CN109709643B - Dual-polarization mode multiplexing-demultiplexing chip based on monolithic integration - Google Patents

Abstract

The invention discloses a dual-polarization mode multiplexing-demultiplexing chip based on monolithic integration. The two input ends of each polarization beam combiner are respectively connected to the respective input single-mode waveguide, the output end is connected to the input end of the mode spot converter, the output end of the mode spot converter is connected to one end of the connecting waveguide, and the other end of the waveguide is connected. One end is connected to the output multi-mode waveguide through a mode multiplexer; as a multiplexer, the optical signal is input from the input single-mode waveguide and output from the output multi-mode waveguide; as a demultiplexer, the optical signal is input from the output multi-mode waveguide and output. Output from input single-mode waveguide. The invention can load multiple signals onto N eigenmodes in the same multi-mode waveguide respectively to form mode multiplexing, and at the same time combine the polarization multiplexing technology to expand the transmission capacity, and has the advantages of convenient connection with the on-chip optical interconnection system, and Few-mode fiber phase coupling and other outstanding advantages.

Description

A dual-polarization mode multiplexing-demultiplexing chip based on monolithic integration

technical field

The invention relates to a multiplexing-demultiplexer, in particular to a dual-polarization mode multiplexing-demultiplexing chip based on monolithic integration.

Background technique

As we all know, long-distance optical communication has achieved great success. Similarly, optical interconnection, as a new interconnection method, can overcome the bottleneck problem of traditional electrical interconnection, and has attracted widespread attention. At present, optical interconnection is constantly advancing towards ultra-short-distance interconnection, and its communication capacity demand is increasing day by day. Aiming at the characteristics of the large data transmission volume of the optical interconnection system, the most direct method is to borrow the wavelength division multiplexing (WDM) technology commonly used in the long-distance optical fiber communication system.

However, the wavelength division multiplexing system requires expensive components or modules such as multiplexed lasers or tunable lasers, so the cost is very high, which will largely limit its wide application in optical interconnection systems. Therefore, there is an urgent need to develop new multiplexing techniques to reduce the cost of wavelength division multiplexing systems. Mode multiplexing technology has been proposed for a long time in multimode fiber communication, but its progress has been slow due to the technical difficulties of fiber mode control (such as conversion and excitation). Recently, it has been proposed that the use of few-mode fiber technology can increase the communication capacity several times and effectively overcome the crosstalk between modes, which has become a current research hotspot. For the mode multiplexing system based on few-mode fiber, the core device is the mode (de)multiplexer, which is used to realize the combination/division of each order mode. The literature [Daoxin Dai, Jian Wang, and Yaocheng Shi, "Silicon mode (de)multiplexer enabling high capacity photonic networks-on-chip with a single-wavelength-carrierlight," Opt.Let.38, 1422-1424, 2013] gives A multi-channel integrated optical waveguide mode multiplexer-demultiplexer based on cascaded asymmetric directional couplers is proposed, which is easy to expand and can realize multi-channel. On this basis, the literature [Jian Wang, Sailing He, and Daoxin Dai, "On-chip silicon 8-channel hybrid (de)multiplexerenabling simultaneous mode- and polarization-division-multiplexing."Laser&Photonics Reviews.8(2):L18 -L22, 2014] presents a dual-polarization mode multiplexer, which can realize the combination/division of the fundamental mode and higher-order modes of two groups of orthogonal polarization states. However, this design requires the asymmetric directional coupler to have significant polarization sensitivity, which is usually realized by using an ultra-high refractive index difference optical waveguide, so it is more suitable for on-chip optical interconnection systems, but there are certain difficulties in coupling with few-mode fibers. . The literature [Nobutomo Hanzawa et al., "Two-mode PLC-based modemulti/demultiplexer for mode and wavelength division multiplexedtransmission." Opt. Express. 21(22): 25752–25760, 2013] gave a SiO ₂ -based optical waveguide Asymmetric directional couplers can realize coupling with few-mode fibers, but have polarization insensitivity characteristics, making it difficult to realize polarization multiplexing, and the large size of _SiO2 waveguide is not suitable for on-chip optical interconnection systems. Therefore, the prior art lacks a multi-channel integrated optical waveguide dual-polarization mode multiplexer-demultiplexer that is both suitable for on-chip optical interconnection systems and convenient for coupling and connection with few-mode fibers.

