This paper investigates the design optimization of digital free-space optoelectronic interconnections with a specific goal of minimizing the power dissipation of the overall link, and maximizing the interconnect density. To this end, we discuss a method of minimizing the total power dissipation of an interconnect link at a given bit rate. We examine the impact on the link performance of two competing transmitter technologies, vertical cavity surface emitting lasers (VCSEL's) and multiple quantum-well (MQW) modulators and their associated driver-receiver circuits including complementary metal-oxide-semiconductor (CMOS) and bipolar transmitter driver circuits, and p-n junction photodetectors with multistage transimpedance receiver circuits. We use the operating bitrate and on-chip power dissipation as the main performance measures. Presently, at high bit rates (>800 Mb/s), optimized links based on VCSEL's and MQW modulators are comparable in terms of power dissipation. At low bit rates, the VCSEL threshold power dominates. In systems with high bit rates and/or high fan-out, a high slope efficiency is more important for a VCSEL than a low threshold current. The transmitter driver circuit is an important component in a link design, and it dissipates about the same amount of power as that of the transmitter itself. Scaling the CMOS technology from 0.5 m down to 0.1 m brings a 50% improvement in the maximum operating bit rate, which is around 4 Gb/s with 0.1 m CMOS driver and receiver circuits. Transmitter driver circuits implemented with bipolar technology support a much higher operating bandwidth than CMOS technology; they dissipate, however, about twice the electrical power. An aggregate bandwidth in excess of 1 Tb/s-cm 2 can be achieved in an optimized free-space optical interconnect system using either VCSEL's or MQW modulators as its transmitters.

Optical transimpedance receivers implemented in CMOS VLSI technologies are modeled and optimized for freespace optoelectronic interconnections. Sensitivity, bandwidth, power dissipation, and circuit area are analyzed for receivers using three different submicron CMOS processes. A comparison with the circuit noise limited optical power indicates that, for digital computing applications, the receiver sensitivity is limited by the gain-bandwidth product of the receiver amplifiers and the necessary noise margin of logic circuits.

We investigate the performance of free-space optical interconnection systems at the technology level. Specifically, three optical transmitter technologies, lead-lanthanum-zirconate-titanate and multiple-quantum-well modulators and vertical-cavity surface-emitting lasers, are evaluated. System performance is measured in terms of the achievable areal data throughput and the energy required per transmitted bit. It is shown that lead-lanthanum-zirconate-titanate modulator and vertical-cavity surface-emitting laser technologies are well suited for applications in which a large fan-out per transmitter is required but the total number of transmitters is relatively small. Multiple-quantum-well modulators, however, are good candidates for applications in which many transmitters with a limited fan-out are needed.

Chatoyant is a tool for the simulation and the analysis of heterogeneous free-space optoelectronic architectures. It is capable of modeling digital and analog electronic and optical signal propagation with mechanical tolerancing at the system level. We present models for a variety of optoelectronic devices and results that demonstrate the system's ability to predict the effects of various component parameters, such as detector geometry, and system parameters, such as alignment tolerances, on system-performance measures, such as the bit-error rate.

Development in late childhood has been associated with microstructural changes in white matter (WM) that are hypothesized to underpin concurrent changes in cognitive and behavioral function. Restriction spectrum imaging (RSI) is a framework for modelling diffusion-weighted imaging that can probe microstructural changes within hindered and restricted compartments providing greater specificity than diffusion tensor imaging for characterizing intracellular diffusion. Using RSI, we modelled voxelwise restricted isotropic, N0, and anisotropic, ND, diffusion across the brain and measured cross-sectional and longitudinal age associations in a large sample (n=8,039) aged 9-13 years from the Adolescent Brain and Cognitive Development (ABCD) StudySM. Participants showed global increases in N0 and ND across WM with age. When controlling for global RSI measures (averaged across WM), we found smaller age-related associations in frontal regions, reflective of more protracted development of frontal WM. Moreover, variability in the development of restricted diffusion in subcortical regions and along particular gray-white matter boundaries was independent of the global developmental effect. Using the ABCD sample, we have unprecedented statistical power to estimate developmental effects with high precision. Our analyses reveal spatially-varying maturational changes for different regions, independent of global changes. This non-uniformity may reflect age-dependent development of distinct cognitive and behavioral processes.