Biomedical Engineering Reference
In-Depth Information
High bit rate communication sources at high-speed pulse code modulation
rates should have the following:
• No modulation distortions (pattern effects)
• Narrow spectral bandwidth
• No high spectral broadening due to modulation
• No self-pulsation (charge storage effects)
Currently at issue are the power requirements for both types of sources that
range from 7 to 100 mA for adequate output in LEDs or threshold operation
in laser diodes. The critical future laser requirements for the optimal usage
of optical interconnections are “development of continuous wave room tem-
perature frequency-selective lasers with high stability (10 Å) and low thresh-
old current (5 ma)” [61] that are capable of being modulated in the tens of
gigahertz just at or above threshold.
2.5.8 Receiver and Detector Technology
Optical interconnect receivers are basically high-speed optoelectronic trans-
ducers that receive incoming optical digital data, transform it to an electrical
signal, amplify and filter it, and restore logic levels. There are several alter-
natives for detectors. Among these are p-i-n photodiodes, avalanche diodes,
and Schottky barrier photodiodes. The p-i-n photodector is essentially a p-n
junction with a controllable depletion region width—the “i” intrinsic region.
Incoming photons generate electron-hole pairs in the intrinsic region and
the carriers are swept out of the region by an applied electric field. The
device speed is chiefly determined by the drift mobility. As a result, the fast-
est response times are achieved using GaAs devices. The voltage require-
ments to deplete the active region are on the order of 5-10 V.
The APD is also a p-n diode; however, it operates in a highly biased mode
near avalanche breakdown. Its operation is similar to the operation of a pho-
tomultiplier tube with improved sensitivity of 3-10 dB. APD detectors can
exhibit rise times in the 30-40 ps range and 80%-90% efficiency [62]. Schottky
barrier photodiodes possess the advantage of being ideally suited for surface
detection. They are constructed by using a thin, transparent layer of metal
for one contact that allows the light to pass very efficiently into the semicon-
ductor contact. With proper construction, any of these detectors are capable
of detecting data rates in the gigahertz range and are compatible with the
wavelengths and modulation rates of laser diodes and LEDs.
2.5.9 Integration of Sources and Detectors
Due to the telecommunications industry requirements, a large num-
ber of sources and detectors are available in single-device packages from
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