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I think you may use LASER

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Posted by HamidAliKhan

Use LASER(light amplification stimulated emission radiation)

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Posted by Hash007

Yes Laser source needs to be used.

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Posted by waqqas1

For single mode fiber, the loss is about 0.5 dB per km for 1300 nm sources.So a laser light could be used to measure it.

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Posted by sagitraz

Laser source needs to be used.

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Posted by dandelo