External cavity feedback type narrow linewidth semiconductor laser

Abstract

This invention belongs to the field of semiconductor laser beam emitters, and particularly relates to an external cavity feedback type narrow linewidth semiconductor laser. The external cavity feedback type narrow linewidth semiconductor laser includes a semiconductor gain chip and a waveguide grating serving as an external cavity mode selector. The waveguide grating is constructed by alternating stacks of high-refractive-index thin films and low-refractive-index thin films to form a multilayer waveguide with a high refractive index difference, and the grating is embedded within it. This design utilizes the strong optical field confinement capability of the multilayer waveguide and the narrowband filtering characteristics of the grating to achieve extremely narrow linewidth, high spectral stability, and precise wavelength control of the laser beam output. This invention is particularly suitable for high-end applications such as space optical communication and laser beam radar, which have stringent requirements for light source performance and environmental adaptability.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor laser beam technology, and particularly relates to an external cavity feedback type narrow linewidth semiconductor laser. Background Technology

[0002] Narrow-linewidth semiconductor laser beam generators have become the core light source for advanced photonic systems such as high-speed coherent optical communication, high-precision laser beam radar, quantum information processing, and high-resolution spectroscopy due to their excellent monochromaticity, low phase noise, and high coherence.

[0003] From the perspective of resonant cavity structure, narrow-linewidth semiconductor laser beamformers are mainly divided into two categories: internal cavity feedback type and external cavity feedback type. Internal cavity laser beamformers have their resonant cavity completely placed inside the gain medium. Although the structure is compact, it has inherent drawbacks: its frequency stability and monochromaticity are easily affected by changes in operating current and temperature; its wavelength tuning range is limited, and mode switching is prone to occur during tuning; when maintaining single-mode operation, output power and beam quality are often constrained. These shortcomings limit its use in applications requiring high precision, wide tuning range, and low noise.

[0004] External cavity feedback laser beamformers enhance mode selectivity through an external resonant cavity, offering advantages such as strong linewidth compression, high side-mode suppression ratio, and flexible wavelength tuning. They have become the mainstream solution for achieving high-performance, narrow-linewidth laser beams. The external cavity mode selector, as the core component of an external cavity feedback laser beamformer, commonly comes in discrete, fiber, and waveguide types. Among these, waveguide-type external cavity mode selectors, due to their ability to be fabricated using integrated processes, exhibit small size, structural stability, and ease of integration with other photonic devices, making them more suitable for building chip-level photonic systems.

[0005] However, existing waveguide-type external cavity mode selectors still have certain limitations in terms of material selection, optical field confinement capability, and resistance to environmental interference. For example, their performance is prone to degradation in harsh environments such as high temperature and radiation, limiting their application in special occasions such as aerospace and nuclear radiation environments. In addition, the limited optical field confinement capability of traditional waveguide structures affects the linewidth compression effect and long-term stability of laser beam generators. Summary of the Invention

[0006] In view of this, the present invention aims to provide an external cavity feedback type narrow linewidth semiconductor laser. This addresses the technical problems of existing waveguide-type external cavity feedback laser beam emitters, such as insufficient stability, limited linewidth compression, weak anti-interference capability, and difficulty in adapting to special environments.

[0007] To achieve the above objectives, the technical solution created by this invention is implemented as follows:

[0008] An external cavity feedback type narrow linewidth semiconductor laser, comprising: Semiconductor gain chip, used to emit laser beams; The first optical lens is used to shape the laser beam; A waveguide grating is used to filter the wavelength of a shaped laser beam and then feed the selected laser beam of a specific wavelength back to a semiconductor gain chip to work with the semiconductor gain chip to achieve linewidth compression. The waveguide grating includes a substrate layer and a cladding layer. The cladding layer is located above the substrate layer and contains a grating layer and a waveguide layer. The waveguide layer includes at least one high-refractive-index thin film and at least one low-refractive-index thin film, with the low-refractive-index thin film and the high-refractive-index thin film stacked alternately. The cladding layer is a low-refractive-index thin film, and the substrate layer is a silicon-based material.

[0009] Furthermore, the high refractive index film is any one of TiO2 film, Ta2O5 film, ZrO2 film, and HfO2 film, the low refractive index film is SiO2 film or MgF2 film, and the silicon-based material is silicon or silicon carbide.

[0010] Furthermore, the waveguide grating has a length of 8 mm, a thickness of 20 micrometers, and a width of 4 mm.

