The various arguments for introducing optical interconnections to silicon CMOS chips are summarized, and the challenges for optical, optoelectronic, and integration technologies are discussed. Optics could solve many physical problems of interconnects, including precise clock distribution, system synchronization (allowing larger synchronous zones, both on-chip and between chips), bandwidth and density of long interconnections, and reduction of power dissipation. Optics may relieve a broad range of design problems, such as crosstalk, voltage isolation, wave reflection, impedence matching, and pin inductance. It may allow continued scaling of existing architectures and enable novel highly interconnected or high-bandwidth architectures. No physical breakthrough is required to implement dense optical interconnects to silicon chips, though substantial technological work remains. Cost is a significant barrier to practical introduction, though revolutionary approaches exist that might achieve economies of scale. An Appendix analyzes scaling of on-chop global electrical interconnects, including line inductance and the skin effect, both of which impose significant additional constraints on future interconnects.

/pdf/rationale-and-challenges-for-optical-interconnects-to-5clh2238g9.pdf

Rationale and challenges for optical interconnects to electronic chips

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.

/pdf/fully-embedded-board-level-guided-wave-optoelectronic-4tl1zzeb4x.pdf

Fully embedded board-level guided-wave optoelectronic interconnects

This paper demonstrates a flexible optical waveguide film with integrated optoelectronic devices (vertical-cavity surface-emitting laser (VCSEL) and p-i-n photodiode arrays) for fully embedded board-level optical interconnects. The optical waveguide circuit with 45/spl deg/ micromirror couplers was fabricated on a thin flexible polymeric substrate by soft molding. The 45/spl deg/ couplers were fabricated by cutting the waveguide with a microtome blade. The waveguide core material was SU-8 photoresist, and the cladding was cycloolefin copolymer. A thin VCSEL and p-i-n photodiode array were directly integrated on the waveguide film. Measured propagation loss of a waveguide was 0.6 dB/cm at 850 nm.

Flexible optical waveguide film fabrications and optoelectronic devices integration for fully embedded board-level optical interconnects

A new and innovative interconnection technology applicable for printed circuit boards is presented. This technology is widely compatible with the existing design and manufacturing processes and technologies for conventional multilayer pc boards and it combines electrical and optical interconnects on pc board level. It provides the potential for on-board bandwidth of several Gb/s and closes the bottleneck caused by the limited performance of electrical interconnection technology. After giving an overview on the most important basic technologies and first available results and engineering samples, the focus of this paper is on the development of appropriate modeling methodologies and simulation algorithms necessary for designing and optimizing optical on-board interconnects within the conventional electrical pc board environment.

A high-performance hybrid electrical-optical interconnection technology for high-speed electronic systems

The present invention entails a volume grating for use as an optical coupler and method for creating the same which comprises a predetermined surface grating pattern having a decreasing surface grating period along a waveguide light propagation direction in the volume grating with a plurality of slanted grating fringes having a variable slant angle along the waveguide light propagation direction to focus coupled light in a first dimension with a predetermined light intensity profile along a grating-cover interface plane of the volume grating. In addition, the predetermined surface grating pattern further includes an increasing radius of curvature along the waveguide light propagation direction to focus the light in a second dimension. The present invention further comprises a system and method for designing the volume grating, as well as a system and method for designing an apparatus for fabricating the volume grating.

Diffractive grating coupler and method

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 1-to-48 waveguide fanout device. 1-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-1 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 tilted 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.

http://ieeexplore.ieee.org/iel3/4820/13321/00609060.pdf

Si CMOS process compatible guided-wave multi-GBit/sec optical clock signal distribution system for Cray T-90 supercomputer

In contrast to volume holographic material where 1-to-many fanouts are realized using multiplexed volume holograms, we report in this paper the first Si-based surface-relief polygonal gratings aiming at optical clock signal distribution application. Surface-relief gratings with 1-/spl mu/m period (0.5 /spl mu/m feature size) were fabricated using reactive ion beam etching (RIE). Both hexagonal and square gratings were demonstrated for 1-to-4 and 1-to-6 fanouts. Surface-normal input and output coupling schemes were carried out with an combined coupling efficiency of 65%. Employment of substrate modes in silicon greatly releases the required grating spacing for the demonstrated two-way surface-normal coupling. 7.5 GHz 1-to-4 clock signal distribution operating at 1.3 /spl mu/m was demonstrated with a signal-to-noise ratio as high as 60 dB. The intensity fluctuation among fanout beams was measured to be within 1 dB. Generalization of 1-to-many fanout can be realized by implementing a polygonal grating with an equivalent number of facets.

Si-based surface-relief polygonal gratings for 1-to-many wafer scale optical clock signal distribution

In this paper, we present our efforts to construct an optoelectronic interconnection layer for high-speed optical clock signal distribution in a Gray T-90 supercomputer board. The optoelectronic interconnection layer under investigation employs optical channel waveguides and cascaded 3 dB 1-to-2 waveguide splitters in conjunction with surface-normal waveguide grating couplers. The planarization requirement for the optical interconnection layer required by multi-layer integration is fulfilled. Furthermore, the difficulties associated with the complicated 3-D multiple alignments are significantly reduced by the surface-normal fanout beams and the unique planarized device feature. An 1-GHz optical clock signal operating at 1.3 /spl mu/m was transmitted through a 45 cm long polymer-based channel waveguide. Some new techniques to fabricate large-area optical channel waveguides and surface-normal waveguide grating couplers are also presented.

1-GHz clock signal distribution for multi-processor super computers

We report the fabrication of tilted gratings on polyimide waveguides. The reported gratings provide an effective unidirectional surface-normal optical couple-in and couple- out for polymer-based optoelectronic interconnects. Such a planarized grating is particularly suitable for wafer-scale MCM optoelectronic interconnects due to its unique non- blocking fanout feature. both surface-normal input and output grating couplers have been demonstrated for the first time on polyimide waveguides on silicon substrate.

Unidirectional surface-normal waveguide grating couplers for wafer-scale MCM interconnect

Wavelength division demultiplexer based on dispersive volume holograms and axial graded index (AGRIN) lenses is presented in this paper. The dispersive volume hologram was fabricated with DuPont photopolymer with two beam interference method. Close to 99 percent center wavelength diffraction efficiency was achieved and the dispersion property of a volume hologram was characterized with a mode-locked femto-second laser source. The experimental data were found to conform well with the theoretical derivations. Axial graded index (AGRIN) lenses have been sued as focusing elements to separate light with different incident angles after dispersive volume holograms. By using dispersive volume holograms and AGRIN lenses, a wavelength division demultiplexer with 1540 nm center wavelength and a 4 nm channel separation was demonstrated. By using two stacked holograms, light dispersion can be further enhanced up to five times. Therefore smaller devices and smaller channel separations can be realized.

Dispersion-enhanced wavelength division multiplexing

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