Communications MCQ Set 1
1. The amount of radiance in planer type of LED structures is
Answer: a [Reason:] Planer LEDs are fabricated using liquid or vapor phase epitaxial processes. Here p-type is diffused into n-type substrate which creates junction. Forward current flow through junction provides Lambertian spontaneous emission. Thus, device emits light from all surfaces. However a limited amount of light escapes the structure due to total internal reflection thus providing low radiance.
2. In optical fiber communication, ______________ major types of LED structures are used
Answer: c [Reason:] Optical fiber communication involves the use of 6 different major LED structure. These are the surface emitter, edge emitter, the super luminescent, the resonant cavity LED, planar LEDs and Dome LEDs.
3. As compared to planar LED structure, Dome LEDs have ______________ External power efficiency, ____________ effective emission area and _____________ radiance.
a) Greater, lesser, reduced
b) Higher, greater, reduced
c) Higher, lesser, increased
d) Greater, greater, increased
Answer: b [Reason:] In Dome LEDs, the diameter of dome is selected so as to maximum the internal emission reaching surface within critical angle of GaAs. Thus, dome LEDs have high external power efficiency. The geometry of Dome LEDs is such that dome is much larger than active recombination area, so it has greater emission era and reduced of radiance.
4. The techniques by Burros and Dawson in reference to homo structure device is to use an etched well in GaAs structure. Determine the given statement is True or false.
Answer: a [Reason:] Burros and Dawson provided a technique to restrict emission to small active region within device thus providing high radiance. Etched well in a GaAs substrate is used to prevent heavy absorption of emitted region and physically accommodating the fiber. These structures provide low thermal impedance allowing high current densities of high radiance.
5. In surface emitter LEDs, more advantage can be obtained by using
a) BH structures
b) QC structures
c) DH structures
d) Gain-guided structure
Answer: c [Reason:] DH structures provide high efficiency from electrical and optical confinement. Along with efficiency, they provide less absorption of emitted radiation.
6. Internal absorption in DH surface emitter Burros type LEDs is
a) Cannot be determined
d) Very low
Answer: d [Reason:] The larger band gap confining layers and the reflection coefficient at the back crystal space is high in DH surface emitter Burros type LEDs. This provides good forward radiance. Thus these structure LEDs have very less internal absorption.
7. DH surface emitter generally give
a) More coupled optical power
b) Less coupled optical power
c) Low current densities
d) Low radiance emission into-fiber
Answer: a [Reason:] The optical power coupled into a fiber depends on distance, alignment between emission area and fiber, SLED emission pattern and medium between emitting area and fiber. All these parameters if considered, reduces refractive index mismatch and increases external power efficiency thus providing more coupled optical power.
8. A DH surface emitter LED has an emission area diameter of 60μm. Determine emission area of source
Answer: d [Reason:] The emission area A of source is given by
A = π(30*10-6) 2= 2.826*10-9cm2.
9. Estimate optical power coupled into fiber of DH SLED having emission area of 1.96*10-5, radiance of 40 W/rcm2, numerical aperture of 0.2 and Fresnel reflection coefficient of 0.03 at index matched fiber surface.
Answer: d [Reason:] The optical power coupler in the step index fiber of SLED is given by
Pc = π(1-r) A RD(NA) 2 = 3.14 (1-0.03)*1.96*10-5*40*(0.2) 2 =9.551*10-5W.
10. In a multimode fiber, much of light coupled in the fiber from an LED is
Answer: c [Reason:] Optical power from an incoherent source is initially coupled into large angle rays falling within acceptance angle of fiber but have more energy than Meridional rays. Energy from these rays goes into the cladding and thus may be lost.
11. Determine the overall power conversion efficiency of lens coupled SLED having forward current of 20 mA and forward voltage of 2 V with 170 μWof optical power launched into multimode step index fiber.
