A broadband photon filter based on thin-film lithium niobate optical waveguide

Abstract

The present invention discloses a broadband photon filter based on a thin-film lithium niobate optical waveguide, which belongs to the field of integrated optoelectronic technology. The broadband photon filter includes an adiabatic filter, a multimode connection waveguide, and a broadband mode converter, wherein the adiabatic filter and the broadband mode converter are both asymmetric directional coupler structures. The adiabatic filter selectively converts the input TE fundamental mode wavelength into a high-order TM mode, and the remaining TE fundamental modes are output from the through-end. The high-order TM mode enters the broadband mode converter through the multimode connection waveguide, and is all converted into the TE fundamental mode and output from the cross-end. The present invention innovatively uses the asymmetric directional coupler structure commonly used in mode division multiplexing as an optical filtering function, and has a simple structure. It is based on the principle of mode adiabatic evolution, is suitable for application in broadband optical filtering operations, and has the advantages of easy design and processing, low loss, etc.

Description

Technical Field

[0001] The present invention belongs to the technical field of integrated optoelectronics, and more specifically, relates to a broadband photon filter based on a thin-film lithium niobate optical waveguide. Background Art

[0002] In recent years, thin-film lithium niobate integrated photonic platforms have attracted considerable attention, offering viable solutions for high-performance modulators thanks to the high electro-optic coefficient of lithium niobate materials, low waveguide loss, and high integration density. Compared to bulk lithium niobate, the platform's high waveguide refractive index difference significantly enhances optical field confinement, which plays a crucial role in improving system integration and reducing energy consumption. The rapid development of big data and artificial intelligence technologies is exponentially increasing society's demand for high-speed information transmission. The compact design of thin-film lithium niobate integrated optical devices enables complex functionality within limited space and reduces production costs, making them crucial in meeting the growing demands of modern communication networks for data processing speed and efficiency. Consequently, designing and manufacturing various functional devices on thin-film lithium niobate platforms has become a key area of ​​research in integrated optics.

[0003] Optical filters are one of the important components in integrated optical systems, which can transmit light of a specific wavelength to a designated port. Narrowband multi-channel optical filters are commonly found in wavelength division multiplexing systems, which use light of different wavelengths to transmit optical information simultaneously, thereby greatly improving the capacity of optical transmission. On-chip optical filters with wide optical bandwidth and large free spectral range are also critical in many applications. In passive optical networks, broadband filters can separate the upstream and downstream channels to achieve single-fiber bidirectional transmission. In online monitoring of optical fiber links, broadband filters can be used to reflect light of a specific wavelength (such as 1625-1675nm), while light of other wavelengths can pass through without loss (such as 1260-1625nm). In on-chip wavelength converters and amplifiers, they can also be used to inject and separate signal light and pump light.

[0004] Currently, a variety of integrated optical filtering structures have been proposed on thin-film lithium niobate platforms. These structures almost all use the principle of beam interference, including arrayed waveguide gratings, tilted multimode interferometers, Mach-Zehnder interferometers, and microring filters. However, these structures are limited by the wavelength-sensitive nature of beam interference and usually have narrow filtering bandwidths or restricted free spectral ranges. Apodized multimode Bragg gratings are also common interferometric filtering structures. Although they can achieve a flat-top spectral response with a certain bandwidth, their upper optical bandwidth limit is limited by the coupling strength between the forward and reverse propagation modes. On the other hand, due to their multimode waveguide structure, the incident light at short wavelengths will undergo unnecessary coupling with the waveguide's higher-order modes, limiting its free spectral range. Therefore, it is difficult to realize on-chip optical filters with wide optical bandwidth and ultra-large free spectral range using filters based on the principle of beam interference. To achieve this goal, it is urgent to propose new filtering structures based on the underlying principles. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention proposes a broadband photonic filter based on a thin-film lithium niobate optical waveguide, based on the principle of adiabatic evolution of waveguide modes and utilizing an asymmetric directional coupler structure. The adiabatic filter selectively converts the input TE fundamental mode wavelength into higher-order TM modes, with the remaining TE fundamental modes output from the through-port. The higher-order TM modes enter the broadband mode converter via a multimode connecting waveguide, where they are all converted to TE fundamental modes and output from the cross-port. This broadband filter is suitable for broadband optical filtering operations and has an extremely large free spectral range.

