A thin-film lithium niobate filter based on periodic phase-shift grating and the corresponding film thickness adaptive method

Abstract

The present invention belongs to the field of integrated optoelectronics and discloses a thin-film lithium niobate filter based on a periodic phase-shift grating and a corresponding film thickness adaptation method. The lithium niobate thin film layer of the device includes a bus connection waveguide, a mode converter, a transition waveguide, a periodic phase-shift grating reflector, a direct-through waveguide, a direct-through connection waveguide, and a branch waveguide. The periodic phase-shift grating reflector is composed of a rectangular waveguide and rectangular grating tooth protrusions and rectangular grating tooth groove depressions of unequal widths located on both sides. For every distance L between the grating teeth distributed on either side of the rectangular waveguide, the width of the corresponding grating tooth will change, while the widths of the other grating teeth remain unchanged, thereby forming a phase shift. By improving the detailed structure of the filter, the present invention can effectively solve the technical problems of the difficulty in finely controlling the grating center reflection wavelength under the limited resolution of electron beam exposure equipment and the difficulty in actually manufacturing narrow channel spacing wavelength division multiplexers.

Description

Technical Field

[0001] The present invention belongs to the technical field of integrated optoelectronics, and more specifically, relates to a thin-film lithium niobate filter based on a periodic phase-shift grating and a corresponding film thickness adaptive method. Background Art

[0002] Unlike single-wavelength communication in optical fibers, wavelength division multiplexing (WDM) technology uses a combination of light with different wavelengths to transmit information. This technology overcomes the current bottleneck of electronic device processing speeds and significantly increases the transmission capacity of optical fibers. Optical modules in data centers are a common application of WDM. Wavelength channels located near the zero-dispersion wavelength of optical fibers are widely used in optical modules because they help extend optical transmission distances. Channels closer to the zero-dispersion wavelength (meaning smaller center-wavelength spacing between channels) have the potential to enable longer-distance optical transmission. Filters are a core component of WDM systems. Currently, most filters in optical modules use discrete components, which are bulky and difficult to assemble, hindering the future trend of highly integrated optical devices. Meanwhile, thin-film lithium niobate integrated photonic platforms, thanks to the high electro-optical coefficient of lithium niobate material, low waveguide loss, and high integration density, offer a potential solution for high-performance WDM systems. Currently, the design and fabrication of modulators and various passive components, including filters, on thin-film lithium niobate platforms has become a mainstream trend in integrated photonics.

[0003] Among the many filter solutions based on the thin-film lithium niobate platform, filters based on grating structures have attracted much attention due to their advantages of low insertion loss, low crosstalk, and small size compared to other structures such as arrayed waveguide gratings and microring filters [1,2,3]. For existing thin-film lithium niobate filters with Bragg grating structures, they achieve selective filtering of different wavelengths by using gratings with different periods. However, when the required filtering wavelength channel spacing becomes narrower, the grating period difference corresponding to adjacent wavelength channels also becomes smaller, and the requirements for processing equipment are increased. For example, the narrowest wavelength channel spacing currently achieved by thin-film lithium niobate filters based on grating structures is about 4.5nm, and the corresponding grating period difference is 1.5nm [3]. If it is necessary to make nWDM wavelength division multiplexing with an adjacent wavelength channel spacing of about 2.3nm on a 400nm lithium niobate film, the corresponding grating period difference may reach 0.8nm, which is very close to the limit resolution of common electron beam exposure equipment (about 0.5nm). Since the grating period generally needs to be an integer multiple of the limit resolution, this significantly increases the processing difficulty and time cost of the device. In addition, waveguide devices prepared on thin films often have the characteristics of high refractive index difference, and are therefore very susceptible to changes in structural dimensions caused by film thickness, resulting in a large difference between the designed center wavelength of the filter and the actual wavelength, which is not conducive to the practical application of the device.