[0011] Furthermore, each high-refractive-index film in the waveguide layer has a thickness of 30 nanometers, a width of 3 micrometers, and a spacing of 1 micrometer between adjacent high-refractive-index films.

[0012] Furthermore, the high refractive index film thickness of the grating layer is 20 nanometers, and the spacing between the grating layer and the bottommost Ta2O5 film of the waveguide layer is 1.5 micrometers.

[0013] Furthermore, the grating has a length of 800 micrometers, a grating period of 560 nanometers, and an etching depth of 20 nanometers.

[0014] Furthermore, the grating contains sampling segments with different sampling periods.

[0015] Furthermore, the waveguide width includes 11 different sampling periods, with the sampling period of each of the 11 sampling segments ranging from 11 micrometers to 13 micrometers. Each of the 11 sampling segments corresponds to a different duty cycle, and each different sampling period is 25 micrometers wide and 25 micrometers apart.

[0016] Furthermore, the semiconductor gain chip uses a III-V group semiconductor gain dielectric.

[0017] Furthermore, a second optical lens is provided in the output direction of the waveguide grating. A portion of the laser beam of a specific wavelength selected by the waveguide grating is reflected back to the semiconductor gain chip, while the other portion is transmitted to the second optical lens, shaped by the second optical lens, and then coupled into the optical fiber.

[0018] Compared with the prior art, the present invention can achieve the following beneficial effects: 1. The strong optical field confinement capability provided by the waveguide layer with high refractive index difference significantly enhances the interaction efficiency between light and the embedded grating. Combined with the external cavity feedback mechanism, efficient mode selection and linewidth compression can be achieved. At the same time, the excellent thermal stability of high refractive index materials such as TiO2, Ta2O5, ZrO2, and HfO2 ensures the long-term stability of the laser beam output spectrum.

[0019] 2. High-refractive-index materials such as TiO2, Ta2O5, ZrO2, and HfO2 possess inherent high hardness, corrosion resistance, high power damage threshold, and outstanding radiation resistance. Waveguide gratings fabricated using these material systems can maintain stable performance in extreme environments such as space radiation and high temperatures, greatly expanding the application of narrow-linewidth laser beams in special environments such as aerospace, space optical communication, and the nuclear industry.

[0020] 3. The grating etched within the waveguide grating provides a flexible and designable spectral response, enabling the laser beam generator to achieve wavelength selection and precise tuning while narrowing the linewidth. The entire external cavity mode selector adopts an integrated waveguide design, resulting in a compact structure and small size. It is easy to couple and package with semiconductor gain chips with low loss, which is beneficial for the miniaturization and mass production of the system. Attached Figure Description

[0021] 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 A schematic diagram of the structure of the external cavity feedback type narrow linewidth semiconductor laser described in the embodiment of the present invention; Figure 2 A three-dimensional structural schematic diagram of the waveguide grating described in an embodiment of the present invention; Figure 3 A schematic diagram of the cross-sectional structure of the waveguide grating described in the embodiment of the present invention; Figure 4 A schematic diagram of the optical field mode distribution of the waveguide grating described in the embodiment of the present invention; Figure 5 A schematic diagram of the design parameters of the grating described in the embodiment of the present invention; Figure 6 A spectral schematic diagram of the reflectance of the grating described in the embodiment of the present invention; Figure 7 A spectral schematic diagram of the transmittance of the grating described in the embodiment of the present invention.

[0022] Explanation of reference numerals in the attached figures: Semiconductor gain chip 1, First optical lens 2, Waveguide grating 3, Substrate layer 31, Grating layer 32, Grating 321, Waveguide layer 33, High refractive index thin film 331, Low refractive index thin film 332, Cladding layer 34, Second optical lens 4. Detailed Implementation

[0023] 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 merely illustrative of the invention and do not constitute a limitation thereof.

[0024] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0025] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," 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.

[0026] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "assembly," "connection," and "joining" 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.

