I think, it is about simplex communication and duplex communication. Communication over a single wavelength is one-way communication. But most of today’s communication systems need to be two-way, that is duplex communication. Two wavelengths are required to be transmitted and received in duplex communication. This can be achieved on the same fiber using different wavelengths or by using two optical fibers. Dual-fiber transceivers are developed to support duplex communication using two optical fibers, while CWDM transceivers handle two different wavelengths in one fiber. CWDM transceivers support wavelengths from 1270nm to 1610nm in steps of 20nm, that is 1270nm, 1290nm, 1310nm, 1330nm and so on…
The difference between CWDM and ordinary dual-fiber optic transceiver mainly consists in the modes of operation.
Ordinary optical transceiver belongs to the photoelectric conversion device and also the active optical module. Each module has one receiving and one transmitting ports, and the transmitting port includes a laser.
CWDM is a light-to-optical conversion device. It belongs to a passive module and does not emit laser light. Generally, it uses light plane wave-guide (PLC) technology to split a beam of light into several beams.
CWDM optical module is able to integrate optical signals with different wavelengths by using CWDM technology and an external wavelength division multiplexer so that they can be transmitted by one fiber. At the same time, the receiving end needs to use the wave division demultiplexer to decompose the complex light signals.