Method and device for realizing PVG preparation based on vector light field generated by spatial light modulator

Abstract

The invention discloses a PVG preparation method and device based on a vector light field generated by a spatial light modulator. The device comprises an illumination system, a phase design system and an exposure system. The lighting system is mainly formed by assembling a laser and a beam expander, and the phase design system is composed of two spatial light modulators (SLMs) which are named as a phase design system 1 and a phase design system 2 respectively. Wherein the lighting system and the phase design system 1 are sequenced on a center line, and the phase design system 2 and the exposure imaging system work on a vertical center line. According to the method, the flexibility of manufacturing the holographic waveguide of the polarization body is greatly improved, phase regulation and control are carried out through the programmable electric control element SLM to control the transverse period, and experimental errors caused by manual adjustment of a reflector in a traditional mode are reduced; meanwhile, the method is different from a traditional interference exposure method, and the whole system has high robustness; in addition, compared with a separated interference light path, one beam expander is used, and the problem of light beam consistency in the beam expanding process is avoided.

Description

Technical Field

[0001] The invention relates to a preparation method and device for realizing PVG (polarization volume holographic grating) by generating a vector light field based on a spatial light modulator, and belongs to the technical field of optical system imaging. Background Art

[0002] As a core technology in the field of optics, the design and fabrication of diffractive optical elements (DOEs) are crucial for the precise manipulation of light wavefronts. In this field, polarization volume holographic gratings have attracted much attention due to their ability to achieve highly precise polarization state control through Pancharatnam-Berry (PB) phase modulation. However, despite the huge application prospects of these holograms, traditional production methods still face challenges in terms of cost, efficiency and stability.

[0003] In particular, when producing high-precision holograms, conventional techniques such as micro-nano processing or methods based on changes in physical medium properties are usually limited by equipment size and environmental sensitivity. In addition, traditional methods often use a separate optical path configuration, which is easily affected by environmental disturbances and leads to unstable experimental settings. In response to the above problems, the present invention introduces an electrically controlled phase element SLM, which can more flexibly control 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 a precise and variable phase to the incident light wavefront through an electrically controlled phase element, the present invention successfully achieves precise control of the holographic pattern and effectively avoids the uncertainty caused by mechanical rotation.

[0004] Although current technology has not broken through the limitation of generating high-precision holographic patterns on large formats (due to the inherent size constraints of SLM), the present invention proposes an innovative solution: by building a single-beam exposure system to enhance the inherent stability of the experimental configuration. This configuration makes the system more resistant to mechanical vibration and other environmental factors, thereby ensuring the quality and consistency of the holographic pattern under long-term operation and the consistency of the produced products.

[0005] In addition, the electrically controlled phase element gives the system extremely high flexibility and fast response capabilities, which marks an important step towards more automated and intelligent hologram generation and opens up new paths for further optimizing and expanding the application of diffractive optical elements. Summary of the invention

[0006] The present invention aims to provide a preparation method and device for realizing PVG by generating a vector light field based on a spatial light modulator, so as to overcome several limitations in the traditional preparation method.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0008] First, azo dyes are used as photo-orientation materials. When the azo dye film is irradiated with polarized light, the azo groups in the molecules undergo cis-trans isomerization, that is, the molecules are rearranged along the polarization direction of the light. This is because the repeated cis-trans isomerization process makes the molecules gradually tend to a stable state, which is consistent with the polarization direction of the incident light. Therefore, the generation of a periodically rotating polarized light field can accurately realize the orientation control of liquid crystal molecules, providing a new technical approach for the preparation of holographic gratings.

[0009] Secondly, this design scheme mainly optimizes the exposure orientation optical path. It is characterized in that the linear polarized light is first converted to 45° and then split by BS. A1-A4 is an effective exposure optical path, and B1-B2 is an invalid optical path that does not affect the polarization state of the final light field. The 45° linear polarized light is phase modulated by SLM1 and SLM2 respectively. The mixed light field modulated by SLM1 is reflected by SLM2 to generate a vector light field with periodic rotation of the polarization state, and a polarization holographic grating is prepared after exposure. This design generates a polarization rotation light field with high uniformity and high stability, which significantly improves the grating performance.

[0010] As a preferred solution, the light source part is configured as a linearly polarized light source of 350-500nm.

[0011] As a preferred solution, the SLM only performs phase control on light in the direction of the director of the liquid crystal molecules.

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

[0013] As a preferred solution, the angle between the polarizer and the horizontal is 45°, and the SLM1 applies a phase φ1 to the incident light wave.

[0014] As a preferred solution, SLM2 is selected to have the same specifications and models as SLM1, and the angle between the liquid crystal directors of SLM1 and SLM2 is 45°. SLM2 applies a phase φ2 to the reflected light beam.