SUMMARY OF THE INVENTION

In view of the problems existing in the background technology, the purpose of the present invention is to provide a dual-polarization mode multiplexing-demultiplexing chip based on monolithic integration, which can load multiple signals to N locals in the same multimode waveguide respectively. In the characterization mode, mode multiplexing is formed, and at the same time, the polarization multiplexing technology is combined to expand the transmission capacity, which is convenient for connecting with the on-chip optical interconnection system and coupling with the few-mode fiber.

The technical scheme adopted by the present invention to solve the technical problem is:

The invention includes: 2N+2 input single-mode waveguides, N+1 polarization beam combiners, N+1 connecting waveguides, mode multiplexers and output multi-mode waveguides, and N>0; and also includes: N+1 A mode spot converter;

The two input ends of each polarization beam combiner are respectively connected to the respective input single-mode waveguides, the output end is connected to the input end of the mode spot converter, and the output end of the mode spot converter is connected to one end of the connecting waveguide, which is connected to the The other end is connected to the output multi-mode waveguide through a mode multiplexer; as a multiplexer, the optical signal is input from the input single-mode waveguide and output from the output multi-mode waveguide; as a demultiplexer, the optical signal is output from the output multi-mode waveguide. Waveguide input and output from input single mode waveguide.

The 2N+2 input single-mode waveguides and the N+1 polarization beam combiners are all of the strongly confined small-section optical waveguide type, the cross-sectional size is nanometer order, and the polarization is sensitive, that is, the waveguide birefringence is greater than 10 ^-4 .

The N+1 connecting waveguides, the mode multiplexer and the output multi-mode waveguide are all weakly confinement large-section optical waveguides with a cross-sectional size of the order of microns and insensitive to polarization, that is, the waveguide birefringence is less than 10 ^-4 .

The N+1 mode spot converters are of an inverted tapered optical waveguide structure, which can adiabatically convert a light spot localized in a strong confinement small cross-section optical waveguide to a light spot in a weakly confined large cross-section optical waveguide through the inverted tapered structure ;

The output end of the polarization beam combiner adiabatically converts its transverse electric fundamental mode and transverse magnetic fundamental mode into the transverse electric fundamental mode and transverse magnetic fundamental mode in the connecting waveguide, respectively, through a mode-spot converter, and vice versa.

The mode multiplexer includes: N+1 connecting waveguides, N coupling waveguides, bus waveguides and output multi-mode waveguides;

One of the connecting waveguides is connected to the input end of the bus waveguide, the other connecting waveguides are connected to their respective coupling waveguides, the coupling waveguides are coupled and connected to the multimode waveguides in the bus waveguide to form each mode coupling area, and the output end of the bus waveguide is connected to the output multimode waveguides .

The bus waveguide includes: N tapered optical waveguides, N multimode optical waveguides, or N mode rotators, and two adjacent multimode waveguides are connected adiabatically by tapered optical waveguides, with the smallest width. The input end of the multimode waveguide is connected with the output end of the mode spot converter through the tapered optical waveguide and the connecting waveguide, the multimode optical waveguide with the largest width is used as the end, and the output end is connected with the output multimode optical waveguide.

The N mode coupling regions adopt the structure of an asymmetric directional coupler.

When the mode multiplexer multiplexes high-order modes with two or more peaks in the height direction, the heights of the multimode optical waveguide and the optical waveguide in the coupling region in the asymmetric directional coupler are not equal or the heights of the asymmetric directional coupler are not equal. The heights of the middle multimode optical waveguide and the coupling region optical waveguide are equal, and the output end of the multimode optical waveguide is connected to the mode rotator.

The N mode rotators introduce a groove in the multi-mode optical waveguide to convert the high-order mode with multiple peaks in the transverse direction into the high-order mode with multiple peaks in the longitudinal direction.

The n-th multi-mode optical waveguide supports at least n+1 eigenmodes.

