We propose a highly sensitive method that can safely measure the Raman gain coefficient spectrum by detecting the spontaneous Raman scattering emitted from the transmission fiber pumped by a single-polarization laser-diode light source in distributed Raman amplification systems. With the proposed method, we have successfully obtained an accurate Raman gain coefficient spectrum with a significantly low pump power of ~0.35 mW at a small Raman gain of 0.01 dB.
