With the development of information technology, the demand for high speed and high capacity data communications is rapidly increasing. What limits the performance of many existing computing, communication, and switching systems is the communication between the devices, rather than the devices themselves. It has been shown that the optical interconnect can be a promising scheme for achieving high speed and high density interconnects.
In light of the above, the aim of this dissertation is to study the design of free-space optical interconnects (FSOIs). Three types of interconnect architectures were studied — lensless FSOIs, FSOIs using microlenses, and FSOIs using monolithically integrated microlenses. A simulation model and an optimal design algorithm for each interconnect architecture were developed, which can provide a constructive guidance for the design of high speed and high density optical interconnects and for estimating the prospective application areas of each interconnect architecture.
Vertical cavity surface emitting lasers (VCSELs), photodetectors, and microlenses are key components for constructing an optical interconnect. Therefore, the structure and characterisation of commercially available VCSELs, the performance and implementation of photodetectors, and the design and implementation of microlenses were presented. To characterise the electrical and optical features of commercially available VCSELs , an automated measurement system was employed. The L-I-V characteristics, threshold currents for the fundamental mode and higher order modes, beam profile, 2-D beam contour, 3-D optical intensity distribution, optical spectrum, and modal content of VCSEL beams were measured using this automated measurement system. It has been shown that VCSELs are ideal sources for optical interconnects. The design of the front-end circuit for FSOIs was presented. A noise model for the front-end circuit was established. The frequency responses of the front-end circuit were analysed. Based on the simulation results, the feature parameters of the front-end circuit have a significant effect on its performance and these parameters can be optimised.
To assist in the design of optical interconnects, the commercial softwares SPICE and Code V were employed. SPICE was used in the design of the front-end circuit and Code V was employed in the design of the transmitter lenses. Using the SPICE simulation engine, the effect of the feature parameters of the front-end circuit on the signal-to-noise ratio (SNR) was studied. Using Code V, the ray aberration, astigmatism, and distortion of the transmitter lenses were studied. In addition, the design process for the transmitter lenses can be extended to the design of the receiver lenses.
Other key factors that will affect the design of FSOIs were also studied, for example, the presence of higher order modes in a VCSEL beam, the beam-clipping ratio of the transmitter lenses and the ratio f/d1 for FSOIs based on microlens relaying, where f is the focal length of transmitter lenses and d1 is the distance from VCSELs to lenses. It has been shown that the presence of higher order modes in a VCSEL beam will cause a deterioration in the SNR or equivalently substantially reduce the interconnect distance. The beam-clipping ratio of the transmitter lenses must be optimised in order to extend the interconnect distance and to keep a high enough SNR. A small variation in the ratio f/d1 will significantly affect the performance of an interconnect.
In conclusion, the design of FSOIs based on three types of interconnect architectures was studied. The simulation model can provide a constructive guidance for the design of high speed and high density FSOIs. Both simulation and experimental results have shown that optical interconnect is a promising scheme for overcoming the bottlenecks encountered in electrical interconnects at the chip and board levels. Here, lensless FSOIs can be applied to chip level interconnects and FSOIs based on microlens relaying can be applied to chip level and board level interconnects. Key Words: Free-space optical interconnect (FSOI), Signal-to-noise ratio (SNR), Vertical cavity surface emitting laser (VCSEL), Higher order transverse mode. Computer aided design. OCIS codes: 200.4650, 200.2610, 250.7260, 220.1140, 220.3620.