A wavelength-mode hybrid multiplexer / demultiplexer based on thin-film lithium niobate

Abstract

The present invention discloses a wavelength-mode hybrid multiplexer / demultiplexer based on thin-film lithium niobate, which includes two optical coupler groups and a mode multiplexer / demultiplexer. Each optical coupler group is composed of four optical auxiliary couplers connected in sequence. The optical auxiliary couplers located at both ends in each optical coupler group are respectively connected to the mode multiplexer / demultiplexer. The hybrid multiplexer / demultiplexer adopts a two-stage cascaded structure, first multiplexing wavelength signals and then multiplexing mode signals. The wavelength multiplexing function is completed by a grating-assisted reverse coupler structure, and the mode multiplexing function is completed by a directional coupler structure. Based on a silicon nitride load layer, the mode waveguide is fabricated using a standard commercial CMOS process. The monolithic integration of this structure with a thin-film lithium niobate electro-optic modulator array is expected to construct a high-speed, large-capacity, and low-cost integrated photonic circuit, expanding the communication capacity of optical interconnections.

Description

Technical Field

[0001] The present invention belongs to the technical field of integrated photonics, and relates to a wavelength-mode hybrid multiplexer / demultiplexer based on thin-film lithium niobate. Background Art

[0002] Lithium niobate has excellent electro-optic, acousto-optic, and optical nonlinear effects, and is a very important material in the field of optical communication. In recent years, with the development of the lithium niobate film-forming process, researchers have successfully fabricated an integrated photonics material platform of lithium niobate on insulator (abbreviated as "thin-film lithium niobate") by peeling off a layer of lithium niobate film from a bulk lithium niobate crystal and bonding it to an insulator substrate. Thin-film lithium niobate not only retains the excellent material properties of lithium niobate but also provides a compact waveguide size through the high refractive index difference between the waveguide core layer and the cladding layer, facilitating the on-chip integration of photonic devices. Currently, a large number of integrated photonic devices based on thin-film lithium niobate have been reported. The representative work is the electro-optic modulator with a CMOS-compatible driving voltage proposed by the research team at Harvard University ("Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages," Nature 562(7725), 101-104(2018)). Subsequently, researchers at HyperLight Corporation in the United States further improved the design of the "traveling-wave electrode" to increase the bandwidth of the thin-film lithium niobate electro-optic modulator to 100 GHz ("Breaking voltage–bandwidth limits in integrated lithium niobate modulators using micro-structured electrodes," Optica 8(3), 357-363(2021)). These works have greatly promoted the development of thin-film lithium niobate integrated photonic devices.

[0003] An optical multiplexer / demultiplexer is an important integrated photonic device that can provide a multiple increase in the communication capacity of optical interconnections, thereby effectively alleviating the pressure on the performance of modulators caused by data growth. For this reason, optical multiplexers / demultiplexers have been widely studied in other integrated photonics material platforms, and the monolithic integration of electro-optic modulators and optical multiplexers / demultiplexers is also the infrastructure for realizing high-speed, large-capacity, and low-cost integrated photonic circuits. Thanks to the high-speed and large-bandwidth characteristics of thin-film lithium niobate electro-optic modulators, it is more attractive to implement optical multiplexing / demultiplexing on this platform.

[0004] There are also some reports on the work of optical multiplexers / demultiplexers based on thin-film lithium niobate. Most of these works are based on a single physical dimension of light (such as wavelength), and utilize the characteristic that carriers do not interfere with each other to achieve the purpose of expanding the number of channels and communication capacity. However, limited by factors such as cost, physical mechanism, and process difficulty, the number of channels for multiplexing and demultiplexing under a single physical dimension is still limited. Summary of the Invention

[0005] The object of the present invention is to provide a wavelength-mode hybrid multiplexer / demultiplexer based on thin-film lithium niobate that uses signals from multiple physical dimensions as carriers, to achieve hybrid multiplexing of wavelength channels and mode channels.

