Saturation in cascaded optical amplifier free‐space optical communication systems

Abstract

The performance of a free-space optical (FSO) communication system in a turbulent atmosphere employing an optical amplifier (OA) cascade to extend reach is investigated. Analysis of both single and cascaded OA FSO communication links is given and the implications of using both adaptive (to channel state) and non-adaptive decision threshold schemes are analysed. The benefits of amplifier saturation, for example in the form of effective scintillation reduction when a non-adaptive decision threshold scheme is utilized at the receiver for different atmospheric turbulence regimes, are presented. Monte Carlo simulation techniques are used to model the probability distributions of the optical signal power, noise and the average bit error rate (BER) due to scintillation for the cascade.The performance of an adaptive decision threshold is superior to a non-adaptive decision threshold for both saturated and fixed gain preamplified receivers and that the ability of a saturated gain OA to suppress scintillation is only meaningful for system performance when a non-adaptive decision threshold is used at the receiver. An OA cascade can be successfully used to extend reach in FSO communication systems and specific system implementations are presented. The optimal cascade scheme with a non-adaptive receiver would use frequent low gain saturated amplification.2

APC‐EDFA‐based scintillation‐suppressed photodetection in satellite optical communication

“…The erbium‐doped fiber amplifier (EDFA) has been widely studied to improve the performance of SOC due to the characteristics of polarization independence, wide spectral band, patterning effects immunity, lower amplified spontaneous emission (ASE) noise, and higher power gain . Preamplifier EDFA is used to mitigate scintillation‐induced received optical signal intensity fluctuation by the feature of gain saturation . When the input power exceeds a critical point, the gains are compressed due to the limitation of erbium ions density in the erbium‐doped fiber (EDF).…”

“…The scintillation effect is mitigated by this gain compression. From the perspective of the operation principle of EDFA, gain saturation can be achieved from automatic gain control (AGC) and automatic current control (ACC) . With regard to AGC‐EDFA, a constant gain is maintained for the various input powers by the variation of pump power.…”

“…Therefore, for the sake of mitigating scintillation sufficiently, a fast gain response time, that is, high dynamic gain frequency of EDFA, is required to be much larger than 10 kHz. However, the high dynamic gain frequency of EDFA is up to around 5 kHz due to the long carrier lifetime of the erbium ions at the upper state . Thus, an assistive technique is required to remove the remnants of scintillation components after scintillation mitigation by an APC‐EDFA.…”

“…8 Preamplifier EDFA is used to mitigate scintillation-induced received optical signal intensity fluctuation by the feature of gain saturation. [9][10][11][12] When the input power exceeds a critical point, the gains are compressed due to the limitation of erbium ions density in the erbium-doped fiber (EDF). The scintillation effect is mitigated by this gain compression.…”

“…From the perspective of the operation principle of EDFA, gain saturation can be achieved from automatic gain control (AGC) and automatic current control (ACC). [9][10][11][12][13] With regard to AGC-EDFA, a constant gain is maintained for the various input powers by the variation of pump power. The purpose of the AGC mode is to maintain a constant gain, so a large received optical signal power is required to have gain saturation.…”