A compact large-bandwidth 90° optical hybrid structure based on silicon nitride waveguides

Abstract

The application discloses a compact large-bandwidth 90-degree optical frequency mixer structure based on a silicon nitride waveguide, which comprises a first butterfly-shaped 2x2 MMI, a second butterfly-shaped 2x2 MMI, a third butterfly-shaped 2x2 MMI, a 1x2 MMI and four curved waveguides; a single-end waveguide of the 1x2 MMI serves as a local reference light LO input end; two output ends of the first butterfly-shaped 2x2 MMI are connected with one input port of the second butterfly-shaped 2x2 MMI and the third butterfly-shaped 2x2 MMI respectively through the curved waveguides, one of the two input ends of the first butterfly-shaped 2x2 MMI serves as a signal light Signal input end, and the other port is vacant; the other input port of the second butterfly-shaped 2x2 MMI and the third butterfly-shaped 2x2 MMI is connected with the two output ends of the 1x2 MMI through the curved waveguides. The application has the advantages that the butterfly-shaped 2x2 MMI is adopted, compared with a traditional rectangular structure, the butterfly-shaped structure is smaller in size and is more conducive to integration; the splitting ratio and the phase difference of the butterfly-shaped 2x2 MMI are more flat and the performance is better.

Description

Technical Field

[0001] The present invention relates to the field of optical communication technology, and in particular to a compact, large-bandwidth, 90-degree optical mixer structure based on silicon nitride waveguide. Background Art

[0002] Coherent optical communication is a crucial research area in the communications field. With its high detection sensitivity, diverse modulation schemes, and strong anti-interference capabilities, it is one of the best options for future high-capacity and long-distance transmission. The 90° optical mixer is a key component in coherent transmission systems. It mixes the input signal with a reference signal (LO) into four orthogonal states, then transmits the four optical signals to two pairs of balanced detectors. The performance of the mixer directly impacts the performance of the coherent receiver. Currently, there are two main types: 1) based on several discrete components, such as 1x2 couplers, 2x2 couplers, and phase shifters; and 2) based on a single device, such as the 4x4 multimode interferometer coupler (MMI).

[0003] The second type of mixer, the 4x4 MMI, is often preferred for materials with high refractive index contrasts. It is passive and requires no external control to achieve a stable 90° phase difference. However, due to the principle of self-imaging, to achieve optimal imaging, the device typically requires hundreds of microns, making integration difficult. Furthermore, due to dispersion, a single device struggles to achieve broadband performance, resulting in strong wavelength and process dependence. These issues are particularly pronounced when applied to silicon nitride platforms.

[0004] The first type of mixer has been verified on silicon and LiNbO3 platforms. This type of mixer requires precise phase control. Due to process tolerances, there are certain variations in waveguide width and thickness, which is difficult for waveguides with high refractive index differences. The refractive index difference between silicon nitride waveguides and the cladding is much smaller than that of silicon waveguides, making it relatively easier to implement. However, the size of each discrete component of silicon nitride waveguides is also larger than that of silicon waveguides. In order to achieve precise phase control, the size of each discrete component needs to be minimized. Summary of the Invention

[0005] The technical problem to be solved by the present invention is that the phase deviation of the first type mixer based on the high refractive index difference waveguide is large due to process tolerance, making it difficult to achieve precise control and making it difficult to expand the working bandwidth;

[0006] To solve the above technical problems, the application provides a compact large-bandwidth 90-degree optical frequency mixer structure based on a silicon nitride waveguide, which is based on a first type of mixer structure, and discrete devices are optimized to replace a conventional rectangular 2x2 MMI with a butterfly 2x2 multi-mode interference coupler (MMI). In the C+L waveband, the light splitting of the butterfly MMI is more flat, the phase difference is smaller, and the length is shorter. The application is based on a silicon nitride platform, has low loss, and adopts three discrete butterfly 2x2 MMIs and a 1x2 MMI, and a compact large-bandwidth 90-degree optical frequency mixer is provided, the common-mode rejection ratio (CMRR) of which can meet the requirement of 30 dB in the C+L waveband; meanwhile, because the device size is smaller and the material refractive index difference is small, the influence of the process tolerance on the phase difference can be effectively reduced.