[0006] To this end, the present invention adopts the following technical solution: A wavelength-mode hybrid multiplexer / demultiplexer based on thin-film lithium niobate, comprising two optical coupler groups and a mode multiplexer / demultiplexer. Each optical coupler group is composed of four optical auxiliary couplers connected in sequence. The optical auxiliary couplers located at both ends in each optical coupler group are respectively connected to the mode multiplexer / demultiplexer; the mode multiplexer / demultiplexer includes three directional couplers connected in sequence. Each directional coupler includes a multimode waveguide and a single-mode waveguide arranged in parallel. The widths of the three multimode waveguides are different, and the three multimode waveguides are connected in sequence through waveguide transition cones. The single-mode waveguide in the first directional coupler and the single-mode waveguide in the second directional coupler are respectively connected to the optical auxiliary couplers located at both ends in one optical coupler group, and the other ends of the multimode waveguide and the single-mode waveguide in the third directional coupler are respectively connected to the optical auxiliary couplers located at both ends in the other optical coupler group.

[0007] The waveguide in the hybrid multiplexer / demultiplexer of the present invention is a silicon nitride-loaded thin-film lithium niobate waveguide. By depositing a layer of silicon nitride film on the surface of thin-film lithium niobate, photolithography, etching and other processes are performed on the silicon nitride layer to form a silicon nitride-lithium niobate ridge waveguide. This waveguide has the following advantages: First, compared with lithium niobate, silicon nitride has a similar but lower refractive index, so most of the mode field will still be distributed in the lithium niobate layer, which is beneficial to directly utilize the excellent material properties of lithium niobate to realize active devices such as electro-optic modulators; Second, compared with lithium niobate, silicon nitride has a similar wide optical transparent window, which is beneficial to the realization of some wavelength conversion and nonlinear processes; Third, compared with lithium niobate, silicon nitride has a more mature and commercialized manufacturing process, which is beneficial to the realization of passive devices based on optical microstructures (such as sub-wavelength gratings) and large-scale waveguide integration.

[0008] The wavelength-mode hybrid multiplexer / demultiplexer of the present invention adopts a two-stage cascaded structure, that is, first multiplexes the wavelength signals and then multiplexes the mode signals. The basic structural unit for wavelength multiplexing is a grating-assisted contra-directional coupler (GACDC, simply referred to as an optical-assisted coupler). This structure can achieve single-band, flat-top, and low-loss wavelength coupling. Compared with the resonant cavity structure (such as a microring resonator), the optical-assisted coupler is not restricted by the free spectral range and can multiplex more wavelength channels within a fixed wavelength range. In addition, the optical-assisted coupler has multiple structural parameters, such as waveguide width, grating width, grating period, and duty cycle. The changes in these structural parameters all have a tuning effect on the coupling wavelength of the device, providing flexibility for design. By cascading multiple optical-assisted couplers, a wavelength division multiplexer / demultiplexer can be formed. The basic structural unit for mode multiplexing is a directional coupler (DC). This structure can achieve low-loss, large-bandwidth, and scalable optical mode coupling. By keeping the width of the single-mode waveguide unchanged and changing the width of the multimode waveguide to make the optical modes satisfy different phase matching conditions, the coupling mode order can be changed. By cascading multiple directional couplers, a mode multiplexer / demultiplexer can be formed.

[0009] The multiplexer / demultiplexer of the present invention is hybrid multiplexing, that is, using signals from multiple physical dimensions as carriers. Since different physical dimensions of light are also independent and do not interfere with each other, more channels can be transmitted in the same link without affecting the performance. The monolithic integration with a thin-film lithium niobate electro-optic modulator array is expected to construct a high-speed, large-capacity, and low-cost integrated photonic circuit, expanding the communication capacity of optical interconnections. Brief Description of the Drawings

[0010] Figure 1 Schematic diagram of the hybrid multiplexer / demultiplexer of the present invention.

[0011] Figure 2 It is a schematic diagram of the optical-assisted coupler in the hybrid multiplexer / demultiplexer of the present invention.

[0012] Figure 3 It is a schematic diagram of the mode multiplexer / demultiplexer in the hybrid multiplexer / demultiplexer of the present invention.

