The principle of lasers is based on stimulated emission, a concept first proposed by Einstein in the early 20th century. The main process is as follows:
- Electron Transition: Atoms or molecules in the working medium gain energy under the influence of a pump source (such as electrical energy, light energy, etc.), transitioning from a low energy level to a high energy level, entering an excited state. Because the high energy level is unstable, the atoms or molecules spontaneously transition back to the low energy level, releasing photons in the process.
- Resonant Cavity Reflection: These photons reflect back and forth within the resonant cavity, interacting with other excited-state atoms or molecules in the working medium, triggering more stimulated emission. This causes the number of photons to increase abruptly, resulting in high-intensity, highly monochromatic, and extremely directional laser light.
Laser mainly consists of three parts: the working medium, the pump source, and the resonant cavity.
- Working Medium: This is the foundation of laser generation. It is composed of an active medium that enables population inversion, such as ruby, neodymium glass, or carbon dioxide gas.
- Pump source: Provides energy to the working medium, inducing stimulated emission. Common methods include electrical excitation and optical excitation.
- Resonant cavity: Composed of total internal reflection mirrors and partial internal reflection mirrors, it provides feedback and an oscillating environment for photons, allowing them to travel back and forth multiple times within the cavity, enhancing the stimulated emission effect and ultimately forming laser output.
The main difference between single-mode and multi-mode lasers lies in the number of modes in the output beam.
- Single-mode laser: Supports only one mode of light propagation. It has high beam quality, good directionality and coherence, a standard circular beam spot, and a small divergence angle. It is suitable for high-precision applications such as laser interferometers and fiber optic communication.
- Multi-mode laser: Supports multiple modes of light propagation. It has a large output beam divergence angle, complex beam shape and intensity distribution, and a shorter coherence length, but high output power. It is suitable for less demanding applications such as materials processing and laser illumination.
Lasers are called Gaussian beams because their intensity distribution across their cross-section approximately conforms to a Gaussian function, meaning the intensity is high at the center and gradually decreases towards the edges, exhibiting a bell-shaped curve.
This distribution characteristic stems from the self-reproducibility of the laser during its formation within the resonant cavity; even after diffraction and propagation, its intensity distribution maintains a Gaussian form. Gaussian beams possess excellent focusing performance and monochromaticity, effectively reducing mode competition and improving beam quality, making them widely used in optical system design, laser processing, and other fields.
Laser Classification Lasers can be classified in many ways, one of which is by the working medium:
- Solid-State Lasers: These use solid materials as the working medium, such as neodymium-doped aluminum garnet (Nd:YAG) lasers. These lasers typically have high power output and good stability, and are widely used in industrial processing, medicine, and scientific research.
- Gas Lasers: These use gases as the working medium, such as helium-neon lasers (He-Ne) and carbon dioxide lasers (CO2). Gas lasers have wide applications in the visible and infrared spectral regions.
- Liquid lasers: Also known as dye lasers, these use organic dye solutions as the working medium. Their wavelength tunability gives them unique advantages in scientific research and biomedicine.
- Semiconductor lasers: These use semiconductor materials as the working medium, such as laser diodes. These lasers offer advantages in miniaturization and integration, and are widely used in optical communication, laser printing, and other fields.
- Free-electron lasers: These use high-speed free electron beams as the working medium. They offer a wide range of output power and wavelengths, making them suitable for high-energy physics and X-ray spectroscopy.
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