Polarization rotator and its design and preparation method

Abstract

The present invention discloses a polarization rotator and its design and preparation method. The polarization rotator comprises a straight waveguide structure with a groove at the top of the waveguide. The width of the waveguide bottom gradually widens / narrows from the starting end to the end. The eccentricity between the midpoint of the waveguide bottom and the midpoint of the groove bottom gradually widens from 0 to a maximum value and then gradually narrows to 0. This structural design can achieve direct conversion between two different polarization fundamental modes, TM0 and TE0, in a straight waveguide without a bend structure. It has high conversion efficiency and excellent performance. The device structure is simple and has small lateral dimensions. The present invention only requires a single round of photolithography and etching process to form a waveguide structure with non-uniform thickness, greatly simplifying the processing flow and reducing processing costs. It also avoids processing errors caused by overlay deviation limitations between multiple rounds of photolithography and its impact on the performance of the polarization rotator. At the same time, the polarization rotator structure is similar to that of a strip waveguide, and the energy loss during the transition between the two is small.

Description

Technical Field

[0001] The present invention relates to the field of integrated optics technology, and in particular to a polarization rotator and a design and preparation method thereof. Background Art

[0002] In recent years, optical chips based on photonic integrated circuit (PIC) technology have been widely used in AI computing chip interconnection, high-speed optical communications, optoelectronic sensing, biomedical testing and other fields, and play an important role as core components.

[0003] Photonic waveguides (PICs) are the basic structure of optical photonic crystals (PICs). The modes they support can be generally divided into transverse electric (TE) (where the electric field is distributed horizontally and the magnetic field is distributed vertically) and transverse magnetic (TM) (where the magnetic field is distributed horizontally and the electric field is distributed vertically), depending on the direction of the electric / magnetic field distribution.

[0004] In practical applications, the geometry of optical waveguides typically has different dimensions in the horizontal and vertical directions. Consequently, the performance of different polarization modes within an optical waveguide varies significantly, and polarization control is a crucial aspect of optical chip design. On the one hand, each device within an optical chip typically operates only in a specific single polarization mode. The introduction of other polarization modes can generate interference signals, impacting the performance of the device and even the entire system. On the other hand, the targeted and proper utilization of different polarization modes can improve the performance of the device and even the entire system. For example, the use of polarization-division multiplexing (PDM) in optical communications can double the number of channels supported by a single wavelength.

[0005] A polarization rotator (also known as a polarization rotator) is an important polarization control device that can convert input light of one polarization mode into light of another polarization mode. In principle, polarization rotation is typically achieved through mode hybridization (also known as mode mixing) between the two converted modes. In practical applications, the two fundamental polarization modes TE0 and TM0 are the most commonly used, but direct conversion between them is difficult and cannot be achieved using ordinary (and easiest to process) strip waveguides. Typically, conversion between the TE0 and TM0 modes requires a two-step process, using the TE1 mode as an intermediary (i.e., "TE0-TE1-TM0"). This results in devices that achieve conversion between the TE0 and TM0 modes being complex in structure and exhibiting suboptimal overall performance. Although some waveguides with special structures can achieve hybridization and direct conversion between TE0 and TM0 modes, these structures are also complex and usually require multiple photolithography and etching processes to form. The processing cost is high, and their actual performance is limited by the overlay (alignment) deviation between multiple photolithography processes. At the same time, these specially structured waveguides are often very different from the commonly used waveguide structures such as strips used before and after the polarization rotator in the system, resulting in significant energy loss of the optical signal during the transition between the two.

[0006] New optical chip materials, such as lithium niobate and lithium tantalate, can be used in the fabrication of low-power, high-bandwidth electro-optical modulators, and their application prospects are promising. However, compared to traditional optical chip materials such as silicon, silicon nitride, and silicon dioxide, dry etching of lithium niobate and lithium tantalate is more challenging. Specifically, achieving vertical (90°) waveguide sidewalls during dry etching is difficult. The maximum sidewall angle achievable with current process capabilities is approximately 75° (materials such as silicon, silicon nitride, and silicon dioxide typically achieve over 85° and approaching 90°). This results in a lithium niobate and lithium tantalate waveguide cross-section that is typically trapezoidal, narrow at the top and wide at the bottom, rather than rectangular. Furthermore, due to the deposition of non-volatile products (such as lithium fluoride) on the material surface during the etching process, the dry etching rate of lithium niobate and lithium tantalate decreases significantly when the etched area is small (this size threshold is typically around hundreds of nanometers, depending on the actual process parameters).

