Optical Communication Lab - Viva Questions

Optical Communication Lab - Viva Questions With Answers

1. What is the basic structure of a modern optical fiber?

The modern optical fiber consists of an optical rod core coated with a cladding. The core and the cladding have different optical characteristics.

2. What are the two principal photo detectors used in a fiber optic line?

 PIN photo diode & Avalanche photo diode (APD)

3. What do you mean by numerical aperture of an optical fiber?

 It is a measure of light collecting (gathering) capacity of the fiber. It is a dimensionless quantity and is less than unity, typical values ranging from 0.14 to 0.5.

4. What are the important specifications of an optical fiber?

(i)Numerical aperture (NA)
(ii) Core diameter
(iii) V-number
(iv) Material of the fiber
(v) Attenuation
(vi) Core refractive index profile (step index or graded index)

5. Which are the different types of fibers?

(i) Plastic fiber
(ii) Glass fiber
(iii) Plastic clad glass fiber
(iv) Halide glass fiber

6. What is a graded index fiber?

 If the refractive index of the core in a fiber is made to vary as a function of the radial distance from the centre of the fiber, it is called graded index fiber. i.e., refractive index decreases as distance increases.

7. What is meant by modes?

The propagation of light energy in an optical fiber takes place at distinct angles of propagation called the modes of propagation or modes.

8. Define internal quantum efficiency of an LED?

The internal quantum efficiency in the active region is the function of electron-hole pairs that recombine radioactively. If the radiative recombination rate per unit volume is RT, and nonradiative recombination rate is Rnr, , the internal quantum efficiency η is the ratio of the radiative recombination rate to the total recombination rate. η = Rr/(Rnr + Rr)

9. What is the difference between intermodal and intramodal dispersion?

Intermodal dispersion : Dispersion caused by the delay between different modes. Typically, it is the delay between the shortest path (zero mode) and the longest path (critical mode).
Intramodal dispersion: Dispersion that is independent of modes, related to the line width of the source and caused by variations in the refractive index as a function of wavelength. 

10. What is the difference between photovoltaic mode and photoconductive mode of operation of photo diode?

PIN photodiode is operated with zero reverse bias called photovoltaic mode of operation.

11. What type of material is used in the detector for regions 800-900 nm and 1100-1600 nm?

800-900 nm : Silicon
1100-1600 nm : InGaAs alloy.

12. Define quantum efficiency of a detector.

Quantum efficiency (η) is defined as the number of electron-hole pairs generated per incident photon energy and is given by = No. of electron-hole pairs generated /No. of incident photons.

13. What are the advantages offered by multimode fiber than a single mode fiber?

The advantages are:
(i) LED can be used as a source
(ii) It convenient for splicing.
(iii) A larger core radii of multimode fibers makes launching optical power into the fiber easier.

14. Define V-number.

V-number is defined by : V = 2πa/λ. N A. It is a dimensionless number gives a measure of the number of modes a fiber can support.

15. What are the advantages of plastic fiber?

The toughness and durability of plastic allow these fibers to be handled without special care. High NA (0.6), large acceptance angle (70°) and large core diameter (110-1400 µm) permit the use of inexpensive LED and make it economically attractive.

16. Define Dispersion.

Dispersion is defined as the pulse spread as function of wavelength. The effect, that causes the output pulse to be wider than the input pulse.

17. What are low loss windows? Give a wavelength for the three low loss windows.

Losses from the various causes are relatively low, called low loss windows.

λ nm
Loss (dB/km)
First window 800-900 nm 
Second window 1100-1350 nm
Third window 1500-1650 nm

18. What are the two main causes of intramodal dispersion ?

Material dispersion
Waveguide dispersion

19. How does material dispersion occur?

Material dispersion occurs because the index of refraction varies as a function of the optical wavelength.

20. What is Lambertian pattern?

In surface emitter LED, the emission pattern is essentially isotropic with 120° half power beamwidth. The isotropic pattern from a surface emitter is called Lambertian pattern. In this pattern, the source is equally bright when viewed from any direction.

21. What are the difference between LED and Laser diode?


(a) Optical output is incoherent
(b) No optical cavity exists for wavelength selectivity
(c) No spatial and temporal coherence
(d) The output radiation has a broad spectral width

Laser Diode

(a) Optical output is nearly coherent
(b) The optical energy is produced in an optical resonant cavity
(c) Optical energy has spatial and temporal coherence
(d) The output beam has very directional and narrow spectral width

22. How is the frequency response of an LED limited?

The frequency response of an LED is limited by its diffusion capacitance because of the storage of injected carriers in the active region of the diode.

23. What are single mode lasers?

Single mode lasers contain only a single longitudinal mode and a single transverse mode. Consequently, the spectral width of the optical emission is very narrow.

24. Define extinction ratio.

It is defined as the ratio of the optical power ouput of a fiber corresponding to binary ‘0’ pulse to power output corresponding to a binary '1’ pulse.

25. What does the term coherent refer to in the optical fiber communication?

 Coherent refers to any technique employing nonlinear mixing between two optical waves.

26. What are the basic performance criteria of the WDM technique ?

Insertion loss
Channel width
Cross talk

27. What are the types of optical amplifiers available?

Semiconductor Laser Amplifier (SLA), Fiber Raman Amplifier (FRA), Fiber Brillouin Amplifier (FBA), Rare Earth ion Doped Fiber Amplifier

28. Why is photodetector called a square law device ?

Because a photodetector converts the received optical power directly into an electric current output (photo current) ?

29. What do you mean by the cut off condition ?

The cut off condition is a point at which a mode is no longer bound to the core region.

30. What is photon life time ?

It is the average time that the photon resides in the lasing cavity before being lost either by absorption or by emission through the facets.

31. Define 3 dB life.

This is the time required for the laser output to drop to one half of its initial value when current passes through the device.

32. What are the characteristics needed for an LED to be used in optical communication?

LED must have a high radiation output, a fast emission response time and a high quantum efficiency.

33. Which material is mostly used for PIN and APDs ?

InGaAs. This material can absorb light with wavelength as long as 1650 nm.

34. What does the laser wavelength depends upon ?

Laser wavelength depends upon the selection of the materials and their energy gap.

Sreejith Hrishikesan

Sreejith Hrishikesan is a ME post graduate and has been worked as an Assistant Professor in Electronics Department in KMP College of Engineering, Ernakulam. For Assignments and Projects, Whatsapp on 8289838099.


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