Physics

               

Key unit Competence 

By the end of this unit I should be able to analyze the application of LASER. 

My goals 

 • Define a laser beam

 • Explain the stimulated emission of light

 • Explain the spontaneous emission of light

 • Analyse the mechanism to produce LASER beam

 • Explain laser properties

 • Explain and describe monochromatic and coherent sources of light 

 • Analyse a LASER light as a source of coherent light.

  • Explain the principle and uses of Laser.

  • Outline applications of LASER

  • Analyse applications and dangers of LASER beam

  • Analyse precautional measures of the negative effects of Laser.

Introductory activity 

A man has tied all forms of advancements from traditional methods of solving problems to advanced methods by use of different technologies. Among other technological advancements, discovery of Laser that is a part of visible light under electromagnetic waves has had a great impact in solving many of our problems.

  a. What do you understand by Electromagnetic waves?

  b. Discuss at least four (4) characteristics of Electromagnetic waves

  c. In your own words, discuss how these electromagnetic waves are produced.

  d. Are all kinds of these electromagnetic waves have the same energy? If Yes why? If No, why not?

  e. Basing on what you know about these electromagnetic waves, what could be positive uses of these waves. Also discuss negative effects of electromagnetic waves.

  f. How are electromagnetic waves related to LASERS?

11.1 CONCEPT OF LASER 

Activity 11.1

    a. From your own understanding, explain how a LASER light is produced.

    b. Does production, need source of energy like electricity. Explain your reasoning.

    c. In energy levels, particles are either in ground or excited states. Is laser formed when particles or                 electrons are in ground or excited states? 

        Explain your reasoning.

The acronym LASER stands for Light Amplifier by Stimulated Emission of Radiation. This expression means that the light is formed by stimulating a material’s electrons to give out the laser light or radiation.

The laser is a device that uses the ability of some substances to absorb electromagnetic energy and re-radiate it, as a highly focused beam of monochromatic and synchronized wavelength radiation. In 1953 Charles H. Townes, with graduate students James P. and Herbert J., produced the first Microwave Amplifier by Stimulated Emission of Radiation (MASER), as a device operating in the same way as a laser, but amplifying microwave radiations. 

This system could release stimulated emissions without falling to the ground state, and thus maintaining a population inversion. A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. That is, the laser is a light source that produces a beam of highly coherent and very nearly monochromatic light because of cooperative emission from many atoms.

11.1.1 Absorption, Spontaneous emission and Stimulated emission 

Activity 11.2 1. Using scientific explanations, 

Explain the meaning of the following terms

  I. Absorption

  II. Stimulated emission

 III. Spontaneous emission

2. Electrons can jump from excited to ground state; does it absorb or radiate energy. 

    Explain your reasoning.

3. Write an equation that would be used to calculate the energy radiated by an electron when it jumps from     one energy level to another. 

    Explain each term used in the equation.

4. What do you understand by the term population inversion?

a.  Absorption 

During the process of absorption, a photon from the source is destroyed and the atom which was at the ground state is promoted to the excited state. 

                                    

If n_o is the  number of atoms per unit volume in the ground state with energy Eg, then the number of atoms per unit volume in the excited state of energy E_ex at temperature T, will depend on both the thermal energy and the difference between the energy levels and is given by

b. Spontaneous emission. 

An atom or an electron can move from one energy level to another. A photon is released when an electron moves from a higher energy level to a lower energy level. The release of photon (a particle of light) is called spontaneous emission. 

At the excited state, an atom will drop to a lower level by emitting a photon of radiation in a process called spontaneous emission. It emits the photon spontaneously after an average time τ called the spontaneous lifetime of the level. This time depends on the atomic species; some levels have long lifetime measured in seconds, whereas others are relatively short on the order of nanoseconds or less. This lifetime determines the ability of the emitting atom to store energy and will affect the efficiency of sources.

