CN116047821A

CN116047821A - Active polarization converter based on liquid crystal cladding slab waveguide

Info

Publication number: CN116047821A
Application number: CN202211725033.3A
Authority: CN
Inventors: 穆全全; 毕泽坤; 刁志辉; 刘永刚; 彭增辉; 杨程亮; 李大禹; 王启东; 张杏云; 鲁兴海; 范维方; 蔚云慧
Original assignee: Changchun Institute of Optics Fine Mechanics and Physics of CAS
Current assignee: Changchun Institute of Optics Fine Mechanics and Physics of CAS
Priority date: 2022-12-30
Filing date: 2022-12-30
Publication date: 2023-05-02

Abstract

The invention relates to an active polarization converter based on a liquid crystal cladding slab waveguide, and belongs to the technical field of active polarization converters. The problem of the active polarization converter based on traditional electro-optic material preparation in prior art exist that the energy utilization is low is solved. The active polarization converter 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 sequentially and parallelly arranged, and a voltage source connected with the first conductive film and the second conductive film. Wherein the directors of the liquid crystal molecules are perpendicular to the first alignment layer and the second alignment layer. The polarization converter has high energy utilization rate, can realize the effective conversion between the transverse electric mode polarized light and the transverse magnetic mode polarized light, and can actively adjust the conversion degree between the transverse electric mode polarized light and the transverse magnetic mode polarized light by applying different driving voltages.

Description

Active polarization converter based on liquid crystal cladding slab waveguide

Technical Field

The invention relates to an active polarization converter based on a liquid crystal cladding slab waveguide, and belongs to the technical field of active polarization converters.

Background

Similar to the route of electronics, photonics is also evolving toward miniaturization and integration. In integrated optics, optical waveguides are the basis of integrated optics. However, the high refractive index contrast and planar geometry provide a large difference in the transmission of transverse electric mode polarized light and transverse magnetic mode polarized light in the optical waveguide, which introduces additional polarization losses to the system. Developing highly integrated polarization converters on optoelectronic chips is an effective way to solve this problem.

Mode coupling technology is one of the main technologies for implementing polarization converters. The mode of realizing the conversion between modes by the off-diagonal tensor depending on the anisotropic film material has been greatly studied due to the advantages of simple preparation, low cost and the like, and the device based on the electro-optic material film can actively adjust the conversion degree of the polarization state, thereby showing great application prospect. However, since the electro-optical response of the conventional electro-optical materials such as lithium niobate, indium phosphide, etc. is small, the driving voltage of the active polarization converter prepared with the lithium niobate material is above hundred volts, and the application thereof is limited by the larger driving voltage. Although there are also improved researches on thin film materials such as lithium niobate and indium phosphide, which reduce the driving voltage and realize active control of polarization conversion, the electrode is usually designed into a periodic structure to realize high conversion efficiency due to weak anisotropy, which introduces additional interface loss and greatly reduces the energy utilization rate of the device.

Disclosure of Invention

In order to solve the problem of low energy utilization rate of an active polarization converter prepared based on a traditional electro-optic material in the prior art, the invention provides an active polarization converter based on a liquid crystal cladding slab waveguide. The active polarization converter utilizes the characteristics of large electro-optic coefficient and strong anisotropism of the liquid crystal material, and can realize dynamic control of polarization conversion under the conditions of lower driving voltage and higher energy utilization rate.

The technical scheme adopted by the invention for solving the technical problems is as follows:

the active polarization converter based on the liquid crystal cladding slab waveguide 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 sequentially and parallelly arranged, and a voltage source for connecting the first conductive film and the second conductive film;

the refractive indexes of the first orientation layer, the liquid crystal layer, the second orientation layer and the substrate layer are smaller than that of the core layer;

the liquid crystal molecules of the liquid crystal layer have initial directors perpendicular to the first alignment layer and the second alignment layer.

Further, a pattern is arranged on the surface of the first conductive film, which is contacted with the first orientation layer.

Further, a first groove and a second groove are formed in the surface, which is in contact with the first orientation layer, of the first conductive film, the first groove and the second groove are parallel, the groove depth is equal to the thickness of the first conductive film, the length is equal to the width of the first conductive film, the first conductive film is divided into a first film area, a second film area and a third film area which are separated from each other, one end of a voltage source is connected with the second film area between the first groove and the second groove, and the other end of the voltage source is connected with the second conductive film.

Further, the material of the liquid crystal layer is a negative liquid crystal material.

Further, the first conductive film and the second conductive film are both ITO conductive films.

Further, the first substrate and the second substrate are both glass substrates.

Further, the material of the substrate layer is silicon dioxide.

Further, the material of the core layer is silicon oxynitride.

Further, the materials of the first orientation layer and the second orientation layer are photo-control orientation agents.

