  Optical cavities have found widespread use in interfacing to quantum
emitters. Concerns about backreflection and resulting loss, however, have
largely prevented the placement of optics such as lenses or modulators within
high-finesse cavities. In this work, we demonstrate a million-fold suppression
of backreflections from lenses within a twisted optical cavity. We achieve this
by quantitatively exploring backscatter in Fabry-P\'erot resonators, separating
the effect into three physical sectors: polarization, mode envelope and spatial
mode profile. We describe the impact of each of these sectors, and demonstrate
how to minimize backreflections within each. This culminates in measured
effective reflectivities below the part-per-billion level for the fundamental
mode. Additionally, we show that beams carrying orbital angular momentum
experience up to $10^{4}$ times additional suppression, limited only by the
density of states of other cavity modes. Applying these ideas to laser
gyroscopes could strongly suppress lock-in, thereby improving sensitivity at
low rotation rates.
