Optical filter, wavelength division multiplexer, and system

Abstract

An optical filter (10), a wavelength division multiplexer, and a system. The optical filter (10) comprises: a mode multiplexer (100), a substrate (203), a cladding (2001), a first grating (2021), a second grating (2022) and a waveguide (201), wherein the mode multiplexer (100) is connected to the waveguide (201); the cladding (2001) is provided on the substrate (203), and the cladding (2001) covers the mode multiplexer (100), the first grating (2021), the waveguide (201) and the second grating (2022); the mode multiplexer (100) is configured to receive an optical signal from an input end (11) of the optical filter (10), and transmit the optical signal to the waveguide (201), the first grating (2021) and the second grating (2022); and the first grating (2021), the waveguide (201) and the second grating (2022) are spaced apart from each other in a first direction, and the first direction is parallel to the surface of the substrate (203). The design of the optical filter (10) can reduce the limitation of a bandwidth on the size of a grating tooth and reduce the difficulty of a production process. Both the spacing between the first grating (2021) and the waveguide (201) and the spacing between the second grating (2022) and the waveguide (201) first decrease and then increase in the direction of light transmission, and the direction of light transmission is parallel to the surface of the substrate (203) and is perpendicular to the first direction, thereby achieving a sidelobe suppression function. Further provided are a wavelength division multiplexer including an optical filter (10), and a system.

Description

Optical filters, wavelength division multiplexers and systems

[0001] This application claims priority to the Chinese patent application filed with the State Intellectual Property Office on June 29, 2023, with application number 202310786785.9 and application name “Optical Filter, Wavelength Division Multiplexer and System”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The embodiments of the present application relate to the field of communications, and in particular to an optical filter, a wavelength division multiplexer, and a system. Background Art

[0003] In the field of optical communications, optical filters can implement wavelength division multiplexing (WDM) to increase communication capacity, while chip-level wavelength division multiplexers can achieve high-density integration in a small size.

[0004] As shown in Figure 3, the optical filter 1 includes a mode multiplexer 10 and a multimode waveguide grating (MWG) filter 20. The MWG filter 20 includes a waveguide 21. The sidewalls of the waveguide 21 are etched to form a grating 22. The grating 22 includes multiple grating teeth. The grating teeth have the same amplitude. A tapered waveguide is provided between the mode multiplexer 10 and the waveguide 21. The width of the waveguide 21 along the z-direction first decreases and then increases, which is a form of amplitude apodization technology that can reduce front-end reflections.

[0005] However, current chip-level wavelength division multiplexers generally use a 180nm process node, so the etching accuracy of the waveguide width and gap is 180nm. The etching depth of the grating teeth in Figure 3, that is, the size of the grating teeth is related to the bandwidth of the optical filter. For example, in an optical filter with a bandwidth of 20nm, the etching depth of the grating teeth δ = 185nm. However, in an optical filter with a bandwidth of 10nm, the etching depth of the grating teeth δ = 160nm. (Figure 3 only shows the position of the grating teeth, and the size is the size of the grating teeth under the same bandwidth and is not used as a reference). In addition, amplitude toeing needs to be performed on the grating teeth to make the characteristic size of the grating smaller, which requires higher processing accuracy and is difficult to be compatible with the current process line.

[0006] Summary of the Invention

[0007] The embodiments of the present application provide an optical filter, a wavelength division multiplexer, and a system, which solve the problem of excessively high precision requirements in the existing optical filter processing technology.

[0008] To achieve the above objectives, the embodiments of the present application adopt the following technical solutions.

[0009] According to a first aspect of an embodiment of the present application, an optical filter is provided, comprising: a mode multiplexer, a substrate, a cladding, a first grating, a second grating, and a waveguide, wherein the mode multiplexer is connected to the waveguide. The cladding is arranged on the substrate, and the cladding covers the mode multiplexer, the first grating, the waveguide, and the second grating. The mode multiplexer is used to receive an optical signal from the input end of the optical filter and transmit the optical signal to the waveguide, the first grating, and the second grating. The first grating, the waveguide, and the second grating are spaced apart along a first direction, wherein the first direction is parallel to the surface of the substrate. The spacing between the first grating and the waveguide, and the spacing between the second grating and the waveguide, both first decrease and then increase along the light transmission direction, and the light transmission direction is parallel to the surface of the substrate and perpendicular to the first direction. Thus, in the present application, the first grating and the second grating are arranged in the cladding, so that the first grating, the second grating, and the waveguide are spaced apart. Compared to the prior art approach of integrally connecting the first and second gratings to the waveguide, this approach reduces bandwidth constraints on grating tooth size, enabling the grating tooth size to meet the requirements of silicon photonics 180nm CMOS process lines. Furthermore, the present invention adjusts the spacing between the first and second gratings and the waveguide, such that the spacing between the first grating and the waveguide, as well as the spacing between the second grating and the waveguide, first decreases and then increases along the direction of light transmission, forming an overall large-small-large pattern, thus achieving sidelobe suppression.

[0010] In an optional implementation, the shape of the waveguide near the first and second gratings can be adjusted, and / or the shapes of the first and second gratings can be adjusted to gradually change the spacing between the first grating and the waveguide, and the spacing between the second grating and the waveguide. This allows the spacing between the first grating and the waveguide, and the spacing between the second grating and the waveguide, to first decrease and then increase along the direction of light transmission, thereby achieving sidelobe suppression.

[0011] Optionally, the first grating and the second grating are both distributed in an arc line. Thus, the shape of the first grating and the second grating can be adjusted so that the first grating and the second grating on both sides of the waveguide are arc-shaped and bend in the direction away from the waveguide respectively. The design of the grating is such that the spacing between the first grating and the waveguide, as well as the spacing between the second grating and the waveguide, first decreases and then increases along the direction of light transmission, thereby achieving the function of sidelobe suppression. Alternatively, the side of the waveguide close to the grating adopts an arc-shaped surface. Thus, the surface shape of the waveguide close to the grating can be adjusted so that the two surfaces of the waveguide close to the first grating and the second grating are respectively arc-shaped and bend in the direction close to the first grating and the second grating respectively. The design of the waveguide is such that the spacing between the first grating and the waveguide, as well as the spacing between the second grating and the waveguide, first decreases and then increases along the direction of light transmission, thereby achieving the function of sidelobe suppression. Alternatively, the first and second gratings on either side of the waveguide can be curved, each curving away from the waveguide. Furthermore, the two surfaces of the waveguide near the first and second gratings can be curved, each curving toward the first and second gratings. The waveguide and gratings are designed so that the spacing between the first grating and the waveguide, as well as the spacing between the second grating and the waveguide, first decreases and then increases along the direction of light transmission, thereby achieving sidelobe suppression.

[0012] In an optional implementation, the grating tooth periods of the first grating and the second grating first decrease and then increase along the light transmission direction, thereby further achieving the function of sidelobe suppression.

[0013] In an optional implementation, the period of the i-th grating tooth of the first grating and the second grating satisfies:

[0014] Where i = 1, 2, ..., n, T0 is the initial period, β i is the propagation constant at the location of the i-th grating, κ i is the coupling coefficient at the position of the i-th grating, is the coupling coefficient of the n / 2th grating. Thus, the grating tooth periods of the first grating and the second grating can be made to decrease first and then increase along the light transmission direction.

[0015] In one optional implementation, the grating teeth of the first grating and the corresponding grating teeth of the second grating are arranged in an offset manner, and the offset displacement between the first grating and the corresponding grating teeth of the second grating is half the grating tooth period. Thus, the first and second gratings are asymmetrically arranged with respect to the waveguide, and the offset displacement is half the grating tooth period, effectively reducing crosstalk between the wavelength band of the filtered optical signal output by the optical filter and adjacent wavelength bands.

[0016] In one optional implementation, the first grating is arranged axially symmetrically about a first axis of symmetry, and the second grating is arranged axially symmetrically about a second axis of symmetry, with both the first and second axes of symmetry being parallel to the first direction. Thus, the first and second gratings are axisymmetric structures, and the spacing between the first grating and the waveguide changes gradually, while the spacing between the second grating and the waveguide changes symmetrically, resulting in symmetrical sidelobe suppression and improved results.

[0017] In an optional implementation, the waveguide, the first grating, and the second grating are all connected to the surface of the substrate, thereby simplifying the manufacturing process.

[0018] In one optional implementation, the first grating and the second grating are spaced apart from the substrate in a second direction, and / or the waveguide is spaced apart from the substrate in a second direction, wherein the second direction is perpendicular to the surface of the substrate. This allows the spacing between the grating and the waveguide and the substrate to be adjusted as needed, providing a more flexible structure.

[0019] In an optional implementation, the waveguide is a ridge waveguide or a strip waveguide, thereby making the structure of the waveguide more flexible.

