Page 337 - Optical Communications Essentials

Test and Measurement

Test and Measurement 327

Figure 19.3. Example of a versatile hand-
held optical power meter. (Model FOT-90A,
provided courtesy of EXFO; www.exfo.com.)

in either tabular or graphic form. The permanent memory registers can store
512 readings manually or 400 readings automatically at a programmable time
interval.
Figure 19.4 shows another handheld tester that also contains optical sources
to carry out more sophisticated optical power measuring. For example, this
instrument can function as a power meter, an optical-loss tester for automat-
ically measuring loss in a fiber in two directions at two wavelengths, an optical
return-loss tester for measuring the quality of optical patch cords, a visual fault
indicator for locating breaks and failures in a fiber cable, and a talk set for full-
duplex communications between field personnel.

19.3.3. Optical power attenuators
In many laboratory or production tests, the characteristics of a high optical sig-
nal level may need to be measured. If the level is very high, such as a strong out-
put from an optical amplifier, the signal may need to be attenuated precisely
before being measured. This is done to prevent instrument damage or to avoid
overload distortion in the measurements. An optical attenuator allows a user to
6
reduce an optical signal level up to, for example, 60dB (a factor of 10 ) in pre-
cise steps at a specified wavelength, which is usually 1310 or 1550nm. The
capabilities of attenuators range from simple tape-cassette-size devices for
quick field measurements that may only need to be accurate to 0.5dB to labora-
tory instruments that have an attenuation precision of 0.001dB.

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