CN110297289B - Indium phosphide-based optical mixer and preparation method thereof - Google Patents

Abstract

The invention discloses an indium phosphide-based optical mixer and a preparation method thereof. The mixer includes an epitaxial wafer device layer formed on an indium phosphide substrate for realizing frequency mixing; an oxide a silicon upper cladding layer formed on the epitaxial wafer device layer for protecting the epitaxial wafer device layer and improving the stability of the epitaxial wafer device layer; and a metal thin film formed on the silicon dioxide upper cladding layer , which controls the polarization. The epitaxial wafer device layer includes a mode spot converter, a 1×2MMI, a first polarization converter, a second polarization converter, a 2×2MMI, a third polarization converter, a waveguide-type polarizer, and a 4×2 MMI sequentially connected by a straight waveguide. 4MMI. The indium phosphide-based optical mixer proposed by the invention has the comprehensive performance of miniaturization, low polarization dispersion, high bandwidth, high responsivity, the preparation process is compatible with the CMOS process, and easy to prepare and integrate.

Description

Indium phosphide-based optical mixer and preparation method thereof

technical field

The invention belongs to the field of coherent optical communication, in particular to an indium phosphide-based optical mixer and a preparation method thereof.

Background technique

In the face of increasing communication speed and bandwidth requirements, people gradually focus on coherent optical communication. Coherent optical communication is a new type of communication system that uses a local oscillator light source to coherently demodulate the received signal at the receiving end. The development of coherent optical communication systems has greatly improved the transmission rate, transmission capacity and transmission distance of traditional optical communication systems. The key to its development lies in the application of coherent optical receivers. The 90° optical mixer is an essential core device in the coherent optical receiver, and it is a decisive factor for the performance of the entire coherent optical receiver. The optical mixer obtains a difference frequency signal carrying amplitude and phase information at the detector end by optically mixing the input light with the local oscillator light that satisfies the phase matching and polarization matching. The output difference frequency signal finally extracts the transmission information by means of the digital signal processing module. The precision of the optical mixer determines the sensitivity of the entire coherent receiver.

Three important indicators to evaluate the excellent performance of a 90° mixer are transmission loss, common mode rejection ratio and phase error. Among them, the phase error is the most critical parameter among the three. The current mainstream optical mixers are based on 4 × 4 multimode interference couplers (multimode interference, MMI) to complete the mixing, which means that the input light must be in a single polarization mode, and there is currently no laser light source that is Single polarization mode output. That is to say, all current optical mixers have more or less polarization mode dispersion, which seriously affects the phase error of the mixer, and eventually leads to the degradation of device performance, which increases the design difficulty for subsequent signal compensation modules.

At present, in the field of coherent optical communication, in order to separately mix the signal light in the Transverse Electric (TE)/Transverse Magnetic (TM) mode, the commonly used method is to use an independent optical instrument (fiber polarization Beam splitter) separates the two eigenstate polarization states (ESOP) in the input signal light, and needs to optimize the design of 4 × 4 MMIs corresponding to different modes of demodulation to realize frequency mixing, which increases the complexity of device design and process, The device size is large and the cost is high. In addition, due to the limitation of CMOS process processing accuracy, no matter what kind of polarization beam splitter can not completely separate the two intrinsic polarization states, it further increases the polarization dispersion of the mixer in coherent optical communication and reduces the frequency mixing. device accuracy.

SUMMARY OF THE INVENTION

(1) Technical problems to be solved

In view of the disadvantages such as large size, low precision, high cost and serious polarization dispersion of the current traditional mixers used for coherent optical communication, an indium phosphide-based optical mixer and a preparation method thereof proposed by the present invention can at least partially Solve the technical problems raised above.

(2) Technical solutions

In order to achieve the above object, the present invention provides an indium phosphide-based optical mixer, comprising: an epitaxial wafer device layer formed on an indium phosphide substrate for realizing frequency mixing; a cladding layer formed on the epitaxial wafer device layer for protecting the epitaxial wafer device layer and improving the stability of the epitaxial wafer device layer; and a metal thin film formed on the silicon dioxide upper cladding layer, using to control polarization.

