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SUMMARY
Now that you have completed this chapter, let's review some of the new terms, concepts,
and ideas that you have learned. You should have a thorough understanding of these
principles before moving on to chapter 8.
A FIBER OPTIC RECEIVER is an electro-optic device that accepts optical signals
from an optical fiber and converts them into electrical signals. A typical fiber optic
receiver consists of an optical detector, a low-noise amplifier, and other circuitry used
to produce the output electrical signal.
RECEIVER SPECTRAL RESPONSE, SENSITIVITY, Frequency response, and DYNAMIC RANGE are key
receiver performance parameters that can affect overall system operation.
RECEIVER SENSITIVITY is the minimum amount of optical power required to achieve a
specific receiver performance. For digital transmission at a given data rate and coding,
this performance is described by a maximum bit-error rate (BER). In analog systems, for a
given modulation and bandwidth, it is described by a minimum signal-to-noise ratio
(SNR).
DYNAMIC RANGE refers to the range of optical power levels over which the receiver
operates within the specified values. It usually is described by the ratio of the maximum
input power to the sensitivity.
A TRANSDUCER is a device that converts input energy of one form into output
energy of another.
An OPTICAL DETECTOR is a transducer that converts an optical signal into an
electrical signal. It does this by generating an electrical current proportional to the
intensity of incident optical radiation.
The semiconductor POSITIVE-INTRINSIC-NEGATIVE (PIN) PHOTODIODE and AVALANCHE
PHOTODIODE (APD) are the principal optical detectors used in fiber optic systems.
A PHOTOCURRENT is generated when photons are absorbed by a photodiode.
RESPONSIVITY is the ratio of the optical detector's output photocurrent in amperes
to the incident optical power in watts.
DARK CURRENT, or reverse-leakage current, is the current that continues to flow in
the photodetector when there is no incident light.
The RESPONSE TIME of a photodiode and its output circuitry depends primarily on
the thickness of the detector active area and the detector RC time constant.
The TRANSIT TIME is the time it takes electrons to travel out of the detector
active area.
The RC TIME CONSTANT is defined by the capacitance (C) of the photodiode and the
resistance (R) of the load. The RC time constant is given by tRC = RC.
A HIGH-SPEED RESPONSE requires short transit times and low capacitance. However,
any change in photodiode parameters to optimize the transit time and capacitance can also
affect quantum efficiency, dark current, and coupling efficiency.
Detector LINEARITY means that the output electrical current (photocurrent) of
the photodiode is linearly proportional to the input optical power.
An AVALANCHE PHOTODIODE (APD) is a photodiode that internally amplifies the
photocurrent by an avalanche process.
In APDs, a large REVERSE-BIAS VOLTAGE, typically over 100 volts, is applied
across the active region.
AVALANCHE MULTIPLICATION occurs when accelerated electrons collide with other
electrons in the semiconductor material, causing a fraction of them to become part of the
photocurrent.
TRADE-OFFS are made in APD design to optimize responsivity and gain, dark current,
response time, and linearity.
The RESPONSE TIME of APDs accounts for the avalanche build-up time in addition
to transit time and RC time constant.
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 detectors electrical signal to a level suitable for further signal
processing.
RECEIVER SENSITIVITY, BANDWIDTH, and DYNAMIC RANGE are key
operational parameters used to define receiver performance.
NOISE is the main factor that determines receiver sensitivity.
RECEIVER NOISE includes thermal noise, dark current noise, and quantum noise.
THERMAL NOISE is the noise resulting from the random motion of electrons in a
conducting medium.
SHOT NOISE is noise caused by current fluctuations due to the discrete nature of
charge carriers.
DARK CURRENT NOISE results from dark current that continues to flow in the
photodiode when there is no incident light.
QUANTUM NOISE results from the random generation of electrons by the incident
optical radiation.
The HIGH-IMPEDANCE AMPLIFIER and the <emphasis
type="b.GIF">TRANSIMPEDANCE AMPLIFIER</emphasis> are the two basic types of
amplifiers used in fiber optic receivers.
The HIGH-IMPEDANCE PREAMPLIFIER provides a high sensitivity, but limits receiver
bandwidth and dynamic range.
The TRANSIMPEDANCE PREAMPLIFIER provides improvements in bandwidth and dynamic
range with some degradation in sensitivity from an increase in noise.
PIN PHOTODIODES are used as the detector in most applications.
AVALANCHE PHOTODIODES are only used in high-speed applications and applications
where detectors with extremely low sensitivities are required.
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