Compact low-loss grating coupler with bottom reflector

Abstract

The invention provides a compact low-loss grating coupler with a bottom reflector, which sequentially comprises a silicon substrate layer, a distributed Bragg reflector, a buried oxide layer, a top silicon layer and a top cladding from bottom to top, and is characterized in that the distributed Bragg reflector is formed by laminating silicon and silicon dioxide; the top silicon layer is composed of a focusing type grating waveguide structure and a strip-shaped waveguide structure connected with the focusing type grating waveguide structure, the focusing type grating waveguide structure is composed of a shallow etching area and a full-height waveguide, optimization design is conducted by evaluating the influence rule of physical parameters, and finally efficient transmission of optical signals in an integrated photoelectric system can be achieved. According to the invention, the substrate loss is reduced, and the efficiency of the grating coupler is improved.

Description

Technical Field

[0001] The present application relates to the field of optical communications, and in particular to a compact low-loss grating coupler with a bottom reflector. Background Art

[0002] As Moore's Law approaches its limit, traditional electronic chips can no longer meet the needs of ultra-high speed and large-capacity storage in high-performance computing and data centers. Optical chips, with their advantages such as large bandwidth and low loss, have been proposed as a solution to replace traditional electronic chips to cope with the growth of communication needs.

[0003] Optical fiber is the main device for carrying and transmitting optical signals, but the diameter of ordinary single-mode optical fiber is generally 8-10um, which is much larger than the width of the silicon-based optical waveguide in the optical chip (300-500nm). This size difference will lead to serious mode mismatch, making it impossible to efficiently load optical signals onto the chip.

[0004] Grating couplers effectively transmit light in free space to the silicon optical waveguide of the optical chip through the diffraction effect of the grating. As an input and output device in the integrated optoelectronic system, it can effectively connect the silicon-based waveguides outside and inside the chip. The main advantage of grating couplers is that they are flexible in position and can be placed anywhere on the chip, so they are suitable for online testing at the wafer level. In addition, they have a large alignment tolerance and are easy to package. However, the slit structure of the grating coupler causes the effective refractive index to change periodically, so that the light is no longer strictly confined to the silicon waveguide; at the same time, traditional grating couplers often consume a lot of optical power in substrate losses. These two factors lead to low coupling efficiency of grating couplers. Summary of the invention

[0005] The purpose of the present invention is to provide a compact low-loss grating coupler with a bottom reflector to solve the problem of low coupling efficiency of the existing uniform grating coupler.

[0006] The technical solution for achieving the purpose of the present invention is: a compact low-loss grating coupler with a bottom reflector, which includes, from bottom to top, a silicon substrate layer, a distributed Bragg reflector, a buried oxide layer, a top silicon layer, and a top cladding, wherein the distributed Bragg reflector is composed of a silicon and silicon dioxide stack; the top silicon layer is composed of a focusing grating waveguide structure and a strip waveguide structure connected to the focusing grating waveguide structure, and the focusing grating waveguide structure is composed of a shallow etched area and a full-height waveguide.

[0007] Furthermore, the thickness of silicon and silicon dioxide in the distributed Bragg reflector is λ c / 4n, where λ represents the central wavelength and n represents the refractive index of the material.

[0008] Furthermore, the coupling efficiency of the grating coupler is related to the period and filling factor of the focusing grating waveguide structure, as well as the incident angle and position of the light source. Based on the Lumerical software of the finite-difference time-domain method, the simulation design of the grating coupler is completed. The specific steps are as follows:

[0009] Step 1: Initialize the filling factor F = 0.5, the incident angle θ is 0°-15°, and calculate the ideal grating period interval through the Bragg condition;

[0010] Step 2: Based on the theoretical grating period interval calculated in step 1, take the grating period as the independent variable, adjust the grating period, perform time-domain finite difference analysis, fit the light field distribution diagram of the grating period and wavelength, and obtain the grating period at which the coupling efficiency of the grating coupler is the highest;

[0011] Step 3: fix the grating period to the grating period when the coupling efficiency of the grating coupler fitted in step 2 reaches the highest, take the filling factor as the independent variable, adjust the filling factor, perform time-domain finite difference method analysis, fit the light field distribution diagram of the grating filling factor and wavelength, and obtain the filling factor with the highest coupling efficiency of the grating coupler;

