# Sheet (5)

```Elect. & Comm. Engineering Department
ECE571: Optical Fiber Communications
Sheet 5: Optical Fiber Link Design
Dr.-Ing. Saleh Hussin & Dr. Mohamed Assed
1. An Optical fiber communication link has the following specified components:
• Laser diode source with output power of 5.1 m watts.
• Coupling loss of 6 dB.
• 200 Km of single mode optical fiber cable with attenuation of 0.17 dB/km.
• One intermediate EDFA amplifier with 11.5 dB gain.
• Splicing and connection loss equals 0.03 dB per km in average.
• The photo detector sensitivity is -30 dBm.
a. Compute the system gain of the optical fiber link.
b. Compute the link power margin.
c. Let the used laser diode has 3dB-power life time of 5 years. Check if the link
budget will stay feasible after 5 years. Give reasons for your answer.
2. Given the following optical fiber communication link specifications:
• Cable attenuation: 0.17 dB/km, Link length: 500 km.
• Laser diode: optical output power = 7 mw,  = 1.55 𝜇m, Spectral width = 1.5
nm.
• Coupling and splicing losses: 1.6 dB, Optical amplifier gain: 11 dB per unit.
•
Photo detector sensitivity: 1.5 𝜇w.
a. The System gain (without amplifiers) and total link losses.
b. The number of optical amplifiers required to achieve system margin of 10 dB or
better.
c. The optical pulse spreading due to material dispersion.
d. The digital transmission information bit rate for NRZ signaling.
MIU / Elect. & Comm. Dep.
Sheet 5
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Elect. & Comm. Engineering Department
ECE571: Optical Fiber Communications
Sheet 5: Optical Fiber Link Design
Dr.-Ing. Saleh Hussin & Dr. Mohamed Assed
3. An optical fiber system uses a LD with  = 1.2 𝜇m, ∆ = 0.4 nm and 10 dBm to
transmit 155 MBits/sec over a distance of 71 km without repeaters. The optical fiber
cable with is available in maximum lengths of 1 km, with attenuation and dispersion
characteristics as shown in fig. 1 (a) and (b) respectively. The connector loss is 0.8
dB, a splice loss is 0.05 dB, and repair power margin is 5 dB.
While the receiver sensitivity for some types of detectors is depicted in fig. 2-a, and
the dispersion penalty for different bit rate options is depicted in fig. 2-b.
MIU / Elect. & Comm. Dep.
Sheet 5
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Elect. & Comm. Engineering Department
ECE571: Optical Fiber Communications
Sheet 5: Optical Fiber Link Design
Dr.-Ing. Saleh Hussin & Dr. Mohamed Assed
Hint: Dispersion Penalty is the required increase in input power the receiver to
eliminate the degradation caused by dispersion.
• Which one of the illustrated detectors can be used for this link?
• If a system upgrade to operation at 622 Mbits/sec is required at a later date
discuss concisely the impact of this requirement on the fiber specification.
4. An STM-4 optical transmission system is to operate at a wavelength of 1550 nm
over an unrepeated distance of 51 km. The transmitter available has an output power
of 2 dBm, while the receiver has a sensitivity of -28 dBm. Two types of fiber with
different specifications are available as shown in Table 1 below. Two connectors are
to be used in the system with a loss of 0.5 dB each, while the splice loss for both
fiber types is 0.05 dB maximum.
Calculate the dispersion penalty associated with the use of each fiber. Prepare a
power budget for each system and decide which fiber type should be used and why?
5. After a number of years of service an existing single mode fiber is upgraded to a
bit rate of 622 Mb/s at 1550 nm, using a new transmitter and receiver. The fiber span
is 81 km and the fiber have a loss of 0.24 dB per km at 1550 nm and a material
dispersion coefficient equal to 0.007.
The fiber was originally operated at 1330 nm, where the fiber loss was 0.7 dB. The
fiber was installed in lengths 700 m long and the maximum splice loss was 0.05 dB.
The original repair margin was 6 dB, but no repairs have been required. Two
MIU / Elect. & Comm. Dep.
Sheet 5
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Elect. & Comm. Engineering Department
ECE571: Optical Fiber Communications
Sheet 5: Optical Fiber Link Design
Dr.-Ing. Saleh Hussin & Dr. Mohamed Assed
connectors were used with a loss of 0.2 dB each. Two different types of transmitter
Transmitter A has an output power of +3 dBm and a spectral width of 0.35 nm.
Transmitter B has an output power of +4 dBm and a spectral width of 0.47 nm. Both
transmitters are capable of operating at 622 Mb/s. The receiver available for the
upgrade has a worst case sensitivity of -32 dBm.
Complete a power budget for the upgraded system using each of the transmitters in
turn to determine which transmitter should be chosen on the basis of the highest
repair margin. State all of your assumptions clearly.
MIU / Elect. & Comm. Dep.
Sheet 5
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