High alumina co-doped silica EDFA and its gain-equalization in long-haul WDM transmission system

In an optical submarine cable transmission system, small size, low consumpt ion power, and high reliability are required for inline repeaters. The stru cture of the inline repeater should be a simple single stage. The design of erbium doped fiber (EDF) itself is very important for the inline repeater to achieve broad bandwidth, high output power, and low noise figure. We des igned and developed high alumina co-doped erbium doped fiber amplifiers (ED FAs) for long-haul, high-capacity WDM transmission systems. We investigated the trade-off relationship between the gain flatness and the output power to optimize the EDF length. We obtained high performance, including a sligh tly sloped gain flatness of +0.04 dB/nm at 1550 nm, a superior noise figure of 4.7dB, and a relatively large output power of +11.5 dBm for an EDF leng th of 5 m using a 1480-nm pumping laser diode. We applied gain-equalizers ( GEQs) using Mach-Zehnder type filters with different FSRs to accurately com pensate for the EDFAs' gain-wavelength characteristics. The main GEQs have free-spectral-ranges (FSRs) of 48-nm, which are about 2 times as long as th e wavelength difference between a 1558-nm EDFA gain peak and a 1536-nm EDFA gain valley. Using a circulating loop with the above EDFAs and GEQs, we pe rformed the broad wavelength bandwidth. The achieved signal wavelength band width after 5,958-km transmission was 20nm. We successfully transmitted 700 -Gbit/s (66 x 10.66-Gbit/s) WDM signals over 2,212 km. The combination of h igh alumina co-doped silica EDFA and large FSR GEQ is attractive for long-h aul, high-capacity WDM transmission systems.