DWDM/CWDM Wavelength ITU Channels Guide

Dense Wavelength Division Multiplexing (DWDM) and Coarse Wavelength Division Multiplexing (CWDM) are fiber optic technologies using different light wavelengths to transmit multiple data streams over a single optical fiber. The International Telecommunication Union (ITU) sets the standards for these channels, which are distinguished by their wavelength spacing, number of channels, and primary use cases.

CWDM ITU Channels

CWDM is a simpler, more cost-effective technology often used for short to medium distances, like in metropolitan or campus networks. Its key features are its wide channel spacing and lower channel count.

• Channel Spacing: CWDM uses 20 nanometers (nm) wide channel spacing. This “coarse” spacing allows for less precise, uncooled lasers, which reduces the cost of the equipment.
• Wavelength Range: The standard CWDM grid, defined by ITU-T Recommendation G.694.2, covers 18 channels from 1270 nm to 1610 nm.
• Wavelength List: The channels are typically defined by their central wavelength and are listed in 20 nm increments. A typical subset of these channels is used in most applications to avoid the “water peak” region (around 1370-1450 nm,) where signal loss is higher in older fiber types.

DWDM ITU Channels

DWDM is designed for high-capacity, long-haul transmission, such as for backbone networks and submarine cables. It’s called “dense” because it packs many more channels into a smaller wavelength range.

• Channel Spacing: DWDM uses a much tighter channel spacing, typically 100 GHz (0.8 nm) or 50 GHz (0.4 nm). This requires more complex, cooled lasers and precise filters to prevent channel interference.
• Frequency Grid: DWDM channels are defined by frequency rather than wavelength to ensure consistent spacing, as the relationship between frequency and wavelength is not perfectly linear. The ITU-T Recommendation G.694.1 specifies a grid centered at 193.1 THz (1552.52 nm).
• Wavelength Range: DWDM primarily operates in the low-loss C-band (1530-1565 nm) and L-band (1570-1610 nm) of the optical fiber, which allows for signal amplification over long distances using erbium-doped fiber amplifiers (EDFAs).
• Channel Example (100 GHz Spacing): Below is a partial list of the C-band’s most commonly used DWDM channels. Each channel is 100 GHz apart, allowing for up to 40 or more channels in this range. For 50 GHz spacing, intermediate channels (e.g., 193.05 THz) are added to double the capacity.