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Fiber Optic System Testing, Troubleshooting & Maintenance – Part 11

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Published on Oct 21, 2014 | News

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There are simple procedures to test, troubleshoot, and maintain a fiber-optic system. For basic procedures only simple inexpensive equipment is required. More sophisticated equipment can be used for advanced analysis.

Measuring Optical Power

The optical power output from a fiber-optic transmitter or fiber cable can be measured with a simple and inexpensive light meter. The light meter is calibrated for each of the three optical windows—850, 1310, and

1550 nm. The meters are available with interchangeable connectors so that systems with any fiber connector type can be tested. The meter gives a reading in milliwatts or dBm.

When troubleshooting a fiber-optic system, the first step is to see if the transmitter unit is sending any optical power. The technician will attach the meter to the transmitter with a fiber patch cord. The output optical power of the transmitter can then be confirmed against the manufacturer’s datasheet. If the transmitter is within specification, the next step is the see if light is making it through the fiber to the receiver side.

If the light level output of the transmitter does not meet specifications, this indicates the source of a possible failure. After reconnecting the transmitter back to the fiber, the optical meter is connected to the receiver side of the fiber. Measure the output fiber from the end of the fiber. The theoretical attenuation for the fiber length can be calculated. Using the theoretical attenuation of the fiber and subtracting it from the transmitter optical output power, the power level that should be present at the fiber end near the receiver can be calculated. As long as the optical power level at the received side is higher than the receiver optical sensi- tivity, the fiber link should operate. If there is a low or no optical signal at the end of the fiber on the receiver side, the fiber may be damaged or have faulty or dirty connectors. Lint-free optical wipes, isopropyl alcohol, and a can of compressed air can be used to clean all optical connectors.

A test of a fiber-optic cable can be performed prior to the purchase of fiber equipment. If a calibrated optical source is not available, handheld calibrated light sources are available as a companion device to the optical power meter. The calibrated light source can be attached to one side of the fiber and the optical meter to the other end.

Optical Time Domain Reflectometer

More extensive tests can be performed with an optical time domain reflectometer (OTDR). An OTDR is a sophisticated device that sends a calibrated light source at a specific wavelength down one end of a fiber. The unit is extremely sensitive and measures the extremely low levels of light reflected back through the fiber.

OTDR works very much like sonar. In sonar an audio tone is bounced off objects. The size of the reflection and the delay determine the size and dis- tance of the objects. As the downstream light beam from the OTDR hits connectors, splices, and other defects in the fiber, it reflects small amounts of light back to the OTDR. Based on the size of the reflection and the time it takes for the reflection to return to the OTDR, the system will provide a calibrated representation of the attenuation and flaws in an optical fiber. The OTDR analysis can be performed at different wavelengths with various modulation schemes. An OTDR analysis is typically only necessary on very long fiber runs with many optical connectors, patch panels, and splices. It is easier to predict optical losses and bandwidth on the majority of fiber-optic cable runs since there will be a minimal number of connectors and splices. Most applications will not require an OTDR.

Cleaning and Maintaining Optical Connectors

Fiber-optic connectors should be cleaned with lint-free optical wipes, and 100% pure isopropyl alcohol should be used with the wipes. Compressed air is also useful to clean any dirt and debris from connectors or receptacles. There are cassette-type cleaning devices that have an advanceable cleaning ribbon. The tips of a male fiber-optic connector are typically ceramic. A protective cap should always be applied to the connector on the fiber cable as well as to the connector on the fiber equipment. This prevents damage and dirt build up.

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