Why is the E band considered undefined for fiber-optic communications?

The E band is often considered undefined for fiber-optic communications primarily due to older fiber-cable usage that suffers from unacceptable water-peak attenuation. Water-peak attenuation refers to the loss of signal strength in optical fibers caused by the presence of hydroxyl (OH-) ions in the glass, which lead to absorption peaks in certain wavelength regions, including parts of the E band. This phenomenon results in excessive signal loss, making it challenging to achieve reliable communication at those wavelengths.

Older generations of optical fiber may not have been optimized to minimize these absorption characteristics, leading to significant limitations in their performance for not just E band usage but also impacting overall transmission quality. Thus, the problematic characteristics associated with the water-peak region render the E band unfit for practical applications in fiber-optic communications at this time.