Waveguide grating

Abstract

This invention belongs to the field of optical waveguides, and particularly relates to a waveguide grating, comprising a substrate and a cladding layer. The cladding layer is located above the substrate layer, and a grating layer and a waveguide layer are embedded within the cladding layer. The waveguide layer includes at least one high-refractive-index thin film and at least one low-refractive-index thin film, with the high-refractive-index and low-refractive-index thin films stacked alternately. The cladding layer is a low-refractive-index thin film, and the substrate layer is a silicon-based material. This invention utilizes a combination of two materials with a high refractive index difference—high-refractive-index and low-refractive-index—and, through a multi-layer stacked structure and an embedded grating design, successfully solves the technical bottlenecks of existing waveguide gratings in terms of efficiency, stability, loss, and radiation resistance. It provides a practical and high-performance key component for high-performance external cavity narrow-linewidth lasers, with broad application prospects in fields such as space optical communication, precision metrology, and photonic integration.

Description

Technical Field

[0001] This invention belongs to the field of optical waveguide technology, and particularly relates to a waveguide grating. Background Technology

[0002] With the rapid development of photonic integration technology, optical waveguide devices, as core components for optical signal transmission and processing, are playing an increasingly important role in fields such as optical communication, photonic computing, precision sensing, and quantum information. Based on the principle of total internal reflection, optical waveguides confine the light field within a high-refractive-index medium for transmission, forming the physical basis for optical path integration and functionalization. Introducing periodic microstructures into waveguide systems to form waveguide gratings can further endow them with key functions such as spectral selection, mode coupling, and dispersion modulation, making them an indispensable basic unit for constructing high-performance photonic integrated circuits.

[0003] Based on the spatial distribution characteristics of grating structures in waveguides, existing waveguide gratings are mainly classified into three types: surface gratings, layered gratings, and bulk gratings. Surface gratings form a periodic undulating structure by etching the waveguide surface, making the process relatively simple. However, the grating area is directly exposed to the environment, making it susceptible to external contamination, temperature and humidity fluctuations, and mechanical vibration, resulting in high scattering loss and insufficient long-term stability. Furthermore, its diffraction efficiency is limited by the surface modulation depth, making it difficult to achieve high-efficiency Bragg diffraction. Layered gratings embed the periodic structure within the waveguide interlayer interface. While this improves the structure's weather resistance to some extent, its performance is extremely sensitive to the interface flatness and mode matching accuracy of the multilayer thin films, limiting the freedom of grating design and presenting bottlenecks in achieving high-efficiency, narrow-linewidth spectral modulation. In contrast, bulk gratings, by constructing three-dimensional periodic refractive index modulation within the waveguide medium, possess excellent wavelength selectivity and spectral resolution. Moreover, the entire grating structure is encapsulated by the waveguide material, significantly improving mechanical reliability and environmental stability, thus becoming a key development direction for high-end waveguide grating devices.

[0004] Based on their periodic structure and modulation characteristics, volume gratings can be further subdivided into uniform Bragg gratings, phase-shifted gratings, interlaced gratings, and apodized gratings. Among these, the uniform Bragg grating, through periodic sampling modulation, forms a comb-shaped reflection spectrum with equally spaced replicas in the frequency domain, where the spacing between reflection peaks is precisely determined by the sampling period. This structure combines narrowband filtering with multi-wavelength synchronous selection capabilities, exhibiting unique advantages in optical communication wavelength locking, distributed feedback, and multi-channel sensing applications. However, traditional gratings are mostly based on conventional material systems such as silicon-based or lithium niobate, and their optical field confinement capabilities, nonlinear effect modulation, and extreme environment adaptability are still insufficient to meet the stringent requirements of next-generation space optical communication, high-power laser systems, and radiation-resistant applications.

[0005] In summary, existing waveguide grating technology still faces the following technical challenges when applied to high-performance external cavity mode selection, narrow linewidth laser linewidth reduction, and space environments: First, the diffraction efficiency and stability of surface gratings and layered gratings are insufficient; second, the optical loss, mode confinement capability, and environmental adaptability of traditional bulk grating material systems need to be improved; and third, existing structures cannot simultaneously achieve strong mode field confinement while ensuring precise wavelength control and radiation resistance. Summary of the Invention

[0006] In view of this, the present invention aims to provide a waveguide grating to solve the technical problems of low diffraction efficiency, poor environmental stability, high optical loss, and insufficient radiation resistance of existing waveguide gratings.

