Polarization rotator and design and preparation method thereof

Abstract

The invention discloses a polarization rotator and a design and preparation method thereof. The polarization rotator comprises a straight waveguide structure, a groove is formed in the top of a waveguide, the width of the bottom edge of the waveguide is gradually widened / narrowed from the starting end to the tail end, and the eccentric distance between the middle point of the bottom edge of the waveguide and the middle point of the bottom of the groove is gradually widened from 0 to the maximum value and then gradually narrowed to 0. According to the structural design, direct conversion between two different polarization basic modes TM0-TE0 can be realized in the straight waveguide without a bending structure, the conversion efficiency is high, and the performance is excellent; the device is simple in structure and small in transverse size; according to the invention, the waveguide structure with non-uniform thickness can be formed only through one round of photoetching and etching process, so that the processing flow is greatly simplified, the processing cost is reduced, and the processing error caused by overlay deviation limitation among multiple rounds of photoetching and the influence on the performance of the polarization rotator are avoided; meanwhile, the polarization rotator is close to the strip-shaped waveguide in structure, and energy loss in the transition process of the polarization rotator and the strip-shaped waveguide is small.

Description

Technical Field

[0001] The present invention relates to the field of integrated optical technologies, and particularly to a polarization rotator and its design and fabrication methods. Background Art

[0002] In recent years, optical chips based on Photonic Integrated Circuit (PIC) technology have been widely used in fields such as AI computing power chip interconnection, high-speed optical communication, photoelectric sensing, biomedical detection, etc., and play an important role as core components.

[0003] A photonic waveguide is the basic structure of a PIC. The modes it supports can generally be divided into two types according to the different directions of the electric field / magnetic field distribution: transverse electric (TE; the electric field is distributed horizontally and the magnetic field is distributed vertically) and transverse magnetic (TM; the magnetic field is distributed horizontally and the electric field is distributed vertically).

[0004] In practical applications, the geometric shape of a photonic waveguide usually has different dimensions in the horizontal and vertical directions. Therefore, the mode performance of different polarizations in a photonic waveguide varies greatly, and polarization control is an important part of the design of an optical chip. On the one hand, each device in an optical chip usually can only work in a specific single polarization mode, and the mixing of other polarization modes will form interference signals, affecting the working performance of the device and even the entire system. On the other hand, targeted and proper utilization of different polarization modes can improve the working performance of the device and even the entire system. For example, using Polarization-Division Multiplexing (PDM) in optical communication can double the number of channels supported by a single wavelength.

[0005] A polarization rotator (also known as a polarization rotator) can convert the input light of one polarization mode into light of another polarization mode and is an important polarization control device. In terms of working principle, polarization rotation is usually achieved through mode hybridization (also known as mode mixing) between the two converted modes. In practical applications, the basic modes TE0 and TM0 of two different polarizations are the most commonly used, but the direct conversion between them is difficult and cannot be achieved through ordinary (the simplest to process) strip waveguides. Usually, the conversion between the TE0 and TM0 modes requires the TE1 mode as an intermediary (i.e., "TE0-TE1-TM0") and can only be achieved in two steps, which results in the devices for realizing the conversion between the TE0 and TM0 modes usually having a relatively complex structure and unsatisfactory comprehensive performance. Although some waveguides with special structures can also achieve the hybridization and direct conversion between the TE0 and TM0 modes, these structures are also complex and usually require multiple photolithography and etching processes to form, resulting in high processing costs and their actual performance being limited by the alignment deviation between multiple photolithography processes; at the same time, these waveguides with special structures also often differ greatly from the commonly used strip waveguides before and after the polarization rotator in the system, resulting in large energy losses during the transition of optical signals between the two.

[0006] New optical chip materials represented by lithium niobate and lithium tantalate can be used in the manufacture of low-power and large-bandwidth electro-optic modulators, and their application prospects are very broad. However, compared with traditional optical chip materials such as silicon, silicon nitride, and silicon dioxide, the dry etching of lithium niobate and lithium tantalate is more difficult. Specifically, it is difficult to obtain vertical (90°) waveguide sidewalls during their dry etching process, and the maximum sidewall angle that can be achieved by the existing general process capabilities is about 75° (materials such as silicon, silicon nitride, and silicon dioxide can usually reach more than 85° and approach 90°), which makes the cross-section of lithium niobate and lithium tantalate waveguides usually a trapezoid with a narrow upper part and a wide lower part rather than a rectangle; at the same time, due to the deposition of difficult-to-volatilize products (such as lithium fluoride) on the material surface during the etching process, the dry etching rate of lithium niobate and lithium tantalate will decrease significantly when the size of the etching area is small (this size threshold is usually about several hundred nanometers, depending on the actual process parameters).

