Ultra-narrow band optical filter and method of designing the same

Abstract

This invention discloses an ultra-narrowband optical filter and its design method, belonging to the field of micro-nano optical device technology. It is a reflective all-dielectric guided-mode resonant filter, comprising, from bottom to top, a substrate, a planar waveguide layer, a single-layer one-dimensional subwavelength grating layer, and a single-layer anti-reflection film. The design method includes initial tuning of the resonant wavelength, optimization of the ultra-narrowband bandwidth, fine-tuning of the resonant wavelength, and synergistic optimization of sideband suppression. It clarifies the priority of adjusting each structural parameter, achieving narrowband reflection filtering through the guided-mode resonance effect of the grating and waveguide layer. Through the synergistic design of the single-layer anti-reflection film and the resonant structure, non-resonant broadband background reflection is suppressed without disrupting the ultra-narrowband resonant peak. At the target filtering wavelength of 1064nm, an ultra-narrowband filter with a peak reflectivity of 100% and a full width at half maximum (FWHM) as low as 0.03nm can be achieved, with a sideband suppression ratio exceeding 30dB. This solves the core defect of existing technologies that cannot simultaneously achieve ultra-narrowband filtering and high sideband suppression ratio.

Description

Technical Field

[0001] This invention relates to the fields of micro-nano optics and metasurface devices, and in particular to an ultra-narrowband optical filter and its design method. Background Technology

[0002] Modern optical systems, particularly in fields such as laser precision machining, quantum information processing, high-density wavelength division multiplexing (WDM) communication, and biofluorescence detection, place unprecedented demands on the high-performance filtering components in the core optical path: while achieving extremely narrow spectral selectivity at a specific wavelength, they must also possess extremely strong suppression capabilities for stray light outside that wavelength. This requirement has directly driven the evolution of optical filter technology from traditional bulk optical elements towards planarization, integration, and high performance.

[0003] Traditional multilayer dielectric filters require stacking a large number of layers to achieve narrow bandwidth, resulting in large thickness, high stress, angle sensitivity, and difficulty in integration. Planar optics, represented by metasurfaces, provides a new approach to device miniaturization and integration. Among them, subwavelength grating structures based on the guided-mode resonance (GMR) principle have become the preferred solution for realizing on-chip integrated ultra-narrowband filters due to their clear physical mechanism and ease of generating resonant responses with extremely high efficiency and extremely narrow linewidth at specific wavelengths.

[0004] However, while achieving picometer-level bandwidth, GMR-based filters face a fundamental and persistent limitation: persistently high sideband background reflectivity due to the inherent Fresnel reflection of the dielectric material constituting the device. This results in a broad-spectrum high-reflectivity plateau superimposed on the reflection spectrum of the GMR filter below the ideal narrowband resonance peak, severely degrading the filter's actual sideband rejection ratio (SRR) and failing to meet the extremely high signal-to-noise ratio requirements of cutting-edge applications such as quantum key distribution and ultra-narrow linewidth laser frequency stabilization. Therefore, how to significantly improve the SRR of GMR filters without sacrificing ultra-narrowband filtering performance has become a pressing technical problem in this field. Summary of the Invention

[0005] To address the core deficiency of existing optical filtering devices in achieving both ultra-narrowband filtering and high sideband suppression ratio, this invention aims to provide an ultra-narrowband optical filter and its design method. Based on the guided mode resonance principle, this filter achieves ultra-narrowband filtering and high sideband suppression at the target filtering wavelength through the synergistic design of a single-layer anti-reflection film and a resonant structure.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] On one hand, the present invention provides an ultra-narrowband optical filter, which is a reflective all-dielectric guided-mode resonant filter, comprising, from bottom to top, a substrate, a planar waveguide layer, a single-layer one-dimensional subwavelength grating layer, and a single-layer anti-reflection film; The refractive index of the planar waveguide layer is greater than the equivalent refractive index of the single-layer one-dimensional subwavelength grating layer, forming a waveguide confinement structure. The single-layer one-dimensional subwavelength grating layer is composed of periodically arranged all-dielectric grating strips. Its grating period is smaller than the target filtering wavelength of the ultra-narrowband optical filter. The grating period, grating strip width, and grating strip height of the single-layer one-dimensional subwavelength grating layer are matched with the target filtering wavelength, satisfying the guided mode resonance phase matching condition, and can generate a resonance reflection peak at the target filtering wavelength.

