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Integrated Chip-to-Chip Optoelectr onic SOP 341
4 × 12-ch laser Tramsmitter- Receiver- 4 × 12-ch
driver IC optochip optochip receiver IC
4 × 12 VCSEL 4 × 12 PD
array array
Optocard
SLC TM -card with 48-waveguide array Silicon carrier with
surface wiring with coupling mirrors surface wiring and vias
FIGURE 6.11 Schematic cross section of the main components in the IBM Terabus showing the
fl ip-chip hierarchy. The microlens array is not clearly shown. See [55] for additional detail.
Finally, the massively parallel optical interconnect Terabus project developed at
IBM has 48 VCSEL channels, each channel having a bit rate of about 20 Gb/s. A sketch
of the architecture is shown in Figure 6.11. Consistent with the rest of the industry,
designers of the Terabus have taken the discrete component integration approach to the
level of arrays of components, but the optical alignment is still accomplished either by
hand or is left to the law of surface tension in the flip-chip array reflow process [55].
6.6 Optoelectronic SOP Thin-Film Components
In this section, we summarize the challenges and opportunities that will be encountered
over the next 15 years toward the implementation of optical interconnects inside the box
as backplanes and as flexible high-speed, high-density chip-to-chip optical interconnects.
Digital signaling over nonequalized microstrip copper transmission lines loses
bandwidth carrying capacity both as the inverse square of the line length and as the
cross-sectional area shrinks [16]. In addition, microstrip copper lines must maintain
impedance matching and shielding, be sufficiently separated to prevent crosstalk, and
be isolated from the power plane in order to prevent coupling to simultaneous switching
noise or ground bounce [56]. Finally, because of these constraints on electrical digital
signaling, high-density and high-speed off-board copper interconnects are difficult to
design. High-performance systems will therefore be the first to migrate to high-density
optical interconnects for high-speed signaling in two and three dimensions in order to
meet performance targets of terabytes per second per unit volume at low power.
The optical solution is sketched in Figure 6.12 in order to point out the necessary
components. A laser driver converts a digital input signal into a series of optical pulses
by the direct modulation of current source I through the laser. The modulated analog
optical signal is then coupled to a waveguide and carried to a photodetector which is
coupled to the receiving end of the same waveguide. The photodetector current is
converted to a voltage signal by a transimpedance amplifier (TIA) which is maintained
at a digital level by a limiting amplifier (LA) with auto gain control. A filter, a clock