Grating unit, grating array and grating coupler

Abstract

The invention relates to the technical field of gratings, and discloses a grating unit, a grating array and a grating coupler.The grating unit comprises a waveguide layer, the waveguide layer is provided with a first groove body and a second groove body, the cross section of the first groove body and the cross section of the second groove body are quadrilateral, and the depth of the first groove body is larger than that of the second groove body; the distance between the first groove body and the second groove body is larger than the negative value of the preset distance. One-way radiation without a bottom reflector is realized in the short-band grating coupler, downward radiation is suppressed, and the grating unit has a relatively high quality factor, so that the upper radiation efficiency loss in a mode field matching process is greatly reduced.

Description

Technical Field

[0001] This invention relates to the field of grating technology, specifically to grating units, grating arrays, and grating couplers. Background Technology

[0002] Mainstream long-wavelength lasers are significantly more expensive than short-wavelength lasers. However, with the rapid development of short-distance optical interconnects, short-wavelength optical carriers exhibit negligible transmission loss and possess considerable application potential. Grating couplers are the core interface for enabling optical signals to enter and exit optical chips. Due to optical bandwidth limitations, traditional long-wavelength grating couplers are unsuitable for the input and output of short-wavelength optical signals.

[0003] Currently, existing short-wavelength grating couplers typically employ a traditional single-slot structure, resulting in low coupling efficiency. Summary of the Invention

[0004] This invention provides a grating unit, a grating array, and a grating coupler to solve the technical problem of low coupling efficiency in short-wavelength grating couplers.

[0005] In a first aspect, the present invention provides a grating unit, including a waveguide layer, on which a first groove and a second groove are provided. The cross-sections of both the first and second grooves are quadrilaterals. The depth of the first groove is greater than the depth of the second groove. The distance w3 between the first and second grooves is greater than a negative value of a preset distance, wherein the preset distance satisfies:

[0006] Where L is the preset distance, h2 is the depth of the second tank, α1 is the bottom angle of the side of the second tank closest to the first tank, and β2 is the bottom angle of the side of the first tank closest to the second tank.

[0007] In some embodiments, the structural parameters of the second tank body satisfy: h2 = (0.001~1) × h1, w2 = (0.001~5) × h2, where w2 is the bottom edge length of the second tank body.

[0008] In some embodiments, the thickness hw of the waveguide layer satisfies: hw = (1~10) × h1, where h1 is the depth of the first groove.

[0009] In some embodiments, the cross-section of the first groove and / or the second groove is an isosceles trapezoid, a right trapezoid, a rectangle, a square, or a parallelogram.

[0010] In some embodiments, the length of the top edge of the first groove and / or the second groove is greater than or equal to the length of the bottom edge.

[0011] In a second aspect, the present invention provides a grating array comprising a plurality of grating units as described in any of the first aspects of the present invention. The plurality of grating units are arranged along a first direction. Among the first preset number of grating units, the bottom edge length of the first groove increases from a first initial value to a first target value, and the bottom edge length of the second groove increases from a second initial value to a second target value. The structural parameters of the grating units arranged in the first preset number and thereafter are the same.

[0012] In some embodiments, the scattering angles of the plurality of grating units arranged first along the first direction increase from the initial scattering angle to the target scattering angle, wherein the initial scattering angle and the target scattering angle satisfy: When theta > 0, 0 < theta0 and theta1 ≤ 3 × theta; When theta < 0, theta0 < 0 and theta1 ≥ 3 × theta; ; period1 = A × B; Where theta0 is the initial scattering angle, theta1 is the target scattering angle, and theta is the preset grating scattering angle. For wave vector, Let π be the mathematical constant, period1 be the first grating period, A be the adjustment coefficient, and B be the sum of the top edge length of the first slot, the top edge length of the second slot, and the distance between the first and second slots in the grating unit.

[0013] In some embodiments, a plurality of grating units are arranged along a first direction with a width of one grating unit equal to the second grating period, wherein the second grating period satisfies: period2=((lam+w1×(nwg-neff1)+w2×(nwg-neff2)) / (nwg–nc×sin(theta))); Where period2 is the second grating period, w1 is the bottom edge length of the first groove, w2 is the bottom edge length of the second groove, lam is the light wavelength, nwg is the effective refractive index of the waveguide layer, neff1 is the effective refractive index of the waveguide layer at the bottom edge depth of the first groove, neff2 is the effective refractive index of the waveguide layer at the bottom edge depth of the second groove, and nc is the refractive index of the cladding layer on the grating unit.