[0007] In addition, lithium niobate and lithium tantalate are birefringent materials. If the optical axis of their thin films is selected to be parallel to the thin film during processing, the thin film will have different refractive indices in different directions. In this case, mode crosstalk and / or energy loss may occur when the optical signal passes through any curved waveguide structure due to the refractive index change.

[0008] These material properties above make it very difficult to design and fabricate devices with complex functions and structures, such as polarization rotators, on lithium niobate and lithium tantalate material platforms. Moreover, due to the limitations of the above-mentioned various factors, the performance is not satisfactory. Summary of the Invention

[0009] An object of the present invention is to provide a polarization rotator and its design and preparation method in view of the deficiencies of the prior art, which can achieve direct conversion between two different polarization fundamental modes, namely TM0 and TE0, in a straight waveguide without a bending structure.

[0010] According to the first aspect of the present specification, a polarization rotator is provided, including a straight waveguide structure. There is a groove at the top of the waveguide. The bottom width of the waveguide gradually becomes wider / narrower from the starting end to the ending end. When the eccentricity between the midpoint of the bottom of the waveguide and the midpoint of the bottom of the groove is constantly 0, the bottom width of the waveguide at the equal effective refractive index of the two modes is denoted as , and the eccentricity gradually becomes wider from 0 until the bottom width of the waveguide is equal to , and then gradually becomes narrower to 0.

[0011] Furthermore, the material of the straight waveguide structure is lithium niobate, lithium tantalate, barium titanate, lead titanate, lead zirconate titanate or sodium bismuth titanate.

[0012] Furthermore, the bottom width of the groove does not exceed 300 nm, and the depth of the groove does not exceed half of the waveguide thickness.

[0013] Furthermore, the top widths of the left and right sides of the straight waveguide structure are not less than 150 nm.

[0014] Furthermore, the relationship between the eccentricity and the bottom width of the waveguide is a piecewise linear relationship, a polynomial relationship, an exponential relationship or a logarithmic relationship.

[0015] Furthermore, the relationship between the bottom width of the waveguide and the waveguide length is a piecewise linear relationship, a polynomial relationship, an exponential relationship or a logarithmic relationship.

[0016] According to the second aspect of the present specification, a design method of a polarization rotator is provided. The design method includes the following steps:

[0017] (1) Determine the working wavelength, waveguide thickness, materials of each layer and the refractive index of each layer at the working wavelength according to the actual application scenario;

[0018] (2) Give the relationship between the etching rate of the groove to be etched and the bottom width of the groove, and the bevel angles of each side according to the pre-calibrated processing technology effect;

[0019] (3) Determine the bottom width of the waveguide at the starting end of the polarization rotator and the values at the ends ; Determine the width of the bottom of the groove and determine the depth of the groove according to the relationship in step (2) ;

[0020] (4) Determine the eccentricity The relationship with the width of the bottom edge of the waveguide ; At the starting end and the ending end of the polarization rotator, ; along with widening / narrowing, it gradually increases, when the value is the largest and is denoted as and then along with widening / narrowing, it gradually decreases;

[0021] (5) The polarization rotator is divided into two sections; the first section, from gradually changes to , increases from 0 to according to the relationship in step (4); the second section, from gradually changes to , decreases from to 0 according to the relationship in step (4).

[0022] Furthermore, in step (4), the relationship between the TE ratio in the waveguide mode and and is obtained through simulation. By increasing or decreasing , the non - hybrid mode region that makes no contribution to polarization conversion is removed.

[0023] Furthermore, when the waveguide material is a birefringent material (such as lithium niobate, lithium tantalate), in step (1), the relationship between the crystal phase of the material and the coordinate system of the device needs to be set.

[0024] Furthermore, in step (5), the optimal lengths of the first and second sections of the polarization rotator are determined through simulation, and the optimization goal is to minimize the total length while meeting the requirements of polarization conversion efficiency.

