Detection sensitivity of laser feedback interferometry using a terahertz quantum cascade laser

We report on the high detection sensitivity of a laser feedback interferometry scheme based on a terahertz frequency quantum cascade laser. We show that variations on the laser voltage induced by optical feedback to the laser can be resolved with reinjection of powers as low as ~−125 dB of the emitted power. Our measurements demonstrate a noise equivalent power of ~1.4 pW/√Hz, although after accounting for reinjection losses we estimate this corresponds to only ~1 fW/√Hz being coupled to the quantum cascade laser active region.