Cavity ring-down lossmeter using a pulsed light emitting diode source and photon counting

Abstract

A new mode of cavity ring-down measurements in which an incoherent, broadband light emitting diode (LED) is used as the spectroscopic source is described. Light from a pulsed LED (570 nm, 12 nm FWHM) was coupled into a 32 cm linear optical resonator and a ring-down waveform obtained on a gated photon counter. Cavity time constants observed were on the order of 3–4 µs, yielding effective optical path-lengths in excess of 1 km. In an effort to demonstrate the function of the instrument we have measured absorption and Rayleigh scattering by gases introduced into the measurement cell. Measurements of Rayleigh scatter by CO2 and 1,1,1,2 tetrafluoroethane (R-134a) were used to calibrate the instrument. The Rayleigh extinction coefficients determined through the LED–CRD method were found to agree with literature values to within 12% on average. In an additional set of experiments, a laboratory-generated vapour containing either iodine or ozone was introduced into the measurement cell in an effort to demonstrate the technique's ability to monitor absorbing gases. The ring-down time constants (τ) observed were found to decrease by 15–75% when the absorbing gases were added to the measurement cell. The observed decrease in cavity time constant was proportional to the quantity of absorbing gas within the measurement cell. Minimum detectable extinction coefficients (2s) of ≈2.5 × 10−7 cm−1 were achieved. The LED–CRD approach may lead to development of inexpensive gas sensors or monitoring systems for atmospheric extinction/visibility.