Modal noise disturbs the received power of optical receivers. It arises in multimode fiber systems and can be modeled by a gamma distribution. In our system, the number of secondary electrons is amplified by an avalanche photodiode (APD) and further Corrupted by the thermal noise df the subsequent linear amplifier. The degradation to the system caused by modal noises is considered along with the signal dependent shot noise and the circuit generated thermal noise. The error probabilities are computed by using the saddlepoint method under the influence of various degrees of modal noise. Methods for searching for the threshold and the optimum APD gain are also presented.
