CN116088205A

CN116088205A - Acousto-optic modulator with unidirectional electrode structure

Info

Publication number: CN116088205A
Application number: CN202310039984.3A
Authority: CN
Inventors: 张维佳; 饶前程; 谷珍杰; 罗雪婷; 刘婷; 刘泽; 刘体辉
Original assignee: Fujian Boer Optoelectronic Technology Co ltd; Tianjin University of Science and Technology
Current assignee: Fujian Boer Optoelectronic Technology Co ltd; Tianjin University of Science and Technology
Priority date: 2023-01-12
Filing date: 2023-01-12
Publication date: 2023-05-09

Abstract

The invention discloses an acousto-optic modulator with a unidirectional electrode structure, which comprises an LN substrate layer, a first buffer layer and an LN film layer which are sequentially stacked, wherein an optical waveguide embedded in the LN film layer is arranged above the first buffer layer, a second buffer layer is arranged above the LN film layer, a piezoelectric transducer is arranged on the second buffer layer, the piezoelectric transducer comprises a separated interdigital electrode with a unidirectional electrode structure, the unidirectional electrode structure comprises an excitation electrode, a short-circuit electrode and an open-circuit electrode, the excitation electrode comprises first electrode strips extending from the interdigital electrode, the short-circuit electrode comprises second electrode strips extending from the unidirectional direction, a second electrode strip and an open-circuit electrode are arranged between two adjacent first electrode strips, the open-circuit electrode is arranged between the adjacent first electrode strips and the second electrode strips, the second electrode strips are arranged between the adjacent open-circuit electrode strips, and the acousto-optic modulator can realize unidirectional propagation of acoustic surface waves and has lower loss and higher modulation efficiency.

Description

Acousto-optic modulator with unidirectional electrode structure

Technical Field

The invention relates to the field of integrated optoelectronics, in particular to an acousto-optic modulator with a unidirectional electrode structure.

Background

Integrated photonics is a science of integrating many high performance multifunctional passive and active devices on a unified substrate material (e.g., SOI wafer) to achieve specific functions, and employing on-chip integration methods to reduce the power consumption and cost of the device system provides superior technical advantages over traditional discrete optical systems. In recent years, with the development of micro-nano processing technology, lithium Niobate On Insulator (LNOI) thin films with excellent acousto-optic, piezoelectric, electro-optic, photorefractive properties and nonlinear optics have been successfully prepared. At present, the lithium niobate thin film technology has become a breakthrough technology in the field of integrated photonics, and the progress of developing active and passive novel photonic integrated circuits with higher performance and larger scale is promoted, and the development of integrated photonics is brought into the surge of new era.

Because the LN film and the silicon dioxide buried layer have larger refractive index difference, the input optical field is limited to be transmitted in the lithium niobate film to the greatest extent, so that the problem of miniaturization size of the LN photon device can be fundamentally solved, and the interaction strength of the square and the LN film can be improved due to the small size, and finally the device performance is improved. The acousto-optic device which can combine the advantages of the lithium niobate film is an acousto-optic modulator, can work in a plurality of different physical fields at present, and can realize the effective coupling and conversion of three physical fields of microwave, sound wave and light wave on the lithium niobate film. And along with successful LNOI preparation, the on-chip integrated acousto-optic modulator becomes a research hot spot, and the acousto-optic modulator can provide more ideal regulation and control devices for on-chip photoelectric integrated circuits, quantum computation and the like.

The acousto-optic modulator is used as an acousto-optic device for controlling the intensity or phase change of laser beams, and mainly comprises interdigital electrodes and an acousto-optic medium. The basic principle is that when a specific power signal of a signal source drives an electrode, a Surface Acoustic Wave (SAW) with the same frequency is generated and coupled into an acousto-optic interaction medium, so that the refractive index of the optical medium is periodically strained, and at the moment, a diffraction phenomenon is generated due to interaction when a light beam passes through the optical medium with the refractive index changed. At present, an acousto-optic modulator is widely applied to the fields of optical signal processing, optical fiber communication switches and the like.

As shown in FIG. 1, the bidirectional uniform interdigital electrode structure often used in SAW-driven acousto-optic devices is widely applied to devices such as acousto-optic frequency shifters, delay lines, surface acoustic wave filters and the like due to the characteristics of simple structure, low requirements on photolithography process precision, easiness in manufacturing and the like, and due to the symmetrical geometric structure, the surface acoustic waves with equal size and opposite directions are generated under the excitation of radio frequency signals to bidirectionally propagate in a piezoelectric medium, and the propagation mode can cause the waste of SAW energy in one direction. In addition, due to the secondary effect and the triple echo effect of the SAW, the passband ripple will be very strong, which will interfere with the normal operation of the acousto-optic device, thereby increasing the insertion loss and further reducing the modulation efficiency of the whole device.

