How does a ruby crystal laser work?

The ruby ​​crystal is a crystal of aluminum oxide A12O3 with a small addition (0.05%) of chromium. When chromium atoms are added, transparent ruby ​​crystals become pink and absorb radiation in two bands of the near ultraviolet region of the spectrum. In total, ruby ​​crystals absorb about 15% of the light from the flash lamp. When the spectrum is absorbed by chromium ions, the ions pass into an excited state. As a result of internal processes, excited chromium ions do not go over to ground states immediately, but through two excited levels. At these levels, the accumulation of ions occurs, and with a sufficiently powerful flash of a neon lamp, an inverse population occurs between the intermediate levels and the ground level of the chromium ion.
The ends of the ruby ​​rod are polished, covered with reflective interference films, while maintaining strict parallelism of the ends to each other.
When the population inversion of the levels of chromium ions in ruby ​​occurs, an avalanche increase in the number of forcedly emitted photons occurs, and feedback on the optical resonator formed by mirrors at the ends of the ruby ​​rod ensures the formation of a narrowly directed beam of red light. The duration of the laser pulse is 0.0001 s, slightly shorter than the duration of the flash of a neon lamp. The pulse energy of the ruby ​​laser is about 1 J.