Polarization independent mode multiplexing / demultiplexing device based on transverse sub-wavelength grating structure

Abstract

The invention discloses a polarization independent mode multiplexing / demultiplexing device based on a transverse sub-wavelength grating structure, and belongs to the technical field of photoelectrons. The device adopts an asymmetric directional coupler structure and mainly comprises a bus waveguide formed by a transverse sub-wavelength grating waveguide and a coupling waveguide parallel to the bus waveguide. The core is that longitudinal sub-wavelength grating waveguides with specific duty ratios are introduced to the coupling side of a bus waveguide and the two sides of a coupling waveguide, and accurate matching of dual-polarization-state effective refractive indexes and coupling lengths is achieved through combined regulation and control of geometric parameters of transverse and longitudinal gratings. And the dual-polarization fundamental mode in the coupling waveguide and the dual-polarization same-order high-order mode in the bus waveguide can realize synchronous conversion in a coupling region. The invention overcomes the defect of strong polarization correlation of the traditional device, has the advantages of polarization independence, low crosstalk, low insertion loss, compact structure, high process tolerance, easiness in mode expansion and the like, and can be used for multi-dimensional multiplexing transmission in optical communication and optical interconnection systems.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of optoelectronics, and relates to a light signal processing device for different modes on chip, which has high application value in the fields of optical interconnection, optical communication, optical computing and the like. BACKGROUND

[0002] The development of semiconductor technology is an important milestone of the third technological revolution, which greatly facilitates people's life and promotes social and technological progress. As its foundation, traditional electronic chips have undergone decades of development, but are facing challenges brought by increasing costs and increasing number of transistors: the complexity of chips is slowing down, and the number of transistors is difficult to increase indefinitely due to physical bottlenecks such as process problems. This makes it increasingly difficult to continue Moore's law, making it difficult to meet the growing demand for information processing.

[0003] Light as an information carrier has significant advantages over electrical signals, such as high speed, large bandwidth, low latency, and multi-dimensional characteristics such as wavelength, mode, and polarization, which can achieve large-capacity information transmission. In the field of optical communication, wavelength division multiplexing (WDM) technology achieves high bandwidth and high capacity by simultaneously transmitting multiple wavelength channels in an optical fiber. However, WDM technology also has limitations: the available frequency spectrum is further reduced by nonlinear effects during transmission; at the same time, the technology requires precise control and management of a large number of wavelengths to maintain channel stability and avoid crosstalk, limiting its further development.

[0004] Mode division multiplexing (MDM) technology based on mode dimension, by utilizing spatial dimension to expand channel number, can be compatible with existing WDM technology. MDM technology can achieve mode conversion during multiplexing / demultiplexing, and only requires a single light source, so it has developed rapidly in the past decade. Mode multiplexer / demultiplexer is a key device for implementing MDM, and its common structures include asymmetric directional coupling (ADC) structure, multimode interference (MMI) structure and Y-branch structure. Among them, the ADC structure is widely used due to its simple design, flexible performance control, and easy manufacturing. For example, Dai Daozinc et al. (Laser Photonics Rev., 2018) based on the ADC principle proposed in 2014, used an adiabatic double-core tapered waveguide structure to realize a dual-polarization 10-channel mode multiplexer / demultiplexer.

[0005] Although the existing ADC structure mode multiplexer / demultiplexer has the above advantages, there is still room for improvement in realizing specific functions or optimizing performance. In particular, how to more efficiently realize the coupling of dual-polarization fundamental mode signals into high-order mode signals of the same order in the same coupling region, while further reducing device size, insertion loss, crosstalk, and improving coupling efficiency and process tolerance, is a problem that needs to be solved in current technology. SUMMARY

[0006] The application aims to provide a polarization-independent mode multiplexer / demultiplexer based on a transverse subwavelength grating, which can realize flexible control of equivalent refractive index and differential mode control freedom for double polarization states by introducing transverse and longitudinal subwavelength gratings, thereby realizing simultaneous conversion of double polarization modes of the same order in a single device, with good device stability and high chip space utilization, and providing a key device basis for realizing a multi-dimensional hybrid multiplexing system.

