The principle of laser generation begins with understanding the atomic structure. It wasn't until the early 20th century that the mysteries of atomic structure were uncovered through experimental methods.
Principle of Laser Generation:
Atoms resemble a miniature solar system, with a nucleus at the center and electrons orbiting around it continuously. The nucleus is composed of protons (positively charged) and neutrons (neutral), giving the atom an overall neutral charge due to an equal number of positive and negative charges from electrons and protons respectively.
In terms of mass, the nucleus contains most of the atom's mass, while the combined mass of all electrons is very small. Within the atomic structure, electrons orbit the nucleus with significant space for movement.
Atoms possess "internal energy," consisting of two parts: the kinetic energy from the electron's orbital velocity and potential energy from the distance between the negatively charged electrons and positively charged nucleus. The sum of all electrons' kinetic and potential energies constitutes the atom's internal energy.
Electrons orbit at varying distances from the nucleus. Those closer have lower energy, while those farther have higher energy. Based on their proximity, electrons are categorized into different "energy levels." Electrons within the same energy level exhibit identical energy levels and orbital spaces.
In summary, an atom may have multiple energy levels, each corresponding to distinct energy states. Some electrons orbit at "lower energy levels," while others orbit at "higher energy levels."
Modern physics textbooks clearly delineate the structural features of certain atoms, including the distribution of electrons across different energy levels.
In an atomic system, electrons generally move across layers, with some atoms at higher energy levels and others at lower energy levels. This constant state is influenced by external factors such as temperature, electricity, and magnetism.
Unstable high-energy electrons spontaneously transition to lower-energy levels, emitting excess energy in the form of photons. This radiation is characterized by independent transitions of each electron and random properties. The resultant light from spontaneous emission is incoherent and scattered in direction. However, each atom's spontaneous emission exhibits unique characteristics, resulting in distinct spectra.
This discussion recalls a fundamental principle in physics: "All objects have thermal radiation capabilities, absorbing and emitting electromagnetic waves continually. The electromagnetic waves emitted by thermal radiation are distributed spectrally and depend on the object's characteristics and temperature." Consequently, the existence of thermal radiation is due to spontaneous emission from atoms.
Stimulated emission occurs when a high-energy electron, under the influence of suitable photons, transitions to a lower energy state and emits a photon of the same frequency as the incident photon. The key characteristic of stimulated emission is that the emitted photon is in the same state (coherent) as the incident photon that induced it. They have identical frequencies, directions, and cannot be distinguished from each other. In this way, through stimulated emission, one photon becomes two identical photons. This means that light is amplified or "amplified". To achieve more frequent stimulated emission, specific conditions are required:
Pumping Source: A pumping source is essential to provide energy to excite more low-energy electrons to higher energy levels. This can be achieved through light, electrical, or chemical reactions. The purpose of the pumping source is to input energy, causing the number of high-energy electrons to exceed those at lower energy levels, thereby achieving population inversion.
Population Inversion: When the number of high-energy electrons exceeds those at lower levels, population inversion occurs. This inversion is fundamental to laser generation, as surplus high-energy electrons rapidly transition to lower levels, emitting photons of the same frequency and phase as the incident photons, thereby increasing the likelihood and frequency of stimulated emission.
Resonance Conditions: The pumping source must resonate with the energy level structure of the target atoms to effectively excite electron transitions. This means the energy of the pumping source must match the transition energies of the target atoms to achieve efficient energy transfer and stimulated emission.
In summary, laser generation depends on exciting atomic transitions through a pumping source to induce frequent stimulated emission. This process results in the emission of coherent photons, known as laser light, characterized by stimulated emission.
Conditions for Laser Generation
Laser generation relies on several key conditions:
Population Inversion: Achieving population inversion is crucial. This means more electrons are in higher energy states than in lower ones. This condition is typically achieved using a pumping source that adds energy to atoms or molecules.
