Conventional low-coherence interferometry (LCI) has been employed in the measurement of polarization mode dispersion (PMD) of fiber-optic components and fibers. However, the smallest PMD, which can be measured using this technique, is limited by the coherence length of the source. We propose and study a biased π-shifted Michelson interferometer where a birefringent crystal (biasing crystal) is inserted in front of the interferometer to introduce a bias differential group delay larger than the coherence time of the source. In this way, the limitation imposed by the source coherence time is overcome and PMDs much smaller than the source coherence time, in the order of several femtoseconds, can be measured.
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