Preparation method for polarization volume holographic grating realizing different diffraction efficiencies, and polarization volume holographic grating

Abstract

A preparation method for a polarization volume holographic grating realizing different diffraction efficiencies, and a polarization volume holographic grating. The method comprises: coating a top surface of a substrate medium (1) with a photo-alignment layer (4); continuing to cover a top surface of the photo-alignment layer (4) with a layer of mask (5), and using two orthogonal circularly polarized light beams to perform interference exposure; removing the mask (5), and coating the top surface of the photo-alignment layer (4) with a layer of liquid crystal solution having a preset concentration; placing the liquid crystal solution in a nitrogen environment, and curing same under ultraviolet light irradiation to obtain a liquid crystal film; using a laser cleaning device to remove the disordered liquid crystal film from a non-grating region of the substrate medium (1), so as to obtain a PVG grating; and covering a surface of the PVG grating with a filter layer having a gradient ultraviolet light transmittance, and placing the filter under ultraviolet light for exposure, so as to obtain a PVG grating region (7) having a uniform and continuous grating medium thickness and a gradually increasing diffraction efficiency. Patterned or gradient regulation and control over the diffraction efficiency is simply and effectively realized, thereby ensuring the continuity and integrity of a grating.

Description

Preparation method of polarization volume holographic grating with different diffraction efficiencies and polarization volume holographic grating TECHNICAL FIELD

[0001] The present application relates to a preparation method of polarization volume holographic grating with different diffraction efficiencies and polarization volume holographic grating, and belongs to the technical field of optical elements. BACKGROUND

[0002] The pupil expansion is one of the most significant features and advantages of the diffractive optical waveguide system. The light satisfies the total reflection condition in the waveguide medium and propagates in the waveguide medium with low loss. In the coupling-out grating area, due to the limitation of the diffraction efficiency of the grating, only part of the light energy can be diffracted each time, and the rest of the light energy continues to propagate along the waveguide. With continuous propagation and repeated diffraction, the light is continuously replicated and coupled out of the waveguide, thereby realizing the expansion of the exit pupil. With the continuous replication and coupling of the light at the coupling-out grating, the remaining light energy is lower and the brightness of the coupled-out light is also reduced, which affects the brightness uniformity of the imaging field of view. Therefore, it is very important to add grating diffraction efficiency regulation in the design of the pupil expansion scheme.

[0003] At present, the diffraction efficiency regulation technology of the polarization volume holographic grating is mainly realized by controlling the thickness of the grating medium layer. In the preparation process of the polarization volume holographic grating, the thickness of the grating medium layer is closely related to the coating method. The main coating methods include spin coating, blade coating and inkjet printing. The equipment cost required by spin coating and blade coating is low, but it cannot realize patterned control of the liquid crystal layer, and the two coatings in adjacent areas will produce obvious boundary lines, which will damage the integrity of the grating. The equipment cost of inkjet printing is extremely high, and it is affected by the thickness regulation scheme. In the process of realizing patterned control of the diffraction efficiency, there is a height difference between the grating medium layers in different areas, which can easily cause the waveguide medium to be uneven and collapse during the subsequent waveguide lamination and packaging process, thereby causing serious effects such as ghosting, color bifurcation and jelly-like image on the imaging. Therefore, it is very important to invent a technology that can realize patterned regulation of the diffraction efficiency of the polarization volume holographic grating at a low cost while ensuring the consistency of the grating thickness. SUMMARY

[0004] The technical problem to be solved by the present application is to overcome the defects of the prior art and provide a preparation method of polarization volume holographic grating with different diffraction efficiencies and polarization volume holographic grating.

[0005] To solve the above technical problems, the present application is realized by using the following technical scheme.

[0006] On the one hand, the present application provides a preparation method of polarization volume holographic grating with different diffraction efficiencies, comprising:

[0007] Step one, coating a light orientation layer on the top surface of the substrate medium;

[0008] Step two, continue to cover a mask on the top surface of the photo-orientation layer, and use two orthogonal circularly polarized light to perform interference exposure;

[0009] Step three, after the exposure, remove the mask, and coat a liquid crystal solution with a preset concentration on the top surface of the photo-orientation layer, the liquid crystal solution is used to prepare a grating medium layer, and the thickness is 100 nm to 10 μm;

[0010] Step four, place the substrate medium covered with the liquid crystal solution in a nitrogen environment and perform curing under ultraviolet light irradiation to obtain a liquid crystal film;

[0011] Step five, use a laser cleaning device to remove the disordered liquid crystal film on the non-grating area of the substrate medium to obtain a PVG grating;

[0012] Step six, cover a filter with gradually changing ultraviolet light transmittance on the surface of the PVG grating, and place the filter under ultraviolet light to expose, to obtain a PVG grating area with consistent and continuous grating medium thickness and gradually increasing diffraction efficiency.

