Volume holographic optical elements and their uniform exposure method and apparatus

Abstract

The scheme belongs to the field of display technology, and discloses a volume holographic optical element and a uniform exposure method and device thereof. The uniform exposure method of the volume holographic optical element comprises the following steps: placing a photosensitive material on one side of a modulation device, making signal light irradiate the photosensitive material through the modulation device, and making the signal light interfere with reference light irradiating the photosensitive material to expose the photosensitive material; after starting the exposure, adjusting the modulation device every unit time T u to make the wave front form and spatial phase distribution of the signal light randomly change. The unit time T u is the time length experienced by the signal light and the reference light in the interference recording of the photosensitive material to form a first amplitude hologram. The diffraction efficiency of the volume holographic optical element obtained in this way is high in spatial distribution uniformity, and the signal light can reproduce a spatial brightness distribution uniformity when irradiated by a light beam along the reference light direction, which is beneficial to improve the imaging quality, uniformity and subsequent application performance of the volume holographic optical element.

Description

Technical Field

[0001] This solution belongs to the field of display technology, specifically involving a volume holographic optical element and its homogenization exposure method and apparatus. Background Technology

[0002] Volume holographic optical elements (VHOEs) are a class of three-dimensional optical structures recorded in photosensitive materials based on the principle of interference. They possess characteristics such as high diffraction efficiency, strong angular selectivity, and good wavelength selectivity, and are widely used in imaging, beamforming, spectral filtering, and augmented reality displays. During the volume holographic exposure process, two spatially coherent laser beams (i.e., reference light and object light) are used to generate interference fringes in the photosensitive material. The intensity of the fringes modulates the refractive index in the photosensitive material, forming a permanent grating structure.

[0003] like Figure 1 As shown, during the interference of beams L1 and L3 and their illumination of the photosensitive material, a holographic grating can be rapidly formed in a short time. For example, the initial holographic grating structure can be formed at time point t1. However, during subsequent exposures where t > t1, the recorded grating gradually acquires a certain diffraction capability, causing the incident beam L1 to produce diffracted light (hereinafter referred to as beam L1-3). Since this beam L1-3 originates from the already formed grating structure, it has a fixed phase difference with the original beam L3. Therefore, the two will coherently superimpose again in the photosensitive material and form new interference fringes. The new interference fringes are spatially consistent with the original interference fringes (i.e., the grating structure that generates beam L1-3), producing a stable constructive or destructive superposition effect at the original interference fringe positions. This results in an uneven distribution of bright and dark light in the outgoing light, forming bright fringes at constructive locations and dark fringes at destructive locations.

[0004] Meanwhile, as the exposure process continues, the photosensitive material inevitably undergoes volume shrinkage or refractive index modulation changes during photochemical reactions and energy absorption, resulting in slight alterations to the period, orientation, or spatial position of the formed grating. These structural changes further cause subtle deviations in the propagation directions between beams L1-3 and the original beam L3, thereby creating a more complex interference fringe distribution within the photosensitive material.

[0005] Similarly, the original beam L3 will also produce diffracted light (hereinafter referred to as beam L3-1) under the action of the formed grating. This beam L3-1 will interfere with beam L1 again and superimpose to form new spatial modulation fringes in the photosensitive material. The above-mentioned multiple and intersecting interference processes are continuously superimposed in the photosensitive material, resulting in significant fluctuations in local exposure intensity, causing the light field, which should be uniformly distributed under ideal conditions, to gradually evolve into a non-uniform distribution.

[0006] Ultimately, the aforementioned non-ideal multiple interference effects introduce significant intensity modulation inhomogeneities into the recorded holograms, manifesting as spatial inconsistencies in diffraction efficiency or brightness (i.e., the formation of dark fringes), which severely affect the imaging quality, uniformity, and subsequent application performance of holographic optical elements. Summary of the Invention

[0007] This solution aims to overcome at least one defect in the existing technology and provide a homogenization exposure method for volume holographic optical elements to solve the problem of non-ideal multiple interference effects in traditional exposure methods.

