Semiconductor laser threshold transition is investigated by high precision power and linewidth measurements giving specific behavior of linewidth at threshold depending on linewidth enhancement alpha factor. An analytical model based on Fokker–Planck equation resolution is proposed for extraction of major laser parameters by fitting simultaneously below and above threshold behaviors. Parameters are extracted for two singlemode semiconductor lasers with different detunings. Comparison with a threshold adapted rate equation model shows identical asymptotic behavior (Schawlow–Townes) for both models but simpler calculations and higher precision at threshold are obtained with the Fokker–Planck method. Detailed investigation of laser phase transition can profit microcavity and surface-emitting laser development by giving unique insight on spontaneous emission evolution and precise noise models.