Method for regulating and controlling nonlinearity of optical waveguide

Abstract

The invention discloses a method for regulating and controlling the nonlinearity of an optical waveguide, and belongs to the technical field of optoelectronic devices, and the method comprises the steps: constructing a waveguide structure with sub-wavelength scale periodicity, and enabling the periodicity size to meet lambda < lt >; lambda / (2n), lambda is the period length, lambda is the incident light wavelength, and n is the effective refractive index of the medium; the sub-wavelength grating waveguide is integrated on an SOI platform or a heterogeneous integrated substrate; by adjusting the unit size, the spacing, the duty ratio and the geometrical shape of the sub-wavelength structure, the effective refractive index of the waveguide is regulated and controlled, the effective mode area is increased, the electric field intensity in the unit area is reduced, and two-photon absorption TPA and free carrier absorption FCA are inhibited; and the nonlinear threshold power of the waveguide is improved by using the equivalent dielectric characteristic of the sub-wavelength structure. The non-linear absorption effect can be effectively reduced, and the power threshold value of the waveguide is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of optoelectronic devices, and particularly to a method for regulating the nonlinearity of an optical waveguide. Background Art

[0002] With the rapid development of integrated photonics technology, silicon photon devices have been widely used in multiple fields such as data communication, optical interconnection, optical computing, and high-performance sensing due to their high refractive index contrast, low-cost mass manufacturing ability, and high compatibility with CMOS processes. Core devices such as modulators, switches, and filters all rely on silicon waveguides to precisely guide and regulate optical signals.

[0003] In the near-infrared communication band (especially at 1.3 μm and 1.55 μm), silicon materials have a relatively high nonlinear coefficient, which causes silicon waveguides to exhibit various nonlinear optical effects under high optical power conditions, including two-photon absorption (TPA), free carrier absorption (FCA), free carrier dispersion (FCD), Kerr effect (light intensity-dependent refractive index change), and stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) that may be excited under specific conditions. These nonlinear mechanisms limit the power handling capacity and transmission performance of the devices to a certain extent.

[0004] Currently, in order to address the power limitation problem faced by silicon waveguides in silicon photon devices, various technical solutions have been proposed. For example, some studies have introduced a PIN structure to apply a reverse bias to clear the free carriers generated by two-photon absorption, thereby reducing free carrier absorption; there are also solutions that attempt to use materials such as silicon nitride (SiN) or aluminum oxide (Al2O3) to replace part of the silicon waveguide structure to weaken the nonlinear loss. At the same time, waveguide geometry optimization (such as wide waveguide design, low-loss boundary design), and multi-channel design to reduce the light intensity distribution density are also gradually being adopted.

[0005] Although various methods for reducing the influence of nonlinear effects in silicon photon devices have been proposed in the prior art, such as increasing the waveguide size, introducing an anti-bias electrode structure, or even replacing the material platform, these methods still have the following deficiencies:

[0006] 1) The increase in device size limits the integration density. For example, by expanding the waveguide cross-section or using a multimode structure to reduce the unit light intensity, although the two-photon absorption and free-carrier absorption are effectively reduced, it will lead to an increase in the occupied area of the device, a decrease in the overall integration density, and is not conducive to large-scale photon integration.

[0007] 2) The manufacturing complexity increases and the cost rises. For example, introducing a PIN structure, electrodes, or a heat sink into the waveguide requires additional doping, metal deposition, and electrical isolation processes, increasing the manufacturing steps and the difficulty of device consistency control.

[0008] 3) The modulation speed is limited. Although introducing an electro-controlled carrier removal method can reduce the influence of free-carrier absorption, the response speed may be limited due to untimely carrier extraction in high-speed applications.

[0009] 4) Compatibility issues are faced when replacing the material platform. Although using low-nonlinear materials such as silicon nitride and alumina can reduce two-photon absorption, problems such as incompatibility with the CMOS process and reduced refractive index contrast are faced, making it difficult to completely replace the silicon platform. Summary of the Invention

[0010] To solve the problems of serious nonlinear loss and low power-carrying capacity existing in silicon waveguides in silicon photon devices in the prior art, the present invention proposes an integrated optical device structure based on a subwavelength grating waveguide (SWG Waveguide), and uses its unique equivalent medium characteristics to effectively reduce the nonlinear absorption effect and significantly improve the power threshold of the waveguide.

