Integrating high-precision and fringe-scale displacement sensing using heterodyne cavity-tracking

We present a heterodyne stabilized cavity-based interferometer scheme that can serve as a compact and high-sensitivity displacement sensor with a fringe-scale operating range. The technique, in principle, can reach a sub-femtometer noise floor and an operating range on the order of one laser wavelength at $\lambda \approx 1\mu m$. With our current experimental setup, we achieve a sensitivity of about $260fm\mathrm{/}\sqrt{\mathrm{H}\mathrm{z}}$ at $1Hz$ and $46fm\mathrm{/}\sqrt{\mathrm{H}\mathrm{z}}$ at around $130Hz$. By probing a length actuated cavity, we demonstrate six orders of magnitude of dynamic range for displacement measurement, reaching a maximum motion of $0.15\mu m$. The tracking bandwidth and displacement range are limited by analog effects in the signal digitization and are extendable in the future.