[0006] According to a first aspect of the present invention, a broadband photon filter based on a thin-film lithium niobate optical waveguide is provided, comprising, from top to bottom, a lithium niobate thin film layer and a silicon dioxide buffer layer, wherein the lithium niobate thin film layer comprises an adiabatic filter, a multimode connection waveguide, and a broadband mode converter connected in sequence;

[0007] The adiabatic filter is an asymmetric directional coupler structure, consisting of a first front single-mode curved waveguide, a first single-mode coupled waveguide, a first rear single-mode curved waveguide, a first front multimode waveguide, a first multimode coupled waveguide, and a first rear multimode waveguide;

[0008] The first multimode coupling waveguide is a tapered waveguide with a width gradually increasing from left to right;

[0009] The first front single-mode curved waveguide, the first single-mode coupled waveguide, and the first rear single-mode curved waveguide are connected in sequence; the first front multi-mode waveguide, the first multi-mode coupled waveguide, and the first rear multi-mode waveguide are connected in sequence; the first front single-mode curved waveguide, the first single-mode coupled waveguide, and the first rear single-mode curved waveguide are respectively opposite to the first front multi-mode waveguide, the first multi-mode coupled waveguide, and the first rear multi-mode waveguide in vertical relation;

[0010] On the left side of the first multimode coupling waveguide and the first single-mode coupling waveguide, the high-order TM mode propagating in the first multimode coupling waveguide and the TE fundamental mode propagating in the first single-mode coupling waveguide have the same effective refractive index at wavelength λ1;

[0011] On the right side of the first multimode coupling waveguide and the first single-mode coupling waveguide, the high-order TM mode transmitted in the first multimode coupling waveguide and the TE fundamental mode transmitted in the first single-mode coupling waveguide have the same effective refractive index at wavelength λ2; and λ2 is greater than λ1;

[0012] The broadband photon filter based on the thin-film lithium niobate optical waveguide can enable the TE fundamental mode entering the first single-mode coupling waveguide from the input end of the broadband filter through the first front single-mode curved waveguide to gradually evolve into a high-order TM mode in the first multimode coupling waveguide when its wavelength is within the range of (λ1, λ2) and further enter the first rear multimode waveguide;

[0013] The broadband photon filter based on the thin-film lithium niobate optical waveguide can allow the TE fundamental mode from the input end of the broadband filter to enter the first single-mode coupling waveguide through the first front single-mode curved waveguide. When its wavelength is not in the range of (λ1, λ2), it directly passes through the first single-mode coupling waveguide and enters the first rear single-mode curved waveguide and is output from the straight-through end of the broadband filter.

[0014] Preferably, the horizontal projection lengths of the first front single-mode curved waveguide and the first rear single-mode curved waveguide are respectively equal to the lengths of the first front multimode waveguide and the first rear multimode waveguide, and the lengths of the first single-mode coupled waveguide and the first multimode coupled waveguide are equal.

[0015] Preferably, the first multimode coupling waveguide and the first single-mode coupling waveguide are arranged close to each other to form a waveguide coupling region; the close arrangement is specifically: the first front single-mode curved waveguide bends from left to right in a direction close to the first front multimode waveguide, and the first rear single-mode curved waveguide bends from left to right in a direction away from the first rear multimode waveguide.

[0016] Preferably, the broadband mode converter is an asymmetric directional coupler structure, consisting of a second front single-mode bent waveguide, a second single-mode coupled waveguide, a second rear single-mode bent waveguide, a second front multimode waveguide, a second multimode coupled waveguide, and a second rear multimode waveguide;

[0017] The second single-mode coupled waveguide is a tapered waveguide with a width gradually increasing from left to right;

[0018] The second front single-mode curved waveguide, the second single-mode coupled waveguide, and the second rear single-mode curved waveguide are connected in sequence; the second front multimode waveguide, the second multimode coupled waveguide, and the second rear multimode waveguide are connected in sequence; the second front single-mode curved waveguide, the second single-mode coupled waveguide, and the second rear single-mode curved waveguide are respectively opposite to the second front multimode waveguide, the second multimode coupled waveguide, and the second rear multimode waveguide in upper and lower directions.