[0008] Furthermore, the ideal refractive index of the single-layer antireflective film satisfies:

[0009] in, The ideal refractive index for a single-layer antireflective film, The refractive index of the incident medium on the incident side of the ultra-narrowband optical filter is defined as follows: the incident medium is a homogeneous medium through which incident light propagates before reaching the ultra-narrowband optical filter. Under normal operating conditions, the incident medium is air at standard atmospheric pressure, and the refractive index is... . The equivalent refractive index is the single-layer one-dimensional subwavelength grating layer, which is calculated using the equivalent medium theory.

[0010] The single-layer antireflective film fully covers the upper surface of the single-layer one-dimensional subwavelength grating layer. The refractive index and thickness of the single-layer antireflective film are synergistically optimized. In the non-resonant broadband band, it reduces the Fresnel background reflection on the device surface through the interference destructive effect, without changing the guided mode resonance phase matching condition between the single-layer one-dimensional subwavelength grating layer and the planar waveguide layer. It retains the ultra-narrowband characteristics and high reflectivity of the resonance reflection peak at the target operating wavelength.

[0011] Furthermore, under the condition of perpendicular incident light, the optical thickness of the single-layer antireflective film is 1 / 4 of the center wavelength of the non-resonant broadband band, satisfying:

[0012] in, The refractive index of a single-layer antireflective film, The thickness of a single-layer antireflective film, This refers to the center wavelength of the non-resonant broadband band. The non-resonant broadband band is a broadband transmission band located on both sides of the target filtering wavelength, which can be selected according to the range of the target filtering wavelength, and is usually close to the target filtering wavelength.

[0013] On the other hand, the present invention provides a design method for an ultra-narrowband optical filter, comprising the following hierarchical optimization steps: S1, Determine the design specifications, including the target filter wavelength, target filter bandwidth, and target sideband rejection ratio; S2, Initial tuning of resonant wavelength: Based on the guided mode resonant phase matching condition, an initial basic structural model of the ultra-narrowband optical filter is constructed. The basic structural model includes, from bottom to top, a substrate, a planar waveguide layer, a single-layer one-dimensional subwavelength grating layer, and a single-layer anti-reflection film. The modulation law of the grating period on the resonant wavelength is quantified by numerical simulation, and the initial grating period that satisfies the target filtering wavelength is determined. S3, Ultra-narrowband bandwidth optimization: With a fixed initial grating period, the influence of grating strip width and height on the filtering bandwidth is quantified by the control variable method. The grating strip width and height are iteratively optimized so that the filtering bandwidth of the ultra-narrowband optical filter at the target filtering wavelength meets the design specifications. S4, Resonance Wavelength Refinement: Fix the optimized grating strip width and height, fine-tune the grating period and planar waveguide layer thickness, and accurately calibrate the resonant reflection peak to the target filtering wavelength while maintaining ultra-narrowband filtering characteristics; S5, Sideband Rejection Ratio Co-optimization: A single-layer anti-reflection film is designed on the surface of the refined grating layer. The refractive index and thickness of the single-layer anti-reflection film are optimized to achieve background reflection suppression in the non-resonant broadband band without destroying the guided mode resonant phase matching condition. Ultimately, the sideband rejection ratio of the device meets the design specifications.

[0014] The beneficial effects of the above-mentioned technical solution of the present invention are as follows: This invention effectively solves the technical bottlenecks of insufficient bandwidth and inadequate sideband suppression ratio in traditional narrowband optical filters by synergistically designing a single-layer antireflective film and a guided-mode resonant metasurface structure.

