A tunable narrow linewidth dual-micro-ring assisted DBR external cavity laser

Abstract

The application relates to the technical field of external cavity lasers, and particularly provides a tunable narrow-line-width double-micro-ring auxiliary DBR external cavity laser, which comprises a reflective semiconductor optical amplifier and a silicon nitride optical waveguide chip, a plurality of phase shifters, at least two cascaded micro-ring resonators with the same physical size and the same power coupling coefficient, and a ridge waveguide Bragg grating are arranged on the silicon nitride optical waveguide chip; the phase shifters are used for adjusting the phase of excited light; the two cascaded micro-ring resonators are used for narrow-band filtering of the excited light; the refractive indexes of the plurality of phase shifters and the cascaded micro-ring resonators are synchronously adjusted based on the thermo-optic effect of silicon nitride; the filtering passband of the micro-ring resonator is moved in alignment with the longitudinal mode of the resonant cavity, and wavelength tuning without mode hopping is realized.

Description

Technical Field

[0001] This invention belongs to the field of external cavity laser technology, and particularly relates to a tunable narrow linewidth dual microring assisted DBR external cavity laser. Background Technology

[0002] With the rapid development of information technology, especially in fields such as optical communication, spectral analysis, precision measurement, and biomedical imaging, the demand for high-performance laser sources is increasing. Among these, external cavity tunable lasers (ECTLs) have become indispensable core components in these fields due to their wide wavelength tuning range, narrow linewidth, and high output power. ECTLs, in particular, capable of achieving wide-range mode-hopping-free tuning, meet the requirements of applications with extremely high wavelength stability requirements. Traditional single-frequency lasers are often limited by their internal structure, providing only fixed or limited wavelength tuning capabilities. In contrast, lasers using external cavity designs can significantly extend their wavelength tuning range. Early external cavity lasers were mainly built based on discrete components, such as using large optical lenses and prisms to construct the cavity. While achieving a wide tuning range, they faced problems such as poor mechanical stability, difficulty in integration, and high cost. In recent years, with advancements in semiconductor materials science and micro / nano fabrication technology, various novel hybrid integrated ECTL schemes have emerged, aiming to overcome these shortcomings. These solutions typically combine components such as semiconductor gain media (e.g., quantum well or quantum dot laser chips), waveguide Bragg gratings (DBRs), microring resonators (MRRs), and phase modulators. By optimizing the coupling relationships and control mechanisms between these components, they achieve more compact, stable, and efficient laser emission systems.

[0003] In practical applications, especially when precise control of laser wavelength is required, "mode hopping" is a problem that urgently needs to be solved. Mode hopping refers to the sudden switching of the laser's operating mode caused by factors such as temperature changes and current fluctuations, resulting in discontinuous jumps in the output wavelength. This instability not only affects the system's accuracy but may also introduce additional noise interference and degrade signal quality. Although there are many successful cases demonstrating the potential of hybrid integrated ECTLs in wide-range wavelength tuning, challenges remain in achieving wide-range continuous tuning without mode hopping. Summary of the Invention

[0004] In view of this, the present invention aims to provide a tunable narrow-linewidth dual-microring assisted DBR external cavity laser, which employs two microring resonators with identical physical dimensions and synchronous tuning. Based on the resonance enhancement effect of the dual-microring resonators, the length of the resonant cavity is significantly increased, while a narrower linewidth is achieved. Furthermore, the filter passband and the resonant cavity longitudinal mode are synchronously aligned and aligned through the thermo-optic effect of silicon nitride, thereby achieving narrow-linewidth, wide-range mode-hopping-free continuous wavelength tuning. This solves the problem that existing vernier filters can only obtain a series of discrete wavelength points and cannot achieve wide-range continuous wavelength tuning.

