Second-order few-mode Raman amplifier for mode-division multiplexed optical communication systems

We experimentally demonstrate and investigate, for first time to our best knowledge, a second-order few-mode Raman amplifier for low noise distributed fiber amplification. The 1455 and 1360 nm pumps are both injected into the few-mode fiber (FMF) in the forms of two degenerate LP₁₁ modes in the backward direction. Within the band from 1542 to 1558 nm, maximum on-off gains of 4 dB are achieved for both LP₀₁ and LP₁₁ modes, and the differential modal gain (DMG) is less than 0.4 dB. The noise figure (NF) improvements at 1550 nm for LP₀₁ and LP₁₁ modes are 1.2 dB and 1.1 dB, respectively, compared with the conventional first-order pumping scheme. The lowest NFs of less than -2 dB are achieved for both modes. We build an optical time-domain reflectometer (OTDR) in the few-mode distributed Raman amplifier (FM-DRA) to measure the signal evolutions, and the results indicate a proof-of-concept low noise amplification for second-order pumping with respect to the conventional first-order pumping case. Due to the second-order pumping, broadened Raman amplification band has been observed with improved gain flatness for both LP₀₁ and LP₁₁ modes, which is also of great importance in the optical communication systems. The second-order FM-DRA can be used potentially in future high capacity mode-division multiplexing (MDM) optical communication systems.