  To meet the growing demand for data traffic, spectrum-rich optical wireless
communication (OWC) has emerged as a key technological driver for the
development of 6G. The resonant beam communication (RBC) system, which employs
spatially separated laser cavities as the transmitter and receiver, is a
high-speed OWC technology capable of self-alignment without tracking. However,
its transmission through the air is susceptible to losses caused by
obstructions. In this paper, we propose an intelligent reflecting surface (IRS)
assisted RBC system with the optical frequency doubling method, where the
resonant beam in frequency-fundamental and frequency-doubled is transmitted
through both direct line-of-sight (LoS) and IRS-assisted channels to maintain
steady-state oscillation and enable communication without echo-interference,
respectively. Then, we establish the channel model based on Fresnel diffraction
theory under the near-field optical propagation to analyze the transmission
loss and frequency-doubled power analytically. Furthermore, communication power
can be maximized by dynamically controlling the beam-splitting ratio between
the two channels according to the loss levels encountered over air. Numerical
results validate that the IRS-assisted channel can compensate for the losses in
the obstructed LoS channel and misaligned receivers, ensuring that
communication performance reaches an optimal value with dynamic ratio
adjustments.
