An active polarization converter based on liquid crystal cladding slab waveguide

Abstract

The present invention relates to an active polarization converter based on a liquid crystal cladding planar waveguide, and belongs to the technical field of active polarization converters. The present invention solves the problem of low energy utilization rate in the active polarization converters prepared based on traditional electro-optical materials in the prior art. The active polarization converter of the present invention comprises a first substrate, a first conductive film, a first orientation layer, a liquid crystal layer, a second orientation layer, a core layer, a substrate layer, a second conductive film, and a second substrate, which are arranged sequentially and in parallel, and a voltage source connecting the first conductive film and the second conductive film. The liquid crystal molecule director vector is perpendicular to the first orientation layer and the second orientation layer. The polarization converter has high energy utilization rate, can realize effective conversion between transverse electric mode polarized light and transverse magnetic mode polarized light, and by applying different driving voltages, can actively adjust the degree of conversion between transverse electric mode polarized light and transverse magnetic mode polarized light.

Description

Technical Field

[0001] The invention relates to an active polarization converter based on a liquid crystal cladding planar waveguide, belonging to the technical field of active polarization converters. Background Art

[0002] Similar to the route of electronics, photonics is also developing towards miniaturization and integration. In integrated optics, optical waveguides are the basis of integrated optical devices. However, the high refractive index contrast and planar geometric structure cause a large difference in the transmission of transverse electric mode polarized light and transverse magnetic mode polarized light in the optical waveguide, which brings additional polarization loss to the system. Developing highly integrated polarization converters on optoelectronic chips is an effective way to solve this problem.

[0003] Mode coupling technology is one of the main technologies for realizing polarization converters. Among them, the method of realizing mode conversion by relying on non-diagonal tensors of anisotropic thin film materials has been widely studied due to its advantages such as simple preparation and low cost. Devices based on electro-optical material films can actively adjust the degree of conversion of polarization states, showing great application prospects. However, due to the small electro-optical response of traditional electro-optical materials such as lithium niobate and indium phosphide, the driving voltage of active polarization converters prepared with lithium niobate materials is above 100 volts, and the large driving voltage limits its application. Although there are also improved studies using thin film materials such as lithium niobate and indium phosphide, which reduce the driving voltage and realize active control of polarization conversion, due to their weak anisotropy, the electrodes are usually designed into periodic structures to achieve high conversion efficiency, which introduces additional interface losses and greatly reduces the energy utilization of the device. Summary of the invention

[0004] In order to solve the problem of low energy utilization of active polarization converters based on traditional electro-optical materials in the prior art, the present invention proposes an active polarization converter based on liquid crystal cladding planar waveguide. The active polarization converter utilizes the large electro-optic coefficient and strong anisotropy of liquid crystal materials to achieve dynamic control of polarization conversion under the conditions of lower driving voltage and higher energy utilization.

[0005] The technical solution adopted by the present invention to solve the technical problem is as follows:

[0006] The active polarization converter based on liquid crystal cladding planar waveguide of the present invention comprises a first substrate, a first conductive film, a first orientation layer, a liquid crystal layer, a second orientation layer, a core layer, a substrate layer, a second conductive film and a second substrate which are arranged sequentially and in parallel, and a voltage source connecting the first conductive film and the second conductive film;

[0007] The refractive indexes of the first alignment layer, the liquid crystal layer, the second alignment layer and the substrate layer are all smaller than the refractive index of the core layer;

[0008] The initial directors of the liquid crystal molecules in the liquid crystal layer are perpendicular to the first alignment layer and the second alignment layer.

[0009] Furthermore, a pattern is provided on the surface of the first conductive film in contact with the first alignment layer.

[0010] Furthermore, a first groove and a second groove are provided on the surface of the first conductive film in contact with the first orientation layer, the first groove and the second groove are parallel, and the groove depth is equal to the thickness of the first conductive film, and the length is equal to the width of the first conductive film, so that the first conductive film is divided into a first thin film area, a second thin film area and a third thin film area which are isolated from each other, one end of the voltage source is connected to the second thin film area between the first groove and the second groove, and the other end is connected to the second conductive film.

[0011] Furthermore, the material of the liquid crystal layer is a negative liquid crystal material.

[0012] Furthermore, the first conductive film and the second conductive film are both ITO conductive films.

