Sub-Doppler spectroscopy of H218O at 1.4 μm

We performed saturated-absorption spectroscopy of H218O at 1.4 μm, using a low-power mirror-extended distributed-feedback diode laser. Although the rotational lines of the ν1+ν3 band investigated in this work had a small transition dipole moment, saturation was possible thanks to a short Fabry–Pérot cavity with a finesse of about 500. We investigated the sub-Doppler spectrum of the 212→111 line for different sample gas pressures. A pressure-broadening coefficient of 32.9±0.7 MHz/Torr was measured. We also evaluated the saturation intensity as well as the transition dipole moment. To our knowledge, the observed Lamb dip represents the first narrow resonance at 1.4 μm.