The beneficial effects that the present invention has are:

1) The present invention can respectively load multiple optical signals onto N eigenmodes in the same multi-mode waveguide to form mode multiplexing, and at the same time combine polarization multiplexing technology to expand transmission capacity, and is suitable for mode multiplexing systems;

2) Using the input waveguide as a type of optical waveguide with strong confinement and small cross-section can easily realize large-scale monolithic integration with other optoelectronic components such as on-chip light sources, modulators, detectors, etc., and is suitable for optical interconnection systems;

3) It is convenient to realize efficient coupling with few-mode fibers by using the output waveguide as a type of weakly constraining large-section optical waveguide.

Description of drawings

Figure 1 is a schematic structural diagram of the present invention.

Figure 2 is a schematic cross-sectional view of a strongly confined small-section optical waveguide.

FIG. 3 is a schematic cross-sectional view of a weakly confinement large-section optical waveguide.

FIG. 4 is a schematic structural diagram of a polarization beam combiner.

Figure 5(a) is a schematic diagram of the structure of the mode spot converter.

Figure 5(b) is a schematic cross-sectional view of the mode spot converter tip.

FIG. 6 is a schematic cross-sectional view of the mode coupling region corresponding to the multiplexing TE ₁₀ and TM ₁₀ modes.

FIG. 7 is a schematic cross-sectional view of the mode coupling region corresponding to the multiplexing TE ₀₁ and TM ₀₁ modes.

Figure 8 is a schematic cross-sectional view of the mode rotator.

Detailed ways

The present invention will be further described below with reference to the accompanying drawings and embodiments.

As shown in FIG. 1, the present invention includes: 2N+2 input single-mode waveguides 10a, 10b,..., 1na, 1nb,..., 1Na, 1Nb, N+1 polarization beam combiners 20,..., 2n,..., 2N, N+1 connecting waveguides 40, . 3n, …, 3N.

The two input ends of each polarization beam combiner are respectively connected to the respective input single-mode waveguides, the output end is connected to the input end of the mode spot converter, and the output end of the mode spot converter is connected to one end of the connecting waveguide, which is connected to the The other end is connected to the output multimode waveguide through a mode multiplexer; as a multiplexer, the optical signal is input from the input single-mode waveguides 1na, 1nb and output from the output multimode waveguide 6; as a demultiplexer, the optical signal is Input from the output multimode waveguide 6 and output from the input single mode waveguides 1na, 1nb.

The 2N+2 input single-mode waveguides 10a, 10b, . . . , 1na, 1nb, . Waveguide type, its cross-sectional size is on the order of nanometers and is polarization sensitive, that is, the waveguide birefringence is greater than 10 ^-4 .

The N+1 connecting waveguides 40, . . . , 4n, . Sensitive, that is, the waveguide birefringence is less than 10-4.

As shown in FIG. 1 , two input waveguides 1na and 1nb are used to form an nth pair of input optical waveguides, and the ends thereof are respectively connected to the two input ends of the nth polarization beam combiner 2n.

The N+1 mode spot converters are of an inverted tapered optical waveguide structure, which can adiabatically convert a light spot localized in a strong confinement small cross-section optical waveguide to a light spot in a weakly confined large cross-section optical waveguide through the inverted tapered structure .

The output end of the polarization beam combiner adiabatically converts its transverse electric fundamental mode and transverse magnetic fundamental mode into the transverse electric fundamental mode and transverse magnetic fundamental mode in the connecting waveguide, respectively, through a mode-spot converter, and vice versa.

The mode multiplexer 5 includes: N+1 connection waveguides, N coupling waveguides, a bus waveguide and an output multimode waveguide 6; one of the connection waveguides 40 is connected to the input end of the bus waveguide, and the remaining connection waveguides are connected to the respective coupling waveguides. connection, the coupling waveguide is coupled and connected to the multimode waveguide in the bus waveguide to form each mode coupling area, and the output end of the bus waveguide is connected to the output multimode waveguide 6,

The bus waveguide includes: N tapered optical waveguides 81, . . ., 8n, . , 10n,...,10N, two adjacent multimode waveguides are connected adiabatically by tapered optical waveguides, the input end of the multimode waveguide with the smallest width is connected to the output end of the mode spot converter through the tapered optical waveguide, connecting waveguide, The multimode optical waveguide with the largest width is used as the end, and its output end is connected with the output multimode optical waveguide.

