Fiber lasers have become the leading optical source in high-power industrial and medical applications, thanks to their unique combination of efficiency, robustness, and high beam quality. These systems rely on active optical fibers doped with rare-earth ions and pumped by semiconductor laser diodes. For their optimal design and operation, accurate numerical models are essential. However, the predictive capability of such models critically depends on the knowledge of fiber parameters, such as emission and absorption cross sections or rare-earth dopant concentration, which vary significantly with fabrication processes and structural design and are rarely disclosed by manufacturers. This lack of reliable parameter data makes calibration procedures indispensable to ensure meaningful simulations and guide optimization efforts.