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Communications MCQ Set 1

1. The threshold temperature coefficient for InGaAsP devices is in the range of
a) 10-40 K
b) 40-75 K
c) 120-190 K
d) 150-190 K

View Answer

Answer: b [Reason:] The threshold temperature coefficient for InGaAsP devices is in between 40 and 75 K. This range shows higher temperature sensitivity due to intrinsic physical properties of InGaAsP material system, Auger recombination, inter-valence band absorption, carrier leakage effects over hetero-junctions.

2. The process where the energy released during the recombination of an electron-hole event getting transferred to another carrier is known as
a) Inter-valence bond absorption
b) Auger recombination
c) Carrier leakage effects
d) Exothermic actions

View Answer

Answer: b [Reason:] Auger recombination is a process where energy is released. This energy is released during the recombination of electron-hole and this released energy is transferred to another electron-hole event. During this process, when a carrier is excited to a higher energy level, it loses its excessive amount of energy by emitting a phonon in order to maintain thermal equilibrium. It consists of number of different processes each process involving three particles (2 electrons and 1 or 2 holes and 1 electron).

3. Auger recombination can be reduced by using
a) Strained MQW structure.
b) Strained SQW structure.
c) Gain-guided strained structure.
d) Strained Quantum dots lasers.

View Answer

Answer: a [Reason:] Auger recombination is a process where energy is released during recombination of electron-hole event is transferred to another event. This loss mechanism can be reduced by using strained by using MCQ laser structure. Strain can be either compressive or tensile, modifying the valence band energy levels of material and therefore can be used to increase energy.

4. High strain in strained MCQ structure should be incorporated. State whether the given statement is true or false.
a) True
b) False

View Answer

Answer: b [Reason:] Strain is introduced in thin layers of quantum wells by making small differences in lattice constants. High strain should be avoided because it causes damage in these thin-quantum layers. Also carrier leakage adds at high temperatures since it represents processes that prevent carrier from recombination thus reducing device efficiency.

5. The parameter that prevents carrier from recombination is
a) Auger recombination
b) Inter-valence band absorption
c) Carrier leakage
d) Low temperature sensitivity

View Answer

Answer: c [Reason:] Carrier leakage is the parameter that prevents carriers (electrons, holes) from recombination. At high temperatures, carrier leakage represents all those processes preventing carriers from recombination. It therefore increases the lasing threshold and thus reduces device efficiency.

6. Determine the threshold current density for an AlGaAs injection laser with T0=180k at 30°C.
a) 6.24
b) 9.06
c) 3.08
d) 5.09

View Answer

Answer: d [Reason:] The threshold current density for a laser is given by- Ith=exp(T/T0) For AlGaAs device, Ith(30)=exp(T/T0) = exp(293/180)= 5.09.

7. The phenomenon occurring when the electron and photon population within the structure comes into equilibrium is known as
a) Auger recombination
b) Inter-valence band absorption
c) Carrier leakage
d) Relaxation oscillations

View Answer

Answer: d [Reason:] Phenomenon occurring when the electron and photon population within the structure comes into equilibrium is known as Relaxation oscillations. The application of a current state to device resulting in a switch delay which is followed by high frequency damped oscillations.

8. When a current pulse reaches a laser having parasitic capacitance after the initial delay time, that pulse will
a) Have no effect
b) Will get vanished
c) Becomes narrower
d) Gets broader

View Answer

Answer: d [Reason:] The pulse will be broadened when it will reach a laser with parasitic capacitance after initial time delay. This is because when a current pulse reaches the laser, the parasitic capacitance of laser provides a source of current over the period when there is high photon density. As electron density is repetitively built up and reduced quickly, there will be several pulses at laser output as photon density will be high resulting in relaxation oscillations.

9. Reducing delay time and ____________ are of high importance for lasers.
a) Auger recombination
b) Inter-valence band absorption
c) Carrier leakage effects
d) Relaxation oscillations

View Answer

Answer: d [Reason:] For lasers generally a switch-on delay time may last for 0.5ns and relaxation oscillations behind twice that period. This behavior can produce serious deterioration in shape of laser pulse at a data rate of 100Mbits. So time delay and Relaxation oscillations are highly desirable for lasers.

