The basic structure of the laser is illustrated in Figure 1 and consists of the following components:
1) Laser active medium
The production of laser light requires a suitable active medium, which can be gas, liquid, solid or semiconductor. In this medium, population inversion can be achieved to create the conditions necessary for the generation of laser light. The existence of metastable energy levels greatly facilitates population inversion.
There are almost a thousand types of active media available, capable of producing laser wavelengths ranging from ultraviolet to far infrared, covering a broad spectrum.
As the heart of the laser, the active medium consists of activating particles (typically metals) and a matrix. The energy level structure of the activating particles determines the spectral characteristics and fluorescence lifetime of the laser, while the matrix mainly determines the physical and chemical properties of the active medium.
Lasers can be divided into three-level systems (such as ruby lasers) and four-level systems (such as Nd:YAG lasers) based on the energy level structure of the activating particles. Commonly used shapes for the active medium are cylindrical (most widely used), flat, disc and tubular.
2) External pumping source
To achieve population inversion in the active medium, atoms must be excited in a certain way to increase the number of particles at higher energy levels. Continuous laser output requires constant “pumping” to maintain a larger population of particles at the higher energy level than at the lower level, therefore the external pumping source is also called the pumping source.
The pumping source provides energy to revert the population between high and low energy levels, with optical pumping being the main method currently used. The pump source must meet two basic conditions: it must have high luminous efficiency and its spectral characteristics must correspond to the absorption spectrum of the active medium. Common sources of bombs include inert gas discharge lamps, solar energy, and diode lasers.
Inert gas discharge lamps are the most commonly used pump sources. Solar power pumping is often used for low-power devices, especially small lasers in space applications that can use solar energy as a permanent power source. Diode pumping represents the future direction of solid-state lasers, combining many advantages and becoming one of the fastest developing lasers.
Diode pumping methods can be divided into two types: transverse pumping (front pumping with coaxial incidence) and longitudinal pumping (side pumping with vertical incidence).
Diode-pumped solid-state lasers have numerous advantages, including long lifetime, good frequency stability, and minimal thermal optical distortion, the most prominent advantage being high pumping efficiency due to precise wavelength matching of pump light and the absorption spectrum of the active medium.
3) Focusing Cavity
The focusing cavity has two functions: it effectively couples the pump source with the active medium and determines the density distribution of the pump light in the active medium, thereby affecting the uniformity, divergence and optical distortion of the output beam.
As both the active medium and the pump source are installed within the focusing cavity, its quality directly impacts the efficiency and performance of the pump. Elliptical cylinder focusing cavities are most commonly used in small solid-state lasers.
4) Optical Resonator
The optical resonator is essentially two highly reflective mirrors placed opposite each other at the ends of the laser. One mirror is fully reflective while the other is partially reflective, allowing most of the light to be reflected back while a small amount is transmitted, producing laser light. Light reflected back into the active medium continues to induce new stimulated emissions, amplifying the light.
The light oscillates back and forth within the resonator, causing an avalanche-like chain reaction and amplification, resulting in the emission of intense laser light from the partially reflective end of the mirror.
The optical resonator not only provides optical feedback to sustain continuous laser oscillation and stimulated emission, but also constrains the direction and frequency of the oscillating light beam to ensure the high monochromaticity and high directivity of the output laser. The simplest and most commonly used optical resonator for solid-state lasers consists of two flat (or spherical) mirrors facing each other.
(5) Cooling and filtration systems
Cooling and filtering systems are indispensable auxiliary devices for a laser. Lasers generate significant heat during operation, thus necessitating cooling measures. The cooling system mainly cools the laser active medium, the pumping source and the focusing cavity to ensure the normal operation of the laser and protect the equipment.
Cooling methods include liquid, gas and conduction, with liquid cooling being the most widely used. Furthermore, to obtain a laser beam with high monochromaticity, it is necessary to filter the output. The filtering system can remove most of the pump light and other interfering light, resulting in a high-quality monochromatic output laser beam.
Let us take the ruby laser as an example to explain the working principle of a laser. The active medium is a ruby rod. Ruby is an aluminum oxide crystal doped with a small amount of trivalent chromium ions, typically a chromium oxide mass ratio of about 0.05%. Because chromium ions absorb green and blue light from white light, the gem appears pink.
The ruby used by Maiman in the first laser invented in 1960 was a cylindrical rod with a diameter of 0.8 cm and a length of about 8 cm. Its ends are a pair of parallel flat mirrors, one coated with a fully reflective film and the other with a transmission rate of 10%, allowing the laser to pass through.
In the ruby laser, a high-pressure xenon lamp is used as a “pump” to excite chromium ions to the E. 3 excited state. Electrons pumped to E 3 quickly transition (in about 10 -8 seconds) to E 2 without radiation. E 2 is a metastable energy level where the probability of spontaneous emission for E 1 is very low, with a lifetime of up to 10 -3 seconds, allowing particles to remain for an extended period.
Consequently, particles accumulate in E 2 achieving a population inversion between energy levels E 2 and E 1 . The stimulated emission of light from E 2 toe 1 is a red laser with a wavelength of 694.3 nm. The pulsed laser obtained from the pulsed xenon lamp lasts less than 1 ms per light pulse, with the energy of each pulse exceeding 10 J and the power of each pulsed laser being capable of exceeding 10 kW.
The process of exciting chromium ions and emitting laser light involves three energy levels, which is why it is called a three-level system. In a three-level system, since the lower energy level E 1 is the ground state and typically accumulates a large number of atoms, achieving population inversion requires a substantial amount of excitation.
