The history of laser development has been more than twenty years since it came out with a brand-new attitude. However, the process of inventing the laser is little known, and even less people are interested in how the inventor made a difficult and tortuous exploration. In fact, every major invention is the crystallization of scientists' wisdom, which contains their sweat and hard work. Naturally, the invention of laser is no exception. More precisely, the research on laser entered a brand-new stage in the late 1950s. Before that, people only studied radio waves and microwaves in depth. Scientists have shortened the wavelength of radio waves to less than ten meters, which makes it possible to communicate all over the world. It was in the 1930s. Later, with the invention of klystron and hole magnetron, scientists studied the properties of centimeter wave. In World War II, due to the development of radio frequency and spectroscopy, the relationship between radiation waves and atoms was re-emphasized. During the war, scientists invented and developed radar. From the technology itself, radar is the product of the development of electromagnetic waves to ultrashort waves and microwaves. After World War II, scientists initiated microwave spectroscopy to explore the microwave range of the spectrum and extend it to shorter wavelengths. At that time, Columbia University had a radiation experimental group led by C.H.Townes, which had been engaged in the research of electromagnetism and millimeter wave radiation. In 195 1, Downes put forward the concept of maser to amplify microwaves through stimulated emission of radiation. After several years' efforts, in 1954, Downs and his assistants Gao Dun and H. Zeger invented the ammonia molecular beam maser and made it work normally. This laid the foundation for the birth of laser in the future. At that time, Downs hoped that the maser could generate microwaves with a wavelength of half a millimeter, but unfortunately, the output wavelength of the maser was 1. 25cm microwave oven. After the advent of maser, scientists hope to make a maser with shorter wavelength output. Downs believes that microwaves can be pushed near the infrared region or even the visible light band. 1958, A.L.Schawlow cooperated with Towns to publish the design scheme and theoretical calculation of maser working in visible light band for the first time. This pushes the laser research to a new stage. Now, it is known that there are two important conditions for generating laser: one is the inversion of particle number; The second is the resonant cavity. It is worth noting that after Einstein put forward the concept of stimulated radiation in 19 16, particle inversion was observed in the experimental study of gas discharge around 1940. According to the experimental technology at that time, it was possible to make a certain type of laser. But why can't it be built? Because no one, including Einstein himself, did not consider stimulated radiation, population inversion and resonant cavity together. Therefore, the invention of laser was delayed for several years. In the process of studying laser, the contribution of introducing resonator should be attributed to Luo Xiao. Luo Xiao has been engaged in spectroscopic research for a long time. The structure of the resonator is inspired by Fabry-Perot interferometer. As Luo Xiao himself said: "When I started to consider the optical resonator, it was natural to start with the structure of two Fabry-Perot interferometers with opposite mirrors." Actually, interferometer is a kind of resonator. During his seven years in Bell Telephone Laboratory, Luo Xiao accumulated a lot of data, and put forward the idea of laser in 1958. Almost at the same time, many laboratories began to study the possible materials and methods of lasers, and the research work of lasers with solid as working substance began at 1958. As Luo Xiao said, "I was completely instilled, which made me believe that what can be done in gas can also be done in solid, and it can be done better in solid." Therefore, I began to explore and look for the materials of solid-state lasers ... "Indeed, in less than a year, at the first international conference on quantum electronics held in September 1959, Luo Xiao proposed to use ruby as the working material of lasers. Soon, Luo Xiao described the structure of the laser in detail: "The structure of the solid maser is relatively simple. In essence, it has a rod (ruby), one end can be totally reflected, the other end is almost totally reflected, and the side is pumped by light. "Unfortunately, Luo Xiao didn't get enough light energy to reverse the number of particles, so he didn't succeed. Fortunately, the scientist T.H.Maiman skillfully used xenon lamp for optical pumping, thus obtaining the inversion of particle number. So, in June of 1960, a conference on the coherence of light was held at the University of Rochester. At the conference, Maiman successfully operated a laser. In July, Maiman unveiled a laser made of ruby. At this point, the world's first laser was born. Laser has a series of excellent characteristics such as monochromaticity and coherence. From the day it was born, people predicted its bright future. For more than 20 years, people have made lasers with different wavelengths, even tunable lasers. The successful development of high power laser has opened up a new field. The mechanism of free electron laser in 1977 is completely different. Its working substance is free electrons with extremely high energy. People can expect to realize continuous high power output through this laser, and the coverage frequency range can develop in both directions. Now, laser has been applied to optics, medicine, atomic energy, astronomy, geography, ocean and other fields, which marks the development of new technological revolution. It is true that if you compare the history of laser development with the history of electronics and aviation development, you have to realize that it is still in the early stage of laser development and more exciting prospects are coming.

