  The line-of-sight (LOS) requirement of free-space optical (FSO) systems can
be relaxed by employing optical intelligent reflecting surfaces (IRSs). In this
paper, we model the impact of the IRS-induced delay dispersion and derive the
channel impulse response (CIR) of IRS-assisted FSO links. The proposed model
takes into account the characteristics of the incident and reflected beams'
wavefronts, the position of transmitter and receiver, the size of the IRS, and
the incident beamwidth on the IRS. Our simulation results reveal that a maximum
effective delay spread of 0.7 ns is expected for a square IRS with area 1
$\mathrm{m}^2$, which induces inter-symbol interference for bit rates larger
than 10 Gbps. We show that the IRS-induced delay dispersion can be mitigated
via equalization at the receiver.
