive indices in different directions is used, the orientation in which the optical path of each MRR is maximized is a consideration in determining the Vernier condition. In some implementations, slightly different FSRs may be achieved, and the Vernier cavity satisfied for a specific wavelength tunability range, by providing the two M RRs with slightly different perimeters, proportionally to the ratio of their corresponding group indices. Nevertheless, it should be noted that the calculation of the waveguide group indices in anisotropic MRRs is nontrivial compared to that in isotropic materials. For an anisotropic MRR, the optical path encounters varying refractive indices due to the bends and curves and different orientations, so the effective group index is obtained by integrating the local group indices over the entire perimeter of the corresponding MRR. Assuming the perimeter of the ithMRR is Si, we define s as the length variable running from 0 to Si. At each point along the path, we define the local group index as a function of orientation angle 0(s), that is

[0091]

[0092] ngi(0(s))- Then, the effective group index, or the path-averaged group index, of ith MRR is defined by:

[0093] _ ;oSzVi(e(s)) c / s (5)7VS-i c

[0094]

[0095] Indeed, the effective group index is obtained by integrating the direction-dependent group index along the path’s length s, with ds being the path differential along the waveguide. Therefore, the value of Ng tdepends both on the shape of the path and on the local orientation at each point along the path, that is, the local tangent of the waveguide route. If the MRR path is given in terms of a parametric equation, r(Z) = (y(Z),z(Z)), where I is a parameter of the interval I e [Zo, L , using the chain rule, the effective group index of ith MRR becomes

[0096]

[0097] Going back to the general definition, therefore, for the ECL design, the integrated group- index path lengths of the anisotropic MRRs can be approximately related as

[0098] fS1r > (m — 1) CS2, . (7) I ng-i\9(s)) ds « - I ng2{9 (s)J ds J

[0099]

[0100] omJo

[0101] This leads to the Vernier FSR for dual-MRR mirror of

[0102] FSRi x FSR, A2(8) FSR ~ ~ > IFS^ - FS^I | JoS1ng^(eCs)) ds - ng 2(0(s)) ds |

[0103]

[0104] where FSRi is the FSR of the ith MRR as FSRi = ^2 / z, SLis the perimeter

[0105]

[0106] / ^ng_i(9^ ds’

[0107] of the ith anisotropic MRR, and ng i(9(s')) is group index function of the waveguide of the ith MRR. For frequency tuning resolution purposes, we may choose one MRR with a much finer FSR than the other MRR, in that case (7) could be modified tong_i(e(s)) ds « — — - ng 2(9(s)) ds

[0108] J

[0109]

[0110] o / v x m jQ

[0111] where N (=1,2, 3,...) is the perimeter reduction factor.

[0112] In an isotropic platform, such as Si, SiN, or Z-cut TFLN, since the group index is irrelevant to the orientation angle 9 (s), the integral in (5) is simply reduced to constant ng. Therefore, the condition (7) would be reduced to Eq. (1), which is the MRR design condition for single-material ECLs. This trivial MRR design is no longer applicable here.

[0113] On an in-plane anisotropic platform like X-cut TFLN, the refractive index and group index of a waveguide strongly depend on the in-plane orientation, denoted 9. Referring to FIG.

[0114] 4A, the refractive index ellipsoid has principal axes along Y (ordinary) and Z (extraordinary). When a MRR has its maximal optical path at an arbitrary angle 9 (FIG.

[0115] 4B), the TE mode experiences a weighted combination of the ordinary and extraordinary indices, leading to distinct refractive index, dispersion and thermo-optic responses. FIGs.

[0116] 40 and 4D illustrate the two special cases in which the MRR is aligned along Y {9 = 0°) or along z 9 = 90°), respectively. In the 9 = 0° racetrack MRR (FIG. 4C), the TE field is predominantly along Z, so the mode sees the extraordinary index neand exhibits a higher TOC. This is therefore an extraordinary MRR. Conversely, in the 6 = 90° racetrack MRR (FIG. 4D), the TE field is primarily along Y, so the mode sees the ordinary index n0with a lower TOC; there is therefore an ordinary MRR.

