We report the generation of gain-switched pulses near 1.06 microm from a Nd(+3)-doped single-mode fiber pumped at 590 nm. For a 5-cm-long fiber, pulses of 100 nsec at a 1-MHz repetition rate were obtained. For a 1-cm-long fiber, the pulse shortened to 40 nsec at repetition rates of up to 2.5 MHz for a single, independently drivable relaxation oscillation.
By using a thick-walled SiO(2) tube (25-mm outside diameter, 1-mm inside diameter), a soft-glass core containing Nd or Yb-Er has its alkali (and to a lesser extent its alkaline earth) components volatilized during drawing to give a good-quality laser fiber with Nd- or Yb-Er-doped cores.
We report the fabrication of extremely low NA preforms (<0.03), highly doped with Yb using a conventional Modified Chemical Vapor Deposition (MCVD) system. Our lowest NA preform (0.025 NA) was drawn to a 52um core step-index double-clad fiber operating in a single mode regime (M 2 =1.04). The fiber had a mode field diameter (MFD) and an effective area (Aeff) greater than 35um and 1000um 2 respectively. In a fiber laser configuration, the efficiency was greater than 85% without any sign of photodarkening. To the best of our knowledge, by using our extremely low NA preforms we have demonstrated the largest MFD and Aeff to date for a single-mode step index double-clad Yb doped fiber without involving any micro-structuration.
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