Chapter 14 LASER

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CHAPTER 14 : LASER

Objective

i) Explain the meaning of the term laser and the principle of its production.

ii) State the meaning of spontaneous emission, stimulated emission and populationinversion.

iii) State the main properties of laser and the advantages of laser.

14.1 Laser and principles of laser production

The word laser is an acronym for light amplification emission of radiation, so that stimulatedemission is the key to laser operation. Einstein introduced this concept in 1917. Although the

word had to wait until 1960 to see an operating laser, the groundwork for its development was put

in place decades earlier.

i) Spontaneous emission

In figure 14.1 a), the atom is in its excited state and no external radiation is present. After a time,

the atom will move of its own accord to its ground state, emitting a photon of energy, hfin the

process. We call this process spontaneous emission-spontaneous because the event was nottriggered by any outside influence. The light from the filament of an ordinary light-bulb is

generated in this way.

Normally, the mean life of excited atoms before spontaneous emission occurs is about10-8 s. However, for some excited states, this mean life is perhaps as much as 10 5 times longer.

We call such long-lived states metastable; they play an important role in laser operation.

ii) Stimulated emission

In figure 14.1 b), the atom is again in its excited state, but this time radiation with a frequency

given by equation 14.1 is present.

hf = E2 E1 (14.1)A photon of energy hfcan stimulate the atom to move to its ground state, during which process

the atom emits an additional photon, whose energy is also hf. We call this process stimulated

emission-stimulatedbecause the event is triggered by an external photon. The emitted photon isevery way identical to the stimulating photon.

Thus, the waves associated with the photons have the same energy, phase, polarizationand direction of travel. Figure 14.1 c) describes stimulated emission for a single atom. Suppose

now that a sample contents a large number of atoms in thermal equilibrium at temperature T.

Before any radiation is directed at the sample, a numberN0 of these atoms are in their groundstate with energy, E0, and a numberNx are in a state of higher energy Ex. Ludwig Boltzmann

showed thatNx is given in terms ofN0 by :

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( ) kTEEx

xeNN/

00

= (14.2)

in which kis Boltzmanns constant. This equation seems reasonable. The quantity kTis the meankinetic energy of an atom at temperature, T. The higher the temperature, the more atoms-on

average-will have been bumped up by thermal agitation (that is, by atom-atom coliisions) to the

higher energy state Ex. Also, because Ex > E0, equation 14.2 requires that Nx < N0; that is, therewill always be fewer atoms, in the excited state than in the ground state. This is what we expect if

the level populations N0 and Nx are determined only by the action of thermal agitation. Figure

14.2 a) illustrates this situation.

If we now flood the atoms of figure 14.2 a) with photons will disappear via absorption by

ground-state atoms, and photons will be generated largely via stimulated emission of excited-state

atoms. Einstein showed that the probabilities per atom for these two processes are identical. Thus,because there are more atoms in the ground state, the net effect will be the absorption of photons.

To produce laser light, we must have more photons emitted than absorbed; that is, we

must have a situation in which stimulated emission dominates. The direct way to bring this aboutis to start with more atoms in the excited state than in the ground state; as in figure 14.2 b).

However, since such a population inversion is not consistent with thermal equilibrium, we mustthink up clever ways to set up and maintain one.

Figure 14.1 : The interaction of radiation and matter in the processes of a) Absorption, b)

Spontaneous emission and c) Stimulated emission. An atom (matter) is represented by the reddor; the atom is in either a lower quantum state with energy E0 or a higher quantum state with

energy Ex. In a) the atom absorbs a photon of energy hffrom a passing light wave. In b) it emits a

light wave by emitting a photon of energy hf. In c) a passing light wave with photon energy hfcauses the atom to emit a photon of the same energy, increasing the energy of the light wave.

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Figure 14.2 : a) The equilibrium distribution of atoms between the ground sate E0 and excited

state Ex, accounted for by thermal agitation. b) An inverted population, obtained by specialmethods. Such an inverted population is essential for laser action.

