In an optical submarine cable transmission system, small size, low consumption power, and high reliability are required for inline repeaters. The structure 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 designed and developed high alumina co-doped erbium doped fiber amplifiers (EDFAs) 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 slightly sloped gain flatness of +0.04 dB/nm at 1550 nm, a superior noise figure of 4.7 dB, and a relatively large output power of +11.5 dBm for an EDF length 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 compensate 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 the 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 performed the broad wavelength bandwidth. The achieved signal wavelength bandwidth after 5,958-km transmission was 20 nm. We successfully transmitted 700-Gbit/s (66 10.66-Gbit/s) WDM signals over 2,212 km. The combination of high alumina co-doped silica EDFA and large FSR GEQ is attractive for long-haul, high-capacity WDM transmission systems.

In long-haul wavelength-division-multiplexed (WDM) transmission systems, signals with shorter and longer wavelengths have self-phase modulation group-velocity-dispersion (SPM-GVD) penalty caused by to the dispersion slope even after the dispersion-compensation at the receiver has been optimized. As a countermeasure, we have already proposed both pre-compensation and post-compensation of chromatic dispersion at the transmitter and receiver for each channel. This method can decrease the channel variation of path-averaged chromatic dispersion along the transmission line, and it can improve the eye opening of the waveform after transmission. We investigated the optimized parameter of chromatic dispersion and chirping at the transmitter. The optimized pre-dispersion compensation parameter R was about 50%. The optimized chirping parameter α was about 3 when the signal wavelength was less than the mean zero-dispersion wavelength. In a single-channel, 5.3-Gbit/s NRZ signal transmission experiment over a 4,760-km straight line, this method decreased SPM-GVD penalty. In a 32-channel, 5.3-Gbit/s WDM transmission experiment over 9,879 km using a circulating loop, this method improved Q-factors for the 1st and 32nd channels by more than 1.5 dB.

We describe 2.5Gb/s 4 channel WDM transmission over 1060km using 18 EDFAs. Gain bandwidth narrowing in concatenated EDFAs has been successfully suppressed using unsaturated EDFAs and a 1.53µm ASE rejection filter.

To expand signal wavelength bandwidth in long-haul, large-capacity WDM transmission systems, we investigated gain-equalizers (GEQs) for Erbium doped fiber amplifiers (EDFAs). We applied GEQs using Mach-Zehnder type filters with two different free-spectral-ranges (FSRs) to accurately compensate for the EDFAs gain-wavelength characteristics. The 1st GEQ with a longer FSR was the main GEQ to compensate for the overall gain-wavelength characteristics, and the 2nd GEQ with a shorter FSR was the secondary GEQ to compensate for the resultant gain undulation after the 1st GEQ. The 2nd GEQ had low maximum loss and long period of equalization-spacing compared to the 1st GEQ. We designed that the FSR for the 1st GEQ was twice the signal wavelength bandwidth, and the FSR for the 2nd GEQ was two thirds of the signal wavelength bandwidth. To compensate for the asymmetry in the EDFAs gain-wavelength characteristics, we designed that the 2nd GEQ minimum-loss wavelength was shorter than the 1st GEQ maximum-loss wavelength. Using a circulating loop with a 21-EDFA chain, we confirmed the signal wavelength bandwidth expanded by the above GEQs. We also investigated the trade-off relationship between the signal wavelength bandwidth and the optical signal-to-noise ratio, as the parameter of the number of the 1st GEQ inserted in the EDFAs chain. The achieved signal wavelength bandwidth after 10,000-km transmission was 12 nm. We successfully transmitted 170 Gbit/s (325. 332 Gbit/s) WDM signals over 9,879 km employing high alumina codoped EDFAs and Mach-Zehnder type filters with long FSRs.