Holographic three-dimensional displays with liquid crystal on silicon spatial light modulator Aneta Michalkiewicz*a, Malgorzata Kujawinskaa, Tomasz Kozackia, Xinghua Wangb, Philip J. Bosb a Institute of Micromechanics and Photonics, Warsaw University of Technology, 8, Sw. A. Boboli Str., 02-525 Warsaw, Poland b Liquid Crystal Institute, Kent State University, POB 5190, Kent, OH44242 ABSTRACT One of the ways to achieve 3D objects visualization is holography. The recent progress of CCD/CMOS cameras provides quick development of digital holographic recording. Optoelectronic reconstruction of digital holograms can be realized by means of variety of spatial light modulators, however each of them suffers several limitations due to big pixel size, low diffraction efficiency and noise. In the paper high efficiency liquid crystal on silicon (LCOS) spatial light modulator is proposed as the novel solution for optoelectronic holographic reconstruction. The system for holograms reconstruction based on LCOS is presented. The results of initial experiments on reconstruction of computer generated and digital holograms of different classes of 2D and 3D objects are shown and discussed. The problems connected with limited resolution of the recording (CCD) and reconstruction (LCOS) devices are considered. The comparison of the results obtained by numerical and optoelectronic reconstruction of digital holograms is presented, together with a discussion of the limitations and further possibilities of these techniques. Keywords: digital holography, digital holographic interferometry, liquid crystal on silicon spatial light modulator, 3D visualization, hologram reconstruction 1. INTRODUCTION Optoelectronic reconstruction of digital holograms has been a challenging task for many years. Conventional holography offers good three dimensional images. It is connected with excellent resolution of holographic films which reaches 5000 lines\mm. The big challenge is to replace the holographic film by digital media and still obtain good quality of reconstruction. The recent achievements in CCD cameras and new generation of high efficiency liquid crystal devices have provided new tools for realization of optoelectronic reconstruction of digital holograms. Several approaches to this task have been made. The most popular and simple approach is the method based on electronically addressed LCDs [1,2]. As alternative, method based on acoustooptic cells was proposed by [3]. Lately the method based on optically addressed LC cells with DLP hologram projection have been proposed. [1]. However non one of this realizations have fulfill satisfactory the requirements connected with optoelectronic reconstruction of digital holograms. Recently substantial progress in the development of liquid crystal on silicon (LCOS) devices has been observed [4,5]. LCOS is characterized by high diffraction efficiency, high ratio of fill factor and gradually decreasing pixel size [6]. Those features identify LCOS as a very promising solution for optoelectronic reconstruction of digital holograms. The goal of this paper is to define the principle limitations and challenges of digital holographic recording and their optoelectronic reconstruction by means of LCOS devices. Also it shows the results of initial experiments using LCOS working in amplitude and phase modes for reconstruction of computer generated structures and digital holograms captured. Their application for 3D objects visualization and deformation analysis (digital holographic interferometry) is discussed. For DH and DHI the experimental results are compared with numerical reconstruction of the same holograms. * a.michalkiewicz@mchtr.pw.edu.pl. phone: (+48) 22 660 86 35, fax: (+48) 22 660 86 01, http://zto.mchtr.pw.edu.pl Interferometry XII: Techniques and Analysis, edited by Katherine Creath, Joanna Schmit, Proceedings of SPIE Vol. 5531 (SPIE, Bellingham, WA, 2004) 0277-786X/04/$15 �� doi: 10.1117/12.560762 85

2. LIMITATION AND CHALLENGES IN DIGITAL HOLOGRAMS RECORDING AND OPTOELECTRONIC RECONSTRUCTION 2.1 Limitations and challenges in DH recording Recording of digital hologram is mainly limited by the spatial resolution of a CCD target which is much lower than that of a holographic plate. This means that the sampling theorem limits the spatial resolution of the intensity distribution to be stored. The maximum spatial frequency fmax to be resolved by the recording medium is determined by the maximum angle ��max between the reference and object wave: ) max max sin(�� 2 �� = f or ��� 2��� max �� �� (1) where �� denotes the wavelength, ��� is the pixel sampling at CCD. Recently CCD cameras pixel size is between 9-5 ��m i.e. up to 200 lines/mm. For that reason the maximum angle between the interfering waves is limited to a few degrees, (Fig. 1a). Without any optical imaging, small objects or objects in large distance from the CCD target may be recorded only. The most popular solution to overcome this restriction is to use a divergent lens in Fresnel set-up (Fig. 1b). This reduces the angle under which the object light incidents the target [7]. The effect is a significant reduction of the spatial frequency spectrum to be resolved, however the detailed information about object structure is lost. The second solution proposed in [8] is to apply multiexposure holograms to code parts of large object or scene in holograms recorded simultaneously at CCD (Fig. 1c). Considering the information about object(s) recorded by means of multiple of reference beams coming from different directions its extended spatial bandwidth may be nicely ���packed��� into the allowed spatial bandwidth of the recording detector . However in this case the complexity of recording and further reconstruction setup increases significantly. a) b) c) Fig. 1 Configuration allowing recording an object defined by angular size ��: a) direct recording, b) recording with demagnified angular size �����, c) multiexposure recording . H ��� hologram plane, Ri ��� reference beam The second limitation of DH recording is related to the size of hologram which is just the product of the number of pixels and distance between them (X=N��Dx, Y=M��Dy). Typically it ranges from 7 mm to 15 mm, which may not allow proper reconstruction of the effects connected with real 3D visualization of an bigger object. The most popular way to overcome this limitation is to extend numerically the aperture by producing of mosaic hologram composed by x and y repetition of recorded hologram. The more complicated but more accurate method is to capture numerous holograms (by CCD matrix shifting) within the plane where interference between object and reference beams occur and combine these holograms into a large synthetic aperture DH [9]. 86 Proc. of SPIE Vol. 5531