Polarization insensitive filter based on sub-wavelength grating structure

Abstract

The invention discloses a polarization insensitive filter based on a sub-wavelength grating structure, and belongs to the technical field of integrated photonic devices. The filter comprises an input transition waveguide, a multimode waveguide grating and an output transition waveguide which are connected in sequence. The multimode waveguide grating is composed of a central sub-wavelength grating waveguide and Bragg gratings symmetrically distributed on the two sides, and the effective refractive indexes of TE polarization and TM polarization are equal by adjusting the waveguide width and duty ratio of the central sub-wavelength grating, so that the polarization sensitivity of the device is eliminated fundamentally. And the input / output transition waveguide adopts a transition mode of mixing a tapered waveguide and a sub-wavelength grating waveguide, so that low-loss mode transmission between a straight waveguide and a multimode grating is realized. According to the invention, the anisotropic equivalent medium characteristic of the sub-wavelength grating is utilized, a function of simultaneously and effectively filtering TE and TM polarization is realized, and the filter has the advantages of polarization insensitivity, flexible design, compact structure, simple process and compatibility with a CMOS (Complementary Metal Oxide Semiconductor) process, and has important application value in the fields of optical communication, optical interconnection and the like.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of integrated photonics, and relates to a wavelength-specific on-chip optical signal processing device, in particular to a polarization-insensitive filter based on a subwavelength grating waveguide. BACKGROUND

[0002] Since the invention of the transistor, microelectronics has made a great leap and become the cornerstone of the modern information society. However, as the feature size of integrated circuits gradually approaches the physical limit, microelectronics is facing multiple bottlenecks such as power consumption, delay and interconnection bandwidth, making it difficult to meet the future demand for high-speed and high-capacity information transmission and processing. Optical interconnection and optical computing technology use photons as information carriers, with inherent advantages such as high bandwidth, fast speed, low power consumption and good parallelism, which is one of the important technical paths to break through the above bottlenecks.

[0003] In optical communication systems, wavelength division multiplexing (WDM) technology is a key technology to improve transmission capacity, which achieves capacity multiplication by simultaneously transmitting multiple different wavelength optical signals in a single optical fiber. As a core device in the WDM system, the filter is used to achieve selective downloading or routing of specific wavelength signals. Common on-chip filter structures include arrayed waveguide gratings, micro-ring resonators, multimode interference couplers and Bragg gratings, etc. Among them, the filter based on Bragg grating has attracted widespread attention due to its simple structure, flat spectral response and no need for curved waveguides.

[0004] However, the polarization state of the optical signal transmitted in the actual optical fiber is usually random and constantly changing. The traditional on-chip Bragg grating filter usually has different effective refractive indices and coupling coefficients for transverse electric (TE) and transverse magnetic (TM) orthogonal polarization modes, resulting in the drift of the filter response (such as center wavelength and bandwidth) with the change of the incident light polarization state, i.e. there is significant polarization sensitivity. This polarization-dependent loss and polarization dependence can degrade system performance, increase bit error rate, and force the system to adopt complex polarization control or polarization diversity reception schemes, thereby increasing the complexity, cost and power consumption of the system.

[0005] To solve the polarization sensitivity problem of the filter, those skilled in the art have proposed various technical solutions. For example, by designing a special asymmetric cladding structure, polarization rotation in the Bragg grating is used to achieve polarization-independent response (see "Polarization-independent photonic Bragg grating filter with cladding asymmetry" by Sangsik Kim et al. published in Optics Letters). However, such methods often rely on complex multi-layer structures or stringent process precision, facing challenges in design flexibility and preparation complexity. Other solutions such as using birefringence compensation or complex grating topography also have problems such as design difficulty, narrow bandwidth, and small process tolerance.

[0006] Therefore, it is urgent to develop a new type of polarization-insensitive filter scheme that should be compatible with standard complementary metal-oxide-semiconductor (CMOS) process, have simple structure, flexible design, stable performance, and other advantages to promote its widespread application in the next generation of integrated photonic systems. SUMMARY

[0007] The purpose of the present application is to overcome the deficiencies and shortcomings of the prior art, and to provide a polarization-insensitive filter based on a subwavelength grating structure. The filter aims to solve the performance degradation problem of traditional on-chip Bragg grating filters caused by polarization sensitivity, while having the advantages of compact structure, flexible design, simple process, and easy integration with standard CMOS process.

