Holographic display system using rotating phase mask and optimization method thereof

Abstract

Disclosed are a holographic display system using a rotating phase mask and an optimization method thereof. The holographic display system comprises: a light source unit for outputting laser light; a spatial light modulator for phase-modulating incident light; and a rotating phase mask for rotating a mask having a pattern adding a phase of a predetermined size, wherein the rotating phase mask is either located between the light source unit and the spatial light modulator or at the output terminal of the spatial light modulator.

Description

Holographic display system using rotational phase mask and optimization method

[0001] An embodiment of the present invention relates to a holographic display system, and more specifically, to a holographic display system using a rotational phase mask and a method for optimizing the system.

[0002] In a typical holographic display system, light emitted from a light source passes through or reflects a phase or amplitude spatial light modulator (SLM) to be restored into an image at a specific location. The phase or amplitude information projected onto the SLM is called a computer-generated hologram (CGH). A representative method for calculating the CGH of a phase SLM is Stochastic Gradient Descent (SGD). SGD is a method that optimizes the phase values ​​on the SLM through backpropagation (e.g., gradient descent) to minimize the loss function between the target image to be projected and the reconstructed holographic image.

[0003] Holographic displays primarily use coherent light sources, such as lasers, because they utilize the phenomenon of light interference. However, when coherent light sources are used within optical systems, speckle noise inevitably occurs. In other words, even if a high-quality CGH is obtained by performing SGD in a simulation, speckle noise caused by the laser is added to the CGH in actual implementation.

[0004] Speckle noise is densely spread, granular point noise that not only significantly degrades image quality but also greatly impairs the user's 3D display viewing experience. To reduce speckle noise, holographic display systems utilizing incoherent light sources, such as LEDs, or rotating diffusers have been proposed. For example, in the field of laser projection, a method is proposed to mitigate speckle noise by introducing a rotating diffuser that rotates along an axis parallel to the optical axis to visual-average the laser light, thereby lowering the degree of coherency. However, if a rotating diffuser is applied directly to a holographic display, the light that should be incident parallel to the pixels of the SLM is incident as diffuse reflection, resulting in a decrease in the quality of the reconstructed image and a significant reduction in resolution. Furthermore, while these conventional methods reduce speckle noise, they simultaneously suffer from the problem of reducing resolution. In addition, there is a method to reduce speckle noise by using time multiplexing techniques to display multiple holographic images in succession. However, this method has limitations in that it increases computational load and requires a spatial light modulator with a high frame rate.

[0005] The technical problem that the embodiments of the present invention aim to solve is to provide a holographic display system and an optimization method thereof that reduce speckle noise while maintaining the resolution using a rotating phase mask.

[0006] An example of a holographic display system according to an embodiment of the present invention for achieving the above technical objectives comprises: a light source unit that outputs laser light; a spatial light modulator that phase-modulates incident light; and a rotating phase mask that rotates a mask having a pattern that adds a phase of a certain magnitude, wherein the rotating phase mask is located between the light source unit and the spatial light modulator or at the output terminal of the spatial light modulator.

[0007] An example of an optimization method for a holographic display system according to an embodiment of the present invention for achieving the above technical problem is an optimization method for a holographic display system using a rotating phase mask in which a mask including a pattern that adds a phase of a certain magnitude is rotated, comprising: a step of obtaining a plurality of mask superimposed images by superimposing the rotating phase mask, which is rotated at different angles, onto an image input from a spatial light modulator or an image output from the spatial light modulator, thereby adding a phase according to the pattern; a step of obtaining a plurality of Fourier transformed images by applying a Fourier transform to the plurality of mask superimposed images; a step of generating a final reconstructed image by time averaging a reconstructed image generated by propagating the plurality of Fourier transformed images over a certain distance; and a step of updating the pattern of the rotating phase mask based on a loss function representing an error between the final reconstructed image and a predefined target image.

[0008] According to an embodiment of the present invention, speckle noise can be reduced while maintaining the resolution by using a rotational phase mask. Since speckle noise removal relies on the physical rotation of the rotational phase mask, there is an advantage that a high frame rate SLM is not required. Furthermore, the system and method of the present embodiment can be used universally in all fields of three-dimensional holographic displays, including near-eye holographic displays.

