---
title: "Short note: Optical source and detector"
subject: "Propagation and Antenna"
chapter: 6
key: "semester_6/propagation_and_antenna:light-source-photo-detector"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/light-source-photo-detector
site: BCT Engineering
---

# Short note: Optical source and detector

Propagation and Antenna · Chapter 6: Optical Fibres

- Asked in: 2080 Chaitra, 3 marks; 2080 Asoj, 3 marks; 2077 Chaitra, 4 marks; 2075 Bhadra, 4 marks; 2075 Baisakh, 4 marks (asked 5 times)
- 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-source-photo-detector
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-6

## Question

Write a short note on light (optical) source and photo (optical) detector.

## Short answer

LED and laser diode are optical sources; LED emits incoherent light for short links, laser diode emits coherent light for long-haul high-speed links. PIN photodiode and avalanche photodiode (APD) are photodetectors; PIN has no gain, low noise; APD provides internal gain for higher sensitivity.

## Answer

An optical fibre link needs a **light source** to convert the electrical signal into light at the transmitter and a **photodetector** to convert light back into an electrical signal at the receiver.

#### Optical sources

Requirements: emission at fibre low-loss wavelengths (850, 1310, 1550 nm), small emitting area, enough power, fast modulation, narrow spectrum, long life and low cost. Two semiconductor sources are used:

- **LED (light emitting diode):** forward-biased p-n junction giving spontaneous emission. Incoherent light, wide spectral width (30–60 nm), low power, bandwidth up to ~100 MHz. Cheap and reliable; used with multimode fibre over short distances (LANs).
- **Laser diode (LD):** stimulated emission in a cavity above a threshold current. Coherent, narrow linewidth (1–3 nm, or < 0.1 nm for DFB lasers), higher power, GHz modulation. Used with single-mode fibre in long-haul high-speed links.

#### Photodetectors

Requirements: high responsivity at the operating wavelength, fast response, low noise, low bias voltage.

- **PIN photodiode:** an intrinsic layer between p and n regions widens the depletion zone, so most photons are absorbed there. No internal gain, low noise, fast, cheap.
- **Avalanche photodiode (APD):** high reverse bias causes impact ionisation, giving internal gain (M ≈ 10–100). More sensitive, used for long links, but needs high voltage and adds excess noise.

| | LED / PIN | LD / APD |
|---|---|---|
| Cost | Low | High |
| Speed | Lower | Higher |
| Use | Short, multimode | Long-haul, single-mode |

## Check yourself

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

1. Which is a feature of an LED compared with a laser diode?
   A. Spontaneous emission with wide spectral width
   B. GHz modulation
   C. Threshold current
   D. Coherent narrow output
   Answer: A. LEDs give incoherent light with 30–60 nm width. (Key point: LED vs laser)

2. Laser diodes are preferred with:
   A. Single-mode long-haul links
   B. Short LAN multimode links only
   C. Copper wire
   D. Low-cost links
   Answer: A. Narrow spectrum, high power and speed suit long-haul single-mode. (Key point: Laser use)

3. An APD differs from a PIN diode in that it:
   A. Is less sensitive
   B. Has no bias
   C. Provides internal gain by impact ionisation
   D. Is cheaper and lower voltage
   Answer: C. High reverse bias causes avalanche gain M ≈ 10–100. (Key point: APD gain)

4. The intrinsic layer in a PIN photodiode:
   A. Emits light
   B. Blocks photons
   C. Widens the depletion zone so more photons are absorbed
   D. Provides gain
   Answer: C. A wide depletion region improves absorption and speed. (Key point: PIN intrinsic layer)

## Related in Propagation and Antenna

- [Short note: Light sources in optical communication](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Alight-sources-short-note) — Propagation and Antenna, chapter 6
- [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
- [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×

## Related in other subjects

- [Optical flat and flatness interferometer](https://www.bctengineer.com/question.md?key=semester_3%2Fmetrology%3Aflatness-interferometer-and-optical-flat) — Metrology, chapter 8
- [TRIAC light dimmer conduction angles and power](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Atriac-light-dimmer) — Advanced Electronics, chapter 7