System for a remote readout of multiple passive sensors using 31 THz quantum cascade laser

We report on an application of a pulsed distributed feedback quantum cascade laser (QCL) for long open path readout system of multiple passive temperature, pressure, humidity or other sensors. Unlike the usually used active sensors, containing radio or optical data transmitters, the presented system does not contains any active transmitters at the sensors side. The QCL and the receiver are displaced on one side of the path. On the opposite path side, a modified Michelson interferometer, based on corner cube retroreflectors, is placed. One retroreflector is fixed and another one is attached to the appropriate sensor, which moves it when the measured parameter is changed. The useful travel range of the movable retroreflector is less than lambda/2 (4.8 mum) to obtain a monotonic transfer function between the measured parameters and the received signals. This system measures distances in the micrometer and sub-micrometer range but displaced up to a few kilometers from the transmitter/receiver station. A relatively long laser pulse of 150-200 ns is used to scan the interferometers and to read out the information. During the pulse, the QCL frequency is swept within a 40 GHz range. This frequency sweeping allows decreasing significantly the influence of the atmospheric turbulence over the accuracy of the data transfer. When the swept radiation illuminates the interferometer, oscillations in the received signal occur due to the interference between the different parts of the beam. The period of the oscillations and the oscillation phase depend on the distance between retroreflectors and the changes in this distance, respectively. The main advantage of the system proposed is that a single transmitter-receiver pair allows reading out multiple sets of completely passive sensors offering high level of data protection due to the low beam divergency