[0015] The present invention realizes the lateral period Λ by controlling the modulation phases φ1 and φ2. x Precise exposure preparation.

[0016] The present invention provides a method for preparing PVG based on generating a vector light field using a spatial light modulator, comprising the following steps:

[0017] Step 1: Apply a photoinduced alignment layer material on the plasma cleaned glass substrate, cover the top surface of the alignment layer with a mask, and use light modulated by SLM1 and SLM2 for exposure to ensure that the periodic rotation characteristics of the light field can be effectively transferred to the alignment layer.

[0018] Step 2: After the exposure is completed, a liquid crystal layer is coated on the alignment layer. The thickness is adjusted according to actual needs and is generally controlled at 1um to 5um to ensure the best optical performance and structural stability.

[0019] Step 3: Place the substrate covered with the liquid crystal solution in a nitrogen environment for UV curing to form a solid polarized holographic grating film layer, wherein the irradiation energy is not less than 1.5 J / cm 2 , to ensure the strength and uniformity of the film layer.

[0020] Step 4: Use laser cleaning equipment to clean the disordered grating area on the substrate material to obtain a polarization holographic grating.

[0021] Beneficial effects: The present invention achieves more precise control over the lateral period of the polarization volume holographic grating by introducing programmable electrically controlled components. Specifically, the spatial light modulator (SLM) can flexibly adjust the periodic structure of the grating as needed, thereby breaking through the bottleneck problem of the difficulty in dynamically adjusting the period in traditional methods such as photolithography and nanoimprinting, and greatly improving the flexibility and precision of the process.

[0022] The core of the solution of the present invention is to generate a single-dimensional periodic rotation of the liquid crystal orientation period, which means that the arrangement of the liquid crystal molecules can show periodic changes in a specific direction. The liquid crystal layer is gradually oriented and modulated by the electric control element, so that a highly consistent and repeatable periodic rotation pattern is achieved. This control method makes the orientation of the liquid crystal molecules more precise and effectively reduces the errors caused by manual adjustment in the traditional method. Another innovation of the present invention is that the generated exposure light path is single-beam exposure, which is much more robust than the traditional dual-path interference. Single-beam exposure avoids the problem of light path alignment in the dual-path interference system, reduces the instability caused by factors such as environmental vibration and light path offset, and thus significantly improves the overall stability and repeatability of the system.

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

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

[0025] Figure 1 The present invention provides a method for preparing a polarized holographic exposure based on a spatial light modulator.

[0026] Figure 2 The liquid crystals of the two SLMs in the optical path of the present invention are Figure 2 The dotted line in (a) is the direction of the SLM1 liquid crystal director, which is parallel to the optical platform. Figure 2 The dotted line in (b) is the direction of the SLM2 liquid crystal director, which is -45° to the director of the optical SLM1.

[0027] Figure 3 Schematic diagram of SLM1 phase control and the change of SLM free space coordinate x.

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

[0029] Figure 5 Schematic diagram of traditional dual-path interference. DETAILED DESCRIPTION

[0030] The following is a clear and complete description of the technical solutions in the examples of the present invention in conjunction with the accompanying drawings in the examples of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the protection scope of the present invention.

[0031] The present invention will be further described below in conjunction with specific embodiments.

[0032] This implementation case introduces a design method for preparing polarization volume holography based on generating vector light field using a spatial light modulator. Figure 1 As shown, the structure includes but is limited to an illumination system, a phase design system, and an exposure system.

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

[0034] Furthermore, the exposure beam passes through a P polarizer, and the angle between the polarization direction of the polarizer and the horizontal is 45°. Polarized light suitable for the needs of the present invention is generated for subsequent phase modulation and polarization control. This configuration can ensure that the characteristics of polarized light are maintained throughout the optical path, thereby achieving high-quality polarization volume holographic grating preparation.

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

[0036]

[0037] As described in the above formula, the A1 light wave is applied in the x direction and y direction respectively. and The phase delay of A2 is determined by the polarization state of the composite beam. Λ x is the lateral period, and x is the spatial coordinate of SLM1 in free space. Figure 3 .

[0038] Furthermore, the reflected light A3 is phase modulated in SLM2, the angle between the liquid crystal molecule director of SLM2 and the x direction is -45°, and the overall phase applied in the direction of the molecule director is The expression of the emitted light is:

[0039]

[0040] Among them, E' x and E' y is the component electric field of A3 in the x and y directions; A is the product of the global phase factor and the amplitude, and the modulus is a constant value and does not affect the polarization state of the light. Further, from formula (3), it can be seen that the polarization state of the vector light field currently generated is periodically changing, and the angle of the polarization state is:

[0041]

[0042] Furthermore, a periodically polarized light field is exposed on the substrate coated with the alignment layer.

