Passive mode locking of commercially available, single mode, Dy3+:fluorozirconate using nonlinear polarization rotation was investigated in a simple, relatively standard cavity configuration incorporating bulk wave-plates, reflectors, and an isolator. Two fiber lengths were employed, 3 m and 4.5 m, pumped in-band at 2.825 µm by a CW Er:fluoride fiber laser supplying up to 10 W. Self-starting, nearly transform-limited, mode locking was observed at threshold powers around 1.7 W, generating pulses of 724 fs and 883 fs at the fundamental frequency of the cavities, with output powers of 66 mW and 72 mW, for 3- and 4.5-m fiber lengths, respectively, and greater than 85 dB dynamic range between spectral peak and noise background. The 4.5-m system operated at a longer wavelength of 3.083 µm. On increasing the output coupling from 22.5% to 55%, 204 mW was obtained for 2.2 W pump power; however, at 2.6 W pumping, although 400 mW output power was achieved, the laser repetition rate operated at higher harmonics.
These lasers exhibited an overall large anomalous dispersion, and consequently there is ample opportunity to optimize and minimize pulse durations from this exciting new addition to the palette of the passively mode-locked fiber laser, and while limited tunability was also observed (~40 nm), optimization of this will also surely add to the attractiveness of this potentially very useful source.
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