SRI RAMAKRISHNA INSTITUTE OF TECHNOLOGY

COIMBATORE – 10
UNIT II
TWO MARKS QUESTION AND ANSWERS
1. What is laser?
LASER is the acronym for Light Amplification by Stimulated Emission of Radiation.
The absorption and emission of light by materials has been very ingeniously and skillfully
exploited in making a device that amplifies light and generates extremely intense, highly
directional, more coherent and monochromatic radiation. This device is called "Laser".

2. What are the coherent sources?

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

3. 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.

4. Define coherent length and coherent time? How are they related to each other?
The maximum length upto which two waves trains have correlation amplitude
and phase is called coherent length and the time upto which they are correlated is
called coherent time.
Coherent length
They are related as, coherent time = ---------------------------
Velocity of light ( C )

5. Distinguish between spontaneous and stimulated emission?

S.No. SPONTANEOUS EMISSION STUMULATED EMISSION

1 The atom in the excited state returns to An atom in the excited state is induced
ground state thereby emitting a photon, to return to ground state thereby
without any external inducement is resulting in two photons of same
called as spontaneous emission. frequency and energy is called as
stimulated emission.

2 The emitted photons can move The emitted photons move in same
randomly. direction and is highly directional.

3 The radiation given out is of less The radiation is highly intense,

intense and are incoherent. monochromatic and coherent.
4 The photons are not in The photons are inphase.
phase.
5 The rate of transition is given by The rate of transition is given by
R 2 1 (S P ) = A21 N2 R21 (ST) = B21 ρv N2.

6. What is Stimulated Emission?

An atom in the excited state is induced to return to ground state thereby resulting in two
photons of same frequency and energy is called as stimulated emission.
7. What is Spontaneous Emission?

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

8. What is stimulated absorption?

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

9. 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 as 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.

10. 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.

11. 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 as pumping.

12. What are different methods of pumping?

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. Chemical process
v. Direct conversion

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 predominant over spontaneous emission.

14 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
---- = e - ( E 2 - E 1 ) /K B T
N2

WhereN 1 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.
15. What is the difference between ordinary and Laser light.

Ordinary Light Laser light

S.No
Light emitted is highly
1 Light emitted is polychromatic monochromatic
Light emitted has not high degree of
2 It has high degree of coherence.
coherence
Emits light only in one direction
3 Emits light in all direction (not directional) (directional)
Laser light is much intense and
4 Light is not intense and bright bright.
In laser beam the angular spread
5. In ordinary light the angular spread is more is less.

16. Define active medium.

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

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

An optical resonator (or) a resonance cavity is a feedback system, which consists of

an active medium kept in between 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

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

i. Active medium
ii. Pumping source.
iii. Optical resonator.

19. What is the use 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.
20. State some of the applications of lasers in engineering and Industry field.
i. High power laser 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.

21. Mention the medical applications of laser.

Laser is used in medical field by following ways.

Treatment of detached retinas.
Performing micro - surgery and bloodless operation.
Treatment of human and animal cancer and skin tumour.
It is used to shatter the kidney stones.
It is used to cut the bones precisely.
22. What are Scientific and Engineering applications of the laser?
Laser beam is used to transmit hundreds messages at a time on radio, television and
telephone.
Communication between the planets is possible with laser.
Since the laser light waves are not absorbed by water, it is possible to establish under
water communication between sub-marines.
Laser is used for forecasting the earth quakes.

23. What are the conditions necessary for stimulated emission of radiation?
All atoms must be in the excited state.
Photon of radiation must strike the atoms in the excited state.

24. What are the difference between Homojunction Laser and Heterojunction Laser?

S. Homojunction Laser Heterojunction Laser

No
1. Homojunction Laser is made by a single Heterojunction Laser is made by
crystalline material. different crystalline materials.
2. Power output is low. Power output is high.
3. Pulsed output ( some-times continuous) Continuous output.
4. It has high threshold current density. It has low threshold current density.

25. What is meant by TEA Laser?

TEA laser is Transversely Excited Atmospheric pressure C0 2 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 C0 2 molecule will be transversely
excited.