[0009] FIG. 1 is a drawing illustrating the configuration of an example of a holographic display system according to an embodiment of the present invention.

[0010] FIG. 2 is a drawing illustrating the configuration of an example of a rotational phase mask according to an embodiment of the present invention.

[0011] FIG. 3 is a drawing illustrating an example of a mask according to an embodiment of the present invention.

[0012] FIGS. 4 and 5 are drawings illustrating an example of a method for optimizing a rotational phase mask according to an embodiment of the present invention, and,

[0013] FIGS. 6 and 7 are drawings illustrating the simulation results of a holographic display system generated through an optimization method according to an embodiment of the present invention.

[0014] Hereinafter, a holographic display system using a rotational phase mask according to an embodiment of the present invention and a method for optimizing the system will be examined in detail with reference to the attached drawings.

[0015] FIG. 1 is a diagram illustrating the configuration of an example of a holographic display system according to an embodiment of the present invention.

[0016] Referring to FIG. 1, the holographic display system includes a light source (100), a rotational phase mask (110), and a spatial light modulator (SLM) (120). In another embodiment, the holographic display system may further include at least one of a beam splitter (130), a 4f filtering system (140), and a Fourier transform unit (150). For convenience of explanation, the following description is based on an embodiment that includes all of the beam splitter (130), the 4f filtering system (140), and the Fourier transform unit (150). In this embodiment, a CCD sensor (160) for observing a reconstructed image is also shown, but it may be omitted.

[0017] The light source unit (100) outputs laser light. In one embodiment, the light source unit (100) may include a laser light source unit (102) that outputs laser light, an objective lens (104), an aperture (105), a condensing lens (106), and a polarizing film (108). The configuration of the light source unit (100) in this embodiment is merely an example to aid understanding, and various existing configurations of the light source unit (100) for holographic displays may be applied to this embodiment. An image to be reproduced as a hologram is output through the laser light source unit (102) of the light source unit (100).

[0018] The rotating phase mask (110) rotates a mask containing a pattern that adds phase to the input light. Although this embodiment illustrates an example where the rotating phase mask (110) is located between the light source unit (100) and the spatial light modulator (120), in other embodiments, the rotating phase mask (110) may be located at the output terminal of the spatial light modulator (120) (e.g., between the 4f filtering system (140) and the beam splitter (130), etc.).

[0019] The size of the mask within the rotational phase mask (100) may be the same as the image size of the spatial light modulator (120). For example, the image output from the spatial light modulator (120) (hereinafter referred to as the 'SLM image') has a resolution of 1080×1080, and the size of the SLM image and the size of the mask may be the same.

[0020] Examples of detailed configurations of a rotational phase mask are shown in FIGS. 2 and FIGS. 3. Additionally, the pattern of the mask can be optimized through the method of FIGS. 4 and FIGS. 5. The method for optimizing the mask pattern is discussed again in FIGS. 4 and FIGS. 5.

[0021] Light output from the light source unit (100) passes through the rotating phase mask (110) and is incident on the spatial light modulator (120). In one embodiment, laser light may be polarized through a polarizing film (108) and incident parallel to the rotating phase mask (110). Of the light incident parallel to the surface, some passes through the area where the pattern of the rotating phase mask (110) exists and the phase is added, while the rest passes through the area where the pattern does not exist and the phase is maintained as is. Since the rotating phase mask rotates in real time, the area where the phase is added in the light incident on the rotating phase mask (110) also changes in real time.

[0022] Light passing through the rotational phase mask (110) is incident on the spatial light modulator (120). The light phase-modulated by the spatial light modulator (120) has its optical path bent 90 degrees through the beam splitter (130). The light redirected by the beam splitter (130) is incident on the 4f filtering system (140).

[0023] The 4f filtering system (140) includes two lenses having the same focal length f1 arranged at an f1 interval and a DC filter at the center. The 4f filtering system (140) can block DC light generated in Fourier holography. Since the DC filter that removes DC noise is a configuration already known, further explanation regarding it is omitted.

