Multimode Amplification and Single‐Mode Transformation Enabled by Thermal Modulation

Abstract Boosting extraction efficiency through multimode amplification while simultaneously generating a fundamental‐mode output represents the ideal paradigm for laser systems. This methodology is particularly advantageous in ultrafast lasers, where increasing output intensity is crucial. Here it is found that the thermo‐optic effect, typically considered detrimental to laser performance, can instead promote mode transformation, enabling single‐mode output from a multimode amplifier. Specifically, through thermal modulation, a 39% increase in output power, reaching 50 W of fundamental‐mode output and a pulse width of 168 fs, in a multimode regenerative amplifier based on a Yb:CaAlGdO 4 medium—establishing a new power benchmark for this system is achieved. In addition, to validate the general applicability, the technique to a Yb:YAG gain medium, achieving a 43% power enhancement and demonstrating effective single‐mode transformation is extended. The demonstrated approach can be generalized to a broad range of laser gain media for power enhancement without additional optical components or design complexities.