Multimode fiber interferometry with and without phase conjugation

It is generally assumed that fiber interferometers must be constructed from single-mode fibers. An alternative is to employ phase conjugation mirrors on the arms of a multimode fiber interferometer so that the spatial scrambling of modes is reversed, and good fringe contrast is maintained. When several interferometers all share the same self-conjugating BaTiO(3) crystal, however, the results show crosstalk between the separate fibers ranging from negligible to prohibitively large depending on the precise spatial configuration within the conjugator. A model accounting for these results is proposed. In addition it was observed in control experiments with conventional Mach-Zehnder configurations that good data could be obtained despite reduced contrast with both graded and step index multimode fibers.