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What is the coherence of the laser? 

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What is the coherence of the laser?
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Laser since the 20th century, science and technology after the relay atomic energy, computers, semiconductors, another major human invention. To understand what coherence 30000mw laser pointer prior to first introduce the basic principles of laser bars.
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Laser, represents a new generation of light by stimulated emission of photons, and then zoom in to get a new light is emitted. The process of stimulated emission of roughly like this: Suppose a certain atom is initially in a high energy level E2, when the photon energy hυ alien just landed on a level equal to the difference between the E2-E1, then the atom can be in addition to photon under low energy level E1 to induce transition from high energy level E2, then emits a photon induced identical photons, not only the frequency (energy) the same, and the emission direction, the direction of polarization and phase of light waves are exactly the same. This means that a photon incident will exit two identical photons, which is the principle of the optical signal is amplified. However, a foreign photon that is capable of causing stimulated emission, but will produce stimulated absorption, absorption of external photons, and then let the atomic energy level transition from E1 to E2 level. Obviously, if you want to generate a 20000mw laser pen, with the proviso that the stimulated radiation effect is greater than the shock absorbing effect by. However, in the thermal equilibrium state, atoms are almost at the lowest level (ground state) on. Therefore, the laser produces a maximum premise: inversion must be formed, so that more of the atoms in the high energy level. Certain materials can make it work inversion formed under specific conditions.
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Then the basic components of the laser roughly a little prospect of a solution, should include three aspects: the working substance, excitation source, a resonant cavity. Working material, we are looking for material for the formation of a population inversion in this material; excitation source is used to excite the atoms of the material system work to achieve this particular condition (increase in the number of particles on the energy level, the number of anti-particles turn); resonator, 10000mw blue laser cavity is formed, generally consists of two mirrors (laser at both ends) composition, so that the light in the resonant cavity oscillate back and forth, causing a chain reaction, avalanche amplification, so that the output of the laser reflector at the exit.
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The principle of laser generation, decided to distinguish four kinds of laser light sources other common characteristics: monochromatic, coherent, directional and high brightness. In fact, these four characteristics essentially be boiled down to one: a coherent laser light with high intensity over a large coherent volumes. Coherence of the laser 3000mw can be divided into spatial coherence and temporal coherence of two kinds, respectively, related to different spatial positions and spatial characteristics of certain light waves between different points in time between wave field correlation. And usually we qualitatively Young's double-slit interference fringes clarity of experiments to determine the degree of coherence of the light beam.
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Temporal coherence and monochromatic light source is directly related. A light source atomic emission time longer, the more observed through a double-slit interference pattern, we say that the longer coherence, and light emission time we called the atomic coherence time, coherence length of the wave train is called the coherence length of time . The longer the coherence length L, the more clear interference fringes, indicating better coherence. Suppose a 2000mw green laser light emitted between the frequency λ1 ~ λ2 wave (preferably monochromatic light does not exist), it can be shown that the coherence length L = λ ^ 2 / Δλ (length of the relationship, proof a little), Δλ = λ2-λ1. When Δλ is smaller, that is, λ2 and λ1 closer, the better the color, the greater the coherence length L, coherence better.
High brightness laser light source is different from the ordinary important feature, and now an important issue to improve the output power and efficiency but also the development of the laser. Use transfer Q, clamping, pulse compression techniques can greatly improve the laser output power of the laser when the laser power is concentrated in one or a few modes, the means to improve the Photonic and degree of coherence , the better. Brightness of a high-power laser can even reach millions of times the Sun's surface brightness.
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In recent years, due to the strong direction, the optical power is concentrated, it is difficult eavesdropping, low cost, fast installation, etc., are increasingly used in laser free space optical communication, but when the light beam is transmitted susceptible to atmospheric fine particles of laser , aerosols, temperature gradient caused by the refractive index of random variations and other factors, so that the light intensity distribution of the laser 5000mw beam, the phase distribution in time and space random fluctuation, wavefront distortion, flicker intensity, beam bending and offset phenomenon. Partially coherent beams in the turbulent atmosphere transport can better overcome the negative impact of atmospheric turbulence and other aspects. Regulation of the degree of coherence of the laser beam size, the same can also be achieved beam shaping produce light intensity distribution hollow, flat, arrays, etc., thus has important applications in the field of laser processing, laser weapons and the like. Laser coherence control, not only as complete as coherent laser, the phase-control produces laser light to form optical tweezers trap, and because some coherence to avoid thermal effect, thereby protecting cells from damage.

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