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

[0008] A waveguide grating includes a substrate and a cladding layer. The cladding layer is located above the substrate layer. A grating layer and a waveguide layer are embedded in the cladding 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 bottom and top layers of the waveguide layer are both high-refractive-index thin films. The top high-refractive-index thin film is in contact with the cladding, and the bottom high-refractive-index thin film is spaced at a predetermined distance from the grating layer.

[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 thin 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 thin films.

[0012] Furthermore, the thickness of the high-refractive-index film in the grating layer is 20 nanometers, and the spacing between the grating layer and the bottommost high-refractive-index film in 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 grating contains 11 sampling segments with different sampling periods, the sampling period of which ranges from 11 micrometers to 13 micrometers, and the 11 sampling segments correspond to 11 different duty cycles.

[0016] Furthermore, 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.

[0017] Furthermore, the silicon-based material is silicon or silicon carbide.

[0018] Compared with the prior art, the present invention can achieve the following beneficial effects: 1. This invention employs alternating stacks of high-refractive-index materials (with a refractive index difference greater than 0.5) and low-refractive-index materials to form a waveguide layer, which strongly confines the optical mode field, greatly reducing optical leakage and scattering loss. Simultaneously, a grating is embedded within the waveguide as a Bragg volume grating, fully utilizing the near-theoretical limit of the high diffraction efficiency of the volume grating. This achieves excellent wavelength selectivity and narrowband filtering functionality while ensuring high coupling efficiency.

[0019] 2. Compared to exposed surface gratings, this invention fully integrates the key grating structure within the waveguide medium, effectively isolating it from interference and damage from the external environment (such as dust, moisture, and physical contact), significantly improving the device's mechanical stability and long-term reliability. Based on the inherent properties of high-refractive-index materials, this structure also possesses high-temperature stability, corrosion resistance, and high hardness, enabling stable operation in harsher environments.

[0020] 3. When this invention is used as an external cavity mode selection element in a semiconductor laser, its embedded high-performance grating can accurately and stably select wavelengths, effectively narrowing the laser linewidth. More importantly, the high refractive index material has excellent radiation resistance properties, enabling the external cavity narrow linewidth semiconductor laser constructed based on this waveguide structure to be reliably applied in special environments with high-energy particle radiation, such as space optical communication and satellite sensing, greatly expanding the application scenarios of such lasers.

[0021] 4. This invention utilizes a waveguide design with alternating stacks of multiple thin films, providing flexible design space for grating layer integration while achieving strong optical field confinement. Compared to layered gratings, which are limited by interface quality, this invention defines the grating within the waveguide, reducing the stringent requirements for interface perfection and offering better process tolerance. Furthermore, by precisely optimizing the thickness and spacing of each layer, as well as grating parameters (such as period, duty cycle, and etching depth), the optical characteristics of the device can be flexibly controlled to meet diverse application needs.

[0022] 5. High-refractive-index materials possess a high power damage threshold characteristic. Combined with the uniform structure design of multilayer thin films, this waveguide structure can stably withstand the transmission and modulation of high-power optical signals, avoiding the device damage problems that are prone to occur with traditional materials under high-power conditions. This advantage makes it irreplaceable in the fields of high-power laser sources and microwave photonics, providing key structural support for the research and development of high-power photonic devices. Attached Figure Description

[0023] 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 waveguide grating structure described in the embodiment of the present invention; Figure 2 This is a schematic diagram of the structure of the grating layer and waveguide layer described in the embodiment of the present invention; Figure 3 A schematic diagram of the optical field mode distribution of the waveguide grating described in the embodiment of the present invention; Figure 4 A schematic diagram of the design parameters of the grating described in the embodiment of the present invention; Figure 5 A spectral schematic diagram of the reflectance of the grating described in the embodiment of the present invention; Figure 6 A spectral schematic diagram of the transmittance of the grating described in the embodiment of the present invention.

[0024] Explanation of reference numerals in the attached figures: Substrate layer 1, grating layer 2, grating 21, waveguide layer 3, high refractive index thin film 31, low refractive index thin film 32, cladding layer 4. Detailed Implementation

[0025] 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.

