Thin film lithium niobate waveguide based on coupling extinction and preparation method thereof

Abstract

The present application relates to the technical fields of micro-nano photoelectronic chip, and especially relates to a thin film lithium niobate waveguide based on coupling extinction and a preparation method thereof.The waveguide comprises lithium niobate, a cladding layer, a substrate and a metal layer; the cladding layer is arranged on the substrate, one end of the lithium niobate is exposed on a first side of the cladding layer, the other end of the lithium niobate is embedded in the inside of the cladding layer, and the metal layer is arranged on the top of the cladding layer.The substrate is an input port, the waveguide is coupled with the exposed end of the lithium niobate, and the top of the cladding layer is an output port.The present application provides a thin film lithium niobate waveguide based on coupling extinction and a preparation method thereof, which is designed according to the coupling condition; since the mode fields of TE and TM are mainly concentrated on the side wall and the top, this makes the TM mode be strongly absorbed by the metal after being coupled to the left waveguide, and the TE mode is weakly absorbed due to the existence of the cladding layer of silicon oxide after being coupled, so that the polarization extinction ratio of the on-chip polarization device can be improved, and the overall loss of the device can be reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of micro-nano optical and electronic chip, and particularly relates to a thin film lithium niobate waveguide based on coupling extinction and a preparation method thereof. BACKGROUND

[0002] Polarization is a degree of freedom for photon encoding information, and polarization control on an optoelectronic chip is important for realizing specific functions. Some optoelectronic devices can utilize the polarization degree of freedom to increase the dimension and speed of information processing, while some optoelectronic devices support single-polarization operation, and the input of other polarization components will produce crosstalk, which will deteriorate the performance indicators of the device. In order to eliminate the interference of stray polarized light, a single-polarization waveguide structure needs to be introduced on the chip. Single-polarization waveguides can be realized by designing the geometric size of the waveguide or introducing high-absorption materials such as metals. By adjusting the geometric size of the waveguide, the effective refractive index and radiation loss of the TE / TM polarization mode (TE mode, Chinese full name: transverse electric mode, English full name: Transverse Electric mode; TM mode, Chinese full name: transverse magnetic mode, English full name: Transverse Magnetic mode) can be changed. The mode field distributions of TE / TM modes are different, and the introduction of high-absorption materials such as metals in the light field concentration area of TE / TM modes can introduce additional loss to realize single-polarization transmission of another polarization mode. Single-polarization waveguide structures have many application scenarios, including electro-optic modulators, on-chip gyroscopes, sensor devices, etc. The performance indicators mainly include insertion loss, size and polarization extinction ratio, etc. Among them, the most important performance is the polarization extinction ratio. For an end-coupled optoelectronic chip, there is an alignment error between the polarization axis of the polarization maintaining fiber in the end-coupling and the crystal direction of the chip, which will excite different polarization components in the optical waveguide, which will cause deterioration to the modulation efficiency or the sensing accuracy of the Sagnac effect in the gyroscope. The previous technical solution mainly ensures the single-polarization operation of the chip by using a high-precision six-axis alignment platform combined with transmission spectrum testing, and then performing ultraviolet curing packaging.

[0003] The existing several schemes for realizing the lithium niobate waveguide with high polarization extinction ratio have the following defects and disadvantages: (1) the scheme for adjusting the equivalent refractive index by using the waveguide geometric size is very dependent on the precision of micro-nano processing, including the line precision of an electron beam exposure system or photolithography in the chip preparation process, which will affect the mode refractive index and further affect the single polarization working characteristics of the device, and the etching depth error in the subsequent chip etching process will also cause the influence; (2) although the direct introduction of metal structure or the like on the waveguide structure or the sidewall can increase the polarization extinction ratio, the overlap with the working mode will introduce additional insertion loss; (3) the refractive index contrast of the traditional lithium niobate waveguide defined based on titanium diffusion or proton exchange is small, and it is difficult to realize the large equivalent refractive index difference between TE / TM modes; (4) if the device itself cannot realize a high polarization extinction ratio, a high-precision six-axis alignment platform needs to be adopted in combination with the transmission spectrum test to ensure the single polarization working of the chip, and then the packaging mode of ultraviolet curing packaging is adopted, and the process flow is relatively complicated, and it is difficult to mass-produce the device. SUMMARY

[0004] The present application provides a thin film lithium niobate waveguide based on coupling extinction and a preparation method thereof, to solve the defects of large insertion loss and low polarization extinction ratio in the prior art, by introducing the principles of directional coupling and metal absorption, a single polarization transmission structure is realized on the thin film lithium niobate optoelectronic chip platform by dry etching, a single structure can realize a TE / TM polarization extinction ratio of more than 20 dB within a propagation length of tens of microns, and the insertion loss is less than 1 dB.

