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Setting up an OTDR for fiber testing

Set up is a critical step of OTDR testing. An expert technician can set it up in manual mode or choose a combination of manual and automatic settings to achieve the most accurate and illustrative OTDR...


September 1, 2008  


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Set up is a critical step of OTDR testing. An expert technician can set it up in manual mode or choose a combination of manual and automatic settings to achieve the most accurate and illustrative OTDR traces.

Important settings for this are fiber type and specifications, wavelengths, averaging time, distance range, and pulse widths. Some other settings that are only available on the most advanced OTDRs include launch fiber compensation, test limits, and the ability to name and identify which end of the fiber the testing originated from.

Fiber type: The most basic setting is the fiber type and the specifications for index of refraction and backscatter coefficient. Some OTDRs allow the user to choose from a menu of optical fibers commonly available today. In this case, the index of refraction (n) and backscatter coefficient are already loaded into the tester. Otherwise, it is necessary to enter the fiber specifications from the manufacturer’s catalog or website. With the right fiber specs plugged into the OTDR, the user is assured that the location and reflectance results are accurate.

Pulse width: This is the most important OTDR setting for advanced OTDR analysis. It determines the dead zone and affects the dynamic range of the instrument. Narrow pulse widths allow a technician to see more detail on the trace and identify events that are close together. Longer pulses allow for maximum distance range. It is important to consider that these short pulse widths often limit distance range and produce noise on the trace.

Wavelengths: Another important setting is selecting the wavelengths at which tests will be performed. Most OTDRs allow tests at multiple wavelengths, and some allow simultaneous tests at two wavelengths. Since light behaves differently at different wavelengths, expert technicians like to compare OTDR traces acquired at more than one wavelength. For instance, a dirty connector will often look OK on a trace acquired at 850nm but show dramatic “tailing” at a longer 1300nm wavelength.

Averaging time: This sets the number of measurements averaged together to create a trace. This can range from a few seconds to more than three minutes. A short averaging time decreases testing time but results in “noisy” traces, while choosing longer averaging time increases dynamic range, accuracy. An expert technician will use a longer averaging time to make cleaner traces and make it possible to detect small events on the trace.

Distance range: This setting helps frame the trace on the OTDR. Maximum distance range does not actually change with this setting, but rather is tied to the dynamic range of the instrument and the fiber that is being tested. A technician performing advanced OTDR testing will usually choose a distance range that is double the distance of the event that he is looking for.

Loss threshold: The loss threshold setting can help identify events that have very low loss or ignore small loss events that the user doesn’t need to see. A low loss threshold setting reduces noise on the trace, which makes small events visible, but tends to increase test time and makes dead zones longer.