High-Power GMA Fiber Laser Based on Resistance Gain Narrowing for Damage-Free Tissue Ablation

High-peak-power femtosecond fiber lasers are ideal for biomedical precision processing, but their performance is often limited by gain narrowing. Here, we report a passive, all-fiber gain-flattening scheme based on an unpumped Yb-doped fiber. By utilizing the reabsorption effect of the unpumped Yb-doped fiber as a spectral shaper within the power amplifier, gain narrowing was effectively suppressed. Based on this scheme, the laser system delivered 23.4 W of average power at a 43.8 MHz repetition rate. The broad spectrum supported compression to 84 fs, yielding 0.53 μJ pulse energy and a high peak power of 6.4 MW. To verify its practical utility, this source was applied to ex vivo myocardial tissue ablation. Histological analysis confirmed the athermal ablation mechanism, characterized by exceptionally clean incisions devoid of carbonization, protein denaturation, or other collateral thermal damage. This work provides an effective and practical architecture for high-peak-power femtosecond fiber lasers and demonstrates its significant potential for "cold" processing applications, such as athermal surgery.