An optical filter based on bound state thin film lithium niobate in quasi-continuum

Abstract

The application discloses a kind of bound state thin film lithium niobate optical filter based on quasi continuum body.The optical filter is sequentially stacked from top to bottom by tunable filter layer, thin film lithium niobate flat plate, dielectric buried oxygen layer and substrate layer, heating electrode and reflective grating are distributed on both sides of waveguide, periodically arranged reflective grating is arranged between heating electrode and waveguide, the width of waveguide is set to realize bound state in continuum, reduce the lateral leakage loss caused by mode leakage, reflective grating is used to reflect the mode leaked into lithium niobate flat plate back to waveguide to further realize bound state in quasi continuum, heating electrode is used to change the effective refractive index of reflective grating.The application is easy to process, has large tolerance, low loss, and can effectively realize low-loss, narrow-bandwidth photonic filtering, by changing the voltage loaded on the heating electrode to control the center wavelength of the filter without changing the physical structure, filters with different reflection center wavelengths can be realized.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optoelectronic devices, and particularly relates to a lithium niobate optical filter based on a bound state in a quasi-continuum. BACKGROUND

[0002] With the development of society, people's demand for network communication capacity and bandwidth is increasing, and photonic integration technology can integrate optical module devices such as active and passive devices in previous communication equipment on a single optical chip, and has the advantages of high integration, low cost, high capacity and large bandwidth, and plays an important role in the development of the next generation of high-speed optical networks.

[0003] At present, various super-compact silicon photonic integrated devices have been successfully developed for optical interconnection and optical sensing applications, and among them, silicon optical filters have been widely used as key devices in wavelength division multiplexing systems and optical spectrum sensing. The traditional grating-based filter has problems such as difficulty in achieving ultra-narrow bandwidth, large transmission loss over long distances, low electrode heating efficiency, and difficulty in achieving large-scale precise and rapid wavelength tuning. SUMMARY

[0004] In order to solve the above key problems, the application provides a lithium niobate optical filter based on a bound state in a quasi-continuum, which effectively suppresses the long-distance transmission loss of the mode in the waveguide, and realizes an ultra-low loss, ultra-narrow bandwidth tunable photonic filter. At the same time, the drift of the reflection center wavelength is controlled by near lateral heating, and the wavelength of the filter is continuously adjustable.

[0005] The technical scheme adopted by the application is as follows:

[0006] The optical filter is mainly composed of a tunable filter layer, a thin film lithium niobate flat plate, a dielectric buried oxygen layer and a substrate layer stacked from top to bottom, the tunable filter layer includes a waveguide, two rows of grating arrays and two heating electrodes, the two heating electrodes are symmetrically distributed on both sides of the waveguide, the two rows of grating arrays are symmetrically distributed on both sides of the waveguide, the waveguide, the grating array and the heating electrode are parallel to each other, and the grating array is arranged between the heating electrode and the waveguide.

[0007] The grating array is mainly composed of a plurality of reflection gratings arranged and distributed in uniform intervals along the long side direction of the waveguide, and the waveguide, the reflection grating and the heating electrode are placed on the upper surface of the thin film lithium niobate flat plate; the dielectric buried oxygen layer is bonded between the thin film lithium niobate flat plate and the substrate layer.

[0008] The optical filter controls the width of the waveguide to realize the bound state in the quasi-continuum, and the width W of the waveguide in the optical filter is obtained by processing according to the following formula:

[0009]

[0010] where W is the width of the waveguide, m is an arbitrary non-zero integer, Δφ is the phase difference between the reflected and transmitted TE modes, λ is the wavelength of the reflection grating, n TE and n TM are the effective refractive indices for the TE and TM modes, respectively.