[0008] To achieve the above-mentioned purpose, the present application adopts the following technical solutions: A filter device based on a subwavelength grating structure, the device includes an input transition waveguide, a multimode waveguide grating, and an output transition waveguide. The input transition waveguide and the output transition waveguide are composed of a straight waveguide, a tapered waveguide, and a subwavelength grating waveguide. The input transition waveguide functions to transmit light in the straight waveguide to the multimode waveguide grating with low loss, or to transmit light in the multimode waveguide grating to the straight waveguide with low loss. The output transition waveguide functions to transmit light in the multimode waveguide grating to the straight waveguide with low loss. The multimode waveguide grating part is composed of a subwavelength grating structure and a Bragg grating structure. The subwavelength grating is located in the middle part of the multimode waveguide grating, and the Bragg grating structure is located on both sides of the multimode waveguide and is distributed in an anti-symmetric manner (the gratings are staggered by half a period in the light propagation direction). This part functions to convert TE0 / TM0 mode of a specific wavelength to TE1 / TM1 mode and reflect it back to the input transition waveguide, while TE0 / TM0 mode at other wavelengths can pass through the multimode waveguide grating part with low loss, i.e. to achieve the function of filtering.

[0009] The polarization-insensitive filter device is a filter based on a multimode Bragg grating structure. By introducing a sub-wavelength grating structure in the multimode Bragg grating structure, the device can simultaneously realize filtering of TE polarization and TM polarization, has polarization insensitivity, and has simple device process, good design flexibility, is easy to manufacture, and has good stability, and has good application prospects in many fields in the future.

[0010] The polarization-insensitive filter introduces a sub-wavelength grating structure, and further first proposes a filter based on a sub-wavelength grating waveguide. By introducing a sub-wavelength grating waveguide in the multimode Bragg grating structure, the anisotropy of the sub-wavelength grating waveguide is utilized to realize the polarization insensitivity of the device. Secondly, a transition mode of mixing a tapered waveguide and a sub-wavelength grating waveguide is adopted in the transition waveguide to realize low-loss transmission of light modes between the straight waveguide and the sub-wavelength grating structure. In addition, the device proposed in the application also has other advantages, for example, simple device process, good design flexibility, good stability, etc.

[0011] The optical filter has the following advantages: 1. The filter is a core device for realizing wavelength division multiplexing technology. The wavelength division multiplexing technology can perform wavelength division multiplexing / demultiplexing on light based on the wavelength of light as an independent dimension, and can expand the capacity of information transmission by several times. The device can be realized on silicon-on-insulator (SOI), and the manufacturing process of the device is compatible with the mature CMOS process today, so the device has a wide application range, low manufacturing cost, high integration, good stability, and good application prospects in the future.

[0012] 2. The sub-wavelength grating structure is introduced in the multimode Bragg grating, the period of the sub-wavelength grating structure satisfies the sub-wavelength condition, and can be equivalent to an anisotropic medium. By utilizing this characteristic, the effective refractive index of the TE polarization and the TM polarization in the waveguide can be adjusted respectively, so that the two are equal, and polarization-insensitive filtering is realized, and the application scenarios of the device are expanded.

[0013] 3. The transition between the straight waveguide and the multimode waveguide grating is realized by using a mixed mode of a sub-wavelength grating waveguide and a tapered waveguide, low-loss transmission of light modes between the straight waveguide and the sub-wavelength grating structure is realized, and the transition zone satisfies the characteristic size, air voids are not easy to produce, and the device is easy to manufacture. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 is a schematic diagram of the optical filter of the application.

[0015] Figure 2 is a schematic diagram of the input transition waveguide part of the optical filter of the application.

[0016] Figure 3 is a schematic diagram of the multimode waveguide grating part of the optical filter of the present application.

[0017] Figure 4 is a schematic diagram of the output transition waveguide part of the optical filter of the present application.

[0018] Figure 5 is a curve of the effective refractive index of each mode in the first subwavelength grating waveguide of the optical filter of the present application as a function of the duty cycle of the first subwavelength grating waveguide when the waveguide width of the first subwavelength grating waveguide is constant.

[0019] Figure 6 is a curve of the effective refractive index of each mode in the first subwavelength grating waveguide of the optical filter of the present application as a function of the waveguide width of the first subwavelength grating waveguide when the waveguide duty cycle of the first subwavelength grating waveguide is constant.

