Grating auxiliary reverse coupler structure based on lithium niobate

Abstract

The invention discloses a grating auxiliary reverse coupler structure based on lithium niobate, and belongs to the technical field of optical waveguides and optical switches. The device is realized based on a Bragg grating, and comprises an optical loop formed by an asymmetric directional coupler and a grating coupler, gratings are etched on the inner side walls of the two waveguides of the middle section to form grating perturbation structures; an optical signal is regulated and controlled by using a grating perturbation structure, so that the TE mode reverse coupling is output from a downloading port, the TM mode is output from a straight-through port, and the polarization beam splitting effect is achieved. According to the definition of the polarization extinction ratio, the TE extinction ratio at the central wavelength reaches 32.4 dB, and the TM extinction ratio reaches 22.6 dB. Compared with a traditional reverse coupler, the reverse coupler is based on the waveguide grating, is not limited by FSR, is large in tuning range and is more beneficial to on-chip integration. According to the invention, the construction of a reconfigurable optical switching network is facilitated, and the flexibility is provided for manufacturing a large-scale optical switch array and an optical interconnection network.

Description

Technical Field

[0001] The present invention relates to the technical field of optical waveguide and optical switch, and in particular to a grating-assisted reverse coupler structure based on lithium niobate, which can be applied to optical communication, optical interconnection and the like. Background Art

[0002] With the explosive growth of interconnected devices and information traffic, information processing technology has gradually reached a bottleneck. In order to achieve faster information processing speed and higher information transmission capacity, silicon-based integrated photonic structures compatible with complementary metal semiconductor (CMOS) processes have begun to be widely studied and applied. In order to build a low-power sustainable data center, researchers have proposed a solution to replace electrical interconnection with optical interconnection and electrical switching with optical switching based on the advantages of low crosstalk, low latency, and large capacity of optical interconnection. The wavelength selective switches used in optical fiber networks are usually based on free optical devices and are not suitable for on-chip integration. In recent decades, people have proposed and implemented a variety of wavelength division multiplexing (WDM) devices, including WDM devices based on ring resonator filters, arrayed waveguide gratings, Mach-Zehnder interferometers, etc. Among them, multi-channel wavelength division multiplexers based on ring resonators and arrayed waveguide gratings are difficult to achieve flat-top response; cascaded multi-channel multiplexers based on Mach-Zehnder interferometers can achieve good performance, but require a large space. Grating-assisted reverse couplers are important optical components in photonic integrated circuits. They can be used as basic components of optical switches, multiplexers (demultiplexers), filters, and splitters. Because optical switches are the core units of silicon-based optoelectronic integrated chips and are also the core devices in optical switching networks, they directly affect chip performance and reliability. In view of the new challenges of multi-directional, low-power, and easy integration of optical switches and optical switch networks under the new generation of information technology, research on the structure of grating-assisted reverse couplers based on lithium niobate is inevitable.

[0003] Optical switches are an important component of the current high-speed and large-capacity information exchange, and are also important devices for on-chip interconnection. The materials used to make waveguide optical switches include various types such as Group IIII-V semiconductors, silicon-based silicon dioxide waveguides, lithium niobate, and organic polymers. Lithium niobate (LN: Lithium Niobate) material has excellent optical properties, obvious linear electro-optical effect, and stable physical and chemical properties. It is one of the crystal materials with good comprehensive performance and the most widely used. By utilizing its excellent electro-optical coefficient, the high-speed and stable switching of the switch state can be achieved under the electro-optical coordinated control. So far, the micromachining process of lithium niobate materials is relatively mature. The many excellent properties of lithium niobate crystals have made waveguide devices based on lithium niobate crystals receive more and more attention, including waveguide gratings. In integrated optical device systems, waveguide gratings play a vital role and are the basic components of many filters and sensors. They have broad application prospects in optical communication systems, optical sensing, lasers and other fields. Summary of the invention

[0004] In view of the new generation of information technology, how to design a reverse coupler suitable for large-scale integration and realize the multi-directional, low-power, and high-reliability characteristics of optical switches and optical switch networks is an important issue in the research of grating-assisted reverse coupler structures based on lithium niobate. In order to solve the above problems, the present invention provides a grating-assisted reverse coupler structure based on lithium niobate, which uses a directional coupler and a waveguide sidewall etched grating to regulate the optical signal, so that the TE mode of the optical signal generates reverse coupling and is output from the download port, and the TM mode is output from the specified through port, which can separate the TE and TM polarization modes to achieve the purpose of polarization splitting. Moreover, the present invention is based on a waveguide grating, has no FSR limitation, and has a large tuning range.

