Method and apparatus for fabricating PVG by generating vector light field using spatial light modulator.

Abstract

This invention discloses a method and apparatus for fabricating PVGs based on a vector light field generated by a spatial light modulator. The apparatus includes an illumination system, a phase design system, and an exposure system. The illumination system is mainly composed of a laser and a beam expander. The phase design system consists of two spatial light modulators (SLMs), named Phase Design System 1 and Phase Design System 2, respectively. The illumination system and Phase Design System 1 are arranged along the center line, while Phase Design System 2 and the exposure imaging system operate along the vertical center line. This invention greatly improves the flexibility of fabricating polarimetric holographic waveguides. Phase modulation and lateral period control are achieved through a programmable electronically controlled SLM, reducing experimental errors caused by manual adjustment of mirrors in traditional methods. Furthermore, unlike traditional interferometric exposure methods, the entire system exhibits strong robustness. Moreover, compared to separate interferometric optical paths, the use of a single beam expander avoids beam consistency issues during beam expansion.

Description

Technical Field

[0001] This invention relates to a method and apparatus for fabricating a PVG (polarizing body holographic grating) based on a vector light field generated by a spatial light modulator, belonging to the field of optical system imaging technology. Background Technology

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

[0003] In particular, conventional techniques such as micro / nano fabrication or methods based on changes in physical medium properties are often limited by equipment size and environmental sensitivity when producing high-precision holograms. Furthermore, traditional methods often employ separate optical path configurations, which are susceptible to environmental disturbances, leading to unstable experimental setups. To address these issues, this invention introduces an electrically controlled phase element (SLM), providing more flexible control over the phase wavefront information by generating the required specific vector light field based on material properties. This design significantly improves the stability and robustness of the polarization holographic grating fabrication process. By applying a precisely variable phase to the incident light wavefront through the electrically controlled phase element, this invention successfully achieves precise manipulation of the holographic pattern and effectively avoids the uncertainties caused by mechanical rotation.

[0004] While current technology has not overcome the limitations of generating high-precision holographic patterns on large surfaces (due to the inherent size constraints of SLMs), this invention proposes an innovative solution: enhancing the inherent stability of the experimental setup by constructing a single-beam exposure system. This configuration makes the system more resistant to mechanical vibrations and other environmental factors, thereby ensuring the quality and consistency of holographic patterns over long-term operation and guaranteeing the consistency of manufactured products.

[0005] Furthermore, the electronically controlled phase element endows the system with extremely high flexibility and rapid response capability, marking an important step towards more automated and intelligent hologram generation and opening up new avenues for further optimization and expansion of the application of diffractive optical elements. Summary of the Invention

[0006] The present invention aims to provide a method and apparatus for fabricating PVG based on a vector light field generated by a spatial light modulator, so as to overcome several limitations in traditional fabrication methods.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0008] Firstly, azo dyes are used as photo-alignment materials. When an azo dye film is irradiated with polarized light, the azo groups in the molecules undergo cis-trans isomerization, meaning the molecules rearrange themselves along the polarization direction of the light. This is because repeated cis-trans isomerization gradually brings the molecules to a stable state, aligning them with the polarization direction of the incident light. Therefore, generating a periodically rotating polarized light field allows for precise control of the alignment of liquid crystal molecules, providing a new technical approach for the fabrication of holographic gratings.

[0009] Secondly, this design primarily optimizes the exposure orientation optical path. Its key feature is that the linearly polarized light is first converted to 45° polarization, and then split by BS (Shape-Based Spectrometer). A1-A4 are the effective exposure optical paths, while B1-B2 are ineffective optical paths that do not affect the final polarization state of the light field. SLM1 and SLM2 are used to phase-modulate the 45° linearly polarized light. The mixed light field modulated by SLM1 is reflected by SLM2 to generate a vector light field with periodically rotating polarization state. After exposure, a polarized holographic grating is fabricated. This design generates a highly uniform and stable polarization rotation light field, significantly improving grating performance.

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

[0011] As a preferred embodiment, the SLM only performs phase modulation on the light pointing in the direction of the liquid crystal molecules.

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

[0013] As a preferred embodiment, the polarizer has an angle of 45° with the horizontal, and the SLM1 applies a phase φ1 to the incident light wave.

