1.What is laser?
It is a device which produces powerful, monochromatic, collimated beam of light in which the light waves are coherent. It is an acronym for Light Amplification by Stimulated Emission of Radiation.
2. What are the characteristics of Laser light? (Or) State the properties of laser beam.
The four important characteristics of the laser beams are:
- It is highly directional.
- It has high intensity.
- The beam is purely monochromatic.
- It has coherence.
3. Distinguish between spontaneous and stimulated emission ?
| S. No | Spontaneous emission | Stimulated emission |
| 1. | The atom in the excited state returns to ground states thereby emitting a photon, without any external inducement is called as spontaneous 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. |
| 2. | The emitted photons can move randomly. | The emitted photons move in same direction and are highly directional. |
| 3. | The radiation given out are of less intense and are incoherent. | The radiation is highly intense, monochromatic and incoherent. |
| 4. | The photons are not in phase. | The photons are in phase. |
4. 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.
5. 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.
6. What are the conditions required for laser action?
- Population inversion should be achieved.
- Stimulated emission should be predominant over spontaneous emission.
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. What are 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.
9. What is the use of Nitrogen and Helium in CO2 laser ?
10. What is meant by TEA Laser?
11. Give some applications of laser in medical field.
- It is used to drill minute holes in cell walls of human body.
- It is used to treat cancer and tumour in human beings and animals.
- It is used for the treatment of detached retina.
- It is used to carry out microsurgery and bloodless operation.
- It is used to shatter the kidney stones.
- It is used to cut the bones precisely.
12. Give four applications of fibre optic sensors.
- Fibre optic sensors are used as optical displacement sensors, which are used to find the displacement of a target along with its position.
- It is used as fluid level detector.
- It is used to sense the pressure, temperature etc., at any environment.
13. What are differences between homojunction and heterojunction laser.
| S. No | Homojunction laser | Heterojunction laser |
| 1. | Homojunction laser is made by a single crystalline material. | Heterojunction laser is made by different crystalline materials. |
| 2. | Power cost is low. | Power output is high. |
| 3. | Pulsed output (sometimes continuous) | Continuous output. |
| 4. | It has high threshold current density. | It has low threshold current density. |
| 5. | Cost is less. | Cost is more. |
| 6. | Life time is less. | Life time is more. |
| 7. | Examples: (i) GaAs (ii) InP | Examples:(i) GaAs / GaA / As (ii) InP / InA / P |
14. Explain the basic principle of fibre optic communication.
Total internal reflection is the principle of fibre optic communication.
Principle : When light travels from a denser to a rarer medium, at a particular angle of incidence called critical angle, the ray emerges along the surface of separation. When the angle of incidence exceeds the critical angle, the incident ray is reflected in the same medium and this phenomenon is called total internal reflection.
15. What are the conditions to be satisfied for total internal reflection?
Light should travel from denser medium to rarer medium.
The angle of incidence ( Φ ) on core should be greater than the critical angle (Φc). i.e. Φ > Φc The refractive index of the core (n1) should be greater than the refractive index of the cladding (n2) . i.e. n1 > n2
16. Define numerical aperture.
Numerical aperture determines the light gathering ability of the fibre. It is a measure of the amount of light that can be accepted by a fibre.
Numerical aperture (NA) can also be defined as the sine of the acceptance angle (θa). if n1 and n2 are the refractive index of the core and cladding respectively. Then
\mathrm{NA}=\sqrt{\mathrm{n}_1^2-\mathrm{n}_2^2}17. Define acceptance angle.
Acceptance angle is the maximum angle to the axis at which light may enter into the fibre so that it can have total internal reflection inside the fibre.
18. Distinguish between step-index and graded index fibres.
| S. No | Step-index fibre | Graded index fibre |
| 1. | The difference in referactive indices between the core and cladding is obtained in a single step and hence called as step – index fibre. | Due to non- uniform refractive indices between the core and the cladding refractive index gradually increases from centre towards interface and hence called graded – index fibre. |
| 2. | The light rays propagate as meridinal rays and pass through fibre axis. | The light rays propagation is in the form of skew rays and does not cross the fibre axis. |
| 3. | It follows a Zig – Zag path of light propagation. | It follows a Helical path (i.e. spiral manner ) of light propagation. |
| 4. | It has a low bandwidth. | It has a high bandwidth. |
| 5. | Distortion is more in multimode step – index fibre. | Distortion is very less and is almost zero due to self focusing effect. |
19. What is meant by fractional index change? What is the relation between fractional index change and numerical aperture?
Fractional index change is the ratio of refractive index difference between the core (n1) and the cladding (n2 ) to the refractive index of core.
(i.e.,) \Delta=\frac{\left(\mathrm{n}_1-\mathrm{n}_2\right)}{\mathrm{n}_1}
The relation between NA and Δ is given by
\mathrm{NA}=\mathrm{n}_1 \sqrt{2 \Delta}20. Differentiate between single mode and multimode fiber.
| S. No | Single mode fibre | Multimode fibre |
| 1. | In single mode fibre only one mode can be propagated. | The fibre in this case allows large number of modes of light to propagate through it. |
| 2. | The single mode fibre has a smaller core diameter and difference in refractive index of core and cladding is small. | Here , since the core diameter is large , the core and cladding refractive index difference is also large. |
21. What are the losses that occurs during optical fiber communication.
During the transmission of light through the optical fiber, three major losses will occur, viz., attenuation, distorsion and dispersion.
Attenuation is mainly caused due to the absorption, scattering and radiation of light inside the fibers.
Distortion and dispersion occurs due to spreading of light and also due to manufacturing defects.
22. What is meant by attenuation?
It is defined as the ratio of the optical power output (Pout) from a fiber of length ’ L ’ to the power input (Pin) .
i.e., Attenuation alpha=\frac{-10}{L} \log \left[\frac{p_{\text {out }}}{P_{\text {in }}}\right] d \mathrm{~dB} / \mathrm{Km}
23. How fibres are used as a sensors?
The fibre optic sensors are used to detect changes in frequency, intensity, temperature, current, polarization of light waves etc. A fibre optic sensor modulates the light passing through it, when it is exposed to change in environment.
24. State some of the applications of optical fibres in medical field.
- Fibre optic endoscopes are used in medical diagnosis.
- It is used to visualize the inner organs of the body.
- Fibres as endoscopes are used in various medical fields such as cardioscopy, laproscopy, cytoscopy etc.
25. What is meant by endoscope?
A medical endoscope is a tubular optical instrument, used to inspect (or) view the internal parts of human body which are not visible to the naked eye. The photograph of the internal parts can also be taken using endoscope.
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