Thin film lithium niobate end-coupler and method of making same

Abstract

The present application relates to the field of integrated optics, and particularly relates to a thin film lithium niobate end face coupler and a preparation method thereof. The coupler comprises, from bottom to top, a substrate, an insulating layer, a lithium niobate coupling layer, a lithium niobate waveguide layer and a silicon dioxide cladding layer. When light is input onto the thin film lithium niobate end face coupler, the thin film lithium niobate end face coupler realizes efficient conversion of the mode field by adjusting the geometric shape and the refractive index distribution of the lithium niobate coupling layer. The thin film lithium niobate end face coupler provided by the present application overcomes the problem of the harsh requirement for the line precision of the processing equipment caused by the change of the mode spot size in the traditional end face coupler by using the way of continuously reducing the waveguide width. The structure uses a double-tapered coupling mode to increase the line width while ensuring that the distance between the double tapers is greater than 500 nm, thereby reducing the resolution requirement for the patterning equipment, so that the lithography machine can be used for large-scale preparation, which is conducive to realizing large-scale photonic integration.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of integrated optics, in particular to a thin-film lithium niobate end-face coupler and a preparation method thereof. BACKGROUND

[0002] In modern optical communication systems, the coupling between chips and optical fibers is an indispensable link, and reducing coupling loss is crucial for improving system performance and energy efficiency. With the rapid development of optical communication, data centers and next-generation communication networks, integrated optoelectronic technology has become a key means to break through the transmission and processing bottlenecks of traditional microelectronic devices due to its high bandwidth, low power consumption and anti-interference advantages. Among numerous optoelectronic devices, end-face couplers, as important components connecting chips and optical fibers, directly affect the efficiency and stability of optical communication systems.

[0003] In recent years, thin-film lithium niobate has attracted widespread attention as a new generation of photonic integration platform material due to its excellent electro-optic effect, high nonlinear coefficient and wideband transparency. Although breakthroughs have been made in the development of photonic devices based on thin-film lithium niobate, the waveguide has a small mode spot size due to high refractive index contrast, making it challenging to match the mode field between the chip and the optical fiber. Therefore, designing a thin-film lithium niobate end-face coupler with large process tolerance and low coupling loss not only enhances the application potential of the thin-film lithium niobate photonic platform, but also provides important support for the performance optimization of optical communication systems. SUMMARY

[0004] Therefore, the present application provides a thin-film lithium niobate end-face coupler and a preparation method thereof to solve the problems of large device size, low bandwidth, high loss and small process tolerance of existing couplers.

[0005] In a first aspect, the present application provides a thin-film lithium niobate end-face coupler, which comprises, from bottom to top: a substrate, an insulating layer disposed on the substrate, a lithium niobate coupling layer disposed on the insulating layer, the lithium niobate coupling layer being mirror-symmetric along its central axis and comprising five regions connected in sequence: a coupling waveguide region, a mode conversion region, a multimode interference coupler region, a mode conversion waveguide region, a flat plate mode conversion region, and a flat plate region.

[0006] The lithium niobate coupling layer is disposed on the insulating layer, and the lithium niobate coupling layer is mirror-symmetric along its central axis and comprises five regions connected in sequence: a coupling waveguide region, a mode conversion region, a multimode interference coupler region, a mode conversion waveguide region, a flat plate mode conversion region, and a flat plate region.

[0007] The lithium niobate waveguide layer is disposed on the surface of the lithium niobate coupling layer, and the lithium niobate waveguide layer comprises: a mode conversion region stacked on the flat plate mode conversion region and a ridge waveguide region stacked on the flat plate region.

[0008] The lithium niobate waveguide layer is disposed on the surface of the lithium niobate coupling layer, and the lithium niobate waveguide layer comprises: a mode conversion region stacked on the flat plate mode conversion region and a ridge waveguide region stacked on the flat plate region.

[0009] When the light field is input to the thin film lithium niobate end-coupler, the thin film lithium niobate end-coupler realizes high-efficiency conversion of the mode field through changes in geometry and refractive index distribution at different positions.

[0010] On the basis of the above technical scheme, preferably, in the lithium niobate coupling layer, the coupling waveguide region is composed of two symmetrical tapered waveguides, and the distance in the vertical direction from one end away from the mode conversion region to one end close to the mode conversion region linearly increases.

[0011] The width of any one of the tapered waveguides linearly decreases from one end close to the mode conversion region to one end away from the mode conversion region, forming a tip.

[0012] On the basis of the above technical scheme, preferably, the width of the tip is not more than 500 nm.

[0013] On the basis of the above technical scheme, preferably, the mode conversion region is composed of two symmetrical frustum structures, and the distance in the vertical direction from one end close to the coupling waveguide region to one end close to the multimode interference coupler region linearly decreases and is not 0.

[0014] The sum of the widths of the two frustum structures close to the multimode interference coupler region is less than the width of the multimode interference coupler region; and the starting width of any one of the frustum structures is equal to the end width of any one of the tapered waveguides.

[0015] On the basis of the above technical scheme, preferably, the widths of the multimode interference coupler regions are equal and are not more than 5 mu m.

