Abnormal residual pump behavior in homemade DSCCP fiber

In a recent experimental study, we investigated a homemade distributed side-coupled cladding-pumped (DSCCP) fiber using a master oscillator power amplifier (MOPA) configuration and tandem-pumping technique. During the experiment, we observed an abnormal behavior of the residual pump power from the counter port of the pump core. This abnormal behavior exhibited a threshold-like characteristic, with both the residual power and corresponding power ratio to injected pump starting to increase exponentially above a specific value. Specifically, when the pump power injected into the system ranged from 7.2 kW to 11.77 kW, the corresponding residual power ratio increased from 4.05% to 11.01%. Simultaneously, the signal optical-to-optical conversion efficiency decreased from 84.11% to 75.33%. This sudden appearance of the phenomenon significantly limits the ability to further scale the power of the system. However, the underlying mechanism causing this abnormal behavior remains unclear and requires further investigation.