At present, a large number of reported acousto-optic modulators still use traditional bidirectional interdigital electrodes, and the acousto-optic modulators manufactured by the electrodes have the problems of overhigh driving power (more than 2W), low modulation efficiency (less than 66 percent) and the like. In 2013, juntao Wang utilized 128 ° Y-cut LN as the piezoelectric material, combined with a bi-directional uniform interdigital electrode, produced an MZ waveguide acousto-optic modulator with modulation efficiency as low as 4.3%. In 2020, an acousto-optic modulator fabricated by using bi-directional uniform interdigital electrodes has a center frequency of 189MHz and a diffraction efficiency of only 3.0%.

Disclosure of Invention

In view of the above technical problems mentioned in the background art, embodiments of the present application provide an acousto-optic modulator with a unidirectional electrode structure to solve the above problems.

In order to achieve the above object, the technical scheme of the present invention is as follows:

the utility model provides an acousto-optic modulator with unidirectional electrode structure, includes LN substrate layer, first buffer layer and LN thin film layer that stacks gradually, first buffer layer top be equipped with imbed in the optical waveguide in the LN thin film layer, LN thin film layer top is equipped with the second buffer layer, be equipped with on the second buffer layer with the adjacent piezoelectric transducer of optical waveguide, piezoelectric transducer is including the split type interdigital electrode that has unidirectional electrode structure, the interdigital direction of split type interdigital electrode is parallel with the length direction of optical waveguide, unidirectional electrode structure includes excitation electrode, short circuit electrode and open circuit electrode, the excitation electrode includes the first electrode strip that a plurality of interdigital extends, the short circuit electrode includes two unidirectionally extending second electrode strips, adjacent two be equipped with a second electrode strip and an open circuit electrode between the first electrode strip of second, and the open circuit electrode is located between adjacent open circuit electrode strip and the first electrode strip.

Preferably, the distance between the piezoelectric transducer and the optical waveguide is half the wavelength of the piezoelectric transducer.

Preferably, the thickness of the optical waveguide is the same as the thickness of the LN thin film layer.

Preferably, the optical waveguide is an MZ waveguide.

Preferably, the thickness of the separated interdigital electrode is 500nm, and the interdigital pair number is 8.

Preferably, the excitation electrode further comprises first connecting strips positioned at the ends of the first electrode strips, and the first electrode strips are respectively connected to the two first connecting strips at a crossing interval.

Preferably, the short-circuit electrode further comprises a second connecting strip positioned at one end of the second electrode strip, and the second connecting strips are respectively connected with two adjacent second electrode strips.

Preferably, the first electrode bar, the open electrode and the second electrode bar are equally spaced.

Preferably, the LN substrate layer, the first buffer layer, and the LN film layer are fabricated using a 128Y cut LNOI film.

Preferably, the first buffer layer and the second buffer layer are SiO ₂ 。

Compared with the prior art, the invention has the following beneficial effects:

(1) The invention manufactures the acousto-optic modulator integrating the piezoelectric transducer and the optical waveguide on the basis of 128-degree Y-cut LNOI, the piezoelectric transducer adopts the separated interdigital electrode to replace the traditional bidirectional uniform interdigital electrode, and the unidirectional SAW can be effectively excited by utilizing the excellent electromechanical coupling coefficient of the 128-degree Y-cut LNOI material and the unidirectional performance of the separated interdigital electrode, so that the modulation efficiency of the acousto-optic modulator can reach 89%.

(2) The invention adopts the separated interdigital electrode with the unidirectional electrode structure to excite the surface acoustic wave, thereby reducing the driving power and the insertion loss of the acousto-optic modulator.

(3) The MZ waveguide is manufactured by adopting the buffer proton exchange technology, so that the exchange time can be delayed, the damage to the piezoelectric crystal is reduced, the loss of the manufactured MZ waveguide is as low as 0.7dB/cm, and the nonlinear optical characteristics of the material are well reserved through annealing treatment.

Drawings

The accompanying drawings are included to provide a further understanding of the embodiments and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments and together with the description serve to explain the principles of the invention. Many of the intended advantages of other embodiments and embodiments will be readily appreciated as they become better understood by reference to the following detailed description.