[0005] The present invention is achieved through the following technical solutions:

[0006] A grating-assisted reverse coupler structure based on lithium niobate, wherein the grating-assisted reverse coupler is implemented based on a double-sidewall etched Bragg grating, and comprises an optical circuit consisting of a silicon dioxide cladding and a waveguide layer prepared based on lithium niobate material, wherein the optical circuit comprises an asymmetric directional coupler and a sidewall etched grating coupler between two waveguides; the upper waveguide width of the asymmetric directional coupler is W a +ΔW a , the lower waveguide width is W b +ΔW b , the spacing between the upper and lower waveguides is set to G, and the inner walls of the two waveguides in the middle section are etched with gratings to form a grating perturbation structure; the grating etching depth in the grating perturbation structure is ΔW a and ΔW b , the duty cycle is d, the period is Λ, determined according to the phase matching condition, the formula is

[0007]

[0008] The grating reverse coupler should be provided with a silicon substrate or a silicon dioxide substrate, and a silicon dioxide protective layer is arranged on the optical waveguide layer made of lithium niobate thin film material, and the refractive index of the protective layer material is lower than the refractive index of the lithium niobate thin film material.

[0009] In the above technical solution, the grating reverse coupler structure is a Bragg grating structure, and the Bragg grating includes an upper and a lower waveguide segment. Periodic rectangular etching is performed on the side walls of the upper and lower waveguide segments so that the effective refractive index of the waveguide presents a periodic change.

[0010] In the above technical solution, the reverse coupler structure includes a waveguide segment a and a waveguide segment b. The first half period and the second half period are arranged on opposite sides of the waveguide segment a and the waveguide segment b, and both are extended half outward to form a grating modulated perturbation structure, which does not directly extend outward or shrink inward on the basis of a straight waveguide.

[0011] In the above technical solution, the reverse coupler structure includes a waveguide segment c and a waveguide segment d on opposite sides, wherein the first half cycle is extended outward by half, and the second half cycle is shortened inward by half, thereby forming a grating modulated perturbation structure.

[0012] The beneficial effects of the present invention are:

[0013] The reverse coupler based on the Bragg grating of the present invention can change the propagation direction of the original light field in the waveguide. The optical signal is regulated by using a directional coupler and a waveguide sidewall etched grating to realize the reverse coupling of the optical signal TE mode and output from the download port, and the TM mode is output from the designated straight-through port, which can separate the TE and TM polarization modes to achieve the purpose of polarization splitting. By selecting the appropriate waveguide gap and grating width, it is easy to meet the condition of not being restricted by the free spectrum range, so that the reverse coupler can accommodate more channels and improve communication efficiency. Compared with the traditional reverse coupler, the present invention is based on a waveguide grating, has no FSR limitation, has a large tuning range, and is more conducive to on-chip integration.

[0014] The present invention is conducive to building a reconfigurable optical switching network. In the later stage, the excellent electro-optical coefficient of lithium niobate material can be used to achieve high-speed and stable switching of switch states under electro-optical coordinated control, providing flexibility for the production of large-scale optical switch arrays and optical interconnection networks. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a schematic diagram of the overall structure of the grating-assisted reverse coupler structure based on lithium niobate of the present invention;

[0016] Figure 2It is a schematic diagram of the overall structure of the multi-directional controllable electro-optical switch structure based on the grating coupler of the present invention;

[0017] Figure 3 A schematic diagram of the structure of the reverse coupler in the present invention;

[0018] Figure 4 A schematic diagram of the structure of the reverse coupler in the present invention;

[0019] FIG5 is a diagram showing the power field distribution at the center wavelength when the TE and TM modes are input respectively according to the present invention, wherein FIG5(a) is the input TE mode, and FIG5(b) is the input TM mode;

[0020] FIG6 is a simulated spectrum diagram of the through end and the download end when the TE and TM modes are input respectively in the present invention, wherein FIG6(a) is the input TE mode, and FIG6(b) is the input TM mode; DETAILED DESCRIPTION

[0021] The present invention is further described in detail below by specific implementation modes in conjunction with the accompanying drawings. The present application can be implemented in a variety of different forms and is not limited to the implementation modes described in this embodiment. The purpose of providing the following specific implementation modes is to facilitate a clearer and more thorough understanding of the content of the invention of the present invention.

