Broadband photonic filter based on thin-film lithium niobate optical waveguide

Abstract

The invention discloses a broadband photonic filter based on a thin-film lithium niobate optical waveguide, and belongs to the technical field of integrated photoelectronics. The broadband photonic filter comprises a heat insulation filter, a multi-mode connection waveguide and a broadband mode converter, wherein the heat insulation filter and the broadband mode converter are both of an asymmetric directional coupler structure. The heat insulation filter selectively converts the input TE fundamental mode wavelength into a high-order TM mode, and the other TE fundamental modes are output from the straight-through end. And the high-order TM mode enters the broadband mode converter through the multi-mode connection waveguide, is completely converted into a TE fundamental mode and is output from the cross end. According to the invention, an asymmetric directional coupler structure commonly used in mode division multiplexing is creatively used as an optical filtering function, and the structure is simple. Based on a mode adiabatic evolution principle, the filter is suitable for being applied to broadband optical filtering operation, and has the advantages of being easy to design and process, low in loss and the like.

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 thin-film lithium niobate optical waveguide. Background Art

[0002] In recent years, thin-film lithium niobate integrated photonic platforms have attracted much attention due to their high electro-optic coefficient, low waveguide loss and high integration, providing a usable solution for high-performance modulators. Compared with bulk lithium niobate, the high waveguide refractive index difference of this platform significantly enhances the light field confinement, which plays an important role in improving system integration and reducing energy consumption. With the rapid development of big data and artificial intelligence technology, the demand for high-speed information transmission in society has increased exponentially. The compact design of thin-film lithium niobate integrated optical devices enables them to realize complex functions in a limited space and reduce production costs. Therefore, they are particularly critical in meeting the growing requirements of modern communication networks for data processing speed and efficiency. Based on this, designing and manufacturing various functional devices on the thin-film lithium niobate platform has become one of the important directions in current 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 specified port. Narrowband multi-channel optical filters are commonly used 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 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 its wavelength passes 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] At present, a variety of integrated optical filtering structures have been proposed on the thin-film lithium niobate platform. 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 characteristics of beam interference and usually have narrow filtering bandwidths or limited free spectral ranges. The apodized multimode Bragg grating is also a common interference filtering structure. Although it can achieve a flat-top spectral response with a certain bandwidth, its upper limit of optical bandwidth is limited by the coupling strength between the forward and reverse transmission modes. On the other hand, due to its multimode waveguide structure, the incident light will be unnecessarily coupled with the waveguide high-order mode at a short wavelength, limiting its free spectral range. It can be seen that it is difficult to realize on-chip optical filters with wide optical bandwidth and ultra-large free spectral range based on the filter principle of beam interference. In order to achieve this goal, it is urgent to propose new filtering structures based on the underlying principles. Summary of the invention

[0005] In view of the shortcomings of the prior art, the present invention proposes a broadband photon filter based on a thin-film lithium niobate optical waveguide based on the principle of adiabatic evolution of waveguide modes and using an asymmetric directional coupler structure. 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 multi-mode connection waveguide, and is completely converted into the TE fundamental mode and output from the crossover end. The broadband filter of the present invention is applied to broadband optical filtering operations and has an ultra-large free spectral range.

[0006] According to a first aspect of the present invention, there is provided a broadband photon filter based on a thin-film lithium niobate optical waveguide, 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 bent waveguide, a first single-mode coupled waveguide, a first rear single-mode bent waveguide, a first front multi-mode waveguide, a first multi-mode coupled waveguide, and a first rear multi-mode 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 effective refractive index of 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 at the wavelength λ1 is equal;

[0011] On the right side of the first multimode coupling waveguide and the first single-mode coupling waveguide, the effective refractive index of 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 at wavelength λ2 is equal; and λ2 is greater than λ1;

[0012] The broadband photon filter based on thin-film lithium niobate optical waveguide can make 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 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 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 through end of the broadband filter.

