100 GHz Spacing
Understanding 100 GHz DWDM Spacing
While RF engineers typically deal with Megahertz over coaxial cables, fiber-optic engineers deal with Terahertz over glass. Light is simply an incredibly high-frequency electromagnetic wave (roughly 193 Terahertz).
If you lay a fiber-optic cable across the Atlantic Ocean, you do not want to use it for just one data stream. You want to inject 80 different lasers into that single fiber at the exact same time. To ensure those lasers do not overlap and blind the receiver, the ITU established a strict grid of frequency slots, separated by exactly 100 GHz.
The ITU Grid Mathematics
The anchor point for the ITU grid is exactly 193.10 Terahertz (which corresponds to a wavelength of roughly 1552.52 nanometers in the optical C-Band).
- Channel 1: 193.10 THz
- Channel 2: 193.20 THz (Exactly 100 GHz higher)
- Channel 3: 193.30 THz
- Channel 4: 193.40 THz
By spacing the lasers 100 GHz apart, the network operator guarantees there is enough mathematical 'empty space' (guard band) between the channels. This allows a passive optical prism (an AWG Multiplexer) at the receiving city to cleanly split the colors of light back apart without cross-talk.
The Push for Narrower Spacing
| Spacing Standard | The Capacity Trade-off | The Engineering Challenge |
|---|---|---|
| 100 GHz Spacing | The legacy standard. Allows roughly 40 to 80 independent channels on a single fiber. Very stable, requires cheaper, less-precise lasers. | Easy to filter. The "wide" channels allow older 10 Gbps signals to fit perfectly without spilling into adjacent lanes. |
| 50 GHz Spacing | Doubles the capacity to 96+ channels. | Requires highly precise, temperature-controlled lasers. If a laser heats up and drifts by even 10 GHz, it will crash into the adjacent channel, destroying the data for both. |
| Flex-Grid (Nyquist) | Abandons fixed spacing entirely. Software dynamically allocates chunks of spectrum in 12.5 GHz slices based on exactly how much bandwidth a specific signal needs at that exact millisecond. | Requires incredibly complex coherent digital signal processing (DSP) and Liquid Crystal on Silicon (LCoS) optical switches to dynamically carve the spectrum. The standard for modern 400G and 800G coherent optics. |
Key Equations
100 GHz Spacing is a critical architectural standard defined by the ITU (International Telecommunication Union) for Dense Wavelength Division Multiplexing (DWDM) in fiber-optic networks. It...
Key specifications:
100 GHz
Power: P(dBm) = 10log(PmW), 0dBm = 1mW
Comparison
| Band | Range | Wavelength | Application | Standard |
|---|---|---|---|---|
| 100 GHz Spacing | 100 GHz region | 3.0 mm | Primary use | ITU allocation |
| Adjacent lower | 90.0 GHz | 3.3 mm | Related band | Shared spectrum |
| Adjacent upper | 110.0 GHz | 2.7 mm | Related band | Guard band |
| Harmonic 2f | 200.0 GHz | 1.5 mm | Spurious | Filter required |
| Sub-harmonic | 50.0 GHz | 6.0 mm | LO option | Mixer design |
Frequently Asked Questions
Why do optical engineers use Wavelength (nm) instead of Frequency (Hz)?
Historically, optical physicists measured the physical length of the light wave (nanometers), while RF engineers measured the oscillation speed (Hertz). In modern DWDM, calculating spacing in nanometers becomes a mathematical nightmare because the spacing is non-linear across the spectrum. Using exact 100 GHz frequency steps is perfectly linear, which is why the telecom industry fully transitioned to frequency-based ITU grids.
What happens if a 100 GHz DWDM laser loses cooling?
A semiconductor laser's emitted frequency is directly tied to its physical temperature. If the micro-peltier cooler fails, the laser will heat up, and its frequency will 'chirp' or slide to the right. It will drift out of its 100 GHz slot and obliterate the neighboring channel. Carrier-grade lasers will instantly self-shutdown if their temperature strays by a fraction of a degree.
Can you run 400 Gigabit data on a 100 GHz spaced grid?
Yes, but it is extremely difficult. A basic 400 Gbps signal naturally wants to occupy more than 100 GHz of spectrum. To squeeze it into the strict 100 GHz pipe, engineers must use extremely dense modulation (like 16-QAM or 64-QAM) and complex polarization multiplexing, which requires an incredibly clean, high-SNR optical fiber.