Impact of Feedback Amplified Spontaneous Emission Peak Power on the Performance of an All‐Optical Automatic Gain‐Controlled Erbium‐Doped Fiber Amplifier in Terrestrial Free‐Space Optical Communication Systems
Chul Han KimABSTRACT
An all‐optical automatic gain‐controlled (AGC) erbium‐doped fiber amplifier (EDFA) is proposed and experimentally demonstrated for suppressing optical signal power fluctuations in terrestrial free‐space optical communication (FSOC) systems. The proposed AGC EDFA employs a simple optical feedback loop consisting of two optical couplers, an optical bandpass filter and a conventional EDFA, eliminating the need for electronic feedback circuitry. The operating characteristics of the amplifier were investigated by measuring the gain and amplified spontaneous emission (ASE) spectra under different feedback ASE peak power levels. Stable AGC operation with gain saturation was achieved when the feedback ASE peak power exceeded approximately −15 dBm, whereas excessive ASE feedback resulted in pump power depletion and reduced amplifier gain. Optical signal fluctuations emulating atmospheric turbulence were generated using an acousto‐optic modulator (AOM), and the fluctuation suppression performance was evaluated under sinusoidal, randomly varying, and square‐wave driving conditions. The proposed AGC EDFA effectively suppressed optical signal fluctuations over an AOM driving frequency range from 1 Hz to 10 kHz and reduced both the peak‐to‐peak signal variation and the scintillation index under randomly varying input conditions. Dynamic response measurements further revealed that the transient response depends strongly on the magnitude of the input optical power fluctuation because of the ASE lasing buildup dynamics and pump power depletion. These results demonstrate that the proposed all‐optical AGC EDFA provides a simple, cost‐effective, and practical approach for mitigating turbulence‐induced optical power fluctuations while offering useful design guidelines for optimizing the dynamic performance of AGC EDFAs in terrestrial FSOC systems.