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Abstract
In this paper, a full color holographic display system based on the intensity matching of the reconstructed image is proposed. The system consists of three color collimated beams, three spatial light modulators (SLMs), three beam splitters, three lenses, three irises, a prism and a receiving screen. The three SLMs are used to load three color holograms, respectively. In order to eliminate the undesirable light in the reconstructed images and adjust the light intensities, the irises which contain two functions of both light intensity attenuation and aperture variation are cleverly produced. Finally, by using the prism, three color images can be coincident on the receiving screen. Experimental results verify the feasibility of the proposed system.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the development of the optoelectronic devices and the improvement of computer performance, holographic display technology based on spatial light modulator (SLM) has attracted more and more attention due to its flexible and easy operation features [1–4]. The holographic display technology records the complex amplitude information of the object by interfering with a laser, and then reconstructs the image of the object by diffraction theory. Therefore, in order to realize color holographic display, it is necessary to reproduce the hologram using three different color lasers [5–8]. The traditional methods of color holographic display usually use one or three SLMs. For example, some researchers use the visual persistence effect to realize color holographic display based on the time multiplexing mechanism [9,10]. Besides, color holographic system is also proposed by using space multiplexing method with three SLMs [11,12]. Although holographic display has always been considered as an ideal way of three-dimensional display, it is still difficult to practically applied in our life. In the color holographic reproduction, due to the difference of wavelengths and the influence of system components, color chromatic aberration exists in the reconstructed image [13–15]. Due to the structure of the SLM, undesirable light such as high-order diffraction images and high-order diffraction light exist in the reconstructed image [16,17]. Moreover, since the human eye is sensitive to different light and the intensities of the lasers are different during use, the color of the reconstructed image of the object may be distorted from the original [18]. For the first two problems, researchers have proposed the corresponding solutions such as using several solid lenses and filters to improve the quality of the reproduction [19,20]. However, the use of multiple components will increase the complexity of the system. In recent years, liquid lenses based on electrowetting effect or mechanical actuation are also produced to realize the color holographic system without undesirable light [21,22]. However, the commercial liquid lens is generally expensive and the mechanical liquid lens has problems such as slow response time [23–26], so the existing structures of the liquid devices suitable for the holographic system need to be optimized. For the third problem, there are still few reports on the related solutions currently. If the holographic display wants to be truly applied, we should not only eliminate the undesirable light in the color reconstructed image, but also keep the color of the reconstructed image realistic.
In this paper, a full color holographic display system based on three SLMs is proposed. Three liquid irises with special structures are designed and used in the holographic display system subtly. When three color lights pass through the irises, high-order diffraction light and high-order images can be eliminated by adjusting the sizes of the irises. By adjusting the voltages applied on the irises, full color holographic image with intensity matching can be reproduced.
2. Structure and operating principle
The schematic diagram of the proposed system is shown in Fig. 1. The system consists of three collimated beams (red laser, blue laser and green laser), three beam splitters (BSs), three lenses, three irises, three SLMs and a prism. In order to observe and record the experimental results easily, a receiving screen is placed behind the system. Three SLMs are used to loaded three color holograms, respectively. When three color lasers illuminate the corresponding BSs, the light is reflected by the BS onto the SLMs. Then the diffraction images can pass through the BSs and lenses. The irises are used to eliminate the undesirable light in the reconstructed images and adjust the light intensities. Finally, by using the prism, three color images can be coincident on the receiving screen.
There are many ways to record and reproduce holograms, and the algorithms for generating the holograms are different accordingly. For example, when the novel-look-up-table (NLUT) algorithm is used to generate the hologram [27], undesirable light can be eliminated by the irises at the focal planes of the lenses. The reconstructed images can be displayed at the diffraction area and the receiving screen does need to be fixed. Then the proposed system is called the holographic display system. In our experiment, phase hologram is produced for the holographic reproduction and we use Gerchberg-Saxton (GS) algorithm to generate the hologram. In order to separate the reconstructed image and the undesirable light, a phase profile for spherical wavefront is loaded on the SLM. Then the proposed system can be called the holographic projection system when the receiving screen is fixed. Either holographic projection or display, the proposed irises can be used to eliminate the undesirable light in the holographic system and realize color matching. When the GS algorithm is used to generate the hologram, the principle of the monochrome holographic reconstruction is shown in Fig. 2(a). d1 is the distance between the SLM and the lens. The iris is placed at the focal plane of the lens. p is the pixel size of the SLM, and d2 is the distance between the lens and the receiving screen. In order to separate the reconstructed image and the undesirable light, a phase profile for spherical wavefront is loaded on the SLM. When diffraction images pass through the iris, the undesirable light can be eliminated by the iris. Since the lens has different refractive indices for different wavelengths of light, when light passes through the lens, the focal lengths are different accordingly, as shown in Fig. 2(b). According to the principle of holographic diffraction, the sizes of the reconstructed images on the receiving screen can be calculated as follows:
where Sr, Sg, Sb are the sizes of the red, green and blue reconstructed images respectively, fr, fg, fb are the focal lengths of the lens for red, green and blue colors, λr, λg, λr are the wavelengths of the corresponding color lasers, and rr, rg, rb are the radiuses of the spherical wavefront for three color holograms respectively. By choosing the appropriate parameters, three color reconstructed images can be coincident on the receiving screen at the same position.