[0016] The effective refractive index of the multimode interference coupler region (33) needs to satisfy the condition shown in formula (I):

[0017] L π = 3 / 4*(n r W e 2 / lambda0) (I);

[0018] The length of the multimode interference coupler region (33) needs to satisfy the condition shown in formula (II):

[0019] L = 3*L π / 4 (II);

[0020] In formula (I) to (II), n r is the equivalent refractive index of the multimode interference coupler region, W e is the equivalent width of the multimode interference coupler region, lambda0 is the optical wavelength, and L πneff is an effective refractive index of the multimode interference coupler region, and L is a length of the multimode interference coupler region.

[0021] On the basis of the above technical scheme, preferably, in the lithium niobate waveguide layer, the mode conversion region is connected with the ridge waveguide region, and the mode conversion region is a tapered structure with a linearly decreasing width, and a tip direction of the tapered structure is consistent with a tip direction of the coupling waveguide region.

[0022] In a second aspect, the present application provides a method for preparing the thin-film lithium niobate end-face coupler, and the method comprises the following steps:

[0023] S1, measuring the thickness of the thin-film lithium niobate wafer sample, and cleaning the sample;

[0024] S2, depositing silicon dioxide or chromium metal on the sample obtained in step S1 as a hard mask for etching the pattern of the lithium niobate end-face coupler;

[0025] S3, spin-coating photoresist on the sample obtained in step S2, and developing and fixing the etching pattern;

[0026] S4, removing the residual photoresist on the sample obtained in step S3, and transferring the pattern from the photoresist to the hard mask;

[0027] S5, removing the hard mask on the sample obtained in step S4, and then transferring the pattern from the silicon dioxide mask to the first part of the lithium niobate waveguide;

[0028] S6, removing the residual hard mask on the sample obtained in step S5 to obtain the pattern of the lithium niobate end-face coupler;

[0029] S7, repeating steps S1-S6 on the sample obtained in step S6 to prepare the second part of the lithium niobate waveguide;

[0030] S8, sequentially subjecting the sample obtained in step S7 to ammonia / hydrogen peroxide water bath heating and concentrated sulfuric acid / hydrogen peroxide water bath heating;

[0031] S9, depositing a silicon dioxide cladding layer on the sample obtained in step S8;

[0032] S10, cutting the sample obtained in step S9 to expose the lithium niobate coupling waveguide;

[0033] S11, polishing the end face of the sample obtained in step S10 to complete the preparation of the thin-film lithium niobate end-face coupler;

[0034] The first part of the lithium niobate waveguide includes a flat plate region and a lithium niobate waveguide layer; and the second part of the lithium niobate waveguide includes a coupling waveguide region, a mode conversion region, a multimode interference coupler region, a mode conversion waveguide region and a flat plate mode conversion region.

[0035] In the step S8, the volume ratio of the ammonia water, the hydrogen peroxide and the deionized water is 1:1:5, the mass concentration of the ammonia water is 25%-28%, the mass concentration of the hydrogen peroxide is 30%, and the water bath heating temperature is 40-80 DEG C.

[0036] The volume ratio of the concentrated sulfuric acid and the hydrogen peroxide is 7:3 or 3:1, the mass concentration of the concentrated sulfuric acid is 98.3%, the mass concentration of the hydrogen peroxide is 30%, and the water bath heating temperature is 40-80 DEG C.

[0037] On the basis of the above technical scheme, preferably, the etching depth of the thin film lithium niobate coupling layer and the lithium niobate waveguide layer is equal, and the etching side wall angle is between 72 DEG and 73 DEG.

[0038] On the basis of the above technical scheme, preferably, the thickness of the ridge waveguide region is less than the thickness of the thin film lithium niobate layer.

[0039] The thin film lithium niobate end face coupler and the preparation method thereof have the following beneficial effects compared with the prior art:

[0040] The thin film lithium niobate end face coupler can match the optical mode of the plane optical fiber with a mode field diameter of 3.2um and 4um, the added multimode interference coupler structure effectively improves the coupling efficiency, the structure matches the mode of the optical fiber and the thin film lithium niobate ridge waveguide by changing the waveguide refractive index, and the conversion of the mode field is completed.

[0041] The thin film lithium niobate end face coupler prepared by the method can be widely applied to the input and output end faces of various thin film lithium niobate devices, such as a thin film lithium niobate electro-optic modulator.

[0042] In the process flow of the thin film lithium niobate end face coupler, the chemical cleaning method is used to effectively remove the re-deposition by-products generated in the dry etching process, and the processing quality of the lithium niobate waveguide is improved. BRIEF DESCRIPTION OF DRAWINGS

[0043] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creating laborious work.

[0044] Figure 1 The top view of the thin film lithium niobate end face coupler provided by the present application is shown in the figure;

[0045] Figure 2 The side view of the thin film lithium niobate end face coupler provided by the present application is shown in the figure;

[0046] Figures 1-2 In the figure, 1 is a substrate; 2 is an insulating layer; 3 is a lithium niobate coupling layer, 31 is a coupling waveguide region, 32 is a mode conversion region a, 33 is a multimode interference coupler region, 34 is a mode conversion waveguide region, 35 is a slab mode conversion region, and 36 is a slab region; 4 is a lithium niobate waveguide layer, 41 is a mode conversion region b, and 42 is a ridge waveguide region;

[0047] Figure 3 The process flow block diagram of the preparation of the thin film lithium niobate end face coupler provided by the present application is shown in the figure;

[0048] Figure 4 The scanning electron microscope images of the thin film lithium niobate end face coupler under different cleaning conditions in step S8 provided by the present application are shown in the figures;

[0049] Figure 5 The coupling effect diagram of the thin film lithium niobate end face coupler under simulation test provided by the present application is shown in the figure. DETAILED DESCRIPTION

[0050] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0051] It should be noted that all directionality indications (such as up, down, left, right, front, back, etc.) in the embodiments of the present application are only used to explain the relative positional relationship, movement condition, etc. between components in a certain specific posture (as shown in the figures). If the specific posture changes, the directionality indications also change accordingly.

