Knowledge Bank

We have demonstrated wavelength modulation spectroscopy of HCl using a difference-frequency-generation (DFG) source and an enhanced-cavity absorption cell. The frequency axis of the spectrum is calibrated by a fiber-based optical frequency comb which is locked to a Rb clock linked with TAI. The pump and signal sources of DFG are a 1.06-$\mu$m Nd:YAG laser and a 1.55-$\mu$m ECLD, and the idler wave is generated in a waveguide-type PPLN. The hyperfine structure caused by the Cl nucleus with the spin 3/2 is resolved for the $R(0)$, $R(1)$, and $R(2)$ transitions in the fundamental vibration band. The hyperfine components of $\Delta F=+1,0,-1$, and the cross-over resonances are observed with a typical line width of about 220 kHz, and the transition frequencies are measured with an uncertainty of less than 10 kHz. The pressure- and power-dependences of the transition frequency and the spectral intensity of the cross-over resonances are also investigated.