Answer: c [Reason:] The overall power conversion efficiency is determined by
η pc= Pc/P= 170*10-6/20*10-3*2
12. The overall power conversion efficiency of electrical lens coupled LED is 0.8% and power applied 0.0375 V. Determine optical power launched into fiber
Answer: a [Reason:] Optical power launched can be computed by
η pc= Pc/P
Pc= η pc* P
= 0.8 * 0.0375
13. Mesa structured SLEDs are used
a) To reduce radiance
b) To increase radiance
c) To reduce current spreading
d) To increase current spreading
Answer: c [Reason:] The planar structures of Burros-type LED allow lateral current spreading specially for contact diameters less than 25 μm.This results in reduced current density and effective emission area greater than contact area. This technique to reduce current spreading in very small devices is Mesa structured SLEDs.
14. The InGaAsP is emitting LEDs are realized in terms of restricted
a) Length strip geometry
c) Current spreading
d) Coupled optical power
Answer: a [Reason:] The short striped structure of these LEDs around 100 μmimproves the external efficiency of LEDs by reducing internal absorption of carriers. These are also called truncated strip E-LEDs.
15. The active layer of E-LED is heavily doped with
Answer: a [Reason:] Zn doping reduces the minority carrier lifetime. Thus this improves the device modulation bandwidth hence active layer is doped in Zn in E-LEDs.
Communications MCQ Set 2
1. Rayleigh scattering and Mie scattering are the types of
a) Linear scattering losses
b) Non-linear scattering losses
c) Fiber bends losses
d) Splicing losses
Answer: a [Reason:] Rayleigh scattering and Mie scattering both result from non-ideal physical properties of the fiber. These losses may be impossible to eradicate. Linear scattering mechanisms cause the transfer of optical power contained within one propagating mode to be transferred linearly into a different mode.
2. Dominant intrinsic loss mechanism in low absorption window between ultraviolet and infrared absorption tails is
a) Mie scattering
b) Rayleigh scattering
c) Stimulated Raman scattering
d) Stimulated Brillouin scattering
Answer: b [Reason:] Rayleigh scattering results from non-ideal physical properties of fiber. It is a type of linear scattering loss and is difficult or impossible to eradicate. Hence, it is termed as dominant intrinsic mechanism.
3. Rayleigh scattering can be reduced by operating at smallest possible wavelengths. State whether the following statement is true or false.
Answer: b [Reason:] Rayleigh scattering results from inhomogeneity of a random nature occurring on a small level compared with the wavelength of light. The Rayleigh scattering is inversely proportional to the wavelength. Thus, as wavelength scattering reduces.
4. The scattering resulting from fiber imperfections like core-cladding RI differences, diameter fluctuations, strains, and bubbles is
a) Rayleigh scattering
b) Mie scattering
c) Stimulated Brillouin scattering
d) Stimulated Raman scattering
Answer: b [Reason:] Linear scattering also occurs at inhomogeneity which are comparable in size with the guided wavelength. These results from non-perfect cylindrical structures of the waveguide and hence caused by fiber imperfections.
5. Mie scattering has in-homogeneities mainly in
a) Forward direction
b) Backward direction
c) All direction
d) Core-cladding interface
Answer: a [Reason:] In Mie scattering, the scattering in-homogeneities size is greater thanλ/10. Also, the scattered intensity has an angular dependence which is very large. The in-homogeneities are mainly in the direction of guided wavelength i.e. in forward direction.
6. The in-homogeneities in Mie scattering can be reduced by coating of a fiber. State whether the following statement is true or false.
Answer: a [Reason:] Mie scattering is a type of linear scattering loss. It results from fluctuations in diameter, differences in core-cladding refractive index, and differences along the fiber length. Therefore, such in-homogeneities can be reduced by controlled extrusion and coating of the fiber.
7. Raman and Brillouin scattering are usually observed at
a) Low optical power densities
b) Medium optical power densities
c) High optical power densities
d) Threshold power densities
Answer: c [Reason:] Raman and Brillouin scattering mechanism are non-linear. They provide optical gain but with a shift in frequency, thus contributing to attenuation for light transmission at a particular wavelength. They can be seen at high optical power densities.
