Picosecond light pulses have been observed to appear spontaneously in ruby and Nd: glass lasers $Q$-switched by means of a rotating mirrot. This striking phenomenon is explained on the basis of coherence effects. The implication is that lasers with broad linewidths will quite generally tend to oscillate as pulse-regenerative oscillators.

• Received 6 September 1967

DOI:https://doi.org/10.1103/PhysRevLett.19.1014