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An Introduction to Lasers and Masers

  1. Jul 9, 2006 #1
    An Introduction to Lasers and Masers​

    Maser

    Maser is the abbreviation for “Microwave amplification by stimulated emission of radiation”. Maser is an amplifier or generator for electromagnetic radiation usually microwaves. The laser is an optical maser. Einstein showed in 1917 how atoms, ions or molecules can emit radiation in the form of energy quanta (photons) through spontaneous (disordered photon emission) or photon emission stimulated through a signal.

    It was unclear for a long time if stimulated emission happens orderly (amplify the signal) or if it also add together the photos disorderedly (increasing the noise). It wasn’t until 1954 when C. H. Townes showed that it amplifies the signal.

    Some gases (ex. ammonium) and solid materials (ex. ruby) can be used in a maser. An example of adaptations is sensitive amplifier for microwaves.

    Laser

    Laser is the abbreviation for “Light amplification by stimulated emission of radiation”. Laser light is emitted when many atoms undergo similar energy transitions at the same time. This is achieved by promoting a large number of atoms to an energy level above the ground state. As an electron in one of the excited atoms jumps down from its higher energy level it emits a photon. As this photon travels past another atom in an excited state, it causes the electron in this atom to jump down to the lower level. The passage of light thus stimulates the emission of radiation from other atoms – producing the intense beam of light characteristic of the laser.

    The laser was first demonstrated by the American Theodore Maiman in 1960 using a ruby laser. The stimulated emission of radiation was initially postulated by Einstein in 1917. The laser’s counterpart in the microwave part of the spectrum paved the way for the production of laser light after two American scientists Schawlow and Townes made a theoretical paper proposing how the maser technology could be widened to fit the visible part of the spectrum.

    A laser consists of an active medium which is placed between two mirrors. The arrangement of the mirrors is called a laser cavity. On of the mirrors is semi-transparent to release the laser that is generated in the cavity. Energy must be supplied from ex. flash bulb. The supplement of energy in the form of a for instance flash bulb is called pumping. The active medium consists of atoms or molecules.

    Normally almost all of the atoms are in their lowest energy level, the so called ground state. From there they can be transferred to a higher, excited energy level through absorption of a light quantum (a photon) with the energy ∆E = hv. The upper state is often very short-lived (microsecond to nanosecond) and the atom returns to the ground state during the emission of radiation. Two radiation processes can happen; spontaneous emission or stimulated emission. The spontaneous emission consist of a random emission of photons in random direction is responsible for the excited state being so short-lived. The stimulated emission can happen if the atom is shone with radiation that has the frequency that is equal to the transition (v). The stimulated photons are emitted with the same frequency as that of the stimulating photons. The probability for the decay of an excited atom is equal to the probability for absorption of a photon by an atom in its ground state and being excited. The stimulated emission can be regarded as a negative absorption.

    Normally a beam of light is weakened by a medium when the atoms in it undergo energy level transitions. If you on the other hand get more atoms in the higher energy level that in the lower (population inversion) the amplification of the beam through stimulated transitions from the higher energy level will dominate over the weakening of the beam through the absorption of light of the atoms in the ground state. The result is amplification by stimulated emission of radiation.

    The two mirrors form a resonator which is an apparatus for identification or amplification of a special frequency around the amplifying active medium. As long as the population inversion stands, there will be an enormous amplification along the passage between the mirrors. For each and every atom that returns to the ground state, the beam gains another photon. The laser is reflected back and forth in the cavity until it has enough intensity to be sent out of the cavity. After that, the flash bulb can again excite enough atoms for the creation of a new pulse. This is the principle behind a pulsating laser. In a continued laser, the laser light is released all the time. If an obstacle is but in the laser cavity the laser discharge is stopped. If the obstacle is removed when the flash bulb has excited a large number of atoms it becomes one gigantic pulse with a today maximum effect of a few terawatt (1012 W).

    Lasers can be modified to have an enormous sharp in frequency, effect in pulse and length of pulse. This can’t be obtained in the same laser system. According to Heisenberg’s uncertainty relationship an ultra short pulse can not be sharp in frequency.

    Different types of laser:

    Gas laser (ex. argon laser)
    Carbon dioxide laser
    Diode laser
    Dye laser

    The ruby laser was the first fully functional type of laser. It has an active medium of a ruby rod with Cr3+ ions in a matrix of Al2O3.

    The technique of laser has a vast number of adaptations for example in:

    - Measuring in the industry workshop.
    - Astronomy (ex. measuring the distance from the earth to the moon)
    - Welding, cutting, hardening of metals etc.
    - Analysis techniques (analytic chemistry, remote analysis of pollutants)
    - Military use
    - Information technology IT (transmissions in fiber optic communication systems, data processing)
    - Medicine (treatment of diseases)

    © Mattara 2006. All Rights Reserved.
     
    Last edited: Jul 9, 2006
  2. jcsd
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