Types of optical fibres

Types of optical fibres

1. Classification Axes

Fibres classified by (a) number of modes and (b) index profile.

2. By Modes — Single vs Multimode

• Single-mode fibre (SMF): core small ($≈9$ µm), $V=(2πa/λ)NA<2.405$ ⇒ only fundamental LP01 mode propagates. No modal dispersion, high bandwidth (THz·km), used for long-haul telecom (1310/1550 nm). Expensive, needs precise alignment.
• Multimode fibre (MMF): large core ($50$–$62.5$ µm) ⇒ $V≫1$ ⇒ many modes (~100s). Each mode travels different path length ⇒ modal dispersion (pulse broadening). Used for short links, LAN (850 nm, cheap).

3. By Index Profile — Step vs Graded

Type	Index in Core	Rays / Modes	Dispersion	Use
Step-index SMF	constant $n_1$ then step to $n_2$	total internal reflection at step	waveguide + material	Telecom long haul
Step-index MMF	constant $n_1$	zig-zag rays, different path lengths	large modal dispersion ~$n_1Δ/(c)$ per km	Short, low bandwidth (historical)
Graded-index MMF (GRIN)	parabolic $n(r)=n_1√[1-2Δ(r/a)²]$	rays bend continuously (sinusoidal), modes have near-equal transit time (self-focusing)	modal dispersion greatly reduced (~ $n_1Δ²/(8c)$)	LAN, 10G Ethernet with 550 m

Graded-index trick: outer rays travel faster (lower $n$) so they catch up with axial rays ⇒ compensates path difference.

Step-index MMF: rays ——> \/\/\/\/ path length depends on angle ⇒ spread
GRIN MMF: rays ~ sinusoidal, focus periodically ⇒ all arrive together
SMF: single ray along axis ⇒ no spread

6. Exam Points

• Distinguish single-mode vs multimode and step vs graded with diagrams — 5-7 marks, very frequent (almost certain table).
• Explain modal dispersion and why GRIN reduces it — 5 marks, very frequent.
• Give core sizes and $V$-number cutoff $2.405$ — 3-5 marks, frequent.
• Applications: SMF long haul, MMF LAN — 3 marks.