  The line-of-sight (LOS) requirement of free-space optical (FSO) systems can
be relaxed by employing optical relays or optical intelligent reflecting
surfaces (IRSs). In this paper, we show that the power reflected from FSO IRSs
and collected at the receiver (Rx) lens may scale quadratically or linearly
with the IRS size or may saturate at a constant value. We analyze the power
scaling law for optical IRSs and unveil its dependence on the wavelength,
transmitter (Tx)-to-IRS and IRS-to-Rx distances, beam waist, and Rx lens size.
We also consider the impact of linear, quadratic, and focusing phase shift
profiles across the IRS on the power collected at the Rx lens for different IRS
sizes. Our results reveal that surprisingly the powers received for the
different phase shift profiles are identical, unless the IRS operates in the
saturation regime. Moreover, IRSs employing the focusing (linear) phase shift
profile require the largest (smallest) size to reach the saturation regime. We
also compare optical IRSs in different power scaling regimes with optical
relays in terms of the outage probability, diversity and coding gains, and
optimal placement. Our results show that, at the expense of a higher hardware
complexity, relay-assisted FSO links yield a better outage performance at high
signal-to-noise-ratios (SNRs), but optical IRSs can achieve a higher
performance at low SNRs. Moreover, while it is optimal to place relays
equidistant from Tx and Rx, the optimal location of optical IRSs depends on the
phase shift profile and the power scaling regime they operate in.