[0117] One should note that TFLN is an example of an anisotropic material used in this embodiment; other anisotropic materials, such as Lithium Tantalate (LiTaO3), Barium Titanate (BaTiO3 or BTO), Electro-Optic Polymers, or, in suitable crystal cuts or film orientations, other uniaxial materials such as aluminum nitride (AIN), aluminum scandium nitride (AIScN), aluminum gallium nitride (AIGaN), and silicon carbide (e.g., 4H-SiC), or the like, could also be employed.

[0118] To demonstrate how to use Eqs. (5) - (8), the core design equations for the anisotropic MRR-based ECL, the anisotropic racetrack MRRs in FIGs. 4C and 4D were used to build an example ECL with the configuration shown in FIG. 3A. A C-band ECL was assumed with waveguide dimensions given in FIG. 5A. The waveguide cross-section design ischosen the same for both MRRs, nevertheless, in other implementation different design and dimensions could be used for both MRRs. FIG. 5B shows the corresponding group index ellipsoid for an X-cut TFLN. The racetrack MRRs have two straight sections (Szfor the ordinary MRR and SYfor the extraordinary MRR) and two half-circles with radii R.

[0119] For the ordinary racetrack MRR 34, then, we have

[0120] rnI ng l(0(s)) ds = 2 ng oSz+ 2 R ng l(0) d0. (10) J

[0121]

[0122] o ~ ” Jo

[0123] Similarly for the extraordinary MRR 36 we have

[0124] [S2f77 / ! Ingj(@(s)) ds = 2 ng_eSY+ 2 R I ng 2(0) d0 (11) J

[0125]

[0126] o J-TT / 2

[0127] where ng £(0) is computed using a standard Mode-Solver versus angle for the corresponding refractive index, which is shown in FIG. 50, which suggests that the changes in neffand ngexhibit a trigonometric behavior, and, in particular, ngcan be approximated by fitting the data as follows

[0128] nfl(0) = ng_ecos20 + ng_0sin20. (12)

[0129] Replacing Eq. 12 into Eqs. (10), we find

[0130] ngi(0(s)) ds = 2 ng 0Sz+ nR (ng e+ ng 0) (13)

[0131]

[0132] Jo

[0133] Similarly for the extraordinary MRR2, we find

[0134] cs2

[0135] ng 20 s)) ds = 2 ng eSY+ nR (ng e+ ng o) (14) Jo

[0136] One can deduce that contribution from the bend regions is equal in both MRRs. Putting together in Eq. (7), we find

[0137] ’ ~ (m~!)ng_e $ 1 (ng e+ng o) (15)

[0138]

[0139] Z m ng_oY 2m ng_o

[0140] The Vernier FSR for this dual-MRR mirror then becomes

[0141] FSRV«n1

[0142]

[0143] 2 | n g o SZrig gSy]FIG. 6A shows the group indices of ordinary and extraordinary waveguides with respect to wavelength, calculated using a standard Mode-Solver. FIGs. 6B and 60 show the simulated transmission spectra of the MRRs and the resultant Vernier mirror. Choosing R = 60 n and SY= lOO n, then, for m = 28, using Eq. (15) we find SY= 86.8iim, which yields a tuning range of approximately 50 nm.

[0144] Because the refractive index in anisotropic materials is tensorial, the TOC is also inherently direction-dependent, varying with the propagation orientation. In particular, TFLN exhibits a significant discrepancy in its TOCs along its principle axes: the extraordinary index changes at approximately T0Ce=

[0145]

[0146] « 4 x 10-5K-1, while the

[0147]

[0148] ordinary index varies much more slowly at around TOC0=

[0149]

[0150] « 2.5 x 10-6K-1. This

[0151]

[0152] means that, depending on the light’s propagation direction, the effective thermal response of the waveguide can differ substantially. In a waveguide where the electric field is not aligned purely along one of the principal axes, the effective TOC then becomes a weighted combination of the extraordinary and ordinary responses. By analogy with the index ellipsoid, one may define a “thermo-optic ellipsoid” to represent how the TOCs vary with direction in the anisotropic material, as shown in FIG. 7A. Mathematically, when the waveguide propagation is at an angle 6 relative to the Y axis (as depicted in FIG. 7B for 6 ranging from 0° to 90°), the effective TOC can be approximated by a trigonometric expression as

[0153] du

[0154] TOC(ff) = — (0) = TOCecos29 + TOC0sin29 I-1]. (17) dT

[0155] This relation accounts for the contributions from both indices, capturing that the overall thermal sensitivity of a MRR based on its waveguide’s propagation orientation.

