GAIN-SWITCHED GAAS VERTICAL-CAVITY SURFACE-EMITTING LASERS

We have gain-switched GaAs vertical-cavity surface-emitting lasers (VC SEL's) using sinusoidal electrical modulation at rates between 1.5 and 8 GHz, using devices with operating wavelengths between 820 and 860 n m. The shortest pulse obtained directly from such a laser was 24 ps. T he time-bandwidth products for the gain-switched VCSEL's was between 0 .6 and 3, which is smaller than the time-bandwidth products observed f or gain-switched, single-frequency, edge-emitting lasers. Some of the excess bandwidth is caused by linear chirp, which was compensated usin g linear dispersion in single-mode optical fiber. The shortest compres sed pulse was 15 ps. The pulses contained significant nonlinear chirp, however, which reduced the expected compression factor for linear dis persion to a factor of 2. The timing jitter for gain-switched pulse tr ains was 4-6 ps, which is comparable to the timing jitter observed for gain-switched, single-frequency, edge-emitting lasers. We discuss dev ice design tradeoffs which affect the duration of the pulses from gain -switched VCSEL's. In particular, to minimize pulse duration, it is ne cessary to minimize the photon lifetime, which generally increases the threshold gain. Such an increase is deleterious because of the signif icant resistive heating in the DBR stacks of the monolithic device. Gi ven the present resistivity of the DBR stacks, the design of the VCSEL 's studied here is about optimal for short pulse generation.