The epitaxial wafer device layer includes: a mode spot converter, which couples the signal light with phase information into the epitaxial wafer device layer; a 1×2 multi-mode interference coupler, which is connected by the straight waveguide. The signal light with information coupled into the epitaxial wafer device layer by the mode spot converter is equally divided into two beams of light with equal power and phase; a first polarization converter, which will be equally divided by the 1×2 multi-mode interference coupler The polarization state of one of the two beams of light with equal power and phase is rotated by 90°; a second polarization converter, which equalizes the power and phase of the two beams equally divided by the 1×2 multimode interference coupler In the light, the polarization state of the other beam of light after a straight waveguide is rotated by 90°; a 2×2 multi-mode interference coupler, which will be converted by the first polarization converter and the second polarization converter The rotated signal light is coupled to output TE and TM mode signal light; a third polarization converter rotates the polarization state of the TM mode signal light output by the 2×2 multi-mode interference coupler by 90° into a TE mode signal light; a waveguide type polarizer for performing TE mode signal on the TE mode signal light output by the 2×2 multi-mode interference coupler and the TE mode signal light rotated by the third polarization converter Light filtering to filter out residual TM mode signal light; and a 4×4 multi-mode interference coupler to perform multi-mode interference between the TE mode signal light filtered by the waveguide polarizer and the TE mode local oscillator light to complete Mixing of the TE mode input signal. Among them, the length of the interference region of the 1×2 multi-mode interference coupler is L=3L _π /8; the length of the interference region of the 2×2 multi-mode interference coupler is L=L _π /2; the 4×4 multi-mode interference coupler The length of the interference region of the interference coupler is L=3L _π /4; the distance L between the first polarization converter and the second polarization converter satisfies L=π/(2(β _TE −β _TM )); wherein , L _π is the beat length, β _TE is the propagation constant of the TE mode, and β _TM is the propagation constant of the TM mode.

Wherein, the thickness of the silicon dioxide upper cladding layer is 1 μm-3 μm; the metal film is located above the connecting straight waveguide between the third polarization converter and the 4×4 multi-mode interference coupler, and the material used is Ag , Au, Cu or Al, with a thickness of 100nm-200nm.

In order to achieve the above object, the present invention also provides a method for preparing an indium phosphide-based optical mixer, comprising:

Step 1, preparing an epitaxial wafer device layer, which is formed on an indium phosphide substrate;

Step 2: growing a silicon dioxide upper cladding layer on the epitaxial wafer device layer;

Step 3: A metal thin film is grown on the silicon dioxide upper cladding layer.

Wherein, on the indium phosphide substrate, an epitaxial wafer device layer is prepared by photolithography, etching and etching processes, and the epitaxial wafer device layer includes a mode spot converter, a 1×2 multi-mode Interference coupler, first polarization converter, second polarization converter, 2×2 multi-mode interference coupler, third polarization converter and 4×4 multi-mode interference coupler; wherein, the etching thickness of the etching process is 4μm-5μm.

The first polarization converter, the second polarization converter and the third polarization converter are realized by the inclined sidewall on one side of the straight waveguide, the inclined sidewall is obtained by wet etching with bromine-methanol solution, and the inclined sidewall is obtained. The inclination angle is 50°-60°; the metal film is grown on the silicon dioxide upper cladding layer above the connecting straight waveguide between the third polarization converter and the 4×4 multimode interference coupler, and is connected with the third polarization converter and the 4×4 multimode interference coupler. Straight waveguides between triple polarization converters and 4×4 multimode interference couplers form waveguide polarizers

(3) Beneficial effects

It can be seen from the above technical solutions that an indium phosphide-based optical mixer and a preparation method thereof provided by the present invention have the following beneficial effects:

(1) The indium phosphide-based optical mixer proposed by the present invention has the characteristics of micron-scale size, which greatly reduces the spatial size of the optical mixer and improves the compactness of the device structure. The selected indium phosphide substrate is easy to integrate with the subsequent balanced photodetectors of the same material, which can further improve the integration degree and coupling efficiency, and lay a solid foundation for the integration and miniaturization of high-performance optical coherent photodetectors in the future.

(2) In the indium phosphide-based optical mixer proposed in the present invention, by means of a Mach-Zehnder interferometer, a third polarization converter and a polarization beam splitter based on a 2×2 MMI, the TE signal light and the TM signal light can be separated from each other. The two signal optical modes are separated, and the signals carried by the TE mode and the TM mode in the signal light are extracted respectively, which further improves the utilization rate of the optical signal and broadens the signal bandwidth, and the device only needs to input the TE mode. Vibrating light can complete the mixing of TE mode and TM mode.

