THEORY OF SUBPICOSECOND PULSE GENERATION BY ACTIVE MODELOCKING OF A SEMICONDUCTOR-LASER AMPLIFIER IN AN EXTERNAL CAVITY - LIMITS FOR THE PULSEWIDTH

Active modelocking of a semiconductor laser amplifier (SCLA) in an external cavity is theoretically modeled, using a cosh-2-description for the optical pulses. Analytical solutions are derived. The model includes finite gainwidth, which is shown to be of the largest importance for a correct description, and gain saturation. A lower limit of the optical pulsewidth is derived in terms of the gainwidth, the injection current, the external loss, and other parameters. In recent experiments Bowers et al. found, surprisingly, pulses being a factor of 10 to 100 broader than the inverse gainwidth, and trailing pulses with an intensity almost of the same order of magnitude as the leading one even for SCLA facet reflectivities as low as 10(-2). Both features are quantitatively explained by our theory.