Editing Tunable diode laser absorption spectroscopy (section)

=== 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}}