Biomedical Engineering Reference
In-Depth Information
advantage of optical interconnects over their electrical counterparts are
the following:
• Freedom from stray capacitance and impedance matching
• Freedom from grounding problems
• Provide relief from the pinout problem
• Lower power requirements
• Increased flexibility for interconnects
• Light weight and small volume
• Planar integration
• Increased effective bandwidth of the system
• Two-way communication over a single transmission path (fiber)
• Passive MUX/DEMUX for high reliability at low cost
• Major system cost reductions
• Simple upgrading of existing systems
• Immunity to RFI, EMI, and EMP effects
The first, and perhaps most important, advantage is the lack of stray capaci-
tance. Optical interconnects suffer no such capacitance effects and crosstalk
can be controlled as long as care is taken to avoid scattering effects in broad-
cast systems. The second advantage is freedom from capacitance loading
effects. The speed of propagation of an electrical signal on a transmission
line depends on the capacitance per unit length. As more and more com-
ponents are attached to a transmission line, the time required to charge the
line increases and the propagation speed of the signal decreases. Optical
interconnects provide a constant signal speed (i.e., the speed of light in the
guiding medium), which is independent of the attached components.
2.5.7 Compatible Source Technology
The telecommunications and optoelectronic industries have pursued
extensive research programs investigating semiconductor optical sources
operating at 0.85, 1.3, and 1.55 μm wavelengths. This has introduced to the
marketplace a wide range of available sources that are capable of achiev-
ing previously unattainable modulation rates with extremely high reliability.
LEDs are generally limited to hundreds of Mb/s up to 500 MHz [59]. In con-
trast, laser diodes, although similar in structure to LEDs, are highly efficient.
An experimental InGaAsP laser has demonstrated an internal quantum
efficiency approaching 100%. This cooled laser has also shown a bandwidth
capability exceeding 26 GHz [60]. Depending on the system implementa-
tion used in the interconnection of LSI components, either of these types of
sources would be adequate for data transmission.
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