what is WDM – Wavelength Division Multiplexing?

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WDM – Wavelength Division Multiplexing


The starting point of wide-area optical networking was the introduction of the optical amplifier in the early 1990s. Amplifiers enabled both optical transport over longer distances and the ability to compensate for the losses in the optical multiplexer and de-multiplexer units that were needed to create multi-channel systems. WDM systems enabled telecommunications companies to expand the capacity of their network without laying additional fiber. The first WDM systems were two-channel ones that used 1310 nm and 1550 nm wavelengths. Shortly afterwards came multi-channel systems that used the 1550 nm region – where the fiber attenuation is lowest. Depending on their wavelength patterns, WDM systems are typically divided into Coarse Wavelength Division Multiplexing (CWDM)  and Dense Wavelength Division Multiplexing (DWDM).

Infringer’s founding vision is to enable an infinite pool of intelligent bandwidth that the next communications infrastructure is built upon and is well-recognized in the technology segments of optical wavelength division multiplexing (WDM) transport and packet OTN switching. In fine ra pioneered a new approach for networks with photonic integration which provides massive WDM capacity in a small power and space footprint to handle growing bandwidth needs.

Wavelength-division multiplexing (WDM) is a method of combining multiple signals on lasers beams at various in-fared (IR) wavelengthfor transmission along fiber optic media. Each laser is modulated by an independent set of signals. Wavelength-sensitive filters, the IR analog of visible-light color filters, are used at the receiving end. WDM is similar to frequency-division multiplexing (FDM). But instead of taking place at radio frequencies (RF), WDM is done in the IR portion of the electro magnetic spectrum.. Each IR channel carries several RF signals combined by means of FDM or time-division multiplexing (TDM). Each multiplexed IR channel is separated, or de-multiplexed, into the original signals at the destination. Using FDM or TDM in each IR channel in combination with WDM or several IR channels, data in different formats and at different speeds can be transmitted simultaneously on a single fiber. In early WDM systems, there were two IR channels per fiber. At the destination, the IR channels were de-multiplexed by a di-chroic (two-wavelength) filter with a cutoff wavelength approximately midway between the wavelengths of the two channels. It soon became clear that more than two multiplexed IR channels could be de-multiplexed using cascaded dichroic filters, giving rise to coarse wavelength-division multiplexing (cwdm) and dense wavelength-division multiplexing (DWDM). In CWDM, there are usually eight different IR channels, but there can be up to 18. In DWDM, there can be dozens. Because each IR channel carries its own set of multiplexer RF signals, it is theoretically possible to transmit combined data on a single fiber at a total effective speed of several hundred giga bits per second (Gbps). The use of WDM can multiply the effective bandwidth of a fiber optic communications system by a large factor, but its cost must be weighed against the alternative of using multiple fibers bundled into a cable. A fiber optic repeater device called the amplifier can make WDM a cost-effective long-term solution.

Explanation  of WDM (wavelength division multiplexing) 

Wavelength division multiplexing systems can combine signals with multiplexing and split them apart with a de-multiplexer. And with the proper fiber cable, the two can be done simultaneously; moreover, these two devices can also function as an add/drop multiplexer (ADM), i.e. simultaneously adding light beams while dropping other light beams and rerouting them to other destinations and devices. Formerly, such filtering of light beams was done with etalons, devices called Fabry–Pérot interferometers using thin-film-coated optical glass. The first WDM technology was conceptualized in the early 1970s and realized in the laboratory in the late 1970s; but these only combined two signals, and many years later were still very expensive. As of 2011, WDM systems can handle 160 signals, which will expand a 10 Gbit/second system with a single fiber optic pair of conductors to more than1.6Tbit/second(i.e.1,600Gbit/s). Typical WDM systems use single-mode optical fiber (SMF); this is optical fiber for only a single ray of light and having a core diameter of 9 millionths of a meter (9 µm). Other systems with multi-mode fiber cables (MM Fiber; also called premises cables) have core diameters of about 50 µm. Standardization and extensive research have brought down system costs significantly. WDM systems are divided according to wavelength categories, generally course WDM (CWDM) and dense WDM (DWDM). CWDM operates with 8 channels (i.e., 8 fiber optic cables) in what is known as the “C-Band” or “erbium window” with wavelengths about 1550 nm (nanometers or billionths of a meter, i.e. 1550 x 10-9 meters). DWDM also operates in the C-Band but with 40 channels at 100 GHz spacing or 80 channels at 50 GHz spacing. Even newer technology, called Raman amplification, is using light in the L-Band (1565 nm to 1625 nm), approximately doubling these capacities.

What is WDM (Wavelength Division Multiplexing)

WDM is the abbreviation for Wavelength-Division Multiplexing and represents one of the optical communication technologies for transmitting high-capacity signals.  Shows a schematic diagram of WDM transmission. The transmitting end of WDM has a plurality of semiconductor lasers that emit light of different wavelengths. Each LD beam is modulated to create a signal light. The created signal lights are combined through the multiplexer and are transmitted over a single optical fiber. At the receiving end, the signals are separated again into light of each wavelength by the de-multiplexer, and are received by the PDs. Since the numbers of transmittable signals are proportional to the numbers of wavelength, the use of many more wavelengths makes possible to transmit many more signals.

CWDM (Coarse WDM) is a WDM with wide wavelength spacing

In the case of WDM transmission, many more signals can be sent with the use of many more wavelengths. However, a certain wavelength band must be divided with narrow spacing in order to use many wavelengths. The narrow spacing requires to use high precision components of LD, filter and etc., so and the system become expensive. On the other hand, wide wavelength spacing reduces the transmission capacity of signals but achieves economical systems. Therefore, the ITU-T (International Telecommunication Union Telecommunication Standardization Sector) defines two types of WDM wavelength spacing so that appropriate systems are available depending on the application. One of the standards is DWDM with narrow wavelength spacing suitable for large-capacity, long-distance transmission. The other is the CWDM with wide wavelength spacing suitable for medium capacity, 50 ~ 80km distance transmission shows the CWDM wavelengths. There are 18 center wavelengths of 20 nm spacing from 1271 nm to 1611 nm, covering the O-, E-, S-, C- and L-band. In fact the case of using all 18 wavelengths is not a lot, but the four wavelengths of 1531nm ~ 1611nm or the 8 wavelength of 1471nm ~ 1611nm are used frequently. This is because the mass production optical components, like a CWDM filter, a Mux/DeMux unit, and an optical add-drop module, can be available in this wavelength range.

DWDM (Dense WDM) is a WDM with narrow wavelength spacing

shows the DWDM wavelengths. The wavelength spacing of DWDM is defined by light frequencies rather than the wavelengths. Because DWDM is premised on long-distance transmission using an optical amplifier, the C and L bands suitable for amplification are used, and the wavelength bands are divided finely to increase the transmission capacity.


uses of a WDM network

shows an image diagram of the optical communication network. Network is classified into three categories, i.e., the core network connecting the major cities, the metro network connecting the main area in the city, and the access network connecting each home and company. In the core network, DWDM suitable for a large-capacity and long-distance transmission is used. CWDM is mainly used in the metro network. CWDM is premised not to use an optical amplifier, and communication distance is about 50km ~ 80km. Though most areas in the city can be covered without an optical amplifier, the CWDM amplifiers developed by Fiber Labs can serve to extend reach if necessary.


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