[0004] References:

[0005] [1] J.He, et.al., High-performance lithium-niobate-on-insulator optical filter based on multimode waveguide gratings, Optics Express, 2022, 30(19): 34140

[0006] [2] J.He, et.al., First Realization of a Three-Channel Lithium-niobate Photonic Filter for 50G Passive Optical networks, ACS Photonics, 2023, 10: 3740-3747

[0007] [3] J.He, et al., Twelve-channel LAN wavelength-division multiplexer on lithium niobate, nanophotonics, 2024, 13(1): 85-93 Summary of the Invention

[0008] In response to the above-mentioned defects or improvement needs of the prior art, the purpose of the present invention is to provide a thin-film lithium niobate filter based on a periodic phase-shift grating and a corresponding film thickness adaptation method. By improving the detailed structure of the filter, it is possible to effectively solve the technical problems of the difficulty in finely controlling the central reflection wavelength of the grating and the difficulty in actually manufacturing a narrow channel spacing wavelength division multiplexer when the resolution of the electron beam exposure equipment is limited.

[0009] To achieve the above objectives, according to one aspect of the present invention, a thin-film lithium niobate filter based on a periodic phase-shift grating is provided. The filter comprises, from top to bottom, a lithium niobate thin film layer and a silicon dioxide buffer layer. The lithium niobate thin film layer comprises a bus connection waveguide, a mode converter, a transition waveguide, a periodic phase-shift grating reflector, a straight-through ferry waveguide, a straight-through connection waveguide, and a branch waveguide. The bus connection waveguide, the mode converter, the transition waveguide, the periodic phase-shift grating reflector, the straight-through ferry waveguide, and the straight-through connection waveguide are connected in sequence, and the branch waveguide and the bus connection waveguide are connected to the same end of the mode converter.

[0010] Let the end of the periodic phase-shift grating reflector close to the transition waveguide be the input end of the periodic phase-shift grating reflector, the other end of the periodic phase-shift grating reflector close to the straight-through transition waveguide be the output end of the periodic phase-shift grating reflector, and the direction in which the input end of the periodic phase-shift grating reflector points to the straight-through transition waveguide is the input transmission direction of the periodic phase-shift grating reflector. Then, the periodic phase-shift grating reflector is composed of a rectangular waveguide arranged along the input transmission direction and rectangular grating tooth protrusions and rectangular grating tooth groove recesses with unequal widths located on both sides of the rectangular waveguide; for any one side, two adjacent grating teeth are separated by a grating tooth groove, and any one grating tooth is spaced from an adjacent one. The total width of the grating grooves is equal to the period Λ0; the grating teeth on one side of the rectangular waveguide are relatively distributed to the grating grooves on the other side, and the width of the grating teeth is less than or equal to the width of the corresponding grating grooves, and the distance between the top of any grating tooth and the bottom of the grating groove on the other side opposite to it is a constant value w0; starting from the input end of the periodic phase-shifted grating reflector, in the input transmission direction of the periodic phase-shifted grating reflector, the grating teeth distributed on any side of the rectangular waveguide are spaced apart by a distance L, and the width of the corresponding grating tooth will change, while the widths of the other grating teeth remain unchanged, thereby forming a phase shift; and the difference between the width of the grating tooth whose width changes and the width of the other grating teeth that do not change is d, where d is the width of the grating tooth whose width changes. Any value in between.

[0011] As a further preference of the present invention, the symmetry line of the rectangular waveguide parallel to the input transmission direction is taken as the center line of the rectangular waveguide. Then, the distance from the top of the grating teeth on both sides of the rectangular waveguide to the center line of the rectangular waveguide changes like a toe-cut function, and the distance from the bottom of the grating tooth groove on both sides to the center line of the rectangular waveguide changes like a toe-cut function.

[0012] As a further preferred embodiment of the present invention, the apodization function is a function with a bell-shaped change, preferably any one of a Gaussian function, a hyperbolic secant function, and a Kaiser function.

[0013] As a further preferred embodiment of the present invention, the periodic phase-shift grating reflector can reversely couple the TE0 mode entering from the input end to the TE1 mode, or can reversely couple the TE1 mode entering from the input end to the TE0 mode, and satisfies the following phase matching conditions:

[0014]

[0015] And L=iΛ0+d

[0016] Among them, n TE0 is the effective refractive index of TE0 mode, n TE1is the effective refractive index of the TE1 mode, λ is the central reflection wavelength of the grating; k is the effective refractive index change rate, which corresponds to the average value of the wavelength derivatives of the effective refractive index of the TE1 mode and the TE0 mode of the rectangular waveguide with a width of w0 at wavelength λ; i is a positive integer.

