# Optical Sources and Transmitters MCQ

## Optical Sources and Transmitters MCQ

Optical Sources and Transmitters MCQ, MCQ on Optical Sources, MCQ on Optical Transmitters, GATE Questions on Optical Source and Transmitter, Objective Questions with answers on Optical Sources and Transmitters, Multiple Choice Questions on Optical Sources and Transmitters, Engineering MCQ

### Multiple Choice Questions

Q.1. In a semiconductor if the charge carriers (electrons or holes) can make a transition from the conduction band to the valence band without any change in the momentum value, the semiconductor material is known as

• Narrow band gap material.
• Wide band gap material.
• Indirect band gap material.
• Direct band gap material.

Q.2. The absolute power level produced by an optical source is 0 dBm. On a linear scale, it will be

• 0 mW
• 1 mW
• 0.1 W
• 1 W

Q.3. If the band gap of a semiconductor material is 1 eV, its emission wavelength will be

• 1300 nm
• 1240 nm
• 1550 nm
• 685 nm

Q.4. Which one of the following statements is true in case of an LED?

• It is an incoherent optical source that emits light over a relatively wider angle with a broad spectrum.
• It is a highly coherent optical source that emits light over a relatively wider angle with a broad spectrum.
• It is an incoherent optical source and emits radiation in a narrow range of angles and contains a narrow spectrum of wavelengths.
• It is a highly coherent optical source and emits radiation in a narrow range of angles and contains a narrow spectrum of wavelengths.

Answer: It is an incoherent optical source that emits light over a relatively wider angle with a broad spectrum.

Q.5. Which one of the following statements is true?

• A laser diode is an incoherent optical source that emits light over a relatively wider angle with a broad spectrum.
• A laser diode is highly coherent optical source that emits light over a relatively wider angle with a broad spectrum.
• A laser diode is an incoherent optical source and emits radiation in a narrow range of angles and contains a narrow spectrum of wavelengths.
• A laser diode is a highly coherent optical source and emits radiation in a narrow range of angles and contains a narrow spectrum of wavelengths.

Answer: A laser diode is a highly coherent optical source and emits radiation in a narrow range of angles and contains a narrow spectrum of wavelengths.

Q.6. _________ is an example of a direct bandgap semiconductor material which is suitable for emission of light.

• Si
• Ge
• GaAs
• GaAlSi

Q.7. For the emission wavelength of 954 nm, the bandgap of a material of which a laser is made, is

• 1 eV
• 1.24 eV
• 1.3 eV
• 1.5 eV

Q.8. When a particular semiconductor device is formed by adding

• higher impurities concentrations into the same crystal, the resultant junction is known as homojunction.
• lower impurities concentrations into the same crystal, the resultant junction is known as homojunction.
• lower impurities concentrations into the same crystal, the resultant junction is known as heterojunction.
• higher impurities concentrations into the same crystal, the resultant junction is known as heterojunction.

Answer: lower impurities concentrations into the same crystal, the resultant junction is known as homojunction.

Q.9. __________ are the advantages of employing double heterojunction type devices.

• Low injection efficiency, improved ohmic contact, optical guidance, and minority carrier confinement
• High injection efficiency, improved ohmic contact, optical guidance, and majority carrier confinement
• Low injection efficiency, majority carrier confinement, optical guidance and improved ohmic contact
• High injection efficiency, improved ohmic contact, minority carrier confinement, and optical guidance

Answer: Low injection efficiency, improved ohmic contact, optical guidance, and minority carrier confinement

Q.10. When an LED is modulated by an electrical signal,

• the output optical power is constant at low modulation frequency but falls off at high modulation frequency.
• the output optical power is constant at high modulation frequency but falls off at low modulation frequency.
• the output optical power falls off at low modulation frequency but is constant at high modulation frequency.
• the output optical power rises at low modulation frequency but falls off at high modulation frequency.

Answer: the output optical power is constant at low modulation frequency but falls off at high modulation frequency.

Q.11. The modulation response of an LED is described by

• $P(f)=\frac{P_{0}}{\sqrt{1+(2\Pi f\tau)^{2} }}$
• $P(f)=\frac{P_{0}}{\sqrt{1+(2\Pi f/\tau)^{2} }}$
• $P(f)=\frac{P_{0}}{1+(2\Pi f\tau)^{2} }$
• $P(f)=\frac{P_{0}}{\sqrt{1+2\Pi f\tau }}$

Answer: $P(f)=\frac{P_{0}}{\sqrt{1+(2\Pi f\tau)^{2} }}$

Q.12. By definition, the internal quantum efficiency is related with radiative and non-radiative lifetimes as per following relationship

