Editing Tunable diode laser absorption spectroscopy (section)

==Limitations and means of improvement==
The main disadvantage of [[absorption spectrometry]] (AS) as well as [[laser absorption spectrometry]] (LAS) in general is that it relies on a measurement of a small change of a signal on top of a large background. Any noise introduced by the light source or the optical system will deteriorate the detectability of the technique. The sensitivity of direct absorption techniques is therefore often limited to an absorbance of ~10<sup>−3</sup>, far away from the shot noise level, which for single pass direct AS (DAS) is in the 10<sup>−7</sup> – 10<sup>−8</sup> range. Since this is insufficient for many types of applications, AS is seldom used in its simplest mode of operation.

There are basically two ways to improve on the situation; one is to reduce the noise in the signal, the other is to increase the absorption. The former can be achieved by the use of a modulation technique, whereas the latter can be obtained by placing the gas inside a cavity in which the light passes through the sample several times, thus increasing the interaction length. If the technique is applied to trace species detection, it is also possible to enhance the signal by performing detection at wavelengths where the transitions have larger line strengths, e.g. using fundamental vibrational bands or electronic transitions.

=== Modulation techniques ===
Modulation techniques make use of the fact that [[pink noise|technical noise]] usually decreases with increasing frequency (which is why it is often referred to as 1/f noise) and improve the signal to noise ratio by encoding and detecting the absorption signal at a high frequency, where the noise level is low. The most common modulation techniques are wavelength modulation spectroscopy (WMS) and frequency modulation spectroscopy (FMS).

In WMS the wavelength of the light is continuously scanned across the absorption profile, and the signal is detected at a harmonic of the modulation frequency.

In FMS, the light is modulated at a much higher frequency but with a lower modulation index. As a result, a pair of sidebands separated from the carrier by the modulation frequency appears, giving rise to a so-called FM-triplet. The signal at the modulation frequency is a sum of the beat signals of the carrier with each of the two sidebands. Since these two sidebands are fully out of phase with each other, the two beat signals cancel in the absence of absorbers. However, an alteration of any of the sidebands, either by absorption or dispersion, or a phase shift of the carrier, will give rise to an unbalance between the two beat signals, and therefore a net-signal.

Although in theory baseline-free, both modulation techniques are usually limited by residual amplitude modulation (RAM), either from the laser or from multiple reflections in the optical system (etalon effects). If these noise contributions are held low, the sensitivity can be brought into the 10<sup>−5</sup> – 10<sup>−6</sup> range or even better.

In general the absorption imprints are generated by a straight line light propagation through a volume with the specific gas. To further enhance the signal, the pathway of the light travel can be increased with [[Multipass spectroscopic absorption cells|multi-pass cells]]. There is however a variety of the WMS-technique that utilizes the narrow line absorption from gases for sensing even when the gases are situated in closed compartments (e.g. pores) inside solid materia. The technique is referred to as [[gas in scattering media absorption spectroscopy]] (GASMAS).

=== Cavity-enhanced absorption spectrometry (CEAS) ===
The second way of improving the detectability of TDLAS technique is to extend the interaction 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 because of all the non-linear processes involved.

External cavities can either be of multi-pass type, i.e. Herriott or [[White cell (spectroscopy)|White cells]], of non- resonant type (off-axis alignment), or of resonant type, most often working as a [[Fabry–Pérot etalon|Fabry–Pérot (FP) etalon]]. Multi-pass cells, which typically can provide an enhanced interaction length of up to ~2 orders of magnitude, are nowaday common together with TDLAS.

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 ~ 10<sup>4</sup> to 10<sup>5</sup>. It should be clear that if all this increase in interaction length can be utilized efficiently, this vouches for a significant increase in detectability. A problem with resonant cavities is that a high finesse cavity has very narrow cavity modes, often in the low kHz range (the width of the cavity modes is given by FSR/F, where FSR is the free-spectral range of the cavity, which is given by  ''c''/2''L'', where ''c'' is the speed of light and ''L'' is the cavity length). Since cw lasers often have free-running linewidths in the MHz range, and pulsed even larger, it is non-trivial to couple laser light effectively into a high finesse cavity.

