The difference in light level between what the optical transmitter puts out to the minimum level of light required by the optical receiver to work correctly is called the optical power budget. If the optical path (including fibres, connectors, splices, and any other components) has less optical loss than allowed in the optical power budget then the communications link will function correctly. If the optical path has more loss than the optical power budget then the link will not work.

When calculating the power budget for a new link it is necessary to allow a margin above the minimum light level required by the receiver to allow for the changes that occur during the life of the link, including equipment aging and optical path changes.

Light Source and Power Meter Tests are done by putting a known optical level (the Light Source) at one end of a link and then measuring the level of light received at the other end with the power level. The difference between the Light Source output and the received power level at the far end is the loss through the optical path, and is often referred to as the link loss or the insertion loss of the path, or simply as the attenuation.

One of the fundaments of Light Source and Power Meter testing is to know the level of light that you Light Source is putting out. This is achieved by placing the Light Source and Power Meter side by side and connecting them together via a patchcord. Light sources have a front connection that is similar (or often the same) as a standard optical connector. Most modern power meters though accept a standard optical connector however is actually only an adapter that holds the fibre in position in front of the detector. The significance of this is that the power meter connection has no connector loss as a standard connection has - the receiver measures the actual light output from the patchcord. When the patchcord is plugged into the link to be tested then the received power measured at the far end will include the loss through the first patchcord/link connection. This is correct as this loss represents the loss that will occur in real loss through the optical transmitter/link connection.

The steps in completing Light Source and Power Meter Measurements are defined in a number of standards or tests procedures and there are some significant variations between them. It is important to ensure that the test method you are using is acceptable to the Manager or client that you are completing the tests for. The following would be a generic test method.

1. Set the equipment up and turn it on. Select the appropriate wavelength for your measurement.
2. Clean connectors and complete the reference measurement and record. On most modern power meters it is possible to set the reference into memory so that all other readings are automatically subtracted from it so that the link loss can be read directly.
3. Connect Light Source at one end and the Power Meter at the other end. Complete measurement.
4. Subtract measured level from reference level to obtain the insertion loss of the link under test. For example, if reference level was -10dBm, and measured level was -24dBm, then there is 14dB loss through the link (Note the different units of dBm and dB).
5. Repeat the test for other fibres as necessary.

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Created: 25 April 1998. Last modified: 25 April 1998.

Copyright ©1998 Edward Hall. All rights reserved.

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