A broadband prism coupling method and structure

Abstract

The present application relates to a kind of broadband prism coupling method and structure, belong to optical waveguide technical field.The broadband prism coupling method of the present application, by introducing optical anisotropy birefringent prism and one-dimensional anisotropic grating in prism coupling waveguide structure, the optical anisotropy birefringent prism and one-dimensional anisotropic grating are made of optical anisotropic material;Optical anisotropy birefringent prism utilizes its different refractive index characteristics to different polarization state, separates into transverse electric mode and transverse magnetic mode with incident light beam, and makes the light beam of two kinds of modes respectively with different exit angles incident to the corresponding one-dimensional anisotropic grating area of prism coupling waveguide structure, realizes different polarization components in the same broadband light beam simultaneously high-efficiency coupling.Solve the actual engineering problem that different polarization state needs different grating period and different incident angle, realize broadband, high-efficiency, polarization-independent light coupling, provide complete technical solution for the broadband light coupling of optical waveguide.

Description

Technical Field

[0001] This invention belongs to the field of optical waveguide technology, specifically relating to a broadband prism coupling method and structure. Background Technology

[0002] As integrated optics develops towards miniaturization and broadband, one of the core issues is how to efficiently couple free-space beams into optical waveguides. Prism coupling has unique advantages such as simple structure and selectable waveguide mode excitation.

[0003] However, traditional prism coupling is limited by phase matching conditions, and its coupling angle varies significantly with wavelength. Furthermore, the transverse electric mode and the transverse magnetic mode have different dispersion characteristics. When the incident angle is fixed, the traditional prism coupling structure can only achieve high coupling efficiency in a narrow band near the center wavelength (usually less than 15 nm), which cannot meet the requirements of broadband spectral analysis systems for broadband, high-efficiency, and polarization-independent coupling.

[0004] The existing patent CN119916524A discloses a polarization-independent prism coupling method and waveguide structure. By introducing optically anisotropic materials into the substrate, core, and / or cladding layers of the prism-coupled waveguide, and adjusting their refractive index distribution, the transverse electric and transverse magnetic modes in the waveguide have the same effective refractive index. This ensures that different polarization states in the same beam can be simultaneously coupled into the optical waveguide, eliminating the polarization sensitivity problem of prism coupling and overcoming the phase-matching constraint of traditional prism coupling methods. This allows light with different polarization states in the same beam to have the optimal coupling angle. However, this technology focuses on the polarization sensitivity problem and does not consider the efficient coupling of different polarization components in a wide-band beam. Summary of the Invention

[0005] This invention addresses the technical problems of polarization sensitivity and narrow operating bandwidth inherent in traditional prism coupling by providing a broadband prism coupling method and structure. The method introduces an optically anisotropic material into the waveguide structure, adjusts the transverse magnetic mode (TM mode) and transverse electric mode (TE mode) at the center wavelength to have the same effective refractive index, and utilizes an optically anisotropic material grating to achieve independent dispersion control for different polarization modes, thereby ensuring that different polarization components in the same broadband beam can be coupled simultaneously and efficiently.

[0006] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows: A broadband prism coupling method is provided by introducing an optically anisotropic birefringent prism and a one-dimensional anisotropic grating into a prism coupling waveguide structure, wherein both the optically anisotropic birefringent prism and the one-dimensional anisotropic grating are made of optically anisotropic materials. Optical anisotropic birefringent prisms utilize their characteristic of having different refractive indices for different polarization states to separate incident beams into transverse electric and transverse magnetic modes. The two modes of beams are then incident at different exit angles onto the corresponding one-dimensional anisotropic grating regions of the prism-coupled waveguide structure, achieving simultaneous and efficient coupling of different polarization components in the same broadband beam.

[0007] Preferably, one or two one-dimensional anisotropic gratings are provided at the incident end face of the prism-coupled waveguide structure to achieve dispersion modulation of the transverse electric mode or the transverse magnetic mode, respectively.