[0027] The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0028] like Figures 1-3As shown in the figure, this invention provides an external cavity feedback type narrow linewidth semiconductor laser, including a semiconductor gain chip 1, a first optical lens 2, and a waveguide grating 3. The first optical lens 2 is disposed between the semiconductor gain chip 1 and the waveguide grating 3. The semiconductor gain chip 1 serves as a light source for emitting a laser beam; the first optical lens 2 is used to shape the laser beam; and the waveguide grating 3 serves as an external cavity mode selection device for wavelength selection of the shaped laser beam, then feeding the selected specific wavelength laser beam back to the semiconductor gain chip 1, working in conjunction with the semiconductor gain chip 1 to achieve linewidth compression and output a narrow linewidth laser beam. This design, through the external cavity feedback mechanism, can effectively suppress mode hopping in the laser beam and improve spectral purity.

[0029] Semiconductor gain chip 1 uses a gain medium based on III-V semiconductor materials to generate a broadband multi-wavelength beam, covering a certain range of spectral width. The III-V semiconductor gain medium can be made of materials such as gallium arsenide or indium phosphide. The specific material selection depends on the target wavelength. For example, indium phosphide-based chips can be used for the 1550 nm wavelength band commonly used in communications.

[0030] The external cavity feedback type narrow linewidth semiconductor laser also includes a second optical lens 4, which is set in the output direction of the waveguide grating 3. A portion of the laser beam filtered by the waveguide grating 3 is reflected back to the semiconductor gain chip 1 for further amplification, while the other portion is transmitted as the output laser beam to the second optical lens 4, and after being shaped by the second optical lens 4, it is coupled into the optical fiber.

[0031] The waveguide grating 3 includes a substrate layer 31 and a cladding layer 34, with the cladding layer 34 located above the substrate layer 31. The substrate layer 31 is made of silicon-based material, specifically silicon or silicon carbide, while the cladding layer 34 is made of a low-refractive-index thin film. A grating layer 32 and a waveguide layer 33 are embedded within the cladding layer 34. The grating layer 32 is located above or below the waveguide layer 33. The grating layer 32 is made of a high-refractive-index thin film, and a grating 321 is etched on the high-refractive-index thin film. The waveguide layer 33 includes at least one high-refractive-index thin film 331 and at least one low-refractive-index thin film 332, with the low-refractive-index thin film 332 and the high-refractive-index thin film 331 stacked alternately.

[0032] In one specific embodiment of the present invention, the bottom and top layers of the waveguide layer 33 are both high-refractive-index thin films. The bottom high-refractive-index thin film is spaced apart from the grating layer 32 by a predetermined distance, and the top high-refractive-index thin film is in contact with the cladding layer 34. The high-refractive-index thin film 331 is any one of TiO2, Ta2O5, ZrO2, and HfO2 thin films. The low-refractive-index thin film 332 in the waveguide layer 3 and the low-refractive-index thin film in the cladding layer 34 are both SiO2 or MgF2 thin films. The following description uses the example of the high-refractive-index thin film 331 being made of Ta2O5 and the low-refractive-index thin film 332 being made of SiO2.

[0033] The cladding layer 34 uses a SiO2 thin film to provide low refractive index support, the grating layer 32 uses a Ta2O5 thin film, and the waveguide layer 33 is formed by alternating stacks of multiple high refractive index films 331 and low refractive index films 332 to create a high refractive index difference structure, thereby effectively confining the optical mode field. A grating 321 is etched within the grating layer 32. When the grating layer 32 is located below the waveguide layer 33, it is equivalent to the grating 321 being embedded within the bottommost high refractive index film 331 inside the waveguide layer 33, allowing for wavelength selection of the incident light through periodically arranged grating segments.

[0034] The substrate 31 serves as the mechanical support and optical isolation base for the entire waveguide grating 3 structure. Its material is silicon, which exhibits extremely low material loss and good surface flatness. The thickness of the substrate 31 is typically selected between 2 and 8 micrometers, with a preferred range of 3 to 5 micrometers. This thickness range ensures sufficient mechanical strength while effectively suppressing leakage of the optical field from the substrate 31.

[0035] A grating layer 32 is formed by depositing Ta2O5 material on the substrate layer 31, and a grating 321 is formed by scriber mapping of the grating layer 32. The Ta2O5 material has a high refractive index, typically around 2.05 to 2.15 in the visible to near-infrared band. This layer performs two important functions: first, it acts as a partial waveguide structure to participate in optical field confinement; second, it achieves the narrowband filtering function of the grating 321 through internal periodic refractive index modulation.

[0036] A high-refractive-index thin film 331 is first deposited on the grating layer 32, followed by a low-refractive-index thin film 332. This process is repeated to form a multilayer alternating stacked structure based on the high-refractive-index thin film 331 and the low-refractive-index thin film 332, which is the waveguide layer 33.

