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## Chapter: Optical Communication and Networking : Fiber Optic Receiver and Measurements

Optical Communication and Networking - Fiber Optic Receiver and Measurements - Important Short Questions and Answers: Fiber Optic Receiver and Measurements

1.  Define minimum detectable optical power.

It is defined as the optical power necessary to produce a photocurrent of the same magnitude as the root mean square of the total current.

2.  Define quantum noise.

It is not possible to predict exactly how many electron-hole pairs are generated by a known optical power incident on the detector is the origin of the type of short noise called quantum noise.

3.  What is meant by error rate?

An  approach  is to  divide  the  number  Ne  of  errors  occurring  over  a certain time interval t by the number Nt of pulses transmitted during this interval. This is called either the error rate or the bit error rate. 4.  Define quantum limit

It is possible to find the minimum received optical power required for a specific bit error rate performance in a digital system. This minimum received power level is known as quantum limit.

5.  Give the classifications of preamplifiers.

o Low impedence(LZ) preamplifier

o High impedence(HZ) preamplifier

o Transimpedence preamplifier

6.  What is meant by excess noise factor?

The ratio of the actual noise generated in an avalanche photodiode to the noise that would exist if all carrier pairs were multiplied by exactly m is called the excess noise factor (F).

7.  What is meant by inter symbol interference (ISI)?

ISI results from pulse spreading in the optical fibre. The presence of this energy in adjacent time slots results in an interfering signal. Hence it is called ISI.

8.     Give the advantages of Pin photodiodes.

Very low reverse bias is necessary

High quantum efficiency

Large bandwidth

Low noise level

9.     What do you mean by thermal noise?

Thermal noise is due to the random motion of electrons in a conductor. Thermal noise arising from the detector load resistor and from the amplifier electronics tend to dominate in applications with low signal to noise ratio.

10. Give the equation for mean square shot noise.

The mean square shot noise is given by 11.            Define multiplication M .

The multiplication M for all carriers generated in  the photodiode is defined by

M=IM/IP

IM : average value of the total multiplied output current

IP : primary unmultiplied photocurrent

12. What is current mode of operation of photodiode?

In photo conducting mode, the photocurrent is  slightly dependent on  the reverse bias. For a constant  reverse  bias, the current  is linear.  This  is called  current mode of operation of the photodiode.

13. What are the system requirements?

The following are the key system requirements.

The desired or possible transmission distance

The data rate or channel bandwidth

Bit error rate (BER)

14.            What are splices? What are the requirements of splices?

The  splices  are  generally  permanent  fiber  joints,  whereas  connectors  are  temporary fiber joints. Splicing is a sort of soldering. The requirements of splices are:

Should cause low attenuation

Should be strong & light in weight

Should have minimum power loss

Should be easy to install

15.            What are the methods of fiber splicing?

There are 3 methods of fiber splicing. They are:

Electric arc fusion splicing or fusion splicing

Mechanical splicing

V-groove splicing or loose tube splicing

16.            What are connectors? What are the types of connectors?

The connectors are used to join the optical sources as well as detectors to the optical fiber temporarily.  They are also used to join two optical fibers.  The 2 major types of connectors are:

Lensed type expanded beam connector

Ferrule type connector

17.            What are the requirements of a good connector?

The requirements of a good connector are as follows:

o   Low loss

o   Repeatability

o   Predictability

o Ease of assembly and use o Low cost & reliability

o   Compatibility

18.            Give the 2 analysis that are used to ensure system performance?

The 2 analysis that are used to ensure system performance are:

Rise time budget analysis

In the optical power loss model for a pt-to-pt link, the optical power rxed at the photo detector depends on the amount of light coupled into the fiber & losses occurring in the fiber at the connectors & splices. The link loss budget is derived from the sequential loss contribution of each element in the link.

Loss=10 log (Pout) / (Pin)

The total optical power loss is,

PT = PS - PR

20. Give the range of system margin in link power budget?

The system margin is usually (6-8) db.  A positive system margin ensures proper operation of the circuit. A negative value indicates that insufficient power will reach the detector to achieve the required bit error rate, BER.

21. What are the system components of system rise time?

The 4 basic system components that contribute to the system rise time are:

o Transmitter (source) rise time

o   Material dispersion time of the fiber

o   Modal dispersion time of the fiber link

All these 4 basic elements may significantly limit system speed.

22. Why the attenuation limit curve slopes downwards to the right?

As the minimum optical power required at the rxer for a given BER becomes higher for increasing data rates, the attenuation limit curve slopes downward to the right.

