A Cavity Ring Down Spectroscopic Setup for High Rep. Rate Real Time Measurment

The cavity ring down spectroscopy gains great sensitivitybecause of its multiple pass in the sample.Its generic difference from white cell and inner cavity methods in not only the long passlegth,but rather its exponential decay signal.From Bear′s law,we can directly link the absorptance α to the cavity ring down time τby τ = 2L/c ln（ R₁R₂）^-1 -2αL). In principle,it is not related to laser pulse to polse power fluctation and other noises by fitting out the τ at each ring down curve.In contrast to previous data acquisition methods,such as two gates of Boxcar on the curve,digital oscilloscope waveform average,and others,we take each individual ring down curve and fit out τat each time.In order to fulfill this goal,we have managed the system to yield a smooth exponential curve at each time,so that we are able to do a reasonable fit.The next challenge is how to do this without sacrifice of rep.rate.We have utilixed an old model Tektronix digitixer（7612D）to do the data acquisition,GPIB interface to transfer data,and PC 486,75MHz to the processing job.The REP command in 7612D allows us to transfer data at a rate of 40Hz for a waveform of 8 bits.2048 points.Therefore,we have set up a system capable of doing high rep.rate real time cavity ring down spectroscopic measurement.We have used this system to record the overtone transition of H₂O vapor in the visible region 620～665nm.The spectrun has demonstrated a detective limit of 5×10^-8 cm ^-1 .