it sounds like a joke that we use a device cost less than 1 $ to test fiber cable. a LED(Low Emitting Diode) with 2 1.5V AAA bateries while solving fiber cable problem..
anybody knows that in information technology filed. there are 2 types of fiber cable, multi-mode fiber and single mode fiber cable.
multimode is what i we use very often as it is suit for low cost, short distance connection. usually less than 5 Kilometer.
it use LED device to send/receive the light.

yesterday some one ask me why dont use a more professionall device to test the fiber /coverter. because i told him that we were using a very chep device: a LED ligher to test if a fiber cable is not broken. if people can see the light from other end.
he suggested some professional device and worries if human eyes may hurt from the light which i don't think so. but since more professional people who do fiber canble engineering also use the LED ligher to test the fiber cable. we will not purchasing that equipment.
it is also possible to see light come from a FX-TX device when checking the work condition.
this article only applies to multi-mode devices. regarding single mode device, it using laser rather than LED to transfer signal. you may get hurt if look into the light.