[0007] To achieve the above-mentioned purpose, the application adopts the following technical scheme: A polarization-independent mode multiplexer / demultiplexer based on a transverse subwavelength grating is a mode multiplexer / demultiplexer based on an asymmetric directional coupler structure, comprising: A bus waveguide is composed of a transverse subwavelength grating waveguide, wherein at least one transverse grating unit adjacent to the coupling waveguide is replaced by a longitudinal subwavelength grating waveguide, and the duty cycle of the longitudinal subwavelength grating waveguide is different from that of the replaced transverse grating unit; A coupling waveguide is arranged in parallel on one side of the bus waveguide, comprising: A transverse subwavelength grating waveguide, and the transverse grating units closest to the nearest end and the farthest end of the bus waveguide are replaced by longitudinal subwavelength grating waveguides, and the duty cycles of the longitudinal subwavelength grating waveguides are the same; A tapered waveguide has a narrow end and a wide end, the wide end is connected with a straight waveguide, and the narrow end is connected with a transverse subwavelength grating waveguide, and the width of the wide end is the same as that of the straight waveguide and the transverse subwavelength grating waveguide; A straight waveguide is used for lossless transmission of double polarization base modes; The duty cycle and width of the longitudinal subwavelength grating waveguide are determined by the phase matching condition, so that the effective refractive index of the double polarization base modes in the coupling waveguide is equal to that of the double polarization modes of the same order in the bus waveguide, and the coupling lengths of the double polarization states tend to be consistent, thereby realizing double polarization-independent mode conversion of the same order in a single coupling area.

[0008] The multiplexer / demultiplexer has multiple coupling areas, and each coupling area corresponds to a specific mode order conversion; wherein the nth coupling area supports n-order mode conversion by increasing the width of the bus waveguide, and the new coupling area structure is arranged in cascade with the adjacent low-order coupling area.

[0009] The multiplexer / demultiplexer is realized based on an SOI material platform, comprising a silicon substrate, a buried oxygen layer and a top layer of silicon arranged in sequence from bottom to top; the bus waveguide and the coupling waveguide are formed in the top layer of silicon, the thickness of the buried oxygen layer is 2-3 μm, and the thickness of the top layer of silicon is 220-340 nm.

[0010] A kind of optical communication system, containing above-mentioned polarization independent mode multiplexer / demultiplexer, for realizing the same order mode multiplexing / demultiplexing of double polarization state in single device synchronously.

[0011] The polarization independent mode multiplexer / demultiplexer based on transverse subwavelength grating structure has the following advantages: 1.Polarization independent mode multiplexer based on transverse subwavelength grating can be used in polarization mixed mode multiplexing system, which can further realize significant expansion of communication capacity by introducing polarization dimension on the basis of mode multiplexing, thereby improving optical interconnection capacity; 2.The manufacturing process of the device can be compatible with the mature CMOS process developed today, so the device manufacturing cost is low, and the device has small size, high integration and high process tolerance, which provides a key device basis for realizing high-dimensional mixed multiplexing system and has good application prospect in the future; 3.After introducing transverse and longitudinal subwavelength grating waveguide in the bus waveguide, the phase matching conditions of the fundamental mode and high-order mode in double polarization are realized by using the transverse grating structure to regulate the effective refractive index of the mode in the waveguide, taking the coupling of the first-order mode as an example, i.e. TE0 =neff TE1 At the same time, neff TM0 =neff TM1 ;The difference of the coupling length of the double polarization is regulated by the longitudinal subwavelength grating structure, so that the coupling length of the double polarization can be accurately matched, and the process tolerance is good. The refractive index difference of the double mode is accurately compensated in the asymmetric waveguide, so that the double polarization state simultaneously satisfies the phase matching condition of coupling, thereby realizing polarization independent mode conversion. Through the joint regulation of grating geometric parameters and waveguide cross-section characteristics, the physical limitation of single refractive index regulation is broken through; 4.The longitudinal subwavelength grating is etched on one side of the bus waveguide and on both sides of the transverse subwavelength grating waveguide, so that the restriction ability of the waveguide to the light field is weakened, the evanescent field of different polarizations is changed, and the coupling of the fundamental mode of the same polarization to the high-order mode is completed in the same coupling area. The same order mode conversion of double polarization can be realized synchronously in single device, and the fixed limitation of polarization-mode coupling in traditional structure is solved; 5.The polarization independent mode multiplexer / demultiplexer based on transverse subwavelength grating proposed in the present application can realize high integration in polarization mixed mode multiplexing system, and the interface architecture is simpler compared with the existing work by introducing transverse and longitudinal subwavelength gratings, which can reduce the coupling length, reduce the number of coupling areas, and reduce the number of input ports. In addition, the polarization crosstalk introduced by polarization dispersion in the process of optical fiber transmission can be solved, and a high-efficiency polarization independent mode multiplexer with the same order can be realized; 6. The polarization-independent mode multiplexer / demultiplexer based on the transverse subwavelength grating has flexible mode expansion characteristics. The mode number supported by the device can be directly expanded by adjusting the width of the bus waveguide, i.e. increasing the width of the bus waveguide. The phase matching can be achieved by adjusting the width of the coupling region, and the coupling requirements of different modes are adapted, so that the limitation of the mode number is further broken through. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 is the overall structure schematic diagram of the mode multiplexer / demultiplexer based on the transverse subwavelength grating structure of the application.

