Modulation-enhanced sensitivity of holographic interferometry

A modulation-demodulation scheme substantially enhances diffusion-dominated-adaptive-interferometric sensitivity. The path length sensitivity is improved by converting a quadratic small-signal response, easily drowned in system noise, to a linear response by mixing with a strong phase modulation. This conversion also shifts low-frequency signals away from 1/f noise. Experimental results show 180 fm/ [square root]Hz displacement sensitivity for a 5 Hz signal with a few milliwatts of optical power, an improvement of 3 orders of magnitude over the unenhanced system.