How Many Laser Generators Are There? （Part 1）

According to the analysis of the principle of laser generation, it is known that the laser is under the action of the "excitation source", the number of high-level electrons of the atom increases, and the transition to the low level after staying for a very short time, and the laser is emitted at the same time. It is not difficult to know that there must be many, many atoms of matter that can emit laser light under the action of the "excitation source".

Roughly speaking, to create a laser generator, there are four elements:

① Select the working medium to generate the laser. It can be a gas, liquid, solid, or semiconductor, as long as the particle population inversion can be achieved in the medium, the laser can be obtained.

② It's important to choose the "motivator". The "excitation source" makes the low-level electrons of the medium effectively transition to the high level, achieving the so-called electron number reversal. The method of gas discharge can be used to use electrons with kinetic energy to excite dielectric atoms, which is called electrical excitation; A Pulse light source can also be used to irradiate the working medium, called light excitation; There are thermal incentives, and chemical incentives and so on. Various modes of excitation are visualized as pumping or pumping. The purpose of "pumping" is to make the number of particles in the high energy level more than the low energy level.

③ It is also important to construct a resonator. Because the laser intensity generated by the "pump" is very weak and cannot be applied practically, the weak laser needs to be resonated with the laser, so that the output laser is enhanced to achieve the degree of practical application.

④ High-energy lasers require cooling systems. Because of the strong light inside the resonator, the resonator needs to be cooled.

According to the working medium of the laser, there are solid lasers, gas lasers, semiconductor lasers, chemical lasers, and now there is a "transparent ceramic laser".

According to the output mode of the laser, there are continuous laser and pulse lasers.

The performance indicators of the laser are mainly concentrated in the following aspects: First, the frequency range of the laser beam, because the laser can do "excitation source", but also can do spectrum analysis light source, so it is necessary to know the spectrum of the laser; The second is the power of the laser beam, especially the maximum power because the power size delineates the application range of the laser; The third is the irradiation area of the laser beam energy concentration, because the irradiation area is different in different applications.

Ⅰ. Solid-state lasers

Lasers can be made from many solid materials. In particular, with synthetic methods, in the process of manufacturing ceramics, you can create crystals containing different components, called "transparent ceramic laser medium", Now, the laser made with artificial crystals is very convenient and practical. Here are three common solid-state lasers.

1. Ruby laser

The first laser was the ruby laser. In July 1960, Maiman successfully made the world's first ruby laser, he shone the light of a flash into the ruby crystal, creating a coherent pulse laser beam, which shocked the world and triggered a boom in the development of lasers.

The analysis shows that ruby is a crystal, and its matrix is Al2O3(aluminum oxide), which contains 0.03-0.4% (weight ratio) of Cr2O3 (chromium trioxide), so you can press these materials powder forming, vacuum sintering, you can produce artificial ruby crystal, with artificial ruby rod production laser performance superior, widely used, This kind of laser medium has also been fully studied; In particular, the "pump source" adopts strong pulsed xenon lamp; The resonator is still the old way, at both ends of the laser, facing each other with two high reflectivity mirrors, one will almost completely reflect the laser to the working medium to participate in the resonance, the other will reflect most of the light back to the resonance, and let a small amount of laser is emitted through the mirror, emitted is a strong laser, is used for practical laser.

Now, the output energy of the ruby laser can be made of different levels, up to thousands of joules.

2. Neodymium-doped yttrium aluminum garnet crystal laser

Neodymium-doped yttrium aluminum garnet crystal is composed of yttrium (Y2O3) and aluminum (Al2O3) according to the ratio of 3:5 incorporation (Nd2O3), pressing molding, sintering crystals in vacuum at 1700℃, often used in near and far infrared solid-state lasers, superior performance. A continuous krypton lamp or tungsten iodide lamp is used as the pump light source, which is just matched with the absorption band of the 3-valent Nd ion. Neodymium-doped yttrium aluminum garnet lasers can be tens to hundreds of watts, and can also make high-power.

3. Nd glass laser

Neodymium is incorporated into high-purity silicate glass as the substrate, and this neodymium glass is used to make lasers, Phosphate is also used as the substrate to incorporate neodymium.

Neodymium glass lasers work similarly to the above crystal lasers.

Low-power neodymium glass lasers are effectively used for optical fiber communication, and the two most critical technologies for the development of optical fiber communication are the modulation of optical signals with semiconductor lasers, and the use of neodymium glass lasers as Repeaters (see popular science article series, No. 61). The signal light is amplified by the Repeaters, and the optical signals can be transmitted over long distances.

High-power neodymium glass lasers are also easy to make. Because the optical uniformity of neodymium glass is good, it is easy to prepare into a large volume material, which is easier to process than crystals, and the production cost of neodymium glass is very low. The larger the volume of the neodymium glass rod, the higher the output laser energy; Therefore, neodymium glass lasers are the preferred solid-state lasers in practice, which can be low power or high power.

High-power neodymium glass lasers have been used in nuclear fusion experiments. In 1974, Shanghai Optical Machinery Institute successfully developed six high-power neodymium glass laser systems at the nanosecond level of 100,000 megawatts, which successfully realized the use of a laser to generate high-temperature and high-density plasma, making a great contribution to the nuclear fusion "ignition" device.

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