Ultra-sensitive focused laser differential interferometer using balanced detection

Balanced detection is demonstrated to be a simple method for achieving shot-noise-limited measurements with a focused laser differential interferometer (FLDI). Both polarization components of the signal beam are separately detected, and differential amplification of the two signals cancels common-mode laser-intensity noise. An ultrasonic source is used to provide a reference signal to compare measurements made by a conventional FLDI with those from the modified instrument. Balanced detection is shown to improve the signal-to-noise ratio by 30 dB and demonstrated to achieve shot-noise-limited measurements.