[0052] In the present application, unless otherwise explicitly specified and limited, the terms "connection", "fixation", etc. should be understood in a broad sense. For example, "fixation" can be fixed connection, or detachable connection, or integral; can be mechanical connection, or electrical connection; can be direct connection, or indirect connection through an intermediate medium; can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise explicitly limited. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0053] The conventional end face coupler usually needs a multi-layer lithium niobate taper structure and a high refractive index cladding to realize the conversion of the light field size, for example, the buried oxygen layer of the thin film lithium niobate is patterned and suspended as a mode spot conversion area, which has a complex process step and poor mechanical stability. In contrast, the structure in the embodiment only needs to etch the lithium niobate layer twice, which has better mechanical stability and simpler process steps than the above scheme.

[0054] Meanwhile, the waveguide width and the pattern spacing of the embodiment are set to be greater than the exposure accuracy of the step-and-repeat photolithography machine, effectively solving the problem of difficult retention of fine lines; the design of the multimode interference coupling region particularly emphasizes high process tolerance, so that even if there is a slight change in size during processing, it will not significantly affect the final device performance. Through these improvements, not only the process tolerance of the device is enhanced, but also the coupling loss is greatly reduced, thereby improving the overall performance. Finally, the thin film lithium niobate end face coupler of the application not only overcomes the shortcomings of the traditional coupler, realizes smaller size, higher bandwidth and lower loss, but also reduces the complexity of device preparation and has good mechanical stability.

[0055] The thin film lithium niobate end face coupler of the application, as shown in Figure 1 from bottom to top, includes: a substrate 1, an insulating layer 2 arranged on the substrate 1;

[0056] A lithium niobate coupling layer 3 is arranged on the insulating layer 2, the lithium niobate coupling layer 3 is mirror-symmetric along its horizontal center line, and includes five regions connected in sequence: a coupling waveguide region 31, a mode conversion region a 32, a multimode interference coupler region 33, a mode conversion waveguide region 34, a flat plate mode conversion region 35 and a flat plate region 36;

[0057] A lithium niobate waveguide layer 4 is arranged on the lithium niobate coupling layer 3, the lithium niobate waveguide layer 4 includes: a mode conversion region b 41 stacked on the flat plate mode conversion region 35 and a ridge waveguide region 42 stacked on the flat plate region 36;

[0058] and a silicon dioxide cladding layer 5 arranged on the surface of the lithium niobate coupling layer 3;

[0059] Wherein, the working principle of the thin film lithium niobate end face coupler of the application is: when light is input to the thin film lithium niobate end face coupler, the thin film lithium niobate end face coupler realizes efficient conversion of the mode field by adjusting the geometric shape and refractive index distribution of the lithium niobate coupling layer 3.

[0060] Specifically, when the light field is input to the thin film lithium niobate end face coupler, the light field is first coupled to the interface of the coupling waveguide region 31, then the light field is uniformly divided into two parts with the increase of the width of each tapered waveguide in the coupling waveguide region 31 and the spacing between the two tapered waveguides, and then the light field size is further increased in the mode conversion region a 32 and high-order modes are excited, and then enters the multimode interference coupler region 33 to occur multimode interference, and finally two beams of light are combined into one beam from the multimode interference coupler region 33, enters the mode conversion waveguide region 34, the width of which gradually decreases to further reduce the light field size, and when injected into the flat plate mode conversion region 35, the light field size and position conversion occur, the light field is widened again after entering the mode conversion region b 41, and finally the optical mode conversion from the optical fiber to the ridge waveguide is realized.

[0061] In some embodiments of the present application, the material of the substrate 1 is selected from silicon or quartz stone for providing mechanical support.

[0062] In some embodiments of the present application, the insulating layer 2 is a thermal oxidation silicon dioxide layer for providing an interface suitable for growing lithium niobate material.

[0063] In some embodiments of the present application, the lithium niobate coupling layer 3 is mirror symmetric along its horizontal center line and includes five regions connected in turn, which are: the coupling waveguide region 31 with tapered width and non-central symmetry, the mode conversion region a 32 with tapered width and symmetry, the multimode interference coupling region 33 with constant width, the mode conversion waveguide region 34 with inverted tapered width, the flat plate mode conversion region 35 with tapered width, and the flat plate region 36 with constant width.

[0064] As any specific embodiment of the present application, the coupling waveguide region 31 is composed of two symmetrical tapered waveguides, the distance in the vertical direction from one end away from the mode conversion region a 32 to one end close to the mode conversion region a 32 linearly increases; and the width of any one of the tapered waveguides from one end close to the mode conversion region a 32 to one end away from the mode conversion region a 32 linearly decreases to form a sharp tip.

[0065] As any specific embodiment of the present application, the coupling waveguide region 1 with tapered width and non-central symmetry has a tip spacing of the line size that can be achieved by the used photoetching machine, which is generally not more than 500 nm.