[0037] Ta2O5 is one of the ideal materials to fill the performance gap between silicon, silicon nitride, and lithium niobate. Ta2O5 possesses excellent properties such as extremely low optical loss (far lower than polycrystalline silicon), high refractive index, wide transparency window, moderate nonlinear effects, excellent chemical stability and physical hardness, no two-photon absorption, corrosion resistance, wear resistance, and radiation resistance. Combined with SiO2, a low-refractive-index material, the two are alternately stacked to form waveguide layer 33. Incident light enters waveguide layer 33 in a direction perpendicular to the multilayer dielectric films, and the refractive index difference between the high-refractive-index film 331 and the low-refractive-index film 332 confines the beam within waveguide layer 33.

[0038] This invention uses a high-refractive-index thin film 331 as the high-refractive-index layer and a low-refractive-index thin film 332 as the low-refractive-index layer. Their alternating arrangement enhances the vertical confinement of the optical mode field, thereby achieving a more compact optical field distribution. This alternating stacking structure differs from traditional single-layer high-refractive-index waveguide cores; instead, it utilizes the periodic refractive index variation of multiple dielectric films to optimize optical transmission characteristics. In one possible implementation, the total thickness of the waveguide layer 33 is determined by the sum of the thicknesses of all the high-refractive-index thin films 331 and low-refractive-index thin films 332.

[0039] The alternating stacking of multiple high-refractive-index thin films 331 and low-refractive-index thin films 332 can effectively improve the waveguide's ability to confine light. Compared to single-material waveguides, multilayer structures can support lower transmission loss within the same cross-sectional area and improve mode field matching with external optical fibers or devices. It should be noted that Ta2O5 material has a high refractive index and extremely low absorption, exhibiting excellent optical performance, especially in the near-infrared band, making it very suitable as a core material for high-performance optical waveguides.

[0040] The refractive index of Ta₂O₅ is typically between 2.0 and 2.3, while that of SiO₂ is approximately 1.45, resulting in a refractive index difference greater than 0.5, sometimes exceeding 0.8. This significant refractive index contrast is a key factor in achieving strong optical field confinement. According to the principle of total internal reflection, when light travels from a high-refractive-index material to a low-refractive-index material, rays with an incident angle greater than the critical angle will undergo total internal reflection at the interface, thus strictly confining the optical mode field to the high-refractive-index layer and its vicinity.

[0041] A larger refractive index difference can significantly reduce the penetration depth of the optical mode field, allowing the light field to be more concentrated within the high-refractive-index thin film 331, reducing leakage to the cladding layer 34 and the substrate layer 31. This strong confinement characteristic is particularly important for realizing waveguides with small bending radii, such as in high-density integrated optical paths, where the bending radius can be reduced to tens of micrometers without causing significant bending losses. For example, at a communication wavelength of 1550 nm, an optical waveguide structure with a multilayer dielectric film and a refractive index difference greater than 0.5 can compress the fundamental mode field diameter to less than 1 micrometer, thereby significantly increasing the integration density.

[0042] It should be noted that although traditional Si3N4 waveguides also have a high refractive index contrast, their refractive index difference is usually around 0.5, and there is a risk of cracking due to high stress. In contrast, the combination of high-refractive-index film 331 and low-refractive-index film 332 not only has a larger refractive index difference, but also a good match in thermal expansion coefficients, resulting in low stress in the deposited film, making it suitable for fabricating multilayer thick film structures.

[0043] The waveguide grating 3 is designed in a cuboid shape to accommodate the requirement of light propagating in a straight line. The cuboid shape facilitates end-face coupling between the waveguide and optical fibers or other optical devices, and also allows for high-density integration on a chip. It should be noted that the dimensions of the cuboid structure can be adjusted according to specific application scenarios; for example, in optical communication modules, the waveguide length is typically between a few millimeters and a few centimeters. Exemplarily, the sides of the cuboid structure can be etched to form flat end faces to reduce light reflection loss during coupling.

[0044] The overall length of the waveguide grating 3 is aligned with the direction of light propagation within the waveguide, ensuring efficient transmission of optical signals along the waveguide axis. This extended length allows the waveguide to support long-distance optical transmission and facilitates integration with other optical components. For example, in an optical communication system, the waveguide's length can be designed to align with the fiber optic axis, achieving low-loss optical signal transmission through precise alignment.

