Mode Locking and Ultrashort Pulses

In this section the influence of the relative phases of modes on the output waveform and on the operation of a laser will be considered. Previously it was shown that the operating frequencies of a laser are to first order determined by the cavity resonances and longitudinal modes (TEM00q) are separated in frequency by Δv = c/2Ln, where L is the cavity length and n is the index of refraction of the medium. We will see that when the phases of the modes are locked the output consists of a series of pulses. In an absorbing medium the pulse velocity has been measured as greater than c. From solid-state lasers pulses on the order of 10–11 sec have been observed.