KEYWORDS: Solar cells, Solar concentrators, Optical components, Reflectivity, Transmittance, Optical design, Prototyping, Integrated optics, Glasses, Monte Carlo methods
A novel non-imaging micro-concentrator concept and its development in Sandia National Lab’s microsystems-enabled photovoltaics (MEPV) program are described in this paper. Key notions of the compact 2-element optical concentrator are toroidal lens surfaces that decentralize the focused beam and a reflective cone structure that enhances light collection and illumination onto micro-scale solar cells (e.g., ~100’s microns in diameter). The optical configuration therefore provides a low-intensity, hot-spot-free illumination pattern on the receiver while achieving a concentration-acceptance angle product (CAP) over 1. Designs taking into account practical factors (such as fabrication capabilities, misalignments) achieve a 400X geometric concentration with a ±2.4° (90% of peak) acceptance angle (CAP = 0.84) and a 600X geometric concentration with a ±2° acceptance angle (CAP = 0.85), allowing low cost, mass production using injection molding. Development and experimental evaluation of a baseline prototype module is also described.
A high bandwidth density chip-to-chip optical interconnect architecture is analyzed. The interconnect design leverages
our recently developed flexible substrate integration technology to circumvent the optical alignment requirement during
packaging. Initial experimental results on fabrication and characterization of the flexible photonic platform are also
presented.
This paper explores issues associated with Photonic Integrated Circuit (PIC) research and development – with an overall
goal of initiating a discussion of how PIC technology should develop and eventually be deployed with high impact.
Significant research and development programs have focused on PICs for routing and switching, and computer
interconnects. Most recently, the application domain of PICs has diversified greatly, and now includes analog signal
processing, remote sensing, biological and chemical sensing, neural interfacing, and solar cells. A key feature of PIC
technology growth has been the exploitation of high-density fabrication and packaging technology originally developed
for the Silicon IC industry. PIC foundry services are emerging – and there has been a natural attempt to ascribe a
“Moore’s Law” to PIC scaling. Analogies to Silicon electronic scaling, however, should be used with caution. PIC
complexity scaling may be driven more by the ability to access the degrees-of-freedom offered by PIC-based optical
domain signal processing, rather than increasing device count. Specific examples of PIC research in chip-scale
computer interconnects and integrated micro-concentrators for solar cells are highlighted.
An optical design for coupling light from off-chip lasers to on-chip surface-normal modulators is proposed in order to implement chip-to-chip free-space optical interconnects. The method uses a dual-prism module constructed from prisms made of two different glasses. The various alignment tolerances of the proposed system were investigated. For the off-chip lasers, vertical cavity surface emitting lasers (VCSEL) are proposed. The rationale behind using on-chip modulators rather than VCSELs is to avoid VCSEL thermal loads on the chip, and because of higher reliability of modulators than VCSELs. Particularly above 10 Gbit/sec (Gbit/sec), an empirical model shows rapid decrease of VCSEL median time to failure vs. data rate. Thus the proposed interconnect scheme which utilizes continuous wave VCSELs that are externally modulated by on-chip multiple quantum well modulators is applicable for chip-to-chip optical interconnects at 20 Gbit/sec and higher line data rates.
High Performance Computing (HPC) systems are putting ever-increasing demands on the throughput efficiency of their
interconnection fabrics. In this paper, the limits of conventional metal trace-based inter-chip interconnect fabrics are
examined in the context of state-of-the-art HPC systems, which currently operate near the 1 GFLOPS/W level. The
analysis suggests that conventional metal trace interconnects will limit performance to approximately 6 GFLOPS/W in
larger HPC systems that require many computer chips to be interconnected in parallel processing architectures. As the
HPC communications bottlenecks push closer to the processing chips, integrated Optical Interconnect (OI) technology
may provide the ultra-high bandwidths needed at the inter- and intra-chip levels. With inter-chip photonic link energies
projected to be less than 1 pJ/bit, integrated OI is projected to enable HPC architecture scaling to the 50 GFLOPS/W
level and beyond - providing a path to Peta-FLOPS-level HPC within a single rack, and potentially even Exa-FLOPSlevel
HPC for large systems. A new hybrid integrated chip-scale OI approach is described and evaluated. The concept
integrates a high-density polymer waveguide fabric directly on top of a multiple quantum well (MQW) modulator array
that is area-bonded to the Silicon computing chip. Grayscale lithography is used to fabricate 5 μm x 5 μm polymer
waveguides and associated novel small-footprint total internal reflection-based vertical input/output couplers directly
onto a layer containing an array of GaAs MQW devices configured to be either absorption modulators or photodetectors.