10. Dynamic line-width broadening under the direct modulation of injection current is known as
a) Auger recombination
b) Inter-valence band absorption
c) Carrier leakage effects
d) Frequency Chirping

View Answer

Answer: d [Reason:] Frequency Chirping is a phenomenon which is due to Dynamic line-width broadening under direct modulation of a single longitudinal mode. Semiconductor laser cause a dynamic shifting of peak wavelength emitted from device. Strong coupling between the free carrier density and refractive index of device present in semiconductor structure results in gain-induced variations which also causes Frequency Chirping.

11. A particular characteristic or parameter that occurs during analog transmission of injection lasers is
a) Noise
b) Mode hopping
c) Carrier leakage effects
d) Frequency Chirping

View Answer

Answer: a [Reason:] During analog transmission, noise behavior of device is main thing that affects the operation of injection laser. This noise may be due to instabilities in kinks in light output versus current characteristics, reflection of light back to device and mode partition noise.

12. Intensity of output from semiconductor injection lasers leading to optical intensity noise is due to
a) Fluctuations in amplitude
b) Mode hopping
c) Carrier leakage effects
d) Frequency Chirping

View Answer

Answer: a [Reason:] Fluctuations in the laser output or intensity of laser output leads to optical intensity noise. These fluctuations are generally caused by temperature variations and spontaneous emission in the laser output. This randomness in fluctuations creates a noise source known as relative intensity noise (RIN).

13. In multimode lasers the optical feedback from unnecessary external reflections affecting stability of frequency and intensity is
a) Remains unaffected
b) Increased gradually
c) Reduced
d) Gets totally vanished

View Answer

Answer: c [Reason:] The effect due to unwanted external reflections in multimode laser is reduced. This is because the reflections are spread along any fiber modes so they are weakly coupled back into laser mode.

14. Reduction in the number of modes in multimode fiber increases the mode partition noise. State whether the given statement is true or false.
a) False
b) True

View Answer

Answer: a [Reason:] Mode partition noise is a result of laser spectral fluctuations and so a reduce in number of modes results in low pulse-width spreading thus providing low values of intermodal dispersion in the fiber. And so, the mode partition noise is decreased in multimode fiber due to reducing the number of modes.

15. The behavior of laser occurring when current is increased above threshold particularly is
a) Mode hopping
b) Auger recombination
c) Frequency chirping
d) Noise

View Answer

Answer: a [Reason:] Mode hopping results in the hopping of modes to a higher wavelength. This mode hopping occurs in all injection lasers and is due to increase in temperature. Mode hopping is not a continuous function of drive current but occurs above 1 to 2 mA. Mode hopping alters characteristics of laser and results in kinks in characteristics of single mode device.

Communications MCQ Set 2

1. Integrated technology for optical devices are developed within optical fiber communication. State whether the given statement is true or false.
a) True
b) False

View Answer

Answer: a [Reason:] Integration of optical devices enable fabrication of the whole system onto a single chip. Integration of such devices has become a confluence of several optical terms.

2. When both active and passive devices are integrated on a single chip, in multilayered form, then these devices are known as _____________
a) IP devices
b) IO devices
c) Wavelength converters
d) Optical parametric amplifiers

View Answer

Answer: a [Reason:] IP technology enables fabrication of subsystems and systems. This is all realized on a single substrate. The integration on a single chip is done in IP technology.

3. _________ is a further enhancement of ________
a) IP, IO
b) IO, IP
c) IO, wavelength converters
d) IP, wavelength converters

View Answer

Answer: a [Reason:] IP seems to be a miniaturization process and integration of optical systems on a single chip. IO devices are formed when both active and passive elements are interconnected. Thus, IP is a developed version of IO.

4. Thin transparent dielectric layers on planar substrates are used in _________ and ______ devices.
a) Wavelength converters and amplification devices
b) IP and IO
c) IP and wavelength converters
d) IO and amplification devices

View Answer

Answer: b [Reason:] IP and IO provide an alternative to conversion of optical signal back to electrical signal. Thin transparent dielectric layers act as optical waveguides to produce small-scale and miniature circuits.

5. __________ did not make significant contribution to earlier optical fiber systems.
a) IO
b) IP
c) Wavelength amplifiers
d) Couplers

View Answer

Answer: a [Reason:] IO is based on single mode optical waveguides. Thus it is incompatible with multimode fiber systems. Thus, IO has less importance than IP.