[0008] To solve the above-mentioned technical problems, the following technical solution is adopted: Firstly, a homogenization exposure method for volume holographic optical elements is proposed. This method includes: placing a photosensitive material on one side of a modulation device, allowing signal light to pass through the modulation device and illuminate the photosensitive material, interfering with reference light illuminating the photosensitive material, thus exposing the photosensitive material; after the exposure begins, at unit time intervals T... u The modulation device is adjusted to cause random changes in the wavefront shape and spatial phase distribution of the signal light. The unit time T... u The length of time it takes for the signal light and the reference light to interfere and record in the photosensitive material to form the first hologram.

[0009] The aforementioned device combines segmented exposure technology with the random adjustment mechanism of the modulation device. By introducing wavefront and phase-independent signal light fields at the critical moment of each hologram formation, it enables multi-segment exposure (each exposure lasts for a unit time T). u The holograms formed by this method do not have uniformity in spatial position, so it is impossible to achieve a stable constructive or destructive superposition effect on existing holograms. This avoids repeated reinforcement of holograms and effectively suppresses the formation of dark fringes. The diffraction efficiency of the resulting volume holographic optical element has high uniformity in spatial distribution. When illuminated by a beam along the reference light direction, it can reproduce a signal light with uniform spatial brightness distribution, which is beneficial to improving the imaging quality, uniformity and subsequent application performance of the volume holographic optical element.

[0010] The modulation device can preferably be a diffuser, a diffuser film, or a microlens array. The surfaces of the diffuser, diffuser film, and microlens array have uneven structures, causing different parts of the signal light to experience different optical path lengths as it passes through, thus disrupting the wavefront and phase of the emitted light. Displacement of the diffuser, diffuser film, or microlens array alters the wavefront shape, spatial phase distribution, and local incident angle of the signal light, thereby achieving spatial averaging of the interference modulation terms in the photosensitive material. Therefore, when adjusting the modulation device, the diffuser, diffuser film, or microlens array can be randomly displaced.

[0011] The modulation device can be a spatial light modulator loaded with a phase-type hologram. The spatial light modulator can load a pre-designed phase-type hologram, with each loaded phase-type hologram being independent of the others. The phase-type hologram possesses the phase characteristics of an equivalent point source or holographic lens, allowing point-by-point control of the wavefront shape and spatial phase distribution of the signal light. After switching the phase-type hologram, the wavefront shape and spatial phase distribution of the signal light will change, thereby achieving decorrelation modulation between the signal light before and after modulation. Therefore, when adjusting the modulation device, the phase-type hologram loaded by the spatial light modulator can be switched.

[0012] Total exposure time T t Satisfy: T t =n T u Where n is an integer and n≥2. In other words, the total exposure time T t The preferred unit of time T u The exposure time should be an integer multiple of T to ensure that the entire exposure process can be based on the unit duration T. u It is divided into n complete exposure periods, and each exposure period can form a complete hologram.

[0013] The signal light and / or reference light are provided by a laser source or controlled by a laser shutter, and the entire exposure process is performed in units of time T. u The exposure is divided into n exposure periods. At the beginning of each exposure period, the laser source or laser shutter is turned on, and at the end of each exposure period, the laser source or laser shutter is turned off. Starting from at least the second exposure period, the modulation device is adjusted before the beginning of each exposure period. As a result, the modulation device will have sufficient time to be adjusted, greatly reducing the difficulty of adjustment and control.

[0014] Secondly, a homogenization exposure apparatus for a volume holographic optical element is proposed, which operates the aforementioned homogenization exposure method. The apparatus includes a light source assembly and a modulation device; the light source assembly provides signal light and reference light, and the modulation device modulates the signal light.

[0015] The aforementioned device combines segmented exposure technology with the random adjustment mechanism of the modulation device. By introducing wavefront and phase-independent signal light fields at the critical moment of each hologram formation, it enables multi-segment exposure (each exposure lasts for a unit time T). u The holograms formed by this method do not have uniformity in spatial position, so it is impossible to achieve a stable constructive or destructive superposition effect on existing holograms. This avoids repeated reinforcement of holograms and effectively suppresses the formation of dark fringes. The diffraction efficiency of the resulting volume holographic optical element has high uniformity in spatial distribution. When illuminated by a beam along the reference light direction, it can reproduce a signal light with uniform spatial brightness distribution, which is beneficial to improving the imaging quality, uniformity and subsequent application performance of the volume holographic optical element.