[0026] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0027] 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.

[0028] 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.

[0029] The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0030] like Figure 1 and Figure 2 As shown, an embodiment of the present invention provides a waveguide grating, including a substrate layer 1 and a cladding layer 4. The cladding layer 4 is located above the substrate layer 1. The substrate layer 1 is a silicon-based thin film, such as a silicon thin film or a silicon carbide thin film, and the cladding layer 4 is a low-refractive-index thin film. A grating layer 2 and a waveguide layer 3 are embedded in the cladding layer 4. The grating layer 2 is located above or below the waveguide layer 3. The grating layer 2 is a high-refractive-index thin film, and a grating 21 is etched on the high-refractive-index thin film. The waveguide layer 3 includes at least one high-refractive-index thin film 31 and at least one low-refractive-index thin film 32, with the low-refractive-index thin film 32 and the high-refractive-index thin film 31 stacked alternately.

[0031] In a specific embodiment of the present invention, the bottom layer and the top layer of the waveguide layer 3 are both high refractive index films. The bottom high refractive index film 31 is spaced apart from the grating layer 2 by a predetermined distance, and the top high refractive index film 31 is in contact with the cladding layer 4.

[0032] The high-refractive-index thin film 31 can be any one of TiO2, Ta2O5, ZrO2, or HfO2. The low-refractive-index thin film 32 in the waveguide layer 3 and the low-refractive-index thin film in the cladding layer 4 are both SiO2 or MgF2 thin films. The following explanation uses the example of high-refractive-index thin film 31 being made of Ta2O5 and low-refractive-index thin film 32 being made of SiO2.

[0033] Substrate 1 serves as the mechanical support and optical isolation base for the entire waveguide grating structure. Its material is silicon or silicon carbide, exhibiting extremely low material loss and excellent surface flatness. The thickness of substrate 1 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 substrate 1.

[0034] A grating layer 2 is formed by directly depositing Ta2O5 material on the substrate layer 1, and a grating 321 is formed by scriber mapping of the grating layer 2. Ta2O5 material has a high refractive index, typically around 2.05–2.15 in the visible to near-infrared band. This layer simultaneously performs two important functions: firstly, it acts as a partial waveguide structure to participate in optical field confinement; secondly, it achieves the narrowband filtering function of the grating 21 through internal periodic refractive index modulation.

[0035] A high-refractive-index thin film 31 is first deposited on the grating layer 2, followed by a low-refractive-index thin film 32. This process is repeated to form a multilayer alternating stacked structure based on the high-refractive-index thin film 31 and the low-refractive-index thin film 32, which is the waveguide layer 3.

[0036] 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 3. Incident light enters waveguide layer 3 in a direction perpendicular to the multilayer thin films, and the refractive index difference between the high-refractive-index film 31 and the low-refractive-index film 32 confines the beam within waveguide layer 3.

[0037] This invention uses a high-refractive-index thin film 31 as the high-refractive-index layer and a low-refractive-index thin film 32 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 thin films to optimize optical transmission characteristics. In one possible implementation, the total thickness of the waveguide layer 3 is determined by the sum of the thicknesses of all high-refractive-index thin films 31 and low-refractive-index thin films 32.

[0038] The alternating stacking of multiple high-refractive-index thin films 31 and low-refractive-index thin films 32 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.

[0039] 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.

[0040] A larger refractive index difference can significantly reduce the penetration depth of the optical mode field, allowing the light field to be more concentrated inside the high-refractive-index thin film 31, reducing leakage to the cladding layer 4 and the substrate layer 1. 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, a multilayer thin film optical waveguide structure with 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.

[0041] The waveguide grating 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. For example, the sides of the cuboid structure can be etched to form flat end faces to reduce light reflection loss during coupling.

[0042] The overall length of the waveguide grating structure 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.

[0043] In one embodiment, a multilayer high-refractive-index thin film 31 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 31 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 31 can be guaranteed through multiple deposition and surface planarization processes.

[0044] The width direction of the overall waveguide grating structure is orthogonal to the vertical stacking direction of the high-refractive-index thin film 31, 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 demarcated using photolithography to ensure that the optical field does not leak to the sides, thereby reducing transmission loss.