[0020] Figure 7 is the transmission efficiency of the TE0 mode, TM0 mode, TE1 mode and TM1 mode in the input / output transition waveguide of the optical filter of the present application.

[0021] Figure 8 is a schematic diagram of the SOI material structure used in the optical filter of the present application.

[0022] Figure 9 is a schematic diagram of the optical filter of the present application realizing the filtering function.

[0023] In the figure: 1. input transition waveguide, 2. multimode waveguide grating, 3. output transition waveguide, 4. input straight waveguide, 5. input tapered waveguide, 6. input subwavelength grating waveguide-1, 7. input subwavelength grating waveguide-2, 8. first subwavelength grating waveguide, 9. first Bragg grating waveguide-1, 10. first Bragg grating waveguide-2, 11. output subwavelength grating waveguide-1, 12. output subwavelength grating waveguide-2, 13. output tapered waveguide, 14. output straight waveguide. DETAILED DESCRIPTION

[0024] The present application will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0025] The overall structure of the optical filter of the present application is shown in Figure 1 and includes an input transition waveguide 1, a multimode waveguide grating 2 and an output transition waveguide 3 connected in sequence.

[0026] As shown in Figure 2As shown, the input transition waveguide 1 of the optical filter of the present application is composed of an input straight waveguide 4, an input tapered waveguide 5, an input subwavelength grating waveguide-1 6, and an input subwavelength grating waveguide-2 7. The input straight waveguide 4 is connected to the input tapered waveguide 5 in sequence, and the input subwavelength grating waveguide-1 6 and the input subwavelength grating waveguide-2 7 are completely identical and symmetrically distributed, and are respectively distributed on both sides of the input tapered waveguide 5.

[0027] The width of the input straight waveguide 4 supports the lossless transmission of TE1 and TM1 modes.

[0028] The shape of the input tapered waveguide 5 is an isosceles trapezoid, the larger width end of which is connected to the input straight waveguide 4 and has the same width as the input straight waveguide 4, and the smaller width end of which is connected to the multimode waveguide grating 2 and has a width equal to the product of the duty cycle and the period of the subwavelength grating waveguide in the multimode waveguide grating 2, i.e. equal to the width of a single waveguide in the first subwavelength grating waveguide 8. The length of the input tapered waveguide 5 is L0, which is determined by simulation to ensure that each optical mode in the input straight waveguide 4 can be transmitted to the multimode waveguide grating 2 with low loss, or to ensure that each optical mode in the multimode waveguide grating 2 can be transmitted to the input straight waveguide 4 with low loss.

[0029] The input subwavelength grating waveguide-1 6 is located on the upper side of the input tapered waveguide 5 and extends in a direction inclined to the upper side of the input tapered waveguide 5, and the direction of the periodic repetition of the input subwavelength grating waveguide-1 6 is approximately perpendicular to the direction of the propagation of the optical mode. The distance between the input subwavelength grating waveguide-1 6 and the input tapered waveguide 5 is constant. The width of the input subwavelength grating waveguide-1 6 is not constant, i.e. the number of periods is not constant. The end of the input subwavelength grating waveguide-1 6 connected to the first subwavelength grating waveguide 8 is the end with a larger number of periods, and this end is also located at the same position as the smaller width end of the input tapered waveguide 5. The end of the input subwavelength grating waveguide-1 6 with a smaller number of periods is away from the first subwavelength grating waveguide 8 and close to the larger width end of the input tapered waveguide 5. The width of the first subwavelength grating waveguide 8 in the multimode waveguide grating 2 minus the width of the smaller width end of the input tapered waveguide 5, and then divided by two, is the width of the larger width end of the input subwavelength grating waveguide-1 6. The width of the input subwavelength grating waveguide-1 6 gradually decreases from the larger width end to the smaller width end, which is specifically manifested in that the number of periods of the input subwavelength grating waveguide-1 6 gradually decreases until zero, i.e. the number of waveguides in the input subwavelength grating waveguide-1 6 gradually decreases until zero. The position where the number of periods of the input subwavelength grating waveguide-1 6 changes is L i(L1 and L2), that is, L2 is the distance between the position where the period number of the input subwavelength grating waveguide-1 is reduced for the first time and the multimode waveguide grating 2, and L1 is the distance between the position where the period number of the input subwavelength grating waveguide-1 is reduced for the second time and the multimode waveguide grating 2, as shown in FIG. 1. Figure 2 The specific numerical value is determined by simulation. i The specific numerical value is determined by simulation.