[0019] Preferably, the horizontal projection lengths of the second front single-mode curved waveguide and the second rear single-mode curved waveguide are respectively equal to the lengths of the second front multimode waveguide and the second rear multimode waveguide, and the lengths of the second single-mode coupled waveguide and the second multimode coupled waveguide are equal.

[0020] Preferably, the second multimode coupling waveguide and the second single-mode coupling waveguide are arranged close to each other to form a waveguide coupling region; the close arrangement is specifically: the second front single-mode curved waveguide bends from left to right in a direction close to the second front multimode waveguide, and the second rear single-mode curved waveguide bends from left to right in a direction away from the second rear multimode waveguide.

[0021] Preferably, the left side of the multimode connection waveguide is connected to the right side of the first rear multimode waveguide; the right side of the multimode connection waveguide is connected to the left side of the second front multimode waveguide.

[0022] Preferably, the broadband photon filter based on thin-film lithium niobate optical waveguide can output from the first rear multimode waveguide, enter the high-order TM mode of the second multimode coupling waveguide through the multimode connecting waveguide and the second front multimode waveguide, gradually evolve into the TE fundamental mode in the second single-mode coupling waveguide, and be output from the cross-end of the broadband filter through the second rear single-mode curved waveguide.

[0023] Preferably, the lithium niobate thin film layer is further covered with an upper cladding layer;

[0024] Preferably, the upper cladding layer is air or silicon dioxide.

[0025] According to another aspect of the present invention, there is provided an application of the broadband photon filter based on thin-film lithium niobate optical waveguide in a cascade multi-channel filter.

[0026] In general, the above technical solutions conceived by the present invention have the following technical advantages compared with the existing technology:

[0027] (1) In the broadband filter of the present invention, the first multimode coupling waveguide is a tapered waveguide with gradually increasing width, so the waveguide cross-sections at both ends of the adiabatic filter have different phase matching wavelengths (λ1 and λ2). For the input TE fundamental mode, when its wavelength is within the range of (λ1, λ2), it will meet the phase matching condition at a certain position in the waveguide coupling region and be converted into a high-order TM mode. When the input TE fundamental mode is not within this wavelength range, since it does not meet the phase matching condition in the entire waveguide coupling region, mode conversion will not occur. The broadband mode converter has a structure similar to that of the adiabatic filter, but its structure ensures that all input high-order TM modes can meet the phase matching condition at a certain position in its waveguide coupling region and are then converted into the TE fundamental mode. Its straight-through end has a band-stop spectral response, and the cross-end has a bandpass spectral response.

[0028] (2) The filter proposed in the present invention is based on the principle of adiabatic evolution of modes, and is therefore suitable for broadband optical filtering operations and has an ultra-large free spectral range, which is significantly different from filters based on the principle of beam interference.

[0029] (3) This invention innovatively utilizes an asymmetric directional coupler structure, commonly used in mode division multiplexing, for filtering. Its structure is simple, and the filter bandwidth and center wavelength can be adjusted simply by changing the waveguide width. Compared to traditional grating filters, it offers advantages such as ease of design and processing, and low loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a schematic diagram of the overall structure of the broadband photon filter based on thin-film lithium niobate optical waveguide of the present invention.

[0031] Figure 2 This is a schematic cross-sectional view of the broadband photon filter waveguide based on thin-film lithium niobate optical waveguide of the present invention.

[0032] Figure 3 This is a schematic diagram of the working principle of the broadband photon filter based on thin-film lithium niobate optical waveguide of the present invention.

[0033] Figure 4 This is the spectral response curve of the broadband photon filter based on thin-film lithium niobate optical waveguide of the present invention.

[0034] Figure 5 This is a schematic diagram of the overall structure of the wavelength division multiplexer of the broadband photon filter based on the thin-film lithium niobate optical waveguide of the present invention.

