What are the main core components of a laser?

From the barcode scanners at supermarket checkout counters to the precision surgical knives in hospitals, from the dazzling light shows on stages to the sparks flying as thick steel plates are cut in factories, lasers have permeated every aspect of modern life. And all of this originated from a device that can "excite" light and concentrate it into a powerful energy source - the laser. 
I. Source of Energy: Pump Source
The pump source is the "engine" of the laser. Its core function is to provide energy to the working substance, laying the foundation for the generation of laser. Just like a water pump draws water from a lower level to a higher level, the pump source "pumps" atoms or molecules from a lower energy level to a higher energy level, creating a particle number inversion (this is the key condition for generating laser). 
Common types of pumping sources include: 
Optical pumping: Using light from a powerful light source (such as a xenon lamp, a krypton lamp) or another laser (such as a laser diode) to irradiate the working substance. This is the most common method used in solid-state lasers (such as YAG lasers). 
Electro-pumping: Directly applying an electric current to the working substance, causing particle excitation through electron collisions. This is the main pumping method for semiconductor lasers (laser diodes) and gas lasers (such as CO₂ lasers). 
Chemical pumping: Utilizing the energy released by chemical reactions to excite particles, which is commonly employed in certain high-power gas lasers. 
The performance of the pumping source directly determines the efficiency and output power of the laser. It is the first step in laser generation and is a crucial one. 
II. Luminous Body: Gain Medium/Working Substance
The gain medium, also known as the working substance, is the "main stage" of the laser and is where the laser actually originates. It determines the core characteristics of the laser, such as the output wavelength (color) and potential power. 
Based on the state of matter, they are mainly divided into four categories: 
Gaseous media: such as carbon dioxide (CO₂), helium-neon (He-Ne), argon ions (Ar⁺), etc. They can generate continuous and high-quality laser beams, and are widely used in cutting, medical treatment and scientific research. 
Liquid medium: Such as organic solvents doped with dyes. It is characterized by the ability to adjust the output wavelength continuously within a certain range, and is commonly used in spectroscopy research. 
Solid media: Such as neodymium-doped yttrium aluminum garnet (Nd:YAG), ruby crystals or neodymium glass. They have a sturdy structure and can generate high-power, high-energy laser pulses, making them the preferred choice in industrial processing and military applications. 
Semiconductor materials: such as gallium arsenide (GaAs) and other compounds. They have small size, high efficiency and are easy to be electrically pumped. They are the absolute main force in fields like optical communication, optical disc reading and laser printing. 
When the particles in the gain medium are excited by the pump source, they will undergo the stimulated emission process and release new photons that are exactly the same as the incident photons, thereby achieving optical amplification. 
III. Resonant Soul: Optical Resonator
The optical resonator is the "quality forging machine" of a laser. It determines the directionality and monochromaticity of the laser. It is usually composed of two carefully placed reflecting mirrors facing each other. One is a total reflection mirror (with a reflection rate close to 100%), and the other is a partial reflection mirror (output coupling mirror, with a reflection rate of approximately 90% - 99%). 
Its core functions are three: 
Positive feedback: It causes the photons generated by stimulated emission to repeatedly reflect between two mirrors, continuously triggering a chain reaction-like stimulated emission, resulting in an exponential increase in light intensity. 
Mode selection: Only the specific wavelengths of light that propagate along the axial direction can stably oscillate and be greatly amplified within the cavity, which significantly improves the monochromaticity (color purity) of the laser. 
Directed output: Eventually, a portion of the extremely intense laser will be transmitted through the partial reflector, forming a highly collimated and narrowly divergent laser beam. 
Without an optical resonator, the working substance merely emits ordinary fluorescence of random direction and varying wavelengths. With it, however, we have forged the precise and pure "laser" that we see. 
IV. Final Touches: Cooling and Control System
For the majority of laser devices (especially those with medium to high power), a cooling system is indispensable. The majority of the energy input by the pump source is converted into heat, causing the temperature of the gain medium to rise sharply, resulting in performance degradation or even damage. An efficient cooling system (such as water cooling, air cooling, or TEC semiconductor cooling) can ensure the stable and continuous operation of the laser. 
Meanwhile, the control system is the core of the laser. It uses precise circuits and software to regulate the current of the pump source, control the operation of the cooling system, and may integrate components such as Q-switches and modulators to achieve precise control of the laser output (such as pulse width and frequency), in order to meet the requirements of various application scenarios. 
In summary, the pump source provides energy, the gain medium is responsible for light amplification, the optical resonator shapes the quality of the laser, and the cooling and control systems ensure its stable operation. These four core components are like a highly coordinated team, indispensable to each other. It is precisely the seamless cooperation among them that transforms ordinary light into a powerful tool capable of changing the world, continuously driving revolutionary progress in technology and industry.