C. Stimulated emission 

Stimulated emission occurs when a photon strikes an atom that is excited state and makes the atom emit another photon
In stimulated emission (Fig. 11.3), each incident photon encounters a previously excited atom. A kind of resonance effect induces each atom to emit a second photon with the same frequency, direction, phase, and polarization as the incident photon, which is not changed by the process. For each atom there is one photon before a stimulated emission and two photons after—thus the name lightamplification. Because the two photons have the same phase, they emerge together as coherent radiation.

In this case the energy difference between the two levels is emitted in the form of electromagnetic wave that adds to the incident one. This is the phenomenon of stimulated emission. 

There is a fundamental difference between the spontaneous and stimulated emission processes because in spontaneous emission one photon is emitted and in stimulated emission both incident and emitted photons are observed.

11.1.2 Laser principle 

The principle of operation remains the same though there is a wide range of lasers. Laser action occurs in three stages: photon absorption, spontaneous emission, and stimulated emission. The particle of the material, which undergoes the process of excitation, might be an atom, molecule, or ion depending on the laser material. This principle is based on the principle of stimulated emission of radiation, the theory that was discussed by Einstein in 1917.The whole concept was discussed in the previous section.

The photon emitted during stimulated emission has the same energy as the incident photon and it is emitted in the same direction as the latter, thus, getting two coherent photons. If these two coherent photons are incident on other two atoms in E2, then it will result in emission of two more photons and hence four coherent photons of the same energy are emitted. The process continues leading to doubling of the present number of photons.

      

If the process is made to go on chain, we ultimately can increase the intensity of coherent radiation enormously. In figure above, such amplification of the number of the coherent photons due to stimulated emission is shown.

The necessary condition for this type of amplification of light intensity by stimulated emission of radiation is that number of atoms in the upper energy state E₂ must be sufficiently increased.

11.1.3 Population inversion 

Population inversion: This is the process of increasing excited electrons in higher energy levels. 

This is the redistribution of atomic energy levels that takes place in a system so that action can occur.

                      

There are different methods of achieving population inversion in atomic states that is essential requirement to produce laser beam. Normally, most of the atoms in a medium are in the ground state of energy E₀. There are four different methods of making these atoms to excited states. 

    I. Excitation with the help of photons. If the atoms are exposed to an electromagnetic radiation of high frequency, then there is selective absorption of energy and thus atoms are raised to excited state. 

   II. Excitation by electrons. This method is used in some gas lasers. Electrons are released from the atoms due to high voltage electric discharge through a gas. These electrons are then accelerated to high velocities due to high electric field inside a discharge tube. When they collide with neutral gas atoms, a fraction of these atoms are raised to excited state

                      

   III. Inelastic collision between atoms. If a gas contains two different two different kinds of atoms X and Y, then during electric discharge through the gas some of the atoms are raised to excited state.

  IV. Excitation by chemical energy. Sometimes, an atom or a molecule can be a product of a chemical reaction and can be produced in its excited state. An example is hydrogen combining with fluorine to form hydrogen fluoride HF that is in excited state.

11.1.4 Laser structure 

Activity 11.3 

From what you know about LASER, what could be the components of laser  Are all parts on laser Light Similar? Explain your reasoning.
         

                      Fig.11. 6: General structure of Laser 

In general case laser system consists of three important parts: Active medium or amplifying medium, the energy source referred to as the pump or pump source and the optical resonator consisting of mirrors or system of mirrors.

Pumping Mechanism. 

Pumping is the process of supplying energy to the laser medium to excite to the upper energy levels. To have this mechanism, it depends on the existence of interactions between light from pump source to constituents of active medium. Usually, pump sources can be: electrical discharges, flash lamps, arc lamps, light from another laser, chemical reactions and even explosive devices. Most common lasers use electrical or optical pumping. The type of pump source used depends essentially on the gain medium.

