Quantum-limited transmission distance of optically repeating systems using a hybrid EDFA/Raman-amplifier

The transmission distance of optically repeating systems is ultimately bounded by quantum noises arising from attenuation and amplification. It is known that fiber Raman amplifiers (FRAs) can achieve longer quantum-limited distances than other amplification schemes owing to their distributed amplification characteristics. However, all-FRA systems are difficult to implement in practice. Therefore, a hybrid erbium-doped fiber amplifier (EDFA)/FRA system may be a reasonable solution for long-haul transmission. In this study, the quantum-limited transmission distance of hybrid EDFA/FRA systems is investigated. A formula for the quantum fluctuation variance during transmission was derived based on quantum mechanical treatment. Calculation results obtained with the derived formula quantitatively indicate that the quantum-limited transmission distance increases as the EDFA/FRA hybrid ratio varies from EDFAs alone to FRAs alone.