[0008] Preferably, when the prism-coupled waveguide structure is applied to non-polarized beam coupling, it needs to be achieved by using an optical anisotropic birefringent prism and a double-layer one-dimensional anisotropic grating; an optical anisotropic birefringent prism needs to be set in front of the one-dimensional anisotropic grating to achieve separation of transverse electric mode and transverse magnetic mode beams and control of incident angle. When the broadband prism coupling method is applied to TE or TM polarized beam coupling, only one layer of one-dimensional anisotropic grating matching the beam mode needs to be set.

[0009] Preferably, the one-dimensional anisotropic grating exhibits a high diffraction efficiency state similar to a blazed grating for the light wave mode it matches, and has high dispersion capability; while for the light wave mode orthogonal to it, it exhibits a parallel plate state and does not have dispersion capability.

[0010] Preferably, by adjusting the refractive index distribution of the optical anisotropic material, the effective refractive index of the transverse electric mode and the transverse magnetic mode at the center wavelength in the prism coupled waveguide structure is made consistent.

[0011] Preferably, the broadband prism coupling method enables simultaneous coupling of different polarization states under the same broadband beam, and this coupling can complete the mutual conversion of the beam in free space and waveguide structure.

[0012] Preferably, the one-dimensional anisotropic grating employs a two-stage optical anisotropic material grating cascade design. The first-stage grating is designed to produce first-order diffraction only for the transverse electric mode, and the grating period is... Angle of incidence The second-stage grating is designed to produce first-order diffraction only for transverse magnetic modes, with a grating period of... Angle of incidence For transverse magnetic mode, the first-stage grating only serves a transmission function; for transverse electric mode, the second-stage grating only serves a transmission function.

[0013] Further preferred, It is 6.345 μm. It is 16.03°; It is 9.879 μm. It is 10.9°.

[0014] A broadband prism coupling structure, which is a prism-grating hybrid coupled waveguide structure, includes: a substrate, a substrate layer, a core layer, an alignment layer, a cladding layer, a conductive layer, and a glass cover plate. One or two one-dimensional anisotropic gratings are disposed on the incident surface of the prism substrate, and it also includes: an optical anisotropic birefringent prism.

[0015] Preferably, one or more of the substrate layer, core layer, and cladding layer are made of optically anisotropic materials.

[0016] The beneficial effects of this invention are: The broadband prism coupling method and structure of the present invention introduces an optical anisotropic material into the waveguide structure, adjusts the transverse magnetic mode (TM mode) and transverse electric mode (TE mode) of the center wavelength to have the same effective refractive index, and uses an optical anisotropic material grating to achieve independent dispersion control of different polarization modes, thereby ensuring that different polarization components in the same broadband beam can be coupled efficiently at the same time.

[0017] The broadband prism coupling method and structure of this invention, by fabricating a one-dimensional anisotropic grating with a specific period on the prism incident surface and matching the optimal incident angle of the beam, actively compensates for the angular drift of the prism-waveguide coupling system using the dispersion of the grating. Furthermore, independent grating parameters are designed for the dispersion differences between transverse electric and transverse magnetic modes. This solves the polarization sensitivity problem in traditional prism coupling within the target wide operating band, while significantly extending the effective operating bandwidth of the coupler. By introducing a cascaded scheme of birefringent prisms and optical anisotropic gratings, the practical engineering problem of requiring different grating periods and incident angles for different polarization states is solved, achieving broadband, efficient, and polarization-independent optical coupling, providing a complete technical solution for broadband optical coupling of optical waveguides. Attached Figure Description

[0018] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0019] Figure 1 This is a schematic diagram of the broadband prism coupling structure according to an embodiment of the present invention.

[0020] Figure 2 This is a comparison curve of the relative coupling efficiency of the TE mode as a function of wavelength in an embodiment of the present invention with that of a traditional single-prism coupling structure.

