In this paper, the expression formula for the total input electrical field of optical receiver in DWDM systems in the case of existence of optical noise and crosstalk, is deduced. The problem of physical coherence and digital logical correlation is stated in detail. The effect of optical extinction ratio for both signal and crosstalk on the system BER performance is calculated, as the background BER for the case of 10 -9 due to optical noise only and optimal decision level are assumed. It is found that the extinction ratio is about 8～12 dB, while the system BER performance is optimal.