In the work, variations in the optical signal parameters in high-density wave- length division multiplexing (HDWDM) optical fiber communication system under strong exter- nal electro-magnetic (EM) field are analyzed for different modulation formats and transmission speeds. It is established that a 8-channel HDWDM system with NRZ modulation is more sen- sitive to the polarization state changes on exposure to external EM field as compared with a system in which the polarization state modulation (POLSK) modulation format is used. The BER analysis shows that the EM effect is one of the factors limiting the minimum for the allowed inter-channel interval. If this allowed minimum is chosen to be 0.3nm for data transmission speed 10 Gbps, in view of the EM influence it should be raised to 0.4nm in order to ensure the necessary BER level an HDWDM system.