We design Tb/s long-haul WDM transmission system with transmission distance of 1,500 km. Types of transmission fiber is a SMF(single mode fiber), a NZDSF(non-zero dispersion shifted fiber), and a LEAF(large effective area fiber). The transmission bit rate of a single channel is 2.5 Gb/s, 10 Gb/s, 20 Gb/s, or 40 Gb/s. We use an analytic expression to determine required OSNR(optical signal to noise ratio)for a given bit error rate $10^{-15}$. The OSNR gives the output power of the in-line ampifier.
For linear transmission, there exists a strict limit in selection of a single channel bit rate, when we do not compensate dispersion slope. However, we have a freedom in selection of a single channel bit rate when we compensate the dispersion and its slope. Since the transmission power is several dBm, we can not neglect the optical nonlinear effects. The self-phase modulation prevents exact dispersion compensation. Thus, Optimization of dispersion compensation is required. For SMF, we cannot transmit 20 Gb/s and 40 Gb/s signal over 1,500 km within 1 dB penalty. For NZDSF and LEAF, we can transmit 2.5 Gb/s, 10 Gb/s, 20 Gb/s signals. In multi-channel transmission systems, cross-phase modulation introduces extra phase distortion and chirping, and four wave mixing brings about cross-talk. These features determine a minimum channel spacing in WDM transmission systems.
When we use SMF as a transmission fiber, minimum channel spacing is 30 GHz for 10 Gb/s signal. In this case, the amplifier spacing is 40 km. If we increase the amplifier spacing to 80 km, the minimum channel spacing increases to 50 GHz. Thus it may be possible to transmit 1.28 Tb/s signal over 1,500 km of SMF, when we use 32 nm bandwidth optical amplifier with 40 km amplifier spacing