[0007] Furthermore, lithium niobate and lithium tantalate are birefringent materials. If thin films are processed with their optical axes parallel to the film, the film will have different refractive indices along different directions. In this case, optical signals passing through any curved waveguide structure may experience modal crosstalk and / or energy loss due to refractive index variations.

[0008] The above material properties make it very difficult to design and process devices with complex functions and structures, such as polarization rotators, on lithium niobate and lithium tantalate material platforms, and the limitations of the above factors lead to poor performance. Summary of the Invention

[0009] The purpose of the present invention is to address the deficiencies in the prior art and provide a polarization rotator and a design and preparation method thereof, which can realize direct conversion between two different polarization fundamental modes, TM0-TE0, in a straight waveguide without a bend structure.

[0010] According to a first aspect of the present specification, a polarization rotator is provided, comprising a straight waveguide structure, wherein the top of the waveguide has a groove, and the bottom width of the waveguide gradually widens / narrows from the starting end to the end. When the eccentricity between the midpoint of the bottom of the waveguide and the midpoint of the bottom of the groove is constant at 0, the bottom width of the waveguide at which the effective refractive index of the two modes is equal is recorded as The eccentricity gradually widens from 0 until the bottom width of the waveguide is equal to Then it gradually narrows 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 thickness of the waveguide.

[0013] Furthermore, the widths of the left and right top sides of the straight waveguide structure are both no less than 150 nm.

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

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

[0016] According to a second aspect of this specification, a method for designing a polarization rotator is provided, the method comprising the following steps:

[0017] (1) The operating wavelength, waveguide thickness, materials of each layer, and the refractive index of each layer at the operating wavelength are determined by the actual application scenario;

[0018] (2) The relationship between the etching rate of the groove to be etched and the width of the groove bottom, as well as the bevel angles of each side edge, are given based on the pre-calibrated processing effect;

[0019] (3) Determine the bottom width of the waveguide The value at the start of the polarization rotator and the end value ; Determine the groove bottom width , and determine the groove depth based on the relationship in step (2) ;

[0020] (4) Determine the eccentricity Waveguide bottom width The relationship between the changes; at the beginning and end of the polarization rotator, ; Follow Widening / narrowing gradually increases, hour The maximum value is recorded as ,after Follow Widening / narrowing gradually decreases;

[0021] (5) The polarization rotator is divided into two sections; the first section, from Gradually changes to , According to the relationship in step (4), the value increases from 0 to ; The second paragraph, from Gradually changes to , According to the relationship in step (4) Decrease to 0.

[0022] Furthermore, in step (4), the TE ratio in the waveguide mode is obtained by simulation. and The relationship between changes, through the increase or reduce to remove the non-hybrid mode regions that do not contribute to the polarization conversion.

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

[0024] Furthermore, 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 meet the polarization conversion efficiency requirements while minimizing the total length.

[0025] According to a third aspect of the present specification, a method for preparing a polarization rotator is provided, the method comprising the following steps:

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

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

[0028] (3) forming a mask pattern structure on the mask layer through photolithography and development. For a hard mask, etching is required after development. The mask pattern structure is composed of two horizontally distributed rectangles with a narrow gap between the two parts of the mask pattern;

[0029] (4) Etching the waveguide core layer to form a straight waveguide structure and the remaining mask, with the etching rate in the narrow gap being slower than that in other areas, thereby forming a groove at the top of the waveguide;

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

[0031] The present invention has the following beneficial effects: the polarization rotator designed in the present invention can achieve direct conversion between two different polarization fundamental modes, TM0 and TE0, in a straight waveguide without a bend structure, with high conversion efficiency and excellent performance. The device has a simple structure and small lateral dimensions. In addition, the present invention fully utilizes the processing characteristics of materials such as lithium niobate and lithium tantalate, and can form a waveguide structure with non-uniform thickness through only a single round of photolithography and etching (traditionally requiring two or more rounds of photolithography and etching). This greatly simplifies the processing process, reduces processing costs, and avoids processing errors caused by overlay deviation limitations between multiple rounds of photolithography, as well as its impact on polarization rotator performance. At the same time, the polarization rotator structure is very similar to that of a strip waveguide, and the energy loss during the transition between the two is very small. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. 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 any creative work.

