Frederic K. Lacroix

Design rules for highly parallel free-Space optical interconnects

Recently, a number of successful free-space chip-to-chip and board-to-board optical interconnects have been demonstrated. Here, we present some of the design rules that can be derived as a result of this work and also as a result of numerical and theoretical analyzes. We draw a number of conclusions. In the area of optoelectronic very large scale integration (VLSI) design, we suggest that differential electrical and optical transceiver designs provide the best performance. In the area of optical design, we present scaling and system partitioning laws for clustered optical relays and determine the interconnect distances at which microlens or macrolens systems are more suitable. We also show that the ease with which two modules can be aligned can be related to the optical invariant of the system and is, thus, a function of the size of the detector and the numerical aperture of the detector optics. Finally, we show that when multiple optical components must be aligned, very high individual component tolerances are required if the system as a whole is to have a high chance of success.

Design and implementation of a modulator-based free-space optical backplane for multiprocessor applications

Design and implementation of a free-space optical backplane for multiprocessor applications is presented. The system is designed to interconnect four multiprocessor nodes that communicate by using multiplexed 32-bit packets. Each multiprocessor node is electrically connected to an optoelectronic VLSI chip which implements the hyperplane interconnection architecture. The chips each contain 256 optical transmitters (implemented as dual-rail multiple quantum-well modulators) and 256 optical receivers. A rigid free-space microoptical interconnection system that interconnects the transceiver chips in a 512-channel unidirectional ring is implemented. Full design, implementation, and operational details are provided.

Experimental and numerical analyses of misalignment tolerances in free-space optical interconnects

A comparison of numerical analyses with experimental measurements suggests that both the ray-tracing and the Gaussian beam-propagation models overestimate the misalignment tolerances for on-axis beams and fail to predict the large longitudinal focal shift that occurs for off-axis beams propagating in free-space optical interconnects.

Electrical, thermal and optomechanical packaging of large 2D optoelectronic device arrays for free-space optical interconnects

Innovative approaches to the design of a high-performance package module accommodating a array of surface-active devices indium bump bonded to a VLSI chip are presented. Electrical, thermal and optomechanical design considerations are discussed and experimental performance results of a prototype implementation are described. The package module supports 139 impedance-controlled signal connections as well as active temperature stabilization of the optoelectronic VLSI chip. The package module is compact, simple to assemble, alignment-tolerant and can be passively inserted into a free-space optical system with no need for further adjustments.

Design, implementation, and characterization of a kinematically aligned, cascaded spot- array generator for a modulator-based free-space optical interconnect

The design and the implementation of a modular spot-array generator for a modulator-based free-space optical interconnect is presented. Two cascaded diffractive optical elements produce 4 x 8 clusters on a 1600 microm x 800 microm pitch, where each cluster is a 4 x 4 array of (1/e(2)) 13.1-microm-radius spots on a 90-microm pitch. The spot-array generator is kinematically aligned to the interconnect system such that no realignment is necessary between removal and reinsertion. Characterization results are presented.

Implementation of a compact, four-stage, scalable optical interconnect for photonic backplane applications

We report on the implementation of a dense 512-beam free-space optical interconnect linking four optoelectronic VLSI chips at the backplane level. The system presented maximizes the positioning tolerances of the components by use of slow f-number (f/16) Gaussian beams and oversized apertures. A beam-clustering scheme whereby a 4 × 4 array of beams is transmitted by each minilens is used to provide a high channel density. A modular approach is used to decrease the number of degrees of freedom in the system and achieve passive alignment of the modules in the final integration phase. A design overview as well as assembly and experimental results are presented.

Optomechanical, electrical, and thermal packaging of large 2D optoelectronic device arrays for free-space optical interconnects

Photonics Systems GroupMcGill University, Department ofElectrical and Computer Engineering3480 University St., Montreal, Quebec, Canada, H3A 2A 7tel: (514) 398 2531, fax: (514) 398 3127, e-mail: mikeaphotonics.ece.mcgill.caI Was with McGill University Electrical and Computer Engineering Department,now with the Department ofPhysics at Heriot-Watt University, Scotland.

Design and implementation of a four-stage clustered free-space optical interconnect

Optical intercotmects have the potential to overcome the limitations encountered in present electronic backplane technology in providing the information throughput required in ever faster multi-processor computers and lelecommunication switching systems[lJ[2]. A major challenge in this area is the design of robust, misalignment tolerant and field-serviceable optomechanical hardware for optical and optoelectronic components. Components often need to be precisely positioned to tolerances in the micron range laterally and fractions of a degree angularly. Alignment must be maintained despite vibrations, temperature variations and the occasional breakdown and maintenance cycle inherent in an industrial environment. This paper describes the implementation of a novel four-stage clustered optical interconnect designed for use in optical backplane applications[l]. The system optical design is first reviewed (more details can be found in[3]) followed with calculated and measured optical power throughputs. A tolerancing analysis illustrates the benefits of partitioning the system in pre-aligned modular building blocks. This is shown to minimize the number of critical alignment steps and considerably simplifies system assembly. It is then shown how proper optomechanical design allows for the passive insertion of modules to complete system assembly.

Design, implementation, and characterization of an optical power supply spot-array generator for a four-stage free-space optical backplane.

The design and implementation of a robust, scalable, and modular optical power supply spot-array generator for a modulator-based free-space optical backplane demonstrator is presented. Four arrays of 8 x 4 spots with 6.47-mum radii (at 1/e(2) points) pitched at 125 mum in the vertical direction and 250 mum in the horizontal were required to provide the light for the optical interconnect. Tight system tolerances demanded careful optical design, robust optomechanics, and effective alignment techniques. Issues such as spot-array generation, polarization, power efficiency, and power uniformity are discussed. Characterization results are presented.

Tolerance stackup effects in free-space optical interconnects

A numerical analysis indicates that tolerance stackup effects in free-space optical interconnects are significant even for short systems containing few components. Results prove that worst-case or root-sum-square analyses are not adequate to predict probable performance accurately. A Monte Carlo analysis must be performed.