Interaction between Laser Radiation and Biological Tissue

The interaction between electromagnetic radiation and biological tissue depends on
The wavelength of light, which determines the energy of each photon of light..
The intensity of radiation.
The shape of irradiation (continuous or pulsed).
For power levels are up to few Watts, the interaction is divided into 3 regions of wavelengths
Short UV region - the photons interact with the proteins, RNA and DNA, and usually kills the biological cells.
Near UV and Short visible range - photochemical reactions such as photosynthesis. Especially with the Excimer laser.
Visible and Near Infra-Red region - Thermal effects due to absorption of the radiation.

Eye Medical Applications with laser

Medical Applications

There are many medical applications of lasers, and there are different ways to classify them into groups:
· According to the organ to be treated by the laser, such as: Eye, General Surgery, Dentistry, Dermatology, Blood vessels, Cardiac, etc.
· According to the type of laser used for treatment, such as: CO2, YAG, and Argon.
· According to the type of treatment, such as diagnostic, surgery, connecting blood vessels.
The classification used here is basically according to the type of treatment, with comments on suitable lasers used for each application:
Lasers in medical surgery.
Lasers in diagnostic medicine, and in combination with drugs.
Lasers for specific applications: Soft lasers.
When using lasers for medical treatments, a good understanding of the interaction between specific laser radiation with specific biological tissue is required.

Applications of Lasers in Chemistry

From the variety of applications of lasers in chemistry, we shall mention:
· Excitation of molecules to specific levels, and examination of the emitted radiation.
· Measurements of the relaxation time of specific excited levels of molecules.
· Disruption of chemical bonds in molecules at specific region - When a laser beam is focused, a very high electric field is created at the focal point (up to 109 V/cm). Such electric fields are larger than the force which holds the valence electrons in an atom. Another possibility is to use wavelengths which are very short (which means that the photons are very energetic) to break the chemical bonds. This is usually done with the Excimer laser.
· Raman spectroscopy: Raman scattering is a process of inelastic scattering of the photon by the molecule. The photon is absorbed by the molecule, and another photon, with a different frequency is emitted. The change in frequency of the photon is connected to the energy transitions in the molecule which absorb the photon. The most important Raman scattering is connected to vibrational transitions of the molecule. By measuring the change in frequency, it is possible to identify the specific molecule.
There are two kind of Raman scattering processes:
o Stokes scattering - when the photon lose energy, and the molecule absorb this energy, and go into excited state. The frequency of the emitted photon is less than the frequency of the incident photon.
o Anti-Stokes scattering - when the photon receives energy from the molecule. The frequency of the emitted photon is higher than the frequency of the incident photon.

Spectral analysis.

the entire lasing process is based on absorption and emission of photons at certain specific wavelengths. The wavelength emitted from the laser is monochromatic, and its linewidth is very narrow.
Thus, the laser can be used for controlled excitation of molecules. Especially useful for this are the tunable lasers, whose wavelength can be precisely tune to excite specific molecule.

The mostly used lasers for material processing are:

CO2 laser - has high power and is highly absorbed in most materials.
Nd-YAG laser - has high power and can be transmitted through optical fibers.

Interaction Mechanism between the Laser Beam and matter

The mechanism of interaction between the laser beam and the processed material:
· Thermal Effects - Most of the applications of lasers in material processing were based on the absorption of the laser radiation inside the material, and the effects were thermal in nature. The absorption process transfers energy to the material. As a result, there is a rise in the temperature in that region to high temperatures.
· Photochemical Effects - Breaking the bonds between the molecules in the material. The Excimer laser emits in the Ultra-Violet (VU) part of the electromagnetic spectrum, and its photons are very energetic. It can be used to cut very delicate and accurate structures without causing thermal damage to surrounding areas.