[0007] The specific technical solutions are as follows:

[0008] A compact large-bandwidth 90-degree optical frequency mixer structure based on a silicon nitride waveguide comprises three butterfly 2x2 MMIs, a 1x2 MMI and four curved waveguides; the three butterfly 2x2 MMIs and the 1x2 MMI are all silicon nitride waveguides, and the four curved waveguides are all silicon nitride curved waveguides.

[0009] The single-end waveguide of the 1x2 MMI serves as a local reference light LO input end; the three butterfly 2x2 MMIs are a first butterfly 2x2 MMI, a second butterfly 2x2 MMI and a third butterfly 2x2 MMI, respectively; the two output ends of the first butterfly 2x2 MMI are connected to one input port of the second butterfly 2x2 MMI and the third butterfly 2x2 MMI through curved waveguides, respectively; one of the two input ends of the first butterfly 2x2 MMI serves as a signal light Signal input end, and the other port is vacant; the other input port of the second butterfly 2x2 MMI and the third butterfly 2x2 MMI is connected to the two output ends of the 1x2 MMI through curved waveguides.

[0010] Further preferably, the bending radius of the curved waveguide is greater than 50 um, so as to ensure low bending loss.

[0011] Further preferably, the middle multi-mode region waveguide of the butterfly 2x2 MMI is a left-right symmetrical two-section tapered waveguide, and is narrow in the middle and wide on both sides, so as to effectively reduce the device size.

[0012] Further preferably, the input / output waveguide of the butterfly 2x2 MMI is a silicon nitride tapered waveguide, and has low transmission loss.

[0013] Further preferably, the silicon nitride thickness of the butterfly 2x2 MMI is 0.3 um.

[0014] Compared with the prior art, the present application has the beneficial effects of:

[0015] The compact 90-degree optical frequency mixer structure provided by the present application adopts a butterfly-shaped 2x2 MMI, and compared with a traditional rectangular structure, the butterfly-shaped structure is smaller in size and more conducive to integration; at the same time, the splitting ratio and phase difference of the butterfly-shaped 2x2 MMI are more flat, and the performance is better.

[0016] The compact 90-degree optical frequency mixer structure provided by the present application is based on a silicon nitride waveguide, covers the C+L waveband in the working waveband, has a large bandwidth, good performance, and good process tolerance. BRIEF DESCRIPTION OF DRAWINGS

[0017] Fig. 1 is a schematic diagram of a compact large-bandwidth 90-degree optical frequency mixer structure based on a silicon nitride waveguide in an embodiment of the present application;

[0018] Fig. 2 is a schematic diagram of a butterfly-shaped 2x2 multimode interference coupler (MMI) structure provided in an embodiment of the present application;

[0019] Fig. 3 is a schematic diagram of an application of the 90-degree optical frequency mixer in an embodiment of the present application;

[0020] Fig. 4 is a simulation result of the splitting ratio under the optimal length of a rectangular and butterfly-shaped 2x2 MMI;

[0021] Fig. 5 is a simulation result of the output end phase difference under the optimal length of a rectangular and butterfly-shaped 2x2 MMI;

[0022] Fig. 6 is a simulation result of the common-mode rejection ratio of a 90-degree optical frequency mixer constructed based on a rectangular and butterfly-shaped 2x2 MMI;

[0023] Fig. 7 is a simulation result of the optical field transmission of the compact large-bandwidth 90-degree optical frequency mixer provided by the present application;

[0024] 1-First butterfly-shaped 2x2 multimode interference coupler (MMI); 101-Upper input waveguide of the 2x2 MMI; 102-Lower input waveguide of the 2x2 MMI; 103-Butterfly-shaped multimode waveguide of the 2x2 MMI; 104-Upper output waveguide of the 2x2 MMI; 105-Lower output waveguide of the 2x2 MMI; 2-Second butterfly-shaped 2x2 MMI; 3-Third butterfly-shaped 2x2 MMI; 4-1x2 MMI; 5-Curved waveguide. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical scheme and advantages of the present application more clear and explicit, the following will combine the accompanying drawings to further describe the present application. Figures 1-7The application will be described in further detail by the embodiments. It should be understood that the specific embodiments described herein merely exemplify the application and should not be used to limit the scope of the application. In addition, the technical features involved in the various embodiments of the application described below can be combined with each other as long as they do not conflict with each other.

