510 CHAPTER 10. COHERENT LIGHTWAVE SYSTEMS
ing a 1.55-μm external-cavity DFB laser whose output was phase-modulated through
a LiNbO 3 external modulator [146]. The receiver sensitivity at 10 Gb/s was 297 pho-
tons/bit. The signal was transmitted through 151 km of dispersion-shifted fiber without
any dispersion-induced power penalty.
Long-haul homodyne systems use optical amplifiers for compensating fiber losses
together with a dispersion-compensation scheme. In a 1993 experiment, a 6-Gb/s PSK
signal was transmitted over 270 km using multiple in-line amplifiers [149]. A mi-
crostrip line was used as a delay equalizer (see Section 7.2) for compensating the fiber
dispersion. Its use was feasible because of the implementation of the single-sideband
technique. In a later experiment, the bit rate was extended to 10 Gb/s by using the
vestigial-sideband technique [150]. The 1.55-μm PSK signal could be transmitted over
126 km of standard telecommunication fiber with dispersion compensation provided
by a 10-cm microstrip line. The design of long-haul homodyne systems with in-line
amplifiers requires consideration of many factors such as phase noise, shot noise, im-
perfect phase recovery, and amplifier noise. Numerical simulations are often used to
optimize the system performance [151]–[154].
10.6.4 Current Status
Any new technology must be tested through field trials before it can be commercial-
ized. Several field trials for coherent lightwave systems were carried out in the early
1990s [155]–[161]. In all cases, an asynchronous heterodyne receiver was used be-
cause of its simplicity and not-so-stringent linewidth requirements. The modulation
format of choice was the CPFSK format. This choice avoids the use of an external
modulator, thereby simplifying the transmitter design. Furthermore, the laboratory ex-
periments have shown that high-sensitivity receivers can be designed at bit rates as high
as 10 Gb/s. A balanced polarization-diversity heterodyne receiver is used to demodu-
late the transmitted signal.
Field trials have included testing of both land- and sea-based telecommunication
systems. In the case of one submarine trial [159], the system was operated at 560 Mb/s
with the CPFSK format over 90 km of fiber cable. In another submarine trial [160],
the system was operated at 2.5 Gb/s with the CPFSK format over fiber lengths of up to
431 km by using regenerators. Both trials showed that the use of polarization-diversity
receivers is essential for practical coherent systems. In addition, the receiver incorpo-
rated electronic circuitry for automatic gain and frequency controls.
In spite of the successful field trials, coherent lightwave systems had not reached
the commercial stage in 2002. As mentioned earlier, the main reason is related to
the success of the WDM technology with the advent of the erbium-doped fiber am-
plifiers. A second reason can be attributed to the complexity of coherent transmitters
and receivers. The integration of these components on a single chip should address the
reliability concerns. Considerable development effort was directed in the 1990s toward
designing optoelectronic integrated circuits (OEICs) for coherent lightwave systems
[162]–[170]. By 1994, a balanced, polarization-diversity heterodyne receiver contain-
ing four photodiodes and made by using the InP/InGaAsP material system, exhibited a
bandwidth in excess of 10 GHz [164]. A tunable local oscillator can also be integrated
on the same chip. Such a tunable polarization diversity heterodyne OEIC receiver was