Structure of optical fibre

Structure of optical fibre

1. What an Optical Fibre Is

Optical fibre = cylindrical dielectric waveguide that guides light by total internal reflection (TIR), with core (higher index $n_1$) surrounded by cladding (lower index $n_2 < n_1$) and jacket/coating (plastic buffer for protection). Typical diameters: core $5$–$100$ µm, cladding $125$ µm, coating $250$ µm.

2. Structure (cross-section)

Centre: Core (silica glass, Ge-doped to raise n1≈1.48) — carries light
Around: Cladding (pure silica, n2≈1.46) — confines light by TIR, reduces loss
Outside: Coating/Buffer (polymer) + Jacket (strength) — mechanical protection, not optical

Step-index: $n_1$ constant in core, abrupt step to $n_2$ at core-cladding interface.

Graded-index: $n$ varies parabolically from $n_1$ on axis to $n_2$ at edge:

, $Δ=(n_1-n_2)/n_1$.

3. Fabrication Note (brief)

Preform (rod) made by MCVD/CVD, then drawn to fibre; doping $GeO_2$ raises $n$, $F$ lowers $n$.

4. Why Core/Cladding Works

TIR at core-cladding interface requires $n_1>n_2$; critical angle

measured from normal. Light incident >$θ_c$ stays trapped.

5. Worked Dimension

Typical single-mode: core 9 µm, cladding 125 µm, n1=1.468, n2=1.462 ⇒ Δ≈0.004, NA≈0.12
Multimode: core 50 or 62.5 µm, NA≈0.2–0.3

6. Exam Points

• Draw cross-section (core, cladding, jacket) with $n_1>n_2$ and dimensions — 5 marks, very frequent.
• State step vs graded index — 3-5 marks, frequent.
• Mention doping (Ge) — 3 marks.