[0011] The embodiments of the present invention provide a method for regulating the nonlinearity of an optical waveguide, including the following steps:

[0012] Construct a waveguide structure with subwavelength-scale periodicity, the period size of which satisfies Λ < λ / (2n), where Λ is the period length, λ is the incident light wavelength, and n is the effective refractive index of the medium. The waveguide structure is composed of high-refractive-index material segments and low-refractive-index medium spacer regions arranged alternately in one or multiple dimensions, and forms a one-dimensional or two-dimensional periodic structure;

[0013] Integrate the subwavelength grating waveguide on an SOI platform or a heterogeneous integration substrate, and the substrate material includes silicon dioxide, silicon nitride, or aluminum nitride;

[0014] By adjusting the unit size, spacing, duty cycle, and geometric shape of the subwavelength structure, regulate the equivalent refractive index of the waveguide, increase the effective mode area, reduce the electric field intensity per unit area, and suppress two-photon absorption (TPA) and free-carrier absorption (FCA);

[0015] Utilize the equivalent medium characteristics of the sub-wavelength structure to increase the non-linear threshold power of the waveguide, enabling the device to maintain stable optical transmission at high input powers, while avoiding the problem of reduced integration due to the increased size of traditional waveguides.

[0016] According to a specific implementation manner of an embodiment of the present invention, the geometric shape of the high refractive index material segment includes at least one of a rectangular strip, a wedge-shaped strip, a circular column, or a rhombus.

[0017] According to a specific implementation manner of an embodiment of the present invention, by regulating the sub-wavelength structure parameters, the regulation of the free carrier lifetime is achieved to further suppress the influence of free carrier absorption on the optical transmission performance.

[0018] According to a specific implementation manner of an embodiment of the present invention, the sub-wavelength grating waveguide structure includes a multi-region design of multi-segment structure splicing, functional area division, or gradient adjustment structure.

[0019] According to a specific implementation manner of an embodiment of the present invention, the heterogeneous integration substrate is prepared by a standard silicon-based process, maintaining compatibility with the CMOS process.

[0020] According to a specific implementation manner of an embodiment of the present invention, the increase in the effective mode area is achieved through mode field distribution regulation, reducing the electric field intensity per unit area by at least 30%.

[0021] According to a specific implementation manner of an embodiment of the present invention, the non-linear threshold power is increased by at least 50% compared to the classical silicon straight waveguide.

[0022] According to a specific implementation manner of an embodiment of the present invention, the method is applied to high-speed modulators, optical switches, optical phased arrays, or high-performance sensors.

[0023] Compared with existing silicon photon devices using an all-silicon waveguide structure, the method for regulating the non-linear effect of an optical waveguide based on a sub-wavelength structure proposed by the present invention has the following beneficial effects:

[0024] 1) Significantly reduce non-linear losses: By regulating the mode field distribution of the SWG structure, effectively increase the effective mode area of the waveguide, significantly reduce the electric field intensity per unit area, thereby suppressing non-linear losses such as two-photon absorption and free carrier absorption induced thereby.

[0025] 2) Improve the power handling capacity of the device: The SWG waveguide has a higher non-linear threshold power, can maintain stable transmission at a higher input power, avoid strong non-linear distortion and thermal drift, and is beneficial for constructing optical modulation and control devices with a large dynamic range.

[0026] 3) Maintain compact size and CMOS compatibility: Compared with the method of directly increasing the size of the silicon waveguide to expand the mode field, the SWG structure does not require a significant increase in device size and can be fabricated by standard silicon-based processes, with good process feasibility and integration compatibility.

