SPECTRAL EFFICIENCY OF OPTICAL FDM SYSTEMS IMPAIRED BY PHASE NOISE

We consider the required frequency spacings between channels in an optical 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 rate (BER) objectives. The limiting factor is adjacent channel interference (ACI), the harmful effects of which are compounded if the optical sources have nonnegligible linewidths. The assumed transmission uses on-off keying (OOK), at a data rate 1/T (in bps), via external modulation of a laser source having linewidth beta (in Hz). The assumed receiver consists of an optical channel selection filter followed by a PIN photodiode and a post-detection integrate-and-dump circuit. Our analysis estimates the ACI-induced floor on BER for the middle of three FDM channels, 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, the required channel spacing ranges from 5.2 to 27.5 bit rates. The multiplying factor associated with using (wide-deviation) frequency shift keying (FSK), coherent (heterodyne) detection, and infinitely many FDM channels, respectively, are estimated to be 2.0, at most 3.0, and at most 1.37.