QCL based WMS
WMS has been combined with QCL to develop a Nitric Oxide gas sensor. The QCL-based NO sensor consists of a continuous-wave distributed feedback quantum cascade laser (CW-DFB QCL) operating in the wavelength region of 5.2 μm (1891 to 1908 cm-1), a multi-pass cell, and a room temperature detector [1].
The multipass cell is an astigmatic Herriott absorption cell, offering a total optical path length of 76 m and which consists of two concave mirrors where light is reflected multiple times, enhancing the effective path length through the gas sample. To reach a fast response of the system, a low pressure is maintained in the cell and a sample flow rate up to 900 ml∕min can be generated. Therefore, the cell is refreshed in less than 2 s, making the sensor suitable for online monitoring application such as exhaled breath analysis.
The detection is performed by a photovoltaic detector working at room-temperature, thus eliminating the need for a highly sensitive liquid nitrogen-cooled IR detector, simplifying daily use of the system and allowing long-term automated operation.
WMS is implemented by modulating the injection current of the laser at a frequency of 100 kHz. The absorption signal is transposed into a frequency domain where noise sources are weaker, and absorption spectra is obtained by demodulating the detected signal at the second harmonic (2f) using a lock-in amplifier. This approach allows a sensitivity of 0.5 ppbv within 1 s averaging.
Figure 1 - Picture of the setup, including the QCL, the multi-pass cell and the room-temperature detector. The whole setup is thermally stabilized.
Figure 2 - Allan variance showing the QCL-based sensor detection limit.
[1] S. M. Cristescu et al., "Laser-based systems for trace gas detection in life sciences," Appl. Phys. B 92(3), 343-349 (2008).