Lasers: Important part-A 2 marks Short Questions and Answers

 

ANNA UNIVERSITY 'PART‒A' QUESTIONS AND ANSWERS

 

1. What are Einstein's A and B coefficients?

The Einstein's Coefficients A and B accounts for spontaneous and stimulated emission / absorption probabilities of light by a system of particles. It also explains the importance of metastable states.

 

2. What are coherent sources.

Coherent sources are the sources which have the same wavelength and frequency. It has correlation with the amplitude and phase at any point with any other point.

 

3. What is meant by population inversion?

In general, the number of atoms in the ground state will be more than that of the atoms in the excited state and it is called usual population. The reverse of this (i.e.) a state of achieving more number of atoms in the higher energy level than that of the lower energy level is called population inversion.

 

4. Define metastable state.

Metastable state is the state for which the life time is more than the excited state, i.e., it is the more stable state, which lies between the excited state and the lower state.

 

5. Distinguish between spontaneous and stimulated emission?

Spontaneous emission

i. The atom in the excited state returns to ground state thereby emitting a photon, without any external inducement is called spontaneous emission.

ii. The emitted photons move in all directions and are random.

iii. The radiation is less intense and is incoherent.

iv. The photons are not in phase (ie.) there is no phase relationship between them.

v. The rate of transition is given by R₂₁(sp) =A₂₁ N₂

Stimulated emission

i. An atom in the excited state is induced to return to ground state, thereby resulting in two photons of the same frequency and energy is called stimulated emission.

ii. The emitted photons move in the same direction and is highly directional.

iii. The radiation is high intense, monochromatic and coherent.

iv. The photons are inphase (i.e.) there is a constant phase difference.

v. The rate of transition is given by R₁₁(st) = B₂₁ ρ_v N₂

 

6. Define coherent length and coherent time? How are they related to each other?

The maximum length upto which two waves trains have correlation with the amplitude and phase is called coherent length and the time upto which they are correlated is called coherent time.

They are related as, coherent time = Coherent length / Velocity of light (c).

 

7. What are the characteristics of Laser light. (Or) State the properties of laser beam.

The four important characteristics of the laser beams are:

(i) It is highly directional.

(ii) It has high intensity.

(iii) The beam is purely monochromatic.

(iv) It has coherence.

 

8. State some of the applications of lasers in engineering and Industry field.

(i) High power lasers are useful to blast holes in diamonds and hard steel.

(ii) They are used to test the presence of pores, cracks flows, blow holes etc in the materials.

(iii) They are used for welding and cutting.

(iv) Laser is used in alloying, cladding, molding and also in microelectronic industries.

 

9. Classify different types of lasers based on active medium, with one example for each.

Type: Example

i) Solid State Laser: Ruby Laser - Active medium is Ruby rod (Al₂O₃ doped with Cr₂O₃)

ii) Gas Laser: Carbon dioxide Laser - Active medium is the mixture of CO₂, N₂ and Helium (or) water vapour.

iii) Liquid Laser: Europium Chelate Laser - Active medium is benzoyl accetonate dissolved in alcohol.

iv) Dye Laser: Coumarin Dye Laser - Active medium is coumarin compound.

v) Semi Conductor Laser: Ga As Laser - Active medium is P‒N junction. diode made up of Ga and As.

 

10. What is the role of nitrogen and Helium in CO₂ laser?

In CO₂ laser the nitrogen helps to increase the population of atoms in the upper level of CO₂ while helium helps to depopulate the atoms in the lower level of CO₂ and also to cool the discharge tube.

 

11. What are the two types of transitions that are possible in a CO₂ gas laser?

i) Transition from Asymmetric mode [00°1] to bending mode [02°0] will emit laser of wavelength 9.6μm.

ii) Transition from Asymmetric mode [00°1] to symmetric mode [10°0] will emit laser of wavelength 10.6μm.

 

12. What are different methods of pumping? (or)

What are the different methods of achieving population inversion?

There are five methods by which the pumping can be made, viz.,

(i) Optical pumping.

(ii) Direct electron excitation (Electron Discharge).

(iii) Inelastic atom‒atom collision.

(iv) Direct conversion.

(v) Chemical process.