14.2 Main properties of laser

i) Laser light is highly monochromatic : Light from an ordinary incandescent light-bulb

is spread over a continuous range of wavelengths and is certainly not monochromatic.The radiation form a fluorescent neon sign is monochromatic, to about 1 part in about

106. However, the sharpness of definition of laser light can be many times greater, as

much as 1 part in 1015.

ii) Laser light is highly coherent: Individual long waves (wave trains) for laser light can

be several hundred kilometers long. When two separated beams that have traveled

such distances over separate paths are recombined, they remember their commonorigin and are able to form a pattern of interference fringes. The corresponding

coherence length for wave trains emitted by a lightbulb is typically less than a meter.

iii) Laser light is highly directional : A laser beam spreads very little; it departs from

strict parallelism only because of diffraction at the exit aperture of the laser. For

example, a laser pulse used to measure the distance of to the Moons generates a spoton the Moons surface with a diameter of only a few meters. Light from an ordinary

bulb can be made into an approximately parallel beam by a lens, but the beam

divergence is much greater than for laser light. Each point on a lightbulbs filamentforms its own separate beam, and the angular divergence of the overall composite

beam is set by the size of the filament.

iv) Laser light can be sharply focused: If two light beams transport the same amount of

energy, the beam can be focused to the smaller spot will have the greater intensity atthat spot. For laser light, the focused spot can be so small that an intensity of 1017

W/cm2 is readily obtained. An oxyacetylene flame, by contrast, has an intensity ofonly about 103 W/cm2.

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Question

1. Explain the meaning of laser?

2. State the properties of laser?

3. Compare spontaneous emission to stimulated emission.

Objective

i) Give examples of laser system and its advantages.

ii) Describe the uses of laser.

14.3 Types of Lasers

There are many different types of lasers. The laser medium can be a solid, gas, liquid or

semiconductor. Lasers are commonly designated by the type of lasing material employed:

i) Solid-state lasers have lasing material distributed in a solid matrix (such as the ruby

or neodymium:yttrium-aluminum garnet "Yag" lasers). The neodymium-Yag laseremits infrared light at 1,064 nanometers (nm). A nanometer is 1 x 10 -9 meters.

ii) Gas lasers (helium and helium-neon, HeNe, are the most common gas lasers) have aprimary output of visible red light. CO2 lasers emit energy in the far-infrared, and are

used for cutting hard materials.

iii) Excimer lasers (the name is derived from the terms excitedand dimers) use reactivegases, 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.

iv) 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.

v) Semiconductor lasers, sometimes called diode lasers, are not 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 orCD players.

14.4 Examples of laser system and its advantages.

14.4.1 A Ruby Laser

A ruby laser (depicted on the previous page) is a solid-state laser and emits at a wavelength of

694 nm. Other lasing mediums can be selected based on the desired emission wavelength (see

table below), power needed, and pulse duration. Some lasers are very powerful, such as the CO 2laser, which can cut through steel. The reason that the CO2 laser is so dangerous is because it

emits laser light in the infrared and microwave region of the spectrum. Infrared radiation is heat,

and this laser basically melts through whatever it is focused upon.

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Other lasers, such as diode lasers, are very weak and are used in todays pocket laserpointers. These lasers typically emit a red beam of light that has a wavelength between 630 nm

and 680 nm. Lasers are utilized in industry and research to do many things, including using

intense laser light to excite other molecules to observe what happens to them.

Shiny lamp

Laser beam

Fullcovered mirror

High Half covered mirrorVoltage

Ruby rod

Figure 14.3 : Ruby laser system

14.4.2 The Helium-Neon Gas Laser

Figure 14.4 shows a type of laser commonly found in student laboratories. It was developed in1961 by Ali Javan and his coworkers. The glass discharge tube is filled with a 20 : 80 mixture of

helium and neon gases, neon being the medium in which laser action occurs. Figure 14.5 shows

simplified energy-level diagrams for the two atoms. An electric current passed through thehelium-neon gas mixture services-through collisions between helium atoms and electrons of the

current-to raise many helium atoms to stateE3, which is metastable.