[0005] To achieve the above objectives, the technical solution created by this invention is implemented as follows: This invention provides a tunable narrow-linewidth dual-microring assisted DBR external cavity laser, comprising: Reflective semiconductor optical amplifiers are used to provide the gain medium; And, a silicon nitride optical waveguide chip, the silicon nitride optical waveguide chip including: multiple phase shifters, at least two cascaded microring resonators with the same physical size and the same power coupling coefficient and a ridge waveguide Bragg grating disposed along the silicon nitride waveguide layer; Among them, the reflective semiconductor optical amplifier is connected to the end face of the silicon nitride optical waveguide chip; The ridge waveguide Bragg grating is etched in the ridge waveguide plate area of ​​the silicon nitride optical waveguide chip. The ridge waveguide Bragg grating serves as an on-chip mirror and forms a Fabry-Perot resonant cavity with the reflective semiconductor optical amplifier, and filters the excitation light. Multi-segment phase shifters are used to adjust the phase of the excitation light; At least two cascaded microring resonators with the same physical size and power coupling coefficient are used for narrowband filtering of the excitation light; Based on the thermo-optical effect of silicon nitride, the refractive index of multiple phase shifters and at least two cascaded microring resonators with the same physical size and power coupling coefficient is adjusted synchronously, so that the filter passband of at least two cascaded microring resonators with the same physical size and power coupling coefficient is aligned and moved with the longitudinal mode of the Fabry-Perot resonator, thereby achieving mode-hopping-free wavelength tuning.

[0006] Preferably, the material layers of the silicon nitride optical waveguide chip include: a silicon substrate, a silicon dioxide buried oxide layer, a silicon nitride waveguide layer, a silicon dioxide upper cladding layer, a heater, and electrodes.

[0007] Preferably, the end-to-end coupling process between the reflective semiconductor optical amplifier and the silicon nitride optical waveguide chip is bonding, micro-transfer printing, or heterogeneous integration.

[0008] Preferably, the 3dB bandwidth of the reflection spectrum of the ridge waveguide Bragg grating is less than twice the free spectral range of at least two cascaded microring resonators with the same physical size and power coupling coefficient.

[0009] Preferably, the multi-segment phase shifter and at least two cascaded microring resonators with the same physical size and power coupling coefficient are all connected to heaters and electrodes. By applying a time-varying voltage or current signal with a specific relationship to each electrode, each heater is driven to heat up. Based on the thermo-optic effect of silicon nitride, the refractive index of the multi-segment phase shifter and at least two cascaded microring resonators with the same physical size and power coupling coefficient is synchronously adjusted.

[0010] Preferably, the refractive index change of the multi-stage phase shifter and at least two cascaded microring resonators with identical physical dimensions and power coupling coefficients needs to satisfy the following: ; in, This represents the effective refractive index change of a multi-stage phase shifter. This represents the effective refractive index change of the microring resonator, where R represents the radius of the microring resonator. This represents the resonance enhancement factor of the microring resonator. Indicates the optical length of the microring resonator. This indicates the mode order of the Fabry-Perot resonator. Indicates the mode order of the microring resonator. This indicates the optical length of a multi-segment phase shifter.

[0011] Preferably, at least two cascaded microring resonators with the same physical size and power coupling coefficient each include: a ring optical waveguide, and straight-bus silicon nitride optical waveguides disposed on the upper and lower sides of the ring optical waveguide, wherein the ring optical waveguide and the straight-bus silicon nitride optical waveguide are coupled by evanescent wave.

[0012] Preferably, the multi-segment phase shifter includes a first phase shifter and a second phase shifter, and at least two cascaded microring resonators with the same physical size and power coupling coefficient include a first microring resonator and a second microring resonator. The first phase shifter is disposed at the input end of the first microring resonator, and the second phase shifter is disposed at the output end of the first microring resonator. The second phase shifter is connected to the input end of the second microring resonator, and the output end of the second microring resonator is connected to the ridge waveguide Bragg grating.

[0013] Compared with the prior art, the present invention can achieve the following beneficial effects: This invention employs two microring resonators with identical physical dimensions and power coupling coefficients, ensuring their filter passbands always overlap during tuning, thus avoiding the limitation of traditional vernier filters that can only be discretely tuned. Simultaneously, by synchronously controlling a phase shifter to compensate for cavity length changes in real time, it ensures that the longitudinal mode of the resonant cavity remains aligned with the center of the filter passband, resolving the mode-hopping problem during tuning and achieving narrow linewidth, wide-range, mode-hopping-free continuous wavelength tuning. Furthermore, the cascaded use of the two microring resonators generates a strong resonance enhancement effect, significantly increasing the effective optical length of the resonant cavity and thus significantly narrowing the laser linewidth.

