## 6 Scanning System Using Pulse-Modulated MEMS SLM  
For viewing zone scanning holographic displays, the viewing zone angle equals the scan angle of the scanning system, and the screen size equals the optical window size of the scanning system. Therefore, a large scan angle and a large optical window are required for the scanning system. Among various scanning systems, this requirement can be satisfied by mechanical scanners, such as the galvano scanner. However, the use of the mechanical system reduces system reliability and stability. The use of the pulse-modulation of laser light to expose the MEMS SLM has been proposed to eliminate the mechanical scanning systems from the holographic display systems employing the MEMS SLM [29]. Figure 13 illustrates the holographic display using the pulse-modulated MEMS SLM. Short-pulse laser light exposes the MEMS SLM. The MEMS SLM consists of two-dimensionally aligned MEMS mirrors. The MEMS mirrors have binary states, i.e., an on state and an off state, and quickly rotate to alter their states from on to off, and vice versa. All MEMS mirrors alter their states simultaneously to display 2D images. The proposed technique uses a rotating state of the MEMS mirrors, which is not conventionally utilized to generate images. The pulse width of the laser is made shorter than the rotating time of the MEMS mirrors. Hologram patterns are sequentially displayed by the MEMS SLM. When the short-pulse laser light exposes the MEMS mirrors in the rotating state, the direction of light reflected by the MEMS mirrors is determined by the timing of the laser pulse exposure. Therefore, 2D images displayed by the MEMS SLM can be scanned by appropriately controlling the timing of the short-pulse generation. Thus, the pulse-modulation of the MEMS SLM enables the scanning of the viewing zone. When the rotation angles of the MEMS mirrors for the on and off states are respectively represented by \(+\alpha\) and \(-\alpha\), the possible scan angle is from \(+2\alpha\) to \(-2\alpha\). Therefore, the use of pulse-modulated MEMS SLM can enlarge the viewing zone angle to \(4\alpha\). Figure 12: 3D images produced by the two-channel display: (**a**) apple, (**b**) head, (**c**) T-REX, and (**d**) castle. The DMD was also used to verify the technique employing the pulse-modulated MEMS SLM. The MEMS mirrors of the DMD had a rotating angle of \(\alpha=12^{\circ}\). Thus, the possible scan angle was from \(+24^{\circ}\) to \(-24^{\circ}\). Thus, the viewing zone angle could be enlarged to \(48^{\circ}\). The rotation time of the MEMS mirrors was 3.83 \(\upmu\)s. The pulse width of the laser light was set to 20 ns. So, the number of scan points was 192. The frame rate of the DMD was 22,727 Hz. Thus, the frame rate of displaying 3D images was 59.2 Hz. An FPGA was used to control the pulse signal generation. Figure 14 shows the photograph of the experimental system. The elimination of the mechanical scanning system greatly simplified the display system. Figure 15 shows the reconstructed images. The effective scan angle was from \(-18^{\circ}\) to \(+24^{\circ}\), because undesirable distributions appeared in the minus-end scan region. The viewing zone angle was enlarged to \(42^{\circ}\). The undesirable light distributions were generated by the small vibration of the MEMS mirrors in the off state when the other MEMS mirrors changed from the off state to the on state. Figure 14: Experimental system of holographic display using pulse-modulated MEMS SLM. Figure 13: Holographic display using pulse-modulated MEMS SLM. The pulse modulation of the DMD was originally proposed for the Lidar system [30]. This technique was also used in the construction of the light field display [31]. Although light scanning using the pulse modulation of the MEMS SLM significantly simplified the display systems, the brightness of 3D images was low because the rotation time of the MEMS mirrors was short. The development of a specialized MEMS SLM for use in 3D displays consisting of MEMS mirrors that have a longer rotation time is important.