EXTERNAL PHASE-MODULATION INTERFEROMETRY

We analyze and test a laboratory benchtop version of a compound interf erometric phase sensor, a Michelson interferometer whose output is com bined coherently with a phase-modulated local oscillator beam tapped o ff the Michelson input beam. This configuration models a whole class o f external-modulation interferometers designed to shift signals, obscu red by low-frequency intensity noise of the light source, into a shot- noise-limited region of the photocurrent spectrum. We find analyticall y that the shot-noise-limited sensitivity achievable with this system is comparable with that obtained by using internal phase modulation, w ith both schemes suffering (for different reasons) approximately a 22% sensitivity penalty compared with ideal shot-noise-limited direct det ection. Experimentally we achieve true shot-noise-limited sensitivity, and we investigate trade-offs necessitated by commonly encountered no nideal features in any external-modulation system. Our analytic model, which specifically accounts for Michelson fringe contrast, electronic receiver noise, phase-modulation depth, and the local oscillator tap- off fraction, is sufficiently accurate to predict the absolute sensiti vity of our benchtop instrument to within 0.5 dB.