[0021] Figure 3 This is a comparison curve of the relative coupling efficiency of the TM mode as a function of wavelength in an embodiment of the present invention with that of a traditional single-prism coupling structure.

[0022] Figure 4 This is a graph showing the change in relative coupling efficiency of TE mode caused by grating constant error in an embodiment of the present invention.

[0023] Figure 5 This is a graph showing the change in relative coupling efficiency of the TM mode caused by grating constant error in an embodiment of the present invention.

[0024] Figure 6 This is a schematic diagram of a cascaded two-layer liquid crystal grating structure applicable to non-polarized beam coupling, according to an embodiment of the present invention.

[0025] Figure 7 This is a schematic diagram of the optical path of an α-BBO crystal as a front anisotropic prism, applicable to non-polarized beam coupling in an embodiment of the present invention.

[0026] The reference numerals in the figure are: 1 is the prism substrate, 2 is the substrate layer, 3 is the waveguide core layer, 4 is the liquid crystal alignment layer, 5 is the liquid crystal cladding layer, 6 is the conductive layer, 7 is the glass cover plate, 8 is the incident light, 9 is the transmission mode in the waveguide, 10 is the liquid crystal grating similar to a blazed grating, and 11 is the α-BBO prism. Detailed Implementation

[0027] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0028] In one embodiment of the present invention, a broadband prism coupling method is provided. This method involves introducing an optically anisotropic birefringent prism and a one-dimensional anisotropic grating into a prism coupling waveguide structure. Both the optically anisotropic birefringent prism and the one-dimensional anisotropic grating are made of optically anisotropic materials. These materials must possess refractive index anisotropy, and include, but are not limited to, liquid crystals and lithium niobate. The optically anisotropic birefringent prism utilizes its characteristic of having different refractive indices for different polarization states to separate the incident beam into a transverse electric mode and a transverse magnetic mode. The two modes of beams are then incident at different exit angles onto the corresponding one-dimensional anisotropic grating regions of the prism coupling waveguide structure, achieving simultaneous and efficient coupling of different polarization components within the same broadband beam.

[0029] One or two layers of one-dimensional anisotropic gratings are disposed at the incident end face of the prism-coupled waveguide structure to achieve dispersion modulation of the transverse electric mode or transverse magnetic mode, respectively. When the prism-coupled waveguide structure is used for unpolarized beam coupling, an optically anisotropic birefringent prism needs to be placed in front of the one-dimensional anisotropic grating to achieve separation of TE mode and TM mode beams and control of the incident angle. The broadband prism coupling method can be applied to unpolarized beam coupling as well as TE or TM polarized beam coupling. When applied to unpolarized light coupling, it requires the cooperation of an optically anisotropic birefringent prism and two layers of one-dimensional anisotropic gratings; when applied to TE or TM polarized beam coupling, only one layer of one-dimensional anisotropic grating matching the beam mode is needed.

[0030] The one-dimensional anisotropic grating exhibits a high diffraction efficiency state similar to a blazed grating and possesses high dispersion capability when matched with its optical wave modes; however, it exhibits a parallel plate state and lacks dispersion capability when matched with optical wave modes orthogonal to it. By adjusting the refractive index distribution of the optical anisotropic material, the effective refractive index of the transverse electric mode and the transverse magnetic mode in the prism-coupled waveguide structure is made consistent at the center wavelength.

[0031] The broadband prism coupling method described above can achieve simultaneous coupling of different polarization states under the same broadband beam, thereby completing the bidirectional transmission and conversion of the beam in free space and waveguide structure.

[0032] One-dimensional anisotropic gratings made of optical anisotropic materials can be designed with different grating periods and incident angles for different polarization modes, and the angular drift in the prism-waveguide coupling system can be compensated by utilizing the grating's dispersion characteristics.

