Optical Fiber Communication Interview Questions and Answers

Get the latest Optical Fiber Communication Interview Questions and Answers with very detailed answers which will be helpful during your interview preparation.

Optical Fiber Communication Interview Questions and Answers

Table of Contents

Que.1 What is an optical communication system?

Answer: The communication system in which electrical information is carried by electromagnetic waves in the optical region ranging from about wavelength of 50 nanometers to approximately 100-micrometer wavelength is called an optical communication system.

Que.2 What are the components of an optical communication system?

Answer: Components of optical communication systems are:

  • Information source: provides an electrical signal to the transmitter comprising of an electrical stage.
  • Transmitter:  drives an optical source to give modulation and processing of carrier light wave.
  • Optical source:  provides electrical to optical conversion. for example- semiconductor laser or LED.
  • Transmission medium: consists of an optical fiber cable.
  • Receiver:  consist of an optical detector and provide optical to electrical conversion. for example- photodiodes, phototransistors, etc.

Que.3 What is fiber cable?

Answer: An optical fiber cable Is a cylindrical dielectric waveguide that operates at optical frequencies. it confines the energy in the form of light within its surface and guides the flow of light in a direction parallel to its axis.

Que.4 What is the working principle of optical fiber?

Answer: Optical fiber works on the principle of Total Internal Reflection. When a light ray encounters a boundary separating two different dielectric mediums, part of the rays is reflected and the remainder is bent or refracted it enters the second medium. so the reflection of light from the boundary when light enters from a denser medium to a rarer medium is known as total internal reflection.

Que.5 What are the advantages of optical fiber communication?

Answer: Advantages of optical fiber communications are

  •  Large bandwidth
  •  Light-weight and small diameter
  •  Low loss.
  •  Less number of repeaters is required.
  •  Low electromagnetic interference.
  •  Low cross-talk.
  •  High security.

Que.6 What are the disadvantages of optical fiber communication?

Answer: Disadvantages are:

  • Not preferable for hilly terrains.
  • initial installation cost is high, hence not preferable for low-density traffic.
  • Lay down the cable in uneven terrain is difficult.

Que.7 What is the significance of the Numerical aperture of optical fiber cable?

Answer: Numerical aperture is the light-gathering capability of the fiber. It is also called a figure of merit of fiber. A high value of numerical aperture is required in order to have a good coupling. If fiber has a low numerical aperture then coupling losses will be large.

Que.8 In what way fiber can be classified?

Answer: Fiber can be classified as- 

  1. On the basis of material used in manufacturing:
    • Glass fibers
    •  Plastic-clad silica fibers
    •  Plastic fibers.
  2. On the basis of the number of modes 
  3. On the basis of the index profile:

Que.9 What is a Step index fiber?

Answer:  Step index fiber is a cylindrical waveguide in which the core is surrounded by uniform cladding and the cladding refraction index is less than that of the core refractive index. As we move from core to cladding there is an abrupt change in the refractive index in the case of step index fiber.

Que.10 How graded index fiber is different from step-index fiber?

Answer:  In graded-index fiber, the core is made up of many layers. the refractive index of the core is highest at the center and decreases as we move away from the core till we reach the cladding.  In this type of fiber, the light waves are bent by refracting towards the core axis to follow the curd path down the fiber length.

 Que.11 Which Optical fiber cable is used for long-distance communication?

Answer: Single-mode step-index fiber is most widely used for long-distance communication because of the minimum dispersion losses and highest transmission bandwidth.

Que.12 What is the application of multimode-graded index fiber?

Answer: Multimode-graded index fiber is used for short-distance communication as they offer less dispersion for propagation.

Que.13 Why the semiconductor materials like germanium and silicon not used for optical communication?

Answer:  Semiconductor materials like germanium and silicon are very useful for electronics but they cannot be used for optical communication because germanium has a high dark current and silicon has a low energy gap so they cannot be used beyond 1100 nanometer wavelength.

Que.14 What are the different optical sources used in optical fiber communication?

Answer:  there are two important optical sources used in optical fiber communication-

  1. Light emitting diode 
  2. Laser diode
  • LED is mostly used in multimode fiber while Laser is used for excitation of mode in single mode fiber.

Que.15 What is the working principle of a Light emitting diode?


  • A light-emitting diode (LED) works on the phenomenon of spontaneous emission when it is forward-biased and conducting current. Radiation from an LED is caused due to the recombination of holes and electrons that are injected into the junction by forwarding bias voltage.
  • One side of the junction is N- type material consisting of mostly free electrons, while another side is P-type material consisting of holes.
  • When no voltage is applied depletion reason is formed at the junction consisting of immobile charge careers. With the application of forwarding bias voltage, Electrons of the N-side move towards the ions near the boundary, and the holes of the P-side move towards the ions just to reduce the width of depletion reason, and also the potential barrier gets reduced. this leads to the flow of conducting current in a LED.

