Spectra of random ZnO lasers pumped by nanosecond pulses

Spectra of random ZnO lasers recorded per pump nanosecond pulse are studied. It is found that these spectra considerably differ from spectra obtained upon picosecond pumping. As a rule, their lines are considerably broadened, the spectra themselves are often smoothed and substantially vary from pulse to pulse. We assume that these features can be explained by the fact that several lasing events at different frequencies occur during one pump pulse. Random variations of laser spectra we explain by fluctuations in the number of spontaneous photons involved in the formation of each lasing mode.