DOE-based beam expander illumination system for splicing holographic displays

Abstract

This invention relates to a DOE-based beam-expanding illumination system for splicing holographic displays, comprising: a laser, a diffractive optical element, a rear lens, and a collimating lens arranged sequentially along the optical path; the laser provides a laser source; the diffractive optical element modulates the laser source wavefront, dividing and shaping the laser source into an M×N two-dimensional array of beam-splitting spots; the rear lens converges and controls the two-dimensional array of beam-splitting spots; the collimating lens collimates the converged beam-splitting spots to obtain M×N collimated illumination spots; and these spots are respectively illuminated onto multiple spatial light modulators arranged in an M×N array, and then synthesized into a complete spliced ​​hologram. This invention employs large-pixel diffractive optical elements with feature sizes in the micrometer range, combined with a lens group, to achieve efficient conversion and beam expansion from a single beam to multiple uniform illumination spots. The system is designed by controlling the output spot intensity distribution, divergence angle, and light energy utilization.

Description

Technical Field

[0001] This invention relates to the fields of holographic display and diffractive optics, and in particular to an illumination system based on diffractive optical element (DOE) beam expansion for splicing holographic displays. Background Technology

[0002] Holographic displays, achieving true 3D vision through wavefront modulation, are considered a core direction for next-generation display technology. Their display area and field of view directly depend on the spatial bandwidth product (SBP) of the spatial light modulator (SLM), i.e., the number of pixels (Nx, Ny). Multi-SLM optical splicing can effectively increase the total number of pixels, thereby expanding the field of view, and is an important path to drive the technology's commercialization. However, traditional Galilean beam expanders require large-aperture optical elements to achieve uniform illumination across the entire target surface, resulting in a lengthy and bulky system. Furthermore, the intensity distribution characteristics of Gaussian beams cause significant attenuation of light intensity received by edge SLM units, severely affecting the uniformity of display brightness.

[0003] DOE beam splitters are used in multi-SLM lighting systems, enabling system weight reduction. A single, thin DOE can replace the lengthy optical path composed of multiple lenses and mirrors in traditional beam expanders, significantly reducing system size and weight. Furthermore, based on the grating principle, DOE beam splitters can efficiently and uniformly divide a single incident laser beam into N outgoing beams, with each beam perfectly maintaining the diameter, divergence angle, and wavefront characteristics of the original beam, only changing its direction. Its beam splitting uniformity is extremely high, with energy differences controllable within 1%. The materials are mostly optical glass or quartz, offering excellent stability. These characteristics make it an ideal choice for realizing compact, lightweight multi-SLM lighting systems.

[0004] It is worth noting that the wavefront modulation capability of a DOE is closely related to its feature size. According to the Nyquist sampling theorem, a smaller pixel size (pitch) means that a higher spatial frequency (fmax) can be tuned, thus theoretically enabling more precise and powerful optical manipulation. However, on the manufacturing side, pursuing nanometer-scale pixels (e.g., optimizing the size from the hundreds of nanometers to the single-digit nanometers) means employing extreme processes such as electron beam direct writing, facing the challenge of a sharp increase in manufacturing costs. Therefore, in system design, a forward-looking balance must be sought between optical performance requirements and manufacturing costs. Summary of the Invention

[0005] The purpose of this invention is to propose a DOE-based beam expander illumination system for splicing holographic displays, which has a lightweight system structure and low manufacturing cost, overcoming the disadvantages associated with the prior art described above.

[0006] To achieve the above objectives, the present invention provides the following solution: A DOE-based beam expander illumination system for splicing holographic displays includes: a laser, and diffractive optical elements, a rear lens, and a collimating lens arranged sequentially along the optical path; The laser is used to provide a laser source; The diffractive optical element is used to perform wavefront modulation on the laser source, dividing and shaping the laser source into an M×N two-dimensional array of beam-splitting spots. The rear lens is used to converge and control the beam splitting spot of the two-dimensional array; The collimating lens is used to collimate the beam-splitting spot after convergence modulation to obtain M×N collimated illumination spots; the M×N collimated illumination spots are used to respectively illuminate multiple spatial light modulators arranged in an M×N array.

[0007] Optionally, the laser is a single-longitudinal-mode solid-state laser that outputs green laser light of a preset wavelength. The laser has a built-in collimation mechanism and emits a beam with a preset beam waist radius and full width at half maximum (FWHM).

[0008] Optionally, the diffractive optical element uses fused silica as the substrate material and has a feature pixel size of 2 to 10 micrometers.