[0013] Figure 2 is the schematic diagram of the second-order mode multiplexer / demultiplexer region in the application.

[0014] Figure 3 is the schematic diagram of the first-order mode multiplexer / demultiplexer region in the application.

[0015] Figure 4 is the working state schematic diagram of the first-order mode multiplexer in the application.

[0016] Figure 5 is the working state schematic diagram of the first-order mode demultiplexer in the application.

[0017] Figure 6 is the schematic diagram of the fundamental mode transmission region in the application.

[0018] Figure 7 is the overall working process schematic diagram of the second-order mode multiplexer / demultiplexer in the application.

[0019] Figure 8 is the structure schematic diagram of the SOI material in the application.

[0020] Figure 9 is the comparative simulation curve diagram of the influence of the etching (b) and non-etching (a) longitudinal subwavelength grating on the coupling length and coupling strength of the coupling of the dual-polarization fundamental mode to the first-order mode.

[0021] Figure 10 is the simulation curve diagram of the variation of the coupling length with the equivalent refractive index of the longitudinal subwavelength grating in the application.

[0022] Figure 11 is the loss and crosstalk simulation performance diagram of the second-order mode coupling region in the C-band 100 nm bandwidth range in the application.

[0023] Figure 12 is the loss and crosstalk simulation performance diagram of the first-order mode coupling region in the C-band 100 nm bandwidth range in the application. DETAILED DESCRIPTION

[0024] The application will be further explained in conjunction with the accompanying drawings.

[0025] Taking a mode multiplexer / demultiplexer of a third-order mode as an example, the overall structure of the device is shown in Figure 1 The device can be divided into three regions, namely a second-order mode coupling region, a first-order mode coupling region and a fundamental mode transmission region, and each region is realized by the combination of transverse and longitudinal subwavelength grating structures to flexibly regulate different mode orders and dual-polarization state signals.

[0026] I. Overall structure of the device and material platform The device is made on an SOI (Silicon-on-Insulator) material platform, which includes a silicon substrate, a buried oxide layer and a top layer of silicon arranged in order from bottom to top. The thickness of the buried oxide layer is 2 μm, and the thickness of the top layer of silicon is 220 nm. All waveguide structures are made in the top layer of silicon. The device mainly includes multiple transverse and longitudinal subwavelength grating waveguides, tapered waveguides and straight waveguides, which realize phase matching and coupling length matching through accurate design of grating duty cycle, width and gap.

[0027] II. Fundamental mode transmission region As shown in Figure 6 The fundamental mode transmission region includes a third straight waveguide, a third tapered waveguide and a fourth transverse subwavelength grating waveguide. The wide end of the third tapered waveguide is consistent with the width of the third straight waveguide, and the narrow end is embedded in the fourth transverse subwavelength grating waveguide, realizing low-loss transition of dual-polarization fundamental mode signals from the straight waveguide to the grating waveguide. This region ensures that the fundamental mode signals are not converted during transmission, and only provides a path and interface function for the signals.