6. Side or edge-emitting or conducting optical devices cannot be integrated on same substrate. State whether the given statement is true or false.
a) True
b) False

View Answer

Answer: b [Reason:] In serial integration of device, different elements of optical chip can be interconnected in a consecutive manner. Thus, integration of side or edge emitting optical devices can be done on a single substrate.

7. Hybrid ________ integration demands _________ IP circuits to be produced on a single substrate.
a) IP, single-layered
b) IO, multilayered
c) IP, multilayered
d) IO, multilayered

View Answer

Answer: c [Reason:] To gain control of optical signals, elements can be directly attached to IP circuit. Both active and passive devices should be on a same substrate. To make devices compatible with 3d structures of other IP/IO devices, hybrid IP integration demands multilayered IP circuits.

8. Using SOI integration technique, __________ components can be coupled to IP devices.
a) Passive
b) Layered
c) Demounted
d) Active

View Answer

Answer: d [Reason:] SOI is used to produce micro-waveguide bends and couplers thereby maintaining compatibility with silicon fabrication techniques. Thus, active components like optical sources, detectors can be coupled to other IP devices using SOI technique.

9. Who invented the IO technology?
a) Albert Einstein
b) Anderson
c) M.S Clarke
d) Robert

View Answer

Answer: b [Reason:] The birth of IO can be traced back to the basic ideas outlined by Anderson in 1966. He suggested the micro-fabrication technology which in turn led to the term integrated optics in 1969.

10. Electronic circuits have a practical limitation on speed of operation at a frequency of around _________
a) 1010Hz
b) 1012Hz
c) 1014Hz
d) 1011Hz

View Answer

Answer: a [Reason:] The speed of operation of electronic devices or circuits results from their use of metallic conductors to transport electronic charges and build up signals. It has a limitation to speed of operation of frequency around 1010Hz.

11. The use of light as an electromagnetic wave of high frequency provides high speed operation around ____________ times the conceivable employing electronic circuits.
a) 108Hz
b) 105Hz
c) 106Hz
d) 104Hz

View Answer

Answer: d [Reason:] The use of light with its property as an electromagnetic wave offers the possibility of high speed operation. For this, the frequency should be high as 1014to 1015Hz.

12. How many layers are possessed by waveguide structures of silica-on-silicon(SOS)?
a) Two
b) Three
c) Four
d) One

View Answer

Answer: b [Reason:] The SOS is a part of IP technology. The waveguide structures provided by it comprises of three layers. They are buffer, the core and the cladding.

13. The ________________ is a versatile solution-based technique for making ceramic and glass materials.
a) SOL gel process
b) SSL gel process
c) SDL gel process
d) SAML gel process

View Answer

Answer: a [Reason:] The SOL gel process involves the transition of system from a liquid to a gel. The SOL gel process along with SOS technique is used for the fabrication of ceramic fibers, film coatings and waveguide based optical amplifiers.

Communications MCQ Set 3

1. Intermodal dispersion occurring in a large amount in multimode step index fiber results in
a) Propagation of the fiber
b) Propagating through the fiber
c) Pulse broadening at output
d) Attenuation of waves

View Answer

Answer: c [Reason:] Pulse broadening due to intermodal dispersion is caused due to difference in propagation delay between different modes in the multimode fiber. As different modes travel with different group velocities, the pulse width at output depends on transmission time of all modes. This creates difference in overall dispersion which results in pulse broadening.

2. After Total Internal Reflection the Meridional ray
a) Makes an angle equal to acceptance angle with the axial ray
b) Makes an angle equal to critical angle with the axial ray
c) Travels parallel equal to critical angle with the axial ray
d) Makes an angle equal to critical angle with the axial ray

View Answer

Answer: d [Reason:] The Meridional ray travels along the axis of the fiber. When the ray is incident, makes an angle equal to acceptance angle and thus it propagates through the fiber. As the propagating ray gets refracted from the boundary, it makes an angle (i.e. critical angle) with the normal.

3. Consider a single mode fiber having core refractive index n1= 1.5.
The fiber length is 12m. Find the time taken by the axial ray to travel along the fiber
a) 1.00μsec
b) 0.06μsec
c) 0.90μsec
d) 0.30μsec

View Answer

Answer: b [Reason:] The time taken by the axial ray to travel along the fiber gives the minimum delay time Tmin= Ln1/c Where L= length of the fiber n1= Refractive index of core c= velocity of light in vacuum.

