I. Introduction We don’t even recognized it, but LASER applications are all around us. In our home, our hand, and eve in

our pocket. For example, you can see the bar code scanner, whenever you pay the bill in convenience store. That is one of the LASER applications. ID card has a hologram on it for protection against the counterfeiting. That could be another example. Besides LASER Technology has been applied to various fields practically, such as LD (Laser Disk), medical appliances and technology. But, we use a word ‘LASER’ easily without knowing what does that exactly mean. Someone think may LASER is just some kind of beam which is showing at the Star Wars, Someone may think LASER is just some kind of electromagnetic wave. But those are all wrong. LASER stands for Light Amplification by Stimulated Emission of Radiation. So, we can say LASER is not one of an electromagnetic (EM) wave. In principle, the LASER is a device which transforms energy from other forms into EM radiation. Base on this fact, we are going to verify ‘General principle and properties of LASER’ and show some examples of LASER applications.

II. General Principle and Properties of LASER Principle of LASER is simple. You can guess it from its original form, Light Amplification by Stimulated

Emission of Radiation. The key word is ‘Amplification’. Atoms and molecules have determinate energetic levels, which can be low or high. The low energetic levels can be excited at high levels, generally by heating. Once they reach the energetic superior levels, they go back to the original state, they return energy in a light form.(fig 1-1) In most cases, the sources of ordinary light which comes from atoms, excited molecules and the light emission, is done in various wavelengths and frequencies.(fig 1-2) But, if atom is excited during the short instant, the atom is influenced by light of a certain wavelength. And this atom can be stimulated to launch radiation that has same phase and wavelength of the light, which influences an atom.(fig 1-3) In this case, we can get much higher energy than first case.(Figure 1-2, and 1-3 show light intensity difference between those two cases) So, when we make a LASER radiation, atoms need to be at the high energy level by energy pumping. And stimulate those atoms. Consequently, radiation from atoms will have a same wave length and phase. This resulting radiation (or beam) is a coherent light beam and it can be high-powered. We call it LASER radiation. And this radiation has several properties.

Laser radiation Properties - Mono-chromaticity Mono-chromaticity means "One color". To understand this term, examine "white light" which is the color interpreted in the mind when we see all colors together. When "white light" is transmitted through a prism, it is divided into the different colors which are in it.(fig 1-4_a) and LASER radiation does not have all those color, because it has only one same wavelength and phase. And Mono-chromaticity also means that LASER has a high intensity of the light within the very small wavelength. So it can have a high energetic level in microscopic region. (fig 1-4_b) Actually, the temperature of the Laser radiation is higher than Sun. (more than 6000K) - Directionality Radiation comes out of the LASER in a certain direction, and spreads at a defined divergence angle. This angular spreading of a LASER beam is very small compared to other sources of electromagnetic radiation, and described by a small divergence angle.(fig 1-5. θ/2 is divergence angle and it is of the order of milli-radians) Since, LASER radiation divergence is of the order of milli-radians, which means almost 0, the beam is almost parallel, and can be send over long distances. - Coherency LASER radiation is composed of waves at the same wavelength, which start at the same time and keep their relative phase as they advance. So, when two or more LASER radiations can make regular interference each other. (fig 1-6_a, b) So, LASER radiation has a coherency.

III. LASER applications

LASER applications in Communication

LASER radiation has been used in space communication system. Due to the directionality of the LASER radiation, it can be send over long distances. A clear atmosphere transparency is similar to fiber one so LASER radiation can keep going through the clear atmosphere. Space is one of a clear atmosphere. So LASER radiation is useful for the Free space satellite laser communication

Holography Hologram is a light wave interference pattern recorded on photographic film or other suitable surface that can produce a 3-dimensional image when illuminated properly. And this is based on the Coherency of LASER radiation. fig2-1 shows the principle of hologram. A laser beam is split into two beams. The reference beam is spread by a lens or curved mirror and aimed directly at the film plate. The object beam is spread and aimed at the object. The object reflects some of the light on the holographic film-plate. The two beams interact forming an interference(fig 2-2) pattern on the film. This is the hologram. Laser light is needed because it is made of coherent waves of same wavelength and phase. This Holography technology has been used in various field. Bar code scanner will be one of good examples. When you go to the E-mart or convenience store, you can see the bar code scanner. When the He–Ne LASER hit on the bar code of the object, reflected light is transformed to the electric signal and get price information from the computer. So the name and Price of that object is showing up on the screen. Another example will be the protection skill. Hologram can not be copied by photo copier because 3-dimensional images can not be copied on 2-dimensional plate that easily. so it is used in credit card, ID card, and bills for protecting against the counterfeiting. And Hologram makes a 3-dimensional images, so it is used in Art.