[0013] Furthermore, the first substrate and the second substrate are both glass substrates.

[0014] Furthermore, the material of the substrate layer is silicon dioxide.

[0015] Furthermore, the material of the core layer is silicon oxynitride.

[0016] Furthermore, the materials of the first alignment layer and the second alignment layer are both photo-controlled alignment agents.

[0017] The preparation method of the active polarization converter based on liquid crystal cladding slab waveguide of the present invention comprises the following steps:

[0018] Step 1: Take a first substrate with a first conductive film and a second substrate with a second conductive film;

[0019] Step 2: Processing or not processing a pattern on the first conductive film;

[0020] Step 3, preparing a substrate layer on the surface of the second conductive film, and then preparing a core layer on the substrate layer;

[0021] Step 4: coating a photo-controlled alignment agent on the first conductive film and the core layer, and curing the coating to obtain a first alignment layer and a second alignment layer;

[0022] Step 5: Assemble the first substrate with the first orientation layer and the second substrate with the second orientation layer into an empty liquid crystal box, and leave an injection port for injecting liquid crystal, inject liquid crystal material into the empty liquid crystal box above the clearing point to obtain a liquid crystal layer, in which the initial directors of the liquid crystal molecules are uniformly arranged and perpendicular to the first orientation layer and the second orientation layer, and cool to room temperature to obtain an active polarization converter based on a liquid crystal cladding planar waveguide.

[0023] Furthermore, spacers are sprayed on the surface of the first alignment layer and the surface of the second alignment layer to ensure the thickness of the empty liquid crystal cell.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The active polarization converter based on liquid crystal cladding planar waveguide of the present invention is based on the large electro-optical response characteristics of liquid crystal. The electrode structure does not need to be prepared into a periodic structure to achieve high-efficiency conversion, avoiding energy loss at each interface of the periodic electrode structure, and significantly improving the energy utilization of the device. In addition, thanks to the huge electro-optic coefficient of liquid crystal, only a very low driving voltage is required.

[0026] The active polarization converter based on liquid crystal cladding planar waveguide of the present invention can realize effective conversion between transverse electric mode polarized light and transverse magnetic mode polarized light, and by applying different driving voltages, the degree of conversion between transverse electric mode polarized light and transverse magnetic mode polarized light can be actively adjusted. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the technical solutions in the specific embodiments of the present invention, the following briefly introduces the drawings required for use in the specific embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0028] Figure 1 It is a schematic structural diagram of an active polarization converter based on a liquid crystal cladding slab waveguide of the present invention.

[0029] Figure 2 It is a schematic diagram of the arrangement of liquid crystal molecules of the active polarization converter based on liquid crystal cladding slab waveguide in the working state of the present invention.

[0030] Figure 3 It is a schematic structural diagram of the first conductive film of the present invention.

[0031] Figure 4 It is a schematic diagram of a polarization conversion test optical path of an active polarization converter based on a liquid crystal cladding slab waveguide according to the present invention.

[0032] In the figure: 1. first substrate, 2. first conductive film, 3. first orientation layer, 4. liquid crystal layer, 5. second orientation layer, 6. core layer, 7. substrate layer, 8. second conductive film, 9. second substrate, 10. voltage source, 11. first thin film region, 12. first groove, 13. second thin film region, 14. second groove, 15. third thin film region, 16. laser, 17. 1 / 4 wave plate, 18. polarizer, 19. coupled input prism, 20. coupled output prism, 21. analyzer, 22. optical power detector. DETAILED DESCRIPTION

[0033] In order to further understand the present invention, preferred embodiments of the present invention are described below. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the present invention.

[0034] The present invention is an active polarization converter based on a liquid crystal cladding planar waveguide, comprising a first substrate 1, a first conductive film 2, a first orientation layer 3, a liquid crystal layer 4, a second orientation layer 5, a core layer 6, a substrate layer 7, a second conductive film 8 and a second substrate 9 arranged sequentially and in parallel, and a voltage source 10 connecting the first conductive film 2 and the second conductive film 8.

[0035] In the above technical solutions, the sequence is usually from top to bottom, but it should be noted that other sequences are also possible for those skilled in the art.

