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  • Laser Fundamentals

    The acronym LASER stands for Light Amplification by Stimulated Emission of Radiation. This is a device designed to produce a beam of monochromatic light in which all the waves are in phase or are coherent.

    Lasers contain four primary components regardless of style, size or application (Fig.1 ).

    Active Medium

    The active medium may be solid crystals such as ruby or Nd:YAG, liquid dyes, gases like CO2 or Helium/Neon, or semiconductors such as GaAs. Active mediums contain atoms whose electrons may be excited to a meta stable energy level by an energy source.

    Excitation Mechanism

    Excitation mechanisms pump energy into the active medium by one or more of three basic methods; optical, electrical or chemical.

    High Reflectance Mirror

    A mirror which reflects essentially 100% of the laser light.

    Partially Transmissive Mirror

     
    A mirror which reflects less than 100% of the laser light and transmits the remainder.

    Figure 1 - Laser Components

     

    Lasing Action

    Light is produced when energy is applied to an atom raising an electron to a higher unstable energy level. This electron will then randomly return to its ground state releasing a photon of light. Light produced in this manner is called incoherent light (Fig.2 ) and has many different wavelengths (colours), directions and phases.

    Figure 2 -  Incoherent Light

    When energy is applied to a laser active medium (Fig.3) electrons are raised to a higher unstable energy level then spontaneously decay to a slightly lower relatively long-lived meta stable state.

    Figure 3 - Energy states of Laser Active Medium

    Electrons in this state will not spontaneously return to their ground energy level; as more energy is applied to the Active Medium more and more electrons are placed in the meta stable state, resulting in a population inversion in which most of the atoms are in a meta stable state.

    Once this population inversion has been achieved, lasing action is initiated by an electron which spontaneously returns to its ground state producing a photon. If the photon released is of exactly the right wavelength it will stimulate an atom in a meta stable state to emit a photon of the same wavelength (Stimulated Emission).

    Many of these stimulated photons will be lost when they hit the side of the lasing medium. However if the photons travel parallel to the long axis of the optical cavity they will continue to stimulate emissions of photons having the same wavelengths which combine coherently (Fig.4) until they reach the mirrored ends of the optical cavity. When the beam strikes the totally reflecting mirror in the optical cavity the beam is reversed and continues to stimulate emissions of photons which increase in intensity until the beam reaches the partially reflecting surface of the optical cavity. A small portion of the coherent light is released while the rest is reflected back through the lasing medium to continue the process of stimulating photons. Laser radiation will continue to be produced as long as energy is applied to the lasing medium.

    Figure 4 - Coherent Light