[0020] In an optional implementation, the method further includes: a connecting waveguide, wherein the mode multiplexer is connected to the waveguide via the connecting waveguide, so that the optical signal mode transmitted by the connecting waveguide does not change inside the connecting waveguide, and the transmission stability is improved.

[0021] A second aspect of an embodiment of the present application provides a wavelength division multiplexer comprising multiple optical filters as described above, cascaded and connected via connecting waveguides. Thus, the wavelength division multiplexer employing the aforementioned optical filters reduces bandwidth limitations on grating tooth size, can utilize 180nm CMOS process lines, and facilitates large-scale production. Furthermore, sidelobe suppression can be achieved.

[0022] In an optional implementation, the wavelength division multiplexer further comprises: a plurality of optical fibers, wherein the plurality of optical fibers are respectively connected to the input ends and the output ends of the plurality of optical filters. Thus, the wavelength division multiplexer is connected to the optical fibers, and signals can be transmitted through the optical fibers.

[0023] A third aspect of the present application provides a passive optical network system, comprising: a plurality of optical line terminals; a wavelength division multiplexer (WDM) connected to the plurality of optical line terminals via optical fibers, the WDM being the WDM described above; an optical distribution network (ODN) connected to the WDM via optical fibers; and a plurality of optical network terminals (ONTs) connected to the ODN via optical fibers. Thus, the use of the WDM in the passive optical network system can improve the WDM performance of the passive optical network system.

[0024] In one optional implementation, some of the multiple optical line terminals have optical modules that are GPON optical modules, another portion has optical modules that are 10G-PON optical modules, and another portion has optical modules that are 50G-PON optical modules. Thus, the wavelength division multiplexer can be used to combine and / or split optical signals.

[0025] The fourth aspect of the embodiments of the present application provides a wavelength division multiplexing system, comprising: a first optical converter, the first optical converter being used to perform optical conversion on a received signal; a multiplexer, the multiplexer comprising the optical filter described above, the multiplexer being used to couple the optical signals of multiple wavelengths converted by the first optical converter to an optical fiber; a demultiplexer, the demultiplexer comprising the optical filter described above, the demultiplexer being connected to the multiplexer via the optical fiber, the demultiplexer being used to separate the coupled optical signals; and a second optical converter, the second optical converter being used to restore the optical signal transmitted by the demultiplexer. Thus, the multiplexer and demultiplexer of the wavelength division multiplexing system use the above-mentioned optical filter, which can reduce the process precision requirements for the optical filter and reduce the difficulty of the production process. It also achieves the function of sidelobe suppression.

[0026] In one optional implementation, the wavelength division multiplexing system further includes an optical amplifier located between the multiplexer and the demultiplexer. The optical amplifier is configured to amplify the power of the optical signal transmitted by the multiplexer and transmit the amplified optical signal to the demultiplexer. Thus, by providing the optical amplifier, optical power amplification can be achieved.

[0027] The embodiments of the present application provide an optical filter, a wavelength division multiplexer, and a system, wherein the optical filter includes: a mode multiplexer, a substrate, a cladding, a first grating, a second grating, and a waveguide, wherein the mode multiplexer and the waveguide are connected, wherein the cladding is provided on the substrate and covers the mode multiplexer, the first grating, the waveguide, and the second grating; the mode multiplexer is used to receive an optical signal from the input end of the optical filter and transmit the optical signal to the waveguide and the first grating and the second grating; wherein the first grating, the waveguide, and the second grating are spaced apart along a first direction, and the first direction is parallel to the surface of the substrate; this can reduce the bandwidth limitation on the grating tooth size and reduce the difficulty of the production process. The spacing between the first grating and the waveguide, as well as the spacing between the second grating and the waveguide, first decreases and then increases along the light transmission direction, and the light transmission direction is parallel to the surface of the substrate and perpendicular to the first direction, thereby achieving the function of sidelobe suppression. Among them, the shape of the waveguide close to the first grating and the second grating can be adjusted, and / or the shape of the first grating and the second grating can be adjusted so that the spacing between the first grating and the waveguide, and the spacing between the second grating and the waveguide are gradually changed. The spacing between the first grating and the waveguide, and the spacing between the second grating and the waveguide, are both reduced first and then increased along the direction of light transmission, thereby achieving the function of sidelobe suppression. In some embodiments, the period of the grating teeth of the first grating and the second grating can also be adjusted so that the period of the grating teeth of the first grating and the second grating is reduced first and then increased along the direction of light transmission. The function of sidelobe suppression can be further achieved. In addition, the grating teeth of the first grating and the grating teeth of the second grating corresponding to the grating teeth of the first grating are arranged in a staggered position, and the displacement of the position of the grating teeth of the first grating and the grating teeth of the second grating corresponding to the grating teeth of the first grating is half of the grating tooth period. Thus, the first grating and the second grating are asymmetrically arranged about the waveguide, and the offset displacement is half of the grating tooth period, which effectively reduces the crosstalk between the band where the filtered light signal output by the optical filter is located and the adjacent band. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a schematic diagram of the uplink and downlink wavelengths of a passive optical network;

[0029] FIG2 is a schematic diagram of the structure of a passive optical network system;

[0030] FIG3 is a schematic structural diagram of an optical filter;

[0031] FIG4 is a transmission spectrum simulation diagram of the optical filter in FIG3 ;

[0032] FIG5 is a schematic structural diagram of an optical filter provided in an embodiment of the present application;

[0033] FIG6 is a perspective view of an optical filter provided in an embodiment of the present application;

[0034] FIG7 is a first AA cross-sectional view in FIG6;

[0035] FIG8 is a second AA cross-sectional view in FIG6;

[0036] FIG9 is a third AA cross-sectional view in FIG6;

[0037] FIG10 is a fourth AA cross-sectional view of FIG6;

[0038] FIG11 is a fifth AA cross-sectional view of FIG6;

[0039] FIG12 is a schematic structural diagram of a waveguide grating provided in an embodiment of the present application;

[0040] FIG13 is a graph showing the variation of the spacing between the waveguide and the grating in FIG12 along the light transmission direction;

[0041] FIG14 is a schematic diagram of a spectrum image of a grating when the spacing between the waveguide and the grating remains unchanged along the light transmission direction;

[0042] FIG15 is a schematic diagram of a spectrum image of the waveguide grating shown in FIG12 ;

[0043] FIG16 is a schematic structural diagram of another waveguide grating provided in an embodiment of the present application;

[0044] FIG17 is a graph showing the variation of the waveguide width along the light transmission direction in FIG16;

[0045] FIG18 is a schematic diagram of a spectrum image of the waveguide grating shown in FIG16 ;

[0046] FIG19 is a schematic structural diagram of another waveguide grating provided in an embodiment of the present application;

[0047] FIG20 is a graph showing the variation of the grating tooth period along the light transmission direction in FIG19;

[0048] FIG21 is a schematic diagram of a spectrum image of the waveguide grating shown in FIG19 ;

[0049] FIG22 is a schematic structural diagram of a wavelength division multiplexing system provided in an embodiment of the present application;

[0050] FIG23 is a schematic structural diagram of a wavelength division multiplexer provided in an embodiment of the present application. DETAILED DESCRIPTION

[0051] In order to make the purpose, technical solutions and advantages of this application clearer, this application will be further described in detail below with reference to the accompanying drawings.

[0052] Hereinafter, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature identified with "first," "second," etc., may explicitly or implicitly include one or more of the features. In the description of this application, unless otherwise specified, "plurality" means two or more.

[0053] In addition, in this application, directional terms such as "upper" and "lower" are defined relative to the orientation of the components in the drawings. It should be understood that these directional terms are relative concepts. They are used for relative description and clarification, and they can change accordingly according to changes in the orientation of the components in the drawings.

[0054] Passive Optical Network (PON): A passive optical network (PON) is an optical distribution network (ODN) between the optical line terminal (OLT) and the optical network terminal (ONT). The OLT provides the network-side interface of the optical access network and is connected to one or more optical network devices (ODNs). The ONT, located directly at the user end, is the device in the access network that provides the user with network services such as high-speed internet access, IPTV, voice, and Wi-Fi. The ODN is a fiber-to-the-home (FTTH) cable network based on PON equipment.

[0055] Wavelength Division Multiplexing (WDM): Wavelength division multiplexing (WDM) is a technology that combines two or more optical carrier signals (carrying various information) of different wavelengths at the transmitting end through a multiplexer (also known as a combiner) and couples them into the same optical fiber for transmission. At the receiving end, a demultiplexer (also known as a wavelength splitter or demultiplexer) separates the optical carriers of different wavelengths, and then an optical receiver further processes them to restore the original signal. This technology of simultaneously transmitting two or more optical signals of different wavelengths in the same optical fiber is called wavelength division multiplexing.

[0056] Optical transmission module: Also known as an optical module, it consists of two main parts: the bi-directional optical sub-assembly (BOSA) and the electrical subassembly (ESA). The optical transmission module is completed by electrically connecting the pins of the BOSA to the peripheral ESA and then installing it into the optical module housing.

