Does the optical fiber have to be used in pairs? Yes, in the second half of the question, do you mean to transmit and receive light on one optical fiber? This is possible. China Telecom's 1600G backbone optical fiber network is like this.

The most fundamental difference between single-mode fiber optic transceivers and multi-mode fiber optic transceivers is the transmission distance. The multi-mode optical fiber transceiver is a multi-node and multi-port signal transmission in the working mode, so the signal distance transmission is relatively short, but it is more convenient, and it is unnecessary to use the construction of the local intranet. Single fiber is a single node transmission, so it is suitable for the transmission of long-distance trunk lines and constitutes the construction of a cross-metropolitan area network. In terms of price, single-mode is more expensive than multi-mode.

Single-mode optical fiber transceiver: transmission distance of 20 kilometers to 120 kilometers

Multimode fiber optic transceiver: transmission distance from 2 kilometers to 5 kilometers

Single-fiber fiber optic transceiver: The data received and sent is transmitted on one fiber

Dual-fiber fiber optic transceiver: The data received and sent is transmitted on a pair of fibers