The first working laser ever constructed used a ruby crystal as its active medium. This historic laser, known as the "Ruby Laser," was developed by physicist Theodore H. Maiman in 1960 at Hughes Research Laboratories in California, USA.

Here's a brief explanation of the active medium and how the Ruby Laser worked:

Active Medium: The active medium in a laser is a material that can amplify light through a process called stimulated emission. It is the heart of a laser device where the laser action occurs. The choice of active medium varies depending on the type of laser.

Ruby Crystal: In the case of the Ruby Laser, the active medium was a synthetic ruby crystal. This crystal was composed of aluminum oxide (Al2O3) with a small amount of chromium (Cr) ions added as an impurity. The chromium ions played a crucial role in the laser action.

Laser Operation: When energy was supplied to the ruby crystal, usually through a high-intensity flash lamp, the chromium ions absorbed this energy and became excited. As the chromium ions returned to their lower energy state, they emitted photons (light particles) in the red part of the visible spectrum. This emission process was characterized by stimulated emission, where an incoming photon triggered the emission of additional photons with the same frequency, phase, and direction.

Optical Resonator: To create a laser beam, the ruby crystal was placed between two mirrors, forming an optical resonator. One mirror was partially reflective, allowing a small portion of the emitted light to pass through, while the other mirror was highly reflective, causing the emitted light to bounce back and forth within the crystal. This bouncing of photons between the mirrors amplified the light through multiple passes.

Output Laser Beam: Eventually, the amplified light escaped through the partially reflective mirror, producing a highly focused, intense, and coherent beam of red laser light.