Agora 20th of July, message from Jeff
Continuing the discussion of cavity ring-down spectroscopy, I will present some results and ideas about configurations for such measurements.
ABSTRACT
Spectroscopic detection of absorption lines in gasses can be obtained using sensitive measurement of absorption coefficients through a long effective path through the medium. The path length can be increased using a cell involving multiple passes through the medium using a folded path, but much longer effective path lengths can be obtained using re-entrant reflections: a resonant cavity. The Q (or finesse) of such a Fabry-Perot cavity is reduced due to optical absorption in the medium, which can be measured using laser pulses and observing the ring-down time constant of the cavity following each excitation.
However a low-loss (high Q) cavity requires mirrors with very high reflectivity, so that very little of the laser pulses’ power enters the cavity. Power measurements with a much higher SNR can be obtained using a CW laser at the exact wavelength of a cavity resonance, in which case a large portion of the laser’s power enters the cavity. Shutting off the laser beam then allows measurement of the ring-down time constant. An alternate method of measuring the cavity Q is to measure the CW power level inside the cavity when resonant at the CW laser’s wavelength. These methods require a control system to match the very narrow cavity resonance to the laser frequency (or visa-versa).
The sensitivity of such measurements is increased by using a cavity with a lower round-trip loss, largely determined by the reflectivity of the mirrors. Using one very high reflection mirror with no transmission can be accomodated by obtaining the cavity ring-down signal through the same mirror through which the laser is injected. This can be done with full power efficiency using a circularly polarized cavity mode.