[0014] This invention etches a Bragg grating onto the planar region of a ridge waveguide, achieving weak coupling by controlling the ripple amplitude and grating period. Compared to traditional grating processes, this design eliminates the need for complex and precise weak coupling control, has lower precision requirements for the etching process, and offers greater process tolerance. Furthermore, the reflection spectrum bandwidth of the ridge waveguide Bragg grating is designed to be less than twice the free spectral range of the microrings, ensuring that only one longitudinal mode receives sufficient feedback within the comb-filtered spectrum provided by the dual microrings. This guarantees a high side-mode suppression ratio and stable single-longitudinal-mode output for the laser. Attached Figure Description

[0015] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a structural diagram of a tunable narrow-linewidth dual-microring assisted DBR external cavity laser provided according to an embodiment of the present invention; Figure 2 This is a cross-sectional structural diagram of a silicon nitride optical waveguide provided according to an embodiment of the present invention. The reference numerals in the figures include: 1. Reflective semiconductor optical amplifier; 2. Silicon nitride optical waveguide chip; 3. First mode converter; 4. Input waveguide; 5. First phase shifter; 6. First micro-ring resonator; 7. Second micro-ring resonator; 8. Second mode converter; 9. Ridge waveguide Bragg grating; 10. Output waveguide; 11. Antireflective coating 101, antireflective coating 102; Ring waveguide 601, straight bus silicon nitride waveguide 602; A. Silicon substrate, B. Silicon dioxide buried oxide layer, C. Silicon dioxide cladding, D. Silicon nitride waveguide layer, E. Heater. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only for explaining the invention and do not constitute a limitation thereof. Similar elements in different embodiments are referred to by associated similar element reference numerals. In the following embodiments, many details are described to facilitate a better understanding of the invention. However, those skilled in the art will readily recognize that some features may be omitted in different situations, or may be replaced by other elements, materials, or methods. In some cases, some operations related to the invention are not shown or described in the specification. This is to avoid obscuring the core parts of the invention with excessive description. For those skilled in the art, detailed description of these related operations is not necessary; the relevant operations can be fully understood based on the description in the specification and general technical knowledge in the art.

[0017] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined to form various implementations. Furthermore, the order of the steps or actions in the method description can be changed or adjusted in a manner readily apparent to those skilled in the art. Therefore, the various orders in the specification and drawings are merely for the clear description of a particular embodiment and do not imply a mandatory order, unless otherwise stated that a particular order must be followed.

[0018] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0019] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0020] The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0021] Please see Figure 1 In one embodiment of the present invention, a tunable narrow-linewidth dual-microring assisted DBR external cavity laser is provided, comprising: A reflective semiconductor optical amplifier 1 is used to provide the gain medium; And, silicon nitride optical waveguide chip 2, the silicon nitride optical waveguide chip 2 includes: multiple phase shifters, at least two cascaded microring resonators with the same physical size and the same power coupling coefficient, and ridge waveguide Bragg grating 10 disposed along the silicon nitride waveguide layer; Among them, the reflective semiconductor optical amplifier 1 is end-to-end coupled to the silicon nitride optical waveguide chip 2; The ridge waveguide Bragg grating 10 is etched in the ridge waveguide plate area of ​​the silicon nitride optical waveguide chip 2. The ridge waveguide Bragg grating 10 serves as an on-chip mirror and forms a Fabry-Perot resonant cavity with the reflective semiconductor optical amplifier 1, and filters the excitation light. Multi-segment phase shifters are used to adjust the phase of the excitation light; At least two cascaded microring resonators with the same physical size and power coupling coefficient are used for narrowband filtering of the excitation light; Based on the thermo-optical effect of silicon nitride, the refractive index of multiple phase shifters and at least two cascaded microring resonators with the same physical size and power coupling coefficient is adjusted synchronously, so that the filter passband of at least two cascaded microring resonators with the same physical size and power coupling coefficient is aligned and moved with the longitudinal mode of the Fabry-Perot resonator, thereby achieving mode-hopping-free wavelength tuning.

[0022] In this design, a reflective semiconductor optical amplifier 1 operates in the C-band as the gain medium, exciting and amplifying the optical signal. During the excitation process within the resonant cavity, photons repeatedly pass through the reflective semiconductor optical amplifier 1, and after gain amplification, laser emission is achieved. The reflective semiconductor optical amplifier 1 is coupled to the silicon nitride optical waveguide chip 2 via end-face coupling. Specific coupling processes can include bonding, micro-transfer printing, or heterogeneous integration. The reflective semiconductor optical amplifier 1 includes two end faces. The front end face is coated with an anti-reflection film with a reflectivity of over 90%. This end face serves as a mirror in the laser resonant cavity, forming a Fabry-Perot resonant cavity together with the ridge waveguide Bragg grating 9 at the other end. The rear end face of the reflective semiconductor optical amplifier 1 is coated with an anti-reflection film with a reflectivity of 0.01%. The anti-reflection film is used to minimize end-face reflection, prevent unnecessary self-oscillations within the resonant cavity, and ensure efficient, unidirectional coupling of light into the silicon nitride optical waveguide chip 2.

