Linewidth measurements of singlemode and multimode InGaAsP lasers were made above and below threshold with adapted interferometers. Linewidth against inverse power plots, Δv = f(1/P), show different Schawlow–Townes slopes above and below threshold. Differences have been observed depending on singlemode or multimode laser behaviour. In the singlemode case the linewidth enhancement factor has been determined (α = 2.6) by extrapolating and correlating results above and below threshold.