A nitride single transverse mode double-pass racetrack micro-ring waveguide for external cavity of visible band FP laser

Abstract

This invention discloses a nitride single-transverse-mode dual-pass racetrack-shaped microring waveguide for the external cavity of a visible-light FP laser. It employs an AlN ridge waveguide system or a GaN strip waveguide system, achieving low-loss transmission of a single transverse mode in the visible-light band through precise control of waveguide cross-sectional parameters. The waveguide structure consists of two main straight waveguides and a racetrack-shaped microring waveguide forming a dual-pass coupling structure. The main waveguides are equally spaced on both sides of the microring straight track, forming three ports: input, pass-through, and download. This invention utilizes the dual-pass structure to feed back specific mode optical signals into the FP laser cavity, achieving self-injection locking and spectral purification. This invention features rigorous design logic and a compact structure, possessing significant monolithic integration application value in the fields of laser displays and precision atomic clocks.

Description

Technical Field

[0001] This invention belongs to the field of visible light self-locking laser design, specifically relating to a nitride single transverse mode dual-pass racetrack-shaped microring waveguide for the external cavity of a visible light FP laser. Background Technology

[0002] Visible-light semiconductor lasers have significant applications in laser projection displays and quantum precision measurement. Currently, most commercially available visible-light semiconductor lasers employ Fabry-Perot (FP) cavity structures. To narrow their GHz-level laser linewidths and achieve single-mode output, self-injection locking technology has become a research hotspot. In photonic integrated circuits, microring resonators are ideal components for constructing external cavity feedback due to their compact structure, high quality factor (Q value), and tunable free spectral range (FSR). For example, microring external cavities based on silicon-on-insulator (SOI) or silicon-nitride (Si3N4) platforms have achieved efficient self-injection locking in the near-infrared band. However, in the visible-light band, these materials face severe reabsorption losses or bandgap limitations. Nitride materials, with their wide bandgap, wide transparency window, and good thermal stability, are ideal carriers for realizing visible-light integrated photonics.

[0003] However, existing nitride microring waveguides still face multiple challenges in practical applications. Firstly, there is the trade-off between mode competition and loss. In the short wavelengths of visible light, waveguides require extremely high fabrication precision. To maintain single transverse mode transmission, the waveguide size needs to be drastically reduced, often leading to increased scattering loss. However, increasing the size easily introduces higher-order mode interference, affecting spectral purification. Secondly, traditional single-pass microring structures often only achieve single-pass feedback. When coupled with FP lasers, the intensity ratio within the feedback cavity is difficult to control precisely, resulting in insufficient feedback efficiency, narrow injection locking range, and poor stability. Furthermore, when fabricating high-quality nitride waveguides on heterogeneous substrates such as sapphire, interlayer stress and lattice mismatch often lead to thin film cracking or interface scattering. Existing waveguide structure designs often lack targeted structural optimization for different nitride systems. Therefore, developing a nitride dual-pass microring waveguide structure that can balance low-loss single transverse mode transmission, high feedback efficiency, and easy integration with FP lasers is of great significance for promoting the development of high-performance integrated light sources in the visible light band. Summary of the Invention

[0004] Objective of the Invention: The objective of the present invention is to provide a nitride single transverse mode double-pass racetrack-shaped micro-ring waveguide for the external cavity of a visible light band FP laser, overcoming the defects such as the wide spectral linewidth and poor frequency stability of the visible light band FP laser in the prior art, as well as the high loss and difficult mode control of the existing nitride external cavity waveguide. Through a specific hetero-stack structure and double-pass racetrack-shaped design, this device aims to achieve low-loss single transverse mode transmission and enhanced feedback efficiency, thereby realizing self-injection locking and spectral purification of the FP laser, and meeting the requirements for high-coherence integrated light sources in the fields of laser display and precision atomic clocks.

