Measurement of relative velocity of independent targets by a quantum cascade laser subject to optical feedback

A novel application of the feedback interferometry in quantum cascade lasers (QCLs) to directly measure the relative velocity of multiple coaxial targets, is proposed. QCLs with optical feedback make extremely compact sensors since they work as both a mixer oscillator and a shot-noise limited quantum detector, thus generating and sensing infrared radiation. In addition, the intrinsic stability of QCLs under strong optical re-injection is exploited here to access the nonlinearity of the QCL active medium. We demonstrate that the cumulative optical reflection from independent semi-transparent targets causes the interferometric signal to exhibit high contrast fringes as a function of the targets displacement along the optical axis. The experimental results using an external dual cavity are in excellent agreement with the simulations based on a Lang-Kobayashi model encompassing multiple target dynamics.