[0156] Since the chosen waveguide mode in the MRR experiences different TOC as it rotates between the principal axes, the local thermo-optic coefficient becomes a function of the propagation direction, 0(s), at each point s along the waveguide, that is T0C(9(s)). Therefore, for this anisotropic MRR, we define an effective TOC as 1 rsT0Ceff= - TOCOS')) ds

[0157]

[0158] (18)

[0159] 5 Jowhere S' is the MRR’s perimeter. For example, for the ECL design example above, we can derive

[0160] > 2 TOC0Sz+ nR(T0Ce+ TOC0)

[0161] 1 ULeff_MRRi- 2 Sz+ 2nR (19) and

[0162] > 2T0CeSY+ nR(T0Ce+ TOC0)

[0163] TOCeff _MRR2= 2 Sv+ 2nR (20)

[0164] Therefore, for the above example, we have TOCeff MRR1= 1.53 x 10-5K-1and TOCeffMRR2= 2.77 x 10-5K-1. Based on design requirements, the difference in effective TOCs TOCeff) between two MRRs can be engineered by tuning the shape and orientation angle 0 of MRRs, thereby achieving values within the range defined by TOCeand TOC0.

[0165] Referring back to FIGs. 3A to 3D, the ECL 20 may further include a tuning mechanism for tuning the spectral resonance of the extraordinary MRR 36, or of both MRRs. In some implementations, the thermo-optic effect is used for tuning and switching between adjacent and non-adjacent wavelength channels. Using the thermo-optic effect in conjunction with the extraordinary MRR 36 takes full advantage of the higher TOC along the extraordinary axis Z. The tuning mechanism may for example include heaters 50 placed above the corresponding MRR 34, 36, to tune their individual resonances. As best seen on FIG. 3C, in the illustrated embodiment each heater 50 includes a metallic heat resistor 52 through which a current is passed to create heat in the resistor, whose temperature modifies the index of semiconductor materials. Metals can be used as heaters which should have a higher resistivity than the Cu and Al which typically is used for routing / pads and metallization. TiN, tungsten (W) lines above the waveguide are typically used. In the illustrated implementation the metallic heat resistor 52 is for example embodied by a TiN arch extending within the cladding 62 over a section of the associated MRR having an arched shape matching the portion of the MRR ring underneath. Metal electrodes 54 are provided over the cladding 62 of the reflector chip and are connected to extremities of the TiN arch 52. Metal vias 56 may provide the electrical connection between the electrodes 54 and the TiN arches 52. The heaters can either be controlled together for continuous wavelength tuning within a single MRR’s FSR, or controlled individually for discrete wavelength tuning, stepping by a single MRR’s FSR.Referring to FIG. 3E, in some implementations, on TFLN-on-Si or BaTiO3-on-Si platforms, the heaters 50 may include N or P -doped silicon elements to change the refractive index of the MRR waveguide. Having a shorter distance between the heater and waveguide may increase the temperature of the MRR waveguide and subsequently change its refractive index while requiring a lower electrical power than metal electrodes. Similarly to metallic heaters, Si-based heater can be biased to create heat and cause a refractive index change in the close-by waveguide through the thermo-optics effect. The Si-doped heater may be positioned next to the MRR waveguide or in implementations, the MRR waveguide itself may act as the resistor. Other schemes using the thermo-optic effect to change the index of the MRR waveguide may also be used.

[0166] In some embodiments, the disparity between the effective TOCs of the anisotropic MRRs may be exploited to achieve thermally stable lasing performance, as detailed below. In the case of an isotropic material MRR-based ECL or an anisotropic ECL in which both MRRs have the same shape and orientation, when a thermal disturbance happens, the response of both MRRs shift by nearly the same amount and in the same direction, as shown in FIGs. 8A and 8B (PRIOR ART). Hence, without using thermal sensors, detecting, measuring and correcting a wavelength shift may not be possible using only PDs. The first row of plots in FIG. 8A shows the individual MRRs’ transfer functions for a resonance frequency at a set temperature Toand +dT temperature variations, while the second row shows the Vernier transfer function for the same temperature changes.

