Reference Guide To Fiber Optics
|Topic: Fiber Optic Network Optical Wavelength Transmission Bands||Table of Contents: The FOA Reference Guide To Fiber Optics|
Fiber Optic Network Optical Wavelength Transmission Bands
As fiber optic networks have developed for longer distances, higher speeds and wavelength-division multiplexing (WDM), fibers have been used in new wavelength ranges, now called "bands," where fiber and transmission equipment can operate more efficiently. Singlemode fiber transmission began in the "O-band" just above the cutoff wavelength of the SM fiber developed to take advantage of the lower loss of the glass fiber at longer wavelengths and availablility of 1310 nm diode lasers. (Originally SM fibers were developed for 850 nm lasers where the fiber core was about half what it is for today's conventional SM fiber (5 microns as opposed to 8-9 microns at 1310 nm.)
To take advantage of the lower loss at 1550 nm, fiber was developed for the C-band. As links became longer and fiber amplifiers began being used instead of optical-to-electronic-to-optical repeaters, the C-band became more important. With the advent of DWDM (dense wavelength-division multiplexing) which allowed multiple signals to share a single fiber, use of this band was expanded. Development of new fiber amplifiers (Raman and thullium-doped) promise to expand DWDM upward to the L-band.
Several low-cost versions of WDM are in use, generally referred to as Coarse WDM or CWDM. Most do not work over long distances so do not require amplification, broading the wavelength choice. The most popular is FTTH PON systems, sending signals downstream to users at 1490 nm and using low cost 1310 nm transmission upstream. Early PON systems also use 1550 downstream for TV, but that is being replaced by IPTV on the downstream digital signal at 1490 nm. Other systems use a combination of S, C and L bands to carry signals because of the lower attenuation of the fiber. Some systems even use lasers at 20 nm spacings over the complete range of 1260 to 1660 nm but only with low water peak fibers.
Manufacturers have been able to make fiber with low-water peaks, opening up a new transmission band (E-band), but it has not yet proven useful except for CWDM. It is probably mostly useful as an extension of the O-band but few applications have been proposed and it is very energy-intensive for manufacture.
DWDM Band Wavelength Range