8. The phonon is a quantum of an elastic wave in a crystal lattice. State whether the given statement is true or false.
Answer: a [Reason:] A phonon is an elastic arrangement of atoms or molecules in condensed matter. This matter maybe solids or liquids. A phonon is a discrete unit of vibrational mechanical energy given by hf joules;
Where h= Planck’s constant
9. A single-mode optical fiber has an attenuation of 0.3dB/km when operating at wavelength of 1.1μm. The fiber core diameter is 4μmand bandwidth is 500 MHz. Find threshold optical power for stimulated Brillouin scattering.
a) 11.20 mw
b) 12.77 mw
c) 13.08 mw
d) 12.12 mw
Answer: b [Reason:] The threshold optical power stimulated Brillouin scattering is given by-
Where, PB= threshold optical power
d= diameter of core
10. 0.4 dB/km, 1.4μm, 6μm, 550MHz. Find threshold optical power for stimulated Raman scattering.
a) 1.98 W
b) 1.20 W
c) 1.18 W
d) 0.96 W
Answer: c [Reason:] The threshold optical power stimulated Raman scattering is given by-
PR= 5.9*10-2d2λαdB Where, PR= optical power for Raman scattering
d= diameter of core
11. Stimulated Brillouin scattering is mainly a
a) Forward process
b) Backward process
c) Upward process
d) Downward process
Answer: b [Reason:] The incident photon in Stimulated Brillouin scattering reduces a phonon of acoustic frequency as well as scattered photon. This produces an optical frequency shift which varies with the scattering angle. This frequency shift is max. in backward direction reducing to zero in forward direction making Stimulated Brillouin scattering a backward process.
12. High frequency optical phonon is generated in stimulated Raman scattering. State true or false
Answer: b [Reason:] An acoustic proton is generated in Stimulated Brillouin scattering. Raman scattering may have an optical power threshold higher than Stimulated Brillouin scattering.
13. Stimulated Raman scattering occur in
a) Forward direction
b) Backward direction
c) Upward direction
d) Forward and backward direction
Answer: d [Reason:] Stimulated Raman scattering is similar to Stimulated Brillouin scattering except that a high frequency phonon is generated in Stimulated Raman scattering. Stimulated Raman scattering can occur in forward and backward direction as it has optical power threshold higher than Stimulated Brillouin scattering.
14. Stimulated Raman scattering may have an optical power threshold of may be three orders of magnitude
a) Lower than Brillouin threshold
b) Higher than Brillouin threshold
c) Same as Brillouin threshold
d) Higher than Rayleigh threshold
Answer: b [Reason:] Stimulated Raman scattering involves generation of high- frequency phonon. Stimulated Brillouin scattering on the other hand, involves generation of an acoustic phonon in a scattering process.
Communications MCQ Set 3
1. The parameters having a major role in determining threshold current of efficiency of injection laser are:
a) Angle recombination and optical losses
b) Frequency chirping
c) Relaxation oscillation
d) Mode hopping
Answer: a [Reason:] Optical losses due to free carrier absorption are more because of their dependence on square of the wavelength. Also irradiative recombination through Auger recombination contributes to it. Both these effects cause more problems in md-infrared wavelengths and so are of much importance art high temperature due to high concentration of free carriers. They also limit maximum operating temperatures.
2. Auger current is mostly ___________________ for material with band gap providing longer wavelength emission.
Answer: c [Reason:] The total current required for injection laser threshold is more than that provided to radioactive recombination as Auger current is added. This current depends on electronic band structure of material and often consists of different Auger transitions. So it is larger for materials with band gaps providing longer wavelength emission.
3. Injection lasers operating in smaller wavelengths are subjected to increased carrier losses. State whether the following statement is true or false?