[0011] The optical filter reflects the waveguide leakage mode by controlling the width of the reflection grating and the distance between the reflection grating and the waveguide, so as to realize the bound state in the quasi-continuum, and the reflection of the waveguide leakage mode by the reflection grating satisfies the following condition:

[0012]

[0013] where R represents the reflectivity of the reflection grating, θ represents the incident angle of the light wave, γ is the imaginary part of the complex effective refractive index of the TM mode in the waveguide, r and t are the Fresnel reflection and transmission coefficients due to the refractive index difference at the ridge wall, respectively, and j represents the imaginary part symbol.

[0014] The reflection grating is used to realize ultra-narrow bandwidth filtering, and the wavelength of the reflection grating satisfies the Bragg condition:

[0015] λ B = 2Λn eff

[0016] where λ B represents the reflection center wavelength of the reflection grating, Λ is the sawtooth period of the grating, and n eff is the effective refractive index of the grating.

[0017] By changing the sawtooth period Λ of the reflection grating, the reflection center wavelength can be changed without loading voltage on the lateral heating electrode.

[0018] The heating electrode changes the effective refractive index of the reflection grating by controlling the loading voltage, and the temperature control mode of the heating electrode is obtained according to the following formula:

[0019]

[0020] where Δλ is the wavelength variation range of the reflection grating, λ is the wavelength of the reflection grating, ξ is the thermo-optic coefficient of the reflection grating material, α is the expansion coefficient of the reflection grating material, and ΔT is the temperature variation range of the reflection grating under the control of the heating electrode.

[0021] The heating electrode is used for laterally heating the reflection grating, different voltages are applied to the heating electrodes on two sides of the reflection grating, so that the temperature of the reflection grating changes, and then the effective refractive index of the reflection grating changes continuously, and then the center wavelength of the reflection grating drifts, and the continuous tunable center wavelength of the reflection grating is realized.

[0022] The waveguide and the N reflection gratings are all made of organic polymers, the heating electrode is made of metal material, the thin film lithium niobate flat plate is made of lithium niobate material, the dielectric buried oxygen layer is made of silicon dioxide material, and the substrate layer is made of silicon material.

[0023] The etching-free lithium niobate optical filter includes a thin film lithium niobate platform and a waveguide, N reflection gratings and heating electrodes on two sides of the reflection gratings on the thin film lithium niobate platform. The waveguide and the reflection grating are made of organic polymer material, and the two-side heating electrode is made of metal material, without etching the lithium niobate. By setting the width of the waveguide to be preferably wide, the bound state in the continuum can be realized, the lateral leakage loss caused by mode leakage is reduced, and the reflection grating placed on both sides of the waveguide can reflect the mode leaked into the lithium niobate flat plate back to the waveguide to further realize the bound state in the quasi-continuum, reduce the lateral loss and achieve narrow-band filtering of the periodic disturbance in the waveguide. Due to the separation of the reflection grating and the waveguide and the suppression of the waveguide lateral leakage, the reflection grating only produces weak periodic disturbance to the mode in the waveguide, thereby producing a super-narrow-band reflection spectrum. In addition, a lateral heating method is adopted, lateral heating electrodes are loaded on both sides of the reflection grating, the effective refractive index of the reflection grating is changed by controlling the voltage loaded on the two-side heating electrodes, the center wavelength of the filter is shifted, and the tuning of the reflection center wavelength is realized. The filter on the etching-free lithium niobate platform has the advantages of easy processing, large tolerance, low loss, free adjustment of bandwidth and reflection center wavelength, high extinction ratio, and can effectively realize low-loss and narrow-band photon filtering. According to specific needs, the center wavelength of the filter can be adjusted by changing the voltage loaded on the heating electrode based on the unchanged physical structure, and filters with different reflection center wavelengths can be realized.

[0024] The width of the waveguide is preferably set to realize the bound state in the quasi-continuum, thereby reducing the leakage loss caused by mode leakage in the waveguide. The width of the reflection grating placed on both sides and the distance between the reflection grating and the waveguide are preferably set to reflect the leaked mode of the waveguide, realize the bound state in the quasi-continuum, and further reduce the lateral leakage loss. Due to the bound state in the quasi-continuum, the reflection grating only produces weak disturbance to the mode in the waveguide, produces a super-narrow-band reflection spectrum, and realizes super-low-loss filtering effect. A lateral heating method is adopted, heating electrodes are loaded on both sides of the reflection grating, the effective refractive index of the reflection grating is changed by controlling the loaded voltage, the reflection center wavelength is shifted, and the tunable filtering wavelength is realized.