[0013] Figure 4 It is a schematic diagram of a silicon nitride-loaded thin-film lithium niobate waveguide in the hybrid multiplexer / demultiplexer of the present invention.

[0014] In the figure: 1. First optical auxiliary coupler, 2. Second optical auxiliary coupler, 3. Third optical auxiliary coupler, 4. Fourth optical auxiliary coupler, 5. Fifth optical auxiliary coupler, 6. Sixth optical auxiliary coupler, 7. Seventh optical auxiliary coupler, 8. Eighth optical auxiliary coupler, 9. First directional coupler, 10. Second directional coupler, 11. Third directional coupler, 12. Straight waveguide of the first optical auxiliary coupler, 13. Grating waveguide of the optical auxiliary coupler, 14. First grating waveguide transition cone, 15. Second grating waveguide transition cone, 16. First curved waveguide, 17. Second curved waveguide, 18. First single-mode waveguide, 19. First multimode waveguide, 20. Third curved waveguide, 21. First waveguide transition cone, 22. Second single-mode waveguide, 23. Second multimode waveguide, 24. Third single-mode waveguide, 25. Fourth curved waveguide, 26. Second waveguide transition cone, 27. Fourth single-mode waveguide, 28. Fifth single-mode waveguide, 29. Third multimode waveguide, 30. Third waveguide transition cone, 31. Fifth curved waveguide, 32. Sixth single-mode waveguide, 33. Seventh single-mode waveguide, 34. Waveguide cladding, 35. Silicon nitride load layer, 36. Lithium niobate thin film layer, 37. Buried silicon dioxide layer, 38. Silicon substrate. Detailed implementation mode

[0015] The present invention will be described in detail below in conjunction with the accompanying drawings and the detailed implementation mode.

[0016] As Figure 1 shown, the hybrid multiplexer / demultiplexer of the present invention includes two optical coupler groups and a mode multiplexer / demultiplexer. Each optical coupler group is composed of four optical auxiliary couplers connected in sequence. The optical auxiliary couplers located at both ends in each optical coupler group are respectively connected to the mode multiplexer / demultiplexer, that is:

[0017] One optical coupler group is composed of the first optical auxiliary coupler 1, the second optical auxiliary coupler 2, the third optical auxiliary coupler 3, and the fourth optical auxiliary coupler 4 connected in sequence; the other optical coupler group is composed of the fifth optical auxiliary coupler 5, the sixth optical auxiliary coupler 6, the seventh optical auxiliary coupler 7, and the eighth optical auxiliary coupler 8 connected in sequence; the first optical auxiliary coupler 1, the fourth optical auxiliary coupler 4, the fifth optical auxiliary coupler 5, and the eighth optical auxiliary coupler 8 are respectively connected to the mode multiplexer / demultiplexer. The first optical auxiliary coupler 1 and the fifth optical auxiliary coupler 5 are arranged oppositely.

[0018] The structures of the eight optical auxiliary couplers are exactly the same. Taking the first optical auxiliary coupler 1 as an example for description.

[0019] As Figure 2, the first optical auxiliary coupler 1 in the hybrid multiplexer / demultiplexer of the present invention includes a first optical auxiliary coupler straight waveguide 12 and an optical auxiliary coupler grating waveguide 13 arranged in parallel. One end of the optical auxiliary coupler grating waveguide 13 is connected to the first bending waveguide 16 through a first grating waveguide transition taper 14. The smaller end of the first grating waveguide transition taper 14 is connected to the optical auxiliary coupler grating waveguide 13. The other end of the optical auxiliary coupler grating waveguide 13 is connected to the second bending waveguide 17 through a second grating waveguide transition taper 15. The smaller end of the second grating waveguide transition taper 15 is connected to the optical auxiliary coupler grating waveguide 13.

[0020] The optical auxiliary coupler straight waveguides in all the optical auxiliary couplers within the same optical coupler group are connected in sequence. The other ends of the optical auxiliary coupler straight waveguides in the optical auxiliary couplers located at both ends within the same optical coupler group are respectively connected to the mode multiplexer / demultiplexer.

