Quantum-cascade laser based spectrometers for pollutant detection
Quantum-cascade (QC) lasers are a novel form of semi-conductor lasers that produce tunable radiation in the 3-25µm spectral window. This wavelength range coincides with the rotational-vibrational absorption bands of many gas phase molecules, such as methane, nitrous oxide, ethylene, formaldehyde and acetylene. These molecules can give information about chemical breakdown pathways in the atmosphere or act as markers as to the quality of an atmosphere.We have recently developed, with both EPSRC and NERC funding, a novel form of spectrometer, the intra-pulse spectrometer, based on a pulsed QC laser, which has been used to detect many key atmospheric gases. This spectrometer will be used in an aeroplane to make in situ measurements of gases in the upper atmosphere.
In principle the operation of this spectrometer is simple. When a current pulse is applied to a QC laser an optical pulse is generated in the time domain. The current pulse also induces a local heating of the QC laser and this causes the operating frequency of the QC laser to change, in our case we see a frequency down-chirp, which gives us a spectral micro-window, see Fig. 1a. If this frequency swept output is then passed through an absorbing species, the absorption profile is then imprinted on the temporal signal, see Fig. 1b.
The QCL spectroscopy kit in flight – a pollutant detection run over Oxford.
Figure 1a: Temporal pulse from a pulsed QC laser (blue trace) with the fringes observed after the pulse has passed through a germanium etalon of free spectral range 0.048 cm-1.
Fig 1b Temporal output from the spectrometer that shows the P9, P10 and P11 lines of nitrous oxide.
In the case of our system we apply a current pulse ranging from 0.3-1.5 µs to the QC laser and obtain spectral micro-windows of 1-3cm-1(30-90 GHz), which is sufficient to identify gases that are found in the atmosphere at natural abundance levels.