[0017] As a further preferred embodiment of the present invention, the straight-through waveguide is a tapered waveguide, the output end of which has the same width as the input end of the mode converter, and the input end has the same width as the output end of the periodic phase-shifted grating reflector;

[0018] The transition waveguide is a tapered waveguide, the output end of which has the same width as the input end of the periodic phase-shift grating reflector, and the input end has the same width as the output end of the straight-through transition waveguide;

[0019] The mode converter is used to output the TE0 mode output from the periodic phase-shift grating reflector from the bus connection waveguide, or to convert the TE0 mode input from the branch waveguide into a TE1 mode and input it from the input end of the periodic phase-shift grating reflector.

[0020] As a further preferred embodiment of the present invention, the lithium niobate thin film layer is further covered with an upper cladding layer;

[0021] Preferably, the upper cladding layer is air or silicon dioxide.

[0022] According to another aspect of the present invention, the present invention provides a method for film thickness adaptation of the above-mentioned thin-film lithium niobate filter based on periodic phase-shift grating, characterized in that it comprises the following steps:

[0023] S1. Using a three-dimensional finite-difference time-domain algorithm, determine the change in the central reflection wavelength W of the periodic phase-shifted grating reflector at d = 0 corresponding to a unit thickness change of the lithium niobate thin film layer.

[0024] S2. Fabricate a periodic phase-shift grating reflector with d = 0 as a reference device and measure the thickness D0 of the original lithium niobate thin film layer before etching and the central reflection wavelength λ0 of the periodic phase-shift grating reflector with d = 0;

[0025] S3. Estimate the central reflection wavelength λ′0 of the periodic phase-shifted grating reflector when d=0 corresponding to any film thickness D based on W, D0, and λ0:

[0026] λ′0=λ0+(D-D0)W

[0027] S4. Based on the estimated value λ′0 of the central reflection wavelength of the periodic phase-shifted grating reflector when d=0, solve the following equation to achieve the target reflection wavelength λ under any film thickness D. cThe corresponding device structural parameter L is the distance between two adjacent phase-shifted grating teeth on either side of the rectangular waveguide.

[0028]

[0029] And L=iΛ0+d

[0030] Wherein, Λ0 is the preset grating period; d is the difference between the width of the preset grating tooth with width change and the width of the other unchanged grating teeth; k is the effective refractive index change rate, which corresponds to the average value of the wavelength derivatives of the effective refractive index of the TE1 mode and TE0 mode at the wavelength λ of the rectangular waveguide with width w0; i is a positive integer.

[0031] According to another aspect of the present invention, the present invention provides a wavelength division multiplexing demultiplexer constructed using the above-mentioned thin-film lithium niobate filter based on periodic phase shift grating, characterized in that it includes at least two of the above-mentioned thin-film lithium niobate filters based on periodic phase shift grating connected in sequence, and each thin-film lithium niobate filter based on periodic phase shift grating has a different central reflection wavelength.

[0032] Compared with the prior art, the present invention can achieve the following beneficial effects:

[0033] The present invention introduces a periodic phase-shifted grating reflector. While maintaining a fixed grating period, the distance L between the phase-shifted grating teeth can be adjusted to achieve control of the grating reflector's center wavelength. This overcomes the limitations of traditional grating filter design and manufacturing, which require specific grating periods for different filter center wavelengths and require that the grating period be an integer multiple of the electron beam exposure equipment resolution. Based on this invention, filters with different center wavelengths can be cascaded, allowing the design and manufacture of wavelength division multiplexers with narrower channel spacing than traditional grating filters without increasing equipment precision, while also reducing processing time.

[0034] The present invention can preferably introduce a toe-cutting function into the periodic phase-shifted grating reflector, and combine it with a mode converter to obtain a filter with high process tolerance, large sideband suppression ratio, flat wavelength response and low insertion loss.

[0035] Furthermore, the present invention can address the problem of central wavelength offset of a grating reflector caused by changes in film thickness during actual device manufacturing. By making a device using a reference sheet, the actual central wavelength can be estimated, thereby determining the structural parameters of the grating reflector required to achieve the target central wavelength under a certain film thickness, compensating for the wavelength change caused by the film thickness, and realizing adaptive film thickness compensation of the device. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1Schematic diagram of the overall structure of the thin-film lithium niobate filter based on periodic phase-shift grating.

[0037] Figure 2 Schematic diagram of grating structure with different grating tooth phase shifts.

