Optical Time Division Multiplexing (OTDM) system technique is the extend time division multiplexing by optically combining a number of lower speed electronic baseband digital channels. In addition, the Semiconductor Optical Amplifiers (SOA's) are important components for optical networks. In this paper, we propose a novel model for an OTDM demultiplexer based on Cross Gain Modulation in a SOA. Analysis are extended to find the output of an optical IM/DD receiver to receive the demultiplexed optical signal and to find the output signal to noise ratio (SNR) taking into account the amplified spontaneous emission (ASE) noise. The results are evaluated in terms of BER, SNR and pump power for various input signal power level. It is found that, at a given signal input power the BER can be reduced by increasing the pump power and the required pump power is higher at higher level of input power.