---
title: "Optical sources and fibre losses"
subject: "Propagation and Antenna"
chapter: 6
key: "semester_6/propagation_and_antenna:optical-sources-fiber-losses"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/optical-sources-fiber-losses
site: BCT Engineering
---

# Optical sources and fibre losses

Propagation and Antenna · Chapter 6: Optical Fibres

- Asked in: 2077 Chaitra, 2+6 marks; 2075 Bhadra, 2+5 marks (asked 2 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/optical-sources-fiber-losses
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-6

## Question

List out different optical sources and explain the losses in optical fiber briefly.

## Short answer

Optical sources include LED and laser diode. Fibre losses arise from absorption (especially OH- ions at 1380 nm), Rayleigh scattering (dominant at short wavelengths, proportional to 1/λ^4), bending (macro and micro), and connector/splice misalignments.

## Answer

#### Optical sources

Light sources used in optical fibre communication:
1. **Light emitting diode (LED):** surface-emitting (Burrus) LED and edge-emitting LED; incoherent, broad spectrum, cheap.
2. **Laser diode (LD):** Fabry–Perot laser, distributed feedback (DFB) laser, distributed Bragg reflector (DBR) laser, and vertical-cavity surface-emitting laser (VCSEL); coherent, narrow spectrum, high power and speed.
3. Other lasers used in special systems: fibre lasers (erbium-doped) and Nd:YAG solid-state lasers.

#### Losses in optical fibre

**Attenuation** is the decrease of optical power along the fibre, measured in dB/km:

```text
α (dB/km) = (10 / L) · log(Pin / Pout)
```

Modern silica fibre has about 0.35 dB/km at 1310 nm and 0.2 dB/km at 1550 nm.

#### 1. Absorption losses

Light energy is absorbed by the glass and converted to heat.
- **Intrinsic absorption:** by the pure silica itself; electronic absorption in the ultraviolet and molecular vibration absorption in the infrared (beyond ~1.6 µm). It sets the long-wavelength limit.
- **Extrinsic absorption:** by impurities such as transition metal ions (Fe, Cu, Cr) and especially **OH⁻ (water) ions**, which give absorption peaks near 950, 1240 and 1380 nm.
- **Atomic defect absorption:** defects in the glass structure, e.g. caused by radiation.

#### 2. Scattering losses

Light is scattered out of the guided path by small non-uniformities in the glass.
- **Rayleigh scattering:** by density and composition fluctuations much smaller than λ; loss ∝ 1/λ⁴. It is the dominant loss at short wavelengths and the main reason long links use 1310 and 1550 nm.
- **Mie scattering:** by larger imperfections (core–cladding irregularities, bubbles), reduced by careful manufacture.
- Non-linear scattering (stimulated Brillouin and Raman) appears only at high optical power.

#### 3. Bending (radiation) losses

- **Macrobending:** bends with radius much larger than the fibre diameter (e.g. cable round a corner). On the outer side of a sharp bend the ray angle falls below the critical angle, so light leaks into the cladding. Loss rises steeply once the bend radius goes below a critical value.
- **Microbending:** small random bends of the fibre axis (µm scale) caused by uneven cabling pressure or temperature changes. They couple light from guided to radiating modes.

#### 4. Coupling, splice and connector losses

Power is lost where light enters the fibre and where fibres are joined, because of lateral, angular and gap misalignment, core size or NA mismatch, and Fresnel reflection at end faces (about 0.05–0.5 dB per joint).

## Check yourself

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

1. Which is a laser type?
   A. DFB laser
   B. Edge-emitting LED
   C. PIN diode
   D. Burrus LED
   Answer: A. DFB, DBR, FP and VCSEL are laser diodes. (Key point: Sources)

2. Rayleigh scattering loss varies as:
   A. 1/λ^4
   B. λ^4
   C. λ
   D. constant
   Answer: A. It dominates at short wavelengths. (Key point: Rayleigh)

3. OH- ions in the glass cause:
   A. Fresnel reflection
   B. Absorption peaks near 950, 1240 and 1380 nm
   C. Bending loss
   D. Rayleigh scattering
   Answer: B. Water is extrinsic absorption. (Key point: OH ions)

4. Macrobending causes loss because:
   A. Cladding absorbs
   B. Fibres are too long
   C. The ray angle falls below the critical angle at the outer side of the bend
   D. The core melts
   Answer: C. Light then leaks into the cladding. (Key point: Macrobending)

## Related in Propagation and Antenna

- [Fibre losses; multimode vs single mode](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afiber-losses-multimode-vs-single) — Propagation and Antenna, chapter 6
- [Dispersion and attenuation in optical fibre](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Adispersion-attenuation) — 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×
- [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
- [Construction and types of optical fibre](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afiber-construction-types) — Propagation and Antenna, chapter 6
- [Properties for selecting optical fibre](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afiber-selection-properties) — Propagation and Antenna, chapter 6

## Related in other subjects

- [Optical fiber advantages and wireless media](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aoptical-fiber-wireless-media) — Telecommunications, chapter 2
- [Optical fiber and its advantages](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aoptical-fiber-advantages) — Basic Electronics Engineering, chapter 5, asked 5×
- [Derive numerical aperture of optical fiber](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Ana-derivation-optical-fiber) — Engineering Physics, chapter 6, asked 4×
- [Advantages of optical fiber over copper](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aoptical-fiber-advantages) — Instrumentation II, chapter 5, asked 3×