Balanced phase-locked loops for optical homodyne receivers: Performance analysis, design considerations, and laser linewidth requirements

Balanced phase-locked loops for optical homodyne receivers are investigated. When a balanced loop is employed in a communications system, a part of the transmitter power must be used for unmodulated residual carrier transmission. This leads to a power penalty. In addition, the performance of the balanced loops is affected by the laser phase noise, by the shot noise, and by the crosstalk between the data-detection- and phase-lock-branches of the receiver. The impact of these interferences is minimized if the loop bandwidth
B
is optimized. The value of B
opt
and the corresponding optimum loop performance are evaluated in this paper. Further, the maximum permissible laser linewidth
\delta\nu
is evaluated and found to be
5.9 \times 10^{-6}
times R
b
, where R
b
(bit/s) is the system bit rate. This number corresponds to
BER = 10^{-10}
and power penalty of 1 dB (0.5 dB due to residual carrier transmission, and 0.5 dB due to imperfect carrier phase recovery). For comparison, decision-driven phase-locked loops require only
\delta\nu = 3.1 \times 10^{-4}. R_{b}
. Thus, balanced loops impose more stringent requirements on the laser linewidth than decision-driven loops, but have the advantage of simpler implementation. An important additional advantage of balanced loops is their capability to suppress the excess intensity noise of semiconductor lasers.