[0024] Light passing through the 4f filtering system (140) enters the Fourier transform unit (150). The Fourier transform unit (150) may be composed of a Fourier transform lens with a focal length of f2. The Fourier transform lens performs an optical Fourier transform on the incident light. The hologram transformed through the Fourier transform unit (150) propagates to multiple different depths and is restored into a corresponding image at each depth.

[0025] FIG. 2 is a diagram illustrating the configuration of an example of a rotational phase mask according to an embodiment of the present invention.

[0026] Referring to FIG. 2, the rotational phase mask (110) includes a mask (210) having a pattern and a rotation element (200) that rotates the mask (210). The pattern on the mask (210) includes a first pattern part (212) that adds a phase to the incident light and a second pattern part (214) that allows the incident light to pass through without a phase change.

[0027] The pattern shape of the mask may vary depending on the embodiment. For example, the mask (210) may be composed of 6.45 mm cells arranged in an N×N (where N is a natural number greater than or equal to 2). The number and size of the cells constituting the pattern may vary depending on the embodiment. Some of these cells belong to a first pattern section (212) that adds phase to the incident light, and the rest belong to a second pattern section (214) that outputs the incident light as is. That is, the mask (210) is a type of binary mask that includes a first cell that adds phase and a second cell that does not add phase. The optimization of the pattern to be examined later is the optimization of the arrangement of the first cell and the second cell. An example of the detailed structure of the mask (210) including the first pattern section (212) and the second pattern section (214) is illustrated in FIG. 3.

[0028] Since the rotation phase mask (110) has a low resolution (e.g., 2×2 cells) compared to the resolution of the SLM image (e.g., 1080×1080 pixels), the scattering effect is not large, so there is less diffuse reflection, and the time-averaged effect of the hologram according to the rotation of the rotation phase mask (110) can be expected. In addition, since it relies on the physical rotation of the rotation phase mask (110) rather than the update of the spatial light modulator (120), there is an advantage that a spatial light modulator (120) with a high frame rate is not required.

[0029] FIG. 3 is a drawing illustrating an example of a mask according to an embodiment of the present invention.

[0030] Referring to FIG. 3, an example of a partial cross-section of a mask (210) is illustrated. In this embodiment, only the pattern layer is illustrated without showing the substrate. The mask (210) includes a substrate and a pattern layer (310) formed by stacking a material of a certain thickness having a refractive index. In one embodiment, the pattern layer (310) may be implemented with SiO2. In addition to this, the pattern layer (310) may be implemented with various other types of materials; however, for convenience of explanation, the following description assumes that the pattern layer (310) is implemented with SiO2.

[0031] In the example of FIG. 2, the first pattern portion (212) of the mask (210) is composed of a substrate and a pattern layer, and the second pattern portion (214) may be composed of only the substrate (300) without a pattern layer. Light incident on the first pattern portion (212) has its phase changed by the pattern layer (310), and light incident on the second pattern portion (214) without a pattern layer (300) is output as is without phase change.

[0032] The mask (210) can adjust the phase difference between the light passing through the first pattern section (212) and the light passing through the second pattern section (214) according to the height (z) of the pattern layer (310) constituting the first pattern section (212). For example, when the difference between the phase of the light passing through the first pattern section (212) and the phase of the light passing through the second pattern section (214) is π, the height of the pattern layer (310) of the first pattern section (212) can be calculated using the following mathematical formula.

[0033]

[0034] Here, jknz represents the phase of light passing through the pattern layer (310), jkz represents the phase of light passing through the area without the pattern layer (300), n represents the refractive index of the pattern layer (310), z represents the thickness of the pattern layer (310), and λ represents the wavelength of the light source.

[0035] If the refractive index (n) of the pattern layer (310) (e.g., SiO2) is 1.42 and the wavelength of the light source is 532 nm, the height (z) of the pattern layer (310) is 0.57826 μm. In other words, if the first pattern portion (212) is deposited with SiO2 having a height of 0.57826 μm in the mask (210), the phase difference between the first pattern portion (212) and the second pattern portion (214) becomes π.

