Category: Optical Communications Objective Questions

Optical Communications Multiple Choice Questions on “Some Injection Laser Structures”.

1. In multimode injection lasers, the construction of current flow to the strip is obtained in structure by __________
a) Covering the strip with ceramic
b) Intrinsic doping
c) Implantation outside strip region with protons
d)Implantation outside strip region with electrons
Answer: c
Explanation: The current flow is realized by implanting the region outside strip with protons. This implantation makes the laser highly resistive and gives superior thermal properties due to absence of silicon dioxide layer.

2. What is the strip width of injection laser?
a) 12 μm
b) 11.5 μm
c) Less than 10 μm
d) 15 μm
Answer: c
Explanation: A strip width less than or equal to 10 μm is usually preferred in injection lasers. This width range provides the lasers highly efficient coupling into multimode fibers as comapred to single mode fibers.

3. Some refractive index variation is introduced into lateral structure of laser.
a) True
b) False
Answer: a
Explanation: Gain guided lasers possess several undesirable characteristics, nonlinearities in light output versus current characteristics, high threshold current, low differential quantum efficiency, movement of optical a;ong junction plane. This problems can be reduced by introducing refractive index variations into lateral structure of lasers so that optical mode is determined along the junction plane.

4. Buried hetero-junction (BH) device is a type of _____________ laser where the active volume is buried in a material of wider band-gap and lower refractive index.
a) Gas lasers.
b) Gain guided lasers.
c) Weak index guiding lasers.
d) Strong index guiding lasers.
Answer: d
Explanation: In strong index guiding lasers, a uniformly thick, planar active waveguide is achieved by lateral variations in confinement layer thickness or refractive index. In Buried hetero-junction (BH) devices, strong index guiding along junction plane introduces transverse mode control in injection lasers.

5. In Buried hetero-junction (BH) lasers, the optical field is confined within __________
a) Transverse direction
b) Lateral direction
c) Outside the strip
d) Both transverse and lateral direction
Answer: d
Explanation: Optical field is strongly confined in both transverse and lateral direction. This provides strong index guiding of optical mode along with good carrier confinement.

6. A double-channel planar buried hetero-structure (DCP BH) has a planar active region, the confinement material is?
a) Alga AS
b) InGaAsP
c) GaAs
d) SiO₂
Answer: b
Explanation: The planar active region made up of InGaAsP can be seen in double-channel planar buried hetero-structure (DCP BH). This material confinement provides a very high power operation with CW output power up to 40 mW in longer wavelength region.

7. Problems resulting from parasitic capacitances can be overcome __________
a) Through regrowth of semi-insulating material
b) By using oxide material
c) By using a planar InGaAsP active region
d) By using a AlGaAs active region
Answer: a
Explanation: The use of reverse-biased current confinement layers introduces parasitic capacitances which reduces high speed modulation of BH lasers. This problem can be reduced by regrowth of semi-insulating material or deposition of dielectric material. This causes increase in modulation speeds of 20 GHz.

8. Quantum well lasers are also known as __________
a) BH lasers
b) DH lasers
c) Chemical lasers
d) Gain-guided lasers
Answer: b
Explanation: DH lasers are known as Quantum well lasers. The carrier motion normal to active layer is restricted in these devices. This results in quantization of kinetic energy into discrete energy levels for carriers moving in that direction. This phenomenon is similar to quantum mechanical problem of one dimensional potential well which is seen in DH lasers.

9. Quantum well lasers are providing high inherent advantage over __________
a) Chemical lasers
b) Gas lasers
c) Conventional DH devices
d) BH device
Answer: c
Explanation: Quantum well lasers exhibit high incoherent advantage over conventional DH lasers. In Quantum well laser structures, the thin active layer results in drastic changes in electronic and optical properties over conventional DH laser structures. This changes are due to quantized nature of discrete energy levels with step-like density and also allow high gain and low carrier density.

