Bismuth-doped fiber saturable absorber for a Q-switched erbium-doped all-fiber laser

The characteristics of a Q-switched erbium-doped all-fiber laser incorporating a bismuth-doped fiber (BDF) as a saturable absorber (SA) are explored for the first time, to the best of our knowledge, using various BDF lengths. The nonlinear optical absorption of the BDF-SA results from excited-state absorption (ESA) and concentration-dependent effects, wherein bismuth ions in higher energy levels absorb photons, causing up-conversion losses. These effects are characterized by modulation depths of 4.2% and 5.6% for 15 and 30 cm BDF lengths, respectively. By integrating 15 cm of BDF into the Er-doped laser cavity, stable, self-starting Q-switched pulses are generated, achieving a maximum pulse energy of 160 µJ, a minimum pulse duration of 2.28 µs, and a repetition rate of up to 99.6 kHz. These performance metrics are constrained by the available pump power of 142 mW from a 980 nm laser diode. The findings provide valuable insights into the design and optimization of Q-switched fiber lasers working in the 1.56 µm wavelength region using BDF as the SA.