[0021] As Figure 3 , the mode multiplexer / demultiplexer in the hybrid multiplexer / demultiplexer of the present invention includes a first directional coupler 9, a second directional coupler 10, and a third directional coupler 11 connected in sequence.

[0022] The first directional coupler 9 includes a first single-mode waveguide 18 and a first multimode waveguide 19 arranged in parallel. The end of the first single-mode waveguide 18 facing away from the second directional coupler 10 is a free end. The end of the first single-mode waveguide 18 facing the second directional coupler 10 is connected to one end of a second single-mode waveguide 22 through a third bending waveguide 20. The other end of the second single-mode waveguide 22 is connected to the other end of the optical auxiliary coupler straight waveguide in the first optical auxiliary coupler 1. The end of the first multimode waveguide 19 facing away from the second directional coupler 10 is a free end. The end of the first multimode waveguide 19 facing the second directional coupler 10 is connected to the second directional coupler 10 through a first waveguide transition taper 21. The smaller end in the first waveguide transition taper 21 is connected to the second directional coupler 10.

[0023] The second directional coupler 10 includes a second multimode waveguide 23 and a third single-mode waveguide 24 arranged in parallel. One end of the second multimode waveguide 23 is connected to the smaller end of the first waveguide transition taper 21. The other end of the second multimode waveguide 23 is connected to the third directional coupler 11 through a second waveguide transition taper 26. The smaller end of the second waveguide transition taper 26 is connected to the third directional coupler 11. The end of the third single-mode waveguide 24 facing the first directional coupler 9 is a free end. The end of the third single-mode waveguide 24 facing away from the first directional coupler 9 is connected to one end of a fourth single-mode waveguide 27 through a fourth bending waveguide 25. The other end of the fourth single-mode waveguide 27 is connected to the other end of the optical auxiliary coupler straight waveguide in the fourth optical auxiliary coupler 4.

[0024] The third directional coupler 11 includes a fifth single-mode waveguide 28 and a third multi-mode waveguide 29 arranged in parallel, wherein one end of the fifth single-mode waveguide 28 facing the second directional coupler 10 is a free end, and the other end of the fifth single-mode waveguide 28 is connected to one end of a seventh single-mode waveguide 33 through a fifth curved waveguide 31, and the other end of the seventh single-mode waveguide 33 is connected to the other end of the straight waveguide of the optical auxiliary coupler in the fifth optical auxiliary coupler 5; one end of the third multi-mode waveguide 29 facing the second directional coupler 10 is connected to the smaller end of the second waveguide transition cone 26, and one end of the third multi-mode waveguide 29 away from the second directional coupler 10 is connected to one end of a sixth single-mode waveguide 32 through a third waveguide transition cone 30, and the smaller end of the third waveguide transition cone 30 is connected to one end of the sixth single-mode waveguide 32, and the other end of the sixth single-mode waveguide 32 is connected to the other end of the straight waveguide of the optical auxiliary coupler in the eighth optical auxiliary coupler 8.

[0025] The free end of the first multimode waveguide 19 is located between the first optical auxiliary coupler 1 and the fifth optical auxiliary coupler 5 .

[0026] The waveguides (including multimode waveguides and single-mode waveguides) in the hybrid multiplexer / demultiplexer of the present invention are all thin-film lithium niobate waveguides loaded with silicon nitride, such as Figure 4 As shown, the waveguide includes a waveguide cladding 34, a lithium niobate film layer 36, a silicon dioxide buried layer 37 and a silicon substrate 38 arranged in sequence from top to bottom, a silicon nitride load layer 35 is constructed on the lithium niobate film layer 36, and the silicon nitride load layer 35 is located in the waveguide cladding 34. The material of the waveguide cladding 34 can be either silicon dioxide or air. Lithium niobate and silicon nitride have relatively high refractive indices compared to the above silicon dioxide and air, so the waveguide has a stronger limiting effect on the optical mode.