10. Strip geometry of a device or laser is important.
a) True
b) False
Answer: a
Explanation: Near fluid intensity distribution corresponding to single optical output power level in plane of junction can be seen in GaAs or AlGaAs lasers. This distribution is in lateral direction and is determined by the nature of lateral waveguide. The single intensity maximum shows the fundamental lateral mode is dominant.

11. Better confinement of optical mode is obtained in __________
a) Multi Quantum well lasers
b) Single Quantum well lasers
c) Gain guided lasers
d) BH lasers
Answer: a
Explanation: As compared to all lasers including single quantum well lasers, multi-Quantum well lasers are having better confinement of optical mode. This results in a lower threshold current density for these devices.

12. Multi-quantum devices have superior characteristics over __________
a) BH lasers
b) DH lasers
c) Gain guided lasers
d) Single-quantum-well devices
Answer: b
Explanation: Lower threshold currents, narrower bandwidths, high modulation speeds, lower frequency chirps and less temperature dependence are parameters determining characteristics of a particular laser. All the above parameters make multi-quantum devices superior over DH lasers.

13. Dot-in-well device is also known as __________
a) DH lasers
b) BH lasers
c) QD lasers
d) Gain guided lasers
Answer: c
Explanation: Quantum well lasers are devices in which device contains a single discrete atomic structure or Quantum-dot. These are elements that contain electron tiny droplets which forms a quantum well structure.

14. A BH can have anything from a single electron to several electrons.
a) True
b) False
Answer: b
Explanation: Quantum-dot lasers are fabricated using semiconductor crystalline materials. They have a particular dimension ranging from nm to few microns. The size, shape of these structures and number of electrons they contain are precisely controlled.

15. QD lasers have a very low threshold current densities of range __________
a) 0.5 to 5 A cm^-2
b) 2 to 10 A cm^-2
c) 10 to 30 A cm^-2
d) 6 to 20 A cm^-2
Answer: d
Explanation: Low-threshold current density between 6 to 20 A cm^-2 is obtained with InAs/InGaAs QD lasers which emit at a wavelength of 1.3 μm and 1.5 μm Such low values of threshold current densities make these lasers possible to create stacked or cascaded QD structures. These structures provide high optical gain for short-cavity transmitters and vertical cavity surface-emitting lasers.

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Optical Communications Multiple Choice Questions on “Non-Linear Effects”.

1. The nonlinear effects in optical fibers are large.
a) True
b) False
Answer: b
Explanation: The nonlinear effect arises from the interactions between light waves and the material transmitting them and thus affects the optical signals. The nonlinear effects are usually small in optical fibers. They have power levels of up to few milliWatts.

2. How many categories of nonlinear effects are seen in optical fibers?
a) One
b) Two
c) Three
d) Four
Answer: b
Explanation: The nonlinear effects are separated on the basis of their characteristics. There are two such categories; one is scattering effect and the other is Kerr effect.

3. Which of the following is not related to Kerr effects?
a) Self-phase modulation
b) Cross-phase modulation
c) Four-wave mixing
d) Stimulated Raman Scattering
Answer: d
Explanation: Stimulated Raman Scattering is related to scattering. The other effects include modulation and mixing which are parts of Kerr effect.

4. Linear scattering effects are _______ in nature.
a) Elastic
b) Non-Elastic
c) Mechanical
d) Electrical
Answer: a
Explanation: Linear scattering effects are elastic because the scattered wave frequency is equal to incident wave frequency. Nonlinear scattering effects are purely inelastic.

5. Which thing is more dominant in making a fiber function as a bidirectional optical amplifier?
a) Core material
b) Pump source
c) Cladding material
d) Diameter of fiber
Answer: b
Explanation: Brillouin gain is always greater than Raman gain. It exists for light propagation in opposite direction to the pump source. Also Brillouin frequency shifts and gain bandwidth are much smaller than Raman. Raman amplification occurs for light propagating in either direction. Thus, pump source is more important in making a fiber function as bidirectional optical amplifier.