[0009] Optionally, the aperture of the rear lens is larger than the aperture of the diffractive optical element.

[0010] Optionally, the transmittance function of the diffractive optical element is: T'=t exp(j Φ); Where t is the transmittance function of the diffractive optical element, Φ is the phase transmittance function, T' is the transmittance function of the diffractive optical element, and j is the imaginary unit. By designing the pixel vector height of the diffractive optical element, the phase transmittance function Φ is changed, thereby modulating the light.

[0011] Optionally, the pixel vector of the diffractive optical element is designed as follows: The output spot distribution, spot spacing, original system design, window efficiency, and 0th-order diffraction rate of the diffractive optical element are constrained, and the pixel vector of the diffractive optical element is designed by optimizing it through an iterative Fourier transform algorithm. The ideal amplitude effect is achieved through a specific phase distribution φ(ξ, η), where ξ is the horizontal coordinate in the spatial domain and η is the vertical coordinate in the spatial domain.

[0012] Optionally, the optimization process using the iterative Fourier transform algorithm is performed iteratively between the DOE plane and the target illumination plane, wherein the target light field on the target plane is set to contain M×N uniformly distributed rectangular bright spots, and the remaining area is a dark field.

[0013] Optionally, the constraints of the Fourier transform algorithm include: maximizing the overall diffraction efficiency and suppressing the zero-order diffracted light intensity to below 0.1%.

[0014] Optionally, a spatial filter is provided after the collimating lens to filter out high-frequency stray light.

[0015] Optionally, an aperture is also provided in the optical path between the laser and the diffractive optical element, the aperture being used to limit and filter stray light.

[0016] The beneficial effects of this invention are as follows: In this invention, the maximum lens aperture is 80mm, reduced to half that of conventional technology. The beam-splitting DOE used, in conjunction with the focusing lens, enables the emission of 4... Four light spots with a fixed spacing were generated. The spot size increased continuously with increasing propagation distance, while the edge spacing remained essentially constant. This DOE achieved a window efficiency of 92.587%, a surface-to-noise ratio (SNR) of 46.406 dB, and a relative zero-order intensity of only 8.3449e-32, essentially completely suppressing zero-order diffraction. After propagation in free space over 1500 mm and collimation using a collimating lens with f2 = 1500 mm, the spatial angular spectrum of the resulting 16 outgoing light spots ranged within 10000 m. -1 Within the range. Corresponding to the divergence angle: λ=532nm; k0=2 pi / λ = 1.181 10 7 (m) -1 )get This can be considered collimated light. After passing through an 80mm collimating lens, the resulting light has a window efficiency greater than 60% and an SNR greater than 14dB. Compared with traditional beam expansion methods, the energy utilization rate is improved, and a 2.49µm feature size DOE is used to control a large spatial frequency range.

[0017] The DOE-based lighting system of the present invention is easy to manufacture using standard manufacturing techniques and therefore provides more efficient and lightweight lighting at a reasonable cost. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1This is a schematic diagram of the diffractive optical element structure according to an embodiment of the present invention; wherein, (a) is the initial structure of the designed diffractive optical element, and (b) is... Figure 1 (a) The DOE periodic pattern after IFT optimization; Figure 2 Embodiments of the present invention Figure 1 The periodic pattern designed in (b) is extended to 5mm. 5mm DOE bit depth map; Figure 3 This is a schematic diagram of the optical path structure according to an embodiment of the present invention; Figure 4 Embodiments of the present invention Figure 1 A schematic diagram of beam splitting at the focal length of the focusing lens obtained from the initial structural design; where (a) is... Figure 1 The initial structural design yields the light intensity distribution at the focal length of the focusing lens, as shown in (b). Figure 1 The spatial spectrum distribution of light splitting at the focal length of the focusing lens, obtained from the initial structural design; Figure 5 This is a schematic diagram of the optimized target light intensity distribution according to an embodiment of the present invention; Figure 6 The light intensity and spatial spectrum distribution obtained from the optical path calculation in this embodiment of the invention; Figure 7 This is a simulated light intensity distribution on an SLM array based on a conventional scheme in an embodiment of the present invention. Detailed Implementation

[0020] The technical solutions of the embodiments 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 scope of protection of the present invention.

