Laser absorption spectrometry (LAS) refers to techniques that use lasers to assess the concentration or amount of a species in gas phase by absorption spectrometry (AS). Optical spectroscopic techniques in general, and laser-based techniques in particular, have a great potential for detection and monitoring of constituents in gas phase. They combine a number of important properties, e.g. a high sensitivity and a high selectivity with non-intrusive and remote sensing capabilities. Laser absorption spectrometry has become the foremost used technique for quantitative assessments of atoms and molecules in gas phase. 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. The most common technique is tunable diode laser absorption spectroscopy (TDLAS) which has become commercialized and is used for a variety of applications.
Direct laser absorption spectrometry The most appealing advantages of LAS is its ability to provide absolute quantitative assessments of species. 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. Direct laser absorption spectrometric (DLAS) techniques are therefore often limited to detection of absorbance ~10−3, which is far away from the theoretical shot noise level, which for a single pass DAS technique is in the 10−7 – 10−8 range. This detection limit is insufficient for many types of applications. The detection limit can be improved by (1) reducing the noise, (2) using transitions with larger transition strengths or (3) increasing the effective path length. The first can be achieved by the use of a modulation technique, the second can be obtained by using transitions in unconventional wavelength regions, whereas the third by using external cavities.
Modulated techniques Modulation techniques make use of the fact that technical noise usually decreases with increasing frequency (often referred to as a 1/f noise) and improves on the signal contrast by encoding and detecting the absorption signal at a high frequency, where the noise level is low. The most common modulation techniques, wavelength modulation spectroscopy (WMS) and frequency modulation spectroscopy (FMS), achieve this by rapidly scanning the frequency of the light across the absorbing transition. 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). The typical sensitivity of WMS and FMS techniques is in the 10−5 range. 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. This gives rise to systems that can run unattended for thousands of hours with minimum maintenance.
Laser absorption spectrometry using fundamental vibrational or electronic transitions The second way of improving the detection limit of LAS is to employ transitions with larger line strength, either in the fundamental vibrational band or electronic transitions. The former, which normally reside at ~5 μm, have line strengths that are ~2–3 orders of magnitude higher than those of typical overtone transition. On the other hand, electronic transitions have often yet another 1–2 orders of magnitude larger line strengths. The transitions strengths for the electronic transitions of NO, which are located in the UV range (at ~227 nm) are ~2 orders of magnitude larger than those in the MIR region. 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. It is more difficult to generate stable cw light addressing electronic transitions, since these often lie in the UV region.
Cavity enhanced absorption spectrometry The third way of improving the sensitivity of LAS is to increase the path length. 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. This has led to a group of techniques denoted as cavity enhanced AS (CEAS). The cavity can either be placed inside the laser, giving rise to intracavity AS, or outside, when it is referred to as an external cavity. Although the former technique can provide a high sensitivity, its practical applicability is limited by non-linear processes. External cavities can either be of multi-pass type, i.e. Herriott or White cells, or be of resonant type, most often working as a Fabry–Pérot (FP) etalon. Whereas the multi-pass cells typically can provide an enhanced interaction length of up to ~2 orders of magnitude, the resonant cavities can provide a much larger path length enhancement, in the order of the finesse of the cavity, F, which for a balanced cavity with high reflecting mirrors with reflectivities of ~99.99–99.999% can be ~104 to 105. A problem with resonant cavities is that a high finesse cavity has narrow cavity modes, often in the low kHz range. Since cw lasers often have free-running line-widths in the MHz range, and pulsed even larger, it is difficult to couple laser light effectively into a high finesse cavity. However, there are a few ways this can be achieved. 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.
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