FIG. 1 is a schematic diagram of a conventional bidirectional uniform interdigital electrode structure;

FIG. 2 is a schematic diagram of split interdigital electrodes of an acousto-optic modulator having a unidirectional electrode structure in accordance with an embodiment of the present application;

FIG. 3 is a schematic cross-sectional view of an acousto-optic modulator with unidirectional electrode structure in accordance with an embodiment of the present application;

FIG. 4 is a schematic diagram of an acousto-optic modulator with unidirectional electrode structure in accordance with an embodiment of the present application;

reference numerals: 1. an excitation bar; 2. a short-circuit electrode; 3. an open-circuit electrode; 4. a grounding bar; 5. separate interdigital electrodes; 6. a second buffer layer; 7.MZ waveguide; 8. an LN film layer; 9. a first buffer layer; 10. an LN substrate layer.

Detailed Description

The present application is described in further detail below with reference to the drawings and examples. It is to be understood that the specific embodiments described herein are merely illustrative of the invention and are not limiting of the invention. It should be noted that, for convenience of description, only the portions related to the present invention are shown in the drawings.

It should be noted that, in the case of no conflict, the embodiments and features in the embodiments may be combined with each other. The present application will be described in detail below with reference to the accompanying drawings in conjunction with embodiments.

Referring to fig. 2-4, an acousto-optic modulator with unidirectional electrode structure is provided in an embodiment of the present invention, which includes an LN substrate layer 10, a first buffer layer 9 and an LN thin film layer 8 stacked in sequence, an optical waveguide embedded in the LN thin film layer 8 is disposed above the first buffer layer 9, a second buffer layer 6 is disposed above the LN thin film layer 8, a piezoelectric transducer adjacent to the optical waveguide is disposed on the second buffer layer 6, the piezoelectric transducer includes a split interdigital electrode 5 with unidirectional electrode structure, specifically, the thickness of the split interdigital electrode 5 is 500nm, the number of interdigital pairs is 8, and each periodic unit is composed of 6 interdigital strips. The interdigital direction of the separated interdigital electrode 5 is parallel to the length direction of the optical waveguide, the unidirectional electrode structure comprises an excitation electrode, a short-circuit electrode 2 and an open-circuit electrode 3, the excitation electrode comprises two first connecting strips and a plurality of first electrode strips which are arranged in parallel, the two first connecting strips respectively extend perpendicularly to form a plurality of first electrode strips which are parallel in an intersecting manner and are arranged in a tooth comb-like manner in an intersecting manner, and the interval distance between the two adjacent first electrode strips is the same. The shorting electrode 2 comprises second connecting strips, unidirectional second electrode strips vertically extend from two ends of the second connecting strips, an open electrode 3 and a first electrode strip are arranged between two adjacent second electrode strips, and a second electrode strip and an open electrode 3 are arranged between two adjacent first electrode strips, that is, the open electrode 3 is located between two adjacent first electrode strips and the second electrode strip, and the second electrode strip is located between two adjacent open electrode 3 and the first electrode strip. In a preferred embodiment, the spacing between the first electrode strips, the open-circuited electrode 3 and the second electrode strips is equal. One of the first connecting strips and the first electrode strip extending above the first connecting strip serve as an excitation strip 1, the other first connecting strip and the first electrode strip extending above the first connecting strip serve as a grounding strip 4, an electric signal is applied to the excitation strip 1, and the grounding strip 4 is grounded, so that the propagation of the SAW is excited, and the SAW can only realize unidirectional propagation due to the existence of the open-circuit electrode 3 and the short-circuit electrode 2.

Specifically, the distance between the piezoelectric transducer and the optical waveguide is half the wavelength of the piezoelectric transducer. The thickness of the optical waveguide is the same as that of the LN thin film layer 8. In a preferred embodiment, the optical waveguide is an MZ waveguide 7. The structure of MZ waveguide 7 is not described in detail herein. The MZ waveguide 7 is prepared by a buffer proton exchange method, the waveguide prepared by the method has low loss, and the optical nonlinear characteristics in the lithium niobate material can be better reserved by annealing treatment after the buffer proton exchange.

Specifically, the LN substrate layer 10, the first buffer layer 9 and the LN thin film layer 8 are processed by adopting a 128 ° Y-cut LNOI thin film, and the 128 ° Y-cut LNOI has high refractive index contrast ratio and higher electromechanical coupling coefficient, so that SAW can be effectively excited. Further improving the modulation efficiency of the device and reducing the insertion loss. The first buffer layer 9 and the second buffer layer 6 are SiO2, and Au is selected as an electrode material.

The working principle of the acousto-optic modulator with the unidirectional electrode structure of the embodiment of the application is as follows:

the excitation electrode is respectively used as a terminal and a grounding end to apply an electric signal, and the electric signal simultaneously excites SAW to spread on the left side and the right side, and at the moment, the potential of the short circuit electrode 2 is zero and the structure is symmetrical, so that the transduction effect is not realized; however, the presence of the adjacent open electrode 3 causes the transduction center to move leftwards, and the open and short electrodes 2 form a distributed reflection source, so that the transduction center and the reflection center are not overlapped any more, which enhances the SAW propagating in the forward direction and weakens the SAW propagating in the reverse direction, thereby realizing unidirectional propagation of the SAW.

When a specific power signal of a signal source drives the separation interdigital electrode 5, the SAW generates the acoustic surface wave with the same frequency and is coupled into the acousto-optic interaction medium, so that the refractive index of the optical medium generates periodic strain, and at the moment, the light beam can generate interaction to generate diffraction phenomenon when passing through the optical medium with the refractive index changed, thereby realizing amplitude or intensity modulation of the optical signal.

According to the acousto-optic interaction theory, there is a corresponding relation between the structure of the electrode, the effect generated by the acoustic surface wave and the acousto-optic interaction intensity, and because the bidirectional uniform IDT devices adopted in the interdigital electrodes based on the traditional optical waveguide design are all in regular symmetrical shapes, compared with the design that the open electrode 3 and the short electrode 2 form a distributed reflection source to cause deflection of a transduction center and a reflection center in the embodiment of the application, the driving power required is higher, the insertion loss is larger, and the coupling efficiency between the mechanical deformation quantity of the light and the optical electromagnetic field is difficult to be effectively enhanced. According to the embodiment of the application, the separated interdigital electrode 5 with the unidirectional electrode structure is introduced above the LN film layer 8, and the deformation displacement field and the optical electromagnetic field are subjected to adjustment and optimization, so that the interaction between the deformation displacement field and the optical electromagnetic field is effectively improved, the acousto-optic interaction effect of the device is obviously improved, and the performance of the acousto-optic modulator is improved. The acousto-optic modulator with the unidirectional electrode structure of the embodiments of the present application has higher acousto-optic interaction efficiency and higher modulation efficiency.

The embodiment of the application also provides a manufacturing method of the acousto-optic modulator with the unidirectional electrode structure, which comprises the following steps:

s1, coating, namely selecting 128-degree Y-cut LNOI as an initial material, wherein the basic structure of the LNOI is an LN film layer 8, a first buffer layer 9 and an LN substrate from top to bottom, and coating a layer of SiO2 film with the thickness of 120nm on the surface of the LN film layer 8 by adopting a high-vacuum magnetron sputtering coating machine.

S2, photoetching, namely depositing a layer of positive photoresist on the SiO2 film on the LN film layer 8 by adopting a photoresist homogenizing machine, baking for 23min at 100 ℃, cooling to room temperature and taking out; placing an optical waveguide mask plate on the positive photoresist, and adopting ultraviolet lithography, wherein the lithography exposure time is 35s; placing the sample in a developing solution for 35s to develop and remove the irradiated positive photoresist, and then rinsing the photoresist residues with deionized water; baking for 10 minutes at 100 ℃, and removing the SiO2 film covered by the photoresist by using a silicon dioxide corrosive liquid; and manufacturing a SiO2 film with an optical waveguide pattern, wherein the line spacing on the optical waveguide pattern is 1.5um, and finally removing the residual photoresist on the SiO2 film by using a photoresist remover to form a second buffer layer 6.

S3, buffering proton exchange, namely cleaning a photoetched sample by using alcohol or acetone reagent, baking the sample at 100 ℃ for 5-10 minutes, placing the cleaned and dried sample into a clean and dried graphite crucible, placing a proper amount of buffering proton exchange powder (5 g of mixture of benzoic acid and lithium benzoate in a specific proportion) into the crucible, sealing, and then placing into a proton exchange furnace for heating to be molten; then exchanging for 5 hours at 245 ℃, then placing the sample into an annealing furnace, annealing for 3 hours at 370 ℃ for 20 minutes, and taking out at room temperature.

S4, cleaning the wafer subjected to the buffer proton exchange by using alcohol or acetone, and baking at 100 ℃ for 10min.