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

[0034] Figure 2 A 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 this disclosure and Schematic diagram of the change with waveguide length;

[0036] Figure 4 The effective refractive index of the mode supported by the waveguide in the polarization rotator application example provided in the second part of this disclosure varies with change( constant at 0);

[0037] Figure 5 The effective refractive index of the mode supported by the waveguide in the polarization rotator application example provided in the second part of this disclosure varies with Change (while according to Figure 3 ) in the case of changes in the relationship shown in ;

[0038] Figure 6 A schematic diagram of the wafer structure provided in the third part of this disclosure;

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

[0040] Figure 8 A schematic diagram of a mask pattern structure formed by a mask layer provided in the third part of the present disclosure;

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

[0042] Figure 10 A schematic diagram of forming a straight waveguide structure and remaining masks provided in the third part of this disclosure;

[0043] Figure 11 A schematic diagram of the structure and various dimensional parameters after etching provided in the third part of this disclosure;

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

[0045] Figure 13 Schematic diagram of depositing the upper cladding layer provided in the third part of this disclosure. DETAILED DESCRIPTION

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

[0047] It should be clear that the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0048] The terms used in the embodiments of the present application are for the purpose of describing specific embodiments only and are not intended to limit the application. The singular forms "a," "an," "the," and "the" 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 embodiment of the present disclosure provides a polarization rotator, which utilizes the mode hybridization occurring during the continuous change of the waveguide cross-sectional structure to achieve direct conversion between the two fundamental modes of different polarizations, TM0 and TE0, in a straight waveguide.

[0050] The waveguide cross-section structure used is as follows Figure 1 As shown, 102 is the lower cladding, 105 is the upper cladding, and 202 is the straight waveguide structure. The waveguide thickness is recorded as , the bottom width of the waveguide is recorded as ; There is a groove on the top of the waveguide, and the groove depth is , the width of the groove bottom is recorded as .

[0051] In the actual processing of waveguides, due to the limitations of process capabilities, it is difficult to make the side of the waveguide completely vertical, especially for some new optical chip materials such as lithium niobate. This makes the waveguide cross section as a whole a trapezoid with a narrow top and a wide bottom, while the top groove is a trapezoid with a wide top and a narrow bottom. The width of the top edge of the waveguide is recorded as , the width of the top edge of the waveguide on the right side is recorded as ; The width of the groove top is recorded as .

[0052] The horizontal distance (eccentricity) between the midpoint of the bottom edge of the waveguide and the midpoint of the bottom of the groove is recorded as ,when When , the waveguide structure is bilaterally symmetrical. In this case, the change in the waveguide width cannot cause hybridization between the TE0 and TM0 modes. When the width of the waveguide changes, 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 width of the bottom side of the waveguide gradually widens / narrows from the starting end (input end) to the end (output end). The following examples are all based on the conversion from TM to TE. Figure 2 As shown in the figure, the bottom width of the waveguide gradually widens (for the case of TE to TM conversion, the bottom width of the waveguide gradually narrows). When the effective refractive index of the two modes is equal (reflected by the intersection of the relationship curve between the effective refractive index of the waveguide mode and the width of the waveguide bottom edge), the width of the waveguide bottom edge is recorded as . It gradually widens from 0 until the bottom width of the waveguide is equal to Then it gradually narrows to 0.

[0054] This structural design enables direct conversion between the two most commonly used fundamental modes, TE0 and TM0. The straight waveguide structure occupies minimal lateral space; the absence of bends eliminates the issue of refractive index changes associated with bends on birefringent materials such as lithium niobate. Furthermore, the waveguide shape closely resembles a commonly used strip waveguide (with only a small groove at the top), enabling extremely low-loss transitions and enhancing practicality.

