Sensitivity Loss of Digital Optical Receivers Caused by Intersymbol Interference

Sensitivity Loss of Digital Optical Receivers Caused by Intersymbol Interference By F. S. CHEN and Y. S. CHEN (Manuscript received July 24, 1980) This paper describes studies of the dependence of optical receiver sensitivity on the launched-pulse width, pulse rise and fall times, and fiber dispersion for nonequalizing receivers. For pulse widths less than a time slot T in the return-to-zero (RZ) format and for small fiber dispersion Of, it is shown that the receiver sensitivity depends principally on the number of photons received per time slot and only weakly on the pulse width and rise and fall times of the launched pulses. For pulse widths equal to or greater than a time slot and/or with increased fiber dispersion (oy/T > 0.2), the receiver sensitivity degrades rapidly. Thus the sensitivity of receivers improves by use of reduced-width launched pulses in RZ format, particularly if the fiber dispersion is large. For example, for a receiver with equal thermalnoise and shot-noise levels, a situation approximating the use of an avalanche photodiode, the measured peak-power sensitivity penalty increased 2.5 dB as the launched-pulse width was increased from 0.6T to 1.1T for Of/T = 0.4. Since the peak-power sensitivity penalty has a broad minimum for pulse width between 0.5T to 1.0T, the use of reduced-width pulses in RZ format is also advantageous if the pulse width varies with temperature or varies among transmitters due to product variations. 1. INTRODUCTION In a binary digital transmission system, the receiver sensitivity is degraded if received data pulses spread into neighboring time slots causing intersymbol interference (ISI).