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    High directivity of laser

    The high directivity of the laser enables it to effectively transmit a long distance while ensuring a very high power density. The laser emitted by the laser is naturally emitted in one direction, and the divergence of the beam is extremely small, about only 0.001 radians, nearly parallel.



    High monochromaticity of laser

    Light color is determined by its wavelength (or frequency). A certain wavelength is corresponding to a certain color. The distribution range of sunlight wavelength is between 0.76 microns to 0.4 microns. Corresponding colors fall into 7 ones, ranging from red to purple. Therefore, sunlight is far from being monochromatic. Any light source emitting light of a single color is called a monochromatic light source, and the wavelength of lightwave emitted from it is unilateral. 

    For instance, krypton lamp, helium lamp, neon lamp, and hydrogen lamp are monochromatic light sources, emitting only light of a certain color. Despite the fact that the wavelength of the monochromatic light source is single, it still has a certain distribution range. For instance, a neon lamp only emits red light and has good monochromaticity, which represents the top of monochromaticity. However, its wavelength distribution still has 0.00001nm. On this account, the red light emitted from the neon lamps, were carefully identified, still contains dozens of red colors,  were carefully identified. Thus, it can be seen that the narrower the wavelength distribution range of light radiation, the better the monochromaticity will be. In view of the high monochromaticity of the laser, the beam can be accurately focused on the focus to obtain a high power density.



    High coherence of laser

    Coherence is the attribute of all fluctuation phenomena. Light has wave-particle duality, that is to say, it comprises photons with particle property on the micro aspect. However, from a macroscopic view, it is also provided with fluctuation. Light is provided with both fluctuation and coherence. 

    As pointed out by Maxwell in Mechanical Theory of Electromagnetic Field in 1865: Light and electromagnetic waves are the expressions of attributes of the same entity; light is an electromagnetic disturbance transmitted in the field according to the law of electromagnetism. Thus, Maxwell revealed the essence of light for the first time in the history of sciences. In other words, light is an electromagnetic wave, an electromagnetic wave of a shorter wavelength. As a frequency, vibration direction, and phase of light are highly uniform, when laser light waves overlap in space, the light intensity distribution in the overlapping area will manifest stable strong and weak phases. This phenomenon is called light disturbance. 

    Therefore, the laser is provided with coherence. Normally, light emitted from the light sources is a disordered mixed beam with different wavelengths other than non-coherent light. As wavelength, phase and vibration amplitude of non-coherent light are extremely inconsistent, their composite wave is also a disordered and irregular curve, from which it is unlikely to find its periodicity. Common light sources, such as sunlight and lamp radiation belong to non-coherent light. Coherence mainly describes the phase relationship of all compositions of light waves.



    Laser generation principles

    Laser has a long development history, of which principles were discovered by Einstein, the renowned physician in 1917. However, "laser" was successively produced for the first time in 1958. Its English name is Laser, the abbreviation of “Light Amplification by the Stimulated Emission of Radiation”. The full English name of laser has fully expressed its production process.



    Laser working medium

    To generate laser, it is essential to select appropriate working mediums, including gas, liquid, solid or semiconductor. It is applicable to realize population inversion in such mediums to create essential conditions for the acquisition of laser. Obviously, the existence of a metastable energy level is favorable for population inversion. Currently, there are nearly one thousand working mediums that can produce extremely extensive laser wavelength, including vacuum ultraviolet and infrared ray.



    Excitation source

    To realize population inversion in a working medium, it is essential to make use of certain methods to excite atomic systems to increase the number of superior particles. Normally, gas discharge can be used to excite dielectric atoms by electrons with kinetic energy. This is called electric excitation. It is also applicable to use a pulsed light source for irradiation of the working medium, this is called light excitation. Moreover, there are also thermal excitation, chemical excitation, and so on. Various excitation modes are vividly referred to as pumping or extraction. To ensure continuous laser output, it is a must to proceed with continuous “pumping"



    Resonant cavity

    It is only to realize population inversion where appropriate working mediums and excitation sources are available. Nevertheless, excited radiation intensity produced in this way is extremely weak, which is unavailable for practical application. For this reason, people begin to consider the use of optical resonant cavities for amplification.

    The so-called optical resonant cavity is actually represented by two opposite mirrors of high reflectivity on both ends of the laser. One is for full reflection, and another one is for partial reflection with limited penetration to make the laser beam penetrate through it. Light back-reflected working medium will continuously induce new exciting radiation and amplified light. As a result, the light will oscillate back and forth in the resonator to produce a chain reaction. After that, a strong laser beam as amplified in a way similar to avalanche is to be obtained for output from one end of the reflector.



    Types of lasers

    There are different methods for classification of lasers. Normally, lasers can be divided into solid, gas, liquid and diode ones as per different working mediums.



    Solid laser

    Generally, solid laser features in small volume, high solidity, easy operation, and high output power. As to the working medium for such laser, a limited amount of activated ions are added into the crystal or glaze that serves as the base material. In addition to ruby and glaze as introduced aforesaid, other ones include the laser with trivalent neodymium ions added into the yttrium aluminum garnet (YAG) crystal. It can emit a 1060nm near-infrared laser beam. Normally, the continuous power of a solid laser is over 100W with pulse peak power up to 109W.

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