LASERS

As a programmer, you would probably have no trouble mastering the electronic level umclidet, the so-called UEU-17… But the quantum umclidet… Hyperfields… transgressive incarnations… Generalized law… — In the laser technology laboratory.

As a programmer, you would probably have no trouble mastering the electronic level umclaidet, the so-called UEU-17… But the quantum umclaidet… Hyperfields… transgressive incarnations… Generalized Lomonosov Law – Lavoisier…

A. Strugatsky, B. Strugatsky. Monday Begins on Saturday

Word “LASER” is an abbreviation of English expressions «Light Amplification by Stimulated Emission of Radiation» – “Light amplification by stimulated emission of radiation.”

The laser emits a coherent (strictly coordinated in space and time), monochromatic and polarized beam. The principle of laser operation can only be described using the theory of quantum mechanics, but it is simple enough for a popular explanation.

Unlike sound waves, electromagnetic waves are transverse and have a more complex structure. They are synchronous harmonic oscillations of magnetic and electric fields in planes perpendicular to the direction of wave propagation at the speed of light (C).

To understand how a laser works, let's turn to physics.

An electromagnetic wave, like any other wave, has quantitative characteristics such as amplitude, frequency, phase and direction of propagation. That is why physicists are accustomed to describing a light wave using equations in vector form. It is important to know that electromagnetic waves are transverse and have two mutually perpendicular directions of oscillation of the magnetic H and electric E components.

Waves maintain the orientation of oscillations throughout the entire propagation path until the moment of interaction with matter. So, we can name another parameter of electromagnetic waves – the orientation of the plane of polarization. The theory of electromagnetic waves was thoroughly developed by James Maxwell.

By studying the radiation spectra of heated bodies, Max Planck came up with a formula that perfectly described this phenomenon. According to this formula, electromagnetic radiation had the properties of an unusual particle, whose minimum energy was equal to the product of a certain constant – Planck's constant (h) to the frequency of electromagnetic oscillation (ν).

In Planck's theory, light is both a wave and a particle with energy that is a multiple of a certain number – Planck's constant (h). Scientists call the smallest and indivisible particle of radiation a quantum.

Taking into account the above formula, we can say that the energy of the red quantum is less than the green one, and the green one is less than the blue one, and so on. Scientists thought hard. What is light then? A particle or a wave? Who is right? Newton or Fresnel? It turns out that everyone is right. That's the story.

Back in 1916, Albert Einstein predicted the possibility of exciting the radiation of atoms by an external electromagnetic field, which has exceptional monochromaticity. To understand what is hidden in the essence of this phenomenon, let us consider in a simplified form the process of interaction of a quantum of light with matter.

Transition of an atom into a metastable state — Interaction of a light quantum with the electron shell of an atom.

Induced emission of atoms — Transition of an atom into a metastable state and induced emission.

And this can happen many times along the path of a spreading avalanche of particles, which is equivalent to an increase in the radiation power proportional to the number of excited atoms encountered along their path.

• In 1939, Valentin Fabrikant, an employee of MPEI, formulated the principle of amplifying electromagnetic radiation in a medium in which an increasing number of excited electrons can be created.

• 1955. Nikolai Basov and Alexander Prokhorov develop a maser whose active medium is ammonia.

• 1957: American scientists Charles Townes and Arthur Schawlow begin developing the principles of the laser.

• 1958. Alexander Prokhorov uses a resonator to create a laser Fabry–Featherwhich consists of two parallel mirrors, one of which is translucent.

In principle, the laser design, like everything ingenious, is extremely simple. Between two strictly parallel mirrors is an optically active medium in which, using a pump lamp or other method of energy supply, excited atoms accumulate.

Any laser consists of a working body in which the radiation is amplified. The working bodies of lasers are a wide variety of substances – solids, liquids and gases.

For the laser to operate successfully, the atoms of the working substance must have special, metastable electron energy levels (E2), shown in the figure.

The laser is “pumped” by bright light from a lamp or another laser, an electric current, an electron beam, or a chemical reaction. With the help of energy pumping, electrons in the laser substance are transferred from the ground energy level (E3) to a higher and more unstable (E1). If the electron energy returns back to the ground state, no effect will be obtained. Therefore, in the active substance of lasers, just below the upper level, there must be another, so-called “metastable” (E2) or “long-lived”, where electrons are retained for a short time. During the pumping action, a population inversion is formed in the laser, in which more electrons accumulate at the metastable level than at the ground level.

Atoms cannot remain in an excited state indefinitely. After some time, excess energy is released in the form of emission of a quantum with normalized energy (and if we recall Planck's formula, frequency).

