The concept of Course WDM (CWDM) emerged from the combination of the production of full spectrum ( low water content) G.652C and G.652D fibers, the development of relatively inexpensive optical sources, and the desire to have low cost optical links operating in access networks and local area networks.
in 2002 the ITU-T released recommendation G.694.2 which defined the spectral grid for CWDM. the CWDM grid is made up of 18 wavelengths defined within the range 1270nm to 1610 nm (O- through L-band) spaced by 20 nm with wavelength drift tolerances of +/-2nm. this can be achieved with inexpensive light sources that are not temperature controlled.
For more detail you can refer –
Y Liu, A R Davies, J D Ingham, R V Penty, and I H White, “uncoded DBR laser directly modulated at 3.125 Gbps as an athermal transmitter for low cost WDM systems,” IEEE Photonics Technol. Lett., vol. 17, pp2026-2028, Oct 2005.
Optical Fiber communications by Gerd Keiser, Mc Graw Hills