[0026] The embodiment proposes a compact and large-bandwidth 90° optical mixer structure based on a silicon nitride waveguide, as shown in Figure 1 from left to right, in order: the first butterfly 2x2 MMI 1, the upper waveguide on the left side of which is a signal light input port, and the lower waveguide is vacant, the upper and lower two output waveguides on the right side of the output end are connected to the second butterfly 2x2 MMI 2 and one input port of the third butterfly 2x2 MMI 3 through two identical curved waveguides 5, respectively, the other input ports of the second butterfly 2x2 MMI 2 and the third butterfly 2x2 MMI 3 are connected to the two output ports of the 1x2 MMI 4, respectively; the single-end input waveguide on the right side of the 1x2 MMI 4 is the input end of the local reference light LO.

[0027] As shown in Figure 1 , the phase changes introduced by the four curved waveguides 5 are 1, 2, 3, 4, due to the existence of process error, for a silicon nitride curved waveguide with a width of 1.2um and a radius of 50um, the sidewall roughness is 5nm, the maximum phase uncertainty is about 7°; assuming that the signal light is Es, input from the first butterfly 2x2 MMI 1, and the reference light is ELO, input from the single-end input of the 1x2 MMI 4, the phase difference of the two output ports of all butterfly 2x2MMIs is , the splitting ratio is , the expressions of the optical fields E1-E4 of the four output ports CH-1 to CH-4 of the second butterfly 2x2MMI 2 and the third butterfly 2x2MMI 3 are as follows:

[0028] .

[0029] Under ideal conditions, 1= 2= 3= 4= , the phase difference of the two output ports of the butterfly 2x2MMI is , the splitting ratio is , and the expressions of the optical fields E1-E4 of CH-1 to CH-4 are as follows:

[0030] .

[0031] Among them, CH-1 and CH-2 have a phase difference of 180°, and both are output in orthogonal directions; CH-3 and CH-4 have a phase difference of 180°, and both are output in the same direction.

[0032] The performance of the 2x2 MMI will directly affect the performance of the entire optical mixer. The embodiment of the present invention proposes a butterfly design, such as Figure 2 shown.

[0033] The input / output waveguides of the butterfly 2x2 MMI are four identical tapered waveguides, namely: 2x2 MMI upper input waveguide 101, 2x2 MMI lower input waveguide 102, 2x2 MMI upper output waveguide 104, and 2x2 MMI lower output waveguide 105.

[0034] The middle multimode waveguide in the butterfly 2x2 MMI waveguide is the 2x2 MMI butterfly multimode waveguide 103, consisting of two symmetrical tapered waveguide sections, narrow in the middle and wide at the edges. Compared to traditional rectangular multimode waveguides, the butterfly structure significantly shortens the multimode waveguide length, while also providing a flatter splitting ratio and smaller phase deviation.

[0035] like Figure 2 As shown in the structure, in this embodiment, the preferred butterfly-shaped 2x2 MMI adopts a silicon nitride thickness of 0.3um, the input / output tapered waveguide narrow end width W1=1.2um, the input / output tapered waveguide wide end width W2=3um, the input / output tapered waveguide length L_Taper=10um, the gap between the two input / output tapered waveguide wide end ports Gap=0.6um, the multi-mode area edge waveguide width W_m1=7um, the multi-mode area middle waveguide width W_m2, the multi-mode area waveguide length L_m, if the multi-mode area adopts a rectangular structure, that is, W_m2=7um, the optimal multi-mode area waveguide length L_m=110um, if the multi-mode area adopts a butterfly structure, that is, W_m2=5um, the optimal multi-mode area waveguide length L_m=80um, and the multi-mode area waveguide length L_m is greatly reduced.