6. _________ semiconductor laser sources generally have broader bandwidths.
a) Injection
b) Pulsed
c) Solid-state
d) Silicon hybrid
Answer: b
Explanation: Pulsed semiconductor lasers have broader bandwidths. Therefore, these sources prove to be inefficient pump sources. They prove inefficient especially for narrow gain spectrum.

7. Nonlinear effects which are defined by the intensity – dependent refractive index of the fiber are called as ________
a) Scattering effects
b) Kerr effects
c) Raman effects
d) Tomlinson effects
Answer: b
Explanation: Kerr effects are nonlinear effects. Nonlinear effects are divided into scattering and Kerr effects. Scattering effects include scattering of phonon whereas Kerr effects include intensity refractive index parameters.

8. Self-phase modulation causes modifications to the pulse spectrum.
a) True
b) False
Answer: a
Explanation: Kerr effect results in different transmission phase for the peak of the pulse compared with leading and trailing edges. Self-phase modulation can broaden the frequency spectrum of the pulse as the time varying phase creates a time varying frequency.

9. Self-phase modulation can be used for _____________
a) Enhancing the core diameter
b) Wavelength shifting
c) Decreasing the attenuation
d) Reducing the losses in the fiber
Answer: b
Explanation: Self phase modulation is related to phase change. It imposes a positive frequency sweep on the pulse which in turn enables wavelength or frequency shifting.

10. The beating between light at different frequencies or wavelengths in multichannel fiber transmission causes ________
a) Attenuation
b) Amplitude modulation of channels
c) Phase modulation of channels
d) Loss in transmission
Answer: c
Explanation: Phase modulation is related to frequency and wavelength shifting. In multichannel fiber transmission, phase modulation causes generation of modulation sidebands at new frequencies. This phenomenon is called as four-wave mixing.

11. What is different in case of cross-phase modulation from self-phase modulation?
a) Overlapping but same pulses
b) Overlapping but distinguishable pulses
c) Non-overlapping and same pulses
d) Non-overlapping but distinguishable pulses
Answer: b
Explanation: In cross phase modulation, variation in intensity of one pulse width modulates the refractive index of the fiber which causes phase modulation of the overlapping phases. In self-phase modulation, this phase modulation broadens the pulse spectrum.

12. When three wave components co-propagate at angular frequency w1, w2, w3, then a new wave is generated at frequency w4, which is given by?
a) w4 = w1 – w2 – w3
b) w4 = w1 + w2 + w3
c) w4 = w1 + w2 – w3
d) w4 = w1 – w2 + w3
Answer: c
Explanation: This type of frequency mixing is called as four-wave mixing. This frequency combination is problematic for multichannel optical communication as they become phase matched if the channel wavelengths are near to zero dispersion wavelengths.

13. _____________ results from a case of nonlinear dispersion compensation in which the nonlinear dispersion compensation in which the nonlinear chirp caused by self-phase modulation balances, postpones, the temporal broadening induced by group velocity delay.
a) Four wave mixing
b) Phase modulation
c) Soliton propagation
d) Raman scattering
Answer: c
Explanation: Soliton propagation is a nonlinear dispersion phenomenon. It limits the propagation distance that can be achieved when acting independently. It balances broadening of light pulse.

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Optical Communications Multiple Choice Questions on “Field Measurements”.

1. ____________ affects both the fiber attenuation and dispersion.
a) Refractive index
b) Micro-bending
c) Connectors
d) Splices
Answer: b
Explanation: Effects such as micro-bending with a resultant mode coupling affect both the fiber attenuation and dispersion. It does not provide the overall characteristics of the transmission link.

2. Which of the following is not included in the optical fiber link measurement test?
a) Attenuation measurement
b) Dispersion measurement
c) Splice loss measurement
d) Receiver sensitivity
Answer: d
Explanation: It is necessary to perform some tests on the optical fiber link to enhance productivity. Apart from receiver sensitivity, other measurement methods are required to test the fiber link.

