In this talk, I will present our work about high-resolution molecular detection methods based on an optical cavity and frequency modulation spectroscopy using 1.28 μm quantum-dot (QD) laser. With a high finesse (>100,000) cavity to reach the saturation limit of the overtone transitions, the sub-Doppler saturation spectroscopy of the nitrous oxide (N2O) was observed with a full width at half-maximum of about 2 MHz at a pressure of several mTorr. 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.