Experimental Investigation on Raman Amplification for Weakly-Coupled MDM Transmission

Weakly-coupled mode division multiplexing (MDM) techniques by suppressing modal crosstalk in the fiber link as much as possible to avoid intermodal multiple-input-multiple-output digital signal processing (MIMO-DSP) have attracted great interest for capacity enhancement of short-reach or even long-haul optical fiber transmission. However, the effective few-mode amplifier scheme has seldom been demonstrated for weakly-coupled MDM systems. In this paper, we firstly propose a few-mode distributed Raman amplifier (FM-DRA) scheme, in which broadband mode multiplexer/demultiplexer (MMUX/MDEMUX) consisting of cascaded all-fiber mode-selective couplers (MSCs) are adopted for simultaneous conversion of both signal and pump light so that weakly-coupled transmission condition will not be broken. Then, the proposed FM-DRA for the LP
₀₁
/LP
₂₁
/LP
₀₂
modes of 60-km few-mode fiber (FMF) supporting 4 LP modes is designed and fabricated with bi-directional Raman pumps. The achieved maximum on-off gain, mode dependent gain (MDG), and wavelength dependent gain (WDG) at the C-band for 60-km FMF transmission is 5.3, 0.6 and 0.7 dB, respectively. A recirculating-loop experimental system is established and weakly-coupled MDM transmission with dual-polarization differential quadrature phase-shift keying (DP-DQPSK) modulation and hybrid FM-DRAs/single-mode Erbium-doped fiber amplifiers (SM-EDFAs) is experimentally investigated for the first time. Simultaneous LP
₀₁
/LP
₂₁
/LP
₀₂
transmission over 360-km FMF is successfully demonstrated only adopting 2 × 2 or 4 × 4 MIMO-DSP. The proposed FM-DRA scheme is beneficial for practical applications of MDM techniques.