A polarization-insensitive silicon-based flattop filter for coarse wavelength division multiplexing systems

Abstract

The application discloses a polarization-insensitive silicon-based flat-top filter for a coarse wavelength division multiplexing system. The output end of a mode demultiplexer is connected with two input connection S-bend waveguides through an adiabatic 1*2 beam splitter. The output ends of the two input connection S-bend waveguides are respectively connected with two input ends of a Bragg grating through one input tapered waveguide respectively. The two output ends of the Bragg grating are respectively connected with two input ends of an output connection S-bend waveguide through one output tapered waveguide respectively. The two output ends of the output connection S-bend waveguide are connected with an output port through an adiabatic 2*1 beam combiner. The application solves the polarization sensitivity problem of the existing CWDM filter on a silicon substrate, is helpful to further simplify the structure on the chip, improve the comprehensive performance of the system on the silicon substrate, and has a wide application prospect.

Description

Technical Field

[0001] This invention relates to the field of integrated optics technology, and more specifically to a polarization-insensitive silicon-based flat-top filter for coarse wavelength division multiplexing (CWDM) systems. Background Technology

[0002] In recent years, with the rapid development of cloud computing and artificial intelligence, the requirements for signal transmission bandwidth and chip computing power have been increasing, making "computing power" a major bottleneck restricting the development of related technologies. As one of the important technical routes to solve the "computing power bottleneck," optical interconnects have received widespread attention and research in recent years, achieving significant progress in both scientific research and commercial applications. Coarse Wavelength Division Multiplexing (CWDM), with its characteristic of multiple wavelength paths running parallel on a single optical fiber, can further improve the bandwidth of optical interconnects and is widely used in various optical interconnect systems.

[0003] The concept of photonic integration dates back to the 1960s and 70s. By integrating various traditional discrete optical devices with different functions onto a single chip, photonic integration can achieve higher density and higher performance integration of complex functions on a single chip, significantly improving the overall performance of the system and reducing production costs. Although the excellent optoelectronic integration characteristics and processing capabilities compatible with existing CMOS processes make silicon-based materials one of the most widely used photonic integration material platforms, its significant birefringence effect presents complex polarization control challenges for silicon-based photonic integration. While controlling the on-chip optical signal to operate in a single polarization state can effectively solve this problem, this usually requires strict control over the polarization state of the optical signal input to the chip, significantly increasing the packaging difficulty and cost in practical applications.

[0004] Taking a silicon-on-a-chip (SiCDM) system as an example, its on-chip receiver first needs to use a filter to separate different wavelength signals input from a single optical fiber into different paths, which are then received by corresponding photodetectors. In this process, traditional filters can only operate on a single polarization, requiring the input fiber to be a polarization-maintaining fiber and precise control of the optical signal polarization. Furthermore, an on-chip polarization beam rotator is needed to further achieve single-polarization operation. These methods further increase system loss, significantly enhance system complexity and packaging difficulty, and cannot fundamentally solve the polarization sensitivity problem of SiCDM systems.

[0005] Another problem with on-chip filters (CWDMs) is temperature sensitivity. In practical applications, the operating temperature of a CWDM system on a silicon substrate needs to cover a wide operating range. To minimize the impact of temperature fluctuations on the filter's center wavelength, the filter typically needs to have flat-top filtering characteristics. Existing silicon-based flat-top filter solutions are mainly based on cascaded Mach-Zehnder (MZ) structures. Although they offer relatively excellent flat-top filtering characteristics and process tolerances, their drawbacks include relatively large size, single-polarization operation, and significant temperature sensitivity. They usually require an additional temperature control module to stabilize the center wavelength, which introduces additional system control costs.

[0006] In summary, how to solve the polarization sensitivity problem of silicon-based photonic integrated filters while maintaining flat-top filtering characteristics is a core technical issue for further improving the overall performance of optical interconnects and expanding the application fields of silicon-based photonic integration. Summary of the Invention

[0007] To address the polarization sensitivity issue of existing on-chip silicon-based flat-top response CWDM filters, this invention proposes a novel filter structure based on a Bragg grating, which can achieve polarization-insensitive filtering while maintaining a flat-top response.