[0035] Figure 6 This is the spectral response curve of the wavelength division multiplexer of the broadband photon filter based on thin-film lithium niobate optical waveguide of the present invention. DETAILED DESCRIPTION

[0036] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0037] like Figure 1 As shown, the present invention provides a broadband photon filter based on a thin-film lithium niobate optical waveguide, comprising: an adiabatic filter 1, a multimode connecting waveguide 2, and a broadband mode converter 3. The adiabatic filter 1, the multimode connecting waveguide 2, and the broadband mode converter 3 are sequentially connected to each other.

[0038] The adiabatic filter 1 comprises a first front single-mode curved waveguide 101, a first single-mode coupling waveguide 102, a first rear single-mode curved waveguide 103, a first front multimode waveguide 104, a first multimode coupling waveguide 105, and a first rear multimode waveguide 106. The first front single-mode curved waveguide 101, the first single-mode coupling waveguide 102, and the first rear single-mode curved waveguide 103 are sequentially connected. The first front multimode waveguide 104, the first multimode coupling waveguide 105, and the first rear multimode waveguide 106 are sequentially connected. The first front single-mode curved waveguide 101, the first single-mode coupling waveguide 102, and the first rear single-mode curved waveguide 103 are vertically opposed to the first front multimode waveguide 104, the first multimode coupling waveguide 105, and the first rear multimode waveguide 106, respectively.

[0039] The horizontal projection lengths of the first front single-mode curved waveguide 101 and the first rear single-mode curved waveguide 103 are respectively equal to the lengths of the first front multimode waveguide 104 and the first rear multimode waveguide 106 . The lengths of the first single-mode coupling waveguide 102 and the first multimode coupling waveguide 105 are equal.

[0040] The first multimode coupling waveguide 105 and the first single-mode coupling waveguide 102 are arranged close to each other, thereby forming a waveguide coupling region. Specifically, the close arrangement includes: the first front single-mode curved waveguide 101 bends from left to right toward the first front multimode waveguide 104, and the first rear single-mode curved waveguide 103 bends from left to right toward the first rear multimode waveguide 106.

[0041] The first multimode coupling waveguide 105 is a tapered waveguide with a gradually increasing width.

[0042] The broadband mode converter comprises a second front single-mode curved waveguide 301, a second single-mode coupling waveguide 302, a second rear single-mode curved waveguide 303, a second front multimode waveguide 304, a second multimode coupling waveguide 305, and a second rear multimode waveguide 306. The second front single-mode curved waveguide 301, the second single-mode coupling waveguide 302, and the second rear single-mode curved waveguide 303 are sequentially connected to one another. The second front multimode waveguide 304, the second multimode coupling waveguide 305, and the second rear multimode waveguide 306 are sequentially connected to one another. The second front single-mode curved waveguide 301, the second single-mode coupling waveguide 302, and the second rear single-mode curved waveguide 303 are vertically opposed to the second front multimode waveguide 304, the second multimode coupling waveguide 305, and the second rear multimode waveguide 306, respectively.

[0043] The effective refractive index of the high-order TM mode on the left side of the second multimode coupling waveguide 305 is greater than the effective refractive index of the TE fundamental mode on the left side of the second single-mode coupling waveguide 302, and the effective refractive index of the high-order TM mode on the right side of the second multimode coupling waveguide 305 is less than the effective refractive index of the TE fundamental mode on the right side of the second single-mode coupling waveguide (302).

[0044] The horizontal projection lengths of the second front single-mode curved waveguide 301 and the second rear single-mode curved waveguide 303 are respectively equal to the lengths of the second front multimode waveguide 304 and the second rear multimode waveguide 306 , and the lengths of the second single-mode coupling waveguide 302 and the second multimode coupling waveguide 305 are equal.

[0045] The second multimode coupling waveguide 305 and the second single-mode coupling waveguide 302 are arranged close to each other to form a waveguide coupling region. Specifically, the second front single-mode curved waveguide 301 bends from left to right toward the second front multimode waveguide 304, and the second rear single-mode curved waveguide 303 bends from left to right toward the second rear multimode waveguide 306.