[0038] Figure 3 Schematic diagram of the cross section of the thin-film lithium niobate filter based on the periodic phase-shift grating of the present invention.

[0039] Figure 4 The figure is a schematic diagram showing the working principle of the thin-film lithium niobate filter based on periodic phase-shift grating in the application of wavelength division multiplexing and demultiplexing device of the present invention. Figure 4 (a) in the figure corresponds to the working principle of wavelength division multiplexer. Figure 4 (b) in the figure corresponds to the working principle of the wavelength division multiplexer.

[0040] Figure 5 The flowchart of the preparation method of thin film lithium niobate filter based on periodic phase-shift grating.

[0041] Figure 6 This is a simulated spectrum diagram of the reference device provided in Example 1 of the present invention.

[0042] Figure 7 This is a simulated spectrum diagram of a four-channel filter provided in Example 1 of the present invention.

[0043] Figure 1 、 Figure 3 , the meanings of the figures are as follows: 101 - bus connection waveguide, 102 - mode converter, 103 - transition waveguide, 104 - periodic phase-shifted grating reflector, 105 - straight-through transition waveguide, 106 - straight-through connection waveguide, 107 - branch waveguide, 201 - upper cladding, 202 - lithium niobate thin film layer, 203 - silicon dioxide buffer layer. DETAILED DESCRIPTION

[0044] 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.

[0045] like Figure 1As shown, the present invention provides a thin-film lithium niobate filter based on a periodic phase-shift grating, comprising: a bus connection waveguide 101, a mode converter 102, a transition waveguide 103, a periodic phase-shift grating reflector 104, a straight-through transfer waveguide 105, a straight-through connection waveguide 106, and a branch waveguide 107, wherein the bus connection waveguide 101, the mode converter 102, the transition waveguide 103, the periodic phase-shift grating reflector 104, the straight-through transfer waveguide 105, and the straight-through connection waveguide 106 are sequentially connected to each other, and the branch waveguide 107 and the bus connection waveguide 101 are connected to the same end of the mode converter 102.

[0046] The side of the periodic phase-shift grating reflector 104 close to the transition waveguide is the input end of the periodic phase-shift grating reflector 104 ; the direction in which the input end of the periodic phase-shift grating reflector 104 points to the straight-through transition waveguide 105 is the input transmission direction of the periodic phase-shift grating reflector 104 .

[0047] The straight-through waveguide 105 is a tapered waveguide, and the width of its output end is the same as the width of the input end of the mode converter 102 , and the width of its input end is the same as the width of the output end of the periodic phase-shift grating reflector 104 .

[0048] The transition waveguide 103 is a tapered waveguide, and the width of its output end is the same as the width of the input end of the periodic phase-shift grating reflector 104 , and the width of its input end is the same as the width of the output end of the mode converter 102 .

[0049] The mode converter 102 outputs the TE0 mode output from the periodic phase-shift grating reflector 104 from the bus connection waveguide 101. The TE0 mode input from the branch waveguide 107 is converted into a TE1 mode and input from the output end of the periodic phase-shift grating reflector 104. It can be composed of a bent directional coupler, a symmetric directional coupler, an asymmetric directional coupler, etc.

[0050] The periodic phase-shifted grating reflector 104 is composed of a rectangular waveguide arranged along the input transmission direction and a series of protruding rectangular grating teeth and recessed rectangular grating slots with unequal widths distributed on both sides of the rectangular waveguide. The grating teeth and grating slots are spaced apart at a period of Λ0. The grating teeth on one side of the rectangular waveguide are arranged relative to the grating slots on the other side, wherein the distance between the grating teeth on one side and the bottom of the grating slots on the other side is a constant value w0, and the distances from the tops of the grating teeth and the bottoms of the grating slots on both sides to the centerline of the rectangular waveguide vary according to a toe-cut function. Starting from the input end of the periodic phase-shifted grating reflector, in the input transmission direction of the periodic phase-shifted grating reflector, the grating teeth distributed on both sides of the rectangular waveguide will undergo a phase shift of a width d for every distance L, where d is the distance between the grating teeth on one side and the bottom of the grating slots on the other side. Any value between; L satisfies the constraint condition: L=iΛ0+d, where i is a positive integer (such as: 1, 2, 3...).

[0051] The apodization function may be a function with a bell-shaped change, such as a Gaussian, a hyperbolic secant, or a Kaiser function.

