1. Fiber Optic Communication Systems
EE411
Fiber Optic Communication Systems
Questions & Answers

2. Give the definition of the refractive index of a material.
This is the relative speed of light as per the light speed in the vacuum.
It is represented by ‘n’
n= Light speed in the vacuum/light speed in a material
n= / light speed in a material (m/s)

3. What are the main differences between MM and SM fibers
What are the main differences between MM and SM fibers? State the main advantages and disadvantages.
In multimode fiber there are more than one mode of propagation. Core sizes are specified as 50 or 62,5, 85, 100, 200µm. Transmission length is limited due to modal dispersion.
In single mode fiber there is only one mode of propagation. Core sizes are limited to less then 10µm. Modal dispersion doesn’t exist, the fiber length is mainly limited by chromatic dispersion.

4. What does dispersion mean
What does dispersion mean? What are the main dispersion problems encountered in MM and SM fibers?
Different modes will travel different distances along the fiber core, so even if they are launched into the fiber at the same time, they will not emerge together. This is called multipath time dispersion.
 Single-mode fiber is the most common choice for long distance communication. It does not have modal dispersion, which distorts the signal pulse at long distances. It still experiences chromatic and material dispersion, and attenuation.

5. What can we do to reduce the modal time dispersion in MM fibers?
We may use graded index fibers instead of step index fibers. Graded-index multimode fiber has the advantage that light rays travel at higher speeds in the outer layers, which have the greater path length.

6. What are the typical cable design used to bundle optical fibers for distribution and installation?
There are two typical cable designs:
Loose-tube cable,
and
Tight-buffered cable, primarily used to distribute fibers inside buildings.

7. What is the reason to perform a cleaving before making any splicing?
The fiber through the center is cleaved and polished during the assembly to improve the light transfer. To connect two fibers, as well as being stripped and totally clean, the end of the fiber must be cut cleanly at right angles. This process is called cleaving. We are looking for the error in this angle to be no more than 1°. Any greater error will give rise to angular losses.

8. What are the main parameters related to the connectors used in fiber optic technology?
Insertion loss
Return loss
Mating durability
Operating temperature
Cable retention
Repeatability

9. What are the polishing styles of the connector ferrules used in fiber optic technology?
Flat finish
Physical contact (PC)
Angled physical contact (APC)

10. What is the advantage of an SC/APC connector polished with a predefined angle?
The APC finish results in very low return losses, It is simply a flat finish set at an angle. The effect of this is that when the Fresnel reflection occurs much of the reflected power is at an angle less than the critical angle and is not propagated back along the fiber.

11. Compare the star and tee type couplers by stating their advantages and disadvantages.(1)
The main advantage of tee couplers is its simplicity. The couplers are readily available and, if required, can be supplied with connectors already fitted.
The disadvantage is the rapid reduction in the power available to each of the workstations as we connect more and more terminals to the network

12. Compare the star and tee type couplers by stating their advantages and disadvantages.(2)
The main advantage of using star couplers is that the losses are lower than a tee coupler for networks of more than three or four terminals
The disadvantage is that the star coupler will normally use much larger quantities of cable to connect the terminals since the star is located centrally and a separate cable is connected to each of the terminals.

13. What are the main parameters that define a light source used as an optic transmitter?
Optical output power,
The center wavelength,
The peak wavelength,
The temperature coefficient of wavelength,
The temperature coefficient of the optical power
The coupling efficiency between a source and a fiber.

14. What are the basic optic detector parameters used as receiver in fiber optic technology? (1)
Responsivity: is the sensitivity of the detector to input flux.
The noise floor : The minimum detectable power, is the ratio of noise current to responsivity.
Quantum efficiency (QE) : is the ratio of the number of electron-hole pairs collected at the detector electrodes to the number of photons in the incident light.
Response time : is the time required for the photodiode to respond to an incoming optical signal and produce an external current.

15. What are the basic optic detector parameters used as receiver in fiber optic technology? (2)
Bandwidth: is the difference between the highest and the lowest frequency that can be transmitted.
Bias voltage: refers to an external voltage applied to the detector.
Active Area and Effective Sensing Area: is the size of the detecting surface of the detection element.
Noise Equivalent Power (NEP): is the amount of flux that would create a signal of the same strength as the RMS detector noise.

16. What are the basic optic detector parameters used as receiver in fiber optic technology? (3)
Wavelength: This is quoted as a range e.g nm to 1600 nm, or by stating the frequency that provides the highest output.
Dynamic range or optical input power : Dynamic range is the ratio of the maximum input power to the lowest.