3. In case of field measurements, the equipment must have ___________ power consumption keeping in mind the battery operation.
a) Low
b) High
c) Negligible
d) Maximum
Answer: a
Explanation: The design criteria allows you to distinguish in parameters required for adaptation of battery operation and equipment handling. The power consumption must be low for an equipment to handle.

4. Which of the following are not considered as environmental conditions required for field measurements?
a) Temperature
b) Humidity
c) Mechanical load
d) Power
Answer: d
Explanation: The equipment must be reliable and provide accurate measurements under extreme environmental conditions such as humidity, temperature and mechanical load. Power is an internal factor.

5. Complicated and involved fiber connection arrangements should be _________ in case of field measurements.
a) Provided
b) Avoided
c) Maximized
d) Minimized
Answer: b
Explanation: The equipment must be connected to the fiber in a simple manner. It should be connected without the need for fine or critical adjustment.

6. Which of the following cannot be used in equipment for field measurements?
a) Fiber
b) Connector
c) External triggering
d) Environmental factor
Answer: c
Explanation: The equipment cannot usually make use of external triggering and regulating circuits between the transmitter and receiver. This is because of their wide spacing on the majority of the optical links.

7. Which sensors are used for alteration of spectral range in equipment?
a) Wide-area photodiodes
b) Circulators
c) Gyrators
d) Photogenic sensors
Answer: a
Explanation: Wide area photodiodes such as silicon, germanium diodes are used for alteration of spectral range. It is generally preferred to have a measurement range from -100 dBm.

8. The handheld optical power meter has a measurement accuracy of ________
a) 0.01 dB
b) 0.25 dB
c) 0.8 dB
d) 1 dB
Answer: b
Explanation: The optical power meter detects the fiber type and switches to optical power measurements. It provides an accuracy of about +(or -) 25 dB.

9. _____________ may be used for measurement of the absolute optical attenuation on a fiber link.
a) Silicon photodiodes
b) InGaAsP photodiodes
c) Optical power meters
d) Gyrators
Answer: c
Explanation: Optical power meter employs cut-back technique. It is used for the measurement of the optical attenuation.

10. A large-area photodiode is utilized in the receiver to eliminate any effects from differing fiber and faces.
a) True
b) False
Answer: a
Explanation: The modulating voltage maintains the equilibrium between the transmitter and the receiver side. A large area photodiode is required to eliminate differing fiber and faces to maintain the equilibrium.

11. Optical time domain reflectometry is also called a backscatter measurement method.
a) False
b) True
Answer: b
Explanation: OTDR technique is used in both on-field and laboratory applications. It is also called a backscatter measurement method as it provides the measurement of attenuation on an optical link down its entire length.

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Optical Communications Multiple Choice Questions on “System Design Considerations”.

1. __________ is the unique property of the glass fiber.
a) Transmission
b) Opaque property
c) Ductile
d) Malleable
Answer: a
Explanation: Glass fibres have a unique property as a transmission medium which enables their use in the communication. The major transmission characteristics are dispersion and attenuation.

2. __________ limits the maximum distance between the optical fiber transmitter and receiver.
a) Attenuation
b) Transmission
c) Equipment
d) Fiber length
Answer: a
Explanation: Attenuation along with dispersion and the conductor size are some of the factors that limit the maximum distance between the optical transmitter and the receiver. The associated constraints within the equipment also affect the distance.

3. The ___________ incorporates a line receiver in order to convert the optical signal into the electrical regime.
a) Attenuator
b) Transmitter
c) Repeater
d) Designator
Answer: c
Explanation: Repeaters are a mediator between transmitter and receiver. The weak signal is strengthened back by the repeaters on its path to the receiver.

4. A regenerative repeater is called as ____________
a) Repetitive repeater
b) Regenerator
c) Attenuator
d) Gyrator
Answer: b
Explanation: When digital transmission techniques are used, the repeater also regenerates the original digital signal in the electrical signal before it is retransmitted as an optical signal via a line transmitter.

