Efficient Harmonic Generation Toward High Power Femtosecond Blue‐Green Laser From a Thulium‐Doped Fiber Amplifier

ABSTRACT Blue‐green ultrafast lasers offer great potential for oceanic optical communication, laser fabrication, and scientific research. We present an efficient cascaded harmonic generation based on an ultrafast high‐power thulium‐doped fiber laser (TDFL) and LiB 3 O 5 (LBO)/β‐BaB 2 O 4 (BBO) crystals. By synergistically optimizing the spatio‐temporal power density of the fundamental laser and the nonlinear crystal length configurations, the system delivers average power/pulse duration of 31.5 W/468 fs at 981.3 nm and 16.1 W/489 fs at 490.8 nm, corresponding to ∼50% conversion efficiency for each harmonic stage. In addition, our work demonstrates the beneficial effects of the intrinsic spatiotemporal walk‐off on suppression of back conversion, while spectral and spatial filtering facilitate beam purification and noise reduction during the second harmonic generation (SHG) process. The multiplicative nature of noise transfer in harmonic generations is substantially counteracted by the spectral and spatial filtering, leading to an integrated relative intensity noise (RIN) of 0.15% (10 Hz–10 MHz) and a timing jitter of 1.8 ps (1 kHz–10 MHz) for the 490.8 nm laser at the maximum output power. To the best of our knowledge, this represents the highest output power ever reported for femtosecond lasers in the 0.9 µm and blue‐green wavelength ranges.