[0045] In one embodiment, a multilayer high-refractive-index thin film 331 extends continuously along the length of the waveguide, forming a strip-shaped high-refractive-index region that runs through the entire waveguide. This extended design ensures that the optical mode field is always confined by the high-refractive-index layer during propagation, avoiding losses due to structural interruptions. It should be noted that the continuity of the high-refractive-index thin film 331 along its length is achieved through deposition and photolithography processes, ensuring the absence of significant defects or breaks. For example, in long-distance waveguides, the continuity of the high-refractive-index thin film 331 can be guaranteed through multiple deposition and surface planarization processes.

[0046] The width direction of the overall waveguide grating 3 is orthogonal to the vertical stacking direction of the high-refractive-index thin film 331, defining the lateral dimensions of the waveguide. The design in the width direction directly affects the lateral confinement capability of the optical mode field, which typically needs to be controlled at the micrometer level to support single-mode transmission. For example, in the width direction, the waveguide can be precisely boundaryed by photolithography to ensure that the optical field does not leak to the sides, thereby reducing transmission loss.

[0047] The height direction of the overall waveguide grating 3 structure is consistent with the multilayer stacking direction of the waveguide layer 33, encompassing the total thickness of the substrate layer 31, grating layer 32, waveguide layer 33, and cladding layer 34. The design in the height direction needs to consider the vertical distribution of the optical mode field, typically enhanced by increasing the cladding thickness. It should be noted that the dimensions in the height direction are usually much smaller than those in the length and width directions to maintain structural compactness.

[0048] It should be noted that the bottom layers of both the grating layer 32 and the waveguide layer 33 are made of Ta2O5 material, and there is no interface transition layer or heterogeneous buffer layer between the grating layer 32 and the waveguide layer 33. They are in direct contact to form a continuous homogeneous Ta2O5 dielectric region. This material continuity design can significantly reduce interface scattering loss and improve the overall waveguide quality factor.

[0049] The refractive index modulation of grating 321 is limited to the interior of grating layer 32, and there are no artificially introduced periodic refractive index modulation structures throughout the entire thickness of waveguide layer 33. That is, from the top of waveguide layer 33 to the boundary with cladding layer 34, this region maintains a uniform bulk material refractive index, without any refractive index modulation processes such as grating etching or ion exchange.

[0050] The grating layer 32 is located directly below the waveguide layer 33, forming a bottom-mounted grating structure. Preferably, the thickness of the waveguide layer 33 is significantly greater than the thickness of the grating layer 32. Typically, the thickness of the waveguide layer 33 is between 1.5 and 4 times the thickness of the grating 321, with a common ratio range of 2.0 to 3.2 times. For example, in a design for the 1550 nm communication band, the thickness of the grating layer 32 can be selected as 80 nm to 180 nm, while the thickness of the waveguide layer 33 is selected as 400 nm to 650 nm. With this thickness ratio, the light field is mainly confined within the thicker waveguide layer 33, and its dominant confinement effect on the light field is significantly stronger than that of the thinner grating layer 32 below.

[0051] Since the optical energy is mainly distributed within the thicker upper waveguide layer 33, while the grating layer 32 is only located in the tail region of the optical energy distribution, the grating 321 has a relatively weak disturbance to the optical field. This weak coupling design can significantly reduce the additional scattering loss and mode conversion loss caused by the grating itself while achieving effective sampling of Bragg reflection.

[0052] In one embodiment of the present invention, the grating 321 is a refractive index modulated sampling Bragg grating, in which all periodic changes in refractive index are confined within the grating layer 32, without extending into the waveguide layer 33 or penetrating the cladding layer 34.

[0053] The grating 321 enables the waveguide grating 3 to function as an external cavity mode selector in narrow-linewidth semiconductor laser beams, achieving mode matching with conventional laser beam chip designs. A waveguide layer 33 is formed by alternating stacks of multiple high-refractive-index thin films 331 and low-refractive-index thin films 332. The grating 321 is embedded within the underlying homogeneous Ta2O5 dielectric region (i.e., the grating layer 32) as an external cavity mode selection element, effectively integrating the grating 321 into the waveguide body. This achieves Bragg diffraction efficiency close to the theoretical limit. This structure effectively isolates external interference to improve stability while simultaneously enabling mode matching with conventional laser beam chips and synergistically achieving narrow linewidth and precise wavelength control. The grating 321 is divided into multiple sampling segments along the waveguide width. Each sampling segment contains sampling periods with different period values, thus forming a grating pattern with multiple reflection peaks.