Stimulated Emission: This process is central to laser operation. When a photon of the right energy encounters an atom in an excited state, it stimulates the emission of another photon with the same energy, phase, polarization, and direction. This results in coherent light amplification.
Optical Cavity: A resonant optical cavity is necessary to enhance stimulated emission. It consists of mirrors that reflect light back and forth through the gain medium (the material that emits light), thereby amplifying the stimulated emission process.
Stimulated Emission and Light Amplification
Stimulated emission enables the geometric increase in the number of photons with identical states (frequency, phase, polarization, and propagation direction). This phenomenon leads to light amplification. When an incoming photon triggers a cascade of emitted photons through stimulated emission, it generates a large number of photons in identical motion states. This is known as stimulated emission light amplification.
When light interacts with atomic systems, three processes simultaneously exist: spontaneous emission, stimulated emission, and stimulated absorption. Whether light is amplified depends on which transition process predominates. To generate laser light, stimulated emission must prevail, resolving two fundamental contradictions: between stimulated emission and stimulated absorption, and between stimulated emission and spontaneous emission.
Activation of Particle Energy Levels
To achieve stable laser emission, there must be light-emitting particles capable of achieving population inversion. These particles, known as active particles, can be molecules, atoms, or ions. Some active particles can exist independently, while others require a host material, known as a matrix, to provide a place for them. Together, the matrix and active particles form the laser working substance.
Not all materials can achieve population inversion, and even among those that can, not every pair of energy levels within a material can achieve it. If particles excited by pumping have a short lifetime in the excited state and cannot accumulate in large numbers over time, population inversion cannot be achieved. Therefore, the working substance needs to have metastable states, meaning it must possess suitable energy level structures.
An effective energy level system for laser operation typically includes upper and lower laser levels, and often includes additional levels relevant to laser production. Common laser working substances consist of atomic systems with a three-level or four-level structure, including metastable states.
Laser's Special Properties
Laser exhibits many unique properties, which have been harnessed to create valuable applications in various fields.
Directional Emission: Ordinary light sources emit light in all directions. To focus light in a specific direction, a spotlight or reflector is needed, such as headlights on cars or searchlights.
Laser beams, however, naturally emit in a single direction with minimal divergence, approximately 0.001 radians, which is nearly parallel. In 1962, humans first used lasers to illuminate the moon from Earth, a distance of about 380,000 kilometers, resulting in a spot on the lunar surface less than two kilometers in diameter. A well-focused laser beam appears parallel when directed towards the moon, covering its entire surface area.
High Brightness: Before lasers were invented, high-intensity pulsed xenon lamps were among the brightest artificial light sources, comparable to the brightness of the Sun. The brightness of laser light, such as that emitted by a ruby laser, can exceed a xenon lamp's brightness by several billion times.
Laser light's high brightness allows it to illuminate objects at great distances. For instance, a ruby laser beam can produce an irradiance of 0.02 units on the moon's surface, appearing bright red and clearly visible. In contrast, the irradiance produced by the most powerful searchlights would be imperceptible to the human eye, at about one trillionth of a unit.
Extreme Purity of Color: The color of light is determined by its wavelength (or frequency). Sunlight spans wavelengths from approximately 0.76 micrometers to 0.4 micrometers, corresponding to seven colors from red to violet, which are not monochromatic.
Common light sources like krypton, helium, neon, and hydrogen lamps emit monochromatic light of specific colors. While monochromatic, these sources still have a certain spectral width. For example, a krypton lamp emits predominantly red light but includes various shades of red upon closer examination.
Laser light, on the other hand, has an extremely narrow wavelength distribution, resulting in highly pure colors. For instance, a helium-neon laser emitting red light can achieve a wavelength distribution narrowed down to nanometers, which is two-ten-thousandths of the wavelength distribution of red light emitted by a krypton lamp. Thus, lasers exhibit far superior monochromaticity compared to any other single-color light source.
High Coherence: According to physics, when two beams of light meet, they can interfere if they have the same frequency, polarization, and constant phase difference.