[0012] Step 4: fix the grating period to the grating period when the coupling efficiency of the grating coupler fitted in step 2 reaches the highest, fix the grating filling factor to the filling factor with the highest coupling efficiency of the grating coupler fitted in step 3, adjust the incident position of the light source, perform time-domain finite difference analysis, fit the light field distribution diagram of the incident position of the light source and the wavelength, and select the incident position of the light source with the highest coupling efficiency of the grating coupler;

[0013] Step 5: Fix the grating period to the grating period when the coupling efficiency of the grating coupler fitted in step 2 reaches the highest, fix the filling factor to the filling factor with the highest coupling efficiency of the grating coupler fitted in step 3, fix the incident position of the light source to the light source position with the highest coupling efficiency of the grating coupler fitted in step 4, take the incident angle as the independent variable, adjust the incident angle, perform time-domain finite difference analysis, fit the light field distribution diagram of the incident angle of the light source and the wavelength, and select the incident angle with the highest coupling efficiency of the grating coupler.

[0014] Further, step 1: initialize the filling factor F = 0.5, the incident angle θ is 0°-15°, and calculate the ideal grating period interval through the Bragg condition, where:

[0015] The Prague conditions are:

[0016]

[0017] where Λ is the ideal grating period, λ c is the central wavelength; θ is the incident angle of the light source; n bgis the refractive index of silicon dioxide; n eff is the effective refractive index of the focusing grating waveguide structure, expressed as:

[0018] n eff =Fn0+(1-F)n E

[0019] F is the filling factor of the focusing grating waveguide structure, n0 and n E are the effective refractive index of the full-height waveguide and the effective refractive index of the shallowly etched area, respectively.

[0020] Furthermore, the effective refractive index of the full-height waveguide in the focusing grating waveguide structure is n0=2.848, and the effective refractive index of the shallow etched area with a thickness of 150 nm is n E =2.534, the effective refractive index of the focusing grating waveguide structure is n eff =2.691, the ideal grating period range is 575nm-668nm.

[0021] Further, step three: fix the grating period to the grating period when the coupling efficiency of the grating coupler fitted in step two reaches the highest, take the filling factor as the independent variable, adjust the filling factor, perform time-domain finite difference method analysis, fit the light field distribution diagram of the grating filling factor and wavelength, and obtain the filling factor with the highest coupling efficiency of the grating coupler, where:

[0022] The filling factor is set to 0.4-0.6, and the light field distribution diagram of the grating filling factor and wavelength is fitted by the time-domain finite difference method.

[0023] Furthermore, when the central wavelength is 1550nm, the thicknesses of silicon and silicon dioxide in the distributed Bragg reflector are 110nm and 270nm respectively, the filling factor in the focusing grating waveguide structure is 0.5, the grating period is 600nm, the light source incident position, i.e., the tail of the strip waveguide structure connected to the focusing grating waveguide structure, is 4.3nm away from the light source, and the incident angle is 5.5°.

[0024] Compared with the prior art, the present invention has the following significant advantages:

[0025] 1) According to the Bragg condition and grating refraction theory, the filling factor F=0.5 of the grating coupler is selected, and the theoretical period range of the grating is accurately calculated to be 575nm-668nm, which provides a scientific theoretical basis for the present invention.

[0026] 2) The finite-difference time-domain method is used to perform parameter scanning on the grating period region, and the adaptability of the filling factor is verified through visual analysis to ensure the accuracy of the coupler design.

[0027] 3) Modify the scanning parameters, perform detailed scanning and analysis on the incident angle and position of the light source, and optimize the above four parameters together to further improve the coupling efficiency of the uniform grating coupler in the present invention.

[0028] 4) On the basis of improving the efficiency of the grating coupler body, the present invention innovatively adds a distributed Bragg reflector, which effectively reduces the base loss and further enhances the coupling efficiency. The design of the reflector adopts a double-layer structure of silicon layer and silicon dioxide layer, and an optimized layout between the buried oxide layer and the substrate. The experimental results prove its significant effect in improving the coupling efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Flowchart for optimizing a grating coupler for the present invention.