[0015] Experimental verification shows that at the target wavelength of 1064nm, this design can achieve picometer-level ultra-narrow bandwidth and near 100% peak reflectivity. At the same time, through the interference cancellation effect of the anti-reflection film, the sideband background reflection is suppressed to more than 30dB, achieving a balance between ultra-narrowband filtering and ultra-high suppression ratio performance.

[0016] The ultra-narrowband optical filter provided by this invention has a simple structure, is entirely planar in dielectric form, and is compatible with standard micro-nano fabrication processes, providing a feasible technical solution for the development of high-performance, highly integrated on-chip filters. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of the structure of an ultra-narrowband optical filter in one embodiment.

[0019] Figure 2 The image shows the reflectivity curve and electric field distribution at the resonant wavelength of an ultra-narrowband optical filter in one embodiment. Detailed Implementation

[0020] The technical solution of the present invention will now be clearly and completely described through specific embodiments. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0021] Reference Figure 1 In one embodiment, an ultra-narrowband optical filter is provided, which is a reflective all-dielectric guided mode resonant filter, comprising, from bottom to top: a substrate 1, a planar waveguide layer 2, a single-layer one-dimensional subwavelength grating layer 3, and a single-layer anti-reflection film 4.

[0022] The refractive index of the planar waveguide layer 2 is greater than the equivalent refractive index of the single-layer one-dimensional subwavelength grating layer 3, forming a waveguide confinement structure. The single-layer one-dimensional subwavelength grating layer 3 is composed of periodically arranged all-dielectric grating strips 3-1. Its grating period is smaller than the target filtering wavelength of the ultra-narrowband optical filter. The grating period, grating strip width, and grating strip height of the single-layer one-dimensional subwavelength grating layer 3 are matched with the target filtering wavelength, satisfying the guided mode resonance phase matching condition, and can generate a resonance reflection peak at the target filtering wavelength.

[0023] A single-layer anti-reflection film 4 is applied to the upper surface of a single-layer one-dimensional subwavelength grating layer 3 to suppress background reflection in non-resonant bands and improve the sideband suppression ratio.

[0024] The substrate 1, planar waveguide layer 2, single-layer one-dimensional subwavelength grating layer 3, and single-layer anti-reflection film 4 are all metal-free all-dielectric materials.

[0025] The substrate 1 described in this invention is located at the bottom layer and provides mechanical support for the entire device. The material selection requirements for substrate 1 are: optical transparency in the near-infrared band at the target filtering wavelength of 1064nm, low loss, compatibility with micro / nano fabrication processes, and a refractive index lower than that of the planar waveguide layer to ensure waveguide confinement effect. The substrate 1 can be made of any of the following materials: quartz glass (… Materials include: (refractive index 1.45-1.5), borosilicate glass, sapphire, calcium fluoride, etc.

[0026] In one embodiment, the substrate 1 is selected as a quartz glass substrate, and the substrate refractive index is... .

[0027] The planar waveguide layer 2 of the present invention is disposed on the substrate 1 and below the single-layer one-dimensional subwavelength grating layer 3, and is used to support and constrain the guided mode, and is the core layer for exciting guided mode resonance.

[0028] The material selection requirements for the planar waveguide layer 2 are: transparency in the target operating wavelength range, refractive index greater than the substrate refractive index and the equivalent refractive index of the single-layer one-dimensional subwavelength grating layer 3, and low optical loss. The optional materials for the planar waveguide layer 2 are any of the following: , Materials include lithium niobate, alumina, and optical polymers, with a refractive index range of 1.8-3.5.

[0029] The thickness design requirements for the planar waveguide layer 2 are as follows: it must meet the waveguide single-mode transmission conditions and avoid high-order mode interference. The selectable thickness range is 100nm-500nm. The thickness of the planar waveguide layer 2 is linearly positively correlated with the resonant wavelength.