[0054] In one specific embodiment of the present invention, the waveguide grating 3 has a length of 8 mm, a thickness of 20 μm, and a width of 4 mm. Each Ta₂O₅ film in the waveguide layer 33 has a thickness of 30 nm and a width of 3 μm, with a spacing of 1 μm between adjacent Ta₂O₅ films. The Ta₂O₅ film in the grating layer 32 has a thickness of 20 nm, and the spacing between the grating layer 32 and the bottommost Ta₂O₅ film in the waveguide layer 33 is 1.5 μm. Figure 4 The diagram shows the optical field mode distribution of waveguide grating 3. The horizontal axis represents the spatial dimensions of the optical waveguide, and the vertical axis represents the energy intensity of the optical field. The optical field energy is highly concentrated in the region between waveguide layer 33 and grating layer 32. In the cladding layer 34 and substrate layer 31, the energy intensity of the optical field is significantly attenuated. The optical field is effectively confined to the region between waveguide layer 33 and grating layer 32, with almost no leakage to cladding layer 34 and substrate layer 31. This verifies the effective confinement effect of waveguide layer 33 and grating layer 32 on the optical mode field. Its fundamental mode effective refractive index is 1.4564, which is very close to the refractive index of SiO2. This is a typical characteristic of strong optical field confinement. The value of 1.4564 falls within the typical effective refractive index range (usually 1.4-1.6) of the output waveguide mode of laser beam generator chips integrated on conventional InP-based or Si waveguides. This lays the physical foundation for achieving low-loss, high-efficiency end-face or side-face optical coupling. Therefore, this invention can be matched with the mode of conventional laser beam generator chips, supporting efficient coupling with laser beam generators.

[0055] In another specific embodiment of the present invention, the parameters of the grating are as follows: Figure 5As shown, the grating length L is 800 micrometers, the grating period a is 560 nanometers, and the etching depth d is 20 nanometers. Along the width of the waveguide grating, the grating comprises 11 sampling segments with different sampling periods. Each period has a grating width of 25 micrometers and a spacing of 25 micrometers. The sampling period m of the 11 sampling segments ranges from 11 micrometers to 13 micrometers, and each of the 11 sampling segments corresponds to 11 different duty cycles. The duty cycle = n / m, where n represents the length of the portion of the grating structure actually etched within one sampling period m.

[0056] The length L of the grating determines the total number of periods participating in Bragg diffraction. The longer the length, the sharper the reflection peak (narrower the linewidth) and the higher the reflectivity. An 800-micrometer length narrows the 3dB bandwidth of the reflection peak to 0.15 nanometers, providing sufficiently strong optical feedback to narrow the linewidth of the laser beam.

[0057] The basic grating period 'a' determines the center wavelength of a single reflection peak. 'a' at 560 nm matches the Bragg diffraction condition (λ = 2·n) for the 1.55 μm communication band. eff •a) Ensure efficient diffraction of light at the target wavelength.

[0058] The etching depth d = 20 nanometers, which is consistent with the thickness of the grating layer 32 (20 nanometers). This means that the etching will completely penetrate the 20-nanometer-thick Ta2O5 film, forming the maximum refractive index contrast and bringing the diffraction efficiency close to the theoretical limit. At the same time, it avoids structural damage and light field leakage caused by excessive etching.

[0059] The sampling period m determines the spacing between adjacent reflection peaks. Repeated modulation within an 800-micrometer length generates multiple reflection peaks in the frequency domain, with a peak spacing Δλ = λ. 2 / (2·n eff (m)≈3.2 nanometers. Eleven different m values ​​demonstrate that the reflection peak spacing can be precisely controlled by adjusting the sampling period, meeting the requirements of multi-wavelength screening.

[0060] The duty cycle n / m affects the intensity bandwidth and side-mode suppression ratio of the reflection peak. While ensuring sufficient reflectivity, an appropriate interval region is preserved to form a clear comb-like spectrum. Eleven different values ​​are matched one-to-one with the sampling period to achieve spectral shaping optimization.

[0061] Eleven different m values ​​correspond to eleven different reflection peak spacings, achieving multi-wavelength coverage in the 1.55–1.65 micrometer band; the matching eleven duty cycles ensure the stability of the reflection peak intensity and bandwidth corresponding to each m value, avoiding performance fluctuations caused by parameter mismatch.

