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Question 5: What is a laser? Explain the principle and operation of a laser. Describe some practical uses of lasers.

ANSWER

Laser

The acronym LASER stands for Light Amplification by Stimulated Emission of Radiations.

Laser light was developed by T.H Maiman in 1960.

Lasers are monochromatic (one wavelength or color), unidirectional (moving straight in one direction) radiations.

Principle of lasers

Laser principle is based in two phenomena.

  • The stimulated emission in which one photon is incident on an excited electron. The excited electron falls back to its ground state and emits a photon of exactly the same frequency and energy as that of the incident photon.
  • The emitted radiation are in steps, which means they have the same wavelength and are coherent with one another. Therefore, they enhance the effect of one another and we get an intense beam of radiation.

Hence, the principle of lasers is the stimulation and amplification of radiations.

Operation of lasers

Laser operation depends upon the existence of meta-stable states in some of substances.(meta means in-between). Let us consider one such substance some of whose atoms are in the excited state by absorbing energy, E3-E1 = hf13 by optical pumping. As there are meta-stable states in the atoms, these electrons will not fall back directly to state E1 like the electrons of other substances that do so after 10-8seconds. Rather, they will fall in the meta-stable state where they remain for a time interval of 10-3 second.

Now we see that excited electrons from state E3 fall more rapidly (after 10-8 s) to the meta stable state and from the meta stable they fall to the ground state E1 after a much longer time of 10-3 seconds. This means that when one electron goes out of meta stable to the ground state of E1 in 10-3 seconds, approximately 3 electrons from the excited state E1 would have jumped to the meta stable state. In this way, the number or population of electrons in the meta stable state will increase with time. This is called population inversion.

After the state of population inversion is achieved, the process of stimulated emission is started. E2 is bombarded by the photons of energy E2-E1=hf12. These photons stimulates the electrons in state E2 to fall down to state E1, emitting one another photon of exactly the same energy and in the same direction. (Note that stimulation does not mean the incident photon is absorbed). So an amplified radiation is obtained.

Usually the emitted photon and the whole assembly are provided with mirrors on the two ends. One mirror is totally reflecting and the other is partially transparent. The emitted photons are reflected back and forth in the assembly and stimulate more and more excited atoms to emit more and more photons.  In this way, an intense beam of coherent radiations is obtained through the partially transparent mirror.

Uses of laser

  • Lasers are used in electrical instruments.
  • Lasers are used for vaporization purposes.
  • Lasers are used in drilling, cutting and welding.
  • Lasers are used in medical treatment like bloodless surgeries, rejoining of detached retinas, and destroying tissues like tumors in a localized area.
  • Lasers can develop hidden finger prints.
  • High power lasers is a potential energy source to produce nuclear fusion reactions.
  • Lasers can be used to generate three dimensional images of objects (called holography).

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