**3.3 Low-diffraction or collimation**

Directionality is a property by which a light beam bends after passing sharp corners of objects. Diffraction or scattering of light at sharp edges increase the distance

**Figure 5.**

*Practical Applications of Laser Ablation*

**3. Properties of laser**

**3.1 Coherence**

analyzing the laser properties.

longitudinal and transverse mode.

**3.2 Monochromatic**

factor for machining.

photons usually generate waste heat and finally lost.

The working of laser is schematically represented in **Figure 3**. A lasing medium contained by a cylindrical glass container is closed using completely (100%) reflecting mirror on one end and partially reflecting mirror on the other end. When the glass vessel is exposed to a light using flash lamps, the photons of light excites the atoms of lasing medium thus population inversion is obtained. Further due to stimulated emission, photons are emitted. These stimulated photons in the longitudinal direction form a high intense, coherent and highly directional laser beam. Most of the stimulated photons would not be in the longitudinal direction and these

The distinctive properties of laser are coherence, highly monochromatic, intensive radiance and directionality. These optical properties can be quantified for

The relationship between magnetic and electronic components of electromagnetic wave refers to coherence property. The light beam is said to be coherent when these components are properly aligned as shown in **Figure 4**. There are two terms of coherence for a laser as spatial coherence and temporal coherence. Coherence is said to be spatial when the correlation of phases happens at different points in a space at a single point of time whereas in temporal coherence, correlation happens at single point in a space over a time period. **Figure 5** shows the concept of coherence. Temporal coherence can be quantified through two important measures such as coherence length and time. This property can be improved by run the laser in single

It is the most important property of laser and it can be measured by spectral line width. When the range of emitted frequencies is small by a light source, it is said to be high monochromatic. Laser beam normally have very few or single spectral lines with highly narrow widths as shown in **Figure 6**. Monochromaticity is most important because wide range of applications depends on this property such as interferometry, velocimetry, holography, separation of isotope and communications which require laser beam content. But this property is a not decisive

**90**

**Figure 4.**

*Components of electromagnetic wave.*

**Figure 7.** *Comparison of radiation from normal light bulb and a laser beam.* from light source therefore certain amount of energy is lost. But laser beams possess very low-diffraction property hence higher energy transfer can be effectively achieved. This directional characteristic is useful when directing the laser beam for machining applications.

## **3.4 Intensive radiance**

The intensive radiance of a light is defined as the amount of power emitted per unit area for a given solid angle. The unit for radiance is watts per square meter per steradian. The angle by which a light beam is focused as a cone is called a solid angle. Since the intensity of photons is high in laser beam, it can have high output powers. Laser light source possess extreme amount of intensive radiance and transmitted through a small solid edge angle. This property makes it very convenient to be used for machining operations. **Figure 7** gives the comparison of power density transmitted by normal light source and a laser [3].