The most important resonant CEAS techniques are [[cavity ring-down spectrometry]] (CRDS), integrated cavity output spectroscopy (ICOS) or cavity enhanced absorption spectroscopy (CEAS), phase-shift cavity ring-down spectroscopy (PS-CRDS) and Continuous wave Cavity Enhanced Absorption Spectrometry (cw-CEAS), either with optical locking, referred to as (OF-CEAS),<ref>D. Romanini, A. A. Kachanav, J. Morville, and M. Chenevier, Proc. SPIE EUROPTO (Ser. Environmental Sensing) 3821(8), 94 (1999)</ref> as has been demonstrated Romanini et al.<ref name=Morville2005>{{cite journal | last1=Morville | first1=J. | last2=Kassi | first2=S. | last3=Chenevier | first3=M. | last4=Romanini | first4=D. | title=Fast, low-noise, mode-by-mode, cavity-enhanced absorption spectroscopy by diode-laser self-locking | journal=Applied Physics B | publisher=Springer Science and Business Media LLC | volume=80 | issue=8 | date=2005-05-31 | issn=0946-2171 | doi=10.1007/s00340-005-1828-z | pages=1027–1038| bibcode=2005ApPhB..80.1027M | s2cid=120346016 | url=https://hal.archives-ouvertes.fr/hal-03841163/file/apb2988rev2c.pdf }}</ref> or by electronic locking.,<ref name=Morville2005 /> as for example is done in the [[Noise-Immune Cavity-Enhanced Optical-Heterodyne Molecular Spectroscopy]] (NICE-OHMS) technique.<ref>{{cite journal | last1=Ma | first1=Long-Sheng | last2=Ye | first2=Jun | last3=Dubé | first3=Pierre | last4=Hall | first4=John L. | title=Ultrasensitive frequency-modulation spectroscopy enhanced by a high-finesse optical cavity: theory and application to overtone transitions of C<sub>2</sub>H<sub>2</sub> and C<sub>2</sub>HD | journal=Journal of the Optical Society of America B | publisher=The Optical Society | volume=16 | issue=12 | date=1999-12-01 | issn=0740-3224 | doi=10.1364/josab.16.002255 | pages=2255–2268| bibcode=1999JOSAB..16.2255M }}</ref><ref>{{cite journal | last1=Taubman | first1=Matthew S. | last2=Myers | first2=Tanya L. | last3=Cannon | first3=Bret D. | last4=Williams | first4=Richard M. | title=Stabilization, injection and control of quantum cascade lasers, and their application to chemical sensing in the infrared | journal=Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy | publisher=Elsevier BV | volume=60 | issue=14 | year=2004 | issn=1386-1425 | doi=10.1016/j.saa.2003.12.057 | pages=3457–3468| pmid=15561632 | bibcode=2004AcSpA..60.3457T }}</ref><ref>{{cite journal | last1=Schmidt | first1=Florian M. | last2=Foltynowicz | first2=Aleksandra | last3=Ma | first3=Weiguang | last4=Lock | first4=Tomas | last5=Axner | first5=Ove | title=Doppler-broadened fiber-laser-based NICE-OHMS – Improved detectability | journal=Optics Express | publisher=The Optical Society | volume=15 | issue=17 | year=2007 | issn=1094-4087 | doi=10.1364/oe.15.010822 | pages=10822–10831| pmid=19547439 | bibcode=2007OExpr..1510822S | doi-access=free }}</ref> or combination of frequency modulation and optical feedback locking CEAS, referred to as (FM-OF-CEAS).<ref>{{cite journal | last1=Kasyutich | first1=Vasili L. | last2=Sigrist | first2=Markus W. | title=Characterisation of the potential of frequency modulation and optical feedback locking for cavity-enhanced absorption spectroscopy | journal=Applied Physics B | publisher=Springer Science and Business Media LLC | volume=111 | issue=3 | date=2013-02-02 | issn=0946-2171 | doi=10.1007/s00340-013-5338-0 | pages=341–349|arxiv=1212.3825| bibcode=2013ApPhB.111..341K | s2cid=253855037 }}</ref>

The most important non-resonant CEAS techniques are off-axis ICOS (OA-ICOS)<ref>{{cite journal | last1=Paul | first1=Joshua B. | last2=Lapson | first2=Larry | last3=Anderson | first3=James G. | title=Ultrasensitive absorption spectroscopy with a high-finesse optical cavity and off-axis alignment | journal=Applied Optics | publisher=The Optical Society | volume=40 | issue=27 | date=2001-09-20 | pages=4904–4910 | issn=0003-6935 | doi=10.1364/ao.40.004904 | pmid=18360533 | bibcode=2001ApOpt..40.4904P }}</ref> or off-axis CEAS (OA-CEAS), wavelength modulation off-axis CEAS (WM-OA-CEAS),<ref>{{cite journal | last1=Kasyutich | first1=V.L. | last2=Canosa-Mas | first2=C.E. | last3=Pfrang | first3=C. | last4=Vaughan | first4=S. | last5=Wayne | first5=R.P. | title=Off-axis continuous-wave cavity-enhanced absorption spectroscopy of narrow-band and broadband absorbers using red diode lasers | journal=Applied Physics B: Lasers and Optics | publisher=Springer Science and Business Media LLC | volume=75 | issue=6–7 | date=2002-11-01 | issn=0946-2171 | doi=10.1007/s00340-002-1032-3 | pages=755–761| bibcode=2002ApPhB..75..755K | s2cid=120045701 }}</ref> off-axis phase-shift cavity enhanced absorption spectroscopy (off-axis PS-CEAS).<ref>{{cite journal | last1=Kasyutich | first1=Vasili L. | last2=Martin | first2=Philip A. | last3=Holdsworth | first3=Robert J. | title=Effect of broadband amplified spontaneous emission on absorption measurements in phase-shift off-axis cavity enhanced absorption spectroscopy | journal=Chemical Physics Letters | publisher=Elsevier BV | volume=430 | issue=4–6 | year=2006 | issn=0009-2614 | doi=10.1016/j.cplett.2006.09.007 | pages=429–434| bibcode=2006CPL...430..429K }}</ref>

These resonant and non-resonant cavity enhanced absorption techniques have so far not been used that frequently with TDLAS. However, since the field is developing fast, they will presumably be more used with TDLAS in the future.

{{Main|Laser absorption spectrometry}}