[0030] In one embodiment, the planar waveguide layer 2 is selected from... Material, refractive index of planar waveguide layer 2 Thickness of planar waveguide layer 2 .

[0031] The single-layer one-dimensional subwavelength grating layer 3 is disposed on the planar waveguide layer 2 and is composed of periodically arranged all-dielectric grating strips 3-1. The grating period is smaller than the target filtering wavelength, which is used to provide momentum matching, couple the incident light to the planar waveguide layer, and excite guided mode resonance.

[0032] The material selection requirements for the single-layer one-dimensional subwavelength grating layer 3 are: compatibility with the planar waveguide layer material, transparency in the target operating wavelength band, and low optical loss.

[0033] In one embodiment, the single-layer one-dimensional subwavelength grating layer 3 is selected from the same materials as the planar waveguide layer 2. Material, refractive index The structural parameters of the single-layer one-dimensional subwavelength grating layer 3 are: grating period p = 706 nm, grating strip width w = 200 nm, and grating strip height g = 50 nm.

[0034] The single-layer anti-reflection film 4 of this invention completely covers the upper surface of the single-layer one-dimensional subwavelength grating layer 3, and is used to suppress background reflection in the non-resonant band and improve the sideband suppression ratio. The material selection requirements for the single-layer anti-reflection film 4 are: transparency in the target operating band, low optical loss, mature fabrication process, and compatibility with micro-nano fabrication processes. The optional materials for the single-layer anti-reflection film 4 are: , , wait.

[0035] Two options are provided below: Option 1: Single-layer anti-reflective film 4 Thin film, refractive index ,thickness ; Option 2: Single-layer anti-reflective film 4 Thin film, refractive index 5. Thickness Option 2 has a higher level of technological maturity and lower cost.

[0036] In this embodiment, the single-layer one-dimensional subwavelength grating layer 3 is a subwavelength periodic structure, with a grating period smaller than the target filtering wavelength. According to the zero-order equivalent medium theory, the formula for calculating the equivalent refractive index of the single-layer one-dimensional subwavelength grating layer 3 for TE-polarized incident light is as follows:

[0037] in, The fill factor of the grating is defined as the ratio of the grating strip width to the grating period, i.e. ; The refractive index of the grating strip material; The refractive index of the medium between the grating strips is denoted as ρ. In this embodiment, the medium between the grating strips is air at standard atmospheric pressure. .

[0038] Substituting the structural parameters from the above embodiment: grating strip width w = 200 nm, grating period p = 706 nm, the fill factor is calculated. The grating strip material is Refractive index in the 1064nm band 837. Substituting the above parameters into the formula, the equivalent refractive index of the single-layer one-dimensional subwavelength grating layer 3 in this embodiment is calculated. .

[0039] Based on the equivalent refractive index of a single-layer one-dimensional subwavelength grating layer 3 The ideal refractive index of the single-layer antireflective film 4 satisfies:

[0040] in, The ideal refractive index for a single-layer antireflective film 4. The refractive index of the incident medium on the incident side of the ultra-narrowband optical filter is defined as follows: the incident medium is a homogeneous medium through which incident light propagates before reaching the ultra-narrowband optical filter. Under normal operating conditions, the incident medium is air at standard atmospheric pressure, and the refractive index is... . Let be the equivalent refractive index of the single-layer one-dimensional subwavelength grating layer 3. Thus, the refractive index of the ideal antireflective coating is calculated to be... Considering the wide range of commercially available, mature optical thin-film materials, this embodiment selects the one with the best process compatibility. (n=1.5) and the theoretically optimal match (n=1.36) was used as the material for the single-layer anti-reflection film 4, and the thickness was optimized by COMSOL full-wave electromagnetic simulation. Finally, the optimal thickness parameters of the single-layer anti-reflection film 4 were determined.