[0025] The reflective grating is separated from the waveguide, and parameters such as the distance between the reflective grating and the waveguide, the grating period, the grating width, the grating length and the like can be changed to realize filtering of different wavelengths. Further, a side heating mode is adopted, different voltages are applied to the heating electrodes on both sides of the reflective grating, the temperature of the reflective grating changes, and then the effective refractive index of the grating changes continuously, the center wavelength drifts, and continuous tunable center wavelength reflection is realized.

[0026] The waveguide leaks the mode scattered to both sides, and the intensity rapidly decays with the increase of the propagation distance. By controlling the distance between the reflective grating and the waveguide, the side scattering can be suppressed on the one hand, and on the other hand, the farther the distance between the reflective grating and the waveguide, the weaker the disturbance to the mode in the waveguide due to the rapid decay of the side scattering mode. Due to the periodic structure of the reflective grating, the light is strongly reflected in a narrow band near the Bragg wavelength. The weaker the disturbance to the mode in the waveguide by the reflective grating, the narrower the reflection bandwidth. That is, by designing the distance between the reflective grating and the waveguide, the reflection bandwidth can be selected under the quasi-continuous domain bound state to produce an ultra-narrow reflection bandwidth.

[0027] The application provides a method for realizing accurate, fast and continuous tunable reflection center wavelength, which can change the distance between the reflective grating and the waveguide, the period, width and length of the reflective grating, and control the filtering of different wavelengths under the condition that the side heating electrode is not loaded with voltage. The application can realize total reflection of the leaked mode of the waveguide by controlling the width of the reflective grating and the distance between the waveguide, and further realize the bound state in the continuum. The application is based on the bound state thin film lithium niobate optical filter in the quasi-continuum, has low loss, adjustable reflection center wavelength bandwidth, and can realize fast and accurate control of the shift of the reflection center wavelength by only changing the loaded voltage on the basis of the unchanged physical structure.

[0028] The bound state thin film lithium niobate optical filter of the application can effectively realize an ultra-low loss and ultra-narrow bandwidth photonic filter, and can effectively realize the near-distance side heating mode to realize efficient and wide-range continuous tunable filtering center wavelength on the basis of the unchanged physical structure.

[0029] The application has the following beneficial effects:

[0030] 1. The application adopts the reflective grating filter, has the advantages of flat spectral response, wavelength selection and flexible adjustable bandwidth, and is easy to meet the application requirements of various optical communications.

[0031] 2、The filter of the present application separates the reflection grating from the waveguide laterally, the separated reflection grating produces extremely weak disturbance to the mode in the waveguide, forms narrow-bandwidth filtering, and controls the waveguide width and the position and shape of the reflection grating, realizes the bound state in the continuum, suppresses the lateral mode leakage loss, and effectively reduces the long-distance transmission loss.

[0032] 3、The present application does not need to change the physical structure, adopts the lateral heating mode, only needs to apply voltage on the heating electrode laterally of the reflection grating, realizes the high efficiency, large range and continuous adjustment of the filter center wavelength.

[0033] 4、The present application adopts the etching-free lithium niobate scheme, only needs one-time exposure, does not need complex dry etching, the process is simple, the cost is low, the loss is small, and has great practical application potential. The present application obtains an optical filter with simple structure, convenient processing and manufacturing, low loss and fast wavelength tuning without changing the physical structure on the thin film lithium niobate platform. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 It is the overall structure diagram of the thin film lithium niobate optical filter based on the bound state in the quasi-continuum of the present application.

[0035] Figure 2 It is a cross-sectional schematic view of the material used in the thin film lithium niobate optical filter based on the bound state in the quasi-continuum of the present application.

