 decreas- ing nonlinear effects. The overall optical gain is smaller and part of it is distributed; thus, less ASE is expected. A single-stage EDFA is sufficient, also implying less ASE at the receiver. 6.3.3.1 Ultra-Long-Haul Transmission at 10 Gbit/s: Draft Design Our scope in this section is to determine the value of the optical SN ratio before detection, given the required system length via a power budget. Transmission Isolator Isolator Transmission fiber fiber Single stage flattening filter Raman pumps FIGURE 6.28 Block scheme of the line site of an ultra-long-haul system integrating a hybrid EDFA-Raman amplifier. Telecommunication Networks In this case, the system design is quite more complex, because hybrid amplifiers are to be optimized contemporary to the dispersion map, since Raman gain mainly occurs in the We will assume that the required length of our system is around 10,000 km. In order to improve the control of dispersion and nonlinear phenomena, we will select RZ modula- tion with a short duty cycle: 25%. This means that the width of the transmitted Gaussian pulses at 1/e is as short as 25 ps. Since the signal spectrum is 40 GHz large (unilateral), this choice forces to enlarge the transmission line bandwidth up to 0GHz (unilateral, see Section 5.8.5.5) SO that = 4. A channel distance of 50 GHz is not feasible with this optical bandwidth; the system will support in the extended C band 38nm) 46 channels at 0GHz of spacing. In order to generate such short pulses, we select a high power external cavity laser with a linewidth of 300 kHz and an emitted power of 16 dBm. The span length is fixed to 60 km to minimize as much as possible the ASE power while remaining within a reasonable number of required intermediate sites. In order to carry out the power budget we have to design the hybrid amplifier. Since we are interested in a draft design that will be confirmed by simulation, the hybrid amplifier will be optimized using the Raman amplifier model reported in Chapter 4, with- out resorting to the much mo