Que.16 What is the working principle of a Laser diode?


  • Laser stands for light amplification by stimulated emission of radiation. it is a highly monochromatic coherent source for optical emission. The laser consists of an active medium that is capable of providing optical amplification and an optical resonator that provides the necessary optical feedback.
  • Laser action is a result of three main processes:
  1. Absorption 
  2. Spontaneous emission
  3. Stimulated emission.
  • When a photon of energy difference impinges on the atom, an electron in the lower energy state can absorb the photon energy and may excite to the higher energy level. This is known as absorption.
  • since the electron at a higher state is unstable, it will try to come down, if it will come without any external stimulation then it is called spontaneous emission. It releases a photon of energy hv.
  • If we externally strike photons of energy difference to the electrons in a higher energy state it gives out two photons that are coherent in nature and thus emit coherent light.

Que.17 Why laser operated near the threshold value of Current?

Answer: The laser operated near the threshold because:

  •  it reduces switch-on delay.
  •  it reduces the heat generated across the junction.
  •  It reduces the nonlinear distortion in the system.
  •  it reduces the noise in the system during ambient conditions.

Que.18 What are the merits of Laser over LEDs?

Answer:  Merits of a Laser over an LED:

  •  The laser diode generates monochromatic coherent light.
  •  Low model dispersion in laser.
  •  They provide higher bandwidth and data rates.
  •  The output power of the laser is much higher hence they can be used for long distances.

Que.19 What are the merits of LEDs over Laser?

Answer: Merits of LEDs over Laser:

  •  Laser is more expensive than LED.
  •  The laser utilizes single-mode fibers which are costlier than multimode fibers and LED uses cheaper multimode fibers.
  •  Lasers have a shorter lifetime than LEDs.
  • LEDs are less temperature sensitive than lasers.

Que.20 What is the working principle of a PIN photodiode?


  • In order to convert the modulated light back into an electrical signal, photodiodes or photodetectors are used. The PIN photodiode is the most widely used photodiode.
  • It works on the principle of photoconduction.
  • An intrinsic layer is lightly doped by N-type material sandwiched between heavily doped p and n regions. light enters through a small window and falls on the intrinsic layer.  when each photon of light falls on the valence band and generates one electron-hole pair also known as charge carriers.  These additional careers enhance the conductivity of the PIN photodiode.

Que.21 Can PIN detectors be operated in both photoconductive and photovoltaic modes?


  • Yes, PIN detectors can be operated in both photoconductive and photovoltaic modes.
  • In photovoltaic mode: no bias is applied to the detector. In this case, the detector works very slowly and the output is approximately logarithmic to the input light. In photoconductive mode, the detector is reverse biased the output, in this case, is currently varying linearly with the input light.

Que.22 How does Avalanche photodiode work?


  •  Avalanche photodiode works on the principle of photoconduction with high reverse bias.
  • because of the high reverse bias, a high electric field is generated causing impact ionization near the breakdown voltage of the junction. During impact ionization, a career gains enough velocity and high kinetic energy to ionize other atoms. This cumulative multiplication of atoms is called avalanche multiplication.
  • The sensitivity of an avalanche photodiode is 10 to  150 times as compared to a PIN photodiode.

Que.23 What are the characteristics of a photodiode?

Answer:  Characteristics of photodiode are:

  1. Dark current: Thermally generated leakage current that flows in the absence of input light is called dark current. It is of the order of nano ampere.
  2. Responsivity: Represents conversion efficiency of the diode in ampere per watts. it is the ratio of output current to the input optical power.
  3. Transit time: Transit time determines the speed of the diode. It decides the maximum bitrate that the diode can detect.

Que.24 What is Wavelength division multiplexing?


  • Optical signals of different wavelengths (1300 to 1600 nm) can propagate without interfering with each other. The scheme of combining a number of wavelengths over a single fiber is called Wavelength division multiplexing.
  • Each input is generated by a separate optical source with a unique wavelength. An optical multiplexer couples light from individual sources to the transmitting fiber. At the receiving end, an optical demultiplexer is required to separate different carriers before the photodetection of the individual signals.

Que.25 What are the Advantages of wavelength division multiplexing?