[0062] The reflectance spectrum of grating 321 is as follows Figure 6 As shown, the transmittance spectrum of grating 321 is as follows: Figure 7 As shown. Figure 6 The mid-reflection spectrum exhibits sharp reflection peaks. Figure 7 The transmission spectrum shows a corresponding narrowband attenuation valley, indicating that the grating achieves efficient reflection (or strong suppression of transmission) only for light of specific wavelengths, while having almost no reflection (or efficient transmission) for light of other wavelengths, thus possessing significant narrowband filtering characteristics.

[0063] Both the reflection peak and the transmission valley exhibit extremely narrow spectral widths (full width at half maximum). This indicates that waveguide grating 3 responds strongly only to light within an extremely narrow wavelength range, exhibiting very high wavelength resolution and enabling single-mode lasing and linewidth narrowing of the external cavity laser beam.

[0064] High contrast (high extinction ratio): At non-resonant wavelengths (the valley of the reflection spectrum and the peak of the transmission spectrum), the reflectivity is extremely low, while the transmittance is close to 1. This extremely high extinction ratio means that the device can select the target wavelength with extremely high purity and almost completely suppress non-target wavelengths, ensuring the spectral purity and stability of the laser beam output.

[0065] The regularly spaced reflection peaks in the reflectance spectrum and the corresponding regularly spaced transmission valleys in the transmittance spectrum together constitute a typical comb-shaped spectrum. This intuitively demonstrates that the grating of this invention is a sampled grating, not a uniform grating. The comb spacing is determined by the sampling period m, which verifies the ability to flexibly adjust the channel spacing by designing m, providing possibilities for applications such as wavelength division multiplexing.

[0066] In an ideal low-loss device, the sum of reflectivity and transmittance should be close to 100% (ignoring absorption and scattering losses). Observation Figure 6 and Figure 7 The complementary shapes of the mid-reflection peak and transmission valley indicate that the inherent optical loss (absorption and scattering) of waveguide grating 3 is extremely low, with most of the light energy distributed between the reflection and transmission channels. This confirms the success of this invention in reducing transmission loss by using a high-quality Ta2O5 / SiO2 material system and an embedded grating.

[0067] The above description details the structure of the waveguide grating provided in the embodiments of the present invention. The fabrication method of the waveguide grating is as follows: First, a first Ta2O5 thin film is deposited on a thoroughly cleaned SiO2 substrate. This first Ta2O5 thin film is used as the grating layer for subsequent formation of the embedded grating.

[0068] The deposition process can employ various methods such as ion beam sputtering, magnetron sputtering, electron beam evaporation, or pulsed laser beam deposition. Among these, ion beam sputtering and reactive magnetron sputtering are currently the most commonly used methods for obtaining high-quality Ta₂O₅ thin films. By precisely controlling the oxygen partial pressure and deposition rate during the deposition process, Ta₂O₅ thin films with low absorption, high density, and surface roughness better than 0.5 nanometers can be obtained.

[0069] After the first layer of Ta2O5 film is deposited, a grating pattern is formed on the first layer of Ta2O5 film and the refractive index is periodically modulated.

[0070] In a preferred process flow, the desired high-precision grating pattern is first formed on the photoresist coated on the surface of the first Ta2O5 thin film using electron beam lithography or deep ultraviolet lithography. Subsequently, ion beam etching or reactive ion etching is used to transfer the grating pattern into the interior of the Ta2O5 material.

[0071] By precisely controlling the etching time and etching bias, the etching depth can be precisely controlled, thereby obtaining the required refractive index modulation amplitude. It should be noted that in the technical solution of this invention, the etching depth is typically controlled between 100% and 200% of the thickness of the first Ta2O5 film, preferably between 110% and 150%. This partial etching method can obtain sufficient refractive index contrast while avoiding mode loss and other problems caused by over-etching. Furthermore, the 331 Ta2O5 film and 332 SiO2 film in waveguide layer 33 have a sufficient refractive index difference to participate in waveguide confinement.

[0072] After etching, photoresist is removed and the surface is cleaned. Then, a second Ta2O5 film is deposited, followed by a SiO2 film. The structure formed by the alternating stacking of Ta2O5 and SiO2 films serves as a waveguide layer.

