8.1 Basic Optical Fiber Links
309
1.3 μm, where signal dispersion is very low, bit-rate-distance products of at least
1500 (Mb/s)·km are achievable with LEDs in multimode fibers. For InGaAsP lasers,
distances of 150 m can be achieved at 100-Gb/s rates in OM4 multimode fiber at
1.3 μm (see Sect. 13.4). A single-mode fiber can provide such data rates over much
longer distances.
Greater repeaterless transmission distances are possible with a laser, laser diodes
typically couple from 10 to 15 dB more optical power into a fiber than an LED. This
advantage and the lower dispersion capability of laser diodes may be offset by cost
constraints. Not only is a laser diode itself more expensive than an LED, but also the
laser transmitter circuitry is much more complex, because the lasing threshold has to
be dynamically controlled as a function of temperature and device aging. However,
mass production techniques and technology innovations have led to a wide variety
of cost-effective laser transmitters being commercially available.
For the optical fiber, there is a choice between single-mode and multimode fiber,
either of which could have a step-index or a graded-index core. This choice depends
on the type of light source used and on the amount of dispersion that can be tolerated.
Light-emitting diodes (LEDs) tend to be used with multimode fibers. The optical
power that can be coupled into a fiber from an LED depends on the core-cladding
index difference , which, in turn, is related to the numerical aperture of the fiber
(for = 0.01, the numerical aperture NA ≈ 0.21). As increases, the fiber-coupled
power increases correspondingly. However, because dispersion also becomes greater
with increasing , a tradeoff must be made between the optical power that can be
launched into the fiber and the maximum tolerable dispersion.
When choosing the attenuation characteristics of a cabled fiber, the excess loss that
results from the cabling process must be considered in addition to the attenuation
of the fiber itself. This must also include connector and splice losses as well as
environmental-induced losses that could arise from temperature variations and dust
or moisture on the connector end faces.
8.1.3 Creating a Link Power Budget
An optical power loss model for a point-to-point link is shown in Fig. 8.3. The optical
power received at the photodetector depends on the amount of light coupled into the
fiber by the transmitter and the losses occurring in the fiber and at the connectors
and splices. The link loss budget is derived from the sequential loss contributions of
each element in the link. Each of these loss elements is expressed in decibels (dB) as
Element loss = 10 log
P out
P in
(8.1)
where P in and P out are the optical powers entering and leaving the link element,
respectively. The loss value corresponding to a particular element generally is called
the insertion loss for that element.
309
1.3 μm, where signal dispersion is very low, bit-rate-distance products of at least
1500 (Mb/s)·km are achievable with LEDs in multimode fibers. For InGaAsP lasers,
distances of 150 m can be achieved at 100-Gb/s rates in OM4 multimode fiber at
1.3 μm (see Sect. 13.4). A single-mode fiber can provide such data rates over much
longer distances.
Greater repeaterless transmission distances are possible with a laser, laser diodes
typically couple from 10 to 15 dB more optical power into a fiber than an LED. This
advantage and the lower dispersion capability of laser diodes may be offset by cost
constraints. Not only is a laser diode itself more expensive than an LED, but also the
laser transmitter circuitry is much more complex, because the lasing threshold has to
be dynamically controlled as a function of temperature and device aging. However,
mass production techniques and technology innovations have led to a wide variety
of cost-effective laser transmitters being commercially available.
For the optical fiber, there is a choice between single-mode and multimode fiber,
either of which could have a step-index or a graded-index core. This choice depends
on the type of light source used and on the amount of dispersion that can be tolerated.
Light-emitting diodes (LEDs) tend to be used with multimode fibers. The optical
power that can be coupled into a fiber from an LED depends on the core-cladding
index difference , which, in turn, is related to the numerical aperture of the fiber
(for = 0.01, the numerical aperture NA ≈ 0.21). As increases, the fiber-coupled
power increases correspondingly. However, because dispersion also becomes greater
with increasing , a tradeoff must be made between the optical power that can be
launched into the fiber and the maximum tolerable dispersion.
When choosing the attenuation characteristics of a cabled fiber, the excess loss that
results from the cabling process must be considered in addition to the attenuation
of the fiber itself. This must also include connector and splice losses as well as
environmental-induced losses that could arise from temperature variations and dust
or moisture on the connector end faces.
8.1.3 Creating a Link Power Budget
An optical power loss model for a point-to-point link is shown in Fig. 8.3. The optical
power received at the photodetector depends on the amount of light coupled into the
fiber by the transmitter and the losses occurring in the fiber and at the connectors
and splices. The link loss budget is derived from the sequential loss contributions of
each element in the link. Each of these loss elements is expressed in decibels (dB) as
Element loss = 10 log
P out
P in
(8.1)
where P in and P out are the optical powers entering and leaving the link element,
respectively. The loss value corresponding to a particular element generally is called
the insertion loss for that element.