Programmable Broadband Polarization-Preserving Optical Delay Line with Faraday Mirrors

We report a broadband optical delay line programmable for delays up to 1
${{\bf \mu s}}$
with nanosecond resolution, while preserving an
arbitrary
and
unknown
polarization state of the original light. The delay line comprises bidirectional pass in cascaded segments of standard single-mode fibers, along with terminal reflection from Faraday mirrors (FMs) at both ends. Each segment contains either a short path or a long path, selected by an optical switch. The FMs auto-compensate for random polarization fluctuations introduced by the optical fibers, thereby restoring the initial polarization state. Broadband operation of the delay line was assessed by investigating dispersion of both the fiber and the FMs. Surprisingly, we found that the two-pass propagation in the fiber can mitigate dispersive distortion introduced by the FM. Numerical experiments demonstrated that the output polarization state of the delay line has a mean fidelity better than 99% over a range of 50-nm deviation from the central wavelength. Remarkably, the mean fidelity often surpasses that in the absence of the fiber itself.