The intensity of laser beam actually depends on the gain mediums. The initial beam is not so important. Most lasers involve cavities where the light bounces back and forward between mirrors and passes through a gain medium like ruby glass. Gain mediums increase the intensity of the beam. Gain mediums increase beam intensity exponentially for low intensity entrance beams but become more linear as the beam intensity increase, There is a saturation point of these gain mediums. The key point is when the losses in beam intensity for a beam cycling round the cavity equals the input of the gain medium, which is the operating point of the laser.

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