[0036] FIGS. 4 and FIGS. 5 are drawings illustrating an example of a method for optimizing a rotational phase mask according to an embodiment of the present invention.

[0037] Referring to FIGS. 4 and FIGS. 5 together, a holographic display optimization device (hereinafter referred to as the "optimization device") superimposes a rotational phase mask onto an image output from a spatial light modulator (hereinafter referred to as the "SLM image") to add a phase according to a pattern. The optimization device may be implemented as a computing device including memory, a processor, and an input / output device. In this case, each step of the present embodiment may be implemented in software, loaded into memory, and then executed by the processor.

[0038] The optimization device can perform an optimization process based on an SLM image in which the phase is in a continuous uniform random distribution state (S500). The target image is predefined. The pattern shape of the first pattern part (212) of the rotation phase mask (110) can also be initially arranged arbitrarily.

[0039] The optimization device rotates the mask (410) by an angle divided into K (where K is a natural number greater than or equal to 2) of 360 degrees and superimposes the rotated mask (410) with the SLM image (400). For example, if 360 degrees is divided into 6 parts, masks (410) rotated to 0 degrees, 60 degrees, 120 degrees, 180 degrees, 240 degrees, and 300 degrees, respectively, can be obtained. Since the mask (410) rotates continuously, in actual implementation, the pattern of the mask (410) rotating in real time is superimposed on the incident light; however, in the optimization process of this embodiment, for the convenience of calculation through simulation, it is assumed that the 360-degree section is divided into multiple (K) sections and masks corresponding to each rotation angle are superimposed.

[0040] The optimization device generates a mask superimposed image (420) with added phase by superimposing K rotated masks and an SLM image (400) (S510). In each mask superimposed image (420), the rotation angle of the pattern is different. In the SLM image (400), the phase of the pixels at positions overlapping with the pattern is added by a predefined amount, and the phase of the pixels at positions not overlapping with the pattern is maintained as is.

[0041] The optimization device generates an image (hereinafter referred to as the 'Fourier transformed image (430)') by performing a Fourier transform on the mask superimposed image (420). (S520) The Fourier transformed image (430) is propagated to various depths (d=1,2,3...D) and reconstructed into an image. The reconstructed image may consist of D reconstructed images focused at D different distances (d). The reconstructed image may be generated through a numerical propagation model.

[0042] At this time, there are K different reconstruction images that exist sequentially according to the rotation state of the mask (410), and the optimization device generates a final reconstruction image by time-averaging the K different reconstruction images (S530). Depth-wise images (u) of the time-averaged final reconstruction image. recon,d) can be calculated as shown in the following mathematical formula.

[0043]

[0044] Here, K is the number of rotation angles of the rotation phase mask, u slm is an SLM image, m k is the mask of the k-th rotation angle, u recon,d represents the reconstructed image propagated to depth d.

[0045] The optimizer calculates a loss function representing the error between the time-averaged final reconstructed image (440) and a predefined target image (450) (S540), and repeats the process of updating the mask pattern so that the loss function decreases (S550). In one embodiment, the optimizer can calculate the loss function by accumulating the error between the depth-wise images of the final reconstructed image (440) and the depth-wise images of the target image (450).

[0046] The optimizer updates the pattern by applying an inverse wave (e.g., gradient descent) to the loss function. Once the pattern update process of the mask (410) is completed, the optimizer may additionally perform a process of updating the phase (φ) of the SLM image (400) after fixing the mask pattern. In one embodiment, the loss function may include an L2 error between the final reconstructed image and the target image, and a Nadam optimizer may be used for the update process. The update process can be expressed mathematically as follows.

[0047]

[0048] Here, u recon,d represents the images by depth of the final reconstructed image, and u targer,d represents the images of the target image by depth.

[0049] In another embodiment, the optimizer can update the pattern (ρ) of the mask and the phase (φ) of the SLM image together by backpropagating the loss function.

[0050] FIGS. 6 and 7 are drawings illustrating the simulation results of a holographic display system generated through an optimization method according to an embodiment of the present invention.

