How is a fibre laser constructed and how does it produce laser light?

In one line

A fibre laser uses an optical fibre whose core is doped with rare-earth ions as the active medium, pumped by diode lasers, with fibre Bragg gratings acting as the cavity mirrors.

Explained simply

Picture a very long, thin glass "pipe" whose centre is sprinkled with special atoms. Pump light is trapped inside and keeps hitting those atoms along metres of fibre, so even a weak pump produces strong amplification. Mirrors are written directly into the fibre itself, so there are no loose optics to align.

How it works

Construction:

• Active medium: silica fibre core doped with rare-earth ions such as erbium (Er³⁺), ytterbium (Yb³⁺) or neodymium (Nd³⁺).
• Pump: semiconductor laser diodes (for example around 980 nm for erbium) coupled into the fibre. In high-power designs a double-clad fibre carries pump light in the inner cladding, which crosses the doped core repeatedly.
• Cavity: fibre Bragg gratings (FBGs), periodic refractive-index patterns written in the core. One acts as a high reflector and the other as a partial output reflector at the lasing wavelength.

Working:

1. Pump light is absorbed by the dopant ions, raising them to higher levels.
2. Ions relax to a long-lived (metastable) level, creating population inversion.
3. Spontaneous photons guided along the core cause stimulated emission.
4. FBGs reflect only the chosen wavelength back and forth, building up oscillation.
5. Laser light exits through the partially reflecting grating: about 1550 nm for erbium, roughly 1030 to 1080 nm for ytterbium.

Quick example

Ytterbium fibre lasers are widely used in industry for cutting and welding sheet metal, and erbium-doped fibres work at 1550 nm, the lowest-loss window of silica fibre used in telecom.

Advantages: high efficiency, excellent beam quality, compact and rugged, cooled easily because the fibre is long and thin, little alignment needed.

Common mistakes

• Saying the fibre cladding is doped; it is the core that holds the active ions.
• Drawing bulk mirrors only; fibre lasers normally use fibre Bragg gratings.
• Forgetting to name the pump source (diode lasers).

How MU asks it

Asked as "Explain construction and working of fibre laser" (5 marks), sometimes with "state its applications". Draw the fibre with pump diode, coupler, two FBGs and output; write the steps above and list two applications and two advantages.