[0023] The silicon nitride optical waveguide chip 2 has ultra-low loss characteristics and serves as the external cavity chip of the laser to achieve wavelength selection and control. For example... Figure 2 As shown, the material layers of the silicon nitride optical waveguide chip 2, from top to bottom, include: a silicon substrate A, a silicon dioxide buried oxide layer B, a silicon dioxide upper cladding layer C, a silicon nitride waveguide layer D, a heater E, and electrodes. The silicon nitride waveguide layer serves as the optical transmission structure. The silicon nitride waveguide layer D is a strip waveguide with a height of 400 nm and a width of 800-1000 nm. The main tuning devices in the silicon nitride optical waveguide chip 2 are all designed along the silicon nitride waveguide layer. Along the silicon nitride waveguide layer D, the silicon nitride optical waveguide chip 2 sequentially includes a first mode converter 3, an input waveguide 4, a first phase shifter 5, a first micro-ring resonator 6, a second phase shifter 7, a second micro-ring resonator 8, a second mode converter 9, a ridge waveguide Bragg grating 10, and an output waveguide 11. The first mode converter 3 serves as a transition structure between the reflective semiconductor optical amplifier 1 and the silicon nitride optical waveguide chip 2, thermally transferring the output optical field of the reflective semiconductor optical amplifier 1 to the silicon nitride optical waveguide chip 2. Since the mode size of the reflective semiconductor optical amplifier 1 differs from that of the input waveguide 4 by orders of magnitude, direct docking would result in significant mode mismatch loss. Therefore, the first mode converter 3 is used to achieve mode matching between the reflective semiconductor optical amplifier 1 and the input waveguide 4, reducing reflection and scattering losses.

[0024] This invention employs a multi-segment phase shifter design, maximizing the maximum phase shift while maintaining chip size. Simultaneously modulating two micro-ring resonators and shifting their filter passbands, the multi-segment phase shifter ensures the longitudinal mode of the resonant cavity remains at the center of the filter passbands of the two micro-ring resonators. Specifically, a first phase shifter 5 and a second phase shifter 7 are designed. Both phase shifters 5 and 7 are connected to a heater E and electrodes. Applying current or voltage to the electrodes drives the heater E to heat the laser. Based on the thermo-optical effect of silicon nitride, the first and second phase shifters 5 and 7 can perform laser phase adjustment and compensation. The first phase shifter 5 is connected to the rear end of the input waveguide 4 and is used for secondary adjustment of the laser optical phase. The rear end of the first phase shifter 5 is connected to the input terminal of the first micro-ring resonator 6, the output terminal of the first micro-ring resonator 6 is connected to the second phase shifter 7, and the rear end of the second phase shifter 7 is connected to the input terminal of the second micro-ring resonator 8. The output terminal of the second micro-ring resonator 8 is connected to the ridge waveguide Bragg grating 10 via a second mode converter 9. The first microring resonator 6 and the second microring resonator 8 have identical physical dimensions, power coupling coefficients, and fabrication processes; both are symmetrical, all-through microring resonators. Both the first microring resonator 6 and the second microring resonator 8 include a ring-shaped optical waveguide 601 based on strip-shaped silicon nitride, and straight-line silicon nitride optical waveguides 602 on the upper and lower sides of the ring-shaped optical waveguide 601.

[0025] The radius of the ring waveguide 601 is 100-120nm, and the distance between the straight bus silicon nitride waveguide 602 and the ring waveguide 601 is 600-950nm. The ring waveguide 601 and the straight bus silicon nitride waveguide 602 are coupled by evanescent wave. The spacing and coupling length of the ring waveguide 601 of the first micro-ring resonator 6 and the straight bus silicon nitride waveguides 602 on both sides are completely consistent, ensuring that the power coupling coefficient is the same.