[0167] By contrast, one significant advantage of anisotropic MRR-based ECL structures is that due to the different effective TOCs, in case of a temperature disturbance, the MRR filter response shift of, for example, the extraordinary MRR is different, i.e. , larger, from that of the ordinary MRR, as shown in FIGs. 8C and 8D. This effect can be exploited to detect the power drop in the laser output using monitoring PDs and adjust the wavelength accordingly. Also, when the cavity mode is selected at a detuned frequency for example when the goal is achieving minimum linewidth, then a single PD in this case may help to detect the direction and amount of the shift to adjust the wavelength accordingly.

[0168] Additionally, by leveraging the disparity in TOCs of both MRRs, the direction of the shift can also be detected using a 90-degree hybrid coupler to measure the phase difference between the ECL’s outputs, as the result is shown in FIG. 8D where, for example, we usedthrough ports of the MRRs. This may not be feasible using single-material ECLs, shown in FIG. 8B. This unique property enables the proposed anisotropic MRR-based ECLs to incorporate an inherent feedback error signal that continuously realigns the wavelength, effectively performing a wavelength locking operation. This not only enhances the stability of the lasing process but also eliminates the need for external wavelength lockers. Moreover, in some implementations, by integrating an internal wavelength locking mechanism the system design may be considerably simplified, reducing both complexity and cost. The built-in error signal facilitates rapid and precise adjustments, ensuring optimal performance even under varying environmental conditions and operational loads. Consequently, this advanced design improves overall reliability and efficiency, making the ECLs a robust solution for high-precision applications.

[0169] FIG. 10 is a schematic representation of an ECL 20 having ordinary and extraordinary MRRs 34 and 36 in a Vernier configuration, shown by way of example in the configuration of FIG. 3A, further including an in-phase / quadrature (IQ) detection module 72 configured to generate one or more cavity-feedback error signals 74 for wavelength stabilization. The IQ detection module 72 includes a 90-degree optical hybrid coupler 76 optically coupled to two monitoring outputs 78a and 78b respectively associated with the ordinary and extraordinary MRRs 34 and 36, such that the 2x4, 90-degree optical hybrid coupler 76 mixes the monitoring output signals 78a and 78b to generate four optical outputs 80a, 80b, 80c and 80d having relative phase shifts of 0°, 90°, 180°, and 270°. These four outputs 80a, 80b, 80c and 80d are directed to two balanced photodiode pairs 82a, 82b and 84a, 84b: one pair 82a, 82b receives the 0° and 180° outputs and produces the in-phase (I) electrical signal by differential detection, while the other pair 84a, 84b receives the 90° and 270° outputs and produces the quadrature (Q) electrical signal. The I and Q electrical signals are then amplified and provided to processing circuitry 86. In some implementations, the processing circuitry 86 computes an error quantity from the relative phase and amplitude of the l / Q signals, thereby indicating both the magnitude and direction of a thermally induced spectral shift between the spectral responses of the ordinary and extraordinary MRR 34 and 36. This enables direct discrimination of the direction of drift, even for small perturbations, and facilitates closed-loop correction of the lasing wavelength without requiring an external wavelength locker.Referring to FIGs. 9A to 9E, different examples of configurations of ECLs based on anisotropic MRRs are shown.

[0170] FIG. 9A shows a variant of the ECL configuration of FIG. 3A in which a separate loop mirror 48 defines a feedback circuit closing the loop formed by the tunable cavity resonator 32. In this configuration, the waveguide structure 33 of the tunable cavity reflector 32 includes a first waveguide branch 43a optically coupled to the gain waveguide 24. No coupler is required at the entrance of the tunable cavity reflector 32 in this variant. A phase shifter 42 is used to tune the cavity modes. The first waveguide branch 43a is further optically to the ordinary MRR 34. A second waveguide branch 43b optically connects the ordinary and extraordinary MRRs 34 and 36. The extraordinary MRR 36 is further optically coupled to the loop mirror 48 through a third waveguide branch 43c and an optical coupler 49.

