High-gain U-band discrete Raman amplifier for multi-band optical transmission systems

We experimentally demonstrate a set of U-band discrete Raman amplifiers using backward incoherent pumping in 1 km HNLF, achieving up to 22.3 dB net gain, and 4.2-5.8 dB NF. Three different types of Raman gain fiber have been investigated, including 1 km HNLF, 0.51 km HNLDSF, 8 km and 7.6 km IDF. Using HNLF achieved the highest gain and the lowest NF, while using 8 km IDF yielded up to 16.2 dB net gain and a minimum of 6.3 dB NF due to its low Raman gain coefficient and higher fiber loss. Using 0.51 km HNLDSF gave up to 21 dB net gain, but at a cost of over 8.1 dB noise figure. These amplifiers were incorporated in a C + L + U-band coherent transmission system using 516 × 24.5GBd DP-64/256QAM channels over 50 km SSMF. We achieved a maximum decoded data rate of 123.5Tb/s across C + L + U bands, with 25.6Tb/s specifically in the U-band (1625-1650 nm).