High-power U-band femtosecond laser pulses generated by nonlinear optical gain modulation

Ultrafast laser pulses in the 1.62-1.68 μm wavelength range (U-band) hold significant potential for applications such as spectroscopy, optical communication, and bio-imaging. However, it is challenging to generate high-energy, high-performance ultrafast pulses in the U-band, since it cannot be covered by the emission spectra of well-developed rare-earth ions of erbium and thulium. Herein, we demonstrate a simple and efficient method to generate U-band femtosecond laser pulses by nonlinear optical gain modulation of a 1645 nm single-frequency (SF) laser through stimulated Raman scattering. Near-transform-limited 1645 nm laser pulses were obtained with 2.3 W average power and 234 fs pulse duration at a 102.3 MHz repetition rate and 63.2% conversion efficiency. By tuning the wavelength of the SF laser, this method is expected to enable wavelength-flexible, high-energy ultrafast pulse generation across the entire U-band.