Thin film lithium niobate end face coupler and preparation method thereof

Abstract

The invention relates to the technical field of integrated optics, in particular to a thin-film lithium niobate end face coupler and a preparation method thereof. The coupler comprises a substrate and an insulating layer from bottom to top, a lithium niobate coupling layer, a lithium niobate waveguide layer and a silicon dioxide cladding layer; 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 a mode field by adjusting the geometrical shape and refractive index distribution of the lithium niobate coupling layer. The thin-film lithium niobate end face coupler provided by the invention overcomes the problem that a traditional end face coupler realizes the change of the size of a spot size by using a mode of continuously reducing the waveguide width, resulting in a strict requirement on the line precision of processing equipment, and the structure improves the line width by using a biconical coupling mode, so that the precision of the thin-film lithium niobate end face coupler is improved, and the precision of the thin-film lithium niobate end face coupler is improved. Meanwhile, it is guaranteed that the distance between the two cones is larger than 500 nm, and the requirement for the resolution ratio of graphical equipment is lowered, so that large-scale preparation can be conducted through a photoetching machine, and large-scale photon integration can be achieved easily.

Description

Technical Field

[0001] The present invention relates to the field of integrated optical technology, and in particular to a thin-film lithium niobate end face coupler and a preparation method thereof. Background Art

[0002] In modern optical communication systems, the coupling between chip and optical fiber is an indispensable link, and reducing coupling loss is crucial to improving system performance and energy efficiency. With the rapid development of optical communications, 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 with its advantages of high bandwidth, low power consumption and anti-interference. Among many optoelectronic devices, end couplers are important components for connecting chips and optical fibers, and their performance directly affects the efficiency and stability of optical communication systems.

[0003] In recent years, thin-film lithium niobate, as a new generation of photonic integrated platform materials, has attracted widespread attention due to its excellent electro-optical effect, high nonlinear coefficient and broadband transparency. Although breakthrough progress has been made in the research and development of photonic devices based on thin-film lithium niobate, its waveguide has a small mode spot size due to the high refractive index contrast, which makes the mode field matching between the chip and the optical fiber face great challenges. Therefore, designing a thin-film lithium niobate end coupler with large process tolerance and low coupling loss can not only enhance the application potential of the thin-film lithium niobate photonic platform, but also provide important support for the performance optimization of optical communication systems. Summary of the invention

[0004] In view of this, the present invention proposes a thin film lithium niobate end face coupler and a preparation method thereof to solve the problems of existing coupling devices such as large size, low bandwidth, high loss, and small preparation process tolerance.

[0005] In a first aspect, the present invention provides a thin-film lithium niobate end-face coupler, which comprises, from bottom to top: a substrate, an insulating layer disposed on the substrate;

[0006] A lithium niobate coupling layer is disposed on the insulating layer, wherein the lithium niobate coupling layer is mirror-symmetrical along its axis and includes five regions connected in sequence: a coupling waveguide region, a mode conversion region, a multimode interference coupler region, a mode conversion waveguide region, a slab mode conversion region, and a slab region;

[0007] A lithium niobate waveguide layer is disposed on the lithium niobate coupling layer, wherein the lithium niobate waveguide layer comprises: a mode conversion region stacked on the slab mode conversion region and a ridge waveguide region stacked on the slab region;

[0008] and a silicon dioxide cladding layer disposed on the surface of the lithium niobate coupling layer;

[0009] Among them, 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 geometric shape and refractive index distribution at different positions.

[0010] On the basis of the above technical solution, 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 of the two tapered waveguides increases linearly from one end away from the mode conversion region to one end close to the mode conversion region;

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

[0012] Based on the above technical solution, preferably, the width of the tip does not exceed 500 nm.

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

[0014] The sum of the widths of the two frustums close to one end of the multimode interference coupler region is smaller than the width of the multimode interference coupler region; the starting width of any frustum is equal to the end width of any tapered waveguide.

[0015] On the basis of the above technical solution, preferably, the widths of the multimode interference coupler regions are equal and do not exceed 5 μm;

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

[0017] L π =3 / 4*(n r W e 2 / λ 0 ) (Ⅰ);

[0018] The length of the multimode interference coupler region (33) must 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, λ 0is the optical wavelength, L π is the effective refractive index of the multimode interference coupler region, and L is the length of the multimode interference coupler region.