[0030] Figure 2 It is a basic structural schematic diagram of the grating coupler of the present invention.

[0031] Figure 3 It is a specific schematic diagram of the light source and the grating coupler of the present invention.

[0032] 1-silicon substrate layer, 2-distributed Bragg reflector, 3-buried oxide layer, 4-top silicon layer (41-focusing grating waveguide, 42-strip waveguide), 5-top cladding, 6-light source. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0034] A new type of compact low-loss grating coupler with a bottom reflector. The basic structure is shown in the side view. Figure 2 As shown, from bottom to top, it includes a silicon substrate layer 1, a distributed Bragg reflector 2, a buried oxide layer 3, a top silicon layer 4, and a top cladding 5, wherein the distributed Bragg reflector 2 is composed of a silicon and silicon dioxide stack; the top silicon layer 4 is composed of a focusing grating waveguide structure 41 and a strip waveguide structure 42 connected to the focusing grating waveguide structure 41, and the focusing grating waveguide structure 41 is composed of a shallow etched area (grating groove) 41-1 and a full-height waveguide (unetched part) 41-2.

[0035] The light path is provided by the light source 6, which is incident into the grating coupler waveguide structure at a certain angle. The schematic diagram of the light source is shown in Figure 3As shown, θ is the tilt angle of the light source entering the grating coupler, and x is the position of the tail (rightmost end) of the strip waveguide structure 42 connected to the focusing grating waveguide structure 41 from the light source. In order to reduce the substrate loss of the grating coupler, a distributed Bragg reflector 2 composed of silicon and silicon dioxide stacking is added under the buried oxide layer 3 structure. Since there is a refractive index difference of about 2.03 between silicon and silicon dioxide, the thickness of each layer is λ / 4n (λ=1550, n is the refractive index of the material, i.e. 3.47 and 1.44), which is 110nm and 270nm respectively.

[0036] Since the coupling efficiency of the grating coupler is closely related to not only the physical properties of the focusing grating waveguide structure 41 itself: period, filling factor (F), but also the manufacturing process parameters: light source incident angle, position factors. Therefore, during the design, the input parameters of the grating coupler are divided into two categories, namely design parameters and process performance parameters. The design parameters mainly include the inherent characteristic parameters of the grating coupler: period and filling factor; the process performance parameters include the influence of external factors such as the incident angle and position of the light source of the grating coupler.

[0037] The present invention uses Lumerical software based on the finite difference time domain method to complete the simulation design of the grating coupler. The software combines the idea of ​​the FDTD method, meshes the three-dimensional space, simulates the calculation in the time domain, and combines the preset boundary conditions to obtain the simulation calculation results for the structure of the target wavelength.

[0038] An optimization design method for a new compact low-loss grating coupler with a bottom reflector is proposed. The grating filling factor, grating period, light source position, and incident angle are usually optimized together to improve the coupling efficiency of the central wavelength. The specific steps are as follows:

[0039] Step 1: Initialize the filling factor F=0.5, the incident angle θ is 0°-15°, and calculate the ideal grating period interval through the Bragg condition.

[0040] The Prague conditions are:

[0041]

[0042] where Λ is the ideal grating period of the grating coupler, λ c is the central wavelength of 1550nm; θ is the incident angle of the light source; n bg The refractive index of silicon dioxide is 1.44; n eff is the effective refractive index of the focusing grating waveguide structure 41, where:

[0043] n eff =Fn0+(1-F)n E

[0044] F is the filling factor of the focusing grating waveguide structure 41. One cycle of the focusing grating waveguide structure consists of the shallow etched area (grating groove) 41-1 and the full-height waveguide (unetched part) 41-2 of the structure. The grating filling factor is the proportion of the shallow etched area (grating groove) to the total period in one cycle. In the initial design, the grating filling factor is temporarily selected as 0.5; n0 and n E are the effective refractive index of the full-height waveguide and the effective refractive index of the shallow-etched area. It is known that the effective refractive index of the full-height waveguide in the focusing grating waveguide structure is n0=2.848, and the effective refractive index of the shallow-etched area (grating groove) with a thickness of 150nm is n E =2.534. The weighted effective refractive index of the grating coupler can be calculated as n eff =2.691. The grating period is calculated to be 575nm-668nm.