[0046] The second single-mode coupling waveguide 302 is a tapered waveguide with a gradually increasing width.

[0047] The left side of the mode connection waveguide 2 is connected to the right side of the first rear multimode waveguide 106 , and the right side of the multimode connection waveguide 2 is connected to the left side of the second front multimode waveguide 304 .

[0048] On the left side of the first multimode coupling waveguide 105 and the first single-mode coupling waveguide 102 , the high-order TM mode propagating in the first multimode coupling waveguide 105 is phase-matched with the TE fundamental mode propagating in the first single-mode coupling waveguide 102 at wavelength λ1.

[0049] On the right side of the first multimode coupling waveguide 105 and the first single-mode coupling waveguide 102 , the high-order TM mode propagating in the first multimode coupling waveguide 105 is phase-matched with the TE fundamental mode propagating in the first single-mode coupling waveguide 102 at wavelength λ2.

[0050] The TE fundamental mode enters the first single-mode coupling waveguide 102 from the input end of the broadband filter through the first front single-mode curved waveguide 101. When its wavelength is within the range of (λ1, λ2), it will gradually evolve into a high-order TM mode in the first multimode coupling waveguide 105 and further enter the first rear multimode waveguide 106.

[0051] The TE fundamental mode enters the first single-mode coupling waveguide 102 from the input end of the broadband filter through the first front single-mode curved waveguide 101. When its wavelength is not in the range of (λ1, λ2), it directly passes through the first single-mode coupling waveguide 102 and enters the first rear single-mode curved waveguide 103 and is output from the through-port of the broadband filter.

[0052] The high-order TM mode output from the first rear multimode waveguide 106 enters the second multimode coupling waveguide 305 through the multimode connecting waveguide 2 and the second front multimode waveguide 304, gradually evolves into the TE fundamental mode in the second single-mode coupling waveguide 302, and is output from the cross-end of the broadband filter through the second rear single-mode curved waveguide 303.

[0053] like Figure 2 As shown, the adiabatic filter 1, the multimode connection waveguide 2, and the broadband mode converter 3 are all made on the lithium niobate thin film layer 402, wherein the lithium niobate thin film layer 402 is covered with an upper cladding layer 401, and the lithium niobate thin film layer 402 is bonded to the top of the silicon dioxide buffer layer 403.

[0054] The upper cladding layer 401 is air or silicon dioxide.

[0055] The principle of the present invention is as follows Figure 3 As shown. In the adiabatic filter 1 of the present invention, since the first multimode coupling waveguide 105 is a tapered waveguide with gradually increasing width, the waveguide cross-sections at both ends of the adiabatic filter 1 have different phase matching wavelengths (λ1 and λ2). Therefore, for the input TE fundamental mode, when its wavelength is within the range of (λ1, λ2), it will meet the phase matching conditions at a certain position in the waveguide coupling region and be converted into a high-order TM mode. When the input TE fundamental mode is not within this wavelength range, since it does not meet the phase matching conditions in the entire waveguide coupling region, mode conversion will not occur. The broadband mode converter 3 has a structure similar to that of the adiabatic filter 1, but its structure ensures that all input high-order TM modes can meet the phase matching conditions at a certain position in its waveguide coupling region and are then converted into the TE fundamental mode. Therefore, for the broadband filter of the present invention, its through end has a band-stop spectral response, and the cross end has a bandpass spectral response.

[0056] The specific embodiments of the present invention are as follows:

[0057] Example 1

[0058] The device is based on a lithium niobate-on-insulator (LNIO) platform with a silicon dioxide cladding layer. The LNIO thin film is 500nm thick, and the silicon dioxide buffer layer is 4700nm thick. The device is fabricated using electron beam exposure followed by dry etching on the LNIO thin film to a depth of 260nm. The waveguide structure features a 60° sidewall tilt.

