Method and apparatus for preparing PVG by generating vector light field on basis of spatial light modulators

Abstract

Disclosed in the present invention are a method and apparatus for preparing a PVG by generating a vector light field on the basis of spatial light modulators (SLMs). The apparatus comprises an illumination system, a phase design system and an exposure system, wherein the illumination system mainly comprises a laser and a beam expander assembled together; the phase design system is composed of two SLMs, which are respectively designated as a phase design system 1 and a phase design system 2; and the illumination system and the phase design system 1 are aligned along a central axis, while the phase design system 2 and an exposure imaging system operate along a vertical central axis. The present invention greatly improves the flexibility of manufacturing a polarization volume grating waveguide; by using programmable electronically controlled components, i.e., SLMs, to perform phase modulation to control a lateral period, experimental errors caused by manual adjustment of mirrors in traditional methods are reduced; moreover, different from traditional interference exposure methods, the entire system exhibits relatively high robustness; in addition, compared with a separated interference optical path, the use of a single beam expander avoids the problem of beam consistency during beam expansion.

Description

Method and device for generating vector light field based on spatial light modulator to realize preparation of PVG TECHNICAL FIELD

[0001] The present application relates to a method and device for generating vector light field based on spatial light modulator to realize preparation of PVG (polarization volume holographic grating), belonging to the field of optical system imaging technology. BACKGROUND

[0002] Diffractive optical elements (DOE) as a core technology in the field of optics, its design and manufacture are essential for precise manipulation of light wavefront. In this field, polarization volume holographic grating is highly regarded because it can realize highly precise polarization state control through Pancharatnam-Berry (PB) phase modulation. However, although these holograms have great application prospects, traditional production methods still face challenges in cost, efficiency and stability.

[0003] In particular, in the production of high-precision holograms, conventional techniques such as micro-nano processing or methods based on changes in physical medium properties are often limited by equipment size and environmental sensitivity. In addition, traditional methods often use separate optical configurations, which are easily affected by environmental disturbances and lead to unstable experimental setups. To address the above problems, the present application introduces an electrically controlled phase element SLM, which more flexibly controls the phase wavefront information, and only needs to generate the required specific vector light field according to the material properties. This design significantly improves the stability and robustness of the polarization volume holographic grating preparation process. By applying an accurate variable phase to the incident light wavefront through the electrically controlled phase element, the present application successfully realizes precise manipulation of the holographic pattern and effectively avoids the uncertainty caused by mechanical rotation.

[0004] Although the current technology does not break through the limitation of generating high-precision holographic patterns on a large scale (due to the inherent size constraints of SLM), the present application proposes an innovative solution: by constructing a single-beam exposure system to enhance the inherent stability of the experimental configuration. This configuration makes the system more resistant to mechanical vibrations and other environmental factors, ensuring the quality and consistency of the holographic pattern over a long period of time, and ensuring the consistency of the production products.

[0005] In addition, the electrically controlled phase element gives the system very high flexibility and fast response capability, which marks an important step towards more automated and intelligent holographic pattern generation, and opens up new avenues for further optimization and expansion of diffractive optical element applications. SUMMARY

[0006] The present application aims to provide a method and device for generating vector light field based on spatial light modulator to realize preparation of PVG, in order to overcome some limitations in traditional preparation methods.

[0007] To solve the above technical problems, the technical scheme adopted by the present application is:

[0008] In the first aspect, azo dye is used as the photo-alignment material. When the azo dye film is irradiated by polarized light, the azo group in the molecule will undergo cis-trans isomerization, that is, the molecule will be rearranged along the polarization direction of the light. This is because the repeated cis-trans isomerization process makes the molecule gradually tend to be stable, consistent with the incident light polarization direction. Therefore, a periodically rotating polarized light field is generated, which can accurately realize the orientation control of liquid crystal molecules, and provides a new technical approach for the preparation of holographic grating.