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

[0022] In a second aspect, the present invention provides a method for preparing the above-mentioned thin film lithium niobate end face coupler, the method comprising the following steps:

[0023] S1. Measure the thickness of thin film lithium niobate wafer samples and clean the samples;

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

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

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

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

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

[0029] S7, repeating steps S1-S6 for the sample obtained in step S6 to prepare a second portion of lithium niobate waveguides;

[0030] S8, heating the sample obtained in step S7 in an ammonia / hydrogen peroxide water bath and a concentrated sulfuric acid / hydrogen peroxide water bath in sequence;

[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 coupled waveguide;

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

[0034] The first part of the lithium niobate waveguide includes a slab region and a lithium niobate waveguide layer; 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 slab mode conversion region.

[0035] On the basis of the above technical solution, preferably, in step S8, the volume ratio of ammonia water, hydrogen peroxide and deionized water is 1:1:5, wherein the mass concentration of ammonia water is 25%-28%, the mass concentration of hydrogen peroxide is 30%, and the water bath heating temperature is 40-80°C;

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

[0037] On the basis of the above technical solution, preferably, the etching depths of the thin-film lithium niobate coupling layer and the lithium niobate waveguide layer are equal, and the etching side wall angle is between 72° and 73°.

[0038] On the basis of the above technical solution, 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 provided by the present invention have the following beneficial effects compared with the prior art:

[0040] The thin-film lithium niobate end face coupler of the present invention can match the optical mode of planar optical fiber with mode field diameters of 3.2um and 4um. The added multi-mode interference coupler structure effectively improves the coupling efficiency. The structure completes the mode field conversion by changing the waveguide refractive index and matching the optical fiber and thin-film lithium niobate ridge waveguide modes.

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

[0042] In the process flow of preparing the thin-film lithium niobate end face coupler of the present invention, a chemical cleaning method is adopted to effectively remove the redeposition byproducts generated during the dry etching process, thereby improving the processing quality of the lithium niobate waveguide. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0044] Figure 1 A top view of the thin-film lithium niobate end face coupler provided by the present invention;

[0045] Figure 2 A side view of the thin film lithium niobate end face coupler provided by the present invention;

[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 A flowchart of the process for preparing a thin-film lithium niobate end coupler provided by the present invention;

[0048] Figure 4 Scanning electron microscope images of thin film lithium niobate end couplers under different cleaning conditions in step S8 provided by the present invention;

[0049] Figure 5 This is a diagram showing the coupling effect of the thin-film lithium niobate end face coupler provided by the present invention under a simulation test. DETAILED DESCRIPTION

[0050] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0051] It should be noted that all directional indications in the embodiments of the present invention (such as up, down, left, right, front, back, etc.) are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0052] In the present invention, unless otherwise clearly specified and limited, the terms "connection", "fixation", etc. should be understood in a broad sense. For example, "fixation" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0053] Traditional end couplers usually require a combination of multi-layer lithium niobate conical structures and high-refractive index cladding to achieve the conversion of light field size. For example, the buried oxide layer of thin-film lithium niobate is patterned and then suspended as a pattern conversion area. This method has complex process steps and poor mechanical stability. In contrast, the structure proposed in this embodiment only requires etching the lithium niobate layer twice, which has better mechanical stability and simpler process steps than the above solutions.

[0054] At the same time, the waveguide width and pattern spacing of this embodiment are set to be greater than the exposure accuracy of the stepper lithography machine, which effectively solves the problem of difficulty in retaining fine lines; the design of the multi-mode interference coupling area particularly emphasizes a higher process tolerance, so that even if there are slight changes in size during the processing process, it will not significantly affect the final device performance. Through these improvement measures, not only the manufacturing 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 present invention not only overcomes the various shortcomings of traditional coupling devices, achieves a smaller size, higher bandwidth and lower loss, but also reduces the complexity of device preparation and has better mechanical stability.