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

[0056] 1. The operating wavelength and waveguide thickness are determined by the actual application scenario , the material of each layer and the refractive index of each layer at the operating wavelength. For birefringent materials (such as lithium niobate and lithium tantalate), it is also necessary to set the coordinate system relationship between the material crystal phase and the device.

[0057] 2. The etching rate and bottom width of the groove to be etched are given by the pre-calibrated processing effect The process effect can be pre-adjusted according to the application scenario requirements.

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

[0059] (1) It must be satisfied that: during the change of the polarization rotator from the starting end to the end (here we only focus on = ( TM0 = TE0 ) = ( TM0 = TE0 ) . The width of the waveguide bottom where the effective refractive index of the two modes TM0 and TE0 is equal is recorded as .

[0060] (2) The groove should not be too large. It is better not to exceed 300nm, and the corresponding No more than Half is ideal. This waveguide structure is still close to a trapezoidal waveguide, with minimal loss during the transition. Larger grooves allow the polarization rotator to complete polarization conversion in a shorter length, but this is more different from a trapezoidal waveguide and results in greater loss during the transition.

[0061] (3) The overall structure of the polarization rotator should not be too small, especially the width of the top edges on the left and right sides of the starting end. 、 The values ​​of should not be too small. Preferably, these two values ​​should not be less than 150nm.

[0062] 4. Confirm Follow First, or hour, , are the starting and ending points of the polarization rotator. Follow Gradually widens, hour, The maximum value is ,after Follow This design of polarization rotator has better performance. Follow The specific relationship of the change can be a piecewise linear relationship, a polynomial relationship, an exponential relationship, or a logarithmic relationship.

[0063] Larger The value can make the polarization rotator complete the polarization conversion in a shorter length, but it should be noted that the overall structure of the polarization rotator should not be too small, especially arrive The width of the top edges on both sides of this section 、 Preferably, these two values ​​cannot be less than 150nm.

[0064] On this basis, the TE ratio in the waveguide mode is obtained through simulation. and The relationship between the change (a ratio of 1 represents TE mode, a ratio of 0 represents TM mode, and a ratio between 0 and 1 represents mixed mode). You can check whether there is a large non-mixed mode area, which does not contribute to polarization conversion. You can increase or reduce By removing it (without modifying other parameters), the polarization rotator can complete the polarization conversion in a shorter length, thereby improving the performance of the polarization rotator.

[0065] 5. The polarization rotator is divided into two sections. The first section has a polarization rotator length of Increase from 0 to , from Change to , According to the relationship in step 4, it increases from 0 to The second section, polarization rotator length from Increase to , from Change to , According to the relationship in step 4, Reduced to 0. In the two sections, Follow The specific relationship of the change can be a piecewise linear relationship, a polynomial relationship, an exponential relationship or a logarithmic relationship. According to the application scenario requirements, it is determined through simulation. 、 The optimal value achieves high polarization conversion efficiency (TE0 output power / TM0 input power) while keeping the total length short. Typically, the polarization conversion efficiency should be greater than 99%. If necessary, adjust the values ​​and relationships between other parameters in the previous steps. In a lithium niobate scenario, >99% polarization conversion efficiency at a wavelength of 1550 nm can be achieved within a total length of 300 μm.

[0066] In one application example, The lower and upper cladding materials are made of silicon dioxide, 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 index 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, the optical axis Z-axis is parallel to the waveguide core layer, and the cross-sectional direction of the polarization rotator is selected so that the optical axis Z-axis is parallel to the horizontal direction of the cross-sectional direction (such as Figure 1 ). The operating wavelength is 1550nm. At this wavelength, the refractive index of lithium niobate along its optical axis Z-axis crystal phase is 2.1376, and the refractive index along its X-axis and Y-axis crystal phase is 2.2111.

[0067] In this application example, , , ; Each side angle is 75°; , the corresponding etching rate is 31% of other general areas (obtained from experimental measurements, see Table 1), and it can be calculated that ; . The first paragraph , Increase linearly from 1000nm to 1300nm, Increase linearly from 0 to 150nm; the second segment , Increase linearly from 1300nm to 1600nm, It decreases linearly from 150nm to 0. In this example, and The specific situation of the change with the waveguide length is as follows Figure 3 As shown, all relationships are linear.

