The emitted photons are confined to a laser cavity to stimulate further emission from the excited atoms. Ruby laser works on the principle of a three level system. The pumping power required for such a system is very high because more than half of the ground state atoms have to be pumped into the upper level to achieve population inversion.
A few additional points to be noted about the three level system are :
As and , once the population inversion is achieved, the power required to maintain it is small.
transition is generally radiationless, the energy being given away to the lattice.
As more than half of the atoms are to be raised to the level , the probability of spontaneous emission is also much higher.
Laser transition occurs from the metastable state to the lowest possible state which are well separated. This leads to low efficiency.
and 
, once the population inversion is achieved, the power required to maintain it is small.
transition is generally radiationless, the energy being given away to the lattice. 
, the probability of spontaneous emission is also much higher. 