An external continuous wave optical "power supply" is coupled into the waveguide links. Contrast ratios were measured
using a test rider chip in place of a Silicon processing chip. The results suggest that sub-pJ/b chip-scale communication
is achievable with this concept. When integrated into high-density integrated optical interconnect fabrics, it could
provide a seamless interconnect fabric spanning the intra-
The "Modulation-Enabled Tapered Remote Coupler" (METRoC) is introduced targeting the integration of on-chip and
off-chip optical interconnects. With Moore's law scaling of CMOS critical dimensions, the computing power of modern
microprocessors has grown rapidly. Future multi-core chips will demand aggregate on- and off-chip bandwidths in the
TBytes/sec regime. However, metal on-chip global interconnections and off-chip communications do not scale
commensurately with the CMOS device sizes, resulting in challenges to meet the growing bandwidth requirements
within acceptable power budgets. Optics is a potential solution to replace the fundamentally limited electrical
interconnects. Recent advancements in optical and optoelectronic component fabrication and manufacturing processes
may enable implementation of optical interconnects at the chip-scale. An optical solution that seamlessly integrates the
two domains is highly desirable.
The METRoC is proposed as a compact optical interconnect fabric that obviates the need for opto-electronic and electrooptic
conversions when signals propagate between the on-chip and off-chip domains. Multiple quantum well (MQW)
devices are chosen as the optical modulators and photodetectors. The key aspect of METRoC is the modulationenhanced
prismatic structures embedded in waveguides, which enable the direct coupling from the integrated MQW
devices to the intra-chip guided-wave fabrics. Additionally, tapered remote couplers can provide free-space
interconnections between chips or remote regions on-chip. Both coupling structures have small footprint areas and hence
are projected to provide high bandwidth densities. The fabrication process is compatible with silicon CMOS processes.
The coupling fabric can also be used to optically interconnect two silicon die within a multichip module.
Microprocessor performance is now limited by the poor delay and bandwidth performance of the on-chip global wiring layers. Although relatively few in number, the global metal wires have proven to be the primary cause of performance limitations - effectively leading to a premature saturation of Moore's Law scaling in future Silicon
generations. Building upon device-, circuit-, system- and architectural-level models, a framework for performance evaluation of global wires is developed aimed at quantifying the major challenges faced by intrachip global communications over the span of six technology generations. This paper reviews the status of possible intra-chip optical interconnect solutions in which the Silicon chip's global metal wiring layers are replaced with a high-density guided-wave or free-space optical interconnection fabric. The overall goal is to provide a scalable approach that is compatible with established silicon chip fabrication and packaging technology, and which can extend the reach of Moore's Law for many generations to come. To achieve the required densities, the integrated sources are envisioned to be modulators that are optically powered by off-chip sources. Structures for coupling dense modulator arrays to optical power sources and to free-space or guide-wave optical global fabrics are analyzed. Results of proof-of-concept experiments, which demonstrate the potential benefits of ultra-high-density optical interconnection fabrics for intra-chip global communications, are presented.
This work describes a dual-rate optical transceiver designed for power-efficient connections within and between modern high-speed digital systems. The transceiver can dynamically adjust its data rate according to the performance requirements, allowing for power-on-demand operation. To implement dual rate functionality, the transmitter and receiver circuits include separate high-speed and low-power datapath modules. The high-speed module is designed for gigabit operation and optimized to achieve the maximum bandwidth. A simpler low-power module is designed for megabit data transmission and optimized for low power consumption. The transceiver was fabricated with a 0.5μm Silicon-on-Sapphire (SOS) CMOS technology. The vertical cavity surface-emitting lasers (VCSELs) and photodetector devices were attached to the transceiver IC using flip-chip bonding. A free-space optical link system was set up to demonstrate power-on-demand capability. Experimental results show reliable link operations at 2Gb/s and 100Mb/s data transfer rates with about 104mW and 9mW power consumption, respectively. The transceiver’s switching time between these two data rates was demonstrated at 10μs which was limited by on-chip register reconfiguration time. Improvement of this switching time can be obtained by using dedicated IO pads for dual-rate control signals. At the circuit level, the incorporation of dual rate functionality into a typical gigabit optical transceiver would require 255 additional MOS transistors.