[0066] As any specific embodiment of the present application, the mode conversion region a32 is composed of two symmetrical frustums, the distance of which in vertical direction from the end close to the coupling waveguide region 31 to the end close to the multimode interference coupler region 33 decreases linearly and is not 0; and the sum of the width of the two frustums close to the end of the multimode interference coupler region 33 is less than the width of the multimode interference coupler region 33, for optimizing the coupling efficiency and the mode field uniformity; the starting width of any of the frustums is equal to the end width of any of the tapered waveguides.

[0067] As any specific embodiment of the present application, the minimum distance of the mode conversion region a32 is the line size that the used photoetching machine can reach, generally not more than 500 nm.

[0068] In the present application, the multimode interference coupler region 33 excites high-order modes under the input of light field, the modes interfere with each other, and the single input light field or multiple input light fields can be periodically reproduced along the direction of the multimode interference coupler region 33, that is, the light field self-images in the multimode interference coupler region 33. For the multimode interference coupler region 33, the width and length are very important, the width determines the number of high-order modes that the multimode interference region can excite, and affects the imaging quality and tolerance of the multimode interference, and the length of the multimode interference region affects the phase difference between the modes, thereby affecting the mode field characteristics of the multimode interference region. As any specific embodiment of the present application, the width of the multimode interference coupler region 33 is equal and not more than 5 μm; the effective refractive index of the multimode interference coupler region 33 is calculated according to the equivalent width, optical wavelength and equivalent refractive index of the multimode interference coupler region 33, and the calculation formula (I) is:

[0069] L π = 3 / 4*(n r W e 2 / λ0);

[0070] According to the effective refractive index of the multimode interference coupler region 33, the length of the multimode interference coupler region 33 is calculated, and the calculation formula (II) is:

[0071] L = 3*L π / 4;

[0072] In formula (I)-(II), n r is the equivalent refractive index of the multimode interference coupler region 33, W e is the equivalent width of the multimode interference coupler region 33, λ0 is the optical wavelength, L π is the effective refractive index of the multimode interference coupler region 33, and L is the length of the multimode interference coupler region 33.

[0073] On the basis of the above, the coupling waveguide region 31 is used to couple and mode convert the fiber output light field, then the light field is injected into the multimode interference coupler region 33 through the mode conversion region a32 to excite high-order modes and generate interference, the interfered light field realizes the size conversion of the light field, and then enters the mode conversion waveguide region 34 to convert the square size, and then the light field is gradually converted to the waveguide layer through the flat plate mode conversion region 35, and finally the conversion from the fiber output light field to the ridge waveguide mode is realized.

[0074] In some embodiments of the application, the thin film lithium niobate waveguide layer 4 comprises: a mode conversion region b41 with a tapered width and a ridge waveguide region 42 with a constant width.

[0075] As any specific embodiment of the application, the mode conversion region b41 is connected with the ridge waveguide region 42, and the mode conversion region b41 is a tapered structure with a linearly decreasing width, and the tip direction is consistent with the tip direction of the coupling waveguide region 31.

[0076] On the basis of the above, the lithium niobate waveguide layer 4 is located directly above the lithium niobate coupling layer 3, which is composed of a mode conversion region 41 and a ridge waveguide 48. Among them, the mode conversion region 41 plays a role in converting the light field of the lithium niobate coupling layer 3 to the waveguide layer. The ridge waveguide region 42 is on the flat plate region, which together constitutes the lithium niobate ridge waveguide, and plays a role in binding the light mode and connecting with other devices.

[0077] In some embodiments of the application, as shown in Figure 2 The thickness of the substrate 1 is 600 nm, the thickness of the flat plate region 36 and the ridge waveguide layer 42 is 300 nm, the thickness of the silicon dioxide cladding layer 5 is 2 μm, and the maximum thickness variation at different positions should not exceed 10 nm.

[0078] In the application, the pattern processing method of thin film lithium niobate includes femtosecond laser direct writing processing, mechanical cutting and wet etching, etc., wherein the pattern size and precision of femtosecond laser processing are limited by laser spot size and mechanical stage displacement precision; the pattern flexibility of mechanical cutting processing is low, which is only suitable for straight waveguide structure; the etching rate of wet etching processing is different for different crystal phases of lithium niobate, which results in that it is only suitable for pattern processing of specific crystal phase; dry etching is suitable for processing of thin film lithium niobate due to its high precision, high flexibility and high yield.

[0079] Therefore, as any specific embodiment of the application, the etching depth of the substrate 1 of the application is 300 nm, and the lithium niobate coupling layer 3 and the lithium niobate waveguide layer 4 of the application are also formed by dry etching, and the etching depth is 300 nm, and the etching side wall angle is between 72°-73°.