[0005] Technical Solution: The present invention provides a nitride single transverse mode double-pass racetrack-shaped micro-ring waveguide for the external cavity of a visible light band FP laser, characterized in that the micro-ring waveguide adopts an AlN ridge waveguide system or a GaN strip waveguide system; Among them, the AlN ridge waveguide system sequentially includes from bottom to top: a sapphire substrate, an AlN flat layer formed on the sapphire substrate, an AlN waveguide layer formed on the AlN flat layer, and a SiO2 cladding layer covering the above structure; The GaN strip waveguide system sequentially includes from bottom to top: a sapphire substrate, an AlN buffer layer formed on the sapphire substrate, an AlGaN transition layer formed on the AlN buffer layer, a GaN waveguide layer formed on the AlGaN transition layer, and a SiO2 cladding layer covering the above structure; where the GaN waveguide layer is formed into a strip waveguide through through-etching; The AlN waveguide layer or the GaN waveguide layer is etched to form a waveguide, and the waveguide includes a bus straight waveguide 1, a bus straight waveguide 2, and a racetrack-shaped micro-ring waveguide. The two bus straight waveguides are arranged at equal intervals on both sides of the straight part of the racetrack-shaped micro-ring waveguide to form a double-pass coupling structure.

[0006] Further, in the AlN ridge waveguide system, the thickness of the AlN flat layer is 200 - 800 nm, the thickness of the AlN waveguide layer is 100 - 500 nm, the waveguide is a ridge waveguide, its ridge width is 200 - 1900 nm, and the sidewall inclination angle is 75 - 85°.

[0007] Further, in the GaN strip waveguide system, the thickness of the AlN buffer layer is 500 - 1000 nm, the thickness of the AlGaN transition layer is 100 - 200 nm, and its material is Al x Ga 1-x N, where 0 < x < 1, the thickness of the GaN waveguide layer is 150 - 300 nm, the waveguide is a strip waveguide, and its strip width is 280 - 370 nm.

[0008] Furthermore, the thickness of the SiO2 cladding is 1000-1500 nm.

[0009] Furthermore, in the AlN ridge waveguide system, the thickness of the AlN planar layer, the thickness of the AlN waveguide layer, and the ridge width are configured such that the ridge waveguide supports single transverse mode transmission in the visible light band with a transmission loss of less than 0.6 dB / cm.

[0010] Furthermore, in the GaN strip waveguide system, the thickness of the GaN waveguide layer and the strip width are configured such that the strip waveguide supports single transverse mode transmission in the visible light band with a transmission loss of less than 0.6 dB / cm.

[0011] Furthermore, the ridge width or strip width and thickness of the bus straight waveguide and micro-ring waveguide, as well as the thickness of the planar layer in the AlN system, must all meet the single transverse mode low-loss transmission conditions in the visible light band.

[0012] Furthermore, the racetrack-shaped microring and the two bus straight waveguides form a dual-channel coupling structure, which together constitutes three functional ports: Input, Through, and Drop.

[0013] Furthermore, by coupling the FP laser source to the input end, after the optical signal enters the bus straight waveguide, a long-distance coupling region is established using the straight section of the racetrack-shaped structure, enabling the optical signal of a specific resonant mode to be coupled into the micro-ring waveguide with high efficiency.

[0014] Furthermore, by utilizing a dual-pass structure, the resonant optical signal within the micro-ring is symmetrically coupled into another straight waveguide, and combined with an end-feedback mechanism, the spectrally purified signal is precisely transmitted back from the Drop end to the FP laser cavity, achieving self-injection locking.

[0015] Furthermore, by appropriately increasing the ridge width or strip width to improve the effective refractive index of the waveguide, the ability to confine the light field is enhanced, thereby reducing scattering loss.

[0016] Furthermore, by reducing the ridge width or stripe width, the space for higher-order transverse modes can be limited, ensuring that the device only supports single-transverse-mode lasing and avoiding spectral instability caused by multi-mode competition.

[0017] The coupling spacing and coupling length between the microring and the bus straight waveguide can be changed.

[0018] Furthermore, the coupling coefficient between the bus straight waveguide and the microring can be increased by reducing the coupling gap or increasing the coupling length of the racetrack-shaped structure. In the AlN system, the ridge width of the microring waveguide is intentionally designed to be smaller than that of the bus straight waveguide, utilizing the mode mismatch compensation effect to further improve the coupling efficiency.

[0019] Furthermore, by reducing the perimeter of the racetrack-shaped microring, the FSR is increased, ensuring that only a single microring resonance peak exists within the gain bandwidth of the FP laser, thereby effectively suppressing the multimode resonance of the FP laser and achieving single-mode output.