The preparation method of the active polarization converter based on the liquid crystal cladding slab waveguide comprises the following steps:

step one, taking a first substrate with a first conductive film and a second substrate with a second conductive film;

step two, processing or not processing patterns on the first conductive film;

preparing a substrate layer on the surface of the second conductive film, and preparing a core layer on the substrate layer;

coating a light-operated orientation agent on the first conductive film and the core layer, and curing to obtain a first orientation layer and a second orientation layer;

step five, assembling a first substrate with a first orientation layer and a second substrate with a second orientation layer into an empty liquid crystal box, reserving an injection port for injecting liquid crystal, injecting liquid crystal materials into the empty liquid crystal box above a clearing point to obtain a liquid crystal layer, uniformly distributing initial directors of liquid crystal molecules in directions perpendicular to the first orientation layer and the second orientation layer, and cooling to room temperature to obtain the active polarization converter based on the liquid crystal cladding slab waveguide.

Further, spacer pads are sprayed on the first alignment layer surface and the second alignment layer surface to ensure the empty cell thickness.

Compared with the prior art, the invention has the beneficial effects that:

the active polarization converter based on the liquid crystal cladding slab waveguide is based on the large electro-optic response characteristic of liquid crystal, the electrode structure can realize high-efficiency conversion without being prepared into a periodic structure, the energy loss at each interface of the periodic electrode structure is avoided, and the energy utilization rate of the device is obviously improved. Furthermore, only a very low driving voltage is required thanks to the huge electro-optic coefficient of the liquid crystal.

The active polarization converter based on the liquid crystal cladding slab waveguide can realize the effective conversion between transverse electric mode polarized light and transverse magnetic mode polarized light, and can actively adjust the conversion degree between the transverse electric mode polarized light and the transverse magnetic mode polarized light by applying different driving voltages.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings that are needed in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.

Fig. 1 is a schematic structural diagram of an active polarization converter based on a liquid crystal cladding slab waveguide according to the present invention.

Fig. 2 is a schematic diagram of the arrangement of liquid crystal molecules of the active polarization converter based on the liquid crystal cladding slab waveguide in the working state.

Fig. 3 is a schematic structural diagram of a first conductive film according to the present invention.

Fig. 4 is a schematic diagram of a polarization conversion test light path of an active polarization converter based on a liquid crystal cladding slab waveguide according to the present invention.

In the figure: 1. the light source comprises a first substrate, 2, a first conductive film, 3, a first alignment layer, 4, a liquid crystal layer, 5, a second alignment layer, 6, a core layer, 7, a substrate layer, 8, a second conductive film, 9, a second substrate, 10, a voltage source, 11, a first film region, 12, a first groove, 13, a second film region, 14, a second groove, 15, a third film region, 16, a laser, 17, a 1/4 wave plate, 18, a polarizer, 19, a coupling-in prism, 20, a coupling-out prism, 21, an analyzer, 22 and a light power detector.

Detailed Description

For a further understanding of the present invention, preferred embodiments of the invention are described below, but it is to be understood that these descriptions are merely intended to illustrate further features and advantages of the invention, and are not limiting of the claims of the invention.

The invention relates to an active polarization converter based on a liquid crystal cladding slab waveguide, which 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 sequentially and parallelly arranged, and a voltage source 10 for connecting the first conductive film 2 and the second conductive film 8.

In the above technical solution, the sequence is usually from top to bottom, but it should be noted that other sequences may be possible for those skilled in the art.

In the above-described technical solution, the initial directors of the liquid crystal molecules of the liquid crystal layer 4 are perpendicular to the first alignment layer 3 and the second alignment layer 5. The material of the liquid crystal layer 4 is 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 generally glass substrates suitable for use in the production of liquid crystal devices. Typically the first conductive film 2 is integrated with the first substrate 1 and the second conductive film 8 is integrated with the second substrate 9. 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 alignment layer 3 and the second alignment layer 5 are photo-alignment agents. The refractive index of the first alignment layer 3, the liquid crystal layer 4, the second alignment layer 5 and the substrate layer 7 is smaller than the refractive index of the core layer 6. It should be noted that, a person skilled in the art may select materials of the first substrate 1, the first conductive film 2, the first alignment layer 3, the liquid crystal layer 4, the second alignment layer 5, the substrate layer 7, the second conductive film 8, and the second substrate 9 according to the requirements of use.

As shown in fig. 1, when no voltage is applied from the voltage source 10, no voltage is applied to the liquid crystal layer 4, and the initial alignment state of the liquid crystal molecules of the liquid crystal layer 4 is a vertical alignment mode, i.e., the directors of the liquid crystal molecules are perpendicular to the first alignment layer 3. In this state, the relative dielectric tensor of the liquid crystal layer 4 can be written as:

wherein ε _r Represents the relative dielectric tensor in the initial alignment state of the liquid crystal, n _e Representing the refractive index of extraordinary rays, n _o Representing the refractive index of the ordinary light.

