Lnoi polarization independent cross waveguide based on subwavelength grating structure

Abstract

The application provides a kind of LNOI polarization independent cross waveguide based on subwavelength grating structure, including silica substrate, lithium niobate waveguide and silica upper cladding, lithium niobate waveguide is located on silica substrate, silica upper cladding covers on lithium niobate waveguide;Lithium niobate polarization independent cross waveguide includes central square multimode interference self-imaging area, four groups of subwavelength grating area and four branches of tapered transition waveguide, four groups of subwavelength grating area is distributed along multimode interference area, is connected with four sides of self-imaging area, four branches of tapered transition waveguide are coincidently connected with subwavelength grating group, the outer end of tapered transition waveguide is connected with input or output waveguide;The application supports TE, TM polarization mode in X cut Y transmission and X cut Z transmission two-way low-loss transmission, and polarization independent, mode crosstalk is small;LNOI material has the characteristics of wide transparent waveband, low loss, high optical damage threshold resistance;Subwavelength grating structure can adjust and control waveguide equivalent refractive index, widen the working bandwidth of device.

Description

Technical Field

[0001] This invention belongs to the field of integrated photonic device technology, specifically relating to an LNOI polarization-independent cross waveguide based on a subwavelength grating structure. Background Technology

[0002] With the development of modern technology, the demand for low-power, small-size, and large-scale devices in the field of communication is increasing. In large-scale integrated photonic chips, cross-waveguides are one of the indispensable key components. Therefore, cross-waveguides are crucial for improving the transmission performance and reducing transmission loss of integrated photonic chips.

[0003] Lithium niobate on insulator (LNOI) utilizes the fact that lithium niobate has a higher refractive index than silicon dioxide, which allows it to effectively confine light within lithium niobate waveguides. Furthermore, thanks to the large electro-optic coefficient and direct modulation characteristics of lithium niobate, modulators and waveguide devices can be easily integrated onto the same material platform.

[0004] Most reported LNOI polarization-independent cross waveguides only achieve polarization insensitivity in a single transmission direction, and are large in size and have narrow operating bandwidth, which cannot meet the requirements of ultra-high bandwidth and high integration applications. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides an LNOI polarization-independent cross-waveguide based on a subwavelength grating structure. By introducing a subwavelength grating structure to control the waveguide's equivalent refractive index and combining it with the principle of multimode interference self-imaging, polarization-independent transmission of TE and TM modes is achieved in both X-cut Y-transmission and X-cut Z-transmission directions. It also has the advantages of large bandwidth, low loss, and low crosstalk.

[0006] A LNOI polarization-independent cross waveguide based on a subwavelength grating structure includes a silica substrate, a lithium niobate waveguide, and a silica cladding. The lithium niobate waveguide is located on the silica substrate, and the silica cladding covers the lithium niobate waveguide.

[0007] The lithium niobate waveguide includes a central cubic multimode interference self-imaging region, four sets of subwavelength grating regions, and four tapered transition waveguides. The four sets of subwavelength grating regions are distributed along the central multimode interference self-imaging region and are connected to the four sides of the self-imaging region. The four tapered transition waveguides coincide with and are connected to the subwavelength grating regions. Input or output waveguides are connected to the outer ends of the tapered transition waveguides.

[0008] There are four subwavelength grating regions, with regions 2 and 4 having symmetrical structures, and regions 7 and 8 having symmetrical structures. Each subwavelength grating region includes a constant-width region and a gradually changing-width region, and the subwavelength grating period width is 0.236 μm.

[0009] In X-cut Y-transmission, the constant-width and gradient-width regions in subwavelength grating regions 2 and 4 are composed of 18 and 20 subwavelength grating units arranged along the optical transmission direction, respectively, with a duty cycle of 0.8. The grating width in the constant-width region is constant at 3 μm, and the grating tooth width is uniformly 0.1888 μm; the grating width in the gradient-width region linearly gradients from 1.5 μm to 3 μm along the optical transmission direction, with a uniform grating tooth width of 0.1888 μm. In X-cut Z-transmission, the constant-width and gradient-width regions in subwavelength grating regions 7 and 8 are composed of 23 and 20 subwavelength grating units arranged along the optical transmission direction, respectively, with a duty cycle of 0.7. The grating width in the constant-width region is constant at 3 μm, and the grating tooth width is uniformly 0.1652 μm; the grating width in the gradient-width region linearly gradients from 1.5 μm to 3 μm along the optical transmission direction, with a uniform grating tooth width of 0.1652 μm.

[0010] The tapered transition waveguide has the same width gradient region length as the subwavelength grating region it is connected to, with the width linearly gradienting from 1 μm to 0.1 μm or from 0.1 μm to 1 μm.

[0011] Furthermore, the material of the lithium niobate waveguide is lithium niobate, which is an anisotropic crystal.

