A fully embedded board-level guided-wave optical interconnection is presented to solve the packaging compatibility problem. All elements involved in providing high-speed optical communications within one board are demonstrated. Experimental results on a 12-channel linear array of thin-film polyimide waveguides, vertical-cavity surface-emitting lasers (VCSELs) (42 /spl mu/m), and silicon MSM photodetectors (10 /spl mu/m) suitable for a fully embedded implementation are provided. Two types of waveguide couplers, titled gratings and 45/spl deg/ total internal reflection mirrors, are fabricated within the polyimide waveguides. Thirty-five to near 100% coupling efficiencies are experimentally confirmed. By doing so, all the real estate of the PC board surface are occupied by electronics, and therefore one only observes the performance enhancement due to the employment of optical interconnection but does not worry about the interface problem between electronic and optoelectronic components unlike conventional approaches. A high speed 1-48 optical clock signal distribution network for Cray T-90 super computer is demonstrated. A waveguide propagation loss of 0.21 dB/cm at 850 nm was experimentally confirmed for the 1-48 clock signal distribution and for point-to-point interconnects. The feasibility of using polyimide as the interlayer dielectric material to form hybrid three-dimensional interconnects is also demonstrated. Finally, a waveguide bus architecture is presented, which provides a realistic bidirectional broadcasting transmission of optical signals. Such a structure is equivalent to such IEEE standard bus protocols as VME bus and FutureBus.

Short distance optical interconnects, which has long promised to solve a number of problems assumed to exist in electronic systems, seems to be at the threshold of providing real value to future processing systems. In many cases, it has been the cost of optical interconnects which has prevented their widespread adoption. However, while data rates required in future computing systems are increasing, and fiber optic component costs have decreased, the distances over which optical interconnects become attractive have been shrinking. In addition the increasing demand for smaller footprints for the I/O at the card edge, plays into one of the strong advantages unique to optics. An effort called Optical Micro-Networks is underway, funded by DARPA, whose objective is to demonstrate cost effective system level benefits in parallel optics for intra- and inter-rack interconnects by leveraging recent advances in VCSEL arrays, high sped CMOS, and low cost glass fiber cabling, connectors and on-board fiber routing. The approach will integrate optical interconnect functionality directly into an ASIC package thus reducing size, cost, and power.

This paper describes the technical approach and progresses of the POINT program. This project is a collaborative effort among GE, Honeywell, AMP, AlliedSignal, Columbia University and University of California at San Diego, sponsored by DARPA/ETO to develop affordable optoelectronic packaging and interconnect technologies for board and backplane applications. In this paper, we report the development of a backplane interconnect structure using polymer waveguides to an interconnect length of 280 mm to demonstrate high density and high speed interconnect, and the related technical development efforts on: (a) a high density and high speed VCSEL array packaging technology that employs planar fabrication and batch processing for low-cost manufacturing, (b) passive alignment techniques for reducing recurrent cost in optoelectronic assembly, (c) low-cost optical polymers for board and backplane level interconnects, and (d) CAD tools for modeling multimode guided wave systems and assisting optoelectronic packaging mechanical design.

Optical interconnects have long promised significant advantages over their electrical counterparts. Specific advantages include increased bandwidths at long (ten meters or more) interconnection distances, immunity to EMI effects, negligible crosstalk, reduced size, and lower weight. Optical interconnects have been developed for, and are being used in, a range of ground based and aircraft applications, however they are only now beginning to gain acceptance in spaceborne systems. In addition to the maturity demanded from components destined for ground-based applications and the wider temperature excursions characteristic of airborne applications, spaceborne components must also be able to survive the radiation environments associated with their intended applications. The additional qualification required has resulted in delayed introduction of photonic interconnects. We describe the tradeoffs involved in implementing for the first time a spaceborne fiber optic data bus with a clock speed of 1.2 Gbps. The tradeoffs include emitter, detectors, fiber, connectors and packaging. We have selected a series of commercial grade optoelectronic devices which were then qualified for use in spaceborne environments and have developed a space qualifiable packaging scheme. We have designed and implemented the optoelectronic subsystem of the data bus and have simulated its operation. We also describe recent advances in Vertical Cavity Surface Emitting Lasers (VCSELs) for spacebourne databuses. VCSELs also offer advantages in simplicity of packaging and electronic control. We summarize available initial radiation data on these devices and project their impact on spaceborne photonic interconnects.