Answer: b [Reason:] Injection lasers operating in longer wavelengths (mid and far infrared) are subjected to increased carrier losses as compared to devices operating up to 1.6μm. This is from nonradiative recombination through Auger interaction. This recombination energy is dissipated as thermal energy to other free carriers. If band gap of semiconductor is increased, occurrence of these events gets increased.
4. Devices based on quaternary PbSnSeTe and their ternary compounds, emit at wavelength ?
a) Between 3-4 μm
b) Longer than 4 μm
c) Between 3.5 to 4.2 μm
d) Between 2 to 3 μm
Answer: b [Reason:] Quaternary devices emit at wavelength longer than 4μm. Auger effects are less in these alloys which provide lower current thresholds and higher maximum operating temperature.
5. Replacing Sn with Eu, Cd or Ge in some _________________ the band gap.
a) Remove the band gap
b) Does not affect
Answer: d [Reason:] When in a particular alloy laser for example PbSnSeTe, if Sn is replaced with Eu, Cd or Ge, there is an increase in band gap. This increase in band gap provides the laser to operate in shorter wavelength
6. Lasing obtained in __________ when 191 mW of pump light at a wavelength of 0.477 μm is launched into laser.
a) Ternary PbSnSeTe alloy laser
b) Quaternary PbSnSeTe alloy laser
c) Doped Fluoro-zirconate fiber
d) Ternary PbEuTe alloy laser
Answer: c [Reason:] When Fluoro-zirconate fiber lasers are doped with Erbium helium or thulium, there are emission at 2-3 μm wavelength range. But lasing was obtained in this doped Fluorozirconate fiber at a wavelength of 0.477μm.
7. The thulium doped fiber laser when pumped with alexandrite laser output at 0.786 μm, the laser emits at
a) 0.6 μm
b) 0.8 μm
c) 2.3 μm
Answer: c [Reason:] The thulium system emits at 2.3 μmwhen subjected to alexandrite laser at 0.786 μm. this system is four levels in which the pump band is upper lasing level at 2.3μm.
8. The diode-cladding-pumped Erbium praseodymium-doped fluoride device operates at wavelength.
a) Around 3 μm
b) 4 μm
c) 2.6 μm
d) 1.04 μm
Answer: a [Reason:] The diode-cladding-pumped Erbium praseodymium-doped fluoride device operates at a wavelength of 3 μm. This laser is capable of producing a very high output power of about 1W or more. It consists of double clad fluoride fiber.
9. A technique based on inter-sub band transition is known as
a) Auger recombination
b) Frequency chirping
c) Inter-valence band absorption
d) Quantum cascading
Answer: d [Reason:] The quantum cascaded laser is a layered semiconductor device having a series of coupled quantum wells grown on GaAs or Imp substrate. This principle of QC lasers provides emission of an optical signal around full wavelength range. Quantum mechanical band structure determines the emitted wavelength.
10. In a QC laser, a same electron can emit number of photons. State the given statement is true or false?
Answer: a [Reason:] The QC laser operates by pumping a energy level and then using the energy in a controlled manner. This gives some energy each time over several steps. And since a QC laser structure includes a series of energy levels the same electron emits a number of photons while cascading down through each energy level.
11. The phenomenon resulting in the electrons to jump from one state to another each time emitting of photon is known as :
a) Inter-valence band absorption
b) Mode hopping
c) Quantum cascading
d) Quantum confinement
Answer: d [Reason:] In Quantum confinement, charge carriers are trapped in a small area and this occurs in quantum wells at nanometer scale. When the quantum layer size raises to a size comparable to emission wavelength, the electron motion becomes perpendicular to plane of layer. Due to this, the electrons jump from one state to another each time from one state to another.
12. A QC laser is sometimes referred as:
a) Unipolar laser
b) Bipolar laser
c) Gain guided laser
d) Non semiconductor laser
Answer: a [Reason:] A QC laser utilizes only n-type of charge carriers. Their operation is entirely based on electrons and holes play no part in this, so they are known as unipolar lasers.
