Abstract: The disclosure is directed to two embodiments for passive and one embodiment for active mode locking of lasers. The passive systems comprise a GaAs/GaAlAs diode laser (810 nm) pumped Nd:YAG (1964 nm) laser sharing the same optical cavity. A KTP crystal is used to produce a sum of the frequencies of the diode and Nd:YAG laser (460 nm). The KTP crystal and trichroic mirror (with a reflectivity at 810 nm less than the reflectivity at the other two wavelengths) combine to create an effective end mirror reflectivity which increases in accordance with the intensity of the pump laser wavelength (810 nm) to mode lock the system. The active embodiment uses successive sum and difference frequency generation to modulate the reflectivity of a cavity output coupling mirror in a continuous wave dye laser or external cavity diode laser.