The N mode coupling regions adopt the structure of an asymmetric directional coupler.

When the mode multiplexer 5 multiplexes high-order modes with two or more peaks in the height direction, the heights of the multi-mode optical waveguide and the optical waveguide in the coupling region in the asymmetric directional coupler are unequal or asymmetric directional coupling The heights of the multimode optical waveguide in the device and the optical waveguide in the coupling region are equal, and the output end of the multimode optical waveguide is connected to the mode rotator.

The N mode rotators introduce a groove in the multi-mode optical waveguide, and can convert the high-order mode with multiple peaks in the transverse direction into the high-order mode with multiple peaks in the longitudinal direction through a certain length.

The n-th multi-mode optical waveguide supports at least n+1 eigenmodes.

The following is the working project of the present invention as a multi-channel integrated optical waveguide mode multiplexer:

2N+2 input single-mode waveguides 10a, 10b, . . . , 1na, 1nb, . mode, the input optical waveguide 10b, ..., 1nb, ..., 1Nb input is the transverse electric fundamental mode.

The two optical signals loaded by the nth pair of input optical waveguides 1na, 1nb are combined by the nth polarization beam combiner 2n and output from the output end of the nth polarization beam combiner 2n.

The optical signal output from the output end of the nth polarization beam combiner 2n enters the input end of the nth mode spot converter 3n, is output from the output end of the nth mode spot converter 3n, and enters the connecting waveguide 4n, n=0 ,...,N.

The two optical signals loaded in the 0th connecting waveguide 40 sequentially pass through the first tapered optical waveguide 81, the first multi-mode optical waveguide 91, or the first mode rotator 101, . . . , the n-th tapered optical waveguide 8n, The n-th multi-mode optical waveguide 9n, or the n-th mode rotator 10n, ..., the N-th tapered optical waveguide 81, the N-th multi-mode optical waveguide 91, or the N-th mode rotator 10N, the final output is the The optical signal of the fundamental mode of the multimode optical waveguide 6 is output. During this transmission process, the two signals are always loaded on the transverse electric fundamental mode and transverse magnetic fundamental mode of the optical waveguide, respectively, and the polarization state remains unchanged.

The two optical signals loaded in the nth connecting waveguide 4n enter the nth coupling waveguide 7n, and are coupled from the fundamental mode of the coupling waveguide 7n to the nth higher-order mode of the nth multimode optical waveguide 9n by means of evanescent wave coupling, and Keep the polarization state unchanged. Then pass through or include the n-th mode rotator 10n, the n+1-th tapered optical waveguide 8n+1, the n+1-th multi-mode optical waveguide 9n+1, or the n+1-th mode rotator 10n+1, . . . , the N-th tapered optical waveguide 8N+1, the N+1-th multimode optical waveguide 9N+1, or the N+1-th mode rotator 10N+1, and the final output is the output multi-mode optical waveguide 6 which is loaded into the output multi-mode optical waveguide 6 The two optical signals of the n-th order transverse electric high-order mode and the n-th order transverse magnetic high-order mode keep the same polarization state as the two optical signals loaded by the n-th pair of input single-mode waveguides 1na and 1nb, n=1,...,N.

A specific embodiment of a multi-channel integrated optical waveguide mode multiplexer used in a mode multiplexing system is given below, otherwise the mode demultiplexer function can be implemented.

The central wavelength of the working band is selected as 1550nm. Considering the case where the number of mode channels is 2 (N+1) = 6, the modes involved are two groups of polarization modes, TE and TM, each of which contains three modes, namely TE ₀₀ , TE ₀₁ , TE ₁₀ , and TM ₀₀ , TM ₀₁ , TM ₁₀ . Here, the subscript 00 represents the fundamental mode (the mode field has only one peak in the lateral and height directions), and the subscript 01 represents the high-order mode in which the mode field has only one peak in the lateral direction and two peaks in the height direction, The subscript 10 indicates a higher-order mode in which the mode field has two peaks in the lateral direction and only one peak in the height direction.

