this kind of optic fiber is probably only plastic so for a full DIY , you can use also fishing wire with a big power led or an old chistmass tree optic fiber lighted.....
@kimot in theory you might be right, but in practice... it just works. The propability of collisions is extremely low and this is, as far as I know, the only problem you can worry about. After all, it was not my idea to use MySensors with RS485:
https://www.mysensors.org/build/rs485
