  We observed that the polarization state of light after round-trip propagation
through a birefringent medium frequently aligns with the employed input
polarization state "mirrored" by the horizontal plane of the Poincare sphere.
In this letter we explore the predisposition for this mirror state and
demonstrate how it constrains the evolution of polarization states as a
function of the round-trip depth into weakly scattering birefringent samples,
as measured with polarization-sensitive optical coherence tomography (PS-OCT).
The constraint enables measurements of depth-resolved sample birefringence with
PS-OCT using only a single input polarization state, which offers a critical
simplification compared to the use of multiple input states.
