Fiber laser frequency stabilization at 780 nm via second-harmonic generation and modulation transfer spectroscopy...

We demonstrated a high-performance frequency stabilization system based on a 1560 nm fiber laser frequency-doubled to 780 nm and locked to the ⁸⁷Rb D₂ line via modulation transfer spectroscopy. By optimizing spectroscopic parameters and suppressing noise, aided by the introduction of an intensity-noise-equivalent laser power (INELP) concept for accurate in-loop noise assessment, the short-term stability reached 3.32 × 10^-13/τ. With additional active power stabilization using a fiber-coupled AOM, the system achieved 2.8 × 10^-13 at an integration time of 20 s and did not exceed 4 × 10^-12 until 10⁴ s. The current stability is primarily limited by the servo bandwidth constrained by the piezoelectric actuator resonance (∼20 kHz); a fast feedback loop via an AOM is envisioned to further improve the short-term stability.