In-situ Doppler-free spectroscopy with pulsed optical fields

We propose a novel pulsed optical field method that alternately switches the pump beam in conventional saturation absorption to time-division multiplex the same probe beam into both probe and reference beams, followed by digital differential processing to achieve deterministic zero-background Doppler-free spectroscopy. This method effectively mitigates Doppler broadening and common-mode optical noise by addressing disturbances such as non-uniform background absorption and environmental noise, thereby offering enhanced accuracy and robustness. Using this technique, we measured the absolute frequency of Yb$^{+}$ isotopes in the $6s^2\ ^{1}S_0\to 6s6p ^{1}P_1$ transition. By employing an error signal derived from the first-derivative demodulated spectrum of $^{174}\mathrm{Yb}^{+}$, we achieved efficient stabilization of a 369.5 nm ultraviolet diode laser, demonstrating a frequency stability of $3 \times 10^{-11}$ over a 1500-second averaging period and a locking point uncertainty of 850 kHz sustained over 10 days. Furthermore, we report the first in-situ observation of Doppler-free Zeeman sub-level spectra, highlighting the precision of this method and its potential application in measuring magnetic field gradients.