It is known that bringing the dielectric tensor into the wave equation supports the non-coupled transverse electric mode polarized light and transverse magnetic mode polarized light, and the transverse electric mode polarized light or the transverse magnetic mode polarized light can be stably transmitted in the waveguide, so that the non-coupled transverse electric mode polarized light and the transverse magnetic mode polarized light are not coupled.

As shown in fig. 2, when the voltage source 10 applies a voltage, the liquid crystal layer 4 applies a voltage, and the liquid crystal molecules that were 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:

wherein ε _r’ Representing the relative dielectric tensor of the liquid crystal layer under applied voltage, n _e Representing the refractive index of extraordinary rays, n _o Representing the refractive index of the ordinary light, θ represents the angle by which the liquid crystal molecules are rotated by the applied electric field, and the magnitude of the angle is related to the magnitude of the voltage applied by the voltage layer 10. Relative dielectric tensor epsilon of liquid crystal layer 4 when no voltage is applied _r Different, the relative dielectric tensor ε of the liquid crystal layer 4 after voltage application _r’ When the off-diagonal terms appear, the off-diagonal elements in the relative dielectric tensor of the liquid crystal layer 4 can be found by bringing the relative dielectric tensor into the wave equation, so that coupling between transverse electric mode polarized light and transverse magnetic mode polarized light in the waveguide can be realized, and further conversion between transverse electric mode polarized light and transverse magnetic mode polarized light in the waveguide can be realized.

It should be noted that, the first conductive film 2 is different from the second conductive film 8, and only a specific length L of the first conductive film 2 may apply a voltage, where the specific length L is given by a coupling mode theory:

L＝π/|β _e -β _o |

wherein beta is _e And beta _o Representing the phase constants of the transmission of transverse electric mode polarized light and transverse magnetic mode polarized light in the waveguide, respectively.

Specifically, as shown in fig. 3, the surface of the first conductive film 2 contacting the first alignment layer 3 is provided with a first groove 12 and a second groove 14, the first groove 12 and the second groove 14 are parallel, 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, that is, the first groove 12 and the second groove 14 penetrate through the first conductive film 2 in the directions vertical and perpendicular to the paper surface, the first conductive film 2 is divided into a first film region 11, a second film region 13 and a third film region 15 which are separated from each other, the voltage source 10 is connected with the second film region 13, and when the voltage source 10 is applied to the first conductive film 2 and the second conductive film 8, the light beam can realize conversion between polarized light in a transverse electric mode and polarized light in a transverse magnetic mode with high efficiency after being transmitted by a distance of a specific length L. By varying the voltage applied to the voltage source 10, beta in the waveguide can be varied _e -β _o The magnitude of the value can realize the adjustment of the conversion distance. When the lengths of the first conductive film 2 and the second conductive film 8 are unchanged, the output ratio between the transverse electric mode polarized light and the transverse magnetic mode polarized light under different voltages is different, and then the dynamic adjustment of the output polarization state can be realized.

etching or laser marking a first groove 12 and a second groove 14 on a first substrate 1 with a first conductive film 2 to form a first film region 11, a second film region 13 and a third film region 15, obtaining electrodes with specific length, and then treating the surfaces of the first substrate 1 with the first conductive film 2 and a second substrate 9 with a second conductive film 8 for later use;

evaporating a substrate layer 7 on the surface of a second conductive film 8 in a thermal evaporation mode, and preparing a core layer 6 on the substrate layer 7 in a sputtering mode; it should be noted that other preparation methods known to those skilled in the art are also suitable for the present invention;

thirdly, coating photo-control alignment agents on the first conductive film 2 and the core layer 6, wherein the initial director direction of liquid crystal molecules is required to be uniformly distributed and is perpendicular to the first alignment layer 2 and the second alignment layer 5, and the photo-control alignment agents are subjected to photo-crosslinking reaction under the curing condition known to those skilled in the art to obtain the first alignment layer 2 and the second alignment layer 5;

step four, assembling the first substrate 1 with the first alignment layer 2 and the second substrate 9 with the second alignment layer 5 into an empty liquid crystal box, wherein the assembling mode is usually glue fixing, an injection port for injecting liquid crystal is reserved, a spacer is arranged between the first alignment layer 2 and the second alignment layer 5, and the thickness of the liquid crystal box is ensured by spraying the spacer on the surface of the first alignment layer 2 and the surface of the second alignment layer 5; and injecting a negative liquid crystal material into the empty liquid crystal box above the clearing point to obtain a liquid crystal layer 4, uniformly distributing the initial director direction of liquid crystal molecules and being perpendicular to the first alignment layer 2 and the second alignment layer 5, and cooling to room temperature to finish the preparation of the active polarization converter based on the liquid crystal cladding slab waveguide.

