OFDM

On a WDM (Wavelength Division Multiplex) network the flows are assigned to paths which are in turn assigned to wavelengths in this case known as light-paths. However, on this model, the full wavelength capacity is assigned to one end-node pair even if the traffic between the pair does not require the wavelength transmission capacity, therefore part of the bandwidth is wasted with no traffic. (ref)

Orthogonal Frequency Division Multiplexing (OFDM)  is one of the possible methods for signal multiplexing. The multiplexing is done based on the division of a signal into several different channels each of those operating in a different frequency. The multiple channels can operate within close frequency levels without suffering interference as cross-talk. (ref)

In Optical networks the research of future application of OFDM is becoming popular because it enables flexibility and scalability on a network, by dividing a wavelength into smaller channels known as sub-wavelengths.

image extracted from Jinno, 2009

Jinno, M.; Takara, H.; Kozicki, B.; Tsukishima, Y.; Sone, Y. & Matsuoka, S. (2009), 'Spectrum-efficient and scalable elastic optical path network: architecture, benefits, and enabling technologies', Communications Magazine, IEEE 47(11), 66--73.