Experimental Demonstration of the Influence of Cooling Temperature on the Thermal Mode Instability in the YB-Doped Fiber Oscillator

Mode instability (MI) has become a major factor that restricts the brightness enhancement of fiber laser. This paper studies the MI threshold characteristics of the fiber oscillators with refrigerated ytterbium-doped fiber (YDF). In the experiments, two fiber lasers that can operate at low temperature were built. We rigorously tested the MI threshold of the lasers with the YDF placed in two different environments. At room temperature environment, the MI threshold of the oscillator was measured with the temperature of the YDF water cooling plate ranging from 25 °C to 5 °C. When the cooling temperature of the YDF drops from 20 °C to 5 °C, the MI threshold of the laser rises gradually, but the increase is small. In another experiment, the YDF is assembled in a constant temperature test chamber, in which the cooling of the YDF is more efficient. It is found that as the cooling temperature drops, the MI threshold of the laser has an obvious upward trend. In the process of the cooling temperature of the YDF dropped from 20 °C to −10 °C, the MI threshold of the laser was increased from 867 W to above 931 W, an increase of nearly 10%. It is the first detailed experimental demonstration which shows that decreasing the cooling temperature of the gain fiber can indeed increase the MI threshold of the fiber oscillator. This work can clarify the influence of cooling temperature on the laser thermal effect, which is conducive to perfecting the theoretical model of the MI effect.