--- title: "WDM and its light sources" subject: "Telecommunications" chapter: 3 key: "semester_8/telecommunications:wdm-light-sources" source: https://www.bctengineer.com/q/semester_8/telecommunications/wdm-light-sources site: BCT Engineering --- # WDM and its light sources Telecommunications · Chapter 3: Signal Multiplexing - Asked in: 2073 Shrawan, 4+4 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 Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/wdm-light-sources - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-3 ## Question What is wave length division multiplexing? Explain its light sources characteristics and differentiate between them. ## Short answer WDM carries multiple optical signals on separate wavelengths over one fibre. LED emits via spontaneous emission with a wide spectrum and poor fibre coupling; laser diode uses stimulated emission, giving narrow spectrum, high power and fast modulation. Only lasers provide the stable narrow wavelengths needed for dense WDM. ## Answer **Wavelength Division Multiplexing (WDM)** is the technique of sending several optical signals at the same time over one fibre, each on a different wavelength (colour) of light. It is FDM at optical frequencies and multiplies the capacity of an installed fibre. ```text l1 -->|\ /|--> l1 l2 -->| MUX ==== fibre ==== DEMUX|--> l2 l3 -->|/ (l1+l2+...+lN) \|--> l3 lN --> --> lN l = wavelength (lambda) ``` Each channel drives its own source at $\lambda_1, \lambda_2, \ldots$; a passive optical multiplexer (grating, prism or thin-film filter) combines them and a demultiplexer separates them. **CWDM** uses 20 nm spacing; **DWDM** uses 0.8 nm (100 GHz) or closer, with EDFAs amplifying all channels together. #### Light sources and their characteristics Two semiconductor sources are used: the **LED** (light-emitting diode) and the **LD** (injection laser diode). A good optical source must: - emit at a wavelength where fibre loss and dispersion are low (850, 1310, 1550 nm); - have a **narrow spectral width**, so dispersion is low and channels can be closely packed in WDM; - couple enough **power** into the small fibre core; - be easy to **modulate** at high speed with a linear light–current characteristic; - be small, reliable, long-lived and stable with temperature. **LED:** works by **spontaneous emission** when electrons and holes recombine in a forward-biased p-n junction. Output is incoherent with a wide spectrum (30–60 nm) and wide beam, so coupling to fibre is poor. It is cheap, simple and reliable, and is used with multimode fibre over short distances. **Laser diode:** works by **stimulated emission** in a p-n junction with an optical cavity. Above the **threshold current** the output rises steeply and is coherent and narrow (1–3 nm; less than 0.1 nm for DFB lasers). It couples much more power into single-mode fibre and can be modulated at many Gbps, so it is the source used for DWDM and long-haul links. ```text Optical power LD / | / | / LED | / .------- | / .-' | _.-/' +--------+-----------> current I_th (threshold of LD) ``` #### Difference between LED and laser diode | Point | LED | Laser diode | |---|---|---| | Emission | Spontaneous | Stimulated | | Light | Incoherent | Coherent | | Spectral width | Wide, 30–60 nm | Narrow, 1–3 nm (DFB < 0.1 nm) | | Output power | Low (µW to ~1 mW into fibre) | High (several mW) | | Beam | Wide angle, poor coupling | Narrow, good coupling | | Modulation speed | Up to ~100–200 Mbps | Several Gbps | | Threshold | None, linear | Threshold current needed | | Temperature sensitivity | Low | High, needs control | | Cost, life | Cheap, long life | Costly, shorter life | | Fibre / use | Multimode, short LAN links | Single mode, long-haul, DWDM | For WDM, laser diodes (especially DFB lasers with temperature control) are needed because only they give the narrow, stable wavelengths that allow many closely spaced channels. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is wavelength division multiplexing? A. Sending several optical signals on different wavelengths over one fibre at the same time B. Sending optical signals in different time slots on one wavelength C. Sending signals through different fibres of one cable D. Spreading all optical signals with unique codes on one wavelength Answer: A. WDM is FDM at optical frequencies: each signal has its own wavelength and all travel together in one fibre. (Key point: WDM principle) 2. Why is the LED output wide in spectrum and incoherent? A. It works by stimulated emission above a threshold B. It works by spontaneous emission as electrons and holes recombine C. It uses an optical cavity with feedback D. It is driven by a threshold current Answer: B. LEDs emit by spontaneous emission, giving incoherent light with a 30-60 nm spread. (Key point: LED emission) 3. Why are laser diodes (especially DFB) needed for DWDM? A. They are cheaper and have no threshold B. They are insensitive to temperature C. They give narrow, stable wavelengths so many channels can be packed closely D. They have a wider spectrum that covers more channels Answer: C. Only the narrow spectral width (1-3 nm, under 0.1 nm for DFB) allows closely spaced channels. (Key point: Why laser for DWDM) 4. Which statement about threshold is correct? A. The LED needs a threshold current; the laser output is linear from zero B. Both need a threshold current C. Neither has a threshold D. The laser needs a threshold current above which output rises steeply; the LED is linear without one Answer: D. Laser output rises steeply only above the threshold current I_th, while the LED output is roughly linear with no threshold. (Key point: LD vs LED threshold) ## Related in Telecommunications - [Need for multiplexing and WDM](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-need-wdm) — Telecommunications, chapter 3 - [Comparison of FDM, TDM and WDM](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-fdm-tdm-wdm) — Telecommunications, chapter 3, asked 2× - [WDM and T carrier system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Awdm-t-carrier-t1) — Telecommunications, chapter 3, asked 2× - [Pulse stuffing; LED vs LASER sources](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Apulse-stuffing-led-laser) — Telecommunications, chapter 2 ## Related in other subjects - [Time division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Atdm-explain) — Data Communication, chapter 6, asked 3× - [Frequency division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afdm-explain) — Data Communication, chapter 6, asked 3× - [Short note: Frequency division multiplexing](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-fdm) — Communication Systems, chapter 5, asked 2× - [Differentiate DWF and WWF; sources](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Adwf-wwf-sources-sewage) — Sanitary Engineering, chapter 2