[0036] Figure 4 and Figure 5 The splitting ratio and phase difference between the two output ports of the rectangular 2x2MMI structure and the butterfly 2x2MMI structure at the optimal multimode waveguide length are shown respectively. The simulation results are shown in Figure 2. Figure 5 The phase difference is expressed as , that is, the phase difference between the 104 waveguide output and the 105 waveguide output light field It can be clearly seen from the figure that the simulation results of the butterfly 2x2MMI structure are better than those of the traditional rectangular 2x2MMI structure, and are more flat in the C+L band. , the splitting ratio is controlled at ; Figure 6 The following figures show the simulated CMRR results for 90° optical mixers using a rectangular 2x2 MMI structure and a butterfly 2x2 MMI structure, respectively. As can be seen from the figure, the 90° optical mixer based on the butterfly 2x2 MMI structure meets the 30dB CMRR requirement across the entire C+L band, exhibiting flatter performance, better performance, and wider bandwidth.

[0037] Figure 3 This is a schematic diagram of the application of the present invention. The reference light and signal light are input from the LO terminal and the Siganl terminal respectively. The output terminals CH-1 and CH-2 are connected to a pair of balanced detectors, and CH-3 and CH-4 are connected to another pair of balanced detectors to obtain a set of orthogonal signals.

[0038] Figure 7 Based on Figure 1 In the structure shown, signal light is input from the Signal port and output from CH1 to CH4 respectively. The simulation results of the light field transmission path within the device show that the light field energy distribution of the four ports CH1 to CH4 is relatively uniform, and the transmission effect is good.

[0039] The compact 90° optical mixer proposed in this embodiment is based on a silicon nitride waveguide, has an operating band covering the C+L band, and has a large bandwidth, good performance, and good process tolerance.

[0040] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A compact large bandwidth 90° optical hybrid structure based on silicon nitride waveguides, characterized in that, The structure comprises three bowtie 2x2 MMIs, one 1x2 MMI (4) and four curved waveguides (5); the three bowtie 2x2 MMIs and the 1x2 MMI (4) are all silicon nitride waveguides, and the four curved waveguides (5) are all silicon nitride curved waveguides; The single-ended waveguide of the 1x2 MMI (4) serves as a local reference light LO input end; the three bowtie 2x2 MMIs are a first bowtie 2x2 MMI (1), a second bowtie 2x2 MMI (2) and a third bowtie 2x2 MMI (3); the two output ends of the first bowtie 2x2 MMI (1) are connected to one input port of the second bowtie 2x2 MMI (2) and the third bowtie 2x2 MMI (3) respectively through the curved waveguides (5); one of the two input ends of the first bowtie 2x2 MMI (1) serves as a signal light Signal input end, and the other port is vacant; the other input port of the second bowtie 2x2 MMI (2) and the third bowtie 2x2 MMI (3) is connected to the two output ends of the 1x2 MMI (4) through the curved waveguides (5); The middle multimode region waveguide of the bowtie 2x2 MMI is a left-right symmetrical two-section tapered waveguide, and is narrow in the middle and wide on both sides; the input / output waveguide of the bowtie 2x2 MMI is a silicon nitride tapered waveguide; The structural parameters of the bowtie 2x2 MMI include an input / output tapered waveguide narrow end width W1=1.2um, an input / output tapered waveguide wide end width W2=3um, an input / output tapered waveguide length L_Taper=10um, a two-input / output tapered waveguide wide end port spacing Gap=0.6um, a multimode region edge waveguide width W_m1=7um, a multimode region middle waveguide width W_m2=5um and a multimode region waveguide length L_m=80um.

2. The compact, large bandwidth 90° optical hybrid based on silicon nitride waveguides structure of claim 1, wherein, The curved radius of the curved waveguide (5) is greater than 50um.

3. The compact high-bandwidth 90° optical hybrid based on silicon nitride waveguides structure of claim 1, wherein, The silicon nitride thickness of the bowtie 2x2 MMI is 0.3um.

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