10.6. SYSTEM PERFORMANCE 507
Four-wave mixing also becomes a limiting factor for WDM coherent systems [113]
and need to be controlled employing high-dispersion locally but keeping the average
dispersion low through dispersion management.
10.6 System Performance
A large number of transmission experiments were performed during the 1980s to demon-
strate the potential of coherent lightwave systems. Their main objective was to show
that coherent receivers are more sensitive than IM/DD receivers. This section focuses
on the system performance issues while reviewing the state of the art of coherent light-
wave systems.
10.6.1 Asynchronous Heterodyne Systems
Asynchronous heterodyne systems have attracted the most attention in practice simply
because the linewidth requirements for the transmitter laser and the local oscillator are
so relaxed that standard DFB lasers can be used. Experiments have been performed
with the ASK, FSK, and DPSK modulation formats [114]–[116]. An ASK experi-
ment in 1990 showed a baseline sensitivity (without the fiber) of 175 photons/bit at
4 Gb/s [116]. This value is only 10.4 dB away from the quantum limit of 40 pho-
tons/bit obtained in Section 10.4.4. The sensitivity degraded by only 1 dB when the
signal was transmitted through 160 km of standard fiber withD≈17 ps/(nm-km). The
system performance was similar when the FSK format was used in place of ASK. The
frequency separation (tone spacing) was equal to the bit rate in this experiment.
The same experiment was repeated with the DPSK format using a LiNbO 3 phase
modulator [116]. The baseline receiver sensitivity at 4 Gb/s was 209 photons/bit and
degraded by 1.8 dB when the signal was transmitted over 160 km of standard fiber.
Even better performance is possible for DPSK systems operating at lower bit rates. A
record sensitivity of only 45 photons/bit was realized in 1986 at 400 Mb/s [114]. This
value is only 3.5 dB away from the quantum limit of 20 photons/bit. For compari-
son, the receiver sensitivity of IM/DD receivers is such thatN ̄ptypically exceeds 1000
photons/bit even when APDs are used.
DPSK receivers have continued to attract attention because of their high sensitivity
and relative ease of implementation [117]–[125]. The DPSK signal at the transmitter
can be generated through direct modulation of a DFB laser [117]. Demodulation of
the DPSK signal can be done optically using a Mach-Zehnder interferometer with a
one-bit delay in one arm, followed by two photodetectors at each output port of the
interferometer. Such receivers are called direct-detection DPSK receivers because they
do not use a local oscillator and exhibit performance comparable to their heterodyne
counterparts [118]. In a 3-Gb/s experiment making use of this scheme, only 62 pho-
tons/bit were needed by an optically demodulated DPSK receiver designed with an
optical preamplifier [119]. In another variant, the transmitter sends a PSK signal but
the receiver is designed to detect the phase difference such that a local oscillator is not
needed [120]. Considerable work has been done to quantify the performance of various