Wave Division Multiplexing (WDM)

The Prism Principle

WDM works exactly like a prism. It takes a composite beam of light containing many different wavelengths and splits them into individual channels using specialized filters or diffraction gratings.

WDM Varieties: CWDM vs. DWDM

The primary difference between standard WDM technologies is the "spacing" between channels.

• CWDM (Coarse WDM): Typically 20 nm spacing. Harder to interfere, cheaper lasers, but limited to 18 channels and shorter distances (no amplification possible).
• DWDM (Dense WDM): Typically 0.8 nm or 0.4 nm spacing. Supports 80+ channels. Uses EDFA (Erbium-Doped Fiber Amplifiers) to travel thousands of kilometers without converting back to electrical signals.

Wavelength Filtration: TFF vs. AWG

How does a Mux/Demux actually separate colors of light that are less than 1 nm apart? There are two primary technologies:

• Thin Film Filters (TFF): These use multi-layer dielectric coatings on a glass substrate. Through constructive and destructive interference, the filter allows only a specific wavelength to pass while reflecting others. TFF is highly stable and used primarily in CWDM and low-channel count DWDM.
• Arrayed Waveguide Grating (AWG): This is a planar lightwave circuit (PLC) that acts like a high-resolution diffraction grating. Light enters an array of waveguides of slightly different lengths, creating a phase shift that causes different wavelengths to interfere at different physical locations at the output. This is the standard for high-density 40, 80, or 96-channel DWDM systems.

The ITU-T Wavelength Grids

Global interoperability in fiber optics is governed by the International Telecommunication Union (ITU). They define the standard frequencies (and corresponding wavelengths); for WDM:

• ITU-T G.694.1 (DWDM): Defines the "C-Band" (1530nm to 1565nm) and "L-Band" (1565nm to 1625nm). It uses a frequency grid with spacings of 100 GHz, 50 GHz (0.4 nm), or even 25 GHz.
• ITU-T G.694.2 (CWDM): Defines 18 channels from 1271 nm to 1611 nm with a 20 nm spacing. The "Water Peak" (around 1383 nm) historically made some of these channels unusable due to high attenuation in older fiber.

Optical Amplification: The EDFA Advantage

One of the reasons DWDM dominates long-distance subsea and terrestrial links is the Erbium-Doped Fiber Amplifier (EDFA). Unlike electrical repeaters that must convert light to electricity, amplify it, and convert it back, an EDFA uses a pump laser to excite erbium ions in the glass, providing pure optical gain to all WDM channels simultaneously.

However, EDFAs introduce Amplified Spontaneous Emission (ASE) noise, which gradually degrades the OSNR. After several thousand kilometers, the cumulative noise makes the signal unreadable, requiring specialized "Regeneration" (3R: Re-amplifying, Re-shaping, Re-timing).

The Mux/Demux Anatomy

At the heart of a WDM link is the Multiplexer (Mux) and De-multiplexer (Demux).

1. Individual lasers transmit at specific, stabilized wavelengths.
2. The Mux combines them into one fiber.
3. The Demux separates them at the receiver.

Conclusion

WDM is what makes the modern internet possible. Without the ability to send terabits of data over a single hair-thin strand of glass, the cost per megabit would remain prohibitively high.