[0027] The basic method for realizing the silicon nitride-loaded thin-film lithium niobate waveguide is as follows: first, a silicon dioxide buried layer 37 is deposited on a silicon wafer, then a lithium niobate film is bonded to the silicon dioxide buried layer 37, and then a layer of silicon nitride is grown on the surface of the lithium niobate film layer 36, and finally the silicon nitride layer is subjected to processes such as photolithography and etching to form a silicon nitride-lithium niobate ridge waveguide structure. The lithium niobate film is prepared by smart cutting technology, and its detailed preparation method can be found in the literature ("Compactlithium niobateelectrooptic modulators," IEEE J. Sel. Top. QuantumElectron. 24(4), 1–14 (2018)); the silicon nitride layer is prepared by magnetron sputtering, and its preparation method can be found in the literature ("Low lossCMOS-compatiblesilicon nitride photonics utilizing reactive sputtered thinfilms," Opt. Express 27(26), 37795-37805 (2019)).

[0028] The working wavelength of the first optical auxiliary coupler 1 and the fifth optical auxiliary coupler 5 is λ0, the working wavelength of the second optical auxiliary coupler 2 and the sixth optical auxiliary coupler 6 is λ1, the working wavelength of the third optical auxiliary coupler 3 and the seventh optical auxiliary coupler 7 is λ2, and the working wavelength of the fourth optical auxiliary coupler 4 and the eighth optical auxiliary coupler 8 is λ3. The first directional coupler 9 is used for multiplexing and demultiplexing of the highest order mode TE3, the second directional coupler 10 is used for multiplexing and demultiplexing of the second highest order mode TE2, the third directional coupler 11 is used for multiplexing and demultiplexing of the first order mode TE1, and the multiplexing and demultiplexing of the lowest order mode TE0 is directly completed by accessing the single-mode waveguide.

[0029] All optical assisted couplers are composed of ordinary waveguides (straight waveguides) and grating waveguides. When the Bloch mode (TE0) in the grating waveguide and the spatial mode (TE0) in the ordinary waveguide meet the phase matching conditions, the Bloch mode in the grating waveguide will be coupled to the spatial mode of reverse propagation in the ordinary waveguide in a specific band. The phase matching condition of reverse coupling can be expressed as follows: n 1+ n 2=λ / Λ, n 1 is the effective refractive index of the Bloch mode, n$n_2$ is the effective refractive index of the spatial modes propagating in the opposite direction in the ordinary waveguide, $\lambda$ is the central wavelength of the coupling band, and $\Lambda$ is the period of the grating waveguide. At both ends of the reverse coupling region, the grating waveguide and the ordinary waveguide are separated by a bent waveguide. To reduce the transmission loss, a grating waveguide transition taper is provided between the grating waveguide and the bent waveguide, and its function is to slowly transform the Bloch mode into the ordinary spatial mode. Thanks to the symmetry of the optical auxiliary coupler, the optical auxiliary coupler can work bidirectionally. That is, when input from port 1, the reverse coupling wavelength is output from port 1, and when input from port 2, the reverse coupling wavelength is output from port 2.

[0030] According to the phase matching condition, when the refractive indices of the Bloch mode in the grating waveguide and the spatial mode in the ordinary waveguide change or the grating period changes, the reverse coupling wavelength also changes. Therefore, the reverse coupling wavelength can be tuned by adjusting the various structural parameters of the optical auxiliary coupler (including the grating waveguide width, grating period, duty cycle, and ordinary waveguide width). Here, the grating period is selected to change the reverse coupling wavelength of each optical auxiliary coupler, and the other structural parameters remain unchanged. Subsequently, the ordinary waveguides of the first optical auxiliary coupler 1, the second optical auxiliary coupler 2, the third optical auxiliary coupler 3, and the fourth optical auxiliary coupler 4 are connected to form the first wavelength division multiplexing / demultiplexing region, and the ordinary waveguides of the fifth optical auxiliary coupler 5, the sixth optical auxiliary coupler 6, the seventh optical auxiliary coupler 7, and the eighth optical auxiliary coupler 8 are connected to form the second wavelength division multiplexing / demultiplexing region.

