What Is Cwdm

FIS Tech Support What is CWDM? CWDM stands for Coarse Wave Division Multiplexing, and is a method of combining multiple wavelengths onto one fiber – such as a fiber optic “backbone”. In order to get the most bandwidth out of a fiber optic network without the costs associated with laying additional fiber, Wave Division Multiplexing (WDM) was developed. Instead of having a separate fiber for each wavelength (for example 1310nm, 1550nm, etc.), WDM was developed to combine multiple wavelengths, or signals, onto a single fiber. These signals can be transmitted and received in both directions on the same fiber, which is called bi-directional communications. Signals are commonly combined and separated from the WDM fiber backbone using one of two methods: “Multiplex/De-multiplex” (MUX/DEMUX) and “Add/Drop”. The MUX/DEMUX method uses a “point-to-point” system that combines multiple wavelengths at the beginning of the fiber backbone (MUX), and removes or separates them at the endof the fiber (DEMUX). This method is typically used in “campus” applications, such as between buildings. An example of this method used in a broadcast application between the city hall and courthouse of Bozeman, MT is on the website of FIS Blue supplier Telecast Fiber Systems1. The “Add/Drop” method, also known as a “loop” or “multi-point” system2, adds different wavelengths at various points along the length of the fiber, as well as the beginning of the fiber, and removes signals from other points along the fiber, as well as the end of the fiber. “Add/Drop” is most commonly used in fiber to the home (FTTH) applications and “Passive CWDM”, which uses the principles of a prism without additional electrical power, is being promoted to deploy FTTH 3. CWDM is a low cost, but lower bandwidth and shorter distance alternative to DWDM – Dense Wave Division Multiplexing. Although CWDM refers to a variety of applications, it has been standardized by the International Telecommunication Union (ITU) to apply to the wavelengths from 1270 nm through 1610 nm with a channel spacing of 20 nm. Using CWDM allows the combination of 18 signals on one fiber, although only the 8 wavelengths above 1470nm are typically used (C-band). An OSX-20 FIS Handheld Optical Spectrum Analyzer can be used to evaluate these signals. In contrast, modern DWDM systems use 50 GHz (0.4nm) or even 25 GHz (0.2nm) channel spacing for up to 160 channel operation. The wider channel spacing in a CWDM system allows the use of low-cost, lower precision, uncooled lasers - which is less expensive and consumes less power than a DWDM system4. So, a CWDM is used in relatively low-cost applications where bandwidth and distance are not critical. CWDM passive multiplexers are manufactured differently than couplers. One hundred percent of each component signal is transferred to the multiplexed signal with minimal loss (<0.5 dB per channel) 5 in a passive multiplexer. In contrast, a coupler must divide each incoming signal to equal 100 percent on the output, M. Kuligowski

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FIS Tech Support resulting in a significant loss to the incoming signal (for example, a 1x3 coupler would only pass 33 percent of each channel). CWDM technology can also be seamlessly integrated into legacy network’s electrical equipment using GBIC or SFP modules available from various network product manufacturers. Consult your customer for the brand of network equipment they are using. References: 1. Hurwitz, J., Telecast Fiber Systems, Bozeman Montana: City Hall to Courthouse on a single strand of Fiber, http://www.telecast-fiber.com/fiberoptic-video-news-perspective/bid/17816/Bozeman-Montana-City-Hall-toCourthouse-on-a-single-strand-of-Fiber 2. Whatis.com, Coarse Wave Division Multiplexing – Definition, http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci1018154,00.ht ml# 3. Wikipedia, Wavelength-Division Multiplexing, http://en.wikipedia.org/wiki/CWDM#Coarse_WDM 4. Fiber-Optics.info, Coarse Wavelength-Division Multiplexing, http://www.fiberoptics.info/articles/coarse_wavelength-division_multiplexing 5. Tech Guy, Fiber-Optic-Forum.com, RE: Viewing 3 Fibers on a 1-Channel OSA, http://www.fiber-opticforum.com/threadView.aspx?id=472&ps=508&hl=cwdm#post508

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