  Polarization maintaining (PM) fibers are meant to maintain linear
polarization along a preferred axis. A PM fiber can be seen as the fiber
version of a very high order waveplate, designed with different refractive
indices along two orthogonal axes. It is shown that monitoring the polarization
of initially circularly polarized light sent through a PM fiber, leads not only
to new sensing methods, but also to power control, saturable absorption, and
optical path stabilization. Even at peak power levels not exceeding a few mW,
nonlinear transmission is detected, with time constants in the microsecond
range.
