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Chapter 1

Laser Basics

Here are some basic characteristics for laser light.

• Lasers produce a very narrow, intense beam of light. Light from a light bulb
spreads out as it travels, so much less light hits a given area as the distance
from the light source increases (the inverse square law). Laser light travels as
a parallel beam spreading very little, so the inverse square law does not apply.

• Laser light is monochromatic and coherent. White light is a jumble of
colored light waves, each color is a different wavelength. If all the wavelengths
or colors except one were filtered out, the remaining light would be monochromatic.
White light propagates in all directions and is a jumble of phases. If
light waves are all parallel and in phase with one another they are said to be
coherent (i.e. the wave crests and troughs coincide). Therefore these waves
reinforce one another.

• Laser beams can be continuous (CW) or pulsed. Pulsed lasers are switched
on and off, sometimes so rapidly that they may appear to be continuously
emitting light.

• Not all lasers emit visible light. Some lasers produce infrared or ultraviolet
light. This light is still capable of producing injuries.
Most laser systems are made of three basic components:

1. A pumping system or energy source: this can be a flash lamp, microwaves,
chemical reaction, another laser, etc.

2. The lasing medium may be a gas, liquid, solid, semiconductor, electron beam, etc.

3. A resonant cavity, which amplifies the intensity of the light.
Lenses, mirrors, absorbers, shutters, and other accessories may be added to the system to obtain more power, shorter pulses, or special beam shapes, but only these three basic components are necessary for laser action.

Lasers use a process called stimulated emission to amplify light waves. Many
substances give off light by spontaneous emission. Consider what occurs when one of the electrons of an atom absorbs energy. While it possesses this energy, the atom is in an excited electronic state. If the orbital electron gives off this energy in the form of electromagnetic radiation, such as light, with no outside impetus,spontaneous emission has occurred. If a wave emitted by one excited atom strikesanother atom, it may stimulate the second atom to emit energy in the form of asecond wave that travels parallel to and in step (or phase) with the first wave.

The stimulated emission results in the amplification of the first wave. If the two waves strike other excited atoms, a very intense coherent beam can be built up. But if these waves strike unexcited atoms, the energy is absorbed and some intensity is lost. In the normal state of matter on the earth, the great majority of atoms are not excited. As more than the usual number of atoms become excited, the probability increases that stimulated emission, rather than absorption, will take place.

Ruby Lasers

The ruby laser was the first laser invented in 1960. Ruby is an aluminum oxide
crystal in which some of the aluminum atoms have been replaced with chromium atoms. Chromium gives ruby its characteristic red color and is responsible for the lasing behavior of the crystal. Chromium emits green and blue light and reflects only red light.

For a ruby laser, a crystal of ruby is formed into a cylinder. A fully reflecting
mirror is placed on one end and a partially reflecting mirror on the other. A highintensity lamp is spiraled around the ruby cylinder to provide a flash of white light that triggers the laser action. The green and blue wavelengths in the flash excite electrons in the chromium atoms to a higher energy level. Upon returning to their normal state, the electrons emit their characteristic ruby-red light. The mirrors reflect some of this light back and forth inside the ruby crystal, stimulating other excited chromium atoms to produce more red light, until the light pulse builds up to high power and drains the energy stored in the crystal.

The laser flash that escapes through the partially reflecting mirror lasts for only
about 300 millionths of a second (roughly the duration of the flash lamp’s flash), but it is very intense. Early lasers could produce peak powers of some ten thousand watts. Modern lasers can produce pulses that are billions of times more powerful.

Ruby Laser
Figure 1.1: Ruby Laser


Helium-Neon Lasers

One of the most common lasers used is the helium-neon laser. Let us review this
system, comparing and contrasting the way that it functions with the more simple
ruby laser we just described. At the heart of the helium-neon laser system is an
optical cavity comprised of a tube which is sealed with mirrors at each end. One
mirror is 100% reflective while the other is greater than 95% reflective but not
quite 100% reflective. A gas discharge in the tube is created by a brief 6 to 15 kV
trigger and maintained with 2 to 6 kV DC, at 4 to 10 milliamps, applied across the
electrodes.

Electrons strike the helium atoms and excite some of them to metastable
states from which their subsequent decay is restricted to processes which do not
produce radiation. Neon possesses several energy levels which lie just below helium’s
decay-restricted states. An excited helium atom which passes very near a neon atom
may transfer its energy, through a form of resonant coupling, to the neon. This
process allows the helium to decay to the ground state where it may, once again,
be excited by the electric field. Meanwhile, the excited neon atoms may loose their
energy in several ways; one such pathway is the spontaneous emission of visible light
at 632.8 nm (orange).

Laser activity becomes possible when a population inversion exists (i.e. when the number of neon atoms capable of 632.8 nm emission exceeds the number of atoms which are in the relaxed state). The helium metastable atoms produce neon’s population inversion. Some of the 632.8 nm radiation will induce other excited neon atoms to emit light, a process called stimulated emission, and that light is coherent with the stimulating light. Energy losses may occur as a result of spontaneous emission, diffraction, scattering, and collisional relaxation (resulting in heat). The mirrors create a optical path along the entire length of the glass tube, which is needed for sufficient amplification, by stimulated emission of radiation, to occur.

If this amplification exceeds energy losses (including the light exiting the laser from the partially reflecting mirror) then energy density at the desired frequency will rise exponentially and the laser quickly enters into oscillation. In this condition the population inversion decreases and so does amplification. When amplification balances energy losses then a stable operating environment is achieved.


Other Types of Lasers

Helium-neon lasers the most common gas lasers. They have a primary output of
visible red light; although other colors - most often green - are also available. CO2
lasers emit energy in the far-infrared region, and are used for cutting hard materials.
Nitrogen lasers emit ultraviolet light.

Excimer lasers (the name is derived from the terms excited and dimers) use
reactive gases, such as chlorine and fluorine, mixed with inert gases such as argon,
krypton or xenon. When electrically stimulated, a pseudo molecule (dimer) is
produced. When lased, the dimer produces light in the ultraviolet range.

Dye lasers use complex organic dyes, such as rhodamine 6G, in liquid solution
or suspension as lasing media. They are tunable over a broad range of wavelengths.

Semiconductor lasers, sometimes called diode lasers, are solid-state lasers. These
electronic devices are generally very small and use low power. They may be built
into larger arrays, such as the writing source in some laser printers or CD players.

Nd:YAG lasers (neodynium doped yittrium aluminum garnet lasing medium)
are solid state lasers taht emit in the infrared region. They are very powerful and
the output can be frequently doubled, tripled, and quadrupled to produce green and
ultraviolet laser beams.

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