[0026] The first microring resonator 6 and the second microring resonator 8 are also equipped with independent heaters E and electrodes. By applying current or voltage to the electrodes, the heater E can be driven to heat up. Based on the thermo-optic effect of silicon nitride, the first microring resonator 6 and the second microring resonator 8 can perform laser bandpass filtering. The two cascaded microring resonators of the same size designed in this embodiment of the invention can significantly narrow the laser linewidth by utilizing the resonance enhancement effect of the double microrings. Furthermore, compared with the existing vernier filter composed of two microring resonators of different sizes, the two cascaded microring resonators of the same size can avoid the inherent characteristic of the vernier effect, which can only obtain a series of discrete wavelength points, and can achieve a wide range of continuous wavelength tuning.

[0027] During laser tuning, a time-varying voltage or current signal with a specific relationship is applied to each electrode of the first phase shifter 5, the first microring resonator 6, the second phase shifter 7, and the second microring resonator 8 to drive the corresponding heater E of each device to heat up, achieving thermo-optic modulation based on the thermo-optic effect of silicon nitride. The thermo-optic effect is used to change the refractive index of the silicon nitride waveguide layer D of the first phase shifter 5, the first microring resonator 6, the second phase shifter 7, and the second microring resonator 8, causing the filter passbands of the first microring resonator 6 and the second microring resonator 8 to align with the longitudinal modes of the Fabry-Perot resonator. Simultaneously, the first phase shifter 5 and the second phase shifter 7 are modulated during the filter passband shift, ensuring that the longitudinal modes of the resonator are always located at the center of the filter passbands of the first microring resonator 6 and the second microring resonator 8. The control requirements for the above tuning are: The resonant wavelengths of the first microring resonator 6 and the second microring resonator 8, and the center wavelength of the filter passband, can be expressed by the following standing wave condition: ; in, This indicates the mode order of the microring resonator (i.e., the first microring resonator 6 and the second microring resonator 8). R represents the resonant wavelength of the microring resonator, and R represents the radius of the microring resonator. This represents the effective emission rate of the microring resonator.

[0028] Therefore, the relationship between the wavelength redshift and the change in refractive index can be expressed as: ; in, The value R represents the change in resonant wavelength of the first microring resonator 6 and the second microring resonator 8, and R represents the radius of the first microring resonator 6 and the second microring resonator 8. This indicates the mode order of the first microring resonator 6 and the second microring resonator 8. This represents the effective emissivity change of the first microring resonator 6 and the second microring resonator 8.

[0029] Similarly, the resonant wavelength of a Fabry-Perot resonator can be expressed as: ; in, This indicates the mode order of the Fabry-Perot resonator. This represents the resonant wavelength of the Fabry-Perot resonator. This represents the effective refractive index of a strip silicon nitride waveguide. This indicates the physical length of the straight-bus silicon nitride optical waveguide 602. This indicates the effective refractive index of the first phase shifter 5 and the second phase shifter 7. This indicates the physical lengths of the first phase shifter 5 and the second phase shifter 7. This represents the resonance enhancement factor of the first micro-ring resonator 6 and the second micro-ring resonator 8. This represents the effective refractive index of the first microring resonator 6 and the second microring resonator 8. This indicates the physical length of the first microring resonator 6 and the second microring resonator 8.

[0030] The relationship between the wavelength redshift of the Fabry-Perot resonator and the changes in refractive index of the first phase shifter 5, the second phase shifter 7, the first microring resonator 6, and the second microring resonator 8 can be expressed as: ; in, This represents the change in the resonant wavelength of the Fabry-Perot resonator. This indicates the order of the Fabry-Perot resonator modes. This represents the effective refractive index change of the first phase shifter 5 and the second phase shifter 7. This represents the resonance enhancement factor of the first micro-ring resonator 6 and the second micro-ring resonator 8. This indicates the physical lengths of the first microring resonator 6 and the second microring resonator 8. This represents the effective refractive index change of the first microring resonator 6 and the second microring resonator 8.

[0031] To achieve mode-hopping-free tuning, the resonant wavelengths of the first microring resonator 6 and the second microring resonator 8 must be kept consistent with the redshift of the resonant wavelength of the Fabry-Perot resonator. Therefore: ; Where R represents the radius of the first micro-ring resonator 6 and the second micro-ring resonator 8. This indicates the mode order of the first microring resonator 6 and the second microring resonator 8. This represents the resonance enhancement factor of the first micro-ring resonator 6 and the second micro-ring resonator 8. This indicates the physical lengths of the first microring resonator 6 and the second microring resonator 8. This indicates the order of the Fabry-Perot resonator modes. This represents the effective refractive index change of the first microring resonator 6 and the second microring resonator 8. This indicates the physical lengths of the first phase shifter 5 and the second phase shifter 7. This indicates the effective refractive index change of the first phase shifter 5 and the second phase shifter 7.

