Wavelength modulation spectroscopy of nitric oxide using a quantum cascade laser in intermittent continuous wave operation.

Operating quantum cascade lasers (QCLs) in intermittent continuous wave (iCW) shows the merit of a broader frequency tuning range and lower heat dissipation compared to the continuous wave (CW) operation. We demonstrate for the first time wavelength modulation spectroscopy (WMS) of a QCL in iCW operation for sensitive gas detection. A strong absorption line of nitric oxide (NO) at 5.18 μm is exploited by a QCL in iCW mode, which periodically switches off the QCL between individual laser scans. The generated thermal chirp dominates the laser frequency tuning, resulting in a broader spectral coverage of more than 2 cm at a scanning rate of 1 kHz. In addition, a high-frequency dither (50 kHz) is supposed onto this iCW injection current to introduce the harmonic signals that arise from gas absorption. At the WMS-iCW operation of the QCL, we have obtained a minimum detection limit of 4.5 ppb at an averaging time of 80 s, which is improved significantly compared to 130 ppb achieved by direct absorption spectroscopy at the same averaging time using the identical optical setup, without external forced air- or water-cooling. Our method provides a promising method for sensor miniaturization and field application.