Tell us about the device and how the ruby laser works.

The ruby ​​laser consists of a ruby ​​crystal, which is a cylindrical rod. The crystal is surrounded by a spiral-shaped xenon lamp. The ends of the ruby ​​rod parallel to each other are carefully ground. One of them is made mirrored, and the other is partially transparent. With a short but powerful flash of the lamp, the electrons in the chromium atoms pass into an excited state with the energy E3. After that, the electrons pass into the state with the energy E2. This level is metastable; therefore, a fairly large number of excited atoms are formed in the crystal. Each spontaneously emitted photon, upon transition from a state with energy E2 to the ground state, can induce the appearance of a “twin”. The photons that move along the crystal form an avalanche of “twins” after repeated reflection. It exits through the partially transparent end of the crystal as a pulse of red light.