[0008] This invention solves the polarization sensitivity problem of existing silicon-based CWDM filters, achieving polarization-insensitive filtering while maintaining a flat-top response, and has broad application prospects. Based on the existing mature SiN material platform, this invention solves the polarization sensitivity problem of on-chip CWDM filters through unique device structure and filter architecture design.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] The present invention includes a mode demultiplexer, an adiabatic 1×2 beam splitter, an input connecting S-bend waveguide, an input tapered waveguide, a Bragg grating, an output tapered waveguide, an output connecting S-bend waveguide, and an adiabatic 2×1 beam combiner. The output of the mode demultiplexer is connected to the input ends of the two input connecting S-bend waveguides via the adiabatic 1×2 beam splitter. The output ends of the two input connecting S-bend waveguides are connected to the two input ends of the Bragg grating via their respective input tapered waveguides. The two output ends of the Bragg grating are connected to the two input ends of the output connecting S-bend waveguide via their respective output tapered waveguides. The two output ends of the output connecting S-bend waveguide are connected to the output port via the adiabatic 2×1 beam combiner.

[0011] The mode demultiplexer includes an asymmetric directional coupler structure comprising a trunk waveguide and an access waveguide of different widths. The trunk waveguide and the access waveguide are arranged relatively parallel and spaced apart, and the spacing between the trunk waveguide and the access waveguide is the same at all points along the waveguide propagation direction. The trunk waveguide is connected to one end of the center waveguide of the adiabatic 1×2 beam splitter, and the access waveguide is connected to the other end of the center waveguide of the adiabatic 1×2 beam splitter.

[0012] The trunk waveguide is wider to support the transmission of first-order modes TE1 / TM1; the access waveguide is narrower and only supports the transmission of fundamental modes TE0 / TM0; and the TE1 / TM1 modes in the trunk waveguide and the TE0 / TM0 modes in the access waveguide always satisfy the mode matching condition.

[0013] The aforementioned 1×2 adiabatic beam splitter includes a central adiabatic conical waveguide, an upper adiabatic conical waveguide, and a lower adiabatic conical waveguide. The central adiabatic conical waveguide is an isosceles trapezoid, which is arranged from the wide end to the narrow end along the waveguide propagation direction. The upper and lower adiabatic conical waveguides are respectively provided on the two sides of the isosceles trapezoid of the central adiabatic conical waveguide. The upper and lower adiabatic conical waveguides are arranged at intervals with the sides of the isosceles trapezoid of the central adiabatic conical waveguide, and the spacing is the same at all points along the waveguide propagation direction.

[0014] The wide end of the central insulated conical waveguide serves as the central input port, while the wide ends of the upper and lower insulated conical waveguides serve as the upper and lower output ports, respectively.

[0015] The thermally insulated 2×1 bundle combiner and the thermally insulated 1×2 bundle splitter have identical structures and are symmetrically distributed.

[0016] The Bragg grating includes a central waveguide and grating teeth; the central waveguide is strip-shaped and arranged along the waveguide propagation direction, and grating teeth are provided on both sides of the central waveguide; the grating teeth are divided into upper and lower groups of grating teeth, which are symmetrically distributed relative to the central axis of the central waveguide; each group of grating teeth includes grating units arranged at equal intervals along the waveguide propagation direction, but with their width gradually changing from small to large to small.

[0017] The grating teeth in the two Bragg gratings are staggered by half a grating period along the waveguide propagation direction.

[0018] The two ends of the center waveguide of the Bragg grating are connected to the narrow ends of the input tapered waveguide and the output tapered waveguide, respectively.

[0019] The central waveguide has a square cross-section, and the grating teeth are sized using a dovetail distribution, with the grating teeth length also designed using a dovetail.

[0020] This invention designs each component as a polarization-insensitive device, combined with a Bragg grating with a flat-top response. Wavelength signals satisfying the Bragg grating reflection condition are reflected and reversed through an adiabatic 1×2 beamsplitter, becoming a first-order mode for reverse transmission. Finally, a mode demultiplexer achieves specific wavelength filtering characteristics. Simultaneously, wavelength signals not satisfying the Bragg grating reflection condition pass losslessly through the grating and are converted into the fundamental mode output by an adiabatic 2×1 beam combiner, ultimately realizing the function of a polarization-insensitive flat-top response filter for CWDM. The operating wavelength and flat-top response bandwidth of the filter can be changed by altering the period and tooth size of the Bragg grating.

