To obtain pico second pulses, the method of mode locking or mode coupling is used. In the laser cavity, there are several resonating modes whose frequency differences are in multiples of c / 2L where L is the length of the cavity and c is the velocity of light. If the phase difference between these modes is random they can not interfere constructively. If however, the phase differences can be locked, we can get several sharp peaks of pico second duration. Mode locking can be achieved by periodically varying the Q factor of the cavity at the frequency c / 2L or by using a saturable dye. If the cavity length is 30 cms, the modes are separated by 2 ns. If 2000 modes can contribute, the pulse width will be 2 ps!
Femtosecond puses can be obtained by using techniques such as colliding pulse mode locking, Kerr lens mode locking and by using chirped laser mirrors and fiber lasers.
The use of these Lasers to study ultrafast relaxation processes is referred to as the pump probe technique. The pump laser pulse excites a molecule from the state |10 > to state |11>. The intensity of state |11> decays as shown in Fig 31.13 (c). The population of |11>, N1(t) is probed by a second Laser pulse called the probe pulse. The time delay between the two pulses, is limited by the pulse width of the two pulses.
Figure 31.13 a) Pump and probe pulses, b) States |10>, |11> and |12> and their populations NO, N1 and N2 and fluorescences of the excited states |11> and |2>, c) Population decay of N1
This method is useful for studying collisional relaxation in liquids, electronic relaxations and well as in real time observations of molecular vibration.
is limited by the pulse width of the two pulses. 