[0014] Thirdly, the present invention provides a grating coupler comprising a grating array as described in any of the second aspects of the present invention.

[0015] The present invention has the following beneficial effects: The grating unit of the present invention has a first groove and a second groove on the waveguide layer. The cross-section of the first groove and the second groove are both quadrilaterals. The depth of the first groove is greater than the depth of the second groove. This applies the unidirectional guided resonance principle based on topological charge splitting-combining to the short-wavelength grating, realizing unidirectional radiation without a bottom reflector and suppressing downward radiation. Furthermore, the distance between the first groove and the second groove is greater than a negative value of a preset distance, which gives the grating unit a high quality factor and significantly reduces the upradiation efficiency loss during the mode field matching process.

[0016] The grating unit of the present invention does not require additional deposition of functional thin films and metal reflective layers when achieving directional radiation, making the process simple and low-cost. Furthermore, the use of a double quadrilateral structure makes it easy to process.

[0017] When grating units are arranged into grating couplers, apodization is performed on the real space (groove width) and momentum space (grating period), and spacing is introduced between adjacent grooves to improve the quality factor. This significantly reduces the upradiation efficiency loss during the mode field matching process and achieves a high degree of matching between the grating coupler and the target light source in terms of the electric field distribution in the real space and the far-field divergence angle in the momentum space. In other words, the optimal balance between "mode field matching" and "upradiation" is achieved. Attached Figure Description

[0018] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the structure of the grating unit when w3 is greater than 0 in an embodiment of the present invention. Figure 2 This is a schematic diagram of the structure of the grating unit when w3 equals 0 in an embodiment of the present invention. Figure 3 This is a schematic diagram of the structure of the grating unit when w3 is less than 0 in an embodiment of the present invention; Figure 4 This is a schematic diagram of the working energy levels of the grating unit in an embodiment of the present invention; Figure 5 This is a cross-sectional view of the grating array according to an embodiment of the present invention; Figure 6 This is a schematic diagram of the mode field distribution of the grating array without apodization in an embodiment of the present invention; Figure 7 This is a schematic diagram of the mode field distribution of the grating array during apodization according to an embodiment of the present invention; Figure 8This is a top view of the non-focusing grating coupler according to an embodiment of the present invention; Figure 9 This is a top view of a focusing grating coupler according to an embodiment of the present invention; Figure 10 This is a schematic diagram of the upper and lower radiation ratio of the grating unit in Example 1 of the present invention; Figure 11 This is an electric field distribution diagram of the grating unit in Example 1 of the present invention; Figure 12 This is a schematic diagram showing the matching between the upper radiating real space mode field of the grating array and the real space mode field of the target light source in Example 1 of this embodiment of the invention; Figure 13 This is a schematic diagram showing the matching between the upper radiative momentum space mode field of the grating array and the momentum space mode field of the target light source in Example 1 of this embodiment of the invention; Figure 14 This is a schematic diagram of the coupling efficiency of the grating coupler in Example 1 of this embodiment of the invention; Figure 15 This is a schematic diagram of the coupling electric field distribution of the grating coupler in Example 1 of this embodiment of the invention; Figure 16 This is an electric field distribution diagram of the grating unit in Example 2 of the present invention; Figure 17 This is a schematic diagram of the coupling electric field distribution of the grating coupler in Example 2 of this embodiment of the invention. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] It should be noted that the terms "installation," "connection," and "linkage" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal connection of two components. The terms "parallel," "perpendicular," and "equal" include the described situation and situations that are similar to the described situation, where the range of similarity is within an acceptable deviation range, which is determined by a person skilled in the art taking into account the measurement under discussion and the error associated with the measurement of a particular quantity (i.e., the limitations of the measurement system). For example, "parallel" includes absolute parallelism and approximate parallelism, where the acceptable deviation range for approximate parallelism can be, for example, within 5°; "perpendicular" includes absolute perpendicularity and approximate perpendicularity, where the acceptable deviation range for approximate perpendicularity can also be, for example, within 5°; "equal" includes absolute equality and approximate equality, where the acceptable deviation range for approximate equality can be, for example, the difference between the two equals being less than or equal to 5% of either one. For a person skilled in the art, the specific meaning of the above terms in this application can be understood according to the specific circumstances.

