Real-time characterization of coherence degradation in a rod-type fiber high-power chirped-pulse amplifier

High-average-power ultrafast fiber lasers have emerged as important sources for cutting-edge applications, as they can deliver state-of-the-art performance metrics over spatio-temporal-spectral domains. Beyond the pursuit of higher average power, the degradation of pulse coherence and the increase in noise have become major challenges in the development of high-power fiber lasers, attracting considerable research interest due to their crucial role in determining performance for advanced applications. To this end, by leveraging real-time analysis via a dispersive temporal interferometer, we examine the evolution of coherence properties in a high-power, all-polarization-maintaining fiber laser system as its average power is scaled up to 253 W. The real-time characterization reveals that the Raman scattering severely degrades phase coherence, increasing the standard deviation of relative phase fluctuations from 26.6 mrad to 360 mrad at higher power levels. This work provides a practical methodology for assessing coherence and diagnosing high-frequency phase noise in high-power amplifiers, offering guidance for phase control optimization in coherent beam combining systems.