Abstract

Double Phase Conjugate Minors (DPCM) are of great interest for the realization of free-space interconnections between single mode optical fibers. However, an alteration of the polarization of the incident beams, which can be easily induced by mechanical constraints on standard fibers, leads to a dramatic reduction of the conjugation efficiency. We present here a polarization-independent DPCM. The polarization insensitive experimental set-up is described in figure 1. A biréfringent calcite crystal is inserted at the output of the monomode fiber. Thus, two orthogonally polarized components are spatially separated. The polarization of the component that would not contribute to the formation of the DPCM in a conventional configuration is rotated in a liquid crystal cell. Both components are then directed into the InP:Fe sample with a lens. The pump beam results from the interference of the two components. Our experiment is performed by manipulating the polarization of only one pump beam to demonstrate the principle of the polarization independent DPCM. Obviously, the other pump beam could be obtained applying the same method. In order to eliminate conical diffraction and instabilities, this elementary set-up was incorporated in a 2Z-DPCM configuration. Experiments were carried out with an InP:Fe photorefractive crystal at the telecommunication wavelength of 1.32 μm. The diffraction efficiency and the depletion rate were measured for different polarization directions of the incident beam labeled A. The results are illustrated in figure 2. The averaged values of the depletion rate and the diffraction efficiency are 50% and 10% respectively. It is worth noting that these results are comparable to those obtained with a classical configuration. The relative variations of the diffraction efficiency are less than 8% when the polarization state of the incident beam varies. Keeping in mind that the relative variations can reach nearly 100% in a conventional DPCM, our device can be considered stable.

© 1998 IEEE