Editorial Feature

The Structure, Physics and Applications of Lasers

This article was updated on the 11th September 2019.

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The word "laser" is an acronym which stands for "Light Amplification by Stimulated Emission of Radiation".

A laser typically emits light which typically has a high coherence, which can be characterized spatially and temporally. High spatial coherence means the resulting light stays well-focused over a large distance and high temporal coherence means a well-defined wavelength of light is produced. A laser is different from light sources such as incandescent light bulb in that light is emitted with a broad spectrum of wavelengths and into a large solid angle.

Structures of Laser

A laser consists of three basic components:

  • Lasing Material
  • Pump Source
  • Optical Cavity

Structure of Laser - Laser Material

The lasing material is also commonly referred to as the laser medium or the gain medium. It is situated inside the optical cavity which is what makes the standing waves of light in the gain medium for the lasing process to occur.

The gain medium can be a solid, liquid, or a gas, though crystalline materials are most commonly used. The gain medium is required to have the appropriate optical properties and controlled size and shape, concentration and purity.

Structure of Laser - Pump Source

Electricity or intense light sources, such as flash lamps, are examples of pump sources. The laser medium absorbs this pump energy, which excites some of the molecules in the gain medium into excited or higher-energy quantum states.

Structure of Laser - Optical Cavity

The optical cavity is a type of cavity resonator. Inside the optical cavity, the coherent light beam is kept between two reflective surfaces. The purpose of the reflective surfaces is to allow light to pass through the laser medium more than once before the light is lost through to diffraction, absorption or emitted from the output aperture.

As the light beam passing through the laser medium passes through the optical cavity multiple times, the power of the circulating light beam can increase exponentially if the laser amplification in the laser medium is greater than the resonator losses.

Physics of Laser

A pumped gain medium will eventually convert its energy into a stimulated emission of radiation – the laser beam. If the frequency of the incident photon on the gain medium matches the difference in energy between the ground states and excited states of an already excited atom, the atom undergoes simulated emission. This is where it returns to its electronic ground state, releasing a second photon with the same frequency. The phase of the emitted photon and its direction.

To achieve the lasing of the gain medium, something called a population inversion must be achieved. A population inversion occurs when a greater number of atoms are in the excited state than their ground state and is driven by electrical or optical pumping of a gain medium. A population inversion is an unstable energetic state for the system, so the excited electrons will remain in the high energy state for a short period of time before decaying back to the lower energy level. The decay or emission can happen in two ways: spontaneous or stimulated.

The light is amplified when the amount of stimulated emission or decay due to light that passes through is greater than the amount of absorption. This amplification will continue until there is a build-up of sufficient energy for a burst of laser to be transmitted through the optical cavity.

Laser Output

The outputted laser beam can either be emitted as a continuous beam, known as continuous wave or CW operation. Or, if higher peak powers are desirable, the output of the laser can be pulsed. This output method used techniques of Q-switching, mode locking or gain-switching.

Applications of Laser

Lasers are used in the medical, defense, and research industries. Here are some of the examples:

  • Medical - surgery, kidney stone treatment, eye treatment
  • Industrial - cutting, welding, material heat treatment
  • Defense - weapons, direct energy weapon, anti-missile
  • Research - spectroscopy, laser annealing, ablation, scattering and interferometers
  • Commercial - laser printers, laser pointers, compact disc players, barcode scanners

Source: AZoOptics

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