Red lasers work by a process called stimulated emission within a gain medium that produces photons of red light. This process involves exciting atoms within the gain medium to a higher energy state and then triggering them to release photons that are coherent (in phase) and monochromatic (of a single color).
Here's a breakdown of the process:
1. The Gain Medium
The gain medium is the heart of the laser. It can be a solid (like a ruby crystal or a semiconductor material such as gallium arsenide), a liquid (like a dye solution), or a gas (like helium-neon). The type of gain medium determines the wavelength (color) of the laser light. In red lasers, semiconductor diodes are commonly used as the gain medium.
2. Energy Excitation (Pumping)
To achieve lasing, the atoms in the gain medium need to be "pumped" to a higher energy level. This can be done by:
- Optical pumping: Shining a bright light onto the gain medium.
- Electrical pumping: Passing an electrical current through the gain medium (common in semiconductor lasers).
The pumping process creates a population inversion, meaning there are more atoms in the excited state than in the ground state. This is crucial for stimulated emission.
3. Spontaneous Emission
Some excited atoms will spontaneously decay back to the ground state, releasing a photon of light in a random direction. This is called spontaneous emission. While spontaneous emission is not essential for lasing, it's the seed that gets the process started.
4. Stimulated Emission
This is the key to how lasers work. When a photon produced by spontaneous emission encounters another excited atom, it can stimulate that atom to also decay and release a photon. The stimulated photon has the same wavelength, phase, polarization, and direction as the stimulating photon. This results in an amplification of light.
5. Optical Resonator (Mirrors)
The gain medium is placed within an optical resonator, which typically consists of two mirrors – one highly reflective and the other partially reflective. The mirrors bounce the photons back and forth through the gain medium, stimulating more and more atoms to emit photons. This amplifies the light beam and ensures that it becomes highly coherent.
6. Laser Beam Output
The partially reflective mirror allows a portion of the amplified light to escape as the laser beam. This beam is:
- Monochromatic: It consists of light of a single, well-defined wavelength (color).
- Coherent: The photons are in phase with each other.
- Collimated: The beam is highly directional and doesn't spread out much.
Red Light Therapy (Related, but Not the Core Mechanism of the Laser)
It's important to distinguish how a red laser works (described above) from how red light therapy is believed to work. As noted in the prompt's reference, red light therapy is thought to affect mitochondria within cells, boosting cellular energy production. This is a separate mechanism from how the red laser itself generates the red light.
In summary, red lasers generate a focused, coherent beam of red light through stimulated emission within a gain medium, amplified by an optical resonator.
