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Cavity ring-down spectroscopy
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<p>Cavity ring-down spectroscopy (CRDS) is a highly sensitive <a href="/facts/Spectroscopy/mj71FuNK">optical spectroscopic</a> technique that enables measurement of absolute <a href="/facts/Optical_extinction/XP4vX360">optical extinction</a> by samples that <a href="/facts/Scattering/nMou7lEH">scatter</a> and <a href="/facts/Absorption_(electromagnetic_radiation)/xBYKt7yS">absorb</a> light. It has been widely used to study gaseous samples which absorb light at specific <a href="/facts/Wavelength/kcJU0ymz">wavelengths</a>, and in turn to determine <a href="/facts/Mole_fraction/nW4EepmJ">mole fractions</a> down to the <a href="/facts/Parts_per_trillion/PozRuoLM">parts per trillion</a> level. The technique is also known as cavity ring-down laser absorption spectroscopy (CRLAS). </p><p>A typical CRDS setup consists of a <a href="/facts/Laser/jk6PTvvT">laser</a> that is used to illuminate a high-finesse <a href="/facts/Optical_cavity/BITPG3ga">optical cavity</a>, which in its simplest form consists of two highly reflective <a href="/facts/Mirror/wMu4u6BB">mirrors</a>. When the laser is in <a href="/facts/Resonance/q7VLALUz">resonance</a> with a cavity <a href="/facts/Normal_mode/XCsvQpuI">mode</a>, <a href="/facts/Intensity_(physics)/KFWkJkJI">intensity</a> builds up in the cavity due to <a href="/facts/Constructive_interference/lI5f78I4">constructive interference</a>. The laser is then turned off in order to allow the measurement of the exponentially decaying light intensity leaking from the cavity. During this decay, light is reflected back and forth thousands of times between the mirrors giving an effective path length for the extinction on the order of a few kilometers. </p><p>If a light-absorbing material is now placed in the cavity, the <a href="/facts/Mean_lifetime/wLB1zYDP">mean lifetime</a> decreases as fewer bounces through the medium are required before the light is fully absorbed, or absorbed to some fraction of its initial intensity. A CRDS setup measures how long it takes for the light to decay to 1/<i>e</i> of its initial intensity, and this "ringdown time" can be used to calculate the concentration of the absorbing substance in the gas mixture in the cavity. </p>