[0016] If the modulation device is a diffuser, diffuser film, or microlens array, it is preferably connected to a moving mechanism. This moving mechanism drives the diffuser, diffuser film, or microlens array to perform random displacement, facilitating convenient control of this random displacement. The moving mechanism is preferably an electric translation stage or an electric lifting stage, allowing for electric control of the modulation device's random displacement, avoiding manual adjustment, and improving the modulation efficiency of the device.

[0017] The light source assembly preferably includes a laser source and a beam splitter. The laser source emits laser light, and the beam splitter divides the laser light into a signal beam and a reference beam. Since the signal beam and the reference beam originate from the same laser source, it is easy to ensure that the signal beam and the reference beam meet the basic conditions for interference, namely, having the same frequency, the same vibration direction, and a constant phase difference in time during each exposure period.

[0018] The light source assembly can also include two laser sources, one providing signal light and the other providing reference light. Since the signal light and reference light are provided by different laser sources, it is convenient to independently control the transmission of the signal light and reference light.

[0019] The laser source is preferably equipped with a laser shutter to precisely control the laser's on and off times, achieving safety protection and precision machining.

[0020] Thirdly, a volume holographic optical element is proposed. This volume holographic optical element is fabricated using the above-mentioned homogenization exposure method or homogenization exposure device. Its diffraction efficiency has high uniformity in spatial distribution. When illuminated by a beam along the reference light direction, it can reproduce signal light with uniform spatial brightness distribution, which is beneficial to improving imaging quality, uniformity and subsequent application performance.

[0021] Compared with existing technologies, this solution has the following advantages: By combining segmented exposure technology with the random adjustment mechanism of the modulation device, and introducing wavefront and phase-independent signal light fields at the critical moment of each hologram formation, multi-segment exposure (each exposure lasting one unit time T) is achieved. u The holograms formed by this method do not have uniformity in spatial position, so it is impossible to achieve a stable constructive or destructive superposition effect on existing holograms. This avoids repeated reinforcement of holograms and effectively suppresses the formation of dark fringes. The diffraction efficiency of the resulting volume holographic optical element has high uniformity in spatial distribution. When illuminated by a beam along the reference light direction, it can reproduce a signal light with uniform spatial brightness distribution, which is beneficial to improving the imaging quality, uniformity and subsequent application performance of the volume holographic optical element. Attached Figure Description

[0022] The accompanying drawings are for illustrative purposes only and should not be construed as limiting the scope of this solution. To better illustrate the solution, some components in the drawings may be omitted, enlarged, or reduced, and do not represent the actual dimensions of the product. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0023] Figure 1 This is a schematic diagram of the traditional exposure optical path for a solid holographic optical element.

[0024] Figure 2 This is a schematic diagram of the homogenization exposure optical path for a volumetric holographic optical element.

[0025] Figure 3 yes Figure 2 Wavefront phase diagram of the middle beam L3.

[0026] Figure 4 yes Figure 2 Wavefront phase diagram of the middle beam L3'.

[0027] Figure 5 This is a flowchart of the uniform exposure process for a solid holographic optical element.

[0028] Figure 6 This is a schematic diagram of the structure of a homogenization exposure device for solid holographic optical elements.

[0029] Figure 7 This is a schematic diagram of the structure of another holographic optical element homogenization exposure device.

[0030] Explanation of reference numerals in the attached figures: photosensitive material 110, transparent substrate 120, modulation device 210, light source assembly 220, laser light source 221, beam splitter 222. Detailed Implementation

[0031] To enable those skilled in the art to better understand this solution, the following detailed description is provided in conjunction with specific embodiments.