[0057] Currently, with the increasing demand for communication capacity, access networks are gradually evolving from GPON and 10G PON to 50G PON. To ensure compatibility and smooth evolution across multiple generations of PON, a low-cost coexistence solution for GPON, 10G PON, and 50G PON is required. The latest revision of the 50G PON standard takes into account the need for coexistence among all three generations, adding the 1284-1288nm band as a 50G uplink option. Figure 1 shows a schematic diagram of the upstream and downstream wavelengths of a passive optical network. As shown in Figure 1, the upstream rates include GPON upstream, 10G PON upstream, and 50G PON upstream wavelengths. The downstream rates include GPON downstream, 10G PON downstream, and 50G PON downstream wavelengths.

[0058] Figure 2 is a schematic diagram of the structure of a passive optical network system. As shown in Figure 2, in some embodiments, a PON network consists of an OLT at the central office, an ONT at the user side, a wavelength division multiplexer, and an optical network (ODN). The OLT is the convergence device in the PON system, providing an optical interface between the service network and the ODN. The ONT, located on the user side, provides both an optical interface between the user and the ODN and an electrical interface on the user side. The ODN is responsible for the transmission, distribution, and multiplexing of data information. The OLT and ONT ends of the PON network employ a point-to-multipoint topology.

[0059] In some embodiments, the OLT side includes OLTs supporting GPON, 10G PON, and 50G PON rates, with each rate corresponding to two wavelengths for upstream and downstream, for a total of six wavelengths. The ONT side includes ONTs supporting GPON, 10G PON, and 50G PON rates, with each rate corresponding to two wavelengths for upstream and downstream, for a total of six wavelengths.

[0060] For example, as shown in FIG2 , the passive optical network system includes three OLTs: GPON OLT, 10G PON OLT, and 50G PON OLT. Each OLT is connected to a WDM through an optical fiber. The WDM is connected to an optical distribution network, and the optical distribution network is connected to the ONT.

[0061] For example, an optical distribution network includes an optical splitter and optical fibers connected to the splitter. The optical splitter is connected to a wavelength division multiplexer (WDM) via optical fibers, and the optical splitter is also connected to the optical network terminals (ONTs) via optical fibers. There are three types of ONTs: GPON ONT, 10G PON ONT, and 50G PON ONT. Each ONT is connected to the optical splitter via an optical fiber. During operation, the six wavelengths on the OLT side are combined in a WDM process, then transmitted to the optical distribution network (ODN), and then distributed to different ONTs via optical splitters.

[0062] The optical communication system shown in the embodiment of the present application can also be an industrial optical network, a data center network, a wavelength division multiplexing network, or an optical transport network (OTN), etc., without specific limitation.

[0063] In some embodiments, the WDM can be used in a 50G PON passive optical network system.

[0064] In some embodiments, the optical modules of some of the optical line terminals are GPON optical modules, the optical modules of another part of the optical line terminals are 10G-PON optical modules, and the optical modules of another part of the optical line terminals are 50G-PON optical modules. For example, the WDM can be integrated into the 50G-PON optical module.

[0065] In some embodiments, the WDM is a packaged chip.

[0066] In some embodiments, the WDM further comprises: a plurality of optical fibers, each of which is connected to the WDM. In the above embodiment, the wavelength division multiplexer can transmit the different wavelengths of multiple signals in a directional manner, achieving simultaneous transmission of multiple signals through optical wave separation and optical wave combination. For example, the wavelength division multiplexer can transmit multiple optical signals on the same optical fiber, thereby improving optical fiber utilization.

[0067] In some embodiments, WDM includes a wavelength splitter, a wavelength combiner, and an optical filter. The wavelength splitter can separate multiple signals into signals of different wavelengths and direct each signal into a different channel, achieving wavelength multiplexing. The wavelength combiner can combine signals of different wavelengths from various channels into a single signal and output it. The optical filter can filter out optical signals of non-target wavelengths while allowing signals of the target wavelength to pass through.

[0068] Figure 3 is a schematic diagram of the structure of an optical filter. As shown in Figure 3, the spacing between the grating teeth on both sides of the waveguide along the z direction is Δs, where Δs is gradually distributed along the light transmission direction (z direction), with the change trend first decreasing and then increasing. This is a phase apodization scheme that can achieve the function of suppressing sidelobes.

[0069] The optical filter operates as follows: The fundamental TE0 mode in the waveguide is input from the left. When the incident wavelength meets the corresponding phase matching condition, (n0 + nm) = λ / Λ, it is reflected as the corresponding transverse electric (TE) m mode. Here, n0 and nm are the modal effective refractive indices of the TE0 and TEm modes, λ is the incident wavelength, and Λ is the grating period.

[0070] In some embodiments, as shown in FIG3 , when a TE0 mode is input into a waveguide and the incident wavelength does not meet the phase matching condition, it is output from the right side of waveguide C in TE0 mode. When the incident wavelength meets the corresponding phase matching condition, it is reflected by the MWG filter 20. The mode multiplexer 10 can convert the TE1 mode in waveguide A, which has been reverse-coupled (reflected) by the MWG filter 20, into the TE0 mode and output it from waveguide B, thus achieving reverse coupling from the TE0 mode to the TE1 mode.

[0071] The device forms an input port at the left end of the A waveguide, a drop port at the left end of the B waveguide, and an output port at the right end of the C waveguide.

[0072] The mode multiplexer 10 can convert the TE1 mode in the A waveguide after reverse coupling by the MWG filter 20 into the TE0 mode, and output it from the B waveguide.

[0073] However, the etching depth of the grating teeth, that is, the size of the grating teeth, is related to the bandwidth of the optical filter. For example, in an optical filter with a bandwidth of 20nm, the etching depth of the grating teeth is δ=185nm. However, in an optical filter with a bandwidth of 10nm, the etching depth of the grating teeth is δ=160nm. (Figure 3 only shows the position of the grating teeth, and the size is the size of the grating teeth under the same bandwidth and is not used as a reference). In addition, it is necessary to perform amplitude toe cutting on the grating teeth to make the characteristic size of the grating smaller, which requires higher processing technology accuracy. The existing silicon photonics 180nm complementary metal oxide semiconductor (CMOS) process line cannot meet the demand and is not suitable for large-scale mass production.

[0074] To this end, the present invention provides an improved optical filter. This optical filter has low processing precision requirements, can be produced on existing silicon photonics 180nm CMOS process lines, is suitable for large-scale mass production, and has improved transmission spectrum performance.

[0075] Figure 5 is a schematic diagram of the structure of an optical filter provided in an embodiment of the present application. Figure 6 is a perspective view of the optical filter provided in an embodiment of the present application. As shown in Figures 5 and 6, the optical filter includes: a mode multiplexer 100, a connecting waveguide 300, and a waveguide grating 200. The mode multiplexer 100 is connected to the waveguide grating 200 via the connecting waveguide 300. The mode multiplexer 100 is used to receive an optical signal from the input port 11 of the optical filter and transmit the optical signal to the waveguide grating 200.

[0076] In some embodiments, as shown in FIG5 , the mode multiplexer 100 includes a single-mode waveguide 101 and a first gradient waveguide 102. One end of the single-mode waveguide 101 is connected to the input port 11, the other end of the single-mode waveguide 101 is connected to one end of the first gradient waveguide 102, the other end of the first gradient waveguide 102 is connected to one end of the connecting waveguide 300, the other end of the connecting waveguide 300 is connected to one end of the waveguide grating 200, and the other end of the waveguide grating 200 is connected to the output port 13.

[0077] The first tapered waveguide 102 gradually increases in size along the first direction (z-direction) to form a multimode waveguide. For example, the single-mode waveguide 101 supports the transmission of the fundamental transverse electric mode (TE0). The first tapered waveguide 102 supports the transmission of at least TE0 and the first transverse electric mode (TE1).

[0078] The TE0 mode transmitted from left to right along the Z-axis may enter the waveguide grating 200 via the first graded-interval waveguide 102 .

[0079] 5 , the mode multiplexer 100 further includes: a curved waveguide 103 and a second gradient waveguide 104. One end of the curved waveguide 103 is connected to the download end 12. The spacing between the single-mode waveguide 101 and the curved waveguide 103 gradually decreases along the optical transmission direction (z direction). The curved waveguide 103 is used to gradually change the single-mode waveguide 101 and the curved waveguide 103 from a weak coupling state at a longer distance to a strong coupling state at a closer distance along the z direction. The other end of the curved waveguide 103 is connected to one end of the second gradient waveguide 104. The other end of the second gradient waveguide 104 does not support TE0 mode transmission and is suspended. The second gradient waveguide 104 is coupled to the first gradient waveguide 102. The dimension of the second gradient waveguide 104 along the z direction gradually decreases along the z direction. The second end of the second gradient waveguide 104 is in a guided mode-free state. The connecting waveguide 300 is connected to the second end of the first gradient waveguide 102. The connecting waveguide 300 extends along the z-direction based on the x-direction dimension of the second end of the first tapered waveguide 102. If the second end of the first tapered waveguide 102 is a multimode waveguide, the connecting waveguide 300 is also a multimode waveguide. For example, if the first tapered waveguide 102 supports TE0 and TE1 transmission, then the connecting waveguide also supports TE0 and TE1 transmission.

