The Rise of the C Band in Fibre Optic Networks

The C band is usually defined as consisting of wavelengths in the range from 1530 to 1565 nanometers. Virtually all long haul fibre optic telecommunication transmission uses exclusively this band for data payloads. The graph below shows why. Optical loss or attenuation in the infrared frequencies used by most lasers achieves a global minimum in the C band. The solid bar labelled 'experimental' shows actual measured db loss performance. The other lines represent a theoretical decomposition of overall intensity loss into different factors. 

• Waveguide imperfections are geometric flaws in the fibre due to the production process. These flaws lead to light scattering. An example are irregular surfaces at the nanoscale. It could be surface of the optical core or the cladding that shields it. Fore example, if the optical core is not perfectly round, and it never is, then some light will either escape or be reflected back down the fibre in the opposite direction. For a detailed analysis, click here for an academic paper. Below is a photo of the geometric imperfections of an optical core manufactured on earth. Most of these imperfections disappear when fibre is produced in orbit. Gravity plays a big role. These waveguide imperfections are estimates to be responsible for about 10% of optical power loss. ZBLAN is a company trademark. 

• Rayleigh scattering refers to light dispersion caused by tiny optical core imperfections that are less than 1/10th of the light's wavelength.. About 96% of optical power loss is due to Rayleigh scattering. As the name suggests, the light is not absorbed in Rayleigh scattering, but deflected either backwards down the fibre or to the sides where it escapes the optical core. OTDRs use Rayleigh scattering to measure optical latency including RTD (round trip delay). 
• Both infrared and ultraviolet absorption are minimized in the C band. 

Because long haul optical fibre traverses long distances, it must be optically amplified using erbium-doped fibre. These regen huts as they called on the telecom street represent significant capex and opex costs. The lower the optical attenuation, the wider the amplifier spacings and the lower is both capex and operating expense. So the C band is ideal for long haul fibre optic networks. 

However, the C band does experience chromatic dispersion. All laser light pulses contain a range of frequencies. Chromatic dispersion means that light traveling in the C band will move at different speeds depending on the frequency. So every laser pulse starts out like a horizontal row of marching soldiers on a battlefield. But as they march they lose alignment and eventually end up as a long single file queue. Optical receivers will be fooled into thinking they are seeing consecutive light pulses when there is just one. Or the light becomes so smeared that all time slots are interpreted as zeros! These digital errors will add up and produce unintelligible digital garbage. The solution, although initially expensive in terms of equipment, was to use computing power to correct the optical receiver's digital errors. Digital signal processing is a group of algorithms hard wired into circuitry that allow these corrections to be made very quickly with no discernible latency penalty. One algorithm is forward error correction which involves padding the data payload with redundant bits that can be used to detect and correct errors. For details on forward error correction, click here. There are many forward error correction versions including ones that use thresholds to determine if a bit is an error and others that assign  probabilities that a given bit is an error. 

There is an extended range of the original C Band that is known as the Super C band and is defined as 1524 nanometers to 1572 nanometers. The original C band had 3.2 Terahertz of spectrum where Super has 6.1 Terahertz. Improvements in networking technology has made the Super C band the effective C band standard for which most long haul DWDM gear is tailored.