[0013] Further, the substrate medium is optical glass or resin glass, and the shape is a flat plate or a free curved surface; the thickness of the grating medium layer is set to meet the requirement of the maximum diffraction efficiency of the grating required in the waveguide scheme.

[0014] Further, the mask is a hollow light shield.

[0015] Further, the liquid crystal solution with a preset concentration is a liquid crystal solution with a concentration of 1% to 50%; the liquid crystal solution with a preset concentration includes a chiral agent, a liquid crystal, an acrylic ester polymerizable monomer, a photoinitiator, and a surfactant.

[0016] Further, in the step four, the prepared grating medium layer is placed in a nitrogen environment and cured under ultraviolet light irradiation of 400 nm to 10 nm.

[0017] Further, the filter is a neutral density filter, which includes a plurality of regions, the ultraviolet light transmittance of each region is different, and the ultraviolet light transmittance of each region is set with a fixed step value from low to high.

[0018] Further, the filter is a wedge-shaped filter that can partially absorb ultraviolet light.

[0019] Further, in the step six, the neutral density filter is placed under 400 nm to 10 nm ultraviolet light for a preset time to obtain a plurality of PVG grating regions with consistent and continuous grating medium thickness and gradually increasing diffraction efficiency.

[0020] Further, in the step six, the wedge-shaped optical glass is placed under 400nm-10nm ultraviolet light for a preset time to obtain a PVG grating region with a gradually changed diffraction efficiency and a consistent grating medium thickness.

[0021] In a second aspect, the present application provides a polarization volume holographic grating prepared by the preparation method.

[0022] The present application has the following beneficial effects:

[0023] 1. The preparation method is simple, and the equipment cost is low. The equipment required by the method can be directly operated by using the ultraviolet light source used in the ultraviolet curing step in the grating preparation process, without adding additional equipment cost.

[0024] 2. The exposure method is used to reduce the diffraction efficiency of the polarization volume holographic grating, which can simply and effectively realize the patterned or gradually changed diffraction efficiency control, and ensure the continuity and integrity of the grating.

[0025] 3. The method ensures the consistency of the grating medium layer thickness, greatly simplifies the operation difficulty in the subsequent waveguide bonding and packaging process, and reduces the risk of serious influence such as ghosting, color bifurcation, and jelly image in imaging. BRIEF DESCRIPTION OF DRAWINGS

[0026] Fig. 1 is a flowchart of the preparation method of the present application;

[0027] Fig. 2 is a schematic diagram of the principle structure of the polarization volume holographic grating;

[0028] Fig. 3 is a schematic diagram of the polarization volume holographic grating with different diffraction efficiencies prepared by the preparation method in Example 1;

[0029] Fig. 4 is a schematic diagram of the polarization volume holographic grating with different diffraction efficiencies prepared by the preparation method in Example 2;

[0030] Fig. 4 is a schematic diagram of the polarization volume holographic grating with different diffraction efficiencies prepared by the preparation method in Example 2; DETAILED DESCRIPTION

[0031] The present application will be further described below with reference to the accompanying drawings. The following examples are only used to more clearly illustrate the technical solutions of the present application, and cannot be used to limit the protection scope of the present application.

[0032] Embodiment 1, the present application is mainly applicable to polarization volume holographic grating (PVG), the high-efficiency diffraction of PVG is mainly because that the periodic anisotropic refractive index distribution generated by the longitudinal liquid crystal structure in the grating satisfies the Bragg condition. For the normally incident light beam, the Bragg condition of PVG can be expressed as follows:

[0033] Where λ B is the Bragg wavelength in vacuum, Λ B is the Bragg period, is the tilt angle of the equal refractive index plane or the tilt angle represented as the grating vector K as shown in Figure 1, n eff is the average refractive index of the anisotropic medium which can be defined as:

