What is the 'water peak' in optical fiber and how does it impact transmission at specific wavelengths?

The 'water peak' in optical fiber refers to the increased absorption of light at specific wavelengths due to the presence of hydroxyl (OH-) ions within the glass material of the fiber. These OH- ions are impurities that are introduced during the manufacturing process. The water peak is most prominent around the 1383 nm wavelength, but also exists to a lesser extent around 950 nm and 1240 nm. These peaks represent wavelengths where the OH- ions strongly absorb light energy. The impact of the water peak on transmission is that it causes a significant increase in attenuation, or signal loss, at the affected wavelengths. This increased attenuation reduces the distance that optical signals can travel before they need to be amplified or regenerated. In older optical fibers, the water peak was a significant obstacle to using the entire low-loss window of the fiber. These fibers were not optimized for transmission in the 1360-1460 nm range due to the high attenuation. Modern optical fibers, known as low-water-peak fibers (e.g., ITU-T G.652.D), are manufactured with a process that significantly reduces the OH- ion content. This reduces the size of the water peak, allowing for transmission in the 1360-1460 nm range. These fibers are suitable for use in coarse wavelength division multiplexing (CWDM) systems, which utilize multiple wavelengths across the entire low-loss window. Therefore, the water peak's presence and magnitude directly impact the usable bandwidth and transmission distance at specific wavelengths, and low-water-peak fibers are preferred in modern applications where wide spectrum utilization is required. For example, attempting to transmit a signal at 1383 nm through standard fiber would result in significant signal degradation and a much shorter transmission distance compared to transmitting at 1310 nm or 1550 nm.