Tag Archives: CWDM

Evolution of Optical Wavelength Bands

As fiber optic networks have developed for higher speeds, longer distances, and wavelength-division multiplexing (WDM), fibers have been used in new wavelength ranges, namely “bands”. Fiber transmission bands have been defined and standardized, from the original O-band to the U/XL-bands. This article will mainly illustrate the evolution of the typical fiber transmission bands used for different optical telecom systems.

Among these bands, the O-band, also called the Original-band, was the first band used in optical telecommunication because of the small pulse broadening (small dispersion); Single-mode 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 availability of 1310nm diode lasers.


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. The E-band (water-peak band) has not yet proven useful except for CWDM. It is probably mostly used as an extension of the O-band but few applications have been proposed and it is very energy-intensive for manufacture. The E-band and U/XL-bands usually are avoided because they correspond to high transmission loss regions.

To take advantage of the lower loss at wavelength of 1550nm, fiber was then developed for the C-band. The C-band is commonly used along with the development of ultra-long distance transmission with EDFA and WDM technologies. As transmission distances 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 enables multiple signals to share a single fiber, the use of C-band was expanded.

With the development of fiber amplifiers (Raman and thullium-doped), DWDM system was expanded upward to 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 multiple wavelengths simultaneously, which may lead to much attenuation. Therefore optical amplification technology is introduced.

Despite great expectations, the number of installed systems using all-Raman solutions worldwide can be counted on one hand. 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 1485nm.

Although CWDM is now considered as a low-cost version of WDM that has been in use, most do not work over long distances. The most popular is FTTH PON system, sending signals downstream to users at 1490nm (in S-band) and using low cost 1310nm transmission upstream. Early PON systems also use 1550 downstream for TV, but that is being replaced by IPTV on the downstream digital signal at 1490nm. Other systems use a combination of S, C and L bands to carry signals because of the lower attenuation of fibers. Some systems even use lasers at 20nm spacing over the complete range of 1260nm to 1660nm but only with low water peak fibers.

Although various wavelength bands of the O-, S-, C- and L- bands have come into use with the explosive expansion of the traffic in recent years, the optical fiber amplifiers for the O- and S-band wavelengths were not realized for many years because of many technical hurdles. C- and L-band most commonly used in fiber optic networks will play more and more important roles in optical transmission system with the growth of FTTH applications.

Introduction for Optical Add-Drop Multiplexer

The evolution of single wavelength point-to-point transmission lines to wavelength division multiplexing optical networks arose the need to separate/route different wavelength channels. What kind of device can meet this need? The answer is optical add-drop multiplexer (OADM). This device is used for multiplexing and routing different channels of light into or out of a single mode fiber (SMF). “Add” means adding one or more new wavelength channels to an existing multi-wavelength WDM signal. “Drop” means dropping or removing one or more channels, passing those signals to another network path. OADM is particularly important in metropolitan WDM lightwave services where offices or sites can be connected by different add-drop channels. It can increase the network flexibility along the optical link.


A traditional OADM has three stages: an optical multiplexer, and optical demultiplexer, and between them a method of reconfiguring the paths and a set of ports for adding and dropping signals. The multiplexer is to couple two or more wavelengths into the same fiber. The reconfiguration can be achieved by a fiber patch or by optical switches which direct the wavelengths to the add ports. The demultiplexer separates wavelengths in a fiber and direct them into many fibers.


Add-Drop Configurations

To realize the configurations performance of adding or dropping functions, planar and fiber technology could be used. No matter what kind of devices, several factors should be considered, such as low insertion loss, high isolation, polarization insensitivity, and the cost. Planar devices provide compact solutions with the possibility of adding or dropping many channels using only one integrated optical circuit with arrayed-waveguide-grating or waveguide-grating-router. But they have the main disadvantages of high insertion loss and polarisation dependence. Comparatively, all-fiber devices are more attractive because of the low insertion losses, polarisation insensitivity and ease of coupling between output and input of the optical network by using simple splices and pigtails. But these devices are sensitive to environmental variations. There is another kind of devices based on free space optics (micro mirrors and gratings) also used to perform add-drop operations well. However, these devices have high insertion losses and cost much. So thin film filter devices have been used for multiplexing/demultiplexing applications.


OADM have passive modes with fixed wavelength (COADM) and active modes with dynamic wavelength (ROADM). In fixed wavelength OADM, the wavelength has been selected and remains the same until people change it. In dynamic wavelength OADM, the wavelengths could be directed selectively without changing its physical configuration. Different types have different functions. For the OADM with fixed wavelengths, the node’s routing is determined but is not flexible. For the OADM with dynamic wavelengths, it’s more flexible but cost too much.

OADM selectively drops a required wavelength from multiple wavelengths in a fiber and adds the same wavelength into the data flow but with different data content. It’s used in both CWDM systems and DWDM systems. CWDM OADM is designed for the CWDM passive optical systems. It can add/drop wavelengths from multiple fibers onto one optical fiber. And DWDM OADM is designed to add/drop one multiple DWDM channels into one or two fibers. Although OADM is good enough to use, it is still under improvement. In the future,  it will be more cost-effective.