  In this paper, we study optical simultaneous lightwave information and power
transfer (SLIPT) systems employing photovoltaic optical receivers (RXs). To be
able to efficiently harvest energy from broadband light, we propose to employ
multi-junction photovoltaic cells at the optical RX. We consider the case,
where the optical RX is illuminated by ambient light, an intensity-modulated
information free space optical (FSO) signal, and since the ambient light may
not be always present, a dedicated energy-providing broadband optical signal.
Based on the analysis of the equivalent electrical circuit of the
multi-junction photovoltaic RX, we model the current flow through the
photovoltaic cell and derive a novel accurate and two novel approximate energy
harvesting (EH) models for the two cases, where the optical RX is equipped with
a single and multiple p-n junctions, respectively. We derive the optimal
distribution of the transmit information signal that maximizes the achievable
information rate. We validate the proposed EH models by circuit simulations and
show that the photovoltaic RXs saturate for high received signal powers. For
single-junction RXs, we compare the proposed EH model with two well-known
baseline models and demonstrate that, in contrast to the proposed EH model,
they are not able to fully capture the RX non-linearity. Since multi-junction
RXs allow a more efficient allocation of the optical power, they are more
robust against saturation, and thus, are able to harvest significantly more
power and achieve higher data rates than RXs employing a single p-n junction.
Finally, we highlight a tradeoff between the information rate and harvested
power in SLIPT systems and demonstrate that the proposed optimal distribution
yields significantly higher achievable information rates compared to uniformly
distributed transmit signals, which are optimal for linear optical information
RXs.