[0080] This embodiment does not impose any restrictions on the length of the connecting waveguide 300 along the z-direction. The structure of the connecting waveguide 300 described in this embodiment is merely an example and is not intended to be limiting. As long as the connecting waveguide 300 supports at least TE0 and TE1 transmission and ensures adiabatic coupling, it will suffice. Adiabatic coupling means that the optical signal mode transmitted by the connecting waveguide 300 does not change within the connecting waveguide 300.

[0081] The second gradient waveguide 104 can couple the TE1 mode transmitted from right to left along the Z-axis into the TE0 mode.

[0082] The curved waveguide 103 can isolate the input end 11 and the download end 12 by a certain distance to avoid optical crosstalk.

[0083] During operation, the input port 11 receives a TE0 mode optical signal, which passes through the single-mode waveguide 101 and the first gradient waveguide 102 and then enters the connecting waveguide 300. This allows the TE0 mode, which propagates from left to right along the Z axis, to enter the waveguide grating 200 via the first gradient waveguide 102. The signal is filtered in the waveguide grating 200 and reflected back as a TE1 mode optical signal. This TE1 mode signal passes through the second gradient waveguide 104, is coupled to the TE0 mode, and then enters the download port 12 through the curved waveguide 103. Furthermore, the TE1 mode, which propagates from right to left along the Z axis, can be coupled to the curved waveguide 103 for output.

[0084] During operation, the mode multiplexer 100 receives multiple optical signals through the input port 11. The input port 11 may be a port on the input waveguide of the mode multiplexer 100 for receiving the multiple optical signals. Each of the multiple optical signals has a TE0 mode. The multiple optical signals are sequentially transmitted through the single-mode waveguide 101 and the first gradient waveguide 102 to the connecting waveguide 300. Because the dimension of the first gradient waveguide 102 in the x-direction gradually increases along the z-direction, the optical signal with a TE0 mode passes directly through the first gradient waveguide 102 to the connecting waveguide 300. The connecting waveguide 300 transmits the first optical signal to the waveguide grating 200.

[0085] The multi-path optical signal includes a multi-path download optical signal whose wavelength is located in the first wavelength band. In the waveguide grating 200, the download optical signal located in the first wavelength band satisfies the phase matching condition. The download optical signal that satisfies the phase matching condition is reflected and transmitted to the connecting waveguide 300 with TE1. The connecting waveguide 300 receives the download optical signal from the waveguide grating 200, and the download optical signal received by the connecting waveguide 300 has TE1. The phase matching condition is (n0+n1)=λ / T, n0 is the effective refractive index of the TE0 mode, and n1 is the effective refractive index of the TE1 mode. λ is the wavelength of the download optical signal. T is the grating period of the waveguide grating 200. The grating period is the sum of the length of any grating tooth included in the waveguide grating 200 and the length of the grating groove adjacent to the grating tooth along the z direction.

[0086] The download optical signal with TE1 reflected by the connecting waveguide 300 is transmitted to the first gradient waveguide 102. TE1 in the first gradient waveguide 102 and TE0 in the second gradient waveguide 104 meet the phase matching condition, so that the download optical signal is coupled to the second gradient waveguide 104 with TE0. The download optical signal with TE0 is sequentially transmitted through the second gradient waveguide 104 and the curved waveguide 103 to the download terminal 12.

[0087] The multiple optical signals received by the input end 11 of the mode multiplexer 100 also include multiple direct optical signals having wavelengths within a second wavelength band. The first wavelength band is different from the second wavelength band. The direct optical signals within the second wavelength band do not meet the phase matching condition. The direct optical signals that do not meet the phase matching condition are transmitted along the waveguide grating 200 to the output end 13. Moreover, the optical signal output via the waveguide grating 200 has TE0. The description of the mode of the optical signal input to the input end 11 and the mode of the optical signal reflected by the waveguide grating 200 to the connecting waveguide 300 in this embodiment is for illustrative purposes only and does not constitute a limitation.

[0088] In some embodiments, as shown in FIG. 5 and FIG. 6 , the waveguide grating 200 includes: a first grating 2021 , a second grating 2022 and a waveguide 201 .

[0089] In some embodiments, as shown in Figures 6, 7, 8, 9, and 11, the optical filter 10 further includes a substrate 203 (shown in Figure 6) and a cladding (not shown in Figure 6). The cladding is disposed on the substrate 203 and covers the first grating 2021, the waveguide 201, the second grating 2022, and the mode multiplexer 100.

[0090] For example, the cladding covering the first grating 2021, the waveguide 201, the second grating 2022 and the mode multiplexer 100 includes: the first grating 2021, the waveguide 201, the second grating 2022 and the mode multiplexer 100 are all in contact with the surface of the substrate 203, at this time, the cladding covers the upper surface and side surfaces of the first grating 2021, the waveguide 201, the second grating 2022 and the mode multiplexer 100.

[0091] The mode multiplexer 100 is in contact with the surface of the substrate 203, and at least one of the first grating 2021, the waveguide 201, and the second grating 2022 is spaced apart from the surface of the substrate 203. In this case, for the structure in contact with the surface of the substrate 203, the cladding covers the upper surface and side surfaces thereof. For the structure spaced apart from the surface of the substrate 203, the cladding covers the upper surface and side surfaces thereof and fills the gap between the structure and the surface of the substrate 203.

[0092] When preparing the waveguide grating 200 , a mask may be placed on a substrate, and the grating 202 and the waveguide 201 may be formed by etching. Then, a cladding may be formed on the substrate so that the grating 202 and the waveguide 201 are wrapped by the cladding.

[0093] In some embodiments, as shown in Figures 5 and 6, the first grating 2021, the waveguide 201 and the second grating 2022 are spaced apart along a first direction (i.e., the x direction in Figure 6), wherein the first direction is parallel to the surface of the substrate (the surface of the substrate is, for example, the xz plane).

[0094] Therefore, the grating is set in the cladding 2001, and the first grating 2021, the second grating 2022 and the waveguide 201 are set at intervals. Compared with the prior art in which the grating and the waveguide are set as one, the grating tooth size is more flexible, so that the minimum grating tooth size can meet the requirements of the existing silicon photonics 180nm CMOS process line, reducing the process accuracy requirements.

[0095] In some embodiments, as shown in Figures 5 and 6, the spacing between the first grating 2021 and the waveguide 201, as well as the spacing between the second grating 2022 and the waveguide 201, first decreases and then increases along the light transmission direction (the z direction shown in Figure 6). The light transmission direction is parallel to the surface of the substrate and perpendicular to the first direction. For example, the spacing between the first grating 2021 and the waveguide 201, as well as the spacing between the second grating 2022 and the waveguide 201, first decreases and then increases along the light transmission direction (the z direction shown in Figure 6), may include:

[0096] The shapes of the first grating 2021 and the second grating 2022 are adjusted to gradually change the distance between the grating and the waveguide 201. Alternatively, the shape of the side of the waveguide 201 close to the grating is adjusted to gradually change the distance between the grating and the waveguide 201. Alternatively, the shape of the side of the waveguide 201 close to the grating is adjusted, and the shapes of the first grating 2021 and the second grating 2022 are adjusted to gradually change the distance between the grating and the waveguide 201.

[0097] As shown in Figures 6 and 12, the first grating 2021 and the second grating 2022 are both distributed in an arc shape. The top view of the waveguide 201 is rectangular, and the side of the waveguide 201 close to the grating is flat. The curvature direction of the arc line is a first direction, and the curvature direction is away from the waveguide. The curvature direction of the arc line refers to the opening direction of the arc line. For example, the opening directions of the arc lines of the first grating 2021 and the second grating 2022 in Figure 6 are indicated by arrows. It can be seen that the opening direction of the arc line of the first grating 2021 is the +x direction, and the opening direction of the arc line of the second grating 2022 is the -x direction, which are perpendicular to the light transmission direction z direction and are curved in a direction away from the waveguide 201.

[0098] In other embodiments, the side of the waveguide 201 near the gratings utilizes a curved surface, and the first grating 2021 and the second grating 2022 are both arranged in a straight line. The curvature direction of the curved surface is a first direction, and the curvature direction is away from the gratings. The curvature direction of the curved surface refers to the opening direction of the curved surface. For example, the opening directions of the curved surface of the waveguide 201 are indicated by arrows. It can be seen that the opening direction of the curved surface of the waveguide 201 near the first grating 2021 is in the +x direction, and the opening direction of the curved surface of the waveguide 201 near the second grating 2022 is in the -x direction. The opening directions of the curved surface of the waveguide 201 are perpendicular to the light transmission direction z, and each curves toward the direction closest to the gratings.