For the strongly confined small-section optical waveguide, a Si ₃ N ₄ buried optical waveguide is selected, and its cross section is shown in Figure 2: the core layer 112 is made of Si ₃ N ₄ material, with a thickness of 250 nm and a refractive index of 2; The layer 111 is a pure SiO ₂ material with a thickness of 8um and a refractive index of 1.444; the upper cladding layer 113 is a germanium-doped SiO ₂ material with a thickness of 10um and a refractive index of 1.4586; the top layer is also covered with a layer 114 of pure SiO ₂ material , with a thickness of 10um and a refractive index of 1.444.

For the weakly confined large-section optical waveguide, SiO ₂ buried optical waveguide is selected, and its cross section is shown in Figure 3: wherein, the core layer 113 is made of germanium-doped SiO ₂ material, with a thickness of 10um and a refractive index of 1.4586; its lower cladding layer 111 It is a pure SiO ₂ material with a thickness of 8um and a refractive index of 1.444; its upper cladding layer 114 is a pure SiO ₂ material with a thickness of 10um and a refractive index of 1.444; its core layer and cladding refractive index difference is 1%.

For a dual-polarization mode multiplexing-demultiplexing chip based on monolithic integration, a sub-module design method is adopted.

1. Design of the polarization beam combiner

For the polarization beam combiner, two asymmetric directional couplers are cascaded. Its top view is shown in Figure 4. The parameters are as follows: single-mode waveguide width W ₁ =1, multi-mode waveguide width W ₂ =2.9, waveguide spacing W _g =1, S-shaped curved optical waveguide length L _x =100, lateral offset L _z =5, coupling The length of the straight waveguide is L _c =40, and the longitudinal offset of the two coupled straight waveguides is L ₀ =50. The width W ₁ of the single-mode waveguide and the width W ₂ of the multi-mode waveguide are selected according to the phase matching principle, so that the TM ₀ mode in the single-mode waveguide and the TM ₁ mode in the multi-mode waveguide are phase-matched. In this way, the TM ₀ mode in the single-mode waveguide can be completely coupled to the TM ₁ mode in the multi-mode waveguide, while the TE ₀ mode in the single-mode waveguide and the TE ₁ mode in the multi-mode waveguide are only out of phase due to phase mismatch. There is a small amount of coupling, and this part of the coupling energy can be ignored by the optimal choice of the waveguide spacing W _g . For the optimized design, when TE ₀ and TM ₀ are respectively input from the two input ports of the polarization beam combiner, both are output from its output port, that is, the function of polarization beam combiner is realized.

2. Design of the mode spot converter:

For the mode spot converter, an inverted tapered optical waveguide structure is used. The schematic diagram of its structure is shown in Figure 5(a), and the cross-sectional view of its tip is shown in Figure 5(b). The parameters are as follows: Si3N4 head-end waveguide width W1=1, Si3N4 tip waveguide width Wtip=0.15, SiO2 waveguide width W _a =4, height 10, tapered waveguide length L=2000.

The width of the tip should be selected so that the overlap integral of the fundamental mode at the tip and the fundamental mode of the _SiO2 waveguide is large enough to reduce the loss caused by mode mismatch. The overlap integrals of are 99.1% and 98.6%, respectively; the length of the tapered waveguide should be selected so that the mode can be converted without loss as much as possible, here it is 2000, which can make the conversion efficiency of the transverse electric fundamental mode and the transverse magnetic fundamental mode reach 97.5 respectively. % and 99.8%; therefore, the total mode-spot conversion efficiencies of the transverse electric fundamental mode and the transverse magnetic fundamental mode are 96.6% and 98.4%:

3. Design of the mode multiplexer:

The parameters of the S-shaped curved optical waveguide in the coupling waveguide are selected as: a lateral offset of 20 and a length of 800; according to adiabatic conditions, the taper of the first tapered optical waveguide 81 and the second tapered optical waveguide 82 is 1/20 radian.