 

13. What are the conditions required for laser action?

The two important conditions required for laser action are:

(i) Population inversion should be achieved.

(ii) Stimulated emission should be predominent over spontaneous emission.

 

14. Can a two‒level system be used for the production of laser? Why?

No, two‒level system cannot be used for the production of Laser, because for population inversion to be achieved, at least three levels are required.

 

15. Distinguish between temporal coherence and spatial coherence.

Temporal coherence

• Temporal coherence refers to the coherence between the two points of same wave at different time.

• Coherence length is small

• Coherence time is less

Spatial coherence

• Spatial coherence refers to the coherence between two points in different waves over a time 't'

• Coherence length is large

• Coherence time is more.

 

16. Give some applications of laser in medical field.

(i) It is used to drill minute holes in cell walls of human body.

(ii) It is used to treat cancer and tumour in human beings and animals.

(iii) It is used for the treatment of detached retina.

(iv) It is used to carry out microsurgery and bloodless operation.

(v) It is used to shatter the kidney stones.

(vi) It is used to cut the bones precisely.

 

17. What is the Principle for Laser action?

Principle: Due to stimulated emission, the photons multiply in each step giving rise to an intense beam of photons that are coherent and moving in the same direction. Hence the Light is Amplified by Stimulated Emission of Radiation, termed as LASER.

Explanation: Let us consider many number of atoms in the excited state. We know the photons emitted during stimulated emission has the same frequency, energy and are in phase as the incident photon. Thus results in 2 photons of similar properties. These two photons induce stimulated emission of 2 atoms in excited state thereby resulting in 4 photons. This 4 photons induce 4 more atoms and give rise to 8 photons etc., as shown in Fig. 10.7

 

18. Differentiate between laser and ordinary light.

Laser beam

1. In laser beam the angular spread is less.

2. They are highly directional.

3. It is highly intense.

4. It is a coherent beam and is in phase.

5. The radiations are monochromatic.

6. Egs: He‒Ne laser, CO₂ laser etc.

Ordinary Light

1. In ordinary light the angular spread is more.

2. They are not directional

3. It is less intense.

4. It is not a coherent beam and is not in phase.

5. The radiations are polychromatic.

6. Egs: Sunlight, Mercury vapour lamp etc.

 

 

ADDITIONAL PART‒A QUESTIONS AND ANSWERS

 

1. What do you understand by the term of "Coherence of a wave"?

The coherence of a wave describes the accuracy with which it can be represented by a pure sine wave throughout the space and time. Thus it tells the purity of spectral line emitted from the light sources. When the wave is said to be in phase and if it has high directionality, it is said to be coherent beam.

 

2. Explain the need of population inversion in the production of lasers.

When population inversion is achieved, the majority of atoms are in the excited state. So, the absorption coefficient will be negative. The negative absorption coefficient causes the amplification of the incident beam by stimulated emission. Thus the laser beam is produced. Hence, population inversion is a must for the production of Laser beam.

 

3. What is meant by pumping?

Naturally, the population inversion is achieved only at negative temperature which is impossible. Thus, the process by which population inversion is achieved by artificial means is called pumping.

 

4. What is the effect of negative temperature in population inversion?

Under the conditions of thermal equilibrium, Boltzmann's distribution function relating N₁ and N₂ is given by

N₁/N₁ = e‒(E₂‒E₁)/K_BT

Where N₁ and N₂ are the number of atoms in lower and higher states respectively. In the above equation, if T is negative, then stimulated emission will dominate over stimulated absorption, (i.e) population inversion will be achieving only if the temperature is negative.

 

5. What are Einstein's coefficients?

In the Einstein's theory of spontaneous and stimulated emission, if N₁ and N₂ are the number of atoms in the lower energy state (E₁) and higher energy state (E₂) respectively, then the rate of stimulated absorption is given by R₁₂ = B₁₂ ρ_v N₁.

The rate of Spontaneous emission is given by R₂₁(SP) = A₂₁N₂

The rate of stimulated emission is given by R₂₁(ST) = B₂₁ ρ_v N₂

Here, ρ_v is the energy density of the incident radiation and the constants A₂₁, B₁₂ and B₂₁ in the above three equations are called Einstein's coefficients.

 

6. Define active medium.

The medium in which the populations inversion can be achieved is called active medium.