[0080] However, in the process of dry etching lithium niobate, products such as niobium fluoride which are difficult to volatilize are generated, which are attached to the surface of the waveguide and are difficult to remove, thus limiting the preparation of high-quality lithium niobate optical structure. Based on this problem, the inventors provide a method for preparing a thin-film lithium niobate end-coupler with large process tolerance, as shown in Figure 3 The method comprises the following steps:

[0081] S1, measuring the thickness of a thin-film lithium niobate wafer sample and cleaning the sample;

[0082] S2, depositing silicon dioxide or chromium metal on the sample obtained in step S1 as a hard mask for etching the pattern of the lithium niobate end-coupler;

[0083] S3, spin-coating photoresist on the sample obtained in step S2 and developing and fixing the etching pattern;

[0084] S4, removing the residual photoresist and transferring the pattern from the photoresist to the hard mask on the sample obtained in step S3;

[0085] S5, removing the hard mask and then transferring the pattern from the silicon dioxide mask to the first part of the lithium niobate waveguide on the sample obtained in step S4;

[0086] S6, removing the residual hard mask on the sample obtained in step S5 to obtain the pattern of the lithium niobate end-coupler;

[0087] S7, repeating steps S1-S6 on the sample obtained in step S6 to prepare the second part of the lithium niobate waveguide;

[0088] S8, sequentially subjecting the sample obtained in step S7 to ammonia / hydrogen peroxide water bath heating and concentrated sulfuric acid / hydrogen peroxide water bath heating;

[0089] S9, depositing a silicon dioxide cladding layer on the sample obtained in step S8;

[0090] S10, cutting the sample obtained in step S9 to expose the lithium niobate coupling waveguide;

[0091] S11, polishing the end face of the sample obtained in step S10 to complete the preparation of the thin-film lithium niobate end-coupler;

[0092] The first part of the lithium niobate waveguide comprises a flat plate region 36 and a lithium niobate waveguide layer 4; and the second part of the lithium niobate waveguide comprises a coupling waveguide region 31, a mode conversion region a 32, a multimode interference coupler region 33, a mode conversion waveguide region 34, and a flat plate mode conversion region 35.

[0093] The inventors measure the thickness of the initial thin film lithium niobate layer to ensure that it meets the design requirements, and clean the thin film to remove surface contaminants and impurities, providing a clean substrate for subsequent processes; a layer of silicon dioxide or chromium metal is deposited on the cleaned substrate 1 as a hard mask, which is used to protect the part that does not need to be etched and define the pattern during lithography; after spin coating photoresist, the required pattern is exposed to photoresist by lithography technology, then developed and fixed to form the required etching pattern; the residual photoresist generated during the lithography process is removed, and the etching process is used to transfer the pattern from the photoresist to the underlying hard mask; the hard mask is removed, and the previously formed pattern is further transferred to the lithium niobate waveguide layer using etching technology to form the required waveguide structure; after the pattern transfer is completed, all remaining hard mask materials are removed to expose the lithium niobate waveguide layer 4 and the flat plate region 36 that have formed patterns; in order to build a multi-layer structure, steps S1 to S6 are repeated to prepare the coupling waveguide region 31, the mode conversion region a 32, the multimode interference coupler region 33, the mode conversion waveguide region 34 and the flat plate mode conversion region 35, increasing the complexity and functionality of the device.

[0094] In the above process, the sample at each step is sequentially subjected to ammonia / hydrogen peroxide water bath heating and concentrated sulfuric acid / hydrogen peroxide water bath heating treatment to remove possible residual organic matter and other impurities, ensuring the cleanliness of the subsequent process. Then, a layer of silicon dioxide is deposited on the treated sample as a cladding layer to surround the lithium niobate waveguide layer, providing optical isolation and helping to control the propagation path of light. The sample with the deposited silicon dioxide cladding layer 5 is cut to expose the internal lithium niobate coupling waveguide, preparing for the final processing step; the cut sample end face is finely polished to ensure smoothness and reduce light scattering loss, thereby improving coupling efficiency. Finally, the preparation of the thin film lithium niobate end coupler is completed.

[0095] In some embodiments of the present application, step S1, the cleaning process includes: first, at room temperature, rinsing the thin film lithium niobate with one of acetone, methanol or isopropanol; then soaking in concentrated sulfuric acid for 5 min, followed by acetone ultrasonic treatment for 15 min; then treating with a hydrofluoric acid buffer solution for 10 s, followed by ultrasonic treatment with a mixed solution of hydrochloric acid: hydrogen peroxide: deionized water at a mass ratio of 1:1:6 at 75℃ for 10 min, then rinsing with one of acetone, methanol or isopropanol at room temperature, and finally cleaning with deionized water and drying.

[0096] In some embodiments of the present application, step S2, silicon dioxide is grown using a plasma-enhanced chemical deposition device or chromium metal is deposited using an electron beam evaporation.

[0097] As any specific embodiment of the present application, the thickness of the metal layer is greater than 200 nm, and the thickness of the silicon dioxide layer is greater than 500 nm.

[0098] In some embodiments of the present application, in step S3, a spin coating method is used, spr955, 4000 rpm, uniform coating, pre-baking temperature 100℃, processing time 90 seconds, post-baking temperature 115℃, processing time 90 seconds.

[0099] In some embodiments of the present application, in step S4, the ashing equipment is set to plasma power of 300W and working time of 120s; the etching conditions are set to plasma power of 1100W, radio frequency power of 300W and working time of 500s.

[0100] In some embodiments of the present application, in step S5, a plasma etching device is used to transfer the pattern from the silicon oxide mask to the waveguide layer.

[0101] In some embodiments of the present application, in steps S6 and S7, a wet process is used to remove the mask.

[0102] As any specific embodiment of the present application, the metal mask is heated in a 40℃ water bath with metal etching solution for 120 seconds; the silicon oxide mask is heated in a 40℃ water bath with hydrofluoric acid buffer for 360 seconds.