[0031] The first multimode waveguide 19, the second multimode waveguide 23, the third multimode waveguide 29, the first waveguide transition taper 21, the second waveguide transition taper 26, the third waveguide transition taper 30, and the sixth single-mode waveguide 32 in the mode multiplexing / demultiplexer form the bus waveguide.

[0032] In the first directional coupler 9, the first single-mode waveguide 18 is coupled to the first multimode waveguide 19, so that the TE0 mode in the single-mode waveguide and the TE3 in the multimode waveguide satisfy the phase matching condition, and the TE0 mode will be coupled to the multimode waveguide along the waveguide transmission direction and transformed into the TE3 mode. The phase matching condition of the directional coupling can be expressed in the following form: n $0 =$ n x , n $n_0$ is the effective refractive index of the TE0 mode, n xis the effective refractive index of the higher-order modes in the multimode waveguide. At one end of the coupling region, the single-mode waveguide is separated from the multimode waveguide by a bent waveguide. Keeping the width of the single-mode waveguide unchanged, by changing the width of the multimode waveguide, the TE0 mode can be phase-matched with different higher-order modes, so as to couple into the multimode waveguide and transform into different higher-order modes. Therefore, the third single-mode waveguide 24 is set to be coupled with the second multimode waveguide 23 and the TE0 mode is phase-matched with the TE2 mode in the second multimode waveguide 23, and the fifth single-mode waveguide 28 is set to be coupled with the third multimode waveguide 29 and the TE0 mode is phase-matched with the TE1 mode in the third multimode waveguide 29. When the phase is mismatched, the mode in the multimode waveguide cannot be effectively coupled with the TE0 mode in the single-mode waveguide. Therefore, the multiplexing and demultiplexing of the TE0 mode can be directly completed by connecting the sixth single-mode waveguide 32 to the bus waveguide, and the TE1, TE2, and TE3 modes generated by the coupling will not be coupled out of the bus waveguide in the other two coupling regions outside this coupling region. In order to reduce the transmission loss of the higher-order modes, waveguide transition cones are arranged between the multimode waveguides with different widths to gradually transform the mode field in the narrow waveguide into the mode field in the wide waveguide.

[0033] Subsequently, the empty end ( Figure 1 left side) of the first optical auxiliary coupler 1 is connected to the second single-mode waveguide 22 in the first directional coupler 9, and the light input from the input 1 port of the first optical auxiliary coupler 1 to the fourth optical auxiliary coupler 4 will first be output from the output 1 port of these four optical auxiliary couplers (the wavelengths are λ0, λ1, λ2, λ3 respectively, and the mode is TE0), and then be coupled into the bus waveguide of the mode multiplexer / demultiplexer and transform into the TE3 mode. The four channels multiplexed in this process are λ0-TE3, λ1-TE3, λ2-TE3, and λ3-TE3. The empty end ( Figure 1 right side) of the fourth optical auxiliary coupler 4 is connected to the fourth single-mode waveguide 27 in the second directional coupler 10, and the light input from the input 2 port of the first optical auxiliary coupler 1 to the fourth optical auxiliary coupler 4 will first be output from the output 2 of these four optical auxiliary couplers (the wavelengths are λ0, λ1, λ2, λ3 respectively, and the mode is TE0), and then be coupled into the bus waveguide of the mode multiplexer / demultiplexer and transform into the TE2 mode. The four channels multiplexed in this process are λ0-TE2, λ1-TE2, λ2-TE2, and λ3-TE2. The empty end ( Figure 1On the left side), it is connected to the seventh single-mode waveguide 33 in the third directional coupler 11. Then, the light input from the input 1 ports of the fifth optical auxiliary coupler 5 to the eighth optical auxiliary coupler 8 will first be output from the output 1 ports of these four optical auxiliary couplers (with wavelengths of λ0, λ1, λ2, and λ3 respectively, and the mode being TE0). Subsequently, it is coupled into the bus waveguide of the mode multiplexer / demultiplexer and transformed into the TE1 mode. The four channels multiplexed in this process are λ0-TE1, λ1-TE1, λ2-TE1, and λ3-TE1. Connect the empty end of the eighth optical auxiliary coupler 8 ( Figure 1 On the right side), it is connected to the sixth single-mode waveguide 32. Then, the light input from the input 2 ports of the fifth optical auxiliary coupler 5 to the eighth optical auxiliary coupler 8 will first be output from the output 2 ports of these four optical auxiliary couplers (with wavelengths of λ0, λ1, λ2, and λ3 respectively, and the mode being TE0). Subsequently, it is input into the bus waveguide of the mode multiplexer / demultiplexer. The four channels multiplexed in this process are λ0-TE0, λ1-TE0, λ2-TE0, and λ3-TE0. Demultiplexing is the reverse process of the above-mentioned process.