[0059] The adiabatic filter is designed with a central wavelength of 1565nm. The widths of the first multimode coupling waveguide on both sides are selected to be 2.21μm and 2.34μm, respectively. The widths of the first front multimode waveguide and the first rear multimode waveguide are 2.21μm and 2.34μm, respectively. The widths of the first front single-mode curved waveguide, the first single-mode coupling waveguide, and the first rear single-mode curved waveguide are all 0.48μm. The spacing between the first multimode coupling waveguide and the first single-mode coupling waveguide is 0.6μm. The maximum spacing between the first front single-mode curved waveguide and the first front multimode waveguide is 2μm. The maximum spacing between the first rear single-mode curved waveguide and the first rear multimode waveguide is 2μm. The lengths of the three sections of the adiabatic filter are 1300μm, 700μm, and 1300μm, respectively.

[0060] The width of the multimode connecting waveguide is 2.34 μm.

[0061] The broadband mode converter uses the widths of the second single-mode coupling waveguide on both sides to be 0.4μm and 0.6μm, respectively. The widths of the second front single-mode waveguide and the second rear single-mode waveguide are 0.4μm and 0.6μm, respectively. The widths of the second front multimode curved waveguide, the second multimode coupling waveguide, and the second rear multimode curved waveguide are all 2.34μm. The spacing between the second multimode coupling waveguide and the second single-mode coupling waveguide is 0.45μm. The maximum spacing between the second front single-mode curved waveguide and the second front multimode waveguide is 2μm. The maximum spacing between the second rear single-mode curved waveguide and the second rear multimode waveguide is 2μm. The lengths of the three sections of the broadband mode converter are 300μm, 1000μm, and 300μm, respectively.

[0062] The device was simulated and verified using the eigenmode expansion (EME) solver method. Figure 4 The device's spectral response curves at the through-port and cross-port are shown. The results show that the device has a box-shaped flat-top spectral response, a 1-dB bandwidth of approximately 40nm at a central wavelength of 1565nm, an additional loss of less than 0.01dB, and a sideband suppression ratio greater than 20dB, demonstrating its excellent performance.

[0063] Example 2

[0064] like Figure 5 As shown, this embodiment includes a cascaded broadband filter 1 and a broadband filter 2, forming a wavelength division multiplexer (WDM). The input of broadband filter 2 is directly connected to the through-port of broadband filter 1. The center wavelength of broadband filter 1 is 1577 nm, while the center wavelength of broadband filter 2 is 1490 nm. Light of different wavelengths is input from the WDM input port. Light with a center wavelength of 1577 nm is output from cross-port 1, light with a center wavelength of 1490 nm is output from cross-port 2, and the remaining light is output from the through-port.

[0065] A lithium niobate on insulator material platform was selected, and its parameters were the same as those in Example 1.

[0066] For broadband filter 1, the widths of the first multimode coupling waveguide on both sides are selected to be 2.25 μm and 2.35 μm, respectively. The widths of the first front multimode waveguide and the first rear multimode waveguide are selected to be 2.25 μm and 2.35 μm, respectively. The remaining parameters are the same as those in Example 1. The width of the multimode connecting waveguide is 2.35 μm. For the broadband mode converter, the widths of the second front multimode curved waveguide, the second multimode coupling waveguide, and the second rear multimode curved waveguide are all selected to be 2.35 μm. The remaining parameters are the same as those in Example 1.

[0067] For broadband filter 2, the widths of the first multimode coupling waveguide on both sides are selected to be 2.05 μm and 2.15 μm, respectively. The widths of the first front multimode waveguide and the first rear multimode waveguide are selected to be 2.05 μm and 2.15 μm, respectively. The remaining parameters are the same as those in Example 1. The width of the multimode connecting waveguide is 2.15 μm. The widths of the second front multimode curved waveguide, the second multimode coupling waveguide, and the second rear multimode curved waveguide of the broadband mode converter are all selected to be 2.15 μm. The remaining parameters are the same as those in Example 1.

[0068] The device was simulated and verified using the eigenmode expansion (EME) solver method. Figure 6 The device's spectral response curves at the through-port and two crossover ports are shown. The results show that the cascaded device exhibits a box-shaped flat-top spectral response, while crossover port 1 has a 1-dB bandwidth of approximately 30 nm and an excess loss of less than 0.01 dB. Crossover port 2 also has a 1-dB bandwidth of approximately 30 nm and an excess loss of less than 0.01 dB. Crosstalk between the two crossover ports is less than 30 dB, demonstrating the device's excellent performance.