[0028] III. First-order mode coupling region As shown in Figure 3 The first-order mode coupling region includes a second bus waveguide, a third transverse subwavelength grating waveguide, a second tapered waveguide and a second straight waveguide.

[0029] The second bus waveguide is composed of a first transverse subwavelength grating waveguide-2, the grating unit closest to the coupling waveguide of which is replaced by a first longitudinal subwavelength grating waveguide-2, and the duty cycles of the two are different; The third transverse subwavelength grating waveguide serves as a coupling waveguide, the grating closest to the second bus waveguide of which is replaced by a third longitudinal subwavelength grating waveguide-1, and the grating farthest from the second bus waveguide is replaced by a third longitudinal subwavelength grating waveguide-2, and the duty cycles of the two longitudinal gratings are the same; The wide end of the second tapered waveguide is connected to the second straight waveguide, and the narrow end is connected to the third transverse subwavelength grating waveguide, realizing mode transition; The second straight waveguide is used for inputting or outputting dual-polarization fundamental modes.

[0030] The duty cycle and width of the first longitudinal subwavelength grating waveguide-2, the third longitudinal subwavelength grating waveguide-1 and the third longitudinal subwavelength grating waveguide-2 are determined by the waveguide width and duty cycle of the first transverse subwavelength grating waveguide-2 and the third transverse subwavelength grating waveguide according to the phase matching condition. The third transverse subwavelength grating waveguide is parallel to the first transverse subwavelength grating waveguide-2, and the dual-polarization fundamental mode in the third transverse subwavelength grating waveguide will be mode-coupled with the dual-polarization first-order mode in the first transverse subwavelength grating waveguide-2, i.e. mode conversion.

[0031] The duty cycle and width of the longitudinal gratings in this area are designed by the phase matching condition, so that the dual-polarization fundamental mode in the third transverse subwavelength grating waveguide is effectively matched with the dual-polarization first-order mode in the second bus waveguide. As a multiplexer, light is input into the dual-polarization fundamental mode from the second straight waveguide, enters the third transverse subwavelength grating waveguide through the second tapered waveguide, is coupled into the dual-polarization first-order mode in the second bus waveguide and is output, as shown in Figure 4 As a demultiplexer, light is input into the dual-polarization first-order mode from the second bus waveguide, is coupled into the dual-polarization fundamental mode and is output from the second straight waveguide, as shown in Figure 5 .

[0032] Four, second-order mode coupling area As shown in Figure 2 , the second-order mode coupling area includes a first bus waveguide, a second transverse subwavelength grating waveguide, a first tapered waveguide and a first straight waveguide.

[0033] The first bus waveguide is composed of the first transverse subwavelength grating waveguide-1, the grating cell closest to the coupling waveguide is replaced by the first longitudinal subwavelength grating waveguide-1, and the duty cycles of the two are different; The second transverse subwavelength grating waveguide serves as a coupling waveguide, the grating closest to the first bus waveguide is replaced by the second longitudinal subwavelength grating waveguide-1, and the grating farthest from the first bus waveguide is replaced by the second longitudinal subwavelength grating waveguide-2, and the duty cycles of the two longitudinal gratings are the same; The first tapered waveguide is connected to the first straight waveguide at the wide end and to the second transverse subwavelength grating waveguide at the narrow end; The first straight waveguide is used for input / output of the fundamental mode.

[0034] The duty cycle and width of the first longitudinal subwavelength grating waveguide-1, the second longitudinal subwavelength grating waveguide-1 and the second longitudinal subwavelength grating waveguide-2 are determined by the waveguide width and duty cycle of the first transverse subwavelength grating waveguide-1 and the second transverse subwavelength grating waveguide, and are obtained from the phase matching condition. The second transverse subwavelength grating waveguide is close to and parallel to the first transverse subwavelength grating waveguide-1, and the dual-polarization fundamental mode in the second transverse subwavelength grating waveguide will be mode-coupled with the dual-polarization second-order mode in the first transverse subwavelength grating waveguide-1, that is, mode conversion.

