Highly efficient Tm3+ doped germanate large mode area single mode fiber laser

We report the fabrication of a low numerical aperture large mode area, high concentration thulium doped germanate glass fiber using a rod-in tube technique. The fiber core is 20 μm in diameter and operates in the single-mode regime in the 2 μm region. The Tm3+ ion concentration is 3×1020/cm3 and the background fiber attenuation is 1.1 dB/m in the near infrared. We also demonstrate a high-power, high-efficiency fiber laser emitting at 1950 nm using a short (21-cm-long) length of the fiber. A slope efficiency of 55.9% with respect to absorbed pump power and a maximum output power of ~1.52W was achieved. This represents both the highest slope efficiency and the highest output power reported so far for Tm3+ doped germanate single mode fiber using in-band core pumping. The large core area, short device length and high efficiency make this fiber attractive for the development of high peak power pulsed fiber amplifiers and single-frequency fiber lasers operating in the 2 um region.