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

[0056] A lithium niobate coupling layer 3 is disposed on the insulating layer 2, wherein the lithium niobate coupling layer 3 is mirror-symmetrical along its horizontal center line and includes five regions connected in sequence: a coupling waveguide region 31, a mode conversion region a32, a multimode interference coupler region 33, a mode conversion waveguide region 34, a slab mode conversion region 35, and a slab region 36;

[0057] A lithium niobate waveguide layer 4 is disposed on the lithium niobate coupling layer 3, wherein the lithium niobate waveguide layer 4 includes: a mode conversion region b41 stacked on the slab mode conversion region 35 and a ridge waveguide region 42 stacked on the slab region 36;

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

[0059] The working principle of the thin-film lithium niobate end face coupler of the present invention is: when light is input to the thin-film lithium niobate end face coupler, the thin-film lithium niobate end face coupler achieves 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 area 31, and then with the increase of the width of each tapered waveguide in the coupling waveguide area 31 and the spacing between the two tapered waveguides, the light field is evenly divided into two parts, and then the light field size further increases in the mode conversion area a32 and excites high-order modes, and then enters the multi-mode interference coupler area 33 to cause multi-mode interference, and finally the two beams of light are combined into one beam and output from the multi-mode interference coupler area 33, entering the mode conversion waveguide area 34, the width of which gradually decreases, causing the light field size to further decrease, and when injected into the flat mode conversion area 35, the light field size and position are converted, and the light field is widened again after entering the mode conversion area b41, finally realizing the optical mode conversion from optical fiber to ridge waveguide.

[0061] In some embodiments of the present invention, the material of the substrate 1 is selected from silicon or quartz stone to provide mechanical support.

[0062] In some embodiments of the present invention, the insulating layer 2 is a thermally oxidized silicon dioxide layer, which is used to provide an interface suitable for growing lithium niobate material.

[0063] In some embodiments of the present invention, the lithium niobate coupling layer 3 is mirror-symmetrical along its horizontal center line, and includes five regions connected in sequence, namely: a coupling waveguide region 31 with a conical width gradient and non-central symmetry, a mode conversion region a32 with a conical linear width variation and symmetry, a multimode interference coupling region 33 with constant width, a mode conversion waveguide region 34 with an inverted conical width gradient, a planar mode conversion region 35 with a conical width, and a planar region 36 with constant width.

[0064] As any specific embodiment of the present invention, the coupling waveguide region 31 is composed of two symmetrical conical waveguides, and the distance in the vertical direction of the two conical waveguides increases linearly from one end far away from the mode conversion region a32 to one end close to the mode conversion region a32; and the width of any one of the conical waveguides decreases linearly from one end close to the mode conversion region a32 to one end far away from the mode conversion region a32, forming a tip.

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

[0066] As any specific embodiment of the present invention, the mode conversion region a32 is composed of two symmetrical frustum structures, and 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 0; and the sum of the widths of the two frustums close to one end of the multimode interference coupler region 33 is less than the width of the multimode interference coupler region 33, so as to optimize the coupling efficiency and mode field uniformity; the starting width of any of the frustums is equal to the end width of any of the conical waveguides.

[0067] As any specific embodiment of the present invention, the minimum spacing of the mode conversion region a32 is the line size that can be achieved by the photolithography machine used, which is generally not more than 500 nm.

[0068] In the present invention, the multimode interference coupler region 33 excites high-order modes at the input of the light field, and interference occurs between the modes. A 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 undergoes self-imaging therein. For the multimode interference coupler region 33, its width and length are very important. The width determines the number of high-order modes that can be excited in the multimode interference region, affecting the imaging quality and tolerance of the multimode interference. 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 invention, the width of the multimode interference coupler region 33 is equal and does not exceed 5μm; the effective refractive index of the multimode interference coupler region 33 is calculated based on 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 / λ 0 );

[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) to (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, λ 0 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 area 31 is used to couple the optical fiber output light field and perform mode conversion. Then the light field is injected into the multimode interference coupler area 33 through the mode conversion area a32 to excite high-order modes and generate interference. The light field after interference realizes the size conversion of the light field, and then enters the mode conversion waveguide area 34 to convert the square size. Then, the light field is gradually converted to the waveguide layer after passing through the flat mode conversion area 35, and finally realizes the conversion from the optical fiber output light field to the ridge waveguide mode.

[0074] In some embodiments of the present invention, the thin-film lithium niobate waveguide layer 4 includes: 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 present invention, the mode conversion region b41 is connected to the ridge waveguide region 42 , and the mode conversion region b41 is a conical structure with a linearly decreasing width, and its 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, and 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 slab region, and together they form a lithium niobate ridge waveguide, which plays a role in binding the light mode and connecting with other devices.