We present the fabrication of polyimide-based H-tree waveguides for a multi-GBit/sec optical clock signal distribution in a Si CMOS process compatible environment. Such a clock distribution system is to replace the existing electronic counterpart associated with high-performance computers. A waveguide propagation loss of 0.21 dB/cm at 850 nm was experimentally confirmed for the l-to-48 waveguide fanout device, l-to-2 splitting loss and bending loss were measured to be 0.25 dB and higher. The planarization requirement of the optical interconnection layer among many electrical interconnection layers makes the employment of tilted grating a choice of desire. Theoretical calculation predicts the 1-to-l free-space to waveguide coupling with an efficiency as high as 95%. Currently, a coupling efficiency of 35% was experimentally confirmed due to the limited index difference between guiding and cladding layers. Further experiments aimed at structuring a larger guiding/cladding layer index differences are under investigation. To effectively couple an optical signal into the waveguide through the tided grating coupler, the accuracy of the wavelength employed is pivotal. This makes the usage of the vertical cavity surface-emitting lasers (VCSELs) and VCSEL arrays the best choice when compared with edge-emitting lasers. Modulation bandwidth as high as 6 GBit/sec was demonstrated at 850 nm. Such a wavelength is compatible with Si-based photodetectors. Temperature dependence of the threshold current up to 155 °C was measured which will determine the power dissipation issue of the optoelectronic packaging. Finally, the first fully monolithic Si-MOSFET integrated receiver was made as the optical clock signal detector. To further enhance the bandwidth of such a detector, a resonant cavity structure with Si/Si02 as the bottom mirror was employed. The measured demodulation bandwidth is over 10 GHz. A fully integrated guided-wave optical clock signal distribution system having planarized grating couplers, H-tree Si- CMOS process compatible waveguides, VCSELs and Si-based photo-receivers will be demonstrated in the near future.

Optical interconnects have long promised to solve a number of problems assumed to exist in electronic systems. A wealth of technology exists in the area, yet few electronic systems today employ optical interconnects. In many cases, it is the cost of optical interconnects which prevents their widespread adoption, as little fundamental advantage is offered by most applications of optical interconnects. The decreasing cost of optoelectronic technology and of its incorporation into electronic systems will enable further penetration of the market. Plastic fiber is likely to provide a competitive solution for intercabinet interconnects, while both high density fiber backplanes and polymer backplanes are likely to impact first high performance computer systems and eventually a broader range of systems, while free space interconnects will facilitate both advanced versions of existing architectures and the implementation of new systems. The key to practical implementation lies in providing transparency to the system integrator to minimize the departure from established practices. Significant progress is also being made in a new direction in which the inherent advantages of optics can be exploited in unique architectures, while still implementing logic functions in electronics, however such systems are unlikely to find widespread use in the near future.

Short distance optical interconnects are under development for a range of applications including local area networks, optical backplanes, and optoelectronic accelerators or signal processors. In some applications, the aggregate bandwidth required cannot be provided with electrical interconnects, offering an obvious advantage for optics, while in others it is the density of available interconnects which motivates the use of optics. In most commercial applications, it is the cost of the interconnect solution which will affect its acceptance by system integrator. For optics to be applied in a broad range of applications, greater transparency must be provided to the system integrator. We describe both intercabinet and intracabinet interconnects in which the addition of optical interconnects has been designed to perturb the overall system as little as possible and yet still take advantage of optics.

