Only the resonant modes get amplified in the cavity. The modes parallel to the axis of the cavity get amplified and have a very narrow range of wavelengths
. The coherence length lc is the range of wavelengths contained in the laser beam and is given by
lc = 2 / (2)
(31.2)
For a strictly monochromatic beam, lc is infinite but in practice, they can be from a few centimeters (for He..Ne laser, 2 pm and lc = 10 cm) to a few meters.
For ordinary light bulbs, the coherence length is very small,
500nm.
Often lasers are made to operate in pulses whenever it is not possible to maintain the high population of the excited states and when it is necessary to cool the medium by discarding the heat generated in the medium during laser action. The Laser output power P is given by the amount of energy released during a pulse.
P = energy released / pulse duration
(31.3)
For a ns pulse, if the energy released is 1 MJ, the power is 1 MW (megawatt).
A standard method of generating pulses is accomplished by Q - switching. The principle involved here is to make the cavity absorb the pump energy causing population inversion when the cavity is not resonant (so that during the time of absorption, there is no laser action) and after the pump energy has caused sufficient population inversion, initiate laser action.
One way to achieve this is to insert a 'saturable dye' in the laser cavity. A saturable dye is a compound which can absorb a range of photons and once its excitations have been saturated it can not absorb any more photons and this makes the cavity transparent and the resonant modes (which were blocked before saturation) become active and laser action is accomplished by opening the switch
Figure 31.10 Non resonant and resonant cavities in Q - switching. Pulses of 2 to 20 ns can be obtained by this method.
. The coherence length lc is the range of wavelengths contained in the laser beam and is given by 
2 / (2
2 pm and lc = 10 cm) to a few meters. 
500nm. 
