Optical Packet Switches (OPS) and Optical Circuit Switches (OCS) provide the needed low latency transmissions in today large data centers and HPC systems. These switches can deliver lower latency and higher bandwidth than traditional electrical-based switches. Although light-based transmission has its advantages over electrical-based transmissions, in optical devices packet collisions are possible and this can generate retransmissions. In this work we present an optical network simulator called DimLightSim. DimLightSim models communication events in optical devices at packet level by replaying real application traces. Different experimental evaluations have been made using DimLightSim in order to compare current datacenter networks with the fully optical Architecture-on-Demand (AoD) proposed in the Lightness project. Initial results helped to foresee the impact in HPC applications execution time. In terms of performance improvement, the AoD architecture can outperform Infiniband-based network up to 19%.