---
title: "Working mechanism of WDM"
subject: "Data Communication"
chapter: 6
key: "semester_5/data_communication:wdm-working"
source: https://www.bctengineer.com/q/semester_5/data_communication/wdm-working
site: BCT Engineering
---

# Working mechanism of WDM

Data Communication · Chapter 6: Multiplexing and Spreading

- Asked in: 2071 Chaitra, 2 marks
- Video: [Wavelength Division Multiplexing WDM (Basics, Architecture, Components, Technologies & Features)](https://www.youtube.com/watch?v=uBAp-zBJho4) — Engineering Funda
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/wdm-working
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-6

## Question

Explain the working mechanism of WDM.

## Short answer

Wavelength Division Multiplexing assigns each signal a distinct laser wavelength. A prism or diffraction grating multiplexer combines them into one optical fiber. At the receiver, a demultiplexer separates the wavelengths to individual detectors, increasing fiber capacity without interference. Dense WDM (DWDM) supports up to160 channels.

## Answer

**Wavelength division multiplexing (WDM)** is an analog multiplexing technique for optical fibre in which signals of different wavelengths (colours) of light are combined and sent together in one fibre. It is the same idea as FDM, but applied to light.

#### Working
1. Each input is carried by its own laser at a different wavelength λ1, λ2, λ3 ...
2. A **multiplexer** (prism or diffraction grating) combines these narrow beams into one beam that goes through a single fibre.
3. At the receiving end a **demultiplexer** splits the beam back into separate wavelengths, and each goes to its own detector.

```text
 λ1 -->|      |                 |      |--> λ1
 λ2 -->| MUX  |==== fibre ======| DMUX |--> λ2
 λ3 -->|      |                 |      |--> λ3
```

Channels do not interfere because their wavelengths differ. Dense WDM (DWDM) carries 40 to 160 channels in one fibre, which multiplies capacity many times.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does WDM combine in one fibre?
   A. Different time slots
   B. Different light wavelengths from separate lasers
   C. Different carrier phases
   D. Different chip codes
   Answer: B. Each input has its own laser at a different wavelength, all sent in one fibre. (Key point: Principle)

2. What does the demultiplexer do at the receiving end?
   A. Adds more wavelengths
   B. Amplifies the electrical signal
   C. Converts light to chips
   D. Splits the beam back into separate wavelengths for separate detectors
   Answer: D. The DMUX separates wavelengths and sends each to its own detector. (Key point: MUX and DMUX)

3. Why do WDM channels not interfere?
   A. They are sent at different times
   B. Their wavelengths differ
   C. They use different codes
   D. They use different polarities of guard bits
   Answer: B. Different wavelengths keep the channels separated, like FDM for light. (Key point: Why no interference)

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