The multi-peak spectrum of Ti: sapphire pulses observed experimentally under ultra-broad bandwidth conditions ( < 15 fs pulse length) has been explained theoretically. It is shown that the self-phase-modulation effect is equivalent to the process of degenerate four-wave frequency-mixing. Based on this theoretical underpinning, the presence or absence of a multipeak spectrum provides a practical measure of whether the intracavity group-velocity-dispersion is ideally compensated for or not. The multi-peak spectrum of Ti: sapphire pulses observed experimentally under ultra-broad bandwidth conditions (<15 fs pulse length) has been explained theoretically. It shows that the self-phase-modulation effect is equivalent to the process of degenerate four- Based on this theory underpinning, the presence or absence of a multipeak spectrum provides a practical measure of whether the intracavity group-velocity-dispersion is ideally compensated for or not.