[0077] In some embodiments of the present invention, Figure 2 As shown, the thickness of the substrate 1 is 600 nm, the thickness of the slab region 36 and the ridge waveguide layer 42 are both 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 present invention, the graphic processing methods of thin-film lithium niobate include: femtosecond laser direct writing processing, mechanical cutting and wet etching, etc., among which the graphic size and accuracy of femtosecond laser processing are limited by the laser spot size and the displacement accuracy of the mechanical stage; the graphic flexibility of mechanical cutting processing is low and is only suitable for straight waveguide structures; the wet etching processing method has different corrosion rates for lithium niobate of different crystalline phases, resulting in graphic processing that is only suitable for specific crystalline phases; dry etching is suitable for the processing of thin-film lithium niobate due to its high precision, high flexibility and high yield.

[0079] Based on this, as any specific embodiment of the present invention, the etching depth of the substrate 1 of the present invention is 300nm, and the lithium niobate coupling layer 3 and the lithium niobate waveguide layer 4 of the present invention are also formed by dry etching, the etching depths are both 300nm, and the etching side wall angles are both between 72°-73°.

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

[0081] S1. Measure the thickness of thin film lithium niobate wafer samples and clean the samples;

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

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

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

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

[0086] S6, removing the residual hard mask from the sample obtained in step S5 to obtain a lithium niobate end face coupler pattern;

[0087] S7, repeating steps S1-S6 for the sample obtained in step S6 to prepare a second portion of lithium niobate waveguides;

[0088] S8, heating the sample obtained in step S7 in an ammonia / hydrogen peroxide water bath and a concentrated sulfuric acid / hydrogen peroxide water bath in sequence;

[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 coupled waveguide;

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

[0092] The first part of the lithium niobate waveguide includes a slab region 36 and a lithium niobate waveguide layer 4; the second part of the lithium niobate waveguide includes a coupling waveguide region 31, a mode conversion region a32, a multimode interference coupler region 33, a mode conversion waveguide region 34 and a slab mode conversion region 35.

[0093] The inventor measures the thickness of the initial thin film lithium niobate layer to ensure that it meets the design requirements, and cleans the film to remove surface pollutants and impurities to provide a clean substrate for subsequent processes; deposits a layer of silicon dioxide or chromium metal as a hard mask on the cleaned substrate 1, which is used to protect the parts that do not need to be etched and define the pattern during the photolithography process; after spin coating the photoresist, the desired pattern is exposed on the photoresist through photolithography technology, and then developed and fixed to form the desired etching pattern; removes the residual photoresist generated during the photolithography process, and uses an etching process to remove the pattern from the The photoresist is transferred to the hard mask below; 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 desired 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 slab area 36 that have been patterned; in order to construct a multi-layer structure, steps S1 to S6 are repeated to prepare a coupled waveguide area 31, a mode conversion area a32, a multimode interference coupler area 33, a mode conversion waveguide area 34 and a slab mode conversion area 35, thereby increasing the complexity and functionality of the device.

[0094] In the above process, the samples of each step are sequentially heated in an ammonia / hydrogen peroxide water bath and concentrated sulfuric acid / hydrogen peroxide water bath to remove possible residual organic matter and other impurities to ensure the cleanliness of subsequent processes. Then, a layer of silicon dioxide is deposited on the treated sample as a cladding to surround the lithium niobate waveguide layer, provide optical isolation and help control the propagation path of light. The sample with the silicon dioxide cladding 5 is cut to expose the internal lithium niobate coupling waveguide, ready for the final processing step; the end face of the cut sample is finely polished to ensure that the end face is smooth and flat, reduce light scattering loss, thereby improving coupling efficiency, and finally complete the preparation of the thin film lithium niobate end face coupler.

[0095] In some embodiments of the present invention, step S1, the cleaning process includes: at room temperature, first, using one of acetone, methanol or isopropanol to rinse the thin film lithium niobate; then soaking in concentrated sulfuric acid for 5 minutes, ultrasonically treating with acetone for 15 minutes; then treating with hydrofluoric acid buffer solution for 10 seconds, and then using a mixed solution of hydrochloric acid: hydrogen peroxide: deionized water with a mass ratio of 1:1:6 at 75°C for 10 minutes, and then rinsing with one of acetone, methanol or isopropanol at room temperature, and finally washing with deionized water and drying.