26. 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.
27. What is holography?
Holography deals with image construction by means of interference
techniques without using lenses. Here the distribution of amplitude and phase is
recorded in 3D manner so as to get complete information of the object to be
photographed.

28. Explain the importance of holographic storage.

i. Each point of an object is recorded on the whole hologram.
ii. From each piece of a hologram we can reconstruct the image.
iii. Laser is used for recording and reconstruction processes.
iv. The phase and amplitude information in the reflected waves from the object are
completely recorded in the hologram.
29. What are the difference between photography and Holography?

S.No. PHOTOGRAPHY HOLOGRAPHY

1. Ordinary light is used. Laser light is used.

2. The variation of amplitude alone is The distribution of amplitude and phase

recorded. is recorded.

3. It gives 2D picture. - It gives 3D picture i.e., Complete

picture.
4. Lens is used. No lens is used.

30. Distinguish between temporal coherence and spatial coherence.

S.No. TEMPORAL COHERENCE SPATIAL COHERENCE

1. Temporal coherence refers to the Spatial coherence refers to the

coherence between the two points of coherence between two points in
same wave at different time. different waves over a time ‘t’
2. Coherence length is small Coherence length is large

3. Coherence time is less Coherence time is more.

4.

31. How is a light emitting diode different from a semiconductor laser?

S.No. LED LASER

1. It requires low current density It requires high current density

2. Junction of diode need not be polished Junctions of the diode should be highly
polished.
3. Minority carrier injection should take Stimulated emission will take place
place
4. Power output is low Power output is high

32. 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
.
33. What are the two types of transitions that are possible in a CO2 gas laser?
Transition from Asymmetric mode [00º1] to bending mode [02 º 0] will emit laser of
wavelength 9.6µm.
Transition from Asymmetric mode [00 º 1] to symmetric mode [10 º 0] will emit laser of
wavelength 10.6µm.
PART-B

1. Explain the propagation of light through optical fiber and the application of
optical fiber as waveguide and sensor.
2. Define numerical aperture and derive an expression for numerical aperture and angle
of acceptance of fiber in terms of refractive index of the core and fiber.
Mention six advantages of optical fiber for communication as a waveguide.
3. Describe the Double-Crucible or Crucible-Crucible technique for manufacturing
An optical fiber.
4. Explain modes in detail the types of optical fibers based on materials and refractive
indices.
5. Write differences between (i) single and multimode (ii) Step index fiber and graded
index (GRIN) fiber.
6. Write notes in detail (i) Splicing (ii)losses in optical fibers.(iii) Distortion and
Dispersion
7. Describe in detail any one of the light source and detector used in fiber optical
communication system.
Note: Light source (i) Light emitting Diode (LED), (ii) Semiconductor Laser diode
(SLD).
Light detector (i) Pin photo diode (PIN Diode), (ii) Avalanche Photo Diode
(APD).
8. Explain in detail with a neat block diagram of fiber optical communication system.
What are the advantages of fiber optical communication over the conventional
methods (Radio wave communication system).
9. Explain with basic principle, the construction and working of an intrinsic sensors
and extrinsic sensors.
Intrinsic Sensors: (i) Temperature / pressure Sensor.
Extrinsic Sensors: (i) Displacement Sensor.
10.(i) Describe the construction and working of a medical endoscope and give its
applications in medical fields.
(ii) Discuss the applications of fiber optics in engineering, medical and industrial
applications.

Problems:

11. A step-index fiber has a numerical aperture of 0.26, a core refractive index of 1.5
and a core diameter of 100µm calculate (a) The refractive index of the cladding (b)
.The acceptance angle i m (c) The maximum number of modes with a wavelength of 1
µm that the fiber can carry.

12.Estimate NA when the core refractive index is 1.48, relative refractive index is 2%.
Also calculate the critical angle.

13. Calculate the total number of guided modes propagating in the multimode step
index fiber having diameter of 60µm and numerical aperture of 0.25,operating at a
wavelength of 2.7µm