  In this paper, we investigate the modeling and design of intelligent
reflecting surface (IRS)-assisted optical communication systems which are
deployed to relax the line-of-sight (LOS) requirement in multi-link free space
optical (FSO) systems. The FSO laser beams incident on the optical IRSs have a
Gaussian power intensity profile and a nonlinear phase profile, whereas the
plane waves in radio frequency (RF) systems have a uniform power intensity
profile and a linear phase profile. Given these substantial differences, the
results available for IRS-assisted RF systems are not applicable to
IRS-assisted FSO systems. Therefore, we develop a new analytical channel model
for point-to-point IRS-assisted FSO systems based on the Huygens-Fresnel
principle. Our analytical model captures the impact of the size, position, and
orientation of the IRS as well as its phase shift profile on the end-to-end
channel. To allow the sharing of the optical IRS by multiple FSO links, we
propose three different protocols, namely the time division (TD), IRS-division
(IRSD), and IRS homogenization (IRSH) protocols. The proposed protocols address
the specific characteristics of FSO systems including the non-uniformity and
possible misalignment of the laser beams. Furthermore, to compare the proposed
IRS sharing protocols, we analyze the bit error rate (BER) and the outage
probability of IRS-assisted multi-link FSO systems in the presence of
inter-link interference. Our simulation results validate the accuracy of the
proposed analytical channel model for IRS-assisted FSO systems and confirm that
this model is applicable for both large and intermediate IRS-receiver lens
distances. Moreover, in the absence of misalignment errors, the IRSD protocol
outperforms the other protocols, whereas in the presence of misalignment
errors, the IRSH protocol performs significantly better than the IRSD protocol.