Active Medium The active medium is the major determining factor of the wavelength of operation, and other properties of the laser. The gain medium is excited by the pump source to produce a population inversion, and it is where the spontaneous and stimulated emission of photons take place, leading to the phenomenon of optical gain or amplification. The gain medium may be a solid crystal like a ruby, a liquid dye, gases like CO₂ or He-Ne or semiconductors. The gain medium for some lasers like gas lasers is closed by a window under the Brewster’s angle to allow the beam to leave the laser tube. 

Optical resonator or Optical cavity 

The optical resonator or optical cavity is a system of two parallel mirrors placed around the gain medium that provide reflection of the light beam. Light from the medium produced by the spontaneous emission is reflected by the mirrors back into the medium where it may be amplified by the stimulated emission. Mirrors are required for most lasers to increase the circulating power within the cavity to the point where gains exceed losses, and to increase the rate of stimulated emission. One of the mirrors reflects essentially 100% of the light, while the other less than 100% and transmits the remainder. Mirrors can be plane, spherical or a combination of both. Here represented are the common cavities configuration that can be used:
                     

11.1.5 Checking my progress

     1. What do you understand by the term LASER? 

     2. Write in full the acronym L.A.S.E.R 

     3. In your own words, explain how laser light is produced.

     4. Explain the meaning of population inversion and discuss how an atom can be put into excited state.

     5. What is the energy of the laser light that propagates with a frequency of 10¹⁰ Hz in gaseous medium.          (Given that the plank’s constant  h=6.67x10^-34J.s ).     

     6. a. What are the three major components of laser?              

         b. Using diagrams, explain all the types of optical cavity.

11.2 PROPERTIES OF LASER LIGHT

Activity 11.4 

       a. Using the ideas about electromagnetic radiations, what are characteristics of laser light?

       b. Do you think all different kinds of laser light have the same properties? 

           Give reasons to support your answer.

The laser light is not like any other light emitted by usual sources found in nature.  This special light emitted by the laser, has three properties according to its usefulness in many applications: Coherence, Monochromaticity and Collimation or Directionality.

                           

11.2.1 Coherence 

Coherence is the most interesting property of laser light. All photons emitted, are exactly in the same phase, they are all crest and valley at the same time. It is brought about by the mechanism of the laser itself in which photons are essentially copied. The good temporal coherence is essentially for Interferometry like in Holography. Coherence is not trivial and is brought about by the amplification mechanism of the laser.

11.2.2 Monochromaticity 

Monochromaticity is the ability of the laser to produce light that is at one wavelength λ. It is a requirement for coherence since photons of different wavelengths cannot be coherent. When white light is dispersed through a prism, you note that it is composed of an infinite number of wavelengths of light covering the entire visible
spectrum as well as into the UV and IR regions. However, no light source is perfectly monochromatic. Lasers tend to be relatively monochromatic and this depends on the type of laser. Monochromatic output, or high frequency stability, is of great importance for lasers being used in Interferometry. 

11.2.3 Collimation or Directionality 

Collimation or directionality is the property of laser light that allows it to stay in one direction at the strait line, confined beam for large distances. This property makes it possible to use the laser as a level in construction or to pinpoint speeders on a highway. This highly directional laser light is determined by the mechanism of the laser itself.

         

11.2.4 Checking my progress

     1. Choose the correct group of terms that are properties of laser light.  

         a. Coherent, unpolarized, monochromatic, high divergence

         b. Monochromatic, low divergence, polarized, coherent

         c. Polychromatic, diffuse, coherent, focused 

         d. Monochromatic, birefringent, nonpolarized, coherent

     2. Which of the following properties of laser light enables us to use it to measure distances with great              precision?

         a. All the light waves emitted by the laser have the same direction

         b. The light waves are coherent

         c. The light waves are monochromatic

         d. The individual waves effectively work like a single wave with very large amplitude.

     3. Explain how coherence, monochromatic and collimation are interconnected.

     4. All light in laser light are produced and found to be in the same phase. How does this help in the                  formation of 3D images?

