Instantaneous Output Power of 0.2 kW Achieved in a Quasi-Continuous-Wave-Pumped Thulium-Doped Fiber Laser

Cladding-pumped thulium-doped fiber lasers (TDFLs) are attractive for their power scalability and the availability of high-power, high-brightness 0.79-μm diode-lasers for pumping. Their output power has reached over 1 kW at wavelengths around 2 μm [1]. A high concentration is generally used to overcome the large quantum defect and thus improve the efficiency through a “two-for-one” cross-relaxation process [2]. This however leads to high pump absorption and short device lengths. The thermal load per unit length becomes high and leads to coating failure in the absence of highly capable thermal management solutions. Measures such as off-peak pumping can reduce the pump absorption and thus distribute the heat over a longer fiber, even if this reduces the efficiency due to a relatively high background loss at 2 μm [3]. An alternative approach is to operate the TDFL quasi-continuous-wave (QCW), which we investigate in this work.