VIVA Questions on Optical fiber.

Hi! Welcome to another post by geekstocode. Today’s topic is another question and answers session. In other words, today we will be looking into Optical Fiber Viva Questions.

Now, this topic is an important concept in electronics and digital communications. Thus, you need to be well versed in this to ace your tests. Lucky for you, we have come up with the important questions you need to prepare for your Lab Viva.

This way, you can learn the topic thoroughly and also prepare for your Viva.

Also go through Microprocessor Viva Questions, Computer Graphics Viva Questions, Digital Communication Viva Questions

So, let us not waste our time and get started Optical Fiber Viva Questions

Table of Contents

Optical Fiber VIVA Questions

1. What do you mean by fiber optic?

We can think of fiber optics as long lenses. In other words, a long cylinder or rod of transparent material forming a core surrounded by an external cladding with a slightly different material.

When light enters the fiber, it rebounds on the outer cladding towards the core. Hence, the light advances through the fiber in bounds or steps, until it exits at the other end.

2. What is Attenuation loss?

We define the Attenuation loss (or path propagation loss) as the reduction in power density (attenuation) of an electromagnetic wave as it propagates through space.

3. Define Bending loss?

Bending loss is the attenuation occurring because of either a bend in an optical fibre that exceeds the minimum bend radius or an abrupt discontinuity in the core/cladding interface.

4. What are the sources that you use in optical communication?

We most commonly use LED’s and LASER diodes as the sources in optical communication systems. It doesn’t matter whether the system transmits digital or analog signal. Therefore, it is often necessary to use a linear Electrical to Optical converter to allow its use in intensity modulation & high quality analog transmission systems.

5. Elaborate the SONET LAYER

The SONET layer consists the following: 

  • Path layer
  • Line layer
  • section layer,
  • Physical layer

6. Tell me about the critical angle.

Critical angle is the minimum angle of incident beyond which the total internal reflection occurs for light travelling from a medium of higher to one of lower refractive index.

7. Define TIR.

At angles of incident greater than the critical angle, the light reflects back into the originating dielectric medium. Hence, we call this phenomenon total internal reflection or TIR. 

8. Give the significance of n2 > n1.

During manufacturing of fiber, certain impurities (dopants) get in the core and cladding, so that the refractive index is slightly higher in core than cladding. Thus, n2>n1.

9. What are leaky modes in optical fiber?

Leaky modes is for quasi-bound waveguide modes with propagation losses due to leakage into the cladding. In other words, a leaky mode or tunneling mode in an optical fiber or other waveguide is a mode having an electric field that decays monotonically for a finite distance in the transverse direction but becomes oscillatory everywhere beyond that finite distance.

10. What are degenerated modes in optical fiber?

If weakly guiding fibers ∆ are small, then HE-EH mode pairs occur which have almost identical propagation constants and electromagnetic patterns such modes are called degenerate modes.

11. How do you define MFD.

MFD or mode field diameter determines the fundamental mode by the radial extend of its electromagnetic field including losses at launching and joining, microbend losses, wave guide dispersion and width of radiation pattern.

12. What are the mechanisms of losses occurs in optical fiber?

The mechanisms of losses that occur in an optical fiber are:

  1. Absorption (fiber material)
  2. Scattering ( fiber material + structural imperfection)
  3. Radiative effects
  4. Pulse broadening.

13. What are macro bend and micro bend losses?

Macrobend losses occurs when radius of curvature is large as compared to fiber diameter. Whereas,  microbend losses are small scale fluctuations in radius of curvature of the fiber axis.

14. Define Lambertian pattern.

Lambertian pattern is the Isotropic pattern from a surface emitter. Also,  in this pattern source is equally bright when viewed from any direction, but power diminishes as cosө.

15. Tell me something about modal or speckle noise.

We define the modal or speckle noise as the speckle pattern observed in multimode fiber as fluctuation which has characteristics times longer than the resolution time of the detector.

16. State the essential components of a laser?

The major components of lasers are:

  • Amplifying medium
  • An optical feedback
  • Finally, the pumping source.

17. Define demountable connectors?

Demountable connectors are the removable joints which allow easy, fast manual coupling and uncoupling of fiber analogous to electrical plugs and sockets.

18. What is polarization?

Polarization is the electric field orientation of a light signal. Also, it can vary significantly along the length of a fiber polarization state. Thus, it is a fundamental property of an optical signal.

19. What are the possible noises that occur in optical receivers?

The possible noises that occur in optical receivers are as follows:

  1. Photo detector noise
  2. Quantum noise
  3. Dark current noise ( bulk dark current, surface dark current )
  4. Amplifier noise
  5. Lastly the thermal noise or Johnson noise.

20. What is a quantum limit?

It is possible that we find the minimum optical power that we receive, required for a specific bit error rate performance in a digital system. Hence,we refer to this minimum received power level as the  quantum limit.

21.Define  LED

LED is a device that converts the electrical signal to optical signal in optical fiber communication

22. What is a photo detector?

A photo detector is an electrical conversion device. Hence, it helps in transmitting the optical signal through optical fiber and retrieves it in its original electrical form.