[0022] Currently, short-wavelength grating couplers typically employ a traditional single-slot structure, resulting in low coupling efficiency. In some research cases, an 850nm grating coupler based on an L-shaped dual-slot structure was designed, but the efficiency was only -4dB. In other research cases, higher coupling efficiency was achieved by additionally depositing a high-refractive-index material layer and a metal reflective thin film, but the process is more complex and costly.

[0023] Furthermore, in order to make the radiation field of the grating coupler present a Gaussian profile, it is easy to exacerbate the downward leakage of light, making it difficult to achieve the best balance between mode field matching and downward leakage.

[0024] Based on this, embodiments of the present invention provide a grating unit, a grating array, and a grating coupler. To enable those skilled in the art to better understand the present application, the present application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0025] According to embodiments of the present invention, a grating unit is provided, suitable for input-output coupling of optical signals with communication wavelengths less than 1600 nm, and particularly suitable for input-output coupling of short-wavelength optical signals with communication wavelengths less than 1000 nm. Figure 1 As shown, the grating unit includes: A waveguide layer is provided with a first groove 1 and a second groove 2. The cross-sections of both the first groove 1 and the second groove 2 are quadrilaterals. The depth of the first groove 1 is greater than the depth of the second groove 2. The distance w3 between the first groove 1 and the second groove 2 is greater than the negative value of a preset distance. The preset distance satisfies:

[0026] Where L is the preset distance, h2 is the depth of the second tank 2, α1 is the bottom angle of the side of the second tank 2 closest to the first tank 1, and β2 is the bottom angle of the side of the first tank 1 closest to the second tank 2.

[0027] Specifically, the first groove 1 and the second groove 2 can be formed by etching and both extend along the Y direction. When the optical signal is along the X direction perpendicular to the Y direction, the left-right relationship of the first groove 1 and the second groove 2 along the X direction is not restricted, that is, the first groove 1 can be to the left or to the right of the second groove 2. Taking the first groove 1 being located to the left of the second groove 2 as an example, the light beam is incident on the grating unit from the side closer to the first groove 1 and output from the Z direction.

[0028] Combination Figure 1 , Figure 2 and Figure 3 As shown, the distance w3 between the first slot 1 and the second slot 2 can be greater than 0, less than 0, or equal to 0. Specifically, the distance w3 needs to be greater than a negative value of a preset distance to create a separation between the first slot 1 and the second slot 2, thus giving the grating unit a high quality factor. A higher quality factor results in weaker scattering ability, making it less likely for light to be scattered when passing through the slot, thereby reducing the scattering ability of the grating unit and allowing the beam to continue propagating within the waveguide. If w3 is less than -L, the grating scattering intensity is too high, and the quality factor is too low. When applied to a grating coupler, it is difficult to achieve the optimal balance between "real space / momentum space mode field matching" and "radiative efficiency."

[0029] The grating unit of this invention has a first groove 1 and a second groove 2 on the waveguide layer. The cross-section of the first groove 1 and the second groove 2 are both quadrilaterals. The depth of the first groove 1 is greater than the depth of the second groove 2. This applies the unidirectional guided resonance principle based on topological charge splitting-combining to the short-wavelength grating, realizing unidirectional radiation without a bottom reflector and suppressing downward radiation. Furthermore, the distance between the first groove 1 and the second groove 2 is greater than a negative value of a preset distance, which gives the grating unit a high quality factor and significantly reduces the upradiation efficiency loss during the mode field matching process.

[0030] The grating unit of the present invention does not require additional deposition of functional thin films and metal reflective layers when achieving directional radiation, making the process simple and low-cost. Furthermore, the use of a double quadrilateral structure makes it easy to process.

[0031] In some embodiments, the structural parameters of the second groove 2 satisfy: h2 = (0.001~1) × h1, w2 = (0.001~5) × h2, where w2 is the length of the bottom edge of the second groove 2.

[0032] Specifically, the structural parameters of both the first tank 1 and the second tank 2 include depth, bottom edge length, and bottom angles. The structural parameters of the first tank 1 can be any initial values, including the depth h1, the two bottom angles β1 and β2, and the bottom edge length w1. The structural parameters of the second tank 2 include the two bottom angles α1 and α2, the depth h2, and the bottom edge length w2. The structural parameters of the second tank 2 are further optimized based on the structural parameters of the first tank 1. That is, h2 = (0.001~1) × h1, w2 = (0.001~5) × h2. Preferably, h2 = 0.66 × h1, w2 = 0.76 × h2.