23. What are the noise effects on system performance?

The main penalties are modal noise, wavelength chirp, spectral broadening, mode- partition noise.

24. Define modal noise?

It arises when the light from a coherent laser is coupled in to a multimode fiber operating at 400Mbps and higher. It mainly occurs due to mechanical vibrations and fluctuations in the frequency of the optical source.

25. What are the measures to avoid modal noise?

The measures are

use LEDs

use LASER having more longitudinal modes

use a fiber with large numerical aperture

use a single mode fiber

26.            Define mode partition noise?

The mode partition noise is associated with intensit y fluctuations in the longitudinal modes of a laser diode. It becomes more pronounced for the higher bit rates.

27. What is meant by chirping?

It means that the dynamic line broadening (line broadening is a frequency chirp) in the laser which oscillates in the single longitudinal mode under CW operation when the injection current is intensity modulated.

28. What is the best way to minimize the chirping?

It is to choose the laser emission wavelength close to the zero-dispersion of the wavelength of the fiber.

29. What is reflection noise?

It is the optical power that gets reflected at the refractive index discontinuities such as in splices, couplers and filters, or connectors. The reflected signals can degrade both the transmitter and receiver performance.

30. What are the effects of reflection noise in high speed systems?

They cause optical feedback which leads to optical instabilities that may lead to inter symbol interference and intensity noise.

GLOSSARY

1.  Minimum detectable optical power.

It is defined as the optical power necessary to produce a photocurrent of the same magnitude as the root mean square of the total current.

2.  Quantum noise.

It is not possible to predict exactly how many electron-hole pairs are generated by a known optical power incident on the detector is the origin of the type of short noise called quantum noise.

3.  Error rate

An approach is to divide the number Ne of errors occurring over a certain time interval t by the number Nt of pulses transmitted during this interval. This is called either the error rate or the bit error rate.

4.  Quantum limit

It is possible to find the minimum received optical power required for a specific bit error rate performance in a digital system. This minimum received power level is known as quantum limit.

5.  Excess noise factor.

The ratio of the actual noise generated in an avalanche photodiode to the noise that would exist if all carrier pairs were multiplied by exactly m is called the excess noise factor (F).

6.  Inter symbol interference (ISI).

ISI results from pulse spreading in the optical fibre. The presence of this energy in adjacent time slots results in an interfering signal. Hence it is called ISI.

7.  Thermal noise.

Thermal noise is due to the random motion of electrons in a conductor. Thermal noise arising from the detector load resistor and from the amplifier electronics tend to dominate in applications with low signal to noise ratio.

8.  Multiplication M .

The multiplication M for all carriers generated in  the photodiode is defined by

M = IM/ IP

IM : average value of the total multiplied output current

IP: primary unmultiplied photocurrent

9.  Current mode of operation of photodiode.

In photo conducting mode, the photocurrent is slightly dependent on the reverse bias. For a constant reverse bias, the current is linear. This is called current mode of operation of the photodiode.

10. Splices.

The splices are generally permanent fiber joints, whereas connectors are temporary fiber joints. Splicing is a sort of soldering.

11. Connectors.

The connectors are used to join the optical sources as well as detectors to the optical fiber temporarily. They are also used to join two optical fibers. The 2 major types of connectors are:

Lensed type expanded beam connector

Ferrule type connector

In the optical power loss model for a pt-to-pt link, the optical power rxed at the photo detector depends on the amount of light coupled into the fiber & losses occurring in the fiber at the connectors & splices.

13. Modal noise.

It arises when the light from a coherent laser is coupled in to a multimode fiber operating at 400Mbps and higher. It mainly occurs due to mechanical vibrations and fluctuations in the frequency of the optical source.

14. Mode partition noise.

The mode partition noise is associated with intensity        fluctuations in the longitudinal modes of a laser diode. It becomes more pronounced for the higher bit rates.

15. Chirping

It means that the dynamic line broadening (line broadening is a frequency chirp) in the laser which oscillates in the single longitudinal mode under CW operation when the injection current is intensity modulated.

16. Reflection noise.

It is the optical power that gets reflected at the refractive index discontinuities such as in splices, couplers and filters, or connectors. The reflected signals can degrade both the transmitter and receiver performance.

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Optical Communication and Networking : Fiber Optic Receiver and Measurements : Important Short Questions and Answers: Fiber Optic Receiver and Measurements |