Difference between CWDM and DWDM modules

CWDM modules-Coarse Wavelength Division Multiplexer

The figure-1 depicts CWDM based fiber system. As shown it is used to transmit 4 different applications using single optical finer instead of pair of fiber cables. Different applications are using different light wavelengths which helps in multiplexing and de-multiplexing applications at transmit and receive end.

DWDM modules -Dense Wavelength Division Multiplexer

The figure-2 depicts DWDM system consisting of transmitter and receiver parts. Typically DWDM consists of MUX, DEMUX, isolators, circulators, EDFA (Erbium Doped Fiber Amplifier) etc. 
DWDM system basics 

Figure-3 depicts spectrum of CWDM and DWDM technologies. As shown DWDM spectrum will have tighter wavelength spacing between the channels compare to CWDM counterpart. Following table compares the two.

Full formCoarse Wavelength Division Multiplexing,WDM system having less than 8 active wavelengths per optical fiberDense Wavelength Division Multiplexing, WDM system having more than 8 active wavelengths per optical fiber
characteristicDefined by wavelengthsDefined by frequencies
Distanceshort range communicationlong range communication
Frequenciesuses wide range frequenciesuses narrow range frequencies
Wavelength spacingmoreless, hence can pack 40+ channels compare to CWDM in the same frequency range
Amplificationlight signal is not amplified herelight signal amplification can be used here
DriftWavelength drift is possiblePrecision lasers are needed to keep channels on the target
Spectrum utilizationBreaks the spectrum into big chunksdices the spectrum into small pieces
No. of active wavelengths per fiberFewer than 8More than 8