[0033] The bottom angles of the first tank 1 and the second tank 2 may be equal or unequal. For example, the bottom angles of the first tank 1 are all 100°, and the bottom angles of the second tank are all 110°, or the two bottom angles of the first tank 1 and the two bottom angles of the second tank are both 110°, or the left bottom angle of the first tank is 90° and the right bottom angle is 100°, and the left bottom angle of the second tank is 100° and the right bottom angle is 110°.

[0034] Based on the unidirectional guided resonance principle of topological charge splitting-merging, the structural parameters of the first slot 1 and the second slot 2 can break the continuum bound state protected by symmetry in the transverse electric (TE) or transverse magnetic (TM) modes of the grating unit, so that the integer topological charges (q=±1) on the upper and lower sides of the grating unit are split into a pair of half-integer topological charges (q=±1 / 2) carried by circular polarization states. The upper half-integer topological charges remain separated, while the lower half-integer charges continue to merge into a continuum bound state after splitting, thereby realizing unidirectional radiation without a bottom reflector and suppressing downward radiation.

[0035] In some embodiments, the thickness hw of the waveguide layer satisfies: hw = (1~10) × h1, where h1 is the depth of the first groove 1.

[0036] like Figure 4 As shown, this parameter range supports the operation of grating units at energy levels of TE-C and below, and maintains a low refractive index abrupt change between waveguide modes and grating modes to reduce backscattering.

[0037] Furthermore, the cross-section of the first groove 1 and / or the second groove 2 is an isosceles trapezoid, a right trapezoid, a rectangle, a square, or a parallelogram, etc. In a preferred example, the cross-section of the first groove 1 and the second groove 2 is an isosceles trapezoid, and the length of the top side is greater than the length of the corresponding bottom side, so as to realize the upward unilateral radiation of the optical signal, and the isosceles trapezoid is easy to process.

[0038] This invention also provides a grating array, such as... Figure 5As shown, it includes several grating units as in any of the above embodiments. The several grating units are arranged along a first direction. Among the first preset number of grating units, the bottom edge length of the first groove 1 increases from a first initial value to a first target value, and the bottom edge length of the second groove 2 increases from a second initial value to a second target value. The structural parameters of the grating units arranged in the first preset number and subsequent numbers are the same.

[0039] Specifically, the grating unit can be an air cladding layer, the waveguide layer can be lithium niobate on insulator (LNOI) material, the waveguide layer is disposed above the buried oxide layer (BOX), the buried oxide layer is disposed above the substrate layer, and the substrate layer is made of silicon (Si) material. It should be understood that the specific materials of the cladding layer, waveguide layer, buried oxide layer, and substrate layer can be set according to actual needs; this embodiment is only for illustrative purposes.

[0040] The first direction is the X direction, and the grating units are arranged from left to right along the first direction. The total number of grating units is N, and the first preset number is n, where 0 < n ≤ N. When n < N, the grating units undergo partial apodization, and when n = N, the grating units undergo complete apodization.

[0041] The first and second initial values ​​are set according to actual needs. In one example, both the first and second initial values ​​are set to 0. That is, the bottom edge of the first slot 1 of the first n grating units increments from 0 as the first initial value to the first target value wn1; the bottom edge of the second slot 2 of the first n grating units increments from 0 as the second initial value to the second target value wn2. The value of n is adjusted so that the matching degree between the upper radiation real space mode field of the grating array and the real space mode field of the target light source is greater than a set value, for example, so that the matching degree between the upper radiation real space mode field of the grating array and the real space mode field of the target light source is greater than 80%.

[0042] Wherein, the first target value wn1 and the second target value wn2 are the bottom edge lengths of the first slot 1 and the second slot 2 in the standard grating unit, respectively. The standard grating unit is obtained through the multiphysics coupling simulation software COMSOL. Its function is to act as a parameter intermediary. By obtaining the structural parameters of the standard grating unit with unidirectional guided resonance (UGR) characteristics, and using the bottom edge lengths of the first slot 1 and the second slot 2 of the standard grating unit as the first target value wn1 and the second target value wn2, the bottom edges of the first n grating units in the grating array are incrementally apodized, thereby constructing the grating array.