[0033] The period of the one-dimensional anisotropic grating The achromaticity condition is satisfied by the following formula: in, For diffraction orders, Let be the angle of incidence of the light beam at the bottom of the prism. The apex angle of the prism. The effective refractive index of the waveguide, The refractive index of the prism, For wavelength, and These are the dispersion rates of the waveguide and the prism, respectively.

[0034] The one-dimensional anisotropic grating of this invention employs a two-stage optical anisotropic material grating cascade design: the first-stage grating only generates first-order diffraction for the transverse electric mode, and the grating period... Angle of incidence The second-stage grating is designed to produce first-order diffraction only for transverse magnetic modes, with a grating period of... Angle of incidence For transverse magnetic mode, the first-stage grating serves only a transmission function; for transverse electric mode, the second-stage grating serves only a transmission function. In this embodiment... It is 6.345 μm. It is 16.03°; It is 9.879 μm. It is 10.9°.

[0035] To achieve two polarization modes incident at their respective optimized incident angles onto the prism, this invention introduces an optically anisotropic birefringent prism as a beam-splitting element. This birefringent prism is made of an optically anisotropic material and utilizes its different refractive indices for different polarization states to separate the incident beam into a transverse electric mode and a transverse magnetic mode. These two modes are then incident at different exit angles onto the corresponding one-dimensional anisotropic grating regions of the prism-grating hybrid coupled waveguide structure (i.e., the prism-coupled waveguide structure). The material of this birefringent prism includes, but is not limited to, α-BBO crystal.

[0036] See Figure 1 and Figure 7 In one embodiment of the present invention, a broadband prism coupling structure is designed according to the above-described broadband prism coupling method. This structure is a prism-grating hybrid coupling waveguide structure, mainly comprising a prism substrate 1, a base layer of a substrate layer 2, a waveguide core layer 3, and a liquid crystal cladding layer 5, as well as functional layers of a liquid crystal alignment layer 4 and a conductive layer 6, and a glass cover plate 7. A one-dimensional anisotropic grating structure is disposed on the incident surface of the prism substrate 1, and an optically anisotropic birefringent prism is also included. The one-dimensional anisotropic grating structure and the optically anisotropic birefringent prism are made of optically anisotropic materials. One or more layers of the substrate layer 2, waveguide core layer 3, and liquid crystal cladding layer 5 can be optically anisotropic materials. The optically anisotropic materials introduced into the waveguide structure must have refractive index anisotropy; such materials include, but are not limited to, liquid crystals and lithium niobate. More specifically, the broadband prism coupling structure of this embodiment includes, in sequence: a prism substrate 1, a conductive layer 6, a substrate layer 2, a waveguide core layer 3, a liquid crystal alignment layer 4, a liquid crystal cladding layer 5, another liquid crystal alignment layer 4, another conductive layer 6, and a glass cover plate 7. One or two one-dimensional anisotropic gratings are disposed on the incident surface of the prism substrate 1. The grating is a liquid crystal grating 10 similar to a blazed grating. It also includes an optical anisotropic birefringent prism, which is an α-BBO prism 11, but is not limited thereto.

[0037] The prism substrate 1 is used to couple incident light 8 from free space into the waveguide structure and excite mode 9 propagating in the waveguide. The prism substrate 1 is made of lanthanum heavy metal flint glass with a refractive index of 1.95. A conductive layer 6 is disposed on the upper surface of the prism substrate 1. This conductive layer 6, together with another conductive layer 6 disposed in conjunction, applies an electric field to control the alignment of liquid crystal molecules within the liquid crystal cladding layer 5. The conductive layer 6 is indium tin oxide (ITO) with a thickness of 30 nm.

