A frameless hologram display module employing a modified resolution redistribution system is proposed in this paper. The frameless display modules can be aligned two-dimensionally to realize a large display screen. The modified resolution redistribution system consists of a one-lens imaging system, a screen lens, and a multiple illumination system. The prototype module was constructed using a spatial light modulator with a resolution of 1,980 × 1,080, and a pixel pitch of 8.0 μm. The horizontal resolution was increased four times and the magnification of the imaging system was 2.88. The horizontal resolution was increased to 7,920 and the horizontal pixel pitch was reduced to 5.8 μm. The screen size of the module was 2.0 inches and the horizontal viewing angle was 6.3°.
©2010 Optical Society of America
Holography is an ideal three-dimensional (3D) display technique because it reconstructs the wavefront emitted from an object. However, an extremely high-resolution spatial light modulator (SLM) is required to produce practical 3D images. A very fine pixel pitch is necessary for a wide viewing angle, and a extremely large pixel count is necessary for a large screen size.
Several techniques have been proposed to enlarge the viewing angle and the screen size of holographic displays. In the hologram display system proposed by MIT [1,2], a high-resolution one-dimensional hologram distribution generated by an acoustooptic modulator is scanned two-dimensionally by a mechanical scanner to reduce the pixel pitch and to increase the pixel count. A technique to double the viewing angle using time-multiplexing was proposed [3,4]. An active tilting technique  was proposed, in which de-magnified images generated by a high-speed SLM are tiled onto an optically addressed SLM in a time-sequential manner. Techniques using multiple SLMs were proposed by several authors [6,7]. A horizontally scanning holographic display  was proposed in order to increase both the viewing angle and the screen size, in which a series of elementary holograms generated by a high-speed SLM are aligned horizontally by a mechanical scanner.
The use of a resolution redistribution (RR) optical system was also proposed in order to increase the viewing angle of a hologram . With this system, the horizontal resolution is increased by sacrificing the vertical resolution. The horizontal resolution can be increased several times. This technique requires neither mechanical scanning nor time-multiplexing. A technique to eliminate a conjugate image was also reported . Nevertheless, an increase in the screen size of a hologram has not been achieved.
In this study, the RR system is modified in order to enlarge the screen size of holographic displays. A frameless hologram display module can be constructed employing the modified RR system. As illustrated in Fig. 1 , a large screen size is realized by seamlessly aligning frameless display modules two-dimensionally. In this study, the structure of the frameless hologram display module is proposed and the prototype of the frameless hologram display module is demonstrated.
2. Modification of RR system
2.1 Previous RR system
The previous RR system  is illustrated in Fig. 2 . It consists of a 4f imaging system and a multiple illumination system for a SLM. The 4f imaging system consists of two Fourier-transform lenses, and it performs Fourier transforms twice. The multiple illumination system contains multiple point light sources. Multiple Fourier transformed images appear on the Fourier plane and are transmitted through a horizontal slit. Figure 3 shows the horizontal cross section of the previous RR system. As shown in this figure, the diameter of the Fourier-transform lens #2 must be larger than the image size. When a hologram display module is constructed using the previous RR system, the maximum cross-section of the module must be larger than the display screen. Therefore, a frameless hologram display module cannot be achieved.
The redistribution of the resolution is performed on the Fourier plane. As shown in Fig. 4 , the multiple Fourier-transformed images are aligned in a diagonal direction on the Fourier plane by aligning the point light sources in a diagonal direction. The number of point light sources is represented by K, and the size of the Fourier-transformed images is represented by w × w. The horizontal slit placed on the Fourier plane has a height of w/2K. The different areas in the original Fourier-transformed image are reallocated in the horizontal direction. The width of the Fourier-transformed image is increased to Kw. In order to eliminate a conjugate image, a complex-conjugate and symmetric distribution of the upper half is added at the lower half of the Fourier transformed image . When the resolution of the SLM is denoted by Nx × Ny and the pixel pitch is denoted by p, the RR system increases the horizontal resolution to K Nx and decreases the pixel pitch to p/K. Therefore, the horizontal viewing angle of a hologram displayed by the RR system is given by 2 sin−1(Kλ/2p). This viewing angle is approximately K times larger than when a hologram is displayed directly by the SLM.
2.2 Modified RR system
The modified RR system proposed in this study is shown in Fig. 5 . The modified system consists of a one-lens imaging system, a screen lens, and a multiple illumination system. The one-lens imaging system images the SLM on the vertical diffuser placed on the image plane. The screen lens is also placed on the image plane of the one-lens imaging system. The focal plane of the screen lens coincides with the focal plane of the imaging lens. This system performs the same operation on the Fourier transformed images as the 4f imaging system does. The horizontal cross section of the modified system is shown in Fig. 6 . The plane waves that illuminate the SLM are concentrated on the focal plane of the imaging lens, and then are converted back to the plane waves by the screen lens. As compared to Fig. 3, the proposed system has the same function as the previous system. The Fourier plane is the focal plane of the one-imaging lens. The magnification of the imaging system is given by M = l’/l.