[0034] In the hybrid multiplexer / demultiplexer of the present invention, there are 4 wavelength channels (λ0, λ1, λ2, λ3) and 4 mode channels (transverse electric mode - fundamental mode TE0, first-order mode TE1, second-order mode TE2, third-order mode TE3). Since the wavelengths and modes are independent of each other and do not interfere with each other, the signals that can be used as carriers are the pairwise combinations of wavelengths and modes, a total of 16: λ0-TE0, λ0-TE1, λ0-TE2, λ0-TE3; λ1-TE0, λ1-TE1, λ1-TE2, λ1-TE3; λ2-TE0, λ2-TE1, λ2-TE2, λ2-TE3; λ3-TE0, λ3-TE1, λ3-TE2, λ3-TE3. Therefore, more channels can be transmitted in the same link without affecting the performance.

[0035] All the optical auxiliary couplers in the hybrid multiplexer / demultiplexer of the present invention work bidirectionally. Each optical auxiliary coupler has two input ends and two output ends and can multiplex two channels.

Claims

1. A wavelength-mode hybrid multiplexer / demultiplexer based on thin-film lithium niobate, characterized in that, It includes two optocoupler groups and a mode multiplexer / demultiplexer. Each optocoupler group is composed of four optical auxiliary couplers connected in sequence. The optical auxiliary couplers at both ends of each optocoupler group are respectively connected to the mode multiplexer / demultiplexer. The mode multiplexer / demultiplexer includes three directional couplers connected in sequence. Each directional coupler includes a multimode waveguide and a single-mode waveguide arranged in parallel. The widths of the three multimode waveguides are different, and the three multimode waveguides are connected in sequence through waveguide transition tapers. The single-mode waveguide in the first directional coupler and the single-mode waveguide in the second directional coupler are respectively connected to the optical auxiliary couplers at both ends of an optocoupler group. The other ends of the multimode waveguide and the single-mode waveguide in the third directional coupler are respectively connected to the optical auxiliary couplers at both ends of the other optocoupler group; One of the two optocoupler groups is composed of a first optical auxiliary coupler (1), a second optical auxiliary coupler (2), a third optical auxiliary coupler (3), and a fourth optical auxiliary coupler (4) connected in sequence; the other optocoupler group is composed of a fifth optical auxiliary coupler (5), a sixth optical auxiliary coupler (6), a seventh optical auxiliary coupler (7), and an eighth optical auxiliary coupler (8) connected in sequence; the first optical auxiliary coupler (1), the fourth optical auxiliary coupler (4), the fifth optical auxiliary coupler (5), and the eighth optical auxiliary coupler (8) are respectively connected to the mode multiplexer / demultiplexer; the first optical auxiliary coupler (1) and the fifth optical auxiliary coupler (5) are arranged opposite to each other; the structures of the eight optical auxiliary couplers are exactly the same; The first optical auxiliary coupler (1) includes a first optical auxiliary coupler straight waveguide (12) and an optical auxiliary coupler grating waveguide (13) arranged in parallel. One end of the optical auxiliary coupler grating waveguide (13) is connected to a first bending waveguide (16) through a first grating waveguide transition taper (14). The smaller end of the first grating waveguide transition taper (14) is connected to the optical auxiliary coupler grating waveguide (13). The other end of the optical auxiliary coupler grating waveguide (13) is connected to a second bending waveguide (17) through a second grating waveguide transition taper (15). The smaller end of the second grating waveguide transition taper (15) is connected to the optical auxiliary coupler grating waveguide (13); the optical auxiliary coupler straight waveguides in all the optical auxiliary couplers within the same optocoupler group are connected in sequence; the other ends of the optical auxiliary coupler straight waveguides in the optical auxiliary couplers at both ends within the same optocoupler group are respectively connected to the mode multiplexer / demultiplexer; The mode multiplexer / demultiplexer includes a first directional coupler (9), a second directional coupler (10), and a third directional coupler (11) connected in