3.1 Multiplexing TE ₁₀ and TM ₁₀ modes:

According to the phase matching principle, the width and height of the multimode optical waveguide and the corresponding optical waveguide in the coupling region are reasonably selected, so that the fundamental mode TE ₀₀ of the optical waveguide in the coupling region matches the higher-order mode TE ₁₀ of the multimode optical waveguide. At the same time, since the weakly confined optical waveguide is not polarization sensitive, the coupling structure is also polarization insensitive, so the fundamental mode TM ₀₀ of the optical waveguide in the coupling region and the higher-order mode TM ₁₀ of the multimode optical waveguide are also automatically matched. According to the calculation, the width and height of the multimode optical waveguide are 10.56 and 10 respectively, while the width and height of the optical waveguide in the corresponding coupling region are 4 and 10 respectively, as shown in FIG. 6 . The distance between the two waveguides is selected to be 4, and the optimal length of the straight waveguide in the coupling region is 2580, so that the fundamental modes TE ₀₀ and TM ₀₀ in the optical waveguide in the coupling region are fully coupled with the higher-order modes TE ₁₀ and TM ₁₀ in the multimode optical waveguide, respectively.

3.2 Multiplexing TE ₀₁ and TM ₀₁ modes:

Technical solution 1:

The TE ₀₁ and TM ₀₁ modes are multiplexed using a highly non-uniform asymmetric directional coupler. According to the phase matching principle, the width and height of the multimode optical waveguide and the corresponding optical waveguide in the coupling region are reasonably selected, so that the fundamental mode TE ₀₀ of the optical waveguide in the coupling region matches the higher-order mode TE ₀₁ of the multimode optical waveguide. At the same time, since the weakly confined optical waveguide is not polarization sensitive, the coupling structure is also polarization insensitive, so the fundamental mode TM ₀₀ of the optical waveguide in the coupling region and the higher-order mode TM ₀₁ of the multimode optical waveguide are also automatically matched. According to the calculation, the width and height of the multimode optical waveguide are: 5.65 and 10, respectively, and the width and height of the optical waveguide in the corresponding coupling region are: 4 and 5, respectively, as shown in FIG. 7 . The distance between the two waveguides is selected to be 4, and the optimal length of the straight waveguide in the coupling region is 4640, so that the fundamental modes TE ₀₀ and TM ₀₀ in the optical waveguide in the coupling region are fully coupled with the higher-order modes TE ₀₁ and TM ₀₁ in the multimode optical waveguide, respectively.

Technical solution 2:

The TE ₀₁ and TM ₀₁ modes are multiplexed by cascading highly consistent asymmetric directional couplers and mode rotators. First use a highly consistent asymmetric directional coupler to multiplex the TE ₁₀ and TM ₁₀ modes, as in 3.1, and then connect a mode rotator behind the multimode optical waveguide, the TE ₁₀ and TM ₁₀ modes can be converted into TE ₀₁ and TM 10 modes. TM ₀₁ mode, thus realizing the function of multiplexing TE ₀₁ and TM ₀₁ modes. The cross-sectional view of the mode rotator is shown in Fig. 8. By reasonably selecting the values of the groove offset t and the groove width s, the symmetry of the waveguide is broken and the introduction of small crosstalk is guaranteed. The groove depth d is selected so that the The TE ₁₀ mode in the multimode waveguide is equal to the mode overlap integral of the first transverse electric high-order mode and the second transverse electric high-order mode in the mode rotator. Since the weakly confined optical waveguide is not polarization sensitive, the TM ₁₀ mode in the multimode waveguide is equal to The mode overlap integrals of the first transverse magnetic high-order mode and the second transverse magnetic high-order mode in the mode rotator are also automatically equal. The width and height of the multi-mode optical waveguide are w=10.56, h=10. According to the calculation, the groove offset, width and depth are t=0, s=1.5 and d=4.3, respectively. The optimal length of the mode rotator Take 1435 to convert E ₁₀ and TM ₁₀ modes to TE ₀₁ and TM ₀₁ modes.

The above-mentioned embodiments are used to explain the present invention, rather than limit the present invention. Within the spirit of the present invention and the protection scope of the claims, any modifications and changes made to the present invention all fall into the protection scope of the present invention.

