Very high sensitivity CW-cavity ring down spectroscopy: Application to the O2 band near 1.58 μm

A few absorption lines of the QQ branch of the a 1 Δ g ( 0 ) – X 3 Σ g - ( 1 ) hot band of pure O2 have been detected near 1.58 μm by CW-cavity ring down spectroscopy based on a fibred DFB laser. A sensitivity of the order of 2 × 10−11 cm−1 has been achieved by averaging a large number of spectra, allowing for the measurement of line intensities as weak as 5 × 10−31 cm/molecule. The integrated band intensity is estimated to be S01 = 4.5(7) × 10−29 cm/molecule which differs by a factor of about 6 from the value provided by the HITRAN database. Our intensity value is supported by a significantly better agreement with the value deduced from the intensity of the a 1 Δ g ( 0 ) – X 3 Σ g - ( 0 ) cold band and the respective Franck–Condon factors.