Bit-Error-Rate Degradation in a Multimode Fiber Optic Transmission Link Due to Modal Noise

A review is given of the statistics of modal noise and of the expressions for the signal-to-noise ratio for the cases of a monochromatic and a nonmonochromatic light source including the effects of source frequency fluctuations. Using the results, a quantitative analysis is made of how modal noise degrades the bit-error-rate performance of a digital transmission link with multimode fiber. Plots of the bit-error rate versus the received optical power show the characteristic asymptotic behavior (BER floor); expressions are derived for the asymptotic BER. The power penalty due to modal noise can be considerably reduced by suitable system modifications, e.g., by optimization of the decision threshold setting or by application of a low coherent light source such as an LED or a laser diode operating with several spectral emission lines of low coherence and low partition noise.