Diode ring-down spectroscopy without intensity modulation in an off-axis multipass cavity

Cavity ring-down spectroscopy with an off-axis multipass cell and space separated detectors is proposed to record absorption spectra without modulation of the diode laser intensity. The spectral resolution is ∼0.0003 cm−1. The whole spectrum is obtained for one continuous tuning of the laser frequency for ∼20 ms. When comparing this method to conventional CRDS the required rise time is 1000 times slower. The recording of the whole spectrum for one measurement gives additional possibilities of signal extraction at relatively high noise. The technique is applied to absorption measurement of NO2 in atmosphere.