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International audience; We demonstrate quantum-noise-limited frequency-comb cavity-enhanced absorption spectroscopy with spectrally multiplexed detection for acquisition times ranging from 10 ms up to 10 min, where a record absorption sensitivity of 7×10-13 cm-1 per spectral element is attained with nW power levels. For this, a widely tuneable frequency-doubled free-running fs Ti:sapphire laser oscillator is coupled to a cavity of extreme finesse (32 000) whose length is modulated and whose output is spectrally dispersed over a linear charge coupled device array. This scheme is robust, as it does not require tight frequency locking or controlling the comb frequency offset, and works with spectrally broad combs, as it is not limited by cavity dispersion.

International audience; Pptv levels of BrO radical have been detected around 338.5-nm wavelength probing a rotationally structured A←X (7,0) electronic transition using mode-locked cavity-enhanced spectroscopy (ML-CEAS). The spectrometer is composed by a widely tunable, broadband frequency-doubled Ti:Sa mode-locked frequency comb laser injected into a high-finesse optical cavity and a high-resolution spectrometer based on a high-order diffraction grating and a high-sensitivity back-thinned CCD camera. A typical minimum detectable absorption coefficient of 1×10-9 cm-1 in 30 s of acquisition has been achieved, leading to a detection limit of 1.7 parts per trillion of BrO at atmospheric pressure. The compact and robust ultrasensitive broadband UV spectrometer is intended to be employed for in situ long-term direct measurements of BrO and other halogenated radicals, thus responding to the lack of analytical techniques to monitor the concentrations of such highly chemically reactive species.