We report a heterodyne beat with a linewidth of 5.6±0.6 Hz between two cavity-stabilized quantum-cascade lasers operating at 8.5 µm . We also present a technique for measuring this beat that avoids the need for extreme isolation of the optical cavities from the environment, that of employing a third servo loop with low bandwidth to force one cavity to track the slow drifts and low-frequency fluctuations of the other. Although it is 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.
© 2002 Optical Society of America
(000.2170) General : Equipment and techniques
(120.4820) Instrumentation, measurement, and metrology : Optical systems
(140.3070) Lasers and laser optics : Infrared and far-infrared lasers
(140.5960) Lasers and laser optics : Semiconductor lasers
Matthew S. Taubman, Tanya L. Myers, Bret D. Cannon, Richard M. Williams, Federico Capasso, Claire Gmachl, Deborah L. Sivco, and Alfred Y. Cho, "Frequency stabilization of quantum-cascade lasers by use of optical cavities," Opt. Lett. 27, 2164-2166 (2002)