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How Laser Is Produced? The Origin Of Laser

How Laser is produced? 

Before talking about the mechanism of laser generation, first talk about stimulated radiation. There are three kinds of radiation processes in the light radiation,


One is that particles in high energy state transition spontaneously to low energy states, which is called spontaneous radiation;

The second is that particles in a high energy state transition to a low energy state under the excitation of external light, which is called stimulated radiation;

The third is that the energy of particles in the low-energy state absorbing foreign light transitions to the high-energy state is called stimulated absorption.

Spontaneous radiation, even if two particles simultaneously transition from a high energy state to a low energy state, their emitted light phase, polarization state, emission direction may also be different, but the stimulated radiation is different, when the particles in the high energy state are in foreign The excitation of a photon transitions to a low-energy state and emits light that is exactly the same as the external photon in terms of frequency, phase, and polarization state. In a laser, the radiation that occurs is stimulated radiation. The laser it emits is exactly the same in terms of frequency, phase, polarization, and so on. Any excited light system,

Laser diode schematic

Laser diode schematic

That is, there is stimulated radiation and stimulated absorption. Only stimulated radiation is dominant, and external light can be amplified to emit laser light. The general light source is dominated by stimulated absorption. Only the particle's equilibrium state is broken, and the number of high energy state particles is greater than the number of low energy state particles (in this case, the number of ions is reversed).

The three conditions for generating a laser are: achieving population inversion, satisfying threshold conditions, and resonant conditions. The primary condition for the generation of stimulated emission of light is the population inversion. In semiconductors, the electrons in the valence band are pumped to the conduction band. In order to achieve the ion number reversal, a heavily doped P-type and N-type material is usually used to form the PN junction. Thus, under the applied voltage, the number of inversions occurs near the junction region—at the high Fermi level EFC. In the following conduction bands, electrons are stored, and holes are stored in the valence band above the EFV of low Fermi level. Implementing population inversion is a necessary condition for laser generation, but it is not a sufficient condition. To produce a laser, there must be a resonator with a very small loss. The main part of the resonator is two mirrors that are parallel to each other, and the stimulated radiation emitted by the activating substance is reflected back and forth between the two mirrors, continuously causing new ones. The stimulated radiation causes it to be continuously amplified. Only the gain of the stimulated radiation amplification is greater than the various losses within the laser, that is, meet certain threshold conditions:

P1P2exp(2G - 2A) ≥ 1

(P1, P2 are the reflectivity of the two mirrors, G is the gain coefficient of the active medium, A is the loss factor of the medium, exp is constant), can output the stable laser, on the other hand, the laser reflects back and forth in the cavity Only the phase difference between these two beams at the output is Δф = 2qπ q = 1, 2, 3, 4. At this time, it can produce enhanced interference at the output and output a stable laser. Let the length of the resonant cavity be L and the refractive index of the active medium be N.

Δф=(2π/λ)2NL=4πN(Lf/c)=2qπ,

The above equation can be changed to f=qc/2NL. This equation is called a resonance condition. It shows that after the cavity length L and refractive index N have been determined, only certain frequencies of light can form optical oscillations and a stable laser can be output. This shows that the resonant cavity has a certain frequency selection effect on the output laser.

development of

A light source that was invented in the 1960s was named laser. LASER is an acronym for “stimulated radiant light amplification” in English.

In the autumn of 1962, a homojunction GaAs laser diode with a stimulated emission at 77K was first developed.

In 1964, its operating temperature was raised to room temperature.

In 1969, a single heterojunction laser diode operated at room temperature was fabricated.

In 1970, Ga1-xAlxAs/GaAs Double Heterojunction (DH) laser diodes were fabricated and operated at room temperature. Since then, laser diodes have developed rapidly. In 1975, the lifetime of Ga1-xAlxAs/GaAsDH laser diodes increased to more than 105 hours. In1-xGaxAs1-yPy/InP long-wavelength DH laser diodes have also made significant progress, which has led to the development of fiber optic communications and other applications. In addition, far-infrared wavelength laser diodes made of IV-VI materials such as Pb1-xSnxTe have appeared.

 

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