This for additional security, effectively hiding the entire internal network from the outside world. This depends on the DRAM of your router. It has a typical core diameter of 5 to 10 microns. The light source is a laser. It has a typical core diameter of 50-100 microns.
Duplex fiber is a two fiber cable suitable for data transmission.
Typically, one fiber is used to transmit and the other is used to receive. It can be a singlemode or multimode fiber. Simplex fiber is composed of a single fiber cable. It can be a singlemode or multimode fiber. It is an older fiber 802.3 standard for connecting only hubs up to 1 KM apart.