[0069] Regarding the waveguide width of the broadband filter in the present invention: The waveguide structures in the present invention can be broadly categorized as single-mode waveguides and multimode waveguides. Single-mode waveguides transmit the fundamental TE mode, while multimode waveguides transmit higher-order TM modes. To achieve phase matching, the effective propagation constants of the fundamental TE mode and higher-order TM modes must be equal. Therefore, the width of the multimode waveguide should be significantly greater than that of the single-mode waveguide. Furthermore, the design should minimize the waveguide width to reduce the waveguide's confinement of the mode field and improve the efficiency of adiabatic evolution.

[0070] Regarding the lengths of various components in the present invention: Each component of the broadband filter should be of sufficient length. This ensures that the waveguide coupling regions within the adiabatic filter and broadband mode converter can fully complete mode conversion. Furthermore, the length of the single-mode curved waveguides on either side of the adiabatic filter directly influences the device's spectral response at the phase-matching wavelength, determining whether it exhibits a steep spectral response.

[0071] Regarding the waveguide spacing of the broadband filter in the present invention: the maximum waveguide spacing on both sides of the adiabatic filter should be large enough to suppress mode crosstalk and improve the spectral response roll-off and sideband suppression ratio. The spacing of the waveguide coupling regions in the adiabatic filter should be appropriately selected. On the one hand, in order to ensure that the waveguide coupling region can efficiently complete the conversion between the TE fundamental mode and the high-order TM mode, the spacing should not be too large. On the other hand, although further reducing the spacing can improve the mode conversion efficiency, this will result in a longer evolution length on both sides of the adiabatic filter to ensure the spectral response roll-off. Similarly, the maximum waveguide spacing on both sides of the broadband mode converter should also be large enough. Since it does not focus on the spectral response roll-off, the spacing of its waveguide coupling regions should be minimized to improve the mode conversion efficiency.

[0072] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A broadband photon filter based on thin-film lithium niobate optical waveguide, characterized in that: From top to bottom, it comprises a lithium niobate thin film layer (402) and a silicon dioxide buffer layer (403), wherein the lithium niobate thin film layer (402) comprises an adiabatic filter (1), a multimode connection waveguide (2), and a broadband mode converter (3) connected in sequence; The adiabatic filter (1) is an asymmetric directional coupler structure, consisting of a first front single-mode curved waveguide (101), a first single-mode coupled waveguide (102), a first rear single-mode curved waveguide (103), a first front multimode waveguide (104), a first multimode coupled waveguide (105), and a first rear multimode waveguide (106); The first multimode coupling waveguide (105) is a tapered waveguide with a width gradually increasing from left to right; The first front single-mode curved waveguide (101), the first single-mode coupling waveguide (102), and the first rear single-mode curved waveguide (103) are connected in sequence; the first front multi-mode waveguide (104), the first multi-mode coupling waveguide (105), and the first rear multi-mode waveguide (106) are connected in sequence; the first front single-mode curved waveguide (101), the first single-mode coupling waveguide (102), and the first rear single-mode curved waveguide (103) are respectively opposite to the first front multi-mode waveguide (104), the first multi-mode coupling waveguide (105), and the first rear multi-mode waveguide (106) in upper and lower directions; On the left side of the first multimode coupling waveguide (105) and the first single-mode coupling waveguide (102), the effective refractive index of the high-order TM mode transmitted in the first multimode coupling waveguide (105) and the TE fundamental mode transmitted in the first single-mode coupling waveguide (102) at the wavelength λ1 is equal; On the right side of the first multimode coupling waveguide (105) and the first single-mode coupling waveguide (102), the high-order TM mode transmitted in the first multimode coupling waveguide (105) and the TE fundamental mode transmitted in the first single-mode coupling waveguide (102) have the same effective refractive index at wavelength λ2; and the λ2 is greater than λ1.

