Introduction to LASER and it's characteristics

Introduction:

The term LASER stands for Light Amplification by Stimulated Emission of Radiation.
The discovery of laser is one of the important discoveries of the last century. The first successful Laser operation was demonstrated by T.Maiman in 1990 using a ruby crystal in USA. In the following year, the first gas laser was fabricated by Ali Javan and co-workers. Since then, different types of lasers using solids, liquids and gasses have been developed. The immense use of laser, from toys to warfare and from welding to surgery have made laser very popular.

Characteristics of Laser Light:

Like Ordinary light Laser light is electromagnetic in nature. However, there are few characteristics not possessed by the normal light. Some of these characteristic of laser are motioned below:

1.Directionality:

The Laser beam is highly directional having no laser divergence(except the diffraction effect). The output beam of Laser has a well defined wavefront and therefore it is highly directional. Due to it's high directionality, a Laser beam can be focused to a point by passing it to a suitable convex lens. If a Laser beam of wavelength 6000 A and beam radius 2 mm is passed through a convex lens of focal length 5 cm, then the area of the spot at the focal plane is extremely small. For a typical Laser beam, the beam divergence is less than 0.01 milliradian, i.e., for a meter of propagation, the spread is less than 0.01 mm. the normal light spreads to about one kilometer for every kilometer of it's propagation. Just imagine how much a normal beam of light would diverge when it reaches moon at a distance of 3,80,000 km while a Laser beam spreads to just a few kilometers on reaching the moon.

2.Monochromatic-city:

The Laser light is nearly monochromatic. In reality, no light is perfectly monochromatic, i.e., it is not characterized by a single wavelength or frequency but instead; it is characterized by spread in frequency about the central frequency.

3.Coherence:

Laser Radiation is characterized by high degree of coherence, both spatial and temporal. In other words, a constant phase relationship exists in the radiation field of Laser light source at different locations and times. It is possible to observe interference effects from two independent Laser beams. In fact, coherence is main feature which distinguishes Laser radiation from ordinary light and other characteristic are related to high degree of coherence. Ordinary light consists of large number of separate waves without any phase relationship, cancelling and reinforcing each other randomly. The wavefront so produced changes from one point to another and from one instant to instant. Thus there is neither spatial nor temporal coherence in ordinary light.

4.Intensity:

The Laser beam is highly intense compared to ordinary light. Since, the Laser power is concentrated in a beam of very small diameter (few mm); even a small Lase can deliver high intensity at the focal plane of the lens.
Note that even a small power of 1 watt can give an intensity of 1000000000 W/m(square), which is extremely large.