4. A 4 km optical link consists of multimode step index fiber with core refractive index of 1.3 and a relative refractive index difference of 1%. Find the delay difference between the slowest and fastest modes at the fiber output.
a) 0.173 μsec
b) 0.152 μsec
c) 0.96 μsec
d) 0.121 μsec

View Answer

Answer: a [Reason:] The delay difference is given by δTs = Ln1/c Where δTs = delay difference n1= core refractive index Δ = Relative refractive index difference c= velocity of light in vacuum.

5. A multimode step-index fiber has a core refractive index of 1.5 and relative refractive index difference of 1%. The length of the optical link is 6 km. Estimate the RMS pulse broadening due to intermodal dispersion on the link.
a) 92.6 ns
b) 86.7 ns
c) 69.3 ns
d) 68.32 ns

View Answer

Answer: b [Reason:] The RMS pulse broadening due to intermodal dispersion is obtained by the equation is given below: σs = Ln1Δ/2√3c Where σs = RMS pulse broadening L = length of optical link C = velocity of light in vacuum n1 = core refractive index.

6. The differential attenuation of modes reduces intermodal pulse broadening on a multimode optical link. State whether the given statement is true or false.
a) True
b) False

View Answer

Answer: a [Reason:] Intermodal dispersion may be reduced by propagation mechanisms. The differential attenuation of various modes is due to the greater field penetration of the higher order modes into the cladding of waveguide. These slower modes exhibit larger losses at any core-cladding irregularities.

7. The index profile of a core of multimode graded index fiber is given by-
a) N (r) = n1 [1 – 2Δ(r2/a)2]1/2; r<a
b) N (r) = n1 [3 – 2Δ(r2/a)2]1/2; r<a
c) N (r) = n1 [5 – 2Δ(r2/a)2]1/2; r>a
d) N (r) = n1 [1 – 2Δ(r2/a)2]1/2; r<a

View Answer

Answer: d [Reason:] In multimode graded index fibers, many rays can propagate simultaneously. The Meridional rays follow sinusoidal trajectories of different path length which results from index grading.

8. Intermodal dispersion in multimode fibers is minimized with the use of step-index fibers. State whether the given statement is true or false.
a) True
b) False

View Answer

Answer: b [Reason:] As multimode graded index fibers show substantial bandwidth improvement over multimode step index fibers. So, inter-modal dispersion in multimode fiber is minimized with the use of multimode graded index fibers.

9. Estimate RMS pulse broadening per km due to intermodal dispersion for multimode step index fiber where length of fiber is 4 km and pulse broadening per km is 80.6 ns.
a) 18.23ns/km
b) 20.15ns/km
c) 26.93ns/km
d) 10.23ns/km

View Answer

Answer: b [Reason:] The RMS pulse broadening per km due to intermodal dispersion for multimode step index fiber is given by (σs(1 km)/L= 80.6/4=20.15 Where L= length of fiber σs= pulse broadening.

10. Practical pulse broadening value for graded index fiber lies in the range of
a) 0.9 to 1.2 ns/km
b) 0.2 to 1 ns/km
c) 0.23 to 5 ns/km
d) 0.45 to 8 ns/km

View Answer

Answer: b [Reason:] As all optical fiber sources have a finite spectral width, the profile shape must be altered to compensate for this dispersion mechanism. The minimum overall dispersion for graded index fiber is also limited by other intermodal dispersion mechanism. Thus pulse broadening values lie within range of 0.2 to 1 ns/km.

11. The modal noise occurs when uncorrected source frequency is
a) δf>>1/δT
b) δf=1/δT
c) δf<<1/δT
d) Negligible

View Answer

Answer: a [Reason:] Modal noise is dependent on change in frequency. Frequency is inversely proportional to time. The patterns are formed by interference of modes from a coherent source when coherence time of source is greater than intermodal dispersion time δT within fiber.

12. Disturbance along the fiber such as vibrations, discontinuities, connectors, splices, source/detectors coupling result in
a) Modal noise
b) Inter-symbol interference
c) Infrared interference
d) Pulse broadening

View Answer

Answer: Disturbance along the fiber cause fluctuations in specific pattern. These speckle patterns have characteristics time longer than resolution time of detector and is known as modal noise.

13. The modal noise can be reduced by
a) Decreasing width of signal longitudinal mode
b) Increasing coherence time
c) Decreasing number of longitudinal modes
d) Using fiber with large numerical aperture

View Answer

Answer: d [Reason:] Disturbances along fiber cause fluctuations in speckle patterns. Fibers with large numerical apertures support the transmission of large number of modes giving greater number of speckle, thereby reducing modal noise.