[0099] In some embodiments, as shown in FIG16 , the waveguide has a curved surface on a side near the grating, and the first grating and the second grating are both distributed as curved lines. The curved surface and the curved line have the same curvature direction, and the curvature of the curved line is greater than the curvature of the curved surface. Curvature refers to the degree of curvature; the greater the curvature, the smaller the radius of the circle encompassed by the curved line.

[0100] As shown in Figure 16, the opening direction of the curved surface of the waveguide 201 close to the first grating 2021 is the +x direction, the opening direction of the curved surface of the waveguide 201 close to the second grating 2022 is the -x direction, the opening direction of the curved line of the first grating 2021 is the +x direction, and the opening direction of the curved line of the first grating 2021 is the -x direction. The opening direction of the curved surface of the waveguide 201 and the opening direction of the curved line of the grating are perpendicular to the light transmission direction z.

[0101] The embodiment of the present application does not restrict the degree or direction of curvature of the grating and waveguide 201; it is sufficient that the spacing between the grating and waveguide first decreases and then increases along the direction of light transmission. Those skilled in the art can adjust the shape of the grating and waveguide as needed, and this application does not impose any restrictions on this. Thus, by adjusting the spacing between the grating and waveguide 201, a better sidelobe suppression effect is achieved on both sides of the flat region of the reflection spectrum.

[0102] In other embodiments, as shown in FIG19 , the grating tooth period of the first grating 2021 and the second grating 2022 first decreases and then increases along the light transmission direction. The grating tooth period is the distance between the centers of two adjacent grating teeth.

[0103] In some embodiments, the first grating 2021 and the second grating 2022 each include multiple grating teeth, each with a duty cycle of 50%. The duty cycle of a grating tooth is the ratio of its width to its period. While the duty cycle of the grating teeth remains constant, the width of the grating teeth varies as the period of the grating teeth changes along the direction of light transmission. Thus, by adjusting the period of the grating teeth, the sidelobe suppression effect of the transmitted wave is improved.

[0104] In some embodiments, as shown in FIG5 , the first gratings 2021 are arranged axially symmetrically about a first symmetry axis O, and the second gratings 2022 are arranged axially symmetrically about a second symmetry axis O'. Both the first symmetry axis O and the second symmetry axis O' are parallel to the first direction. Thus, the first gratings 2021 and the second gratings 2022 are both axially symmetrical structures. The spacing between the first grating 2021 and the waveguide gradually changes, and the spacing between the second grating 2022 and the waveguide also gradually changes symmetrically, resulting in symmetrical sidelobe suppression and better results.

[0105] The embodiments of the present application do not limit the materials of the cladding 2001, substrate 203, waveguide 201, and grating 202. In this application, the same materials are represented by the same fill symbols, and different materials are represented by different fill symbols. Among them, the refractive index of the cladding is n1, the refractive index of the substrate 203 is n2, the refractive index of the waveguide 201 is n3, and the refractive index of the grating 202 is n4. n1, n2, n3, and n4 satisfy: the refractive index n3 of the waveguide 201 is greater than the refractive index n2 of the substrate 203, and the refractive index n3 of the waveguide 201 is greater than the refractive index n1 of the cladding 2001. The refractive index n4 of the grating 202 is greater than the refractive index n2 of the substrate 203, and the refractive index n4 of the grating 202 is greater than the refractive index n1 of the cladding 2001.

[0106] In some embodiments, the cladding 2001 and substrate 203 are both made of silicon dioxide, and the waveguide 201 and grating 202 are made of the same material. For example, the waveguide 201 and grating 202 are both made of silicon and have the same thickness. In other embodiments, the waveguide 201 and grating 202 are made of different materials, for example, the waveguide 201 is made of silicon and the grating 202 is made of silicon nitride.

[0107] The embodiment of the present application does not limit the height of the grating 202 and the waveguide 201 from the substrate 203 in the y direction. The cladding is provided on the substrate 203 as shown in FIG6 , and the cladding covers the mode multiplexer 100 , the first grating 2021 , the waveguide 201 , and the second grating 2022 .

[0108] In some embodiments, as shown in FIG7 , the first grating 2021, the second grating 2022, and the waveguide 201 are all disposed on the surface of the substrate 203. In other embodiments, as shown in FIG9 , the first grating 2021, the second grating 2022, and the waveguide 201 are all disposed in the cladding 2001 and spaced apart from the substrate 203 along the y-direction. In still other embodiments, as shown in FIG8 , the first grating 2021 and the second grating 2022 are disposed in the cladding 2001 and spaced apart from the substrate 203 along the y-direction, and the waveguide 201 is disposed on the surface of the substrate 203. In other embodiments, as shown in FIG10 , the waveguide 201 is disposed in the cladding 2001 and spaced apart from the substrate 203 along the y-direction, and the first grating 2021 and the second grating 2022 are disposed on the surface of the substrate 203.

[0109] In the above embodiment, the waveguide 201 is a strip waveguide. In other embodiments, as shown in FIG11 , the waveguide 101 is a ridge waveguide.

[0110] The structures and materials of the first grating 2021 , the second grating 2022 and the waveguide 201 are described below with reference to FIG. 7 to FIG. 11 .

[0111] FIG7 is an AA cross-sectional view of FIG6. As shown in FIG7, the waveguide 201 is a strip waveguide. The waveguide 201, the first grating 2021, and the second grating 2022 are all disposed on a substrate 203. The waveguide 201 and the grating 202 are spaced apart along the x-direction. The cladding 2001 covers the upper surface and side surfaces of the waveguide 201, the first grating 2021, and the second grating 2022.

[0112] In some embodiments, referring to FIG. 7 , the waveguide 201 and the first and second gratings 2021 and 2022 have the same height.

[0113] In some embodiments, the waveguide 201 and the first and second gratings 2021, 2022 are made of the same material. For example, they are all made of silicon. The thicknesses of the waveguide 201, the first and second gratings 2021, 2022 along the y-direction may differ. In other embodiments, the waveguide 201, the first and second gratings 2021, 2022 are made of different materials. For example, the waveguide 201 is made of silicon, while the first and second gratings 2021, 2022 are made of silicon nitride. The thicknesses of the waveguide 201, the first and second gratings 2021, 2022 along the y-direction may differ.

[0114] When preparing the waveguide grating 200 shown in FIG7 , a mask can be set on the substrate, and the grating 202 and the waveguide 201 can be formed by etching, and then a cladding is formed on the substrate so that the grating 202 and the waveguide 201 are wrapped by the cladding.

[0115] Figure 8 is another AA cross-sectional view of Figure 6 . As shown in Figure 8 , waveguide 201 is a strip waveguide disposed on the first surface of substrate 203. A first grating 2021 and a second grating 2022 are spaced apart from waveguide 201 in the x-direction, and the gratings (first grating 2021 and second grating 2022) are spaced apart from substrate 203 in the z-direction. A cladding 2001 covers the top and side surfaces of waveguide 201, first grating 2021, and second grating 2022, and fills the gaps between first grating 2021, second grating 2022 and substrate 203.

[0116] In some embodiments, as shown in FIG8 , the height of the waveguide 201 in the z direction is lower than the height of the gratings (the first grating 2021 and the second grating 2022) in the z direction, that is, the distance between the surface of the waveguide 201 close to the substrate and the first surface of the substrate is smaller than the distance between the surface of the grating 202 close to the substrate and the first surface of the substrate. For example, in FIG8 , the waveguide 201 is disposed on the first surface of the substrate 203, and the distance between the surface of the waveguide 201 close to the substrate and the first surface of the substrate is 0, while the gratings (the first grating 2021 and the second grating 2022) are spaced apart from the substrate 203 in the z direction, and the distance between the two is greater than 0. Then, the distance between the surface of the waveguide 201 close to the substrate and the first surface of the substrate is smaller than the distance between the surface of the gratings (the first grating 2021 and the second grating 2022) close to the substrate and the first surface of the substrate.

[0117] In some embodiments, the waveguide 201 and the first and second gratings 2021, 2022 are made of the same material. For example, they are all made of silicon and have the same thickness. In other embodiments, the waveguide 201 and the first and second gratings 2021, 2022 are made of different materials. For example, the waveguide 201 is made of silicon, while the first and second gratings 2021, 2022 are made of silicon nitride.

[0118] When preparing the waveguide grating 200 shown in Figure 8, a mask layer can be set on the substrate first, and the waveguide 201 can be formed by etching. After growing the corresponding material, a second mask layer is then formed, and the grating 202 is formed by etching.

