Superluminal and subluminal laser employing polarization and wavelength-selective cavity mirrors

A Raman laser, which can be either superluminal or subluminal, with applications to precision metrology, is constrained by the fact that its polarization must be orthogonal to that of the pump, which has to co-propagate with the laser over the gain medium. As such, inherent imperfections of state-of-the-art polarizing beam splitters (PBSs) used inside the cavity for injecting and ejecting the pump limit the effective finesse for the laser, which in turn reduces the sensitivity for various sensing applications. Here, we show how polarization selective reflectors (PSRs) can be used to mitigate this limitation. PSRs can be highly reflective for one polarization and highly transparent for the orthogonal polarization for a certain choice of angle and wavelength. The PSR is based on a distributed Bragg reflector (DBR) structure while operating at an unconventional point where one polarization is at the edge of the bandgap, and the orthogonal polarization is outside the bandgap. We present both simulation results and measurements of the PSR's reflection and transmission spectra, for parameters that may yield a potential improvement of ∼32 in the minimum measurable rotation rate for a superluminal ring laser gyroscope utilizing this technique.