Study of intensity pulsations in proton-bombarded stripe-geometry double-heterostructure AlxGa1−xAs lasers

Abstract

The pulsations in the laser emission from several lasers have been studied as a function of temperature, cavity length, external mirror illumination, and effect of mirror etching. Selected DH lasers with 12-μm-wide stripes delineated by proton bombardment and 380-μm-long cavities were used. The modulation depth of the pulsations at 300 °K increases approximately linearly with current above a threshold value and becomes fully modulated at high output powers. At intermediate temperatures, the modulation exhibits a similar initial slope but is not complete at the highest measured output. At low temperatures, well-developed pulsations are not observed; the modulation reaches a maximum at a low amplitude level and then decreases. The frequency dependence of the pulsations indicates that when the modulation level is increasing, the frequency f is relatively constant, and above the maximum moduation f 2 increases approximately linearly with output power. The dependence on cavity length was studied using a series of 380-μm-long unbonded chips which showed strong pulsations under pulsed operation and were then cleaved into ∼125-μm-long sections. Of the short devices which lased, both center and end segments showed pulsations, indicating that unpumped end regions are not acting as saturable absorbers, resulting in the pulsations. The effects of illuminating the mirrors with 4880- and 6328-Å laser radiation and injecting 1.06-μm radiation into the active region were studied. Illuminating the mirrors increased the threshold and decreased the external differential quantum efficiency. The pulsations had the same light power threshold and had qualitatively the same dependence on output power. The possibility that an oxide layer over the facet is associated with the pulsations was studied by stripping the oxide with a dilute NH4OH solution. Up to four 5-sec strippings did not appreciably affect the pulsations. Hence, oxide layers and related nonradiative centers at the mirrors are not responsible for pulsations. Most lasers had kinks in the L-I characteristic. The pulsations were quenched if the lasers were operated in a region near the kink. The models used previously to describe the pulsations do not appear to apply to the lasers studied here. Much of the pulsation behavior can be explained by a simple nonlinear-gain model in the electron-photon density equations of the laser. Such a nonlinear gain can result from spatial nonlinearities in the gain within the stripe.