Active mode locking of double heterostructure lasers in an external cavity

Abstract

Proton bombarded stripe geometry lasers have been actively mode locked in an external cavity consisting of a lens, a Fabry-Perot etalon, and a mirror. Under optimized conditions, pulses as short as 5.3 psec have been observed from measurement of the second harmonic autocorrelation function. The dependence of the pulse width on external parameters was studied in detail. The pulse width t1/2 decreases with increasing direct current. The functional dependence of the pulse width on current indicates that a saturable absorption mechanism is present in the laser which causes significant pulse shortening. The pulse width decreases with increasing rf current. The pulse width is a sensitive function of the frequency detuning of the applied rf current. The shortest pulses are observed when the applied frequency is less than the round trip frequency of the pulse in the cavity. The frequency dependence of the pulse width was obtained using a long cavity with multiple pulses in the cavity. The shortest pulses occur between 0.9 and 1.4 GHz. The Fabry-Perot etalon limits the emission to a 1.9-Å spectral band. At optimum mode locking the band is ∼30 Å below the peak of the multimode emission obtained without the etalon. The shift of the gain curve to lower enery indicates that additional loss is introduced by the feedback from the external cavity. The experimental results are shown to be in good agreement with a theoretical description of mode locking.