566
13 Fiber Optic Communication Networks
end equipment is a driving factor in PON implementations, so this is not a viable
solution. Thus a major challenge for a WDM PON implementation is to have a
low-cost, high-output optical source at each ONT.
One method is to use spectral splicing of a single broadband relatively inexpensive
light source. Various techniques are being explored to achieve this. One idea is to
have each ONT contain a source with a broad optical output spectrum within the
transmitter, such as a superluminescent light emitting diode (SLED). The broad
spectral output of the source at the ONT is connected to one port on a local WDM
device, such as a thin-film filter or an AWG. Only those optical spectral components
from the SLED that can pass through the WDM channel are transmitted through to the
central office. Although all the ONTs have identical SLEDs, each user is connected
to a different port on the local WDM device, so it is possible to slice a different part
of the available optical spectrum for each ONT. This scheme thereby provides each
ONT with a different transmitting wavelength.
Another concept is to use a broadband source (for example, a superluminescent
laser diode or a broadband EDFA source) at the central office and send the source
output downstream through an AWG. Unique spectrally sliced wavelengths travel
to individual ONTs and seed a relatively inexpensive source at each ONT, such as a
Fabry-Perot (FP) laser diode. The seeding action forces the FP laser to operate in a
quasi single mode, which is one of the oscillating modes shown in Fig. 4.23. Because
a different seed wavelength arrives at each ONT, the FP laser located at that ONT
can transmit data upstream at its uniquely locked wavelength.
13.9 Summary
Various types of optical networks have been conceived, designed, and built to satisfy
a wide range of transmission capacities and speeds. The link lengths between users
can vary from short localized connections within a building or a campus environment to networks that span continents and run across oceans. A major motivation
for developing sophisticated communication networks has been the rapid proliferation of information exchange desired by institutions involved in fields such as
commerce, finance, education, scientific and medical research, health care, national
and international security, and entertainment.
The main network topologies for optical networks are the linear bus, ring, star,
and point-to-point mesh configurations. Each configuration has its own particular
advantages and limitations in terms of reliability, expandability, and performance
characteristics. Some of these configurations use basic point-to-point links, which
are simple connections between individual users or between a pair of multiplexing
equipment.
Linear bus networks using passive couplers typically are limited to connections
between several nodes because of the rapid accumulation of optical coupler losses
from node to node along the bus.
13 Fiber Optic Communication Networks
end equipment is a driving factor in PON implementations, so this is not a viable
solution. Thus a major challenge for a WDM PON implementation is to have a
low-cost, high-output optical source at each ONT.
One method is to use spectral splicing of a single broadband relatively inexpensive
light source. Various techniques are being explored to achieve this. One idea is to
have each ONT contain a source with a broad optical output spectrum within the
transmitter, such as a superluminescent light emitting diode (SLED). The broad
spectral output of the source at the ONT is connected to one port on a local WDM
device, such as a thin-film filter or an AWG. Only those optical spectral components
from the SLED that can pass through the WDM channel are transmitted through to the
central office. Although all the ONTs have identical SLEDs, each user is connected
to a different port on the local WDM device, so it is possible to slice a different part
of the available optical spectrum for each ONT. This scheme thereby provides each
ONT with a different transmitting wavelength.
Another concept is to use a broadband source (for example, a superluminescent
laser diode or a broadband EDFA source) at the central office and send the source
output downstream through an AWG. Unique spectrally sliced wavelengths travel
to individual ONTs and seed a relatively inexpensive source at each ONT, such as a
Fabry-Perot (FP) laser diode. The seeding action forces the FP laser to operate in a
quasi single mode, which is one of the oscillating modes shown in Fig. 4.23. Because
a different seed wavelength arrives at each ONT, the FP laser located at that ONT
can transmit data upstream at its uniquely locked wavelength.
13.9 Summary
Various types of optical networks have been conceived, designed, and built to satisfy
a wide range of transmission capacities and speeds. The link lengths between users
can vary from short localized connections within a building or a campus environment to networks that span continents and run across oceans. A major motivation
for developing sophisticated communication networks has been the rapid proliferation of information exchange desired by institutions involved in fields such as
commerce, finance, education, scientific and medical research, health care, national
and international security, and entertainment.
The main network topologies for optical networks are the linear bus, ring, star,
and point-to-point mesh configurations. Each configuration has its own particular
advantages and limitations in terms of reliability, expandability, and performance
characteristics. Some of these configurations use basic point-to-point links, which
are simple connections between individual users or between a pair of multiplexing
equipment.
Linear bus networks using passive couplers typically are limited to connections
between several nodes because of the rapid accumulation of optical coupler losses
from node to node along the bus.