13. In QC lasers, it is possible to obtain different output signal wavelengths. This can be achieved by
a) Inter-valence band absorption
b) Mode hopping
c) Quantum cascading
d) Selecting layers of different thickness
Answer: d [Reason:] In QC laser, electrons emit energy. This energy emitted at this stage determines wavelength of radiation and it depends only on thickness of the layer. Thus output signal wavelength is dependent on thickness of lasers.
14. QC lasers ______________ the performance characteristics.
a) Have negligible effects
b) Does not affects
Answer: c [Reason:] QC lasers are based on inter sub band transition techniques. They have ability of carrying large amount of currents. A single electron is enough to generate number of photons. Thus, provides an increase in output signal power which is greater than thousands at same wavelength due to large number of cascaded stages.
15. An MQW cascaded laser is more advantageous because of:
a) Mode hopping
b) Auger recombination
c) Control over layers of material
d) Properties of material
Answer: c [Reason:] In MQW cascaded layers, cascading creates number of injector/collector and active region in single stage. Each region contains a single quantum wells. Such structures permit maximum injection/collection of current and thereby produce large number of photons. This formation of any injector/collector and active regions is achieved through precise control of several hundreds of layers of the material, where each layer should only be few nanometers thick.
Communications MCQ Set 4
1. In the development of photodiodes for mid-infrared and far-infrared transmission systems, lattice matching has been a problem when operating at wavelengths ____________
a) 1 µm
b) Greater than 2 µm
c) 2 µm
d) 0.5 µm
Answer: b [Reason:] Lattice matching for alloy materials is obtained at wavelengths above 2 µm. For example, a lattice-matched alloy material system (GaSb) was utilized in a p-i-n photodiode for high speed operation at wavelengths up to 2.3 µm.
2. What is generally used to accommodate a lattice mismatch?
c) Graded buffer layer
d) APD array
Answer: c [Reason:] The use of indium alloy cause inherent problems of dislocation-induced junction leakage and low quantum efficiency. To avoid these problems, a compositionally graded buffer layer is used to accommodate a lattice mismatch.
3. HgCdTe material system is utilized to fabricate long-wavelength photodiodes. State whether the given statement is true or false.
Answer: a [Reason:] HgCdTe family alloys allow resonant characteristics via hole ionization. Its band gap energy variation enables optical detection to far-infrared. Thus, this material can be used for fabrication of long-wavelength photodiodes.
4. Avalanche photodiodes based on HgCdTe are used for ______________ in both the near and far infrared.
d) Array applications
Answer: d [Reason:] Avalanche photodiodes based on HgCdTe are used for array applications. The materials of APDs based on HgCdTe possess uniform avalanche gain across an array. This variation in gain is variation in gain is lower in HgCdTe as compared with silicon.
5. The detection mechanism in ____________ relies on photo excitation of electrons from confined states in conduction band quantum wells.
a) p-i-n detector
b) Quantum-dot photo detector
c) p-n photodiode
d) Avalanche photodiodes
Answer: b [Reason:] Quantum-dot photo detector’s detection mechanism involves photo excitation of electrons. This process of photo excitation in photo detectors is similar to that in the Quantum-dot semiconductor optical amplifier. The dots-in-well in Quantum-dot detector is called as DWELL structure.
6. When determining performance of a photo detector ___________ is often used.
a) No. of incident photon
b) No. of electrons collected
d) Absorption coefficient
Answer: c [Reason:] The expression for quantum efficiency does not include photon energy. Thus for characterizing performance of photo detector, Responsivity is used.
7. The important parameter for exciting an electron with energy required from valence band to conduction band is
b) Absorption coefficient
d) Band gap energy
Answer: a [Reason:] As wavelength of incident photon becomes longer, the photon energy is less than energy required to excite electron. Mostly parameters of photodiode are dependent on wavelength.
