  Here we introduce interferometric devices by combining optical feedback
(cavities) with unbiased multiports, which unlike traditional beam dividers,
allow light to reflect back out of the port from which it originated. By
replacing the traditional, directionally-biased beam-splitter in a Michelson
interferometer with an unbiased multiport, the functional dependence of the
scattering amplitudes changes. As a result, the derivative of transmittance
with respect to an external phase perturbation can be made substantially large.
This significantly enhances the resolution of phase measurement, and allows the
phase response curves to be altered in real time by tuning an
externally-controllable phase shift.