[0032] Figures 2 to 7 Possible volume holographic optical elements and their homogenization exposure methods and apparatus are illustrated. For example... Figure 2 , Figure 6 and Figure 7 As shown, the volume holographic optical element is formed at least of a photosensitive material 110. In addition to the photosensitive material 110, the volume holographic optical element may also include a transparent substrate 120 on which the photosensitive material 110 is formed. A homogenization exposure method and apparatus are used to expose the photosensitive material 110 to complete the fabrication of the volume holographic optical element, enabling the volume holographic optical element to possess diffraction capability and uniform spatial distribution of diffraction efficiency, brightness, etc.

[0033] like Figures 2 to 4As shown, the homogenization exposure method follows the principle of two-beam interference, using reference light (such as...) that is separately irradiated onto the photosensitive material 110 and interferes on the photosensitive material 110. Figure 2 The light beam L1) and signal light (such as Figure 2 The light beams L3 and L3' expose the photosensitive material 110, forming a hologram in the photosensitive material 110, ultimately forming a volume holographic optical element. Specifically, in this method, the photosensitive material 110 is placed to one side of the modulation device 210, and the signal light is irradiated onto the photosensitive material 110 via the modulation device 210. After the exposure begins, the light beams are emitted at intervals T. u By adjusting the modulation device 210, the wavefront shape and spatial phase distribution of the signal light are randomly changed. Wherein, the unit time T... u The time it takes for the signal light and reference light to interfere and record in the photosensitive material 110 to form the first hologram mainly depends on the exposure threshold energy of the photosensitive material. In practical engineering, it is usually combined with real-time diffraction efficiency monitoring, or by designing experimental schemes with different exposure energy thresholds, to more accurately determine the unit time T. u The value of .

[0034] Because the wavefront morphology and spatial phase distribution of the signal light change randomly after each hologram is formed, at a certain moment (e.g., t=T)... u 2T u 3T u The phase difference distribution between the diffracted light of the old hologram formed by the current signal light and the diffracted light is different from the phase difference distribution between the diffracted light and the original signal light (i.e., the signal light that interferes to form the old hologram). The new hologram formed by the diffracted light and the current signal light no longer has the same spatial position as the old hologram. Therefore, the new hologram cannot achieve a stable coherent (constructive or destructive) superposition effect on the old hologram. Only incoherent energy superposition is performed, thereby avoiding the repeated enhancement of the old hologram and effectively suppressing the formation of dark fringes. The diffraction efficiency of the resulting volume holographic optical element has high uniformity in spatial distribution. When illuminated by a beam along the reference light direction, it can reproduce a signal light with uniform spatial brightness distribution, which is beneficial to improving the imaging quality, uniformity and subsequent application performance of the volume holographic optical element.

[0035] The modulation device 210 can be a displaceable diffuser, diffuser film, or microlens array, or a spatial light modulator (SLM) capable of loading a phase-type hologram. The diffuser, diffuser film, and microlens array have uneven surfaces; when the signal light passes through, different parts of the beam experience different optical path lengths, causing the wavefront and phase of the emitted light to be scattered. After the diffuser, diffuser film, and microlens array are displaced, the wavefront shape, spatial phase distribution, and local incident angle of the signal light will change, for example, from... Figure 2The beam L3 in the image is transformed into beam L3', thereby achieving spatial averaging of the interference modulation term in the photosensitive material 110. The spatial light modulator can load a pre-designed phase-type hologram. Each loaded phase-type hologram is independent of the others. The phase-type hologram possesses the phase characteristics of an equivalent point source or holographic lens, allowing point-by-point control of the wavefront shape and spatial phase distribution of the signal light. After switching phase-type holograms, the wavefront shape and spatial phase distribution of the signal light will change, for example, from... Figure 2 The beam L3 in the signal light is transformed into beam L3', thereby achieving decorrelation modulation between the signal light before and after modulation. Therefore, if the modulator 210 is a displaceable diffuser, diffuser film, or microlens array, the modulator 210 can be adjusted by randomly displacing the diffuser, diffuser film, or microlens array; if the modulator 210 is a spatial light modulator capable of loading a phase-type hologram, the modulator 210 can be adjusted by switching the phase-type hologram loaded by the spatial light modulator.