5. The wavelength range of __________ will be fruitful for the operating wavelength of the system referring to the system performance.
a) 0.8 – 0.9 μm
b) 1.1 – 2 μm
c) 5.2 – 5.7 μm
d) 3.1 – 3.2 μm
Answer: a
Explanation: It is useful if the operating wavelength of the system is established to range of 0.8-0.9 μm. This will be dictated by the overall requirements for the system performance, cost, etc.

6. How many encoding schemes are used in optical fiber communication system design requirements?
a) Three
b) One
c) Two
d) Four
Answer: c
Explanation: Encoding schemes are used for digital transmission of data. These are bi-phase and delay modulation codes. They are also called as Manchester and Miller codes respectively.

7. In ________ the optical channel bandwidth is divided into non-overlapping frequency bands.
a) Time division multiplexing
b) Frequency division multiplexing
c) Code division multiplexing
d) De-multiplexing
Answer: b
Explanation: In FDM, the non-overlapping frequency bands are divided to the individual frequencies. These individual signals can be extracted from the combined FDM signal by electrical filtering at the receiver terminal.

8. A multiplexing technique which does not involve the application of several message signals onto a single fiber is called as _________
a) Time division multiplexing
b) Frequency division multiplexing
c) Code division multiplexing
d) Space division multiplexing
Answer: d
Explanation: In SDM, each signal channel is carried on a separate fiber within a fiber bundle or multi-fiber cable form. The cross coupling between channels is negligible.

9. Which of the following is not an optical fiber component?
a) Fiber
b) Connector
c) Circulator
d) Detector
Answer: c
Explanation: Circulator is a device used in electromagnetic theory. All others are optical components.

10. ________technique involves an increase in the number of components required.
a) Time division multiplexing
b) Space division multiplexing
c) Code division multiplexing
d) Frequency division multiplexing
Answer: b
Explanation: SDM involves good optical isolation due to the negligible cross coupling between channels. It uses separate fiber and thus requires more number of components.

11. Time division multiplexing is inverse to that of frequency division multiplexing.
a) True
b) False
Answer: a
Explanation: TDM involves distribution of channels in time slots whereas FDM involves bands that are run on different frequencies. Both of these techniques improve accuracy and reduce complexity.

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Optical Communications Multiple Choice Questions on “Mid Infrared and Far Infrared Photodiodes”.

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
Explanation: 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?
a) Alloys
b) Attenuator
c) Graded buffer layer
d) APD array
Answer: c
Explanation: 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.
a) True
b) False
Answer: a
Explanation: 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.
a) Dispersion
b) Dislocation
c) Ionization
d) Array applications
Answer: d
Explanation: 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
Explanation: 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
c) Responsivity
d) Absorption coefficient
Answer: c
Explanation: 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?
a) Wavelength
b) Absorption coefficient
c) Responsivity
d) Band gap energy
Answer: a
Explanation: 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
c) Responsivity
d) Quantum efficiency
Answer: d
Explanation: 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.
a) Detector
b) Responsivity
c) Absorption Coefficient
d) Band gap energy
Answer: a
Explanation: 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.

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Optical Communications Multiple Choice Questions on “The Semiconductor Injection Laser”.

1. Stimulated emission by recombination of injected carriers is encouraged in __________
a) Semiconductor injection laser
b) Gas laser
c) Chemist laser
d) Dye laser
Answer: a
Explanation: Stimulated emission by use of optical cavity in crystal structure is used in semiconductor injection diodes. This provides the feedback of protons which gives injection laser many important advantages over other sources like LED’s.

2. In semiconductor injection laser, narrow line bandwidth is of the order?
a) 1 nm or less
b) 4 nm
c) 5 nm
d) 3 nm
Answer: a
Explanation: A narrow line bandwidth of order 1 nm or less is used. This narrow bandwidth is useful in minimizing the effects of material dispersion.

