Frequency stabilization of quantum cascade lasers using optical cavities

We report a heterodyne beat with a line width of 5.6 ± 0.6 Hz between two cavity-stabilized quantum cascade lasers operating at 8.5 micron. We also present a technique for this measurement that avoids the need for extreme isolation of the optical cavities from the environment; that of employing a third serve loop with a low bandwidth to force one cavity to track the slow drifts and low frequency fluctuations of the other. 

While not fully independent, this technique greatly facilitates heterodyne beat measurements for evaluating the performance of cavity- locked lasers above the bandwidth of the third loop.