  The polarisation Sagnac speedmeter interferometer has the potential to
replace the Michelson interferometer as the instrumental basis for future
generations of ground-based gravitational wave detectors. The quantum noise
benefit of this speedmeter is dependent on high-quality polarisation optics,
the polarisation beam-splitter (PBS) and quarter-waveplate (QWP) optics that
are key to this detector configuration and careful consideration of the effect
of birefringence in the arm cavities of the interferometer. A PBS with an
extinction ratio of better than 4000 in transmission and 700 in reflection for
a $41^{\circ}$ angle of incidence was characterised along with a QWP of
birefringence of $\frac{\lambda}{4} + \frac{\lambda}{324}$. The cavity mirror
optics of a 10m prototype polarisation Sagnac speedmeter were measured to have
birefringence in the range $1\times10^{-3}$ to $2\times10^{-5}$ radians. This
level of birefringence, along with the QWP imperfections, can be canceled out
by careful adjustment of the QWP angle, to the extent that the extinction ratio
of the PBS is the leading limitation for the polarisation Sagnac speedmeter in
terms of polarisation effects.