Faneng 1954 made the first microwave quantum amplifier and obtained a highly coherent microwave beam. 1958a.l. Sholo and C.H. Downs extended the principle of microwave quantum amplifier to the optical frequency range, and pointed out the method of generating laser. 1960, t.h. maiman and others made the first ruby laser. In 196 1 year, Wen Jia and others made a He-Ne laser. 1962, R.N. Hall and others invented the gallium arsenide semiconductor laser. In the future, there will be more and more types of lasers. According to the working medium, lasers can be divided into four categories: gas lasers, solid-state lasers, semiconductor lasers and dye lasers. Recently, a free electron laser has been developed, whose working medium is a high-speed electron beam moving in a periodic magnetic field, and the laser wavelength can cover a wide band from microwave to X-ray. According to the working mode, there are several types, such as continuous, pulse, Q-switched and ultrashort pulse. High-power lasers usually have pulse output. There are thousands of laser wavelengths emitted by different kinds of lasers, the longest wavelength in microwave band is 0.7 mm, and the shortest wavelength in far ultraviolet region is 2 10 angstrom, and lasers in X-ray band are also under study.

Except for free electron lasers, the basic working principles of various lasers are the same. The basic components of the device include excitation (or pumping), working medium with metastable energy level and resonant cavity (see optical resonant cavity). Excitation means that the working medium is excited to the excited state after absorbing external energy, which creates conditions for realizing and maintaining the inversion of particle number. There are light excitation, electric excitation, chemical excitation and nuclear energy excitation. The metastable energy level of the working medium makes stimulated radiation dominant, thus realizing optical amplification. The resonant cavity can make the photons in the cavity have the same frequency, phase and running direction, and make the laser have good directivity and coherence.

Laser working substance refers to the substance system used to realize population inversion and generate stimulated radiation amplification of light, sometimes called laser gain medium, which can be solid (crystal, glass), gas (atomic gas, ionic gas, molecular gas), semiconductor and liquid. The main requirements for laser working materials are to achieve a large degree of particle number inversion between the specific energy levels of their working particles as much as possible, and to maintain this inversion as effectively as possible during the whole laser emission process; Therefore, the working fluid is required to have appropriate energy level structure and transition characteristics.

Excitation (pumping) system refers to the mechanism or device that provides energy source for laser working medium to realize and maintain particle number inversion. According to different working substances and laser working conditions, different excitation methods and devices can be used, and the following four are common. ① Optical excitation (optical pump). The working substance is irradiated by light from an external light source to realize the inversion of the number of particles. The whole excitation device usually consists of gas discharge light source (such as xenon lamp and krypton lamp) and condenser. ② Gas discharge excitation. The number of particles is reversed by using the gas discharge process in the gas working substance. The whole excitation device usually consists of a discharge electrode and a discharge power supply. ③ Chemical stimulation. The inversion of particle number is realized by using the chemical reaction process in the working medium, which usually requires suitable chemical reactants and corresponding initiation measures. (4) Nuclear energy incentives. It uses fission fragments, high-energy particles or radiation produced by small-scale nuclear fission reaction to excite the working medium and realize the inversion of particle number.

There are many kinds of lasers. Next, the working substance, excitation mode, working mode and output wavelength range of the laser will be introduced respectively.

