In fiber-optic communication, what is WDM (Wavelength Division Multiplexing) and how is it related to Frequency Division Multiplexing?

Introduction / Context:
Wavelength Division Multiplexing (WDM) is a key technology in modern fiber-optic networks. It dramatically increases the capacity of fiber links by sending multiple optical channels at different wavelengths through the same physical fiber. Interviewers often ask what WDM is and how it relates to Frequency Division Multiplexing (FDM), since both concepts involve separating signals by frequency.

Given Data / Assumptions:

• We are using fiber-optic cables to transport data via light.
• Different wavelengths (colors) of light correspond to different optical frequencies.
• We want to carry multiple independent data channels on one fiber pair.
• Optical components can combine and separate these wavelengths.

Concept / Approach:
WDM can be viewed as optical Frequency Division Multiplexing. Instead of assigning distinct RF frequency bands, it assigns different wavelengths of light within the fiber's operating window. Each wavelength carries its own independent data stream, often at high bit rates like 10 Gbps or 100 Gbps. Multiplexers combine the different wavelengths into a single composite optical signal, and demultiplexers at the far end split them back into separate channels. Variants like CWDM (Coarse WDM) and DWDM (Dense WDM) define how closely the wavelengths are spaced, allowing from a handful to dozens or even hundreds of channels per fiber.

Step-by-Step Solution:
Step 1: Define WDM as Wavelength Division Multiplexing, a technique for sending several optical channels through one fiber by using different wavelengths. Step 2: Explain that each channel uses a specific wavelength (color) of light, which corresponds to a unique optical frequency. Step 3: Describe how an optical multiplexer combines signals from multiple transmitters into one fiber. Step 4: At the receiving end, an optical demultiplexer separates the composite signal back into individual wavelengths and sends each to its own receiver. Step 5: Note that this is analogous to FDM in the frequency domain, but implemented with optical wavelengths rather than RF bands.

Verification / Alternative check:
Studying fiber network diagrams for long haul backbone systems or high capacity data center links shows multiple colored lines (representing wavelengths) travelling over a single physical fiber pair. Equipment specifications for DWDM systems list channel spacing in nanometers or gigahertz and the number of supported wavelengths. This evidence confirms that WDM is essentially frequency division applied to light, with each wavelength carrying a separate signal on one fiber.

Why Other Options Are Wrong:
Option B describes time division multiplexing, not WDM, which relies on simultaneous channels at different wavelengths. Option C wrongly claims that WDM is wireless; WDM is specifically an optical fiber technology. Option D focuses on encryption, which may be used in optical systems but is unrelated to the multiplexing method itself.

Common Pitfalls:
A common misconception is to treat each fiber as carrying only one data stream, ignoring the huge capacity gains from WDM. Another pitfall is confusing CWDM and DWDM or assuming they are entirely different technologies; they are simply different wavelength spacing schemes within WDM. Recognizing WDM as optical FDM helps build intuition about how operators scale network capacity without pulling new fiber.

Final Answer:
WDM is a form of Frequency Division Multiplexing for fiber-optic systems, where multiple optical signals at different wavelengths are sent simultaneously over a single fiber.