In traditional optical communication, duplexity is achieved by using two fibers, each having a transmitter and a receiver. Economically, bidirectional wavelength division multiplexing (WDM) transmission systems utilizing a single fiber will be more attractive not only reducing the use of the fiber by a factor of two, but also the number of components. Duplex transmissions over a single fiber can double the capacity of an installed unidirectional link. The idea of this paper is to study another approach using the subcarrier multiplexing (SCM)-based optical network and evaluate the physical transmission quality of analog and digital signal using SCM approach and the characteristic of fiber nonlinear crosstalk such as stimulated Raman scattering, Cross phase modulation and four-wave mixing in the SCM externally modulation optical link. A suitable bandwidth of 890 – 950 MHz is selected for subcarriers and channel bandwidth of 200 KHz and carrier. By measuring the optical bit interference (OBI) performance limitations of the subcarrier multiplexing WDM optical transmission system is investigated. The OBI for 10 channels for input power 1 dB is -40 dB whereas for 110 channels the OBI is -20 dB separation of 250 KHz are considered.