Page "Laser" Paragraph 30

If the gain ( amplification ) in the medium is larger than the resonator losses, then the power of the recirculating light can rise exponentially.

But each stimulated emission event returns an atom from its excited state to the ground state, reducing the gain of the medium.

With increasing beam power the net gain ( gain minus loss ) reduces to unity and the gain medium is said to be saturated.

In a continuous wave ( CW ) laser, the balance of pump power against gain saturation and cavity losses produces an equilibrium value of the laser power inside the cavity ; this equilibrium determines the operating point of the laser.

If the applied pump power is too small, the gain will never be sufficient to overcome the resonator losses, and laser light will not be produced.

The minimum pump power needed to begin laser action is called the lasing threshold.

The gain medium will amplify any photons passing through it, regardless of direction ; but only the photons in a spatial mode supported by the resonator will pass more than once through the medium and receive substantial amplification.