Re: It's about wavelength as opposed to transceivers.
"That sounds interesting. Are you meaning there are rental agreements for running light through fibre?"
Yes - any modern DWDM system should have the ability to accept what are known as 'alien' wavelengths.
These are 'coloured' optical signals supplied from another carrier and then, after level adjustment, are combined in the optical filters with wavelengths generated by the host system and sent to line. The only processing done to these alien wavelengths by the host system is analogue adjustments, eg., attenuation, amplification, filtering. The host system has no access to the data carried on the alien wavelengths as they are never demodulated into an electrical signal within the host.
This feature is useful if you need to get several wavelengths between data centres in different cities, but don't want to go to the trouble of laying and maintaining your own cross-country fibre network and duplicate what some other carrier has already built.
So, in your data centres you install a base level DWDM box with one or more transponders and no amplifiers. The coloured output from these travels on dark fibre to a national carrier's site, where they are loaded onto a cross-country DWDM system as alien wavelengths - there obviously is a point at which you might decide it's better economics to build your own full DWDM system and lease dark fibre cross-country, but adding optical filters, amplifiers and intermediate amplifier sites adds considerably to the cost.