[0036] Figure 3 It is a simulation result diagram of adjusting the waveguide width to realize the propagation loss of the bound state in the continuum of the present application.

[0037] Figure 4 It is a simulation result diagram of using the reflection grating to realize the bound state in the continuum and reduce the lateral leakage loss of the present application.

[0038] Figure 5 It is a filter effect diagram obtained by simulation in the specific embodiment of the present application.

[0039] Wherein: 1, waveguide; 2, reflection grating; 3, heating electrode; 4, thin film lithium niobate flat plate; 5, dielectric buried oxygen layer; 6, substrate layer. DETAILED DESCRIPTION

[0040] The present application will be described in detail below in combination with specific implementation examples, and the following implementation examples will help those skilled in the art to further understand the present application, but do not limit the present application in any form.

[0041] As Figures 1-2As shown, the optical filter is mainly composed of a tunable filter layer, a thin film lithium niobate flat plate 4, a dielectric buried oxygen layer (BOX layer) 5 and a substrate layer 6 stacked from top to bottom in turn, the tunable filter layer includes a waveguide 1, two rows of grating arrays and two heating electrodes 3, the two heating electrodes 3 are symmetrically distributed on the two sides of the waveguide 1 respectively, the two rows of grating arrays are symmetrically distributed on the two sides of the waveguide 1 respectively, the waveguide 1, the grating array and the heating electrode 3 are parallel to each other, and the grating array is arranged between the heating electrode 3 and the waveguide 1.

[0042] The grating array is mainly composed of a plurality of reflective gratings 2 arranged and distributed in sequence and uniformly spaced along the long side direction of the waveguide 1, the waveguide 1, the reflective grating 2 and the heating electrode 3 are placed on the upper surface of the thin film lithium niobate flat plate 4; the dielectric buried oxygen layer 5 is bonded between the thin film lithium niobate flat plate 4 and the substrate layer 6. In a specific implementation, N reflective gratings 2 are periodically arranged between the waveguide 1 and the heating electrode 3 to form a grating array; the heating electrode 3 is located on the side of the N reflective gratings 2 to achieve wavelength tuning.

[0043] By setting a waveguide with a suitable width, a bound state in a continuum is achieved, and lateral leakage loss is reduced. At the same time, the reflective gratings 2 placed on both sides of the waveguide 1 can reflect the laterally leaked modes, further reducing the lateral loss. Due to the suppression of lateral leakage, the reflective gratings 2 only produce very weak perturbations to the modes in the waveguide 1, thereby forming a super-narrow bandwidth reflection spectrum. By using a lateral heating method, heating electrodes 3 are installed on both sides of the reflective gratings 2, and by controlling the applied voltage, the refractive index of the reflective gratings 2 is changed, thereby changing the effective refractive index and causing the reflected center wavelength to shift, achieving the purpose of continuous tuning in the quasi-continuous domain. The present application can effectively achieve rapid and accurate control of the shift of the reflected center wavelength by only changing the applied voltage without changing the physical structure.

[0044] As shown in FIG. 1, Figure 3 The transmission loss of the waveguide mode in the waveguide 1 will change periodically with the change of the width of the waveguide 1, when the width of the waveguide 1 is near the odd multiple of the half wavelength of the propagation mode, there is a phase difference of π between the two sides of the leaked mode, the two interfere destructively to suppress the lateral scattering, realizing the bound state in the continuous domain, thereby effectively reducing the long-distance transmission loss of the mode in the waveguide. When the width of the waveguide 1 is near the even multiple of the half wavelength of the propagation mode, the leaked mode on both sides of the ridge waveguide 1 has a phase difference of 2π, the two interfere to enhance the lateral scattering, thereby increasing the transmission loss. By controlling the width of the ridge waveguide 1 to be near the odd multiple of the half wavelength of the propagation mode, the leaked mode on both sides of the ridge waveguide 1 has a phase difference of π in the lateral direction, the two interfere destructively to suppress the lateral scattering, thereby realizing the bound state in the continuous domain, controlling the transmission loss of the mode in the waveguide, and effectively reducing the long-distance transmission loss of the mode in the waveguide.