[0119] Figure 9 is another AA cross-sectional view of Figure 6 . As shown in Figure 9 , the first grating 2021 and the second grating 2022 are spaced apart from the waveguide 201 in the x-direction. The gratings (first grating 2021 and second grating 2022) are at the same height as the waveguide 201 in the z-direction, and the waveguide 201 is spaced apart from the substrate in the z-direction. The cladding 2001 covers the top and side surfaces of the waveguide 201, the first grating 2021, and the second grating 2022, and fills the gaps between the waveguide 201, the first grating 2021, and the second grating 2022 and the substrate 203.

[0120] In some embodiments, as shown in FIG9 , the waveguide 201 is a strip waveguide, the height of the waveguide 201 in the z-direction is the same as the height of the grating (the first grating 2021 and the second grating 2022) in the z-direction, and the distance between the surface of the waveguide 201 close to the substrate and the first surface of the substrate is equal to the distance between the surface of the grating 202 close to the substrate and the first surface of the substrate. For example, in FIG9 , the waveguide 201 and the grating (the first grating 2021 and the second grating 2022) are both spaced apart from the substrate 203 in the z-direction, and the distance between the two is greater than 0, and the distance between the surface of the waveguide 201 close to the substrate and the first surface of the substrate is equal to the distance between the surface of the grating 202 close to the substrate and the first surface of the substrate.

[0121] In some embodiments, the waveguide 201 and the first and second gratings 2021, 2022 are made of the same material, for example, silicon, and have the same thickness. In other embodiments, the waveguide 201 and the first and second gratings 2021, 2022 are made of different materials, for example, the waveguide 201 is made of silicon, while the first and second gratings 2021, 2022 are made of silicon nitride.

[0122] Figure 10 is another AA cross-sectional view of Figure 6 . As shown in Figure 10 , waveguide 201 is a strip waveguide. A first grating 2021 and a second grating 2022 are disposed on the first surface of a substrate 203. The first grating 2021 and the second grating 2022 are spaced apart from the waveguide 201 in the x-direction, and the waveguide 201 is spaced apart from the substrate 203 in the z-direction. A cladding 2001 covers the top and side surfaces of the waveguide 201, the first grating 2021, and the second grating 2022, and fills the gap between the waveguide 201 and the substrate 203.

[0123] In some embodiments, as shown in FIG10 , the height of the waveguide 201 in the z-direction is higher than the height of the gratings (the first grating 2021 and the second grating 2022) in the z-direction, that is, the distance between the surface of the waveguide 201 close to the substrate and the first surface of the substrate is greater than the distance between the surface of the grating 202 close to the substrate and the first surface of the substrate. For example, in FIG10 , the gratings (the first grating 2021 and the second grating 2022) are disposed on the first surface of the substrate 203, and the distance between the surface of the gratings (the first grating 2021 and the second grating 2022) close to the substrate and the first surface of the substrate is 0, while the waveguide 201 is spaced apart from the substrate 203 in the z-direction, and the distance between the two is greater than 0. Therefore, the distance between the surface of the waveguide 201 close to the substrate and the first surface of the substrate is greater than the distance between the surface of the gratings (the first grating 2021 and the second grating 2022) close to the substrate and the first surface of the substrate.

[0124] In some embodiments, the waveguide 201 and the first and second gratings 2021, 2022 are made of the same material. For example, they are all made of silicon and have the same thickness. In other embodiments, as shown in FIG10 , the waveguide 201 and the first and second gratings 2021, 2022 are made of different materials. For example, the waveguide 201 is made of silicon, while the first and second gratings 2021, 2022 are made of silicon nitride.

[0125] Figure 11 is another AA cross-sectional view of Figure 6. As shown in Figure 11, waveguide 201 is a ridge waveguide. Waveguide 201 and first and second gratings 2021 and 2022 are all disposed on the first surface of substrate 203. Waveguide 201 is connected to the substrate via a slab 204. Cladding 2001 covers the surface and sides of first and second gratings 2021 and 2022, as well as the waveguide, and also covers the exposed portion of slab 204. Slab 204 is made of the same material as waveguide 201, and slab 204 and waveguide 201 form a ridge waveguide.

[0126] In some embodiments, as shown in FIG11 , the waveguide 201 and the grating 202 have the same height and are made of the same material. For example, the waveguide 201 and the grating 202 are both made of silicon and have the same thickness. In other embodiments, the waveguide 201 is a ridge waveguide structure, the waveguide 201 and the grating 202 have different heights, and the grating 202 and the slab 204 are spaced apart.

[0127] In Figure 3, the z-direction spacing Δs between the grating teeth on either side of the waveguide varies gradually along the optical transmission direction. However, the spacing between the grating teeth on both sides of the waveguide input and output is almost zero, resulting in a nearly symmetrical distribution. This makes reverse coupling between the TE0 mode and even-order modes more likely to occur. For example, as shown in Figure 4, the target optical filter is located at 1270nm, but a concave peak appears at 1380nm, affecting the transmission spectrum performance at the through-port.

[0128] To this end, the embodiments of the present application further improve the optical filter to optimize the transmission spectrum performance.

[0129] In some embodiments, the grating teeth of the first grating 2021 and the grating teeth of the second grating 2022 corresponding to the grating teeth of the first grating 2021 are staggered, so that the grating teeth of the first grating 2021 and the second grating 2022 are asymmetrically distributed on both sides of the waveguide 201.

[0130] In some embodiments, the relative displacement between the grating teeth of the first grating 2021 and the grating teeth of the second grating 2022 corresponding to the grating teeth of the first grating 2021 remains unchanged along the propagation direction. The relative displacement between the grating teeth of the first grating 2021 and the grating teeth of the second grating 2022 corresponding to the grating teeth of the first grating 2021 may be the spacing Δsi between the i-th grating tooth 2021i of the first grating 2021 and the i-th grating tooth 2022i of the second grating 2022 along the z-direction. The displacement of the relative position misalignment between the grating teeth of the first grating 2021 and the grating teeth of the second grating 2022 corresponding to the grating teeth of the first grating 2021 remains unchanged along the propagation direction, which means that the spacing Δs1 between the first grating tooth of the first grating 2021 and the first grating tooth of the second grating 2022 along the z direction, the spacing Δs2 between the second grating tooth of the first grating 2021 and the second grating tooth of the second grating 2022 along the z direction... the spacing Δsi between the i grating teeth of the first grating 2021 and the i grating tooth of the second grating 2022 along the z direction... the spacing Δsn between the n grating tooth of the first grating 2021 and the n grating tooth of the second grating 2022 along the z direction are all equal.

[0131] For example, as shown in Figure 5, the first grating tooth of the first grating 2021 is 2021a, and the first grating tooth of the second grating 2022 is 2022a, wherein the spacing between 2021a and 2022a along the z direction is Δs1, the seventh grating tooth of the first grating 2021 is 2021g, and the seventh grating tooth of the second grating 2022 is 2022g, wherein the spacing between 2021g and 2022g along the z direction is Δs7, wherein Δs1 is equal to Δs7.

[0132] The present application does not impose any restrictions on the specific numerical values ​​of the displacement of the grating teeth of the first grating 2021 and the grating teeth of the second grating 2022 corresponding to the grating teeth of the first grating 2021. It is only necessary to make the displacement of the grating teeth of the first grating 2021 and the second grating 2022 close to half of the grating tooth period. Those skilled in the art can adjust the displacement of the grating teeth of the first grating 2021 and the second grating 2022 as needed, and all of these are within the scope of protection of the present application. Among them, the displacement of the grating teeth position is close to half of the grating tooth period, and there may be errors. For example, the displacement of the grating teeth position can be 20%-80% of the grating tooth period.

[0133] For example, with the two sides of the straight waveguide as a reference, the displacement of adjacent grating teeth can be half a grating period when the gratings on both sides are projected onto the sides of the straight waveguide. That is, the displacement of the grating teeth of the first grating 2021 and the second grating 2022 can be half a grating period. The grating tooth period is the distance between the centers of two adjacent grating teeth.

[0134] For example, the first grating 2021 and the second grating 2022 each include a plurality of grating teeth, the duty cycle of each grating tooth is 50%, and the displacement of the grating teeth of the first grating 2021 and the second grating 2022 can be half of the grating tooth period, that is, the width of one grating tooth.

[0135] For example, as shown in FIG5 , at the first position, the spacing Δs1 between the first grating tooth 2021 a of the first grating 2021 and the first grating tooth 2022 a of the second grating 2022 along the z direction is one grating tooth, ie, half of the grating tooth period.

[0136] Continuing with FIG5 , at the second position, the spacing Δs7 along the z-direction between the seventh grating tooth 2021g of the first grating 2021 and the seventh grating tooth 2022g of the second grating 2022 is one grating tooth, or half the grating tooth period. In other words, the spacing between adjacent grating teeth of the first grating 2021 and the second grating 2022 remains unchanged, while the second grating 2022 is shifted rightward by half a period relative to the first grating 2021.