[0012] Furthermore, the lithium niobate waveguide has a thickness of 600 nm and an etching depth of 300 nm.

[0013] Furthermore, the widths of both the input and output waveguides are 1 μm.

[0014] Furthermore, the length of the tapered transition waveguide in the X-to-Y tangent direction is 4.72 μm, and the lengths of subwavelength grating regions 2 and 4 are both 8.968 μm; the length of the tapered transition waveguide in the X-to-Z tangent direction is 4.72 μm, and the lengths of subwavelength grating regions 7 and 8 are both 10.148 μm.

[0015] Furthermore, the refractive index of the lithium niobate waveguide is [nx, ny, nz] = [2.2111, 2.211, 2.1376].

[0016] Furthermore, a subwavelength grating is introduced to adjust the effective refractive index of the waveguide.

[0017] The beneficial effects of this invention are as follows: 1. It supports low-loss propagation in both X-axis and X-axis Z-axis directions in TE and TM modes, with extremely low polarization crosstalk and small polarization correlation. 2. The structure is symmetrical and regular, with strong stability. The device size is only about 20.9μm × 23.3μm, making it compact and small, which is convenient for large-scale photonic integrated chip applications. 3. It uses LNOI material. Compared with silicon, lithium niobate has advantages such as wide transparency band, low absorption loss, and strong electro-optic effect, while also having high nonlinear optical, electrical, acoustic, and thermo-optic coefficients. 4. The introduction of a subwavelength grating structure and a tapered transition waveguide can significantly widen the operating bandwidth compared with the traditional rectangular cross-shaped waveguide. In the 415nm ultrawideband, the insertion loss in the Y-axis direction is less than 0.31dB and the crosstalk is less than -36dB, while the insertion loss in the Z-axis direction is less than 0.47dB and the crosstalk is less than -34dB, resulting in high transmission efficiency and a large operating bandwidth. 5. The composite structure of multimode interference and subwavelength grating has better mode matching and more flexible effective refractive index control compared with conventional MMI cross waveguides. It can achieve polarization independence at both ports, and has a large process tolerance and strong processing robustness, making it more suitable for practical fabrication. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of an LNOI polarization-independent cross waveguide based on a subwavelength grating structure proposed in this invention;

[0019] Figure 2 (a) A schematic diagram of the specific structure of the input waveguide 1, the input tapered waveguide, and the subwavelength grating region 2 in the transverse Y-direction; (b) A schematic diagram of the specific structure of the input waveguide 6, the input tapered waveguide, and the subwavelength grating region 7 in the longitudinal Z-direction.

[0020] Figure 3 This is a schematic diagram of the LNOI waveguide cross-section;

[0021] Figure 4 The diagram shows the electric field simulation of the device in the transverse Y-direction under (a) TE and (b) TM modes; and the electric field simulation of the device in the longitudinal Z-direction under (c) TE and (d) TM modes.

[0022] Figure 5 The insertion loss and crosstalk of the TE and TM modes in the transverse Y direction are (a) and (b) respectively; the insertion loss and crosstalk of the TE and TM modes in the longitudinal Z direction are (c) and (d) respectively. Detailed Implementation

[0023] The present invention will now be described in detail with reference to the accompanying drawings. Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention. The directional terms such as left, center, right, top, and bottom in the embodiments of the present invention are only relative concepts or referenced to the normal use state of the product, and should not be considered restrictive.

[0024] Example: Refer to Appendix Figure 1 A polarization-independent cross-waveguide based on a subwavelength grating structure is proposed, consisting of two identical and mutually perpendicular cross-sectional parts: a transverse (X-cut Z-transmission) section and a longitudinal (X-cut Y-transmission) section. The transverse section comprises an input waveguide 1, an input tapered waveguide connected to a subwavelength grating region 2, a self-imaging region 3, an output tapered waveguide connected to a subwavelength grating region 4, and an output waveguide 5, arranged in a left-right symmetrical manner. The longitudinal section comprises an input waveguide 6, an input tapered waveguide connected to a subwavelength grating region 7, a self-imaging region 3, an output tapered waveguide connected to a subwavelength grating region 8, and an output waveguide 9, arranged in a top-bottom symmetrical manner. After light enters the subwavelength grating waveguide region from the input tapered waveguide, the mode field width changes gradually, generating a stable multimode interference effect and periodically forming self-imaging points within the waveguide. The subwavelength grating waveguide described in this invention precisely controls the phase matching conditions of multimode interference through differentiated design of grating period, duty cycle, and period number, enabling the optical field to self-reconstruct at the center of the self-imaging region 3, significantly suppressing mode crosstalk at waveguide intersections. After the optical field passes through the subwavelength grating waveguide region, the second self-imaging point falls at the output tapered waveguide for low-loss output, greatly reducing transmission loss.