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

[0097] As any specific embodiment of the present invention, 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 invention, in step S3, spin coating is performed using spr955 at a speed of 4000, a pre-bake temperature of 100° C. for 90 seconds, and a post-bake temperature of 115° C. for 90 seconds.

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

[0100] In some embodiments of the present invention, 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 invention, in steps S6 and S7, a wet process is used to remove the mask.

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

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

[0104] In some embodiments of the present invention, in step S9, plasma enhanced chemical vapor deposition is used at a temperature of 300°C.

[0105] In some embodiments of the present invention, in steps S10 and S11, the cutting needs to cut through the thickness of the entire wafer to make it into independent units without scratching the waveguide; and the polishing requires polishing until the entire end surface is smooth.

[0106] The following are specific embodiments of the present invention.

[0107] Example 1

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

[0109] Step S1, using a wafer film thickness meter to measure the thickness of the thin film lithium niobate layer, and then using a wafer cleaning machine to clean the thin film lithium niobate layer to remove organic and inorganic impurities on the wafer as a substrate 1; the cleaning process includes: at room temperature, first use one of acetone, methanol or isopropanol to rinse the thin film lithium niobate; then soak in concentrated sulfuric acid for 5 minutes, and then ultrasonically treat with acetone for 15 minutes; then treat with hydrofluoric acid buffer solution for 10 seconds, 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 minutes, and then rinse with one of acetone, methanol or isopropanol at room temperature, and finally rinse with deionized water and dry.

[0110] Step S2: growing silicon dioxide with a thickness greater than 500 nm on the substrate 1 obtained in step S1 by using a plasma enhanced chemical deposition device, as a hard mask for etching of lithium niobate patterns.

[0111] S3. Spin-coat the sample obtained in step S2 with spr955 at 4000 rpm to evenly coat the photoresist. Pre-bake at 100° C. for 90 seconds and post-bake at 115° C. for 90 seconds. Use a photolithography machine to define the etching pattern and perform development and fixing.

[0112] S4. On the sample obtained in step S3, use an ashing device to remove the residual glue, wherein the plasma power of the ashing device is set to 300W, and the working time is 120s; then use an etching device, and the etching conditions are: plasma power is 1100W, RF power is 300W, and the working time is 500s, to transfer the pattern from the photoresist to the hard mask.

[0113] S5. On the sample obtained in step S4, remove the hard mask, and then use plasma etching equipment 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 the flat plate area 36 and the lithium niobate waveguide layer 4.

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

[0115] S7. Repeat steps S1-S6 for the sample obtained in step S6 to prepare a second layer of lithium niobate waveguide, wherein the second part of the lithium niobate waveguide includes a coupling waveguide region 31, a mode conversion region a32, a multimode interference coupler region 33, a mode conversion waveguide region 34 and a slab mode conversion region 35.

[0116] S8. Chemically treat the sample obtained in step S7, use an ammonia water / hydrogen peroxide water bath to remove the redeposition products generated by etching, and then use a concentrated sulfuric acid / hydrogen peroxide water bath to remove impurities; the volume ratio of the ammonia water, hydrogen peroxide and deionized water is 1:1:5, wherein the ammonia concentration is 25%, the hydrogen peroxide concentration is 30%, the water bath heating temperature is 40°C, and the time is 10 minutes; 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 minutes.

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

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

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

[0120] Example 2

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

[0122] Step S1, using a wafer film thickness meter to measure the thickness of the thin film lithium niobate layer, and then using a wafer cleaning machine to clean the thin film lithium niobate layer to remove organic and inorganic impurities on the wafer as a substrate 1; the cleaning process includes: at room temperature, first use one of acetone, methanol or isopropanol to rinse the thin film lithium niobate; then soak in concentrated sulfuric acid for 5 minutes, and then ultrasonically treat with acetone for 15 minutes; then treat with hydrofluoric acid buffer solution for 10 seconds, 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 minutes, and then rinse with one of acetone, methanol or isopropanol at room temperature, and finally rinse with deionized water and dry.

[0123] Step S2: growing silicon dioxide with a thickness greater than 500 nm on the substrate 1 obtained in step S1 by using a plasma enhanced chemical deposition device, as a hard mask for etching of lithium niobate patterns.

