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\text{fm}/\sqrt{\text{Hz}} displacement sensitivity for a 5\text{\hspace{0.17em} Hz} signal with a few milliwatts of optical power, an improvement of 3 orders of magnitude over the unenhanced system.