sequence; The first directional coupler (9) includes a first single-mode waveguide (18) and a first multimode waveguide (19) arranged in parallel. One end of the first single-mode waveguide (18) facing away from the second directional coupler (10) is a free end. One end of the first single-mode waveguide (18) facing the second directional coupler (10) is connected to one end of a second single-mode waveguide (22) through a third bent waveguide (20). The other end of the second single-mode waveguide (22) is connected to the other end of the optical auxiliary coupler straight waveguide in the first optical auxiliary coupler (1); One end of the first multimode waveguide (19) facing away from the second directional coupler (10) is a free end. One end of the first multimode waveguide (19) facing the second directional coupler (10) is connected to the second directional coupler (10) through a first waveguide transition taper (21). The smaller-size end in the first waveguide transition taper (21) is connected to the second directional coupler (10); The second directional coupler (10) includes a second multimode waveguide (23) and a third single-mode waveguide (24) arranged in parallel. One end of the second multimode waveguide (23) is connected to the smaller-size end of the first waveguide transition taper (21). The other end of the second multimode waveguide (23) is connected to the third directional coupler (11) through a second waveguide transition taper (26). The smaller-size end of the second waveguide transition taper (26) is connected to the third directional coupler (11); One end of the third single-mode waveguide (24) facing the first directional coupler (9) is a free end. One end of the third single-mode waveguide (24) facing away from the first directional coupler (9) is connected to one end of a fourth single-mode waveguide (27) through a fourth bent waveguide (25). The other end of the fourth single-mode waveguide (27) is connected to the other end of the optical auxiliary coupler straight waveguide in the fourth optical auxiliary coupler (4); The third directional coupler (11) includes a fifth single-mode waveguide (28) and a third multimode waveguide (29) arranged in parallel. One end of the fifth single-mode waveguide (28) facing the second directional coupler (10) is a free end. The other end of the fifth single-mode waveguide (28) is connected to one end of a seventh single-mode waveguide (33) through a fifth bent waveguide (31). The other end of the seventh single-mode waveguide (33) is connected to the other end of the optical auxiliary coupler straight waveguide in the fifth optical auxiliary coupler (5); One end of the third multimode waveguide (29) facing the second directional coupler (10) is connected to the smaller-size end of the second waveguide transition taper (26). The other end of the third multimode waveguide (29) facing away from the second directional coupler (10) is connected to one end of a sixth single-mode waveguide (32) through a third waveguide transition taper (30). The smaller-size end of the third waveguide transition taper (30) is connected to one end of the sixth single-mode waveguide (32). The other end of the sixth single-mode waveguide (32) is connected to the other end of the optical auxiliary coupler straight waveguide in the eighth optical auxiliary coupler (8).

2. The wavelength-mode hybrid multiplexer / demultiplexer based on thin-film lithium niobate according to claim 1, wherein The first single-mode waveguide (18), the first multimode waveguide (19), the second multimode waveguide (23), the third single-mode waveguide (24), the fifth single-mode waveguide (28), and the third multimode waveguide (29) are all silicon nitride-loaded thin-film lithium niobate waveguides.

3. The wavelength-mode hybrid multiplexer / demultiplexer based on thin-film lithium niobate according to claim 2, wherein The described thin-film lithium niobate waveguide includes a waveguide cladding layer (34), a lithium niobate thin-film layer (36), a silicon dioxide buried layer (37), and a silicon substrate (38) arranged in sequence from top to bottom. A silicon nitride load layer (35) is constructed on the lithium niobate thin-film layer (36), and the silicon nitride load layer (35) is located within the waveguide cladding layer (34).

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