According to the classification of working substances, all lasers can be divided into the following categories according to the different states of working substances: ① solid-state (crystal and glass) lasers, and the working substances used in such lasers are made by doping metal ions that can generate stimulated radiation in the crystal or glass matrix to form luminous centers; ② Gas lasers, whose working substance is gas, can be further divided into atomic gas lasers, ionic gas lasers, molecular gas lasers and excimer gas lasers. According to the different properties of working particles that actually produce stimulated emission in gas; (3) Liquid laser mainly includes two working substances, one is organic fluorescent dye solution, and the other is inorganic compound solution containing rare earth metal ions, in which metal ions (such as nd) play the role of working particles and inorganic compound liquid (such as SeOCl) plays the role of matrix; (4) Semiconductor laser, which takes a certain semiconductor material as the working substance and produces stimulated emission. Its principle is that unbalanced carriers are excited by a certain excitation mode (electric injection, optical pump or high-energy electron beam injection), so that the number of particles between energy bands of semiconductor materials or between energy bands and impurity levels is reversed, thus generating stimulated emission of light; ⑤ Free electron laser is a special type of new laser. Its working principle is a directional free electron beam moving at high speed in a periodically changing magnetic field. As long as the speed of the free electron beam is changed, the tunable coherent electromagnetic radiation can be generated. In principle, its coherent radiation spectrum can transition from X-ray band to microwave band, so it has a very attractive prospect.

According to the excitation mode, ① classify the optically pumped lasers. Refers to the laser excited by optical pump, including almost all solid-state lasers and liquid lasers, as well as a few gas lasers and semiconductor lasers. ② Electro-excited laser. Most gas lasers are excited by gas discharge (DC discharge, AC discharge, pulse discharge and electron beam injection), while most common semiconductor lasers are excited by junction current injection, and some semiconductor lasers can also be excited by high energy electron beam injection. ③ Chemical laser. This refers to the laser that uses the energy released by chemical reaction to excite the working substance. The chemical reactions produced by reflection can be initiated by light, discharge and chemistry respectively. (4) nuclear pump laser. It refers to a special laser that uses the energy released by small nuclear fission reaction to excite working substances, such as nuclear pump He-Ar laser.

Classification by working mode Due to the difference of working substance, excitation mode and application purpose, the working mode and working state of lasers are also different, which are mainly divided into the following categories. (1) CW laser is characterized by the excitation of working substance and the corresponding laser output, which can be continuously carried out in a long time range. Solid-state lasers excited by continuous light sources, gas lasers and semiconductor lasers working in continuous electric excitation mode all belong to this category. Because the device will inevitably produce overheating effect in the process of continuous operation, most devices need to take appropriate cooling measures. (2) Single pulse laser. For this kind of laser, the excitation of the working substance and the corresponding laser emission are both a single pulse process in time. General solid-state lasers, liquid lasers and some special gas lasers all work in this way. At this time, the thermal effect of the device can be ignored, so no special cooling measures can be taken. (3) Repetitive pulse laser, characterized in that its output is a series of repeated laser pulses. Therefore, the device can be excited by repetitive pulses, or it can be excited in a continuous manner, but the laser oscillation process is modulated in a certain way to obtain repetitive pulse laser output. Usually, the equipment also needs effective cooling measures. ④ Modulated laser refers to a pulse laser that uses certain switching technology to obtain higher output power. Its working principle is that laser oscillation does not occur after the number of particles in the working medium is reversed (the switch is in the closed state). When the number of particles accumulates to a high enough level, the switch is suddenly turned on instantaneously, thus forming very strong laser oscillation and high output power in a short time (for example, 10 ~ 10 second). Laser tuning technology). ⑤ Mode-locked laser is a special type of laser using mode-locked technology, which is characterized by a definite phase relationship between different longitudinal modes in the * * resonant cavity, so a series of laser ultrashort pulse sequences (pulse width 10 ~ 10 second) with equal time intervals can be obtained. If the special fast optical switching technology is further adopted, a single ultrashort laser pulse can be selected from the above pulse sequence. ⑥ Single mode and frequency stabilized lasers. Single-mode laser refers to a laser that works in a single transverse mode or a single longitudinal mode after adopting certain mode-limiting technology. Frequency-stabilized laser refers to a special laser device that uses certain automatic control measures to stabilize the output wavelength or frequency of the laser within a certain precision range. In some cases, they can also be made into special laser devices with single-mode operation and automatic frequency stabilization control ability (see laser frequency stabilization technology). ⑦ Tunable lasers, generally speaking, the output wavelength of lasers is fixed, but after adopting special tuning technology, the output laser wavelength of some lasers can be continuously and controllably changed within a certain range. This kind of laser is called tunable laser (see laser tuning technology).

