635 nm femtosecond fiber laser oscillator and amplifier

Abstract

Although visible femtosecond lasers based on nonlinear frequency conversion of Ti:sapphire femtosecond oscillators or near-infrared ultrafast lasers have been well developed, limitations in terms of footprint, cost, and efficiency have called for alternative laser solutions. The fiber femtosecond mode-locked oscillator as an ideal solution has achieved great success in the 0.9 to 3.5 μm infrared wavelengths, but remains an outstanding challenge in the visible spectrum (390 to 780 nm). Here, we tackle this challenge by introducing a visible-wavelength mode-locked femtosecond fiber oscillator along with an amplifier. This fiber femtosecond oscillator emits red light at 635 nm, employs a figure-nine cavity configuration, applies a double-clad Pr 3þ -doped fluoride fiber as the visible gain medium, incorporates a visible-wavelength phase-biased nonlinear amplifying loop mirror (PB-NALM) for mode locking, and utilizes a pair of customized high-efficiency and high-groove-density diffraction gratings for dispersion management. Visible self-starting mode locking established by the PB-NALM directly yields red laser pulses with a minimum pulse duration of 196 fs and a repetition rate of 53.957 MHz from the oscillator. Precise control of the grating pair spacing can switch the pulse state from a dissipative soliton or a stretched-pulse soliton to a conventional soliton. In addition, a chirped-pulse amplification system built alongside the oscillator immensely boosts the laser performance, resulting in an average output power over 1 W, a pulse energy of 19.55 nJ, and a dechirped pulse duration of 230 fs. Our result represents a concrete step toward high-power femtosecond fiber lasers covering the visible spectral region and could have important applications in industrial processing, biomedicine, and scientific research.