[0021] The innovation of this invention lies in the use of a novel architecture based on Bragg gratings to achieve polarization-insensitive flat-top filter functionality. This is the first time that polarization-insensitive CWDM filtering functionality has been achieved on a large-scale commercial silicon-based platform. It can overcome the inherent polarization sensitivity problem of silicon-based platforms, avoid the use of expensive polarization-maintaining fibers in the packaging stage, and thus significantly reduce the packaging cost of silicon photonic chips.

[0022] The filter of this invention consists of four parts connected sequentially: a mode demultiplexer, a 1×2 power divider, a Bragg grating filter, and a 2×1 beam combiner. All four parts are polarization-insensitive devices. The Bragg grating operates only at the target wavelength, achieving a flat-top reflection response; the other three parts operate across a wide spectrum. The target wavelength signal is reflected by the Bragg grating filter and then passed back through the 1×2 power divider and mode demultiplexer before being output at the download port, achieving polarization-insensitive filtering for a specific wavelength. Other wavelength signals pass normally through the Bragg grating filter, are combined by the 2×1 beam combiner, and are then output normally.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] This invention innovatively proposes a polarization-insensitive silicon-based flat-top filter for CWDM. This design is based entirely on existing commercial process platforms and requires no special process or material support. It can provide the same flat-top filtering response for on-chip TE and TM modes, thereby significantly simplifying the polarization-dependent complexity of on-chip CWDM systems, reducing system packaging and testing requirements, and promoting the further development of photonic integration technology.

[0025] This invention solves the polarization sensitivity problem of existing silicon-based CWDM filters, helps to further simplify the on-chip structure, improve the overall performance of silicon-based systems, and has broad application prospects. Attached Figure Description

[0026] 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.

[0027] Figure 1 This is a schematic diagram of the structure of a polarization-insensitive flat-top response silicon-based flat-top filter for a CWDM system provided in the embodiment.

[0028] Figure 2 This is a schematic diagram of the pattern (de-) multiplexer provided in the embodiment.

[0029] Figure 3 This is a schematic diagram of the structure of the 1×2 heat-insulating beam splitter provided in the embodiment.

[0030] Figure 4 This is a schematic diagram of the structure of the Bragg grating provided in the embodiment.

[0031] Figure 5 This is provided in the embodiment. Figure 1 Simulation results of optical field transmission between the 1×2 adiabatic beam splitter 5 and the 2×1 adiabatic beam combiner 11.

[0032] Figure 6 This is a simulation result diagram of the optical field transmission of the mode demultiplexer 4 provided in the embodiment.

[0033] Figure 7 This is a schematic diagram of the 4-channel polarization-insensitive CWDM filter structure provided in the embodiment.

[0034] In the diagram: 1-Input port, 2-Download port, 3-Output port, 4-Mode demultiplexer, 5-Adiabatic 1×2 beam splitter, 6-Input S-bend waveguide, 7-Input tapered waveguide, 8-Bracket grating, 9-Output tapered waveguide, 10-Output connection S-bend waveguide, 11-Adiabatic 2×1 beam combiner, 41-Trunk waveguide, 42-Access waveguide, 51-Center adiabatic tapered waveguide, 52-Upper adiabatic tapered waveguide, 53-Lower adiabatic tapered waveguide, 54-Center input port, 55-Upper output port, 56-Lower output port, 81-Center waveguide, 82-Grate teeth. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Modifications or equivalent substitutions made by those skilled in the art based on their understanding of the technical solutions of this invention, without departing from the spirit and scope of the invention, should be covered within the protection scope of this invention.

[0036] like Figure 1 As shown, this embodiment provides a polarization-insensitive silicon-based flat-top filter for CWDM. Its waveguide portion is made of silicon nitride, and the cladding and substrate materials are silicon dioxide. It includes a mode demultiplexer 4, an adiabatic 1×2 beam splitter 5, an input connection S-bend waveguide 6, an input tapered waveguide 7, a Bragg grating 8, an output tapered waveguide 9, an output connection S-bend waveguide 10, and an adiabatic 2×1 beam combiner 11. These components are connected sequentially and are all designed to be polarization-insensitive. Specifically:

[0037] The output of mode demultiplexer 4 is connected to the input of two input S-bend waveguides 6 via an adiabatic 1×2 beam splitter 5. The outputs of the two input S-bend waveguides 6 are connected to the two inputs of Bragg grating 8 via their respective input tapered waveguides 7. The two outputs of Bragg grating 8 are connected to the two inputs of output S-bend waveguide 10 via their respective output tapered waveguides 9. The two outputs of output S-bend waveguide 10 are connected to the output port 3 via an adiabatic 2×1 beam combiner 11.