Medical devices LASER radiation has been used in surgery in stead of mess, because of its directionality. LASER radiation has small divergence angle and because of its mono-chromaticity, high powered energy from the light can be concentrated on microscopic region. And LASER radiation has a certain wavelength, it can be go through the cell tissues. For example, He –Ne LASER which has 632.8nm wavelength, can pass the surface tissues of the human body, and hit the inner tissues which has a cancel. These properties make it possible to operate an eye surgery, operation on the stomach cancel without cutting out the surface skin of human body. LASER radiation is also used on freckle remove operation.

LASER process Because of its good directionality, LASER light can be focused in very small area by using typical lens. And we can get high energy intensity, use it for cutting, welding, drilling, and thermal processing. Since the LASER light has very high temperature, when this LASER light hit the surface of some material, contact part will be evaporate because of the high temperature of LASER light. LASER light is easier to control and has a greater accuracy than other devices.(fig 2-3 and 2-4 )Besides, LASER doesn’t make any forces on the surface of the material directly. So LASER light has a great advantages in various forms of drilling, cutting and welding. Practically, LASER light is used for cutting hard materials like diamond or Titanium.

IV. Laser appliances for the future Wireless communication on the ground

LASER is expected to be an alternative in wireless communication on the ground. Due to interference and security, the radio spectrum is highly regulated all over the world. And as user and users for wireless communications have increased, radio frequencies for the communication has become full. To solve these problem, wireless LASER communication system is suggested. The high data rate and large information throughput available with laser communications are many times greater than in Radio Frequency systems. But this appliances has critical problems. In bad weathers like fog, snow and rain conditions its transparency becomes thousands times lower. And strong light scattering appears in bad weather conditions. The air line can not hold the high level of light power for a long distance because of laser beam spreading, its scattering and absorption in air. LASER radiation spreading in clear air depends on a LASER radiation divergence. It is difficult to keep a pointing of narrow LASER radiation exactly within receiver lens aperture.

Optical computer There are whole bunch of cables inside of the computer. All those cables are for the information transfer between the computer devices. Computer science reached a lot of progress in CPU and memory speed. But there are not much progress in speed of information transfer between CPU and memory, CPU and other hard ware devices. Optical computer is one of the progress form which has been improved of transfer speed. It is using optical fiber instead of common cables. So, it is predicted that speed of transfer between hard ware devices will be 1000 times faster than normal cable using computer.

V. Conclusion In this work, we investigated regarding the LASER as a device which transforms energy from other forms

into EM radiation. And LASER radiation have some useful properties which can be used practically in many ways. Directionality is one of the most powerful property that can be use for the communication and information transfer. Mono-chromaticity is useful for the LASER processing in the industrial. And Coherency makes hologram possible. Those three properties are not natural EM wave properties. But they still means a lot in our world. By the Light amplification by stimulated emission of Radiation, EM wave got very useful properties.

References [1] free space satellite laser communication ; by Adnan Cora-Hacer Atar, Karadenziz Technical University Department of Electrical and electronics Engineering 61080-Trabzon/Turkey [2] Laser cross-link systems and technology ; David L. Begley, Ball Aerospace & Technologies corp [3] http://physica.gsnu.ac.kr [4] http://www.colorado.edu/physics/2000/index.pl [5] scrap of magazine ‘Forbes’ ; July 2004 [6] Field and wave Electromagnetics ; by David K. Cheng E2

W

E1 E1 : Low energetic level E2 : H igh energetic level W : Light wave

fig 1-1 on left of the picture, light wave energy is absorbed so that atom can have a high energetic level. And on the right, light has been generated with certain wavelength and phase when the atom lost its energy.

fig 1-2 fig 1-3 E1 : Low energetic level E2 : High energetic level In fig 1-2, fig 1-3, there are many numbers of atom. (let’s assume that number of atom = N) In case of fig 1-2 , generated light intensity is N. because the wavelength doesn’t fit each other. In case of fig 1-3, all of generated light has same wavelength and phase, the light intensity will be N²

fig 1-4_a fig1-4_b Mono-chromaticity of LASER radiation

fig 1-5. Light from the Lamp diverges to every directions. But Divergence angle of LASER radiation (θ/2) is very small.

fig 1-6_a fig 1-6_b fig 1-6_a : Before the interfere between the light sources which have same wavelength and phase fig 1-6_b : After the interfere between the light sources which have same wavelength and phase

fig 2-1 fig 2-2

fig 2-1 principles of the hologram : Two different beams(Object beam and reference beam) make a 3-D on the Holographic plate fig 2-2 by the interference of those two beams, 3-D images are developing on the plate.

fig 2-3 fig 2-4 LASER device which is used in Memory construction