[0027] Through the optimization means at the structural level, the present invention effectively improves the threshold power that can be borne by silicon photonic devices under high-power conditions without sacrificing integration and process feasibility, providing important support for realizing high-performance and scalable silicon photonic systems. Description of the Drawings

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0029] Figure 1 Schematic diagram of the diversity of the subwavelength structure used in the present invention;

[0030] Figure 2 Graph of the relationship between the threshold output power of a silicon straight waveguide and the waveguide width;

[0031] Figure 3 Basic structure of the SWG waveguide;

[0032] Figure 4 Mode field distribution diagram of the SWG waveguide;

[0033] Figure 5 Comparison diagram of the input and output powers of a silicon straight waveguide and an SWG waveguide;

[0034] Figure 6 Graph of the relationship between the threshold output power of a 1-cm long SWG waveguide and the waveguide width. Detailed Description of the Embodiments

[0035] The embodiments of the present invention will be described in detail below with reference to the drawings.

[0036] The method for regulating the nonlinearity of an optical waveguide proposed by the present invention is based on a subwavelength structure, such as Figure 1 shown, and this structure is composed of the following functional units:

[0037] 1. Sub-wavelength structural unit: The sub-wavelength structure has sub-wavelength scale periodicity, that is, its period size is much smaller than the effective wavelength of the incident light in the medium, usually satisfying Λ < λ / (2n), where Λ is the period length, λ is the wavelength of the incident light, and n is the effective refractive index of the medium. Its basic unit includes one-dimensional or multi-dimensional alternately arranged high-refractive-index material (such as silicon, silicon nitride, etc.) segments and low-refractive-index medium (such as silica, air, polymer, etc.) spacer regions. The structural units of the sub-wavelength structure can exhibit various geometric shapes, including but not limited to rectangular bars, wedge-shaped bars, circular columns, rhombuses, etc., and can form one-dimensional or two-dimensional periodic structures in different dimensions (such as horizontal and vertical) according to design requirements, and adjust the structural spacing in each direction ( Figure 1 d1 to d3 in the figure). In addition, the arrangement and shape combination of the structural units can be flexibly configured to achieve fine control of parameters such as the equivalent refractive index.

[0038] 2. Substrate layer structure: The sub-wavelength structure can be integrated on an SOI (Silicon-On-Insulator) platform or other heterogeneous integration substrates. The substrate materials include but are not limited to: silica, silicon nitride, aluminum nitride, etc.

[0039] Figure 1 This is a schematic diagram of the diversity of the sub-wavelength structure used in the present invention, showing the combination methods of periods in multiple dimensions, different structural unit sizes and shape types, reflecting its flexibility and regulation potential in structural design.

[0040] The present invention not only covers Figure 1 the sub-wavelength structures shown in the figure, but also widely covers various variants and improved structures derived from this basic structure. These structures include but are not limited to: different material systems; structural designs with different shapes, duty cycles, and period lengths; multi-region designs including multi-segment structure splicing, functional area division, or gradient adjustment structures; composite structures integrated with other silicon photon devices; special layouts and integration methods for power distribution, mode field control, and nonlinear compensation. In particular, the present invention also advocates precise regulation of structural parameters (including structural unit size, spacing, duty cycle, etc.) to achieve adjustment of the free carrier lifetime, thereby suppressing the influence of free carrier absorption on optical transmission performance and improving the power threshold of the device. In addition, the present invention also protects composite integration schemes that further combine new materials or other enhanced functional modules on the above basic structure to achieve systematic optimization of problems such as nonlinear loss in the optical field transmission process.

[0041] To more clearly illustrate the technical solution and its effectiveness of the present invention, first, a brief analysis is made on the power limit and its physical mechanism of the silicon waveguide under high-power conditions. In a classical silicon optical waveguide, light is affected by nonlinear effects during propagation. Especially when a relatively high power is input in the communication band (around 1550 nm), two-photon absorption will trigger free-carrier absorption, resulting in optical power attenuation. The above effects can be described by the nonlinear propagation equation:

[0042]

[0043] where E is the electric field strength along the waveguide propagation direction z, β TPA is the two-photon absorption coefficient, σ FCA is the free-carrier absorption coefficient, N C is the free-carrier density, and α is the linear loss coefficient. The free-carrier density can be described by the following equation:

[0044]

[0045] Considering the case of continuous-wave input into the waveguide, the left end of Equation (2) can be taken as 0, and the solution is where τ C is the free-carrier lifetime. The power of the input and output waveguide is represented by P = E 2 A eff denotes, and A eff is the effective mode area of the waveguide, and the expression is:

[0046]

[0047] where F(x, y) is the optical intensity distribution function perpendicular to the propagation direction. By structurally adjusting the waveguide design to increase A eff , the optical field distribution in the waveguide is more dispersed, weakening the nonlinear effect, and improving the power-carrying capacity of the waveguide.