[0051] Referring to FIGS. 6 and 7, the peak signal-to-ratio (PSNR) and structural similarity index measure (SSIM) are described between a conventional system that does not use a rotational phase mask and the system of the present embodiment that uses a rotational phase mask. Each value indicates similarity to a target; the higher the PSNR and the closer the SSIM is to 1, the more similar the image generated by the system can be determined to be to the target image.

[0052] It can be confirmed that the speckle noise, which is noticeably observed in the existing system, is significantly reduced in the system of the present embodiment. The PSNR and SSIM are also 30.629 dB and 0.902, respectively, indicating an improvement of 5 dB and 0.2 or more compared to the existing system. To evaluate the level of speckle noise more quantitatively, speckle contrast was measured. Speckle contrast was calculated as (intensity standard deviation) / (average intensity) in a specific region, and a value closer to 1 indicates severe noise. The speckle contrast in the system of the present embodiment is 0.068, which is a decrease of approximately 41.4% compared to the speckle contrast of 0.116 in the existing system. Through this, it can be confirmed that the rotational phase mask proposed in the system of the present embodiment effectively reduces speckle noise.

[0053] The present invention can also be implemented as computer-readable program code on a computer-readable recording medium. A computer-readable recording medium includes all types of recording devices in which data that can be read by a computer system is stored. Examples of computer-readable recording media include ROM, RAM, CD-ROM, magnetic tape, floppy disk, optical data storage device, etc. Additionally, computer-readable recording media can be distributed across networked computer systems, allowing computer-readable code to be stored and executed in a distributed manner.

[0054] The present invention has been described above with reference to its preferred embodiments. Those skilled in the art will understand that the present invention may be embodied in modified forms without departing from the essential characteristics of the invention. Therefore, the disclosed embodiments should be considered in an illustrative rather than a restrictive sense. The scope of the invention is defined by the claims, not by the foregoing description, and all variations within the scope of the claims should be interpreted as being included in the invention.

Claims

1. A light source unit that outputs laser light; A spatial light modulator that phase-modulates incident light; and It includes a rotational phase mask that rotates a mask having a pattern that adds a phase of a certain size, and A holographic display system characterized in that the above-mentioned rotating phase mask is located between the light source and the spatial light modulator or at the output terminal of the spatial light modulator.

2. In claim 1, the rotational phase mask is, A mask having the same resolution as the spatial light modulator and including the pattern; and A holographic display system characterized by including a rotating element that rotates the above mask.

3. In Paragraph 2, the above mask is, Substrate; and A holographic display system characterized by including a pattern layer formed by stacking a material of a certain thickness having a refractive index on the substrate.

4. A method for optimizing a holographic display system using a rotating phase mask in which a mask including a pattern that adds a phase of a certain size rotates, A step of obtaining a plurality of mask superposition images by superimposing the rotation phase mask, rotated at different angles, onto an image input to a spatial light modulator or an image output from the spatial light modulator, and adding a phase according to the pattern; A step of obtaining multiple Fourier transformed images by applying a Fourier transform to the above multiple mask superposition images; A step of generating a final reconstructed image by time-averaging the reconstructed images generated by propagating the plurality of Fourier transform images over a certain distance; and Optimization method of a holographic display system characterized by including the step of updating the pattern of the rotation phase mask based on a loss function representing the error between the final reconstructed image and a predefined target image.

5. In Paragraph 4, the above-mentioned updating step is, Optimization method of a holographic display system characterized by including the step of updating the pattern of the rotational phase mask by backpropagating the loss function representing the above error using gradient descent.

6. In Paragraph 4, the above-mentioned updating step is, Optimization method of a holographic display system characterized by including the step of calculating a loss function that accumulates the error between the depth-wise images of the final reconstructed image and the depth-wise images of the target image.

7. In Paragraph 4, the above-mentioned updating step is, A method for optimizing a holographic display system characterized by including the step of fixing the pattern after the update of the pattern of the rotational phase mask is completed, and then updating the phase of the spatial light modulator.

8. A computer-readable recording medium storing a computer program for performing the method described in paragraph 4.

Images