[0137] Therefore, the grating teeth of the first grating 2021 and the second grating 2022 are misaligned, and the displacement Δs of the misalignment between the grating teeth of the first grating 2021 and the second grating 2022 remains unchanged, which can avoid reverse coupling of the TE0 and TE2 modes and achieve optimization of transmission spectrum performance.

[0138] The present application does not impose any specific restrictions on the specific shape of the grating in the top view, for example, it can be rectangular, circular, hexagonal, etc.

[0139] The solution of this application is described below with reference to Figures 12 to 21.

[0140] The cross-section of the waveguide grating in the embodiment shown below adopts the structure shown in Figure 7, where the waveguide 201 and grating 202 are both made of silicon nitride with a thickness of 250 nm, and the cladding 2001 and substrate 203 are both made of silicon dioxide. The projected shape of the waveguide grating 200 is, for example, a rectangle.

[0141] The spacing between the waveguide 201 and the grating 202 can be adjusted, and the distribution of the coupling coefficient can be adjusted to match the target apodization function and achieve a sidelobe suppression effect.

[0142] Figure 12 is a schematic diagram of the structure of a waveguide grating provided in an embodiment of the present application. In some embodiments of the present application, as shown in Figure 12, waveguide 201 is a rectangular waveguide with a width of W. The grating structure is rectangular with a period of T, a width of D, a duty cycle of 50%, and the number of periods as N. The minimum spacing between waveguide 201 and grating 202 is d1, and the maximum spacing is d2. The minimum dimensions of grating 202 meet the requirements of a 180nm process line.

[0143] In some embodiments, the waveguide 201 includes two operating modes: TE0 mode and TE1 mode. In some embodiments, the waveguide 201 does not support other high-order modes.

[0144] The period of the grating 202 is T. In some embodiments, T satisfies the following formula: (n0+n1)=λ / T (1)

[0145] Where n0 is the effective refractive index of the TE0 mode, n1 is the effective refractive index of the TE1 mode, and λ is the target filtering wavelength.

[0146] In some embodiments, W, d1, d2, D, T, and N may satisfy: W=1.1um, d1=0.2um, d2=0.5um, D=0.4um, T=0.41um, and N=400.

[0147] The light propagation direction is the z direction. The spacing between the waveguide 201 and the grating 202 (the spacing between the waveguide 201 and the first grating 2021, or the spacing between the waveguide 201 and the second grating 2022) is denoted as d(z). d(z) exhibits a large-small-large distribution in the propagation direction. In some embodiments, d(z) satisfies the following formula: d(z) = d1 + c(z - Lg / 2) 2 Formula (2)

[0148] c in formula (2) satisfies: c = 4 (d2 - d1) / Lg 2 Formula (3)

[0149] Lg in formula (2) and formula (3) satisfies: Lg=N*T Formula (4)

[0150] Wherein, d1 is the minimum distance between the waveguide 201 and the grating 202 , and d2 is the maximum distance between the waveguide 201 and the grating 202 .

[0151] FIG13 is a graph showing the variation in the spacing between the waveguide and grating in FIG12 along the direction of light transmission. As shown in FIG13 , the spacing d between the waveguide 201 and the grating 202 exhibits a large-small-large distribution along the z direction. FIG14 is a schematic diagram of the spectrum image of the grating when the spacing between the waveguide and the grating remains unchanged along the direction of light transmission, i.e., the transmission spectrum under the structure with d2 = d1 = 200 nm. As shown in FIG14 , the sidelobes on both sides of the flat region of the reflection spectrum are strong, and the sidelobe suppression ratio (SLSR) of the first sidelobe is less than 5 dB. FIG15 is a schematic diagram of the spectrum image of the waveguide grating shown in FIG12 . As shown in FIG15 , the SLSR on the right side of the flat region of the reflection spectrum is greater than 20 dB, while the SLSR on the left side maintains the original level. Therefore, by providing a grating with a gradually varying spacing between the waveguide 201 and the grating 202, a better suppression effect of the right-side sidelobe can be achieved.

[0152] In the above embodiment, sidelobe suppression can be achieved by adjusting the distance between the waveguide 201 and the grating 202. In other embodiments of the present application, the shape of the waveguide 201 can be adjusted to further achieve sidelobe suppression.

[0153] Figure 16 is a schematic diagram of the structure of another waveguide grating provided in an embodiment of the present application. The difference between Figure 16 and Figure 9 is that the waveguide 201 has a curved sidewall on the side close to the grating 202. The waveguide 201 adopts a curved surface structure with a width of W. The grating structure is rectangular with a period of T, a width of D, a duty cycle of 50%, and the number of periods is recorded as N. Among them, the light transmission direction is the z direction, and the spacing between the waveguide 201 and the grating 202 is recorded as d, and d presents a large-small-large distribution in the transmission direction. The minimum spacing between the waveguide 201 and the grating 202 is d1, and the maximum spacing is d2. The minimum size of the grating 202 meets the 180nm process line requirements.

[0154] The light transmission direction is the z direction. In some embodiments, the change W(z) of the width W of the waveguide 201 along the light transmission direction z satisfies the following formula: W(z)=1100+p*(500 q -d(z) q ) / m Formula (5)

[0155] Among them, p = 1.607, q = -0.004298, m = 7.088*10 -5 .

[0156] Figure 17 is a schematic diagram showing the variation in the width W of waveguide 201 along the z-direction. As shown in Figure 17, the width W(z) of waveguide 201 exhibits a large-small-large distribution along the z-direction. Figure 18 is a schematic diagram showing the spectrum image of the waveguide grating shown in Figure 16. As shown in Figure 18, the SLSR of the first sidelobe on the left is greater than 25dB, and the SLSR of the first sidelobe on the right is greater than 10dB. Compared to the waveguide grating shown in Figure 12, the waveguide grating shown in Figure 16 has a better suppression effect on the side lobes of the transmission wave. Therefore, by providing a grating with a gradually varying spacing between waveguide 201 and grating 202, the side lobes on both sides of the passband can be effectively suppressed.

[0157] Figure 19 is a structural schematic diagram of another waveguide grating 200 provided in an embodiment of the present application. As shown in Figure 19, the difference between the waveguide grating 200 and the waveguide grating shown in Figure 12 is that the period T of the grating 202 of the waveguide grating 200 shown in Figure 19 is no longer a constant, but is distributed in a large-small-large pattern along the transmission direction z.

[0158] In some embodiments, the grating period satisfies the following formula:

[0159] Among them, T I is the period of the i-th grating tooth, T0 is the initial period set, β i is the propagation constant at the location of the i-th grating, κ i is the coupling coefficient at the position of the i-th grating, is the coupling coefficient of the n / 2th grating.

[0160] In some embodiments, T0=400 nm, and the variation pattern of T with z can be obtained according to the structural parameters determined by the above formula.

[0161] As shown in Figure 20, the grating period is approximately 402.5 nm at both ends of the transmission direction, and 400 nm at the center. The grating period is symmetrical about the grating center.

[0162] Figure 21 is a schematic diagram of the spectrum image of the waveguide grating shown in Figure 19. As shown in Figure 21, the side lobes on both sides of the passband are effectively suppressed. The SLSR of the first side lobe on the left is greater than 25dB, and the SLSR of the first side lobe on the right is greater than 10dB. Compared with the waveguide grating in Figure 12, the side lobes on both sides of the transmission wave of the waveguide grating shown in Figure 19 have a better suppression effect. Figure 22 is a structural schematic diagram of a wavelength division multiplexing system provided in an embodiment of the present application. As shown in Figure 22, the wavelength division multiplexing system includes: a first optical transponder unit (OTU) 1, a multiplexer (MUX), a demultiplexer (DMUX) and a second optical transponder unit OTU2.

[0163] The first optical converter unit (OTU1) is configured to perform optical conversion on the received signal and transmit the converted optical signal to the multiplexer. The multiplexer (MUX) includes the above-mentioned optical filter and is configured to couple the optical signals of multiple wavelengths converted by the first optical converter (OTU1) to an optical fiber. The demultiplexer (DMUX) includes the above-mentioned optical filter and is connected to the multiplexer (MUX) via the optical fiber to separate the coupled optical signals.

[0164] In some embodiments, referring to Figure 22, the wavelength division multiplexing system also includes: an optical amplifier (OA), which is located between the multiplexer MUX and the demultiplexer DMUX. The optical amplifier is used to amplify the optical signal transmitted by the multiplexer and send the optical signal with amplified optical power to the demultiplexer.

[0165] The present application also provides a wavelength division multiplexer. In some embodiments, the wavelength division multiplexer includes multiple optical filters as shown in Figure 5. The wavelength division multiplexer formed by cascading multiple optical filters can realize the functions of combining and demultiplexing multiple wavelengths.

[0166] For example, as shown in FIG23 , the wavelength division multiplexer includes a first optical filter 10 a , a second optical filter 10 b , and a third optical filter 10 c .