[0025] This invention combines Finite-Difference Time-Domain (FDTD) simulation with Particle Swarm Optimization (PSO) algorithm to perform global parameter optimization, obtaining optimal structural parameters that meet the requirements of wide bandwidth, low loss, low crosstalk, and polarization independence. The PSO algorithm has advantages such as fast convergence speed, few parameters, and simple implementation, and can efficiently approximate the optimal solution in high-dimensional parameter optimization, but it relies on reasonable parameter initialization. Based on the PSO algorithm principle, a subwavelength grating waveguide model is first constructed in the simulation software, with the grating period fixed at 0.236 μm. Key parameter sampling points are uniformly selected along the propagation direction, using the number of periods, duty cycle, and subwavelength waveguide width in each subwavelength grating region as optimization variables, and setting reasonable parameter value ranges. During the optimization process, the objective functions are maximizing the normalized transmission power of TE and TM modes and minimizing insertion loss and polarization-dependent loss. The structural parameters are updated and FDTD simulation is performed in each iteration. If the performance of the objective function improves, the parameter update is accepted; otherwise, the change is discarded. After multiple rounds of iteration and convergence, the optimal parameter combination is obtained as follows.

[0026] See attached document Figure 2 (a) A schematic diagram of the specific structure of the input waveguide 1 and the input tapered waveguide and subwavelength grating region 2 in the transverse Y-direction. The width of the input tapered waveguide at the front end is selected as W1=1μm, and it narrows to W2=0.1μm at the end to suppress mode reflection. The grating period length of the subwavelength grating region 2 is set as Λ=0.236μm, the duty cycle is f1=0.8, the number of periods of the subwavelength grating region 2 in the constant width region and the gradually changing width region are N1=18 and N2=20, respectively, and the width of the gradually changing width region gradually changes from W3=1.5μm to W4=3μm. The output tapered waveguide, subwavelength grating region 4 and output waveguide 5 at the other end are consistent with the input tapered waveguide, subwavelength grating region 2 and input waveguide 1 in terms of structural parameters, and can be obtained by the left and right mirror symmetry of the input waveguide 1 and the input tapered waveguide and subwavelength grating region 2 about the self-imaging region 3.

[0027] See attached document Figure 2 (b) A schematic diagram of the specific structure of the input waveguide 6 and the input tapered waveguide and subwavelength grating region 7 in the longitudinal Z-direction. The width of the input tapered waveguide at the front end is selected as W5=1μm, and it narrows to W6=0.1μm at the end to suppress mode reflection. The grating period length of the subwavelength grating region 7 is set as Λ1=0.236μm, the duty cycle is f2=0.8, the number of periods of the subwavelength grating region 7 in the constant width region and the gradually changing width region are N3=23 and N4=20, respectively, and the width of the gradually changing width region gradually changes from W7=1.5μm to W8=3μm. The output tapered waveguide, subwavelength grating region 8 and output waveguide 9 at the other end are consistent with the input tapered waveguide, subwavelength grating region 7 and input waveguide 6 in terms of structural parameters, which can be obtained by the symmetry of the input waveguide 6 and the input tapered waveguide and subwavelength grating region 7 with respect to the upper and lower mirrors of the self-imaging region 3.

[0028] The overall device size is approximately 20.9 μm × 23.3 μm, with a compact structure that facilitates large-scale on-chip integration. The self-imaging region 3 is enclosed by the ends of four subwavelength grating waveguides, with a side length equal to the width of the constant subwavelength grating region (3 μm), ensuring structural continuity and the integrity of the optical field self-imaging.

[0029] The invented device is based on X-cut LNOI material. The substrate, from bottom to top, consists of a silicon (Si) substrate, a 4.7 μm thick silicon dioxide (SiO2) buried layer, and a 600 nm thick lithium niobate layer. The waveguide etching depth is 300 nm. At a working wavelength of 1550 nm, the refractive index of the SiO2 cladding is 1.444. The lithium niobate material is an anisotropic crystal, and the refractive index of the lithium niobate waveguide is [nx, ny, nz] = [2.2111, 2.2111, 2.1376]. The mode field distributions of TE and TM modes differ significantly in different propagation directions, making it difficult for traditional symmetrical structures to simultaneously achieve bidirectional polarization-independent transmission. This invention effectively compensates for mode mismatch caused by material anisotropy by differentiating the grating duty cycles (0.8 laterally and 0.7 longitudinally) and their respective period numbers in the lateral and longitudinal directions. This ensures low insertion loss transmission in both TE and TM modes while significantly reducing polarization-dependent loss and crosstalk.

[0030] Reference Figure 3 Silica was selected as the substrate material. A 0.3 μm thick lithium niobate plate layer was placed on the substrate, and a 0.3 μm thick lithium niobate sheet was placed on top of the plate layer. The refractive indices of the silica substrate and the lithium niobate waveguide were 1.48 and 2.2, respectively. Photolithography was performed on the lithium niobate sheet, which was then buried in a 4.7 μm thick silica cladding.