[0041] Under perpendicular incident light conditions, the optical thickness of the single-layer antireflective film 4 is 1 / 4 of the center wavelength of the non-resonant broadband band, satisfying:

[0042] in, The refractive index of the single-layer antireflective film 4 is... The thickness of the single-layer antireflective film 4, The center wavelength of the non-resonant broadband band is the broadband transmission band located on both sides of the target filtering wavelength.

[0043] The above embodiments use zero-order equivalent medium theory to complete the initial calculation of the equivalent refractive index of the single-layer one-dimensional subwavelength grating layer 3. Those skilled in the art can also use conventional methods in the field, such as first-order equivalent medium theory, rigorous coupled-wave analysis, and finite element numerical simulation, to calculate the equivalent refractive index of the single-layer one-dimensional subwavelength grating layer 3 more accurately without affecting the implementation of the present invention.

[0044] In one embodiment, the target filtering wavelength is 1064 nm. To achieve a peak reflectance of 100% at the target filtering wavelength for the ultra-narrowband optical filter, the full width at half maximum (FWHM) is 0.03 nm, and the sideband suppression ratio (SPR) is ≥30 dB. The substrate 1 is selected as a quartz glass substrate with a refractive index of [missing information]. The planar waveguide layer 2 is selected from... Material, refractive index of planar waveguide layer 2 The thickness of planar waveguide layer 2 The single-layer one-dimensional subwavelength grating layer 3 is made of the same material as the planar waveguide layer 2. Material, refractive index The structural parameters of the single-layer one-dimensional subwavelength grating layer 3 are: grating period p = 706 nm, grating strip width w = 200 nm, and grating strip height g = 50 nm. The single-layer anti-reflective film 4 is... Thin film, refractive index 1.36, thickness .

[0045] In another embodiment, the target filtering wavelength is 1064 nm. To achieve a peak reflectance of 100% at the target filtering wavelength for the ultra-narrowband optical filter, the full width at half maximum (FWHM) is 0.03 nm, and the sideband suppression ratio is ≥30 dB. The substrate 1 is a quartz glass substrate with a refractive index of [missing information]. The planar waveguide layer 2 is selected from... Material, refractive index of planar waveguide layer 2 The thickness of planar waveguide layer 2 The single-layer one-dimensional subwavelength grating layer 3 is made of the same material as the planar waveguide layer 2. Material, refractive index The structural parameters of the single-layer one-dimensional subwavelength grating layer 3 are: grating period p = 706 nm, grating strip width w = 200 nm, and grating strip height g = 50 nm. The single-layer anti-reflective film 4 is... Thin film, refractive index 1.5, thickness .

[0046] In one embodiment, a design method for the aforementioned ultra-narrowband optical filter is provided, detailing the collaborative optimization process of device parameters, including the following steps: S1, Determine the design specifications, including the target filter wavelength, target filter bandwidth, and target sideband rejection ratio; S2, Initial tuning of resonant wavelength: Based on the guided mode resonant phase matching condition, an initial basic structural model of the ultra-narrowband optical filter is constructed. The basic structural model includes, from bottom to top, a substrate, a planar waveguide layer, a single-layer one-dimensional subwavelength grating layer, and a single-layer anti-reflection film. The modulation law of the grating period on the resonant wavelength is quantified by numerical simulation, and the initial grating period that satisfies the target filtering wavelength is determined. S3, Ultra-narrowband bandwidth optimization: With a fixed initial grating period, the influence of grating strip width and height on the filtering bandwidth is quantified by the control variable method. The grating strip width and height are iteratively optimized so that the filtering bandwidth of the ultra-narrowband optical filter at the target filtering wavelength meets the design specifications. S4, Resonance Wavelength Refinement: Fix the optimized grating strip width and height, fine-tune the grating period and planar waveguide layer thickness, and accurately calibrate the resonant reflection peak to the target filtering wavelength while maintaining ultra-narrowband filtering characteristics; S5, Sideband Rejection Ratio Co-optimization: A single-layer anti-reflection film is designed on the surface of the refined grating layer. The refractive index and thickness of the single-layer anti-reflection film are optimized to achieve background reflection suppression in the non-resonant broadband band without destroying the guided mode resonant phase matching condition. Ultimately, the sideband rejection ratio of the device meets the design specifications.