2. The broadband photon filter based on thin-film lithium niobate optical waveguide according to claim 1, characterized in that: The direction of the first single-mode coupling waveguide (102) is defined as a horizontal direction; the projected lengths of the first front single-mode curved waveguide (101) and the first rear single-mode curved waveguide (103) in the horizontal direction are respectively equal to the lengths of the first front multimode waveguide (104) and the first rear multimode waveguide (106); and the lengths of the first single-mode coupling waveguide (102) and the first multimode coupling waveguide (105) are equal.

3. The broadband photon filter based on thin-film lithium niobate optical waveguide according to claim 1, characterized in that: The first multimode coupling waveguide (105) and the first single-mode coupling waveguide (102) are arranged close to each other, thereby forming a waveguide coupling region; the close arrangement is specifically as follows: the first front single-mode curved waveguide (101) bends from left to right in a direction close to the first front multimode waveguide (104), and the first rear single-mode curved waveguide (103) bends from left to right in a direction away from the first rear multimode waveguide (106).

4. The broadband photon filter based on thin-film lithium niobate optical waveguide according to claim 1, characterized in that: The broadband mode converter (3) is an asymmetric directional coupler structure, consisting of a second front single-mode curved waveguide (301), a second single-mode coupled waveguide (302), a second rear single-mode curved waveguide (303), a second front multimode waveguide (304), a second multimode coupled waveguide (305), and a second rear multimode waveguide (306); The second single-mode coupling waveguide (302) is a tapered waveguide with a width gradually increasing from left to right; The second front single-mode curved waveguide (301), the second single-mode coupling waveguide (302), and the second rear single-mode curved waveguide (303) are connected in sequence; the second front multi-mode waveguide (304), the second multi-mode coupling waveguide (305), and the second rear multi-mode waveguide (306) are connected in sequence; the second front single-mode curved waveguide (301), the second single-mode coupling waveguide (302), and the second rear single-mode curved waveguide (303) are respectively opposite to the second front multi-mode waveguide (304), the second multi-mode coupling waveguide (305), and the second rear multi-mode waveguide (306) in upper and lower directions; The effective refractive index of the high-order TM mode on the left side of the second multimode coupling waveguide (305) is greater than the effective refractive index of the TE fundamental mode on the left side of the second single-mode coupling waveguide (302), and the effective refractive index of the high-order TM mode on the right side of the second multimode coupling waveguide (305) is less than the effective refractive index of the TE fundamental mode on the right side of the second single-mode coupling waveguide (302).

5. The broadband photon filter based on thin-film lithium niobate optical waveguide according to claim 4, characterized in that: The direction of the second single-mode coupling waveguide (302) is defined as a horizontal direction; the projected lengths of the second front single-mode curved waveguide (301) and the second rear single-mode curved waveguide (303) in the horizontal direction are respectively equal to the lengths of the second front multimode waveguide (304) and the second rear multimode waveguide (306); and the lengths of the second single-mode coupling waveguide (302) and the second multimode coupling waveguide (305) are equal.

6. The broadband photon filter based on thin-film lithium niobate optical waveguide according to claim 4, characterized in that: The second multimode coupling waveguide (305) and the second single-mode coupling waveguide (302) are arranged close to each other, thereby forming a waveguide coupling region; the close arrangement is specifically as follows: the second front single-mode curved waveguide (301) bends from left to right in a direction close to the second front multimode waveguide (304), and the second rear single-mode curved waveguide (303) bends from left to right in a direction away from the second rear multimode waveguide (306).

7. The broadband photon filter based on thin-film lithium niobate optical waveguide according to claim 4, characterized in that: The left side of the multimode connection waveguide (2) is connected to the right side of the first rear multimode waveguide (106); and the right side of the multimode connection waveguide (2) is connected to the left side of the second front multimode waveguide (304).

8. The broadband photon filter based on thin-film lithium niobate optical waveguide according to claim 4, characterized in that: The lithium niobate thin film layer (402) is also covered with an upper cladding layer (401).

9. The broadband photon filter based on thin-film lithium niobate optical waveguide according to claim 8, characterized in that: The upper cladding layer (401) is air or silicon dioxide.

10. Use of the broadband photon filter based on thin-film lithium niobate optical waveguide according to any one of claims 1 to 9 in a cascade multi-channel filter.

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