10.6. SYSTEM PERFORMANCE 509
Figure 10.15: BER curves for a 4-Gb/s PSK homodyne transmission experiment with 5 m
(empty squares) and 167 km (filled squares) of fiber. The quantum limit is shown for com-
parison. The inset shows the eye diagram after 167 km of fiber at−44 dBm received power.
(After Ref. [145];©c1990 IEEE; reprinted with permission.)
of such systems requires an optical phase-locked loop [136]–[141]. Many transmis-
sion experiments have shown the potential of PSK homodyne systems using He–Ne
lasers, Nd:YAG lasers, and semiconductor lasers [142]–[150]. The receiver sensitivity
achieved in these experiments depends on the bit rate. At a relatively low bit rate of
140 Mb/s, receiver sensitivities of 26 photons/bit at 1.52μm [143] and 25 photons/bit
at 1.32μm [144] have been obtained using He–Ne and Nd:YAG lasers, respectively.
In a 1992 experiment, a sensitivity of 20 photons/bit at 565 Mb/s was realized using
synchronization bits for phase locking [147]. These values, although about 4 dB away
from the quantum limit of 9 photons/bit, illustrate the potential of homodyne systems.
In terms of the bit energy, 20 photons at 1.52μm correspond to an energy of only 3
attojoules!
The sensitivity of PSK homodyne receivers decreases as the bit rate increases. A
sensitivity of 46 photons/bit was found in a 1-Gb/s experiment that used external-cavity
semiconductor lasers operating near 1.5μm and transmitted the signal over 209 km of
a standard fiber [142]. Dispersion penalty was negligible (about 0.1 dB) in this experi-
ment, as expected from Fig. 10.15. In another experiment, the bit rate was extended to
4 Gb/s [145]. The baseline receiver sensitivity (without the fiber) was 72 photons/bit.
When the signal was transmitted over 167 km of standard single-mode fiber, the re-
ceiver sensitivity degraded by only 0.6 dB (83 photons/bit), indicating that dispersion
was not a problem even at 4 Gb/s. Figure 10.15 shows the BER curves obtained in this
experiment with and without the fiber together with the eye diagram (inset) obtained
after 167 km of fiber. Another experiment increased the bit rate to 10 Gb/s by us-