[0030] The unit structure parameters such as the period and the duty cycle of the input subwavelength grating waveguide-1 6 are completely the same as and constant as those of the first subwavelength grating waveguide 8. The period number of the larger end of the input subwavelength grating waveguide-1 6 is determined by the period number of the first subwavelength grating waveguide 8, specifically: the period number of the larger end of the input subwavelength grating waveguide-1 6 is obtained by subtracting one from the period number of the first subwavelength grating waveguide 8 and then dividing by two.

[0031] The input subwavelength grating waveguide-2 7 is completely the same as the input subwavelength grating waveguide-1 6, and the positions are different, that is, the input subwavelength grating waveguide-2 7 is symmetrical to the input subwavelength grating waveguide-1 6 about the input tapered waveguide 5.

[0032] As shown in FIG. 1, the multimode waveguide grating 2 of the optical filter of the present application is composed of the first subwavelength grating waveguide 8, the first Bragg grating waveguide-1 9 and the first Bragg grating waveguide-2 10. The first Bragg grating waveguide-1 9 and the first Bragg grating waveguide-2 10 are respectively located on the two sides of the first subwavelength grating waveguide 8. Figure 3 The one end of the first subwavelength grating waveguide 8 is connected with the input transition waveguide 1, and the other end is connected with the output transition waveguide 3; the direction of the period repetition of the first subwavelength grating waveguide 8 is perpendicular to the direction of the light mode propagation; the waveguide width and the duty cycle of the first subwavelength grating waveguide 8 are constant, and the determination of the waveguide width and the duty cycle of the first subwavelength grating waveguide 8 should make the multimode waveguide grating 2 support low-loss transmission of TE1 mode and TM1 mode, and make the effective refractive indexes of the TE mode and the TM mode in the multimode waveguide grating 2 equal; the length of the first subwavelength grating waveguide 8 is determined by simulation, and the length should make the light at the filtering wavelength be almost completely reflected, and the light of the remaining wavelengths pass through the multimode waveguide grating 2 with low loss; the period of the first subwavelength grating waveguide 8 satisfies the subwavelength grating condition, and the period is constant.

[0033]

[0034] ​The first Bragg grating waveguide-1 9 is located on the upside of the first subwavelength grating waveguide 8 and extends completely horizontally; the direction of periodical repetition of the first Bragg grating waveguide-1 9 is the same as the direction of light mode propagation; the interval of the first Bragg grating waveguide-1 9 and the first subwavelength grating waveguide 8 is constant; the width of the first Bragg grating waveguide-1 9 is constant, which is equal to the product of the duty cycle and the period of the first subwavelength grating waveguide 8, that is, equal to the width of a single waveguide in the first subwavelength grating waveguide 8; the duty cycle of the first Bragg grating waveguide-1 9 is constant; the length of the first Bragg grating waveguide-1 9 is equal to the length of the first subwavelength grating waveguide 8, and the two ends are in the same horizontal position; the first Bragg grating waveguide-1 9 is a rectangular sawtooth structure Bragg grating, which is periodically distributed, and the period of the first Bragg grating waveguide-1 9 satisfies the following formula: Wherein, n eff0 is the effective refractive index of TE0 / TM0 mode, n eff1 is the effective refractive index of TE1 / TM1 mode, λ is the filter center wavelength, and Λ is the period of the first Bragg grating waveguide-1.

[0035] The first Bragg grating waveguide-2 10 is completely the same as the first Bragg grating waveguide-1 9 in terms of various parameters, and is different in position, and the two are symmetric about the first subwavelength grating waveguide 8, and are anti-symmetrically distributed, that is, the two gratings are staggered by half a period on the axis of light propagation direction; the first Bragg grating waveguide-2 10 is located on the downside of the first subwavelength grating waveguide 8.

[0036] As shown in Figure 4 , the output transition waveguide 3 of the optical filter is completely the same as the input transition waveguide 1 in terms of various parameters, and the output transition waveguide 3 is symmetric about the multimode waveguide grating 2 with the input transition waveguide 1. The output transition waveguide 3 is composed of an output straight waveguide 14, an output tapered waveguide 13, an output subwavelength grating waveguide-1 11, and an output subwavelength grating waveguide-2 12.