[0036] In the above technical solution, the initial director of the liquid crystal molecules of the liquid crystal layer 4 is perpendicular to the first orientation layer 3 and the second orientation layer 5. The material of the liquid crystal layer 4 is a negative liquid crystal. The first conductive film 2 and the second conductive film 8 are preferably both ITO conductive films. The first substrate 1 and the second substrate 9 are not particularly limited and are usually glass substrates suitable for preparing liquid crystal devices. Usually, the first conductive film 2 and the first substrate 1 are integrated together, and the second conductive film 8 and the second substrate 9 are integrated together. The material of the substrate layer 7 is preferably silicon dioxide. The material of the core layer 6 is silicon oxynitride. The materials of the first orientation layer 3 and the second orientation layer 5 are both photo-controlled orientation agents. The refractive index of the first orientation layer 3, the liquid crystal layer 4, the second orientation layer 5 and the substrate layer 7 are all less than the refractive index of the core layer 6. It should be noted that those skilled in the art can select the materials of the first substrate 1, the first conductive film 2, the first orientation layer 3, the liquid crystal layer 4, the second orientation layer 5, the substrate layer 7, the second conductive film 8 and the second substrate 9 according to the use requirements.

[0037] like Figure 1 As shown, when the voltage source 10 does not apply voltage, the liquid crystal layer 4 does not apply voltage, and the initial arrangement state of the liquid crystal molecules in the liquid crystal layer 4 is a vertical arrangement mode, that is, the director of the liquid crystal molecules is perpendicular to the first alignment layer 3. In this state, the relative dielectric tensor of the liquid crystal layer 4 can be written as:

[0038]

[0039] In the formula, ε r represents the relative dielectric tensor of the liquid crystal in the initial arrangement state, n e represents the refractive index of extraordinary light, n o Represents the refractive index of ordinary light.

[0040] Substituting the above dielectric tensor into the wave equation, it can be seen that at this time, the waveguide supports uncoupled transverse electric mode polarized light and transverse magnetic mode polarized light. Transverse electric mode polarized light or transverse magnetic mode polarized light can be stably transmitted in the waveguide, and there is no coupling between the two.

[0041] like Figure 2 As shown, when the voltage source 10 applies voltage and the liquid crystal layer 4 applies voltage, the liquid crystal molecules originally perpendicular to the first alignment layer 3 gradually rotate in a direction perpendicular to the paper surface and parallel to the first alignment layer 3. In this state, the dielectric tensor of the liquid crystal layer 4 can be written as:

[0042]

[0043] In the formula, ε r’ represents the relative dielectric tensor of the liquid crystal layer under applied voltage, n e represents the refractive index of extraordinary light, n o represents the refractive index of ordinary light, θ represents the angle of rotation of the liquid crystal molecules under the influence of the external electric field, and the angle is related to the voltage applied by the voltage layer 10. The relative dielectric tensor ε of the liquid crystal layer 4 when no voltage is applied is r Different, the relative dielectric tensor ε of the liquid crystal layer 4 after applying voltage r’ Non-diagonal terms appear. At this time, by substituting the above relative dielectric tensor into the wave equation, it can be found that the non-diagonal elements in the relative dielectric tensor of the liquid crystal layer 4 can realize the coupling between the transverse electric mode polarized light and the transverse magnetic mode polarized light in the waveguide, and then realize the conversion between the transverse electric mode polarized light and the transverse magnetic mode polarized light in the waveguide.

[0044] It should be noted that the first conductive film 2 is different from the second conductive film 8. The voltage can be applied to the first conductive film 2 only over a specific length L. The specific length L is given by the coupled mode theory:

[0045] L=π / |β e -β o |

[0046] Among them, β e and β o They represent the phase constants of transverse electric mode polarized light and transverse magnetic mode polarized light transmitted in the waveguide respectively.

[0047] Specifically, Figure 3 As shown, a first groove 12 and a second groove 14 are provided on the surface of the first conductive film 2 in contact with the first orientation layer 3. The first groove 12 and the second groove 14 are parallel, and the groove depth is equal to the thickness of the first conductive film 2, and the length is equal to the width of the first conductive film 2, that is, the first groove 12 and the second groove 14 penetrate the first conductive film 2 in the vertical direction and perpendicular to the paper surface, and divide the first conductive film 2 into a first film area 11, a second film area 13 and a third film area 15 separated from each other. The voltage source 10 is connected to the second film area 13. When the voltage source 10 is applied to the first conductive film 2 and the second conductive film 8, after the light beam is transmitted over a distance of a specific length L, a high-efficiency conversion between transverse electric mode polarized light and transverse magnetic mode polarized light can be achieved. By changing the voltage applied to the voltage source 10, the |β in the waveguide can be changed. e -β o When the lengths of the first conductive film 2 and the second conductive film 8 remain unchanged, the output ratios of the transverse electric mode polarized light and the transverse magnetic mode polarized light are different under different voltages, that is, the output polarization state can be dynamically adjusted.

