Coarse WDM and its applications
Coarse Wavelength Division Multiplexing (CWDM), a WDM technology, is characterized by wider channel spacing than Dense WDM (DWDM) as defined in ITU-T Rec. G.671. CWDM systems can realize cost-effective applications, through a combination of uncooled lasers, relaxed laser wavelength selection tolerances and wide passband filters. CWDM systems can be used in transport networks in metropolitan areas for a variety of clients, services and protocols. Appendix I provides an explanation of the rationale for choosing the central wavelength spacing and the determining factors of the wavelength variation.
Nominal central wavelengths for coarse WDM systems
The nominal central wavelengths for coarse WDM systems are used as a reference to define, for each channel used, an upper wavelength bound and a lower wavelength bound. These bounds define limits for the wavelength of the transmitter under all conditions and at the same time the wavelength limits over which the specifications of the optical multiplexers and demultiplexers must be met.
The upper wavelength bound is the central wavelength of the channel plus the central wavelength deviation found in the Recommendation defining the application. The lower wavelength bound is the central wavelength of the channel minus the central wavelength deviation found in the Recommendation defining the application. The CWDM grid wavelengths within the range 1271 nm to 1611 nm are shown in Table 1. The value of “c” (speed of light in vacuum) that should be used for converting between wavelength and frequency is 2.99792458 × 108 m/s.
Nominal central wavelengths (nm), for spacing of 20 nm, 1271, 1291, 1311, 1331, 1351, 1371, 1391, 1411, 1431, 1451, 1471, 1491, 1511, 1531, 1551, 1571, 1591, 1611, NOTE – The endpoints of this table are illustrative only
Effective CWDM realizations with uncooled lasers and wide passband filters require a nominal central wavelength spacing of not less than 20 nm. Total source wavelength variation of the order of ±6-7 nm is expected to be compatible with current filter technologies. As for the guardband, one third of the minimum channel spacing is sufficient. Therefore, in order to maximize the number of channels, 20 nm has been chosen.
Specific values and allocations of this variation will be defined in individual applications. The wavelength variation is determined by mainly two factors. First, the laser manufacturer is allowed a wavelength variation around the nominal wavelength in order to achieve a higher yield and/or relax fabrication tolerances. Second, the use of uncooled lasers will cause the wavelength to change with temperature within the specified temperature range of the laser.