[0045] Step 2: Based on the theoretical grating period interval calculated in step 1, take the grating period as the independent variable, adjust the grating period, perform time-domain finite difference analysis, fit the light field distribution diagram of the grating period and wavelength, and obtain the grating period at which the grating coupler coupling efficiency is the highest.

[0046] After comparison, when the grating period is 600nm, the coupling efficiency of the grating coupler reaches the highest, which is 45%.

[0047] Step 3: Fix the grating period to the grating period when the coupling efficiency of the grating coupler fitted in step 2 reaches the highest, take the filling factor as the independent variable, adjust the filling factor, perform time-domain finite difference analysis, fit the light field distribution diagram of the grating filling factor and wavelength, and obtain the filling factor with the highest coupling efficiency of the grating coupler.

[0048] The grating period is fixed to 600nm, the filling factor is set to 0.4-0.6, and the light field distribution diagram of the grating filling factor and wavelength is fitted by the time-domain finite difference method. It is found that when the filling factor is 0.5, the coupling efficiency of the grating coupler is the highest, which proves that the filling factor F=0.5 selected in step 1 is feasible.

[0049] Step 4: fix the grating period to the grating period when the coupling efficiency of the grating coupler fitted in step 2 reaches the highest, fix the grating filling factor to the filling factor with the highest coupling efficiency of the grating coupler fitted in step 3, adjust the incident position of the light source, perform time-domain finite difference analysis, fit the light field distribution diagram of the incident position of the light source and the wavelength, and select the incident position of the light source with the highest coupling efficiency of the grating coupler;

[0050] The grating period is fixed at 600nm and the grating filling factor is F=0.5. The light field distribution diagram of the incident position of the light source and the wavelength is fitted by the time-domain finite difference method. The incident position of the light source x is the position of the tail (rightmost end) of the strip waveguide structure 42 connected to the focusing grating waveguide structure 41 from the light source. The emitted energy is mainly concentrated at the central wavelength of 1550nm. When the incident position of the light source is x=4.3nm, the coupling efficiency of the grating coupler is the highest. x=4.3nm is selected.

[0051] Step 5: Fix the grating period to the grating period when the coupling efficiency of the grating coupler fitted in step 2 reaches the highest, fix the filling factor to the filling factor with the highest coupling efficiency of the grating coupler fitted in step 3, fix the incident position of the light source to the light source position with the highest coupling efficiency of the grating coupler fitted in step 4, take the incident angle as the independent variable, adjust the incident angle, perform time-domain finite difference analysis, fit the light field distribution diagram of the incident angle of the light source and the wavelength, and select the incident angle with the highest coupling efficiency of the grating coupler.

[0052] With a fixed period of 600nm, a filling factor of F=0.5, and a light source incident position x=4.3nm, the light field distribution diagram of the incident angle and wavelength of the light source is fitted by the time-domain finite difference method. The coupling efficiency of the grating coupler is the highest when the incident angle is 5.5° in the central wavelength band of 1550nm. Finally, the incident angle θ=5.5° is selected.

[0053] Step 6: The optimized grating coupler is combined with the bottom reflector to finally obtain a new compact low-loss grating coupler with a bottom reflector. According to the above steps and parameter optimization, the efficiency of the focusing uniform grating coupler is finally increased to 77% at the target wavelength.

[0054] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0055] The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the present application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

Claims

1. A compact low-loss grating coupler with a bottom reflector, characterized in that: From bottom to top, it includes a silicon substrate layer, a distributed Bragg reflector, a buried oxide layer, a top silicon layer, and a top cladding layer, wherein the distributed Bragg reflector is composed of a silicon and silicon dioxide stack; the top silicon layer is composed of a focusing grating waveguide structure and a strip waveguide structure connected to the focusing grating waveguide structure, and the focusing grating waveguide structure is composed of a shallow etched area and a full-height waveguide.

2. The compact low-loss grating coupler with bottom reflector according to claim 1, characterized in that: The thickness of silicon and silicon dioxide in the distributed Bragg reflector is λ c / 4n, where λ c represents the central wavelength and n represents the refractive index of the material.

