Hello Gurinder singh,
Your system work well for 4 channels so To make 8 channel WDM system you can simply add 4 more channels and 8×1 multiplexer instead of 4×1.And 1×8 Demux at the receiver side simply by matching the frequency on the both sides.you can start frequency as you are taking in current system channels of frequencies 193.05,193.1,193.15,193.2,193.25,193.3,193.35 and 193.4.For coherent reception you have to place 4 more LO with same frequency as the channel frequency.Let me know if it helps
Regards