Semiconductor diode lasers with wavelengths longer than 1.1 micrometers ( ?m) are used primarily for fiber-optic communications. The only III-V material system used for commercial lasers is InGaAsP, a quaternary compound which can be lattice-matched to InP substrates for laser wavelengths between about 1.1 and 1.65 ?m. Researchers are studying other III V systems, but none are offered commercially. Lead salt semiconductors are used in long-wavelength lasers which have different applications and are treated separately in Chap. 21.

Development of InGaAsP lasers has followed a pattern somewhat different from that for shorter-wavelength diode lasers. Although InGaAsP lasers can be made at a broad range of wavelengths, most work has concentrated on two windows of silica optical fibers: 1.31 ?m, where step-index single-mode fibers have zero wavelength dispersion and loss of about 0.5 decibel per kilometer (dB/km), and 1.55 ?m, where silica fibers have their lowest loss, about 0.15 dB/km. InGaAsP lasers have been designed with properties such as high modulation bandwidth and narrow spectral linewidth needed for high-performance long-distance fiber-optic systems.

Unit sales of InGaAsP lasers are modest compared to those of shorter-wavelength diode lasers only about 100,000 in 1990. However, the total has been increasing by some 20 percent a year, and at an average price near $500, InGaAsP lasers cost much more than their shorter-wavelength counterparts. That price reflects modest production, difficult technology, exacting performance requirements, and their vital role in expensive high-capacity communication systems.