Interpretation
Under normal circumstances, most particles are in the ground state. In order for these particles to produce radiation, the particles in the ground state must be excited to a higher energy level. Due to the difference in the internal structure of the atom, the same external conditions make the probability of the atom excited from the ground state to each higher energy level different. The possibility of an atom, molecule or ion being excited to a certain energy level is usually called the "excitation probability" of this energy level.
Theoretical research shows that there are two types of light emission processes. One is that in the absence of external photons, an atom at a higher energy level E2 spontaneously transitions to a lower energy level E1 and emits an energy For E2-E1 photons, this process is called "spontaneous transition"; the light waves emitted by the spontaneous transition of atoms are called spontaneous radiation.
Another kind of emission process is that atoms at a higher energy level E2, under the action of a radiation field with a frequency of ν, transition to a lower energy level E1 and radiate a photon of energy. This process is called induction Radiation transition; the light wave emitted by the induced radiation transition is called induced radiation.
Difference from spontaneous radiation
The most important difference between induced radiation and spontaneous radiation is interference. Spontaneous radiation is the spontaneous process of atoms that are not controlled by the external radiation field. The phase of the spontaneous radiation field of a large number of atoms does not interfere. The propagation direction and polarization state of the radiation field are also randomly distributed, while the induced radiation is radiated from the outside world. Lighting process under field control. Therefore, the frequency, phase, propagation direction and polarization state of the induced radiation field are exactly the same as the external radiation field. Laser is a kind of interference light that induces radiation.
Concept understanding
It is necessary to understand the concept of the energy level of atoms before inducing radiation. The most important thing to emit light is the so-called transition.
Atomic structure
Atom is basically composed of nucleus and electrons. If there is external energy that increases the distance between the electron and the nucleus, the internal energy increases; otherwise, it decreases.
Atomic Energy Level
Bohr Hypothesis: Atoms have certain stationary states. In these stationary states, they do not emit or absorb electromagnetic radiation. The stationary state energy of atoms can only adopt certain discrete states. The values of E1, E2, etc., these steady-state energy values are called energy levels.
Electrons can change their orbits through energy step transitions, and the orbits farther from the nucleus have higher energy levels. When an electron transitions from an orbit farther from the nucleus (high energy level) to an orbit closer to the nucleus (low energy level), a photon is emitted. Conversely, the absorption of photons or phonons can make electrons transition from lower energy orbits to higher energy orbits. Each transition corresponds to a specific energy and wavelength.
The high-level energy E2 and the low-level energy E1 corresponding to the transition satisfy the relationship:
In the above formula, c refers to the speed of light in vacuum, γ is the wavelength, ν is the frequency, and h is Planck's constant.
Background information
It is based on a new set of theories put forward by the great scientist Einstein in 1916. This theory is that in the atoms that make up matter, there are different numbers of particles (electrons) distributed at different energy levels, and the particles at high energy levels are excited by a certain photon and jump from the high energy level to (transition). At a low energy level, light of the same nature as the light that excites it will be radiated at this time, and in a certain state, there can be a phenomenon that a weak light excites a strong light. This is called "light amplification of induced radiation", or laser for short. Laser has four main characteristics: high brightness, high directivity, high monochromaticity and high coherence (monochromaticity and coherence have the same meaning).
Application
Nowadays laser has been widely used in laser welding, laser cutting, laser drilling (including oblique holes, different holes, plaster drilling, tipping paper drilling, steel plate drilling). Hole, packaging, printing and punching, etc.), laser quenching, laser heat treatment, laser marking, glass engraving, laser trimming, laser lithography, laser film making, laser film processing, laser packaging, laser repair circuit, laser wiring technology, laser Cleaning etc.
After more than 30 years of development, laser is almost everywhere nowadays. It has been used in all aspects of life and scientific research: laser acupuncture, laser cutting, laser cutting, laser welding, laser hardening, laser recording, Laser rangefinder, laser gyroscope, laser plumbometer, laser scalpel, laser bomb, laser radar, laser gun, laser cannon, etc. In the near future, lasers will certainly have wider applications.
Laser weapon is a directed energy weapon that uses a directed laser beam to directly damage or invalidate a target. According to different combat purposes, laser weapons can be divided into two categories: tactical laser weapons and strategic laser weapons. The weapon system is mainly composed of lasers, tracking, aiming, and launching devices. The commonly used lasers nowadays include chemical lasers, solid-state lasers, and CO2 lasers. Laser weapons have the advantages of fast attack speed, flexible steering, accurate strikes, and immunity to electromagnetic interference, but they also have weaknesses such as being vulnerable to weather and environmental influences. Laser weapons have a development history of more than 30 years, and breakthroughs have been made in their key technologies. The United States, Russia, France, Israel and other countries have successfully conducted various laser targeting tests. Nowadays, low-energy laser weapons have been put into use. They are mainly used to interfere with and blind photoelectric sensors at relatively close distances, as well as to attack human eyes and some enhanced observation equipment; high-energy laser weapons mainly use chemical lasers. According to the current level, 5～ It is expected to be deployed and used on ground and air platforms within 10 years for tactical air defense, theater anti-missile and anti-satellite operations.