[0005] The present application provides a thin film lithium niobate waveguide based on coupling extinction, comprising: lithium niobate, a cladding layer, a substrate and a metal layer.

[0006] The cladding layer is arranged on the substrate, one end of the lithium niobate is exposed on a first side of the cladding layer, the other end of the lithium niobate is embedded in the inside of the cladding layer, and the metal layer is arranged on the top of the cladding layer; wherein,

[0007] The substrate is an input port, the waveguide is coupled with the exposed end of the lithium niobate, and the top of the cladding layer is an output port.

[0008] According to the thin film lithium niobate waveguide based on coupling extinction provided by the present application, the lithium niobate comprises: a first strip-shaped lithium niobate and a second strip-shaped lithium niobate parallel to each other; the metal layer comprises: a first metal layer and a second metal layer, the first metal layer is located on the top of the cladding layer and at one end of the first strip-shaped lithium niobate embedded in the inside of the cladding layer, and the second metal layer is located on the top of the cladding layer and at one end of the second strip-shaped lithium niobate embedded in the inside of the cladding layer.

[0009] The thin film lithium niobate waveguide based on coupling extinction provided by the application further comprises: a third strip-shaped lithium niobate, the third strip-shaped lithium niobate penetrates through the cladding layer, and one end of the third strip-shaped lithium niobate is exposed to the second side of the cladding layer, and the other end of the third strip-shaped lithium niobate is exposed to the third side of the cladding layer.

[0010] The thin film lithium niobate waveguide based on coupling extinction provided by the application further comprises: the extending direction of the third strip-shaped lithium niobate is perpendicular to the extending directions of the first strip-shaped lithium niobate and the second strip-shaped lithium niobate.

[0011] The thin film lithium niobate waveguide based on coupling extinction provided by the application further comprises: the cladding layer and the substrate are in an integrated structure.

[0012] The thin film lithium niobate waveguide based on coupling extinction provided by the application further comprises: the cladding layer comprises a silicon dioxide cladding layer; and the substrate comprises a silicon dioxide substrate.

[0013] The thin film lithium niobate waveguide based on coupling extinction provided by the application further comprises: the metal layer comprises one of metal chromium, metal nickel and metal platinum.

[0014] The application further provides a preparation method of the thin film lithium niobate waveguide based on coupling extinction according to the application, comprising the following steps:

[0015] S1, performing electron beam lithography exposure on a thin film lithium niobate substrate;

[0016] S2, depositing a metal layer by magnetron sputtering for use as a hard mask in subsequent etching;

[0017] S3, placing the chip as a whole in a reagent capable of washing away the electron beam photoresist, lifting off the EB photoresist and the metal layer thereon, and retaining the metal layer defined with the etching pattern;

[0018] S4, using trifluoromethane and argon as etching gases, introducing chemical reaction and physical bombardment in etching by using the two gases, and simultaneously achieving high etching depth-width ratio and smooth waveguide sidewall; and etching through the lithium niobate layer by using inductively coupled plasma etching to form a strip-shaped lithium niobate waveguide;

[0019] S5, after etching, performing chemical polishing treatment on the chip;

[0020] S6, depositing silicon dioxide;

[0021] S7, performing electron beam lithography overlay, metal deposition and lift-off processing to obtain a metal layer required for single polarization operation at the top of the waveguide;

[0022] S8, cleaning the chip, and further depositing a cladding layer of the chip for protecting the chip;

[0023] S9, a facet polishing process is performed to form a facet coupler structure for coupling input and output light.

[0024] The preparation method of the thin-film lithium niobate waveguide based on coupling extinction provided by the application adopts FDTD simulation to perform parameter scanning, scans the equivalent refractive index of TE / TM and coupling strength, and thus determines the coupling distance, the cladding thickness, the substrate thickness and the size of the metal layer.

