--- title: "Short note: Light sources in optical communication" subject: "Propagation and Antenna" chapter: 6 key: "semester_6/propagation_and_antenna:light-sources-short-note" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/light-sources-short-note site: BCT Engineering --- # Short note: Light sources in optical communication Propagation and Antenna · Chapter 6: Optical Fibres - Asked in: 2076 Baisakh, 4 marks - Video: [Introduction of Optical Sources](https://www.youtube.com/watch?v=Iz-httvcFIQ) — Dopamine - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/light-sources-short-note - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-6 ## Question Write a short note on light sources in optical communication. ## Short answer Semiconductor sources convert electrical signals to optical power at 850, 1310 or 1550 nm. LEDs use spontaneous emission: incoherent, wide spectrum (30-100 nm), low cost, suitable for multimode short links. Laser diodes (FP, DFB, VCSEL) use stimulated emission: coherent, narrow spectrum (<1 nm), high modulation speed and power, used with single-mode long-haul links. ## Answer A **light source** in optical communication converts the electrical signal into optical power for launching into the fibre. Semiconductor sources are used because they are small, efficient, directly modulated by current and emit at fibre wavelengths (850, 1310, 1550 nm). #### Requirements - Emission wavelength in the low-loss windows of the fibre - Enough output power and efficient coupling (small emitting area) - Narrow spectral width (less chromatic dispersion) - High modulation speed, linearity, long life, low cost #### Light emitting diode (LED) - Forward-biased p-n junction (GaAlAs, InGaAsP); light by **spontaneous emission**. - Types: surface-emitting (Burrus) and edge-emitting. - Incoherent, wide spectrum (30–100 nm), wide beam, low power (µW to a few mW), bandwidth up to ~100 MHz. - Simple, cheap, reliable, less temperature-sensitive. Used with multimode fibre for short links. #### Laser diode (LD) - Light by **stimulated emission** in an optical cavity (Fabry–Perot, DFB, DBR, VCSEL) once current exceeds a threshold. - Coherent, narrow spectrum (1–5 nm; DFB < 0.1 nm), narrow beam, high power (tens of mW), modulation to tens of GHz. - Costlier, needs temperature and power control. Used with single-mode fibre in long-haul, high-speed links. | Property | LED | Laser diode | |---|---|---| | Emission | Spontaneous | Stimulated | | Spectral width | Wide | Narrow | | Power / speed | Low | High | | Cost | Low | High | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does an LED generate light? A. Stimulated emission in a cavity above threshold B. Reverse-bias avalanche C. Thermal heating of silica D. Spontaneous emission in a forward-biased p-n junction Answer: D. LEDs are incoherent and wide-spectrum because emission is spontaneous. (Key point: LED emission) 2. What is required for a laser diode to emit? A. Stimulated emission in an optical cavity once current exceeds a threshold B. Any forward current C. Reverse bias D. An external amplifier Answer: A. Above threshold, stimulated emission gives coherent, narrow, high-power output. (Key point: Laser emission) 3. Which pairing is correct? A. LED with single-mode long-haul; laser with multimode short links B. LED with multimode fibre for short links; laser with single-mode fibre for long-haul C. Both only with plastic fibre D. LED for high speed, laser for low cost Answer: B. LEDs are cheap, low power, wide spectrum; lasers are costly but high power and fast. (Key point: LED versus laser) 4. Why is a narrow spectral width desirable? A. It increases modal dispersion B. It increases the NA C. It reduces chromatic dispersion D. It lowers the threshold current Answer: C. A narrow spectrum means less pulse spreading from wavelength-dependent delay. (Key point: Spectral width) ## Related in Propagation and Antenna - [Optical sources and fibre losses](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aoptical-sources-fiber-losses) — Propagation and Antenna, chapter 6, asked 2× - [Elements of optical communication system](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aoptical-communication-elements) — Propagation and Antenna, chapter 6, asked 3× - [Light propagation; define NA and acceptance angle](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Alight-propagation-define-na-acceptance) — Propagation and Antenna, chapter 6 - [Short note: Optical source and detector](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Alight-source-photo-detector) — Propagation and Antenna, chapter 6, asked 5× - [Types of optical fibre: construction and propagation](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aoptical-fiber-types-construction) — Propagation and Antenna, chapter 6, asked 2× - [Optical fibre system vs wireless](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aofc-working-vs-wireless) — Propagation and Antenna, chapter 6 ## Related in other subjects - [Light sources and specular reflection principle](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Alight-sources-specular-reflection) — Industrial Power Distribution and Illumination, chapter 6 - [WDM and its light sources](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Awdm-light-sources) — Telecommunications, chapter 3 - [Communication system; optical fiber merits](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Adefine-comm-optical-merits) — Basic Electronics Engineering, chapter 5 - [Light sources and indoor lighting schemes](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Alight-sources-indoor-schemes) — Industrial Power Distribution and Illumination, chapter 6