What is the source of photon energy found in the center of the laser?

• A. Power Pumping Station
• B. Active Medium
• C. Optical Cavity
• D. Laser Output

Answer: (B)

The photons come from the active medium. Inside the gain medium, atoms or molecules are excited (population inversion) by pumping energy into the system. When they return to a lower energy state, they emit photons, and through stimulated emission these photons stimulate more emissions in the same medium. The energy of each photon equals the energy gap of that transition, so the photon energy is determined by the active medium itself. The pumping supplies the energy to reach the inverted population, while the optical cavity provides feedback to amplify and shape the beam, and the laser output is just the light we extract.