[0047] Furthermore, in steps S2 to S4, finite element numerical simulation is performed using COMSOL Multiphysics software. The simulation boundary conditions are periodic boundaries in the x / y directions, a perfectly matched layer in the z direction, and the incident light is TE-polarized perpendicularly incident.

[0048] In one embodiment, the design specifications determined in step S1 are as follows: target filtering wavelength is 1064nm, full width at half maximum (FWHM) is ≤0.05nm, sideband suppression ratio is ≥30dB, peak reflectivity is ≥99%, and TE polarization is perpendicular to the incident wavelength.

[0049] To achieve the design specifications determined in step S1, the resonant wavelength is first initially tuned by optimizing the grating period parameter. The grating period is positively linearly correlated with the resonant wavelength, making it the most crucial first parameter determining the resonant wavelength. While it has no significant impact on the filtering bandwidth, the target filtering wavelength is preferentially locked through the grating period. Specifically, in step S2, the controlled variable method is used, keeping other initial parameters constant, and different grating period values ​​p are set. The reflection spectra under different grating periods are obtained using COMSOL Multiphysics finite element simulation software. Quantification shows that the grating period is positively linearly correlated with the resonant wavelength, exhibiting the greatest control over the resonant wavelength. Therefore, the grating period is the first core parameter determining the resonant wavelength. Based on the target filtering wavelength of 1064 nm, the initial grating period is determined to be 680 nm, completing the initial tuning of the resonant wavelength.

[0050] The grating strip width and height are also core parameters determining the filtering bandwidth. Therefore, under the premise of a fixed initial grating period, the filtering bandwidth is compressed by optimizing the grating strip width and height. Specifically, in step S3, the grating strip width and height are quantified using the controlled variable method to show a positive linear correlation with the filtering bandwidth. These two parameters are the core parameters determining the filtering bandwidth. By reducing the grating strip width and height, the filtering bandwidth is compressed, and the impact on the resonant wavelength is relatively weak.

[0051] For optimizing the grating strip width w: with the initial grating period p=680nm fixed and other initial parameters unchanged, different grating strip widths were set. Simulation and quantization showed that the grating strip width is weakly positively correlated with the resonant wavelength and strictly positively linearly correlated with the filtering bandwidth. The smaller the grating strip width, the narrower the filtering bandwidth. Based on this, the optimal grating strip width was determined. In this embodiment, the optimal grating strip width is 100nm.

[0052] For optimizing the grating strip height g: with a fixed initial grating period p = 680nm and grating strip width w = 100nm, and other initial parameters unchanged, different grating strip heights g were set. Simulation quantization revealed that the grating strip height is weakly positively correlated with the resonant wavelength and strictly positively linearly correlated with the filtering bandwidth; the smaller the grating strip height, the narrower the filtering bandwidth. Considering both the bandwidth compression effect and the feasibility of the fabrication process, the optimal grating strip height was determined. In this embodiment, the optimal grating strip height is 50nm.

[0053] After optimizing both the grating strip width and height, a slight shift in the resonant wavelength occurs. Therefore, precise calibration is performed using the planar waveguide layer thickness and grating period. Step S4 achieves resonant wavelength refinement by fine-tuning the planar waveguide layer thickness and grating period. Quantification using the controlled variable method reveals a positive linear correlation between the planar waveguide layer thickness and the resonant wavelength. Its adjustment range on the resonant wavelength is smaller than that of the grating period, serving as an auxiliary refinement parameter for the resonant wavelength.