[0021] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0022] like Figure 3 As shown, this embodiment proposes a DOE-based beam expander illumination system for splicing holographic displays, including: a laser, and diffractive optical elements, a rear lens, and a collimating lens arranged sequentially along the optical path; Laser, used to provide a laser source; Diffractive optical elements are used to modulate the wavefront of a laser source, dividing and shaping the laser source into an M×N two-dimensional array of beam-splitting spots; The rear lens is used to converge and control the beam splitting spot of the two-dimensional array; The collimating lens is used to collimate the beam after it has been focused and modulated, so as to obtain M×N collimated illumination spots. The M×N collimated illumination spots are used to illuminate the multiple spatial light modulators arranged in an M×N array, respectively.

[0023] Specifically, the illumination system in this embodiment includes a Gaussian light source, an aperture stop, a DOE designed according to an 1f device and optimized by the IFT algorithm, a corresponding focusing lens, a second aperture stop, and a collimating lens. After the light source output passes through the aperture stop, DOE, and first focusing lens, it is split into beams. After free-space propagation and collimation by the second collimating lens, mutually parallel and collimated light sources are obtained for illuminating the SLM; the aperture stop is used to limit and filter stray light. Traditional beam-expanding illumination systems typically expand the light spot into a larger-aperture Gaussian spot, requiring a large-aperture collimating lens, which is bulky. Furthermore, the light is projected onto 16 spatial light modulators after passing through the beam splitter, resulting in significant energy waste and varying illumination intensity for different spatial light modulators due to the intensity distribution of the Gaussian spot. According to this invention, the DOE beam-splitter beam-expanding and homogenizing illumination system with an 1f design can reduce the size of the illumination system for spliced ​​SLM holographic displays and the waste of light energy, while improving the uniformity of light energy illuminating different SLMs. Furthermore, this design improves the modulation of the DOE to a wider range of spatial frequencies by using a rear-mounted telephoto collimating lens, reducing the requirements for the DOE's feature size, thereby achieving low-cost and high-efficiency lighting.

[0024] Furthermore, the laser is a single-longitudinal-mode solid-state laser that outputs green laser light of a preset wavelength. The laser has a built-in collimation mechanism to emit a beam with a preset beam waist radius and full width at half maximum (FWHM). Specifically, in this embodiment, the output wavelength is 532 nm, and the built-in collimation mechanism results in an output beam with a beam waist radius and full width at half maximum (FWHM) of 2.2 mm.

[0025] Diffractive optical elements (DOEs) are a class of components that modulate light waves based on the principle of light diffraction, rather than refraction or reflection. A DOE is typically a flat or nearly flat optical substrate with discontinuous, multi-level steps or continuously undulating micro- and nano-scale fine relief structures etched or molded onto its surface. These structures modulate the phase of the incident light wave, thereby producing a specific light intensity distribution in the far field. This is completely different from traditional lenses (spherical or aspherical continuous curved surfaces).

[0026] Furthermore, in this embodiment, the diffractive optical element uses fused silica as the substrate material, and the feature pixel size is 2 to 10 micrometers. Specifically, in this embodiment, the substrate material of the diffractive optical element is fused silica, its aperture is 5mm × 5mm, and the feature pixel size of the diffractive optical element is 2 to 5 micrometers; the free space propagation distance of the beam-splitting spot is 1500mm, and even more preferably, the feature pixel size of the diffractive optical element is 2.49 micrometers.

[0027] Furthermore, the aperture of the rear lens is larger than that of the diffractive optical element. The focal length f1 of the rear lens is 150mm. The focal length f2 of the collimating lens is 1500mm, and the aperture is 80mm.

[0028] Furthermore, the M×N array is preferably a 4×4 array, corresponding to 16 spatial light modulators.

[0029] Furthermore, the phase distribution of the diffractive optical element is optimized using an iterative Fourier transform algorithm to achieve the desired beam splitting and shaping effects.

[0030] The goals of optimization design include: (a) Sixteen separate illumination spots are formed on the target surface, with the intensity uniformity within each spot being greater than 80%; (b) Suppress the zero-order diffraction light so that its relative intensity is less than 0.1%; (c) Improve the overall diffraction efficiency to be greater than 50%.

[0031] The overall diffraction efficiency of the diffractive optical element is optimized to 55%-65%, and the relative intensity of the zero-order diffracted light is suppressed to less than 0.01%.

[0032] Furthermore, a spatial filter is also provided in the optical path between the collimating lens and the spatial light modulator array to filter out high-frequency stray light and further improve the quality of the illumination spot.

