Rebecca Berman

An intra-chip free-space optical interconnect

Continued device scaling enables microprocessors and other systems-on-chip (SoCs) to increase their performance, functionality, and hence, complexity. Simultaneously, relentless scaling, if uncompensated, degrades the performance and signal integrity of on-chip metal interconnects. These systems have therefore become increasingly communications-limited. The communications-centric nature of future high performance computing devices demands a fundamental change in intra- and inter-chip interconnect technologies. Optical interconnect is a promising long term solution. However, while significant progress in optical signaling has been made in recent years, networking issues for on-chip optical interconnect still require much investigation. Taking the underlying optical signaling systems as a drop-in replacement for conventional electrical signaling while maintaining conventional packet-switching architectures is unlikely to realize the full potential of optical interconnects. In this paper, we propose and study the design of a fully distributed interconnect architecture based on free-space optics. The architecture leverages a suite of newly-developed or emerging devices, circuits, and optics technologies. The interconnect avoids packet relay altogether, offers an ultra-low transmission latency and scalable bandwidth, and provides fresh opportunities for coherency substrate designs and optimizations.

3-D integrated heterogeneous intra-chip free-space optical interconnect

This paper presents the first chip-scale demonstration of an intra-chip free-space optical interconnect (FSOI) we recently proposed. This interconnect system provides point-to-point free-space optical links between any two communication nodes, and hence constructs an all-to-all intra-chip communication fabric, which can be extended for inter-chip communications as well. Unlike electrical and other waveguide-based optical interconnects, FSOI exhibits low latency, high energy efficiency, and large bandwidth density, and hence can significantly improve the performance of future many-core chips. In this paper, we evaluate the performance of the proposed FSOI interconnect, and compare it to a waveguide-based optical interconnect with wavelength division multiplexing (WDM). It shows that the FSOI system can achieve significantly lower loss and higher energy efficiency than the WDM system, even with optimistic assumptions for the latter. A 1×1-cm2 chip prototype is fabricated on a germanium substrate with integrated photodetectors. Commercial 850-nm GaAs vertical-cavity-surface-emitting-lasers (VCSELs) and fabricated fused silica microlenses are 3-D integrated on top of the substrate. At 1.4-cm distance, the measured optical transmission loss is 5 dB, the crosstalk is less than -20 dB, and the electrical-to-electrical bandwidth is 3.3 GHz. The latter is mainly limited by the 5-GHz VCSEL.

A 3-D Integrated Intrachip Free-Space Optical Interconnect for Many-Core Chips

This letter presents a new optical interconnect system for intrachip communications based on free-space optics. It provides all-to-all direct communications using dedicated lasers and photodetectors, hence avoiding packet switching while offering ultra-low latency and scalable bandwidth. A technology demonstration prototype is built on a circuit board using fabricated germanium photodetectors, micro-lenses, commercial vertical-cavity surface-emitting lasers, and micro-mirrors. Transmission loss in an optical link of 10-mm distance and crosstalk between two adjacent links are measured as 5 and -26 dB, respectively. The measured small-signal bandwidth of the link is 10 GHz.

Recent progress on 3-D integrated intra-chip free-space optical interconnect

In this paper, the authors compare the energy efficiency of a WDM-based optical interconnect similar to and the proposed intra-chip free-space optical interconnect FSOI system to illustrate the advantages of the approach. The operation wavelength of the WDM-based systems is chosen to be 1550 nm, while the FSOI system still uses 980 nm. To emphasize the effects of photonic devices and optics design, the transceiver circuits is excluded from the calculation, which would be similar in both cases. To simplify the calculation, the PDs in both systems are assumed to have 100% quantum efficiency and adequate bandwidth to support 10-Gbps data rate, which gives the WDM system unfair advantage.

Heterogeneous 3-D circuits

A three-dimensional (3-D) integrated circuit combining free-space optical interconnects (FSOI) with CMOS devices has been developed. An overview of the combined optical and CMOS system is described. Experimental data and simulated results are provided. A 3-D integrated test circuit merging the free-space optical network with gallium arsenide based vertical cavity surface emitting lasers (VCSELs) and germanium based photodetectors has been experimentally tested. The prototype 3-D IC test circuit exhibits a 3.3GHz bandwidth and a 5.1mW total power consumption per link. The bandwidth is limited due to the use of 5GHz bandwidth commercial VCSELs and the large inductive impedance of the wirebonds attaching the VCSELs and photodetectors to the I/O pads. The design of transmitter circuits for the 3-D integrated free-space optical interconnect system is discussed, and simulated extrapolated results on operating frequency and bandwidth are provided. The microprocessor operates at 333MHz and includes a bus width of 64bits, requiring a FSOI bandwidth of 10.65Gbps after serialization.

Optical design study of a VIS-SWIR 3X zoom lens

A design study is compiled for a VIS-SWIR dual band 3X zoom lens. The initial first order design study investigated zoom motion, power in each lens group, and aperture stop location. All designs were constrained to have both the first and last lens groups fixed, with two middle moving groups. The first order solutions were filtered based on zoom motion, performance, and size constraints, and were then modified to thick lens solutions for the SWIR spectrum. Successful solutions in the SWIR were next extended to the VIS-SWIR. The resulting nine solutions are all nearly diffraction limited using either PNNP or PNPZ (“Z” indicating the fourth group has a near-zero power) design forms with two moving groups. Solutions were found with the aperture stop in each of the four lens groups. Fixed f-number solutions exist when the aperture stop is located at the first and last lens groups, while varying f-number solutions occur when it is placed at either of the middle moving groups. Design exploration included trade-offs between parameters such as diameter, overall length, back focal length, number of elements, materials, and performance.

Chip-scale demonstration of 3D integrated intrachip free-space optical interconnect

This paper presents the first chip-scale demonstration of an intra-chip free-space optical interconnect (FSOI) we recently proposed. This interconnect system uses point-to-point free-space optical links to construct an all-to-all intra-chip communication network. Unlike other electrical and waveguide-based optical interconnect systems, FSOI exhibits low latency, high energy efficiency, and large bandwidth density with little degradation for long distance transmission, and hence can significantly improve the performance of future many-core chips. A 1x1-cm2 chip prototype is fabricated on a germanium substrate with integrated photodetectors. A commercial 850-nm GaAs vertical-cavity-surface-emitting-laser (VCSEL) and fabricated fused silica micro-lenses are 3-D integrated on top of the germanium substrate. At a 1.4-cm distance, the measured optical transmission loss is 5 dB and crosstalk is less than -20 dB. The electrical-to-electrical bandwidth is 3.3 GHz, limited by the VCSEL.

Design Forms and Pupil Mangement for a High Resolution, Long Working Distance Zooming Microscope

A high resolution, long working distance zooming microscope of two design forms is compared: 1. a zoomed afocal system is paired with a fixed focal length objective and a fixed focal length tube lens and 2. an objective is zoomed and paired with a fixed focal length tube lens.

Optimal power distribution for minimizing pupil walk in a 7.5X afocal zoom lens

An extensive design study was conducted to find the optimal power distribution and stop location for a 7.5x afocal zoom that controls the pupil walk through zoom such that the lens can be coupled to a high-resolution microscope objective and tube lens with minimal vignetting and performance loss.