DISPERSION OF MICROWAVE-MODULATED OPTICAL SIGNALS

Digital transmission of light in single-mode fiber at gigabit rates is of increasing interest. This work describes experimental and analytical methods that permit the evaluation of dispersion effects that control intersymbol interference in digital systems. Experimentally, the method uses sinusoidal modulation of the light at a microwave frequency, e.g., 5 GHz, and the subsequent measurement of signal gain or loss in a dispersive fiber. Both techniques of direct modulation of DFB lasers and external modulation by Mach-Zehnder modulators are studied experimentally and analytically. For DFB lasers, the transmission measurements are used to derive the transient and the adiabatic components of chirp. In addition, it is shown that for external modulation at 5 GHz the 1.55-mum optical signals suffer relatively little dispersion loss for transmission distances up to 60 km of standard fiber. Analytical solutions of waveform distortion are given for external modulation and show good agreement with measurements. Finally, the analytical results for signal loss under ''information bandwidth'' limited transmission are used as a benchmark to estimate the dispersion penalty in a digital system. The dependence of transmission distance on bit rate in the NRZ format is derived and compared with available digital transmission experimental results.