Ge-doped Raman fiber amplifier with enhanced signal-to-noise ratio using second Stokes control pulses and effect of double Rayleigh back-scattering

Raman fiber lasers and amplifiers (RFA) are again receiving interest due to development of high-power fiber laser and activation of DWDM optical fiber communications. The basis of the RFA is a stimulated Raman scattering (SRS) in the fiber that can be widely applied to Raman fiber lasers and amplifiers, and a nonlinear-optical modulator. Recently new applications are a hybrid optical amplifier using both Er-doped and Raman fiber amplifiers which has a very wide 3-dB bandwidth of 90.5 ns. A large signal-to-noise ratio (SNR) is an essential factor in high-speed optical transmission to get a low bit-error-rate (BER). During optical amplification, noise is also amplified due to the amplified spontaneous emission (ASE) and other effects. Specially, the performance of DWDM optical transmission system is often limited by the optical amplifier noise. To reduce the noise level, a mid-stage isolator, an attenuator, or a fiber Bragg grating was applied in some optical amplifiers. But it can work only in a spectral regime. Usually the SNR is reduced less than half of the input SNR and a noise figure is larger than 3 dB. Therefore a new idea to reduce the noise level is proposed with help of the Ge-doped Raman fiber amplifier (GDRA) and second Stokes pulses in the temporal domain.