[0052] like Figure 2 As shown in FIG, when the grating tooth phase shift is d<0, the grating teeth become narrower; when d=0, the grating tooth width remains unchanged; when d>0, the grating teeth become wider.

[0053] The periodic phase-shifted grating reflector 104 reversely couples the TE0 mode to the TE1 mode and satisfies the following phase matching conditions:

[0054]

[0055] Among them, n TE0 is the effective refractive index of TE0 mode, n TE1 is the effective refractive index of the TE1 mode, λ is the central reflection wavelength of the grating, and k is the average dispersion slope of the TE1 mode and TE0 mode near λ in a waveguide with a width of w0.

[0056] In a specific implementation, the grating teeth phase shift d of the periodic phase-shifted grating reflector 104 can be kept unchanged, and an integer grating period Λ0 that is easy to process can be used to adjust the distance L between the grating teeth with phase shift to achieve the control of the central wavelength of the grating reflector.

[0057] like Figure 3 As shown, the bus connection waveguide 101, mode converter 102, transition waveguide 103, periodic phase-shifted grating reflector 104, straight-through transition waveguide 105, straight-through connection waveguide 106, and branch waveguide 107 are all made on a lithium niobate thin film layer 202, wherein the lithium niobate thin film layer is covered with an upper cladding layer 201, and the lithium niobate thin film layer is bonded to the top of the silicon dioxide buffer layer 203.

[0058] The upper cladding layer is composed of air or silicon dioxide.

[0059] like Figure 4 As shown, the method for preparing the thin-film lithium niobate filter based on the periodic phase-shift grating comprises the following steps:

[0060] S01. Determine, by computer simulation using a three-dimensional finite-difference time-domain algorithm, a change in the central reflection wavelength W of a non-phase-shifted grating reflector (i.e., a periodic phase-shifted grating reflector when d=0) corresponding to a unit film thickness change;

[0061] S02, making a phase-shift-free grating reflector as a reference device, and measuring the film thickness D0 before making the device, and the central reflection wavelength λ0 of the phase-shift-free grating reflector;

[0062] S03, estimate the central reflection wavelength λ′0 of the phase-shift-free grating corresponding to any film thickness D based on W, D0, and λ0;

[0063] S04. Based on the estimated value λ′0 of the central reflection wavelength of the phase-shift-free grating, solve the equation to achieve the target reflection wavelength λ under any film thickness D. c The corresponding device structure parameters.

[0064] The non-phase-shift grating reflector is a periodic phase-shift grating reflector with a grating tooth phase shift d=0.

[0065] The estimated value of the central reflection wavelength λ′0 of the phase-shift-free grating is:

[0066] λ′0=λ0+(D-D0)W

[0067] The device structural parameter is that the distance between two adjacent grating teeth with phase shift is L.

[0068] The equation for solving the device structural parameters is:

[0069]

[0070] And L=iΛ0+d

[0071] In the specific implementation, the grating tooth width change d, the grating period Λ0, and the effective refractive index change rate k are determined to be constant values, and the target reflection wavelength λ is achieved under any film thickness D. c The distance L between two adjacent phase-shifted grating teeth on the same side of the corresponding rectangular waveguide is used to align the center wavelength of the actually manufactured reflection grating with the designed center wavelength.

[0072] In addition, the present invention also provides a wavelength division multiplexing demultiplexer, comprising: at least two sequentially connected thin-film lithium niobate filters based on periodic phase shift gratings, each of the thin-film lithium niobate filters based on periodic phase shift gratings having a different central reflection wavelength.

[0073] Working Principle of WDM Demultiplexer Figure 4 As shown, Figure 4 (a) is a schematic diagram of the filter in the wavelength division multiplexing scenario. Figure 4 (b) is a schematic diagram of the filter in the wavelength division multiplexing scenario.

[0074] In the wavelength division multiplexing scenario, such as Figure 4As shown in (a) of FIG. 1 , light of a specific wavelength is input from branch waveguide 107 in TE0 mode and converted to TE1 mode by mode converter 102. Light of the specific wavelength in TE1 mode is coupled to the TE0 mode in the reverse direction by periodic phase-shift grating reflector 104 and ultimately output from bus connection waveguide 101. In practical applications, multiple filters can be cascaded, each of which outputs light of different wavelengths from the same waveguide through wavelength-selective reflection, thus achieving wavelength division multiplexing.