[0048] To further verify the influence of the waveguide geometric structure on the nonlinear propagation performance, the present invention conducts a simulation analysis on the optical power transmission characteristics of silicon straight waveguides under different waveguide width conditions. In the case of continuous-wave input into the waveguide, taking 90% of the maximum output power within the range of 0 - 6 W of the input power as the threshold output power, the image of the length of 1 cm straight waveguide width and the threshold output power is plotted as Figure 2 shown, Figure 2 is the relationship between the threshold output power of the silicon straight waveguide and the waveguide width, as shown by Figure 2It can be seen that, when the fixed carrier lifetime remains unchanged, the threshold output power of the waveguide increases with the increase of the waveguide width. Increasing the waveguide width can theoretically increase the output threshold power, but it will introduce problems such as increased process complexity and increased device volume, which is not conducive to integration, and there are certain difficulties in the actual device processing.

[0049] To solve the above problems, the present invention proposes a structural design based on a subwavelength grating waveguide as Figure 3 shown. By introducing a subwavelength structure into the waveguide design and designing a subwavelength grating waveguide, the modulation of the mode field distribution is realized, the effective mode area is effectively increased, and thus the power limitation effect caused by non-linear absorption is reduced. Figure 3 This is the basic structure of the SWG waveguide.

[0050] The mode field distribution diagram of the SWG waveguide is as Figure 4 shown, which respectively shows the mode distribution of the silicon material region (left figure) and the spacer region (right figure) of the SWG waveguide. The boxed part in the figure is the waveguide boundary. Figure 4 This is the mode field distribution diagram of the SWG waveguide.

[0051] In the case where the fixed carrier lifetime of the present invention is 1 ns, the input-output power comparison diagrams of silicon straight waveguides and SWG waveguides with widths of 0.5 μm and 1 μm are as Figure 5 shown, Figure 5 This is the input-output power comparison diagram of the silicon straight waveguide and the SWG waveguide.

[0052] In the case where the fixed carrier lifetime is 1 ns, compared with the classical silicon straight waveguide structure, the SWG waveguide shows a higher non-linear threshold power when the input power continuously increases, and the rising trend of its output power is more gentle, indicating that it has lower non-linear loss and stronger power carrying capacity under high power conditions, further verifying its structural advantages in non-linear loss suppression.

[0053] In order to further explore the influence of the subwavelength grating waveguide structure parameters on the non-linear loss characteristics, the present invention conducts a numerical simulation analysis on the results of the change of the non-linear threshold output power of different SWG waveguides with the waveguide width under the condition of a fixed carrier lifetime. It should be noted that in the actual structure, the free carrier lifetime of the SWG waveguide is not a fixed value and may change with the change of the waveguide geometry (such as width, periodic structure size, etc.). An increase in width will affect the spatial distribution of the optical field and the carrier recombination path, resulting in a dynamic adjustment of the lifetime. Therefore, in the simulation of the present invention, the lifetime is set to a fixed value to facilitate a clear analysis of the influence trend of the width on the threshold power under different lifetime conditions.

[0054] Figure 6 This is the relationship diagram between the threshold output power and the waveguide width of a 1 cm long SWG waveguide, asFigure 6 As shown, as the width of the SWG waveguide increases, the ability of the device to maintain stable output when the input power continuously increases is significantly enhanced, and the corresponding non-linear threshold output power shows an obvious upward trend. This indicates that by reasonably designing the lateral structural dimensions of the SWG waveguide, its power-bearing capacity can be effectively improved, and its performance in high-power optical phased array systems can be further optimized.

[0055] Compared with existing silicon photon devices using all-silicon waveguide structures, the present invention creatively uses the following technical means:

[0056] 1) Introduction of sub-wavelength grating waveguide structure: Sub-wavelength structures are introduced in waveguide design. By designing sub-wavelength grating waveguides, the optical field distribution is changed to achieve a reduction in non-linear loss while maintaining a compact size.