[0020] Furthermore, the target quality factor is determined based on the required spectral purification accuracy. Then, by adjusting the coupling gap, the microring is made to operate in a critically coupled or slightly undercoupled state to obtain an extremely narrow resonant bandwidth, thereby achieving the effect of narrowing the laser linewidth.

[0021] In AlN waveguide systems, appropriately increasing the ridge width can increase the effective refractive index and reduce waveguide transmission loss, while decreasing the ridge width can limit the generation of higher-order modes; the same principle applies to waveguide width in GaN waveguide systems. Reducing the microring coupling length increases the free spectral range, prevents the excitation of FP multimode resonance, and simultaneously reduces the overall device size, facilitating integration. By analyzing the relationship between the microring quality factor and the coupling efficiency between the main straight waveguide and the microring, the coupling efficiency corresponding to the desired quality factor is obtained. Then, by analyzing the relationship between the coupling efficiency of the main straight waveguide coupled to the microring and the coupling gap, the coupling gap parameter is determined. This enables self-injection locking of visible-light FP lasers, possessing significant monolithic integration application value in laser displays and precision atomic clocks.

[0022] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) The design of this invention has extremely high platform compatibility. By providing two optional heterogeneous stacking systems, AlN ridge waveguide and GaN strip waveguide, it can cover the visible light band and meet the monolithic integration requirements of FP lasers of different wavelengths, which has important industrial application value.

[0023] (2) The present invention significantly improves feedback efficiency and locking stability through a dual-channel racetrack-shaped structure. Compared with the traditional single-bus microring, the dual-channel coupling structure establishes an active physical feedback path. By precisely adjusting the racetrack-shaped coupling length and coupling gap, the quantitative control of the feedback light intensity ratio can be achieved, effectively solving the problems of narrow self-injection locking range and poor stability caused by insufficient feedback in the traditional structure.

[0024] (3) This invention has excellent spectral purification and mode suppression capabilities. By using the precise control of the circumference and bending radius of the racetrack-shaped micro-ring to increase the longitudinal mode spacing, combined with the single transverse mode waveguide cross-section design, it can effectively suppress the multimode resonance and high-order mode interference of the FP laser, significantly narrow the laser linewidth, and improve the spectral purity.

[0025] (4) This invention excels in reducing transmission loss and enhancing optical field confinement. By synergistically optimizing the thickness of the planar layer and the ridge width in the AlN waveguide system, the effective refractive index of the waveguide is improved, which not only reduces sidewall scattering loss but also enhances the confinement of short-wavelength visible light fields, ensuring high-performance operation of the device in a compact integrated environment.

[0026] (5) The present invention has a compact structure and is easy to manufacture. Compared with the ring structure, the racetrack-shaped design increases the coupling tolerance and reduces the sensitivity to nanoscale processing precision. At the same time, the overall design logic is conducive to monolithic or hybrid integration with FP lasers, effectively reducing the size of the entire photonic integrated chip and reducing the system packaging cost. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the structure of the present invention.

[0028] Figure 2 This is a schematic cross-sectional view of the ridge waveguide structure of the AlN waveguide system that satisfies single transverse mode transmission according to the present invention.

[0029] Figure 3 This is a schematic cross-sectional view of the strip waveguide structure of the GaN waveguide system that satisfies single transverse mode transmission according to the present invention.

[0030] Figure 4 The electric field energy distribution diagrams of the TE fundamental mode of the two waveguide systems that satisfy the single structural parameter limit value for single transverse mode transmission according to the present invention are as follows: (a) Electric field energy distribution diagram of the TE fundamental mode of the AlN waveguide with a total thickness of 700nm, a planar layer of 600nm, and a ridge width of 1900nm; (b) Electric field energy distribution diagram of the TE fundamental mode of the GaN waveguide with a total thickness of 280nm and a stripe width of 370nm.

[0031] Figure 5 The graphs show the relationship between the waveguide bending radius and bending loss of the AlN waveguide system that satisfies single transverse mode transmission according to the present invention; (a) the curve showing the relationship between the propagation loss of the bent waveguide and the bending radius, and (b) the curve showing the relationship between the total loss of the quarter-circle of the bent waveguide and the bending radius, where the total loss includes the total propagation loss and the first straight-bend waveguide coupling loss.