[0038] The four parts of the mode demultiplexer 4, the adiabatic 1×2 beam splitter 5, the Bragg grating 8, and the adiabatic 2×1 beam combiner 11 are all polarization-insensitive devices. Among them, the Bragg grating 8 only operates at the target wavelength to achieve a flat-top reflection response; the other three parts are broadband devices.

[0039] The target wavelength signal is reflected by the Bragg grating 8 filter and then reversed through the 1×2 power divider 5 and mode demultiplexer 4 before being output at the download port, achieving polarization-insensitive filtering for a specific wavelength. Other wavelength signals pass normally through the Bragg grating 8 filter, are combined by the 2×1 beam combiner 11, and are then output normally.

[0040] The input terminal of the mode demultiplexer 4 is connected to input port 1, and a download port 2 is also arranged parallel to and at intervals next to input port 1.

[0041] Apart from the Bragg grating, all of the above structures have a large operating bandwidth, which can cover the entire band required by the CWDM system, and ultimately achieve polarization-insensitive flat-top filtering.

[0042] like Figure 2As shown, the mode demultiplexer 4 includes an asymmetric directional coupler structure comprising a trunk waveguide 41 and an access waveguide 42 of different widths, used to separate the TE0 and TE1 modes in the waveguide. The trunk waveguide 41 and the access waveguide 42 are relatively close to parallel and spaced apart, located beside the trunk waveguide 41, and the spacing between the trunk waveguide 41 and the access waveguide 42 is the same at all points along the waveguide propagation direction.

[0043] The trunk waveguide 41 is connected to one end of the center waveguide of the adiabatic 1×2 beamsplitter 5, and the access waveguide 42 is connected to the other end of the center waveguide of the adiabatic 1×2 beamsplitter 5. The trunk waveguide 41 is wider, satisfying the transmission of first-order modes TE1 / TM1; the access waveguide 42 is narrower, supporting only the fundamental mode TE0 / TM0. By setting their widths, the TE1 / TM1 modes in the trunk waveguide 41 and the TE0 / TM0 modes in the access waveguide 42 always satisfy the mode matching condition, thus enabling mutual coupling between the first-order modes TE1 / TM1 in the trunk waveguide 41 and the fundamental modes TE0 / TM0 in the access waveguide 42. Simultaneously, the fundamental modes TE0 / TM0 in the trunk waveguide 41, due to their larger mode mismatch, can transmit normally without loss, unaffected by the access waveguide 42. The widths of the main waveguide 41 and the access waveguide 42 can be obtained through software simulation optimization, and the following conditions must be met: the effective refractive index of the TE1 / TM1 mode at the wide end of the main waveguide 41 is greater than the effective refractive index of the TE0 / TM0 mode at the narrow end of the access waveguide 42, and the effective refractive index of the TE1 / TM1 mode at the narrow end of the main waveguide 41 is less than the effective refractive index of the TE0 / TM0 mode at the wide end of the access waveguide 42.

[0044] In practice, the thermally adiabatic 1×2 bundle splitter 5 and the thermally adiabatic 2×1 bundle combiner 11 have the same structure and are mirror-symmetric.

[0045] like Figure 3As shown, the adiabatic 1×2 beam splitter 5 includes a central adiabatic conical waveguide 51, an upper adiabatic conical waveguide 52, and a lower adiabatic conical waveguide 53. The central adiabatic conical waveguide 51 is an isosceles trapezoid, with the height of the isosceles trapezoid arranged along the waveguide propagation direction. The isosceles trapezoid extends from the wide end where the lower base is located to the narrow end where the upper base is located along the waveguide propagation direction. The upper adiabatic conical waveguide 52 and the lower adiabatic conical waveguide 53 are respectively provided on the two sides of the isosceles trapezoid of the central adiabatic conical waveguide 51. The side-insulated conical waveguide 52 and the lower side-insulated conical waveguide 53 are arranged from the narrow end to the wide end along the waveguide propagation direction. The upper side-insulated conical waveguide 52 and the lower side-insulated conical waveguide 53 are arranged at intervals between the waists of the isosceles trapezoid of the central insulated conical waveguide 51 and the spacing is the same at all points along the waveguide propagation direction. The insulated 1×2 beam splitter 5 itself is symmetrically distributed from top to bottom and is used to evenly distribute the optical signal input from the central input port 54 to the upper output port 55 and the lower output port 56 through insulated coupling.

