124 CHAPTER 3. OPTICAL TRANSMITTERS
mitters can be improved considerably by using monolithic integration of the laser
with the driver. Since optical and electrical devices are fabricated on the same chip,
such monolithic transmitters are referred to asoptoelectronic integrated-circuit(OEIC)
transmitters. The OEIC approach was first applied to integration of GaAs lasers,
since the technology for fabrication of GaAs electrical devices is relatively well es-
tablished [129]–[131]. The technology for fabrication of InP OEICs evolved rapidly
during the 1990s [132]–[136]. A 1.5-μm OEIC transmitter capable of operating at
5 Gb/s was demonstrated in 1988 [132]. By 1995, 10-Gb/s laser transmitters were fab-
ricated by integrating 1.55-μm DFB lasers with field-effect transistors made with the
InGaAs/InAlAs material system. Since then, OEIC transmitters with multiple lasers
on the same chip have been developed for WDM applications (see Chapter 8).
A related approach to OEIC integrates the semiconductor laser with a photodetec-
tor [137]–[139] and/or with a modulator [117]–[120]. The photodetector is generally
used for monitoring and stabilizing the output power of the laser. The role of the modu-
lator is to reduce the dynamic chirp occurring when a semiconductor laser is modulated
directly (see Section 3.5.2). Photodetectors can be fabricated by using the same mate-
rial as that used for the laser (see Chapter 4).
The concept of monolithic integration can be extended to build single-chip trans-
mitters by adding all functionality on the same chip. Considerable effort has been
directed toward developing such OEICs, often calledphotonic integrated circuits[6],
which integrate on the same chip multiple optical components, such as lasers, detectors,
modulators, amplifiers, filters, and waveguides [140]–[145]. Such integrated circuits
should prove quite beneficial to lightwave technology.
3.6.5 Reliability and Packaging
An optical transmitter should operate reliably over a relatively long period of time (10
years or more) in order to be useful as a major component of lightwave systems. The
reliability requirements are quite stringent for undersea lightwave systems, for which
repairs and replacement are prohibitively expensive. By far the major reason for failure
of optical transmitters is the optical source itself. Considerable testing is performed
during assembly and manufacture of transmitters to ensure a reasonable lifetime for
the optical source. It is common [95] to quantify the lifetime by a parametertFknown
asmean time to failure(MTTF). Its use is based on the assumption of an exponential
failure probability [PF=exp(−t/tF)]. Typically,tFshould exceed 10^5 hours (about
11 years) for the optical source. Reliability of semiconductor lasers has been studied
extensively to ensure their operation under realistic operating conditions [146]–[151].
Both LEDs and semiconductor lasers can stop operating suddenly (catastrophic
degradation) or may exhibit a gradual mode of degradation in which the device effi-
ciency degrades with aging [147]. Attempts are made to identify devices that are likely
to degrade catastrophically. A common method is to operate the device at high temper-
atures and high current levels. This technique is referred to as burn-in oraccelerated
aging[146] and is based on the assumption that under high-stress conditions weak de-
vices will fail, while others will stabilize after an initial period of rapid degradation.
The change in the operating current at a constant power is used as a measure of de-
vice degradation. Figure 3.28 shows the change in the operating current of a 1.3-μm