Transmission of the image signal with the use of a multimode fiber.pdf
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1 February 1996 OPTICS COMMUNIC TIONS Optics ommun ications 123 (1996) 629-636 Transmis sion of the image signal with the use of a multimode fiber Maxim A. Bolshtyansky, Boris Ya. Zel’dovich CREOL: Center for Research and Education i n Opt ics and Lasers, ni versity of Cent ral Flori da, 12424 Research Parkway, Suit e 4GQ , Or lando, FL 32836-327 1, USA Received 17 May 199 5 Abstract ‘ho schemes of the image acquisition through a multimode fiber are suggested. Both are based on the assumption that the input-to-output propagation Green function is stabilized by some or other technique. In both schemes the object is character ized by the transverse distribution of reflection pattern, and the reflected signal is to be collected by a photode tector for further computer processing. The applications for endoscopy are discussed. 1. Introdu ction The problem of getting pictorial information from some or other closed space is important not only from the application point of view but also as an intel- lectual challenge. Multimode optical fibers in princi- ple can transmit rather large number of information- bearing pixels: it essentially coincides with the num- fiber with the optical core diameter about 0.3 mm and the step of refractive index An = 0.01. Small diame- ter allows for safe penetration of a monofiber endo- scope into the most intricate parts of the human body or of a complicated mechanical or electronic device. The difficulties connected with the use of multimode monofiber are also well known: the image is scrambled at the first millimeter of in-fiber propagation. Even in pure theoretical investigation of a near-optimal case o f parabolic-index fiber with a resolution 100 x 100 el- ements, the scrambling distance does not exceed few meters [ 11. However, the interferenc e-type mixing of the waves in a fiber is quite different from the thermo- dynamically irreversible processes in particle mechan- its. Just those specific features of wave propagation - reversibility and reproducibility - allow in principle to overcom e the scrambling and to restore the encoded image. New group o f ideas has emerged in connection with the discovery of such phenomena as “Optical Phase Conjugation” and “Dynamic Holography” [ l-31. In particular, it was suggested [4] by Yariv that the pro- duction of phase conjugated (or time reversed) replica of the scrambled pattern of the laser radiation and its subsequent b ack propagation through an identical fiber allows to get the unscrambled image, just due to the time-reversibility of the wave propagation process. Up to now the reconstruction of the image via non- linear optical phase conjugation was realized [ 51 not through an identical fiber, but through the original one. While itself it was an important achievement, the im- age appeared in [S] at the “wrong” end of the en- doscope (i.e. inside the body for the case of med- ical endoscopy). The reason was the practical im- possibility of creation of two identical fibers that are placed and bent identically with inter ferometric accu- racy. Another disadvant age is the requirement of a sta- 0030~4018/%/%12.00 0 1996 Ekevier Science B.V. All rights reserved SSDI 0030-4018(95) 00589-7