[0046] All three waveguides are tapered waveguides with gradually varying widths, consistent lengths, and aligned ends, forming a symmetrical overall structure.

[0047] The wide end where the bottom of the central insulated conical waveguide 51 is located serves as the central input port 54, and the wide ends where the bottom of the upper insulated conical waveguide 52 and the lower insulated conical waveguide 53 are located serve as the upper output port 55 and the lower output port 56, respectively.

[0048] The thermally adiabatic 1×2 beam splitter 5 has a symmetrical structure and is used to evenly divide the optical signal input from the central thermally adiabatic conical waveguide into two paths, upper and lower. The central thermally adiabatic conical waveguide is flush with the left and right ends of the upper and lower thermally adiabatic conical waveguides. The thermally adiabatic 2×1 beam combiner 11 has the same structure and is symmetrically distributed from left to right.

[0049] The input connection S-bend waveguide 6 is connected to the output port of the adiabatic 1×2 beam splitter and the wide end of the input tapered waveguide, respectively, to maintain low-loss transmission while separating the signals of the two output ports of the adiabatic 1×2 beam splitter by a sufficient distance; the output connection S-bend waveguide has the same structure and is symmetrically distributed.

[0050] The wide end of the input tapered waveguide is connected to the input S-bend waveguide 6, and the narrow end is connected to the Bragg grating, which is used to realize lossless transmission of optical signals; the output tapered waveguide has the same structure and is symmetrically distributed.

[0051] The Bragg grating 8 is used to reflect light of a specific wavelength (satisfying the Bragg reflection condition) in the input light field, while light of other wavelengths is transmitted normally; the transmitted light is output from one port, and the reflected light returns along the same path.

[0052] like Figure 4As shown, the Bragg grating 8 includes a central waveguide 81 and grating teeth 82; the central waveguide 81 is strip-shaped and arranged along the waveguide propagation direction, and grating teeth 82 are provided on both sides of the central waveguide 81; the grating teeth 82 are divided into upper and lower groups of grating teeth, which are symmetrically distributed relative to the central axis of the central waveguide 81, that is, symmetrically arranged on both sides of the central waveguide 81; each group of grating teeth includes grating units that are equally spaced along the waveguide propagation direction, but whose width gradually changes from small to large to small.

[0053] The grating teeth 82 in the upper and lower Bragg gratings 8 are staggered by half a grating period along the waveguide propagation direction, i.e., the horizontal direction. Thus, the upper and lower Bragg gratings are staggered by half a grating period in the horizontal direction, thereby generating a phase difference of π between the reflected signals at the upper and lower Bragg gratings.

[0054] The two ends of the center waveguide 81 of the Bragg grating 8 are connected to the narrow ends of the input tapered waveguide 7 and the output tapered waveguide 9, respectively. The two ends of the center waveguide 81 are connected to the adiabatic 1×2 beam splitter / adiabatic 2×1 beam combiner 11 via the input tapered waveguide 7 / output tapered waveguide 9 and the input connecting S-bend waveguide 6 / output S-bend waveguide 10, respectively.

[0055] The central waveguide 81 has a square cross-section to meet the polarization insensitivity requirement. The grating teeth 82 are sized using a dovetail distribution, and the length of the grating teeth is designed with a dovetail.

[0056] The grating teeth 82 are periodic structures. By adjusting the geometric parameters of the grating teeth 82, light of a specific wavelength can satisfy the Bragg condition. This wavelength can be used as the working wavelength, and the fundamental mode TE0 / TM0 of the working wavelength input can be reflected.