 The advantages of wavelength division multiplexing are: 

  1. Capacity upgrades:  since all propagating wavelengths support independent data rates in Gbps, by combining all these overall capacity of the system is upgraded.
  2. Transparency:  wavelength division multiplexing can carry fast asynchronous, slow asynchronous, synchronous analog and digital data.
  3. Wavelength switching:  devices like add-drop multiplexers, connectors, and wavelength converters help with wavelength switching in a WDM network.

Que.26 What are Optical couplers? What are the functions of Couplers?


  • Optical couplers are designed to either branch one optical signal into 2 ports or couple 2 optical signals into one port.
  • Functions of optical couplers –
  • Couplers are needed for signal distribution whereas splicers or connectors are used for jointing two fibers.
  • Couplers are used to divide or combine optical signals for application within an optical fiber distribution system. parameters characterizing a coupler are coupling ratio and excess loss.
    • The coupling ratio: is defined as the optical power of the output port divided by the total input power.
    • Excess loss: is defined as the total input power to the total output power.

Que.27 What is the difference between spontaneous emission and stimulated emission?


  • Spontaneous emission:  In this process, the electrons in a higher state release their energy on their own when it comebacks from a higher energy state to a lower energy state naturally. LEDs work on the process of spontaneous emission.
  • Stimulated emission: In this process, a photon having energy equal to the energy between two states, interacts with electrons in the upper state, causing it to return to the lower state with the creation of a second photon. for example- Laser diode.

Que.28  What are the different types of losses that occur in connecting two fibers?


  • Different types of losses that over in connecting fibers are:
  1. Splicer losses: occur when 2 fibers are joined on a permanent basis. Losses should be between 0.05 and 0.1 DB for the splice.
  2. Connector losses: connectors are used to provide removable joints so as to allow easy fast manual coupling and coupling of fibers. connector losses are high and could be 0.2 to 0.3 dB per connector.
  • Fresnel reflection loss-It is associated with the step change and refractive index of the jointed surface.
  • To connect the end of two different fibers their ends must be perfectly smooth and their core must be aligned to each other.

Que.29  How will you define Attenuation in optical fibers?

Answer: Attenuation means the loss of signal power due to propagation. Attenuation takes place in optical fiber due to various reasons like couplers, splicers, and connectors.

Que.30 What are the reasons for attenuation in optical fiber?

Answer: The attenuation inside the fiber is due to various reasons:

  • Intrinsic attenuation: it is the loss of optical power over a distance due to fiber only. it is mainly due to absorption and scattering. Absorption is a natural property of glass. Scattering is due to the loss of optical energy Due to structural imperfections in fiber. scattering losses decrease with an increase in wavelength.
  • Extrinsic attenuation is the loss of optical power due to external sources. it is mainly caused by the bending of a fiber. It occurs due to two reasons:
    • Micro bending losses:  these losses are caused during the cabling process or manufacturing. these losses are not visible to human eyes and can be reduced by a good design.
    • Macro bending: It refers to the loss due to the excessive bending of a fiber. The loss may occur when wrapping the fiber on a spoor or pulling fiber cable around the cable.

Que.31 What is Dispersion?  what are the various types of dispersion?


  • Dispersion is defined as the phenomenon of the spreading of light as it travels from one end of the fiber to another end. A pulse of light with a given width and amplitude when travels through a fiber, its amplitude decreases, and its width increases which lead to intersymbol interference inside the fiber.
  • Dispersion can be classified as:
  1. Intermodal dispersion: This is caused because different rays take a different path in a fiber. When a light pulse is launched in fiber, it is a combination of different rays. If all rays are incident at the same time at the input of fiber, they will reach at different times at the output. This is called Intermodal dispersion. Intermodal dispersion is less in multimode graded-index fiber than that in multimode step index fiber.
  2. Intra-model or chromatic dispersion:  This type of dispersion occurs due to the fact that group velocity is a function of wavelength. Also, the refractive index depends on the wavelength hence it is also called chromatic dispersion.
  • There are two reasons for chromatic dispersion that are:
  1. Material dispersion: This arises because of the variation of the refractive index of the core material as a function of wavelength. Material this person increases with fiber length.
  2. Waveguide dispersion:  This is due to the guiding characteristics of the wave that 80% of the power is confined to the core while 20% in cladding as the core is having higher refractive index and cladding so the wave travels faster in cladding than the core which leads to waveguide dispersion.

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Hello friends, my name is Trupal Bhavsar, I am the Writer and Founder of this blog. I am Electronics Engineer(2014 pass out), Currently working as Junior Telecom Officer(B.S.N.L.) also I do Project Development, PCB designing and Teaching of Electronics Subjects.

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