I believe that the term laser is familiar to everyone. In our daily life, we often come into contact with lasers, such as the laser pen we use in class, and the CD-ROM drive used to read CD-ROM materials in computers or audio sets. In industry, lasers are often used for cutting or micromachining. Militarily, lasers are used to intercept missiles. Scientists also used lasers to measure the distance between the earth and the moon very accurately, with an error of only a few centimeters. Lasers are widely used. What are its characteristics and how did it come into being? Next, we will explain the basic characteristics and principles of laser.
Characteristics of laser
High brightness, high directivity, high monochromaticity and high coherence are the four characteristics of laser.
(1) High laser brightness: solid-state laser brightness can reach1011w/cn2sr. Moreover, after the high-brightness laser beam is focused by the lens, it can produce thousands or even tens of thousands of degrees of high temperature near the focus, making it possible to process almost all materials.
(2) High directivity of laser: The high directivity of laser enables it to effectively transmit a long distance, and at the same time, it can ensure the high power density of focusing, both of which are important conditions for laser processing.
(3) High monochromaticity of laser: Due to the high monochromaticity of laser, the beam can be accurately focused on the focus and high power density can be obtained.
(4) High coherence of laser: Coherence mainly describes the phase relationship of each part of light wave.
It is the singular characteristics of laser that make it widely used in life, industrial processing, military, scientific research and other fields.
Laser generation principle
The development of laser has a long history. Its principle was discovered by the famous physicist Einstein as early as 19 17, but it was not until 1958 that the laser was successfully manufactured for the first time. The English name of Laser is laser, which is the abbreviation of light amplification caused by stimulated emission of radiation. The English full name of laser has fully expressed the main process of making laser. But before explaining this process, we must first understand the structure of matter and the principle of radiation and absorption of light.
Matter consists of atoms. Figure 1 is a schematic diagram of carbon atoms. The center of an atom is the nucleus, which is composed of protons and neutrons. Protons are positively charged, while neutrons are uncharged. The periphery of an atom is covered with negatively charged electrons, which move around the nucleus. Interestingly, the energy of electrons in atoms is not arbitrary. Quantum mechanics, which describes the microscopic world, tells us that these electrons will be in some fixed "energy levels", and different energy levels correspond to different electron energies. For simplicity, we can imagine these energy levels as some orbits around the nucleus, as shown in figure 1. The farther away from the nucleus, the higher the orbital energy. In addition, the maximum number of electrons that can be accommodated in different orbits is also different. For example, the lowest orbit (also the orbit of the nearest nucleus) can only hold two electrons at most, and the higher orbit can hold eight electrons. In fact, this oversimplified model is not completely correct [1], but it is enough to help us explain the basic principle of laser.
Electrons can jump from one energy level to another by absorbing or releasing energy. For example, when an electron absorbs a photon [2], it may jump from a lower energy level to a higher energy level (Figure 2A). Similarly, an electron at a high energy level will jump to a low energy level by emitting a photon (Figure II B). In these processes, the photon energy absorbed or released by electrons is always equal to the energy difference between these two energy levels. Because photon energy determines the wavelength of light, the absorbed or released light has a fixed color.
When all the electrons in an atom are at the lowest possible energy level, the energy of the whole atom is the lowest, and we call this atom in the ground state. Figure 1 shows the arrangement of electrons when carbon atoms are in the ground state. When one or more electrons are at a higher energy level, we say that an atom is in an excited state. As mentioned earlier, electrons can jump between energy levels by absorption or release. Transition can be divided into three forms:
(1) Spontaneous absorption-electrons jump from a low energy level to a high energy level through absorbed photons (Figure 2A).
(2) Spontaneous emission-electrons spontaneously jump from high energy level to low energy level by releasing photons (Figure II b).
(3) Stimulated radiation-photons are injected into matter, which induces electrons to jump from high energy level to low energy level and releases photons. The incident photon and the released photon have the same wavelength and phase, and this wavelength corresponds to the energy difference between the two energy levels. A photon induces an atom to emit a photon, which eventually becomes two identical photons (Figure 2 c).