As IC densities grow to 100's of millions of devices per chip and beyond, the inter-chip link bandwidth becomes a critical performance-limiting bottleneck in many applications. Electronic packaging technology has not kept pace with the growth of IC I/O requirements. Recent advances in smart pixel technology, however, offer the potential to use 3-D optical interconnects to overcome the inter-chip I/O bottleneck by linking dense arrays of Vertical Cavity Surface Emitting Lasers (VCSELs) and photodetectors, which are directly integrated onto electronic IC circuits. Many switching and parallel computing applications demand multi-chip interconnection fabrics that achieve high-density global I/O across an array of chips. Such global interconnections require a high degree of space-variance in the interconnection fabric, in addition to high inter-chip throughput capacity. This paper reviews the architectural and optical design issues associated with global interconnections among arrays of chips. The emphasis is on progress made in the design and implementation of the second generation Free-space Accelerator for Switching Terabit Networks (FAST-Net) prototype. The FAST-Net prototype uses a macro-optical lens array and mirror to effect a global (fully connected) fabric across a 4 X 4 array of smart pixel chips. Clusters of VCSELs and photodetectors are imaged onto corresponding clusters on other chips, creating a high- density bi-directional data path between every pair of smart pixel chips on a multi-chip module. The combination of programmable intra-chip electronic routing and the fixed global inter-chip optical interconnection pattern of the FAST- Net architecture has been shown to provide a low latency, minimum complexity fabric, that can effect an arbitrary interconnection pattern across the chip array. Recent experimental results show that the narrow beam characteristics of VCSELs can be exploited in an efficient optical design for the FAST-Net optical interconnection module. A new design combines micro-, mini-, and macro-optical elements to achieve the required high registration and resolution accuracy while minimizing the packaging and alignment complexity.
Premanand Chandramani, Jeremy Ekman, P. Gui, Xiaoqing Wang, Fouad Kiamilev, K. Driscoll, B. Vanvoorst, Fred Rose, Jim Nohava, J. Allen Cox, Marc Christensen, Predrag Milojkovic, Michael Haney
The system architecture and the first prototype demonstrator system for the VCSEL-based Interconnects in VLSI Architectures for Computational Enhancement (VIVACE) program is described. The main goal of the VIVACE program is to build a high bi-section bandwidth free-space optically interconnected switch and to demonstrate it in a system of multiple compute nodes running a distributed algorithm. The prototype demonstrator system developed is a stand alone first-generation VIVACE Optical Network Interface Card (VONIC) communicating to another VONIC through a parallel- data fiber link. This system was developed to test the signal integrity and Bit Error Rate between two VONICs.
The FAST-Net (Free-space Accelerator for Switching Terabit Networks) concept uses an array of wide field-of-view imaging lenses to realize a high-density shuffle interconnect across an array of smart-pixel integrated circuits. This paper presents a design approach for these lenses that achieves the minimum complexity required to meet the demands of the FAST-Net concept's off-axis multi-chip environment. Generalized eikonals for arbitrary surfaces were examined to determine the performance bounds for the FAST-Net optical system. Then an analysis provided an estimate of 6 for the number of spherical surfaces needed to achieve good optical resolution and distortion performance across an array of 10-micron diameter VCSEL sources that are imaged onto a array of 50-micron wide detectors. A ray trace simulation confirmed this number. Subsequent analysis evaluated the achievable efficient of replacing spherical surfaces with aspherical ones. By exploiting the mismatch between the low numerical aperture VCSELs and relatively higher numerical aperture interconnection optics, it was found that 3 aspherical surfaces could replace 6 spherical surfaces in the FAST-Net system for the specified performance criteria. A lens design that utilizes 3 aspherical surfaces and achieves necessary registration and resolution of the FAST-Net system was determined. The results provide a general framework for the design of wide field-of-view free space interconnection systems that incorporate high-density VCSEL arrays.