[0036] During the exposure of the photosensitive material 110 by the reference light and the signal light, any hologram (including the first hologram) can be rapidly formed in a very short time. The time taken for the hologram to form from scratch (i.e., the unit time T) is... u ) often only accounts for T of the total exposure time t Part of it, namely the total duration T of the entire exposure process. t Typically, it is the unit of time T. u The value should be n times the unit exposure time T, where n is preferably an integer, to ensure that the entire exposure process can be based on the unit exposure time T. u The exposure is divided into n complete exposure periods, each of which can form a complete hologram. In other words, the total exposure time T is... t Satisfy: T t =n T u n is an integer and n≥2.

[0037] Throughout the exposure process, the signal light and reference light can continuously illuminate the photosensitive material 110, or they can be temporarily stopped illuminating the photosensitive material 110 while adjusting the modulation device 210. If both the signal light and reference light continuously illuminate the photosensitive material 110, the speed of adjusting the modulation device 210 must be fast enough to ensure that the signal light interfering to form a hologram does not interfere with the diffracted light of that hologram. If at least one of the signal light and reference light temporarily stops illuminating the photosensitive material 110 while adjusting the modulation device 210, i.e., at t=T... u 2T u 3T u…Illumination of the photosensitive material 110 is stopped at certain times, and irradiation resumes only after the modulation device 210 has been adjusted. This allows sufficient time for the modulation device 210 to be adjusted, significantly reducing the difficulty of adjustment control. The signal light and reference light can be provided by a laser source 221, which can be controlled by a laser shutter. Therefore, the laser source 221 or the laser shutter (if any) can be turned on at the beginning of each exposure period and turned off at the end of each exposure period. The modulation device 210 is adjusted before the start of each exposure period, starting at least from the second exposure period. For example… Figure 5 As shown, the exposure process may include the following steps: S101. Turn on the laser light source 221 or the laser shutter so that the signal light and reference light illuminate the photosensitive material 110 for a duration of T. u A hologram is formed by interference on the photosensitive material 110; S102. Turn off the laser light source 221 or the laser shutter, so that at least one of the signal light and the reference light stops illuminating the photosensitive material 110. S103. Adjust the modulator 210 to cause random changes in the wavefront shape and spatial phase distribution of the signal light transmitted through the modulator 210. S104. Determine whether the actual exposure time has reached the preset exposure time T. t If yes, then end the exposure; if no, then repeat steps S101 to S104.

[0038] The above-described homogenization exposure method can be operated on a homogenization exposure apparatus. For example... Figures 6 to 7 As shown, the homogenization exposure apparatus includes a light source assembly 220 and a modulation device 210. The light source assembly 220 provides signal light and reference light, which respectively illuminate the photosensitive material 110 and interfere on the photosensitive material 110. The modulation device 210 modulates the signal light so that the wavefront shape and spatial phase distribution of the signal light are at the critical moment for hologram formation (i.e., t=T). u 2T u 3T u …time) undergoes random changes.

[0039] The modulator 210 can be a diffuser, a diffuser film, or a microlens array, or it can be a spatial light modulator. If it is the former, the modulator 210 can be mounted on a moving mechanism (not shown) to achieve random displacement. The moving mechanism can be an electric translation stage, an electric lifting stage, or other electric moving mechanism to electrically control the random displacement of the modulator 210, avoiding manual adjustment and improving the adjustment efficiency of the modulator 210.

[0040] The light source assembly 220 is equipped with a laser light source 221. The signal light and reference light can be provided by the same laser light source 221, or they can be provided by different laser light sources 221. Figure 6 As shown, if the signal light and reference light are provided by the same laser source 221, the light source assembly 220 is also equipped with a beam splitter 222, which splits the laser emitted from the laser source 221 into signal light and reference light. Figure 7 As shown, if the signal light and reference light are provided by different laser sources 221, the light source assembly 220 is configured with two laser sources 221, namely a first laser source and a second laser source. The first laser source provides the signal light, and the second laser source provides the reference light. For the former, since the signal light and reference light come from the same laser source 221, it is easy to ensure that the signal light and reference light meet the basic conditions for interference, namely, the same frequency, the same vibration direction, and a constant phase difference in time during any exposure period. For the latter, since the signal light and reference light are provided by different laser sources 221, it is convenient to independently control the transmission of the signal light and reference light.