[0032] Figure 6 This is a graph showing the relationship between the Q value and the coupling efficiency of the coupling region in the AlN waveguide system of the microring waveguide of the present invention.

[0033] Figure 7 The graph shows the relationship between the coupling efficiency of the coupling region and the coupling gap in the AlN waveguide system of the microring waveguide of the present invention.

[0034] Figure 8The output spectra of the Through and Drop ends of the AlN waveguide system of the microring waveguide of the present invention at visible wavelengths are shown.

[0035] Figure reference numerals: 1. Sapphire substrate, 2. AlN buffer layer, 3. AlGaN transition layer, 4. AlN planar layer, 5. Bus straight waveguide one, 6. Bus straight waveguide two, 7. Raceway-shaped micro-ring waveguide, 8. Coupling length, 9. Coupling spacing, 10. Light source input port, 11. Through end, 12. Drop end, 13. SiO2 cladding. Detailed Implementation

[0036] The technical solution of the present invention will be described in detail below with reference to specific embodiments and accompanying drawings.

[0037] like Figure 1 The diagram shows a schematic of a nitride single-mode dual-pass racetrack-shaped microring waveguide structure for the external cavity of a visible-light FP laser provided by this invention. This device, based on a nitride heterostructure stacking system and a racetrack-shaped dual-pass coupling structure, aims to achieve low-loss transmission, self-injection locking, and spectral purification in the visible-light band. Its specific layered heterostructure, from the substrate upwards, includes: a sapphire substrate 1, an AlN buffer layer 2, an AlGaN transition layer 3, an AlN planar layer 4, an AlN or GaN waveguide layer (this waveguide layer is etched to form the waveguide structures indicated by marks 5, 6, and 7), and a SiO2 cladding layer 13 covering it.

[0038] On this heterogeneous stacking platform, a dual-channel coupling structure consisting of a first bus straight waveguide 5, a second bus straight waveguide 6, and a central racetrack-shaped micro-ring waveguide 7 was constructed using micro-nano fabrication techniques. This structure forms three key functional ports: a light source input port 10, a through port 11, and a drop port 12. The racetrack-shaped micro-ring waveguide 7, the first bus straight waveguide 5, and the second bus straight waveguide 6 achieve mutual coupling of the optical field through a preset coupling length 8 and coupling spacing 9.

[0039] This invention ensures that the device supports only single transverse mode low-loss transmission in the visible light band by precisely controlling waveguide cross-sectional parameters, such as ridge width or stripe width, thickness, and planar layer thickness, thereby effectively suppressing higher-order mode interference. In the workflow, the optical signal generated by the FP laser enters through input port 10, establishing a long-distance coupling region through the straight section of the racetrack-shaped structure, allowing the optical signal of a specific resonant mode to efficiently enter the micro-ring. Utilizing the physical feedback mechanism of the dual-pass structure, the resonant signal, after spectral purification by the micro-ring, enters the second straight waveguide through symmetrical coupling and is precisely fed back to the FP laser cavity from the Drop terminal 12.

[0040] Specifically, by reducing the perimeter of the microring to increase the FSR, a single resonant peak can be ensured within the laser gain bandwidth, thereby suppressing multimode lasing. Simultaneously, by optimizing the coupling gap 9 to keep the microring in a critically coupled or undercoupled state, extremely high Q values ​​can be obtained and the lasing spectral linewidth can be narrowed. This invention utilizes the wide bandgap characteristics of AlN or GaN materials and the coupling tolerance advantages of the racetrack-shaped design to provide a highly coherent, monolithically integrated light source solution for visible light laser displays and precision atomic clocks.

[0041] Example 1

[0042] like Figure 2 The diagram shows a cross-sectional view of the ridge waveguide structure of the AlN waveguide system that satisfies single transverse mode transmission according to the present invention. This embodiment demonstrates that the AlN ridge waveguide for achieving single transverse mode transmission in the visible light range has a certain range for its ridge width, etching depth, and planar layer thickness. If these ranges are exceeded, higher-order modes may appear or no modes may be able to propagate, which does not meet the self-injection locking requirements of FP lasers. Furthermore, the sidewalls of the ridge waveguide are not designed vertically because vertical etching is difficult and has poor precision in AlN etching processes. This 75-85° sidewall tilt angle design is easier to achieve, has smaller process errors, and can effectively reduce propagation loss.