[0032] That is, the refractive index changes of the first phase shifter 5, the second phase shifter 7, the first microring resonator 6, and the second microring resonator 8 must satisfy the following relationship: ; in, This represents the effective refractive index change of the first phase shifter 5 and the second phase shifter 7. This represents the effective refractive index change of the first microring resonator 6 and the second microring resonator 8, where R represents the radius of the first microring resonator 6 and the second microring resonator 8. This represents the resonance enhancement factor of the first micro-ring resonator 6 and the second micro-ring resonator 8. This indicates the optical lengths of the first microring resonator 6 and the second microring resonator 8. This indicates the order of the Fabry-Perot resonator modes. This indicates the mode order of the first microring resonator 6 and the second microring resonator 8. This indicates the optical length of the first phase shifter 5 and the second phase shifter 7.

[0033] The download end of the second microring resonator 8 is connected to the ridge waveguide Bragg grating 10 via the second mode-spot converter 9, which also serves to achieve mode-spot matching. The ridge waveguide Bragg grating 10 and the front end face of the reflective semiconductor optical amplifier 1 form a Fabry-Perot resonant cavity, and the ridge waveguide Bragg grating 9 provides secondary wavelength selection. The ridge waveguide Bragg grating 9 is formed by etching the planar region of the ridge waveguide, creating a weak perturbation to the optical field to achieve a low coupling coefficient, thereby realizing a narrow-band reflection spectrum. The reflection spectrum of the ridge waveguide Bragg grating 9 is located in the C-band with a duty cycle of 30%-50%, achieving lower process requirements and larger process tolerances. The 3dB bandwidth of the reflection spectrum of the ridge waveguide Bragg grating 9 is at least less than twice the free spectral range of the first microring resonator 6 and the second microring resonator 8, to achieve single-mode output.

[0034] During laser operation, the reflective semiconductor optical amplifier 1, acting as the gain region, provides gain to the laser. The generated light is adiabatically transitioned to the input waveguide 4 via the first mode converter 3, then modulated by the first phase shifter 5, the first micro-ring resonator 6, the second phase shifter 7, and the second micro-ring resonator 8. After being adiabatically transitioned to the ridge waveguide Bragg grating 10 via the second mode converter 9, the ridge waveguide Bragg grating 10 reflects part of the light, which returns along the original path, forming oscillations. Part of the light is then transitioned to the output waveguide 11 via another mode converter. The first phase shifter 5 and the second phase shifter 7 are used to adjust the phase of the light wave within the cavity; the first micro-ring resonator 6 and the second micro-ring resonator 8 have a filtering function, selecting a series of longitudinal modes. The ridge waveguide Bragg grating 10 then returns only one longitudinal mode within its operating bandwidth, forming a single-mode oscillation. During the tuning process, the first phase shifter 5, the first micro-ring resonator 6, the second phase shifter 7, and the second micro-ring resonator 8 are synchronously thermo-optically modulated with a specific injection current or voltage ratio. The thermo-optic effect is used to change the refractive index of the waveguide so that the amount of refractive index change satisfies the above-mentioned preset relationship, thereby achieving narrow linewidth, wide range, and mode-skipping continuous wavelength tuning.

[0035] In summary, the above description is merely a preferred embodiment of this specification and is not intended to limit the scope of protection of this specification. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this specification should be included within the scope of protection of this specification.

[0036] The systems, apparatuses, modules, or units described in one or more of the above embodiments may be implemented by a computer chip or entity, or by a product having a certain function. A typical implementation device is a computer. Specifically, a computer may be, for example, a personal computer, a laptop computer, a cellular phone, a camera phone, a smartphone, a personal digital assistant, a media player, a navigation device, an email device, a game console, a tablet computer, a wearable device, or any combination of these devices.

[0037] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0038] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to interchangeably. Each embodiment focuses on describing the differences from other embodiments. In particular, the system embodiments are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments.