[0009] In the second aspect, the design scheme is mainly optimized for the exposure orientation light path. The feature is that the linearly polarized light is first converted to 45°, and then split by BS. A1-A4 are effective exposure light paths, and B1-B2 are invalid light paths that do not affect the polarization state of the final light field. The 45° linearly polarized light is phase-modulated by SLM1 and SLM2 respectively. The mixed light field after SLM1 modulation is reflected by SLM2 to generate a vector light field with periodically rotating polarization state, and a polarization volume holographic grating is prepared after exposure. This design generates a high-uniformity and high-stability polarized rotating light field, which significantly improves the performance of the grating.

[0010] As a preferred scheme, the light source part is set to a linearly polarized light source of 350-500nm.

[0011] As a preferred scheme, the SLM only phase controls the light in the direction of the liquid crystal molecule director.

[0012] As a preferred scheme, the exposure light path is collimated and expanded by an expander to generate a plane wave with uniform intensity and constant frequency.

[0013] As a preferred scheme, the polarizing plate has a horizontal included angle of 45°, and SLM1 applies a phase φ1 to the incident light wave.

[0014] As a preferred scheme, SLM2 is selected to have the same specifications and model as SLM1, and the liquid crystal director included angle of SLM1 and SLM2 is 45°. SLM2 applies a phase φ2 to the reflected light beam.

[0015] The present application controls the modulation phases φ1 and φ2, and then realizes the accurate exposure preparation of the transverse period Λ x .

[0016] The present application provides a method for generating a vector light field based on a spatial light modulator to realize the preparation of a PVG, which comprises the following steps:

[0017] Step one, after plasma cleaning, the glass substrate is coated with a photoalignment layer material, a mask is covered on the top surface of the orientation layer, and exposure is performed using light modulated by SLM1 and SLM2 to ensure that the periodic rotation characteristics of the light field can be effectively transmitted to the orientation layer.

[0018] Step two, after exposure, the orientation layer is coated with a liquid crystal layer, the thickness is adjusted according to actual requirements, generally controlled within 1-5um, to ensure the best optical performance and structural stability.

[0019] Step three, the substrate covered with liquid crystal solution is placed in a nitrogen environment for ultraviolet curing to form a solid polarization body holographic grating film layer, wherein the irradiation energy is not less than 1.5J / cm 2 , to ensure the strength and uniformity of the film layer.

[0020] Step four, the disordered grating area on the substrate material is removed by using a laser cleaning device to obtain a polarization body holographic grating.

[0021] Beneficial effects: The present application realizes more accurate control of the lateral period of the polarization body holographic grating by introducing programmable electric control elements. Specifically, the spatial light modulator (SLM) can flexibly adjust the period structure of the grating as needed, thereby breaking through the bottleneck problem of dynamic adjustment of the period in traditional photolithography and nanoimprint, and greatly improving the flexibility and precision of the process.

[0022] The core of the scheme of the present application is to generate a single-dimensional periodic rotation liquid crystal orientation period, which means that the arrangement of liquid crystal molecules can exhibit periodic changes in a specific direction. By step-by-step orientation modulation of the liquid crystal layer by the electric control element, a highly consistent and repeatable periodic rotation mode is realized. This control method makes the orientation of the liquid crystal molecules more accurate, effectively reducing the errors caused by manual adjustment in traditional methods. Another innovation of the present application is that the exposure light path generated is single-beam exposure, which has great robustness compared to traditional double-path interference. Single-beam exposure avoids the light path alignment problem in the double-path interference system, reduces the instability caused by environmental vibration, light path deviation and other factors, thereby significantly improving the overall stability and repeatability of the system.

[0023] This design not only simplifies the optical path configuration, but also greatly reduces the complexity and demand for precision equipment in the experimental process, which is very suitable for application in industrial production.