Generated by spontaneous re-emission, the quanta fly off in different directions. In order to create a laser, a medium operating in the positive feedback mode is needed. As a positive feedback in a light-amplifying medium, scientists began to use the Fabry interferometer – A pen, which is two parallel flat mirrors with a very high reflectivity. This device has another name – “resonator”.

In such a resonator, from the huge number of quanta generated as a result of amplification, there will always be at least one that will move along the axis of the resonator (parallel to the axis of the installed mirrors). Such a quantum has a huge advantage over the others. Repeatedly reflected from the parallel installed mirrors, it will encounter on its way the maximum number of atoms of the active substance in an excited state. Imagine what an avalanche of its “clones” this single quantum will carry along with itself, repeatedly flying from one mirror to another!

One of the resonator mirrors is semi-transparent. This means that some of the quanta still escape from the captivity of the mirror resonator. Thus, a narrow parallel beam of twin quanta emerges from our wonderful device called a laser. Each of them has the same frequency, phase, and orientation of the polarization plane. No other light source can compare with a laser in the orderliness of the emitted radiation.

A.M. Prokhorov, C. Townes and N.G. Basov (left to right), 1965.

In 1960, American physicist Theodore Maiman, an employee of the company "Hughes Aircraft"designed the first ruby laser (wavelength 0.69 µm) based on the work of Basov, Prokhorov and Townes. 1 – ruby rod; 2 – flash lamp; 3 – 100% reflection mirror; 4 – 95% reflection mirror; 5 – reflector; 6 – flash lamp power supply. — In 1960, American physicist Theodore Maiman, an employee of Hughes Aircraft, constructed the first ruby laser (wavelength 0.69 µm) based on the work of Basov, Prokhorov and Townes. 1 – ruby rod; 2 – flash lamp; 3 – 100% reflection mirror; 4 – 95% reflection mirror; 5 – reflector; 6 – flash lamp power supply.

Soon Charles Townes and Arthur Schawlow patented the laser design. In principle, everything is clear and simple, but in fact, lasers are high-tech products. In 1962, Nikolai Basov proposed the idea of a laser based on a semiconductor crystal, and the Americans Javan, Bennett and Garriott developed a gas laser. With the advent of lasers, holography began to rapidly develop in the most unexpected directions.

Properties of laser beam

Laser light is exceptionally monochromatic. By using additional devices to prevent the laser from generating several frequencies at the same time, it is possible to achieve a coherence length of hundreds of meters and a radiation spectrum width of several kilohertz.

The laser beam front has a shape close to an ideal plane. Therefore, the angular divergence of the laser beam is hundredths of a degree.

With the help of a focusing lens, the laser beam can be collected into a spot of very small diameter and obtain an exceptionally high energy density. The distribution of radiation intensity in the cross-section of the laser beam tends to the Gauss law. This circumstance determines its geometry in the focusing zone.

d = 4λf/πA, where d is the diameter of an ideal Gaussian beam at the focus of the lens; λ is the wavelength of the laser radiation; A is the beam size at the input of the lens; f is the focus of the lens

Near the focal plane of the lens, the light beam has the shape of a “waist” with a flat front in the focal zone. The shorter the focus of the lens (f) and the larger the diameter of the laser beam (A), the smaller the focal spot (d) will be.

Due to the listed properties, it is possible to create high laser radiation densities, which are used for rapid heating of matter, cooling or acceleration of charged particles. The laser beam cuts and welds refractory materials, it is used to measure distances, speeds and accelerations of objects, sizes and concentrations of particles in liquids and gases. Engineers have found many useful applications for lasers.

New generation of lasers

A little more than half a century has passed since the first solid-state and gas lasers appeared. During this time, science has not stood still and made many discoveries in the field of semiconductor materials, nonlinear optical media and artificial crystals. As a result of the wide front of research, a new generation of laser diodes and so-called DPSS lasers was developed.

DPSS abbreviation ( diode-pumped solid-state laser) , means – diode pumped solid state laser.

Pumping a laser crystal with laser diode radiation allows for high generation efficiency and radiation quality with a relatively simple and compact design. The advantage of diode-pumped lasers is that laser diode radiation spectrally matches well with the absorption bands of activator ions with metastable levels in the generating crystal. The pumping efficiency exceeds 80%, and the thermal load is reduced to a minimum.

DPSS lasers with high coherent radiation power are compact and very economical.

DPSS lasers can emit over a very wide frequency range.

The simplest DPSS laser is designed as follows:

Coherent radiation of a laser diode with a wavelength of 808 nm is focused in a Nd:YVO4 crystal and excites in it stimulated radiation with a wavelength of 1064 nm. Then a nonlinear KTP crystal converts the infrared beam into a green beam with a wavelength of 532 nm. Then the radiation is converted into a narrow beam by collimation lenses. An IR filter separates the green beam from a fairly powerful infrared beam, which is still present after conversion in the KTP crystal.