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Physical Principles of Laser Therapy

  • Physical principles of laser therapy

    Coherence, polarization and monochromicity are essential features in laser radiation

    “Laser” is above all, an acronym that means “Light Amplification by Stimulated Emission Radiation”, that is an emission stimulated by radiation of amplified light. The name reflects the operative principle of laser technology.
    In common substances, light is absorbed by the transformation of energy into heat and consequently, it gets reflected or dispersed. However, inside the body of the laser this light is intensified due to the energy that comes from the exterior, like we see in other sources of light. For example, the difference between laser and florescent light lies on the fact that the induced emission of photons is caused by the photons themselves, in which a sort of “reproduction” of photons happens, resulting in the amplification of light. Any photon newly created is an absolute copy of the photon that produced it. From this process, the unique features of laser radiation emerge: coherence, polarization and monochromicity (absorption of the laser ray by one single colour. For example, the red ray is absorbed by the organisms red areas, like the blood vessels).

    Laser radiation characteristics

    Laser radiation presents itself with very specific characteristics.
    The laser is the amplifier or the coherent quantic emitter of electromagnetic radiation within the visible spectrum of light.
    Laser radiation is the electromagnetic radiation of the visible region of the spectrum whose characteristics are coherence, polarization and monochromicity. These characteristics allow the enhancement of high local concentration of energy.

    Coherence (from the latin “cohaerens”, which means “connected”) is developed from various wave-like processes that share the same frequency, coordinated in time. This characteristic is related to two or more wave-like processes and when they are overlapped, this feature defines the ability to mutually weaken or strengthen those processes. The overlapping of waves in space results in an interference phenomenon (in here we can distinguish between spatial coherence and temporal coherence). In other words, coherence is the propagation in the same direction of photons with similar frequency. The radiation that contains these types of photons is called coherent.

    Monochromicity is the feature of radiation that has a certain kind of frequency or wave length. More specifically, it´s the feature of radiation that holds a sufficient small spectrum width that allows it to have only one color from the whole spectrum of colors. We consider that radiation is monochromatic when the spectrum width is inferior to 5nm (nanometers). The pulsed semiconductor laser has exactly this spectrum width, but the spectrum width of the continuous monomode laser is inferior to 0.3 nm.

    Polarization is the symmetry (or lack of it) in the distribution of the orientation of the intensity vector in the electric or magnetic field of the electromagnetic wave, in relation with the direction of its propagation. If two mutually perpendicular components of the intensity vector of the electric field oscillate, and the difference of their phases in that oscillation is constant in time, the wave created is called polarized. If the oscillation is chaotic (for example in the case of propagation of electromagnetic waves in anisotropic environments, reflection, refraction, diffusion, etc), the wave is no longer polarized.