Large and fast peak is, probably, an “unclamped” action potential. What was the holding potential? Playing with holding potential could give some hints regarding this matter.

Thank you for your feedback. The EPSC measurement was clamped at -70 mV. Thank you.

I agree with Maksim, the fast event is unclamped action potential. You have it as your optically evoked EPSCs are large enough to trigger neuronal firing from patched cell. Try to add QX-314 to your intracellular pipette solution. It is intracellular inhibitor of voltage-gated sodium channels. It has been used a lot to avoid that sort of a problem with both optically and electrically evoked EPSCs.

In addition to the fast event, the "EPSC" goes on for >100 ms. This would be highly unusual for a single synapse, and probably represents a torrent of synaptic activity. It seems possible that you are activating more complex circuitry with the optogenetic stimulus. For example, you could be activating additional cells besides the one you are recording from, which are themselves triggering EPSCs and IPSCs in adjacent cells, perhaps including your cell. Studying the monosynaptic connection could be tricky. You may need to play with conditions, for example, lowering Ca in the bath or adding low concentrations of synaptic antagonists to prevent network effects.