According to the output wavelength range and the different output laser wavelength range, various lasers can be divided into the following categories. (1) far infrared laser, the output wavelength range is between 25 ~ 1000 micron, and the laser output of some molecular gas lasers and free electron lasers belong to this region. ② Mid-infrared laser refers to a laser device whose output laser wavelength is in the mid-infrared region (2.5 ~ 25 microns), which represents CO molecular gas laser (10.6 microns) and CO molecular gas laser (5 ~ 6 microns). (3) Near-infrared laser refers to a laser device whose output laser wavelength is in the near-infrared region (0.75 ~ 2.5 microns), represented by Nd-doped solid-state laser (1.06 microns), CaAs semiconductor diode laser (about 0.8 microns) and some gas lasers. ④ Visible laser refers to a kind of laser device whose output laser wavelength is in the visible spectrum region (4,000 ~ 7,000 angstrom or 0.4 ~ 0.7 micron), including ruby laser (6,943 angstrom), He-Ne laser (6,328 angstrom), argon ion laser (4,880 angstrom, 5 145 angstrom) and krypton ion laser (4,743 angstrom). ⑤ Near ultraviolet laser, whose output laser wavelength is in the near ultraviolet spectrum (2000 ~ 4000 angstrom), which represents nitrogen molecular laser (337 1 angstrom), xenon fluoride (XeF) excimer laser (351angstrom, 353 1 angstrom). ⑥ Vacuum ultraviolet laser. Its output laser wavelength range is in the vacuum ultraviolet spectrum (50 ~ 2000 angstrom), and typical examples are (h) molecular laser (1644 ~ 1098 angstrom), Xenon (xe) excimer laser (1730 angstrom) and so on. ⑦X-ray laser refers to a laser system whose output wavelength is in the X-ray spectral region (0.0 1 ~ 50 A). At present, soft X-ray has been successfully developed, but it is still in the exploration stage.

[Edit this paragraph] The invention of laser.

The invention of laser is a great achievement of science and technology in the 20th century. Finally, people can drive the luminescence process of molecules and atoms with extremely small scale, large quantity and chaotic motion, so as to obtain the ability to generate and amplify coherent infrared rays, visible rays and ultraviolet rays (even X rays and γ rays). With the rise of laser science and technology, human understanding and utilization of light has reached a new level.

The birth history of laser can be roughly divided into several stages, among which the concept of stimulated radiation put forward by Einstein in 19 16 is an important theoretical basis. This theory points out that a matter particle in a high-energy state will be transformed into a low-energy state under the action of a photon whose energy is equal to the energy difference between two energy levels, and a second photon will be generated and emitted at the same time as the first photon, which is stimulated radiation. The light output by this radiation is amplified and coherent, that is, the emission direction, frequency, phase and polarization of multiple photons are exactly the same.

Since then, the establishment and development of quantum mechanics have made people have a deeper understanding of the microstructure and motion law of matter, and the energy level distribution, transition and photon radiation of microscopic particles have also been more strongly proved, objectively perfecting Einstein's stimulated radiation theory and further laying a theoretical foundation for the generation of laser. After its birth in the late 1940s, quantum electronics was quickly applied to study the interaction between electromagnetic radiation and various micro-particle systems, and many corresponding devices were developed. The rapid development of these scientific theories and technologies has created conditions for the invention of laser.

