3.6. TRANSMITTER DESIGN 125
Figure 3.28: Change in current as a function of time for a 1.3-μm InGaAsP laser aged at 60◦C
with 5 mW of output power. (After Ref. [148];©c1985 AT&T; reprinted with permission.)
InGaAsP laser aged at 60◦C under a constant output power of 5 mW from each facet.
The operating current for this laser increases by 40% in the first 400 hours but then
stabilizes and increases at a much reduced rate indicative of gradual degradation. The
degradation rate can be used to estimate the laser lifetime and the MTTF at the elevated
temperature. The MTTF at the normal operating temperature is then extrapolated by
using an Arrhenius-type relationtF=t 0 exp(−Ea/kBT), wheret 0 is a constant andEa
is the activation energy with a typical value of about 1 eV [147]. Physically, grad-
ual degradation is due to the generation of various kinds of defects (dark-line defects,
dark-spot defects) within the active region of the laser or LED [2].
Extensive tests have shown that LEDs are normally more reliable than semicon-
ductor lasers under the same operating conditions. The MTTF for GaAs LEDs easily
exceeds 10^6 hours and can be> 107 hours at 25◦C [147]. The MTTF for InGaAsP
LEDs is even larger, approaching a value∼ 109 hours. By contrast, the MTTF for In-
GaAsP lasers is generally limited to 10^6 hours at 25◦C [148]–[150]. Nonetheless, this
value is large enough that semiconductor lasers can be used in undersea optical trans-
mitters designed to operate reliably for a period of 25 years. Because of the adverse
effect of high temperatures on device reliability, most transmitters use a thermoelectric
cooler to maintain the source temperature near 20◦C even when the outside temperature
may be as high as 80◦C.
Even with a reliable optical source, a transmitter may fail in an actual system if the
coupling between the source and the fiber degrades with aging. Coupling stability is an
important issue in the design of reliable optical transmitters. It depends ultimately on
the packaging of transmitters. Although LEDs are often packaged nonhermetically, an
hermetic environment is essential for semiconductor lasers. It is common to package
the laser separately so that it is isolated from other transmitter components. Figure
3.25 showed two examples of laser packages. In the butt-coupling scheme, an epoxy