[0057] like Figure 5 As shown in the simulation test, when the optical signal is input in the fundamental mode TE0 / TM0 mode from the center input port 54 of the adiabatic 1×2 beamsplitter 5, the spectral transmission of the optical field in the reflected first-order mode TE1 / TM1 mode at the center input port 54 and in the fundamental mode TE0 / TM0 mode at the middle output port of the adiabatic 2×1 beam combiner 11 is as follows: It can be seen that in the operating wavelength range of 1305-1315nm, the fundamental mode TE0 / TM0 mode input to the adiabatic 1×2 beamsplitter 5 is mainly reflected as the first-order mode TE1 / TM1 mode, while the transmitted light field intensity is below -20dB. However, in the wavelength range greater than the operating wavelength range, the input optical signal maintains its fundamental mode form and passes through the filter with almost no loss.

[0058] like Figure 6As shown, when the optical field is input from the trunk waveguide 41 in TE1 or TM1 mode, the mode (de)multiplexer can achieve ultra-low loss first-order mode TE1 / TM1 to fundamental mode TE0 / TM0 mode conversion and output from the access waveguide 42 within a broadband range of 1260-1340nm. At the same time, the residual optical field in the trunk waveguide is below -12dB.

[0059] like Figure 7 As shown, using the polarization-insensitive silicon-based flat-top filter proposed in this invention as a unit, a standard four-channel CWDM filter can be constructed to achieve the decomposition of four CWDM wavelength channels. Three filter units are connected in series. By changing the Bragg grating period of the three filter units, their filtered center wavelengths correspond to 1271nm, 1291nm, and 1311nm respectively. Signals of the corresponding wavelengths are output from the download port of the corresponding filter unit, while signals of the remaining wavelengths can continue to be output from the output port with almost no loss, thus realizing the four-channel CWDM filtering function.

[0060] The following provides a specific example of a filter.

[0061] In the specific implementation, the left ports of the trunk waveguide 41 and the access waveguide 42 of the mode demultiplexer 4 are used as input port 1 and download port 2, respectively. The right port of the trunk waveguide 41 of the mode demultiplexer 4 is connected to the center input port 54 of the adiabatic 1×2 beam splitter 5, and the right port of the access waveguide 42 of the mode demultiplexer 4 is left unused.

[0062] The intermediate output port of the adiabatic 2×1 combiner 11 serves as the output port 3 of the entire filter.

[0063] S1. When the input signal is in TE0 mode:

[0064] A specific wavelength (λ) input from input port 1 sThe signal light (TE0 mode) passes losslessly through the mode demultiplexer 4 and enters the center input port 54 of the adiabatic 1×2 beamsplitter 5. The signal light is then uniformly split into upper and lower signals by the adiabatic 1×2 beamsplitter 5, and enters the Bragg grating 8 through the input connecting S-bend waveguide 6 and the input tapered waveguide 7, respectively. At this point, the intensity and phase of the two signal lights are completely identical. The signal light entering the upper and lower Bragg gratings 8 is then reflected along its original path. Due to a half-cycle misalignment in the transverse direction between the upper and lower Bragg gratings 8, a phase difference of π occurs between the two reflected signal lights. Furthermore, the upper and lower reflected signals enter the upper and lower output ports 55 and 56 of the adiabatic 1×2 beamsplitter, respectively, after passing through the input tapered waveguide and the input connecting S-bend waveguide. At this time, the adiabatic 1×2 beam splitter reverses to generate the function of the adiabatic 2×1 beam combiner. The two reflected signal beams are re-beamed into TE1 mode and reach the right port of the main waveguide 41 of the mode demultiplexer 4. They are further coupled into TE0 mode and enter the access waveguide 42, and finally output from the download port 2.

[0065] Other wavelengths input from input port 1 (λ>λ) s The signal light (TE0 mode) passes almost losslessly through mode demultiplexer 4 and adiabatic 1×2 beam splitter 5, and is split into two signals with the same intensity and phase. These signals then pass sequentially through input connecting S-bend waveguide 6, input tapered waveguide 7, Bragg grating 8, output tapered waveguide 9, and output connecting S-bend waveguide 10, respectively, to the upper and lower input ports of adiabatic 2×1 beam combiner 11 (see upper output port 55 and lower output port 56 of the adiabatic 1×2 beam splitter). Since the intensity and phase of the upper and lower signals remain consistent at this point, the two signals are re-bundled into TE0 mode after passing through the adiabatic 2×1 beam combiner 11 and arrive at output port 3.

