Volumetric display created by holographic laser drawing

Volumetric displays have received much attention for use as 3D displays in optics and computer graphics. These displays can produce 3D images that can be observed from any point of view without physical discomfort or the use of any special devices.¹ Volumetric displays based on laser irradiation have been constructed using plasma,^{2, 3} quantum dots,^{4, 5} and rare earth elements.^{6, 7} These displays have a wide angle of view because they require no physical connection between the light source and the display volume. With previous approaches, however, the number of voxels in a volumetric display is not large enough to generate practical volumetric images because of limitations imposed by the repetition frequency of the laser and the speed of the 3D scanning system.

We have generated a volumetric display using a holographic laser drawing technique and a multilayer fluorescent screen.⁸ We carry out holographic laser drawing by means of a computer-generated hologram (CGH), which is displayed on a liquid-crystal spatial light modulator (LCSLM) to increase the number of voxels in the volumetric display per unit time. We use the fluorescent screen to produce a multicolored display.

Figure 1 shows the experimental setup of our volumetric display system. The light source was an amplified femto second laser (Micra and Legend Elite Duo, Coherent) with a center wavelength of 800nm, a repetition frequency of 1kHz, a pulse duration of <130fs, and an output power of 7W. A voxel was formed by two-photon absorption excited by a focused femtosecond laser pulse. We changed the position of the focus using a 3D beam scanner and a Fourier CGH. The 3D beam scanner comprised a Canon GM-1010 2D galvanometer scanner and an Optotune EL-10-30-C varifocal lens. The galvanometer scanner controlled the position of the focus in the horizontal direction. The maximum deflection angle was ±20 degrees, the step response time was 280μs per 0.1 degrees, and the resolution was 20 bits. The varifocal lens, which controlled the focus in the axial direction, had a focal length that could be tuned in the range from +200 to +80mm, a response time of <2.5ms at a percentage step size of 10–90%, and an aperture of 10mm.

Figure 1. A holographic volumetric display is generated using a femtosecond laser and a multilayer fluorescent screen. SLM: Spatial light modulator.

The CGH was displayed on a Hamamatsu X8267 LCSLM with 1024×768 pixels. The parallel beams used to generate the CGH for holographic parallel optical access⁹ were designed by an optimal-rotation-angle method.¹⁰ The 3D beam scanner and LCSLM were controlled by programs in C++ on a PC running the Windows 7 operating system.

Figure 2(a) shows a volumetric display rendered by 3D scanning with a single focused beam. This image was taken by a camera with an exposure time of 1/8 second. The repetition frequency of the laser pulses was 1kHz and the irradiation energy was 1.8μJ. The screen comprised eight fluorescent layers containing coumarin 481. The layers had dimensions of 20×20×0.5mm and were stacked with gaps of 0.5mm between them. Figure 2(b) shows a volumetric display obtained by 3D scanning using holographic laser beams with four optical accesses. The total pulse energy was 3.2μJ. The brightness and number of voxels of the volumetric images were increased using the parallel optical access.

Figure 2. Volumetric displays are generated using (a) a single optical access and (b) holographic laser with four accesses.

Figure 3 (center) shows a volumetric display composed of both red and blue-green voxels, which was generated using a fluorescent screen with three layers: two containing coumarin 481 and one containing rhodamine B. Figure 3 (left) and (right) show the fluorescence spectra of coumarin 481 and rhodamine B, respectively. The pulse energy was 1.6μJ.

Figure 3. Left: Fluorescence excitation and emission spectra of coumarin 481. Center: Volumetric display with fluorescent layers of different colors obtained by using coumarin 481 and rhodamine B. Right: Fluorescence excitation and emission spectra of rhodamine B.

In summary, the holographic laser drawing technique that we have developed not only increases the number of voxels that the display can render but also enables the formation of brighter volumetric images. In future work, this system must permit multiple fluorescent plates to be accessed in parallel. This can be achieved using the ability of the hologram to easily control the focal length for separate spots of light. The maximum number of holographic parallel optical accesses in our system was 88, which we could increase with a more-sophisticated CGH optimization method;¹¹ it should be easy to produce more than 1000 parallel pulses. We have demonstrated a two-color volumetric display, but we need improvements in materials and screen structure to obtain a full-color display. Here, we have used fluorescent voxels formed by two-photon absorption in a polymer plate, but our system also has the potential to render aerial graphics¹² and bubble graphics via femtosecond laser-induced breakdown.