Hi Dhiman,Mai i know for which scheme you are using coherent detection.If you are using the qam only the there is qam demodulator after photo detectors which gives you one output.If you are using QAM for example as in OFDM then you can get the idea from optisystem ofdm samples
Advanced modulation > ofdm samples
Rest for better information here attach paper or sample file you made so far .Thank you