FUNDAMENTAL LIMITS OF SUB-PS PULSE GENERATION BY ACTIVE-MODE LOCKING OF SEMICONDUCTOR-LASERS - THE SPECTRAL GAIN WIDTH AND THE FACET REFLECTIVITIES

We present a numerical simulation of active mode locking of a semiconductor laser amplifier (SCLA) in an external cavity, which includes the finite spectral gain width. This is shown to be essential for introducing a lower limit of the pulse width, although the gain linewidth is about a factor of 10-50 broader than the inverse pulse width. Our numerical treatment consistently explains the surprising experimental findings that even SCLA facet reflectivities as low as 10(-4) lead to trailing pulses with an intensity almost of the same order of magnitude as the leading one. Our theory makes detailed predictions about the influence of the facet reflectivities, the spectral gain width, and the injection current on the ps pulse generation. The numerical results are compared to an analytical expression for the obtainable pulsewidth which is based on the assumption of a cosh-2 shape for the optical pulses.