Dozens of channels on a 100, 50, 25 or 12.5 GHz grid anchored at 193.1 THz — the capacity workhorse of long-haul.
A Grid Defined in Frequency
G.694.1 anchors the grid at 193.1 THz and allows spacings of 100, 50, 25 or 12.5 GHz. Channels on this site are numbered by the convention f = 190.0 + n × 0.1 THz, so channel 34 is 193.4 THz and channel 61 is 196.1 THz.
Frequency is the natural unit for two reasons. Laser stability is a frequency property — a wavelength locker holds an optical frequency, not a wavelength — and equal frequency steps let one filter design serve the whole band, which equal wavelength steps would not.
Doing the Arithmetic
Converting is λ = c / f, or in convenient units λ in nanometres equals 299792.458 divided by the frequency in terahertz. Channel 34 is 1550.12 nm, channel 17 is 1563.86 nm and channel 61 is 1528.77 nm.
Spacing in wavelength is not constant. Because Δλ = λ² Δf / c, a 100 GHz step is 0.80 nm at 1550 nm but 0.78 nm at 1530 and 0.82 nm at 1565. It matters whenever you check a filter passband against a laser datasheet quoted in the other unit. The wavelength and frequency converter and the grid calculator do both directions.
Why It All Lives in the C-Band
Nothing in G.694.1 requires the C-band; the grid is defined across the S, C and L bands. Everything sits in the C-band anyway because that is where an EDFA has gain, roughly 1530 to 1565 nm.
One amplifier lifts every channel at once with no per-channel electronics, and that single fact is what made long-haul WDM economic. L-band systems exist and roughly double the capacity of a route, at the cost of a second, separate amplifier chain and a band splitter at every site.
Fixed, Tunable, or Coherent
A fixed-wavelength DWDM SFP+ or SFP28 is the cheapest way onto the grid and the most tedious to stock: forty-eight part numbers, one per channel, each of which has to be spared. Tunable optics collapse that into a single part number set by software.
At 400G and above the channel is usually a coherent pluggable, where the tunable laser is part of the modem and the DWDM line is driven straight out of a router port. See our 100 GHz and 200 GHz mux/demux modules.
What Goes Wrong
Three recurring faults. A laser drifting off channel because the cooler or its control loop has failed, which presents as a link that works cold and fails once the chassis is warm. A mux ordered on the wrong plan — a 200 GHz mux passes only every second 100 GHz channel, so half a channel list lands on a blocked port.
And overloading an amplified span: too much power per channel drives four-wave mixing and cross-phase modulation between neighbours, which no amount of receiver sensitivity will fix. Per-channel launch power and OSNR are the two numbers worth measuring before anything else.
DWDM grid reference
| Standard | ITU-T G.694.1 |
|---|---|
| Anchor | 193.1 THz |
| Channel formula (this site) | f = 190.0 + n × 0.1 THz |
| Worked example | C34 = 193.4 THz = 1550.12 nm |
| Conversion | λ (nm) = 299792.458 / f (THz) |
| 100 GHz spacing | 0.80 nm at 1550 nm |
| 50 GHz spacing | 0.40 nm at 1550 nm |
| Usable channels | roughly 40 to 48 at 100 GHz, twice that at 50 GHz |
| Laser | TEC-cooled DFB or tunable, held to about ±0.1 nm |
Related Terms
CWDM, OADM, EDFA, OSNR and coherent optics. For O-band DWDM in fronthaul see the O-band DWDM module.