[0038] The substrate layer 2 is made of silicon dioxide with a refractive index of 1.46 and a thickness that linearly transitions from 0 to 2 μm with a linear transition region length of 7 mm. It supports the waveguide structure on top of the substrate. The waveguide core layer 3 is made of silicon oxynitride with a refractive index of 1.85 and a thickness of 580 nm. This waveguide core layer 3 is the main channel for light transmission in the waveguide structure, used to achieve total internal reflection and guidance of light. The liquid crystal alignment layer 4 is a film alignment structure with a refractive index of 1.58 and a thickness of 50 nm. The liquid crystal alignment layer 4 is mainly used to control the alignment direction of liquid crystal molecules in the liquid crystal cladding layer 5 to achieve modulation and control of light. The liquid crystal used in the liquid crystal cladding layer 5 is a negative liquid crystal with a refractive index of 1.48 for ordinary light and 1.69 for extraordinary light, and the liquid crystal layer is 6.5 μm thick. The liquid crystal cladding layer 5 is used to ensure the effective transmission of incident light 8 to achieve mode 9 in the waveguide. The glass cover plate 7 is made of quartz glass with a refractive index of 1.46. The glass cover plate 7 serves a protective and support function. The operating wavelength range within the waveguide is 1495 nm to 1605 nm. The one-dimensional anisotropic grating period for dispersive modulation of the TE mode is 6.354 μm, and the incident angle is 16.03°. The one-dimensional anisotropic grating period for dispersive modulation of the TM mode is 9.879 μm, and the incident angle is 10.9°. The apex angle of the α-BBO prism 11 is 37.72°. Utilizing its characteristic of having different refractive indices for different polarization states, the α-BBO prism 11 separates the incident light 8 into a transverse electric mode and a transverse magnetic mode. The two modes of beams are then incident at different exit angles onto the corresponding blazed grating-like liquid crystal grating 10 region of the prism-grating hybrid coupled waveguide structure, achieving simultaneous and efficient coupling of different polarization components within the same broadband beam.

[0039] Please refer to Figure 2 The graph shows a comparison of the relative coupling efficiency of the broadband prism coupling structure and the traditional single prism structure under TE mode as a function of wavelength. As can be seen from the graph, the wavelength bandwidth under single prism coupling is approximately 20 nm. In contrast, the broadband prism coupling structure of this invention, namely the prism-grating hybrid coupled waveguide structure, can cover a wavelength bandwidth from 1495 nm to 1605 nm.

[0040] Please refer to Figure 3 The graph shows a comparison of the relative coupling efficiency of the broadband prism coupling structure and the traditional single prism structure under TM mode as a function of wavelength. As can be seen from the graph, the wavelength bandwidth under single prism coupling is approximately 30 nm. In contrast, the broadband prism coupling structure of this invention, namely the prism-grating hybrid coupled waveguide structure, can cover a wavelength bandwidth from 1495 nm to 1605 nm.

[0041] Please refer to Figure 4 The graph shows the change in relative coupling efficiency of the TE mode caused by grating constant error in the broadband prism coupling structure. As can be seen from the graph, a 50% decrease in relative coupling efficiency corresponds to a grating constant tolerance of 18 nm.

[0042] Please refer to Figure 5 The graph shows the change in relative coupling efficiency of the TM mode caused by grating constant error in the broadband prism coupling structure. As can be seen from the graph, a 50% decrease in relative coupling efficiency corresponds to a grating constant tolerance of 45 nm.

[0043] Please refer to Figure 6 The diagrams show the liquid crystal molecule arrangement that functions as a blazed grating only in TE mode and the liquid crystal molecule arrangement that functions as a blazed grating only in TM mode. As shown in the diagrams, for TE mode, the liquid crystal molecules rotate periodically along the z-axis in the xy-plane. For TM mode, the liquid crystal molecules rotate periodically along the z-axis in the xz-plane.

[0044] Please refer to Figure 7 In the application of non-polarized light coupling, to meet the different incident angles of the two polarization modes, a schematic diagram of the system optical path using an α-BBO crystal as a pre-element is shown. As can be seen from the figure, the crystal refracts the TE and TM modes at different angles, thus generating different incident angles for the subsequent grating.

