CN103901563B - A grating coupler with adjustable refractive index and its manufacturing method - Google Patents

Abstract

The invention relates to the technical field of photonic devices, in particular to a method for manufacturing a grating coupler with adjustable refractive index, which is characterized in that: the grating grooves are sub-wavelength structures with different transverse duty ratios, and the gratings are different in the longitudinal direction, that is, in the x direction. The x position has a grating with different transverse or y-direction duty ratios f _y ; the vertical duty ratio f _x of the grating is a fixed value between (0, 1); it constitutes a grating slot with different x positions in the longitudinal direction The subwavelength structure of , the value of its lateral duty cycle f _y is determined by the leakage factor distribution α(x) required for the output Gaussian field distribution G(x). The grating coupler with this structure can effectively reduce the coupling loss when optical fiber and waveguide are coupled.

Description

A grating coupler with adjustable refractive index and its manufacturing method

technical field

The invention relates to the fields of optical communication and optical interconnection, in particular to the design and preparation of an optical waveguide coupling structure with adjustable refractive index. The grating coupler with this structure can effectively reduce the coupling loss when optical fiber and waveguide are coupled.

Background technique

Driven by low cost and low power consumption, silicon photonics technology is based on the SOI (silicon-on-insulator) platform. So far, a variety of silicon photonic devices have been prepared and photonic integration has been realized. Silicon photonics technology is considered to be the solution to the current integration problems. The most promising technique for power consumption and bandwidth issues in circuits. An important challenge facing silicon photonics technology at present is how to introduce laser light sources into and out of the photonic integrated circuit. Due to its high coupling efficiency, the grating coupler is low in cost and compatible with the CMOS process. It does not require chip end polishing and can be placed on the chip surface. The advantages of realizing optical signal input and output anywhere have become the most effective laser light source coupling scheme in silicon photonic integrated circuits.

There are many ways to improve the coupling efficiency of the grating coupler, such as adding a bottom multilayer dielectric mirror, flip-chip bonding metal mirror and cover layer, etc. These methods either enhance the directivity, or reduce the distance between the grating coupler and the fiber. Reflection loss, but because the grating on the grating coupler is usually uniformly distributed, and the optical power distribution of the uniformly distributed grating coupler diffracted outwards decreases exponentially along the propagation direction, that is, P=P ₀ exp(-2αx), P ₀ is the incident optical power, and 2α is the power leakage factor (constant) of the grating. There is a mode mismatch between the attenuated diffraction mode field and the Gaussian mode field of the single-mode fiber, which limits the further improvement of the coupling efficiency of the grating coupler. In order to match the upward diffraction mode field of the grating with the Gaussian mode field of the single-mode fiber, the non-uniform distribution of the leakage factor can be realized through the longitudinal non-uniform grating to obtain the Gaussian output field distribution G(x), the power leakage factor of the grating Must meet:

$\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&alpha;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; G</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; /</span><span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; x</span>}}{\mathrm{<span\; class="notranslate">\; G</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; dt</span>}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

In the formula It is a normalized Gaussian distribution, where w ₀ =5.2μm, and the size of w ₀ corresponds to the mode field radius of a single-mode fiber of 5.2μm. Using the energy normalization condition, it can be obtained

The current non-uniform grating is a grating with non-uniform duty ratio in the longitudinal direction by shallow etching on the top silicon layer of SOI, or it is fabricated by using the hysteresis effect in the inductively coupled plasma etching process to have non-uniform duty ratio and non-uniform grating at the same time. The grating with uniform groove depth can realize the non-uniform distribution of the leakage factor. The disadvantage of the current solution is that the refractive index of the material is fixed; in addition, secondary etching is required, that is, after the silicon waveguide is formed by etching through the top silicon layer of SOI, it is necessary to etch shallowly on the surface of the silicon waveguide to form linear grating grooves. The process is complicated and the preparation cost High; in addition, there is a problem of poor process repeatability in the preparation of non-uniform gratings by using the hysteresis effect.