The main advantages of lasers for material processing are:

The main advantages of lasers for material processing are:
· Very high accuracy in the final processed products that can be obtained without the need for polishing.
· No wearing of mechanical tools. Mechanical tools change their dimensions during the working process, and require constant measurements and feedback to adapt their position to original plan in computerized instrumentation. Material processing includes many kinds of processes. A partial list includes:
· Cutting - The laser can be a very precise cutting tool. High power lasers are used for cutting steel, while other lasers are used to cut fabrics, rubber, plastic, or any other material.
· Welding - Combining (fusing) two materials together. By heating the materials near the connecting region, the materials melt locally, and fuse together.
· Hardening - By heating specific areas of the material, most metals can be hardened most of the metals. Even local hardening of specific part of a tool can be done by local irradiation.
· Melting - Absorption of laser beams caused a rise in temperature. Since very high power can be transferred to materials in a very short time, melting can be easily done.
· Evaporating - Used to ablate material (transfer it into the gas phase).
· Photolithography - especially in the semiconductor industry. Very delicate shapes can be created in materials which are used for masks in photolithography. Special materials respond to light at specific wavelength by changing their properties. Thus it is possible to remove parts of the material with very high precision (in micrometer range).
· 3D Laser measurements - With the help of a scanning laser, it is possible to obtain the information about a shape of a three-dimensional object and put it in the computer.
· 3D Stereo lithography - Similar to photolithography, but the laser is used to create three dimensional sculpture of the information stored within a computer. A combination of the last two applications enables creating 3-D models. Even statue of people were build with high accuracy using these techniques.

Straight line marking, or plan of reference.

Many daily applications require a precise reference line for alignment. Examples are:
· Laying pipes of gas, water, electricity, etc.
· Digging tunnels under-ground (such as the one under the English Channel between England and France).
· Alignment of mechanical systems.
· Marking spots for pointing invisible radiation from another laser (such as Nd-YAG or CO2 lasers). The visible laser radiation is aligned parallel to the invisible radiation, such that it mark the place where the invisible beam is pointing.
· Marking a reference plane for construction:
By using a vibrating (or rotating) mirror to reflect a visible laser light, a perfect plane is defined in space. The mirror is vibrating around one axis, so the light is reflected into consecutive angles continuously, thus defining a perfect plane. Since the vibration of the mirror is at a frequency greater than the persistence of vision in the brain, the viewer sees a plane of light. This plane helps aligning walls, sealing, etc. in industrial construction.

Straight line marking, or plan of reference.

Many daily applications require a precise reference line for alignment. Examples are:
· Laying pipes of gas, water, electricity, etc.
· Digging tunnels under-ground (such as the one under the English Channel between England and France).
· Alignment of mechanical systems.
· Marking spots for pointing invisible radiation from another laser (such as Nd-YAG or CO2 lasers). The visible laser radiation is aligned parallel to the invisible radiation, such that it mark the place where the invisible beam is pointing.
· Marking a reference plane for construction:
By using a vibrating (or rotating) mirror to reflect a visible laser light, a perfect plane is defined in space. The mirror is vibrating around one axis, so the light is reflected into consecutive angles continuously, thus defining a perfect plane. Since the vibration of the mirror is at a frequency greater than the persistence of vision in the brain, the viewer sees a plane of light. This plane helps aligning walls, sealing, etc. in industrial construction.

Accurate measurements (Distance, Movement, Interferometry

Since laser radiation is electromagnetic radiation, traveling at the speed of light, very accurate measurements can be performed with lasers. Because of its high speed (the speed of light (c) is the ultimate speed …), measurements of high speed moving objects are not a problem, and the information is available in (almost) real time.
· Interferometric measurements (which give the highest resolution known today) .
· Range-finder for military applications is described in chapter Based on the same principles for measuring distances, industrial measuring devices have been developed.

Industrial Applications

Industry accepted the laser as a tool soon after the laser was invented in 1960. At first the laser was used for alignment and measurements, but with time applications using high power laser beams became more common. The main industrial applications are:
Accurate measurements (Distance, Movement, Interferometry).
Straight line marking, or plan of reference.
Material working: cutting, welding, hardening, melting, evaporating, photolithography, etc. Spectral analysis

Laser Applications

The number of applications of lasers is enormous, and it is not possible to explain all of them here. In this chapter, the applications are divided into groups, and our hope is that with time we will fill the missing information on most of the well known applications of lasers. Some applications are already described in details, such as:
· Compact Disk (CD).
· Laser Printer.
· Bar Code Scanner.
· Inertial Fusion.