[0043] Specifically, the real-space mode field of the target light source generally exhibits a Gaussian distribution. Without an increasing apodization structure, when a beam is incident on the grating array from the left, the beam intensity decreases due to successive scattering. The beam intensity on the right side of the grating array is less than that on the left. Since each grating unit has the same scattering capability, the upper radiating real-space mode field of the grating array is as follows: Figure 6 As shown, the radiation on the left and right sides is unbalanced. By setting an incremental apodization structure, the scattering ability of the grating unit on the left side can be made less than that of the grating unit on the right side, thus making the radiation on both sides relatively balanced. In fact, the spatial mode field distribution is as follows: Figure 7 As shown.

[0044] The grating array of this invention introduces a spacing between the first slot 1 and the second slot 2 to improve the quality factor. By using the grating units with the above-mentioned high quality factor for apodization arrangement, the upradiative efficiency loss during the mode field matching process is greatly reduced.

[0045] In some embodiments, the scattering angles of the plurality of grating units arranged first along the first direction increase from the initial scattering angle to the target scattering angle, wherein the initial scattering angle and the target scattering angle satisfy: When theta > 0, 0 < theta0 and theta1 ≤ 3 × theta; When theta < 0, theta0 < 0 and theta1 ≥ 3 × theta; ; period1 = A × B; Where theta0 is the initial scattering angle, theta1 is the target scattering angle, and theta is the preset grating scattering angle. For wave vector, Let π be the mathematical constant, period1 be the first grating period, A be the adjustment coefficient, and B be the sum of the top edge length of the first slot 1, the top edge length of the second slot 2, and the distance between the first slot 1 and the second slot 2 in the grating unit.

[0046] Specifically, the preset grating scattering angle is the scattering angle of a standard grating unit, and the first grating period period1 = A × B. When the cross-section of the first slot 1 and the second slot 2 is an isosceles trapezoid, the parameter B is the length of each top edge plus the spacing obtained according to the geometric relationship of the quadrilateral. The specific calculation formula is as follows: B=hn1 / tan(π-β1)+hn1 / tan(π-β2)+wn1+hn2 / tan(π-α1)+hn2 / tan(π-α2) +wn2+wn3); Wherein, hn1 is the depth of the first slot 1 in the standard grating unit, hn2 is the depth of the second slot 2 in the standard grating unit, wn1 is the first target value, which is also the bottom edge length of the first slot 1 in the standard grating unit, wn2 is the second target value, which is also the bottom edge length of the second slot 2 in the standard grating unit, and wn3 is the distance between the first slot 1 and the second slot 2 in the standard grating unit. period1 is obtained in the multiphysics coupling simulation software COMSOL, and its function is to serve as a parameter intermediary to obtain the structural parameters of the standard grating unit under UGR.

[0047] It should be understood that the number of grating elements performing scattering angle apodization is less than or equal to the total number of grating elements in the grating array. Furthermore, the number of grating elements performing scattering angle apodization can be the same as or different from the number of grating elements performing bottom edge apodization. For example, partial apodization can be performed on the bottom edge while full apodization is performed on the scattering angle.

[0048] The preset grating scattering angle theta is calculated by the first grating period period1, and then the range of values ​​for the initial scattering angle and the target scattering angle is determined. Based on the above values, the scattering angle in momentum space is adjusted, which can improve the matching degree between the upper radiation momentum space mode field (far-field divergence angle) of the grating array and the momentum space mode field of the target light source, so that the momentum space mode field matching degree reaches the set value, for example, 80%.

[0049] Furthermore, the optimal structural parameters can be obtained by performing cross-iterative optimization on the real space and momentum space.

[0050] In some embodiments, a plurality of grating units are arranged along a first direction with a width of one grating unit equal to the second grating period, wherein the second grating period satisfies: period2=((lam+w1×(nwg-neff1)+w2×(nwg-neff2)) / (nwg–nc×sin(theta))); Where period2 is the second grating period, w1 is the bottom edge length of the first groove 1, w2 is the bottom edge length of the second groove 2, lam is the light wavelength, nwg is the effective refractive index of the waveguide layer, neff1 is the effective refractive index of the waveguide layer at the bottom edge depth of the first groove 1, neff2 is the effective refractive index of the waveguide layer at the bottom edge depth of the second groove 2, and nc is the refractive index of the cladding layer on the grating unit.