[0043] Example 2

[0044] like Figure 3 The diagram shown is a cross-sectional schematic of the strip waveguide structure of the GaN waveguide system that satisfies single transverse mode transmission according to the present invention. This embodiment demonstrates that the GaN strip waveguide that achieves single transverse mode transmission in the visible light range has a certain range in its strip width and thickness. Exceeding this range, it is highly likely that higher-order modes will appear or no modes will be able to propagate, which also fails to meet the self-injection locking requirement of FP lasers.

[0045] like Figure 4 The figure shows the electric field energy distribution of the TE fundamental mode of a straight waveguide that satisfies the single structural parameter limit value for two waveguide systems of the present invention, which meet the single transverse mode transmission requirements. Figure 4As shown in (a) and (b), both embodiments demonstrate that the energy is highly concentrated in the central region of the waveguide, and the optical field attenuates rapidly at the edge when entering the SiO2 cladding and the bottom planar layer. This proves that the structural design provides a sufficient effective refractive index difference, achieving strong field confinement in the visible light band. At the upper limit of the ridge width or stripe width dimension, only the TE fundamental mode exists, proving that the design successfully suppresses the existence space of higher-order transverse modes and avoids multimode competition. In addition, there is no significant energy leakage from the guided mode to the sapphire substrate, thus ensuring extremely low transmission loss. This pure single-transverse-mode optical field distribution ensures efficient and high-purity mode coupling between the bus straight waveguide and the racetrack-shaped microring, which is the physical basis for subsequent precise feedback intensity control and high-stability self-injection locking of FP lasers.

[0046] like Figure 5 Figures (a) and (b) show the relationship between the waveguide bending radius and bending loss of the AlN waveguide system that satisfies single transverse mode transmission according to the present invention. Figure 5 (a) shows the propagation loss as a function of the bending radius. As the bending radius gradually decreases from 115 μm to 105 μm, the loss remains at a very low level. However, at a bending radius of approximately 102 μm, a significant and sharp loss peak appears on the curve. This phenomenon is due to the phase matching between the guided mode in the ridge waveguide and the mode in the underlying planar layer at this specific radius, resulting in resonant coupling and causing a significant energy leakage to the planar layer. When the radius continues to decrease to below 100 μm, the loss returns to the baseline level because the resonant coupling condition is no longer met. Figure 5 (b) shows the variation of the total loss (including propagation loss and primary straight-to-bend waveguide coupling loss) with radius in a quarter-circle ring. While a smaller bend radius helps increase FSR and reduce device size, Figure 5 (b) shows that the total loss increases as the radius decreases, and there is also a local loss disturbance peak caused by resonance at 102um.

[0047] In this embodiment, the key dimensions of the micro-ring waveguide were determined by the above method: in order to ensure that the racetrack-shaped micro-ring has an extremely high Q value to achieve spectral purification, the resonant loss range near 102um must be avoided.

[0048] like Figure 6 The figure shows the relationship between the Q-value and the coupling efficiency of the coupling region in an AlN waveguide system for a nitride single-transverse-mode dual-pass racetrack-shaped microring waveguide used in the external cavity of a visible-light FP laser according to the present invention. Combined with simulation data, it can be seen that the Q-value of the microring exhibits a significant inverse-proportional nonlinear decay relationship with the coupling efficiency. When the coupling efficiency is extremely low, the optical energy is highly confined within the microring, and the Q-value of the microring reaches the theoretical upper limit determined by the intrinsic loss of the waveguide, approximately 2.1 × 10⁻⁶. 5As coupling efficiency gradually increases, the energy transfer rate from the resonant optical field inside the microring to the main straight waveguide accelerates, leading to a shortened photon lifetime within the cavity and a nonlinear decrease in the Q value. For example, when the coupling efficiency reaches 10%, the Q value is approximately 71,700; when the coupling efficiency increases to 50%, the Q value further decreases to approximately 15,600. When the coupling efficiency exceeds the extreme overcoupling region of 90%, the resonance effect is severely disrupted, and the Q value experiences a precipitous drop.

