9.2 Raman-assisted systems and experiments 255 provides enough optical bandwidth to support many optical channels using WDM techniques. Since the first optical undersea transmission systems were installed, the cost of transmission capacity in undersea systems has declined by four orders of magnitude [4]. Both total cost of transmission capacity and "first cost" of deployment are principle drivers of system design. Initial cost of long-haul systems can be reduced by further increasing the distance between repeaters. However, simply increasing span length degrades the optical signal-to-noise ratio (OSNR) at the receiver. Motivated by this fundamental restriction, distributed Raman amplification (DRA) and remote optically pumped amplifiers (ROPA), either alone or used in more advanced amplifier schemes together with EDFAs, have been popular subjects of investigation for a number of years [5,6]. Both technologies offer the attractive possibility of increasing the length of the spans between repeaters and making the system more linear as well as enhancing the received OSNR. Both technologies offer a positive synergy when used with many of the other technologies listed in Figure 9.1, particularly large effective area fibers, dispersion flattened fibers, and differential phase shift keying modulation format. This chapter will address the recent history of the development of the DRA and