Laser Concept

The main ingredient of the first quantum revolution, wave-particle duality, has led to inventions such as the transistor and the laser that are at the root of the information society.

Professor Dr. Alain Aspect

Experimental Physicist

Light Amplification by the Stimulated Emission of Radiation or LASER is an artificial light generated by a device that produces intense beams of light which are monochromatic, coherent, directional, and highly non-divergent. The wavelength of laser light is extremely pure (monochromatic or single color) when compared to other sources of light, and all of the photons that make up the laser beam have a fixed phase relationship (coherence) with respect to one another. Laser light typically have very low divergence, they can travel over great distances and can also be focused into a very small spot with a very high energy density. Figure 1.1 expresses these properties of laser light.

Figure 1.1 Laser and Characteristics of Lasers 

As mentioned, laser light is not found in nature so it is made artificially by a device which contains three components (Figure 1.2):

Figure 1.2: A Sample Laser Device

  1. Lasing material or gain or active medium usually solid (crystals, glasses and semiconductors); liquid (organic solvents and dyes; example: dye lasers); or gas. The type of laser is usually named after the lasing medium, and this is also the main factor determining the type of pump required and the wavelength of the resulting laser light.)
  2. External energy source or pump (which may be an electric current or discharge, flash lamp, light from another laser, or a chemical reaction).
  3. Optical resonator (this is laser’s simplest form consists of two parallel mirrors: a highly reflective mirror and a partially reflective mirror, also called output coupler.)

To explain internal mechanism of laser generation we need to take help of atomic theory, moreover, to avoid complexity we will describe using classical atomic theory which is sufficient to explain the principle of laser operation rather than modern atomic theory. According to classical theory atomic structure as a central nucleus composed of protons and neutrons surrounded by a cloud of electrons that orbit the nucleus in a series of discrete orbitals. When energy is supplied to the atom, the electrons move from their low-energy orbitals near the nucleus to high-energy orbitals further away. The atom is said to absorb energy, and move from the ground state energy level (E1) to an excited level (E2). An electron in a high-energy orbital eventually returns to the low-energy orbital. As it does so, the difference in energy is released in the form of a photon, which has random phase and direction. This process is called spontaneous emission, and is the same process which causes an incandescent bulb, neon light, fluorescent tube, cathode ray tube or heating element to produce light.
The photon will have a frequency v and energy hv given by E2E1 = hv, where h is Planck’s constant 6.63 × 10–24 J/s Hence, the wavelength of the light produced is determined by the amount of energy released when the electron returns to a lower orbital, and may be within the visible spectrum or beyond it (i.e. infrared or ultraviolet). If the photon collides with other excited electrons in the lasing medium, it will cause a second photon to be released which is identical to the original photon in its direction, phase, polarization and energy (wavelength). This is called stimulated emission. A cascade effect occurs as photons stimulate the emission of more photons, resulting in amplification or optical gain. The photons are initially released in random directions. However, as the chain reaction progresses, photons are reflected back and forth between the mirrors, and soon all atoms emit light along this axis. The output coupler is partially reflective, and allows a small number of photons to escape from the lasing medium. This is the usable laser light, which is directed to the target via a delivery system of fiber-optic light guides for visible light or a series of mirrors for infrared. For sustained laser action to occur, the majority of atoms must be maintained in the excited state (population inversion) by the continuous input of energy from the pump. If pumping is intermittent, a pulsed laser will result (Silfvast, 2004; Williams, 2008).

Reference: 

Silfvast, W. T. (2004). Laser Fundamentals. Cambridge University Press.

Williams, D. (2008). Laser basics. Anaesthesia & Intensive Care Medicine, 9(12), 550-552

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