SPECTRAL EFFICIENCY OF OPTICAL FDM SYSTEMS IMPAIRED BY PHASE NOISE

We consider the required frequency spacings between channels in an opt ical frequency division multiplexed (FDM) network. With an eye on the promise of spectrally efficient fiber distribution networks, we derive the minimum permissible spacings consistent with meeting bit error ra te (BER) objectives. The limiting factor is adjacent channel interfere nce (ACI), the harmful effects of which are compounded if the optical sources have nonnegligible linewidths. The assumed transmission uses o n-off keying (OOK), at a data rate 1/T (in bps), via external modulati on of a laser source having linewidth beta (in Hz). The assumed receiv er consists of an optical channel selection filter followed by a PIN p hotodiode and a post-detection integrate-and-dump circuit. Our analysi s estimates the ACI-induced floor on BER for the middle of three FDM c hannels, as a function of frequency spacing and linewidth-to-bit rate ratio (betaT). For BER = 10(-9) and betaT ranging from 0.32 to 5.12, t he required channel spacing ranges from 5.2 to 27.5 bit rates. The mul tiplying factor associated with using (wide-deviation) frequency shift keying (FSK), coherent (heterodyne) detection, and infinitely many FD M channels, respectively, are estimated to be 2.0, at most 3.0, and at most 1.37.