During the last three decades of the twentieth century, it has been found out useful<br />applications of laser rayin the elds of medicine, surgery, ophthalmology, industry, automatic<br />control, communications, chemistry andthe like. To this point, laser became a tool that is used<br />to treat the serious disease (cancer) by using photic-dynamictreatment, where a sensitive<br />substance of light is injected into the injured body and then laser with wave length of630 nm<br />can be used for the sake of making chemical reaction that produces poisonous substance to<br />kill tumors.Laser can also be used in sterilization of blood in blood bank which puries blood<br />from bacteria and virouses.[1-9]<br />The Action of Tissue on Laser<br />The Action of Tissue on Laser LightWhen a laser beam is an incident on a tissue, four basic<br />.physical phenomena can occur as shown in gure I<br />Reection and refraction, Absorption, Scattering and Transmission , and<br />The relative and absolut emagnitudes of these phenomenathe wavelength of the laser light<br />and the physical phenomenous drprnd on the eave length of the lasrrsnd the properties of the<br />. tusses<br />The Effect of Laser on Bio substance<br />When the laser light is absorbed by a bio substance ( a tissue, bacterial suspension, ...etc.) it<br />can result indifferent types of effects depending on the wavelength of the laser radiation, power<br />.density, pulse duration, andthe nature of the bio substance<br />Laser effects on the bio substance can be classied according to their wavelengthdependency<br />to wavelength dependent and wavelength independence<br />I.Photochemical interaction<br />At low laser intensities, irradiation of cells at certain wavelength can activate some of the native<br />componentsin bio substance. In this way specic biochemical reactions, as well as whole<br />cellular metabolism can be altered.This reaction is believed to form the basis of low power<br />laser effect (bio stimulation) Photochemical ablationinvolves photo dissociation or direct<br />breaking of intramolecular bonds in biopolymers, caused by absorption oncident photons and<br />subsequent release of bio substance[10]. This occur when the energy of the incident photonis<br />of the order of the bonding energy of a biomolecules then chemical decomposition occurs<br />leading to productsoccupying rapidly a volume larger than the initial one forming the ablation<br />.process[11, 12]<br />II Photo thermal interaction<br />The term thermal interaction refers for a large group of interaction type, where the increase<br />local temperatureis the signicant parameter change. Thermal effects can be induced by either<br />CW or pulsed laser radiation. At60oC, denaturation of proteins and collagen occurs which<br />leads to coagulation of tissue and necrosis of cell. At100oC, water molecules contained in<br />.most tissues start to vaporize<br />The large vaporization heat of water isadvantageous, since the vapor generated carries away<br />excess heat and helps to prevent any increase in thetemperature of adjacent tissue. Due to the<br />large increase in volume during this phase transition, gas bubbles arefound inducing<br />.mechanical ruptures and thermal decomposition of tissue fragments<br />At temperatures exceeding150oC, carbonization takes place which is observable by the<br />.blackening of adjacent tissue and the escape of smoke<br />.To avoid carbonization, the tissue is usually cooled with either water or gas<br /> Finally, beyond 300c, melting canoccur,[13,14] depending on the target materia