The invention provides a grating coupler with adjustable refractive index and a manufacturing method thereof. The coupler etches sub-wavelength rectangular grooves (or square, circular and other shapes of grooves) with different transverse duty ratios at different grating groove positions in the longitudinal direction to form grating groove distributions with different equivalent refractive indices in the longitudinal direction, which can then be adjusted Due to the diffractive properties of the grating, the output field distribution is a Gaussian distribution that matches the fiber mode. The non-uniform grating in the present invention is a grating groove formed by engraving the top silicon layer of SOI to form a sub-wavelength structure, and the waveguide structure and the sub-wavelength structure can be produced by only one etching process. At the same time, the method in the invention overcomes the disadvantage that the refractive index of the traditional optical medium thin film material cannot be adjusted.

Contents of the invention

In order to solve the problem of mode mismatch between the output mode field of the ordinary uniform grating coupler and the mode field of the single-mode fiber existing in the prior art, the object of the present invention is to provide an adjustable refractive index for realizing high-efficiency broadband coupling A grating coupler and a manufacturing method thereof, which can realize coupling between a single-mode optical fiber and a planar optical waveguide.

In order to obtain α(x) related to the propagation direction (longitudinal) x to obtain a Gaussian-shaped output field distribution G(x), it is realized by using a non-uniform grating groove distribution in the x direction. At different x positions in the longitudinal direction, different transverse (y direction) duty ratios f _y are used to adjust the distribution of the equivalent refractive index n _L of the grating groove, so that the upward radiation of the grating is a Gaussian beam that matches the fiber mode.

According to the equivalent medium theory of the subwavelength structure, when light waves pass through the subwavelength structure, the subwavelength structure can be regarded as an equivalent uniform medium. Taking the distribution of non-uniform grating grooves under TE polarization as an example, the refraction of the equivalent medium The rate is:

${\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; sub</span>}}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; [</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; the\; air</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; the\; y</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

In formula (2), n _air =1 is the refractive index of the waveguide etched area (air), and n ₁ is the refractive index of the unetched area of the waveguide core layer. The period of the fixed transverse direction (y direction) is sub-wavelength order, by adjusting the transverse duty cycle f _y at different longitudinal (x direction) positions (that is, the waveguide core layer is in the xoy plane, and at different x coordinate positions on the core layer surface The corresponding duty cycle of etching the sub-wavelength structure in the y-axis direction is f _y , and different x-coordinate positions have different f _y ), so that grating groove distributions with different equivalent refractive indices can be obtained in the longitudinal direction.

According to the volume fraction of grating grooves and grating teeth, the equivalent refractive index of the grating layer can be calculated:

n _eq =(1-f _x )n _sub +f _x n ₁ (3)

(3) In the formula, f _x is the longitudinal (x direction) duty cycle of the grating, and the value of f _x is between (0, 1).

According to the eigenequation of the TE mode of the planar waveguide:

${<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; eq</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; eff</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}\mathrm{<span\; class="notranslate">\; h</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; m\&pi;</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; the\; tan</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; eff</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; eq</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; eff</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; the\; tan</span>}}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; eff</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; eq</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; eff</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

Where H is the thickness of the core layer of the waveguide, n ₂ is the refractive index of the lower confinement layer of the waveguide, n ₃ is the refractive index of the refractive index matching liquid added above the waveguide (in order to reduce the reflection of the grating upward diffracted light on the fiber end face and improve the upward diffraction The coupling efficiency between the light and the fiber, in the experimental test, it is necessary to add a refractive index matching liquid on the surface of the grating, and the end face of the fiber is immersed in the matching liquid), n _eff is the effective refractive index of the grating waveguide with a subwavelength structure, m=0 is the mode Order. When λ is the wavelength of the incident light in vacuum, the effective refractive index n _eff of the TE mode grating waveguide can be calculated according to equation (4).

The grating period can be obtained from the following grating diffraction Bragg condition,

qλ=Λ _x [n _eff -n ₁ sin(θ)](5)

Where q=-1 is the diffraction order, Λ _x is the period of the grating in the propagation direction x, and θ is the angle of the optical fiber from the vertical direction.

The leakage factor expression of the grating coupler is:

2α=(P _up +P _down )/P _in (6)

(6) In the formula, P _in is the optical power of the input optical waveguide, P _up and P _down are the optical power of upward and downward diffraction respectively, in the simulation of the grating coupler, the normalization of P _in is equal to 1, and P _up and P _down can be calculated by simulation. α can be calculated through the outward leakage power P _up and P _down of a certain wavelength light of a uniform grating coupler under different grating groove refractive indices.

