Light Amplification by Stimulated Emission of Radiation

A laser is a device that uses optical amplification to produce light through the stimulated emission of electromagnetic radiation. The term LASER is an acronym for light amplification by stimulated emission of radiation. A laser is distinguished from other light sources as it is monochromatic and coherent light. Coherence allows a laser to be concentrated to a small area, allowing it to be used for laser cutting and lithography. Laser coherence also permits a laser beam to remain narrow across long distances, allowing for applications such as laser pointers and lidar.

Light Amplification by Stimulated Emission of Radiation

When electrons in particular glasses, crystals, or gases absorb energy from an electrical current or another laser, they become excited and shift from a lower energy orbit to a higher energy orbit. The electrons emit photons when they return to their ground state. These photons all have the same wavelength and are coherent, which means the light waves’ crests and troughs are all in sync. Ordinary visible light, on the other hand, has various wavelengths and is not coherent.

What is laser?

A laser is a device that causes atoms or molecules to emit light at specific wavelengths and amplifies them thus resulting in a highly narrow beam of radiation. The emission only covers a small range of visible, infrared, and ultraviolet wavelengths. There have been many different types of lasers developed, each with its own set of characteristics. “Light amplification by stimulated emission of radiation” is what laser stands for.

Light Amplification by Stimulated Emission of Radiation

Working of laser

In a laser, photons of ordinary light interact with atoms in three ways:

  • Absorption of radiation
  • Spontaneous emission
  • Stimulated emission

What is absorption of radiation?

It is a process in which electrons in ground state gain energy in the form of photons and then jump to the higher energy levels. Consider, we have two energy states E1 and E2, which are ground state and excited state respectively. When a photon of energy equal to the energy difference of these 2 levels is incident, then electrons from lower state jump to excited state.

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What is spontaneous emission?

A process by which electrons in excited state return to ground state by emitting photons. The emitted photons have energy equal to the difference of energy of the respective levels. This happens due to the reason that electrons in excited state are not stable, so after 10-8 sec, they return back, by radiating. One interesting point is that the emitted electrons in spontaneous emission do not flow in same direction, moreover their phase is also different. Spontaneous emission is a natural process.

Light Amplification by Stimulated Emission of Radiation

What is stimulated emission in laser?

The basic laser principle is stimulated emission which is defined as a process of the interaction between excited electrons and incident photons, by which excited electrons return to the ground state. In contrast to spontaneous emission, light energy is directly provided to the excited electrons instead of ground state electrons. It is an artificial process in which electrons are made back to ground state before they complete their lifetime in excited state.

In a stimulated emission, two photons are released; one is due to the incident photon and other is due to the energy release from excited electron. All emitted photons are in phase and have same wavelength, so all travel in same direction. Simulated emission is a faster process than spontaneous emission.

What is metastable state?

Light Amplification by Stimulated Emission of Radiation

Metastable states are the excited states, such as those of atoms or ions that have a long lifetime despite modest spontaneous emission. While an excited atom can often make a transition to one of its lower states by spontaneous emission in nanoseconds, in some situations such transitions are “forbidden” due to symmetries that prevent the ordinarily dominant dipole interaction with the electromagnetic field. As a result, level lives can be significantly longer than normal, such as several milliseconds rather than a few nanoseconds. Usually the lifetime of electron in a metastable state is 10-3 sec, which is much longer than that of excited state.

What is population inversion?

Light Amplification by Stimulated Emission of Radiation

Population inversion is the redistribution of atomic energy levels in a system to allow laser action to occur. A system of atoms is normally in thermal equilibrium, with more atoms in low-energy states than in higher-energy states. Despite the fact that energy absorption and emission are ongoing processes, the population of atoms in different energy states remains constant. When this distribution is disrupted by injecting energy into the system, a population inversion occurs, with more atoms in higher energy levels than in lower energy states.

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This is a type of stimulated emission that occurs when the number of particles in an excited state exceeds the number of particles in a lower energy state. The quantity of stimulated emission that light can cause when the laser achieves population inversion will be larger than the amount of absorption from the mirrors. This produces an optical amplifier, which can be used to make a laser oscillator when placed inside a resonant optical cavity.

