Electronic holography at terrahertz and infrared frequencies

N. George
The Institute of Optics, University of Rochester, USA
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We describe a new holography system for recording holograms that is practical from 1 mm through 3 µm wavelengths. These hitherto inaccessible bands of wavelength can now be recorded with excellent sensitivity and resolution using microbolometer arrays that have improved greatly in the past few years. These spectral bands are useful for biomedical imaging as an alternative to x-rays in some situations, for astronomy, for industry and for night vision devices. In holography following Leith and Upatnieks, one records an interference pattern using a coherent source; and thereafter, reconstructs a wavefront which reproduces the phases and amplitudes of the original object waves. For our first experiments we will describe the verification of our assertion that one can truly record interference patterns. We use a simple Mach–Zehnder interferometer with laser illumination at 10.6 µm. It is interesting that one can record the carrier frequency fringe of a hologram using a thermal detector even with a time constant of tens of milliseconds. For reconstructing the object wavefront in holography there are several well-known techniques. First, one needs to choose between an original illumination beam and a reversed beam. Then one can consider scaling the hologram to the visible band. Finally, with the hologram digitally recorded, one can use modern computer-generated visual displays. We will describe our experiments in which we use the efficient, direct sampling of the carrier frequency fringes [9,10]. Using well-known theories for the inverse scattering and the digital computer, one can calculate the field and the intensity back in the object space. Details of the phase unwrapping process will be described. This talk is dedicated to Professors Emmett N. Leith and Stephen A. Benton, eminent scholars and beloved colleagues.