Posted in July 2011

Alessandro Polo will discuss Rhinoceros, an application to make 3D drawings for posters etc on 27th July 2011. It will be mainly an introduction session of one hour.

We will  also have an elaborate session of Rhinoceros, and Adobe illustrator for Agora in the month of September along with the introduction session. This is because of the vacation period now.

After having had a chance to go through the math more carefully (and in less of a rush) I fixed a number of mistakes in the slides. This includes some minor scaling errors noted during the session, one which brings my expression for the cavity Finesse into line with the accepted definition, correction of one expression which confused field amplitude and intensity, and (most importantly) reworking the expression for the power reflected from the input port of the cavity. None of the qualitative conclusions discussed are altered, but of course I wanted to get the math right! I posted the reworked slides up at this web address:
    http://www.strw.leidenuniv.nl/~meisner/RingDown.ppt

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.