Important Short Questions and Answers: Sources and Detectors of Optical

SOURCES AND DETECTORS

1.     What are the advantages of optical communication?

1.     Low transmission loss.

2 Small size and weight.

3.     No electromagnetic interference.

4.     Electrical isolation.

2.     Define direct band gap materials and indirect band gap materials.

In  direct band  gap   materials        direct transition    is           possible         from  valence     band  to

conduction band.e.g.GaAs,InP,InGaAs In indirect band gap materials direct transition is not possible from valence band to conduction.e.g.silicon,germanium.

3.     What are the advantages of LED?

1.     LEDs are less complex circuits than Laser diodes.

2.     Fabrication is easier.

3.     They have long life.

4.     What are the two types of confinement used in LEDs?

1.     Optical confinement.

2.     Carrier confinement.

5.     What are the two types of LED configurations?

1.     Homo junction

2.     Single and double hetero junction.

6.     What are the three requirements of Laser action?

1.     Absorption

2.     Spontaneous emission

3.     stimulated emission.

7.     What are the three types of Laser diode structures?

1.     Gain indexed guide

2.     Positive indexed guide

3.Negative indexed guide

8.     What are the fundamental structures of Index guided lasers?

1.     buried hetero structure.

2.     Selectively diffused construction

3.     Varying thickness structure

4.     Bent layer configuration.

9.     What are the three basic methods of current confinement?

1.     Preferential dopant diffusion.

2.     Proton implantation

3.     Inner strip confinement

4.     Re growth of back biased PN junction.

10.            Define modulation.

The process of imposing information on a light stream is called modulation. This can be achieved by varying the laser drive current.

11. Define external quantum efficiency.

The external quantum efficiency is defined as the number of photons emitted per radiative electron-hole pair recombination above threshold.

12. Define threshold current.

The threshold current is conventionally defined by extrapolation of the lasing region of the power-versus-current curve. At high power outputs, the slope of the curve decreases because of junction heating.

13. Define longitudinal modes.

Longitudinal modes are associated with the length of the cavity and determine the typical spectrum of the emitted radiation.

14. Define lateral modes.

These modes lie in the plane o f t he p n junction. They depend on the side wall preparation and the width of the cavity. It determines the shape of the lateral profile of the laser beam.

15. Define transverse modes.

Transverse modes are associated with the electromagnetic field and beam profile in the direction perpendicular to the plane of the pn junction. They determine the laser characteristics as the radiation pattern and the threshold current density.

16. Define population inversion.

Stimulated emission will exceed absorption only if the population of the excited states is greater than that of the ground state. This condition is called as population inversion

17. Define internal quantum efficiency.

The internal quantum efficiency is the fraction of the electron-hole pairs that recombine radiatively. If the radiative recombination rate is R and the non-radiative recombination rate is Rnr, then the internal quantum efficiency is the ratio of the ratio of the radiative recombination rate to the total recombination rate.

18. Differentiate LEDs and Laserdiodes.

LED

1.  The output obtained is incoherent.

2.  Less expensive and less complex

3.  Long lifetime.

Laser diode

1.  The output obtained is coherent.

2.  More expensive and more complex.

3.  Less lifetime.

Where p--   concentration of holes.

n   concentration of electrons.

Ni intrinsic concentration.

20.            What is an intrinsic and extrinsic semiconductor material?

Intrinsic semiconductors have no impurities.

Extrinsic semiconductors contain impurities like boron and phosphorus.

21.            Define responsivity

The performance o f an avalanche photodiode is characterized by its responsivity 

R_APD= ηqM = Ro M hv

where

Ro is the unity gain responsivity.

22. Define long wavelength cut off related to photodiode.

The upper wavelength cutoff (λc) is determined by the band gap energy Eg of the material. If Eg is expressed in units of electron volts(eV),then λc is given in units of micrometers (µm) by

23.            Give some types of photodetectors.

•         Photomultipliers

•         Pyroelectric detectors

•         Semiconductor- based detectors

•         Phototransistors

•         Photodiodes

24.            What are the advantages of photodiodes?

a.     Small size

b.     Suitable material c. High sensitivity

d.     Fast response time

25.            What are the types of photodiodes?

•         PIN  photodetector

•         Avalanche photodiode(APD)

26.            Define photocurrent.

The high electric field present in the depletion region causes the carriers to separate and be collected across the reverse-biased junction. This gives to a current flow in the external circuit, with one electron flowing for every carrier pair generated. This current flow is known as photocurrent.