According to the formula (6), on the premise that other structural parameters of the grating coupler are fixed, changing the parameter combination of different transverse duty ratio f _y and longitudinal period Λ _x can be calculated by simulation software to obtain different α values, and the power of the grating can be adjusted Leakage factor α, while the value of Λ _x parameter changes with the change of n _eff in formula (5), n _eff changes with the change of n _eq in formula (4), and n _eq changes with n _sub in formula (3) , and nsub changes with the change of f _y in formula (2). Therefore, according to this series of change relations, it can be deduced backwards: by adjusting f _y (the period of the subwavelength structure in the y direction is fixed to any value of the subwavelength order), the leakage factor distribution α(x) is adjusted to realize the diffraction output. The field distribution is a Gaussian type field distribution G(x).

According to the α distribution corresponding to the uniform grating coupler under different grating groove refractive indices, combined with the distribution of the grating groove equivalent refractive index corresponding to different lateral duty ratios f _y and in order to obtain the Gaussian shape of the output field distribution G(x) necessary The power leakage factor α distribution of the grating can be obtained by adjusting the refractive index of the grating grooves with different lateral duty ratios f _y to achieve a Gaussian output field distribution.

To sum up the specific adjustment ideas are as follows: adjust f _y in equation (2) to adjust n _sub , then adjust n eq in equation (3) by n _sub , then adjust n eff in equation (4) by n _eq , and then adjust n _eff in equation (4) by n _eq n _eff adjusts Λ _x in equation (5), and a value of α can be obtained from a set of f _y and Λ _x parameters. Different combinations of f _y and Λ _x parameters can obtain different α values, and the power of the grating can be adjusted The leakage factor α, which in turn is distributed by Equation (1) according to a specific α(x) in the x direction, results in a Gaussian-shaped output field distribution G(x).

For achieving the above object, technical solution of the present invention is:

A grating coupler with adjustable refractive index, characterized in that it comprises:

a base layer;

a lower confinement layer, the lower confinement layer overlying the substrate layer;

A waveguide core layer, the waveguide core layer is above the lower limit value layer, including a submicron waveguide core layer, a tapered waveguide core layer, and a strip-shaped waveguide core layer, and the connection between the submicron waveguide core layer is a tapered waveguide core The narrow end of the tapered waveguide core layer and the wide end of the tapered waveguide core layer are connected by a strip waveguide core layer, and a grating with non-uniform grating groove distribution is fabricated on the surface of the strip waveguide core layer. Outcoupling of incident light in the core layer;

An optical fiber, the optical fiber is directly above the grating, and the optical fiber is used to receive upward diffracted light.

The waveguide core layer (the grating layer is etched on the surface of the core layer), the confinement layer, and the substrate layer. The waveguide structure is formed by connecting a strip waveguide, a tapered waveguide and a submicron waveguide in the horizontal direction. A tapered waveguide is connected with the strip waveguide, a submicron waveguide is connected with the tapered waveguide, and there is an optical fiber above the grating for receiving upwardly diffracted light.

The invention utilizes the grating to diffract the light passing through the core layer of the waveguide out of the waveguide, and the diffracted light is received by the optical fiber placed above the grating.

A method for manufacturing a grating coupler with adjustable refractive index, the specific steps are:

Step 1: Fabricate an upper confinement layer and a waveguide core layer sequentially on the substrate;

Step 2: cleaning the sheet structure made in step 1, and drying;

Step 3: Put the dried sheet into the glue coater, spin coat the photoresist layer, and dry;

Step 4: Exposing the photoresist on the surface of the waveguide core layer by electron beam exposure technology to form photoresist mask patterns of strip waveguides, tapered waveguides and submicron waveguides;

Step 5: using inductively coupled plasma etching to form the core layers of the strip waveguide, the tapered waveguide and the submicron waveguide;

Step 6: Put the film into the glue homogenizer, and spin-coat the photoresist on the surface of the core layer;

Step 7: Exposing the surface of the strip waveguide core layer by an electron beam exposure process to form a photoresist mask pattern of the grating;

Step 8: developing and fixing the exposed device structure;

Step 9: using inductively coupled plasma etching to form a grating;

Step 10: cleaning the film on which the grating has been formed, and drying.