[0045] The structure adopts a structure in which the reflective grating 2 is separated from the waveguide 1. The reflective grating 2 on both sides is separated from the waveguide 1. The reflective grating 2 can reflect the lateral leakage mode of the waveguide 1, further reducing lateral scattering and reducing the long-distance transmission loss of the waveguide.

[0046] like Figure 4 As shown, structurally, the reflective grating 2 is separated from the waveguide 1. Compared with the traditional lateral etching grating, the reflective grating of this invention can suppress lateral leakage of modes in the waveguide. The structure of this invention can suppress lateral leakage loss of modes in the waveguide by optimizing the distance between the reflective grating 2 and the waveguide 1.

[0047] By separating the reflective grating from the waveguide, parameters such as the spacing between the grating and the waveguide, the grating period, the grating width, and the grating length can be changed to achieve filtering of different wavelengths. Furthermore, by employing lateral heating, different voltages are applied to the heating electrodes on both sides of the reflective grating, causing a temperature change in the grating. This, in turn, continuously alters the effective refractive index of the grating, resulting in a shift in the center wavelength and achieving continuous tunability of the reflected center wavelength.

[0048] The optical filter achieves bound states in a quasi-continuum by controlling the width of waveguide 1. The width W of waveguide 1 in the optical filter is obtained according to the following formula:

[0049]

[0050] Where W is the width of waveguide 1, m is any non-zero integer (in practice, it can be an integer 2), Δφ is the phase difference between the reflection and transmission TE modes (transverse electric field modes), λ is the wavelength of the reflection grating 2, and n TE and n TM The effective refractive indices are TE mode and TM mode (transverse magnetic field mode), respectively, with π set to 3.14.

[0051] The optical filter reflects the leakage modes of waveguide 1 by controlling the width of the reflection grating 2 and the spacing between the reflection grating 2 and waveguide 1, thereby realizing the bound states in the quasi-continuum. The reflection of the leakage modes of waveguide 1 by the reflection grating 2 satisfies the following conditions:

[0052]

[0053] Where R represents the reflectivity of the reflection grating 2, θ represents the incident angle of the light wave, γ is the imaginary part of the complex effective refractive index of the TM mode in waveguide 1, r and t are the Fresnel reflection and transmission coefficients caused by the difference in refractive index at the ridge wall, respectively, and j is the symbol for the imaginary part.

[0054] Reflection grating 2 is used to achieve ultra-narrow bandwidth filtering, and the wavelength of reflection grating 2 satisfies the Bragg condition:

[0055] λ B = 2Λn eff

[0056] wherein λ B represents the reflection center wavelength of the reflection grating 2, Λ is the sawtooth period of the grating, n eff is the effective refractive index of the grating.

[0057] By changing the sawtooth period Λ of the reflection grating in the case that the lateral heating electrode is not loaded with voltage, the reflection center wavelength can be changed.

[0058] The heating electrode 3 changes the effective refractive index of the reflection grating 2 by controlling the loaded voltage to cause the reflection center wavelength to shift, so as to realize the tunable filtering wavelength. The temperature control mode of the heating electrode 3 is processed according to the following formula:

[0059]

[0060] wherein Δλ is the wavelength change range of the reflection grating 2, λ is the wavelength of the reflection grating 2, ξ is the thermo-optic coefficient of the material of the reflection grating 2, α is the expansion coefficient of the material of the reflection grating 2, and ΔT is the temperature change range of the reflection grating 2 controlled by the heating electrode 3.

[0061] The heating electrode 3 is used to laterally heat the reflection grating 2. By applying different voltages to the heating electrodes 3 on both sides of the reflection grating 2, the temperature of the reflection grating 2 changes, and then the effective refractive index of the reflection grating 2 changes continuously, and then the center wavelength of the reflection grating 2 drifts, so as to realize the continuous tunable of the reflection center wavelength of the reflection grating 2.

