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Field measurements differ from laboratory measurements because they measure the transmission properties of installed fiber optic components. Laboratory measurements can only attempt to simulate the actual operating conditions of installed components. Fiber optic component properties measured in the laboratory can change after the installation of these components on board ship. End users must perform field measurements to evaluate those properties most likely affected by the installation or repair of fiber optic components or systems.

The discussion on field measurements is limited to optical fiber and optical connection properties. Optical fiber and optical connection field measurements evaluate only the transmission properties affected by component or system installation or repair. Because optical fiber geometrical properties, such as core and cladding diameter and numerical aperture, are not expected to change, there is no need to remeasure these properties. The optical fiber properties that are likely to change include fiber attenuation (loss) and bandwidth. Bandwidth changes in the field tend to be beneficial, so field bandwidth measurement is generally not performed. If field bandwidth measurements are required, they are essentially the same as laboratory measurements so they will not be repeated. The optical connection properties that are likely to change are connection insertion loss and reflectance and return loss.

The installation and repair of fiber optic components on board ship can affect system operation. Microbends introduced during installation can increase fiber attenuation. Modal redistribution at fiber joints can increase fiber attenuation in the fiber after the joint. Fiber breaks or faults can prevent or severely disrupt system operation. Poor fiber connections can also increase insertion loss and degrade transmitter and receiver performance by increasing reflectance and return loss. End users should perform field measurements to verify that component performance is within allowable limits so system performance is not adversely affected.

There are additional differences in measuring optical fiber and optical connection properties in the field than in the laboratory. Field measurements require rugged, portable test equipment, unlike the sophisticated test equipment used in the laboratory. Field test equipment must provide accurate measurements in extreme environmental conditions. Since electrical power sources may not always be available in the field, test equipment should allow battery operation. In addition, while both fiber ends are available for conducting laboratory measurements, only one fiber end may be readily available for field measurements. Even if both fiber ends are available for field measurements, the fiber ends are normally located some distance apart. Therefore, field measurements may require two people.

The main field measurement technique involves optical time-domain reflectometry. An optical time-domain reflectometer (OTDR) is recommended for conducting field measurements on installed optical fibers or links of 50 meters or more in length. An OTDR requires access to only one fiber end. An OTDR measures the attenuation of installed optical fibers as a function of length. It also identifies and evaluates optical connection losses along a cable link and locates any fiber breaks or faults.

End users can also measure fiber attenuation and cable plant transmission loss using an optical power meter and a stabilized light source. End users use this measurement technique when optical time-domain reflectometry is not recommended. Measurements obtained with a stabilized light source and power meter are more accurate than those obtained with an OTDR. Measuring fiber attenuation and transmission loss using a power meter and light source requires access to both ends of the fiber or link. An optical loss test set (OLTS) combines the power meter and source functions into one physical unit.

Q.25 Is it essential for end users to remeasure optical fiber geometrical properties after installation in the field?
Q.26 When is an OTDR recommended for conducting field measurements on installed optical fibers or links?

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