27. Define quantum efficiency.

It is defined as the number of the electron – hole pairs generated per incident photon of energy hv and is given by

n=No.of electron-hole pairs generated / No. of incident photons

28. Define impact ionization.

In order for carrier multiplication to take place, the photo generated carriers must traverse a region where a very high electric field is present. In this high field region, a photo generated electron or hole can gain energy so that it ionizes bound electrons in the valence band upon colliding with them. This carrier multiplication mechanism is known as impact ionization.

29. Define avalanche effect.

The newly created carriers are accelerated by the high electric field, thus gaining enough energy to cause further impact ionization. This phenomenon is called avalanche effect.

30. What is p+ ∏ p n+ reach- through structure?

The reach –through avalanche photodiode (RAPD) is composed of a high resistivity p-type material deposited as an epitaxial layer on a p+ substrate. A p- type diffusion is then made in the high resistivity material, followed by the construction of an n+ layer. The configuration is called p+ ∏ p n+ reach- through structure.

GLOSSARY

1.  Direct band gap materials and indirect band gap materials.

In direct band gap materials direct transition is possible from valence band to conduction band.e.g.GaAs,InP,InGaAs In indirect band gap materials direct transition is not possible from valence band to conduction.e.g.silicon,germanium.

2.     Advantages of LED.

1.     LEDs are less complex circuits than Laser diodes.

2.     Fabrication is easier.

3.     They have long life.

3.     Two types of confinement used in LEDs.

1.     Optical confinement.

2.     Carrier confinement.

4.     Modulation.

The process of imposing information on a light stream is called modulation. This can be achieved by varying the laser drive current.

5.  External quantum efficiency.

The external quantum efficiency is defined as the number of photons emitted per radiative electron-hole pair recombination above threshold.

6.  Threshold current.

The threshold current is conventionally defined by extrapolation of the lasing region of the power-versus-current curve. At high power outputs, the slope of the curve decreases because of junction heating.

7.  Longitudinal modes.

Longitudinal modes are associated with the length of the cavity and determine the typical spectrum of the emitted radiation.

8.  Lateral modes.

These modes lie in the plane of the p n junction. They depend on the side wall preparation and the width of the cavity. It determines the shape of the lateral profile of the laser beam.

9. Transverse modes.

Transverse modes are associated with the electromagnetic field and beam profile in the direction perpendicular to the plane of the pn junction. They determine the laser characteristics as the radiation pattern and the threshold current density.

10. Population inversion.

Stimulated emission will exceed absorption only if the population of the excited states is greater than that of the ground state. This condition is called as population inversion

11. Internal quantum efficiency.

The internal quantum efficiency is the fraction of the electron-hole pairs that recombine radiatively. If the radiative recombination rate is R and the non-radiative recombination rate is Rnr, then the internal quantum efficiency is the ratio of the ratio of the radiative recombination rate to the total recombination rate.

12. Intrinsic and extrinsic semiconductor material.

Intrinsic semiconductors have no impurities.

Extrinsic semiconductors contain impurities like boron and phosphorus.

13. Responsivity

The performance o f an avalanche photodiode is characterized by its responsivity 

RAPD= ηqM = Ro M hv

where

Ro is the unity gain responsivity.

14. Long wavelength cut off related to photodiode.

The upper wavelength cutoff (λc) is determined by the band gap energy Eg of the material. If Eg is expressed in units of electron volts(eV),then λc is given in units of micrometers (µm) by

15. Photocurrent.

The high electric field present in the depletion region causes the carriers to separate and be collected across the reverse-biased junction. This gives to a current flow in the external circuit, with one electron flowing for every carrier pair generated. This current flow is known as photocurrent.

16. Quantum efficiency.

It is defined as the number of the electron – hole pairs generated per incident photon of energy hv and is given by

n=No.of electron-hole pairs generated /No. of incident photons

17. Impact ionization.

In order for carrier multiplication to take place, the photo generated carriers must traverse a region where a very high electric field is present. In this high field region, a photo generated electron or hole can gain energy so that it ionizes bound electrons in the valence band upon colliding with them. This carrier multiplication mechanism is known as impact ionization.

18. Avalanche effect.

The newly created carriers are accelerated by the high electric field, thus gaining enough energy to cause further impact ionization. This phenomenon is called avalanche effect.