A fiber optic data link carries signals for communications, security, control and similar purposes by using transceivers and optical fibers. Designed to protect the fibers, an optical fiber cable should be installed, spliced and terminated with the proper hardware to mate the data link transceivers, and included in a fiber optic cable plant. This cable plant must be selected and installed to withstand the environment, and typically terminated at outlets or patch panels near the communications equipment. It’s connected to the transceiver by short fiber optic jumper. Last blog introduces fiber optic data links: parts, signals and power budget. Today’s blog details another device used in data links: fiber optic cable plant.
Cable Plant Basics
Since the fiber optic cable plant consists of the optical cable which is terminated with the transceiver, this cable plant must be compatible with the performance parameters of the transceivers for the link to operate properly. This includes types of fiber capped with different connectors (e.g. LC to SC fiber patch cable), optical loss and bandwidth of the cable plant. For the cable plant, a loss budget must be calculated to estimate its loss and a power budget to determine if the planned communications system will operate over the cable plant.
Cable Plant Performance Factors
For a fiber optic data link performances, the parameters are those that define the communications signals to be carried on the link or bandwidth at which the link operates, the length of the link and the specifications (bandwidth and optical loss) of the fiber optic cable plant. These factors determine the types of transceivers and cable plant components that must be chosen for a communications system. (Among these factors, the loss of the cable plant and the bandwidth have effects on the link design and testing after installation.)
The loss of the cable plant is determined by the summation of the loss in the cable plant because of fiber attenuation, splice loss and connector loss. In some cases, the fiber attenuation may be increased due to improper installation of the cable. As a signal travels down the fiber, the signal will be attenuated by the optical fiber and reduced by the loss in connectors and splices.
For each cable plant designed, the loss budget must be calculated. Then according to the loss budget, the loss of the fiber in the cable plant can be estimated by multiplying the length (km) by the attenuation coefficient (dB/km), then adding the loss from connectors and/or splices determined by the number of connectors and/or splices times the estimated loss each to get the total estimated loss of the cable plant. The cable plant loss budget must be lower than the power budget of the link transceivers (see below) for the link to work properly.
Dispersion or pulse spreading limits the bandwidth of the link. Transceivers have some dispersion caused by the limitations of the electronics and electro-optical components, but most of the dispersion results from the limited bandwidth of the fiber in the cable plant.
Dispersion in multi-mode fiber (MMF) occurs by modal dispersion or chromatic dispersion. Modal dispersion is caused by the different velocities of the various modes being transmitted in the fiber. Chromatic dispersion is caused by the different velocities of light at different wavelengths.
Single-mode fiber (SMF) also causes dispersion, but generally only in very long links. Chromatic dispersion has the same cause as MMF, the differences in the speed of light at different wavelengths. SMF may also suffer from polarization-mode dispersion causes by the different speeds of polarized light in the fibers.
The transceiver must be chosen to offer proper performance to the communications system’s requirements for bandwidth or bitrate, and to supply an optical transmitter output of sufficient power and receiver of adequate sensitivity to operate over the optical loss caused by the cable plant of the communications system. The difference in the transmitter output and receiver sensitivity defines the optical power budget of the link.
The cable plant components, optical fiber, splices and connectors, are chosen to allow sufficient distance and bandwidth performance with the transceivers to meet the communications system’s optical power budget requirements. The power budget of the link defines the maximum loss budget for the cable plant. The maximum link length will be determined by the power budget and loss budget for low bit rate links that will be derated for dispersion for higher bandwidth links.
Most communications systems with short links have options for both MMF and SMF, while longer links use only SMF. All networks may provide guidance as to the types or grades of fiber needed to support certain applications.
Every manufacturer of data links components and systems specifies their link for receiver sensitivity (perhaps a minimum power required) and minimum power coupled into the fiber from the source. In order for a manufacturer or system designer to test them properly, it is necessary to know the test conditions. For data link components, that includes input data frequency or bitrate and duty cycle, power supply voltages and the type of fiber coupled to the source. For systems, it will be the diagnostic software needed by the system.
Fiber optic cable plant is an integral part of a fiber optic data link, and it should be managed in the exact path that every fiber in each cable follows, including intermediate connections and every connector type.