Sensitive all-single-mode-fiber resonant ring interferometer

A new type of fiber interferometer is presented which utilizes a high-finesse all-single-mode-fiber ring resonator. The measured sensitivity to periodic phase shifts induced in the fiber ring is
1.0 \times 10^{-6}
rad/
\sqrt{Hz}
or better over the 100-Hz-10-kHz frequency range with maximum sensitivity of
1.9 \times 10^{-7}
rad/
\sqrt{Hz}
at 10 kHz. This sensitivity, while high, is not shot-noise limited and appears to be determined by the spectral bandwidth of the single-frequency laser source.