Research on Quality Enhancement Techniques for Computational Holographic Reproduction

With the development of three-dimensional display technology, the research of three-dimensional display technology is changing day by day, and people want to get a more realistic visual experience. As a true three-dimensional display technology, holographic display can provide all the depth information needed by the human eye to perceive three-dimensional objects, giving people a comfortable and realistic three-dimensional visual sense. Holographic technology has a wide range of applications in military, medical, commercial and other fields.

The development of computational holographic display technology so far still exists with poor reproduction image quality, slow computation speed and holographic reproduction image of small size and narrow field of view and other key issues, of which, the scattering noise as a computational holographic display of the inherent problems and constraints on the further development of its in this paper from the perspective of inhibiting scattering noise and expanding the field of view (FOV) to make the quality of the image to improve.

Research on Quality Enhancement Techniques for Computational Holography Reproduction

First of all, clarify what is optical holography?

Optical holography is an imaging technology that records and reproduces information about an object. Its principle is: the light waves emitted by the object and the light of known amplitude and phase are interfered, and they are recorded on the light-sensitive medium, and then, through the principle of light diffraction, the recorded light-sensitive medium is illuminated by a specific illumination, and all the information of the original object can be reproduced in the reproduction process.

What is the basic principle of computational holography compared to optical holography?

The basic principle of computational holography is that the phase distribution of light waves is realized by computer algorithms, and then this information is converted into digital signals and stored in the computer. Then, these phase information are converted into a series of control signals through digital signal processing technology, and then these control signals are converted into optical holograms through optical devices (e.g., liquid crystal screens, gratings, etc.). Unlike optical holography, computational holography does not require an optical development process, so it can realize high-speed, high-precision holographic image generation.

Fig. 1 Schematic diagram of computational holographic optical reproduction

The spatial light modulator used in this experiment is our FSLM-2K55-P, and its parameters and specifications are as follows:

Calculating the source of scattering noise in holographic displays

The phenomenon of laser scattering in holographic displays is regarded as an optical interference that affects the quality of the holographic reproduced image and is called scattering noise. Pure phase holograms can be obtained with high quality reproduced images. However, flaws in the acquisition algorithm of pure phase holograms and the high coherence of the reproduced light source can lead to the presence of scattering noise, so measures must be taken to suppress scattering noise.

The inclusion of an initial random phase in the hologram computation is necessary because it allows the high-frequency information of the object to be transmitted and reconstructed. When the initial random phase is not added to the object surface, only part of the high-frequency information can be transmitted to the holographic surface, which leads to the loss of low-frequency information and affects the quality of the object reconstruction during reproduction.

Fig. 2 Schematic diagram of the recording and reproduction process of holograms

There are other sources of scattering noise in the hologram reproduction image, which are mainly divided into the following four parts:

a) In the process of hologram encoding, the loss of amplitude information of the object surface will lead to the generation of scattering noise.

b)In the holographic reproduction system, the aperture limitation of the SLM makes the reproduced light extra diffracted, leading to scattering noise.

c)In the recording process of hologram, the object light wave on the holographic surface will be limited by the size of the holographic surface and receive part of the information, which makes the appearance of scattering noise.

d) Surface defects in the optics of the holographic display system can cause the formation of rough surfaces, and the irradiation of a highly coherent reproduction light source can lead to the generation of scattering noise.

In order to suppress scattering noise, methods such as time averaging and pixel separation can be used. Some scattering noise suppression methods are briefly described below.

1. GS algorithm

The Grechberg-Saxton (GS) algorithm is currently the more mature algorithm in obtaining pure phase holograms. This algorithm requires several iterations of Fourier transform and inverse transform calculations under the constraints set by the object plane and the holographic plane to obtain phase holograms with high diffraction rates. The flowchart of the algorithm is shown below.

Fig. 3 Flowchart of GS algorithm

2. Pixel separation method

In the holographic reproduction system, a high coherence laser is needed as the reproduction light source, and some optical devices are used in the process of generating the reproduction image after diffraction. Due to the limited aperture size of the optics, an additional diffraction effect occurs when the coherent reproduction light is diffracted into the reproduction. This diffraction effect causes the reproduced image points to take on the form of Airy spots. For any one Airy spot, there are overlapping Airy spots in its area, in which random interference occurs under the influence of the reproduced light, leading to the further generation of scattering noise, and the area of random interference also increases with the increase of the overlapping area of the Airy spots, and the speckle noise becomes more and more serious.

The pixel separation method is the solution proposed to suppress this speckle noise. In the pixel separation method, the object can be separated into N² object point groups by taking a specific pixel separation interval N, so that the neighboring object points in the object are spatially separated into different object point groups. Each object point group will correspondingly generate a sub reconstructed image, and the overlap area between Airy spots in the sub image varies with different values of N as shown in the figure below.

Fig. 4 Overlap of Airy spots in the subimage when the pixel separation interval N takes different values

Fig. 5 The final reproduced image and the corresponding magnified area for different values of the pixel separation interval N

3. Time averaging

In order to suppress scattering noise, time averaging can be used to improve the quality of holographic reproduction images. It has been shown that the scattering contrast is reduced to the original when N independent uncorrelated scattering patterns are superimposed. Therefore, this method can significantly suppress the scattering noise in holographic reproduction images.

The basic principle of the time-averaging method is to compute a sequence of multiple holograms, introduce a different random initial phase in the computation of each image, and obtain sub-reproduced images with different scattering distributions after reproducing the image sequence. The holographic reproduction image with scattering suppression is obtained by the principle of time multiplexing.

Fig. 6 Realization process of time averaging method

In the article, a large field-of-view holographic display with speckle noise suppressed is proposed. Unlike the traditional method, this method can generate multiple sub-CGHs with large sizes. by pixel separation, the recorded objects are separated into multiple object-point groups, and the information of each object-point group is recorded on different holograms with independent initial random phase. In holographic reconstruction, three SLMs with a linearly arranged structure in space are used to load the sub-holograms and used to reconstruct the image by time multiplexing. Where the FOV is amplified because the size of the light distribution at each image point is enlarged. At the same time, the speckle noise of the image is reduced by averaging effect and separation of neighboring image points.

Fig. 7 Schematic diagram of the proposed method

Fig. 8 Structure of the reconstruction system

Fig. 9 Optically reconstructed “3D” images taken from the left viewpoint (A1,A2) and the right viewpoint (A3,A4) when the proposed method focuses on “3” and “D” separately; 3D” images taken from the left viewpoint (B1,B2) and the right viewpoint (B3,B4) when focusing on ‘3’ and ‘D’ respectively by the pixel separation method; when Optically reconstructed “3D” images taken at the left (C1,C2) and right (C3,C4) viewpoints when “3” and “D” are focused by the GS method respectively.

Fig. 10 Optically reconstructed images captured from the left viewpoint (A) and right viewpoint (B), respectively, by the proposed method; optically reconstructed images captured at the left viewpoint (C) and right viewpoint (D) by the GS method

Summary

In the paper, a large field-of-view holographic display method based on the suppression of scattering noise is proposed. The method ensures an increase in the size of the light distribution at each image point through the generation of sub-CGHs, thus realizing the expansion of the FOV. In addition, based on time-division multiplexing, the speckle noise is effectively suppressed by averaging the noise and separating neighboring image points. The method is simple and easy to operate and has some practical value. Compared with the GS method, the proposed method in this paper increases the FOV size by 40 times and reduces the scattering contrast by 54.55% at the viewing distance R = 600 mm. Compared with the pixel separation method, the scattering contrast is reduced by 19%.