Ultra-Low-Noise Mode-Division Multiplexed WDM Transmission Over 100-km FMF Based on a Second-Order Few-Mode Raman Amplifier

We demonstrate, for the first time, an ultra-low-noise mode-division multiplexed transmission with eight wavelength-division multiplexed channels over a 100-km few-mode fiber based on a second-order few-mode Raman amplifier. To minimize the mode dependent gain, two backward depolarized pumps at the wavelengths of 1455 nm (75 mW) and 1360 nm (1890 mW) are equally launched both into the LP11a and LP11b modes. The Raman ON-OFF gains of the LP01 and LP11 modes are both above 8 dB and the mode dependent gain is less than 0.7 dB. An amplified spontaneous emission noise improvement of 1.8 dB is achieved in the second-order few-mode Raman amplifier, with respect to the first-order Raman amplification. The effective noise figure of the second-order few-mode Raman amplifier is about -3 dB, which is similar to 1.4 dB lower than that in the first-order pumping case. In the transmission experiments, a single-mode transmission over LP01 mode and a mode-division multiplexed transmission over LP11a and LP11b modes are performed separately. The bit-error-rates for all the transmission channels are significantly improved due to the low-noise figure amplification of the second-order few-mode Raman amplifier.