Exact laser line-width requirements evaluation for optical PSK homodyne communication systems with Dither loop

The laser line-width required in PSK homodyne communication systems with Dither phase-locked loop receivers are exactly evaluated. It is shown that second-order phase-locked loops require at least 0.2 pW of signal power per every Hz of laser line-width (this number refers to the system with the detector responsivity 0.94 A/W, damping ratio 0.707, and the phase error standard deviation 10°). This signal power is used for phase locking, and is, therefore lost from the data receiver. This study for the first time shows the imperfect-phase-recovery-induced power penalty as a function of laser line-width. It can be estimated from the theoretical result that an optimal dither phase locked-loop receiver requires the laser line-width as Δυ ≤ 3.14 × 10−5 × Rb, where Rb is the system bit rate. This number refers to the system with power penalty of 1 dB at BER = 10−10.