High sensitivity detection of molecular oxygen using cavity-enhanced frequency modulation spectroscopy

In recent years, high finesse optical cavities have been used for high sensitivity spectroscopy by exploiting the very long absorption path lengths, 10 to 100 times greater than in traditional multipass cells. Cavity Ring Down Spectroscopy applications have been widely performed with both pulsed [1] and CW laser sources [2]. Direct absorption spectroscopy with an optical cavity has also been demonstrated by recording the peak transmitted power around an absorption line [3]. Another relevant application is represented by saturated absorption spectroscopy of overtone molecular transitions. In this case, high finesse and high build-up provide enough intracavity power for saturation of the absorbing gas [4]. More recently, the “NICE-OHMS” method, using frequency modulation techniques, has been used to detect extremely weak saturated signals in a high finesse cavity [5]. In this novel method, the laser beam is frequency modulated at exactly the cavity free-spectral-range (FSR) frequency. Using heterodyne detection of the transmitted power, the phase shift of the central carrier produced by optical dispersion gives rise to the signal. The NICE-OHMS method has reported detection sensitivities for absorption of 10−12 cm−1 [5]. In this work, we demonstrate how the cavity enhanced frequency modulation technique can be used to perform high sensitivity measurements of small absorption signals, corresponding to weak absorption lines in molecular oxygen.