[0049] For FP lasers in the visible light band, to achieve high-stability self-injection locking and linewidth narrowing, the external cavity system must strike a delicate balance between an extremely narrow resonant bandwidth and sufficient feedback injection intensity, i.e., a trade-off between high Q value and coupling efficiency. Figure 6 The data results intuitively reveal this physical trade-off mechanism: in order to maintain a high quality factor that meets the requirements of spectral purification, the single optical field exchange ratio between the bus straight waveguide and the micro-ring must be strictly limited through structural design, so that the device operates in the low coupling efficiency range.

[0050] This curve provides a crucial quantitative mapping basis for the device design of this invention. In actual fabrication, based on this curve, designers can first deduce the required target Q value according to the specific linewidth narrowing target required for the FP laser, and then locate the target coupling efficiency corresponding to that Q value on this curve. This correspondence clarifies the physical boundary of optical field coupling, providing a rigorous theoretical guide for further adjustments to physical geometric parameters, such as a racetrack-shaped coupling gap, to achieve this target coupling efficiency.

[0051] like Figure 7 The figure shows the relationship between the coupling efficiency of the coupling region and the coupling gap in an AlN waveguide system for a nitride single-transverse-mode dual-pass racetrack-shaped microring waveguide used in the external cavity of a visible-light FP laser according to the present invention. Figure 7 Simulation data curves show that the coupling efficiency between the bus straight waveguide and the racetrack-shaped microring is extremely sensitive to the coupling gap, exhibiting a classic exponential decay characteristic. When the coupling gap is narrow, such as 100 nm, the Gaussian evanescent field outside the waveguide has a high overlap, resulting in strong optical energy exchange, and the coupling efficiency is at its highest, approximately 6.5%. As the coupling gap gradually increases, the evanescent field decays exponentially, leading to a sharp decrease in coupling efficiency. When the gap increases to 150 nm, the coupling efficiency drops to approximately 1.1%; when the gap is further increased to 200 nm or more, the coupling efficiency drops below 0.2%, at which point almost no optical energy transfer occurs between the bus straight waveguide and the microring.

[0052] Figure 7 As mentioned above Figure 6Together, these constitute the complete parameter design closed loop of this invention for achieving specific spectral purification targets. In actual device development, to meet the requirement of high coherence self-injection locking in the visible light band, the target microring quality factor Q value needs to be maintained at 10. 5 Level, through Figure 6 The mapping relationship leads to the conclusion that the system requires extremely low coupling efficiency, for example, around 1%, to maintain the weak or critical coupling state of the micro-rings. Substituting this target coupling efficiency into... Figure 7 By corresponding to the curve, the required physical coupling gap parameter can be directly and accurately located, which is around 150-160nm.

[0053] also, Figure 7 The exponential distribution curve also provides crucial tolerance selection for photolithography. In the extremely small gap range, such as less than 120nm, a process deviation of a few nanometers can lead to drastic fluctuations in coupling efficiency. However, in the lower coupling efficiency operating region preferred in this invention, such as the 150-200nm range, the curve slope is relatively gentle. This means that when the device of this invention achieves the high Q value required for narrowing the linewidth, the corresponding physical coupling gap is wider. This not only greatly reduces the limit resolution requirements for electron beam lithography (EBL) or deep ultraviolet (DUV) lithography, but also significantly improves the yield and performance consistency of the device in actual mass production and fabrication.

[0054] like Figure 8 The image shows the output spectra at the Through and Drop ends at visible wavelengths in an AlN waveguide system for a nitride single-transverse-mode dual-pass racetrack-shaped microring waveguide used in the external cavity of a visible-light FP laser according to the present invention. Figure 8 The spectral response curves show that, under phase-matching conditions, a light field of a specific wavelength successfully couples from the bus straight waveguide into the racetrack-shaped microring, undergoes strong resonance enhancement within the microring, and is finally smoothly output from the Drop end through a symmetrical dual-pass coupling structure. Extraction Figure 7 As can be seen from the adjacent resonance peak data, the racetrack-shaped microring resonator designed in this embodiment achieves an FSR of approximately 0.181 nm. This value is large enough to ensure that only a single microring resonance peak exists within the gain bandwidth of a typical visible light FP laser, suppressing multi-longitudinal mode lasing and providing a reliable frequency domain filtering guarantee for realizing the single-mode output of the laser.

