This issue is subtle and will be difficult to understand. You will notice that in the Optical Receiver component there is a Downsampling tab. You will also notice that in your attached pics of the spectrum the two optical signals (red curves) do not connect and are separate signals. (Two red curves, not one continuous one).
Now the problem, the software only has a finite number of sampled time values to represent the signal and therefor there is a maximum bandwidth that it can represent as one signal. This is the number of samples value in the Layout parameters. In OptiSystem, since optical channels are usually widely spaced in the frequency domain there is the ability to have multiple channels like the two red curves in your figure. In electrical signals there is no such concept and the entire signal must be represented as one channel. When the receiver component is calculating the electrical signal from two optical channels it must make a decision which frequency to center it’s available bandwidth and this is set to the maximum power by default. You can change this if you desire.
The solution of this problem is to increase the Samples per Bit in the Layout parameter so that the signal can be represented by one continuous curve or channel. I tried a Samples per Bit of 16384 and I changed the Downsampling rate to be 1*Sample rate in the receiver. The eye diagram is very noisy from thermal effects and the additional optical signal but I can see it appear in the RF Spectrum now.