Laser

• Light waves are made up of tiny packets of energy called photons.
• Normal light emission happens when atoms or molecules release photons when they transition from higher energy states to lower ones. These emitted photons have random directions and energies, creating a diffuse light.
• The concept that makes lasers unique is stimulated emission. This occurs when an incoming photon interacts with an excited atom in the laser material. The photon’s energy triggers the excited atom in the material to emit a new photon with identical characteristics. The new photon has the same wavelength and so colour, phase and direction as the original.
• Laser material refers to the medium that is used to generate the laser light.
• This phenomenon creates a cascade effect. The newly emitted photon can itself stimulate another excited atom, leading to two identical photons travelling in the same direction. This process repeats, rapidly amplifying the initial light within the laser cavity.
• The cavity comprises two mirrors strategically positioned at the opposite ends of the laser material. One mirror is fully reflective, while the other partially reflects.
• As the amplified light bounces between the mirrors, it continues to stimulate more emissions, resulting in an intense beam of identical photons. The partially reflective mirror allows a portion of this intense light to escape as the laser beam, while the rest continues to contribute to the amplification within the cavity.

How lasers work

• Here’s how a laser works in simplified terms:
  • Energy Source: A laser needs an energy source to “pump” the material it will interact with. This can be electricity, chemical reactions, or even sunlight.
  • Material: The type of material varies depending on the desired properties and application. Examples include:
    • Gas: Examples include helium-neon (HeNe) lasers used commonly in laboratories or carbon dioxide (CO2) lasers used for industrial cutting and welding.
    • Solid: Solid-state lasers are becoming increasingly common, with materials like neodymium-doped yttrium aluminium garnet used in various applications like medical procedures and material processing.
    • Liquid: Dye lasers, where the gain medium is a liquid solution containing organic dyes, offer tunable wavelengths making them suitable for research and spectroscopy.
    • Semiconductor: Diode lasers, also known as semiconductor lasers, like the ones used in CD/DVD players and laser pointers, utilize a p-n junction in a semiconductor material to create light.
  • Laser cavity: The cavity sometimes includes lenses or prisms to control the light path and manipulate its properties, such as focusing and directing the beam. They are also chosen to be as transparent as possible at the laser’s operating wavelength to minimize light absorption within the cavity.
  • Mirrors: Two mirrors at the ends of the laser cavity reflect the light back and forth, further amplifying it before one mirror allows a small portion to escape as the laser beam.
  • Excited State: The energy source excites the atoms or molecules in the laser material, bringing them to a higher energy level.
  • Stimulated Emission: A photon enters the material and “stimulates” an excited atom to release its energy as another photon.
  • Amplification: This new photon can then stimulate other excited atoms to release photons as well, creating a chain reaction that amplifies the light.

Properties of laser light

• This process leads to several unique properties of laser light:
  • Coherence: All the photons in a laser beam have the same wavelength and travel in sync, creating a very pure and concentrated light source.
  • Monochromaticity: Laser light has a single, monochromatic, pure colour (wavelength), unlike normal light, which is a mixture of many colours.
  • Collimation: The light is focused into a very narrow beam, giving it high intensity and precision.