What to consider when doing 10G over CWDM ?
Today, the majority of new installed links operate at 10G Ethernet. Here CWDM becomes a little bit more tricky. Beside the fiber attenuation you also have to include the chromatic dispersion of your fiber in your calculation. Chromatic dispersion is the time variance which occurs to a single pulse of your signal - the signal is stretched on the fiber transmission path due the dispersion characteristics of the transporting fiber (Chromatic Dispersion is "the spreading of a light pulse per unit source spectrum width in an optical fibre caused by the different group velocities of the different wavelengths composing the source spectrum." )
Chromatic dispersion exists always but it becomes more and more important the higher the link speed is. If we do our math and take a 15km G.652 fiber  we will end up with the following result for the dispersion (chromatic dispersion value * link length):
|wavelength||chromatic dispersion||fiber attenuation|
|1310nm||0 ps/nm||5,25 dB|
|1490nm||200 ps/nm*||3,6 dB|
|1550nm||270 ps/nm||3 dB|
; Dλ = S0 / 4 * ( λ - (λ04 / λ3))
matching this with the dispersion tolerance of the CWDM lasers used in various CWDM SFP+
|partnumber||description||powerbudget||dispersion tolerance||max. distance||limiting factor|
|DFB based CWDM SFP+||13dB||0 - 200ps/nm||8 to 30km||dispersion|
|EML based CWDM SFP+||14dB||0 - 800ps/nm||40km||attenuation|
|EML based CWDM SFP+||23dB||0 - 1600ps/nm||70km||attenuation|
we come to the following conclusion for your 15km G.652 fiber:
- you can only use CWDM channel 1270nm to 1490nm when taking a DFB based CWDM SFP+
- you can use the EML based CWDM SFP+ with 14dB power budget and have the full 1270nm to 1610nm CWDM support.
Disclaimer: I did not use any insertion loss of the CWDM Mux/Demux in my calculation - a rule of thumb for a 8-channel CWDM Mux is 2dB insertion loss per end.
 G.652 compliant Corning fiber datasheet
 ITU-T G.652 Characteristics of a single-mode optical fibre and cable