8. __________ is less than or unity for photo detectors.
a) Absorption coefficient
b) Band gap energy
d) Quantum efficiency
Answer: d [Reason:] Quantum efficiency determines the absorption coefficient of semiconductor material of photo detector. It is not all incident photons are absorbed to create electron-hole pairs. Thus quantum efficiency must be less than unity.
9. There must be improvement in __________ of an optical fiber communication system.
c) Absorption Coefficient
d) Band gap energy
Answer: a [Reason:] If proper and improved and highly efficient detector is utilized, it will then reduce the repeated stations. It will also lower down both capital investment and maintenance cost.
Communications MCQ Set 5
1. _____________ is essentially a crude form of Amplitude shift keying.
a) Analog modulation
b) Digital intensity modulation
d) Receiver structure
Answer: b [Reason:] Many techniques have been developed to amplitude modulate an optical signal. Digital intensity modulation used in direct detection systems is essentially a crude form of ASK in which the received signal is detected using square law detector.
2. Almost _________ of the transmitter power is wasted in the use of external modulators.
Answer: a [Reason:] All external modulators suffer the drawback that around half of the transmitted power is wasted. To avoid this, non-synchronous detection can be employed.
3. The line width in the range ________ of bit rate is specified for ASK heterodyne detection.
b) 2 to 8%
c) 10 t0 50%
Answer: c [Reason:] The ASK modulation scheme can be used with laser sources exhibiting the line widths comparable with the bit transmission rate. For ASK heterodyne detection, line width range of 10 to 50% is usually specified.
4. ______________ is also referred to as on-off keying (OOK).
Answer: Amplitude shift keying (ASK) involves the locking and assembling of the amplitude of the wave. It involves the carrier wave along with the amplitude wave or transmitted wave and hence referred to as on-off keying.
5. ________ does not require an external modulator.
Answer: a [Reason:] FSK involves the frequency deviation property of the directly modulated semiconductor laser used in wideband systems. Unlike ASK, it does not require an external modulator, which in turn, avoids the wastage of transmitted power.
6. The frequency deviation at frequencies above 1 MHz is typically ____________
a) 10 to 20 mA-1
b) 100 to 500 mA-1
c) 1000 to 2000 mA-1
d) 30 to 40 mA-1
Answer: b [Reason:] The carrier modulation effect occurs at the frequencies above 1 MHz. At the phase of carrier modulation, the frequency deviation is about 100 to 500 mA-1.
7. ___________ offers the potential for improving the coherent optical receiver sensitivity by increasing the choice of signalling frequencies.
Answer: a [Reason:] Multilevel FSK includes 4-level or 8-level FSK. It improves the receiver sensitivity by reducing the deviation and increasing the usage of signalling frequencies.
8. Eight level FSK and binary PSK yields an equivalent sensitivity. State whether the given statement is true or false.
Answer: b [Reason:] Binary PSK and 8-level FSK provides an equivalent sensitivity. The main drawback of 8-level FSK is that it yields an equivalent sensitivity to binary PSK at the expense of a greater receiver bandwidth requirement.
9. External modulation for ________ modulation format allows the most sensitive coherent detection mechanism.
Answer: c [Reason:] External modulation for PSK is usually straightforward. It is therefore utilized to provide the modulation format which allows the most sensitive coherent detection mechanism.
10. _________ can potentially provide spectral conservation through the use of multilevel signalling.
a) M-ary PSK
Answer: a [Reason:] In M-ary schemes, the spectral efficiency is increased by the factor log2 M.this is purely for M-level schemes which can provide multilevel signalling patterns.
11. The digital transmission on implementation of polarization modulation which involves polarization characteristics of the transmitted optical signal is known as _____________
a) Frequency shift keying
b) Amplitude shift keying
c) Phase shift keying
d) Polarization shift keying
Answer: d [Reason:] Polarization shift keying is abbreviated as PolSK. PolSK requires additional receiver complexity than other modulation formats.