[0124] S3. Spin-coat the sample obtained in step S2 with spr955 at 4000 rpm to evenly coat the photoresist. Pre-bake at 100° C. for 90 seconds and post-bake at 115° C. for 90 seconds. Use a photolithography machine to define the etching pattern and perform development and fixing.

[0125] S4. On the sample obtained in step S3, use an ashing device to remove the residual glue, wherein the plasma power of the ashing device is set to 300W, and the working time is 120s; then use an etching device, and the etching conditions are: plasma power is 1100W, RF power is 300W, and the working time is 500s, to transfer the pattern from the photoresist to the hard mask.

[0126] S5. On the sample obtained in step S4, remove the hard mask, and then use plasma etching equipment 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 the flat plate area 36 and the lithium niobate waveguide layer 4.

[0127] S6. Remove the residual hard mask from the sample obtained in step S5, heat the metal mask with a metal etching solution in a 40° C. water bath for 120 seconds; heat the silicon oxide mask with a hydrofluoric acid buffer solution in a 40° C. water bath for 360 seconds; and obtain a lithium niobate pattern.

[0128] S7. Repeat steps S1-S6 for the sample obtained in step S6 to prepare a second layer of lithium niobate waveguide, wherein the second part of the lithium niobate waveguide includes a coupling waveguide region 31, a mode conversion region a32, a multimode interference coupler region 33, a mode conversion waveguide region 34 and a slab mode conversion region 35.

[0129] S8. Chemically treat the sample obtained in step S7, use ammonia water / hydrogen peroxide water bath heating to remove the redeposition products generated by etching, and then use concentrated sulfuric acid / hydrogen peroxide water bath heating to remove impurities; the volume ratio of ammonia water, hydrogen peroxide and deionized water is 1:1:5, wherein the concentration of ammonia water is 28%, the concentration of hydrogen peroxide is 30%, the water bath heating temperature is 80°C, and the time is 30 minutes; the volume ratio of concentrated sulfuric acid and hydrogen peroxide is 3:1, wherein the concentration of concentrated sulfuric acid is 98.3%, the concentration of hydrogen peroxide is 30%, the water bath heating temperature is 80°C, and the time is 30 minutes.

[0130] S9, growing silicon dioxide as an upper cladding layer of the end coupler on the sample obtained in step S8 at 300° C. using a plasma enhanced chemical deposition device;

[0131] S10, cutting the sample obtained in step S9 by using a wafer dicing machine to expose the lithium niobate coupled waveguide;

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

[0133] Comparative Example 1

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

[0135] Comparative Example 2

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

[0137] Comparative Example 3

[0138] The difference from Example 1 is that in step S7, only 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 couplers prepared in Example 1 and Comparative Examples 1 to 3. Figure 4 shown.

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

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

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

[0143] In summary, the thin-film lithium niobate end coupler with high process tolerance provided by the embodiment of the present invention overcomes the problem that the traditional end coupler uses the method of continuously reducing the waveguide width to achieve the change of the pattern spot size, which leads to stringent requirements on the line accuracy of the processing equipment. The structure increases the line width by using a double-conical coupling method, while ensuring that the spacing between the double cones is greater than 500nm, reducing the resolution requirements of the patterning equipment. Therefore, it can be prepared on a large scale using a lithography machine, which is conducive to the realization of large-scale photonic integration.

[0144] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A thin film lithium niobate end coupler, characterized in that: The invention comprises, from bottom to top: a substrate (1), an insulating layer (2) arranged on the substrate (1); A lithium niobate coupling layer (3) is disposed on the insulating layer (2), wherein the lithium niobate coupling layer (3) is mirror-symmetrical along its axis and comprises 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 slab mode conversion region (35) and a slab region (36); A lithium niobate waveguide layer (4) disposed on the lithium niobate coupling layer (3), the lithium niobate waveguide layer (4) comprising: a mode conversion region b (41) stacked on the slab mode conversion region (35) and a ridge waveguide region (42) stacked on the slab region (36); and a silicon dioxide cladding layer (5) arranged on the surface of the lithium niobate coupling layer (3); Among them, 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 geometric shape and refractive index distribution at different positions.