(3) In the indium phosphide-based optical mixer proposed by the present invention, the two 4×4 MMIs used for frequency mixing do not need to be designed to be the size of the TE mode and the TM mode respectively, which further reduces the design and process difficulty.

(4) The indium phosphide-based optical mixer proposed by the present invention further improves the suppression of the residual TM mode in the TE mode by introducing a waveguide polarizer containing a metal coating, and greatly reduces the polarization in the mixer Modal dispersion reduces the phase error of the mixer and improves the sensitivity of the coherent optical communication system.

(5) The indium phosphide-based optical mixer proposed by the present invention, on the basis of the CMOS process, combines the mode spot converter, 1×2MMI, first polarization converter, second polarization converter, 2×2MMI, The three polarization converters, the waveguide-type polarizer and the 4×4 MMI are integrated on the same substrate, which is easy to fabricate and integrate, reduces the cost and can be mass-produced.

Description of drawings

1 is a schematic structural diagram of an epitaxial wafer device layer in an indium phosphide-based optical mixer proposed by the present invention;

FIG. 2 is a schematic cross-sectional structure diagram of the indium phosphide-based optical mixer proposed by the present invention taken along the line A-A in FIG. 1;

FIG. 3 is a schematic cross-sectional structure diagram of the indium phosphide-based optical mixer proposed by the present invention taken along the line B-B in FIG. 1;

4 is a flow chart of the preparation process of the indium phosphide-based optical mixer proposed by the present invention;

5 is a schematic perspective view of a device structure fabricated on an epitaxial wafer in the indium phosphide-based optical mixer proposed by the present invention.

[Description of reference numerals]

1: Indium phosphide substrate

2: Epitaxial wafer device layer

3: Silica upper cladding

4: Metal thin film

201: Mode Spot Converter

202: Straight Waveguide

203: 1×2 Multimode Interference Coupler

204: First Polarization Converter

205: Second Polarization Converter

206: 2×2 Multimode Interferometric Coupler

207: Third Polarization Converter

208: Waveguide Polarizer

209: 4×4 Multimode Interferometric Coupler

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be described in further detail below with reference to specific examples and accompanying drawings. It should be noted that, in the drawings or descriptions in the specification, the same drawing numbers are used for the same or similar parts. Implementations not shown or described in the drawings are in the form known to those of ordinary skill in the art. Additionally, although examples of parameters including specific values may be provided herein, it should be understood that the parameters need not be exactly equal to the corresponding value, but rather approximate the corresponding value within acceptable error tolerances or constraints involved.

According to an aspect of the present invention, in an exemplary embodiment of the present invention, an indium phosphide-based optical mixer with low polarization dispersion and high bandwidth is provided, please refer to FIG. 1 , the indium phosphide-based optical mixer device includes:

The epitaxial wafer device layer 2 is formed on the indium phosphide substrate 1, and is used to realize frequency mixing;

The silicon dioxide upper cladding layer 3 is formed on the epitaxial wafer device layer 2 for protecting the epitaxial wafer device layer 2 and improving the stability of the epitaxial wafer device layer; and

The metal thin film 4 is formed on the silicon dioxide upper cladding layer 3 for controlling polarization.

The epitaxial wafer device layer 2 includes a mode spot converter 201 , a 1×2 MMI 203 , a first polarization converter 204 , a second polarization converter 205 , a 2×2 MMI 206 , and a third polarization converter 207 , which are connected in sequence through a straight waveguide 202 . , waveguide polarizer 208 and two 4×4MMI 209:

The mode spot converter 201 couples the signal light with phase information into the epitaxial wafer device layer;

1×2MMI 203, which divides the signal light with information entering the epitaxial wafer device layer into two beams of equal power and phase;

the first polarization converter 204, which rotates the polarization state polarization of one of the two beams of light equally divided by the 1×2 MMI 203 with equal power and phase by 90°;

The second polarization converter 205 rotates the polarization state polarization of the other beam after passing through a straight waveguide in the two beams of equal power and phase divided by the 1×2 MMI 203 by 90°;

2×2 MMI 206, which couples the signal light rotated by the first polarization converter 204 and the second polarization converter 205 to output two modes of signal light, TE and TM;

The third polarization converter 207 rotates the polarization state polarization of the TM mode signal light output by the 2×2 MMI 206 by 90° into the TE mode signal light;

The waveguide polarizer 208 filters the TE mode signal light output by the 2×2 MMI 206 and the TE mode signal light rotated by the third polarization converter 207;

Two 4×4 MMIs 209 perform multi-mode interference between the TE mode signal light filtered by the waveguide polarizer 208 and the TE mode local oscillator light to complete the frequency mixing of the TE mode input signal.