3. The compact low-loss grating coupler with bottom reflector according to claim 1, characterized in that: The coupling efficiency of the grating coupler is related to the period and filling factor of the focusing grating waveguide structure, as well as the incident angle and position of the light source. The simulation design of the grating coupler is completed based on the Lumerical software of the finite-difference time-domain method. The specific steps are as follows: Step 1: Initialize the filling factor F = 0.5, the incident angle θ is 0°-15°, and calculate the ideal grating period interval through the Bragg condition; Step 2: Based on the theoretical grating period interval calculated in step 1, take the grating period as the independent variable, adjust the grating period, perform time-domain finite difference analysis, fit the light field distribution diagram of the grating period and wavelength, and obtain the grating period at which the coupling efficiency of the grating coupler is the highest; Step 3: fix the grating period to the grating period when the coupling efficiency of the grating coupler fitted in step 2 reaches the highest, take the filling factor as the independent variable, adjust the filling factor, perform time-domain finite difference method analysis, fit the light field distribution diagram of the grating filling factor and wavelength, and obtain the filling factor with the highest coupling efficiency of the grating coupler; Step 4: fix the grating period to the grating period when the coupling efficiency of the grating coupler fitted in step 2 reaches the highest, fix the grating filling factor to the filling factor with the highest coupling efficiency of the grating coupler fitted in step 3, adjust the incident position of the light source, perform time-domain finite difference analysis, fit the light field distribution diagram of the incident position of the light source and the wavelength, and select the incident position of the light source with the highest coupling efficiency of the grating coupler; Step 5: Fix the grating period to the grating period when the coupling efficiency of the grating coupler fitted in step 2 reaches the highest, fix the filling factor to the filling factor with the highest coupling efficiency of the grating coupler fitted in step 3, fix the incident position of the light source to the light source position with the highest coupling efficiency of the grating coupler fitted in step 4, take the incident angle as the independent variable, adjust the incident angle, perform time-domain finite difference analysis, fit the light field distribution diagram of the incident angle of the light source and the wavelength, and select the incident angle with the highest coupling efficiency of the grating coupler.

4. The compact low-loss grating coupler with bottom reflector according to claim 1, characterized in that: Step 1: Initialize the fill factor F = 0.5, the incident angle θ is 0°-15°, and calculate the ideal grating period interval through the Bragg condition, where: The Prague conditions are: where Λ is the ideal grating period, λ c is the central wavelength; θ is the incident angle of the light source; n bg is the refractive index of silicon dioxide; n eff is the effective refractive index of the focusing grating waveguide structure, expressed as: n eff =Fn0+(1-F)n E F is the filling factor of the focusing grating waveguide structure, n0 and n E are the effective refractive index of the full-height waveguide and the effective refractive index of the shallowly etched area, respectively.

5. The compact low-loss grating coupler with bottom reflector according to claim 4, characterized in that: The effective refractive index of the full-height waveguide in the focusing grating waveguide structure is n0 = 2.848, and the effective refractive index of the shallow etched area with a thickness of 150nm is n E =2.534, the effective refractive index of the focusing grating waveguide structure is n eff =2.691, the ideal grating period range is 575nm-668nm.

6. The compact low-loss grating coupler with bottom reflector according to claim 4, characterized in that: Step 3: Fix the grating period to the grating period when the coupling efficiency of the grating coupler fitted in step 2 reaches the highest, take the filling factor as the independent variable, adjust the filling factor, perform time-domain finite difference analysis, fit the light field distribution diagram of the grating filling factor and wavelength, and obtain the filling factor with the highest coupling efficiency of the grating coupler, where: The filling factor is set to 0.4-0.6, and the light field distribution diagram of the grating filling factor and wavelength is fitted by the time-domain finite difference method.

7. The compact low-loss grating coupler with bottom reflector according to claim 4, characterized in that: When the central wavelength is 1550nm, the thicknesses of silicon and silicon dioxide in the distributed Bragg reflector are 110nm and 270nm respectively, the filling factor in the focusing grating waveguide structure is 0.5, the grating period is 600nm, the light source incident position, i.e., the tail of the strip waveguide structure connected to the focusing grating waveguide structure, is 4.3nm away from the light source, and the incident angle is 5.5°.

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