[0103] In some embodiments of the present application, in step S8, the volume ratio of the ammonia, hydrogen peroxide and deionized water is 1:1:5, the concentration of the ammonia is 25%-28%, the concentration of the hydrogen peroxide is 30%, the water bath heating temperature is 40-80℃, and the time is 10-30min; the volume ratio of the concentrated sulfuric acid and the hydrogen peroxide is 7:3 or 3:1, the concentration of the concentrated sulfuric acid is 98.3%, the concentration of the hydrogen peroxide is 30%, the water bath heating temperature is 40-80℃, and the time is 10-30min.

[0104] In some embodiments of the present application, in step S9, a plasma enhanced chemical vapor deposition is used, and the temperature is 300℃.

[0105] In some embodiments of the present application, in steps S10 and S11, the cutting needs to cut through the entire thickness of the wafer to make it an independent unit, and the waveguide should not be scratched; the polishing requires polishing the entire end surface smooth.

[0106] The following is a specific embodiment of the present application.

[0107] Embodiment 1

[0108] Based on the structural requirements of the thin film lithium niobate end-coupler of the present application, the embodiment provides a method for preparing a thin film lithium niobate end-coupler based on the requirements, which comprises the following steps:

[0109] Step S1, using a wafer film thickness meter to measure the thickness of the thin film lithium niobate layer, then using a wafer cleaning machine to clean the thin film lithium niobate layer, remove organic and inorganic impurities on the wafer as substrate 1; the cleaning process includes: at room temperature, first select one of acetone, methanol or isopropanol to flush the thin film lithium niobate; after soaking in concentrated sulfuric acid for 5 min, ultrasonic treatment with acetone for 15 min; then use hydrogen fluoride acid buffer solution for 10 s, and then use a mixed solution of hydrochloric acid: hydrogen peroxide: deionized water with a mass ratio of 1:1:6 at 75°C for 10 min, and then flush with one of acetone, methanol or isopropanol at room temperature, and finally use deionized water to clean and dry.

[0110] Step S2, using a plasma enhanced chemical deposition device to grow silicon dioxide on the substrate 1 obtained after step S1, with a thickness greater than 500 nm, as a hard mask for lithium niobate pattern etching.

[0111] S3, on the sample obtained after step S2, use spin coating method, spr955, 4000 rpm uniform photoresist, pre-baking temperature 100°C for 90 seconds, post-baking 115°C for 90 seconds, use a photoetching machine to define the etching pattern and develop and fix.

[0112] S4, on the sample obtained after step S3, use ashing equipment to remove residual glue, where the ashing equipment plasma power is set to 300W, and the working time is 120s; then use etching equipment, etching conditions: plasma power is 1100W, radio frequency power is 300W, working time is 500s, transfer the pattern from the photoresist to the hard mask.

[0113] S5, on the sample obtained after step S4, remove the hard mask, and then use a plasma etching device to transfer the pattern from the silicon oxide mask to the first part of the lithium niobate waveguide; the first part of the lithium niobate waveguide includes a flat plate region 36 and a lithium niobate waveguide layer 4.

[0114] S6, remove the residual hard mask on the sample obtained after step S5, heat the metal mask in a 40°C water bath with metal etching solution for 120 seconds; heat the silicon oxide mask in a 40°C water bath with hydrogen fluoride acid buffer solution for 360 seconds; obtain a lithium niobate pattern.

[0115] S7, repeat steps S1-S6 on the sample obtained in step S6 to prepare a second layer of lithium niobate waveguide, the second part of the lithium niobate waveguide includes a coupling waveguide region 31, a mode conversion region a 32, a multimode interference coupler region 33, a mode conversion waveguide region 34 and a flat plate mode conversion region 35.

[0116] S8, the sample obtained in step S7 is subjected to chemical treatment, and re-deposition products generated by etching are removed by using an ammonia water / hydrogen peroxide water bath heating, followed by using a concentrated sulfuric acid / hydrogen peroxide water bath heating to remove impurities; the volume ratio of the ammonia water, hydrogen peroxide and deionized water is 1:1:5, wherein the ammonia water concentration is 25%, the hydrogen peroxide concentration is 30%, the water bath heating temperature is 40°C, and the time is 10 min; the volume ratio of the concentrated sulfuric acid and hydrogen peroxide is 7:3, wherein the concentrated sulfuric acid concentration is 98.3%, the hydrogen peroxide concentration is 30%, the water bath heating temperature is 40°C, and the time is 10 min.

[0117] S9, silicon dioxide is grown on the sample obtained in step S8 by using a plasma enhanced chemical deposition device at 300°C as an upper cladding layer of the end face coupler;

[0118] S10, the sample obtained in step S9 is cut by using a wafer dicing machine to expose the lithium niobate coupling waveguide;

[0119] S11, the sample obtained in step S10 is subjected to end face polishing by using a polishing machine to complete the preparation of the thin film lithium niobate end face coupler.