[0171] In this configuration, light entering the tunable cavity reflector 32 first travels into the first waveguide branch 43a, from which it is directly coupled into the ordinary MRR 34. It can then travel from the ordinary MRR 34 to the second layer waveguide branch 43b, from which it can enter the extraordinary MRR 36. From the extraordinary MRR 36 light travels to the third waveguide branch 43c into the loop mirror 48 through the coupler 49. Light is then reflected back through the same path in reverse, until it exits the tunable cavity reflector 32 through the first waveguide branch 43a. By way of example, the optical output 45 may be provided at the optical coupler 49.

[0172] It will be noted that in the illustrated example of FIG. 9A the fixed cavity mirror 28 is also embodied by a Sagnac loop mirror, although in other variants a RSOA design may also be used.

[0173] Referring back to FIGs. 3A and 9A, there is shown two typical basic configurations of anisotropic material-based ECLs in which, on the one hand, both MRRs are placed inside a loop mirror (see FIG. 3A), and on the other hand, a separate loop mirror added to the end of the cascaded MRRs (see Fig. 9A). FIG. 9B shows a third basic configuration which brings together the configurations and advantages of both, especially when using the anisotropic material MRR-based ECL concept. In this configuration, the waveguide structure 33 is similar to that of FIG. 9A, in that light from the gain chip 22 first encountersa first waveguide branch 43a, is coupler into the extraordinary MRR 36 and exits through a second waveguide branch 43b. The second waveguide branch 43b leads to a coupler 49. Here, the ordinary MRR 34 is provided after the coupler 49 and forms part of the loop mirror 48. Light reaching the coupler 49 is therefore split into clockwise and counterclockwise light portions, respectively coupled into a third and fourth waveguide branch 43c, 43d, both optical coupled to the ordinary MRR 34. In effect, one MRR is placed outside of the loop mirror, and the other is added inside the loop mirror.

[0174] Referring to FIG. 90, there is shown a more generalized case for two dual MRR-ECLs where anisotropic MRRs are oriented at angle

[0175]

[0176] and 02, respectively, allowing for an optimal superposition of the properties from both the ordinary and extraordinary axes of the anisotropic material. This flexible design strategy enables fine-tuning of the ECL’s performance by balancing the contributions of both refractive index systems. The laser’s optical output, whether as the main output or a monitoring output, can be extracted from various points within the laser structure, such as the fixed cavity reflector 28 on the RSOA, the ports of the MRRs 34 or 36, and the tunable coupler 49. As will be readily understood by one skilled in the art, even though to generalized case of FIG. 90 is applied to a loop mirror configuration similar to that of FIG. 9A, similar generalizations of the closed loop configuration of FIG. 3A or the combined configuration of FIG. 9B may also be envisioned. For the purpose of enhancing performance, such as achieving wider wavelength tunability, multiple anisotropic MRRs can be incorporated within the feedback circuit.

[0177] In some implementations, as for example shown in the embodiments of FIGs. 9A to 9E, the phase shifter 42 may be a thermo-optic phase shifter provided along the Y propagation axis such that TE light experiences the extraordinary index newhen propagating therethrough, which can advantageously reduce the electrical power required for cavity mode wavelength tuning. Moreover, as the largest EOG is associated with the extraordinary axis, in some variants an electro-optic phase modulator 46 may be implemented in either the same phase shifter 42 or another waveguide segment predominantly aligned with the Y axis for fast frequency modulation, as shown in FIGs. 9D and 9E. Additionally, in some embodiments, an electro-optic modulator 47 may be implemented in the extraordinary MRR 36 (see FIGs. 9D and 9E), in the ordinary MRR 34 (see FIG. 9E) or in a tunable coupler arrangement 70 (see FIG. 9E) to enable fast amplitude modulation.Additionally, in other variants, an intracavity periodically-poled lithium niobate (PPLN) waveguide section 51 may be incorporated in one of the MRRs (shown in the ordinary MRR in FIG. 9D and in the extraordinary MRR in FIG. 9E by way of example) to enable second harmonic generation (SHG).