Laser is basically generated by the third transition mechanism.
There is another ingenious thing about generating laser, which is to realize the so-called inversion of particle number. Take ruby laser as an example (Figure 3). Atoms first absorb energy and jump to excited states. The time for an atom to be in an excited state is very short. After about 10-7 seconds, it will fall into an intermediate state called metastable state. The atom is in metastable state for a long time, about 10-3 seconds or more. Electrons are in metastable state for a long time, resulting in more atoms in metastable state than in ground state. This phenomenon is called population inversion. The inversion of particle number is the key to generate laser, because it makes more atoms pass through stimulated radiation from metastable state to ground state than through spontaneous absorption from ground state to metastable state, thus ensuring that photons in the medium can be increased to the output laser.
Structure of laser
A laser usually consists of three parts.
1, laser working medium
The generation of laser must choose a suitable working medium, which can be gas, liquid, solid or semiconductor. In this medium, the number of particles can be reversed, creating the necessary conditions for obtaining laser. Obviously, the existence of metastable energy levels is very beneficial to realize the reverse cycle of particle numbers. At present, there are nearly a thousand kinds of working media, and the laser wavelength that can be generated includes far infrared from vacuum ultraviolet channel, which is very extensive.
2. Incentive source
In order to reverse the number of particles in the working medium, it is necessary to stimulate the atomic system by certain methods and increase the number of particles in the upper energy level. Generally, gas discharge can be used to excite dielectric atoms with electrons with kinetic energy, which is called electrical excitation; The working medium can also be irradiated by pulsed light source, which is called light excitation; There are also thermal and chemical incentives. Various excitation methods are vividly called pumping or pumping. In order to continuously obtain laser output, it is necessary to continuously "pump" to maintain more particles with high energy level than with low energy level.
3. Resonant cavity
With suitable working substance and excitation source, the number of particles can be reversed, but the intensity of stimulated radiation generated by this method is too weak to be applied in practice. So people thought of using optical resonator to amplify. The so-called optical resonator is actually two mirrors with high reflectivity installed face to face at both ends of the laser. One piece is almost totally reflected, and the other piece is mostly reflected and slightly transmitted, so that the laser can be emitted through this mirror. The light reflected back to the working medium continues to induce new stimulated radiation, and the light is amplified. Therefore, the light oscillates back and forth in the resonant cavity, causing a chain reaction, being amplified like an avalanche, and generating intense laser light, which is output from one end of a partial mirror.
Let's take ruby laser as an example to explain the formation of laser. The working substance is a ruby stick. Ruby is an alumina crystal doped with a small amount of trivalent chromium ions. In fact, chromium oxide with a mass ratio of about 0.05% is doped. Because chromium ions absorb green light and blue light in white light, the gem is pink. 1960 The laser-generated ruby invented by Mayman is a round bar with a diameter of 0.8cm and a length of about 8cm. The two end faces are a pair of parallel plane mirrors, one end of which is coated with a total reflection film, and the transmittance of the other end is 10%, allowing the laser to pass through.
In the ruby laser, the high-pressure xenon lamp is used as the "pump", and the strong light emitted by the xenon lamp excites chromium ions to reach the excited state E3, and the electrons pumped to E3 quickly transition to E2 (~ 10-8s) by non-radiation. E2 is a metastable energy level, and the probability of spontaneous emission from E2 to E 1 is very small, and the lifetime is as long as 10-3s, that is, particles are allowed to stay for a long time. Therefore, particles accumulate on E2, and the number of particles in E2 and E 1 levels is reversed. From E2 to E 1, red laser with wavelength of 694.3nm is stimulated. Pulsed laser is obtained by pulsed xenon lamp. The duration of each light pulse is less than 1ms, and the energy of each light pulse is above 10J. That is to say, the power of each pulse laser can exceed the order of 10kW. It is noted that the above chromium ion involves three levels from excitation to laser emission, so it is called a three-level system. In a three-level system, the lower level E 1 is the ground state, which usually accumulates a large number of atoms and requires strong excitation to realize the inversion of the number of particles.