[0062] The waveguide 1 and the N reflection gratings 2 are made of organic polymers, the heating electrode 3 is made of metal material, the thin film lithium niobate flat plate 4 is made of lithium niobate material, the dielectric buried oxygen layer 5 is made of silicon dioxide material, and the substrate layer 6 is made of silicon material.

[0063] Specifically, in terms of structure, the lateral heating structure is adopted, and the lateral heating electrode 3 is placed on both sides of the reflection grating 2. Compared with vertical heating, because the thermal conductivity of silicon material is higher than that of silicon dioxide, the lateral heating method can more efficiently realize the temperature control of the waveguide. When the voltage is applied to the lateral heating electrodes 3 placed on both sides of the reflection grating 2, the temperature T of the reflection grating changes. Since the refractive index n of the reflection grating 2 is a function of the temperature T of the reflection grating, when the temperature T of the reflection grating changes, the refractive index n of the reflection grating 2 also changes. Because the effective refractive index n eff of the reflection grating 2 is a function of the refractive index n, the effective refractive index also changes. According to the reflection condition equation of the reflection grating 2:

[0064]

[0065] wherein Δλ B represents the reflection center wavelength variation range of the reflection grating 2, and ΔΛ is the sawtooth period variation range of the grating;

[0066] When the physical structure is fixed, only the influence of the reflection grating temperature T is considered, and then the reflection center wavelength λ B of the reflection grating 2 is a function of the effective refractive index n eff of the reflection grating 2, the period Λ of the reflection grating 2, and only the reflection grating temperature T, that is,

[0067]

[0068] wherein ξ is the thermo-optic coefficient of the reflection grating material, and α is the expansion coefficient of the reflection grating material, since the thermo-optic coefficient is two orders of magnitude higher than the expansion coefficient, the drift of the reflection center wavelength of the reflection grating 2 is largely dependent on the thermo-optic coefficient of silicon, and the effective refractive index and the geometric size of the waveguide will change slightly with the temperature, thereby causing the drift of the Bragg center reflection wavelength, and the continuous wavelength tuning can be realized.

[0069] In particular, by adopting the lateral heating 3 mode, and utilizing the quasi-continuous domain bound state of the waveguide, the distance between the electrode and the waveguide is further reduced, the absorption loss of the light field by the electrode metal is reduced, and high heating efficiency and wide-range continuous wavelength tuning can be realized.

[0070] A specific embodiment of the present application is as follows:

[0071] The organic polymer waveguide structure based on the lithium niobate material on the insulator is selected: the structure layer is an organic polymer ZEP520A, the thickness is 500 nm, the refractive index is 1.532 at a wavelength of 1550 nm. The waveguide below is a 300 nm thick lithium niobate film, the refractive index is n o = 2.21, n e = 2.14 at a wavelength of 1550 nm. The lower cladding layer is silicon dioxide (SiO2), and the thickness is 2.7 μm; the upper cladding layer is air.

[0072] The filter design is carried out for the 1550 nm waveband, and at this time the reflection grating period is 1.44 μm. The waveguide width of the grating is selected to be 1.8 μm, the distance between the reflection grating on both sides and the waveguide is 120 nm, the width of the reflection grating is 490 nm, the duty cycle of the reflection grating is 0.5, and the number of the reflection grating periods is 1000. The TM mode reflection spectrum of the device is simulated and verified by the eigenmode expansion algorithm. Figure 5 The reflection peak when the center wavelength is 1550 nm is shown. From Figure 5It can be seen that the device of the application can obtain a 3dB bandwidth of about 0.6nm at a center wavelength of 1556nm for TM mode.