[0073] To ensure interface quality and material continuity, the deposition process parameters of the second Ta2O5 film should be kept as consistent as possible with those of the first layer, so as to obtain an interface bonding quality close to that of the bulk material.

[0074] After the waveguide layer is grown, ridge photolithography and etching are required to ensure that all Ta2O5 films in the waveguide layer are 3 micrometers wide.

[0075] Finally, after the waveguide layer deposition is completed, the cladding is deposited. The cladding can be formed by various methods such as plasma-enhanced chemical vapor deposition, low-pressure chemical vapor deposition, or sputtering. Among them, PECVD is widely used due to its low temperature, high deposition rate, and good coverage.

[0076] In one embodiment of the invention, the cladding and waveguide layer maintain a completely flat interface structure, without any form of periodic refractive index perturbation or surface relief grating structure. This design ensures that the refractive index remains uniformly distributed both longitudinally and laterally from the top of the waveguide layer to the entire upper surface region in contact with air or subsequent cladding layers, thereby avoiding additional scattering, polarization-dependent loss, and mode coupling problems that may be introduced by upper surface gratings.

[0077] It should be understood that the various forms of processes shown above can be used to reorder, add, or delete steps. For example, the steps described in this invention disclosure can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution disclosed in this invention can be achieved, and this is not limited herein.

[0078] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. An external cavity feedback type narrow linewidth semiconductor laser, characterized in that, include: Semiconductor gain chip, used to emit laser beams; The first optical lens is used to shape the laser beam; A waveguide grating is used to filter the wavelength of a shaped laser beam and then feed the selected laser beam of a specific wavelength back to a semiconductor gain chip to work with the semiconductor gain chip to achieve linewidth compression. The waveguide grating includes a substrate layer and a cladding layer. The cladding layer is located above the substrate layer and contains a grating layer and a waveguide layer. The waveguide layer includes at least one high-refractive-index thin film and at least one low-refractive-index thin film, with the low-refractive-index thin film and the high-refractive-index thin film stacked alternately. The cladding layer is a low-refractive-index thin film, and the substrate layer is a silicon-based material.

2. The external cavity feedback type narrow linewidth semiconductor laser according to claim 1, characterized in that, The high-refractive-index film is any one of TiO2 film, Ta2O5 film, ZrO2 film, and HfO2 film, and the low-refractive-index film is SiO2 film or MgF2 film. The silicon-based material is silicon or silicon carbide.

3. The external cavity feedback type narrow linewidth semiconductor laser according to claim 1, characterized in that, The waveguide grating is 8 mm long, 20 micrometers thick, and 4 mm wide.

4. The external cavity feedback type narrow linewidth semiconductor laser according to claim 1, characterized in that, Each high-refractive-index film in the waveguide layer is 30 nanometers thick and 3 micrometers wide, with a spacing of 1 micrometer between adjacent high-refractive-index films.

5. The external cavity feedback type narrow linewidth semiconductor laser according to claim 1, characterized in that, The high-refractive-index film of the grating layer is 20 nanometers thick, and the spacing between the grating layer and the bottommost high-refractive-index film of the waveguide layer is 1.5 micrometers.

6. The external cavity feedback type narrow linewidth semiconductor laser according to claim 1, characterized in that, The grating is 800 micrometers long, has a grating period of 560 nanometers, and an etching depth of 20 nanometers.

7. The external cavity feedback type narrow linewidth semiconductor laser according to claim 6, characterized in that, The grating contains sampling sections with different sampling periods.

8. The external cavity feedback type narrow linewidth semiconductor laser according to claim 7, characterized in that, The waveguide width includes 11 different sampling periods. The sampling period of the 11 sampling segments ranges from 11 micrometers to 13 micrometers, and the 11 sampling segments correspond to 11 different duty cycles. Each different sampling period is 25 micrometers wide and 25 micrometers apart.

9. The external cavity feedback type narrow linewidth semiconductor laser according to claim 1, characterized in that, The semiconductor gain chip uses a III-V group semiconductor gain dielectric.

10. The external cavity feedback type narrow linewidth semiconductor laser according to claim 1, characterized in that, A second optical lens is provided in the output direction of the waveguide grating. A portion of the laser beam of a specific wavelength selected by the waveguide grating is reflected back to the semiconductor gain chip, and the other portion is transmitted to the second optical lens. After being shaped by the second optical lens, it is coupled into the optical fiber.

Images