Why is the E band considered undefined in fiber-optics communications?

The E band in fiber-optic communications is considered undefined primarily because it operates at wavelengths that historically have not been standardized for use in optical communication systems. The E band corresponds roughly to wavelengths between 360 nm to 750 nm, which fall under the ultraviolet and visible light spectrum. This range was not originally allocated for optical telecommunication purposes, leading to a lack of suitable infrastructure and equipment designed to operate efficiently within this band.

Over time, fiber optic technology has progressed, and the industry has largely favored other established bands such as the O, E, S, C, and L bands for data transmission due to their well-defined properties and extensive research backing their use. The E band tends to have limited practical application and support, primarily due to the prevalence of older fiber cables that were installed using standards focused on the bands that are now more commonly used.

This is why the presence of substantial outdated fiber cabling impacts the definition and usability of the E band, reinforcing that it lacks the robustness required for modern communication needs.