[0178] Embodiments of the anisotropic MRR-based designs for ECLs described herein may offer a range of applications across diverse fields, supported by its potential combination of performance characteristics. In communication, its lower linewidth, larger output power and short-and long-term stability could position it as a strong candidate for integrated highspeed Intensity-Modulation-Direct Detection (IM-DD) and optical coherent transceivers, potentially operating in various spectral ranges, including the C, L, and O bands, the visible, and the near IR. For LiDAR, the ECL's potential high power and rapid tuning could contribute to accurate, high-resolution distance measurements, a promising prospect for autonomous vehicles and 3D mapping. In quantum optics, the SHG using the embedded PPLN structures could be exploited to generate a tunable quantum light source that can enhance quantum information processing systems, alternatively, the SHG response could be used as a nonlinear frequency discriminator to generate a feedback signal for laser stabilization. The proposed ECL may cater to the unique needs of both polarisations, possibly enabling precise manipulation of quantum states of light. Finally, to seamlessly realize such ECLs in practice, we can leverage existing fabrication processes offered by various foundries, showing the proposed approach could provide a practical and accessible ECL, potentially advancing quantum computing, communication, and sensing technologies.

[0179] It will be readily understood that while the examples shown herein have two MRRs, an ordinary one and an extraordinary one, additional MRRs may be provided in the anisotropic waveguide structure without departing from the scope of protection. For example, the photonic device may include two extraordinary MRRs and one ordinary MRR, or any other combinations, in as much as both the ordinary and extraordinary combinations are provided.

[0180] While the designs disclosed herein leverage the TE mode of polarisation direction in X-cut TFLN, the same concept can be applied to other polarisation directions, such astransverse magnetic (TM) or quasi-TM, by carefully engineering the waveguide geometry to exploit the anisotropic refractive indices behaviour of the TFLN in either X-cut, Z-cut or other orientations. By way of example, in some embodiments, the ordinary and extraordinary MRRs are configured to operate in a TM mode. For example, in an X-cut anisotropic platform, the extraordinary axis Z lies in the plane of the wafer, while the ordinary axis Y is orthogonal thereto. For a guided TM-like mode, the electric-field vector has a dominant component normal to the wafer (in the X direction), together with an in-plane component whose orientation depends on the local propagation direction of the waveguide. In this embodiment, the oblong shape of the ordinary MRR 34 is oriented such that, over more than half of its total optical-path length, the in-plane component of the TM-like mode electric field is aligned with the ordinary axis Y, causing the guided mode to experience the ordinary refractive index to a greater extent. Conversely, the oblong shape of the extraordinary MRR 36 is oriented such that, over more than half of its total optical-path length, the in-plane component of the TM-like mode electric field is aligned with the extraordinary axis Z, causing the guided mode to experience the extraordinary refractive index to a greater extent.

[0181] Of course, numerous additional modifications could be made to the embodiments described above without departing from the scope of protection as defined in the appended claims.

Claims

CLAIMS:

1. An external cavity laser, comprising:a photonic integrated platform comprising a gain chip providing a gain medium and at least one reflector chip coupled to the gain medium to provide a resonant cavity, the at least one reflector chip comprising a waveguide structure made of an anisotropic material, the anisotropic material having an ordinary refractive index along an ordinary axis and an extraordinary refractive index along an extraordinary axis, the waveguide structure comprising;- an ordinary MRR having an oblong shape oriented such that a polarisation direction of polarised light guided therealong extends in majority along the ordinary axis of the anisotropic material; and- an extraordinary MRR having an oblong shape oriented such that the polarisation direction of the polarised light guided therealong extends in majority along the extraordinary axis of the anisotropic material, the ordinary MRR and extraordinary MRR coupled together in a Vernier configuration.

2. The external cavity laser according to claim 1, wherein the anisotropic material comprises X-cut thin-film lithium niobate (TFLN).

3. The external cavity laser according to claim 1, wherein the anisotropic material comprises Lithium Tantalate (LiTaO3), Barium Titanate (BTO), or Electro-Optic Polymers.