14. Digital transmission is more likely to be affected by modal noise. State true or false
a) True
b) False

View Answer

Answer: b [Reason:] Analog transmission is more affected by modal noise due to higher optical power levels which is required at receiver when quantum noise effects are considered. So it is important to look into design considerations.

Communications MCQ Set 4

1. Intrinsically _________________ are a very linear device.
a) Injection lasers
b) DH lasers
c) Gain-guided
d) LEDs

View Answer

Answer: d [Reason:] The ideal light output power against current characteristics for an LED linear. This tends to be more suitable for analog transmission where several constraints are put in linearity of optical source.

2. Linearizing circuit techniques are used for LEDs. State whether the given statement is true or false.
a) True
b) False

View Answer

Answer: a [Reason:] In practice, LEDs exhibit nonlinearities depending on configuration used. Thus, to allow its used in high quality analog transmission system and to ensure linear performance of device, linearizing circuit techniques is used.

3. The internal quantum efficiency of LEDs decreasing _______________ with ________________ temperature.
a) Exponentially, decreasing
b) Exponentially, increasing
c) Linearly, increasing
d) Linearly, decreasing

View Answer

Answer: b [Reason:] The light emitted from LEDs decreases. This is due to increase in p-n junction temperature. Thus, this results in exponentially decreasing internal quantum efficiency with temperature increment.

4. To utilize _____________________ of SLDs at elevated temperatures, the use of thermoelectric coolers is important.
a) Low-internal efficiency
b) High-internal efficiency
c) High-power potential
d) Low-power potential

View Answer

Answer: c [Reason:] The output characteristics of SLDs are typically of nonlinear in nature. This is observed with a knee becoming apparent at an operating temperature around 20 degree c. Thus, to utilize high-power potential of these devices at elevated temperature, thermoelectric coolers are necessarily used.

5. For particular materials with smaller bandgap energies operating in _____________ wavelength, the linewidth tends to ______________
a) 2.1 to 2.75 μm, increase
b) 1.1 to 1.7 μm, increase
c) 2.1 to 3.6 μm, decrease
d) 3.5 to 6 μm, decrease

View Answer

Answer: b [Reason:] For materials with smaller bandgap, linewidth increases to 50 to 160 nm. This increases in band gap is due to increased doping levels and formation of bandtail states.

6. The active layer composition must be adjusted if a particular center wavelength is desired.State whether the given statement is true or false.
a) True
b) False

View Answer

Answer: a [Reason:]There is a difference in output spectra between surface and edge emitting LEDs when devices have generally heavily doped and lightly doped active layers by reduction in doping.

7. In optical fiber communication, the electrical signal dropping to half its constant value due to modulated portion of optical signal corresponds to _______
a) 6 dB
b) 3 dB
c) 4 dB
d) 5 db

View Answer

Answer: b [Reason:] Modulation bandwidth in optical communication is often defined in electrical/optical terms. So when considering electrical circuitry in optical fiber system, electrical 3 dB point or frequency at which output electrical power is reduced by 3 dB bandwidth with respect to input electrical power.

8. The optical 3 dB point occurs when currents ratio is equal to
a) ⅝
b) ⅔
c) ½
d) ¾

View Answer

Answer: c [Reason:] In optical regime, the bandwidth is defined by frequency at which output current has dropped to ½ output input current system.

9. The optical bandwidth is _____________ the electrical bandwidth.
a) Smaller
b) Greater
c) Same as
d) Zero with respect to

View Answer

Answer: b [Reason:] The difference between optical and electrical bandwidth In terms of frequency depends on the shape of the frequency response of the system. If the system response is assumed to be Gaussian, then optical bandwidth is a factor of √2 greater than electrical bandwidth.

10. When a constant d.c. drive current is applied to device, the optical o/p power is 320 μm. Determine optical o/p power when device is modulated at frequency 30 MHz with minority carrier recombination lifetime of LED i.e. 5ns.
a) 4.49*10-12
b) 6.84*10-9
c) 1.29*10-6
d) 2.29*10-4

View Answer

Answer: d [Reason:] The output o/p at 30 MHz is Pc( 30 MHz ) = Pdc/ (1+(wΓi)2)1/2 = 320*10-6/(1+(2π*30*10-6*5*10-9)2)1/2 = 2.29*10-4W.

