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FIBER OPTIC RECEIVERS
In fiber optic communications systems, optical signals that reach fiber optic receivers
are generally attenuated and distorted (see figure 7-5). The fiber optic receiver must
convert the input and amplify the resulting electrical signal without distorting it to a
point that other circuitry cannot use it.
Figure 7-5. - Attenuated and distorted optical signals.
As stated previously, a fiber optic receiver consists of an optical detector, an
amplifier, and other circuitry. In most fiber optic systems, the optical detector is a PIN
photodiode or APD. Receiver performance varies depending on the type of detector used. The
amplifier is generally described as having two stages: the preamplifier and the
postamplifier. The preamplifier is defined as the first stage of amplification
following the optical detector. The postamplifier is defined as the remaining
stages of amplification required to raise the detector's electrical signal to a level
suitable for further signal processing. The preamplifier is the dominant contributor of
electrical noise in the receiver. Because of this, its design has a significant influence
in determining the sensitivity of the receiver.
The output circuitry processes the amplified signal into a form suitable for the
interfacing circuitry. For digital receivers, this circuitry may include low-pass filters
and comparators. For analog receivers, this circuitry may also include low-pass filters.
Receiver sensitivity, bandwidth, and dynamic range are key operational parameters used
to define receiver performance. One goal in designing fiber optic receivers is to optimize
receiver sensitivity. To increase sensitivity, receiver noise resulting from
signal-dependent shot noise and thermal noise must be kept at a minimum. A more detailed
discussion of receiver shot and thermal noise is provided later in this chapter.
In addition to optimizing sensitivity, optical receiver design goals also include
optimizing the bandwidth and the dynamic range. A receiver that has the ability to operate
over a wide range of optical power levels can operate efficiently in both short- and
long-distance applications. Because conflicts arise when attempting to meet each goal,
trade-offs in receiver designs are made to optimize overall performance.
Q.19 Which amplifier stage (the preamplifier or the
postamplifier) is a dominant
contributor of noise and significantly influences the sensitivity of the receiver?
Q.20 List the key operational parameters used to define receiver performance.
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