From the above description, we notice that a laser must have three basic conditions to work, namely, laser substance, optical resonator and pump source, and its basic structure is shown in Figure 4.
Energy is input into the laser material through the pump source, so that the number of particles is reversed, and the weak light generated by spontaneous radiation is amplified in the laser material. Because mirrors are placed at both ends of the laser material, some qualified light can be fed back to participate in the excitation, and then the excited light will oscillate. After repeated excitation, the light projected by the right-hand mirror is a high-brightness laser with good monochromaticity, directivity and coherence. Different types of lasers have different materials in luminescent materials, mirrors, pumping sources, etc. The lasers mentioned below are classified according to these differences.
Type of laser
There are different types of lasers. Generally, according to different working media, lasers can now be divided into solid-state lasers, gas lasers, liquid lasers and semiconductor lasers.
1, solid-state laser
Generally speaking, solid-state lasers have the characteristics of small devices, robustness, convenient use and high output power. The working medium of this kind of laser is that a small amount of activating ions are uniformly doped in the crystal or glass as the matrix material. In addition to the ruby and glass mentioned above, there are also commonly used lasers doped with trivalent neodymium ions in YAG crystals, which emit 1060nm near-infrared laser. Generally, the continuous power of a solid-state laser can reach more than 100W, and the pulse peak power can reach 109W. ..
2. Gas laser
The gas laser has simple structure and low cost; Convenient operation; Uniform working medium and good beam quality; And can work continuously for a long time. This is also a laser with the largest variety and the widest application at present, accounting for about 60% of the market. Among them, He-Ne laser is the most commonly used one.
3. Semiconductor laser
Semiconductor lasers use semiconductor materials as working media. At present, GaAs laser is relatively mature, emitting 840nm laser. There are also lasers such as aluminum-doped gallium arsenide, chromium sulfide and zinc sulfide. Excitation methods include optical pumping and electrical excitation. This kind of laser has the advantages of small volume, light weight, long service life and simple and firm structure, and is especially suitable for airplanes, vehicles and spaceships. In the late 1970s, the development of optical fiber communication and optical disc technology greatly promoted the development of semiconductor lasers.
4. Liquid laser
Dye laser is commonly used, and organic dye is used as working medium. In most cases, organic dyes are dissolved in solvents (ethanol, acetone, water, etc.). ), and some of them work in steam state. Laser with different wavelengths (in the visible range) can be obtained by using different dyes. Dye lasers generally use lasers as pumping sources, such as argon ion lasers. The working principle of liquid laser is complicated. Its advantages are continuous adjustable output wavelength and wide coverage, which makes it widely used.
A brief history of laser and laser technology in China
It has been 40 years since Einstein put forward the concept of stimulated radiation in 19 17. It was not until 1958 that two American scientists in the microwave field, C.H.Townes and A.I.Schawlaw, broke their silence and published the famous paper Infrared and Optical Lasers. The historical scientists working in the field of optics got excited at once and put forward various experimental schemes to realize the inversion of particle number, thus opening up a brand-new field of laser research.
In the same year, Soviet scientists Basov and prokhorov published the paper "Suggestions on Realizing Three-level Particle Number Inversion and Semiconductor Laser",1In September, 959, Downs put forward the suggestion of manufacturing ruby laser ...1In May, 960, T.H.Maiman of Hughes Laboratory in California, USA, manufactured the world's first ruby laser and obtained the wavelength of 60. Maiman uses ruby as luminescent material and pulse xenon lamp with high luminous density as excitation light source (pictured). In fact, his research began as early as 1957, and years of efforts finally activated the first laser beam in history. 1964 Downs, Basov and Prokhov shared the Nobel Prize in Physics for their contributions to laser research.
The first ruby laser in China was successfully developed at Changchun Institute of Optics and Mechanics, China Academy of Sciences in August 196 1. This kind of laser has a new improvement in structure than that designed by Maiman, especially when the industrial level of China was far lower than that of the United States, and the development conditions were very difficult. These are all designed and manufactured by the researchers themselves. After that, China's laser technology also developed rapidly and was widely used in various fields. 1June, 1987,10/2w high-power pulsed laser system, Shen Guang device, was successfully developed in Shanghai Institute of Optics and Mechanics, Chinese Academy of Sciences, and has made a good contribution to laser fusion research in China for many years.
