Over the past 15 years, the capacity of optical fiber transmission systems has risen by more than three orders of magnitude. At the same time, the maximum achievable distance for a single link is 10 times greater than what it was over a decade ago. Innovative optical technology has resulted in a significant drop in the cost-per-bit for transport. Essentially, operators want this cost to continue to fall, as this will help them to meet the constantly growing demand for bandwidth. Applying intensity modulation (a relatively simple transmission scheme compared with those of modern radio systems) in spectral direct-detection systems is one method that has been used to increase network capacity. This method uses dense wavelength division multiplexing (DWDM) and optical amplification, to achieve capacities as high as 800Gbps on a single fiber, corresponding to 80×10Gbps DWDM channels, over link distances of up to 3,000km without the need for signal regeneration.
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