A mode-tracking system that includes a mode-controlling subsystem has been incorporated into an external-cavity (EC) quantum cascade laser that operates in a mid-infrared wavelength range. The mode-tracking system makes it possible to perform mode-hop-free wavelength scans, as needed for high-resolution spectroscopy and detection of trace gases. The laser includes a gain chip, a beam-collimating lens, and a diffraction grating. The grating is mounted on a platform, the position of which can be varied to effect independent control of the EC length and the grating angle. The position actuators include a piezoelectric stage for translation control and a motorized stage for coarse rotation control equipped with a piezoelectric actuator for fine rotation control. Together, these actuators enable control of the EC length over a range of about 90 μm with a resolution of 0.9 nm, and control of the grating angle over a coarse-tuning range of ±6.3° and a fine-tuning range of ±520 μrad with a resolution of 10 nrad. A mirror mounted on the platform with the grating assures always the same direction of the output laser beam.This work was done by Gerard Wysocki, Frank K. Tittel, and Robert F. Curl of Rice University for Johnson Space Center.
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