[0014] Preferably, the projected lengths of the first front single-mode curved waveguide and the first rear single-mode curved waveguide in the horizontal direction 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 coupling waveguide and the first multimode coupling 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 specifically includes: 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 coupling waveguide is a tapered waveguide whose width gradually increases 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 multi-mode waveguide, the second multi-mode coupled waveguide, and the second rear multi-mode 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 multi-mode waveguide, the second multi-mode coupled waveguide, and the second rear multi-mode waveguide in upper and lower directions.

[0019] Preferably, the projected lengths of the second front single-mode curved waveguide and the second rear single-mode curved waveguide in the horizontal direction 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 coupling waveguide and the second multimode coupling 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 specifically includes: 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 curved 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 enable the high-order TM mode output from the first rear multimode waveguide to enter 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 bent waveguide.

[0023] Preferably, the lithium niobate thin film layer is also 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 solution conceived by the present invention has the following technical advantages compared with the prior art:

[0027] (1) In the broadband filter of the present invention, the first multimode coupling waveguide is a tapered waveguide with a 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 then be 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) The present invention innovatively uses the asymmetric directional coupler structure commonly used in mode division multiplexing as a filtering function. Its structure is simple, and the bandwidth and center wavelength of the filter can be adjusted by simply changing the waveguide width. Compared with traditional grating filters, it has the advantages of easy design and processing, low loss, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 The figure 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 a broadband photon filter waveguide based on a thin-film lithium niobate optical waveguide according to the present invention.

[0032] Figure 3 The figure 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 The figure 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 The wavelength division multiplexer spectral response curve 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 purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain 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 can 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 thin-film lithium niobate optical waveguide, comprising: an adiabatic filter 1, a multimode connection waveguide 2, and a broadband mode converter 3. The adiabatic filter 1, the multimode connection waveguide 2, and the broadband mode converter 3 are sequentially connected to each other.

[0038] The adiabatic filter 1 is composed of 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 connected in sequence. The first front multimode waveguide 104, the first multimode coupling waveguide 105, and the first rear multimode 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 multimode waveguide 104, the first multimode coupling waveguide 105, and the first rear multimode waveguide 106 in upper and lower directions.

[0039] 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.

[0040] The first multimode coupling waveguide 105 and the first single-mode coupling waveguide 102 are arranged close to each other to form a waveguide coupling region; the close arrangement specifically includes: the first front single-mode curved waveguide 101 is bent from left to right in a direction close to the first front multimode waveguide 104, and the first rear single-mode curved waveguide 103 is bent from left to right in a direction away from 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 is composed of 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 each other. The second front multimode waveguide 304, the second multimode coupling waveguide 305, and the second rear multimode waveguide 306 are sequentially connected to each other. 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 multimode waveguide 304, the second multimode coupling waveguide 305, and the second rear multimode waveguide 306 in upper and lower directions.

[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 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.

[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; the close arrangement is specifically as follows: the second front single-mode curved waveguide 301 is bent from left to right in a direction close to the second front multimode waveguide 304, and the second rear single-mode curved waveguide 303 is bent from left to right in a direction away from 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 multimode connection waveguide 2 is connected to the right side of the first rear multimode curved 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 a 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 a 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 end 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 a 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 a 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 in 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 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 condition at a certain position in its waveguide coupling region, and then be 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 lithium niobate material platform on insulator is selected, with silicon dioxide as the upper cladding, the lithium niobate film layer thickness is 500nm, and the silicon dioxide buffer layer thickness is 4700nm. The device is formed by electron beam exposure and then dry etching on the lithium niobate film layer, with an etching depth of 260nm, and the side wall of the waveguide structure has a 60° inclination angle.

[0059] The design center wavelength of the adiabatic filter is 1565nm, and the widths of the first multimode coupling waveguide on both sides are selected to be 2.21μm and 2.34μm, respectively, and 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 selects the widths of the second single-mode coupling waveguide on both sides to be 0.4μm and 0.6μm, respectively, and the widths of the second front single-mode waveguide and the second rear single-mode waveguide to be 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 spectral response curves of the device at the through end and the cross end are shown. The results show that the device has a box-shaped flat-top spectral response, and has a 1-dB bandwidth of about 40nm at the central wavelength of 1565nm, an additional loss of less than 0.01dB, and a sideband suppression ratio of more than 20dB, which proves the good performance of the device.