[0055] Furthermore, by extracting the drop-end transmission spectrum, the high load quality factor of this dual-pass racetrack-shaped micro-ring device was obtained, achieving a significant spectral purification effect and meeting the stringent requirements of high-coherence laser displays and precision atomic clocks for ultra-narrow linewidth light sources.

Claims

1. A nitride single-transverse-mode dual-pass racetrack-shaped microring waveguide for the external cavity of a visible-light band FP laser, characterized in that, The micro-ring waveguide adopts an AlN ridge waveguide system or a GaN strip waveguide system; The AlN ridge waveguide system comprises, from bottom to top: a sapphire substrate (1), an AlN planar layer (4) formed on the sapphire substrate (1), an AlN waveguide layer formed on the AlN planar layer (4), and a SiO2 cladding layer (13) covering the above structure. The GaN strip waveguide system comprises, from bottom to top: a sapphire substrate (1), an AlN buffer layer (2) formed on the sapphire substrate (1), an AlGaN transition layer (3) formed on the AlN buffer layer (2), a GaN waveguide layer formed on the AlGaN transition layer, and a SiO2 cladding layer (13) covering the above structure. The AlN waveguide layer or the GaN waveguide layer is etched to form a waveguide. The waveguide includes a first bus straight waveguide (5), a second bus straight waveguide (6), and a racetrack-shaped micro-ring waveguide (7). The first bus straight waveguide (5) and the second bus straight waveguide (6) are equally spaced on both sides of the straight section of the racetrack-shaped micro-ring waveguide (7) to form a dual-pass coupling structure.

2. The microring waveguide according to claim 1, characterized in that, In the AlN ridge waveguide system, the thickness of the AlN planar layer (4) is 200-800nm, the thickness of the AlN waveguide layer is 100-500nm, the waveguide is a ridge waveguide with a ridge width of 200-1900nm and a sidewall tilt angle of 75-85°.

3. The microring waveguide according to claim 1, characterized in that, In the GaN stripe waveguide system, the thickness of the AlN buffer layer (2) is 500-1000 nm, the thickness of the AlGaN transition layer (3) is 100-200 nm, and the material thereof is Al x Ga 1-x N, wherein 0 < x < 1, the thickness of the GaN waveguide layer is 150-300 nm, and the waveguide is a stripe waveguide with a stripe width of 280-370 nm.

4. The microring waveguide according to claim 1, characterized in that, The thickness of the SiO2 cladding (13) is 1000-1500 nm.

5. The microring waveguide according to claim 1 or 2, characterized in that, In the AlN ridge waveguide system, the thickness of the AlN planar layer (4), the thickness of the AlN waveguide layer, and the ridge width are configured such that the ridge waveguide supports single transverse mode transmission in the visible light band and the transmission loss is less than 0.6 dB / cm.

6. The microring waveguide according to claim 1 or 3, characterized in that, In the GaN strip waveguide system, the thickness of the GaN waveguide layer and the strip width are configured such that the strip waveguide supports single transverse mode transmission in the visible light band with a transmission loss of less than 0.6 dB / cm.

7. The microring waveguide according to claim 1, characterized in that, The perimeter of the racetrack-shaped microring waveguide (7) is configured such that its free spectral range is greater than the gain bandwidth of the Fabry-Perot laser.

8. The microring waveguide according to claim 1, characterized in that, The coupling gap (9) and coupling length (8) between the first bus straight waveguide (5) and the second bus straight waveguide (6) and the racetrack-shaped micro-ring waveguide (7) are configured to make the racetrack-shaped micro-ring waveguide (7) operate in a critically coupled or undercoupled state.

9. The microring waveguide according to claim 8, characterized in that, In the AlN ridge waveguide system, the coupling gap (9) is 150-160 nm.

10. The microring waveguide according to claim 1, characterized in that, In the AlN ridge waveguide system, the bending radius of the racetrack-shaped micro-ring waveguide (7) avoids the radius value that would cause it to resonate with the planar layer mode.

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