[0025] After the etching in S5 is completed, the chip is subjected to chemical polishing treatment, which specifically comprises the following steps:

[0026] After the etching is completed, a mixed solution is prepared from hydrogen peroxide, ammonia and water in a mass ratio of 2:2:1, and the chip is subjected to chemical polishing treatment by using a water bath heating method.

[0027] The thin-film lithium niobate waveguide based on coupling extinction provided by the application comprises lithium niobate, a cladding, a substrate and a metal layer. The cladding is arranged on the substrate, one end of the lithium niobate is exposed on a first side of the cladding, the other end of the lithium niobate is embedded in the interior of the cladding, and the metal layer is arranged on the top of the cladding. The substrate is an input port, the waveguide is coupled to the exposed end of the lithium niobate, and the top of the cladding is an output port. The thin-film lithium niobate waveguide based on coupling extinction and the preparation method thereof are designed for coupling conditions. Since the mode fields of TE and TM are mainly concentrated on the sidewall and the top, this makes the TM mode be strongly absorbed by the metal after being coupled to the left waveguide, and the TE mode is weakly absorbed due to the existence of the silicon oxide cladding after being coupled, so that the polarization extinction ratio of the on-chip polarization device can be improved and the overall loss of the device can be reduced. BRIEF DESCRIPTION OF DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the application or prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are some embodiments of the application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0029] Figure 1 is a cross-sectional view of the thin-film lithium niobate waveguide based on coupling extinction provided by the application.

[0030] Figure 2 is a top view of the thin-film lithium niobate waveguide based on coupling extinction provided by the application.

[0031] Figure 3 is a light field propagation diagram of the thin-film lithium niobate waveguide based on coupling extinction provided by the application under input TE mode.

[0032] Figure 4 is the light field propagation map of the thin film lithium niobate waveguide based on coupling extinction provided by the application in the input TM mode.

[0033] Figure 5 is the transmittance of the thin film lithium niobate waveguide based on coupling extinction provided by the application in the TE mode.

[0034] Figure 6 is the transmittance of the thin film lithium niobate waveguide based on coupling extinction provided by the application in the TM mode.

[0035] Reference signs:

[0036] 1: cladding layer; 2: substrate; 3: metal layer; 41: first strip-shaped lithium niobate; 42: second strip-shaped lithium niobate; 43: third strip-shaped lithium niobate. DETAILED DESCRIPTION

[0037] In order to make the objects, technical solutions and advantages of the present application clearer, the technical solutions in the present application will be described clearly and completely below in combination with the drawings in the present application. Obviously, the described embodiments are some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application.

[0038] In the description of the present embodiment, it should be understood that the orientations or positional relationships indicated by the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like are based on the orientations or positional relationships shown in the drawings, and are only for the convenience of describing the present embodiment and simplifying the description, and therefore cannot be understood as indicating or implying that the devices or elements indicated must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present embodiment.

[0039] In addition, the terms "first" and "second" are only for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined with "first" and "second" can explicitly or implicitly include at least one of the features. In the description of the present embodiment, the meaning of "a plurality of" is at least two, such as two, three, etc., unless otherwise specifically limited.

[0040] In this embodiment, unless otherwise clearly specified and limited, the terms "arranged", "mounted", "connected", "linked", "fixed" and the like should be interpreted broadly, for example, can be fixed connection, can also be detachable connection, or integral; can be mechanical connection, can also be electrical connection; can be directly connected, can also be indirectly connected through an intermediate medium, can be the internal communication of two elements or the interaction relationship of two elements, unless otherwise clearly limited. For those skilled in the art, the specific meaning of the above terms in this embodiment can be understood according to the specific circumstances.

[0041] In the embodiments of the present application, unless otherwise clearly specified and limited, the first feature is "on" or "under" the second feature. The first and second features can be in direct contact or indirectly contact through an intermediate medium. Moreover, the first feature can be directly above or obliquely above the second feature, or it can only mean that the horizontal height of the first feature is higher than that of the second feature. The first feature can be directly below or obliquely below the second feature, or it can only mean that the horizontal height of the first feature is lower than that of the second feature.