• $\eta _{int}=\frac{1}{1-(\tau_{r}/\tau _{nr})}$
• $\eta _{int}=\frac{1}{1+(\tau_{r}/\tau _{nr})}$
• $\eta _{int}=\frac{1}{1+(\tau_{nr}/\tau_{r})}$
• $\eta _{int}=\frac{1}{1+(\tau_{r}\times \tau_{nr})}$

Answer: $\eta _{int}=\frac{1}{1+(\tau_{r}/\tau _{nr})}$

Q.13. When an LED is modulated by an electrical signal,

• the output optical power is constant at low modulation frequency but falls off at high modulation frequency.
• the output optical power is constant at high modulation frequency but falls off at low modulation frequency.
• the output optical power falls off at low modulation frequency but is constant at high modulation frequency.
• the output optical power rises at low modulation frequency but falls off at high modulation frequency.

Answer: the output optical power falls off at low modulation frequency but is constant at high modulation frequency.

Q.14. In a quarter wave shifted DFB laser diode, ___________ is introduced at the center of the laser cavity.

• a phase shift of π/4 radians for a path difference of l/2
• a phase shift of π/2 radians for a path difference of l/2
• a phase shift of π/2 radians for a path difference of l/4
• a phase shift of π/4 radians for a path difference of l/4

Answer: a phase shift of π/2 radians for a path difference of l/4

Q.15. An LED is applied with an input electric power of 170 mW. If the optical power launched into the fiber by this LED is typically 25 μW, then the percent efficiency is

• 1.5
• 0.15
• 0.015
• 0.0015

Q.16. Statement I – Laser operation occurs only above a certain threshold current.

Statement II – Below threshold current, the laser diode acts as a conventional LED.

• Statement I and Statement II are correct.
• Only statement I is correct.
• Statement I is incorrect but Statement II is correct.
• Both statements I and II are incorrect.

Answer: Statement I and Statement II are correct.

Q.17. In a heterostructure geometry for semiconductor optical source,

• The active region has a relatively smaller refractive index as compared to that of adjoining p- and n-type semiconductor cladding layers due to a smaller bandgap.
• The active region has a relatively larger refractive index as compared to that of adjoining p- and n-type semiconductor cladding layers due to a larger bandgap.
• The active region has a relatively smaller refractive index as compared to that of adjoining p- and n-type semiconductor cladding layers due to a larger bandgap.
• The active region has a relatively larger refractive index as compared to that of adjoining p- and n-type semiconductor cladding layers due to a smaller bandgap.

Answer: The active region has a relatively larger refractive index as compared to that of adjoining p- and n-type semiconductor cladding layers due to a smaller bandgap.

Q.18. A laser device as optical source has a very narrow angular light beam spread as compared with LEDs, which permits coupling efficiency of the order of _________ into single mode fibers.

• 100%
• 70%
• 50%
• 30%

Q.19. A relatively narrow emitted light spectral width by semiconductor laser diodes enables operation at approximately data rates.

• 10 Gbps
• 1 Gbps
• 500 Mbps
• 100 Mbps

Q.20. DFB laser structures exhibit

• lower mode partition noise, lower mode stability and narrower linewidths.
• lower mode partition noise, higher mode stability and narrower linewidths.
• lower mode partition noise, higher mode stability and wider linewidths.
• lower mode partition noise, lower mode stability and wider linewidths.

Answer: lower mode partition noise, higher mode stability and narrower linewidths.

Q.21. Assuming the effective refractive of the waveguide as 3.3, the grating period in a DFB laser to obtain single-mode operation at 1550 nm will be

• 85 nm
• 177 nm
• 235 nm
• 360 nm

Q.22. The main function of an optical fiber transmitter is

• to amplify optical signals, followed by launching them to an optical fiber.
• to convert an electrical signal into an optical signal, followed by launching them to an optical fiber.
• to convert an optical signal into an electrical signal.
• to convert an electrical signal into an optical signal, followed by launching them through the air.

Answer: to convert an electrical signal into an optical signal, followed by launching them to an optical fiber.

Q.23. In comparison with a surface-emitting LED (SLED), an edge-emitting LED (ELED) possesses all the properties except that it

• Emits light in a relatively narrower angle of emission.
• Couples relatively more power into optical fibers having small numerical aperture.
• Emits optical signal over a relatively wider spectral range.
• Is relatively more sensitive to operating temperature.

Answer: Couples relatively more power into optical fibers having small numerical aperture.

Q.24. The term lasing threshold means

• the minimum possible value of the drive current at which the laser output will be primarily from stimulated emissions.
• the front step of the laser chip.
• the shortest wavelength that the laser emits.
• the cut or polished surfaces at either edge of the active region in the laser.

Answer: the minimum possible value of the drive current at which the laser output will be primarily from stimulated emissions.

Q.25. In order to operate the lasers under varying thermal conditions, one of the following devices is generally included in the standard laser package

• An optical isolator