3. Injection laser have a high threshold current density of __________
a) 10⁴Acm^-2 and more
b) 10²Acm^-2
c) 10^-2Acm^-2
d) 10^-3Acm^-2
Answer: a
Explanation: Injection laser have a high threshold current density of 10⁴Acm^-2due to lack of matter and in-efficient light sources. These high current densities are largely utilized in pulse mode in order to minimize junction and thus avert damage.

4. η_T is known as slope quantum efficiency.
a) True
b) False
Answer: b
Explanation: η_D is known as slope quantum efficiency. It gives a measure rate of optical output power change with current and thus it determines slope of output characteristics in the region. So, η_T is referred to as slope quantum efficiency.

5. The total efficiency of an injection laser with GaAs active region is 12%. The applied voltage is 3.6 V and band gap energy for GaAs is 2.34 eV. Determine external power efficiency.
a) 7.8 %
b) 10 %
c) 12 %
d) 6 %
Answer: a
Explanation: The total external power efficiency is defined as
η = η_T(Eq/V)*100
= 0.12 (2.34/3.6) *100
= 7.8 %.

6. In a DH laser, the sides of cavity are formed by _______________
a) Cutting the edges of device
b) Roughening the edges of device
c) Softening the edges of device
d) Covering the sides with ceramics
Answer: b
Explanation: In a DH laser, the sides of cavity are formed by the roughening edges of the device. This is done so as to reduce the unwanted emission in these directions and limit the number of horizontal transversal modes.

7. A particular laser structure is designed so that the active region extends the edges of devices.
a) True
b) False
Answer: a
Explanation: Laser structures are particularly designed so that the active region does not extend beyond the edges. This is done to reduce problems like difficult heat sinking, lasing from multiple filament in wide active areas, unsuitable light output geometry for efficient coupling and also to reduce required threshold current.

8. Gain guided laser structure are __________
a) Chemical laser
b) Gas laser
c) DH injection laser
d) Quantum well laser
Answer: c
Explanation: DH injection lasers are known as gain guided laser structure. This is because the optical mode distribution along the junction plane is decided by optical gain.

9. Laser modes are generally separated by few __________
a) Tenths of micrometer
b) Tenths of nanometer
c) Tenths of Pico-meter
d) Tenths of millimeter
Answer: b
Explanation: The spacing in between modes is about a few tenths of nanometer. The spacing of the modes depends on optical cavity length where each one corresponds to an integral number of lengths.

10. The spectral width of emission from the single mode device is __________
a) Smaller than broadened transition line-width
b) Larger than broadened transition line-width
c) Equal the broadened transition line-width
d) Cannot be determined
Answer: a
Explanation: Single mode device has a smaller spectral width as compared to that of broadening transition line-width. This is because for a single-mode operation, the laser optical output must have only a single longitudinal and single transverse mode.

11. Single longitudinal mode operation is obtained by __________
a) Eliminating all transverse mode
b) Eliminating all longitudinal modes
c) Increasing the length of cavity
d) Reducing the length of cavity
Answer: d
Explanation: Single longitudinal mode operation is obtained by reducing the length L of cavity. Length must be reduced until the frequency separation of adjacent modes is given in the equation
δf = c/2nL is larger than gain curve. Then only single mode falling in transition line width will oscillate in laser cavity.

12. A correct DH structure will restrict the vertical width of waveguide region is?
a) 0.5μm.
b) 0.69 μm
c) 0.65 μm
d) Less than 0.4 μm
Answer: d
Explanation: The vertical width DH structure should be less than 0.4μm. This allows only fundamental transverse mode and removes any interference of higher order transverse modes on emitted longitudinal waves.

13. The external power efficiency of an injection laser with a GaAs is 13% having band gap energy of 1.64 eV. Determine external power efficiency.
a) 0.198
b) 0.283
c) 0.366
d) 0.467
Answer: a
Explanation: The external power efficiency of an injection laser is given by
η_ep = η_T (Eq/V)*100
η_T = η_ep/100 (v/Eg)
= 13/100 (2.5/1.64)
= 0.198.

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