There are several windows in fiber optic communication

Optical transmission windows are specific wavelength ranges where light travels through fiber with minimal attenuation (signal loss) and dispersion (distortion). To fully leverage its capabilities, it's essential to understand three foundational concepts: Bandwidth, Wavelength, and Optical Windows. Bandwidth refers to the capacity of a fiber optic cable to transmit data — much. Generally speaking, Silica based glass optical fibers can transmit 250nm to 2000nm wavelengths. We have heard about the O-bands, E-bands, L-bands etc. [pdf]