4. The external cavity laser according to any one of claims 1 to 3, wherein the oblong shape of the extraordinary MRR is oriented such that the polarisation direction of the guided TE mode is predominantly aligned with the extraordinary axis of the anisotropic material.

5. The external cavity laser according to any one of claims 1 to 4, wherein the oblong shape of the ordinary MRR is oriented such that the polarisation direction of the guided TE mode is predominantly aligned with the ordinary axis of the anisotropic material.

6. The external cavity laser according to any one of claims 1 to 5, wherein the oblong shapes of the ordinary and extraordinary MRRs are each defined by a pair of parallelelongated waveguide segments joined by a pair of bent waveguide segments at opposite ends thereof.

7. The external cavity laser according to claim 6, wherein the elongated waveguide segments of the extraordinary MRR extend along the ordinary axis.

8. The external cavity laser according to claim 7, wherein the elongated waveguide segments of the ordinary MRR extend along the extraordinary axis.

9. The external cavity laser according to claim 6, wherein the elongated waveguide segments of the ordinary MRR extend at a non-zero angle with respect to the elongated waveguide segments of the extraordinary MRR.

10. The external cavity laser according to any one of claims 1 to 9, configured to guide said polarised light in a Transverse Electric (TE) mode.

11. The external cavity laser according to any one of claims 1 to 9, configured to guide said polarised light in a Transverse Magnetic (TM) mode.

12. The external cavity laser according to any one of claims 1 to 11, further comprising a tuning mechanism for tuning a spectral resonance of the extraordinary MRR.

13. The external cavity laser according to claim 12, wherein the tuning mechanism is a thermo-optic and electro-optic tuning mechanism.

14. The external cavity laser according to claim 12 or 13, wherein the tuning mechanism is further used for tuning a spectral resonance of the ordinary MRR.

15. The external cavity laser according to any one of claims 1 to 14, further comprising an in-phase / quadrature (IQ) detection module configured to generate a cavity-feedback error signal for wavelength stabilization, the IQ detection module including a 90-degree optical hybrid coupler optically coupled to two monitoring outputs respectively associated with the ordinary and extraordinary MRRs.

16. The external cavity laser according to claim 15, wherein the 90-degree optical hybrid coupler is configured to mix signals from said monitoring outputs to generate fouroptical outputs in quadrature, the IQ detection module further comprising two balanced photodiode pairs producing respective in-phase (I) and quadrature (Q) electrical signals by differential detection of said four optical outputs.

17. A photonic device, comprising:a photonic integrated platform comprising at least one reflector chip comprising a waveguide structure made of an anisotropic material, the anisotropic material having an ordinary refractive index along an ordinary axis and an extraordinary refractive index along an extraordinary axis, the waveguide structure comprising;- an extraordinary MRR having an oblong shape oriented such that a transverse mode guided around said extraordinary MRR experiences the extraordinary refractive index for greater than 50% of one round trip around the extraordinary MRR; and- an ordinary MRR having an oblong shape oriented at a non-zero angle with respect to the oblong shape of the extraordinary MRR.

18. The photonic device according to claim 17, wherein the anisotropic material comprises X-cut thin-film lithium niobate (TFLN), Lithium Tantalate (LiTaO3), Barium Titanate (BTO), and / or Electro-Optic Polymers.

19. The photonic device according to claim 17 or 18, wherein the oblong shape of the ordinary MRR is oriented such that the transverse mode when guided around said ordinary MRR experiences the ordinary refractive index for greater than 50% of one round trip around the ordinary MRR.

20. The photonic device according to any one of claims 17 to 19, wherein the oblong shape of the extraordinary MRR is oriented such that the transverse mode guided around said extraordinary MRR experiences the extraordinary refractive index for greater than about 80% of one round trip around the extraordinary MRR.

21. The photonic device according to any one of claims 17 to 20, wherein the oblong shape of the ordinary MRR is oriented such that the transverse mode guided around said ordinary MRR experiences the ordinary refractive index for greater than about 80% of one round trip around the ordinary MRR.

22. The photonic device according to any one of claims 17 to 21, wherein the non-zero angle of the oblong shape of the ordinary M RR with respect to the oblong shape of the extraordinary MRR is about 90 degrees.

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