Extensive characterization of the optical feedback cavity enhanced absorption spectroscopy (OF-CEAS) technique: ringdown-time calibration of the absorption scale

Mid-infrared sensing of CO at saturated absorption conditions using intracavity quartz-enhanced photoacoustic spectroscopy

“…Also, the volume of the gas cell can be drastically reduced using the Brewster window as a window of the gas cell. Exploiting optical feedback, light can be efficiently coupled into cavities of significantly higher finesse than presented herein, yielding even higher intracavity powers [38]. The resulting intracavity intensities exceed the saturation intensities of many molecules.…”

“…For the Brewster window cavity, rather than the transmission of the in-coupling mirror, the reflectivity R B of the Brewster window determines how much light is coupled into the cavity. R B and hence the power buildup can be adjusted conveniently by changing the angle at which the window is placed in the cavity, which is a unique feature of the Brewster cavity design [31,38]. The bidirectional power P intra circulating inside the cavity on resonance assuming perfect mode matching as a function of R B is given by (see Appendix A for derivation) Herein, P 0 is the incident laser power, R M is the mirror reflectivity (assumed equal for both mirrors) and αL are the combined half round-trip losses excluding reflective losses.…”

“…6). This is due to a decrease in intracavity power P intra caused by absorption by CO [38] [increasing losses αL in (2)]. To correct for this, P intra was measured for each concentration using the demodulated detector signal and the known value P intra = 6.3 W for c = 0 and the results are shown in Fig.…”

“…Second, in a single cavity, the finesse and power buildup can be varied by orders of magnitude by adjusting the window's angle (compare Sect. 2.3) [38] or by changing the polarization to s, e.g. via a half-wave plate.…”

Cavity-based absorption spectroscopy techniques

“…Such OF-CRDS approaches have been used to measure absorption spectra (Morville et al , 2004;Motto-Ros et al , 2007;van Helden et al , 2009;Tan et al , 2010), aerosol-particle extinction (Butler et al , 2007(Butler et al , , 2009) and ultra-high refl ectivity (Gong et al , 2008). In addition to timedomain OF-CRDS measurements, its closely related amplitude-domain analogue, CEAS/ICOS (as in Section 6.4.1 ), has also been applied in the OF mode to absorption spectroscopy (Morville et al , 2005;Courtillot et al , 2006;Kerstel et al , 2006;Romanini et al , 2006;Motto-Ros et al , 2008;Wehr et al , 2008;Baran et al , 2009;van Helden et al , 2009;Ventrillard-Courtillot et al , 2009;Hamilton et al , 2010;Habig, et al , 2012). We note that analogous passive feedback between an AR-coated diode laser and a simple three-mirror V-shaped optical build-up cavity was demonstrated in early frequency-locking work by King and Pittaro (1998), providing an excitation source for measurements of spontaneous Raman spectra.…”