23. .Why is cladding necessary for an optical fiber?

Cladding is necessary for an optical fiber for the following reasons:

  • Provide proper light guidance inside the core.
  • And to avoid leakage of light from the fiber.

24. State some uses of optical fiber. 

  • Firstly, to transmit the information which are in the form of coded signals of the telephone communication, computer data.
  • And to transmit the optical images. For example, endoscopy.

25.What is an acceptance angle?

Acceptance angle is the maximum angle ɸmax with which a ray of light can enter through the entrance end of the fiber and still be totally internally reflected.

26. What are meridional rays?

Meridional rays are the rays following the zigzag path when they travel through fiber and for every reflection it will cross the fiber axis.

27. Say something about skew rays.

Skew rays are rays following the helical path around the fiber axis when they travel through the fiber and they would not cross the fiber axis at any time.

28. What is mie scattering?

Mie scattering is the scattering created by such inhomogeneities is mainly in the forward direction.

29. What is the significance of dispersion?

Dispersion of the transmitted optical signal causes distortion for both digital and analog transmission along optical fiber

30. What is dispersion flattening? 

Dispersion flattening is the reduction of fiber dispersion by spreading the dispersion minimum out over a wide range of approaches.

31. What is the cutoff wavelength of fiber?

We define the cutoff wavelength as the minimum value of wavelength that can be transmitted through the fiber. Additionally, the wavelength greater than the cutoff wavelength can be transmitted.

32. State the losses responsible for attenuation in optical fibers.

  1. Absorption
  2. scattering
  3. Bending

33. What is fiber splicing? 

Fiber splicing is a Permanent joint formed between two individual optical fibers in the field or factory.

34. What is a fiber coupler?

Optical fiber coupler is a device that distributes light from a main fiber into one or more branch fibers. Thus, we use a directional coupler to combine and split optical signals in an optical network. 

35. State some advantages of LED.

The advantages of LED are as follows:

  1. It has simpler fabrication.
  2. Since, it has simple construction, the cost is low.
  3. It is a very reliable source of transmission. 

36. Why do you prefer laser diode over LEDs for communication applications? 

  • Firstly, It allows high intensity radiation.
  • Because laser diode has narrow spectral width of its laser source which is preferable. 

37. What is dark current noise?

The dark current noise arises from electrons/holes which are thermally generated in the PN junction of the photodiode. Thus, this current continues to flow through the bias circuit of the device when no light is incident on the photodiode. 

38. What are the requirements of an Optical fiber?

The requirements of Optical fiber

  • Light detector
  • Equalizer
  • Also, signal discriminator circuits.

39. What is the threshold level?

A Decision circuit compares the signal in each time slot with a certain reference voltage. This time slot is what we refer to as the threshold level. 

40. What is BER?

It is an approach  to divide the number .No .of. errors occurring over a certain time interval t by the number N of pulses transmitted during this interval.

41. State the advantages of preamplifiers. 

  1. Low noise level
  2. High bandwidth
  3. And, High gain

42. .Tell me something about SONET/SDH.

SONET is a set of standards defining the rates and formats for optical networks. Similarly, Europe has also established a standard. This is the synchronous digital hierarchy (SDH). 

43. What are anomalies?

An anomaly is the smallest discrepancy that we can observe between the actual and desired characteristics of an item.

44. What are the advantages of SDH?

  • High transmission rate
  • Reliability
  • High availability and capacity

45. What is broadcast and select network?

A broadcast and select network is a second generation network in which each transmitter sends messages at a fixed frequency to the central star coupler through a fiber path.

46. What is a single hop network?

In communication, all network nodes connect to the central hub node. We base these single hope WDM networks on central passive star coupler PSC or array waveguide granting(AWG)

47. What is a light path?

A light path is a connection between two end nodes whose traffic we switch only in the optical domain at intermediate nodes.

48. What do you mean by WDM?

WDM is wavelength division multiplexing .The optical beam consists of different wavelengths and several channel information is transmitted over a single channel.

49. Give some important photo detector materials.

The important materials are:

  1. InGaAs
  2. And, InAlGaAs.

50. What are the various error sources?

  • Quantum noise
  • Bulk dark current noise
  • Also, surface leakage current noise.

51. State Snell’s law.

Snell’s law states that the relationship between the angle of incident and angle of refraction for a wave impinging on an interface between two media with different incidences of angle is always constant. 

In other words, it gives a relationship between angle of incident and refraction for a wave impinging on an interface between two different media with different refractive index.


Today we have covered all the optical fiber Viva Questions. Therefore, if you have thoroughly read these questions, you are prepared for your Viva Questions on Optical fiber.

Since we have looked into all the topics around the concept, you can also think that you are ready with the concept of Optical fiber. Thus, make sure to study these properly in order to ace your test.

Also, the internet has made many materials and additional questions available for you ease on the subject. Hence, make sure to go through them. Learning never stops at one destination. 

All the best for your exams and VIVA

We will see you in our next post!

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