Types of optical fibre: construction and propagation

An optical fibre is a thin cylindrical waveguide of glass or plastic that guides light by total internal reflection. Fibres are classified by refractive index profile (step or graded) and number of modes (single or multi).

Construction (common to all types)

   +-------------------------------+
   |  jacket / buffer (plastic)    |
   |  +-------------------------+  |
   |  |  cladding  n2 (125 µm)  |  |
   |  |  +-------------------+  |  |
   |  |  |   core  n1 > n2   |  |  |
   |  |  +-------------------+  |  |
   |  +-------------------------+  |
   +-------------------------------+

• Core: high-index glass (silica doped with GeO₂) that carries the light.
• Cladding: lower-index glass that confines light by TIR and protects the core surface.
• Buffer coating / jacket: plastic layers for mechanical protection; cables add strength members (Kevlar) and an outer sheath.

Propagation mechanism

Light travels in an optical fibre by total internal reflection (TIR). The core has a higher refractive index (n1) than the cladding (n2). When a ray inside the core meets the core–cladding boundary at an angle (from the normal) greater than the critical angle φc = sin⁻¹(n2/n1), it is totally reflected back into the core with no refraction loss. Repeated TIR guides the ray along the fibre, even around gentle bends. Only rays entering within the acceptance angle θa = sin⁻¹√(n1² − n2²) are guided. Each allowed ray direction is a mode.

1. Step-index multimode fibre

 n          cladding ______________________
 |  ____                  /\    /\    /\
 | |    |    core  ______/__\__/__\__/__\__
 |_|    |_  r       zig-zag rays

• Core diameter 50–200 µm (typically 100 µm), cladding 125–400 µm; index changes abruptly from n1 to n2.
• Many modes travel in zig-zag paths of different lengths, so pulses spread (intermodal dispersion). Bandwidth is low (~20–50 MHz·km).
• Easy to couple light from LEDs; cheap connectors.
• Applications: short data links, sensors, illumination, plastic optical fibre in cars and home networks.

2. Step-index single-mode fibre

 cladding  ____________________________
 core      -----------------------------> one straight mode
 cladding  ____________________________

• Very small core (8–10 µm) and small Δ (~0.3%), so only one mode propagates (V < 2.405).
• No intermodal dispersion; very high bandwidth (tens of GHz·km and more) and lowest loss (~0.2 dB/km at 1550 nm).
• Needs a laser source and precise splicing.
• Applications: long-haul telephone and internet backbones, submarine cables, FTTH, CATV trunks.

3. Graded-index multimode fibre

 cladding  ____________________________
 core        _--_      _--_      _--_
           _-    -_  _-    -_  _-    -_   curved rays
 cladding  ____________________________

• Core index highest at the axis and falls gradually (near parabolic) to n2 at the cladding; core 50 or 62.5 µm, cladding 125 µm.
• Rays bend smoothly (helical/sinusoidal paths). Outer rays travel longer paths but in lower-index glass at higher speed, so all modes arrive nearly together; modal dispersion is much lower (bandwidth ~ 1 GHz·km).
• Applications: LANs, data centres, campus backbones and medium-distance links with LED or VCSEL sources.

Comparison