[0037] The specific design process of the device is introduced as follows. Firstly, the period of the first subwavelength grating waveguide 8 should satisfy the subwavelength condition, so that the period of the subwavelength grating can be determined.

[0038] The waveguide width and the duty cycle of the first subwavelength grating waveguide 8 should satisfy that the effective refractive indexes of TE polarization and TM polarization in the first subwavelength grating waveguide 8 are equal. Figure 5 As shown in the figure, the figure is the curve of the effective refractive index of each mode in the first subwavelength grating waveguide 8 changing with the duty cycle thereof calculated by using the finite difference algorithm, the horizontal axis is the duty cycle, and the vertical axis is the effective refractive index of the mode; Figure 5The corresponding first subwavelength grating waveguide 8 is realized based on a 220nm-thick SOI platform, the waveguide width is 3μm, the period is about 200nm, the wavelength of input light is 1550nm, and the duty cycle varies from 0.4 to 0.7; the effective refractive index of each mode in the first subwavelength grating waveguide 8 is calculated by using the finite difference algorithm, and the curve of the effective refractive index of each mode in the first subwavelength grating waveguide 8 with respect to the waveguide width is shown in the figure, the horizontal axis represents the waveguide width, and the vertical axis represents the effective refractive index of the mode; Figure 5 It can be known that, when other parameters remain unchanged, the effective refractive index of TE polarization and TM polarization changes to different degrees with the change of the duty cycle. Figure 6 The curve of the effective refractive index of each mode in the first subwavelength grating waveguide 8 with respect to the waveguide width is shown in the figure, the horizontal axis represents the waveguide width, and the vertical axis represents the effective refractive index of the mode; Figure 6 The corresponding first subwavelength grating waveguide 8 is realized based on a 220nm-thick SOI platform, the duty cycle is 0.55, the period is about 200nm, the wavelength of input light is 1550nm, and the waveguide width varies from 2.5μm to 3μm; the effective refractive index of each mode in the first subwavelength grating waveguide 8 is calculated by using the finite difference algorithm, and the curve of the effective refractive index of each mode in the first subwavelength grating waveguide 8 with respect to the waveguide width is shown in the figure, the horizontal axis represents the waveguide width, and the vertical axis represents the effective refractive index of the mode; Figure 6 It can be known that, when other parameters remain unchanged, the effective refractive index of TE polarization and TM polarization changes to different degrees with the change of the waveguide width. In combination with the above two figures, Figure 5 , Figure 6 It can be known that, by independently optimizing the duty cycle and the waveguide width, the effective refractive index of TE polarization and TM polarization can be equal, and meanwhile the first subwavelength grating waveguide 8 supports low-loss transmission of TE1 and TM1 modes.

[0039] The period of the first Bragg grating waveguide-1 9 and the first Bragg grating waveguide-2 10 should be selected so that the forward transmission TE0 / TM0 mode is converted into the reverse transmission TE1 / TM1 mode at the filtering wavelength, and meanwhile the TE0 / TM0 mode at other wavelengths continues to be forward transmitted, and low-loss passes through the multimode waveguide grating 2. The period Λ is determined by the above formula.

[0040] The length of the first subwavelength grating waveguide 8 should be selected so that the filtering wavelength can be effectively filtered out, and meanwhile other wavelengths can pass through with low loss. The parameters L i (L1 and L2) of the input / output subwavelength grating waveguide-1 / 2 (6, 7, 11, 12) and the length L0 of the input / output tapered waveguide (5, 13) should be selected so that the light in the input / output straight waveguide (4, 14) can be transmitted into the multimode waveguide grating 2 with low loss, and the light in the multimode waveguide grating 2 can be transmitted into the input / output straight waveguide (4, 14) with low loss. Figure 7 The transmission efficiency diagram of the input / output transition waveguide is shown in the figure, the L0 of the input / output transition waveguide is 20μm, the L1 is 15μm, and the L2 is 10μm; it can be known from the figure that the input / output transition waveguide can realize low-loss transmission.

[0041] The optical filter is realized based on silicon-on-insulator (SOI) material. The structure of the SOI material is generally composed of a substrate silicon layer, a silicon dioxide film as a buried oxygen layer, and a top silicon layer, as shown in Figure 8 .

