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High speed smart pixel arrays (SPAs) hold great promise as an enabling technology for board-to-board interconnections in digital systems. SPAs may be considered an extension of a class of optoelectronic components that have existed for over a decade that of optoelectronic integrated circuits (OEICs). The vast majority of development in OEICs has involved the integration of electronic receivers with optical detectors and electronic drivers with optical sources or modulators. In addition, very little of this development has involved more than a single optical channel. But OEICs have underpinned much of the advancement in serial fiber links. SPAs encompass an extension of these optoelectronic components into arrays in which each element of the array has a signal processing capability. Thus, a SPA may be described as an array of optoelectronic circuits for which each circuit possesses the property of signal processing and, at a minimum, optical input or optical output (most SPAs will have both optical input and output).?
The name smart pixel is combination of two ideas, "pixel" is an image processing term denoting a small part, or quantized fragment of an image, the word "smart" is coined from standard electronics and reflects the presence of logic circuits. Together they describe a myriad of devices. These smart pixels can be almost entirely optical in nature, perhaps using the non-linear optical properties of a material to manipulate optical data, or they can be mainly electronic, for instance a photoreceiver coupled with some electronic switching.
Smart pixel arrays for board-to-board optical interconnect may be used for either backplane communications or for distributed board-to-board communications, the latter known as 3-D packaging. The former is seen as the more near-term of the two,
Figure 1
employing free-space optical beams connecting SPAs located on the ends of printed circuit boards in place of the current state-of-the-art, multi-level electrical interconnected boards. 3-D systems, on the other hand, are distributed board-to-board optical interconnects, exploiting the third dimension and possibly employing holographic interconnect elements to achieve global connectivity (very difficult with electrical interconnects).
Most work in high speed SPAs has involved the use of either multiple-quantum-well (MQW) modulators or vertical-cavity surface-emitting lasers (VCSELs) as the optical source, and each of these has taken one of two approaches, monolithic and hybrid (e.g., monolithic VCSELs/GaAs and hybrid VCSELs/Si). The hybrid approaches are rapidly gaining popularity since they can take advantage of mainstream silicon microelectronics for the pixel logic circuitry, thereby leveraging the 30 billion dollar silicon semiconductor industry.
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