August 30, 2011
A receiver is generally designed with a transmitter. Both are modules within the same package. The light detection is carried out by a photodiode, which senses light and converts it into an electrical current. However, the optical signal from the fiber-optic cable and the resulting electrical current will have a small amplitude. Consequently, the photodiode circuitry must be followed by one or more amplification stages. There might even be filters and equalizers to shape and improve the information-bearing electrical signal.
August 30, 2011
Span analysis is the calculation and verification of a fiber-optic system's operating characteristics. This encompasses items such as fiber routing, electronics, wavelengths, fiber type, and circuit length. Attenuation and nonlinear considerations are the key parameters for loss-budget analysis. Before implementing or designing a fiber-optic circuit, a span analysis is recommended to make certain the system will work over the proposed link. Both the passive and active components of the circuit have to be included in the loss-budget calculation. Passive loss is made up of fiber loss, connector loss, splice loss, and losses involved with couplers or splitters in the link. Active components are system gain, wavelength, transmitter power, receiver sensitivity, and dynamic range.
August 30, 2011
Receiver sensitivity and dynamic range are the minimum acceptable value of received power needed to achieve an acceptable BER or performance. Receiver sensitivity takes into account power penalties caused by use of a transmitter with worst-case values of extinction ratio, jitter, pulse rise times and fall times, optical return loss, receiver connector degradations, and measurement tolerances.
August 30, 2011
Power measured in dBm at a particular wavelength generated by the transmitter LED or LD used to launch the signal is known as the transmitter launch power. Generally speaking, the higher the transmitter launch power, the better. However, one must be wary of receiver saturation, which occurs when the received signal has a very high power content and is not within the receiver's dynamic range.
August 29, 2011
Fiber-optic cables might have to be spliced together for a number of reasons—for example, to realize a link of a particular length. Another reason might involve backhoe fade, in which case a fiber-optic cable might have been ripped apart due to trenching work. The network installer might have in his inventory several fiber-optic cables, but none long enough to satisfy the required link length. Situations such as this often arise because cable manufacturers offer cables in limited lengths—usually 1 to 6 km. A link of 10 km can be installed by splicing several fiber-optic cables together. The installer can then satisfy the distance requirement and avoid buying a new fiber-optic cable. Splices might be required at building entrances, wiring closets, couplers, and literally any intermediate point between a transmitter and receiver.
August 29, 2011
Many factors must be considered when designing a fiber-optic cable plant.