In subcarrier multiplexed optical transmission systems the modulation linearity of the transmitter plays an important role because that nonlinearity gives rise to serious harmonic and intermodulation distortions. Therefore the modulation linearity of the laser has been significantly improved by applying an active matching technique. For that purpose, the microwave impedance of the laser has been measured as a function of the bias current and frequency. Based on the measured data, a nonlinear equivalent circuit has been derived that was used in the matching process. As a result, a significant improvement in the laser modulation linearity has been attained.