[0031] Reference Figure 4 Optical field analysis was performed on the LNOI polarization-independent cross waveguide based on the subwavelength grating structure for both TE and TM modes. It can be seen that the color changes of the optical field in TE and TM modes are regular. The density of the light spot changes from dense to sparse and then back to dense. This is because multimode interference is generated from single-mode light, and then periodically changes back to single-mode light output due to its self-imaging effect.

[0032] Reference Figure 5 FDTD simulations were performed on the transmission of TE and TM modes in the transverse and longitudinal directions of the subwavelength grating-controlled two-port polarization-independent cross-waveguide based on the LNOI platform. Within the wavelength range of 1260nm to 1675nm (i.e., a bandwidth of 415nm), the insertion loss of both TE and TM modes in the Y-direction was less than 0.31dB and the crosstalk was less than -36dB, while in the Z-direction, the insertion loss of both TE and TM modes was less than 0.47dB and the crosstalk was less than -34dB. Excellent low-loss and polarization-independent characteristics were observed at both ports.

[0033] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A LNOI polarization-independent cross waveguide based on a subwavelength grating structure, characterized in that: It includes a silicon dioxide substrate, a lithium niobate waveguide, and a silicon dioxide top cladding. The lithium niobate waveguide is located on the silicon dioxide substrate, and the silicon dioxide top cladding covers the lithium niobate waveguide and the silicon dioxide substrate. The lithium niobate waveguide includes a central cubic multimode interference self-imaging region, four sets of subwavelength grating regions, and four tapered transition waveguides. The four sets of subwavelength grating regions are distributed along the central multimode interference self-imaging region and are connected to the four sides of the self-imaging region. The four tapered transition waveguides coincide with and are connected to the subwavelength grating regions. Input or output waveguides are connected to the outer ends of the tapered transition waveguides. There are four subwavelength grating regions. Regions 2 and 4 are structurally symmetrical, and regions 7 and 8 are structurally symmetrical. Each subwavelength grating region includes a constant width region and a gradually changing width region. The period width of each subwavelength grating region is 0.236 μm.

2. The LNOI polarization-independent cross waveguide based on a subwavelength grating structure according to claim 1, characterized in that: In X-cut Y-transmission, the constant-width and gradient-width regions in subwavelength grating regions 2 and 4 are composed of 18 and 20 subwavelength grating units arranged along the optical transmission direction, respectively, with a duty cycle of 0.

8. The grating width in the constant-width region is constant at 3 μm, and the grating tooth width is uniformly 0.1888 μm. In the gradient-width region, the grating width linearly gradients from 1.5 μm to 3 μm along the optical transmission direction, with a uniform grating tooth width of 0.1888 μm. In X-cut Z-transmission, the constant-width and gradient-width regions in subwavelength grating regions 7 and 8 are composed of 23 and 20 subwavelength grating units arranged linearly along the optical transmission direction, respectively, with a duty cycle of 0.

7. The grating width in the constant-width region is constant at 3 μm, and the grating tooth width is uniformly 0.1652 μm. In the gradient-width region, the grating width linearly gradients from 1.5 μm to 3 μm along the optical transmission direction, with a uniform grating tooth width of 0.1652 μm. The tapered transition waveguide has the same width gradient region length as the subwavelength grating region it is connected to, with the width linearly gradienting from 1 μm to 0.1 μm or from 0.1 μm to 1 μm.

3. The LNOI polarization-independent cross waveguide based on a subwavelength grating structure according to claim 1, characterized in that: The material of the lithium niobate waveguide is lithium niobate, which is an anisotropic crystal.

4. The LNOI polarization-independent cross waveguide based on a subwavelength grating structure according to claim 1, characterized in that: The lithium niobate waveguide has a thickness of 600 nm and an etching depth of 300 nm.

5. The LNOI polarization-independent cross waveguide based on a subwavelength grating structure according to claim 1, characterized in that: The width of both the input and output waveguides is 1 μm. The lengths of subwavelength grating regions 2 and 4 in the X-to-Y tangent direction are 8.968 μm, and the lengths of subwavelength grating regions 7 and 8 in the X-to-Z tangent direction are 10.148 μm.

6. The LNOI polarization-independent cross waveguide based on a subwavelength grating structure according to claim 1, characterized in that: The refractive index of the lithium niobate waveguide is [nx, ny, nz] = [2.2111, 2.2111, 2.1376].

7. The subwavelength grating-controlled dual-port polarization-independent cross waveguide based on the LNOI platform according to claim 1, characterized in that: The effective refractive index of the waveguide is adjusted by introducing a subwavelength grating.

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