[0120] Example 2

[0121] The embodiment provides a method for preparing a thin film lithium niobate end face coupler based on the requirement, comprising the following steps:

[0122] Step S1, the thickness of the thin film lithium niobate layer is measured by using a wafer film thickness meter, and then the thin film lithium niobate layer is cleaned by using a wafer cleaning machine to remove organic and inorganic impurities on the wafer as a substrate 1; the cleaning process comprises the following steps: at room temperature, first, one of acetone, methanol or isopropanol is selected to flush the thin film lithium niobate; then, the wafer is soaked in concentrated sulfuric acid for 5 min, and then treated by ultrasonic wave in acetone for 15 min; then, the wafer is treated by hydrogen fluoride buffer solution for 10 s, and then treated by ultrasonic wave in a mixed solution of hydrochloric acid, hydrogen peroxide and deionized water with a mass ratio of 1:1:6 at 75°C for 10 min; then, the wafer is flushed by one of acetone, methanol or isopropanol at room temperature, and finally cleaned by deionized water and dried.

[0123] Step S2, silicon dioxide is grown on the substrate 1 obtained in step S1 by using a plasma enhanced chemical deposition device, and the thickness is greater than 500 nm, serving as a hard mask for etching the lithium niobate pattern.

[0124] S3, on the sample obtained in step S2, a spin coating method is adopted, spr955 is uniformly coated at a speed of 4000 rpm, the front baking temperature is 100°C, the treatment time is 90 s, the back baking temperature is 115°C, the treatment time is 90 s, the etching pattern is defined by using a photoetching machine, and then developed and fixed.

[0125] S4. On the sample obtained in step S3, the residual photoresist is removed by using an ashing device, wherein the ashing device is set at 300 W of plasma power and 120 s of working time; then the pattern is transferred from the photoresist to the hard mask by using an etching device, wherein the etching device is set at 1100 W of plasma power, 300 W of radio frequency power and 500 s of working time.

[0126] S5. On the sample obtained in step S4, the hard mask is removed, and then the pattern is transferred from the silicon oxide mask to the first part of the lithium niobate waveguide by using a plasma etching device; the first part of the lithium niobate waveguide comprises the flat plate region 36 and the lithium niobate waveguide layer 4.

[0127] S6. On the sample obtained in step S5, the residual hard mask is removed, the metal mask is heated in a 40℃ water bath with a metal etching solution for 120 seconds, and the silicon oxide mask is heated in a 40℃ water bath with a hydrofluoric acid buffer for 360 seconds; thus the lithium niobate pattern is obtained.

[0128] S7. The sample obtained in step S6 is subjected to steps S1-S6 again to prepare a second layer of lithium niobate waveguide; the second part of the lithium niobate waveguide comprises the coupling waveguide region 31, the mode conversion region a 32, the multimode interference coupler region 33, the mode conversion waveguide region 34 and the flat plate mode conversion region 35.

[0129] S8. The sample obtained in step S7 is subjected to chemical treatment, wherein the re-deposition product generated by etching is removed by using an ammonia / hydrogen peroxide water bath, and then impurities are removed by using a concentrated sulfuric acid / hydrogen peroxide water bath; the volume ratio of the ammonia, hydrogen peroxide and deionized water is 1:1:5, the concentration of the ammonia is 28%, the concentration of the hydrogen peroxide is 30%, the water bath heating temperature is 80℃, and the time is 30 min; the volume ratio of the concentrated sulfuric acid and the hydrogen peroxide is 3:1, the concentration of the concentrated sulfuric acid is 98.3%, the concentration of the hydrogen peroxide is 30%, the water bath heating temperature is 80℃, and the time is 30 min.

[0130] S9. On the sample obtained in step S8, a silicon dioxide is grown as an upper cladding layer of the end face coupler by using a plasma enhanced chemical deposition device at 300℃;

[0131] S10. The sample obtained in step S9 is cut by using a wafer dicing machine to expose the lithium niobate coupling waveguide.

[0132] S11. The sample obtained in step S10 is subjected to end face polishing by using a polishing machine to complete the preparation of the thin film lithium niobate end face coupler.

[0133] Comparative Example 1

[0134] The difference from Example 1 is that step S7 is omitted, and the other steps remain unchanged.

[0135] Comparative Example 2

[0136] The difference from Example 1 is that in step S7, only ammonia / hydrogen peroxide water bath heating wet treatment is performed.

[0137] Comparative Example 3

[0138] The difference from Example 1 is that in step S7, only the concentrated sulfuric acid / hydrogen peroxide water bath heating wet treatment is performed.

[0139] Scanning electron microscopy was performed on the thin-film lithium niobate end-face couplers prepared in Example 1 and Comparative Examples 1-3 above, as shown in... Figure 4 As shown.

[0140] Depend on Figure 4 It can be seen that: From Figure 4 (A) It can be seen that there are etching byproducts on the upper surface of the etched lithium niobate waveguide; from Figure 4 (B) It can be seen that the waveguide treated with ammonia / hydrogen peroxide wet process has improved cleanliness, but etching byproducts are still present; from Figure 4 (C) It can be seen that after the concentrated sulfuric acid / hydrogen peroxide wet process, the cleanliness is improved, but etching byproducts are still present; from Figure 4 (D) It can be seen that the lithium niobate waveguide after being treated with ammonia / hydrogen peroxide wet process and then with concentrated sulfuric acid / hydrogen peroxide wet process has a smooth and delicate surface.

