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Cavity Enchanced Absorption Spectroscopy (CEAS) or Integrated Cavity Output Spectroscopy (ICOS)

A new variation of the Cavity Ringdown Spectroscopy (CRDS) technique was developed independently in 1998 in two different groups by O'Keefe et al. [1] and Engeln et al. [2]: Integrated Cavity Output Spectroscopy (ICOS) and Cavity Enhanced Absorption Spectroscopy (CEAS). This approach enables absorption spectra to be obtained through the integration of the total light signal transmitted through the high finesse optical cavity, either by oscillating the cavity length [3] or by anti-mode-matching the laser with the cavity such that the laser couples to a large number of transverse cavity modes [2].

Within ICOS [3,4], a gas-detection cell is used with two highly reflective end mirrors (>99:9%), resulting in a very high finesse cavity [ratio between the free spectral range (FSR) and  FWHM of the cavity mode: F > 8000]. In contrast to cw CRDS [5], the laser light is not locked on each cavity mode but swept over the gas absorption line. Moreover, opposite to on-axis alignment at the center of the optical cavity off-axis injection of the laser light (OA-ICOS) into the cavity will generate spatially separated, multiple reflections within the cavity before the reentrant condition of the optical beam is fulfilled. Therefore, OA-ICOS is effectively lowering the FSR of the cavity, producing a dense mode structure [6,7]. In a successful off-axis alignment, many cavity modes exist underneath every molecular transition and the laser linewidth should be broad with regard to the FSR of the cavity, but narrow with regard to the molecular line. Even narrowband lasers couple to many cavity modes simultaneously, which gives alignment robustness at the cost of cavity throughput power. One big advantage of OA-ICOS as compared with CRDS is that it can be used without limitations concerning ring-down time or mode matching between the laser frequency and the FSR.


Mode spectra shown here contrast the on-axis mode structure with the off-axis mode structure showing how the free spectral range effectively collapses, thereby lowering the finesse while maintaining the same cavity time constant by effectively enlarging the cavity length. The lower panels illustrate the resultant signals from each alignment. In a successful off-axis alignment, many cavity modes should exist underneath every molecular transition, and the laser line width should be broad with regard to the free spectral range of the cavity but narrow with regard to the molecular line [6].

Despite the advantages that OPOs have over other mid-IR coherent sources, such as high power, narrow linewidth and broad tunability, ICOS was not until 2010 combined with an OPO [8]. The high output power of an OPO (>1 W) gives superiority in using of OA-ICOS.  For the first time, our group demonstrated that 50 pptv ethane (C2H6) in nitrogen, at 2997 cm-1, can be recorded in just 0.25 seconds. In addition, we showed real-time breath sampling of methane and water with the time resolution of 0.1 s.

1. O'Keefe, "Integrated cavity output analysis of ultra-weak absorption," Chem. Phys. Lett. 293, 331-336 (1998).
2. R. Engeln, G. Berden, R. Peeters, and G. Meijer, "Cavity enhanced absorption and cavity enhanced magnetic rotation spectroscopy," Rev. Sci. Instr. 69, 3763-3769 (1998).
3. A. O'Keefe, J. J. Scherer, and J. B. Paul, "cw Integrated cavity output spectroscopy," Chem. Phys. Lett. 307, 343-349 (1999).
4. E. J. Moyer, D. S. Sayres, G. S. Engel, J. M. St. Clair, F. N. Keutsch, N. T. Allen, J. H. Kroll, and J. G. Anderson, "Design considerations in high-sensitivity off-axis integrated cavity output spectroscopy," Appl. Phys. B 92, 467-474 (2008).
5. J. Morville, S. Kassi, M. CHenevier, and D. Romanini, "Fast, low-noise, mode-by-mode, cavity-enhanced absorption spectroscopy by diode-laser self-locking," Appl. Phys. B 80, 1027-1038 (2005).
6. G. S. Engel, W. S. Drisdell, F. N. Keutsch, E. J. Moeyr, and J. G. Anderson, "Ultrasensitive near-infrared integrated cavity output spectroscopy technique for detection of CO at 1.57 um: new sensitivity limits for absorption measurements in passive optical cavities," Appl. Opt. 45, 9221-9229 (2006).
7. D. S. Baer, J. B. Paul, M. Gupta, and A. O'Keefe, "Sensitive absorption measurements in the near-infrared region using off-axis integrated-cavity-output spectroscopy," Appl. Phys. B 75, 261-265 (2002).
8. D. D. Arslanov, M. Spunei, A. K. Y. Ngai, S. M. Cristescu, I. D. Lindsay, S. T. Persijn, K. J. Boller, and F. J. M. Harren, "Rapid and sensitive trace gas detection with continuous wave optical parametric oscillator-based wavelength modulation spectroscopy," Appl. Phys. B 103, 223-228 (2011).