[0034] In order to establish the Bragg high-efficiency diffraction, the longitudinal refractive index variation period of the PVG grating medium needs to be sufficient to complete the superposition of interlayer reflection of the refractive index plane. The more the number of Bragg diffraction periods contained in the internal structure of the PVG grating, the higher the diffraction energy that can be diffracted with the superposition of interlayer reflection, and vice versa. Therefore, under the condition of ensuring the consistency of the thickness of the liquid crystal layer of the PVG grating and without considering the change of the diffraction angle and the Bragg wavelength, we can realize the diffraction efficiency control of the PVG by changing the number of Bragg periods in its internal structure. By using ultraviolet light exposure, the optical properties of the liquid crystal molecules can be changed, the refractive index modulation Δn can be reduced, and the Bragg period can be increased. Under the condition of consistent thickness, the number of Bragg periods contained in the internal medium of the PVG is reduced, thereby realizing the reduction of the diffraction efficiency of the PVG. Through the way of ultraviolet light exposure, the patterned control or even the gradual change control of the diffraction efficiency can be conveniently realized.

[0035] Specifically, the embodiment introduces a preparation method of a polarization volume holographic grating with different diffraction efficiencies, as shown in Figures 1 and 3, which comprises the following steps:

[0036] Step one, coating a photo-alignment layer 4 on the top surface of the substrate medium 1;

[0037] Step two, continuing to cover a mask plate 5 (the mask plate 5 is a hollow light shield, the masked part is opaque, and the middle part is hollow, and the hollow area forms a grating transverse structure under laser exposure) on the top surface of the photo-alignment layer 4, and using two orthogonal circularly polarized lights for interference exposure;

[0038] Step three, after the exposure is completed, the mask plate 5 is removed, and a liquid crystal solution with a predetermined concentration is coated on the top surface of the photo-alignment layer 4, and the thickness of the liquid crystal solution is 5 μm, which is used to prepare a grating medium layer;

[0039] Step four, the liquid crystal solution is cured under the irradiation of ultraviolet light in a nitrogen environment to obtain a liquid crystal film;

[0040] Step five, the disordered liquid crystal film on the non-grating area of the substrate medium is removed by using a laser cleaning device to obtain a PVG grating (incoupling grating 6 and outcoupling grating with consistent diffraction efficiency);

[0041] Step six, a filter with gradually changing ultraviolet transmittance is covered on the surface of the PVG grating, and the filter is exposed to ultraviolet light to obtain a PVG grating region with consistent and continuous grating medium thickness and gradually increasing diffraction efficiency, that is, an outcoupling grating 7.

[0042] The polarization volume holographic grating is prepared on an optical medium with a certain refractive index and transmittance, the refractive index ranges from 1.5 to 2.2, the substrate medium material is optical glass or resin glass, and the shape is a flat plate or a free curved surface.

[0043] The grating medium layer thickness can be controlled to be 100 nm to 10 μm, and the thicker the grating medium layer thickness, the higher the grating diffraction efficiency. A suitable thickness is selected to meet the requirement of the maximum diffraction efficiency of the grating in the waveguide scheme.

[0044] The liquid crystal solution with a preset concentration is a liquid crystal solution with a concentration of 1% to 50% (in this embodiment, a liquid crystal solution with a concentration of 38% is used); the liquid crystal solution with a preset concentration includes a chiral agent, a liquid crystal, an acrylic ester polymerizable monomer, a photoinitiator, and a surfactant. The higher the refractive index modulation of the used liquid crystal composition and acrylic ester polymerizable monomer material, the better. The refractive index modulation of the commonly used material is 0.1 to 0.3.

[0045] The ultraviolet exposure uses ultraviolet light with a wavelength in the range of 400 nm to 10 nm, and the greater the ultraviolet light intensity, the shorter the required exposure time. The regional regulation and control of the diffraction efficiency can be realized by controlling the ultraviolet light intensity in the same time, or the diffraction efficiency of the grating sample can be reduced to a target value by controlling the exposure time under the same ultraviolet light intensity.

[0046] In this embodiment, the filter is a neutral density filter 2, which is composed of nine parts (the number of parts can be changed as needed, and in this embodiment, there are nine parts). The ultraviolet transmittance of each region is controlled to be different, the lowest transmittance is 10%, the ultraviolet transmittance of each region is set with a step value of 10%, and the highest transmittance is 90%. The filter is exposed to 365 nm ultraviolet light for 8 hours, and the ultraviolet light intensity is controlled to be 20 J / cm2, so that nine PVG grating regions with consistent and continuous grating medium thickness and gradually increasing diffraction efficiency can be obtained.

