Hi Joseph,
I’ll be back in my office next week. The signal power will increase because OptiSystem is adding the noise bins to it. In a lab setting generally you don’t have as much knowledge about how to distinguish the noise power from the signal power, so generally the peak power is compared to a power far from the peak frequency. Possibly in between channels or if the channels are too close then outside the channel frequencies range. If you want to get an SNR the same as on you might measure in a lab then convert the noise bins and choose an interpolation point in between channels or far from the channel frequencies.
Regards