There is a straightforward approach to understanding laser processing which is relevant for every situation. This approach considers the only processes that can occur when a laser beam strikes a surface. Both science and intuition give us the answer, all of the beam, be it a laser or any light beam, is either reflected or absorbed or transmitted. Expressed very simply: Reflection + Absorption + Transmission = 1

In almost all practical situations, some of all three processes occur. All laser processes can be considered in this way, and our understanding of many of the phenomena of laser processing can be helped by this approach. Furthermore, considering the relative amounts of each of these processes that occur in a particular process is also useful.

The next assumption that can be made is that again in almost all practical situations, some of this pure light beam - some of the photons, are absorbed by the target, and these photons are converted into thermal energy at the target, heat.

Next, we should consider what happens to this heat. Back to basics, it may be either conducted, radiated or convected away from the area. Intuition again, and mathematical models if you can use them, come into play and show that in most practical situations, convection and radiation play a very small part. So conduction is the dominating process. If the rate of heat input to the targetzone is greater than the rate of conduction of heat away from the target zone, then the temperature rises locally and either melting or vaporization or a combination of both occurs, although some scientists will of course point out the possibility of sublimation.

So, let us assume that a significant proportion of the beam is absorbed by the target, as this is usually the case in the majority of industrial laser processes. It is widely accepted that there are three dominating factors:

• average power of the laser beam
• intensity of the laser spot on the target
• wavelength of the laser beam.

Although these are all interrelated in ways that are beyond the scope of this introduction, to make their effects easier to understand they will be treated separately. Of these three, perhaps the average power in watts is the easiest to understand and the intensity and wavelength need some explanation.

Intensity

The laser industry has other more technically correct definitions for this term, but intensity (or irradiance), is usually calculated approximately by dividing the power of the beam by the spot area. Another widely used related term is ‘brightness'. Physicists again carefully define this but in general it is a measure of the ability of a laser source to be focussed to a small spot. Of course, some lasers are brighter than others are.

Wavelength

For non-specialists to understand the importance of the wavelength, it may be useful to refer back to Physics 101 and recall the electromagnetic spectrum. This contains a huge range of wavelengths stretching from short wavelength ultra violet waves to very much longer wavelength radar waves. We can then consider a laser beam as a very pure stream of such electromagnetic waves or particles (or photons of light) all going in the same direction. For a fixed wavelength, the laws of physics tell us that all the photons have the same energy. For a shorter wavelength beam, the energy of each photon is higher.

So what happens on an atomic scale when this stream of pure light or photons are absorbed by a surface? Usually, the energy is converted into vibration of the atoms which in effect heats up the surface, although in some cases this can be on the scale of a few thousandths of a millimeter (or microns). Under certain very special conditions these photons may, if their energy is high enough, break an atomic bond. The majority of laser processes are however, thermal in nature, they are the result of the generation of very well controlled melting and vaporization processes. Most importantly, the laser is controllable both spatially and temporally; that is to say we can control exactly where and when the heat is being put in to the target. Once applications engineers have established how to perform a particular process, the reliability and repeatability of the laser should ensure that the process does not change.

Average power

This initial reaction may be then sustained or not sustained according to the heat input or average power in watts (W) of the laser beam. The average power of a laser beam is very easy to understand and measure, especially for a continuous wave (CW) beam. For a pulsed beam, the product of the pulse energy and the pulse repetition rate gives us the average power.

There are many charts available which show the relationship between power or intensity and interaction time (the time for which any part of the surface is exposed to the laser spot), all of which are difficult to use or to relate to every day laser processing situations. There are also a number of intensity thresholds that are discussed in the scientific literature. For particular ranges of intensity and for certain target materials, it has been shown that very non linear behavior may occur, producing step changes in processing performance. Perhaps the best known example of this is ‘keyhole' welding of metals using high power CO2 lasers which relies on producing a vapor filled keyhole to produce high efficiency deep welds on metals. Another important complicating factor is that the physical properties of many materials such as the thermal conductivity changes as the materials absorb energy and heat up.

Added to what is already developing into a rather complex picture is the fact that lasers which produce these different wavelengths not only look physically very different, but also produce beams which are also very different optically. Although there are many formulae that allow us to quantify the behavior of laser beams, these require a knowledge of optics that is not assumed here.

There are two other generalizations which may also help: we may visualize the wavelength of the laser beam and the intensity of the laser spot on the target as controlling the initial reaction of the target to the beam and the average power as determining the rate of the process. Also, it is safe to assume that the shorter the wavelength, the more expensive each photon at the workpiece! If very small amounts of material are concerned, then this is not always an issue. Small parts are not necessarily cheap parts, especially in the field of microelectronics or microsurgery. If the requirement is to cut ½" steel, then the cheaper the photons the better.

Now that we have explored this mechanistic approach to laser processing, and have introduced some of the more common terms used in laser processing, the next question is which materials can be processed by lasers? The answer is of course, all materials! Think of a material and the chances are that at some stage in the short history of laser usage, somebody has tried to process it with a laser. Metals, diamond, tungsten, cloth, plastics, human tissue – the list is endless.  Our laser finder on the right can help you find what materials can be processed per application.