Physics of Laser

Fig. 3.1

Electromagnetic spectrum of light from gamma rays to radio wave wavelengths, UV ultra-violet or ultraviolet, IR infra-red or infrared

3.3 Characteristic of Ordinary Light and Property of Laser Light

Light is an electromagnetic radiation which besides its wave characteristic has a photonic corpuscular characteristic that transports a defined amount of energy (quantum).
The ordinary visible light, both natural (solar) and artificial, has several characteristics that must be defined to understand the laser light properties.
The ordinary light is disorderly emitted, because of the absence of coherence in the time and space of the photons; consequently the light is scattered in all the directions and diffuses homogeneously. The ordinary light is multichromatic, being composed of all the wavelengths and colors within the visible spectrum and also called white or achromatic light.
The basic colors of the visible light are violet, blue, cyan, green, yellow, orange, and red (Table 3.1).

Table 3.1

Basic colors of the visible light
Violet 380–450 nm
Blue 450–475 nm
Cyan 476–495 nm
Green 495–570 nm
Yellow 570–590 nm
Orange 590–620 nm
Red 620–750 nm
When the sunlight diffuses through the water droplets remained in suspension after a meteorological phenomenon such as a storm or a natural phenomenon such as a waterfall, the light can scatter and refract itself within the water droplets, producing the phenomenon of rainbow, that described the colors of the visible spectrum of the light.
Primary colors (or absolute colors) are the basic colors from which are obtained, combining them, all the others: they are red, blue, and yellow. The additive or subtractive combination of primary colors is used to obtain secondary colors; this concept helps to understand the use of complementary colors in photodynamic therapy and photoactivated dental bleaching (read ahead). The blue (420–470 nm) is the complementary color of yellow. The cyan (480 nm) is the complementary color of red (630–760 nm), and the magenta (that is a secondary color produced by combination of blue and red) is the complementary color of green (490–570 nm).
The laser light is different from the ordinary light, being able to aim a high amount of energy in a limited space in the form of light radiation.
Several characteristics differentiate laser light from ordinary light:

  • Laser light is collimated, that is to say that the waves, thanks to the spatial coherence of emitted photons, have only one very focused direction.
  • Laser light is coherent, that is to say that each wave/photon has the same phase with the other emitted waves/photons that are kept in time and space.
  • It is monochromatic, that is to say that each photon has just one wavelength and just one color (if visible) (Table 3.2).

    Table 3.2

    Property of laser and ordinary light
    Characteristics
    Laser light
    Ordinary light
    Direction
    Collimated
    One very focused direction
    Noncollimated or multidirectional
    Temporal phase
    Coherent
    Each wave/photon has the same phase with the other emitted waves/photon that are kept in time
    Noncoherent or disorganized
    Color
    Monochromatic
    One wavelength and just one color (if visible)
    Multichromatic from 400 to 700 nm

3.4 Basic Component of Lasers

Several components are necessary to constitute a dental laser unit:

1.

The optical cavity (or resonator) that includes the active medium
 
2.

The active medium which characterizes different wavelengths of specific lasers
 
3.

The pumping source (or energy source) to supply the energy necessary for the stimulation
 
4.

A controller that is a software that controls the modality and parameter of laser emission and a cooler, necessary for cooling the laser system
 
5.

The delivery system that transports the laser energy to a terminal handpiece and tips and finally to the tissue (Table 3.3)

Table. 3.3

Basic components of a laser
Optical cavity
Active medium
Pumping source
Controller
Cooler
Delivery system
Handpiece and tips
 

3.4.1 Optical Cavity

It is a hollow cavity that contains the active medium and two mirrors located at its extremities; one is completely reflective, while the other one is partially reflective and permeable.
The excitement of the active medium by an external source of energy produces the stimulated emission of photons that, reflecting on the mirrors of the cavity and passing through the active medium many times, add their energy via an amplification phenomenon before coming out from the partially permeable mirror.

3.4.2 Active Medium

It is the heart of the laser and can be a solid, a liquid, a gas, or a semiconductor in the case of diode laser. The active medium determines the specific wavelength of different lasers and its name identifies different lasers. Table 3.4 reports the active medium of lasers mostly used in dentistry.

Table 3.4

Active media, hosting media, and doping atoms of the most used lasers in dentistry
Laser
Abbreviation
Active medium
Hosting medium
Doping atom
Wavelength (nm)
Argon
Ar
Gas
488 and 514
Carbon dioxide
CO2
Gas
9,300, 9,600, and 10,600
Diode
Semiconductor
445, 635–810, 940–970, 1,064
Potassium titanyl phosphate
KTP
Solid
YAG crystal
Neodymium frequency doubled
532
Neodymium-doped yttrium-aluminum-garnet
Nd:YAG
Solid
YAG crystal
Neodymium
1,064
Neodymium-doped yttrium-aluminum-perovskite
Nd:YAP
Solid
YAP crystal
Neodymium
1,340
Erbium-doped yttrium-scandium-gallium-garnet
Er,Cr:YSGG
Solid
YSGG crystal
Erbium and chromium
2,780
Erbium-doped yttrium-aluminum-garnet
Er:YAG
Solid
YAG crystal
Erbium
2,940
The active medium supplies the electrons for the production of the laser photons.

3.4.3 Pumping Source (or Energy Source)

The pumping source excites the atoms of the active medium producing the inversion of the population of electrons. This source of energy is usually represented by an electric coil or a diode laser or a flash lamp. The characteristics of pumping source are important for the generation of the laser pulse, especially for short-duration pulses (high peak power).

3.4.4 Controller Subsystem and Cooler

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Nov 22, 2015 | Posted by in Dental Materials | Comments Off on Physics of Laser
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