Claims (8)

1. A dual-polarization mode multiplexing-demultiplexing chip based on monolithic integration, comprising: 2N+2 input single-mode waveguides (10a, 10b, ..., 1na, 1nb, ..., 1Na, 1Nb), N+ 1 polarization beam combiner (20,…, 2n,…, 2N), N+1 connecting waveguides (40,…, 4n,…, 4N), mode multiplexer (5) and output multimode waveguide (6 ), and N>0; it is characterized in that, it also includes: N+1 mode-spot converters (30, . . . , 3n, . . , 3N); The two input ends of each polarization beam combiner are respectively connected to the respective input single-mode waveguides, the output end is connected to the input end of the mode spot converter, and the output end of the mode spot converter is connected to one end of the connecting waveguide, which is connected to the The other end is connected to the output multi-mode waveguide through a mode multiplexer; as a multiplexer, the optical signal is input from the input single-mode waveguide and output from the output multi-mode waveguide; as a demultiplexer, the optical signal is output from the output multi-mode waveguide. waveguide input and output from input single mode waveguide; The 2N+2 input single-mode waveguides and the N+1 polarization beam combiners are all of the strongly restricted small-section optical waveguide type, and the cross-sectional size is in the nanometer order, and is polarization-sensitive, that is, the waveguide birefringence is greater than 10 ^-4 ; The N+1 connecting waveguides, the mode multiplexer (5) and the output multi-mode waveguide (6) are all of the weakly confinement large-section optical waveguide type, the cross-sectional size of which is in the order of microns, and is insensitive to polarization, that is, the waveguide double-section. The refraction is less than 10 ^-4 . 2. A dual-polarization mode multiplexing-demultiplexing chip based on monolithic integration according to claim 1, wherein the N+1 mode spot converters are inverted tapered optical waveguide structures, which can Adiabatically convert the light spot localized in the strong confinement small cross-section optical waveguide to the light spot in the weakly confinement large cross-section optical waveguide through the inverted tapered structure, and vice versa; The output end of the polarization beam combiner adiabatically converts its transverse electric fundamental mode and transverse magnetic fundamental mode into the transverse electric fundamental mode and transverse magnetic fundamental mode in the connecting waveguide, respectively, through a mode-spot converter, and vice versa. 3. A dual-polarization mode multiplexing-demultiplexing chip based on monolithic integration according to claim 1, wherein the mode multiplexer (5) comprises: N+1 connecting waveguides (40 , ..., 4n, ..., 4N), N coupling waveguides (71, ..., 7n, ..., 7N), bus waveguides and output multimode waveguides (6); One of the connecting waveguides is connected to the input end of the bus waveguide, the other connecting waveguides are connected to their respective coupling waveguides, and the coupling waveguides are coupled to the multimode waveguides in the bus waveguide to form each mode coupling region, and the output end of the bus waveguide is connected to the output multimode waveguide ( 6) Connect. 4. The dual-polarization mode multiplexing-demultiplexing chip based on monolithic integration according to claim 3, wherein the bus waveguide comprises: N tapered optical waveguides (81, . . . , 8n , . The mode waveguides are connected adiabatically by tapered optical waveguides. The input end of the multimode waveguide with the smallest width is connected to the output end of the mode spot converter through the tapered optical waveguide and the connecting waveguide. The multimode optical waveguide with the largest width is used as the end, and its output end Connect with the output multimode optical waveguide. 5 . The dual-polarization mode multiplexing-demultiplexing chip based on monolithic integration according to claim 4 , wherein the N mode coupling regions adopt the structure of an asymmetric directional coupler. 6 . 6 . The dual-polarization mode multiplexing-demultiplexing chip based on monolithic integration according to claim 3 , wherein the mode multiplexer (5) has two in the multiplexing height direction. 7 . or high-order modes with more than two peaks, the heights of the multimode optical waveguide in the asymmetric directional coupler and the optical waveguide in the coupling region are not equal, or the heights of the multimode optical waveguide and the optical waveguide in the coupling region in the asymmetric directional coupler are equal and multimode The optical waveguide output end is connected to the mode rotator. 7. The monolithic integration-based dual-polarization mode multiplexing-demultiplexing chip according to claim 4, wherein the N mode rotators are for introducing a groove in the multimode optical waveguide, Convert a higher-order mode with multiple peaks in the transverse direction to a higher-order mode with multiple peaks in the longitudinal direction. 8 . The dual-polarization mode multiplexing-demultiplexing chip based on monolithic integration according to claim 4 , wherein the n-th multi-mode optical waveguide supports at least n+1 eigenmodes. 9 .

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