We were suspicious that the reason the transfer function was decreasing with an increase in power was actually due to the monitoring circuit, specifically the MAX2470 RF Buffer. To test this hypothesis I took a power sweep with an EOM driver (SN07) which only contained the 5V regulator and the Buffer, along with all the components associated with these devices. There was no transistor, as well as no diodes at the input power supply. The overall change from -5 dBm to 15 dBm was only around 0.1dB. This indicates the monitoring circuit does not provide an explanation for the reduction in gain seen at higher power levels.