Frequency-agile, rapid scanning spectroscopy: absorption sensitivity of 2 3 10 212 cm 21 Hz 21/2 with a tunable diode laser D. A. Long • G.-W. Truong • R. D. van Zee • D. F. Plusquellic • J. T. Hodges Received: 26 February 2013 / Accepted: 4 June 2013 Ó Springer (outside the USA) 2013 Abstract 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 ele- ctro-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 9 10 -11 cm -1 after only 20 ms of averaging, and a noise- equivalent absorption coefficient of 1.7 9 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. 1 Introduction At present, there are many problems in trace gas sens- ing and chemical physics that require ultrasensitive spectrometers with a fast response and the ability to pre- cisely measure absorption spectra. Cavity-enhanced laser absorption measurements exploit the long effective path lengths of high-finesse optical resonators to yield low detection limits and high spectral resolution [1–8]. How- ever, in typical implementations, these instruments have relatively slow scanning rates. In the case of a continuously swept probe laser and a fixed-length resonator, this situa- tion effectively reduces the measurement duty cycle and can lead to spectral distortion [9, 10]. Alternatively, when the resonator’s length (and hence resonant frequency) is swept, mechanical elements limit both the rate and range of tuning [11]. In summary, for cavity-enhanced spectro- scopic techniques, there are important tradeoffs with regard to optimizing sensitivity, duty cycle, spectral fidelity, and wavelength coverage. We have recently presented a new approach to cavity- enhanced spectroscopy in which high-bandwidth electro- optics are utilized to step a selected laser sideband from mode-to-mode of a resonant cavity and thus scan across a molecular resonance [12]. This technique, which we refer to as frequency-agile, rapid scanning spectroscopy (FARS), removes the dead time associated with tuning the laser frequency between the resonances of a high-finesse cavity and thus yields scanning rates that are limited only by the cavity response time. While our initial demonstrations achieved high sensitivities and scanning rates, we utilized distributed feedback and ultranarrow linewidth fiber lasers that offer tuning ranges of only a few nanometers. Herein, we describe a realization of FARS cavity ring- down spectroscopy (FARS-CRDS) that utilizes a com- mercially available and widely tunable external-cavity diode laser (ECDL). This experiment uses two orthogonal, linear polarizations of the probe laser: one providing a continuous high-bandwidth lock to the ring-down cavity, D. A. Long (&)  G.-W. Truong  R. D. van Zee  D. F. Plusquellic  J. T. Hodges National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899, USA e-mail: david.long@nist.gov G.-W. Truong Frequency Standards and Metrology Research Group, School of Physics, The University of Western Australia, Perth, WA 6009, Australia 123 Appl. Phys. B DOI 10.1007/s00340-013-5548-5