ITU-T G.694.1 DWDM Frequency Grid: Fixed Spacing, Flexible Grid & C-Band

1. Why Dense Wavelength Division Multiplexing (DWDM) Exists

In optical communications, laying new underground or undersea single-mode fiber cables costs tens of millions of dollars. To multiply the carrying capacity of existing glass without laying new physical fiber, telecommunications engineers deploy Dense Wavelength Division Multiplexing (DWDM).

By modulating distinct data streams onto closely spaced laser colors (optical wavelengths) and launching them into the same physical glass core, a single fiber pair can transport over 96 independent channels, delivering aggregate capacities exceeding 40 Terabits per second.

To ensure global interoperability between transponders, optical multiplexers, and reconfigurable optical add-drop multiplexers (ROADMs) from different equipment manufacturers, the International Telecommunication Union standardized the ITU-T Recommendation G.694.1 optical frequency grid.

2. The Mathematical Anchor: 193.100 THz

Unlike radio frequency bands that are referenced by wavelength or band limits, ITU-T G.694.1 references the DWDM grid in pure optical frequency (THz) because optical frequency remains invariant when passing through optical filters, whereas wavelength changes as the refractive index of optical glass varies.

The grid is anchored to a universal physical constant: 193.100 THz (which corresponds to an optical vacuum wavelength of approximately 1552.52 nm, right at the center of the optical C-Band). All channel frequencies f_channel are defined by integer steps n from this anchor:

Where Δf is the standardized channel spacing:

• 100 GHz Grid (Δf = 0.1 THz): Standard spacing (~0.8 nm separation), supporting 48 channels in C-Band.
• 50 GHz Grid (Δf = 0.05 THz): High-density spacing (~0.4 nm separation), supporting 96 channels in C-Band.
• 25 GHz Grid (Δf = 0.025 THz): Ultra-dense spacing, supporting 192 channels across combined C+L bands.

3. The Evolution to Flexible Grid (Flex-Grid)

As transmission speeds advanced to 400 Gbps, 800 Gbps, and 1.2 Tbps using higher-order coherent modulation (64-QAM) and ultra-high baud rates (exceeding 100 Gbaud), the physical spectral width of an optical signal expanded beyond the rigid 50 GHz boundary.

To accommodate wide, high-baud-rate channels without leaving stranded bandwidth, ITU-T G.694.1 introduced the Flexible Grid (Flex-Grid). Under Flex-Grid, channels are allocated with a variable slot width:

This allows modern ROADM networks to allocate exactly 75 GHz for a 400G carrier or 112.5 GHz for an 800G carrier, maximizing total spectral efficiency across the C-Band.