As for changing the channel spacing value, all you need to do is to change the frequency of your CW light sources. For instance, the channel spacing in the one you have just designed is 100GHz. if you want to make it 50GHz spacing, your second CW laser would be 193.05 THz, the third 193.1, and the fourth is 193.15 THz etc. That means the channel spacing with the available channels is 50GHz. etc. I hope this is beneficial.