I would expect to see some lower frequency effects. i.e. we should look at the timeseries of the demod with the excitation on and off.
I would guess tat the exc on should show us the variations in the optical gain below 3 Hz, whereas the exc off would not show it.
Maybe you should do some low pass filtering on the time series you have to see the ~DC effects? Also, reconsider your AA filter design: how do you quantitatively choose the cutoff frequency and stopband depth?