[0045] In summary, the broadband prism coupling method and structure provided by this invention introduces a birefringent prism and a one-dimensional grating of optical anisotropic material into a traditional prism coupling waveguide structure. Independent polarization-selective gratings are designed for the different dispersion rates of the transverse electric mode (TE mode) and the transverse magnetic mode (TM mode). The grating dispersion is used to compensate for the angular dispersion in the broadband coupling of the prism, so that the prism coupling structure can be extended to polarization-independent broadband beams, significantly expanding the working bandwidth of the prism coupling structure.

[0046] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A broadband prism coupling method, characterized in that, By introducing an optically anisotropic birefringent prism and a one-dimensional anisotropic grating into a prism-coupled waveguide structure, both the optically anisotropic birefringent prism and the one-dimensional anisotropic grating are made of optically anisotropic materials. Optical anisotropic birefringent prisms utilize their characteristic of having different refractive indices for different polarization states to separate incident beams into transverse electric and transverse magnetic modes. The two modes of beams are then incident at different exit angles onto the corresponding one-dimensional anisotropic grating regions of the prism-coupled waveguide structure, achieving simultaneous and efficient coupling of different polarization components in the same broadband beam.

2. The broadband prism coupling method according to claim 1, characterized in that, One or two one-dimensional anisotropic gratings are set at the incident end face of the prism-coupled waveguide structure to achieve dispersion modulation of the transverse electric mode or the transverse magnetic mode, respectively.

3. The broadband prism coupling method according to claim 1, characterized in that, When the prism-coupled waveguide structure is applied to non-polarized beam coupling, it needs to be achieved by using an optical anisotropic birefringent prism and a double-layer one-dimensional anisotropic grating. An optical anisotropic birefringent prism needs to be placed in front of the one-dimensional anisotropic grating to achieve separation of transverse electric mode and transverse magnetic mode beams and control of the incident angle. When the broadband prism coupling method is applied to TE or TM polarized beam coupling, only one layer of one-dimensional anisotropic grating matching the beam mode needs to be set.

4. The broadband prism coupling method according to claim 1, characterized in that, The one-dimensional anisotropic grating exhibits a high diffraction efficiency state similar to a blazed grating and has high dispersion capability when paired with a light wave mode; however, it exhibits a parallel plate state and does not have dispersion capability when paired with a light wave mode orthogonal to it.

5. The broadband prism coupling method according to claim 1, characterized in that, By adjusting the refractive index distribution of the optical anisotropic material, the effective refractive index of the transverse electric mode and the transverse magnetic mode at the center wavelength in the prism-coupled waveguide structure is made consistent.

6. The broadband prism coupling method according to claim 1, characterized in that, The broadband prism coupling method enables simultaneous coupling of different polarization states under the same broadband beam, and this coupling can complete the mutual conversion of the beam in free space and waveguide structure.

7. The broadband prism coupling method according to any one of claims 1-6, characterized in that, The one-dimensional anisotropic grating employs a two-stage cascaded optical anisotropic material grating design. The first-stage grating is designed to produce first-order diffraction only for the transverse electric mode, and the grating period is... Angle of incidence The second-stage grating is designed to produce first-order diffraction only for transverse magnetic modes, with a grating period of... Angle of incidence For transverse magnetic mode, the first-stage grating only serves a transmission function; for transverse electric mode, the second-stage grating only serves a transmission function.

8. The broadband prism coupling method according to claim 7, characterized in that, It is 6.345 μm. It is 16.03°; It is 9.879 μm. It is 10.9°.

9. A broadband prism coupling structure, which is a prism-grating hybrid coupled waveguide structure, comprising: Prism substrate, substrate layer, core layer, alignment layer, cladding layer, conductive layer and glass cover plate; Its characteristic is that one or two layers of one-dimensional anisotropic gratings are provided on the incident surface of the prism substrate, and it also includes: an optical anisotropic birefringent prism.

10. The broadband prism coupling structure according to claim 9, characterized in that, One or more of the substrate, core, and cladding layers are made of optically anisotropic materials.

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