[0054] With a fixed optimized grating strip width w=100nm and grating strip height g=50nm, simulation quantization revealed a positive linear correlation between the planar waveguide layer thickness and the resonant wavelength. The adjustment range of the planar waveguide layer on the resonant wavelength was less than that of the grating period, serving as an auxiliary parameter for refining the resonant wavelength. By fine-tuning the planar waveguide layer thickness to 150nm and the grating period to 706nm, the resonant reflection peak was precisely calibrated to the target filtering wavelength of 1064nm. At this point, the device achieved an ultra-narrowband filtering performance with a peak reflectivity of 100% and an FWHM of only 0.03nm, completing the optimization of the core structural parameters.

[0055] After optimizing the core structural parameters, the device has the problem of excessively high broadband background reflection in the non-resonant band. Adjusting the grating structure parameters alone cannot effectively improve the sideband suppression ratio. Therefore, a single-layer anti-reflection film 4 is introduced for synergistic optimization. The design of the single-layer anti-reflection film parameters is as follows.

[0056] Initial parameter calculation: Based on the classical formula for a single-layer antireflective coating, the incident medium is air, and the equivalent refractive index of the grating layer is... The calculated ideal refractive index is 1.14; for the non-resonant center wavelength of 1064nm, the calculated MgF2 ( The ideal thickness of SiO2 is 196 nm. The ideal thickness is 175nm.

[0057] Co-simulation verification: Substitute the above initial parameters into the model for simulation, and refer to... Figure 2 The figures show the reflectivity curve and electric field distribution at the resonant wavelength of the ultra-narrowband optical filter. The results show that after adding a SiO2 monolayer antireflection film, the device still maintains 100% peak reflectivity and 0.03nm bandwidth at 1064nm, and the sideband suppression ratio is as high as 1167 (about 30dB), which fully meets the design specifications and verifies the rationality of the parameters.

[0058] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

[0059] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An ultra-narrowband optical filter, characterized in that, It is a reflective all-dielectric guided-mode resonant filter, which, from bottom to top, includes a substrate, a planar waveguide layer, a single-layer one-dimensional subwavelength grating layer, and a single-layer anti-reflection film; The refractive index of the planar waveguide layer is greater than the equivalent refractive index of the single-layer one-dimensional subwavelength grating layer, forming a waveguide confinement structure. The single-layer one-dimensional subwavelength grating layer is composed of periodically arranged all-dielectric grating strips. Its grating period is smaller than the target filtering wavelength of the ultra-narrowband optical filter. The grating period, grating strip width, and grating strip height of the single-layer one-dimensional subwavelength grating layer are matched with the target filtering wavelength, satisfying the guided mode resonance phase matching condition, and can generate a resonance reflection peak at the target filtering wavelength.

2. The ultra-narrowband optical filter according to claim 1, characterized in that, The ideal refractive index of the single-layer antireflective film satisfies: in, The ideal refractive index for a single-layer antireflective film, The incident medium is the refractive index of the incident medium on the incident side of the ultra-narrowband optical filter. The incident medium is a uniform medium through which the incident light propagates before it is incident on the ultra-narrowband optical filter. The equivalent refractive index is the single-layer one-dimensional subwavelength grating layer, which is calculated using the equivalent medium theory.

3. The ultra-narrowband optical filter according to claim 2, characterized in that, The incident medium is air at standard atmospheric pressure, with a refractive index of [missing information]. .

4. The ultra-narrowband optical filter according to claim 1, 2, or 3, characterized in that, Under perpendicular incident light conditions, the optical thickness of the single-layer antireflective film is 1 / 4 of the center wavelength of the non-resonant broadband band, satisfying: in, The refractive index of a single-layer antireflective film, The thickness of a single-layer antireflective film, The center wavelength of the non-resonant broadband band is the broadband transmission band located on both sides of the target filtering wavelength.

5. The ultra-narrowband optical filter according to claim 4, characterized in that, The target filtering wavelength is 1064nm. The ultra-narrowband optical filter has a peak reflectivity of 100% at the resonant reflection peak at the target filtering wavelength, a full width at half maximum (FWHM) of 0.03nm, and a sideband suppression ratio of ≥30dB.