Fig. 2 Process of the holographic reproduction. (a) Principle of the monochrome holographic reconstruction; (b) focal lengths of the lens for different wavelengths.
The proposed iris contains two functions of light intensity attenuation and aperture variation. As shown in Fig. 3, the chamber is filled with two immiscible liquids, where liquid-1 is the non-conductive liquid and liquid-2 is the conductive liquid. A dielectric layer and an electrode are attached to the bottom substrate. A black mask is embedded in the bottom substrate. When the voltage is applied on the electrode and liquid-2, the conductive liquid will move to the center of the bottom substrate due to the electrowetting effect. When the voltage is different, the range of movement is different accordingly. In this way, the iris can achieve the function of aperture variation. The relationship between the voltage U and the contact angle θ can be expressed as follows:
Fig. 3 Structure of the proposed iris. (a) Iris without the voltage; (b) iris with voltage; (c) LC cell without voltage; (d) LC cell with voltage.
where ε is dielectric constant, θ is the contact angle between conductive liquid and the bottom substrate, d is the thickness of the insulating layer, γ1, γ2, and γ12 are the interfacial tensions between the bottom substrate and non-conductive liquid, the bottom substrate and conductive liquid and the two liquids respectively. Besides, two orthogonal polarizers are stuck on the top and bottom substrates respectively, as shown in Figs. 3(c)-3(d).
Figure 4 is the principle of the light intensity attenuation. A positive liquid crystal (LC) layer is initially aligned in the 90° twisted-nematic (TN) cell and the cell gap is uniform throughout all LC area. The planar indium tin oxide (ITO) electrodes are on the inner sides of the top and bottom substrates, respectively. Two orthogonal polarizers are laminated on the top and bottom substrates of the TN cell respectively, and their corresponding transmission axes are parallel to the direction of rubbing of the polyimide (PI) film, respectively. In the voltage-off state, the LC molecules are rotated by 90° along the helical axis from the top to bottom substrates, enabling the proposed iris to transmit light under crossed-polarizers, as shown in Fig. 5(a). In the voltage-on state, the LC directors are reoriented, as shown in Figs. 5(b)-5(d). In the 0° twist state in which the LC molecules are perpendicular to the substrates, the LC molecules do not provide any phase retardation for the incident light, so all incident light is blocked by the crossed-polarizers.
Fig. 5 State of the LC molecules. (a) State when U = 0; (b)-(c) the states when different voltages are applied.
3. Experiments and results
In the experiment, the pixel size of the SLM is 6.4µm and its resolution is 1920 × 1080. The SLM is produced by Xi’ an CAS Microstar Optoelectronic Technology Co., Ltd and it is a phase type SLM. The wavelengths of red, green and blue lasers are λr = 632 nm, λg = 532 nm, λb = 471 nm, respectively. The size and the transmission of the BS are 25.4 mm × 25.4 mm × 25.4 mm and >80%, respectively. The output powers of the lasers are >20mw. The number of the iterations is set to 40 in the GS algorithm. In order to realize the ideal holographic display, the irises based on electrowetting effect are produced. The positive LC material used in the experiment is E7 LC (the viscosity constant is η = 29 Pa·s, the refractive indexes of o and e waves are ne = 1.741 and no = 1.517 respectively, the dielectric anisotropy is Δε = 11.4). Figure 6 presents the photomicrographs of the 90° and 0° twist states of the TN cell in the voltage-off and on states respectively. The results are observed under a transmission-polarizing optical microscope, and the TN cell are in the while and dark state respectively. Figure 7 shows the transmittance of the TN cell, where the measured and simulated parameters are realized by commercial simulation software TechWiz LCD 3D (Sanayi System Co., Ltd., Korea) and LC electro-optic effect tester (LCDEO-2 + Z, ZKY, China), respectively. As illustrated in Fig. 7, the transmittance of the TN cell decreases with the increases of the applied voltage. For the experiment and simulation results, the threshold voltages are both 2.7 Vrms and the LC cell reaches dark state at 4.8 Vrms. The 0° twist state is non-dispersive and exhibits excellent dark-state transmittance. The results of the light intensity attenuation are shown in Fig. 8. The response time is determined by the thickness of the LC layer and it has been tested in the experiment. When the thickness is 3 µm, the response time can reach ~9 ms. The thickness of the LC layer can be designed according to the requirements.