[0063] Example 2

[0064] like Figure 5 As shown, this embodiment includes a broadband filter 1 and a broadband filter 2 which are cascaded to each other to form a wavelength division multiplexer. Among them, the input end of the broadband filter 2 is directly connected to the straight-through end of the broadband filter 1, the central wavelength of the broadband filter 1 is 1577nm, and the central wavelength of the broadband filter 2 is 1490nm. Lights of different wavelengths are input from the input end of the wavelength division multiplexer, the light with a central wavelength of 1577nm is output from the cross end 1, the light with a central wavelength of 1490nm is output from the cross end 2, and the rest of the light is output from the straight-through end.

[0065] A lithium niobate material platform on an insulator is selected, and its parameters are 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, and the other parameters are the same as those in Example 1. The width of the multimode connection waveguide is 2.35 μm. The broadband mode converter selects the widths of the second front multimode bending waveguide, the second multimode coupling waveguide, and the second rear multimode bending waveguide to be 2.35 μm, and the other 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, and the other parameters are the same as those in Example 1. The width of the multimode connection waveguide is 2.15 μm. The broadband mode converter selects the widths of the second front multimode bending waveguide, the second multimode coupling waveguide, and the second rear multimode bending waveguide to be 2.15 μm, and the other 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 spectral response curves of the device at the through end and the two cross ends are shown. The results show that the cascaded device has a box-shaped flat-top spectral response, while the cross end 1 has a 1-dB bandwidth of about 30nm and an additional loss of less than 0.01dB, and the cross end 2 has a 1-dB bandwidth of about 30nm and an additional loss of less than 0.01dB. The crosstalk between the two cross ends is less than 30dB, which proves the good performance of the device.

[0069] Regarding the waveguide width of the broadband filter in the present invention: The waveguide structure in the structure of the present invention can be roughly summarized into two types: single-mode waveguide and multi-mode waveguide. Among them, the single-mode waveguide transmits the TE fundamental mode, and the multi-mode waveguide transmits the high-order TM mode. In order to achieve the phase matching condition, the effective propagation constants of the TE fundamental mode and the high-order TM mode are equal. Therefore, the width of the multi-mode waveguide should be much larger than the width of the single-mode waveguide. In addition, the design should also select the smallest possible waveguide width to reduce the waveguide's constraint on the mode field and improve the adiabatic evolution efficiency.

[0070] Regarding the length of each part in the present invention: each part of the broadband filter should have a sufficient length. On the one hand, it can ensure that the waveguide coupling region in the adiabatic filter and the broadband mode converter can fully complete the mode conversion. On the other hand, the length of the single-mode curved waveguide on both sides of the adiabatic filter directly affects the spectral response characteristics of the device at the phase matching wavelength, and determines whether it can show 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 region 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 of 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 pay attention to 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 only 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 protection scope 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 includes a lithium niobate thin film layer (402) and a silicon dioxide buffer layer (403), wherein the lithium niobate thin film layer (402) includes 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, and is composed of a first front single-mode bent waveguide (101), a first single-mode coupled waveguide (102), a first rear single-mode bent waveguide (103), a first front multi-mode waveguide (104), a first multi-mode coupled waveguide (105), and a first rear multi-mode waveguide (106); The first multimode coupling waveguide (105) is a tapered waveguide whose width gradually increases 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 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 a wavelength λ2 is equal; 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 specifically comprises: the first front single-mode curved waveguide (101) is bent from left to right in a direction close to the first front multimode waveguide (104), and the first rear single-mode curved waveguide (103) is bent 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, and is composed 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 whose width gradually increases from left to right; The second front single-mode curved waveguide (301), the second single-mode coupled 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 coupled 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 coupled 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 coupled 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 specifically comprises: the second front single-mode curved waveguide (301) is bent from left to right in a direction close to the second front multimode waveguide (304), and the second rear single-mode curved waveguide (303) is bent 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 bending 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. Application of the broadband photon filter based on thin-film lithium niobate optical waveguide as claimed in any one of claims 1 to 9 in a cascade multi-channel filter.

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