Faculty of Information and
Engineering Technology
Prof. Dr. Abdelmegid Allam
Bar Code
OFCS (NETW 702)
Three hours
Jan 2011
Final Examination
Please read carefully before proceeding.
1. The duration of this exam is 3 hours
2. Point value of this exam is 40%
3. Only calculators are permitted for this exam
4. This exam booklet contains 18 pages including this one.
Good Luck!
Problem Number
1
2
3
Total
Possible Marks
15
20
34
69
Final Marks
1
NETW702-OFCS Formula Sheet:
OPTICAL FIBER PROPERTIES:
ωo /a = 0.65 + 1.619V -3/2 + 2.879V -6
for SI fiber
2
1/2
ωo /a = (1/n1k)(2/Δ)
for GI near parabolic profile
L=1.7(0.85/λ)4 dB/km , NA = (n12- n22) , ∆ = (n1-n2) /n1
P2 / P1 = 1- ((α+2)/2α∆)[2a/R +(3λ/4πn2R)2/3]
ηSI = (NA)2 min[1,(a /rs )2 ]
ηGI =2n12 ∆[1-(2/(α+2))(rs /a)α]
Lt = 0.75y/a
Losscd= -10log(dR/dT )2
for dR< dT otherwise =0dB
Lossα = -10log (αR(αT+2))/ (αT (αR+2))] for dR< dT
otherwise =0 dB
2
LossNA= -10log(NAR/NAT )
for NAR< NAT otherwise =0 dB
Losssz= -10log[4(ωoR /ωoT+ ωoT /ωoR ) -2 ] dB
σnGI =
M=(λ/c)|d2n1/dλ2
|
σnGI
=
20 √3 c
σ = σ = σ LM
m c λ
2
RECEIVER PERFORMANCE:
ρ = ηeλ/hc [A/W] h= 6.626x10-34 J.sec e=1.602x10-19C
eP / hf  R 
MeP / hf  R 
2
2
SNR 
k= 1.38 x 10-23 J /K
SNR 
L
2eI D  eP / hf f .RL  4kTf
MeP / hf  R 
(M
L
2
2
SNR 
(M
L
n
SNR 