Fig. 6 Photomicrographs of 90° and 0° twist states of the TN cell. (a) Voltage-off state; (b) voltage-on state.
The aperture size of the iris can be varied by applying voltages on the electrode and liquid-2. In the experiment, liquid-1 is the phenylmethyl silicone oil with a density of 1.03 g/cm3, and the propyl alcohol mixed with ink with a density of ~1.03 g/cm3 is used as liquid-2. The aperture changes under different voltages are shown in Fig. 9. In the initial state, liquid-2 is clung to the sidewall of the substrate and the iris shows the largest aperture. When the voltage U<35VDC, liquid-2 cannot be driven because of the existed threshold voltage. When U>35 VDC, liquid-2 begins to rush to the center substrate. It can be seen clearly that the aperture shrinks when the applied voltage increases from 35 VDC to 80 VDC, as shown in Figs. 9(b)-9(d). The measured aperture sizes are 12.4 mm, 10.9 mm, 9.8 mm and 7.6 mm, respectively. Such an experiment shows the aperture sizes can be changed variably. The measured aperture sizes under different driving voltages are also depicted in Fig. 10. Repeated experiments have been conducted to verify the repeatability and uniformity of the liquid iris, as shown in Fig. 10. From Fig. 10 we can see the that the proposed liquid iris has a reasonable uniformity. Moreover, the uniformity of the iris can be further improved by using an annular ITO electrode to make the liquid flow into the center regularly.
Fig. 9 Results of the aperture size changes of the iris when we apply voltages on the electrode. (a) U = 0 V; (b) U = 40 V; (c) U = 50 V; (d) U = 70 V.
In order to measure the transmittance, a spectrometer is used to test the iris, as shown in Fig. 11. From the result we can see that the transmittance of the iris is > 88%. The light intensity can be controlled by both liquid iris and LC cell. Actually, the light intensity in each unit area is the same when we only use the liquid iris. While the LC-based attenuator can control the light intensity with per unit area. In the experiment, the liquid iris is used to control the total luminous area and the LC-based attenuator is used to control the light intensity of per unit area. Thus, the liquid iris and LC attenuator can be controlled separately to meet various requirements of holographic display.
In order to verify the effect of the iris, the monochromatic optical path is used for the contrast. Figure 12(a) is the traditional system without using the iris and Fig. 12(b) is the system by using the proposed iris. Figure 13(a) is the reconstructed image without using the proposed iris, and we can see that there is a zero-order diffraction light in the middle of the image. Besides, there are many high-order reconstructed images around the zero-order spot. However, only the first-order diffraction image is needed. When the proposed iris is used, the black dot of the iris can eliminate the zero-order diffraction light and the aperture can be used to eliminate high-order reconstructed images around the zero-order spot. Then the quality of the reconstructed image can be improved effectively. The size of the aperture can range from ~7 mm to ~14.5 mm. Compared with the traditional system, the phase profile for spherical wavefront is loaded on the SLM in the proposed system in order to separate the reconstructed image and the undesirable light. So, the undesirable light can be eliminated by the liquid iris. In the experiment, the settings for the camera are the same. The results show that the system by using the liquid iris can provide better reproduction. By adjusting the size of the aperture, reconstructed image without undesirable light can be seen on the receiving screen. However, when the aperture continues to change, only a portion of the image can be displayed on the receiving screen, as shown in Fig. 13. In the experiment, the size of the aperture can be adjusted according to the requirements.
Fig. 12 Monochromatic reconstruction of the holographic system. (a) Traditional system without using the iris; (b) system by using the proposed iris.
Fig. 13 Results of the green image. (a) Results of the reconstructed image by using the traditional holographic system; (b)-(d) results when the size of the iris changes by using the proposed system.
By adjusting the voltage applied on the LC cell, the function of light intensity attenuation can be realized, as shown in Fig. 14. From the results we can see that the light intensity of the reconstructed image can be changed by varying the voltages. Figures 15-16 are the red and blue results with different light intensities.
In order to achieve the desired color effect, the irises are used to adjust the intensity of the three color lights. As shown in Fig. 17, when three color lasers illuminate the SLMs respectively, the light intensities of the zero-order lights are different accordingly. So, when three zero-order lights coincide, the colored light is not pure white, as shown in Figs. 17(d)-17(e). By adjusting the light intensities, ideal white light can be realized, as shown in Fig. 17(f). Then the yellow teapot can be reconstructed on the receiving screen, as shown in Fig. 17(g).