[0042] The present application will be described below Figure 1 and Figure 2 A thin film lithium niobate waveguide based on coupling extinction is described in the present application. The thin film lithium niobate waveguide based on coupling extinction comprises: lithium niobate, cladding 1, substrate 2 and metal layer 3.

[0043] The cladding 1 is arranged on the substrate 2, one end of the lithium niobate is exposed on the first side of the cladding 1, the other end of the lithium niobate is embedded in the inside of the cladding 1, and the metal layer 3 is arranged on the top of the cladding 1. The substrate 2 is an input port, the waveguide is coupled with the exposed end of the lithium niobate, and the top of the cladding 1 is an output port.

[0044] Specifically, the lower side of the waveguide of the present application (i.e. the substrate 2) serves as an input port, and is coupled with the left waveguide (i.e. the lithium niobate) during propagation. Due to the high refractive index and strong absorption of the metal, the polarization components of TE and TM have different coupling strengths, so the absorption intensity of the metal layer 3 deposited above the left waveguide is different, thereby realizing the function of high polarization extinction. The upper side of the waveguide of the present application (i.e. the metal layer 3) serves as an output port after extinction. The waveguide size, coupling distance, silicon oxide layer thickness and chromium metal strip size of the present application are designed respectively, wherein the waveguide structure is mainly designed for its coupling condition. Since the mode fields of TE and TM are mainly concentrated on the side wall and the top, the TM mode is strongly absorbed by the metal after being coupled to the left waveguide, while the TE mode is weakly absorbed after being coupled due to the existence of the cladding 1 of silicon oxide, which is the working principle of realizing high polarization extinction ratio in the present application.

[0045] Specifically, the waveguide structure of the present application is roughly composed of three parts: a lithium niobate device layer, a silicon oxide cladding layer 1 and a top chromium metal strip structure. The structure of the device upper layer is a bus waveguide and a coupling waveguide path part, the bus waveguide is the input and output port of the device, and single polarization transmission of the working polarization state (TE mode) can be realized. The coupling waveguide structure and the chromium metal strip above it are auxiliary structures for realizing single polarization on-chip, which do not correspond to the input and output ports. The structure of coupling waveguide plus metal absorption extinction can change the coupling conditions of TE and TM modes, and couple the TM mode to the coupling waveguide path on the left side of the figure, reduce the energy coupled back to the bus waveguide through metal absorption, thereby realizing high polarization extinction ratio TE mode transmission, while the TE mode is far away from the metal absorption structure, and the insertion loss of the working mode is also small. It can be seen that the working principle of the present application is different from the existing single polarization waveguide device, and the single polarization transmission is realized by polarization-sensitive directional coupling and metal strip.

[0046] Simulation results of the thin film lithium niobate waveguide based on coupling extinction of the present application:

[0047] The simulation results are shown in Figure 3 , Figure 4 , Figure 3 is the light field propagation diagram under input TE mode, it can be seen that the intensity of TE mode coupled to the left waveguide is low and there is basically no attenuation along the bus waveguide, while Figure 4 shows that the TM mode light field is mainly coupled into the left waveguide and is absorbed. Figure 5 and Figure 6 respectively give the transmittance of TE and TM modes, in which two curves are the transmittance of light field through one group of structures (solid line) and two groups of structures (dashed line) in the wavelength range of 1530-1570 nm, in which the lower curve is the transmittance of two groups of structures, and the higher curve is the transmittance of one group of structures. For TE mode, the insertion loss of this structure is 0.45 dB, while for TM mode, it is 23.6 dB. In the case of 0.45 dB insertion loss, a polarization extinction ratio of 23 dB can be realized. At the same time, the coupling area is 25 microns long, and the small device size and low insertion loss make it possible to cascade multiple devices to realize higher polarization extinction ratio.

[0048] Generally, the metal layer 3 adopts chromium metal strip, and can also be replaced by metal titanium and metal aluminum, etc.

[0049] Specifically, the FDTD simulation is used for parameter scanning, mainly scanning the equivalent refractive index and coupling strength of TE / TM, to ensure that the equivalent refractive index of the TE mode in the bus waveguide and the coupling waveguide is quite different, and the TE mode is basically not coupled to the coupling waveguide, while the equivalent refractive index of the TM mode is close, and the coupling strength is high, so as to determine the waveguide size, coupling distance, silicon oxide layer thickness (thickness of cladding layer 1 and substrate 2), and size of the chromium metal strip. The structure size parameters of one embodiment provided are as follows: the left waveguide width is 0.75 um, the right waveguide width is 0.8 um, the etching depth is 0.3 um, and the thickness of lithium niobate is 0.4 um.

