How to test the light path impassability in wlan project?

Generally speaking, an optical fiber can send and receive data at the same time. Generally, only one of the two optical fibers is connected, and the other is for standby. In WLAN project, it is generally possible to test whether there is an optical signal at ONU end, and the optical value is also very important. If the light is too attenuated, it will not be used. The light value is generally between-15 and -20.