Wherein the grating is a grating whose lateral period is of sub-wavelength order and has different lateral (y direction) duty ratios f _y at different x positions in the longitudinal direction (x direction).

Description of drawings

Fig. 1 is a schematic structural diagram of a grating coupler with adjustable refractive index according to the present invention.

Fig. 2 is a schematic cross-sectional view of a grating coupler structure with adjustable refractive index.

Figure 3(a)-Figure 3(f) are schematic diagrams of the preparation method of the grating coupler with tunable refractive index. Fig. 3(a) is a diagram of sequentially fabricating a confinement layer 6 and a silicon waveguide core layer 7 on a silicon substrate 8 in step 1 of the embodiment to form an SOI sheet; The SOI sheet of the core layer of the shaped waveguide, tapered waveguide and sub-micron waveguide is put into the coating machine, and the figure after the photoresist layer 9 is spin-coated; Figure 3 (c) is the electron beam used in step 7 of the embodiment The exposure process exposes the surface of the core layer of the strip waveguide 3 to form the figure of the grating 5; Figure 3 (d) is a figure after developing and fixing the exposed SOI sheet in step 8 of the embodiment; Figure 3 (e) is In step 9 of the embodiment, inductively coupled plasma etching is used to form the grating 4 on the surface of the silicon waveguide core layer 7; FIG. Core layer 7, after drying.

Fig. 4 is the normalized Gaussian distribution G(x) and the grating coupler leakage factor distribution α(x) required to make the output field distribution of the grating coupler G(x).

Figure 5 shows the grating coupler with adjustable refractive index in the present invention. Taking the distribution of non-uniform grating grooves under TE polarization as an example, the transverse duty cycle f _y and longitudinal grating period Λ _x corresponding to the equivalent refractive index n _sub of the grating grooves and leakage factor α. According to the curve in Fig. 5, when realizing different output field distributions (such as Gaussian field distribution), the required x-direction leakage factor distribution α(x) can find the corresponding value of grating coupler parameters, that is, with α (x) The value distribution of the horizontal duty cycle f _y and the vertical grating period Λ _x of the corresponding non-uniform grating groove.

Fig. 6 is a schematic diagram of the relationship between the coupling efficiency and the wavelength of the grating coupler with adjustable refractive index in a specific embodiment of the present invention.

Fig. 7 is a schematic diagram of the distribution of electromagnetic wave electric field components in each layer of the grating coupler with adjustable refractive index in a specific embodiment of the present invention when the incident light wavelength is 1550 nm.

detailed description

The structure and features of the present invention will be further described in detail below in conjunction with the drawings and embodiments. As shown in Figure 1 and Figure 2, taking SOI material as an example, a grating coupler with adjustable refractive index includes:

A substrate silicon layer 8;

A lower confinement layer 6 of a silicon waveguide, the confinement layer 6 is silicon dioxide material;

A silicon waveguide core layer 7, the waveguide core layer 7 is made under the upper confinement layer 6; the waveguide core layer 7 includes: the core layer of the submicron waveguide 1, the tapered waveguide 2 and the strip waveguide 3; the submicron waveguide 1 is a cuboid; the narrow end of the tapered waveguide 2 is connected to the submicron waveguide 1, and the other end is connected to the strip waveguide 3; the width of the tapered waveguide 2 is between the submicron waveguide 1 and the strip waveguide 3, for Reduce the transmission loss between the submicron waveguide 1 and the strip waveguide 3; a grating 4 is fabricated on the surface of the strip waveguide 3;

An optical fiber 5, the optical fiber 5 is used to receive the upward diffracted light of the grating 4, and the axis of the optical fiber 5 deviates from the surface normal of the grating 4 by 10°.

The grating described in the present invention is a grating whose transverse period is of sub-wavelength order and has different transverse (y direction) duty ratios f _y at different x positions in the longitudinal direction (x direction).