Fig. 17 (a)-(c) zero-order lights of three colors; (d)-(f) white light when the intensities of three color lights changes; (g) color reconstructed image of the teapot.
The most common mechanical iris is based on the movable blade. Although this design does not introduce optical loss in the optical path, it requires a complex rotation mechanism. So, the mechanical iris is difficult to be miniaturized. Besides, the mechanical iris also has disadvantages such as polygonal aperture and high cost. The traditional method is using two polarizers (or “wave-plate + PBS combination”) to adjust the light intensity. In order to achieve color matching, six polarizers may be required to adjust the intensity of the three colors. This kind of mechanical operation is not convenient. Color holographic system is often complex, and these polarizers will make the system more complex. Besides, the traditional method of eliminating the undesirable light in the reconstructed image is using several solid lenses and filters (see the dotted box in Fig. 18). So, the holographic system with color matching and no undesirable light will be very complex. Due to the presence of zero-order spot and high-order diffraction images in the holographic reconstructed image, it is necessary to eliminate not only the high-order diffraction images, but also the middle zero-order light. Although the electrically modulation devices have been widely used, mechanical apertures of this type are not common. In this paper, the adjustable iris is proposed in order to eliminate the undesirable light in the reconstructed images and adjust the light intensity. The advantage of the proposed system is the multifunctional integration of the iris. The iris integrates the functions of the diaphragm and the optical attenuation. In our experiment, the electrowetting-actuated liquid iris can only be used to control the amount of light by adjusting the aperture size of the iris and it cannot control the light intensity. Thus, an LC cell is employed to attenuate the light intensity. The LC-based attenuators have the advantages of fast response time (~9 ms) and low driving voltage (~5 V). Besides, these devices are simple to fabricate and easy to integrate with other optoelectronic devices, such as liquid apertures and liquid lenses. The size of the whole iris is only a few centimeters, which greatly simplifies the traditional holographic system.
In this paper, in order to separate the reconstructed image and the undesirable light, a phase profile for spherical wavefront is loaded on the SLM. From Eqs. (1)-(3) we know that the sizes of the reconstructed image change with the radiuses. When the size of the reconstructed image changes, the aperture needs to be adjusted accordingly. Due to the limitation of the SLM, the size of the reconstructed image is typically only a few centimeters. In order to enlarge the size of the reconstructed image, many methods have been proposed by the researchers. To further illustrate the advantages of the proposed iris, a holographic zoom system is built, as shown in Fig. 18. When the size of the reconstructed image changes, the aperture size of the iris is adjusted to eliminate the undesirable light. The red laser is used as an example for the experiment and the results are shown in Fig. 19. From the results we can see that the reconstructed image with high quality can be displayed when the image size changes. Holographic zoom systems have been studied a lot in order to meet the display requirement (such as Refs [17], [21]. and [26]). The liquid iris integrates the functions of the diaphragm and the optical attenuation, and it has the advantages of small size and ease of integration. When the iris is integrated with the liquid lens, holographic zoom system without undesirable light can be realized more easily.
In this paper, the liquid iris with the unique structure is proposed and we use the iris in holographic display to simplify the traditional holographic system. At present, there are many problems in the holographic display that need to be solved. Through the continuous efforts of researchers, we hope to make holography closer to the application. Compared with the traditional method by using two polarizers or “wave-plate + PBS combination” to adjust the light intensity, the LC-based attenuators have small size and fast response time. Compared with the traditional method by using several solid lens and filters, the liquid iris can effectively simplify the system. So, compared with the traditional system, the proposed system can realize ideal color reconstruction easily. By controlling the voltage, the intensity of the reconstructed image can be changed conveniently. In this way, full color holographic display system based on intensity matching of reconstructed image is realized. However, there are still some problems to be solved. Due to the absorption of the liquid itself and the LC cell, the transmittance is affected. Besides, the aperture shapes are irregular when the driving voltage is too high. That may result from the simple structure of the ITO electrode which has no limitation to the dyed liquid when it stretches into the center. We can design a multilayer annular sheet on the bottom substrate to make the liquid flow into the center regularly. We believe that with the optimization of the system, the holographic display effect will become better and the holographic display can be truly applied in the future.
4. Conclusion
In this paper, a full color holographic display system is proposed based on three SLMs. Three special structure of liquid irises are designed and used in the holographic display system subtly. When three color lights pass through the irises, high-order diffraction light and high-order images can be eliminated by adjusting the sizes of the irises. By adjusting the voltages applied on the irises, full color holographic image with intensity matching can be reproduced. The proposed holographic system has a simple structure. We hope that the proposed system can make a small contribution to the further development of the holographic display.
Funding
National Natural Science Foundation of China, 61535007, 61805169 and 61805130.
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