Among them, the multimode interference region length calculation of 1×2MMI, 2×2MMI and 4×4MMI is based on the multimode interference principle of MMI and the guided mode transmission analysis method, and the beat length L _π is L _π =4n _r w _e ² / 3λ ₀ , where n _r is the effective refractive index in the corresponding mode of the multi-mode interference region; we _e is the effective width in the corresponding mode, and λ ₀ is the operating wavelength of the device; therefore, the length of the interference region L of 1×2MMI 203 = _3Lπ /8; the length of the interference region of the 2x2 MMI 206 is L= _Lπ /2; the length of the interference region of the 4x4 MMI 209 is L= _3Lπ /4.

At the same time, the combination of four components, 1×2 MMI 203 , first polarization converter 204 , second polarization converter 205 , and 2×2 MMI 206 , realizes the function of polarization beam splitter. In order to achieve the most ideal beam splitting effect, the phase difference between the two beams is required to satisfy ±π/2, that is, the distance between the first polarization converter 204 and the second polarization converter 205 must satisfy L=π/(2(β _TE −β _TM )), where β _TE is the propagation constant of the TE mode; β _TM is the propagation constant of the TM mode.

In addition, the waveguide polarizer 208 realizes the elimination of a certain mode through the difference of the transmission loss of the metal film 4 to the TE mode and the TM mode; in an embodiment of the present invention, a thickness of 100nm-200nm is deposited on the surface of the straight waveguide The Al metal thin film can realize the loss of TE mode is controlled at 0.5dB-1dB, while the loss of TM mode is as high as 20dB-30dB; this can achieve the effect of further filtering the residual TM mode, reducing the frequency of the mixer. Polarization dispersion improves device accuracy.

In addition, due to the indium phosphide substrate selected for the indium phosphide-based optical mixer provided by the present invention, it is convenient to integrate with subsequent balanced photodetectors of the same material, which can further improve the integration degree and coupling efficiency, and provide future high-performance optical coherence. The integration and miniaturization of photodetectors have laid a solid foundation.

Referring to FIGS. 2 and 3 , the epitaxial wafer device layer 2 is located above the indium phosphide substrate 1 , and the entire epitaxial wafer is grown by MOCVD.

FIG. 2 is a schematic cross-sectional structure diagram of the indium phosphide-based optical mixer shown in FIG. 1 along the line A-A. The device structures 201-207 and 209 in the epitaxial wafer device layer are formed by etching on the epitaxial wafer.

Among them, the straight waveguide structure is a deep ridged InP waveguide with a ridge height of 4 μm and a width of 2.6 μm, which can ensure the single-mode transmission of the C-band signal light in the straight waveguide. In addition, the deep ridge is beneficial to further reduce the transmission loss of the straight waveguide and improve the responsivity of the device;

Among them, the thickness of the silicon dioxide upper cladding layer 3 is about 2 μm, and the refractive index at the wavelength of 1.55 μm is about 1.44, which plays a key role in protecting the core layer device. As indium phosphide chip is a fragile and easily damped integrated chip, it is very important to deposit silicon dioxide upper cladding to protect the device.

FIG. 3 is a schematic cross-sectional structure diagram of the indium phosphide-based optical mixer shown in FIG. 1 along the line B-B. The metal Al thin film 4 has a thickness of about 100 nm, and is deposited above the straight waveguide connected between the third polarization converter 207 and the 4×4 MMI 209 by electron beam evaporation. Implements further filtering of TM patterns.

Based on the phosphide optical mixer shown in Figures 1-3, the present invention also provides a method for preparing the indium phosphide optical mixer shown in Figures 1-3, the preparation method comprising:

Step 1, preparing an epitaxial wafer device layer, which is formed on an indium phosphide substrate;

Step 2: growing a silicon dioxide upper cladding layer on the epitaxial wafer device layer;

Step 3: A metal thin film is grown on the silicon dioxide upper cladding layer.

