IL, or insertion loss, is the loss of signal power resulting from inserting a device in a transmission line or optical fiber. RL, or return loss, refers to the loss of signal power reflected to the light source.
No matter for the manufacturing process or installation, IL & RL test matters a lot. For optical cable vendors, the insertion loss and return loss tested should be conformed to a series of corresponding standards. For example, TIA standards specify a maximum fiber connector insertion loss of 0.75dB, which is considered as the worst-case situation. For most fiber connectors in the market, they have a range of 0.3dB to 0.5dB for standard loss and 0.15dB to 0.2dB for low loss. Manufacturers use IL & RL tester/meter to check if the values are within the normal limits so that the end-users could receive qualified products.
For end-users, apart from taking the IL & RL value written in the product specification lists for reference to design the optical link and choose other devices and assemblies based on the reference value, it is also possible to test by yourself if testing tools are available. It helps installers troubleshoot and identify faulty system components. OTDR, OFDR are frequently used techniques used for measuring return loss.