The light source required for optical fiber communication should be one that can be modulated at 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, so as to carry information.
In 1960, Maimen invented the ruby laser. The main difference between Laser and ordinary light is that the optical frequency of laser is very simple, with linear spectral lines, known as coherent light in optics, which is the most suitable source for fiber communication. Normal light, on the other hand, has a jumble of wavelengths. Normal light, on the other hand, has a jumble of wavelengths. Coherent light is characterized by concentrated energy, small divergence Angle and approximate parallel light. After the ruby laser was invented, various lasers came into being: gas laser, such as He-ne laser; There are solid lasers, such as YAG iridium aluminum garnet laser; There are chemical lasers; Dye laser, etc. Among them, semiconductor laser is the most suitable for optical fiber communication light source, its small size, high efficiency, its wavelength is suitable for low loss window of optical fiber.
However, the manufacturing process of semiconductor laser is very complicated. It requires the epitaxial growth of five layers of doped semiconductor on a very high purity and flawless substrate material, and then the photolithography of micrometer-sized optical waveguide on the substrate, which is even more difficult than that of optical fiber. In the late 1970s, a long - life semiconductor laser operating continuously at room temperature was finally made. In 1976, the world's first practical optical fiber communication line was built between Atlanta and Washington. At this time the semiconductor laser has not passed, the light source is the use of semiconductor light emitting tube. In the early 1980s, single-mode fiber and laser have been mature, since the advantages of fiber communication large capacity gradually play.
The light emitted by the semiconductor laser has a very pure spectral line, concentrated energy and very thin beam, which can be efficiently emitted into the single-mode fiber with a core diameter of only 8 microns. Semiconductor laser is used as light source in high - speed optical fiber communication system.
The structure of the simplest semiconductor laser is shown in Figure 1. It consists of five layers of semiconductors, with an active layer in between mixed with an active substance. When an electric current is injected into the two electrodes, electrons in the atoms of the active substance in the active layer are excited from a lower energy state to a higher energy state. These electrons emit light when they are reduced from a higher energy state to a lower energy state, which is called spontaneous radiation. Spontaneous radiation light is not very pure, that is, it contains a wide spectrum of lines. If the spontaneous radiation is strong, the light is reflected back and forth between the two mirrors of the semiconductor. In this process, for each light energy with the same phase, the energy becomes larger and larger. For each light energy with different phases, the energy weakens each other and becomes smaller and smaller. The energy is converted into a specific wavelength of light in a cavity made of two mirrors in a semiconductor, which, when oscillated, forms a laser. Laser light is produced by what is called stimulated radiation. The mirror of the semiconductor is smooth and translucent, and the laser can be output through the mirror. The purpose of the limiting layer is to concentrate light energy within the active layer to improve efficiency. The wavelength of light emitted by a semiconductor laser depends mainly on the distance between the semiconductor material and the mirror.
Application of laser
With its excellent performance and low price, laser has been increasingly widely used in optical fiber communication, optical fiber sensing, industrial processing, medical treatment, military and other fields.
In terms of communication, the 1.30 micron and 1.55 micron laser bands provided by the laser are two low loss Windows for communication. The laser can not only produce continuous laser output, but also realize the generation of ps-fs ultra-short optical pulses, which has a huge potential application in DWDM system. Laser makes communication system have higher transmission speed, longer transmission distance, plays an irreplaceable role.
In terms of sensing, lasers are used in phase, wavelength, intensity and polarization mode fiber sensing. Temperature and pressure can be measured in oil or gas Wells; Strain can be measured in roads, Bridges and ship hulls; In-flight health monitoring in the wings of aircraft; It can also be used in fiber optic hydrophones and current sensing.
In the industry, laser has been in metal and non-metallic materials processing and treatment, laser engraving, laser product marking, laser welding, welding seam cleaning, precision drilling and laser graphic art imaging and other aspects are very significant.
In the medical field, lasers have been widely used because of their small size, good fiber softness, good beam quality, and do not require cooling systems. Fiber laser can shorten the surgical time of tissue shedding and photocoagulation, and greatly improve the success rate of curing eye diseases such as keratoplasty, myopia, hyperopia, etc. It also plays an important role in plastic surgery, tumor removal, cancer treatment and skin diseases.
In the military aspect, high power laser for its high brightness, small irradiation area, small size and popular favor. As a weapon, it can accurately target and destroy the target. Besides, it has important significance in positioning, ranging, remote sensing, tracking and guidance, laser radar system sensing technology and space technology.
Characteristics of laser
Fiber laser has attracted much attention in recent years and has become the focus of our research. This is because it has many advantages that other lasers cannot match, which are mainly shown in:
(1) The beam quality is good, with very good monochromatic, directional and stability;
(2) The fiber is both the laser gain medium and the guiding wave medium of light, so the disaster efficiency of the pump light is quite high, the core diameter is small, the high power density is easy to form 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 is more than 60%.
(3) The matrix material is Si02, which has excellent temperature stability; The cylindrical structure of the optical fiber has a high surface area/volume ratio, fast heat dissipation, the ambient temperature allows in -20-+7000C, the heat load of the working material is quite small, without cooling system, can produce high brightness and peak power, has reached 140mw/cm2;
(4) small size, simple structure, working material for flexible medium, can be designed to be quite small and flexible, easy to use, easy system integration, cost-effective;
(5) As a laser medium doped fiber, doped rare earth ions have extremely rich energy level structure, energy level transition covers a wide band from ultraviolet to infrared, laser oscillation can achieve a lot of transition levels. It can be designed and operated in a wide spectral range (455-3500nm), and the fluorescence spectrum of the glass fiber is quite wide. The tunable fiber laser can be obtained by inserting the appropriate wavelength selector, and the tuning range has reached 80nm.
(6) The technology of silicon fiber is now very mature, so high precision, low loss fiber can be produced, greatly reducing the cost of laser.
(7) It has natural accommodation and compatibility with conventional transmission fiber in terms of material and geometric size, so it is easy to integrate fiber, low disaster loss and easy to use.
(8) can work in harsh environmental conditions, such as high impact, high vibration, high temperature, etc.
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