An embodiment of the present invention provides more detailed preparation steps, as shown in Figure 4, which specifically includes the following steps:

Step S401: providing an indium phosphide substrate;

Step S402: preparing an epitaxial wafer device layer, which specifically includes:

Step S4021: epitaxially growing an InP-InGaAsP-InP epitaxial wafer on the indium phosphide substrate;

Step S4022: growing a silicon dioxide mask on the InP-InGaAsP-InP epitaxial wafer;

Step S4023: after spin-coating photoresist on the silicon dioxide mask, exposing and developing;

Step S4024: etching out a square groove for etching;

Step S4025: removing the glue, and etching out the inclined sidewall of the polarization converter with the aid of an etchant;

Step S4026: growing a top layer of silicon dioxide on the epitaxial wafer device layer;

Step S4027: spin-coating photoresist on the top layer silicon dioxide, exposing, developing and fixing;

Step S4028: using the top layer of silicon dioxide as a hard mask, etching the pattern area structure, and completing the transfer of the pattern from the photoresist to the epitaxial wafer;

Step S4029: After heating, degumming, and cleaning, the mode spot converter, the 1×2 multi-mode interference coupler, the first polarization converter, the second polarization converter, the 2 ×2 multi-mode interference coupler, the third polarization converter, the 4 × 4 multi-mode interference coupler;

Step S403: growing a silicon dioxide upper cladding layer;

Step S404: growing a metal Al thin film to form the waveguide polarizer;

Slice, and grind and polish the end face, and finally complete the preparation of indium phosphide-based optical mixer.

Among them, the epitaxial wafer is obtained by MOCVD epitaxy;

Wherein, the inclined sidewall is obtained by wet etching of bromine-methanol solution, and the inclination angle of the inclined sidewall is 50°-60°:

Wherein, in the step of growing the metal film, it specifically includes:

spin-coating a negative glue on the silica upper cladding;

On the negative glue, above the connecting straight waveguide between the third polarization converter and the 4×4 multi-mode interference coupler, a hole is lithographically opened;

At the opening, a metal film is deposited by evaporation;

The negative adhesive is peeled off and cleaned to prepare the desired waveguide type polarizer.

It should be noted that when cleaning the indium phosphide substrate and the device layer of the epitaxial wafer during the preparation process, since inorganic cleaning will corrode the chip, use: heating in a water bath in an acetone solution; heating with an ethanol water bath to remove residual acetone ; Rinse residual ethanol with deionized water; Cleaning method by nitrogen blowing.

Based on the indium phosphide-based optical mixer shown in Figures 1-3, the frequency mixing process of the indium phosphide-based optical mixer provided by the present invention for signal light is as follows:

The signal light is coupled into the straight waveguide 202 through the mode-spot converter 201 , and is divided into two beams of light with equal intensity and phase through the 1×2 MMI multi-mode interference coupler 203 . Then it passes through a straight waveguide structure similar to a Mach-Zehnder interferometer, in which the first polarization converter and the second polarization converter are located on the two sides of the two arms of the Mach-Zehnder delay line, respectively. The light needs to pass through a fixed length difference before being input to the first polarization converter 204 to achieve efficient polarization beam splitting of the entire device. The light of the two arms of the MZI is respectively input into the 2×2 MMI multi-mode interference coupler 206 to complete the polarization beam splitting therein, and the two output lights are respectively TE and TM modes. The output light of the TE mode is further filtered by the waveguide type polarizer 208 to filter out the residual TM mode and then input to the 4×4 MMI multi-mode interference coupler 209 for mixing the TE mode to complete the frequency mixing with the local oscillator light of the TE mode and output the signal light The signal carried by the TE mode in . The other TM mode output light is converted into TE mode by the third polarization converter 207 and further filtered by the waveguide polarizer 208 to filter the residual TM mode and then input to the 4×4 MMI multi-mode interference coupler 209 for mixing the TE mode with The local oscillator light in the TE mode completes the frequency mixing and outputs the signal carried by the TE mode in the signal light.

The specific embodiments described above further describe the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above descriptions are only specific embodiments of the present invention, and are not intended to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be should be included within the protection scope of the present invention.