[0066] In summary, the specific wavelength (λ) input from input port 1 s The signal light will be output from download port 2; other wavelengths (λ>λ) input from input port 1 will be output. s The signal light will be output from output port 3.

[0067] S2. When the input signal is in TM0 mode:

[0068] because Figure 1 All components are designed to be polarization insensitive, therefore the TM0 input signal will have the same signal response as the TE0 input signal, i.e., the specific wavelength (λ) input from input port 1 will have the same response. s The signal light will be output from download port 2; other wavelengths (λ>λ) input from input port 1 will be output. s The signal light will be output from output port 3.

[0069] The embodiments of the present invention are as follows:

[0070] Using a silicon nitride platform, the designed mode demultiplexer has a trunk waveguide 41 with widths of 1.85 μm and 2.2 μm at both ends, and an access waveguide 42 with widths of 0.9 μm and 0.7 μm at both ends. The coupling region length is 60 μm, and the waveguide spacing is 300 nm. The adiabatic 1×2 beamsplitter 5 is 100 μm long. The central adiabatic tapered waveguide 51 has widths of 1.4 μm and 0.12 μm at both ends, while the upper and lower adiabatic tapered waveguides 52 and 53 have widths of 0.12 μm and 0.8 μm at both ends, respectively. The waveguide spacing is 0.3 μm. The central waveguide 81 of the Bragg grating 8 has a width of 0.4 μm, the grating teeth 82 have a period of 0.428 μm, a duty cycle of 0.5, and a spacing of 0.4 μm between them and the central waveguide 81.

[0071] Figure 5 This diagram illustrates the spectral transmission of the fundamental mode TE0 / TM0 at output port 3 and the first-order modes TE1 and TM1 at output port 54 when optical signals are input from the central input port 54 of the adiabatic 1×2 beamsplitter 5 in TE0 and TM0 modes, respectively. It can be seen that near the center wavelength of 1311 nm, regardless of whether the input signal is in TE or TM mode, the input optical signal can be reflected back to the central input port 54 of the adiabatic 1×2 beamsplitter 5 in the form of the first-order mode TE1 / TM1, and this reflection has a flat-top response spectral range exceeding 14 nm, thus effectively mitigating the temperature drift problem during practical application. Meanwhile, for wavelengths between 1285-1300 nm and wavelengths greater than 1325 nm, the input signal will be transmitted normally with almost no loss, reaching output port 3 in the fundamental mode TE0 / TM0. However, for input signals with wavelengths less than 1285 nm, a significant amount of loss will occur during transmission as a radiating mode.

[0072] Figure 6 The diagram shows the spectral transmission at the left port of the trunk waveguide 41 and the left port of the access waveguide 42 when an optical signal is input in first-order mode TE1 / TM1 from the right port of the mode demultiplexer 4. It can be seen that throughout the entire O-band, regardless of whether it is TE or TM mode, the first-order mode optical signal input at the right port of the trunk waveguide 41 can be coupled to the left port of the access waveguide 42 with almost no loss, thus enabling the download of the corresponding signal.

[0073] Figure 7The diagram shows a typical 4-channel cascaded CWDM filter structure. As can be seen, the four CWDM wavelength optical signals input from the single port on the left are output sequentially from the four ports at wavelengths of 1271nm, 1291nm, 1311nm, and 1331nm, respectively, achieving separation of the four wavelength channels. In this process, each filter only needs to process wavelengths not less than its operating wavelength (λ). s The light signal, therefore, is like Figure 5 Radiation loss in the 1260-1285nm wavelength range will not affect the normal operation of the filter.

[0074] In summary, through innovative mode demultiplexer, adiabatic beam splitter, and Bragg grating design, this invention can realize a polarization-insensitive CWDM flat-top filter.