[0051] Specifically, the second grating period is the width of each grating unit along the first direction. The second grating period is determined according to the above formula and used as the width of each corresponding grating unit, which can maintain phase matching between each grating unit in the case of apodization.

[0052] The present invention also provides a grating coupler comprising a grating array as described in any of the above embodiments.

[0053] like Figure 8 and Figure 9 As shown, grating couplers are mainly composed of grating arrays that extend longitudinally in a straight strip shape to form a non-focused grating coupler, or extend in an arc shape to form a focused grating coupler.

[0054] Preferably, the depth h1 of the first slot 1 of the first 3 grating units in the grating coupler is 0.

[0055] The embodiments of the present invention will be described below with reference to specific circumstances.

[0056] Example 1: Taking lithium niobate as an example, the communication wavelength of the grating unit proposed in this invention is 850nm, and an air cladding is used; The waveguide layer has a thickness hw = 350 nm; both the first groove 1 and the second groove 2 are isosceles trapezoids with base angles α1, α2, β1, and β2 all equal to 97°; the depth of the first groove 1 is h1 = 260 nm, and the length of its base is w1 = 110 nm; the spacing between the first groove 1 and the second groove 2 is w3 = 0. The depth of the second groove 2 is h2 = 171 nm, and the length of its base is w2 = 130 nm; the first grating period is period1 = 432 nm; this grating unit is used as a standard grating unit, and its scattering angle is the preset grating scattering angle theta = -12.6°.

[0057] Combination Figure 10 and Figure 11 As shown, the wave vector of the grating unit is kx = -0.111; the normalized frequency is 0.51, and it operates in the TE-A band. The up-down radiation ratio is 63.7dB. It has good up-radiation characteristics for short-wavelength optical signals, effectively prevents down-leakage, and can significantly improve the radiation efficiency of short-wavelength optical signals.

[0058] Correspondingly, the grating array in the non-focused grating coupler has a total of 55 grating elements, nc=1, and the specific apodization is as follows: The bottom edge length of the first slot 1 of the first 30 grating units is w1 = 110 × (x1 - 1) / 29; the bottom edge length of the second slot 2 of the first 30 grating units is w2 = 130 × (x1 - 1) / 29, where 0 < x1 ≤ 30 and is an integer; The scattering angle of all grating units is theta_n = (-0.2735 × x2 - 4.7265)°, where 0 < x2 ≤ 55 and is an integer.

[0059] Combination Figure 12 and Figure 13As shown, this grating coupler achieves a high degree of matching between the grating coupler and the target light source in terms of the electric field distribution in real space and the far-field divergence angle in momentum space.

[0060] Combination Figure 14 and Figure 15 As shown, this grating coupler has high coupling efficiency for short-wavelength beams and achieves unidirectional radiation without a bottom mirror.

[0061] Example 2: Taking lithium niobate as an example, the communication wavelength of the grating unit proposed in this invention is 860nm, and an air cladding is used; The waveguide layer has a thickness hw = 350 nm; both the first groove 1 and the second groove 2 are rectangular, with base angles α1, α2, β1, and β2 all equal to 90°; the depth of the first groove 1 is h1 = 236 nm, and the length of its base is w1 = 136 nm; the spacing between the first groove 1 and the second groove 2 is w3 = 0. The depth of the second groove 2 is h2 = 142 nm, and the length of its base is w2 = 176 nm; the first grating period is period1 = 426 nm; this grating unit is used as a standard grating unit, and its scattering angle is the preset grating scattering angle theta = -10.9°.

[0062] like Figure 16 As shown, the wave vector of the grating unit is kx = -0.113; it operates in the TE-A band; and its up-down radiation ratio is 65.2dB. It has good up-radiation characteristics for short-wavelength optical signals, effectively preventing down-leakage and significantly improving the radiation efficiency of short-wavelength optical signals.

[0063] Correspondingly, the grating array in the non-focused grating coupler has a total of 55 grating elements, nc=1, and the specific apodization is as follows: The bottom edge length of the first groove 1 of the first 20 grating units increases linearly from 50nm to w1; the bottom edge length of the second groove 2 of the first 20 grating units increases linearly from 70nm to w2. The scattering angle of all grating units is theta_n = (-0.2735 × x2 - 4.7265)°, where 0 < x2 ≤ 55 and is an integer.

