WHAT IS LASER?

LASER , the name is very common nowadays. Past scientific friction become today's reality because of the invention of Laser. Laser , the ultimate ruler, applications exist throughout our society, and new uses are discovered almost daily.

laser?

The word LASER is an acronym whose letters stand for " Light Amplification by Stimulated Emission of Radiation." Einstein's entirely theoretical prediction was made 43 years before the first laser was made. At that time when electronics was unheard of, transistors didn't exist and vacuum valves were still a novelty. Einstein predicted in 1917 that, under certain circumstances, an incident photon will generate another one , of exactly the same energy and hence the same frequency. Einstein added that in this type of emission both photons, old and new, will be in phase, will have the same polarisation and will propagate in the same direction.

The phenomenon of stimulated emission was first used by Townes in 1954 in the construction of a micro wave amplification device called the MASER which is an acronym for Microwave Amplification by Stimulated Emission of Radiation. At about the same time a similar device was also proposed by Prochorov and Basov. The maser principle was later extended to the optical frequencies by Schawlow and Townes in 1958, which led to the realisation of the device now known as the Laser. The first successful operation of a laser device was demonstrated by Maiman in 1960 using ruby crystal.

Why the laser is such a SPECIAL light source !" :Light Amplification by Stimulated Emission of Radiation."

 

These words describe a process that generates an intense beam of light.

The light is very pure -- that is, all the light rays in the beam are nearly the same colour.

the light is extremely well collimated -- that is , all the rays are headed in almost exactly the same direction.

The laser is a very special light source because the light it produces is coherent. COHERENCE concerns more than the frequency of the radiation, not only is laser light monochromatic but the PHASE of all its constituent photons is also the same. They work in unison, which makes them so effective These characteristics make the light in a laser beam very special.

Thus lasers are often termed as monochromatic and Coherent sources of light.

Applications in manufacturing:

Laser machining: CO2 laser beam cutting a 5 mm thick stainless steel sheet. The beam itself is, of course, invisible. Laser tools can cut a variety of materials, from high carbon steel through titanium alloys, ceramics and reinforced rubber to wood, cloth and cardboard. Other advantages are : low noise, dust, fume and vibration levels, the possibility of operating through a glass shield, no fraying (fabrics), the ease of starting a cut in the middle of a work piece, and the elimination of the need for a wide range of cutting tools.

 

Figure 15. : Schematic diagram for beam focusing head design for laser welding when using a shielding gas

 

When it comes to industrial hole drilling, the laser proves of value mostly in working materials at both extremes of the hardness scale: diamonds and rubies on the one hand and polythene and rubber on the other. Making holes in general metalwork by laser, however, is not an economic proposition (yet), as the removal of large quantities of material involves the latent heat not only of melting, but also of evaporating the material. This can be very great indeed, especially for large holes. However, when small holes have to be made either in inaccessible places or at very accurate angles and depth, such as in a vane for a jet engine, laser drilling can be invaluable. Laser welding (Fig.15) has already found its way to the motor car industry. In conclusion, the use of laser beams in manufacturing, although not massive, is highly diverse. Wherever applied it leads not only to higher yields but also to superior product quality.

Physics of Laser Cutting

Laser Machining Processes

Several major laser machining processes are illustrated below.

This diagram illustrates the cutting process in cross section. Note that a gas assist can be used to speed cutting of metals.

Drilling Rates

A simple method of analysis is proposed in (Wilson, 1987).

The energy to vaporize a mass m of solid material at intial temperature T is

Ev = m(CS(Tm-T)+CL(Tv-Tm)+Lf+Lv)	CS = solid specific heat capacity
	CL = liquid specific heat capacity
	Tm = melting point
	Tv = boiling point
	Lf = latent heat of fusion
	Lv = latent heat of vaporization

Usually, L_f << L_v and T << T_v , and C_s � C_L = C. We then have the simple form E_v = m(CT_v+L_v). Now, consider a circular laser beam of area A boring into the surface of such a material with a velocity v_s directed into the material. It must remove a section of mass v_sr A per unit time. Ignoring reflectance from the material surface, the heat flow is equal to the beam power P. Assuming a beam with diameter d and equal power over A, we have P = v_s(p d²/4)(CT_v+L_v).

If v_s exceeds the normal rate of heat diffusion into the material, this equation is fairly accurate for estimating drilling rates or hole depths. To find hole depths, solve for the quantity v_st, where t is the duration of the beam pulse. For example, consider a 100-msec pulse from a 10W laser with a beam diameter of 1mm. If this were to strike a Perspex (methyl methacrylate) sheet, the resultant hole would have a depth of 1.6mm.

Note the inverse-square dependence of hole depth on beam diameter. Halving the beam diameter results in a hole four times deeper. This highlights the importance of beam focusing in laser machine design.

Cutting Rates

This model can be used to estimate cutting rates as well. Consider the laser scanning over the surface of the material with velocity v_b. As it scans, it cuts through the material to a depth z = v_sd/v_b. We now have

P = (p /4)zv_br d(CT_v+L_v)

The following table can then be used to approximate the laser power necessary to cut a given material.

(Chryssolouris, 1991) describes a model that accounts for the material absorptivity as well.

	s = cutting depth
	a = material absorptivity
	P = laser beam power
	p = material density
	v = scanning velocity
	d = beam spot diameter
	cp = specific heat
	Tv = temperature at surface (melting temp.)
	T = temperature of ambient
	L = latent heat of fusion

Note the following:

• The cutting depth is proportional to P/vd, which is the energy input per area of workpiece.
• Cutting depth is small for materials with a high melting point and a high latent heat of evaporization.