17. What are limitations of the received power of a fiber optic receiver?
The received power must be high enough to keep the BER to a low value.
The received power must be low enough to avoid damage to the receiver.

18. Optical repeaters or switches. Optical power amplifiers.
How can we extend the transmission distance of a fiber optic link? What are the main units that we can use for this purpose?
Optical repeaters or switches.
Optical power amplifiers.
Preamplifiers.
Mid-span or line amplifiers.

19. What are the reasons to use optical amplifiers and repeaters ?
They are used to overcome the following effects:
Signal degradation.
Pulse spreading due to group velocity dispersion (GVD)
Polarization mode dispersion (PMD)
Signal distortion.
Jitter for data rates above 10 Gbit/s
Amplified spontaneous emission (ASE) noise.

20. What is the meaning of 3R while identifying an optical repeater?
There are three levels of functionality: 
Regeneration (also known as amplification, this insures that the outgoing signal has sufficient power to reach its next destination),
Reshaping (removes pulse shape distortion such as those caused by dispersion),
Retiming (removes timing jitter to improve clock recovery at the receiver).

21. What does EDFA mean? What is the purpose of using it?
This means :
RARE EARTH DOPED OPTICAL FIBER AMPLIFIERS
They are used to amplify optical signals without converting them to electrical signals and converting back again to optical.

22. What are the main types of the WDM systems used in fiber optic networks? What are the main differences between them? (1)
Coarse WDM systems (CWDM) were first developed with only 2–3 wavelengths widely spaced, for example 1300 nm and 1550 nm. This may be useful for networks that require a low cost solution for bi-directional communication on a single fiber. Subsequently, CWDM systems with 4, 8, or 16 channels were developed.
Wide spectrum WDM (WWDM) systems can support up to 16 channels, using wavelengths which are spaced relatively far apart; there is no standardized wavelength spacing currently defined for such systems, although spacing of 1 to 30 nm have been employed.

23. What are the main types of the WDM systems used in fiber optic networks? What are the main differences between them? (2)
Dense WDM (DWDM) employed wavelengths spaced much closer together, with wavelengths near 1550 nm and a minimum wavelength spacing of 0.8 nm (100 GHz).
This may be further subdivided as follows:
First generation DWDM systems typically employed up to eight full duplex channels multiplexed into a single duplex channel.
Second generation DWDM systems employ up to 16 channels.
Third generation DWDM systems employ up to 32 channels..
Fourth generation or Ultra-dense WDM are expected to employ 40 channels or more, channel spacing as small as 0.4 nm (50 GHz) have been selected. This is the largest system currently in commercial production for data communication applications

24. What are the main test instruments used for fiber optic measurements?
These are :
OPM : Fiber optic power meter
OLTS : Optical loss test set. (OPM + optical source)
OTDR : Optical Time Domain Reflectometer.
BERT : Bit Error Rate Test Set
DSO : Digital sampling Oscilloscope

25. What are the main problems while measuring cable loss of a fiber optic link using light source and an OPM? How can we solve them?
Stability problems: the power output of the light must be very stable over the period of the test, typically within 0.1 dB over 1 hour. Reference cables are used for calibration.
When we shine light down an optic fiber, the light source is wider than the core of the fiber and much of the light will enter the cladding. This additional light will give a false indication when we come to measure the losses on a fiber, as the light escapes from the cladding surface. Mode strippers or Mode filters are used to overcome.
Periodic calibration is made for accurate results.

26. Draw the block diagram of a OTDR and explain how does this equipment work?

27. What type of measurements we can perform in an optical link using an OTDR?
Length and attenuation
Fault location
Measuring distances
Measuring refractive indexes of F/O cables.

28. What are the main problems while making measurements with an OTDR
What are the main problems while making measurements with an OTDR? How can we overcome these difficulties?
Dead zones in the display.
To overcome this problem, we add our own patchcord at the beginning of the system.
Ghost echoes (false reflections)
Ghost reflections can be recognized by their even spacing. So they are omitted during evaluation.

29. What are the effects of changing the pulse width while using an OTDR?
Long pulses prevent to detect different events closely located. The minimum distance separating two events that can be displayed separately is called the range discrimination of the OTDR.
Low pulse widths mean good separation of events but the pulse has low energy content, so the maximum range is very poor.

30. What is the purpose to see an ‘’eye diagram’’ through a DSO.
The eye diagram contains the following information about the transmitter:
the relative separation between the logic levels,
rise and fall times,
jitter,
eye height and width,
extinction ratio.

31. END of the QUESTIONS & ANWERS