Edge-Lit Coherent Backlight for Flat-Panel Holographic Displays

Coherent backlight is an essential component for holographic displays. In this paper, a compact design of edge-lit coherent backlight featuring two holographic optical elements for two-dimensional beam expansion is presented. In experiments, the input laser beam is magnified by a factor of 89 with a diffraction efficiency of 4.3%. More importantly, holographic images can be successfully reconstructed with this backlight. INTRODUCTION Holographic displays are usually associated with a set of optical elements, which need a spacious room to be mounted. Among other things, the lasers as the coherent light source are too bulky for the holographic displays to be applied in the consumer electronics. In order to meet the industry trend known as the flat-panel, a backlight with a compact form factor is desired. Although some efforts have been made on compact expanders for lasers using optical folding method or edge-illuminated holograms [1], they can only expand the beam in one dimension. In 2002, Shechter et al. introduced a planar beam expander with multiple gratings [2], the input narrow beam could be magnified by a factor of 25. In 2013, Xiong et al. [3] proposed a two-dimensional (2D) coherent backlight design using a scattered wave to read out a reflection hologram. Nevertheless, its diffraction efficiency (DE) is only 0.3%, which is too low for practical applications. In this paper, we present an edge-lit coherent backlight design with DE being improved up to 4.3%, which is one order in magnitude higher than its former design [3]. PROPOSED BACKLIGHT DESIGN The schematic drawing of the proposed coherent backlight design is depicted in Fig. 1. It consists of two HOEs, which are essentially two linear gratings. The first HOE (H1) is used to expand a laser beam along the vertical direction as an elongated beam, which will be further expanded along the horizontal direction by the second HOE (H2). Therefore, 2D beam expansion is realised. The slant angles of the HOEs, i.e. α and β, determine the width and thickness of the backlight, respectively. The propagation direction of undiffracted wave should be perpendicular to the diffracted direction to reduce disturbance. All the other higher diffraction orders can be negligible if both HOEs are transmission volume holograms. For achieving high DE, Bragg mismatch [4] should be avoided by firmly fixing the angles of the reading beam, H1, and H2. It should be noticed that H1 and H2 are not equivalent to two mirrors, upon the reflection of which, the shape and size of the laser beams will keep unchanged. Fig. 1. Schematic drawing of the edge-lit coherent backlight. EXPERIMENTS The material used to fabricate the foregoing HOEs is polymer dispersed liquid crystal (PDLC) [5], which comprises 50 wt%: 35.4 wt%: 13 wt%: 1 wt%: 0.6 wt% of liquid crystal 5CB (HCCH): TMPTA monomer (Aldrich): N-vinylpyrrollidone (Aldrich): N-phenylglycine (Aldrich): Rose Bengal (Aldrich). The uniform PDLC mixture is injected into an empty cell, whose cellgap is controlled by 30-μm Mylar spacers. Figures 2(a) and 2(b) illustrate the optical setups for H1 during recording and reconstruction, respectively. In Fig. 2(a), the recording light derived from a 488-nm laser is set to s-polarization to ensure high interference efficiency as the recording angle is almost 90°. The reference beam is emitted from the laser with a diameter 0.1 cm and the area of the spot is 0.0079 cm 2 . The slant angle α of H1 is 6°, which enables the reference beam to illuminate an elongated elliptic area (0.075 cm 2 ) on H1. In addition, the object beam is expanded to 1 cm in diameter by a conventional beam expander. The intensities of both recording beams are 2 mW/cm 2 on H1, and the exposure duration is 1 minute. In Fig. 2(b), the object plane wave is blocked by a black board, and the reference beam reconstructs a collimated elongated beam, which will later interfere with another plane wave during recording of H2.