[0041] Regardless of the number of laser light sources 221 configured in the light source assembly 220, each laser light source 221 can be equipped with a laser shutter (not shown). A laser shutter is an electrically controlled optical shutter device that blocks or allows a laser beam to pass through mechanically or electronically, thereby controlling the laser transmission in the optical path. It can precisely control the opening and closing time of the laser to achieve safety protection and precision machining.

[0042] Obviously, the above embodiments of this solution are merely examples for clearly illustrating this solution, and are not intended to limit the implementation of this solution. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this solution should be included within the scope of protection of the claims of this solution.

Claims

1. A method for homogenizing exposure of a volume holographic optical element, characterized in that, The method includes: placing a photosensitive material on one side of a modulation device, illuminating the photosensitive material with signal light through the modulation device, causing interference with reference light illuminating the photosensitive material, and exposing the photosensitive material; after the exposure begins, exposing the photosensitive material at unit time T intervals. u The modulation device is adjusted to cause random changes in the wavefront morphology and spatial phase distribution of the signal light; the unit time T u The time taken for the signal light and reference light to interfere and record in the photosensitive material to form the first hologram; the modulation device is a diffuser, a diffuser film, a microlens array, or a spatial light modulator loaded with a phase-type hologram, and during adjustment, the diffuser, diffuser film, or microlens array is randomly displaced or the phase-type hologram loaded by the spatial light modulator is switched.

2. The homogenization exposure method for the volume holographic optical element according to claim 1, characterized in that, The total exposure time T t Satisfy: T t =n T u Where n is an integer and n≥2.

3. The homogenization exposure method for the volume holographic optical element according to claim 2, characterized in that, The signal light and / or the reference light are provided by a laser source or controlled by a laser shutter, and the entire exposure process is performed according to the unit time T. u The exposure is divided into n exposure periods. At the beginning of each exposure period, the laser light source or the laser shutter is turned on, and at the end of each exposure period, the laser light source or the laser shutter is turned off. The modulation device is adjusted before the start of each exposure period, starting at least from the second exposure period.

4. A homogenization exposure apparatus for a volume holographic optical element, characterized in that, The apparatus includes a light source assembly and a modulation device, the light source assembly providing signal light and reference light, the modulation device modulating the signal light, and the apparatus operates the homogenization exposure method of the volume holographic optical element according to any one of claims 1 to 3.

5. The homogenization exposure apparatus for a volume holographic optical element according to claim 4, characterized in that, The modulation device is a diffuser, a diffuser film, or a microlens array. The diffuser, diffuser film, or microlens array is connected to a moving mechanism, which is used to drive the diffuser, diffuser film, or microlens array to perform random displacement.

6. The homogenization exposure apparatus for a volume holographic optical element according to claim 5, characterized in that, The moving mechanism is an electric translation platform or an electric lifting platform.

7. The homogenization exposure apparatus for a volume holographic optical element according to claim 4, characterized in that, The light source assembly includes a laser source and a beam splitter. The laser source emits laser light, and the beam splitter divides the laser light into signal light and reference light.

8. The homogenization exposure apparatus for volume holographic optical elements according to claim 4, characterized in that, The light source assembly includes two laser light sources, one of which provides signal light and the other provides reference light.

9. The homogenization exposure apparatus for a volume holographic optical element according to claim 7 or 8, characterized in that, The laser source is equipped with a laser shutter.

10. A volume holographic optical element, characterized in that, The volume holographic optical element is manufactured by the homogenization exposure method of the volume holographic optical element according to any one of claims 1 to 3 or the homogenization exposure apparatus of the volume holographic optical element according to any one of claims 4 to 9.

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