Question: 1. What's the difference between laser and ordinary light in our life?
Please list the places where you use lasers in your life.
Chapter II Optical Resonator
This chapter mainly teaches the composition and function of optical resonator; Mode of optical resonator; Geometric analysis and diffraction theory analysis of optical resonator; Iterative solution of parallel plane cavity mode: stabilizing the focal cavity of spherical mirror; General stable spherical cavity and equivalent focal cavity; Unstable resonator. Key points: learn to write some propagation matrices of optical systems; It can be judged whether the cavity is stable; Master the method of realizing multi-longitudinal mode and single longitudinal mode oscillation; Choose FP method, compound cavity method and other single-mode methods, and give the corresponding mode spacing; Understand the establishment process of open die. Difficulties: the concept of aperture transmission line.
I. Composition of Optical Resonator
The simplest optical resonator consists of two mirrors with high reflectivity plated at both ends of the active medium. Common basic concepts:
Optical axis: the axis perpendicular to the mirror surface in the middle of the optical resonator.
Aperture: Optical resonant cavity plays a role in limiting the size and shape of light beam. In most cases, the aperture is the two end faces of the active substance, but in some lasers, additional elements are placed to limit the beam to an ideal shape.
Second, the types of optical resonator
Openness of resonant cavity, closed cavity, open cavity and gas waveguide cavity
Open optical resonator (open cavity) can usually be divided into stable cavity and unstable cavity.
Mirror-shaped, spherical and aspheric cavities, end-face reflection cavities and distributed feedback cavities.
The number of mirrors, two-mirror cavity and multi-mirror cavity, simple cavity and compound cavity.
Third, the role of optical resonator
Provide optical positive feedback: when the oscillating beam travels once in the cavity, it can not only reduce the loss in the cavity and the beam energy caused by the output of the laser beam through the mirror, but also ensure that the beam with enough energy is repeatedly amplified by the stimulated radiation of the active medium in the cavity to maintain continuous oscillation. There are two factors that affect the optical feedback of the resonant cavity: the reflectivity of the two mirrors that make up the resonant cavity; The geometric shapes of mirrors and their combinations.
Control of oscillating beam: effectively control the number of actual oscillating modes in the cavity to obtain coherent light with good monochromaticity and strong directivity, directly control the transverse distribution characteristics, spot size, resonance frequency and beam divergence angle of the laser beam, control the loss of the beam in the cavity, and control the output power of the laser beam at a certain gain.
4. Mode (mode) of optical resonator
1. longitudinal mode-longitudinal stable field distribution, the longitudinal mode appearing in the laser is determined by two factors. The larger the fluorescence linewidth of spontaneous emission of working atoms, the more longitudinal modes may appear; The longer the laser cavity is, the smaller the frequency interval between adjacent longitudinal modes is, and the more longitudinal modes can be accommodated in the same fluorescence linewidth.
2. The stable field distribution in transverse mode-transverse X-Y plane. Transverse mode (self-reproducing mode): A stable field distribution that can "self-reproduce" after propagating back and forth on the cavity mirror.
3. Measurement method of laser mode. Measurement method of transverse mode: place a light screen in the light path; Take pictures; The intensity distribution of laser beam is obtained by pinhole or knife-edge scanning method, and the distribution shape of laser transverse mode is determined by the measurement method of longitudinal mode: Fabry-Perot F-P scanning interferometer, and spherical scanning interferometer is used in the experiment.
5. Fox-Li numerical iteration method for parallel plane cavity.
The advantages of parallel plane cavity are: good beam directivity, large mode volume and easy to obtain single-mode oscillation. The disadvantages are: high adjustment precision, large diffraction loss and geometric loss, and its stability is between stable cavity and unstable cavity, which is not suitable for small gain devices and is still widely used in lasers with medium power or above.