).2eI D  eP / hf f .RL  4kTf
m / 2.MeP / hf  R 
).2eI  eP / hf f .R  4kTf
2
2
(M

).2eI D  eP / hf f .RL  4kTf
L
n
D
L
2
SNR 
PES
PNS  PNT
 e 
2 RL PS PL  
 hf 


ePL  PS 
1    4kT (f )
2eRL (f )  I D 
hf  PL 

PNA  4kTA f
NEP 
2
2
iNSD
 iNT
 f
PNout  GPN  G 4k (Te )f  G 4k ( FT )f
3
Problem 1
[15 Marks]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
a) Put (√) or ( X) in the above table for the corresponding following
Statements
1 Optical receiver is a device that converts, amplifies then processes the
signal to extract the information from the optical signal
2 Shot noises is due to the random generation of the discrete carriers
3 For photodiode with internal gain M, the signal power will increase by a
factor of M
4 Noise equivalent powers is the maximum detectable power which indicates
how sensitive a receiver is
5 The propagation of light in optical fiber can be analyzed using ray analysis if the wave
length is very small relative to the fiber dimensions
6 The power density of an optical beam is proportional to the light intensity
7 Waveguide dispersion is due to dispersive properties of the waveguide material which
causes difference in group velocity between spectral components coupled to a fiber from
an optical source
8 In fused biconical taper couplers, the splitting ratio is independent on the
interaction length and the cladding thickness.
b) Choose the correct answer
9 In baseband D-IM
a)base band signal is modulating the optical source intensity.
b) an electrical demodulation is required.
c) is expensive and difficult to implement.
d)all of the above
10 In coherent transmission
a) the source is multimode spectra
b) the polarization of the optical signal should matches that of the local
oscillator optical signal
c) low LO signal power is required.
d) all of the above
11 Optical demodulation process is required for:
(a) subcarrier intensity modulation
(b) coherent transmission
(c ) D-IM
(d) all of the above
4
12 The shot noise limited SNR is proportional to:
(a) double of optical power
(b) square of optical power
(c) square root of optical power
(d) optical power
13 A fiber line has core index of refraction 1.45, diameter 50μm and its refractive index
fractional difference is 0.012(consider the outer media is air), then the cladding index is
(a) 1.4326
(b) 1.3426
(c) 1.4526
(d) 1.54326
14 The Electric and magnetic field patterns in the transverse plane in a
dielectric slab waveguide are:
(a) A travelling wave pattern
(b) A standing wave pattern
(c) A combination of the a,b
(d) none of the above.
15 The reason for selecting 1.55um for most optical communication system is
(a) best for lowest pulse spread
(b) best λ for laser generation
(c) best λ for lowest attenuation
(d) all of the above
5
Problem 2
[20 Marks]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
a) A PIN detector has responsivity 0.5A/W and 2nA dark current is connected
to 2000Ω load when the system bandwidth is 50 MHz at a temperature 40oC:
1-the value of the received optic power at which the thermal and shot noise
are equal is:
a)54uW
b) 45uW
c) 25uW
d)46.78uW
2-the signal to noise ratio at this power level is:
a) 95.3dB
b)59.3dB
c)63.2dB
d)61.3dB
3-the value of the shot noise power at this power level is:
a)846x10-12W b) 932x10-10W c) 468x10-15W d) 864x10-15W
b) An electronic amplifier has a power gain of 8 and a noise figure of 3 dB
follows a photodetector of responsivity 0.5 A/W and a load resistor of
100 Ω. If the receiver optic power is 0.5uW and bandwidth of 1 MHz at a
temperature 300K then:
4-the signal power flow at the load resistor is:
a) 6.25x10-12W
b) 9.32x10-10W c) 8.64x10-15W d) 4.68x10-15W
5- the signal power at the amplifier output is:
a) 1.5x10-11W
b) 8.9x10-11W
c) 5.0x10-11 W d)1.3x10-11W
6-the thermal noise power at the load resistor is:
a) 5.2x10-14 W
b) 3.5x10-14W
c) 1.55x10-14W d) 1.66x10-14W
7-the thermal noise power at amplifier output is:
a) 2.65x10-13W
b) 1.66x10-12W c) 1.6x10-12 W d)3.5x10-14W
8- the amplifier noise temperature is :
a) 297K
b)320K
c)250K
d)300K
9- the equivalent input thermal noise power is:
a) 3.5x10-13W
b) 2.66x10-12W
c) 365x10-13W d) 0.33x10-13 W
10-the SNR at the amplifier input (neglect the shot noise) is:
a) 30.3dB
b)25.8dB
c) 3.2dB
d)6.13dB
11-the SNR at the amplifier output (neglect the shot noise) is:
a) 21.9dB
b) 22.7dB
c) 23.8dB
d)24.13dB
6
c)An optical source emitting 10 mW at 0.85 um and its gain is 160. A fiber cable with
20dB loss is connected to APD photodetector of responsivity 0.5 A/W and n=2.5. The
detector's dark current is 2nA. The load resistor is 50, the receive bandwidth
is 10 MHz and its temperature is 27oC. The system has 14 dB power loss due
to source coupling and 10 dB power loss due to splices and connectors.
Consider no excess noise, determine the following:
12- the receiver optic power is:
a) 4.68x10-6W
b) 9.32x10-6W
c) 8.64x10-7W d) 0.4x10-6W
13- the detected signal current is:
a) 35uA
b) 32uA
c)25uA
d)30uA
14- the detected signal power is:
a) 468x10-10W
b) 480x10-11W
c) 512x10-10W d) 319x10-12W
15- the shot noise power is:
a) 4.68x10-12W
b)1 19.32x10-13W c) 112.5x10-12W d) 103.62x10-13 W
16- the thermal noise power is:
a)3.68x10-12W
b) 0.32x10-13W
c) 1.66x10-13W d) 2.64x10-13W
17- the signal to noise ratio is:
a) 5196
b) 5969
c) 4863
d) 5296
7
8
9
Problem 3:
[34 Marks]
A) Compute the core diameter for SMSI fiber given the cutoff wavelength is 1250 nm, the
operating wavelength is 1550 nm , the core index is 1.465 and the fractional refractive
index change is 0.0034. Also compute the spot size at the operating wavelength
[5 Marks]
10
B) A multimode step index fiber has a relative refractive index difference of 1% and a core
refractive index of 1.48. The maximum optical bandwidth that may be obtained with a
particular source on a 4.5 km link is 3.1 MHz.
(a) Determine the rms pulse broadening per kilometres resulting from dispersion
mechanisms considering that the light pulses at the output are Gaussian in shape
(b)Assuming waveguide dispersion may be ignored, estimate the rms spectral width of
the source used, if the material dispersion parameter for the fiber at the operating
wavelength is 90 ps nm-1 km-1.
[5 Marks]
11
C) A single mode step index fiber has a core diameter of 7 µm and a core refractive index
of 1.49. Estimate the shortest wavelength of the light which allows single-mode operation
when the relative refractive index difference for the fiber is 1%. It is required to increase
the fiber core diameter to 10 µm while maintaining the single-mode operation at the same
wavelength. Estimate the maximum possible refractive index difference for the fiber
[4 Marks]
12
D) To measure the attenuation coefficient of an optical fiber, the output power from an
optical source is coupled onto the fiber and measured at the other end. If a 10 km long
spool of fiber is used, the received optical power is -20dBm and under identical conditions
but with a 20 km long the received power is -23dBm. Determine the attenuation
coefficient in dB/km and the input power. If an additional attenuation of the source fiber
coupling is 3dB and fiber detector coupling is 1 dB, determine the input power in mW
that keep the output power -20 dBm for 10 km long.
[8 Marks]
13
14
E) Consider the full duplex network shown in the figure. If the directional couplers each
have 3-dB coupling and 1.5 dB of excess loss and all connectors (one at each directional
coupler port, one at the transmitter, and one at the receiver) have 0.8 dB of loss. The
fiber has a 4 dB loss. Compute the total loss (in decibels) from the transmitter to the
receiver.
[4 Marks]
15
16
F) A star network in the figure connects six terminals. The excess loss of the star
coupler is 1.8 dB, connector losses are 0.6 dB, and splice losses are 0.3 dB. Terminal 1,
2, 3, are 80 m from the star coupler using fiber of loss 30dB/km. Terminal 4, 5 and 6
are 30 m from the star coupler using fiber of losses 40db/km .
(a) Sketch the network (include all the connectors and the splices you think you need).
(b) If terminal 1 transmits, compute the total transmission loss (in decibels) to each of
the receivers.
[8 Marks]
17
18