6. The ultra-narrowband optical filter according to claim 5, characterized in that, The refractive index of the planar waveguide layer The thickness of the planar waveguide layer is d = 150 nm; the refractive index of the substrate is... ; The grating period of a single-layer one-dimensional subwavelength grating layer is p=706nm, and the refractive index of the grating strips is... 1.837, grating strip width w=200nm, grating strip height g=50nm.

7. The ultra-narrowband optical filter according to claim 6, characterized in that, The single-layer anti-reflective film is Thin film, refractive index 1.36, thickness .

8. The ultra-narrowband optical filter according to claim 6, characterized in that, The single-layer anti-reflective film is Thin film, refractive index 1.5, thickness .

9. The ultra-narrowband optical filter according to claim 1, characterized in that, The substrate, planar waveguide layer, single-layer one-dimensional subwavelength grating layer, and single-layer antireflection film are all metal-free all-dielectric materials.

10. A design method for an ultra-narrowband optical filter, characterized in that, The method for designing the ultra-narrowband optical filter as described in any one of claims 1, 2, 3, 5, 6, 7, 8, or 9 includes the following hierarchical optimization steps: S1, Determine the design specifications, including the target filter wavelength, target filter bandwidth, and target sideband rejection ratio; S2, Initial tuning of resonant wavelength: Based on the guided mode resonant phase matching condition, an initial basic structural model of the ultra-narrowband optical filter is constructed. The basic structural model includes, from bottom to top, a substrate, a planar waveguide layer, a single-layer one-dimensional subwavelength grating layer, and a single-layer anti-reflection film. The modulation law of the grating period on the resonant wavelength is quantified by numerical simulation, and the initial grating period that satisfies the target filtering wavelength is determined. S3, Ultra-narrowband bandwidth optimization: With a fixed initial grating period, the influence of grating strip width and height on the filtering bandwidth is quantified by the control variable method. The grating strip width and height are iteratively optimized so that the filtering bandwidth of the ultra-narrowband optical filter at the target filtering wavelength meets the design specifications. S4, Resonance Wavelength Refinement: Fix the optimized grating strip width and height, fine-tune the grating period and planar waveguide layer thickness, and accurately calibrate the resonant reflection peak to the target filtering wavelength while maintaining ultra-narrowband filtering characteristics; S5, Sideband Rejection Ratio Co-optimization: A single-layer anti-reflection film is designed on the surface of the refined grating layer. The refractive index and thickness of the single-layer anti-reflection film are optimized to achieve background reflection suppression in the non-resonant broadband band without destroying the guided mode resonant phase matching condition. Ultimately, the sideband rejection ratio of the device meets the design specifications.

11. The design method according to claim 10, characterized in that, In step S2, the grating period is positively linearly correlated with the resonant wavelength by quantization using the controlled variable method. The grating period is the first core parameter that determines the resonant wavelength.

12. The design method according to claim 10, characterized in that, In step S3, the grating strip width and height are positively linearly correlated with the filtering bandwidth by quantization using the controlled variable method. These two parameters are the core parameters that determine the filtering bandwidth. The filtering bandwidth can be compressed by reducing the grating strip width and height.

13. The design method according to claim 10, characterized in that, In step S4, the thickness of the planar waveguide layer is positively linearly correlated with the resonant wavelength by the controlled variable method. Its adjustment range on the resonant wavelength is smaller than that of the grating period, which serves as an auxiliary refinement parameter for the resonant wavelength.

14. The design method according to any one of claims 11 to 13, characterized in that, In steps S2 to S4, finite element numerical simulation is performed using COMSOL Multiphysics software. The simulation boundary conditions are periodic boundaries in the x / y directions, a perfectly matched layer in the z direction, and the incident light is TE-polarized perpendicularly incident.

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