[0047] As another embodiment, as shown in Fig. 4, embodiment 2 is different from embodiment 1 in that the optical glass 3 is wedge-shaped; the optical glass itself has certain hindrance to the transmission of ultraviolet light, and we can control the transmission of ultraviolet light intensity by controlling the thickness of the optical glass, so that the optical glass is ground into a wedge shape, so that the ultraviolet light intensity transmitted through the optical glass is distributed in a stepped gradient. Similarly, it is placed under 365 nm ultraviolet light for 8 h, and the ultraviolet light intensity is controlled at 20 J / cm 2 Thus, a PVG grating area with a consistent grating medium thickness and a gradually distributed diffraction efficiency can be obtained.

[0048] Embodiment 3, based on the same inventive concept as embodiment 1 or embodiment 2, introduces a polarization volume holographic grating prepared by the preparation method described in embodiment 1 or embodiment 2.

[0049] The above only describes the preferred embodiments of the present application, and it should be pointed out that for ordinary skilled persons in the art, without departing from the technical principles of the present application, a number of improvements and modifications can be made, and these improvements and modifications should be considered as the protection scope of the present application.

Claims

1. A method for fabricating polarizing holographic gratings with different diffraction efficiencies, characterized in that, include: Step 1: Apply a photo-alignment layer to the top surface of the substrate medium; Step 2: Cover the top surface of the light alignment layer with another mask and use two orthogonal circularly polarized beams for interference exposure; Step 3: After exposure, remove the mask and coat a layer of liquid crystal solution of a preset concentration on the top surface of the photoalignment layer. This liquid crystal solution is used to prepare the grating dielectric layer and has a thickness of 100nm to 10μm. Step four: The substrate medium covered with the liquid crystal solution is placed in a nitrogen environment and cured under ultraviolet light to obtain a liquid crystal film. Step 5: Use laser cleaning equipment to remove the disordered liquid crystal film in the non-grating area of ​​the substrate medium to obtain the PVG grating; Step 6: Cover the PVG grating surface with a filter that has a gradually changing ultraviolet light transmittance, and expose the filter under ultraviolet light to obtain a PVG grating region with a uniform grating medium thickness and a continuous diffraction efficiency that increases sequentially.

2. The method for fabricating polarizing holographic gratings with different diffraction efficiencies according to claim 1, characterized in that, The substrate medium is optical glass or resin glass, and its shape is a flat plate or a free-form surface; the thickness of the grating medium layer is set to meet the requirement of the maximum diffraction efficiency of the grating required in the waveguide scheme.

3. The method for fabricating polarizing holographic gratings with different diffraction efficiencies according to claim 1, characterized in that, The mask is a perforated light-blocking sheet.

4. The method for fabricating polarizing holographic gratings with different diffraction efficiencies according to claim 1, characterized in that, The liquid crystal solution of the preset concentration is a liquid crystal solution of 1% to 50%; the liquid crystal solution of the preset concentration includes a chiral agent, liquid crystal, polymerizable acrylate monomer, photoinitiator, and surfactant.

5. The method for fabricating polarizing holographic gratings with different diffraction efficiencies according to claim 1, characterized in that, In step four, the prepared grating dielectric layer is placed in a nitrogen environment and cured under ultraviolet light irradiation of 400nm to 10nm.

6. The method for fabricating polarizing holographic gratings with different diffraction efficiencies according to claim 1, characterized in that, The filter is a neutral density filter, which includes several regions, each with a different ultraviolet light transmittance. The ultraviolet light transmittance of each region is set sequentially from low to high in a fixed step value.

7. The method for fabricating polarizing holographic gratings with different diffraction efficiencies according to claim 1, characterized in that, The filter is a wedge-shaped filter that can partially absorb ultraviolet light.

8. The method for fabricating polarizing holographic gratings with different diffraction efficiencies according to claim 6, characterized in that, In step six, the neutral density filter is exposed to ultraviolet light at a wavelength of 400nm to 10nm for a preset time to obtain several PVG grating regions with uniform grating medium thickness and continuously increasing diffraction efficiency.

9. The method for fabricating polarizing holographic gratings with different diffraction efficiencies according to claim 7, characterized in that, In step six, a wedge-shaped optical glass is exposed to ultraviolet light of 400nm to 10nm for a preset time to obtain a PVG grating region with a uniform thickness and a gradually varying diffraction efficiency.

10. A polarizing holographic grating, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 8.

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