Sub-Doppler resolution near-infrared spectroscopy at 1.28 μm with the noise-immune cavity-enhanced optical heterodyne molecular spectroscopy method

We report on the sub-Doppler saturation spectroscopy of the nitrous oxide (N₂O) overtone transition at 1.28 μm. This measurement is performed by the noise-immune cavity-enhanced optical heterodyne molecular spectroscopy technique based on the quantum-dot (QD) laser. A high intra-cavity power, up to 10 W, reaches the saturation limit of the overtone line using an optical cavity with a high finesse of 1.145 × 10⁵. At a pressure of several mTorr, the saturation dip is observed with a full width at half-maximum of about 2 MHz and a signal-to-noise ratio of 71. To the best of our knowledge, this is the first saturation spectroscopy of molecular overtone transitions in the 1.3 μm region. The QD laser is then locked to this dispersion signal with a stability of 15 kHz at 1 s integration time. We demonstrate the potential of the N₂O as a marker because of its particularly rich spectrum at the vicinity of 1.28–1.30 μm where there are several important forbidden transitions of atomic parity violation measurements and the 1.3 μm O-band of optical communication.