This application note presents the time measurements of the optical gain in a semiconductor laser using the Apex Technolgoies High Resolution Optical Spectrum Analyser AP2040/AP2050 series. The high resolution allows for accurate gain measurements close to the lasing threshold. This is demonstrated by gain measurements on a bulk InGaAsP 1.5 μm Fabry-Perot laser. Combined with direct measurement of transparency carrier density values, parameters were determined for characterising the gain at a range of wavelengths and temperatures. The necessity of the use of a logarithmic gain model is shown.