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

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1 Author(s)
Kazovsky, Leonid G. ; Bell Communications Research, Red Bank, NJ, USA

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 Rbis 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 Rb. At the same time, homodyne receivers impose much more stringent requirements on the laser linewidth ( \Delta \nu < 3 \cdot 10^{4} R_{b} ).

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Lightwave Technology, Journal of  (Volume:4 ,  Issue: 4 )