System Design

Fiber optic system design can be simple point-to-point data links or can involve more complex topologies. However, it is generally necessary only to refer to point-to-point data links when discussing the process of link design. Fiber optic systems that incorporate complex architectures can be simplified into a collection of point-to-point data links before beginning the design process.

Fiber optic system design is a complicated process that involves link definition and analysis. The design process begins by providing a complete description of the communication requirements. This information is used to develop the link architecture and define the communications links. Systemdesigners must decide on the operational wavelength and types of components to use in the system. These decisions affect numerous system and link design parameters, such as launched power, connection losses, bandwidth, cost, and reliability.

Once a system design has been formulated, each link is analyzed to determine its viability. Link analysis involves calculating each link's power budget and risetime budget. Calculating a power budget involves identifying all of the sources of loss in the fiber optic link. These losses and an additional safetymargin are then compared to the difference between the transmitter output power and the receiver sensitivity. The difference between the transmitter output power and the receiver sensitivity is referred to as the available power. If the sources of loss plus the safety margin are less than the available power in the link, the design is viable.

Once a system design has been formulated, each link is analyzed to determine its viability. Link analysis involves calculating each link's power budget and risetime budget. Calculating a power budget involves identifying all of the sources of loss in the fiber optic link. These losses and an additional safetymargin are then compared to the difference between the transmitter output power and the receiver sensitivity. The difference between the transmitter output power and the receiver sensitivity is referred to as the available power. If the sources of loss plus the safety margin are less than the available power in the link, the design is viable.

Calculating the risetime budget involves calculating the risetimes of the link transmitter and the optical fiber. The composite optical transmitter/fiber risetime is referred to as the fiber exit risetime. If the fiber exit risetime is less than the maximum input risetime specified for the link receiver, then the link design is viable.

If a proposed link design is not viable, the system designer will reevaluate various decisions made earlier in the system design. These reevaluations may include using a different transmitter or receiver or may involve redesigning the physical configuration of the link. Because there are many variablesinvolved in link design and analysis, it may take several iterations before the variables are combined in a manner that ensures link operation.

SYSTEM INSTALLATION

The military has a standard to provide detailed information and guidance to personnel concerned with the installation of fiber optic cable plants on naval surface ships and submarines. The fiber optic cable plant consists of all the fiber optic cables and the fiber optic interconnection equipment within the ship,including connectors, splices, and interconnection boxes. The fiber optic cable plant installation standard consists of a basic standard and six numbered parts dealing with the following:

· Cables-provides detailed methods for cable storage and handling, end-sealing, repair, and splicing

· Equipment-provides detailed methods for fiber optic equipment installation and cable entrance to equipment

· Penetrations-provides detailed methods for cable penetrations within the ship's structure

· Cableways-provides detailed methods to install fiber optic cables in cableways

· Connectors and interconnections-provides detailed methods for installing fiber optic connectors and other interconnections, such as splices

· Tests-identifies and provides detailed methods for testing fiber optic cable plants before, during, and after installation and repair

There are other standards that discuss fiber optic system installation. Many of these standards incorporate procedures for repair, maintenance, and testing. The techniques developed for installing fiber optic hardware are not much different than for installing hardware for copper-based systems. However, the primary precautions that need to be emphasized when installing fiber optic systems on board ships are as follows:

· Optical fibers or cables should never be bent at a radius of curvature less than a certain value, called the minimum bend radius. Bending an optical fiber or cable at a radius smaller than the minimum bend radius causes additional fiber loss.

· Fiber optic cables should never be pulled tight or fastened over or through sharp corners or cutting edges. Extremely sharp bends increase the fiber loss and may lead to fiber breakage.

· Fiber optic connectors should always be cleaned before mating. Dirt in a fiber optic connection will significantly increase the connection loss and may damage the connector.

· Precautions must be taken so the cable does not become kinked or crushed during installation of the hardware. Extremely sharp kinks or bends increase the fiber loss and may lead to fiberbreakage.

· Only trained, authorized personnel should be allowed to install or repair fiber optic systems.

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