a method and apparatus for stabilizing the output of a mode - locked laser by monitoring the temporal behavior of the pulse train profile and controlling the laser cavity optical length and / or loss modulation frequency accordingly . a mismatch of the cavity optical length and the loss modulation frequency will yield a first characteristic noise condition on the laser beam pulse train when the optical length is too short for a given loss modulation frequency and a second , different noise condition when the optical length is too long . the laser beam is monitored and analyzed to determine which noise condition is present . the cavity optical length is adjusted accordingly by movement of one or more optical elements or by changing the index of refraction of one or more optical elements . in the alternative , or additionally , the loss modulation frequency can be adjusted to bring the laser back into mode lock .