It is also a widely used technique for a variety of other applications, e.g. within the field of optical frequency metrology or in studies of light matter interactions.
[1] Its biggest disadvantage is that it relies on a measurement of a small change in power from a high level; any noise introduced by the light source or the transmission through the optical system will deteriorate the sensitivity of the technique.
Both techniques have the advantage that the demodulated signal is low in the absence of absorbers but they are also limited by residual amplitude modulation, either from the laser or from multiple reflections in the optical system (etalon effects).
The most frequently used laser-based technique for environmental investigations and process control applications is based upon diode lasers and WMS (typically referred to as TDLAS).
Due to their good tunability and long lifetime (> 10,000 hours), most practical laser-based absorption spectroscopy is performed today by distributed feedback diode lasers emitting in the 760 nm – 16 μm range.
[citation needed] The recent development of quantum cascade (QC) lasers working in the MIR region has opened up new possibilities for sensitive detection of molecular species on their fundamental vibrational bands.
This can be obtained by placing the species inside a cavity in which the light bounces back and forth many times, whereby the interaction length can be increased considerably.
One such method is Vernier Spectroscopy, which employs a frequency comb laser to excite many cavity modes simultaneously and allows for a highly parallel measurement of trace gases.
While independent of laser amplitude noise, this technique is often limited by drifts in the system between two consecutive measurements and a low transmission through the cavity.
Off-axis ICOS (OA-ICOS) improves on this by coupling the laser light into the cavity from an angle with respect to the main axis so as to not interact with a high density of transverse modes.
[12][13][14] However, all attempts to directly combine CEAS with a locking approach (DCEAS) have one thing in common; they do not manage to use the full power of the cavity, i.e. to reach LODs close to the (multi-pass) shot-noise level, which is roughly 2F/π times below that of DAS and can be down to ~10−13.
There is, however, one technique that so far has succeeded in making full use of the cavity by combining locked CEAS with FMS so as to circumvent both of these problems: Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS).