Abstract: A method and apparatus are disclosed for predicting the failure of semiconductor lasers. To predict the failure of a particular semiconductor laser, operational characteristics that are predictive of a laser's health are computed while the laser is in use (e.g., while the laser is transmitting a signal or pumping an optical amplifier or solid state laser). This is done by modulating the injection current of the semiconductor laser and observing changes in laser parameters such as output power and junction voltage. From these observations, various laser characteristics can be computed including current threshold, slope efficiency and dynamic resistance. By carefully selecting the injection current modulation frequency and degree, the system in which the laser is used is not significantly disturbed by the changes in output power.