[0075] In the wavelength division multiplexing scenario, such as Figure 4 As shown in (b) of FIG, light of different wavelengths is input from bus connection waveguide 101 in TE0 mode. In periodic phase-shifted grating reflector 104, light of a specific wavelength within the input TE0 mode is coupled into the TE1 mode, which propagates in the opposite direction. This light is converted to TE0 mode by mode converter 102 and then output from branch waveguide 107. In practical applications, multiple filters can be cascaded to selectively reflect light of different wavelengths, outputting them from separate waveguides to achieve wavelength division multiplexing.

[0076] The specific embodiments of the present invention are as follows:

[0077] Example 1

[0078] The device is based on a lithium niobate-on-insulator (LNIO) platform with an air cladding layer. The LNIO thin film is 400nm thick, and the SiO2 buffer layer is 2700nm thick. The device is fabricated using electron beam exposure followed by dry etching on the LNIO thin film to a depth of 200nm. The waveguide structure features a 60° sidewall tilt.

[0079] In this embodiment, a four-channel nWDM wavelength division multiplexer with a wavelength channel spacing of approximately 2.3 nm is constructed using thin-film lithium niobate filters based on periodic phase-shift gratings. Each of these thin-film lithium niobate filters is cascaded sequentially, with the center reflection wavelengths set to λ1 = 1304.5 nm, λ2 = 1306.9 nm, λ3 = 1309.1 nm, and λ4 = 1311.4 nm. If the center wavelength is controlled by directly adjusting the grating period, the corresponding grating periods are Λ1 = 348.9 nm, Λ2 = 349.7 nm, Λ3 = 350.3 nm, and Λ4 = 350.9 nm, respectively. The period difference between each grating is less than 1 nm. However, the grating period generally needs to be an integer multiple of the limiting resolution of the electron beam lithography equipment, which significantly increases the difficulty and time cost of device manufacturing. The mode converter is composed of an asymmetric directional coupler. The waveguide width of the periodic phase-shifted grating reflector is w0 = 2000 nm, and the teeth and grooves are staggered with a period of Λ0 = 350 nm. The grating tooth width is 100 nm, the grating groove width is 250 nm, and the grating period is 1400. The apodization function is a Gaussian function. The grating tooth width variation is a preset constant value d = -100 nm (that is, the width of the grating tooth that changes becomes 0 nm; the width of the other grating teeth that do not change remains 100 nm).

[0080] Next, a three-dimensional time-domain finite-difference algorithm simulation is performed on a computer to simulate the device film thickness adaptation process when the film thickness changes.

[0081] Through computer simulation of the three-dimensional finite-difference time-domain algorithm, it was found that the average value of the wavelength derivative of the effective refractive index of the TE1 mode and TE0 mode of the lithium niobate waveguide with a width of w0 = 2000nm at a wavelength of 1310nm is approximately k = -0.00059 / nm.

[0082] The three-dimensional time-domain finite-difference algorithm was used to simulate the phase-shift-free grating reflector made on a lithium niobate film with a thickness of D0 = 400 nm by computer to simulate the actual production of the reference device, and the central reflection wavelength of the grating reflector was obtained to be λ0 = 1308.1 nm. Figure 6 The simulation results of the grating reflector show that the device has a 1dB bandwidth of about 2nm, an insertion loss of 0.12dB, and a side mode suppression ratio of 22dB, proving that the device has the potential to be used in wavelength division multiplexing systems.

[0083] A three-dimensional finite-difference time-domain (FDTD) simulation revealed that the center wavelength shift of a phase-shift-free grating reflector per unit thickness change is W = 0.56. Therefore, for a phase-shift-free grating reflector fabricated on a lithium niobate film of any thickness, the estimated center reflection wavelength is λ′0 = λ0 + (D - D0)W = 1084.1 + 0.56D.

[0084] Taking a lithium niobate film with a thickness of D = 409 nm as an example, the corresponding central reflection wavelength of the phase-shift-free grating reflector is λ′0 = 1313.24 nm. To fabricate a filter on a lithium niobate film with a thickness of D = 409 nm, in order to achieve the ideal central wavelength of the reflective grating in the filter, the device structural parameters of the grating reflectors in the four filter units need to be calculated from the equation: L1 = 12350 nm, L2 = 17250 nm, L3 = 24250 nm, and L4 = 48400 nm. This indicates that the differences in the structural parameters L of each filter unit are on the order of microns, far exceeding the exposure resolution of the electron beam equipment. This achieves adaptive compensation for film thickness while reducing processing difficulty.