[0042] The working mode of the device is specifically described below. As shown in Figure 9 , the TE0 / TM0 mode is input in the input transition waveguide 1, the TE0 / TM0 mode is transmitted to the multimode waveguide grating 2 part with low loss, the forward transmission TE0 / TM0 mode at the filter wavelength is reversely coupled into the reverse transmission TE1 / TM1 mode, the reverse transmission TE1 / TM1 mode is transmitted to the input straight waveguide 4 with low loss, and the subsequent will be demultiplexed out of the filter by the mode demultiplexer; the forward transmission TE0 / TM0 mode at other wavelengths will continue to be transmitted forward, and the low-loss TE0 / TM0 mode will pass through the multimode waveguide grating 2 and the output transition waveguide 3 to reach the output straight waveguide 14.

Claims

1. A polarization insensitive filter based on subwavelength grating structure, characterized in that, The input transition waveguide (1), the multimode waveguide grating (2) and the output transition waveguide (3) are sequentially connected along the light path direction; The input transition waveguide (1) is used for receiving incident light and coupling the fundamental mode light field into the multimode waveguide grating (2) with low loss, or for receiving reflected light from the multimode waveguide grating (2) and coupling the high-order mode light field out with low loss; The multimode waveguide grating (2) comprises a first subwavelength grating waveguide (8) and first Bragg grating waveguide-1 (9) and first Bragg grating waveguide-2 (10) which are asymmetrically distributed on both sides of the first subwavelength grating waveguide (8); the waveguide width and duty cycle of the first subwavelength grating waveguide (8) are configured to equalize the effective refractive index of TE polarization and TM polarization to achieve polarization insensitivity; the first Bragg grating waveguide-1 (9) and the first Bragg grating waveguide-2 (10) are configured to reflect TE0 / TM0 mode of a specific wavelength and convert it into TE1 / TM1 mode for reverse transmission, and allow low-loss transmission of light of other wavelengths. The output transition waveguide (3) is used for outputting light and coupling the light field passing through the multimode waveguide grating (2) out with low loss.

2. The polarization-insensitive filter of claim 1, wherein, The input transition waveguide (1) comprises an input straight waveguide (4), an input tapered waveguide (5), and input subwavelength grating waveguide-1 (6) and input subwavelength grating waveguide-2 (7) which are symmetrically distributed on both sides of the input tapered waveguide (5) and are sequentially connected; The input tapered waveguide (5) is in the shape of an isosceles trapezoid, the wider end is connected with the input straight waveguide (4), and the narrower end is connected with the first subwavelength grating waveguide (8); The widths of the input subwavelength grating waveguide-1 (6) and the input subwavelength grating waveguide-2 (7) gradually increase from one end close to the input straight waveguide (4) to one end close to the first subwavelength grating waveguide (8).

3. The polarization-insensitive filter of claim 2, wherein, The periods and duty cycles of the input subwavelength grating waveguide-1 (6) and the input subwavelength grating waveguide-2 (7) are the same as those of the first subwavelength grating waveguide (8).

4. The polarization-insensitive filter of claim 1, wherein, The output transition waveguide (3) is structurally symmetrical to the input transition waveguide (1) and comprises an output straight waveguide (14), an output tapered waveguide (13), an output subwavelength grating waveguide-1 (11) and an output subwavelength grating waveguide-2 (12).

5. The polarization-insensitive filter of claim 1, wherein, The period Λ of the first Bragg grating waveguide-1 (9) and the first Bragg grating waveguide-2 (10) is determined by the following formula: where n eff0 is the effective refractive index for TE0 / TM0 mode, n eff1 is the effective refractive index for TE1 / TM1 mode, and λ is the filter center wavelength.

6. The polarization-insensitive filter of claim 5, wherein, By adjusting the period Λ of the Bragg grating waveguide, the tuning of the center wavelength λ of the filter can be realized.

7. The filter of claim 1, wherein, The period of the first subwavelength grating waveguide (8) satisfies the subwavelength condition, and the waveguide width and duty cycle thereof can be independently adjusted to equalize the effective refractive index of TE polarization and TM polarization mode.

8. The filter of claim 1, wherein, The filter is prepared based on SOI, lithium niobate or silicon nitride material platform.

9. An optical communication system, characterized by A polarization-insensitive filter as claimed in any one of claims 1-8.

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