Spectrally resolved measurements of unclamped spontaneous emission above threshold in 1.3 µm InGaAsP bulk double heterostructure and compressively strained multiquantum-well lasers are analyzed using steady state rate equations. Both types of structures are characterized by nonlinear gain coefficients in the range of (2.7 -3.0) × 10-17 cm3at 20 °C. The nonlinear gain coefficient decreases with increasing temperature due to thermionic emission of carriers out of the active region and decreasing differential gain. The former effect was found to be more pronounced in quantum-well lasers. Excellent agreement with the rate equation based model is obtained.