Claims (8)

1. An indium phosphide-based optical mixer comprising:

an epitaxial wafer device layer formed on an indium phosphide substrate for realizing frequency mixing;

the upper cladding layer of the silicon dioxide is formed on the epitaxial wafer device layer and used for protecting the epitaxial wafer device layer and improving the stability of the epitaxial wafer device layer; and

a metal film formed on the silica upper cladding for controlling polarization;

wherein, epitaxial wafer device layer includes that it connects gradually through straight waveguide:

a spot size converter optically coupling the signal with the phase information into the epitaxial wafer device layer;

a 1 x 2 multimode interference coupler, which divides the signal light with phase information coupled into the epitaxial wafer device layer by the spot size converter into two beams of light with equal power and phase;

a first polarization converter for rotating the polarization state polarization of one of the two light beams equally divided by the 1 × 2 multimode interference coupler with equal power and phase by 90 °;

the second polarization converter rotates the polarization state polarization of the other light beam which passes through the straight waveguide by 90 degrees from the two light beams with equal power and phase which are equally divided by the 1 multiplied by 2 multimode interference coupler;

a 2 x 2 multimode interference coupler, which couples the two beams of signal light rotated by the first polarization converter and the second polarization converter to output two modes of TE and TM signal light;

a third polarization converter for rotating the polarization state of the TM mode signal light output from the 2 × 2 multimode interference coupler by 90 ° to become a TE mode signal light;

a waveguide type polarizer for filtering the TE mode signal light outputted from the 2 × 2 multimode interference coupler and the TE mode signal light rotated by the third polarization converter to filter out residual TM mode signal light; and

and the 4 x 4 multimode interference coupler performs multimode interference on the TE mode signal light filtered by the waveguide type polarizer and the TE mode local oscillator light to complete frequency mixing of the TE mode input signal.

2. The indium phosphide-based optical mixer of claim 1 wherein the 1 x 2 multimode interference coupler has an interference zone length L-3L_π8; the length of the interference zone of the 2 x 2 multimode interference coupler is L ═ L_π2; the length of the interference zone of the 4 multiplied by 4 multimode interference coupler is L-3L_π(ii)/4; wherein L is_πIs a beat length, L_π＝4n_rw_e ²/3λ₀In the formula, n_rEffective refractive index of the multimode interference region in a corresponding mode; w is a_eTo correspond to the effective width in the mode, λ₀Is the operating wavelength of the device.

3. The indium phosphide-based optical mixer of claim 1 wherein the distance L between the first polarization converter and the second polarization converter satisfies L ═ pi/(2 (β)_TE-β_TM) In which beta is_TEIs the propagation constant of the TE mode, beta_TMIs the propagation constant of the TM mode.

4. The indium phosphide-based optical mixer of claim 1 wherein the silica overclad layer is 1 μm to 3 μm thick.

5. The indium phosphide-based optical mixer of claim 1 wherein the thin metal film is disposed over a straight waveguide between the third polarization converter and the 4 x 4 multimode interference coupler and is made of Ag, Au, Cu or Al with a thickness of 100nm to 200 nm.

6. A method of making the indium phosphide-based optical mixer of any one of claims 1 to 5, comprising:

step 1: preparing an epitaxial wafer device layer on an indium phosphide substrate;

step 2: forming a silicon dioxide upper cladding layer on the epitaxial wafer device layer; and

and step 3: forming a metal film on the silica upper cladding layer; the preparation of the epitaxial wafer device layer on the indium phosphide substrate specifically comprises the following steps:

preparing an epitaxial wafer device layer on an indium phosphide substrate through photoetching, etching and corrosion processes, wherein the epitaxial wafer device layer comprises a spot size converter, a 1 × 2 multi-mode interference coupler, a first polarization converter, a second polarization converter, a 2 × 2 multi-mode interference coupler, a third polarization converter and a 4 × 4 multi-mode interference coupler which are sequentially connected through a straight waveguide; wherein the etching thickness of the etching process is 4-5 μm.

7. The method for preparing the optical waveguide according to claim 6, wherein the first polarization converter, the second polarization converter and the third polarization converter are realized by inclined sidewalls on one side of the straight waveguide, the inclined sidewalls are obtained by bromine-methanol solution wet etching, and the inclination angle of the inclined sidewalls is 50 ° to 60 °.

8. The method of claim 6, wherein in step 3, the metal film is grown on top of a silica upper cladding over a straight waveguide between the third polarization transformer and the 4 x 4 multimode interference coupler, forming a waveguide-type polarizer with the straight waveguide between the third polarization transformer and the 4 x 4 multimode interference coupler.

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