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, and are not limited thereto; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that the technical solutions recorded in the foregoing embodiments can still be modified, or some or all of the technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A confined-state thin-film lithium niobate optical filter based on a quasi-continuum, characterized in that: It is mainly composed of a tunable filter layer, a thin-film lithium niobate plate (4), a dielectric buried oxide layer (5) and a substrate layer (6) stacked from top to bottom; the tunable filter layer includes a waveguide (1), two grating arrays and two heating electrodes (3), the two heating electrodes (3) are symmetrically distributed on both sides of the waveguide (1), the two grating arrays are symmetrically distributed on both sides of the waveguide (1), the waveguide (1), the grating arrays and the heating electrodes (3) are parallel to each other, and the grating arrays are disposed between the heating electrodes (3) and the waveguide (1); The grating array is formed by a number of reflective gratings (2) arranged at uniform intervals along the long side of the waveguide (1). The waveguide (1), reflective gratings (2) and heating electrode (3) are all placed on the upper surface of the thin film lithium niobate plate (4). The dielectric buried oxide layer (5) is bonded between the thin film lithium niobate plate (4) and the substrate layer (6). The optical filter achieves bound states in a quasi-continuum by controlling the width of the waveguide (1). The width W of the waveguide (1) in the optical filter is obtained according to the following formula: Where W is the width of waveguide (1), m is any non-zero integer, Δφ is the phase difference between the reflection and transmission TE modes, λ is the wavelength of the reflection grating (2), and n TE and n TM These are the effective refractive indices for TE mode and TM mode, respectively.

2. The optical filter based on bound-state thin-film lithium niobate in a quasi-continuum according to claim 1, characterized in that: The optical filter reflects the leakage mode of the waveguide (1) by controlling the width of the reflection grating (2) and the spacing between the reflection grating (2) and the waveguide (1). The reflection of the leakage mode of the waveguide (1) by the reflection grating (2) satisfies the following conditions: Where R represents the reflectivity of the reflective grating (2), θ represents the incident angle of the light wave, γ is the imaginary part of the complex effective refractive index of the TM mode in the waveguide (1), r and t are the Fresnel reflection and transmission coefficients caused by the difference in refractive index at the ridge wall, respectively, and j is the symbol for the imaginary part.

3. The optical filter based on bound-state thin-film lithium niobate in a quasi-continuum according to claim 1, characterized in that: The wavelength of the reflective grating (2) satisfies the following Bragg condition: l B =2Λn eff Where, λ B The reflection center wavelength of the reflection grating (2) is represented by Λ, where Λ is the sawtooth period of the grating, and n is the number of wavelengths. eff is the effective refractive index of the grating.

4. The optical filter based on bound-state thin-film lithium niobate in a quasi-continuum according to claim 1, characterized in that: The heating electrode (3) controls the applied voltage to change the effective refractive index of the reflection grating (2). The temperature control of the reflection grating (2) by the heating electrode (3) is obtained according to the following formula: Wherein, Δλ is the wavelength variation range of the reflective grating (2), λ is the wavelength of the reflective grating (2), ξ is the thermo-optic coefficient of the reflective grating (2) material, α is the expansion coefficient of the reflective grating (2) material, and ΔT is the temperature variation range of the reflective grating (2) under the temperature control of the heating electrode (3).

5. The optical filter based on bound-state thin-film lithium niobate in a quasi-continuum according to claim 1, characterized in that: The heating electrode (3) is used to heat the reflective grating (2) laterally. By applying different voltages to the heating electrodes (3) on both sides of the reflective grating (2), the temperature of the reflective grating (2) changes and the effective refractive index of the reflective grating (2) changes continuously, thereby causing the center wavelength of the reflective grating (2) to drift and realizing the continuous tunability of the center wavelength of the reflective grating (2).

6. The optical filter based on bound-state thin-film lithium niobate in a quasi-continuum according to claim 1, characterized in that: The waveguide (1) and N reflective gratings (2) are all made of organic polymer, the heating electrode (3) is made of metal, the thin-film lithium niobate plate (4) is made of lithium niobate, the dielectric buried oxide layer (5) is made of silicon dioxide, and the substrate layer (6) is made of silicon.

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