LASERS

1 INTRODUCTION

Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. In 1917, based on thermodynamic equilibrium between atoms and radiation, Albert Einstein predicted that there are two kinds of light emission from matter, namely spontaneous and stimulated emissions.

He further proved that both spontaneous emission and stimulated emission are necessary to derive Plan ck’s Quantum theory of radiation, which is the basis for theoretical prediction of Laser. In 1960, Charles Towner demonstrated experimentally stimulated emission for first time at Microwave frequencies as MASER and received Noble prize in 1964. In the same year, Theodore Maiman demonstrated stimulated emission based LASER in optical frequencies using Ruby rod as lasing medium, and Ali Javan and his co-workers constructed laser device using He-Ne gas as lasing medium. In 1962, lasing action using semiconductor medium was invented. Since then a variety of materials were used to demonstrate lasing action using liquids, ionized gases, dyes etc.

2 Characteristics of Laser

Some of the unique characteristics of lasers which are different from ordinary incoherent light are:

(1) Directionality

(2) High intensity

(3) Monochromacity and

(4) High degree of coherence

3 Directionality

Any conventional light source like incandescent light emits radiations in all direction whereas a laser source emits radiation only in one direction. The directionality of the laser beam is generally expressed in terms of full angle beam divergence which is twice the angle that the outer edge of the beam makes with the axis of the beam. The outer edge is defined as a point at which the intensity (I) of the beam drops to 1/e times its value at the centre.

4 Intensity

A laser emits light radiation into a narrow beam, and its energy is concentrated in a small region. This concentration of energy both spatially and spectrally accounts for the great intensity of lasers. It can be shown that even a one-watt laser would appear many thousand times more intense than a 100 watt ordinary lamp. If we compare the number of photons emitted in one second from a square centimetre of a surface of a laser source with those from an ordinary source, the ratio is of the order of l028 to 1012.

Monochromacity

The light from a laser source is highly monochromatic compared to light from a conventional incoherent monochromatic source. The monochromacity is related to the wavelength spread of radiation.

Coherence

Laser radiation is characterized by a high degree of ordering of the light field compared to radiation from other sources. In other words, laser light has a high degree of coherence, both spatial and temporal.

Spatial coherence, also called transverse coherence, describes how far apart two sources or two portions of the same source can be located in a direction transverse to the direction of observation and still exhibit coherent properties over a range of observation points. The high degree of coherence of laser radiation makes it possible to realise a tremendous spatial concentration of light power such as 1013 watt in a space with linear dimensions of only 1 m.

The temporal coherence on the other hand, normally refers to the relative phase or the coherence of two waves at two separate locations along the propagation direction of the two beams. It is sometimes referred to as longitudinal coherence. If we assume that two waves are exactly in phase at the first location, then they will maintain the same phase at the second location up to a distance lC where lC is defined as the coherence length. For white light the coherence length is of the order of hundred nm while for monochromatic incoherent light its value is of hundred microns. For lasers the value of coherence length is of the order of several metres.

Types of Lasers

Lasers are divided into different types based on the lasing materials used. Accordingly the important types of lasers are Solid state lasers, gas lasers, and semiconductor lasers. Most of the lasers emit light in IR or visible region, work in Continuous wave (CW) mode or in pulsed mode. Table 5.1 gives some important types of Lasers with examples.

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