Claims

1. A tunable narrow-linewidth dual-microring assisted DBR external cavity laser, characterized in that, include: Reflective semiconductor optical amplifiers are used to provide the gain medium; And, a silicon nitride optical waveguide chip, the silicon nitride optical waveguide chip comprising: multiple phase shifters, at least two cascaded microring resonators with the same physical size and power coupling coefficient, and a ridge waveguide Bragg grating disposed along the silicon nitride waveguide layer; The reflective semiconductor optical amplifier is coupled face-to-face with the silicon nitride optical waveguide chip. The ridge waveguide Bragg grating is etched into the ridge waveguide plate region of the silicon nitride optical waveguide chip. The ridge waveguide Bragg grating serves as an on-chip mirror and forms a Fabry-Perot resonant cavity with the reflective semiconductor optical amplifier, and filters the excitation light. The multi-segment phase shifter is used to adjust the phase of the excitation light; The at least two cascaded microring resonators with the same physical size and power coupling coefficient are used to perform narrowband filtering on the excitation light; Based on the thermo-optical effect of silicon nitride, the refractive index of the multi-segment phase shifter and the at least two cascaded microring resonators with the same physical size and power coupling coefficient is adjusted synchronously, so that the filter passband of the at least two cascaded microring resonators with the same physical size and power coupling coefficient is aligned and moved with the longitudinal mode of the Fabry-Perot resonator, thereby achieving mode-skipping wavelength tuning. The refractive index change of the multi-segment phase shifter and the at least two cascaded microring resonators with the same physical dimensions and power coupling coefficients needs to satisfy the following: ； in, This represents the effective refractive index change of the multi-segment phase shifter. This represents the effective refractive index change of the microring resonator, where R represents the radius of the microring resonator. This represents the resonance enhancement factor of the microring resonator. Indicates the optical length of the microring resonator. This indicates the mode order of the Fabry-Perot resonator. Indicates the mode order of the microring resonator. This indicates the optical length of the multi-segment phase shifter.

2. The tunable narrow-linewidth dual-microring assisted DBR external cavity laser according to claim 1, characterized in that, The material layers of the silicon nitride optical waveguide chip include: a silicon substrate, a silicon dioxide buried oxide layer, a silicon nitride waveguide layer, a silicon dioxide top cladding layer, a heater, and electrodes.

3. The tunable narrow-linewidth dual-microring assisted DBR external cavity laser according to claim 1, characterized in that, The end-to-end coupling process between the reflective semiconductor optical amplifier and the silicon nitride optical waveguide chip is bonding, micro-transfer printing, or heterogeneous integration.

4. The tunable narrow-linewidth dual-microring assisted DBR external cavity laser according to claim 1, characterized in that, The reflection spectrum 3dB bandwidth of the ridge waveguide Bragg grating is less than twice the free spectral range of the at least two cascaded microring resonators with the same physical size and power coupling coefficient.

5. The tunable narrow-linewidth dual-microring assisted DBR external cavity laser according to claim 1, characterized in that, The multi-segment phase shifter and the at least two cascaded microring resonators with the same physical size and power coupling coefficient are all connected to heaters and electrodes. By applying a time-varying voltage or current signal with a specific relationship to each electrode, each heater is driven to heat up. Based on the thermo-optic effect of silicon nitride, the refractive index of the multi-segment phase shifter and the at least two cascaded microring resonators with the same physical size and power coupling coefficient is synchronously adjusted.

6. The tunable narrow-linewidth dual-microring assisted DBR external cavity laser according to claim 1, characterized in that, The at least two cascaded microring resonators with the same physical size and power coupling coefficient each include: a ring optical waveguide and a straight-bus silicon nitride optical waveguide disposed on the upper and lower sides of the ring optical waveguide, wherein the ring optical waveguide and the straight-bus silicon nitride optical waveguide are coupled by evanescent wave.

7. The tunable narrow-linewidth dual-microring assisted DBR external cavity laser according to claim 1, characterized in that, The multi-segment phase shifter includes a first phase shifter and a second phase shifter. The at least two cascaded microring resonators with the same physical size and power coupling coefficient include a first microring resonator and a second microring resonator. The first phase shifter is located at the input end of the first microring resonator, and the second phase shifter is located at the output end of the first microring resonator. The second phase shifter is connected to the input end of the second microring resonator, and the output end of the second microring resonator is connected to the ridge waveguide Bragg grating.

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