[0025] According to the third aspect of this specification, a method for manufacturing a polarization rotator is provided, and this manufacturing method includes the following steps:

[0026] (1) Prepare a wafer, whose structure from bottom to top is a substrate, a lower cladding, and a waveguide core layer;

[0027] (2) Form a mask layer on the surface of the wafer;

[0028] (3) Through photolithography and development, a mask pattern structure is formed on the mask layer. For a hard mask, etching is also required after development. The mask pattern structure consists of two horizontally distributed rectangles, and there is a narrow gap between the two parts of the mask pattern;

[0029] (4) Through etching, a straight waveguide structure and a remaining mask are formed on the waveguide core layer. The etching rate at the narrow gap is slower than that in other regions, so as to form a groove on the top of the waveguide;

[0030] (5) After removing the remaining mask, the upper cladding is deposited.

[0031] The beneficial effects of the present invention are as follows: The polarization rotator designed by the present invention can achieve direct conversion between the fundamental modes of two different polarizations, TM0 and TE0, in a straight waveguide without a bending structure, with high conversion efficiency and excellent performance; the device structure is simple and the lateral size is very small; in addition, the present invention makes full use of the characteristics of materials such as lithium niobate and lithium tantalate in the processing technology, and a waveguide structure with a non-uniform thickness can be formed only through one round of photolithography and etching processes (traditionally, two or more rounds of photolithography and etching are required), which greatly simplifies the processing flow, reduces the processing cost, and avoids the processing errors caused by the interlayer registration deviation limitations between multiple rounds of photolithography and their impact on the performance of the polarization rotator; at the same time, the structure of the polarization rotator is very close to that of the strip waveguide, and the energy loss during the transition between the two is very small. Description of the Drawings

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0033] Figure 1 Schematic diagram of the waveguide cross-section structure of the polarization rotator provided in the first part of the present disclosure;

[0034] Figure 2 Schematic diagram of the straight waveguide structure of the polarization rotator provided in the first part of the present disclosure;

[0035] Figure 3 In the application example of the polarization rotator provided in the second part of the present disclosure and Schematic diagram of the change with the waveguide length;

[0036] Figure 4 In the application example of the polarization rotator provided in the second part of the present disclosure, the effective refractive index of the mode supported by the waveguide varies with variation ( The case where it is constantly 0);

[0037] Figure 5 For the effective refractive index of the mode supported by the waveguide in the polarization rotator application example provided in the second part of the present disclosure, which varies with while according to Figure 3 the relationship shown in);

[0038] Figure 6 Schematic diagram of the wafer structure provided in the third part of the present disclosure;

[0039] Figure 7 Schematic diagram of forming a mask layer on the wafer surface provided in the third part of the present disclosure;

[0040] Figure 8 Schematic diagram of the mask layer forming a mask pattern structure provided in the third part of the present disclosure;

[0041] Figure 9 Schematic diagram of the mask pattern structure and various dimensional parameters provided in the third part of the present disclosure;

[0042] Figure 10 Schematic diagram of forming a straight waveguide structure and the remaining mask provided in the third part of the present disclosure;

[0043] Figure 11 Schematic diagram of the structure and various dimensional parameters after etching completion provided in the third part of the present disclosure;

[0044] Figure 12 Schematic diagram of removing the remaining mask provided in the third part of the present disclosure;

[0045] Figure 13 Schematic diagram of depositing an upper cladding provided in the third part of the present disclosure. Detailed implementation manners

[0046] The present invention will be further described in detail below with reference to the accompanying drawings and specific implementation manners.

[0047] It should be clear that the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without creative efforts belong to the scope of protection of the present application.

[0048] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the application. The singular forms of "a", "the", and "said" used in the embodiments of the present application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0049] The first part of the embodiments of the present disclosure provides a polarization rotator, which realizes the direct conversion between the fundamental modes of TM0 and TE0 with two different polarizations in a straight waveguide by using the mode hybridization that occurs during the continuous change of the waveguide cross-sectional structure.

[0050] The adopted waveguide cross-sectional structure is as Figure 1 shown, where 102 is the lower cladding, 105 is the upper cladding, and 202 is the straight waveguide structure. The waveguide thickness is denoted as t, and the bottom width of the waveguide is denoted as w; the waveguide has a groove at the top, and the groove depth is denoted as d, and the bottom width of the groove is denoted as w1.

