A proposed electro-optical image-processing system would enable remote viewing of an object through a multimode optical fiber, as though the fiber were a conventional image-transmitting optic like a lens or prism. Ordinarily, it would be impractical to use a multimode optical fiber as an imaging probe or imaging optic because image information would become distorted (scrambled) during propagation along the fiber. The proposed system would provide compensation for this scrambling. Fiber-optic imaging probes of this type could be made very thin and could be particularly useful as minimally invasive probes in medical diagnosis.

An Unscrambled Image of the Object could be generated at point B by exploiting phase conjugation at point C to reverse the scrambling that occurs during propagation along the optical fibers.

The system (see figure) would include two multimode optical fibers, which would be terminated side by side at one end (point A) facing the object that one seeks to view. The tips of the fibers would lie at a short distance,s, from the object. A source of light at point C would illuminate the object via fiber 2. An observer at point B would attempt to view the illuminated object through fiber 1. The problem is to predistort the illumination (prescramble the amplitudes and phases of the fiber-optic waveguide modes of the illuminating electromagnetic field) in such a way as to compensate for the scrambling that occurs during transmission of the image along fiber 1 from point A to point B, so that the image of the object would arrive unscrambled at point B.

The solution would involve the generation and use of a hologram in a phase-conjugating crystal at point C. First, a flat mirror would be placed facing the tips of the optical fibers, where the object would later be placed for viewing. A source of light would be placed at point A (where the observer would later be stationed). Light from this source would travel through fiber 1 to point A, where it would be reflected into fiber 2. Upon emerging from fiber 2 at point C, the light would enter the crystal. At the same time, the crystal would be illuminated with a reference (plane-wave) beam of light. Interference between the reference beam and the light emerging from fiber 2 would produce the desired hologram, which would encode the information about scrambling in both fibers 1 and 2.

Once the hologram had been generated, one could exploit the phase-conjugation principle to reverse the propagation of the optical signal and thus reverse scrambling. The crystal would be illuminated with the phase conjugate of the reference beam (in essence, a beam of the same wavelength propagating along the reverse of the path of the reference beam); this would cause reverse-propagation with unscrambling of light from point C back to point A, then back to point B. If the mirror were replaced by the object to be viewed, then the reverse-propagating light would illuminate the object and the image of the object would spatially modulate the reverse-propagating beam, such that an undistorted image of the object would appear at the completion of reverse propagation and unscrambling at point B.

This work was done by Deborah Jackson of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category, or circle no. 177 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).


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Multimode Optical Fiber as Imaging Probe

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This article first appeared in the February, 1998 issue of Photonics Tech Briefs Magazine.

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