[0141] Simulation tests were performed on the thin-film lithium niobate end-face coupler prepared in Example 1. The test method was as follows: modeling was performed using numerical simulation software, where the refractive index of lithium niobate was set to 2.21, the refractive index of silicon oxide was set to 1.444, the refractive index of silicon was set to 3.4792, the refractive index of air was set to 1, the mode field diameter of the optical fiber was set to 3.2 μm, the thickness of the lithium niobate waveguide layer was set to 300 nm, the thickness of the lithium niobate coupling layer was set to 300 nm, the tip spacing of the lithium niobate coupling layer 1 was set to 500 nm, and the width of the lithium niobate ridge waveguide was set to 1.5 μm. The coupling loss results were obtained in the wavelength range of 1500 nm-1600 nm. The test results are as follows. Figure 5 As shown.

[0142] Depend on Figure 5 It can be seen that the coupling loss of the thin-film lithium niobate end coupler prepared by the present invention is less than 0.5dB from 1500nm to 1600nm.

[0143] In summary, the high-process-tolerance thin-film lithium niobate end-coupler provided by the embodiment of the present application overcomes the problem of the harsh requirement for the line precision of the processing equipment caused by the use of the waveguide width continuously narrowing mode to realize the change of the mode spot size, the structure improves the line width by using the double-tapered coupling mode, ensures that the distance of the double taper is greater than 500nm, reduces the resolution requirement of the patterning equipment, and therefore can be prepared on a large scale by using the photolithography, which is beneficial to realize large-scale photonic integration.

[0144] The above merely describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A thin film lithium niobate end coupler, characterized by, From bottom to top, it comprises: a substrate (1), an insulating layer (2) arranged on the substrate (1); A lithium niobate coupling layer (3) arranged on the insulating layer (2), the lithium niobate coupling layer (3) is mirror symmetric along its axis, and comprises five regions connected in turn: a coupling waveguide region (31) with tapering width and non-central symmetry, a mode conversion region a (32) with linearly tapering width and symmetry, a multimode interference coupler region (33) with constant width, a mode conversion waveguide region (34) with reverse tapering width, a flat mode conversion region (35) with tapering width and a flat region (36) with constant width; A lithium niobate waveguide layer (4) arranged on the lithium niobate coupling layer (3), the lithium niobate waveguide layer (4) comprises: a mode conversion region b (41) stacked on the flat mode conversion region (35) and a ridge waveguide region (42) stacked on the flat region (36); And a silica cladding (5) arranged on the surface of the lithium niobate coupling layer (3); In the lithium niobate coupling layer (3), the coupling waveguide region (31) is composed of two symmetrical tapering waveguides, the distance in the vertical direction of the two tapering waveguides increases linearly from one end away from the mode conversion region a (32) to one end close to the mode conversion region a (32); The outer side of the output end of the mode conversion region a (32) is flush with the outer side of the input end of the multimode interference coupler region (33); Wherein, the mode conversion region a (32) is composed of two symmetrical frustum structures, the distance in the vertical direction of the two frustum structures decreases linearly from one end close to the coupling waveguide region (31) to one end close to the multimode interference coupler region (33), and is not equal to 0; The sum of the widths of the two frustums close to the multimode interference coupler region (33) is less than the width of the multimode interference coupler region (33); The starting width of any frustum is equal to the end width of any tapering waveguide; Any tapering waveguide decreases linearly in width from one end close to the mode conversion region a (32) to one end away from the mode conversion region a (32), forming a sharp end. When the light field is input to the thin film lithium niobate end face coupler, the thin film lithium niobate end face coupler realizes efficient conversion of the mode field by changing the geometry and refractive index distribution at different positions, which is specifically manifested as: when the light field is input to the thin film lithium niobate end face coupler, the light field is first coupled to the interface of the coupling waveguide region (31), then with the increase of the width of each tapered waveguide in the coupling waveguide region (31) and the spacing between the two tapered waveguides, the light field is uniformly divided into two parts, then the light field size is further increased in the mode conversion region a (32) and high-order modes are excited, then enters the multimode interference coupler region (33) to occur multimode interference, finally two beams of light are combined into one beam from the multimode interference coupler region (33) and enter the mode conversion waveguide region (34), the width of the region gradually decreases so that the light field size is further reduced, and when injected into the flat plate mode conversion region (35), the light field size and position conversion occur, the light field is widened again after entering the mode conversion region b (41), and finally the optical mode conversion from the optical fiber to the ridge waveguide is realized.

2. The thin-film lithium niobate end-coupler of claim 1, wherein, The width of the tip is not more than 500 nm.

3. The thin-film lithium niobate end-coupler of claim 1, wherein, The width of the multimode interference coupler region (33) is equal and not more than 5 μm; The effective refractive index of the multimode interference coupler region (33) needs to satisfy the condition as shown in formula (I): （Ⅰ）； The length of the multimode interference coupler region (33) needs to satisfy the condition as shown in formula (II): L = 3 x L π / 4 (II); In formula (I)~(II), n r is the effective refractive index of the multimode interference coupler region (33), W e is the effective width of the multimode interference coupler region (33), λ0is the optical wavelength, L π is the effective refractive index of the multimode interference coupler region (33), L is the length of the multimode interference coupler region (33).

4. The thin-film lithium niobate end-coupler of claim 1, wherein, In the lithium niobate waveguide layer (4), the mode conversion region b (41) is connected with the ridge waveguide region (42), and the mode conversion region b (41) is a tapered structure with linearly decreasing width, and the tip direction is consistent with the tip direction of the coupling waveguide region (31).

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