[0045] The height direction of the overall waveguide grating structure is aligned with the multilayer stacking direction of waveguide layer 3, encompassing the total thickness of substrate layer 1, grating layer 2, waveguide layer 3, and cladding layer 4. The height direction design must 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.

[0046] It should be noted that the bottom layers of both grating layer 2 and waveguide layer 3 are made of Ta2O5 material, and there is no interface transition layer or heterogeneous buffer layer between grating layer 2 and waveguide layer 3. 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.

[0047] The refractive index modulation of grating 21 is limited to the interior of grating layer 2, and there are no artificially introduced periodic refractive index modulation structures throughout the entire thickness of waveguide layer 3. That is to say, from the top of waveguide layer 3 to the boundary with cladding layer 4, this region maintains a uniform bulk material refractive index, without any refractive index modulation processes such as grating etching or ion exchange.

[0048] The grating layer 2 is located directly below the waveguide layer 3, forming a bottom-mounted grating structure. Preferably, the thickness of the waveguide layer 3 is significantly greater than the thickness of the grating layer 2. Typically, the thickness of the waveguide layer 3 can be between 1.5 and 4 times the thickness of the grating 21, 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 2 can be selected as 80 nm to 180 nm, while the thickness of the waveguide layer 3 can be selected as 400 nm to 650 nm. With this thickness ratio, the light field is mainly confined within the thicker waveguide layer 3, and its dominant confinement effect on the light field is significantly stronger than that of the thinner grating layer 2 below.

[0049] Since the optical energy is mainly distributed within the thicker upper waveguide layer 3, while the grating layer 2 is only located in the tail region of the optical energy distribution, the grating 21 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.

[0050] In one embodiment of the present invention, the grating 21 is a refractive index modulated sampling Bragg grating, in which all periodic changes in refractive index are confined within the grating layer 2, without extending into the waveguide layer 3 or penetrating the cladding layer 4.

[0051] The grating 21 enables the waveguide grating to be used as an external cavity mode selector in narrow-linewidth semiconductor lasers, achieving mode matching with conventional laser chips. A waveguide layer 3 is formed by alternating stacks of multiple high-refractive-index thin films 31 and low-refractive-index thin films 32. The grating 21 is embedded within the underlying homogeneous Ta2O5 dielectric region (i.e., grating layer 2) as an external cavity mode selection element, effectively integrating the grating 21 into the waveguide body. This achieves Bragg diffraction efficiency close to the theoretical limit. This structure effectively isolates external interference to improve stability, achieves mode matching with conventional laser chips, and synergistically enables narrowing of the linewidth and precise wavelength control. The grating 21 is divided into multiple sampling segments along the waveguide propagation direction. Each sampling segment contains sampling periods with different period values, thus forming a grating pattern with multiple reflection peaks.

[0052] In one specific embodiment of the present invention, the waveguide grating has a length of 8 mm, a thickness of 20 μm, and a width of 4 mm. Each Ta₂O₅ film in waveguide layer 3 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 grating layer 2 has a thickness of 20 nm, and the spacing between grating layer 2 and the bottommost Ta₂O₅ film in waveguide layer 3 is 1.5 μm. Figure 3The diagram shows the optical mode distribution of the waveguide grating. 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 energy is highly concentrated in the region between waveguide layer 3 and grating layer 2. In cladding layer 4 and substrate layer 1, the energy intensity of the optical field is significantly attenuated, effectively confining the optical field within the region between waveguide layer 3 and grating layer 2, with almost no leakage to cladding layer 4 and substrate layer 1. This verifies the effective confinement effect of waveguide layer 3 and grating layer 2 on the optical mode field. The effective refractive index of its fundamental mode is 1.4564, very close to the refractive index of SiO2, which is a typical characteristic of strong optical field confinement. This value of 1.4564 falls precisely within the typical effective refractive index range (usually 1.4-1.6) of the output waveguide mode of laser 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 modes of conventional laser chips, supporting efficient coupling with lasers.

[0053] In another specific embodiment of the present invention, the parameters of the grating are as follows: Figure 4 As 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, it 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 a different duty cycle, where the duty cycle = n / m, and n represents the length of the portion of the grating structure actually etched within a sampling period m. The grating length L determines the total number of periods participating in Bragg diffraction. A longer length results in a sharper reflection peak (narrower linewidth) and higher reflectivity. The 800-micrometer length narrows the 3dB bandwidth of the reflection peak to 0.15 nanometers, providing sufficiently strong optical feedback to narrow the laser linewidth.

