Development of a frequency-stabalized mid-infrared external Cavity-QCL cavity ringdown spectrometer

External cavity quantum cascade lasers (EC-QCLs) provide significantly enhanced wavelength tunability while maintaining the high output powers of traditional QCLs throughout the mid-infrared portion of the spectrum. However, the inclusion of wavelength-selective moving parts allows vibrational and acoustic noise to be coupled into the laser as frequency and power noise. This can be particularly troublesome for cavity-enhanced spectroscopy, as attempts to increase vibrational isolation may interfere with stable cavity alignment. Here, we discuss our efforts to improve the frequency stability of our EC-QCL while maintaining tunability and consistent cavity alignment. A mid-infrared hollow silica waveguide is used to vibrationally isolate the laser from the cavity while maintaining alignment. To further increase frequency stability, the laser is side-of-fringe locked to an uncoated solid germanium etalon. Tunability is maintained by incrementing the angle of incidence upon the etalon using a piezo-driven mirror. Angle of incidence tuning and automated periodic re-locking are managed using a BeagleBone Black development board. This allows us to maintain essentially continuous frequency tuning between 1140-1220 \wn while stabilizing the laser's frequency to within 30 MHz. Other mid-infrared frequencies could easily be accessed by swapping QCL chips