Temporal and spectral development of mode locking was investigated in a ring-cavity Nd : glass laser. It was found that whenever a mode-locked pulse formed in the ring cavity a satellite pulse would develop traveling in the opposite direction. The varying delay of the satellite pulse permitted time-resolved spectroscopy of the early stages of mode-locked oscillation. It was found that the spectral distribution broadens from 3 to 80 Å because of selfphase modulation inside the laser cavity.