We develop an analytic model of Coherent Optical Orthogonal Frequency Division Multiplexing (OFDM) propagation and detection over multi-span long-haul fiber links, comprehensively and rigorously analyzing the impairments due the combined effects of FWM, Dispersion and ASE noise. Consistent with prior work of Innoe and Schadt in the WDM context, our new closed-form expressions for the total FWM received power fluctuations in the wake of dispersive phase mismatch in OFDM transmission, indicate that the FWM contributions of the multitude of spans build-up on a phased-array basis. For particular ultra-long haul link designs, the effectiveness of dispersion in reducing FWM is far greater than previously assumed in OFDM system analysis. The key is having the dominant FWM intermodulation products due to the multiple spans, destructively interfere, mutually cancelling their FWM intermodulation products, analogous to operating at the null of a phased-array antenna system. By applying the new analysis tools, this mode of effectively mitigating the FWM impairment, is shown under specific dispersion and spectral management conditions, to substantially suppress the FWM power fluctuations. Accounting for the phased-array concept and applying the compact OFDM design formulas developed here, we analyzed system performance of a 40 Gbps coherent OFDM system, over standard G.652 fiber, with cyclic prefix based electronic dispersion compensation but no optical compensation along the link. The transmission range for 10-3 target BER is almost tripled from 2560 km to 6960 km, relative to a reference system performing optical dispersion compensation in every span (ideally accounting for FWM and ASE noise and the cyclic prefix overhead, but excluding additional impairments).
Coherent homodyne detection using local laser oscillator is an important technique for applications requiring high receiver sensitivity. Conventional homodyne detection uses a continuous-wave (CW) local laser oscillator (LO) in which the only adjustable parameter is its average power. A pulsed LO with repetition rate same as the data symbol rate is proposed which provides new degree of freedom in receiver design, namely, its pulse shape and duty cycle. It is shown that pulsed LO is overall effective in enhancing receiver sensitivity compared with CW LO. We have investigated and compared the performance of coherent detection of 12.5 Gb/s binary phase-shift-keyed signals using integrated LiNbO 3 optical 90 o hybrid with pulsed and CW LO for different receiver bandwidths. Our results showed that pulsed LO provide at least 2 dB in sensitivity improvement. We also observed that pulsed LO is effective in reshaping broadened signal pulses. Our simulation results agreed well with experiment and predicted that for a given signal and receiver bandwidth there is an optimal LO pulse width that gives maximum eye opening. Our simulation results also showed that pulsed LO is potentially effective in reducing penalty of pulse smearing as a result of beam steering impairment in free-space laser communications.
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