Enhancing long-distance optical link performance in MDM systems with multi-stage amplification and systematic design

Abstract

Few-mode erbium-doped fiber amplifiers (EDFAs) are crucial for long-distance transmission and modal power management. Current research primarily focuses on gain equalization within individual amplifiers. However, the gain of the amplifier is closely related to the input power. Variations in modal loss result in power disparities among different signal modes, causing gain instability and adversely impacting the system's overall performance. In multi-stage links, even a single amplifier with minimal gain discrepancy might create unequal amplification in the subsequent stage due to difference modal losses. After cascading multiple amplifiers, the imbalance will become even more severe. We have designed a target loss compensation erbium-doped fiber for multi-stage amplification. By considering both gain differences and fiber losses, we manipulate the doping structure to intentionally introduce differential modal gain to compensate for various modal losses. With a power disparity threshold set at 2 dB, the tolerance of cascaded inline amplifiers has been enlarged from 2 to 15, significantly extending the permissible relay distance from 250 km to 2000 km. The method no longer focuses solely on the performance of individual amplifiers but examines the power management of the entire link. The results clearly demonstrate the effectiveness of the loss compensation strategy. It provides a robust and practical method for mode power management without increasing the complexity.