Interferometers with internal and external phase modulation: experimental and analytical comparison

Optical intensity noise in a light source easily degrades the sensitivity of a shot-noise-limited interferometer which is directly detecting low frequency phase or displacement variations. This paper describes and compare two experimental methods in which high frequency optical phase modulation was used to shift low frequency phase signals in an interferometer to a shot noise limited region of the photocurrent spectrum. This phase modulation is applied either within the interferometer arms internal modulation or in a local oscillator beam tapped off the main interferometer and coherently recombined with the interferometer output external modulation. The photocurrent is mixed electronically with the high frequency modulating waveform to extract the signal information free from laser intensity noise. It was possible to detect interferometrically low frequency signals with true shot-noise-limited sensitivity and found, theoretically and experimentally, that the interferometric sensitivity achievable in each scheme depends critically on non-ideal factors, such as imperfect interferometric fringe contrast and electronic noise in the detectors or amplifiers. The relative merits and operating requirements of both modulation schemes in practical interferometers are examined. 42 refs., 13 figs