CWCAVITYRINGDOWNSPECTROSCOPY-Sibor.ppt

CAVITY RING DOWN SPECTROSCOPY AYSENUR BICER Outline What is CRD spectroscopy A simple CRDS experiment Pulsed laser CRDS versus CW-CRDS CW- CRDS experimental schema Experimental results Knife edge method What is CRD spectroscopy CRDS is a sensitive absorption technique in which the rate of absorption in an optical cavity is measured It has significantly high sensitivity The effective absorption path length is very long The sensitivity is independent of intensity fluctuations of the light source Small fractional absorptions sub- ppm levels CO2 400 ppm (open air) A Simple CRDS Experiment A laser pulse coupled into an optical cavity The decay time is determined by measuring the time dependence of the light leaking out of the cavity By measuring the decay time the rate of absorption is determined directly providing the losses on an absolute scale After one pass-through the cavity the intensity of the first optical pulse (Beer-Lambert’s law) The intensity of the second pulse After n complete round trip the pulse intensity behind the cavity will be Pulsed laser CRD spectroscopy Pulsed lasers promise Fourier transform limited line widths of the order of 100 MHz, in practice it is difficult to archive The length of the cavity, L, and the radius of the mirrors curvature of the mirrors should be chosen such that cavity is optically stable They are rather bulky, require massive amounts of electricity to run, and cost several hundred thousand dollars The pulsed lasers have the advantage of broad wavelength coverage Continuous Wave CRD spectroscopy The main advantage of using CW laser radiation sources in any spectroscopic system is the increased resolution in the frequency domain (Trigger event ) In order to observe a ring down transits CW have to be switched of The bandwidth of these lasers is very small so can be only scanned over small wavelength regions each mode can have various allowed longitudinal modes associated with it The frequency spacing be