We present a Herriott-type multipass laser absorption spectrometer enhanced by optical heterodyne detection. The proposal is demonstrated by measuring the spectra of water vapor molecule in the region from 12247.6873 to 12249.6954 cm^-1. Compared with direct absorption spectroscopy, the signal-to-noise ratio is improved nearly one magnitude of factor by combining with the optical heterodyne spectroscopy and extra weak absorption lines are observed. The minimum detectable absorption is estimated at 4.36×10^-8 cm^-1 and the measured line shape dominated by Doppler broadening can be precisely recovered by direct transformation of experimental optical heterodyne spectral profile.
The absorption spectrum of N2＋ has been studied using optical-heterodyne velocity mod- ulation spectroscopy in the near-infrared region. The observed spectral lines were assigned to the （3,1）, （4,2）, （5,3）, （8,5） bands of the A2Ⅱu-X2∑g＋ system and the line lists were pro- vided. The （5,3） band was studied for the first time. Fourteen rotational-resolved bands in literatures were fitted together with our observed bands and the molecular constants were obtained for VA=0-9 and vx=0-5.