The idea of iterative solution of resonant cavity: 1. Assuming that there is an initial field distribution on a mirror, it is substituted into the iterative formula to calculate the field generated on the second mirror after passing through the cavity for the first time. 2. Using the substitution iteration formula obtained by (1), the field generated by the second passage in the cavity on the first mirror is calculated; 3. After repeated operations for many times, observe whether the steady-state field distribution is formed.
Symmetrical rectangular (square mirror) parallel plane mirror cavity means that the mirrors of the resonant cavity are parallel, and the dimensions in the vertical and optical axis directions are limited. The parallel plane cavity of the strip mirror means that the dimension of the mirror is limited in one direction and infinite in the other direction. The iterative solutions of symmetric rectangle, strip mirror parallel plane cavity, circular mirror parallel plane cavity and parallel plane cavity are analyzed.
Sixth, the difference between * * focal cavity and parallel plane cavity
1. Distribution of the fundamental mode field of the mirror: the fundamental mode of the parallel plane cavity is distributed evenly and symmetrically on the whole mirror, and the amplitude is the largest in the center of the mirror, and gradually decreases at the edge of the mirror; * * * The distribution of the fundamental mode of the focal cavity on the mirror surface is based on Hermite-Gaussian approximation, which has nothing to do with the transverse geometric size of the mirror, but only with the cavity length; In general, the focal cavity mode is concentrated near the center of the mirror;
2. The mirror with a plane cavity with parallel phase distribution is not isoplane; * * * The mirror surface of the focal cavity is equal plane;
3. The diffraction loss of unidirectional loss parallel plane cavity is much higher than that of * * focal cavity;
4. The frequency change caused by the change of transverse mode order M and N in a plane cavity with unidirectional phase shift parallel to the resonant frequency is much smaller than that caused by the change of longitudinal mode order Q; In a * * * focal cavity, the changes of m, n or q have the same order of magnitude at the resonant frequency.
Seven. Amplitude and phase distribution of mirror modes in symmetric focal cavity of circular mirror
The amplitude distribution of the fundamental mode on the mirror is Gaussian. On the whole mirror, there is no nodal line in the center of the mirror (r=0), and the amplitude is the largest. The spot radius of the fundamental mode on the mirror (the distance from the center of the mirror when the amplitude of the fundamental mode drops to 1/e of the central value): for high-order modes, there are pitch lines along the radial direction, and the number of pitch lines is p; There are pitch circles along the radius direction, and the number of pitch circles is L; With the increase of p and l, the spot radius of the mode increases, and the spot radius increases faster with the increase of l than with the increase of p; Spot radius of higher-order mode: the distance between the point where the amplitude decreases to the outermost maximum at 1/e and the mirror center; The circular focal mirror itself is also an isophase plane.
Eight, generally stable spherical mirror cavity
General spherical mirror cavity: a cavity composed of two spherical mirrors with different curvature radii at arbitrary intervals.
Mode theory of general stable spherical mirror cavity: it can be strictly established from the diffraction integral equation of optical cavity, and based on the mode theory of * * * focal cavity, it is equivalent to * * * focal cavity.
Equivalence between general stable spherical cavity and * * focal cavity: According to the mode theory of * * focal cavity, any * * focal cavity is equivalent to infinite stable spherical cavities; And any stable spherical mirror cavity is uniquely equivalent to the focusing cavity. The equivalence between a generally stable spherical cavity and a * * focal cavity means that they have the same traveling wave field.
Nine, unstable resonator
Advantages of unstable resonator: 1. The controllable mode has a large volume, and the transverse size of the mode is enlarged by increasing the size of the mirror; 2. Controllable diffraction coupling output, and the output coupling rate is related to the geometric parameter G of the cavity; 3. It is easy to identify and control the lateral mode; 4. It is easy to obtain single-ended output and collimated parallel beams. Disadvantages of unstable resonator: 1. The output beam part is annular; 2. The beam intensity distribution is uneven, showing some kind of diffraction ring.