[0022] like Figure 1As shown, the present invention designs a grating-assisted reverse coupler structure based on lithium niobate, including an asymmetric directional coupler, an upper waveguide 1, a lower waveguide 2, a grating perturbation structure, a lithium niobate waveguide layer 3 and a silicon dioxide substrate 4. The left end of the lower waveguide 2 is the optical signal input end of the reverse coupler, the right end of the lower waveguide 2 is the through end of the reverse coupler, and the left end of the upper waveguide 1 is the download end of the reverse coupler, forming a 2x2 asymmetric directional coupler; Bragg gratings are etched on the inner side walls of the two middle waveguide parts, and the first half period is extended outward by half on the opposite side, and the second half period is reduced inward by half to form a grating perturbation structure. The grating-assisted reverse coupler structure based on lithium niobate can be formed according to the above scheme. When the TE mode of the optical signal is input from the left side end of the lower waveguide 2, i.e., the input port, the two waveguides have different widths and do not meet the phase matching condition. The mismatch is serious and no co-directional coupling will be generated. Based on the existence of the grating perturbation, the reverse coupling meets the phase matching condition, so the TE mode will be output from the left side end of the upper waveguide 1, i.e., the download port of the reverse coupler, through the action of the grating; when the TM mode of the optical signal is input from the left side end of the lower waveguide 2, i.e., the input port, the co-directional coupling does not meet the phase matching condition and is cut off. At the same time, the period of the grating is for the TE mode, and is calculated by the formula according to the phase matching condition of the reverse coupling. Therefore, the TM mode does not meet the phase matching condition of the reverse coupling either, so the TM mode can only be output from the right side end of the lower waveguide 2, i.e., the straight-through port of the reverse coupler. The optical signal is regulated by using a directional coupler and a grating etched on the side wall of the waveguide, so that the TE mode of the optical signal generates reverse coupling and is output from the download port, and the TM mode is output from the specified straight-through port, which can separate the TE and TM polarization modes and realize the function of polarization beam splitting.

[0023] like Figure 2 As shown, a top view of a grating-assisted reverse coupler structure based on lithium niobate, in which the waveguide widths of the grating-assisted upper waveguide 1 and the grating-assisted lower waveguide 2 are different. Due to the different widths, the propagation constants of the two are also different. When the propagation constants of the two are greatly different, the efficiency of the co-directional coupling will be particularly low, and the co-directional coupling will be cut off. In the grating perturbation structure, the ridge height of the lithium niobate film is 350nm, the plate height is 100nm, the lengths of the input and output waveguides are both 10μm, and the width of the upper waveguide 1 is W a +ΔW a , the width of the lower waveguide 2 is W b +ΔW b , the grating etching width is ΔW a and ΔW b , the duty cycle is d, the period is Λ, the spacing between the upper and lower waveguides is set to G, and the number of grating periods is 500.

[0024] like Figure 3As shown, the reverse coupler includes a waveguide section a and a waveguide section b, which are arranged parallel to each other up and down, and a periodic tooth structure is arranged on the opposite sides of the waveguide section a and the waveguide section b, and the first half period and the second half period are arranged on the opposite sides of the waveguide section a and the waveguide section b, and both extend half of the period outward to form a grating. The length of the grating period is related to the propagation wavelength and the waveguide material, and is generally several hundred nanometers, wherein the depth and width of a single tooth structure are designed according to the specific situation.

[0025] like Figure 4 As shown, the reverse coupler includes a waveguide segment c and a waveguide segment d, the waveguide segment c and the waveguide segment d are arranged parallel to each other, and a periodic tooth structure is arranged on the opposite sides of the waveguide segment c and the waveguide segment d, and the first half period is extended half outward and the second half period is shortened half inward on the opposite sides of the waveguide segment c and the waveguide segment d to form a grating. The structure of this embodiment can maintain the average effective refractive index of the grating perturbation zone within one period to be not much different from that of the original straight waveguide, thereby ensuring that the Bragg wavelength drift is small and a result closer to the theoretical value can be obtained.

