A relation for the spectral width of the envelope of lasing emission of gain-guided lasers is derived which contains only parameters that can be easily measured, such as near and far field, gain width, and facet reflectivity. For GaAlAs

-groove lasers, one typically obtains for the spectral width

, in relation to the astigmatism factor

, a

Å at 5 mW optical power, while GaInAsP

-groove lasers at 1.3 μm exhibit a

Å.