In the present invention, since the grating 4 is fabricated on the silicon waveguide layer, the light can be coupled out of the silicon waveguide into an external optical fiber. The incident light in the submicron waveguide 1 enters the grating area in the strip waveguide 3 through the tapered waveguide 2, and is diffracted by the grating 4. The diffracted light is divided into: upward diffraction and downward diffraction caused by negative first-order diffraction, of which the upward The diffracted light is received by the optical fiber 5 , and a part of the downward diffracted light is reflected by the interface between the buried oxide layer and the substrate, and then goes upward and is received by the optical fiber 5 . Through the above measures, a grating coupler with high coupling efficiency can be obtained.

The present invention provides a method for manufacturing a grating coupler with adjustable refractive index, as shown in Figure 3, comprising the following steps:

Step 1: Fabricate an upper confinement layer 6 and a silicon waveguide core layer 7 sequentially on a silicon substrate 8 to form an SOI sheet, as shown in Figure 3(a);

Step 2: cleaning the silicon waveguide core layer 7 on the surface of the SOI sheet, and drying;

Step 3: Put the dried SOI sheet into a homogenizer, spin coat the photoresist layer, and dry;

Step 4: Exposing the photoresist on the surface of the SOI sheet by electron beam exposure technology to form the photoresist mask pattern of the strip waveguide 3, the tapered waveguide 2 and the submicron waveguide 1;

Step 5: using inductively coupled plasma etching to form the core layers of the strip waveguide 3, the tapered waveguide 2 and the submicron waveguide 1, as shown in FIG. 1;

Step 6: Put the SOI sheet with strip waveguide 3, tapered waveguide 2 and sub-micron waveguide 1 into the coater, and spin-coat the photoresist layer 9, as shown in Figure 3(b);

Step 7: Exposing the surface of the strip waveguide 3 by using an electron beam exposure process to form a pattern of the grating 4, as shown in Figure 3(c); wherein the grating 4 is a two-dimensional non-uniform grating on the order of submicron;

Step 8: Develop and fix the exposed SOI sheet, as shown in Figure 3(d);

Step 9: using inductively coupled plasma etching to form a grating 4 on the surface of the silicon waveguide layer 7, as shown in FIG. 3(e);

Step 10: cleaning and drying the silicon waveguide layer 7 on the surface of the SOI sheet on which the grating 4 has been formed, as shown in FIG. 3( f ).