[0026] As shown in FIG5 , the power field distribution at the center wavelength obtained when the TE and TM modes are input from the left input port of the lower waveguide 2 respectively. It can be seen from the figure that the TE mode does not meet the phase matching condition of the co-directional coupling and has a serious mismatch. Based on the existence of the grating perturbation, the reverse coupling meets the phase matching condition and can obtain effective coupling output from the left end of the upper waveguide 1, i.e., the download end of the reverse coupler. The TM mode is cut off because the co-directional coupling does not meet the phase matching condition and does not meet the phase matching condition of the reverse coupling, so the TM mode can only be output from the right end of the lower waveguide 2, i.e., the through port of the reverse coupler.

[0027] As shown in Figure 6, the simulated spectra of the through end and the download end are obtained when the TE and TM modes are input from the left input port of the lower waveguide 2. According to the definition of polarization extinction ratio, it can be found from the simulation results that the TE extinction ratio at the center wavelength reaches 32.4dB, and the TM extinction ratio reaches 22.6dB. The download end has a higher extinction ratio, and for the through end, the extinction ratio can be further improved by increasing the number of grating periods.

[0028] The above is only a preferred specific implementation of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with the technical field within the technical scope disclosed in the present invention should be covered within the protection scope of the present invention.

Claims

1. The present invention proposes a grating-assisted reverse coupler structure based on lithium niobate. Features: The grating-assisted reverse coupler is realized based on a double-sidewall etched Bragg grating, and includes an optical circuit consisting of a silicon dioxide cladding and a waveguide layer prepared based on a lithium niobate material. The optical circuit includes an asymmetric directional coupler and a sidewall etched grating coupler between two waveguides; the width of the waveguide 1 on the asymmetric directional coupler is W a +ΔW a , the width of the lower waveguide 2 is W b +ΔW b , the spacing between the upper and lower waveguides is set to G, and the inner walls of the two waveguides in the middle section are etched with gratings to form a grating perturbation structure; the grating etching depth in the grating perturbation structure is ΔW a and ΔW b , the duty cycle is d, the period is Λ, which is determined according to the phase matching condition; the grating reverse coupler should be provided with a silicon substrate or a silicon dioxide substrate, and a silicon dioxide protective layer is provided on the optical waveguide layer made of lithium niobate thin film material, and the refractive index of the protective layer material is less than the refractive index of the lithium niobate thin film material.

2. The grating-assisted reverse coupler structure based on lithium niobate according to claim 1, Features: The waveguide widths of the grating-assisted upper waveguide and the grating-assisted lower waveguide in the grating reverse coupler are different. Due to the different widths, the propagation constants of the two are also different. When the propagation constants of the two are greatly different, the efficiency of the co-directional coupling will be particularly low, resulting in co-directional coupling cutoff.

3. The grating-assisted reverse coupler structure based on lithium niobate according to claims 1 and 2 is based on a grating perturbation structure, Features: The grating reverse coupler structure is a Bragg grating structure, which includes two waveguide sections, upper and lower. Periodic rectangular etching is performed on the side walls of the upper and lower waveguide sections to make the effective refractive index of the waveguide change periodically. In the grating perturbation structure, the ridge height of the lithium niobate film is 350nm, the plate height is 100nm, the length of the input and output waveguides is 10μm, and the width of the upper waveguide is W. a , the width of the lower waveguide is W b , the grating etching width is ΔW a and ΔW b , the duty cycle is d, the period is Λ, the spacing between waveguides is set to G, and the number of grating periods is 500.

4. The grating-assisted reverse coupler structure based on lithium niobate according to claims 1-3, Features: The reverse coupler structure includes a waveguide section a and a waveguide section b. On opposite sides of the waveguide section a and the waveguide section b, the first half period and the second half period are both extended halfway outward to form a grating modulated perturbation structure, which is not directly extended outward or directly reduced inward on the basis of a straight waveguide. Alternatively, the reverse coupler structure includes arranging the first half cycle to extend half outward and the second half cycle to shrink half inward on opposite sides of the waveguide segment c and the waveguide segment d, thereby forming a grating modulated perturbation structure.

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