If there are more particles in high energy state than in low energy state in a system, there will be particle number inversion. So as long as there is a photon, it will force an atom in a high energy state to emit a photon like it, and these two photons will cause other atoms to emit stimulated radiation, thus realizing the amplification of light; If the feedback effect of the appropriate resonant cavity is added, optical oscillation will be formed and laser will be emitted. This is how lasers work. 195 1 year, American physicists purcell and Pound successfully caused the inversion of the number of particles in the experiment and obtained stimulated radiation of 50 kHz per second. Later, American physicist Charles Downes and Soviet physicists Massoff and ProHohloff successively put forward the design of generating and amplifying microwave by using the principle of atomic and molecular stimulated radiation.

However, most of the above theoretical and experimental studies of microwave spectroscopy belong to "pure science", and whether the laser can be developed successfully was still very slim at that time.

But the scientists' efforts finally paid off. 1954, Thomas, the American physicist mentioned earlier, finally made the first ammonia molecular beam maser, which successfully set a precedent for using molecular and atomic systems as coherent amplifiers or oscillators for microwave radiation.

The maser developed by Downs et al. only produces microwaves with the wavelength of 1.25 cm, and the power is very small. With the development of production and technology, scientists are urged to explore new luminous mechanisms to produce new light sources with excellent performance. 1958, Downs and his brother-in-law, Arthur Sholow, combined the maser with the theoretical knowledge of optics and spectroscopy, and put forward the key proposal of adopting an open resonator, which prevented the characteristics of laser such as coherence, directivity, linewidth and noise. At the same time, Basov, ProHohloff and others also put forward the principle scheme to realize the optical amplification of stimulated radiation.

Since then, many laboratories in the world have participated in a fierce development competition to see who can successfully manufacture and operate the world's first laser.

1960, American physicist Theodore Mayman won the global development competition with a slight advantage in his research laboratory in Miami, Florida. He used a high-intensity flash tube to stimulate the chromium atoms in the ruby crystal, thus producing a fairly concentrated slender red beam, which can reach a higher temperature than the sun when it hits a certain point.

The "Meman design" has aroused the shock and suspicion of the scientific community, because scientists have been waiting and looking forward to the He-Ne laser.

Although Maiman was the first scientist to introduce laser into the practical field, the debate in court about who invented this technology once caused great controversy. One of the competitors is Gordon Gould, who invented "laser" (short for stimulated emission optical frequency amplifier). This word was put forward when he was a Ph.D. student at 1957 Columbia University. At the same time, the inventors of maser Towns and Sholow also developed the concept of laser. After the final judgment of the court, Downs became the winner, because the written work of the research was nine months earlier than Gould's. However, the invention right of Maiman Laser has not been shaken.

196065438+In February, an American scientist of Iranian origin, Jia Wan, led people to finally successfully manufacture and operate the world's first gas laser-He-Ne laser. 1962, three groups of scientists invented the semiconductor laser almost simultaneously. 1966, scientists developed an organic dye laser with continuously adjustable wavelength in a certain range. In addition, there are chemical lasers with large output energy, high power and independent of power grid.

Because of its outstanding characteristics, laser has been rapidly applied to industry, agriculture, precision measurement and detection, communication and information processing, medical treatment, military and other aspects, and has caused revolutionary breakthroughs in many fields. For example, people can use the concentrated and extremely high energy of laser to process various materials and drill 200 holes in a needle; As a means to stimulate, mutate, cauterize and vaporize organisms, laser has achieved good results in practical applications of medicine and agriculture. In the field of communication, a light-guiding cable that uses a laser column to transmit signals can carry information equivalent to that carried by 20 thousand telephone copper wires; Laser is not only used in communication, night vision, early warning and ranging, but also various laser weapons and laser-guided weapons have been put into practical use.

In the future, with the further development of laser technology, the performance of laser will be further improved and the cost will be further reduced, but its application scope will continue to expand and it will play an increasingly huge role.