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342    Cha pte r  S i x






                                                                Laser
                                                                driver
                                                                           Digital in
                                                  I


                                                                  Decision
                                          TIA                      circuit  Digital out
                                                  L/A    Filter
                           Light in
                                                                    Clock
                                                 Auto gain         recovery
                                                  control
                                      Front end    Signal conditioning  Data recovery
                    FIGURE 6.12  Key components employed in optical interconnects. A digital signal is converted
                    into an analog signal by the direct modulation of current I. Light pulses are emitted in accordance
                    with the digital modulation. The light signal is introduced into and transmitted along a waveguide
                    to a photodetector that absorbs the light and generates a proportional current.


                    recovery, and decision circuits may form part of the digital data recovery portion of the
                    circuit. The circuitry is virtually identical to that of data optical communications. The
                    requirements on the bit error rate are more stringent for computing applications, being
                            –15
                    at least 10 .
                    6.6.1  Passive Thin-Film Lightwave Circuits
                    Planar lightwave circuits are used to convey information by manipulating packets of
                    photons. The idea is similar to electric circuits that make use of packets of electric charge
                    toward the same end. Each approach has fundamental and practical strengths and
                    limitations. Planar lightwave circuits consist of light-confining and light-guiding
                    functions (waveguides), power dividing functions (MMI, Y-splitters), power combining
                    functions (Y-combiners), and wavelength combining and spreading functions (waveguide
                    phase array gratings). Ultimately, a useful lightwave circuit also includes interfaces to
                    electrical-optical devices such as sources of light; optical amplifiers; photodetectors; and
                    light amplitude, phase, and wavelength modulators. Thus a planar or nonplanar
                    lightwave circuit consists of two parts, the passive lightwave circuit and the active
                    optoelectronic components for producing, detecting, and encoding information on the
                    light carrier. How well the optical interface between these two parts is designed and
                    constructed determines the optical interconnect reliability, performance, and cost.
                       In this section we will consider only some aspects of lightwave circuits as they apply
                    directly to optoelectronic SOP technology and will indicate sources of more general
                    treatments along the way.
                       The prevailing state-of-the-art for implementing optical-digital signaling among
                    processors is exemplified by the optical electric circuit board (OECB) [51] of which the
                    IBM Terabus is an extension [55]. In both cases, active optical components—lasers, laser
                    drivers, photodetectors, and associated amplifiers—are bonded to a peripheral ball
                    grid array (PBGA) package that may eventually also contain a memory controller with
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