A Dual Laser Frequency-Locking Module for NIR and Telecom Transitions in Alkali Atoms

Precise laser frequency stabilization is critical to many atom-based applications including metrology, sensing, and quantum networking. Warm vapors of alkali atoms are attractive candidates for long-distance quantum networking because they possess transitions in the telecom wavelength that are naturally suited for transmitting over existing telecom infrastructure. However, unlike the well-developed saturated-absorption spectroscopy that provides a stable frequency reference for D-line transitions, accessing the telecom line (e.g., 5P to 6S) requires a two-photon transition, and a reliable locking method is yet to be developed. Here, we present a compact, polarization-agnostic module that provides spectra for both ground-state hyperfine transitions and two-photon telecom transitions in alkali atoms, allowing us to lock laser frequencies simultaneously to relevant NIR and telecom transitions with sub-linewidth precision. We report the locking mechanism (double-resonance optical pumping (DROP)), basic setup and operation, and locking characteristics including linewidth, precision, and long-term stability.