[0085] Furthermore, we simulated the four-channel filter made on the lithium niobate film with a thickness of D = 409nm using a three-dimensional finite-difference time-domain algorithm. Figure 7 As shown in the figure, it can be seen that the central reflection wavelengths of the four channels are λ′1=1304.3nm, λ′2=1306.9nm, λ′3=1308.7nm, and λ′4=1311.0nm, which are basically consistent with the ideal central wavelength. The 1dB bandwidth of each channel is greater than 1.6nm, the insertion loss is less than 0.5dB, and the crosstalk of different channels at the central reflection wavelength is less than <-20dB.

[0086] In this embodiment, a grating reflector with a period of 350nm that is easy to process is used, and a periodic phase shift is applied to the grating teeth to adjust the central reflection wavelength of the grating reflector. Compared with the method of directly adjusting the grating period, this method greatly reduces the requirements for exposure equipment in device manufacturing, improves the process tolerance of the device, and reduces the exposure time required to manufacture the device. Furthermore, the embodiment predicts the central reflection wavelength of the actual device by making a reference device, and compensates for the film thickness by adjusting the structure. The results show that the channel center wavelength of the manufactured four-channel narrowband filter is basically consistent with the design value, and has the characteristics of large sideband suppression ratio and flat wavelength response.

[0087] 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 thin film lithium niobate filter based on a periodic phase shift grating, characterized in that: From top to bottom, the lithium niobate film layer (202) and the silicon dioxide buffer layer (203) are included, wherein the lithium niobate film layer (202) includes a bus connection waveguide (101), a mode converter (102), a transition waveguide (103), a periodic phase-shifted grating reflector (104), a straight-through ferry waveguide (105), a straight-through connection waveguide (106), and a branch waveguide (107), wherein the bus connection waveguide (101), the mode converter (102), the transition waveguide (103), the periodic phase-shifted grating reflector (104), the straight-through ferry waveguide (105), and the straight-through connection waveguide (106) are connected in sequence, and the branch waveguide (107) and the bus connection waveguide (101) are connected to the same end of the mode converter (102); The end of the periodic phase-shift grating reflector (104) close to the transition waveguide (103) is the input end of the periodic phase-shift grating reflector (104), the other end of the periodic phase-shift grating reflector (104) close to the straight-through transition waveguide (105) is the output end of the periodic phase-shift grating reflector (104), and the direction in which the input end of the periodic phase-shift grating reflector (104) points to the straight-through transition waveguide (105) is the input transmission direction of the periodic phase-shift grating reflector (104). Then, the periodic phase-shift grating reflector (104) is composed of a rectangular waveguide arranged along the input transmission direction and rectangular grating tooth protrusions and rectangular grating tooth groove recesses with unequal widths located on both sides of the rectangular waveguide; for any side, an optical gap is formed between two adjacent grating teeth. The grating tooth grooves are spaced such that the total width of any grating tooth and an adjacent grating tooth groove is equal to the period Λ0; the grating teeth on one side of the rectangular waveguide are relatively distributed with the grating tooth grooves on the other side, and the width of the grating teeth is less than or equal to the width of the corresponding grating tooth grooves, and the distance between the top of any grating tooth and the bottom of the grating tooth groove on the other side opposite to it is a constant value w0; starting from the input end of the periodic phase-shift grating reflector (104), in the input transmission direction of the periodic phase-shift grating reflector (104), the grating teeth distributed on any side of the rectangular waveguide are spaced apart by a distance L, and the width of the corresponding grating tooth will change, while the widths of the other grating teeth remain unchanged, thereby forming a phase shift; and the difference between the width of the grating tooth with a changed width and the width of the other grating teeth that have not changed is d, wherein d is the difference between the width of the grating tooth with a changed width and the width of the other grating teeth that have not changed. Any value in between.

2. The thin-film lithium niobate filter based on a periodic phase-shift grating according to claim 1, wherein: Let the symmetry line of the rectangular waveguide parallel to the input transmission direction be the centerline of the rectangular waveguide. Then, the distance from the top of the grating teeth on both sides of the rectangular waveguide to the centerline of the rectangular waveguide changes like a toe-cutting function, and the distance from the bottom of the grating teeth on both sides to the centerline of the rectangular waveguide changes like a toe-cutting function.

