Q-switching is a technique to produce an extremely short duration high-energy laser pulses of the order of gigawatts, which is often called giant pulse formation.
Power is defined as :
Power: Energy/ Time (1 Watt= l Joule/ Second)
For example, Peak power in a ruby laser operating without Q-switch creates a pulse that lasts 0.5-mile sec, and its energy is 5 Joule. The same laser operating with the Q-switch creates a pulse that lasts 10 nano sec, and have the energy of l Joule. ’ Short pulses from gas lasers. and solid-state lasers, are usually created by using switching inside the laser cavity to change the quality (Q) factor of the laser cavity. Such switching is called Q-switch.
In a laser without Q-switch, the atoms are excited to the lasing level at a particular rate. Lasing starts as soon as ‘population inversion’ is created.
In a laser with Q-switch, the feedback which helps in establishing the population inversion is blocked, thus as long as the switch is ‘on’ there is no lasing. The continuous pumping transfers more and more atoms into the excited state. At the moment the Q-switch opens, all the excited state atoms create a short laser pulse with high energy. In a Q-switched laser, the high value of the Q-factor is maintained throughout the excitation of the active medium, until high energy is stored inside the laser cavity. Then the Q-factor is lowered quickly, and all the energy stored in the cavity is emitted as a short pulse. Fig. describes schematically the structure of a Q–switched laser.
Q Switch acts as a shutter that may be open suddenly inside the laser cavity. once the switch is closed, the laser radiation cannet move between the mirrors. Lasing cannot occur, but the excitation continues to pump energy into the cavity. The Q-switch is timed to Open when the gain of the active medium reaches its maximum value. All the energy keep in the active medium is emitted in a single pulse of electromagnetic radiation with high power
Optimal operation of Q-switch is achieved by fulfilling the following requirements :
(i) The transmission should be maximum (negligible losses) when the switch is open.
(ii) The transmission of radiation through the switch should be minimum when in its closed position.
(iii) There should be rapid switching between the closed and open positions.