[0051] In the actual processing of the waveguide, limited by the process capabilities, it is difficult to make the side of the waveguide completely vertical, especially for some new optical chip materials represented by lithium niobate. This makes the overall waveguide cross-section a trapezoid with a narrow upper part and a wide lower part, while the top groove is a trapezoid with a wide upper part and a narrow lower part. The width of the left top edge of the waveguide is denoted as w2, and the width of the right top edge of the waveguide is denoted as w3; the width of the top of the groove is denoted as w4.

[0052] The horizontal distance (eccentricity) between the midpoint of the bottom of the waveguide and the midpoint of the bottom of the groove is denoted as e. When e = 0, the waveguide structure is symmetric left and right. In this case, the change in the width of the waveguide cannot cause hybridization between the TE0 and TM0 modes; when e ≠ 0, the left-right symmetry of the waveguide structure is broken. In this case, the change in the width of the waveguide can cause hybridization between the TE0 and TM0 modes, thereby realizing the conversion between the two polarization modes based on mode hybridization.

[0053] The bottom width of the waveguide gradually widens / narrows from the starting end (input end) to the ending end (output end). Hereinafter, the conversion from TM to TE is taken as an example. As Figure 2 shown, the bottom width of the waveguide gradually widens (for the case of converting TE to TM, the bottom width of the waveguide gradually narrows). When Δ is constantly 0, the waveguide bottom width at the point where the effective refractive indices of the two modes are equal (reflected by the intersection of the relationship curve between the effective refractive index of the waveguide mode and the bottom width of the waveguide) is denoted as w0. It gradually widens from 0 until the bottom width of the waveguide is equal to w0, and then gradually narrows to 0.

[0054] Through such a structural design, direct conversion between the two most commonly used basic modes, TE0 and TM0, can be achieved. Among them, the lateral occupied space of the straight waveguide structure is extremely small; there is no turning structure, and on the birefringent material platform represented by lithium niobate, there is no problem of material refractive index change caused by turning; and the waveguide shape is very close to the common strip waveguide (only having a small groove at the top), and it can be transitioned with extremely low loss, with good practicability.

[0055] The second part of the embodiments of the present disclosure provides a design method for a polarization rotator, and the specific steps are as follows:

[0056] 1. Determine the working wavelength, waveguide thickness , each layer of material and the refractive index of each layer at the working wavelength. For birefringent materials (such as lithium niobate and lithium tantalate), the coordinate system relationship between the material crystal phase and the device also needs to be set.

[0057] 2. Give the relationship between the etching rate of the groove to be etched and the bottom width of the groove , as well as the bevel angles of each side, according to the pre-calibrated processing effect. The processing effect can be adjusted in advance according to the application scenario requirements.

[0058] 3. Determine the bottom width of the waveguide at the starting end of the polarization rotator and the value at the end ; determine the bottom width of the groove , and determine the groove depth according to the relationship in step 2. These three values are determined simultaneously, mainly considering:

[0059] (1) It must be satisfied that during the change process of the polarization rotator from the starting end to the end (here only focusing on the case where is constantly 0), TM0 changes from the first mode (the mode effective refractive index is the highest) to the second mode (the mode effective refractive index is the second highest) and TE0 changes from the second mode to the first mode (taking the TM to TE conversion as an example, for the TE to TM conversion case, the input end and the output end can be inverted). The bottom width of the waveguide at the place where the mode effective refractive indices of TM0 and TE0 are equal is denoted as .

[0060] (2) The groove part should not be too large. It is advisable not to exceed 300 nm, and correspondingly should not exceed half. In this way, the waveguide structure is still close to the trapezoidal waveguide, and the loss during the transition is very small. A larger groove can enable the polarization rotator to complete polarization conversion within a shorter length, but it is more different from the trapezoidal waveguide, and the loss during the transition is greater.

[0061] (3) There are no overly small structural dimensions in the overall structure of the polarization rotator, especially the widths of the top edges on the left and right sides of the starting end. 、 These values should not be too small. Preferably, these two values should not be less than 150 nm.

[0062] 4. Determine the relationship that varies with . First, or When , they are the starting end and the ending end of the polarization rotator. varies with and gradually increases as the width widens. When , the value is the largest, i.e., . After that, varies with and decreases as the width widens. Designing the polarization rotator in this way results in better performance. The specific relationship that varies with can be a piecewise linear relationship, a polynomial relationship, an exponential relationship, or a logarithmic relationship.

