Pulsed Full-Color Digital Holography with a Raman Shifter

Abstract

Holography deals with processes involving the transformation of waves by interference structures that are formed when coherent electromagnetic waves interact with matter (Andreeva, 2002). As an optical technique it was introduced in the seminal works of Gabor, Leith, Denisyuk and others in the 1960’s (Benton, 2005). By succeeding to reconstruct the complete wave (full amplitude and phase information), holography yields depth information of a volume (three-dimensional) scene. The main goal of holography in imaging is to reconstruct wavefronts from 3-D colored and moving objects in real time. Sustained success however, is still hampered by a number of technical hurdles that need to be overcome. Full-color reconstruction requires a multiwavelength light source and a wide bandwidth light-sensitive recording medium. Conventional color holography utilizes three separate continuous-wave (CW) lasers as illuminator which is more expensive and difficult to operate and maintain. The narrow temporal bandwidth of CW lasers also restrict their applications only to holographic imaging investigations involving stationary objects. Here we discuss the use of the hydrogen Raman shifter as a pulsed color holographic light source. The Raman shifter is an attractive holographic light source because it is pulsed, compact, inexpensive, and multi-wavelength (Almoro et al., 2004; Almoro et al., 2007). Pulsed color light sources extend the range of possible applications of color holography to fast moving objects and rapid events. Due to significant advances in photodetector and computer technology, holograms can now be recorded with high-resolution digital cameras and reconstruction could be carried out numerically in a fast manner (Schnars & Juptner, 2002). Digital holography is suitable for industrial applications since it does not involve the cumbersome development of photographic films. In full-color digital holography, a minimum set of three holograms are captured corresponding to the primary color channels (i.e., red, green and blue color channels). For a proper fit of the superposition of the reconstructions, it is important to characterize and control the wavelength dependences of the image size, lateral resolution and depth of focus. These image variations result in the dispersion of the color hologram reconstructions making the final image blurred and unacceptable. Here we discuss the experimental evidence of the said effects and describe a technique to control the chromatic dispersion of full-color holograms.