How laser beam is produced?

Light Amplification by Stimulated Emission of Radiation

Lasers are produced by a variety of devices, including optical cavities. These are the chambers that reflect light back to themselves from a substance that generates electromagnetic radiation. They’re usually comprised of two mirrors, one at each end of the tube, so that when light is reflected, the light beams become brighter. The enhanced signals exit the laser cavity through a clear lens at the end.

The substance that generates electromagnetic radiation emits the light of the laser at various energy states when an energy source, such as an external battery delivers current. Higher-energy electron states in the material are more likely to be unstable and the laser will emit these through its light.

Light Amplification by Stimulated Emission of Radiation, unlike other lights such as flashlight light, emit light in periodic steps with itself. That is, the crest and trough of each laser wave line up with the crest and trough of the waves that come before and after it, resulting in coherent light.

Lasers are built in such a way that they emit light at specified frequencies in the electromagnetic spectrum. The light is often in the form of tiny, discrete beams emitted at specific frequencies by lasers, however some lasers do emit broad, continuous ranges of light.

Properties of laser light

Laser light differs from ordinary light in a number of ways.

  • Laser light has only one wavelength (one specific color). The amount of energy released as an excited electron falls to a lower orbit determines the wavelength of light.

Light Amplification by Stimulated Emission of Radiation

  • Laser light has a certain direction. A flashlight provides diffuse light, whereas a laser produces an extremely narrow beam. Because laser light is coherent, it can travel great distances while remaining focused, even to the moon and back.

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Application of laser

A laser’s highly collimated beam can be concentrated even more to a minuscule dot with an exceptionally high energy density. As a result, it can be used as a cutting and cauterizing tool. Photocoagulation of the retina to stop retinal hemorrhaging and tacking of retinal tears are both done with lasers. If the supporting membrane surrounding the implanted lens turns milky after cataract surgery, higher-powered lasers are used. When the membrane is damaged by light, it often draws back like a shade, restoring vision almost instantaneously. For delicate surgery, a concentrated laser can behave as a razor-sharp scalpel, cauterizing as it cuts. The cauterizing action is particularly important for surgical procedures in blood-rich tissue such as the liver. They are being used in heat treatments for annealing and hardening the metal parts. Laser is used to communicate in space and underwater. They are also utilized to cut glass and quartz.

Light Amplification by Stimulated Emission of Radiation

There are many other uses of laser in daily life. Some of them are given below:

  • Laser cooling
  • Barcode scanners
  • Spectroscopy
  • CDs and optical discs
  • Laser printing
  • Communication
  • Laser nuclear fusion
  • Surveying
  • Welding and Cutting
  • Medical applications

Lasers in Military

  • To estimate the distance to an object, laser range finders are utilized.
  • The ring laser gyroscope can detect and measure the extremely small angles of rotation in moving objects.
  • Lasers can be employed as high-precision hidden illuminators for nighttime reconnaissance.
  • Lasers are employed to detonate a warhead’s energy by destroying the rocket.
  • LIDARs use laser light to precisely determine the distance to an item.

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Medical uses of laser

Laser is used for

  • Destroying kidney stones
  • Diagnosis and therapy of cancer
  • Bloodless surgery
  • Eye lens curvature correction
  • Treating lung and liver diseases, acne and hair removal
  • Detecting ulcers in intestines
  • Creating plasma
  • Studying structure of micro-organisms
  • Removing tumors and decayed part of teeth

Lasers in Technology

  • Laser helps to study the Brownian motion of gases.
  • He-Ni laser firstly confirmed that speed of light in all directions is same.
  • Number of atoms can be counted in certain substance by laser.
  • To retrieve the stored data from compact disk, lasers are used.
  • CR-ROM can store much data with the help of laser.
  • They are used to determine the rate of rotation of earth.
  • Computer printers use lasers.
  • 3D pictures in space without laser are possible with them.
  • Underwater nuclear blasts and earthquakes can be predicted by them.

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