The invention relates to a testing and using method of an active polarization converter based on a liquid crystal panel cladding waveguide, which comprises the following steps:

the test light path is shown in fig. 4, and comprises a laser 16, a 1/4 wave plate 17, a polarizer 18, a coupling-in prism 19, a coupling-out prism 20, an analyzer 21 and an optical power detector 22. The laser 16 emits a near infrared wavelength linearly polarized light beam, which is converted into circularly polarized light after passing through the 1/4 wave plate 17, and the desired 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 waveguide through the coupling-in prism 19 and transmitted to the coupling-out prism 20 to be coupled out into free space, then an analyzer 21 with an optical axis orthogonal to the polarizer 18 is placed, and finally the output light is received through the optical power detector 22 and the power of the output light beam is recorded. It was tested that there was no coupling of transverse electric mode polarized light and transverse magnetic mode polarized light in the waveguide when no voltage was applied. At this time, the polarization direction of the output beam is the same as the optical axis direction of the polarizer 18, and an extinction phenomenon occurs after passing through the analyzer 21. When a voltage is applied, coupling exists between transverse electric mode polarized light and transverse magnetic mode polarized light in the waveguide, the polarization state of the output light beam is related to the polarization conversion degree in the waveguide, and extinction phenomenon is not generated after the output light beam passes through the analyzer 21. In addition, by applying different voltages, the relationship between the power level and the applied voltage can be obtained after passing through the analyzer 21, and the active tuning characteristic of the device can be obtained.

It should be apparent that the above embodiments are merely examples for clarity of illustration and are not limiting of the invention. Other variations or modifications of the above teachings will be apparent to those of ordinary skill in the art. It is not necessary here nor is it exhaustive of all embodiments. And obvious variations or modifications thereof are contemplated as falling within the scope of the present invention.

Claims

1. The active polarization converter based on the liquid crystal cladding slab waveguide is characterized by 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) which are sequentially and parallelly arranged, and a voltage source (10) for connecting the first conductive film (2) and the second conductive film (8);

the refractive indexes of the first orientation layer (3), the liquid crystal layer (4), the second orientation layer (5) and the substrate layer (7) are smaller than the refractive index of the core layer (6);

the liquid crystal molecules of the liquid crystal layer (4) have initial directors perpendicular to the first alignment layer (3) and the second alignment layer (5).

2. Active polarization converter based on liquid crystal cladding slab waveguide according to claim 1, wherein the first conductive film (2) is provided with a pattern on the surface in contact with the first alignment layer (3).

3. Active polarization converter based on liquid crystal cladding slab waveguide according to claim 2, characterized in that the surface of the first conductive film (2) contacting the first alignment layer (3) is provided with a first groove (12) and a second groove (14), the first groove (12) and the second groove (14) are parallel, the groove depths are 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 film region (11), a second film region (13) and a third film region (15) which are separated from each other, one end of the voltage source (10) is connected with the second film region (13) between the first groove (12) and the second groove (14), and the other end is connected with the second conductive film (8).

4. Active polarization converter based on a 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. 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 a liquid crystal cladding slab waveguide according to claim 1, wherein the first substrate (1) and the second substrate (9) are both glass substrates.

7. 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 the liquid crystal cladding slab waveguide according to claim 1, wherein the materials of the first alignment layer (3) and the second alignment layer (8) are photo-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, comprising the steps of:

step one, taking a first substrate (1) with a first conductive film (2) and a second substrate (9) with a second conductive film (8);

step two, processing or not processing patterns on the first conductive film (2);

preparing a substrate layer (7) on the surface of the second conductive film (8), and preparing a core layer (6) on the substrate layer (7);

coating a light-operated orientation agent on the first conductive film (2) and the core layer (6), and curing to obtain a first orientation layer (2) and a second orientation layer (5);

step five, assembling a first substrate (1) with a first orientation layer (2) and a second substrate (9) with a second orientation layer (5) into an empty liquid crystal box, reserving an injection port for injecting liquid crystal, injecting liquid crystal materials into the empty liquid crystal box above a clear point to obtain a liquid crystal layer (4), uniformly distributing initial director directions of liquid crystal molecules and being perpendicular to the first orientation layer (3) and the second orientation layer (5), and cooling to room temperature to obtain the active polarization converter based on the liquid crystal cladding slab waveguide.

10. The method for manufacturing an active polarization converter based on a liquid crystal clad slab waveguide according to claim 9, wherein spacer pads are sprayed on the surface of the first alignment layer (2) and the surface of the second alignment layer (5) to ensure the empty cell thickness.