Network application and system demonstration of WDM systems with very large spans (error-free 32×10 Gbit/s 750 km transmission over 3 amplified spans of 250 km)

Abstract

Network applicationsSubmarine systems are usually categorized into two types : very long trans-oceanic links with submerged optical amplifiers, and short distance unrepeatered systems, limited to around 300 to 400 km. There is actually some room for and some interest in submarine systems of medium haul which would use land-based in-line optical amplifiers. In the scope of global networks, such systems can be deployed for example to cross seas with intermediate sites on islands or to create coastal festoons, like in typical examples of Figure 1. Intermediate stations can simply re-amplify the signals or can also add-and-drop some wavelengths. Such systems are cheaper than systems with electrical inline regeneration and do not involve submerged optical amplifiers. These systems are now becoming of interest because of the advent of technologies which allow to reach very large spans. Enabling technologiesVery long spans can be basically achieved by using the same technologies as for long distance unrepeatered systems [1,2,3] : low loss line fiber, large launch signal power, Raman preamplification and forward error correction. Low loss line fiber can be standard fiber (0.2 dB/km) or pure silica core fiber (0.18 dB/km) for even longer spans. The large chromatic dispersion (+17 to +20 ps/nm.km) of these fibers is beneficial for WDM systems because it reduces interactions between channels, which enables the use of a narrow channel spacing (50 GHz) and to launch large powers [4]. Optical amplifiers with very large power are now available and the launch power is actually limited by nonlinearities in the line fiber, such as Kerr effect, which induces pulse distortions, and Raman effect, which causes signal depletion and seems to be the ultimate limit [l]. Raman preamplification [5] is a further way to increase the span length, owing to the advent of powerful pumps, in particular Raman fiber lasers which can deliver more than 1 Watt. Forward Error Correction (FEC) is also an efficient solution to increase the span power budget and to improve the transmission quality. System demonstrationIn the scope of these concepts, a laboratory demonstration has been achieved, as depicted in Figure 2.The experiment consists in the error-free transmission of 32 channels at 10 Gbit/s over 3 spans of 250 km. The line fiber is pure silica core fiber with an effective area of 75 pm2, a chromatic dispersion around +18.5 ps/nm.km and a mean loss of 0.1 78 dB/km. Each span has an average loss of 44.5 dB and the total launch power in each span is +21.5 dBm. Raman preamplification is achieved in each span by launching 1 Watt of 1455-nm pump power in the line fiber from the end of the span. The Raman amplification unflatness and the Raman depletion between channels are corrected with some

Ultrabroad-band Raman amplifiers pumped and gain-equalized by wavelength-division-multiplexed high-power laser diodes

“…Also, Raman amplifiers play an important role in transoceanic multi-terabit-per-second WDM transmission [23]. An interesting use of the power budget margin is to extend the fiber span distance between amplifiers up to 250 km [24], 140 km [25], and 160 km [26]. Some attempts have been made to increase the bit rate of single channel up to 320 Gb/s [27].…”

High-capacity Raman-amplified long-haul transmission and the impact of optical fiber properties

“…Raman amplification also can be used to increase span lengths to 160 to 250 km [59][60][61], thus allowing network operators to skip amplifier huts and significantly reduce operating expenses for medium-and long-haul systems. We have investigated the system performance of a forty channel, 10-Gb/s system with 200-km spans, at least twice the conventional span length used in terrestrial systems.…”