Michael Haney, Marc Christensen, Predrag Milojkovic, Jeremy Ekman, Premanand Chandramani, Richard Rozier, Fouad Kiamilev, Yue Liu, Mary Hibbs-Brenner, Jim Nohava, Edith Kalweit, Sommy Bounnak, Terry Marta, B. Walterson
Highly interconnected multiprocessor systems are now performance limited by the backplane interconnection bottleneck associated with planar interconnection technologies. Smart pixel throughput capabilities are projected to exceed I Thitls/cm2 [1] and offer the promise of overcoming the bottlenecks of planar technologies for many types of interconnection-limited multiprocessor problems. Systems that use smart pixel-based free space optical interconnects (FSOI) provide two general dense interconnection capabilities: intelligent parallel data transfer and intelligent parallel data interchange. Optical imaging provides a high throughput approach to linking smart pixel planes for data transfer. In this case the high 110 density of smart pixels may provide a power consumption and size advantage over electronics [2,3]. For data interchange, FSOI provides the additional ability to perform the data partitioning and interleaving useful in space variant link interconnection patterns like the perfect shuffle (PS) [41,which are inherently difficult to implement in planar interconnection technologies. Such patterns are characterized by high BSBW [51. In multi-processor architecture design, there is a direct trade-off between minimum BSBW and latency in a network. It is therefore generally desirable to implement networks with the largest minimum BSBW that can be practically achieved to solve a given problem. The ability of optical elements to interconnect large arrays in space-variant patterns, without crosstalk in the medium, suggests that FSOI techniques are particularly promising for problems with high BSBW. For problems with greater than 1 ThitJsec BSBW (i.e., greater than the capabilities of a single chip) free space optical interconnects have a marked advantage [6,71. Therefore, globally interconnected multi-chip smart pixel based architectures have the potential to reap the full benefits of FSOI. This paper describes the experimental demonstration of a smart pixel based optical interconnection prototype currently being developed under the Free-space Accelerator for Switching Terabit tworks (FAST-Net) project, sponsored by the U.S. Defense Advanced Research Projects Agency. The prototype system incorporates 2-D arrays of monolithically integrated high-bandwidth vertical cavity surface emitting lasers (VCSELs) and photodetectors (PDs). A key aspect of the FAST-Net concept is that all smart pixels are distributed across a single multi-chip plane. This plane is connected to itself via an optical system that consists of an array of matched lenses (one for each smart pixel chip position) and a mirror. The optical interconnect system implements a global point-to-point shuffle pattern. The interleaved 2-D arrays of VCSELs and PDs in the prototype are arranged on a clustered self-similar grid pattern with a closest element pitch of 100 tm. The circular VCSEL elements have a diameter of 10 pm and the square PDs have an active region that is 50 jim wide. These arrays are packaged and mounted on printed circuit boards along with CMOS driver, receiver, and FPGA controller chips. Micro-positioning mounts are used to effect alignment that is consistent with current MCM chip placement accuracy. Shuffled optical data links between the multiple ICs have been demonstrated in preliminary evaluation of this system. These results suggest that a multi-Terabit optically interconnected MCM module is feasible.
Michael Haney, Marc Christensen, P. Milojkovik, Jeremy Ekman, Premanand Chandramani, Richard Rozier, Fouad Kiamilev, Yue Liu, Mary Hibbs-Brenner, Jim Nohava, Edith Kalweit, Sommy Bounnak, Terry Marta, B. Walterson
This paper reports progress toward the experimental demonstration of a smart pixel based optical interconnection prototype currently being developed under the Free-space Accelerator for Switching Terabit Networks (FAST-Net) project. The prototype system incorporates 2D arrays of monolithically integrated high- bandwidth vertical cavity surface emitting lasers (VCSELs) and photodetectors (PDs). A key aspect of the FAST-Net concept is that all smart pixels are distributed across a single multi-chip plane. This plane is connected to itself via an optical system that consists of an array of matched lenses (one for each smart pixel chip position) and a mirror. The optical interconnect system implements a global point-to-point shuffle pattern. The interleaved 2D arrays of VCSELs and PDs in the prototype are arranged on a clustered self-similar grid pattern with a closest element pitch of 100 micrometers . The circular VCSEL elements have a diameter of 10 micrometers and the square PDs have an active region that is 50 micrometers wide. These arrays are packaged and mounted on circuit boards along with the CMOS driver, receiver, and FPGA controller chips. Micro-positioning mounts are used to effect alignment that is consistent with current MCM chip placement accuracy. Shuffled optical data links between the multiple ICs have been demonstrated in preliminary evaluation of this system. These results suggest that a multi-Terabit optically interconnected MCM module is feasible.