[0063] A relatively large value can enable the polarization rotator to complete polarization conversion within a shorter length. However, it should be noted that there are no overly small structural dimensions in the overall structure of the polarization rotator, especially to the widths of the top edges on the left and right sides of this section 、 . Preferably, these two values should not be less than 150 nm.

[0064] On this basis, the relationship between the TE ratio in the waveguide mode and and is obtained through simulation (a ratio equal to 1 represents the TE mode, a ratio equal to 0 represents the TM mode, and a value between 0 and 1 represents a mixed mode). It is possible to check whether there is a relatively large non - mixed mode region, which makes no contribution to polarization conversion. By increasing or decreasing (without modifying other parameters), it is possible to enable the polarization rotator to complete polarization conversion within a shorter length, thereby improving the performance of the polarization rotator.

[0065] 5. The polarization rotator is divided into two sections. In the first section, the length of the polarization rotator increases from 0 to , varies from to , and increases from 0 to according to the relationship in step 4. In the second section, the length of the polarization rotator From increase to , From vary to , According to the relationship in step 4, from decrease to 0. In both paragraphs, along with The specific relationship of the variation can be a piecewise linear relationship, a polynomial relationship, an exponential relationship or a logarithmic relationship. According to the requirements of the application scenario, determine , the optimal values through simulation, so that the polarization conversion efficiency (output TE0 power / input TM0 power) is relatively high and the total length is relatively short. Usually, the polarization conversion efficiency needs to be higher than 99%; if necessary, the values of other parameters and the relationships between parameters in the previous steps can be adjusted. In the lithium niobate scenario, a polarization conversion efficiency of >99% at a wavelength of 1550 nm can be achieved within a total length of 300 um.

[0066] In an application example, , the materials of the lower cladding and the upper cladding are silica, and the waveguide material is lithium niobate. Lithium niobate is a birefringent material, and the refractive index along its Z-axis crystal phase is different from the refractive indices along its X-axis and Y-axis. In this example, the lithium niobate crystal phase is X-cut, that is, the X-axis is perpendicular to the waveguide core layer, and the optical axis Z-axis is parallel to the waveguide core layer, and the cross-section direction of the polarization rotator is selected so that the optical axis Z-axis is parallel to the horizontal direction of the cross-section (as shown in Figure 1 ). The working wavelength is 1550 nm. At this wavelength, the refractive index of lithium niobate along its optical axis Z-axis crystal phase is 2.1376, and the refractive indices along its X-axis and Y-axis crystal phases are 2.2111.

[0067] In this application example, , , ; each side angle is 75°; , the corresponding etching rate is 31% of that of other general areas (obtained by experimental measurement, see Table 1), and it can be calculated that ; . The first paragraph , linearly increases from 1000 nm to 1300 nm, linearly increases from 0 to 150 nm; the second paragraph , linearly increases from 1300 nm to 1600 nm, linearly decreases from 150 nm to 0. In this example, and The specific situation of the variation with the waveguide length is as shown in Figure 3 , and each relationship is a linear relationship.

[0068] Figure 4 shows the case where the effective refractive index of the modes supported by the waveguide varies with varying ( constant at 0). It can be seen that the TE0 mode and the TM0 mode cross (crossing) near , that is, they have the same effective refractive index. Figure 5 shows the case where the effective refractive index of the modes supported by the waveguide varies with varying (while changing according to the relationship shown in Figure 3 ). It can be seen that the TE0 mode and the TM0 mode undergo anti-crossing near , resulting in mode hybridization, and thus direct conversion between these two modes can be achieved.

[0069] In this application example, TM0 mode optical field is input from the input end of the polarization rotator (waveguide length ), and most of its energy is converted into TE0 mode optical field and output from the output end of the polarization rotator (waveguide length ). Its polarization conversion efficiency reaches 99.4% at the working wavelength of 1550 nm. At the same time, the energy loss when the TM0 mode in the trapezoidal cross-section waveguide with the same thickness, bottom width, and side angle transitions to the input end of the polarization rotator is only 0.8%, and the energy loss when the TE0 mode in the output end of the polarization rotator transitions to the trapezoidal cross-section waveguide with the same thickness, bottom width, and side angle is only 1.2%.

[0070] The third part of the present disclosure provides a method for fabricating a polarization rotator.

