Coherent phase lock of a 9 mu m quantum cascade laser to a 2 mu m thulium optical frequency comb

We demonstrate coherent phase locking of a room-temperature continuous-wave quantum cascade laser (QCL) at 9.1 mu m to a Tm-fiber laser frequency comb centered at 2 mu m, with an integrated residual phase error of 0.9 rad (30 mHz to 1.5 MHz). This resulted in a QCL linewidth reduction from 525 to 25 kHz at 1 ms observation time, limited by the linewidth of the free-running frequency comb. (C) 2012 Optical Society of America