If an optical device is inserted into a setup, some of the optical power may be lost in the device or at optical interfaces. Some examples:
- A fiber connector, a mechanical splice or a fusion splice may be used to connect two fibers, instead of having a single continuous fiber. Some of the optical power will be lost due to non-perfect interfaces, not exactly matching effective mode areas or similar factors.
- The larger amount of insertion loss may be intentionally inserted in the form of a fiber-optic attenuator.
- A Faraday isolator is inserted after the output of a laser in order to prevent it against back-reflections. Some power is lost at imperfect anti-reflection coatings of the isolator and possibly by parasitic absorption or scattering in the optical elements.
The insertion loss (or attenuation) is usually specified in decibels, calculated as 10 times the logarithm of base 10 of the ratio of input and output powers. For fiber connectors, for example, it is often of the order of 0.5 dB. High-quality fusion splices may reach values like 0.02 dB.
For high-power devices, a high insertion loss is often unwanted not only due to the power loss but also because of possibly strong heating effects resulting from absorbed light.
Optical Loss Testers
For use mainly in optical fiber communications, there are optical loss testers with which insertion losses can be quite conveniently measured.
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