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The use of optical fibers in holographic set-ups allows the recording of holograms of objects which are not directly accessible to conventional optical technics. The problems for recording holograms through optical fibers are of the same nature as for producing optical fiber sensors. But the optimal use of the two technics corresponds to opposite conditions: sensors require a great sensitivity to phase variations while the hologram quality is disturbed by a very low variation. There are two possibilities of freezing the phases during the recording of the hologram: 1) in cwl.laser holography it is necessary to avoid any variation of physical parameters (temperature, pressure, electromagnetic field, vibrations, twisting, mechanical strain, air turbulence, etc...) influencing the phase or the polarisation of the light transmitted by the fiber; 2) use of a pulse laser with a very short time of emission. Unfortunately in this case non-linearity phenomena may occur in the fiber: reduction of the coherence length, light frequency variation. The focusing of the light beam may also ionize the air and damage the input face of the fiber. At last, the very high powers may destroy the fiber. In spite of those drawbacks holograms of small objects have been recorded through multi-mode and monomode fibers with the help of ruby and YAG lasers. A) Ruby laser: with a ruby laser giving a pulse of 20 mJ in 20 ns at 0.6943 pm the following set-ups have been tested: a) The object beam passes through a 1 mm-multimode step-index fiber; no fiber for the reference beam. b) Same arrangement for the object beam, the reference passing through a monomode fiber. The beamsplitter is located between the oscillator and the amplifier. B) YAG-laser: with a YAG-laser giving a pulse of 20 mJ in 20 ns at 0.53 pm the following set-ups have been tested: a) A 1mm-multimode step-index fiber in the object beam and a 133 pm-multimode step-index fiber in the reference beam. b) Same arrangement for the object beam and a monomode fiber in the reference beam. In each case single- and double-exposure holograms have been recorded. Some typical results are presented and discussed.
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