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We present a new method of optical frequency comb spectroscopy that combines cavity enhancement with frequency modulation to obtain immunity to laser frequency-to-amplitude noise conversion by the cavity modes and, thus, high absorption sensitivity over a broad spectral range. A frequency comb is locked to a cavity with a free spectral range (FSR) equal to 4/3 times the repetition rate of the laser, and phase-modulated at a frequency equal to the cavity FSR. The transmitted light is analyzed by a Fourier transform spectrometer with a high bandwidth detector. Phase-sensitive detection of the interferogram yields a noise-immune cavity-enhanced optical frequency comb spectroscopy (NICE-OFCS) signal. In the first demonstration, we record NICE-OFCS signals from the overtone CO2 band at 1575 nm with absorption sensitivity of 4.3×10(-10) cm(-1) Hz(-1/2) per spectral element, close to the shot noise limit.

We demonstrate near-infrared cavity-enhanced optical frequency comb spectroscopy of water in a premixed methane/air flat flame. The detection system is based on an Er:fiber femtosecond laser, a high finesse optical cavity containing the flame, and a fast-scanning Fourier transform spectrometer (FTS). High absorption sensitivity is obtained by the combination of a high-bandwidth two-point comb-cavity lock and auto-balanced detection in the FTS. The system allows recording high-temperature water absorption spectra with a resolution of 1 GHz and a bandwidth of 50 nm in an acquisition time of 0.4 s, with absorption sensitivity of 4.2 × 10(-9) cm(-1) Hz(-1/2) per spectral element.

We report an instrument designed for trace gas measurement of highly reactive halogenated radicals, such as bromine oxide and iodine oxide, as well as for nitrogen dioxide and formaldehyde. This compact and robust spectrometer relies on an alternated injection of a frequency-doubled femtosecond radiation at 338 and 436 nm into two parallel high-finesse cavities, for measuring BrO + H(2)CO, and IO + NO(2), respectively. The transmission of the broadband radiation through the cavity is analyzed with a high resolution, compact spectrograph consisting of an echelle grating and a high sensitivity CCD camera. The transportable instrument fits on a breadboard 120 × 60 cm size and is suitable for in situ and real time measurements of these species. A field campaign at the Marine Boundary Layer in Roscoff (in the northwest of France, 48.7°N, 4.0°W) during June 2011 illustrates the outstanding performance of the instrument, which reaches a bandwidth normalized minimum absorption coefficient of 1.3 × 10(-11) cm(-1) Hz(-1/2) per spectral element, and provides detection levels as low as 20 parts per quadrillion of IO in 5 min of acquisition.