We investigate the effect of transmitter and receiver array configurations on the stray-light and diffraction-caused crosstalk in free-space optical interconnects. The optical system simulation software (Code V) is used to simulate both the stray-light and diffraction-caused crosstalk. Experimentally measured, spectrally-resolved, near-field images of VCSEL higher order modes were used as extended sources in our simulation model. Our results show that by changing the square lattice geometry to a hexagonal configuration, we obtain the reduction in the stray-light crosstalk of up to 9 dB
and an overall signal-to-noise ratio improvement of 3 dB.
Free-space optical interconnects (FSOIs) utilize arrays of vertical-cavity surface emitting lasers (VCSELs), microlenses, and photodetectors to effectively overcome the communication bottleneck caused by the poor performance of electrical interconnects. We derived a comprehensive FSOI link equation which can be used to determine the interconnect performance parameters, such as the receiver carrier-to-noise ratio. The link equation includes both optical and electrical noise components. The optical noise component is caused mainly by laser beam diffraction. We have simplified the modeling of optical noise by using the recently introduced Mode Expansion Method. The optical noise component strongly depends on the modal content of the incident VCSEL beam. The models used in the literature assume that the cross-sectional profile of the emitted laser beam resembles the fundamental Gaussian mode. Our link equation takes into account the modal structure of a multimode VCSEL beam. We have investigated the FSOI performance and we found that for each merit function there exists a single set of design parameters yielding the optimal performance. We have also found that the presence of higher-order modes negatively affects the performance. Our results show that FSOIs based on multimode VCSELs can be utilized in chip-level interconnects despite increased beam diffraction.
In this paper we investigate for the first time the effect of the crosstalk introduced due to laser beam imaging in a free-space optical interconnect (FSOI) system. Due to the overfill of the transmitter microlens array by the vertical cavity surface emitting laser (VCSEL) beam, one part of the signal is imaged by the adjacent microlens to another channel, possibly far from the intended one. Even though this causes increase in interchannel and intersymbol interference, to our knowledge this issue has been neglected so far. The numerical simulation has been performed using a combination of exact ray tracing and the beam propagation methods. The results show that some characteristics of stray-light crosstalk are similar to that of diffraction-caused crosstalk, where it is strongly dependent on the fill factor of the microlens, array pitch, and the channel density of the system. Despite the similarities, the stray-light crosstalk does not affect by an increase in the interconnection distance. As simulation models for optical crosstalk are numerically intensive, we propose here a crosstalk behavioral model as a useful tool for optimization and design of FSOIs. We show that this simple model compares favorably with the numerical simulation models.