An optical laser works by using a gain medium to amplify light through stimulated emission, then bouncing it back and forth between mirrors to further amplify and emit a coherent beam.
Here's a breakdown of the process:
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Gain Medium: This is the heart of the laser. It can be a solid, liquid, or gas that contains atoms, molecules, or ions that can be excited to a higher energy level. Examples include ruby crystals, helium-neon gas, and semiconductor materials.
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Energy Pumping: Energy is supplied to the gain medium (often via electricity, another laser, or flash lamps) to excite the atoms/molecules/ions to a higher energy level. This process is known as "pumping."
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Spontaneous Emission: Some of the excited atoms/molecules/ions will spontaneously decay back to their ground state, releasing a photon (a particle of light) in the process. This photon travels in a random direction.
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Stimulated Emission: The key to laser operation! When a photon produced by spontaneous emission encounters another excited atom/molecule/ion, it can stimulate that atom/molecule/ion to also decay back to its ground state, releasing a second photon. This second photon has the exact same wavelength, phase, polarization, and direction as the first photon. This is how light amplification occurs.
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Optical Cavity (Resonator): The gain medium is placed between two mirrors. One mirror is highly reflective (close to 100%), while the other is partially reflective (typically a few percent transmission). Photons traveling along the axis of the cavity (the space between the mirrors) are reflected back and forth through the gain medium, stimulating more emission with each pass. This dramatically amplifies the light. The mirrors select for light that matches the resonant frequencies of the cavity.
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Coherent Beam Emission: Because all the stimulated photons are identical, the resulting light within the cavity is coherent (all the waves are in phase). A portion of this coherent light escapes through the partially reflective mirror, forming the laser beam.
In simpler terms: Think of the mirrors as creating a light amplifier. The gain medium is the "engine" that generates more light, and the mirrors ensure that only light traveling in the correct direction gets amplified. This results in a powerful, focused, and coherent beam of light. The mirror on one side of the laser's optical material bounces the photon back toward the electrons, stimulating more photons to be emitted and amplifying the light signal. The cavity is crucial for this process.
