Two Marks Questions with Answers

ANNA UNIVERSITY

PART - A

‘2’ Marks Q & A

1. What is stimulated emission?

The process of induced emissions of photons caused by the incident photons is called stimulated emission. This process is a key factor for the operation of a laser.

2. What are the conditions necessary for stimulated emission of radiation?

(i) The atoms must be in the excited state. 

(ii) The photon of light radiation must strike the atoms in the excited state.

3. Write the differences between spontaneous emission and stimulated emission

4. What is meant by population inversion and how is it achieved?

The establishment of a situation in which the number of atoms in higher energy level is more than that in lower energy level is called population inversion. It is an essential requirement for producing a laser beam. It is achieved by pumping action.

5. Explain the need for population inversion in the production of laser?

Stimulated emission process is a key factor for the production of laser. For stimulated emission, more number of atoms must be in the excited state.

Establishing a situation in which number of atoms in higher energy state is more than that in lower energy state is called population inversion. Hence, population inversion is needed in the production of laser.

6. What is pumping action?

The process of creating a population inversion in the atomic states is known as pumping action. It is essential requirement for producing a laser beam.

7. What are the methods commonly used for pumping action?

(i) Optical pumping (excitation by photons) 

(ii) Electrical discharge method (excitation by electrons) 

(iii) Direct conversion 

(iv) Inelastic collision between atoms.

8. What is optical pumping?

When the atoms are exposed to light radiations (of energy hv), atoms in the lower energy state absorb these light radiations and go to excited state. This method of pumping is called optical pumping.

It is used in solid state lasers like ruby laser and Nd - YAG laser.

9. What is meant by active material in laser?

A material in which population inversion can be achieved is called as active material.

10. What are the characteristics of the laser? (or) What are the properties of the laser beam?

• Laser light is highly coherent

• It is highly powerful and intense.

• It is directional and monochromatic.

• It is capable of travelling over long distance without any energy loss.

• It is extremely bright.

• Laser beam is not easily absorbed by the water.

11. Under which conditions a set of laser beams is said to be coherent?

A set of laser beams is said to be coherent if they have same frequency and constant phase difference among them with respect to space and time.

12. Compare the characteristics of laser with ordinary light.

13. What is optical resonant cavity? 

It is a pair of mirrors with active material in between them. One of the mirrors of the resonant cavity is made partially reflecting to serve as an output element passing the light (laser) out of the resonator. The other mirror is a highly reflecting one.

14. What is the function of resonator cavity in laser?

Resonator cavity is made up of a pair of fully reflecting plate and a partially reflecting plate. Both of them are optically plane and accurately parallel. The active medium is placed between these mirrors.

The photons emitted along the axial direction during stimulated emission travel back and forth across the active medium and grow in strength. After enough strength is attained, laser beam emerges out from the partial reflector.

15. What is the principle of laser action?

Stimulated emission process is a key factor for the laser action. This can be multiplied through chain reaction. This multiplication of photons through stimulated emission leads to coherent, powerful, monochromatic, collimated beam of light-emission.

16. What are the three important components of any laser device?

(i) Active medium

(ii) Pumping source 

(iii) Optical resonator

17. What are the conditions required for laser action?

• Population inversion should be achieved.

• Stimulated emission should be predominant over spontaneous emission.

18. What are Einstein's coefficients?

In 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), then the number of stimulated absorption transition is given by

The number of spontaneous emission transition is given by

The number of stimulated emission transition is given by

where Q is the energy density of the incident radiation. A21, B12 and B21 in the above three equations are called Einstein's Coefficients.

19. How lasers are classified? or Mention the various types of lasers.

(i) Solid state lasers 

(ii) Gas lasers

(iii) Liquid lasers

(iv) Dye lasers 

(v) Semiconductor lasers.

20. What is Nd - YAG laser?

Nd - YAG is a neodymium based laser.

Nd - Neodymium (rare earth element Nd3+).

YAG - Yittrium Aluminium Garnet (Y3Al5O12).

It is a four level solid state laser.

21. What are the applications of Nd-YAG laser ?

(i) It finds many applications in range finders and illuminators

(ii) It finds applications in resistor trimming, scribing, micro machining operations such as welding, drilling etc.

