Relative frequency stabilization of two 8.5-micron quantum cascade lasers to 5.6 Hz

Summary from only given. We demonstrate the successful frequency stabilization of two QCLs to separate optical cavities using highly optimized servo loops based on the Pound-Drever-Hall technique. We present 5.6 Hz FWHM beat notes between these two systems that are stable for periods up to a second. This represents an effective reduction from the recently recorded free-running linewidth of these same lasers of 150 kHz of over 25,000.