We achieve a quantum-noise-limited absorption sensitivity of 1.7×10-12 cm -1per spectral element at 400 s of acquisition time with cavity-enhanced frequency comb spectroscopy, the highest demonstrated for a comb-based technique. The system comprises a frequency comb locked to a high-finesse cavity and a fastscanning Fourier transform spectrometer with an ultra-low-noise autobalancing detector. Spectra with a signal-to-noise ratio above 1000 and a resolution of 380MHz are acquired within a few seconds. The measured absorption lineshapes are in excellent agreement with theoretical predictions.PACS numbers: 42.62.Fi, 42.50.Lc, 85.60.Gz, 42.79.Gn Continuous wave (cw) laser absorption spectroscopy is a well-established technique for quantitative measurements of various constituents in gas phase. The sensitivity of the technique can be improved either by employing modulation techniques, which shift the signal to audio [1, 2] or radio [3] frequencies, where the technical noise is reduced, or by implementing an external high-finesse enhancement cavity [4, 5], which increases the interaction length of the light with the sample, and thus the absorption signal. Successful combination of these two approaches has led to impressive shot-noise-limited absorption sensitivities [6]. The main constraint of cavity-2 enhanced cw techniques, however, is their inability to measure broadband spectra in short acquisition times. The optical frequency combs extend the benefits of cw-laser-based techniques to thousands of laser lines and remove the bandwidth limitation [7, 8]. The discrete spectrum of equidistant comb lines matches that of an external enhancement cavity, enabling efficient coupling of the comb light into the cavity [9,10]. Several detection schemes have been proposed for cavity-enhanced direct frequency comb spectroscopy (CE-DFCS), but none was able to suppress the technical noise down to the quantum limit. In most previous realizations of the technique, the light transmitted through the cavity has been detected using a dispersive element and a multidetector array [11][12][13]. With this approach the laser frequency-to-amplitude (FM-to-AM) noise conversion caused by the narrow cavity modes can be efficiently reduced, by measuring either the cavity ringdown time [11] or integrated cavity output [12]. However, the sensitivity is limited by the technical noise in the detector array because of the low optical power in each spectrally resolved element. An alternative is to use Fourier transform spectrometry (FTS), either with a mechanical Michelson interferometer [14,15] or with the dual comb approach [16]. The advantage of using FTS for CE-DFCS is that the spectral bandwidth of the detection system is limited only by the cavity dispersion, and not by the number of detection elements [17]. However, a photodetector and data acquisition board with a large dynamic range are necessary to obtain high signal-to-noise ratios [18]. Moreover, the use of FTS requires constant transmission through the cavity and thus the comb to b...

Optical frequency combs offer enormous potential in the detection and control of atoms and molecules by combining their vast spectral coverage with the extremely high spectral resolution of each individual comb component. Sensitive and multiplexed trace gas detection via cavity-enhanced direct frequency comb spectroscopy has been demonstrated for various molecules and applications; however, previous demonstrations have been confined to the visible and near-infrared wavelength range. Future spectroscopic capabilities are created by developing comb sources and spectrometers for the deep ultraviolet and mid-infrared spectral regions. Here we present a broadband high resolution mid-infrared frequency comb-based Fourier transform spectrometer operating in the important molecular fingerprint spectral region of 2100-3600 cm(-1) (2.8-4.8 microm). The spectrometer, employing a multipass cell, allows simultaneous acquisition of broadband, high resolution spectra (down to 0.0035 cm(-1) of many molecular species at concentrations in the part-per-billion range in less than 1 min acquisition time. The system enables precise measurements of concentration even in gas mixtures that exhibit continuous absorption bands. The current sensitivity, 2 x 10(-8) cm(-1) Hz-1/2 per spectral element, is expected to improve by two orders of magnitude with an external enhancement cavity. We have demonstrated this sensitivity increase by combining cavity-enhanced frequency comb spectroscopy with a scanning Fourier transform spectrometer in the near-infrared region and achieving a sensitivity of 4.7 x 10(-10) cm(-1) Hz(-1/2). A cavity-enhanced mid-infrared comb spectrometer will provide a near real-time, high sensitivity, high resolution, precisely frequency calibrated, broad bandwidth system for many applications.

We present direct observation of the velocity-selective optical pumping of the Rb ground state hyperfine levels induced by 5S(1/2) --> 5P(1/2) femtosecond pulse-train excitation. A modified direct frequency comb spectroscopy based on the fixed frequency comb and a weak cw scanning probe laser was developed. The femtosecond pulse-train excitation of a Doppler-broadened Rb four-level atomic vapor is investigated theoretically in the context of the density matrix formalism and the results are compared with the experiment.

Quantum-mechanical simulations of the excitation spectra of KRb from the lowest vibrational level of the lowest triplet and singlet electronic states have been performed using recently calculated interaction potential curves and corresponding transition dipole moments. The obtained spectra can be used for a comparison with experimental absorption spectra of KRb molecules produced in their vibronic ground state or attached to cold helium droplets. In addition, we compare the semiclassically simulated spectra with absorption measurements in dense K-Rb vapour at high temperatures, which helped us to identify three diffuse bands as 1 transitions. The first may be observable in an excitation spectrum of KRb dimer formed on cold helium droplets.

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