  Resonant laser spectroscopy is essential for the characterization, operation,
and manipulation of single quantum systems such as semiconductor quantum dots.
The separation of the weak resonance fluorescence from the excitation laser is
key for high-quality single- and entangled photon sources. This is often
achieved by cross-polarization laser extinction, which is limited by the
quality of the optical elements. Recently, it was discovered that
Fresnel-reflection birefringence in combination with single-mode filtering
counteracting spin-orbit coupling effects enables a three-order of magnitude
improvement of polarization extinction [PRX 11, 021007 (2021)]. Here, we first
investigate multiple reflections and analyze beam reshaping, and observe that
the single-reflection extinction enhancement is optimal. We then demonstrate
this method for cross-polarization extinction enhancement for a resonantly
excited semiconductor quantum dot in a birefringent optical micro cavity, and
observe a 10x improvement of single-photon contrast.