Progress made during the previous 12 months toward the fabrication and test of a flight demonstration prototype of the acousto-optic time- and space-integrating real-time SAR image formation processor is reported. Compact, rugged, and low-power analog optical signal processing techniques are used for the most computationally taxing portions of the SAR imaging problem to overcome the size and power consumption limitations of electronic approaches. Flexibility and performance are maintained by the use of digital electronics for the critical low-complexity filter generation and output image processing functions. The results reported for this year include tests of a laboratory version of the RAPID SAR concept on phase history data generated from real SAR high-resolution imagery; a description of the new compact 2D acousto-optic scanner that has a 2D space bandwidth product approaching 106 sports, specified and procured for NEOS Technologies during the last year; and a design and layout of the optical module portion of the flight-worthy prototype.
Progress toward a flight demonstration of the acousto-optic time- and space- integrating real-time SAR image formation processor program is reported. The concept overcomes the size and power consumption limitations of electronic approaches by using compact, rugged, and low-power analog optical signal processing techniques for the most computationally taxing portions of the SAR imaging problem. Flexibility and performance are maintained by the use of digital electronics for the critical low-complexity filter generation and output image processing functions. The results reported include tests of a laboratory version of the concept, a description of the compact optical design that will be implemented, and an overview of the electronic interface and controller modules of the flight-test system.
The technical approach and recent experimental results for the acousto-optic time- and space- integrating real-time SAR image formation processor program are reported. The concept overcomes the size and power consumption limitations of electronic approaches by using compact, rugged, and low-power analog optical signal processing techniques for the most computationally taxing portions of the SAR imaging problem. Flexibility and performance are maintained by the use of digital electronics for the critical low-complexity filter generation and output image processing functions. The results include a demonstration of the processor's ability to perform high-resolution spotlight-mode SAR imaging by simultaneously compensating for range migration and range/azimuth coupling in the analog optical domain, thereby avoiding a highly power-consuming digital interpolation or reformatting operation usually required in all-electronic approaches.
An optical outer product architecture is presented which performs residue arithmetic operations via position-coded look-up tables. The architecture can implement arbitrary integer- valued functions of two independent variables in a single gate delay. The outer product configuration possesses spatial complexity (gate count) which grows linearly with the size of the modulus, and therefore with the system dynamic range, in contrast to traditional residue look-up tables which have quadratic growth in spatial complexity. The use of linear arrays of sources and modulators leads to power requirements that also grew linearly with the size of the modulus. Design and demonstration of a proof-of-concept experiment are also presented.
The time-and-space integrating (TSI) acousto-optic approach for real-time synthetic aperture radar (SAR) imaging was developed for applications with severe power and size constraints. Compactness and low power consumption are achieved by performing the computationally intensive operations in the analog optical domain. The required SAR imaging filters are updated rapidly through electronic programmability and downloaded to the optical system via acousto-optic Bragg cells. Under the DARPA TOPS (Transition of Optical Processing into Systems) Program, a rugged prototype of this concept is being developed for integration with high performance SAR platforms. In this paper the TSI approach is reviewed and simulation results are presented which demonstrate the concept's ability to efficiently focus high resolution SAR phase history data across a large target area, while compensating for large amounts of range migration.
Consideration is given to an advanced version of the acoustooptic time-and-space integrating (TSI) approach to real-time SAR imaging for severe power and size constraints applications. The concept incorporates electronic programmability and has the flexibility to compensate for the anomalies of the SAR imaging problem in a dynamically changing data collection scenario. Particular attention is given to a computationally intensive spotlight mode and real-time acoustooptic programmable imaging and display for SAR. It is concluded that all of the electronically programmed signals are straightforward to calculate and can be handled by standard digital processors, at real-time rates without taxing their capabilities. The effective distribution of the computational load between optics and electronics indicates that this SAR architecture is potentially compact and has low power requirements for both the electronic and optical portions of the processor.
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