[0050] The thin film lithium niobate waveguide based on coupling extinction provided by the application comprises: lithium niobate, a cladding layer 1, a substrate 2, and a metal layer 3. The cladding layer 1 is arranged on the substrate 2, one end of the lithium niobate is exposed on a first side of the cladding layer 1, the other end of the lithium niobate is embedded in the inside of the cladding layer 1, and the metal layer 3 is arranged on the top of the cladding layer 1. The substrate 2 is an input port, the waveguide is coupled to the exposed end of the lithium niobate, and the top of the cladding layer 1 is an output port. The thin film lithium niobate waveguide based on coupling extinction provided by the application is designed for coupling conditions. Since the mode fields of TE and TM are mainly concentrated on the side wall and the top, this makes the TM mode be strongly absorbed by the metal after being coupled to the left waveguide, while the TE mode is weakly absorbed after being coupled due to the existence of the cladding layer 1 of silicon oxide, so as to improve the polarization extinction ratio of the on-chip polarization device and reduce the overall loss of the device.

[0051] In one embodiment of the application, the lithium niobate comprises: a first strip-shaped lithium niobate 41 and a second strip-shaped lithium niobate 42 parallel to each other; the metal layer 3 comprises: a first metal layer 3 and a second metal layer 3, the first metal layer 3 is located on the top of the cladding layer 1 and at one end of the first strip-shaped lithium niobate 41 embedded in the inside of the cladding layer 1, and the second metal layer 3 is located on the top of the cladding layer 1 and at one end of the second strip-shaped lithium niobate 42 embedded in the inside of the cladding layer 1. In this embodiment, since the cladding layer 1 wraps the first metal layer 3 and the second metal layer 3, and the cladding layer 1 provides a gap between the first metal layer 3 and the second metal layer 3, the absorption is weak after the TE mode is coupled due to the existence of the gap.

[0052] In one of the embodiments of the present application, the lithium niobate further comprises a third strip-shaped lithium niobate 43, the third strip-shaped lithium niobate 43 passes through the cladding layer 1, and one end of the third strip-shaped lithium niobate 43 is exposed to the second side of the cladding layer 1, and the other end of the third strip-shaped lithium niobate 43 is exposed to the third side of the cladding layer 1. Preferably, the extending direction of the third strip-shaped lithium niobate 43 is perpendicular to the extending direction of the first strip-shaped lithium niobate 41 and the second strip-shaped lithium niobate 42, that is, the third strip-shaped lithium niobate 43 is arranged perpendicular to the first strip-shaped lithium niobate 41 and the second strip-shaped lithium niobate 42. Specifically, the width of the first strip-shaped lithium niobate 41 and the second strip-shaped lithium niobate 42 is 0.75 um, the width of the third strip-shaped lithium niobate 43 is 0.8 um, the etching depth is 0.3 um, and the thickness is 0.4 um.

[0053] In one of the embodiments of the present application, the cladding layer 1 and the substrate 2 are in an integrated structure. In this embodiment, the cladding layer 1 and the substrate 2 can be in an integrated structure, that is, the bottom of the cladding layer 1 has the function of the substrate 2. Whether the cladding layer 1 or the substrate 2 is in an integrated structure or a separate structure, the cladding layer 1 and the substrate 2 can adopt silicon dioxide as the material, that is, the cladding layer 1 comprises a silicon dioxide cladding layer 1; the substrate 2 comprises a silicon dioxide substrate 2.

[0054] In one of the embodiments of the present application, the metal layer 3 comprises one of metal chromium, metal nickel and metal platinum. Preferably, the metal layer 3 adopts metal chromium, and the shape of the metal layer 3 is a strip-shaped metal chromium.

[0055] The present application also provides a preparation method of the thin film lithium niobate waveguide based on coupling extinction according to the above-mentioned embodiments of the present application. The preparation method generally comprises the following steps: EBL electron beam lithography-Cr deposition-lift-off-physical and chemical ICP etching-washing the chip-chemical polishing-depositing silicon dioxide-depositing metal-etching metal strips-end face polishing.