[0068] Figure 4 It is shown that the effective refractive index of the modes supported by the waveguide varies with change( It can be seen that the TE0 mode and TM0 mode are Crossing occurs near the surface of the substrate, meaning that the two have the same effective refractive index. Figure 5 It is shown that the effective refractive index of the modes supported by the waveguide varies with Change (while according to Figure 3 It can be seen that the TE0 mode and the TM0 mode are Anti-crossing occurs nearby, generating mode hybridization, which allows direct conversion between the two modes.

[0069] In this application example, the polarization rotator input (waveguide length ) input TM0 mode light field, most of its energy is converted into TE0 mode light field and output from the polarization rotator (waveguide length ) output, achieving a polarization conversion efficiency of 99.4% at an operating wavelength of 1550nm. Furthermore, the energy loss of the TM0 mode in a trapezoidal waveguide with the same thickness, base width, and side angles as it transitions to the polarization rotator input is only 0.8%, while the energy loss of the TE0 mode at the polarization rotator output as it transitions to a trapezoidal waveguide with the same thickness, base width, and side angles is only 1.2%.

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

[0071] For some materials used in integrated optics, such as lithium niobate, the etching process produces non-vertical sides (lateral etching occurs), and the etch rate decreases significantly as the size of the etched opening decreases due to the material's properties. This variable etch rate allows structures with two or more etch depths to be directly formed through a single photolithography + etching process. In addition to reducing processing costs, since only a single photolithography step is involved, the relative positions of these structures are not subject to deviations due to alignment accuracy (overlay accuracy) issues encountered during multiple photolithography steps. Table 1 shows an example of the relationship between etch rate and opening size obtained in actual processing. The specific relationship can vary depending on processing conditions and can be fitted using linear, logarithmic, or polynomial functions.

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

[0073]

[0074] Based on this, the present invention provides a method for preparing a polarization rotator, and the specific steps are as follows:

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

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

[0077] 3. If Figure 8 As shown, the mask layer 104 is formed into a mask pattern structure 201 through photolithography and development. For the hard mask, etching is required after development. Figure 9 The mask pattern structure 201 and its size parameters are shown. It consists of two horizontally distributed rectangles. The left mask pattern width is , the width of the mask pattern on the right is , the spacing between the two mask patterns is , the gap opening is narrow, and the etching rate here is significantly lower than other general areas. The above parameters also determine the bottom width of the waveguide .

[0078] 4. If Figure 10 As shown, the waveguide core layer 103 is etched to form a straight waveguide structure 202 and a remaining mask 203 . Figure 11 The structure and dimensional parameters after etching are shown. It should be pointed out that in the actual processing of waveguides, it is difficult for the sides of the waveguide to be completely vertical. The angles between the sides of the waveguide and the horizontal plane are as follows: Figure 11 As shown, they are respectively (generally ).

[0079] Since the opening in the middle gap between the left and right parts of the mask is small, the etching speed is slow, and the etching depth on both sides reaches When the etching depth is only , thus forming a shallow groove in the middle of the waveguide. and The actual ratio depends on the material type of the waveguide core layer 103, as well as the specific etching process type and process parameters used. 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 is required to form the straight waveguide structure 202. Due to the existence of lateral etching, the side edges are not vertical, and the width of the groove top is expanded to ; The width of the top edge of the left side of the waveguide is , the width of the top edge of the waveguide on the right is .

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

[0082] 6. If Figure 13 As shown, deposit an upper cladding layer 105. Preferably, the material of the upper cladding layer 105 is silicon dioxide.

[0083] The polarization rotator fabrication method provided by the present invention utilizes the significantly lower etching rate of regions with narrow openings and large aspect ratios compared to regions with larger openings and smaller aspect ratios. This allows the formation of a waveguide structure with non-uniform thicknesses through a single round of photolithography and etching. In contrast, conventional waveguide structures with such non-uniform thicknesses require a single photolithography and etching step for each thickness. For example, the waveguide structure with a central groove in the present invention typically requires two photolithography and etching steps.

