What is the ‘favorite’ color for optical communications? The answer is…there are several. Depending on the application, there are three common ‘color bands’. The ‘colors’ used in fiber optic communications are typically identified by their wavelength. Short length fiber spans typically use one color band, medium distance another, and long distance links yet another band.
You may remember the color spectrum from science class when using a prism or simply observing a rainbow. This spectrum is the visible light observable by the human eye. Light is just one type of energy represented on what is known as the “electromagnetic spectrum.” This spectrum also includes x-rays, ultraviolet radiation, microwaves, radio, TV, cell phones, and all the other wireless signals. They are simply electromagnetic (EM) radiation of different wavelengths.
The upper band in the illustration above shows the entire sweep of the EM spectrum, from very long wavelengths (more than 1 kilometer!) on the left to extreme short wavelength Gamma and Cosmic rays on the right. The portion of the spectrum composed of light energy, referred to as the Optical Spectrum, is expanded in the lower band of the picture.
The optical spectrum ranges from infrared (IR), at the long wavelength end, to ultraviolet (UV) at the short. The light we are most familiar with is what we can see, known as the visible spectrum. Our eyes are sensitive to light in the range of about 400 nanometers to 700 nanometers, ranging from blue/violet to red. The human eye is designed to operate in the same region as the brightest output of the sun. Perhaps if the planet we call home revolved around a star with a different bright region, we would be able to see more of the IR or UV spectrum.
COLOR ‘WINDOWS’ IN FIBER OPTICS
For communications applications in glass optical fibers, light in the infrared region, which has wavelengths longer than visible light, is used. This portion of the optical spectrum is used because the attenuation in fiber is lowest in this range. Attenuation of glass optical fiber is caused by two factors: scattering and absorption.
Scattering (referred to as Rayleigh Scattering) is caused by light bouncing off atoms or molecules in the glass. As shown in the graphic below, attenuation due to scattering is much lower at the long wavelength end of the optical spectrum than the short wavelength end.
Absorption occurs due to small amounts of water components, hydroxide ions and hydroxyl groups (typically referred to as OH–) captured within the glass. The most significant OH– losses occur around 950nm, 1250nm and 1380nm.
There are three popular wavelength ranges or ‘windows’ used for fiber optic communications: 850nm, 1310nm and 1550nm. The determining factors for these popular wavelengths for fiber optic transmission are:
- Longer wavelengths in the infrared for lower loss in the glass fiber
- Avoidance of absorption peaks
- Practicality of building optical sources and receivers at various wavelengths
Factor 1, above, would suggest that all transmission is best done in the 1550nm window, where the Rayleigh scattering losses are lowest. However, LED and VCSEL (vertical cavity surface emitting laser) sources can currently be manufactured in high volume at very low cost in the 850nm window. The higher attenuation due to scattering limits the reach of transceivers in this window, therefore, 850nm wavelength transmission dominate short reach applications. Long distance applications (50km+) are owned by transceivers utilizing the 1550nm window as the scattering losses are simply too high to achieve those distances at the shorter wavelengths. Achieving these distances comes at a price. Sources such as Distributed FeedBack (DFB) and Externally Modulated Lasers (EML) are not easily fabricated in anything but an edge-emitting semiconductor structure, greatly increasing the manufactured cost of these devices. On the receiving side, sophisticated receivers such as Avalanche Photo-Diodes (APDs) also drive up cost. There is a middle ground…the 1310nm window. The availability of inexpensive Fabry-Perot laser sources and simple Positive Intrinsic Negative (PIN) receivers to provide reliable transmission covering a range of 1km to about 50km has made the 1310nm window the third popular alternative.
This article has focused on the three major transmission windows used for fiber optic communications. In the past decade, use of wavelength-division multiplexing (WDM), either DWDM (dense WDM) or CWDM (coarse WDM) has become popular. These systems use multiple ‘hues’ within each the 1310nm or 1550nm window. Each of these shades of color can be multiplexed onto a single fiber. Look for a future article from FluxLight going into more detail about these richly colored alternatives.
For More Information
For more information about FluxLight’s many colored transceivers, please visit https://www.fluxlight.com/transceivers-1/?cat=1071