[0056] Specifically, the preparation method specifically comprises:

[0057] S1, performing electron beam lithography exposure on the thin film lithium niobate substrate 2;

[0058] S2, depositing a metal layer 3 by magnetron sputtering for use as a hard mask in subsequent etching;

[0059] S3, placing the whole chip in a reagent capable of washing away the electron beam resist, lifting off the EBL resist and the metal layer 3 thereon, and retaining the metal layer 3 defined with the etching pattern;

[0060] S4, using trifluoromethane and argon as etching gases, utilizing the two gases to respectively introduce chemical reaction and physical bombardment in etching, while achieving high etching depth-width ratio and smooth waveguide sidewall; using inductively coupled plasma etching to etch through the lithium niobate layer to form a strip-shaped lithium niobate waveguide;

[0061] S5, after etching is completed, the chip is subjected to chemical polishing treatment;

[0062] S6, depositing silicon dioxide;

[0063] S7, electron beam lithography, metal deposition and lift-off processing of the top single-polarization waveguide required metal layer 3;

[0064] S8, cleaning the chip, further depositing the cladding layer 1 of the chip for protecting the chip;

[0065] S9, end face polishing processing of the end face coupler structure for coupling input and output light.

[0066] It can be seen that the preparation method of the thin film lithium niobate waveguide based on coupling extinction provided by the application is different from the titanium diffusion and proton exchange lithium niobate chip, and the whole device is processed by the dry etching process preparation process.

[0067] In one embodiment of the application, FDTD simulation is used for parameter scanning, and the equivalent refractive index and coupling strength of TE / TM are scanned to determine the coupling distance, the thickness of the cladding layer 1, the thickness of the substrate 2 and the size of the metal layer 3. Specifically, FDTD simulation is used for parameter scanning, mainly scanning the equivalent refractive index and coupling strength of TE / TM to ensure that the equivalent refractive index of the TE mode in the bus waveguide and the coupling waveguide is quite different, and the TM mode is basically not coupled to the coupling waveguide, and the equivalent refractive index of the TM mode is close and the coupling strength is high. The simulation result after design Figure 3 and Figure 4 The above effects can also be obviously verified.

[0068] In one embodiment of the application, the chemical polishing treatment of the chip after etching is completed in S5 specifically includes: after etching is completed, a mixed solution is prepared by using hydrogen peroxide, ammonia and water in a mass ratio of 2:2:1, and the chip is subjected to chemical polishing treatment by using water bath heating.

[0069] The application provides a specific embodiment of a preparation method of a thin film lithium niobate waveguide based on coupling extinction:

[0070] First, the e-beam lithography exposure is performed on the cleaned thin film lithium niobate substrate 2. Then, about 120 nm of Cr layer is deposited by magnetron sputtering as a hard mask for subsequent etching. In the lift-off process, the chip as a whole is placed in a reagent capable of washing away the e-beam photoresist (the cleaning reagent for each photoresist is different, typically acetone, butanone, etc.), the EB photoresist and the upper Cr metal layer 3 are lifted off, and the Cr metal layer 3 with the defined etching pattern is retained. In the physical and chemical ICP step, CHF3 and Ar are used as etching gases, which can introduce chemical reactions and physical bombardment in etching, respectively, while achieving high etching depth-width ratio and smooth waveguide sidewall. The 400 nm thick lithium niobate layer is etched through to form a lithium niobate strip waveguide using an inductively coupled plasma etching device. During the etching process, appropriate parameters such as gas ratio, gas pressure, temperature and etching time need to be selected. After the etching is completed, a chemical polishing treatment is performed on the chip using a mixed solution of hydrogen peroxide, ammonia and water in a ratio of 2:2:1, and heated in a water bath at 80 degrees for 1 hour. This step can clean the small amount of reaction products left on the sidewall, reducing the roughness and scattering loss of the sidewall. After depositing 200 nm thick silicon oxide by PECVD, EBL overlay, metal deposition and lift-off processing are performed to obtain the metal chromium strip required for single polarization operation at the top of the waveguide. After cleaning the contaminants on the surface of the chip, the cladding layer 1 of the chip is further deposited to protect the chip. After polishing the end face to process the end face coupler structure for coupling input-output light, the overall chip is obtained.

