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Quantum noise intrinsically restricts the ultimate transmission distance in fiber communication systems that use optical repeating amplifiers. This study theoretically investigates the quantum-noise-limited distance of optical coherent transmission using fiber Raman amplifiers. An analytical formula for the upper bound of the transmission distance is derived based on full quantum mechanics, considering the amplification noise and intrinsic fluctuation of a coherent state (or vacuum noise), and calculations are performed using the derived formula. The results quantitatively indicate that the Raman amplified systems potentially achieve considerably longer transmission distances, more than ten times, than a system with erbium-doped fiber amplifiers under ideal conditions. |
