Coarse wavelength-division multiplexing (CWDM) is used to increase the bandwidth of an optical fiber by multiplexing several wavelengths (or colors) onto it—just like dense wavelength-division multiplexing (DWDM).
CWDM uses up to 18 wavelengths, while DWDM can go up to 80 or even 160 channels. The ITU-T recommendation G.694.2 specifies the wavelengths used in CWDM are from 1271 nm to 1611 nm, with a channel spacing of 20 nm.
Typically, only the upper 8 of the 16 wavelengths are used. The very wide channel spacing of CWDM, compared to DWDM, allows for the use of cheaper CWDM network components, such as uncooled lasers or lower-quality multiplexers and demultiplexers. However, this broad spectral range prevents using erbium-doped fiber amplifiers, that work best around 1550 nm, and accordingly limits the maximum range of CWDM deployments to about 60 or 80 km.
CWDM: ideal for metro networks
These two facts make CWDM a very attractive option in metro networks that require additional bandwidth, but where deployment costs should be kept at a minimum. Furthermore, CWDM networks are scalable to DWDM networks by replacing some transceiver cards (typically 1530 nm or 1550 nm) by DWDM cards, thus enhancing the bandwidth of the fiber.
CWDM channel power measurement
As any DWDM network, CWDM networks must be tested for loss, connector cleanliness, dispersion and spectral quality. EXFO not only offers CWDM OTDRs and probes, but also a scalable optical spectrum analyzer, the FTB-5230S or FTB-5235, and a cost-effective channel analyzer, the FOT-5200, which allows for extremely fast CWDM channel power measurement.
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