The development of multimode passive polymer optical waveguide components for board and backplane interconnect applications, such as in the the DARPA-sponsored, ''Polymer Optical Interconnect Technology (POINT)'' program,(1,2) require several optics design issues to be addressed including efficiency and modal noise. For example, the mating of arrays of sources, detectors, and fibers requires appropriate fanout structures to match the component pitch. Here we consider designs for such structures employing multimode polymer waveguides, including both abrubt and smooth bending elements. We investigate these structures using a new multimode BPM simulation CAD tool, and consider the bend losses as a function of geometry, angle, and source condition. The results are compared with experimental observations on devices fabricated for use in the POINT demonstration module. The simulation closely matches the experiment, demonstrating the utility of such efforts in practical component development.

We report the formation of polyimide-based H-tree waveguides for a multi-GBit/sec optical clock signal distribution in a Si CMOS process compatible environment. Such a clock distribution system is to replace the existing electronic counterpart associated with high-speed supercomputers such as Cray T-90 machine. A waveguide propagation loss of 0.21 dB/cm at 850 nm was experimentally confirmed for the l-to-48 waveguide fanout device. l-to-2 splitting loss and bending loss were measured to be 0.25 dB and higher. The planarization requirement of the optical interconnection layer among many electrical interconnection layers makes the employment of tilted grating a choice of desire. Theoretical calculation predicts the l-to-l free-space to waveguide coupling with an efficiency as high as 95%. Currently, a coupling efficiency of 35% was experimentally confirmed due to the limited index difference between guiding and cladding layers. Further experiments aimed at structuring a larger guiding/cladding layer index differences are under investigation. To effectively couple an optical signal into the waveguide through the titled grating coupler, the accuracy of the wavelength employed is pivotal. This makes the usage of the vertical cavity surface-emitting lasers (VCSELs) and VCSEL arrays the best choice when compared with edge-emitting lasers. Modulation bandwidth as high as 6 GHz was demonstrated at 850 nm. Such a wavelength is compatible with Si-based photodetectors. Temperature dependence of the threshold current up to 155 "C was measured which will determine the power dissipation issue of the optoelectronic packaging. Finally, the first fully monolithic Si-MOSFET integrated receiver was made as the optical clock signal detector. To further enhance the bandwidth of such a detector, a resonant cavity structure with Si/SiO, as the bottom mirror was employed. The measured demodulation bandwidth is over 10 GHz. A fully integrated guided-wave optical clock signal distribution system having planarized grating couplers, H-tree Si-CMOS process compatible waveguides, VCSELs and Si-based photoreceivers will be demonstrated in the near future.

Both multichip modules (MCMs) and optical interconnects are expected to play a pivotal role in the development of high performance electronic systems. Only by packaging optoelectronic components within the multichip modules can the advantages of both technologies be realized. We have demonstrated the incorporation of optoelectronic components into two different MCM technologies. In the first, we have used existing equipment and proven polymer materials to implement optoelectronic interfaces in a high density 'chip-first' technology. In the second, we have demonstrated optoelectronic packaging in 'chip-and-wire' packaging schemes. In both cases, the optical characteristics are compatible with commercial vertical cavity surface emitting lasers and optoelectronic receivers, allowing the implementation of practical MCM-to-MCM interconnects.

The Polymer Optical interconnect Technology (POINT) is a collaborative program among GE, Honeywell, AMP, AlliedSignal, Columbia University and University of California at San Diego (UCSD), sponsored by ARPA, in developing affordable optoelectronic packaging and interconnect technologies for board- and backplane-level optical interconnect applications. The POINT program leverages on the existing electronic design, processing, fabrication and MCM packaging technologies to optoelectronic packaging. The POINT program also incorporates several state-of-the-art optoelectronic technologies that include: high speed VCSEL for multi-channel data transmission; flexible optical polymer waveguides and low-loss polymers for board and backplane interconnects; low-cost diffractive optical elements (DOE) for board-to-backplane interconnect; and use of molded MT-type connectors to reduce weight and size. In addition, to further reduce design and fabrication cycle times, CAD tools for multimode optical waveguide modelling, and for mechanical modelling of optoelectronic packaging will be employed to aid the technology development.