[0075] The specific embodiments described above illustrate the technical solution and beneficial effects of the present invention in detail. It should be understood that the above description is only the most preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, additions, and equivalent substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A polarization-insensitive silicon-based flat-top filter for coarse wavelength division multiplexing systems, characterized in that, The device includes a mode demultiplexer (4), an adiabatic 1×2 beam splitter (5), an input connection S-bend waveguide (6), an input tapered waveguide (7), a Bragg grating (8), an output tapered waveguide (9), an output connection S-bend waveguide (10), and an adiabatic 2×1 beam combiner (11). The output of the mode demultiplexer (4) is connected to the input of the two input connection S-bend waveguides (6) via the adiabatic 1×2 beam splitter (5). The outputs of the two input connection S-bend waveguides (6) are connected to the two inputs of the Bragg grating (8) via their respective input tapered waveguides (7). The two outputs of the Bragg grating (8) are connected to the two inputs of the output connection S-bend waveguide (10) via their respective output tapered waveguides (9). The two outputs of the output connection S-bend waveguide (10) are connected to the output port (3) via the adiabatic 2×1 beam combiner (11). The mode demultiplexer (4) includes an asymmetric directional coupler structure consisting of a trunk waveguide (41) and an access waveguide (42) of different widths. The trunk waveguide (41) and the access waveguide (42) are arranged relatively parallel and spaced apart. The spacing between the trunk waveguide (41) and the access waveguide (42) is the same at all points along the waveguide propagation direction. The trunk waveguide (41) is connected to one end of the center waveguide of the adiabatic 1×2 beam splitter (5), and the access waveguide (42) is connected to the other end of the center waveguide of the adiabatic 1×2 beam splitter (5). The thermally adiabatic 1×2 beam splitter (5) includes a central thermally adiabatic conical waveguide (51), an upper thermally adiabatic conical waveguide (52), and a lower thermally adiabatic conical waveguide (53). The central thermally adiabatic conical waveguide (51) is an isosceles trapezoid, which is arranged from the wide end to the narrow end along the waveguide propagation direction. The upper thermally adiabatic conical waveguide (52) and the lower thermally adiabatic conical waveguide (53) are respectively provided on the two sides of the isosceles trapezoid of the central thermally adiabatic conical waveguide (51). The upper thermally adiabatic conical waveguide (52) and the lower thermally adiabatic conical waveguide (53) are arranged at intervals with the waist of the isosceles trapezoid of the central thermally adiabatic conical waveguide (51), and the spacing is the same at all points along the waveguide propagation direction. The thermally insulated 2×1 bundle combiner (11) and the thermally insulated 1×2 bundle splitter (5) have the same structure and are symmetrically distributed; The Bragg grating (8) includes a central waveguide (81) and grating teeth (82); the central waveguide (81) is strip-shaped and arranged along the waveguide propagation direction, and grating teeth (82) are provided on both sides of the central waveguide (81); the grating teeth (82) are divided into upper and lower groups of grating teeth, which are symmetrically distributed relative to the central axis of the central waveguide (81); each group of grating teeth includes grating units arranged at equal intervals along the waveguide propagation direction, but with their width gradually changing from small to large to small; The Bragg grating (8) operates only at the target wavelength to achieve a flat-top reflection response; the other three components operate across a wide spectrum.

2. The polarization-insensitive silicon-based flat-top filter for coarse wavelength division multiplexing systems according to claim 1, characterized in that: in, The main waveguide (41) is wider, which satisfies the transmission of the first-order mode TE1 / TM1; the access waveguide (42) is narrower, which only supports the transmission of the fundamental mode TE0 / TM0; and the TE1 / TM1 mode in the main waveguide (41) and the TE0 / TM0 mode in the access waveguide (42) always satisfy the mode matching condition.

3. A polarization-insensitive silicon-based flat-top filter for coarse wavelength division multiplexing systems according to claim 1, characterized in that: The wide end of the central insulated conical waveguide (51) serves as the central input port (54), and the wide ends of the upper insulated conical waveguide (52) and the lower insulated conical waveguide (53) serve as the upper output port (55) and the lower output port (56), respectively.

4. A polarization-insensitive silicon-based flat-top filter for coarse wavelength division multiplexing systems according to claim 1, characterized in that: The grating teeth (82) in the two Bragg gratings (8) are arranged to be offset by half a grating period along the waveguide propagation direction.

5. A polarization-insensitive silicon-based flat-top filter for coarse wavelength division multiplexing systems according to claim 1, characterized in that: The two ends of the center waveguide (81) of the Bragg grating (8) are connected to the narrow ends of the input tapered waveguide (7) and the output tapered waveguide (9), respectively.

6. A polarization-insensitive silicon-based flat-top filter for coarse wavelength division multiplexing systems according to claim 1, characterized in that: The central waveguide (81) has a square cross-section, and the grating teeth (82) are sized using a dovetail distribution, with the grating teeth length using a dovetail design.

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