Abstract

We have studied experimentally and theoretically the temporal and spectral characteristics of an actively mode-locked laser diode array in an external cavity as it evolves to the steady state. The steady-state pulse width of the laser output was 45 ps, assuming Gaussian pulse shape. The average output power was 15 mW at 433 MHz. During the first 40 ns after the onset of diode laser action, the laser pulse width sharpened quickly and finally reached the steady state in ~100 ns. This corresponded to forty-five round trips. The number of clusters of the longitudinal-mode spectrum reduces during the evolution and finally approaches a steady-state spectral distribution with the pulse energy mainly distributed among a few clusters near the line center. The narrowing of the spectrum as time goes by is a manifestation of homogeneous broadening. Unlocked, broad spectrum translates into random fast fluctuations in time. As the pulses become mode-locked, the spectrum becomes narrower and at the same time the pulsewidth or envelope can sharpen.

© 1991 Optical Society of America