Laser light emitted by a laser exhibits high coherence because its frequency, polarization, and phase are highly consistent. When two laser beams overlap in space, they exhibit stable interference patterns of alternating intensity. Therefore, laser light is coherent, whereas light from ordinary sources lacks coherence because its frequency, polarization, and phase vary.
Extremely Short Flash Duration: Ordinary light sources cannot achieve very short flash durations. For instance, a camera flash typically lasts about one-thousandth of a second. In contrast, pulsed lasers can achieve very short flash durations, as short as 6 femtoseconds (1 femtosecond = 10^-15 seconds).
Frequency of Laser: Lasers emit light with frequencies ranging between infrared and ultraviolet, depending on the type of laser.
3.Laser
The basic components of a laser include the pump source, resonator cavity, and laser medium. Here is a brief introduction, with detailed discussions of each component to follow in subsequent chapters.
Pump Source: The pump source is the energy supplier needed to activate the laser medium. It can provide energy to the laser medium through electricity, light, or chemical reactions, thereby exciting electrons to higher energy levels. The input of energy from the pump source is crucial for achieving population inversion.
Resonator Cavity: The resonator cavity consists of two mirrors or mirrors arranged in a space, designed to reflect laser light and create laser resonance. The resonator cavity enhances the stimulated emission process of the laser, enabling the laser to achieve amplification and stable output.
Laser Medium: The laser medium typically consists of active particles such as molecules, atoms, or ions, along with a medium in which these particles reside. The active particles undergo population inversion, leading to stimulated emission and resulting in laser output within the resonator cavity.
These components work together to form a complete laser system. Subsequent chapters will delve into the working principles, characteristics, and roles of each component in laser functionality.
Pump Source: It excites the laser working material, pumping active particles from the ground state to the higher energy level to achieve population inversion. It can be optical excitation, gas discharge excitation, chemical excitation, nuclear excitation, etc. The choice of excitation source depends on the characteristics of the working material. Therefore, different working materials often require different pump sources. For example, pulsed xenon lamps, iodine tungsten lamps, and other optical excitation methods are generally used for solid-state lasers, while electric excitation methods are used for gas lasers by directly exciting the working material through discharge. In addition, the selection of the excitation source should also consider issues such as excitation efficiency.
Resonator Cavity: It increases the effective length of the working medium, selects the directionality of the beam, and chooses the laser frequency, etc. The optical resonator cavity is the most critical factor determining the output characteristics of laser devices such as monochromaticity, directionality, and coherence. More specific details will be discussed later.
Working Medium: It is the material system that achieves population inversion and produces stimulated emission amplification of light. Laser media can be gases, liquids, solids, and semiconductors, requiring the existence of metastable energy levels. To select a laser working material, spectral analysis of the material must be performed. Based on this analysis, selections are made according to different requirements. This is a complex issue that involves many considerations. The most important issue is whether a material has suitable transition levels, i.e., whether population inversion can be achieved between certain energy levels.
Here's a brief introduction to several types of lasers:
Ruby Laser: The ruby laser was the first laser to demonstrate laser action, emitting a 694.3 nm red laser. Its working medium is a ruby rod made of chromium-doped aluminum oxide, achieving stimulated emission through trivalent chromium.
Nd
Laser: The Nd
laser is currently one of the best-performing lasers in medium-to-low-power solid-state lasers, capable of emitting light at several wavelengths. Its working material is a neodymium-doped garnet rod, operating as a four-level system and achieving stimulated emission through trivalent neodymium.
Helium-Neon (HeNe) Laser: The helium-neon laser was one of the earliest gas lasers developed, capable of emitting lasers at wavelengths of 632.8 nm, 1150 nm, and 3390 nm. It has good monochromaticity, a simple structure, and stable output power, making it widely used. Laser spectral lines occur between different excited states of neon atoms, with helium atoms playing a supportive role. Neon's energy level structure is complex but can also be viewed as a four-level system.
