  We observed periodic optical pulses from an Er:YSO crystal during irradiating
with an continuous-wave excitation laser. We refer to this new phenomenon as
"periodic super-radiance". This periodicity can be understood qualitatively by
a simple model, in which a cyclic process of a continuous supply of population
inversion and a sudden burst of super-radiance is repeated. The excitation
power dependences of peak interval and the pulse area can be interpreted with
our simple model. In addition, the linewidth of super-radiance is much narrower
than an inhomogeneous broadening in a crystal. This result suggests that only
Er3+ ions in a specific environment are involved in super-radiance.
