This work investigates the response of optical fiber communication systems to variations of the launch power for data rates between 2.5 and 40 Gb/s. A simulation model with OptiSystem is developed, where launch power and transmission distance are varied systematically for each data rate. The Q-factor and bit error rate (BER) are used to evaluate the maximum QoS-qualified transmission distance and the first-failing distance. The sensitivity to launch power was quantified by the relative change in Q-factor with launch power. The results indicate that the maximum transmission reach is decreasing from 103 km at 2.5 Gb/s to 4 km at 40 Gb/s. In contrast, the relative Q-factor variation decreases from 89.94% to 2.69%, which means that the sensitivity of the transmission quality to the launchpower variation decreases significantly with the increase of the data rate. The study finds different sensitivity regions and shows the correlation between data rate, transmission reach and launch-power sensitivity. These results provide practical guidelines for launch-power adjustment in multi-rate optical networks, particularly showing that power tuning is most effective in the 2.5–10 Gb/s range.