1. Stimulated radiation: the first condition, is also a necessary condition, but not a sufficient condition.

2. The working substance must have a metastable energy level

If you let these by the laser photon one by one to launch, is not able to form a strong energy. (Spontaneous emission) In general, when electrons are excited to high energy levels, the time to stay at high energy levels is short. And some of the material in the second level of electrons longer, second only to the ground level. This level is called metastable energy level.

3. Number of particles reversed

External photons can excite photons, produce stimulated radiation, but may also be absorbed by low energy levels. In the laser working medium, both the stimulated and stimulated absorption processes exist simultaneously. At room temperature, more absorption than the launch. Select the appropriate material, so that the high (sub) level of electronic than the low level of electronic more, that is, the formation of particles inversion, so that stimulated emission more than absorption.

4. resonant cavity

The photons generated in the laser are generated by spontaneous emission, and their frequency and direction are chaotic. To make the

frequency simple, the direction of concentration, it must have a resonant cavity

5. The gain is greater than the loss

Laser working materials and resonators can cause photon loss. Only when the photon oscillates once in the cavity produces more photons than the loss of photons, in order to have a magnifying effect.