[0048] The preparation method of the active polarization converter based on liquid crystal cladding slab waveguide of the present invention comprises the following steps:

[0049] Step 1: Etching or laser marking the first groove 12 and the second groove 14 on the first substrate 1 with the first conductive film 2 to form the first thin film area 11, the second thin film area 13 and the third thin film area 15 to obtain electrodes of a specific length, and then treating the surfaces of the first substrate 1 with the first conductive film 2 and the second substrate 9 with the second conductive film 8 to be clean and ready for use;

[0050] Step 2: depositing the substrate layer 7 on the surface of the second conductive film 8 by thermal evaporation, and then preparing the core layer 6 on the substrate layer 7 by sputtering; it should be noted that other preparation methods known to those skilled in the art are also applicable to the present invention;

[0051] Step 3: coating the first conductive film 2 and the core layer 6 with a photo-controlled alignment agent, ensuring that the initial director direction of the liquid crystal molecules is uniformly arranged and perpendicular to the first alignment layer 2 and the second alignment layer 5, thermally curing or photo-curing, the curing conditions are well known to those skilled in the art, and the photo-controlled alignment agent undergoes a photo-crosslinking reaction to obtain the first alignment layer 2 and the second alignment layer 5;

[0052] Step 4: Assemble the first substrate 1 with the first orientation layer 2 and the second substrate 9 with the second orientation layer 5 into an empty liquid crystal box. The assembly method is usually to fix them with glue, and an injection port for injecting liquid crystal is left. A spacer is provided between the first orientation layer 2 and the second orientation layer 5. Usually, the spacer is sprayed on the surface of the first orientation layer 2 and the surface of the second orientation layer 5 to ensure the thickness of the liquid crystal box; inject negative liquid crystal material into the empty liquid crystal box above the clearing point to obtain a liquid crystal layer 4. The initial director directions of the liquid crystal molecules are uniformly arranged and perpendicular to the first orientation layer 2 and the second orientation layer 5. Cool to room temperature to complete the preparation of an active polarization converter based on a liquid crystal cladding planar waveguide.

[0053] The testing and use method of the active polarization converter based on liquid crystal slab cladding waveguide of the present invention:

[0054] Test optical path Figure 4 As shown, it includes a laser 16, a quarter wave plate 17, a polarizer 18, a coupling input prism 19, a coupling output prism 20, an analyzer 21, and an optical power detector 22. The laser 16 emits a near-infrared wavelength linear polarized light beam, which is converted into a circularly polarized light after passing through the quarter wave plate 17. The required transverse electric mode polarized light or transverse magnetic mode polarized light can be generated by adjusting the optical axis of the polarizer 18. The transverse electric mode polarized light or transverse magnetic mode polarized light is coupled into the interior of the waveguide through the coupling input prism 19 and transmitted to the coupling output prism 20 to be coupled out to the free space. Then, an analyzer 21 with an optical axis orthogonal to the polarizer 18 is placed. Finally, the output light is received by the optical power detector 22 and the power of the output light beam is recorded. According to the test, when no voltage is applied, there is no coupling between the transverse electric mode polarized light and the transverse magnetic mode polarized light in the waveguide. At this time, the polarization direction of the output light beam is the same as the optical axis direction of the polarizer 18, and extinction will occur after passing through the analyzer 21. When voltage is applied, the transverse electric mode polarized light and the transverse magnetic mode polarized light in the waveguide are coupled, and the polarization state of the output light beam is related to the degree of polarization conversion inside the waveguide, and no extinction phenomenon occurs after passing through the analyzer 21. In addition, by applying different voltages, the relationship between the power size and the applied voltage can be obtained after passing through the analyzer 21, and the active tuning characteristics of the device can be obtained.