11. The optical power at 20 MHz is 246.2 μW. Determine dc drive current applied to device with carrier recombination lifetime for LED of 6ns.
a) 3.48*10-4
b) 6.42*10-9
c) 1.48*10-3
d) 9.48*10-12

View Answer

Answer: a [Reason:] The optical output power at 20 MHz is Pe(20 MHz) = Pdc/(1+(WTi)2)1/2 246.2*10-6= Pdc/(1+(2π*20*10-6*5*10-9)2)1/2 Pdc = 3.48*10-4.

12. Determine the 3 dB electrical bandwidth at 3 dB optical bandwidth Bopt of 56.2 MHz.
a) 50.14
b) 28.1
c) 47.6
d) 61.96

View Answer

Answer: b [Reason:] The 3dB electrical bandwidth is given by B = Bopt/ √2 = 56.2/2 = 28.1 MHz.

13. The 3 dB electrical bandwidth B is 42 MHz. Determine 3dB optical bandwidth Bopt:
a) 45.18
b) 59.39
c) 78.17
d) 94.14

View Answer

Answer: b [Reason:] The 3dB electrical bandwidth is B = Bopt/ √2 Bopt = B* √2 = 42* √2 = 59.39 MHz.

14. Determine degradation rate βrif constant junction temperature is 17 degree celsius.
a) 7.79*10-11
b) 7.91*10-11
c) 6.86*10-11
d) 5.86*10-11

View Answer

Answer: a [Reason:] The degradation rate βris determined by βr= β0exp (-Ea/KT) = 1.89*107exp (-1*1.602*10-19/1.38*10-23*290) = 7.79*10-11 h-1.

15. Determine CW operating lifetime for LED with βrt = -0.58 and degradation rate βr= 7.86*10-11 h-1.
a) 32.12
b) 42
c) 22.72
d) 23.223

View Answer

Answer: c [Reason:] The CW operating lifetime is given by t = Ln 0.58/7.86*10-11 = 22.72h-1.

Communications MCQ Set 5

1. The absence of _______________ in LEDs limits the internal quantum efficiency.
a) Proper semiconductor
b) Adequate power supply
c) Optical amplification through stimulated emission
d) Optical amplification through spontaneous emission

View Answer

Answer: c [Reason:] The ratio of generated electrons to the electrons injected is quantum efficiency. It is greatly affected if there is no optical amplification through stimulated emission. Spontaneous emission allows ron-radiative recombination in the structure due to crystalline imperfections and impurities.

2. The excess density of electrons Δnand holes Δpin an LED is
a) Equal
b) Δpmore than Δn
c) Δn more than Δp
d) Does not affects the LED

View Answer

Answer: a [Reason:] The excess density of electrons ΔnandΔp (holes) is equal. The charge neutrality is maintained within the structure due to injected carriers that are created and recombined in pairs. The power generated internally by an LED is determined by taking into considering the excess electrons and holes in p- and n-type material respectively.

3. The hole concentration in extrinsic materials is _________ electron concentration.
a) much greater than
b) lesser than
c) equal to
d) negligible difference with

View Answer

Answer: a [Reason:] In extrinsic materials, one carrier type will be highly concentrated than the other type. Hence in p-type region, hole concentration is greater than electron concentration in context of extrinsic material. This excess minority carrier density decays with time.

4. The carrier recombination lifetime becomes majority or injected carrier lifetime. State whether the given statement is true or false.
a) True
b) False

View Answer

Answer: b [Reason:] The initial injected excess electron density and τrepresents the total carrier recombination time. In most cases, Δnis a small fraction of majority carriers and contains all minority carriers. So in these cases, carrier recombination lifetime becomes minority injected carrier lifetime τi.

5. In a junction diode, an equilibrium condition occurs when
a) Δngreater than Δp
b) Δnsmaller than Δp
c) Constant current flow
d) Optical amplification through stimulated emission

View Answer

Answer: c [Reason:] The total rate at which carriers are generated in sum of externally supplied and thermal generation rates. When there is a constant current flow in this case, an equilibrium occurs in junction diode.

