Answer: ACC mode - automatic current control: The EDFA pump operating current is set by the user and automatically locked by the EDFA to maintain a constant pump current. Even if the input optical power fluctuates, the pump current will not respond to changes, so the output power will also fluctuate. EDFA does not interfere with this power fluctuation, and ACC mode can be used for all EDFA models. Small signal EDFA amplifier, only ACC mode.

APC mode - automatic power control: The user sets the signal light output power of EDFA, PD automatically monitors and feedbacks the output power, EDFA controls and adaptively adjusts the pump to achieve stable output signal. The advantage of APC mode is that when the input optical power fluctuates, EDFA will minimize the fluctuation of output power as much as possible, which is suitable for power type and line type EDFAs.

Use the following diagram to facilitate understanding: