---
title: "Short note: Splicing"
subject: "Propagation and Antenna"
chapter: 6
key: "semester_6/propagation_and_antenna:splicing-short-note"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/splicing-short-note
site: BCT Engineering
---

# Short note: Splicing

Propagation and Antenna · Chapter 6: Optical Fibres

- Asked in: 2079 Chaitra, 3 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/splicing-short-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-6

## Question

Write a short note on splicing.

## Short answer

Splicing permanently joins two optical fibres. Fusion splicing melts fibre ends with an electric arc (loss ~0.02–0.1 dB), while mechanical splicing aligns ends using a V-groove and index-matching gel (loss ~0.1–0.5 dB). Loss sources include misalignment and Fresnel reflections.

## Answer

**Splicing** is the permanent (or semi-permanent) joining of two optical fibres end to end so that light passes from one to the other with minimum loss. It is needed because cables come in limited lengths (2–6 km) and for repairs.

#### Types of splicing

1. **Fusion splicing:** the cleaned and cleaved fibre ends are aligned (often automatically using cameras) and fused by an **electric arc**, so the glass melts into one piece.
   - Loss: about 0.02–0.1 dB; very low back-reflection; strong and stable.
   - Needs an expensive fusion splicer; used in backbone and long-haul links.
2. **Mechanical splicing:** fibre ends are held in alignment by a mechanical structure such as a **V-groove**, an elastomeric splice or a precision tube, with **index-matching gel** to reduce reflection.
   - Loss: about 0.1–0.5 dB; quick, cheap tools; used for field repairs and drops.

#### Causes of splice loss

- **Extrinsic:** lateral (offset) misalignment, longitudinal gap, angular tilt, poor end-face cleave.
- **Intrinsic:** mismatch in core diameter, NA, index profile or core ellipticity between the two fibres.
- Fresnel reflection at a glass–air gap (≈ 0.17 dB per surface) in mechanical joints without gel.

Splices are protected with heat-shrink sleeves and kept in splice trays inside joint closures.

## Check yourself

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

1. Why is splicing needed in fibre links?
   A. Cables come in limited lengths and fibres must be joined permanently, and for repairs
   B. To increase the numerical aperture of the fibre
   C. To convert multimode fibre into single mode
   D. To amplify the optical signal
   Answer: A. Splicing joins two fibres end to end with minimum loss, since cable lengths are only 2-6 km, and is used for repairs. (Key point: Purpose of splicing)

2. How does fusion splicing join two fibres?
   A. The ends are glued with index-matching gel only
   B. The aligned, cleaved ends are melted together by an electric arc
   C. The ends are held in a V-groove without melting
   D. The ends are twisted together and heat-shrunk
   Answer: B. Fusion splicing welds the glass with an electric arc, giving about 0.02-0.1 dB loss and low back-reflection. (Key point: Fusion splicing)

3. What is the role of index-matching gel in a mechanical splice?
   A. It melts the fibre ends together
   B. It increases the core diameter
   C. It reduces Fresnel reflection at the fibre-end gap
   D. It protects the splice from tensile force
   Answer: C. The gel fills the glass-air gap, avoiding the reflection loss of about 0.17 dB per surface. (Key point: Mechanical splice)

4. Which of these is an intrinsic cause of splice loss?
   A. Lateral offset of the fibre ends
   B. Angular tilt between the fibres
   C. Poor cleave of the end face
   D. Mismatch in core diameter or NA between the two fibres
   Answer: D. Intrinsic losses come from differing fibre properties (core diameter, NA, index profile, ellipticity); offset, gap, tilt and cleave are extrinsic. (Key point: Splice loss causes)