The reason laser light is highly coherent, stable, and focused in one direction is due to a phenomenon called stimulated emission and the design of laser systems.
In a laser (an acronym for Light Amplification by Stimulated Emission of Radiation), the process of stimulated emission plays a crucial role. Stimulated emission occurs when an excited atom or molecule, already in an excited state, is stimulated by an incoming photon of the same energy. This stimulation causes the excited atom or molecule to emit a second photon with the same energy, phase, and direction as the stimulating photon.
The emitted photons in stimulated emission are coherent, meaning they have the same frequency, phase, and direction. This coherence is maintained because the stimulated emission process ensures that the emitted photons maintain the same properties as the incident photons.
Inside a laser system, this process is amplified and controlled to create a coherent and focused beam of laser light. The laser typically consists of an active medium, such as a crystal, gas, or semiconductor, placed between two mirrors. One mirror is partially reflective, allowing a small fraction of light to pass through, while the other mirror is highly reflective.
The active medium is excited using an external energy source, such as an electrical discharge or another laser. As the excited atoms or molecules in the active medium undergo stimulated emission, the emitted photons bounce back and forth between the mirrors, being amplified with each pass through the medium.
The highly reflective mirror at one end forms a resonant cavity, allowing only specific wavelengths to be amplified and constructive interference to occur. This amplification process causes the photons to align in phase and travel parallel to each other, resulting in a well-collimated and focused beam of laser light that emerges through the partially reflective mirror.
The design of the laser system and the feedback mechanism provided by the mirrors ensure that the light waves constructively interfere, reinforcing each other and maintaining a narrow beam. Additionally, the active medium and the resonant cavity are carefully designed to minimize scattering and absorption of light, further enhancing the coherence and stability of the laser beam.
It's important to note that while laser light can be highly focused and collimated, it can still undergo diffraction and scattering to some extent, especially as it propagates over longer distances. However, compared to conventional light sources, laser light exhibits significantly less divergence and can be more effectively concentrated and directed over longer distances.