[0064] like Figure 17 As shown, this grating coupler exhibits high coupling efficiency for short-wavelength beams and achieves unidirectional radiation without a bottom mirror. Therefore, the grating coupler of this embodiment achieves an optimal balance between mode field matching and upward radiation, significantly improving the coupling efficiency of short-wavelength grating couplers.

[0065] The embodiments of the present invention have the following technical effects: Preventing downward leakage: By applying the unidirectional guided resonance principle based on topological charge splitting-merging to the short-wavelength band, a double quadrilateral grooved grating structure is proposed to realize unidirectional radiation without a bottom mirror in the short-wavelength grating coupler and suppress downward radiation. Reduced process complexity: The directional radiation grating unit does not require additional deposition of functional thin films and metal reflective layers, and its double quadrilateral structure makes it easy to process; Reduce the difficulty of mode field matching: When arranging grating units into grating couplers, apodization is performed on the real space (groove width) and momentum space (grating period), and spacing is introduced between adjacent grooves to improve the quality factor. This significantly reduces the upradiation efficiency loss during the mode field matching process and achieves a high degree of matching between the grating coupler and the target light source in the real space electric field distribution and the momentum space far-field divergence angle, that is, achieving the best balance between "mode field matching" and "upradiation".

[0066] Although embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A grating unit, characterized in that, The waveguide layer includes a first groove and a second groove. Both the first and second grooves have quadrilateral cross-sections. The depth of the first groove is greater than the depth of the second groove. The distance w3 between the first and second grooves is greater than a negative value of a preset distance, which satisfies the following: Where L is the preset distance, h2 is the depth of the second groove, α1 is the bottom angle of the side of the second groove closest to the first groove, and β2 is the bottom angle of the side of the first groove closest to the second groove.

2. The grating unit according to claim 1, characterized in that, The structural parameters of the second tank body satisfy: h2 = (0.001~1) × h1, w2 = (0.001~5) × h2, where w2 is the bottom edge length of the second tank body.

3. The grating unit according to claim 1, characterized in that, The thickness hw of the waveguide layer satisfies: hw = (1~10) × h1, where h1 is the depth of the first groove.

4. The grating unit according to claim 1, characterized in that, The cross-section of the first tank and / or the second tank is an isosceles trapezoid, a right trapezoid, a rectangle, a square, or a parallelogram.

5. The grating unit according to claim 1, characterized in that, The length of the top edge of the first groove and / or the second groove is greater than or equal to the length of the bottom edge.

6. A grating array, characterized in that, It includes a plurality of grating units as described in any one of claims 1 to 5, wherein the plurality of grating units are arranged along a first direction, and in the first predetermined number of grating units, the bottom edge length of the first groove increases from a first initial value to a first target value, and the bottom edge length of the second groove increases from a second initial value to a second target value, and the structural parameters of the grating units arranged in the first predetermined number and thereafter are the same.

7. The grating array according to claim 6, characterized in that, The scattering angles of the first few grating units arranged along the first direction increase from the initial scattering angle to the target scattering angle, and the initial scattering angle and the target scattering angle satisfy: When theta > 0, 0 < theta0 and theta1 ≤ 3 × theta; When theta < 0, theta0 < 0 and theta1 ≥ 3 × theta; ； period1 = A × B; Where theta0 is the initial scattering angle, theta1 is the target scattering angle, and theta is the preset grating scattering angle. For wave vector, Let π be the mathematical constant, period1 be the first grating period, A be the adjustment coefficient, and B be the sum of the top edge length of the first slot, the top edge length of the second slot, and the distance between the first slot and the second slot in the grating unit.

8. The grating array according to claim 7, characterized in that, A plurality of the grating units are arranged along a first direction with a second grating period as the width of one grating unit, wherein the second grating period satisfies: period2=((lam+w1×(nwg-neff1)+w2×(nwg-neff2)) / (nwg–nc×sin(theta))); Wherein, period2 is the second grating period, w1 is the bottom edge length of the first groove, w2 is the bottom edge length of the second groove, lam is the light wavelength, nwg is the effective refractive index of the waveguide layer, neff1 is the effective refractive index of the waveguide layer at the bottom edge depth of the first groove, neff2 is the effective refractive index of the waveguide layer at the bottom edge depth of the second groove, and nc is the refractive index of the cladding layer on the grating unit.

9. A grating coupler, characterized in that, Including the grating array as described in any one of claims 6-9.

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