8.1 Basic Optical Fiber Links
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performs electrical-to-optical (E/O) conversion to change the electrical signal into
an optical format. At the far end of the fiber a photodiode receiver performs optical-toelectrical (O/E) conversion to recover the electrical information signal for processing
by the following electronic circuits. This type of link places the least demand on
optical fiber technology and thus sets the basis for examining more complex network
architectures.
The design of an optical link involves many interrelated variables among the
fiber, light source, and photodetector operating characteristics, so that the actual
link design and analysis may require several iterations before they are completed
satisfactorily. Because performance and cost constraints are very important factors in
fiber optic communication links, the designer must carefully choose the components
to ensure that the desired performance level can be maintained over the expected
system lifetime without over specifying the component characteristics.
The following key system requirements are needed in analyzing a link:
1. The desired (or possible) transmission distance
2. The data rate or channel bandwidth
3. The bit-error rate (BER)
To fulfill these requirements, the designer has a choice of the following
components and their associated characteristics:
1. Multimode or single-mode optical fiber
(a) Core size
(b) Core refractive-index profile
(c) Bandwidth or signal dispersion
(d) Attenuation
(e) Numerical aperture or mode-field diameter
2. LED or laser diode optical source
(a) Emission wavelength
(b) Spectral linewidth
(c) Optical output power
(d) Effective radiating area
(e) Emission pattern
(f) Number of emitting modes
3. Pin or avalanche photodiode
(a) Responsivity
(b) Operating wavelength
(c) Speed
(d) Sensitivity
Two analyses usually are carried out to ensure that the desired system performance
can be met: these are the link power budget and the system rise-time budget analyses.
In the link power budget analysis one first determines the power margin between the
305
performs electrical-to-optical (E/O) conversion to change the electrical signal into
an optical format. At the far end of the fiber a photodiode receiver performs optical-toelectrical (O/E) conversion to recover the electrical information signal for processing
by the following electronic circuits. This type of link places the least demand on
optical fiber technology and thus sets the basis for examining more complex network
architectures.
The design of an optical link involves many interrelated variables among the
fiber, light source, and photodetector operating characteristics, so that the actual
link design and analysis may require several iterations before they are completed
satisfactorily. Because performance and cost constraints are very important factors in
fiber optic communication links, the designer must carefully choose the components
to ensure that the desired performance level can be maintained over the expected
system lifetime without over specifying the component characteristics.
The following key system requirements are needed in analyzing a link:
1. The desired (or possible) transmission distance
2. The data rate or channel bandwidth
3. The bit-error rate (BER)
To fulfill these requirements, the designer has a choice of the following
components and their associated characteristics:
1. Multimode or single-mode optical fiber
(a) Core size
(b) Core refractive-index profile
(c) Bandwidth or signal dispersion
(d) Attenuation
(e) Numerical aperture or mode-field diameter
2. LED or laser diode optical source
(a) Emission wavelength
(b) Spectral linewidth
(c) Optical output power
(d) Effective radiating area
(e) Emission pattern
(f) Number of emitting modes
3. Pin or avalanche photodiode
(a) Responsivity
(b) Operating wavelength
(c) Speed
(d) Sensitivity
Two analyses usually are carried out to ensure that the desired system performance
can be met: these are the link power budget and the system rise-time budget analyses.
In the link power budget analysis one first determines the power margin between the