Laser Linewidth it is the width of the power spectral density of the emitted electric field in terms of frequency, wavenumber or wavelength. The linewidth of a laser is strongly (but non-trivially) related to the temporal coherence, characterized by the coherence time or coherence length. In this article the concept of laser linewidth can have varied theoretical descriptions here, it also provides a simple experimental description. One of the first methods used to measure the coherence of a laser was interferometry. An alternative approach is the use of spectrometry.