[0084] Specifically, in areas with narrow openings and large aspect ratios, the physical and chemical reactions involved in etching are less likely to fully contact the etched material, and the reaction products are difficult to remove quickly, resulting in a significant reduction in the local etch rate. The extent of the etch rate reduction is closely related to the type of etched material, the specific opening size, aspect ratio, the etching mechanism used (such as reactive ion etching (RIE) and inductively coupled plasma reactive ion etching (ICP-RIE)), the type of reactants, and other etching process parameters (such as the flow rate of the reactant gas, the pressure in the reaction chamber, and the plasma energy). For a specific type of etched material and the desired opening size and aspect ratio, the degree of local etch rate reduction can be adjusted by adjusting the etching mechanism, the type of reactants, and the etching process parameters, thereby precisely controlling the specific thickness values ​​of the resulting non-uniform thickness waveguide structure.

[0085] The above description is merely 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 modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of one or more embodiments of this specification shall be included in the scope of protection of one or more embodiments of this specification.

Claims

1. A polarization rotator, characterized in that: It is used to realize direct conversion between TM0 and TE0 modes, including a straight waveguide structure with a groove on the top of the waveguide. For the case of TE to TM conversion, the width of the bottom side of the waveguide gradually narrows from the starting end to the end, and for the case of TM to TE conversion, the width of the bottom side of the waveguide gradually widens from the starting end to the end. When the eccentricity between the midpoint of the bottom side of the waveguide and the midpoint of the bottom of the groove is constant at 0, the width of the bottom side of the waveguide where the effective refractive index of the two modes is equal is recorded as The eccentricity gradually widens from 0 until the bottom width of the waveguide is equal to Then it 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 thickness of the waveguide.

4. The polarization rotator according to claim 1, wherein The widths of the top sides on the left and right sides of the straight waveguide structure are both no less than 150 nm.

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

6. A method for designing a polarization rotator according to any one of claims 1 to 5, characterized in that: The following steps are involved: (1) The operating wavelength, waveguide thickness, materials of each layer, and the refractive index of each layer at the operating wavelength are determined by the actual application scenario; (2) The relationship between the etching rate of the groove to be etched and the width of the groove bottom, as well as the bevel angles of each side edge, are given based on the pre-calibrated processing effect; (3) Determine the bottom width of the waveguide The value at the start of the polarization rotator and the end value ; Determine the groove bottom width , and determine the groove depth based on the relationship in step (2) ; (4) Determine the eccentricity Waveguide bottom width The relationship between the changes; at the beginning and end of the polarization rotator, ; Follow Widening / narrowing gradually increases, hour The maximum value is recorded as ,after Follow Widening / narrowing gradually decreases; (5) The polarization rotator is divided into two sections; the first section, from Gradually changes to , According to the relationship in step (4), the value increases from 0 to ; The second paragraph, from Gradually changes to , According to the relationship in step (4) Decrease to 0.

7. The method for designing a polarization rotator according to claim 6, wherein: In step (4), the TE ratio in the waveguide mode is obtained by simulation. and The relationship between changes, through the increase or reduce to remove the non-hybrid mode regions that do not contribute to the polarization conversion.

8. The method for designing a polarization rotator according to claim 6, wherein: When the waveguide material is a birefringent material, it is necessary to set the coordinate system relationship between the material crystal phase and the device in step (1).

9. The method for designing a polarization rotator according to claim 6, wherein: 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 ensure that the total length is the shortest while the polarization conversion efficiency meets the requirements.

10. A method for preparing the polarization rotator according to any one of claims 1 to 5, characterized in that: The following steps are involved: (1) Prepare a wafer whose structure from bottom to top is substrate, lower cladding, and waveguide core layer; (2) forming a mask layer on the wafer surface; (3) forming a mask pattern structure on the mask layer through photolithography and development. For a hard mask, etching is required after development. The mask pattern structure is composed of two horizontally distributed rectangles with a narrow gap between the two parts of the mask pattern; (4) Etching the waveguide core layer to form a straight waveguide structure and the remaining mask, with the etching rate in the narrow gap being slower than that in other areas, thereby forming a groove at the top of the waveguide; (5) After removing the remaining mask, the upper cladding layer is deposited.

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