What is the structure of a ruby laser?

With the help of a powerful pulse of a flash lamp, chromium ions are transferred to an excited state. Transferring part of the energy to the crystal lattice, ions move to a metastable energy level, where they begin to accumulate. A random photon can cause an avalanche of induced coherent photons. The induced radiation is rapidly reflected from its ends many times. One of the ends of the ruby ​​rod is made mirrored, and the other – transparent. A powerful pulse of coherent monochromatic red radiation emerges through the partially transparent end.