[0014] As a preferred option, SLM2 uses the same specifications and model as SLM1, and the angle between the liquid crystal pointing vectors of SLM1 and SLM2 is 45°. SLM2 applies a phase φ2 to the reflected light beam.

[0015] This invention achieves transverse periodicity Λ by controlling the modulation phases φ1 and φ2. x Precise exposure preparation.

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

[0017] Step 1: Coat the photo-alignment layer material onto the plasma-cleaned glass substrate, cover the top surface of the alignment layer with a mask, and expose it with light modulated by SLM1 and SLM2 to ensure that the periodic rotation characteristics of the light field can be effectively transferred to the alignment layer.

[0018] Step two: After exposure, a liquid crystal layer is coated on the alignment layer. The thickness is adjusted according to actual needs, generally controlled between 1µm and 5µm, to ensure optimal optical performance and structural stability.

[0019] Step 3: Place the substrate covered with the liquid crystal solution in a nitrogen environment for ultraviolet curing to form a robust polarizing holographic grating film layer, wherein the irradiation energy is not less than 1.5 J / cm². 2 This is to ensure the strength and uniformity of the film layer.

[0020] Step 4: Use laser cleaning equipment to remove the disordered grating areas on the substrate material to obtain a polarizing holographic grating.

[0021] Beneficial effects: By introducing programmable electronically controlled components, this invention achieves more precise control over the lateral period of a polarizing holographic grating. Specifically, the spatial light modulator (SLM) can flexibly adjust the periodic structure of the grating as needed, thereby overcoming the bottleneck problem of the difficulty in dynamically adjusting the period in traditional photolithography and nanoimprinting methods, and greatly improving the flexibility and precision of the process.

[0022] The core of this invention lies in generating a single-dimensional, periodically rotating liquid crystal alignment period, meaning that the arrangement of liquid crystal molecules can exhibit periodic changes in a specific direction. By progressively modulating the alignment of the liquid crystal layer using electronically controlled components, a highly consistent and repeatable periodic rotation pattern is achieved. This control method makes the alignment of liquid crystal molecules more precise, effectively reducing errors caused by manual adjustments in traditional methods. Another innovation of this invention is that the generated exposure optical path is a single-beam exposure, which has significantly greater robustness compared to traditional dual-beam interference. Single-beam exposure avoids the optical path alignment problems in dual-beam interference systems, reducing instability caused by environmental vibrations, optical path offsets, 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 of the experimental process and the need for precision equipment, making it very suitable for industrial production applications.

[0024] This invention significantly improves the stability of the fabrication process. By applying different voltages to the SLM, different response phases can be controlled, thereby regulating the lateral period of the grating. It offers high operability and effectively avoids phase errors caused by mechanical rotation in traditional methods, while also improving the yield of grating fabrication. Attached Figure Description

[0025] Figure 1 This invention provides a method for preparing polarizing body holographic exposure based on a spatial light modulator.

[0026] Figure 2 These are two SLM liquid crystals in the optical path of this invention, wherein... Figure 2 In (a), the dashed line represents the direction of the SLM1 liquid crystal pointer, which is parallel to the optical platform; Figure 2 In the middle (b), the dashed line represents the direction of the SLM2 liquid crystal pointer, which is -45° to the pointer of the optical SLM1.

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

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

[0029] Figure 5 This is a schematic diagram of a 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 volume holograms based on a vector light field generated by a spatial light modulator, such as... 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 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]

[0037] The above formula describes applying pressure in the x and y directions of the A1 light wave, respectively. and The phase delay. The polarization state of the A2 composite beam is determined by δ. Λ x The horizontal period is denoted by x, and x represents the spatial coordinates of SLM1 in free space. For example... Figure 3 .

[0038] Furthermore, the reflected light A3 undergoes phase modulation in SLM2. The angle between the liquid crystal molecule direction vector of SLM2 and the x-direction is -45°, and the entire direction of the molecule direction vector is subjected to a phase modulation. The expression for the emitted light is:

[0039]

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

[0041]

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

[0043] Furthermore, after exposure, a liquid crystal solution of a predetermined concentration is coated onto the alignment layer, typically controlling the grating thickness to be 1–5 μm. The coating of the predetermined liquid crystal solution is not elaborated upon in this invention.

[0044] Furthermore, the substrate is placed in a nitrogen atmosphere for ultraviolet curing.