[0035] The region realizes the phase matching of the dual-polarization fundamental mode and the dual-polarization second-order mode through longitudinal grating regulation, and completes the second-order mode conversion. The working principle is similar to that of the first-order mode coupling region, and high-efficiency coupling in the dual-polarization state is realized through accurate design of grating parameters.

[0036] Five, multiplexing and demultiplexing process of multi-order signals As shown in Figure 7 , the multiplexing / demultiplexing process is as follows: Multiplexing process: the fundamental mode signals are input from the first, second and third straight waveguides. The fundamental mode in the third straight waveguide is guided into the fourth transverse subwavelength grating waveguide through the third tapered waveguide, and still transmits in the form of the fundamental mode; the fundamental modes in the first and second straight waveguides are coupled into the second-order mode in the first bus waveguide and the first-order mode in the second bus waveguide through the corresponding tapered waveguides and transverse grating waveguides, respectively. Finally, the multi-order mode signals are merged and output in the bus waveguide.

[0037] Demultiplexing process: the multi-order mode signals are input from the bus waveguide, and are decomposed into each order of mode through the reverse coupling of each coupling region, and finally the fundamental mode signals are output through the corresponding straight waveguide.

[0038] Six, simulation verification and performance analysis Take the SOI platform with a top silicon thickness of 220 nm as an example for simulation verification: 1. Regulation effect of longitudinal subwavelength grating on coupling characteristics As shown in Figure 9 (a), it is the coupling length and coupling strength curve of the coupling of two polarized fundamental modes to the first-order mode without etching longitudinal subwavelength grating, Figure 9 (b) is the coupling length and coupling strength curve of the coupling of two polarized fundamental modes to the first-order mode after etching longitudinal subwavelength grating, from Figure 9 (a), it can be seen that there is a significant difference in the coupling length of TE and TM polarizations, and it is difficult to realize synchronous coupling; Figure 9 (b) is the result after etching the longitudinal grating. When the length of the coupling region is 14 μm, the coupling strength of the two polarizations is more than 90%, and the coupling length is consistent, indicating that the longitudinal grating effectively balances the coupling behavior of the dual-polarization state, which is the key to realizing polarization-independent operation.

[0039] 2. The matching relationship between the coupling length and the equivalent refractive index Figure 10 The simulation of the change of the coupling length with the equivalent refractive index of the longitudinal subwavelength grating is shown. As can be seen from the figure, when the equivalent medium refractive index of the longitudinal subwavelength grating is 2.45 and 2.9, the coupling lengths of TM0-TM1 and TE0-TE1 are the same, and the base modes of the two polarizations can be simultaneously coupled into the first-order mode, and the two coupling lengths are 42 μm and 26 μm, respectively. This result provides a clear basis for the phase matching condition in the design, and ensures that the double polarization states realize efficient mode conversion in the same coupling area.

[0040] 3. Performance exploration of the second-order mode coupling area The structure parameters of this area are as follows: The width of the second transverse subwavelength grating waveguide is 1.0 μm, and the duty cycle is 0.58; the width of the first bus waveguide is 4.3 μm, and the duty cycle is 0.54; the gap between the second transverse subwavelength grating waveguide and the first bus waveguide is 200 nm. The duty cycles of the second longitudinal subwavelength grating waveguide-1 and the second longitudinal subwavelength grating waveguide-2 etched laterally are 0.5, and the length of the coupling area is 50.0 μm.

[0041] The simulation results in the C-band 100 nm bandwidth are shown in FIG. 5. Figure 11 As shown in FIG. 5, the maximum loss of TE0-TE2 in the C-band 100 nm bandwidth range is -1.75 dB, and the maximum crosstalk is -11.54 dB. The maximum loss of TM0-TM2 is -1.10 dB, and the maximum crosstalk is -17.45 dB.

[0042] 4. Performance exploration of the first-order mode coupling area The width of the second bus waveguide is 2.75 μm, and the duty cycle is 0.54; the width of the third transverse subwavelength grating waveguide is 1.08 μm, and the duty cycle is 0.58; the gap between the third transverse subwavelength grating waveguide and the second bus waveguide is 200 nm; and the length of the coupling area is 45 μm.