[0071] For some materials in the field of integrated optics represented by lithium niobate, due to material characteristics, the sidewalls are not perpendicular during the etching process (there is lateral etching), and the etching rate decreases significantly as the etching opening size decreases. Utilizing this variation characteristic of the etching rate, a structure with two or more etching depths can be directly formed through one lithography + etching. In addition to reducing processing costs, since only one lithography is involved, the relative positions between these structures will not deviate due to the alignment accuracy (overlay accuracy) problem in multiple lithographies. Table 1 shows an example of the relationship between the etching rate and the opening size obtained in actual processing. Affected by the processing conditions, the specific relationship can vary and can be fitted by linear / logarithmic / polynomial functions.

[0072] Table 1 Example of the relationship between the etching rate and the opening size

[0073]

[0074] Based on this, the preparation method of the polarization rotator provided by the present invention is as follows:

[0075] 1. Prepare a wafer. As Figure 6 shown, its structure from bottom to top is a silicon substrate 101, a lower cladding 102, and a waveguide core layer 103. Preferably, the material of the lower cladding 102 is silica. Preferably, the waveguide core layer 103 is 600 nm thick lithium niobate with an X-cut crystal phase, that is, its X-axis is perpendicular to the waveguide core layer, the optical axis Z-axis is parallel to the waveguide core layer 103, and the cross-section direction of the polarization rotator is selected such that the optical axis Z-axis is parallel to the horizontal direction of the cross-section.

[0076] 2. As Figure 7 shown, form a mask layer 104 on the surface of the wafer.

[0077] 3. As Figure 8 shown, through photolithography and development, make the mask layer 104 form a mask pattern structure 201. For a hard mask, etching is also required after development. Figure 9 Shows the formed mask pattern structure 201 and its various dimensional parameters. It consists of two horizontally distributed rectangles. Among them, the width of the left mask pattern is , the width of the right mask pattern is , and the spacing between the two parts of the mask pattern is . This gap opening is relatively narrow, and the etching rate here is significantly lower than that of other general areas. The above parameters also determine the waveguide bottom width .

[0078] 4. As Figure 10 shown, through etching, make the waveguide core layer 103 form a straight waveguide structure 202 and the remaining mask 203. Figure 11 Shows the structure after etching and its various dimensional parameters. It should be noted that during the actual processing of the waveguide, it is difficult to make the sides of the waveguide completely vertical. The angles between the sides of the waveguide and the horizontal plane are as Figure 11 shown, and are respectively denoted as (usually ).

[0079] Since the opening at the middle gap between the left and right parts of the mask is small, the etching rate at this place is slow. When the etching depth on both sides reaches , the etching depth at this place is only , thus forming a relatively shallow groove in the middle of the waveguide. The actual ratio of depends on the type of material of the waveguide core layer 103, as well as the specific etching process type and process parameters. The specific value of this ratio can be changed by adjusting the etching process parameters.

[0080] In this way, only one photolithography (step 3) and etching (step 4) process are required to form the straight waveguide structure 202. Due to the existence of lateral etching, the side edges are not perpendicular, and the width at the top of the groove is enlarged to ; the width of the top edge on the left side of the waveguide is , and the width of the top edge on the right side of the waveguide is .

[0081] 5. As shown in Figure 12 , the remaining mask 203 is removed by etching and cleaning.

[0082] 6. As shown in Figure 13 , the upper cladding 105 is deposited. Preferably, the material of the upper cladding 105 is silicon dioxide.

[0083] The preparation method of the polarization rotator provided by the present invention utilizes that the etching rate in the region with a narrow opening and a large aspect ratio is significantly lower than that in the region with a generally large opening and a small aspect ratio. Only through one round of photolithography and etching processes, a waveguide structure with a non-uniform thickness can be formed. In contrast, usually, for such a waveguide structure with a non-uniform thickness, one photolithography and etching step is required for each thickness. For example, for the waveguide structure with an intermediate groove in the present invention, usually 2 photolithography and etching steps are required.

[0084] Specifically, in the region with a narrow opening and a large aspect ratio, the participants in the physical and chemical reactions involved in etching are not easily in full contact with the material to be etched, and the reaction products are not easily removed quickly, resulting in a significant reduction in the local etching rate. The degree of reduction in the etching rate is closely related to the type of the material to be etched, the specific opening size and aspect ratio, the etching mechanism adopted (such as reactive ion etching (RIE), inductively coupled plasma reactive ion etching (ICP-RIE), etc.), the types of reactants, and other process parameters in etching (such as the flow rate of the reactant gas introduced, the pressure in the reaction chamber, the plasma energy, etc.). For a specific type of material to be etched and the required opening size and aspect ratio, the degree of reduction in the local etching rate can be adjusted by adjusting the etching mechanism, the types of reactants and the process parameters in etching, so as to precisely control the specific values of each thickness in the formed waveguide structure with a non-uniform thickness.