[0024] The present application greatly improves the stability of the preparation process. By applying different voltages to the SLM, different response phases can be controlled to control the lateral period of the grating. It has high flexibility and effectively avoids the phase error caused by mechanical rotation in traditional methods, while improving the yield of the prepared grating. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 illustrates a method for preparing polarizing body holographic exposure based on a spatial light modulator, as provided in this invention.

[0026] Figure 2 shows the liquid crystals of two SLMs in the optical path of the present invention. In Figure 2(a), the dashed line is the direction of the pointer of the SLM1 liquid crystal, which is parallel to the optical platform; in Figure 2(b), the dashed line is the direction of the pointer of the SLM2 liquid crystal, which is -45° to the pointer of the optical SLM1.

[0027] Figure 3 is a schematic diagram showing the changes in SLM1 phase modulation and SLM free space coordinate x.

[0028] Figure 4(a) is a schematic diagram of the liquid crystal structure with the alignment layer, and (b) is a schematic diagram of the liquid crystal structure with the thickness direction of the polarizer hologram. The arrows point in the direction of the long axis of the liquid crystal molecules.

[0029] Figure 5 is a schematic diagram of traditional dual-path interference. Detailed Implementation

[0030] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0031] The present invention will be further described below with reference to specific embodiments.

[0032] This implementation case introduces a design method for preparing polarized body holograms based on a vector light field generated by a spatial light modulator, as shown in Figure 1. The structure includes, but is limited to, an illumination system, a phase design system, and an exposure system.

[0033] The illumination system includes a laser and a beam expander / collimation system. The laser provides a stable light source. The beam expander / collimation system provides a uniform light spot, ensuring that the equiphase plane of the exposure beam is perpendicular to the beam's propagation direction.

[0034] Furthermore, the exposure beam passes through a P-polarizer, the polarization direction of which makes a 45° angle with the horizontal. This generates polarized light suitable for the requirements of this invention, used for subsequent phase modulation and polarization control. This configuration ensures that the characteristics of the polarized light are maintained throughout the optical path, thereby achieving high-quality fabrication of polarized volume holographic gratings.

[0035] The beam A1, after passing through BS, illuminates SLM1, and the beam is set. The phase difference between the o-ray and e-ray (inside the liquid crystal) on SLM1 is δ. Therefore, the Jones matrix of the beam after passing through SLM1 can be expressed as:

[0036] The above formula describes that the phase delay of A1 light wave x direction and y direction is respectively applied and . The polarization state of A2 combined light beam is determined by δ. Wherein, Λ x is the transverse period, and x is the spatial coordinate of SLM1 in free space. As shown in Figure 3.

[0037] Further, the reflected light A3 is phase-modulated at SLM2, the angle between the director of the liquid crystal molecules of SLM2 and the x direction is-45°, and the overall phase applied to the molecule director direction is The expression of the outgoing light is:

[0038] Wherein, E′ x and E′ y are the component electric fields of A3 in x and y directions; A is the global phase factor and amplitude product, and the modulus is a constant value, which does not affect the polarization state of the light. Further, from formula (3), it can be seen that the current generated is a vector light field, and the polarization state is periodically changed, and the angle between the polarization states is:

[0039] Further, the periodic polarization light field is exposed on the substrate coated with an orientation layer.

[0040] Further, after exposure, a liquid crystal solution with a preset concentration is coated on the orientation layer, and the grating thickness is generally controlled to be 1-5 μm. The coating of the preset liquid crystal solution is not expanded in the present application.

[0041] Further, the substrate is placed in a nitrogen environment for ultraviolet curing.

[0042] Further, the non-grating area is cleaned to obtain a usable polarization volume holographic grating.

[0043] In the present example, the spatial light modulator used has a resolution of 1920*1080, and the minimum pixel point is 6.4 μm. In theory, the smaller the physical size of the spatial light modulator, the higher the phase precision applied, thereby making the characteristics of the prepared volume holographic grating more stable.