[0057] 2) Implementation path of high non-linear threshold power devices: It is clearly proposed to improve the non-linear threshold power through SWG waveguide design; simulation data support for the influence of different SWG waveguide parameters on the output power is provided; it can be widely applied to silicon photon devices such as high-speed modulators and optical switches that require high optical power-bearing capacity.

[0058] 3) Structural optimization and matching based on non-linear transmission mechanism: Taking the non-linear transmission equation as the theoretical basis, the power limit of silicon photon devices is broken through through structural design rather than process or material modification, which has general applicability.

[0059] The method for regulating the non-linear effect of optical waveguides based on sub-wavelength structures proposed by the present invention has the following beneficial effects:

[0060] 1) Significantly reduce non-linear loss: By regulating the mode field distribution of the SWG structure, the effective mode area of the waveguide is effectively increased, and the electric field intensity per unit area is significantly reduced, thereby suppressing non-linear losses such as two-photon absorption and free carrier absorption induced thereby.

[0061] 2) Improve the power handling capacity of the device: The SWG waveguide has a higher non-linear threshold power, can maintain stable transmission at a higher input power, and avoid strong non-linear distortion and thermal drift, which is beneficial for constructing optical modulation and control devices with a large dynamic range.

[0062] 3) Maintain a compact size and CMOS compatibility: Compared with the method of directly increasing the size of the silicon waveguide to expand the mode field, the SWG structure does not require a significant increase in the device size and can be fabricated by standard silicon-based processes, with good process feasibility and integration compatibility.

[0063] In summary, through the optimization means of the structural hierarchy, the present invention effectively improves the threshold power that can be borne by the silicon photon device under high-power conditions without sacrificing the integration degree and process feasibility, providing important support for the realization of a high-performance and scalable silicon photon system.

[0064] As described above, the above are only specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A method for regulating the nonlinearity of an optical waveguide, characterized in that, Comprising the following steps: Construct a waveguide structure with sub-wavelength scale periodicity, whose period size satisfies Λ < λ / (2n), where Λ is the period length, λ is the incident light wavelength, and n is the effective refractive index of the medium. The waveguide structure is composed of one-dimensional or multi-dimensional alternately arranged high refractive index material segments and low refractive index medium spacer regions, and forms a one-dimensional or two-dimensional periodic structure; Integrate the sub-wavelength grating waveguide on an SOI platform or a heterogeneous integration substrate, and the substrate material includes silicon dioxide, silicon nitride, or aluminum nitride; By adjusting the unit size, spacing, duty cycle, and geometric shape of the sub-wavelength structure, regulate the equivalent refractive index of the waveguide, increase the effective mode area, reduce the electric field intensity per unit area, and suppress two-photon absorption (TPA) and free carrier absorption (FCA); Utilize the equivalent medium characteristics of the sub-wavelength structure to enhance the nonlinear threshold power of the waveguide, enabling the device to maintain stable optical transmission at high input powers, while avoiding the problem of reduced integration due to the increase in the size of traditional waveguides.

2. The method according to claim 1, wherein The geometric shape of the high refractive index material segments includes at least one of a rectangular strip, a wedge-shaped strip, a circular column, or a rhombus.

3. The method according to claim 2, characterized in that, By regulating the sub-wavelength structure parameters, realize the adjustment of the free carrier lifetime to further suppress the influence of free carrier absorption on the optical transmission performance.

4. The method according to claim 3, wherein The sub-wavelength grating waveguide structure includes a multi-region design with multi-segment structure splicing, functional area division, or gradient adjustment structure.

5. The method according to claim 1, characterized in that, The heterogeneous integration substrate is prepared by standard silicon-based processes, maintaining compatibility with CMOS processes.

6. The method according to claim 5, wherein The increase in the effective mode area is achieved through mode field distribution regulation, reducing the electric field intensity per unit area by at least 30%.

7. The method according to claim 1, characterized in that, The nonlinear threshold power is increased by at least 50% compared to a classical silicon straight waveguide.

8. The method according to claim 7, wherein The method is applied to high-speed modulators, optical switches, optical phased arrays, or high-performance sensors.

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