[0033] More specifically, SLMs can dynamically control the optical path of light waves by dynamically controlling the long axis direction of liquid crystal elements on the target surface, thereby achieving dynamic holographic display. Typical SLM-based holographic display systems use a single SLM to achieve small-area displays, which is suitable for near-eye displays. However, in the case of naked-eye holographic displays, the resolution of SLM-based holographic displays depends on the spatial bandwidth product of the system, i.e., the spatial bandwidth product of the SLMs. For SLMs, Where ax is the side length of the target surface and p is the pixel value. This represents the highest spatial frequency that an SLM can modulate. Therefore, it can be simplified to: SBP = The effectiveness depends entirely on the number of pixels in the SLM (Spatial Laser Mediar). Therefore, by stitching together multiple SLMs and optimizing the hologram on each SLM, the spatial bandwidth product of the system can be fundamentally improved, thereby achieving a breakthrough in the spatial resolution and field of view of naked-eye holographic displays. Because the entire target surface needs to be covered, the aperture of the illumination source required for stitching SLMs is larger. The current Galilean beam expander system expands the light spot emitted from the built-in collimated solid-state laser by nearly a hundred times. This results in a bulky beam expander system, and the gaps between different SLMs also cause low light energy utilization. Furthermore, since the emitted light spot after beam expansion is still a Gaussian spot, its intensity exhibits a normal distribution in one dimension. Therefore, the illumination intensity of the SLM on the optical axis will be weaker, leading to uneven distribution of high-frequency and low-frequency components in the final stitched holographic display pattern. This invention innovatively introduces diffractive optical elements to improve the illumination system. The beam splitter based on the diffractive optical elements has a high power threshold, which can directly and efficiently split a single beam of light into multiple beams, weakening the light intensity while keeping the wavefront unchanged. It has accurate angle separation, good consistency between energy and wavefront after beam splitting, and the energy difference between beams is <1%. It is also extremely thin and light, and the position of the output spot can be designed according to the target position of the SLM. While providing precise illumination, it avoids the influence of the normal distribution of Gaussian light intensity, thus achieving the purpose of uniform light.

[0034] The transmittance function of a diffractive optical element is the core of the element. ’ =t exp(j Φ), where t is the transmittance function of the diffractive optical element (DOE), and Φ is the phase transmittance function, which depends on the pixel height of the DOE. By designing the pixel height of the diffractive optical element, Φ can be changed, thus modulating the light. During the design process, constraints are imposed on the outgoing light spot distribution, spot spacing, original system design, window efficiency, and 0th-order diffraction rate of the diffractive optical element. The pixel height of the DOE is then optimized using an iterative Fourier transform algorithm. A specific phase distribution φ(ξ, η) is used to achieve the ideal amplitude effect, thereby realizing efficient, uniform, and low-noise multipath illumination. The constraints on the outgoing light spot distribution and spot spacing are: setting an ideal light intensity distribution in the optimization target; window efficiency and 0th-order diffraction efficiency are: optimizing the phase distribution using IFTA (Gerchberg-Saxton et al.), minimizing the 0th-order energy, and maximizing the uniformity and efficiency of the target diffraction order.

[0035] The phase distribution φ(ξ, η) of the DOE is optimized using an iterative Fourier transform algorithm. The optimization process iterates between the DOE plane and the target illumination plane, where the target light field U_target(x, y) on the target plane is set to contain M×N uniformly distributed rectangular bright spots, with the remaining area being a dark field. The algorithm's constraints include maximizing the overall diffraction efficiency and suppressing the zero-order diffraction intensity to below 0.1%.

[0036] The simulation system in this embodiment includes: a solid-state laser source with a wavelength of 532nm, a divergence angle of 0.5mrad, and built-in collimation; a beam splitter DOE with a pixel size of 2.49um and an aperture of 5mm; a focusing lens with a focal length of 150mm; and a collimating lens with a focal length of 1500mm and an aperture of 80mm. The focusing lens and the collimating lens have a free propagation distance of 1500mm.

[0037] The key metrics to be calculated in the simulation are: Overall diffraction efficiency: the ratio of total light energy irradiated to the incident light energy in 16 target regions.

[0038] Zero-order diffraction efficiency: the proportion of stray light intensity that is not split at the center.

[0039] Light spot uniformity: the ratio of the maximum to the minimum light intensity within each illumination spot.

[0040] Crosstalk: The interval between light spots.

[0041] Diffractive optical elements with a specific phase distribution φ(ξ, η) can be manufactured for small-batch testing using grayscale photolithography direct writing or for mass production using multi-mask overlay binary etching. Fabrication on a fused silica substrate ensures a high power threshold. Its 2.49-micron feature size and 8th-order phase modulation depth ensure the feasibility of the designed performance while controlling manufacturing costs.

