The Spectral Width is the output wavelength range of a laser. Lasers are not completely monochromatic i.e. they do not produce a single wavelength of light and so the laser output is generally a range of wavelengths.

The spectral width curve of a laser is usually Gaussian and so it is difficult to point out the beginning and end of the curve. Thus Spectral Width is usually quoted as FWHM (Full Width at Half Maximum). FWHM of a laser is the width of its output optical spectrum, i.e. the width of the optical power spectral density of the emitted optical output in terms of frequency or wavelength. It is measured between the points on the curve where the power has decayed to one half of its peak value.