[0045] Furthermore, the non-grating areas are cleaned to obtain a usable polarizing holographic grating.

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

[0047] In this example, single-beam exposure is used, which enhances the system's stability and significantly improves its robustness to the environment. For example... Figure 5 The diagram shown is a two-path interference pattern. Figure 5 θ rThe specific principles for controlling the lateral period are not detailed in this patent; in this case, the SLM can be edited to replace θ. r To conduct horizontal cyclical regulation.

[0048] In this example, the substrate can be optical glass or resin glass, and its shape and size are generally controlled within 10mm*15mm. The thickness of the grating dielectric layer can be adjusted between 1µm and 10µm.

[0049] This embodiment uses a 30% liquid crystal solution. This liquid crystal solution contains a chiral agent, RM257 liquid crystal, polymeric monomers, a photoinitiator, and a surfactant. The higher the refractive index modulation of the liquid crystal composition, the higher the bandwidth of the received visible light reflection, and the more uniform and saturated the color of the emitted light. Currently, commonly used materials have refractive index modulation between 0.1 and 0.3.

[0050] In this example, ultraviolet light exposure uses an ultraviolet light source with wavelengths ranging from 10 nm to 400 nm. Increasing the intensity of ultraviolet light can shorten the exposure time, thereby flexibly controlling the distribution area of ​​diffraction efficiency.

[0051] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. An apparatus for fabricating a polarizing holographic grating, characterized in that, include: Illumination system, phase design system, exposure system; The illumination system generates a horizontally oriented light beam and includes a laser and a beam expander / collimation system, which are coaxial in the horizontal direction. The laser has a wavelength of 350-500 nm, and the beam expander / collimation system generates a spot diameter of 20 mm × 20 mm. 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 polarization direction of the polarizer is at a 45° angle to the horizontal direction, used to convert the light beam into linearly polarized light in a specific direction. The liquid crystal molecule director of SLM1 is at a 45° angle to the horizontal direction, and the liquid crystal molecule director of SLM2 is at a -45° angle to the horizontal direction. The response bandwidth of SLM1 and SLM2 is... The light source has a wavelength of 350nm~500nm, a bit depth of 8 bits or more for phase modulation from 0 to 2π, a fill factor of not less than 94%, a resolution of not less than 1920×1200, and a reflectivity of not less than 90%. During operation, the linearly polarized light passes through the beam splitter, undergoes a first phase modulation by the SLM1, is reflected by the beam splitter to the SLM2, undergoes a second phase modulation by the SLM2, and is reflected to the exposure system, thereby generating a vector light field with a periodically rotating polarization state. The exposure system carries a substrate coated with a photo-alignment layer material and uses the vector light field to expose the photo-alignment layer to form a latent image with a periodic liquid crystal alignment pattern on the substrate. The apparatus also includes a post-processing unit for coating a liquid crystal layer on the exposed alignment layer, performing ultraviolet curing, and cleaning, wherein the ultraviolet curing irradiation energy is not less than 1.5J / cm². 2 .

2. The preparation apparatus according to claim 1, characterized in that, The subsequent processing unit includes a nitrogen environment control device and an ultraviolet light source, used to place the substrate coated with the liquid crystal layer in a nitrogen environment for ultraviolet curing to form a robust polarizer holographic grating film layer.

3. A method for fabricating a polarizing holographic grating based on the fabrication apparatus of claim 1, characterized in that, The process includes the following steps: Step 1, providing a cleaned substrate and coating it with a photo-alignment layer material to form an alignment layer; Step 2, placing the substrate in the exposure system, activating the illumination system and phase design system, and exposing the alignment layer using a vector light field with periodically rotating polarization states; Step 3, after exposure, coating a preset liquid crystal layer onto the exposed alignment layer; Step 4, placing the substrate covered with the liquid crystal layer in a nitrogen environment for ultraviolet curing to form a robust PVG film layer, wherein the ultraviolet irradiation energy is not less than 1.5 J / cm². 2 Step 5: Clean the areas on the substrate where no ordered grating structure has been formed to obtain the final polarizing holographic grating.

4. The preparation method according to claim 3, characterized in that, The thickness of the liquid crystal layer is 1μm to 10μm.

5. A polarizing holographic grating, characterized in that, Prepared by the method described in claim 3 or 4.

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