定义：插入某一器件产生的功率损耗。

如果将某一光学器件插入到装置中，可能会在装置中或者光学表面产生一些损耗。例如： 

• 光纤连接器，是一个机械接头或者熔接接头来连接两个光纤，而不是只用一根连续的光纤。由于有效模式面积不匹配或者类似效应，在非理想的表面会引起一些光学损耗。 
• 在激光器的输出后面放置法拉第隔离器，是为了消除背向反射。在隔离器非理想抗反射涂层中会损耗一些功率，通常是由于寄生吸收或者散着。 

插入损耗通常由分贝表征，即输出功率与输入功率比值取以10为底的对数后再乘以10. 例如，光纤连接器的插入损耗在0.5 dB左右。高质量的熔接接头可以使该值降为0.02 dB。 

对于高功率装置，通常不希望插入损耗很大，一方面功率存在损耗，另外也会由于吸收的光会产生很强的热效应。 

Definition: power losses due to insertion of a device

Units: %, dB

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.

A convenient method for measuring insertion loss is optical time-domain reflectometry. With that, the insertion loss of multiple optical elements along a fiber can be measured separately.

Optical Loss Testers

For use mainly in optical fiber communications, there are optical loss testers with which insertion losses can be quite conveniently measured.