Cavity leak-out spectroscopy

Fig. 1.21 Experimental setup of cavity leak-out spectroscopy with mode-matching optics and detector (P. Hering, Institute of Laser Medicine, University of Diisseldorf)...

Many bioactive molecules have absorption bands in the infrared, which are accessible by existing infrared lasers or OPOs (Fig. 10.50). When enhancement cavities and cavity ringdown or cavity leak out spectroscopy are used detection sensitivities down into the ppb or even ppt range can be achieved [1545], This is important for sensitive measurements of breathing gases, where the composition of the exhaled gas gives information about possible diseases. One famous example is the presence of the bacteria Helicobacter pylori in the stomach, which can cause gastritis or even stomach cancer. Their presence and concentration can be monitored by the spec-... 

A. Popp et al.. Ultrasensitive mid-infrared cavity leak-out spectroscopy using a cw optical parametric oscillator. Appl. Phys. B 75, 751 (2003)... 

D. Halmer, G. von Basum, P. Hering, M. Miiitz, Mid-infrared cavity leak-out spectroscopy for ultrasensitive detection of carbonyl sulfide. Opt. Lett. 30, 2314 (2005)... 

M. Miirtz, B. Freeh, W. Urban, High-resolution cavity-leak-out absorption spectroscopy in the 10 pm region. Appl. Phys. B 68, 243 (1999)... 

Output resonance is reached (for a droplet with a fixed radius) when specific wavelengths within the inelastic emission profile correspond to MDR s with different n,t values. For those wavelengths, the droplet can be envisioned as an optical cavity with a large Fresnel number and Q-factors which are dependent on the specific n and I values. The portion of the inelastic radiation detected is that allowed to "leak" out of the droplet cavity. However, it is the internal field distributions of the electromagnetic waves at X, and X, which are best described by spherical harmonic functions and not by plane waves as in the case of an extended medium, that affect the nonlinear optical interactions. Such interactions in droplets can be illustrated by several well known examples in nonlinear spectroscopy of liquids in an optical cell. 

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