[0043] The simulation results in the 100 nm bandwidth are shown in FIG. 6. Figure 12 As shown in FIG. 6, the maximum loss of TE0-TE1 in the 100 nm bandwidth is -1.35 dB, and the maximum crosstalk is -28.87 dB. The maximum loss of TM0-TM1 is -1.22 dB, and the maximum crosstalk is -18.20 dB.

[0044] The simulation results show that the high efficiency, low crosstalk and same order mode conversion of the double polarization states are realized by the synergistic design of the transverse and longitudinal subwavelength gratings while the length of the compact coupling region is maintained.

[0045] Based on the same principle, the application can be realized on other material platforms, and has high design flexibility and process compatibility.

[0046] In summary, the application realizes the polarization-independent, low-loss and low-crosstalk multi-order mode multiplexing / demultiplexing function by the joint design of the transverse and longitudinal subwavelength gratings, has the advantages of compact structure, easy integration and strong expansibility, and is suitable for future high-speed optical communication and optical interconnection systems.

Claims

1. A polarization independent mode multiplexer / demultiplexer based on transverse subwavelength grating structures, characterized in that, The multiplexer / demultiplexer is based on an asymmetric directional coupler structure, comprising: a bus waveguide composed of transverse subwavelength grating waveguides, wherein at least one transverse grating unit adjacent to the coupling waveguide is replaced by a longitudinal subwavelength grating waveguide, and the duty cycle of the longitudinal subwavelength grating waveguide is different from that of the replaced transverse grating unit; a coupling waveguide arranged in parallel on one side of the bus waveguide, comprising: a transverse subwavelength grating waveguide, and the transverse grating units at the nearest end and the farthest end of the bus waveguide are replaced by longitudinal subwavelength grating waveguides, and the duty cycles of the longitudinal subwavelength grating waveguides are the same; a tapered waveguide having a narrow end and a wide end, the wide end of which is connected to a straight waveguide, and the narrow end of which is connected to the transverse subwavelength grating waveguide, and the width of the wide end is the same as the width of the straight waveguide and the transverse subwavelength grating waveguide; a straight waveguide for lossless transmission of dual-polarization base modes; wherein the duty cycle and width of the longitudinal subwavelength grating waveguide are determined by the phase matching condition, so that the effective refractive index of the dual-polarization base modes in the coupling waveguide is equal to that of the dual-polarization same-order high-order modes in the bus waveguide, and the coupling lengths of the dual-polarization states tend to be consistent, thereby realizing dual-polarization independent same-order mode conversion in a single coupling region.

2. The polarization independent mode multiplexer / demultiplexer according to claim 1, characterized in that, The multiplexer / demultiplexer has multiple coupling regions, each corresponding to a specific mode order conversion; wherein the nth coupling region supports n-order mode conversion by increasing the width of the bus waveguide, and the newly added coupling region structure is arranged in a cascading manner with the adjacent low-order coupling region.

3. The polarization independent mode multiplexer / demultiplexer according to claim 1 or 2, characterized in that, The multiplexer / demultiplexer is realized based on an SOI material platform, comprising a silicon substrate, a buried oxygen layer and a top layer of silicon arranged from bottom to top; the bus waveguide and the coupling waveguide are formed in the top layer of silicon.

4. The polarization independent mode multiplexer / demultiplexer according to claim 3, characterized in that, The thickness of the buried oxygen layer is 2-3 μm, and the thickness of the top layer of silicon is 220-340 nm.

5. The polarization independent mode multiplexer / demultiplexer according to claim 1, wherein, By adjusting the duty cycle and width of the longitudinal subwavelength grating waveguide, the equivalent refractive index and coupling length of the TE and TM polarization base modes and the same-order high-order modes are independently regulated to meet the phase matching condition.

6. An optical communication system, characterized by, A polarization-independent mode multiplexer / demultiplexer as claimed in any one of claims 1 to 5 is used to simultaneously realize the same-order mode multiplexing and demultiplexing functions of dual-polarization states in a single device.

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