Performance analysis and laser linewidth requirements for optical PSK heterodyne communications systems

An optical PSK heterodyne communications receiver is investigated. The receiver is based on the decision-directed phase-locked loop. The performance of the phase-locked loop subsystem is analyzed taking into account both shot noise and laser phase noise. It is shown that for reliable phase locking (rms phase error less than 10°), heterodyne second-order loops require at least 6771 electrons/s per volt every hertz of the laser linewidth. This number corresponds to the limit when the loop dumping factor η is infinitely large; if
\eta = 0.7
, then the loop needs 10 157 electrons/(s . Hz). If the detector has a unity quantum efficiency and
\lambda = 1.5 \mu
m, the above quoted numberers give 0.9 pW/ kHz for
\eta \rightarrow \infty
and 1.35 pW/kHz for
\nu = 0.7
. The loop bandwidth required is also evaluated and found to be
155 \Delta\nu
, where
\Delta\nu
is the laser linewidth. Finally, the linewidth permitted for PSK heterodyne recievers is evaluated and found to be
2.26 \cdot 10^{-3} R_{b}
where R
b
is the system bit rate. For
R_{b}=100
Mbit/s, this leads to
\Delta\nu < 226
kHz. Such and better linewidths have been demonstrated with laboratory external cavity lasers. For comparison, ASK and FSK heterodyne receivers are much more tolerant to phase noise,-they can tolerate
\Delta\nu
up to 0.09 R
b
. At the same time, homodyne receivers impose much more stringent requirements on the laser linewidth (
\Delta\nu < 3 \cdot 10^{4} R_{b}
).