3. The thin-film lithium niobate filter based on a periodic phase-shift grating according to claim 2, wherein: The apodization function is a function having a bell-shaped variation.

4. The thin-film lithium niobate filter based on a periodic phase-shift grating according to claim 3, wherein: The apodization function is any one of a Gaussian function, a hyperbolic secant function, and a Kaiser function.

5. The thin-film lithium niobate filter based on a periodic phase-shift grating according to claim 1, wherein: The periodic phase-shift grating reflector (104) can reversely couple the TE0 mode entering from the input end to the TE1 mode, or reversely couple the TE1 mode entering from the input end to the TE0 mode, and satisfies the following phase matching conditions: And L=iΛ0+d Among them, n TE0 is the effective refractive index of TE0 mode, n TE1 is the effective refractive index of the TE1 mode, λ is the central reflection wavelength of the grating; k is the effective refractive index change rate, which corresponds to the average value of the wavelength derivatives of the effective refractive index of the TE1 mode and the TE0 mode of the rectangular waveguide with a width of w0 at wavelength λ; i is a positive integer.

6. The thin-film lithium niobate filter based on a periodic phase-shift grating according to claim 1, wherein: The straight-through waveguide (105) is a tapered waveguide, the width of its output end is the same as the width of the input end of the mode converter (102), and the width of its input end is the same as the width of the output end of the periodic phase-shift grating reflector (104); The transition waveguide (103) is a tapered waveguide, the width of its output end is the same as the width of the input end of the periodic phase-shift grating reflector (104), and the width of its input end is the same as the width of the output end of the straight-through transition waveguide (105); The mode converter (102) is used to output the TE0 mode output from the periodic phase-shift grating reflector (104) from the bus connection waveguide (101), or to convert the TE0 mode input from the branch waveguide (107) into the TE1 mode and input it from the input end of the periodic phase-shift grating reflector (104).

7. The thin-film lithium niobate filter based on a periodic phase-shift grating according to claim 1, wherein: The lithium niobate thin film layer (202) is also covered with an upper cladding layer (201).

8. The thin-film lithium niobate filter based on a periodic phase-shift grating according to claim 7, wherein: The upper cladding layer (201) is air or silicon dioxide.

9. The film thickness adaptive method of a thin film lithium niobate filter based on a periodic phase shift grating according to any one of claims 1 to 8, characterized in that: The following steps are involved: S1. Using a three-dimensional finite-difference time-domain algorithm, determine the change in the central reflection wavelength W of the periodic phase-shifted grating reflector at d = 0 corresponding to a unit thickness change of the lithium niobate thin film layer. S2. Fabricate a periodic phase-shift grating reflector with d = 0 as a reference device and measure the thickness D0 of the original lithium niobate thin film layer before etching and the central reflection wavelength λ0 of the periodic phase-shift grating reflector with d = 0; S3. Estimate the central reflection wavelength λ′0 of the periodic phase-shifted grating reflector when d=0 corresponding to any film thickness D based on W, D0, and λ0: λ′0=λ0+(D-D0)W S4. Based on the estimated value λ′0 of the central reflection wavelength of the periodic phase-shifted grating reflector when d=0, solve the following equation to achieve the target reflection wavelength λ under any film thickness D. c The corresponding device structural parameter L is the distance between two adjacent phase-shifted grating teeth on either side of the rectangular waveguide. And L=iΛ0+d Wherein, Λ0 is the preset grating period; d is the difference between the width of the preset grating tooth with width change and the width of the other unchanged grating teeth; k is the effective refractive index change rate, which corresponds to the average value of the wavelength derivatives of the effective refractive index of the TE1 mode and TE0 mode at the wavelength λ of the rectangular waveguide with width w0; i is a positive integer.

10. A wavelength division multiplexing / demultiplexing device constructed using the thin film lithium niobate filter based on a periodic phase shift grating as claimed in any one of claims 1 to 8, characterized in that: The invention comprises at least two sequentially connected thin-film lithium niobate filters based on periodic phase-shift gratings as claimed in any one of claims 1 to 8, each of which has a different central reflection wavelength.

Images