The energy of helium state E3 (20.61 eV) is very close to the energy of neon state E2(20.66 eV). Thus, when a metastable (E3) helium atom and a ground-state (E0) neon atom collide,

the excitation energy of the helium atom is often transferred to the neon atom, which then moves

to stateE2. In this manner, neon levelE2 in figure 14.5 can become more heavily populated thanneon levelE1. This population inversion is relatively easy to set up because (1) initially there are

essentially no neon atoms in state E1, (2) the metastability of helium level E3 ensures a ready

supply of neon atoms in level E2, and (3) atoms in level E1 decay rapidly (through intermediatelevels not shown) to the neon ground state E0.

Suppose now that a single photon is spontaneously emitted as a neon atom transfers fromstate E2 to state E1. Such a photon can trigger a stimulated emission event, which, in turn, can

trigger other stimulated emission events. Through such a chain reaction, a coherent beam of red

laser light, moving parallel to the tube axis, can build up rapidly. This light, of wavelength 632.8

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nm, moves through the discharge tube many times by successive reflections from mirrors M 1 and

M2 (Figure 14.4), accumulating additional stimulated emission photons which each passage. M 1 istotally reflected but M2 is slightly leaky so that a small fraction of the laser light escapes to

form a useful external beam.

Figure 14.4 : The elements of a helium-neon gas laser. An applied potential Vdc sends electrons

through a discharge tube containing a mixture of helium gas and neon gas. Electrons collide withhelium atoms, which then collide with neon atoms, which emit light along the length of the tube.

The light passes through transparent windows W and reflects back and forth through the tube

from mirrors M1 and M2 to cause more neon atom emissions. Some of the light leaks throughmirror M2 to form the laser beam.

Figure 14.5 : Four essential energy levels for helium and neon atoms in a helium-neon gas laser.Laser actions occurs between levelsE2andE1 of neon when more atoms are at theE2 level than at

theE1 level.

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Here are some typical lasers and their emission wavelengths:

Laser Type Wavelength (nm)

Argon fluoride (UV) 193

Krypton fluoride (UV) 248

Nitrogen (UV) 337

Argon (blue) 488

Argon (green) 514

Helium neon (green) 543

Helium neon (red) 633

Rhodamine 6G dye (tunable) 570-650

Ruby (CrAlO3) (red) 694

Nd:Yag (NIR) 1064

Carbon dioxide (FIR) 10600

14.5 The uses of laser

Bar-codes

- The products in a supermarket are marked with a series of parallel black bars.- The laser scans across the bar-code and the change in intensity of the reflected light is

detected by a photo-detector to identify the product.

Laser communication

- Signals are generated by semiconductor lasers.The signals are transmitted by the OpticsFibers.

- More signals can be transmitted and also loss in signals is reduced compared with the

transmission with radio waves.

Medical laser

- Lasers are good for microsurgery because of its high power density at the pointabsorption. The tissue vaporizes rapidly before heat conducts to the surrounding healthy

tissues.

Uses :

- To remove benign tumors

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- Ablation of the cornea

Laser printers

- Fine toner powder will stick to the drum and then transfer to a paper by contact.

- The image is fixed by heating.

Laser used for information

- Laser is used to read compact disc (CD) and digital versatile disc (DVD)

- Laser printers are quieter, faster and sharper the inkjet printers.

- Laser light is used to form an electromagnet image of the picture on a light sensitive drum- Information is stored on track and can contain a series of pits.

- The laser is focused on the track. The reflected light is measured by a photodiode where

the pattern of low and high intensity of light is converted to a binary digital electronicsignal.

Question

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