[0167] As shown in FIG23 , taking three grating optical filters in cascade as an example, the wavelength division multiplexer is composed of a first optical filter 10a, a first connecting waveguide 300a, a second optical filter 10b, a second connecting waveguide 300b, a third optical filter 10c, and the Nth connecting waveguide 300n, which are sequentially connected end to end.

[0168] The N optical filters, such as the first optical filter 10a, the second optical filter 10b and the third optical filter 10c, can be designed for different wavelengths and bandwidths and have different structural parameters.

[0169] The embodiment of the present application does not limit the structure of the connecting waveguide. The connecting waveguide can be designed in various forms, such as a straight waveguide, a curved waveguide, or a wedge-shaped waveguide.

[0170] This wavelength division multiplexer supports both wavelength download and upload functions. Taking the structure of the wavelength division multiplexer shown in Figure 23 as an example, a first wavelength can be input through the first input port 11a, passed through the first optical filter 10a, and downloaded from the first output port 12a. A second wavelength can be input from the first input port 11a, sequentially pass through the first optical filter 10a, the first connecting waveguide 300a, and the second optical filter 10b, and downloaded from the second output port 12b. A third wavelength can be input from the second input port 11b, reflected by the third optical filter 10c, and enter the second connecting waveguide 300b. It then passes through the second optical filter 10b, the first connecting waveguide 300a, and enters the first optical filter 10a, before being output from the first output port 12a.

[0171] This wavelength division multiplexer employs the aforementioned optical filter, which reduces bandwidth limitations on grating tooth size and can be fabricated using 180nm CMOS process lines, facilitating mass production. Furthermore, it can achieve sidelobe suppression.

[0172] The embodiments of the present application provide an optical filter, a wavelength division multiplexer, and a system. The optical filter in the present application includes: a mode multiplexer, a substrate, a cladding, a first grating, a second grating, and a waveguide, wherein the mode multiplexer and the waveguide are connected, wherein the cladding is arranged on the substrate and covers the mode multiplexer, the first grating, the waveguide, and the second grating; the mode multiplexer is used to receive an optical signal from the input end of the optical filter and transmit the optical signal to the waveguide and the first grating and the second grating; wherein the first grating, the waveguide, and the second grating are spaced apart along a first direction, and the first direction is parallel to the surface of the substrate; this can reduce the bandwidth limitation on the grating tooth size and reduce the difficulty of the production process. The spacing between the first grating and the waveguide, as well as the spacing between the second grating and the waveguide, first decreases and then increases along the light transmission direction, and the light transmission direction is parallel to the surface of the substrate and perpendicular to the first direction, thereby achieving the function of sidelobe suppression. Among them, the shape of the waveguide close to the first grating and the second grating can be adjusted, and / or the shape of the first grating and the second grating can be adjusted so that the spacing between the first grating and the waveguide, and the spacing between the second grating and the waveguide are gradually changed. The spacing between the first grating and the waveguide, and the spacing between the second grating and the waveguide, are both reduced first and then increased along the direction of light transmission, thereby achieving the function of sidelobe suppression. In some embodiments, the period of the grating teeth of the first grating and the second grating can also be adjusted so that the period of the grating teeth of the first grating and the second grating is reduced first and then increased along the direction of light transmission. The function of sidelobe suppression can be further achieved. In addition, the grating teeth of the first grating and the grating teeth of the second grating corresponding to the grating teeth of the first grating are arranged in a staggered position, and the displacement of the position of the grating teeth of the first grating and the grating teeth of the second grating corresponding to the grating teeth of the first grating is half of the grating tooth period. Thus, the first grating and the second grating are asymmetrically arranged about the waveguide, and the offset displacement is half of the grating tooth period, which effectively reduces the crosstalk between the band where the filtered light signal output by the optical filter is located and the adjacent band.

[0173] The above is only a specific embodiment of the present application, but the scope of protection of this application is not limited thereto. Any changes or substitutions within the technical scope disclosed in this application should be included in the scope of protection of this application. Therefore, the scope of protection of this application should be based on the scope of protection of the claims.

Claims

1. An optical filter, characterized in that: include: A mode multiplexer, a substrate, a cladding, a first grating, a second grating, and a waveguide, wherein: The mode multiplexer is connected to the waveguide, wherein the cladding is disposed on the substrate, and the cladding covers the mode multiplexer, the first grating, the waveguide and the second grating; The mode multiplexer is used to receive an optical signal from an input end of the optical filter and transmit the optical signal to the waveguide and the first grating and the second grating; The first grating, the waveguide and the second grating are spaced apart along a first direction, wherein the first direction is parallel to the surface of the substrate; the spacing between the first grating and the waveguide, and the spacing between the second grating and the waveguide, both first decrease and then increase along a light transmission direction, and the light transmission direction is parallel to the surface of the substrate and perpendicular to the first direction.

2. The optical filter according to claim 1, characterized in that The first grating and the second grating are both distributed in arc lines.

3. The optical filter according to claim 1 or 2, characterized in that The side of the waveguide close to the grating adopts an arc-shaped surface.

4. The optical filter according to any one of claims 1 to 3, characterized in that: The grating tooth periods of the first grating and the second grating first decrease and then increase along the light transmission direction.

5. The optical filter according to claim 4, characterized in that The period of the i-th grating tooth of the first grating and the second grating satisfies: Where i = 1, 2, ..., n, T0 is the initial period, β i is the propagation constant at the i-th grating location, κ i is the coupling coefficient at the i-th grating location, is the coupling coefficient of the n / 2th grating.

6. The optical filter according to any one of claims 1 to 5, characterized in that: The grating teeth of the first grating and the grating teeth of the second grating corresponding to the grating teeth of the first grating are arranged in a staggered position, and the grating teeth of the first grating and the grating teeth of the second grating corresponding to the grating teeth of the first grating have a positional staggered displacement of half a grating tooth period.

7. The optical filter according to any one of claims 1 to 6, characterized in that: The first gratings are distributed in an axisymmetric manner based on a first symmetry axis, and the second gratings are distributed in an axisymmetric manner based on a second symmetry axis. Both the first symmetry axis and the second symmetry axis are parallel to the first direction.

8. The optical filter according to any one of claims 1 to 7, characterized in that: The waveguide, the first grating and the second grating are all connected to the surface of the substrate.

9. The optical filter according to any one of claims 1 to 7, characterized in that: The first grating and the second grating are spaced apart from the substrate in a second direction, and / or the waveguide is spaced apart from the substrate in a second direction, wherein the second direction is perpendicular to a surface of the substrate.

10. The optical filter according to any one of claims 1 to 9, characterized in that: The waveguide is a ridge waveguide or a strip waveguide.

11. The optical filter according to any one of claims 1 to 10, characterized in that: Also includes: A connecting waveguide, wherein the mode multiplexer is connected to the waveguide via the connecting waveguide.

12. A wavelength division multiplexer, characterized in that: The optical filter comprises a plurality of optical filters as claimed in any one of claims 1 to 11, wherein the plurality of optical filters are cascaded and connected via connecting waveguides.

13. The wavelength division multiplexer according to claim 12, characterized in that: The wavelength division multiplexer further comprises: a plurality of optical fibers, wherein the plurality of optical fibers are respectively connected to the input ends of the plurality of optical filters and the output ends of the plurality of optical filters.

14. A passive optical network system, characterized in that: include: multiple optical line terminals; A wavelength division multiplexer, the wavelength division multiplexer is connected to the plurality of optical line terminals via optical fibers, the wavelength division multiplexer adopts the wavelength division multiplexer according to claim 12 or 13; an optical distribution network connected to a wavelength division multiplexer via an optical fiber; A plurality of optical network terminals are connected to the optical distribution network via optical fibers.

15. The passive optical network system according to claim 14, characterized in that: The optical modules of some of the multiple optical line terminals are GPON optical modules, the optical modules of another part of the optical line terminals are 10GPON optical modules, and the optical modules of another part of the optical line terminals are 50G-PON optical modules.

16. A wavelength division multiplexing system, characterized in that: include: a first optical converter, the first optical converter being used to optically convert a received signal; A multiplexer, the multiplexer comprising the optical filter according to any one of claims 1 to 11, the multiplexer being used to couple the optical signals of multiple wavelengths converted by the first optical converter to an optical fiber; A demultiplexer, the demultiplexer comprising the optical filter according to any one of claims 1 to 11, the demultiplexer being connected to the multiplexer via the optical fiber, and the demultiplexer being used to separate the coupled optical signal; A second optical converter, wherein the second optical converter is used to restore the optical signal transmitted by the demultiplexer.

17. The wavelength division multiplexing system according to claim 16, characterized in that: Also includes: An optical amplifier is located between the multiplexer and the demultiplexer, and is used to amplify the power of the optical signal transmitted by the multiplexer and send the optical signal with amplified optical power to the demultiplexer.

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