In the optical fiber communications system, several transmission bands have been defined and standardized, from the original O-band to the U/XL-band. The E- and U/XL-bands have typically been avoided because they have high transmission loss regions. The E-band represents the water peak region, while the U/XL-band resides at the very end of the transmission window for silica glass.
Intercity and metro ring fiber already carry signals on multiple wavelengths to increase bandwidth. Fibers entering the home will soon do the same. Now several types of optical telecom systems have been developed, some based on time division multiplexing (TDM) and others on wavelength division multiplexing (WDM). This article may represent the evolution of optical wavelength bands mainly by describing these two high-performance systems.
Wavelength Division Multiplexing (WDM)
Dense Wavelength Division Multiplexing
DWDM systems were developed to deal with the rising bandwidth needs of backbone optical networks. The narrow spacing (usually 0.2 nm) between wavelength bands increases the number of wavelengths and enables data rates of several Terabits per second (Tbps) in a single fiber.
These systems were first developed for laser-light wavelengths in the optical C-band, and later in the L-band, leveraging the wavelengths with the lowest attenuation rates in glass fiber as well as the possibility of optical amplification. Erbium-doped fiber amplifiers (EDFAs, which work at these wavelengths) are a key enabling technology for these systems. Because WDM systems use many wavelengths at the same time, which may lead to much attenuation. Therefore optical amplification technology is introduced. Raman amplification and erbium-doped fiber amplifiers are two common types used in WDM systems.
In order to meet the demand for “unlimited bandwidth,” it was believed that DWDM would have to be extended to more bands. In the future, however, the L-band will also prove to be useful. Because EDFAs are less efficient in the L-band, the use of Raman amplification technology will be re-addressed, with related pumping wavelengths close to 1485 nm.
Coarse Wave Division Multiplexing
CWDM is the low-cost version of WDM. Generally these systems are not amplified and therefore have limited range. They typically use less expensive light sources that are not temperature stabilized. Larger gaps between wavelengths are necessary, usually 20 nm. Of course, this reduces the number of wavelengths that can be used and thus also reduces the total available bandwidth.
Current systems use the S-, C- and L-bands because these bands inhabit the natural region for low optical losses in glass fiber. Although extension into the O and E-band (1310 nm to 1450 nm) is possible, system reach (the distance the light can travel in fiber and still provide good signal without amplification) will suffer as a result of losses incurred by use of the 1310 nm region in modern fibers.
Time Division Multiplexing
TDM systems use either one wavelength band or two (with one wavelength band allocated to each direction). TDM solutions are currently in the spotlight with the deployment of fiber-to-the-home (FTTH) technologies. Both EPON and GPON are TDM systems. The standard bandwidth allocation for GPON requires between 1260 and 1360 nm upstream, 1440 to 1500 nm downstream, and 1550 to 1560 nm for cable-TV video.
To meet the rise in bandwidth demand, these systems will require upgrading. Some predict that TDM and CWDM (or even DWDM) will have to coexist in the same installed network fibers. To achieve this, work is underway within the standardization bodies to define filters that block non-GPON wavelengths to currently installed customers. This will require the CWDM portion to use wavelength bands far away from those reserved for GPON. Consequently, they will have to use the L-band or the C- and L-bands and provided video is not used.
In each case, sufficient performance has been demonstrated to ensure high performance for today’s and tomorrow’s systems. From this article, we know that the original O-band hasn’t satisfied the rapid development of high bandwidth anymore. And the evolution of optical bands just means more and more wavelength bands will be called for. In the future, with the growth of FTTH applications, there is no doubt that C- and L-bands will play more and more important roles in optical transmission systems.