 

7. What is meant by optical resonator (or) Resonance cavity?

An optical resonator (or) a resonance cavity is a feed back system, which consists of an active medium kept inbetween a 100% mirror and a partial mirror. Here, the intensity of light produced in the active medium is increased by making the light to bounce back and forth between the mirrors. Finally the laser beam comes through the partial mirror.

 

8. Draw the flowchart for laser action.

The flowchart for laser action is as shown in Fig. 10 .9.

 

9. What is principle used in Nd‒YaG cases?

The term "Doped Insulator Laser" refers to the active medium, yttrium aluminium garnet doped with neodymium Nd³⁺. The neodymium ion has many energy levels. Due to optical pumping these ions are raised to excited levels. During the transition from metastable state to E₁ state, the laser beam of wavelength 1.064 μm is emitted.

 

10. Explain inelastic atom‒atom collision.

In this type of pumping method, a combination of two types of gases are used say A and B both having the same (or) nearly coinciding excited states A* and B*.

During elastic discharge, the A atom gets excited A* (meta stable state) due to collision with electrons. The excited A* atoms now, collide with B atom, so that B goes to excited state B* as shown below:

 e^‒ + A → A*

 A*+B → B* + A

eg. Helium ‒ Neon laser

 

11. What is meant by TEA Laser?

TEA laser is Transversely Excited Atmospheric pressure CO₂ laser. Here the gas flow is maintained along the axis of the discharge tube and the current in the arc, flows at right angles to the axis of the discharge tube. Hence the CO₂ molecule will be transversely excited.

 

12. What is the Principle used in Rhodamine ‒ 6G Laser.

Organic dye such as Rhodamine‒6G (or) Xanthene dye dissolved in solvents like water or alcohol is placed in the Dye cell. The pumping source excites these Dye molecules to excited state. After achieving population inversion, these molecules emit photons of the same wavelength, leading to laser action.

 

13. Mention any two advantages and disadvantages of Xanthene dye laser.

Advantages

(i) Dye lasers possess wide range of tunable wavelengths and hence it is used for various applications.

(ii) These lasers will achieve very high power output in both pulsed and continuous wave modes.

(iii) The dye lasers will have high repetition rates and will allow efficient cooling.

Disadvantages / Limitations

(i) Dye molecules will degrade in due course of time and hence the lifespan of Dye laser is less.

(ii) The experimental setup for Dye laser is complex in nature and hence maintenance cost will be high.

 

14. List out any four applications of a dye laser.

(i) Dye lasers are widely used in spectroscopy for the study of properties of materials.

(ii) In Astronomy, dye lasers are used to create artificial guide stars.

(iii) Since Dye lasers are Tunable lasers, they are used to selectively excite molecules and study their reactions in research field.

(v) Dye lasers are used to separate different isotopes of an element.

(v) Dye lasers are used to do cosmetic procedures like hair removal, skin rejuvenation and tattoo removal.

(vi) In medical field dye lasers plays a vital role in treating eye diseases and studying the effects of laser light on eye structures.

 

15. What are the applications of laser in communication and military fields.

(a) Communication field

(i) Laser beam is useful in underwater communication between submarines and warships.

(ii) They are very useful in long haul and short haul communication network.

(iii) Lasers are used in microwave communication due to its narrow band width.

(b) In military field

(i) High power and directional laser beam can be used to annihilate objects like aeroplanes, missiles etc., by pointing the laser beam on them and hence its is called death ray.

(ii) It is useful in producing laser guns. It can also serve as a war weapon.

 

16. What is meant by LIDAR? Give its use.

LIDAR stands for Light Detection And Ranging. It is used in Laser Remote Sensing applications such as

i. Measuring atmospheric pollutant concentration.

ii. Ozone concentration.

iii. Vapour (water) concentration etc.

 

17. Why do we call laser as a Non‒material knife?

In laser surgery, without knife (or) hammer, bloodless operation, cutting tissues etc. can be made, hence laser is called as non‒material knife.

 

18. Give the applications of LASER in material processing.

In material processing laser is used in

(i) Micro electronics

(ii) Cutting

(iii) Welding

(iv) Drilling

(iv) Perforating holes

(v) Surface hardening

(vi) Alloying etc.