Polarization-Insensitive FOPA Based on a Polarization-Diverse Bidirectional PM Fiber Loop

A polarization-insensitive fiber optic parametric amplifier (FOPA) is demonstrated based on a polarization diverse bidirectional loop consisting of two equal-length highly nonlinear polarization maintaining fiber (PMF) sections and a 4-port polarization beam splitter (PBS). With equal pump power in the clockwise and counterclockwise directions, polarization sensitivity of OPA gain can be avoided without the need of polarization controlling in the loop. By 90° axis rotation between the two PMF sections the impact of polarization mode dispersion is minimized. Because the states of polarization of the counter propagating pumps in the loop are mutually orthogonal, stimulated Brillouin scattering (SBS) generated by the pump in each direction is not amplified by the parametric process of the pump in the opposite direction, which effectively eliminated the interference noise caused by pump/SBS mixing. System performance is investigated by inserting the FOPA in front of a dual-polarization 400Gb/s (16-QAM) coherent receiver as a preamplifier. Good agreement between measured and calculated BER versus received signal power indicated negligible polarization-dependent gain of the FOPA.