     5. Laser light can be used as a level. Which special feature that makes it be used

11.3 APPLICATIONS AND DANGERS OF MISUSE OF LASER 

11.3.1 Applications of lasers.

Activity 11.5

   

     a. Having studied LASERS, where do you think in real life LASERS are helpful? 

     b. From your experience, have you ever used LASER light?

     c. Other than using it by yourself, what are other places where laser light is applied

There are many interesting uses for lasers, depending on the special characteristic being applied. Laser Diodes are used in a wide range of applications. Partial lists of those applications include: 

      I. They are used in common consumer devices such as DVD players, bar code scanners; CD ROM drivers; laser disc and other optical storage drivers; laser printers and laser fax machines; sighting and alignment scopes; measurement equipment; free space communication systems; pump source for other lasers; high performance imagers; and typesetters. CD players have lasers. Light from the laser in CD player reflects off patterns on CD’s surface. The reflected light is converted to a sound wave.

      II. Laser beams can be used in diverse fields of science and technology. Like in the control of motion of moving objects like aircrafts or missiles. This method thus makes it possible for a missile to hit a certain target.

      III. Because of high directional property, lasers are used to measure distances accurately. A laser beam is sent and the time taken for it to be reflected back is measured. Using this idea, the distance can thus be measured.

      IV. Because laser beam can be focused into a small spot, it can thus be used to cut minute holes onto a material.

       V. The very high intensity of laser beam means that the amplitude of the corresponding electromagnetic wave is very large. So it is possible to investigate the non linear optical properties of different materials with the help of laser light.

      VI. Lasers are also used in industry for cutting materials such as metal and cloths. and welding materials 

      VII.  Doctors use lasers for surgery and various skin treatments 

      VIII. They are used in military and law enforcement devices for marking targets and measuring range and speed.

        IX.   Laser lighting displays use laser light as an entertainment medium.

         X.   Lasers also have many important applications in scientific research .

In a tabular way, we can have a summary of different types of lasers and their applications. 

The following are types of lasers and their Applications 

11.3.2 Dangers of lasers 

Activity 11.6 

   a. Laser light is used in many areas like industries, offices, airports and many other places. Do you think long exposure of laser light is harmful?

   b. Why do you think so?

   c. What makes these lasers harmful if mis-used? 

       Give a scientific reasoning

You should be careful when dealing with lasers, because they can have a negative impact when exposed to your body. Among other negative effects, some of them are discussed below .

       I. If directly exposed to our skin, it burns the skin

      II. When absorbed by skin, Laser light reacts with body cells causing cancer.

     III. Because of their high energy, it affects eyes if exposed to them

     IV. Lasers can affect cells of a human being. This leads to mutation

Because of the negative effects of lasers, care must be taken to avoid all the risks of being affected by lasers.

11.3.3 Precaution measures to avoid negative effects of lasers

Activity 11.7

           

       a. Observe the picture above clearly. Using scientific reasoning explain why the people performing the activity above are putting on protective wear as shown.

       b. Building on what you have discussed in a) above, what precautional measures can you take to avoid negative effects of LASERS if at all you were working in a place exposed to them.

The following are some of the measures one can take to avoid the negative effects of lasers.

      I. For any one working in places where there are incidences of being exposed to laser light, one should wear protective clothes, glasses and shoes so that there is no direct exposure of these radiations on to the body. 

      II. One should minimize the time of working with lasers.

     III. Areas that are exposed to these radiations should be warning signs and labels so that one can be aware of places/areas where laser light is used.

     IV. Safe measures like Use of remote control should be used to avoid direct exposure of these radiations (LASER light).

     V. People should be given trainings on how to handle lasers.

     VI. There should also access restrictions to laboratories that use laser

11.3.4 Checking my progress

       1. Discuss all the negative effects of laser light.

       2. Using vivid examples, explain how one can prevent him or herself of all dangers caused by laser light. 

       3. We have seen that laser light is good and at the same time bad. Using your personal judgement, which side outweighs the other. Give scientific reasons.

