repeater.
It was clear during the 1970s that the repeater spacing could be increased consid-
erably by operating the lightwave system in the wavelength region near 1.3 μm, where
fiber loss is below 1 dB/km. Furthermore, optical fibers exhibit minimum dispersion in
this wavelength region. This realization led to a worldwide effort for the development
of InGaAsP semiconductor lasers and detectors operating near 1.3 μm. The second
generation of fiber-optic communication systems became available in the early 1980s,
but the bit rate of early systems was limited to below 100 Mb/s because of dispersion in
multimode fibers [14]. This limitation was overcome by the use of single-mode fibers.
A laboratory experiment in 1981 demonstrated transmission at 2 Gb/s over 44 km of
single-mode fiber [15]. The introduction of commercial systems soon followed. By
1987, second-generation lightwave systems, operating at bit rates of up to 1.7 Gb/s
with a repeater spacing of about 50 km, were commercially available.