[0054] 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.

[0055] The etching depth d = 20 nanometers, which is consistent with the thickness of grating layer 2 (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.

[0056] 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 for multi-wavelength selection.

[0057] The duty cycle n / m affects the intensity bandwidth and side-mode suppression ratio of the reflection peak. The parameters of grating 21 ensure a suitable refractive index while retaining appropriate spacing regions to form a clear comb-like spectrum. Eleven different values ​​are matched one-to-one with the sampling period to achieve spectral shaping optimization.

[0058] 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.

[0059] The reflectance spectrum of grating 21 is as follows Figure 5 As shown, the transmittance spectrum of grating 21 is as follows: Figure 6 As shown. Figure 5 The mid-reflection spectrum exhibits sharp reflection peaks. Figure 6 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.

[0060] Both the reflection peak and the transmission valley exhibit extremely narrow spectral widths (full width at half maximum). This indicates that the waveguide grating responds strongly only to light within an extremely narrow wavelength range, possessing very high wavelength resolution and enabling single-mode lasing and linewidth compression of the external cavity laser.

[0061] 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 output.

[0062] 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.

[0063] In an ideal low-loss device, the sum of reflectivity and transmittance should be close to 100% (ignoring absorption and scattering losses). Observation Figure 5 and Figure 6The complementary shapes of the mid-reflection peak and transmission valley indicate that the inherent optical loss (absorption and scattering) of the waveguide grating 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 using a high-quality Ta2O5 / SiO2 material system and an embedded grating.

[0064] 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.

[0065] The deposition process can employ various methods such as ion beam sputtering, magnetron sputtering, electron beam evaporation, or pulsed laser 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.

[0066] 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.

[0067] 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.

[0068] 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 the complete etching through the entire grating layer, ensuring that the Ta2O5 film in the waveguide layer has sufficient refractive index difference to participate in waveguide confinement.

[0069] 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.

[0070] 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. In fact, it can be deposited directly and continuously in the same deposition equipment without breaking the vacuum, thereby obtaining an interface bonding quality close to that of bulk materials.

[0071] 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.

[0072] 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.

[0073] Specifically, after waveguide layer deposition, surface planarization is typically performed, such as using chemical mechanical polishing or ion beam polishing, to control the surface roughness of the waveguide layer to below 0.3 nanometers. Then, the process proceeds directly to cladding deposition, without any photolithography, etching, or ion implantation steps that would alter the surface refractive index distribution. This deliberate avoidance in the process path is precisely to ensure the integrity and uniformity of the interface region on the waveguide layer.

[0074] 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.

[0075] 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. A waveguide grating, characterized in that, It includes a substrate layer and a cladding layer. The cladding layer is located above the substrate layer and has a grating layer and a waveguide layer embedded in it. 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 waveguide grating according to claim 1, characterized in that, The bottom and top layers of the waveguide layer are both high-refractive-index thin films. The top high-refractive-index thin film is in contact with the cladding, and the bottom high-refractive-index thin film is spaced at a predetermined distance from the grating layer.

3. The waveguide grating according to claim 1, characterized in that, The waveguide grating is 8 mm long, 20 micrometers thick, and 4 mm wide.

4. The waveguide grating according to claim 1, characterized in that, Each high-refractive-index thin film in the waveguide layer is 30 nanometers thick and 3 micrometers wide, with a spacing of 1 micrometer between adjacent high-refractive-index thin films.

5. The waveguide grating according to claim 1, characterized in that, The thickness of the high-refractive-index film in the grating layer is 20 nanometers, and the spacing between the grating layer and the bottommost high-refractive-index film in the waveguide layer is 1.5 micrometers.

6. The waveguide grating 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 waveguide grating according to claim 6, characterized in that, The grating contains sampling sections with different sampling periods.

8. The waveguide grating according to claim 7, characterized in that, The grating contains 11 sampling segments with 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.

9. The waveguide grating 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.

10. The waveguide grating according to claim 1, characterized in that, Silicon-based materials are silicon or silicon carbide.

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