      4. Depending on your judgement in (3) do you think man should continue using laser light?

11.4 END UNIT ASSESSMENT

11.4.1 Multiple choice 

Copy the questions below to your exercise and chose the best alternative that answers the question.
      1. What does the acronym LASER stand for? 

          a. Light Absorption by Stimulated Emission of Radiation

          b. Light Amplification by Stimulated Emission of Radiation

          c. Light Alteration by Stimulated Emission of Radiation

       2. The acronym MASER stands for?

            a. Microwave Amplification by Stimulated Emission of Radiation 

            b. Molecular Absorption by Stimulated Emission of Radiation

            c. Molecular Alteration by Stimulated Emission of Radiation

            d. Microwave amplification by Stimulated Emission of Radio waves

        3. What is one way to describe a Photon? 

            a. Solid as a rock

            b. A wave packet 

            c. A torpedo

            d. Electromagnetic wave of zero energy

        4. Which of the following determines the color of light?

            a. Its intensity

            b. Its wavelength

             c. Its source

             d. Some information missing

         5. Among the three examples of laser listed below, which one is considered “eye safe”?

              a. Laser bar-code scanners

              b. The excimer laser

              c. Communications lasers 

              d. YAG

          6. Why are lasers used in fiber optic communications systems

              a. The government has mandated it

              b. They can be pulsed with high speed data

              c. They are very inexpensive

              d. They are not harmful

         7. Lasers are used in CDs and DVDs. What type of laser is used in these players?

               a. Semiconductor

               b. YAG

               c. Alexandrite

               d. All the above

          8. The best reason why lasers used in “Laser Printers” is

               a. They can be focused down to very small spot sizes for high resolution

               b. They are cheap

               c. They are impossible to damage

               d. They are locally available

          9. As wavelength gets longer, the laser light can be focused to… 

             a. Larger spot sizes

             b. Smaller spot sizes 

             c. Large and small spot sizes

             d. None of the above

        10. Among the following, which color of laser has the shortest wavelength?

             a. Yellow 

             b. Red

             c.      Blue    

             d.     Green 

        11. What property of laser light is used to measure strain in roadways?

             a. Intensity

             b. Power

             c. Coherence

             d. All the above

       12. What is the type of laser used most widely in industrial materials processing applications?

             a. Dye Laser 

             b. Ruby Laser

             c.      YAG laser  

             d.      Carbon Dioxide Laser     

       13. Why are lasers used for cutting materials

             a. It never gets dull  

             b. Accuracy

             d. It has a small “heat affected zone”

             e. Smoother cuts c. Repeatability

             f. All of the above

       14. The Excimer laser produces light with what wavelength?

              a. Visible

              b. Ultraviolet

              c. Infrared 

              d. All the above.

       15. Most lasers are electrically inefficient devices. 

            a. True

            b. False 

      16. Chemical lasers use………………. to produce their beams. 

            a. Excessive amounts of electrical power  

             b. Small amounts of electrical power

             c. No electrical power 

             d. Other lasers

      17. What type of laser could cause skin cancer if not used properly?

            a. Red semiconductor laser

            b. Excimer laser

            c.     Blue semiconductor

            d.     YAG laser.

11.4.2 Structured questions 

       18. a. What do you understand by term LASER?

             b. Depending on the nature and what laser is made of, Laser is classified into different types.                            Discuss at least 5 types of lasers. 

       19 The following are basic characteristics of laser light. With clear explanation, what does each imply               as connected to laser light.

           a. Coherence

           b. Monochromaticity

           c. Collimation\

       20. a. With the aid of diagram Explain the meaning of the following terms 

                 I. Spontaneous Absorption of light

                 II. Stimulated Emission

                III. Spontaneous Emission

                IV. Population inversion 

             b. Laser light have been employed in different areas. This has helped man in solving different                          problems. 

                 What are some of the areas where laser light is employed.

              c. Though laser light is very important in different activities, it can also cause harm if mis-used In                       what ways is laser light harmful.