When the magnification of the one-lens imaging system is larger than unity, i.e., M > 1, the screen size can be made larger than the diameter of the imaging lens. Therefore, a frameless hologram display module can be constructed employing the modified RR system.
3. Frameless hologram display module
The proposed frameless hologram display module employing the modified RR system is illustrated in Fig. 7 . A reflection-type SLM is used. Therefore, the modified RR system is folded by the SLM. The imaging lens of the one-lens imaging system is also used as the condenser lens of the multiple illumination system for the SLM. An optical fiber array is used as the multiple point light sources. The use of the optical fiber array enables a compact implementation. A beam splitter is used to bend light emitted from the fiber array to illuminate the SLM. A horizontal slit is attached to one surface of the beam splitter. A rectangular lens is used as the screen lens to achieve the frameless display screen.
When the SLM performs amplitude modulation, it has to be illuminated by linearly polarized light. Therefore, polarization-maintaining single-mode fibers are used to construct the optical fiber array. In addition, light reflected by the SLM has to be passed through a polarizer, which means the polarizer has to be positioned somewhere between the SLM and the vertical diffuser. In the module structure shown in Fig. 7, the polarizer is placed on the surface of the screen lens.
4. Prototype module
A prototype of the proposed frameless hologram display module was constructed. For the reflection-type SLM, a Pluto manufactured by HoloEye Corporation was used. The resolution was 1,920✕1,080. The pixel pitch was 8.0 μm. The screen size was 0.7 inch. Pluto was originally designed for phase modulation. In this study, it was used for amplitude modulation by using linearly polarized light for illumination, and placing a polarizer on the module screen.
Figure 8(a) shows a fabricated fiber array that consists of four optical fibers, i.e., K = 4. The numerical aperture (N.A) of the optical fibers was 0.11. Unfortunately, this N.A. is too small to illuminate the entire display area of the SLM. A micro-lens array consisting of four micro-lenses was attached to the optical fiber array to increase the N.A. to 0.23. The fabricated N.A. converter lens array is shown in Fig. 8(b). A fiber-coupled laser diode was used as a light source. The wavelength was 635 nm, and the optical power was 23 mW. The output optical fiber was divided into four optical fibers using fiber couplers.
The constructed prototype frameless hologram display module is shown in Fig. 9 . The SLM was attached to the back of the module. The polarizer and the vertical diffuser were attached to the front of the module. For the vertical diffuser, a lenticular sheet was used. The imaging lens consisted of four lenses. The N.A. converter lens array was attached to the optical fiber array. The horizontal slit was attached to the beam splitter. The magnification of the imaging system was M = 2.88.
The screen size of the constructed module was 2 inches. The resolution on the module screen was 7,680 × 135. The horizontal pixel pitch was decreased to 5.76 μm. The horizontal viewing angle was increased to 6.3°. The module size was 27 × 46 × 241 mm3.
The photographs of reconstructed images produced by the prototype module are shown in Fig. 10 . The reconstructed 3D images change smoothly depending on the horizontal viewing position. Because the horizontal viewing angle is increased by the RR technique, the 3D images can be observed by both eyes. However, undesirable light distributions are observed. They are caused by light reflection on the surfaces of the optical components, because some of the optical components did not have adequate antireflection coatings. Higher-order light diffraction components also degraded the reconstructed images.
The proposed frameless display module is compared with the hologram displays using an SLM and the previous RR system. The magnification of the previous system was unity, and that of the frameless display module is larger than unity. Therefore, the screen size of the frameless display module is larger than that of the previous system. The horizontal pixel pitch of the frameless module is also larger than that of the previous system. Therefore, the horizontal viewing angle of the module is smaller compared with the previous system.
A frameless hologram display module employing a modified RR system was proposed. The modified RR system consisted of a one-lens imaging system, a screen lens, and a multiple illumination system. We constructed a prototype of a frameless hologram display module. The screen size was 2.0 inches. A frameless display screen was achieved. The RR technique increased the horizontal resolution to 7,920 and decreased the horizontal pixel pitch to 5.76 μm. The horizontal viewing angle was enlarged to 6.3°. The reconstructed images had smooth motion parallax and could be observed by both eyes. In a future study, a technique to remove higher-order diffraction components will be developed, and multiple display modules will be constructed to demonstrate the increase in the display screen size.
This research is partly supported by the National Institute of Information and Communications Technology (NICT), Japan.
References and links
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