S5, photoetching a pattern of the separated interdigital electrode 5 on the surface of the LN film layer 8, wherein the photoresist is negative photoresist, and baking for 15min at 100 ℃. Depositing a layer of negative photoresist on the LN film layer 8 by adopting a photoresist homogenizing machine, baking for 15min at 100 ℃, taking out and cooling to room temperature; fixing a designed mask plate on the negative photoresist, and adopting contact exposure; treating the sample in a developing solution for 35s, and then rinsing photoresist residues with deionized water; the contour of the split interdigital electrode 5 is etched on the surface of the LN thin film layer 8.

S6, controlling the vacuum degree of the electron beam vacuum coating machine to be 5E-4, controlling the beam current to firstly coat a Cr film with the thickness of 100nm on the surface of the LN layer, and then coating an Au film with the thickness of 500nm in a vacuum environment.

And S7, soaking the sample after coating in acetone, stripping off the redundant metal film, and cleaning.

And S8, finally polishing the end face of the sample so as to measure the transmission loss of the device.

In the acousto-optic modulator with the unidirectional electrode structure, the open-circuit electrode 3 and the short-circuit electrode 2 form a distributed reflection source, so that the forward propagation SAW is enhanced, and the reverse SAW is weakened, thereby realizing the unidirectional propagation of the SAW. The incorporation of electrode unidirectionality into the acousto-optic modulator will reduce excessive device loss due to SAW bi-directional propagation. In addition, the 128-degree Y-cut LNOI film not only has high refractive index contrast ratio, but also has higher electromechanical coupling coefficient, and can effectively excite SAW. Further improving the modulation efficiency of the device and reducing the insertion loss. The embodiment of the application provides an acousto-optic modulator with a unidirectional electrode structure, which has the loss as low as 0.7dB/cm and the diffraction efficiency as high as 89%.

While the present invention has been described with reference to the specific embodiments thereof, the scope of the present invention is not limited thereto, and any changes or substitutions will be apparent to those skilled in the art within the scope of the present invention, and are intended to be covered by the present invention. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.

Claims

1. The utility model provides an acousto-optic modulator with unidirectional electrode structure, its characterized in that includes LN substrate layer, first buffer layer and LN thin film layer that stacks gradually, first buffer layer top be equipped with the embedding in LN thin film layer in the light waveguide, LN thin film layer top is equipped with the second buffer layer, be equipped with on the second buffer layer with the adjacent piezoelectric transducer of light waveguide, piezoelectric transducer is including the split type interdigital electrode that has unidirectional electrode structure, the interdigital direction of split type interdigital electrode is parallel with the length direction of light waveguide, unidirectional electrode structure includes excitation electrode, short circuit electrode and open circuit electrode, the excitation electrode includes the first electrode strip that a plurality of interdigital extends, the short circuit electrode includes two unidirectionally extending second electrode strips, is equipped with a second electrode strip and an open circuit electrode between two adjacent first electrode strips and the second electrode strip, and the open circuit electrode is located between adjacent first electrode strip and the second electrode strip, the second electrode strip is located between adjacent open circuit electrode strip and the first electrode strip.

2. The acousto-optic modulator with unidirectional electrode structure according to claim 1, wherein a spacing between said piezoelectric transducer and said optical waveguide is half a wavelength of said piezoelectric transducer.

3. The acousto-optic modulator with unidirectional electrode structure according to claim 1, wherein the thickness of the optical waveguide is the same as the thickness of the LN thin film layer.

4. The acousto-optic modulator with unidirectional electrode structure according to claim 1, wherein said optical waveguide is an MZ waveguide.

5. The acousto-optic modulator with unidirectional electrode structure of claim 1, wherein the thickness of the split interdigital electrode is 500nm and the number of interdigital pairs is 8.

6. The acousto-optic modulator with unidirectional electrode structure according to claim 1, wherein said excitation electrode further comprises first connection bars at ends of said first electrode bars, and a plurality of said first electrode bars are respectively connected to two first connection bars at a crossing interval.

7. The acousto-optic modulator with unidirectional electrode structure according to claim 1, wherein said shorting electrode further comprises a second connecting bar at one end of said second electrode bars, said second connecting bar being connected to two adjacent second electrode bars, respectively.

8. The acousto-optic modulator with unidirectional electrode structure of claim 1, wherein the spacing between the first electrode stripe, the open electrode and the second electrode stripe is equal.

9. The acousto-optic modulator with unidirectional electrode structure of claim 1, wherein said LN substrate layer, first buffer layer and LN thin film layer are fabricated using a 128 ° Y cut LNOI thin film.

10. The acousto-optic modulator with unidirectional electrode structure of claim 1, wherein said first and second buffer layers are SiO ₂ 。