[0071] The device embodiments described above are merely illustrative, wherein the units described as separate components can or can not be physically separate, and the components shown as units can or can not be physical units, i.e., they can be located in one place, or distributed on multiple units. Some or all of the modules can be selected to achieve the purpose of the embodiment scheme according to actual needs. Those skilled in the art can understand and implement without creative labor.

[0072] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for some technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A thin-film lithium niobate waveguide based on coupled extinction, characterized in that, include: Lithium niobate, cladding (1), substrate (2), and metal layer (3); The cladding layer (1) is disposed on the substrate (2), and one end of the lithium niobate is exposed on a first side of the cladding layer (1), while the other end of the lithium niobate is embedded inside the cladding layer (1). The metal layer (3) is disposed on top of the cladding layer (1); wherein, The substrate (2) is the input port, the waveguide is coupled to the exposed end of the lithium niobate, and the top of the cladding (1) is the output port; The lithium niobate includes: a first strip-shaped lithium niobate (41) and a second strip-shaped lithium niobate (42) that are parallel to each other; the metal layer (3) includes: a first metal layer (3) and a second metal layer (3), the first metal layer (3) being located on top of the cladding (1) and at one end where the first strip-shaped lithium niobate (41) is embedded inside the cladding (1), and the second metal layer (3) being located on top of the cladding (1) and at one end where the second strip-shaped lithium niobate (42) is embedded inside the cladding (1); The lithium niobate further includes: a third strip-shaped lithium niobate (43) that passes through the cladding (1), with one end of the third strip-shaped lithium niobate (43) exposed on the second side of the cladding (1) and the other end of the third strip-shaped lithium niobate (43) exposed on the third side of the cladding (1); The extension direction of the third strip-shaped lithium niobate (43) is perpendicular to the extension directions of the first strip-shaped lithium niobate (41) and the second strip-shaped lithium niobate (42).

2. The thin-film lithium niobate waveguide based on coupled extinction according to claim 1, characterized in that, The cladding (1) and the substrate (2) are an integral structure.

3. The thin-film lithium niobate waveguide based on coupled extinction according to claim 1, characterized in that, The cladding (1) includes a silicon dioxide cladding (1); the substrate (2) includes a silicon dioxide substrate (2).

4. The thin-film lithium niobate waveguide based on coupled extinction according to claim 1, characterized in that, The metal layer (3) includes one of chromium, nickel and platinum.

5. A method for fabricating a thin-film lithium niobate waveguide based on coupled extinction according to any one of claims 1 to 4, characterized in that, include: S1. Electron beam lithography exposure is performed on a thin-film lithium niobate substrate (2); S2, A metal layer (3) is deposited by magnetron sputtering and used as a hard mask in subsequent etching; S3. Place the entire chip in a reagent that can wash away the electron beam photoresist, lift the EB photoresist and the metal layer (3) on it, and retain the metal layer (3) with the etched pattern defined. S4. Trifluoromethane and argon are used as etching gases. These two gases introduce chemical reactions and physical bombardment respectively during etching, thereby achieving a high etching aspect ratio and smooth waveguide sidewalls. The lithium niobate layer is etched through using inductively coupled plasma etching to form a lithium niobate strip waveguide. S5. After etching is completed, the chip is chemically polished. S6, depositing silicon dioxide; S7. Perform electron beam lithography overlay, metal deposition and lift-off processing to create the metal layer required for single polarization operation at the top of the waveguide (3); S8. Clean the chip and further deposit the chip cladding (1) to protect the chip; S9. The end face is polished to produce an end face coupler structure for coupling input and output light.

6. The method for fabricating a thin-film lithium niobate waveguide based on coupled extinction as described in claim 5, characterized in that, FDTD simulation was used to perform parameter scanning, scanning the equivalent refractive index and coupling strength of TE / TM, thereby determining the coupling spacing, cladding (1) thickness, substrate (2) thickness and metal layer (3) dimensions.

7. The method for fabricating a thin-film lithium niobate waveguide based on coupled extinction as described in claim 5 or 6, characterized in that, After etching is completed in S5, the chemical polishing process for the chip specifically includes: After etching, a mixed solution of hydrogen peroxide, ammonia and water in a mass ratio of 2:2:1 is prepared, and the chip is chemically polished by water bath heating.

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