Hi mohammed ridha al-shamapti,
The principle of a laser is based on three separate features: a) stimulated emission within an amplifying medium,
b) population inversion of electronics and c) an optical resonator.According to the quantum mechanics, an electron within an atom or lattice can have only certain values of energy, or energy levels.
There are many energy levels that an electron can occupy, but here we will only consider two.
If an electron is in the excited state with the energy E2 it may spontaneously decay to the ground state, with energy E1, releasing the difference in energy between the two states as a photon.
This process is called spontaneous emission, producing fluorescent light.
As we have to know Unique properties of lasers
Lasers range in size from tiny diode lasers to large systems the size of a football field. All of these have three basic properties in common, which separate lasers from ordinary light sources:

1.Monochromaticity: conventional light sources emit light consisting of a broad range of wavelengths (i.e. colours); a laser, on the other hand, emits only a very narrow range of wavelengths.

2.Directionality: conventional light sources, like a light bulb, emit light in all directions, while lasers can emit light that spreads (‘diverges’) only very little with distance. However, all laser beams eventually diverge as they move through space.

3.Coherence: Some consider coherence to be the most fundamental property of laser light, i.e. where all parts of the electromagnetic waves are in phase.

All laser are working on same principle either three or four levels.

Thanks,