LASER: Light Amplification by Stimulate Emission of
Radiation
The general properties of the laser beam:
- Monochromatic: it is a unique light source emits a narrow beam of
light of a single wave length.
- Coherent: the laser waves are in phase with each other.
- Collimated: the laser beam can be focused to a spot only a few
microns in diameter.
Sun Light
LED Light
Laser
The principle of laser production
Laser components:
1- Active medium: solid, liquid or gas where the absorption, stimulation
emission take place.
2- Inversion population:
a. Atoms in the ground state stimulated by an incident photon to the excited
-3
state (metastable state) in 10 sec.
b. The no. of excited atoms in the metastable state > ground state.
c. The atoms tend to decay back to the ground level by spontaneous
-6
-3
emission in (10 -10 )sec in each decay a photon released.
d. The spontaneous emission leads to the amplification of the beam by the
resonator.
3- resonator: which contain two parallel mirrors, the active medium placed
between them. One of the mirrors cause total reflection, while the
other cause a partial reflection.
- The photon incident on the first mirror reflected totally along the axis of
the resonator then hit the partially reflected mirror permits continuous
amplification.
4- The generated photons are identical to the incident photons.
Essential elements of lasers
Resonator
Mirror
100% reflectivity
Laser medium
Mirror
<100% reflectivity
Laser
output
Pumping
Types of laser
a. Pulsed Laser
solid lasers:
Ruby laser (λ= 694 nm)
Nd:YAG
Semiconductor
b. Continuous Laser
Gas lasers:
Co
2
He-Ne
argon laser
Liquid lasers: dye, excimer.
Ruby laser
- In (1960) T.H. Maiman produced a laser from ruby crystal.
- The ruby rod is aluminum oxide with some chromium atoms
through it.
- One end of the rod is covered completely with silver to allow light
to escape.
- The bright flash light causes the electrons in the chromium atoms of
the rod to gain energy, which is released inside as a red light.
- The red light is reflected back and forth between the mirrored
surfaces at the end of the rods.
- The light causes the chromium atoms to release more light
photons.
- The intensity of the red light beam increases and leaves the end
of the half-coated end of the rod as a laser beam
Laser Tissue Interactions:
The wavelength and power output of a particular laser determines its
medical applications, because the interaction between laser radiation and
biological tissue depends on important parameters such as :
- Laser Parameters
- Tissue Parameters
Irradiation Dose and power density of Laser
Energy (J)
• Power (W) = ----------------Time (sec)
Irradiance; also called power density can be calculated as follows
Power output (W)
Power density (Irradiance) (W/cm²) = -------------------Impact spot Area (cm²)
Treatment dose is the same as the energy density. Doses refer to the amount
of energy per unit area, the dose is the most important treatment parameter
Dose (J/cm²) = power density (W⁄cm²) × Time (sec)
Energy density (J/ cm²) = power (W)× time (sec)/Area (cm²) of spot
Skin Parameters: Skin components
Epidermis - the outermost layer serves as the first layer of defense against
the outside world. Cells at the surface are dead, and they fall off and away
as upward pressure is applied by continuously multiplying cells in the
lower epidermis
Dermis - provides the skin's structural integrity, elasticity, and resilience;
includes fibroblasts, capillaries, lymph nodes, sweat glands, hair glands,
but only a few nerves and muscle cells
Subcutaneous tissue - the thickness varies greatly; mostly fat or adipose
cells, protects the body from cold and mechanical trauma
Selective Absorption of Chromospheres
Melanin: gives skin its tone or color. Darker skin has more melanin.
Patients with darker skin may experience some destruction of melanin
when the 1064nm light is absorbed, but it will re-generate by itself.
Hemoglobin (blood): absorption of energy especially by 532nm
H20 (water): absorption low for 532nm, very moderate absorption for
1064nm
Classification of Lasers
• By laser medium: gas (He-Ne, CO2, nitrogen lasers), liquid (dye
laser), solid lasers (Ruby, Nd:YAG, Nd-glass, Er:YAG lasers)
• By pumping method: flashlamp, electrical discharge, chemical
actions
• By operating mode: continuous-wave (CW), pulsed lasers
• By wavelength: infrared, visible, ultraviolet, x-ray lasers
Types of medical laser:
1- High power Lasers
They are used in many surgical procedures to cut, coagulate and evaporate
tissues. The power range for the surgical lasers is between (3000 to 10,000)
mW, also it is known as ''surgical lasers''. It must be powerful enough to
heat up the tissues to temperatures over 50 degrees Celsius. The most
widely used are Nd:YAG and Co2 lasers.
2- Low Level Lasers (LLL)
• The output power is between (1 to 500) mW
• The typical wavelength is in the range 600-1000 nm (red to near
infrared)
• short-pulse-width devices are in the range of 1-100 W with typical
pulse-widths of 200 ns
• It is applied for relieving chronic joint disorders, neck pain,
rheumatoid arthritis and wound healing
•
This low power will not damage the hydrogen bonds in the tissues
and
does not cause any change but the photochemistry effect. Stimulating a cell
(increased in cellular metabolism) and the effects are biochemical, not
thermal. Photons, which are particles of electromagnetic energy, are
emitted from the low power laser. These particles enter the tissues and are
absorbed in the mitochondria, which are tiny structures within the
substance of each individual cell. The energy is converted to chemical
energy within the cell. The permeability of the cell membrane changes
which in turn produces various physiological effects. These physiological
changes affect a variety of cell types including macrophages, fibroblasts,
endothelial cells and mast cells.
•