Vertical cavity surface emitting lasers (VCSELs) offer substantial advantages in performance and simplicity of packaging over the edge emitting lasers currently being applied to state-of-the-art photonic interconnects. We have demonstrated operation of VCSELs at cryogenic temperatures and at temperatures as high as 200 degrees Celsius, with a single device operating from minus 55 degrees Celsius to plus 125 degrees Celsius. The devices operate to 14 GHZ and can be operated in excess of 1 GHZ with bias-free operation. Initial radiation tests indicate an order of magnitude improvement in hardness with respect to neutron damage over an LED which is currently used in spaceborne photonic interconnect modules. We also describe the packaging of VCSELs in compact multichip modules. By using passive alignment techniques, optoelectronic devices can be packaged in established multichip module fabrication schemes without adding costly high precision assembly techniques.

As large factories, chemical plants and oil refineries become more and more automated, it becomes necessary to relay information from a large number of sensors to central control sites. In addition, it has also become important to transmit large amoutns of information in the form of data, voice, video, etc., to workers around the plant or factory. A unique technique based on optical microwave cellular system is described which is highly suitable for such an application.

Recent advances in optical devices, polymeric materials, and in the electronic MCM packaging and interconnect technologies could bring the cost of optical interconnect to a level affordable for module, board, and backplane level interconnect applications. Specifically, we discuss how the development in (1) vertical-cavity surface-emitting-laser devices, (2) multichip module packaging technologies, (3) optical polymers, and (4) adaptive interconnect can be applied to benefit optoelectronic packaging and interconnect. We show how these advancements will allow widely used planar processes and the already developed packaging technology in electronics to be applicable to optoelectronic packaging to reduce both recurring and nonrecurring engineering costs for this new technology insertion into computers and advanced electronic systems.

Acceptance of optical interconnects into spaceborne and ground-based military systems will be limited by the risk and maturity of the technology. Multimode fibers are currently the interconnect medium of choice for military application due to the availability of qualified parts. Multimode optical interconnects also offer lower assembly cost than singlemode interconnects, making them attractive for use in commercial computers. In both applications, modal noise may limit the attainable bit error rate in a digital system. We report on an investigation of the validity of simple expressions for determining modal noise in multimode systems, and consider in particular waveguides supporting fewer modes than typical multimode fibers and waveguides with incomplete modal excitation. We conclude that the simple expressions are valid for losses greater than approximately 0.5 dB per interface, but that the actual signal-to-noise ratio is significantly poorer than that predicted by theory for large displacements of waveguides in which a small fraction of the total modal volume is excited. We describe two simulation techniques for determining the statistics of the transmission associated with a waveguide bend, and find that the simple formula widely used for straight waveguide intersections do not describe curved waveguide interconnects accurately. We also describe a practical demonstration of two optical interconnect systems in which multimode interconnect media incorporating several imperfect interfaces and excited with coherent sources can achieve useful bit error rates in digital systems. In the first interconnect, a fiber optica data bus for satellite use employs six multimode connectors and multimode fiber to transmit data at 1.2 Gbps over distances from 1 meter to 100 meters. In the second interconnect, passive multimode polymide waveguides compatible with intracabinet optical interconnects are used to implement a multichip module (MCM)-to-MCM interconnect in which optoelectronic die are incorporated in a 'chip first' multichip module technology.

The determination of the signal-to-noise ratio degradation of optical signals in the presence of modal noise is important to the design of multimode data links with coherent sources. To date, most of the analyses put forth are based upon the assumption of a uniform distribution of power over all of the guided modes of a fiber/waveguide.! This assumption, while making the analysis more straight forward, may not always be justifiable.