As we have discussed in working of ruby laser that the terminus of laser action is the ground state E1 in ruby laser. Therefore it is difficult to maintain the population inversion. This will result in the depletion of upper laser level E2 population (due to stimulated emission) more rapidly than it can be restored by the flash light that is optical pumping source. The laser emission is made up of spikes of high intensity emissions. This phenomenon is called spiking of the laser.
After the depletion of E2 state, the laser action ceases for a few microseconds. Since the flash lamp is still active, it again pumps the ground state chromium ions to upper level and again laser action begins. A series of such pulses is produced until the intensity of the flash light has fallen to such a level that it can no linger rebuild the necessary population inversion. So the output laser will be in the form of pulse in ruby laser or in other words, it will not be continuous.
Reference: This article is referred from my authored book “optics and lasers” having ISBN 81-272-2948-2. In case of any doubt, post in the comment section.