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.