Check your light source for power drift

July 1, 1995
An optical-loss test set is used to test end-to-end loss through optical fiber. It is impossible to obtain an accurate measurement of optical loss, however, if the power level of the light source drifts. This drift can occur when the unit is first turned on, and when there are sharp changes in ambient temperature.

Larry Johnson, The Light Brigade Inc.

Problem

An optical-loss test set is used to test end-to-end loss through optical fiber. It is impossible to obtain an accurate measurement of optical loss, however, if the power level of the light source drifts. This drift can occur when the unit is first turned on, and when there are sharp changes in ambient temperature.

Solution

Manufacturers often specify the amount of drift to which their units are subject, and the time it takes them to stabilize. However, a simple test will validate the manufacturer?s specifications.

Procedure

1) With the light source turned off, connect it to the power meter using a fiber jumper. Set both light source and power meter to the same wavelength.

2) Turn on the power meter. Then turn on the light source, and watch the power meter carefully.

3) Record how much the light source drifts and how long it takes to stabilize. Compare the results with the manufacturer?s specifications.

4) Turn off the light source for five minutes and repeat the test. Have the results changed? If so, the power level of the light source is inconsistent from use to use.

5) If possible, move the test set from a warm to a cold environment (or from cold to warm) and repeat the test. Does this affect the results? If so, the light source is not stable in its output over the temperature range where it is likely to be used.

There are several other points about light sources to keep in mind:

- Some light sources can be adjusted to maintain a specific power level. Testing for drift is still necessary with these

devices.

- Be careful to examine the jumper running from the light source to the crossconnect panel. If the jumper is under

tension, the light level being transmitted may change.

- When disconnecting the light source, disconnect at the power-meter end. The photo detector in the power meter is larger than the fiber, and so the measurement is not affected. The coupling at the light-source end is critical, however; to obtain a repeatable measurement, the alignment between light source and fiber must not be disturbed.

- Have a spare set of batteries; low battery power can affect readings.

Click here to enlarge image

By connecting an optical light source to a power meter via a jumper, you can test for power drift when you turn on the light source.

Larry Johnson is president at The Light Brigade Inc., Kent, WA.

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