Mode-locked cavity-enhanced absorption spectroscopy

We demonstrate the principle of cavity enhanced absorption with femtosecond modelocked lasers. The wide spectral coverage allowed by these sources makes this a promising high sensitivity linear absorption technique. The uniformity of the modelocked frequency comb is the feature allowing effective injection of a high finesse cavity. The smooth and stable laser spectral pro le guarantees a good background for the intracavity sample absorption spectrum, recorded by a spectrograph and a linear detector array. With a modelocked Ti: Sa laser and a cavity of finesse F similar or equal to 420 (F/pi is the enhancement factor) we obtain a 4 nm section of a weak overtone band in 40 ms with 0.2 cm(-1) resolution, and a detection limit of 2 x 10(-7) /cm/rootHz. (C) 2002 Optical Society of America.