(iii) It finds applications in medical field like endoscopy, urology, neurosurgery, ENT, gynaecology, dermatology, dental surgery and general surgery.

22. What is CO2 laser ?

It is a four level molecular gas laser. The active medium of this laser is CO2 gas. Laser transition takes place between the vibrational energy states of the CO2 molecules. It is a very useful and efficient laser.

23. What is the active medium in CO2 laser ?

A gas mixture consisting of CO2, nitrogen and helium is the active medium.

24. What are the applications of CO2 laser ?

(i) High Power CO2 lasers find applications in materials processing, welding, drilling, cutting, soldering etc., because of their very high output power. 

(ii) It is used in laser remote sensing. 

(iii) It has found wide applications in neurosurgery and general surgery. 

(iv) It is used to perform micro-surgery and bloodless operations.

(i) Welding 

(ii) Cutting 

(iii) Drilling 

(iv) Heat treatment of metallic and non metallic (plastic, ceramic, glass) materials. 

(v) Non - Destructive Testing (NDT): Testing the materials for flaws or defects without damaging them.

25. What is semiconductor laser?

Semiconductor diode laser is a specially fabricated pn junction device. It emits laser light when it is forward biased

26. What is homo-junction laser?

Homo-junction means that a pn junction is formed from a single crystalline material

Example: Gallium Arsenide (GaAs)

27. What are the drawbacks of homojunction laser diodes?

● The output beam has large divergence

● Coherence and stability are poor

● Optical confinement is very poor

28. What are the applications of semiconductor laser?

(i) It is mostly used in optical fiber communications.

(ii) It is used to heal the wounds by means of infrared radiation

(iii) It is used in computer laser printers and for writing and reading CD's

29. What is laser material processing?

Material processing involves cutting, welding, drilling and surface treatment using laser beams

When the material is exposed to laser light, then light energy is converted into heat energy. Due to heating effect, the material is heated then melted and vapourised. Also a fine beam of laser acts like a machine tool to do cutting, welding etc.

30. Mention the applications of lasers in industry.

Nd:YAG and CO₂ lasers are very much used in industries for the following processes

31. What is laser welding ?

In this technique, a focussed laser beam is incident on spot where the two parts are to be welded. The spot-contact points get welded due to heating affect of fine laser beams. 

32. What are the advantages of laser welding ?

(i) Laser welding is contactless, therefore there is no possibility of introduction of harmful impurities. 

(ii) Laser welding can be performed faster in atmospheric pressure unlike electron beam welding where vacuum is a must. 

(iii) Dissimilar materials can be welded

33. What is heat treatment of laser ?

A powerful laser beam is incident on a metal surface. That portion at which laser light is incident gets heated. As the beam is moved away to other areas, the heated spot cools down rapidly. This procedure is used for heat treatment of metal surfaces which enhances the strength of the metal.

34. What are advantages of laser cutting ? 

(i) Laser cutting is used for wide range of processed materials (paper, cloth, plywood, glass, ceramics, sheet metal); 

(ii) This laser cutting introduces minimum mechanical distortion and minimum thermal damage in the material being cut 

(iii) This cutting process has high chemical purity.

ADDITIONAL Q&A

1. What is laser?

Laser stands for Light Amplification by Stimulated Emission of Radiation. 

Laser is a light source. It produces a powerful, monochromatic, collimated beam of light in which the light waves are coherent.

2. What is stimulated absorption?

An atom in the ground state with energy E1 absorbs a photon of energy hv and go to an excited state (higher state) with energy E2 provided that the photon energy hv is equal to the energy difference (E2 - E1). This process is called stimulated absorption or simply absorption.

3. What is spontaneous emission?

The atom in the excited state E2 (higher energy state) returns to the ground state E1 (lower energy state) by emitting a photon of energy hv without the action of an external agency. Such an emission of radiation which is not triggered by an external influence is called spontaneous emission

4. What is gas laser?

Gas laser is a type of laser, in which gases such as CO2, Nitrogen and He-Ne are used as active medium for laser operations. CO2, Nitrogen and He-Ne are the important gas lasers.