[0043] Furthermore, after exposure, a liquid crystal solution of a preset concentration is coated on the alignment layer, and the grating thickness is generally controlled to be 1-5 μm. The coating of the preset liquid crystal solution is not elaborated in the present invention.

[0044] Furthermore, the substrate is placed in a nitrogen environment for UV curing.

[0045] Furthermore, the non-grating area is cleaned to obtain a usable polarization volume holographic grating.

[0046] In this example, the resolution of the spatial light modulator used is 1920*1080, and the minimum pixel is 6.4 μm. In theory, the smaller the physical size of the spatial light modulator, the higher the applied phase accuracy, making the prepared volume holographic grating characteristics more stable.

[0047] In this example, single beam exposure is used. This method enhances the stability of the system and significantly improves its robustness to the environment. Figure 5 The figure shows the double-path interference diagram. Figure 5 Medium θ rControl the horizontal cycle. The specific principle will not be elaborated in this patent. At this time, you can edit SLM to replace θ r Carry out horizontal cycle regulation.

[0048] 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 10μm.

[0049] In this embodiment, a 30% liquid crystal solution is used. The liquid crystal solution contains a chiral agent, RM257 liquid crystal, a polymerizable monomer, a photoinitiator, and a surfactant. The higher the refractive index modulation of the liquid crystal composition, the higher the bandwidth of receiving visible light reflection, and the color of the emitted light will be more uniform and saturated. The refractive index modulation of the commonly used materials is between 0.1 and 0.3.

[0050] In this example, the UV light exposure uses a UV light source with a wavelength in the range of 10 nm to 400 nm. Increasing the UV light intensity can shorten the exposure time, thereby flexibly controlling the distribution area of ​​the diffraction efficiency.

[0051] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A device for preparing a polarization holographic grating, characterized in that: include: Illumination system, phase design system, exposure system; the phase design system includes a polarizer, a beam splitter, and two spatial light modulators SLM1 and SLM2; The illumination system, polarizer, beam splitter, and SLM1 are coaxial in the horizontal direction; the exposure system, beam splitter, and SLM2 are coaxial in the vertical direction; the illumination system is used to generate a light beam in the horizontal direction; the polarizer is used to convert the light beam into linearly polarized light; the linearly polarized light is transmitted through the beam splitter, and after being phase modulated by SLM1, it is reflected by the beam splitter to SLM2, and after being phase modulated by SLM2, it is reflected to the exposure system, thereby generating a vector light field with a periodic rotational change in polarization state; the exposure system is used to prepare a polarization volume holographic grating after exposing the vector light field.

2. The device for preparing a polarization holographic grating according to claim 1, characterized in that: The illumination system comprises a laser and a beam expansion and collimation system, the laser and the beam expansion and collimation system are coaxial in the horizontal direction, the wavelength of the laser is 350-500nm, and the diameter of the light spot generated by the beam expansion and collimation system is 20mm*20mm.

3. The device for preparing a polarization holographic grating according to claim 1, characterized in that: The angle between the polarization direction of the polarizer and the horizontal direction is 45°.

4. The device for preparing a polarization holographic grating according to claim 3, characterized in that: The angle between the director of the liquid crystal molecules of the SLM1 and the horizontal direction is 45°, and the angle between the director of the liquid crystal molecules of the SLM2 and the horizontal direction is -45°.

5. The device for preparing a polarization holographic grating according to claim 2, characterized in that: The response bandwidth of SLM1 and SLM2 includes but is not limited to 350nm~500nm, has a bit depth control degree of 8bit or above and realizes phase control of 0~2π, a filling degree of not less than 94%, a panel resolution of not less than 1920*1200, and a reflectivity of not less than 90%.

6. A method for preparing PVG based on the preparation device of any one of claims 1 to 5, wherein the method comprises: The steps include: Step 1: Apply a photoinduced alignment layer material on the cleaned glass substrate, cover the top surface of the alignment layer with a mask, and perform exposure using light modulated by SLM1 and SLM2; Step 2: After the exposure is completed, a modulated liquid crystal layer is preset on the alignment layer; Step 3: Place the substrate covered with the liquid crystal solution in a nitrogen environment for UV curing to form a solid PVG film layer, wherein the irradiation energy is not less than 1.5 J / cm 2 ; Step 4: clean the disordered grating area on the substrate material to obtain a polarization volume holographic grating.

7. The method according to claim 6, characterized in that The thickness of the liquid crystal layer is 1 um to 10 um.

8. A polarization holographic grating, characterized in that: Prepared by the method according to claim 6.

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