D. Grosse¹, T. Schlauch¹, J.C. Balzer¹, A. Klehr², G. Erbert², G. Tränkle² and M.R. Hofmann¹

Published in:

Semiconductor and Integrated Optoelectronics (SIOE) conference, Cardiff, Wales, United Kingdom, Apr. 18-21 (2011).

Abstract:

So far, all concepts for three dimensional biomedical imaging rely on scanning in at least one dimension. Single-shot holography [1], in contrast, stores three-dimensional information encoded in an electro-magnetic wave scattered back from a sample in one single hologram. Single-shot holography operates with simultaneous recordings of holograms at different wavelengths. While the lateral sample information is stored in the lateral interference patterns of individual holograms, the depth information is obtained from the spectral distribution at each lateral image point, similar to Fourier-domain optical coherence tomography [2]. Consequently, the depth resolution of the reconstructed image is determined by the bandwidth of the light source, so that a broadband light source is needed to obtain high depth resolution. Additionally, the holographic material, in which the holograms are stored, restricts the useable bandwidth. Standard holographic materials, like photoplates, can only store one hologram at a time. A thick photorefractive crystal, in contrast, can store several holograms of different wavelengths at once. As the crystal works best when using a source with a discrete spectrum, a light source is needed that has a spectrum with well distinguishable laser lines. Thus, a comb-like laser spectrum is most suitable.
In a proof-of-principle experiment, we use colliding pulse mode-locked (CPM) laser diodes [3] as light sources. Theses diodes consist of two gain segments placed to each side of a saturable absorber segment. The gain segments are forward biased whereas the absorber segment is reverse biased. Passive mode-locking is achieved by counter propagating pulses which meet in the absorber section and efficiently saturate it. Thus, short pulses with ps duration and high repetition rates can be generated. This creates the desired comb-like spectrum. We use this comb-like spectrum to demonstrate the concept of single-shot holography by storing multiple holograms at the same time in a photorefractive Rh:BaTiO₃ crystal.

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