Rami Arieli: "The Laser Adventure" Chapter 5 Section 1, page 7

1. Natural broadening.

This broadening is always present, and comes from the finite transition time from the upper laser level to the lower laser level.

Natural linewidth is narrow: 104 - 108 [Hz], compared to the radiation frequency of visible light: 1014 [Hz].

Each energy level has a specific width (Dn), and specific lifetime (Dt).

Natural broadening results from the Heisenberg uncertainty principle:

DE*Dt > h
DE = h* Dn
Dn > 1/ Dt

Numerical examples:

Dt = 10-8 [s] = = > Dn = 108 [Hz]
Dt = 10-4 [s] = = > Dn = 104 [Hz]

The longer the specific energy level transition lifetime, the narrower is its linewidth.