03-10-2016, 10:29 AM
EXPERIMENTAL INVESTIGATION AND OPTIMIZATION ON DIFFICULT-TO- LASER-CUT MATERIAL USING GAS LASER
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Introduction to Laser
LASER is the acronym for Light Amplification by Stimulated Emission of Radiation.
Albert Einstein first explained the theory of stimulated emission in 1917, which became the basis of Laser. He postulated that, when the population inversion exists between upper and lower levels among atomic systems, it is possible to realize amplified stimulated emission and the stimulated emission has the same frequency and phase as the incident radiation. However, it was in late 1940s and fifties that scientists and engineers did extensive work to realize a practical device based on the principle of stimulated emission. Notable scientists who pioneered the work include Charles Townes, Joseph Weber, Alexander Prokhorov and Nikolai G Basov.
Initially, the scientists and engineers were working towards the realization of a MASER (Microwave Amplification by the Stimulated Emission of Radiation), a device that amplified microwaves for its immediate application in microwave communication systems. Townes and the other engineers believed it to be possible create an optical maser, a device for creating powerful beams of light using higher frequency energy to stimulate what was to become termed the lasing medium. Despite the pioneering work of Townes and Prokhorov it was left to Theodore Maiman in 1960 to invent the first Laser using ruby as a lasing medium that was stimulated using high energy flashes of intense light.
The development of Lasers has been a turning point in the history of science and engineering. It has produced a completely new type of systems with potentials for applications in a wide variety of fields. During sixties, lot of work had been carried out on the basic development of almost all the major lasers including high power gas dynamic and chemical lasers. Almost all the practical applications of these lasers in defense as well as in industry were also identified during this period. The motivation of using the high power lasers in strategic scenario was a great driving force for the rapid development of these high power lasers. In early seventies, megawatt class carbon dioxide gas dynamic laser was successfully developed and tested against typical military targets. The development of chemical lasers, free electron and X-ray lasers took slightly longer time because of involvement of multidisciplinary approach.
1.2 Types of Laser
A laser is composed of an active laser medium, or gain medium, and a resonant optical cavity. The Laser gain medium transfers external energy into the laser beam. It is a material with controlled purity, size, concentration, and shape, which amplifies the beam by the process of stimulated emission. The laser gain medium, in general, is pumped, by an external energy source including a flash lamp, another laser source, electric gas discharge, exothermic chemical reactions etc. The pump energy is absorbed by the laser medium, exciting some of its particles into high-energy state where these can interact with light both by absorbing photons and by emitting photons. Under certain conditions, as mentioned in earlier sections, the amount of stimulated emission due to light that passes exceeds the amount of absorption resulting in amplification. Thus the basic components of a laser are:
• Lasing material e.g. crystal, glass, gas, semiconductor, dye, etc.
• Pump source that adds energy to the lasing material, e.g. flash lamp, electrical current to cause electron collisions, radiation from a laser, chemical reactions etc.
• Optical cavity, which consists of reflectors, acts as the feedback mechanism for light amplification.
In this section, we would like to discuss various types of lasers like, solid state lasers, semiconductor lasers, dye lasers, excimer lasers, gas lasers, gas dynamic lasers, chemical lasers, X-Ray lasers, Free Electron lasers etc. Our intention is to provide salient features of various systems, without going into intricate details. The reader is advised to go through the various references for details at the end of the section.
Solid Laser
The Fig. 1.2 shown below is the schematic diagram of a solid laser. In this a ruby like crystal is used which acts as an active medium. It is basically cylindrical in shape. This crystal is surrounded by a xenon flash lamp T. This flash lamp is of helical shape. In this arrangement this lamp acts as a pumping arrangement. Both the ends E1 and E2 of the crystal are properly polished. Similar to the gas lasers, the surface M1 will do the complete reflection but on the other hand M2 will reflect partially. Whenever we will pass the current through the arrangement a laser beam of red color having large intensity will come out.
Liquid Lasers: - In liquid lasers organic dyes are used as active medium inside the glass tube. The complete circulation of dye is done in the tube with the help of a pump. From this organic dye laser light will emerge out.
1.2.4 Semi conductor Lasers: - In these lasers junction diodes are used. The doping of p-n junction diode is done. Both the acceptors and donors are doped. These are known as ILD (Injection Laser Diodes). Whenever the current is passed then the light modulation from the ILD can be seen. This is used in various electronic equipments.
1.3 Advantages of Laser
Advantages of laser cutting over mechanical cutting include easier work holding and reduced contamination of work piece (since there is no cutting edge which can become contaminated by the material or contaminate the material). Precision may be better, since the laser beam does not wear during the process. There is also a reduced chance of warping the material that is being cut, as laser systems have a small heat-affected zone. Some materials are also very difficult or impossible to cut by more traditional means.
Laser cutting for metals has the advantages over plasma cutting of being more precise and using less energy when cutting sheet metal, however, most industrial lasers cannot cut through the greater metal thickness that plasma can. Newer lasers machines operating at higher power (6000 watts, as contrasted with early laser cutting machines' 1500 watt ratings) are approaching plasma machines in their ability to cut through thick materials, but the capital cost of such machines is much higher than that of plasma cutting machines capable of cutting thick materials like steel plate.
Limitations of Laser
The main disadvantage of laser cutting is the high power consumption. Industrial laser efficiency may range from 5% to 15%. The power consumption and efficiency of any particular laser will vary depending on output power and operating parameters. This will depend on type of laser and how well the laser is matched to the work at hand. The amount of laser cutting power required, known as heat input, for a particular job depends on the material type, thickness, process (reactive/inert) used, and desired cutting rate.
1.5 Laser Applications
These applications include such as;
i. Medical (Bleed less Surgery, Laser healing, Surgical treatment, Kidney stone treatment, Eye treatment, dentistry),
ii. Industrial (Cutting, Welding, Material heat treatment),
iii. Defense (Battle field, Anti-missile, Directed Energy Weapon (DEW), Electro Optic Counter Measures (EOCM)),
iv. Research tool (Spectroscopy, Laser ablation, Laser annealing, Laser scattering, Laser interferometers, LIDARS),
v. Product development / Commercial (Laser Printers, Compact disc, Barcode scanners, Laser pointers, Holograms).
Experts in the area would be approached to contribute and emphasis will be on the practical aspects suggesting the type of laser for a specific application, its specifications, potentials, alternatives, availability etc.