[0055] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. The obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. An active polarization converter based on a liquid crystal cladding slab waveguide, characterized in that: The invention comprises a first substrate (1), a first conductive film (2), a first orientation layer (3), a liquid crystal layer (4), a second orientation layer (5), a core layer (6), a substrate layer (7), a second conductive film (8) and a second substrate (9) which are arranged in sequence and in parallel, and a voltage source (10) connecting the first conductive film (2) and the second conductive film (8); The first conductive film (2) is different from the second conductive film (8). The first conductive film (2) can only apply voltage to a specific length L. The specific length L is given by the coupled mode theory: L＝π / |β e -β o | In the formula, β e and β o They represent the phase constants of transverse electric mode polarized light and transverse magnetic mode polarized light transmitted in the waveguide respectively; The refractive indexes of the first alignment layer (3), the liquid crystal layer (4), the second alignment layer (5) and the substrate layer (7) are all smaller than the refractive index of the core layer (6); The initial directors of the liquid crystal molecules of the liquid crystal layer (4) are perpendicular to the first orientation layer (3) and the second orientation layer (5).

2. The active polarization converter based on liquid crystal cladding slab waveguide according to claim 1, characterized in that: A pattern is provided on the surface of the first conductive film (2) in contact with the first orientation layer (3).

3. The active polarization converter based on liquid crystal cladding slab waveguide according to claim 2, characterized in that: A first groove (12) and a second groove (14) are provided on the surface of the first conductive film (2) in contact with the first orientation layer (3); the first groove (12) and the second groove (14) are parallel, and the groove depth is equal to the thickness of the first conductive film (2); the length is equal to the width of the first conductive film (2); the first groove (12) and the second groove (14) divide the first conductive film (2) into a first thin film area (11), a second thin film area (13) and a third thin film area (15) which are isolated from each other; one end of the voltage source (10) is connected to the second thin film area (13) between the first groove (12) and the second groove (14), and the other end is connected to the second conductive film (8).

4. The active polarization converter based on liquid crystal cladding slab waveguide according to claim 1, characterized in that: The material of the liquid crystal layer (4) is a negative liquid crystal material.

5. The active polarization converter based on liquid crystal cladding slab waveguide according to claim 1, characterized in that: The first conductive film (2) and the second conductive film (8) are both ITO conductive films.

6. The active polarization converter based on liquid crystal cladding slab waveguide according to claim 1, characterized in that: The first substrate (1) and the second substrate (9) are both glass substrates.

7. The active polarization converter based on liquid crystal cladding slab waveguide according to claim 1, characterized in that: The material of the substrate layer (7) is silicon dioxide, and the material of the core layer (6) is silicon oxynitride.

8. The active polarization converter based on liquid crystal cladding slab waveguide according to claim 1, characterized in that: The materials of the first alignment layer (3) and the second alignment layer (8) are both photo-controlled alignment agents.

9. The method for preparing an active polarization converter based on a liquid crystal cladding slab waveguide according to any one of claims 1 to 8, characterized in that: Here are the steps: Step 1: Take a first substrate (1) with a first conductive film (2) and a second substrate (9) with a second conductive film (8); Step 2: Processing or not processing a pattern on the first conductive film (2); Step 3: preparing a substrate layer (7) on the surface of the second conductive film (8), and then preparing a core layer (6) on the substrate layer (7); Step 4: coating a photo-controlled alignment agent on the first conductive film (2) and the core layer (6), and curing the coating to obtain a first alignment layer (2) and a second alignment layer (5); Step 5: Assemble the first substrate (1) with the first orientation layer (2) and the second substrate (9) with the second orientation layer (5) into an empty liquid crystal box, and leave an injection port for injecting liquid crystal, inject liquid crystal material into the empty liquid crystal box above the clearing point to obtain a liquid crystal layer (4), the initial director direction of the liquid crystal molecules is uniformly arranged and perpendicular to the first orientation layer (3) and the second orientation layer (5), and cool to room temperature to obtain an active polarization converter based on a liquid crystal cladding planar waveguide.

10. The method for preparing an active polarization converter based on a liquid crystal cladding slab waveguide according to claim 9, characterized in that: Spacers are sprayed on the surface of the first alignment layer (2) and the surface of the second alignment layer (5) to ensure the thickness of the empty liquid crystal cell.

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