Fig. 4 is the normalized Gaussian distribution G(x) and the grating coupler leakage factor distribution α(x) required to make the output field distribution of the grating coupler G(x). Figure 5 shows the grating coupler with adjustable refractive index in the present invention. Taking the distribution of non-uniform grating grooves under TE polarization as an example, the transverse duty cycle f _y and longitudinal grating period Λ _x corresponding to the equivalent refractive index n _sub of the grating grooves and leakage factor α. Using Figure 5, the non-uniform grating coupler with adjustable refractive index of the grating groove can be designed for any leakage factor distribution between 0 and 0.5 μm ^-1 in the longitudinal direction, and then the leakage factor distribution α( x), the Gaussian output field distribution G(x) in Figure 4 can be obtained. Fig. 6 is a simulation curve of the coupling efficiency of a specific embodiment of the grating coupler with adjustable refractive index according to the present invention. The horizontal axis of the curve is the wavelength, and the vertical axis is the coupling efficiency, that is, the optical power coupled into the optical fiber 5 when the input optical power in the silicon waveguide core layer 7 is 1. When the wavelength is 1550nm, the coupling efficiency is up to 62%, 3dB The bandwidth is 87nm. Fig. 7 is the distribution of the electric field components of the electromagnetic wave in each layer of the grating coupler when the incident light wavelength is 1550nm in the specific embodiment of the grating coupler with adjustable refractive index of the present invention. The incident light is incident from the left side of the silicon waveguide core layer 7 and passes through the grating 4 Realize upward diffraction. It can be seen from Figure 7 that most of the incident light is coupled upward, and the upward diffraction mode field distribution is in the form of Gaussian distribution. The simulation tool used in Fig. 6 and Fig. 7 is the full-vector open source software Camfr based on the eigenmode expansion method. The specific structural parameters used in the simulation are: the refractive index n ₁ =3.476 of the substrate silicon layer 8, and the thickness is 1 μm (this layer The thickness has no effect on the simulation results, and the value in the simulation is much smaller than the thickness of the actual substrate silicon layer); the refractive index n ₂ of the silicon dioxide layer 6 =1.444, and the thickness is 2.2 μm; the refractive index n ₁ of the silicon waveguide core layer 7 =3.476, thickness 220nm; the refractive index matching layer on the waveguide core layer 7 (corresponding to the refractive index matching liquid to be added on the surface of the grating in order to reduce the reflection of the optical fiber end face to the upwardly diffracted light in the actual experimental test) refractive index n ₃ =1.46; the etching depth of the grating 5 is 220nm, the vertical duty ratio is f _x =0.5, and the vertical period is 606nm, 608nm, 611nm, 616nm, 620nm, 625nm, 631nm, 637nm, 644nm, 651nm, 658nm, 667nm , 675nm, 683nm, 692nm, 702nm, 716nm, 726nm, 742nm, 753nm, 735nm, 708nm, 677nm, 651nm, 632nm, 619nm, 609nm, 605nm; the period of the lateral subwavelength structure is 400nm, the etching depth is 220nm, and the longitudinal grating The equivalent refractive indices of the slots (the corresponding lateral duty cycle f _y ) are 3.4 (0.95), 3.3824 (0.94), 3.356 (0.93), 3.3161 (0.9), 3.2794 (0.88), 3.24 (0.86), 3.1916 ( 0.83), 3.143(0.8), 3.0897(0.78), 3.0406(0.75), 2.9853(0.71), 2.9231(0.68), 2.8649(0.65), 2.8062(0.62), 2.7448(0.59), 2.6734(0.55), 2.585 0.51), 2.5188(0.48), 2.4211(0.44), 2.3531(0.41), 2.4626(0.46), 2.6359(0.53), 2.8479(0.64), 3.0404(0.75), 3.1887(0.82), 3.2929(0.891), 3.3 0.94), 3.4112 (0.96).

The above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto, anyone familiar with the technology can also make some improvements and improvements without departing from the technical principles of the present invention. Modifications, these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims (1)

1. a manufacture method for the adjustable grating coupler of refractive index, grating groove is the sub-wavelength structure of different horizontal dutycycle, and grating is have different transverse directions and y direction dutycycle f at the x position that longitudinal direction and x direction are different_yGrating；Grating is at longitudinal dutycycle f_xValue is a definite value between (0,1)；Constitute the sub-wavelength structure of longitudinally upper different x position grating groove, its horizontal dutycycle f_yValue by export required for Gaussian field distribution G (x) leakage factor distribution α (x) determine；

It is characterized in that: in order to obtain Gaussian field distribution G (x) in formula (1), need to regulate leakage factor distribution α (x), output field distribution G (x) of gaussian shape, the Power leakage factor of grating must is fulfilled for:

$2\mathrm{\&alpha;}\left(x\right)=G^2\left(x\right)/\&lsqb;1-{\&Integral;}_0^x{G}^2\left(t\right)dt\&rsqb;---\left(1\right)$

In formulaFor normalized Gauss distribution, wherein w₀=5.2 μm, w₀Size is corresponding with single-mode fiber spot size 5.2 μm；

According to formula (6) under the premise that other structural parameters of grating coupler are fixed, change different horizontal dutycycle f_yWith longitudinal periods lambda_xParameter be combined through simulation software and calculate and obtain different α value, i.e. the Power leakage factor-alpha of adjustable light barrier,

The leakage factor expression formula of grating coupler is:

2 α=(P_up+P_down)/P_in(6)

(6) P in formula_inFor the luminous power of input waveguide, P_upAnd P_downThe respectively luminous power of diffraction up and down, in the analog simulation to grating coupler, P_inNormalization is equal to 1；By to the uniform grating bonder under different grating groove refractive indexs to the outside leakage power P of certain wavelength light_upAnd P_downCalculating obtains α；

By regulating f_y, f_yFor the fixing sub-wavelength structure any value that the cycle is sub-wavelength magnitude in y direction, regulate leakage factor distribution α (x) and then to realize the field distribution of diffraction output be Gaussian field distribution G (x).