[0042] In this embodiment, the diffractive optical element structure is as follows: Figure 1 As shown, Figure 1 (a) shows the initial structure of the diffractive optical element designed in this embodiment; Figure 1 (b) shows the vector distribution S(ξ, η) of the optimized DOE, corresponding to the modulation phase (0 to 2π). Figure 2 Showing for Figure 1 (b) is periodically extended and combined to form a sag distribution map that can be produced using DOE. Figure 4 This embodiment is illustrated. Figure 1 A schematic diagram of beam splitting at the focal length of the focusing lens, obtained from the initial structural design; where... Figure 4 (a) shows the light intensity distribution obtained by beam splitting at the focal length of the focusing lens based on the initial structural design. Figure 4 (b) is the spatial spectrum distribution obtained by beam splitting at the focal length of the focusing lens based on the initial structural design; Figure 5 A schematic diagram illustrating the optimized target light intensity distribution of this embodiment is shown; Figure 6 This embodiment demonstrates the light intensity and spatial spectral distribution obtained from the optical path calculation; Figure 7 The simulated light intensity distribution of a conventional method illuminating an SLM array is shown.

[0043] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A DOE-based beam expander illumination system for splicing holographic displays, characterized in that, include: A laser, and diffractive optical elements, a rear lens, and a collimating lens arranged sequentially along the optical path; The laser is used to provide a laser source; The diffractive optical element is used to perform wavefront modulation on the laser source, dividing and shaping the laser source into an M×N two-dimensional array of beam-splitting spots. The rear lens is used to converge and control the beam splitting spot of the two-dimensional array; The collimating lens is used to collimate the beam-splitting spot after convergence modulation to obtain M×N collimated illumination spots; the M×N collimated illumination spots are used to respectively illuminate multiple spatial light modulators arranged in an M×N array, and synthesize them into a complete stitched hologram.

2. The DOE-based beam expander illumination system for splicing holographic displays according to claim 1, characterized in that, The laser is a single-longitudinal-mode solid-state laser that outputs green laser light of a preset wavelength. The laser has a built-in collimation mechanism and emits a beam with a preset beam waist radius and full width at half maximum (FWHM).

3. The DOE-based beam expander illumination system for splicing holographic displays according to claim 1, characterized in that, The diffractive optical element uses fused silica as the substrate material and has a feature pixel size of 2 to 10 micrometers.

4. The DOE-based beam expander illumination system for splicing holographic displays according to claim 1, characterized in that, The aperture of the rear lens is larger than the aperture of the diffractive optical element.

5. The DOE-based beam expander illumination system for splicing holographic displays according to claim 1, characterized in that, The transmittance function of the diffractive optical element is: T’=t exp(j Φ); Where t is the transmittance function of the diffractive optical element, Φ is the phase transmittance function, T' is the transmittance function of the diffractive optical element, and j is the imaginary unit. By designing the pixel vector height of the diffractive optical element, the phase transmittance function Φ is changed, thereby modulating the light.

6. The DOE-based beam expander illumination system for splicing holographic displays according to claim 5, characterized in that, The pixel vector height for designing diffractive optical elements includes: The output spot distribution, spot spacing, original system design, window efficiency, and 0th-order diffraction rate of the diffractive optical element are constrained, and the pixel vector of the diffractive optical element is designed by optimizing it through an iterative Fourier transform algorithm. The ideal amplitude effect is achieved through a specific phase distribution φ(ξ,η); where ξ is the horizontal coordinate in the spatial domain and η is the vertical coordinate in the spatial domain.

7. The DOE-based beam expander illumination system for splicing holographic displays according to claim 6, characterized in that, The optimization process using the iterative Fourier transform algorithm is performed iteratively between the DOE plane and the target illumination plane. The target light field on the target plane is set to contain M×N uniformly distributed rectangular bright spots, with the remaining area being a dark field.

8. The DOE-based beam expander illumination system for splicing holographic displays according to claim 6, characterized in that, The constraints of the Fourier transform algorithm include: maximizing the overall diffraction efficiency and suppressing the zero-order diffracted light intensity to below 0.1%.

9. The DOE-based beam expander illumination system for splicing holographic displays according to claim 1, characterized in that, A spatial filter is also provided after the collimating lens to filter out high-frequency stray light.

10. The DOE-based beam expander illumination system for splicing holographic displays according to claim 1, characterized in that, An aperture is also provided in the optical path between the laser and the diffractive optical element, and the aperture is used to limit and filter out stray light.

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