Frequency-agile, rapid scanning spectroscopy: absorption sensitivity of 2 x 10(-12) cm(-1) Hz(-1/2) with a tunable diode laser

We present ultrasensitive measurements of molecular absorption using frequency-agile rapid scanning, cavity ring-down spectroscopy with an external-cavity diode laser. A microwave source that drives an electro-optic phase modulator with a bandwidth of 20 GHz generates pairs of sidebands on the probe laser. The optical cavity provides for high sensitivity and filters the carrier and all but a single, selected sideband. Absorption spectra were acquired by stepping the tunable sideband from mode-to-mode of the ring-down cavity at a rate that was limited only by the cavity decay time. This approach allows for scanning rates of 8 kHz per cavity resonance, a minimum detectable absorption coefficient of 1.7 × 10−11 cm−1 after only 20 ms of averaging, and a noise-equivalent absorption coefficient of 1.7 × 10−12 cm−1 Hz−1/2. By comparison with cavity-enhanced laser absorption spectrometers reported in the literature, the present system is, to the best of our knowledge, among the most sensitive and has by far the highest spectrum scanning rate.D. A. Long, G.-W. Truong, R. D. van Zee, D. F. Plusquellic, J. T. Hodge

Frequency-agile, rapid scanning spectroscopy

LetterChallenging applications in trace gas measurements require low uncertainty and high acquisition rates. Many cavity-enhanced spectroscopies exhibit significant sensitivity and potential, but their scanning rates are limited by reliance on either mechanical or thermal frequency tuning. Here, we present frequency-agile, rapid scanning spectroscopy (FARS) in which a high-bandwidth electro-optic modulator steps a selected laser sideband to successive optical cavity modes. This approach involves no mechanical motion and allows for a scanning rate of 8 kHz per cavity mode, a rate that is limited only by the cavity response time itself. Unlike rapidly frequency-swept techniques FARS does not reduce the measurement duty cycle, degrade the spectrum's frequency axis or require an unusual cavity configuration. FARS allows for a sensitivity of ~2 × 10−12 cm−1 Hz−1/2 and a tuning range exceeding 70 GHz. This technique shows promise for fast and sensitive trace gas measurements and studies of chemical kinetics.G.-W. Truong, K. O. Douglass, S. E. Maxwell, R. D. van Zee, D. F. Plusquellic, J. T. Hodges & D. A. Lon