6. Determine the total carrier recombination lifetime of a double heterojunction LED where the radioactive and nonradioactive recombination lifetime of minority carriers in active region are 70 ns and 100 ns respectively.
a) 41.17 ns
b) 35 ns
c) 40 ns
d) 37.5 ns

View Answer

Answer: a [Reason:] The total carrier recombination lifetime is given by τ= τrτnrrnr = 70× 100/70 + 100 ns = 41.17 ns Where τr= radiative recombination lifetime of minority carriers τnr= nonradioactive recombination lifetime of minority carriers.

7. Determine the internal quantum efficiency generated within a device when it has a radiative recombination lifetime of 80 ns and total carrier recombination lifetime of 40 ns.
a) 20 %
b) 80 %
c) 30 %
d) 40 %

View Answer

Answer: b [Reason:] The internal quantum efficiency of device is given by ηint= τ/τr= 40/80 ×100 = 80% Where τ= total carrier recombination lifetime τr= radiative recombination lifetime.

8. Compute power internally generated within a double-heterojunction LED if it has internal quantum efficiency of 64.5 % and drive current of 40 mA with a peak emission wavelength of 0.82 μm.
a) 0.09
b) 0.039
c) 0.04
d) 0.06

View Answer

Answer: b [Reason:] The power internally generated within device i.e.double-heterojunction LED can be computed by Pint= ηint hci/eλ = 0.645×6.626×10-34×3×108×40×10-3/ 1.602×10-19× 0.82 × 10-6 = 0.039 W Where ηint= internal quantum efficiency h = Planck’s constant c = velocity of light i = drive current e = electron charge λ= wavelength.

9. The Lambertian intensity distribution __________ the external power efficiency by some percent.
a) Reduces
b) Does not affects
c) Increases
d) Have a negligible effect

View Answer

Answer: a [Reason:] In Lambertian intensity distribution, the maximum intensity I0is perpendicular to the planar surface but is reduced on the sides in proportion to the cosine of θ i.e. viewing angle as apparent area varies with this angle. This reduces the external power efficiency. This is because most of the light is tapped by total internal refraction when radiated at greater than the critical angle for crystal air interface.

10. A planar LED fabricated from GaAs has a refractive index of 2.5. Compute the optical power emitted when transmission factor is 0.68.
a) 3.4 %
b) 1.23 %
c) 2.72 %
d) 3.62 %

View Answer

Answer: c [Reason:] The optical power emitted is given by Pe = PintFn2/4nx2= Pint (0.680×1/4×(2.5)2)= 0.0272 Pint. Hence power emitted is only 2.72 % of optional power emitted internally. Where, Fn2= transmission factor nx= refractive index.

11. A planar LED is fabricated from GaAs is having a optical power emitted is 0.018% of optical power generated internally which is 0.018% of optical power generated internally which is 0.6 P. Determine external power efficiency.
a) 0.18%
b) 0.32%
c) 0.65%
d) 0.9%

View Answer

Answer: d [Reason:] Optical power generated externally is given by ηcp= (0.018Pint/ 2Pint)*100 Where, Pint= power emitted ηcp=external power efficiency.

12. For a GaAs LED, the coupling efficiency is 0.05. Compute the optical loss in decibels.
a) 12.3 dB
b) 14 dB
c) 13.01 dB
d) 14.6 dB

View Answer

Answer: c [Reason:] The optical loss in decibels is given by- Loss= -10log10 ηc Where, ηc= coupling efficiency.

13. In a GaAs LED, compute the loss relative to internally generated optical power in the fiber when there is small air gap between LED and fiber core. (Fiber coupled = 5.5 * 10-4Pint)
a) 34 dB
b) 32.59 dB
c) 42 dB
d) 33.1 dB

View Answer

Answer: b [Reason:] The loss in decibels relative to Pint is given by- Loss = -10log10Pc/Pint Where, Pc= 5.5 * 10-4Pint.

14. Determine coupling efficiency into the fiber when GaAs LED is in close proximity to fiber core having numerical aperture of 0.3
a) 0.9
b) 0.3
c) 0.6
d)0.12

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Answer: a [Reason:] The coupling efficiency is given by ηc= (NA)2= (0.3)2= 0.9.

15. If a particular optical power is coupled from an incoherent LED into a low-NA fiber, the device must exhibit very high radiance . State whether the given statement is true or false.
a) True
b) False

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Answer: a [Reason:] Device must have very high radiance specially in graded index fiber where Lambertian coupling efficiency with same NA is about half that of step index fibers. This high radiance is obtained when direct bandgap semiconductors are fabricated with DH structure driven at high current densities.