[0044] In the present example, single-beam exposure is adopted, which enhances the stability of the system and significantly improves the robustness to the environment. As shown in Figure 5, a double-path interference chart is shown, and θ r controls the transverse period, and the specific principle is not expanded in the present patent, at this time, the transverse period can be controlled by editing SLM to replace θ r to control the transverse period.

[0045] In this example, the substrate can be optical glass or resin glass, and the shape and size are generally controlled to be 10mm*15mm. The thickness of the grating medium layer can be adjusted between 1um and 10um.

[0046] In this example, a 30% liquid crystal solution is used. The liquid crystal solution contains a chiral agent, RM257 liquid crystal, a polymer monomer, a photoinitiator, and a surfactant. The higher the refractive index modulation of the liquid crystal composition, the higher the bandwidth of the reflected visible light, and the more uniform and saturated the color of the outgoing light. The refractive index modulation of the commonly used material is between 0.1 and 0.3.

[0047] In this example, the ultraviolet light exposure uses an ultraviolet light source with a wavelength in the range of 10nm to 400nm. Increasing the intensity of the ultraviolet light can shorten the exposure time, thereby flexibly controlling the distribution area of the diffraction efficiency.

[0048] The above only describes the preferred embodiments of the present application, and it should be noted that for ordinary skilled persons in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, and these improvements and refinements should also be considered within the scope of protection of the present application.

Claims

1. An apparatus for fabricating a polarizing holographic grating, characterized in that, Comprise: The illumination system, the phase design system, the exposure system; the phase design system comprises a polarizer, a beam splitter, two spatial light modulators SLM1 and SLM2; The illumination system, the polarizer, the beam splitter, SLM1 are coaxial in the horizontal direction; the exposure system, the beam splitter, SLM2 are coaxial in the vertical direction; the illumination system is used to generate a horizontal light beam; the polarizer is used to convert the light beam into linearly polarized light; the linearly polarized light is transmitted through the beam splitter, phase modulated by SLM1, reflected by the beam splitter to SLM2, phase modulated by SLM2, and reflected to the exposure system to generate a vector light field with periodically rotating polarization state; the exposure system is used to expose the vector light field to prepare a polarization volume holographic grating.

2. The apparatus according to claim 1, wherein The illumination system comprises a laser and a beam expansion collimation system, the laser and the beam expansion collimation system are coaxial in the horizontal direction, the wavelength of the laser is 350-500nm, and the beam expansion collimation system generates a spot diameter of 20mm*20mm.

3. The apparatus of claim 1, wherein the apparatus is configured to produce a polarization volume holographic grating. The angle between the polarization direction of the polarizer and the horizontal direction is 45°.

4. The apparatus according to claim 3, wherein the apparatus is characterized by: The angle between the liquid crystal molecule director of SLM1 and the horizontal direction is 45°, and the angle between the liquid crystal molecule director of SLM2 and the horizontal direction is-45°.

5. The apparatus according to claim 2, wherein The response bandwidth of SLM1 and SLM2 includes but is not limited to 350nm-500nm, has 8bit and above bit depth control degree and realizes 0-2π phase control, the filling degree is not less than 94%, the panel resolution is not less than 1920*1200, and the reflectivity is not less than 90%.

6. A method for generating a PVG based on a spatial light modulator based device according to any of claims 1-5, characterized in that, Comprise the following steps: Step one, after cleaning the glass substrate, apply a photoalignment layer material on the top surface of the orientation layer, cover a mask on the top surface of the orientation layer, and use light modulated by SLM1 and SLM2 for exposure; Step two, after exposure, a liquid crystal layer is prepared on the orientation layer; Step three, the substrate covered with liquid crystal solution is placed in a nitrogen environment for UV curing, forming a solid PVG film layer, wherein the irradiation energy is not less than 1.5 J / cm 2 ; Step four, clean the disordered grating area on the substrate material to obtain a polarization volume holographic grating.

7. The method of claim 6, wherein, The thickness of the liquid crystal layer is 1um-10um.

8. A polarization volume holographic grating, characterized in that, Prepared by the method of claim 6.

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