Talking about the origin, development and application of laser devices

The origin and development of laser devices

The light source required for optical fiber communication should be a light source that can be modulated at a high speed in order to carry large-capacity information. Such as lasers and light-emitting tubes. The so-called "modulation" is to change the intensity of light according to the information to be transmitted to carry information.

In 1960, Maimen invented the ruby laser. The difference between laser and ordinary light is mainly that the light frequency of laser is very simple and has linear spectrum. It is called coherent light in optics and is the most suitable light source for optical fiber communication. The light frequency of ordinary light is very messy, and it contains many wavelengths. The light frequency of ordinary light is very messy, and it contains many wavelengths. The characteristic of coherent light is that the light energy is concentrated, the divergence angle is very small, and it is similar to parallel light. After the invention of the ruby laser, a variety of lasers were born: gas lasers, such as helium-neon lasers; solid-state lasers, such as YAG iridium aluminum garnet lasers; chemical lasers; dye lasers, etc. Among them, the semiconductor laser is the most suitable light source for optical fiber communication. It is small in size, high in efficiency, and its wavelength is compatible with the low loss window of optical fiber.

However, the manufacturing process of semiconductor lasers is very complicated. It is necessary to epitaxially grow 5 layers of doped semiconductors on extremely high-purity and defect-free substrate materials, and then lithographically engrave micron-sized optical waveguides on it. Compared with optical fibers, it is more difficult. Nothing less. At the end of the 1970s, a long-life semiconductor laser with continuous working at room temperature was finally made. In 1976, the world's first practical optical fiber communication line was established from Atlanta to Washington. At this time, the semiconductor laser has not yet passed the test, and the light source is a semiconductor light-emitting tube. In the early 1980s, single-mode fibers and lasers had matured, and since then the superiority of large-capacity optical fiber communications has gradually been brought into play.

The light emitted by the semiconductor laser has a pure spectrum, concentrated energy, and a very thin beam, which can efficiently shoot into a single-mode fiber with a core diameter of only 8 microns. Today's high-speed optical fiber communication system uses semiconductor lasers as light sources.

The principle of semiconductor laser light emission

The simplest structure of a semiconductor laser is shown in Figure 1. It is composed of 5 layers of semiconductors, and the active layer in the middle is doped with active materials. After the current is injected into the two electrodes, the electrons in the atoms of the active material in the active layer are excited from a low-energy state to a high-energy state. These high-energy state electrons emit light when they are reduced from a high-energy state to a low-energy state, which is called spontaneous emission. The spontaneously radiated light is not very pure, that is, it contains a wide spectrum. If the spontaneously radiated light is strong, the light is reflected back and forth in the two mirrors of the semiconductor. In this process, for the light energies with the same phase, the energies are superimposed and become larger and larger; for the light energies with inconsistent phases, the energies weaken each other and become smaller and smaller. The energy is converted into light energy of a certain wavelength according to the cavity formed by the two mirrors of the semiconductor, and when it oscillates, it forms laser light. Laser light is generated by so-called stimulated radiation. The mirror surface of the semiconductor is smooth and translucent, and the laser light can be output from the mirror surface. The function of the confinement layer is to concentrate light energy in the active layer to improve efficiency. The wavelength of the laser light emitted by a semiconductor laser mainly depends on the distance between the semiconductor material and the mirror surface.

Laser application

With its excellent performance and low price, lasers have been widely used in optical fiber communications, optical fiber sensing, industrial processing, medical treatment, military and other fields.

In terms of communication, the 1.30 micron and 1.55 micron lasers provided by the laser are two low-loss windows for communication. The laser can not only produce continuous laser output, but also can realize the generation of ps-fs ultra-short optical pulse, which has huge potential applications in DWDM systems. The laser enables the communication system to have a higher transmission speed and a longer transmission distance, which plays an irreplaceable role.

In terms of sensing, lasers are used in phase, wavelength, light intensity, and polarization fiber sensing. Temperature and pressure can be measured in oil or natural gas wells; strain can be measured in roads, bridges and ship hulls; flight health monitoring in aircraft wings; and it can also be used in fiber optic hydrophones and current sensing.

In industry, lasers have done a lot in the processing and processing of metal and non-metal materials, laser engraving, laser product marking, laser welding, weld seam cleaning, precision drilling, and laser graphic art imaging.

In the medical field, lasers have been widely used because of their small size, good fiber flexibility, good beam quality, and no cooling system. The fiber laser can shorten the operation time of tissue shedding and photocoagulation: at the same time, the success rate of curing ophthalmic diseases such as keratoplasty, myopia, and hyperopia is greatly improved. It also plays an important role in plastic surgery, tumor removal, cancer treatment, and skin diseases.

In the military, high-power lasers are popular for their high brightness, small irradiation area, and small size. As a weapon, it can accurately aim and strike and destroy the target. In addition, it has important significance in positioning, ranging, remote sensing, tracking and guidance, lidar system sensing technology and space technology.

Features of laser

Fiber lasers have attracted much attention in recent years and have become the focus of everyone's research. This is because it has long-standing advantages that other lasers cannot match, mainly in:

(1) The beam quality is good, with very good monochromaticity, directivity and stability;

(2) The optical fiber is both a laser gain medium and a light guiding medium, so the coupling efficiency of the pump light is quite high, the core diameter is small, the high power density is easily formed in the fiber, and the fiber laser can easily extend the gain length , So that the pump light is fully absorbed, and the total light-to-light conversion efficiency exceeds 60%;

(3) The matrix material is SiO2, which has excellent temperature stability; while the cylindrical structure of the optical fiber has a high surface area/volume ratio, fast heat dissipation, and the ambient temperature is allowed to be -20-+7000C, which is the heat load of the working material It is quite small, no cooling system is needed, and it can produce high brightness and high peak power, which has reached 140mw/cm2;

(4) Small size, simple structure, and the working material is a flexible medium, which can be designed to be quite small and flexible, easy to use, easy to system integration, and cost-effective;

(5) As the doped fiber of laser medium, the doped rare earth ion has a very rich energy level structure, and the energy level transition covers a wide range from ultraviolet to infrared, which can achieve many transition energy levels of laser oscillation. It can be designed and operated in a wide spectrum range (455-3500nm). In addition, the fluorescence spectrum of the glass fiber is quite wide. A tunable fiber laser can be obtained by inserting an appropriate wavelength selector, and the tuning range has reached 80nm;

(6) The silicon fiber technology is now very mature, so it is possible to produce high-precision, low-loss fiber, which greatly reduces the cost of the laser.

(7) It has natural flexibility and compatibility with conventional transmission fibers in terms of materials and geometric dimensions, so it is easy to integrate optical fibers, has low coupling loss, and is easy to use.

(8) It can work under harsh environmental conditions, such as high impact, high vibration, high temperature, etc.