X. Mold Selection Technology
The advantages of laser are good monochromaticity, directionality and coherence. The ideal laser output beam should have only one mode, but for the actual laser, if mode selection is not adopted, its working state is often multimode. The light intensity distribution of high-order transverse mode laser beam is uneven and the beam divergence angle is large. The monochromaticity and coherence of multi-longitudinal mode and multi-transverse mode lasers are poor. Laser collimation, laser processing, nonlinear optics, laser remote ranging and other fields all need basic transverse mode laser beams. In the application of precision interferometry, optical communication and large area holography, the laser beam is required to be single transverse mode and single longitudinal mode. Therefore, it is an important subject to design and improve the laser resonator to obtain single-mode output.
Selection of transverse mode: In a stable cavity, the diffraction loss of fundamental mode is the smallest, and it will increase rapidly with the increase of transverse mode order. Different transverse modes in the resonator have different diffraction losses, which is the physical basis of transverse mode selection. In order to improve the resolution of modes, the diffraction loss ratio of higher-order modes and fundamental modes should be increased as much as possible, and the proportion of diffraction loss in the total loss should also be increased as much as possible. Diffraction loss and pattern recognition ability are related to the cavity type and Fresnel coefficient of the resonator.
Selection of longitudinal modes: In a general resonant cavity, the loss of different longitudinal modes is the same, so different gains of different longitudinal modes should be used for mode identification and selection. At the same time, artificial loss difference can also be introduced. Insert the F-P etalon into the cavity: by adjusting the parameters of the F-P etalon, there is only one transmission peak in the range of gain linewidth, and only one mode starts to vibrate in the range of transmission peak linewidth, so that the single longitudinal mode can be realized. That is, the mode selection condition is: 1. Select the appropriate optical path of etalon, so that the free spectral range of etalon is equivalent to the gain linewidth of laser. The transmission peaks of two or more etalons are avoided within the gain linewidth. 2. Select an appropriate etalon interface reflectivity, so that the adjacent longitudinal modes of the selected longitudinal mode are suppressed due to low transmittance and large loss.
Types and functions of optical resonator
Author: opticsky date: September 2006-16
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The optical resonator is composed of two or more optical mirrors, which can provide optical positive feedback. The two mirrors can be plane mirrors or spherical mirrors, which are placed at both ends of the laser working substance. The distance between the two mirrors is the cavity length. One of the mirrors has a reflectivity close to 100%, which is called a total reflector. The reflectivity of the other mirror is slightly lower, and the laser is output from this mirror, so it is called an output mirror. They are sometimes called high mirrors and low mirrors, respectively.
According to the shape and relative position of the two mirrors, the optical resonator can be divided into parallel plane cavity, flat concave cavity, symmetrical concave cavity, convex cavity and so on. If the focus of concave mirror falls on the plane mirror, the plano-concave cavity is called a semi-focal cavity. If concave mirror's center of the ball falls on the plane mirror, it will form half a heart cavity. The radius of curvature of the two reflecting spherical mirrors in the symmetrical concave cavity is the same. If the focal points of the mirrors are all located in the middle of the cavity, it is called a symmetrical focusing cavity. If the spherical centers of two spherical mirrors are in the center of the cavity, it is called a * * * heart cavity.
If the light beam propagates in the cavity for any long time without escaping from the cavity, it is called a stable cavity, otherwise it is called an unstable cavity. The resonant cavities listed above are all stable cavities. The resonant cavity composed of two convex mirrors is unstable. If the cavity length in the plano-concave cavity is too long, so that the spherical center of the concave spherical surface falls in the cavity, then the light beams in other directions will inevitably escape from the cavity except the light beams along the optical axis after many reflections, so it is also an unstable cavity. In a symmetrical concave cavity, if the cavity length is too long, the centers of two spherical surfaces will fall on the side near the center of the cavity, which is also an unstable cavity.
If the distance between any paraxial beam and its optical axis in the optical resonator of a stable cavity does not increase infinitely during its back-and-forth reflection, the cavity must be a stable cavity. If L represents the cavity length, and R 1 and R2 are the radii of curvature of two spherical mirrors, the stable cavity shall meet the following conditions:
According to the first inequality, only when R 1 and R2 are both larger or smaller than the cavity length can a stable cavity be formed. According to the second inequality, R 1 and R2 must be smaller than the cavity length and not too small.