[0085] The above description is only a preferred embodiment of one or more embodiments of this specification, and is not intended to limit one or more embodiments of this specification. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of one or more embodiments of this specification shall be included within the scope of protection of one or more embodiments of this specification.

Claims

1. A polarization rotator, characterized in that, For realizing the direct conversion between the TM0 and TE0 modes, including a straight waveguide structure with a groove at the top of the waveguide, the width of the bottom edge of the waveguide gradually widens / narrows from the starting end to the ending end. When the eccentricity between the midpoint of the bottom edge of the waveguide and the midpoint of the bottom of the groove is constantly 0, the width of the bottom edge of the waveguide at the equal effective refractive index of the two modes is denoted as , the eccentricity gradually widens from 0 until the width of the bottom edge of the waveguide is equal to , and then gradually narrows to 0.

2. The polarization rotator according to claim 1, wherein The material of the straight waveguide structure is lithium niobate, lithium tantalate, barium titanate, lead titanate, lead zirconate titanate or sodium bismuth titanate.

3. The polarization rotator according to claim 1, wherein The bottom width of the groove does not exceed 300 nm, and the depth of the groove does not exceed half of the waveguide thickness.

4. The polarization rotator according to claim 1, wherein The top widths of the left and right sides of the straight waveguide structure are both not less than 150 nm.

5. The polarization rotator according to claim 1, wherein The relationship between the eccentricity and the waveguide bottom width, and the relationship between the waveguide bottom width and the waveguide length are piecewise linear relationships, polynomial relationships, exponential relationships or logarithmic relationships.

6. A design method for a polarization rotator according to any one of claims 1-5, characterized in that, It includes the following steps: (1) Determine the working wavelength, waveguide thickness, materials of each layer and the refractive index of each layer at the working wavelength according to the actual application scenario; (2) Give the relationship between the etching rate of the groove to be etched and the bottom width of the groove, and the oblique angles of each side according to the pre-calibrated processing effect; (3) Determine the width of the bottom edge of the waveguide The value at the starting end of the polarization rotator and the value at the end ; Determine the width of the bottom of the groove , and determine the depth of the groove according to the relationship in step (2) ; (4) Determine the eccentricity along with the waveguide bottom width varying relationship; at the starting end and the ending end of the polarization rotator, ; along with the widening / narrowing gradually increases, when the value is maximum and denoted as and then along with the widening / narrowing gradually decreases; (5) The polarization rotator is divided into two sections; the first section, ranging from gradually changing to , increasing from 0 to according to the relationship in step (4); The second paragraph, gradually changes from to , and decreases to 0 according to the relationship in step (4) from ​ 7. The design method of the polarization rotator according to claim 6, characterized in that, In step (4), the relationship between the TE ratio in the waveguide mode and and changing is obtained through simulation. By increasing or decreasing the non - hybrid mode region that makes no contribution to polarization conversion is removed.

8. The design method of the polarization rotator according to claim 6, characterized in that, When the waveguide material is a birefringent material, the relationship between the material crystal phase and the device coordinate system needs to be set in step (1).

9. The design method of the polarization rotator according to claim 6, characterized in that, In step (5), the optimal lengths of the first and second sections of the polarization rotator are determined by simulation, and the optimization goal is to minimize the total length while meeting the requirements of the polarization conversion efficiency.

10. A method for preparing the polarization rotator according to any one of claims 1-5, characterized in that, It includes the following steps: (1) Prepare a wafer, the structure of which from bottom to top is a substrate, a lower cladding layer, and a waveguide core layer; (2) Form a mask layer on the wafer surface; (3) Through photolithography and development, make the mask layer form a mask pattern structure. For a hard mask, etching is also required after development. The mask pattern structure is composed of two horizontally distributed rectangles, and there is a narrow gap between the two parts of the mask pattern; (4) Through etching, make the waveguide core layer form a straight waveguide structure and the remaining mask. The etching rate at the narrow gap is slower than that in other areas, so as to form a groove on the top of the waveguide; (5) After removing the remaining mask, deposit an upper cladding layer.

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