2. The thin film lithium niobate end coupler according to claim 1, characterized in that: In the lithium niobate coupling layer (3), the coupling waveguide region (31) is composed of two symmetrical tapered waveguides, and the distance between the two tapered waveguides in the vertical direction increases linearly from an end away from the mode conversion region a (32) to an end close to the mode conversion region a (32); The width of any of the tapered waveguides decreases linearly from an end close to the mode conversion region a (32) to an end away from the mode conversion region a (32), thereby forming a tip.

3. The thin film lithium niobate end coupler according to claim 2, characterized in that: The width of the tip does not exceed 500 nm.

4. The thin film lithium niobate end coupler according to claim 3, characterized in that: The mode conversion region a (32) is composed of two symmetrical frustum structures, and the distance in the vertical direction of the two frustum structures from one end close to the coupling waveguide region (31) to one end close to the multimode interference coupler region (33) decreases linearly and is not 0; The sum of the widths of the two cones close to one end of the multimode interference coupler region (33) is smaller than the width of the multimode interference coupler region (33); and the starting width of any cone is equal to the end width of any tapered waveguide.

5. The thin film lithium niobate end coupler according to claim 4, characterized in that: The widths of the multimode interference coupler regions (33) are equal and do not exceed 5 μm; The effective refractive index of the multimode interference coupler region (33) must satisfy the condition shown in formula (I): L π =3 / 4*(n r W e 2 / λ0) (Ⅰ); The length of the multimode interference coupler region (33) must satisfy the condition shown in formula (II): L=3×L π / 4 (Ⅱ); In formula (I) to (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).

6. The thin film lithium niobate end coupler according to claim 1, characterized in that: In the lithium niobate waveguide layer (4), the mode conversion region b (41) is connected to the ridge waveguide region (42), the mode conversion region b (41) is a conical structure with a linearly decreasing width, and the tip direction of the conical structure is consistent with the tip direction of the coupling waveguide region (31).

7. A method for preparing the thin film lithium niobate end coupler according to any one of claims 1 to 6, characterized in that: The method comprises the following steps: S1. Measure the thickness of thin film lithium niobate wafer samples and clean the samples; S2, depositing silicon dioxide or chromium metal on the sample obtained in step S1 as a hard mask for etching the lithium niobate end face coupler pattern; S3, spin coating photoresist on the sample obtained in step S2, and developing and fixing the etched pattern; S4, on the sample obtained in step S3, removing the residual glue and transferring the pattern from the photoresist to the hard mask; S5, on the sample obtained in step S4, removing the hard mask, and then transferring the pattern from the silicon oxide mask to the first part of the lithium niobate waveguide; S6, removing the residual hard mask from the sample obtained in step S5 to obtain a lithium niobate end face coupler pattern; S7, repeating steps S1-S6 for the sample obtained in step S6 to prepare a second portion of lithium niobate waveguides; S8, heating the sample obtained in step S7 in an ammonia / hydrogen peroxide water bath and a concentrated sulfuric acid / hydrogen peroxide water bath in sequence; S9, depositing a silicon dioxide cladding layer (5) on the sample obtained in step S8; S10, cutting the sample obtained in step S9 to expose the lithium niobate coupled waveguide; S11, performing end surface polishing on the sample obtained in step S10 to complete the preparation of the thin film lithium niobate end surface coupler; The first portion of the lithium niobate waveguide includes a slab region (36) and a lithium niobate waveguide layer (4); the second portion 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 slab mode conversion region (35).

8. The method for preparing a thin film lithium niobate end coupler according to claim 7, characterized in that: In step S8, the volume ratio of ammonia water, hydrogen peroxide and deionized water is 1:1:5, wherein the mass concentration of ammonia water is 25%-28%, the mass concentration of hydrogen peroxide is 30%, and the water bath heating temperature is 40-80°C; The volume ratio of concentrated sulfuric acid to hydrogen peroxide is 7:3 or 3:1, wherein the mass concentration of concentrated sulfuric acid is 98.3%, the mass concentration of hydrogen peroxide is 30%, and the water bath heating temperature is 40-80°C.

9. The method for preparing a thin film lithium niobate end coupler according to claim 8, characterized in that: The etching depths of the thin-film lithium niobate coupling layer (3) and the lithium niobate waveguide layer (4) are equal, and the etching side wall angle is between 72° and 73°.

10. The method for preparing the thin film lithium niobate end coupler according to claim 9, characterized in that: The thickness of the ridge waveguide region (42) is less than the thickness of the thin film lithium niobate layer.

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