Wednesday, February 19, 2020

Fiber optic power meters

The power measurement requires a power meter with an adapter that fits the fiber optic connector on the cable being tested, and if you are testing a transmitter, a good fiber optic cable (having a size of adequate fiber, since the coupled power depends on the size of the fiber core) and some help from the network electronics to turn on the transmitter. Remember that when measuring power, the meter must be set at the appropriate wavelength and range (generally dBm, sometimes microwatts, but never "dB" [this unit of measurement is a relative power range that is used only to test the lost]).
In order to measure the power, connect the meter to the cable attached to the source that has the output you want to measure. It can be in the receiver to measure the power of the receiver, or you can use a patch cord reference test lead (which has been tested and known to work) that is connected to the transmitter to measure the output power. Turn on the transmitter/source and allow a few minutes for it to stabilize. Set the power meter for the compatible wavelength and observe the power indicated by the meter. Compare it to the power specified for the system and make sure it is sufficient, but not too much.

Optical loss or insertion loss

Optical loss is the main performance parameter of most fiber optic components. For fiber, it consists of the loss per unit length or attenuation coefficient. For connectors, it involves the loss of connection when it joins another connector. For the cables, it consists in the total loss of the cable components, among which are the connectors, the fibers, the splices and any other component in the cable run being tested. We will use wires to illustrate the insertion loss, and then we will look at other components.
Cable loss is the difference between the power coupled in a cable to the end of the transmitter and what comes out to the end of the receiver. The loss test requires the measurement of the total amount of optical power lost in a cable (including fiber attenuation, connection loss and splice loss) with a light source and power meter (LSPM ) fiber optic or optical loss test equipment (OLTS). The loss test is performed at wavelengths suitable for the fiber and its use. Generally, multimode fiber is tested at 850 nm, and optionally, at 1300 nm with LED sources.
Most tests are performed on preconnected cables, either connection cables (patchcords) or installed cable networks. But fiber manufacturers test each fiber to check for loss, in order to calculate its attenuation coefficient. Connector manufacturers test many connectors to obtain an average value of the loss that the connector will have when it is terminated in the fibers. Manufacturers of other components also test the loss of their components to verify their performance.

The measurement of the insertion loss is made by connecting the cable under test to good reference cables with a calibrated launch power that becomes the loss reference "0 dB". Why do you need reference cables to measure the loss? The test with reference cables at each end stimulates the network of cables with connection cables that are connected to a transmission device. You need a cable to measure the output power of the source for the calibration of the loss reference "0 dB".
Also, in order to measure the loss of the connectors at the end of a cable, you must attach them to a similar connector and know that it is good. This is an important point that is often not fully understood. When we talk about connector loss, we really mean loss of "connection", that is, the loss of a pair of connected connectors. Therefore, to measure the connectors, these must be attached to reference connectors, which must be high quality connectors so as not to negatively affect the measured loss when attached to an unknown connector.

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