  The LISA telescopes must exhibit an optical path length stability of
$\frac{\mathrm{pm}}{\sqrt{\mathrm{Hz}}}$ in the mHz observation band to meet
mission requirements. The optical truss interferometer is a proposed method to
aid in the ground testing of the telescopes, as well as a risk-mitigation plan
for the flight units. This consists of three Fabry-Perot cavities mounted to
the telescope which are used to monitor structural displacements. We have
designed and developed a fiber-based cavity injection system that integrates
fiber components, mode-matching optics, and a cavity input mirror into a
compact input stage. The input stages, paired with return mirror stages, can be
mounted to the telescope to form the optical truss cavities. We performed a
thorough sensitivity analysis using various simulation methods to support the
fabrication and assembly of three first-generation prototype cavities, each of
which exhibited a satisfactory performance based on our models.
