Principle and Application of Laser
How the laser works
A laser is a device that emits laser light. According to the working medium, the laser can be divided into four categories: gas laser, solid laser, semiconductor laser and dye laser. Recently, free electron lasers have been developed. High-power lasers are usually pulsed output.
Except for free electron lasers, the basic working principles of various lasers are the same. The indispensable condition for generating laser light is that the population inversion and gain are greater than the loss, so the indispensable components of the device are the excitation (or pumping) source and the working medium with the metastable energy level. Excitation is the excitation of the working medium to excite the excited state, creating conditions for achieving and maintaining the population inversion. The incentive methods include optical excitation, electrical excitation, chemical excitation and nuclear energy excitation.
The metastable energy level of the working medium is such that the stimulated radiation dominates, thereby achieving optical amplification. A common component of a laser is a resonant cavity, but the resonant cavity (see optical cavity) is not an indispensable component. The resonant cavity allows the photons in the cavity to have a consistent frequency, phase, and direction of travel, thereby enabling the laser to have Good directionality and coherence. Moreover, it can shorten the length of the working substance well, and can also adjust the mode of the generated laser by changing the length of the cavity (ie, mode selection), so the laser generally has a resonant cavity.
Three components of the laser
First, the working substance
At the core of the laser, only substances that can achieve energy level transitions can be used as working substances for the laser.
Second, incentive energy
Its role is to energize the working substance, the atom is excited from the low energy level to the external energy of the high energy level. Usually there are light energy, thermal energy, electric energy, chemical energy and so on.
Third, the role of optical cavity:
First, the stimulated radiation of the working substance is continuously performed;
The second is to constantly accelerate the photon;
The third is to limit the direction of the laser output.
The simplest optical cavity consists of two mutually parallel mirrors placed at the ends of the HeNe laser. When some deuterium atoms transition between two energy levels that achieve particle inversion, and emit photons parallel to the direction of the laser, these photons will reflect back and forth between the two mirrors, thus constantly causing stimulated radiation. A very strong laser is produced very quickly.
The pure light and stable spectrum of the laser can be applied in many ways.
Ruby laser
The original laser was rubbed by a bright flash bulb, and the laser produced was a "pulsed laser" rather than a continuously stable beam. The quality of the light produced by this laser is essentially different from the laser produced by the laser diode we use today. This intense light emission, which lasts only a few nanoseconds, is ideal for capturing objects that are easy to move, such as portraits of holographic portraits. The first laser portrait was born in 1967. Ruby lasers require expensive rubies and can only produce short bursts of light.
Helium laser
In 1960 scientists Ali Javan, William R. Brennet Jr. and Donald Herriot designed the HeNe laser. This is the first gas laser that is commonly used in holographic photographers.
Two advantages: 1. Produce continuous laser output; 2. No need for flash bulb to perform light excitation, but use electric excitation gas.
Laser diode
Laser diode is one of the most commonly used lasers. The phenomenon of spontaneous recombination of electrons and holes on both sides of the PN junction of a diode is called spontaneous emission. When the photons generated by spontaneous emission pass through the semiconductor, once they pass through the emitted electron-hole pairs, they can be excited to recombine to produce new photons, which induce the excited carriers to recombine and emit new photons. The phenomenon is called stimulated radiation.
If the injection current is large enough, a carrier distribution opposite to the thermal equilibrium state is formed, that is, the population number is reversed. When the carriers in the active layer are in a large number of reversals, a small amount of spontaneously generated photons generate inductive radiation due to reciprocal reflection at both ends of the resonant cavity, resulting in selective feedback of the frequency selective resonance, or gain for a certain frequency. When the gain is greater than the absorption loss, a coherent light with a good spectral line, the laser, can be emitted from the PN junction. The invention of laser diodes enables the rapid application of laser applications, various types of information scanning, fiber optic communication, laser ranging, laser radar, laser discs, laser pointers, supermarket collections, etc., and various applications are being continuously developed and popularized.