Holographic projection based on tapered lasers and nematic liquid crystal on silicon devices N. Collings* a , M. Reufer b , R.V. Penty a , B. Sumpf c , M. Safer d , D.P. Chu a , W.A. Crossland a a Dept of Engineering, Cambridge, UK; b OSRAM Opto Semiconductors GmbH Regensburg, 93055 Germany; c Ferdinand-Braun-Institut für Höchstfrequenztechnik Berlin, 12489 Germany; d FISBA OPTIK AG St. Gallen, 9016 Switzerland. ABSTRACT Holographic laser projection is high efficiency when using analogue phase spatial light modulators and high brightness laser sources. The experimental work reported will describe the use of digitally addressed nematic liquid crystal on silicon devices for the analogue phase holograms. The laser sources are based on the tapered laser concept and have been provided by a European project called WWW.BRIGHTER.EU. Keywords: Spatial light modulator, liquid crystal on silicon device, tapered laser. 1. INTRODUCTION Holographic projection is an alternative to conventional projection display where the projected image is formed by beam propagation from the hologram plane. The principal advantage of this alternative is that a phase hologram can be used , and the resulting display is more efficient than a conventional projection display. It is additionally important that, since there is not a 1:1 correspondence between the pixels on the display device and the image on the screen, then faults in the display device are not readily apparent in the projected image. The use of beam propagation to form the image places a stricter demand on the light source which is used than does a conventional display. It is important that the light incident on the display device or spatial light modulator (SLM) is spatially coherent. The image will then be formed in the Fourier plane of the lens which receives the beam reflected from the SLM. Lasers are efficient sources of spatially coherent light, and a green laser was employed in the early holographic projectors , 1,2 . Three lasers, red, green, and blue, are used in full colour projectors 1,3 . These provide a colour gamut which is more extended and richer than that provided by filtered white light. A second advantage of a solid state light source is that it can be temporally modulated. This is especially important when the luminosity of the projected image changes due to the scene content. Although low power lasers can be modulated at the speed required for this application, this is not the case for higher power lasers. The provision of high power, modulable red and green lasers for holographic projection was one of the goals of WWW.BRIGHTER.EU. This paper reports their use in this application. 2. CONSTRUCTION OF HOLOGRAPHIC PROJECTOR 2.1 Principle The tapered lasers developed in the Brighter project emit a high power single mode beam of narrow linewidth. The taper amplifier integrated with the laser provides a single mode output beam, which can be propagated to the display device using anamorphic optics. The latter produces an elliptic spot profile which is well matched to the 16:9 aspect ratio of the SLM used in the projector. The SLM is a digitally addressed nematic liquid crystal on silicon (LCOS) device, HEO 1080P, from Holoeye Photonics AG. The device has 1920 x 1080 square 8 μm pixels with a fill factor of 87% and an overall reflectivity of approx. 60%. It operates in phase-only mode when the incident polarization is along the alignment direction of the liquid crystal. The optimum beam waist at the device is a trade off between overfilling and underfilling the device area 4 . Overfilling results in loss of efficiency due to the light outside the display area being lost. Underfilling results in a less efficient holographic replay. The optics was designed so that the visible edge of the beam profile along the major axis filled the long axis of the display. The LCOS device displays an analogue phase hologram which is Invited Paper Liquid Crystals XIV, edited by Iam Choon Khoo, Proc. of SPIE Vol. 7775 777504 · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.858819 Proc. of SPIE Vol. 7775 777504-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 07/31/2013 Terms of Use: http://spiedl.org/terms