We report measurements of the spectral linewidth of InGaAsP lasers emitting at 1.3 &mgr;m as a function of the optical mode power. The spectral linewidth is of the order of 0.1–2.0 GHz for both gain‐ and index‐guided lasers. The linewidth can be described by a modified Schawlow–Townes formula taking into account a power‐independent contribution to the linewidth. This power‐independent contribution is attributed to mode intensity fluctuations due to mode coupling.