Exam design of optical communication systems

Examination Questions Design of Optical Communication Systems
© Haris Hassan, Sant ’Anna School of Advanced Studies Page 2 of 5
Multiple questions (check all correct answers)
1) Chromatic dispersion
a) can be theoretically modeled as a filter having a parabolic phase in the frequency domain
b) Can be theoretically modeled as a filter having linear phase in the frequency domain.
c) Cannot be negative (D < 0).
d) depends on signal power
e) Depends on the effective area value.
f) depends on the signal wavelength
g) Indicates that the transit time in a given fiber is a function of the signal wavelength/frequency.
h) is always positive, i.e., D > 0
2) Chromatic dispersion impairments
a) Are the same on one channel if it is alone or if other WDM channels are also transmitted in the same fiber
b) Depend on the channel spacing
c) Depend on the signal chirp
d) Are typically higher is the transmitter is using direct modulation
3) Chromatic Dispersion impairments in an optical system:
a) Cannot be compensated, must be accounted for by statistical means
b) Must be compensated periodically, so that accumulated dispersion never exceeds the maximum acceptable
dispersion
c) Can be compensated periodically or non periodically; it is enough that the accumulated dispersion does not
exceed the maximum acceptable dispersion at the receiver
d) Determined a narrowing of the signal spectrum, therefore the pulses become broader
4) Chromatic dispersion in single mode fibers (SMF)
a) depends on the signal phase
b) can be theoretically modeled as parabolic phase superimposed to the pulse shape
c) is higher than in DSF or NZDF (Dispersion Shifted Fibers and Non Zero Dispersion Fibers, respectively)
5) Chromatic dispersion is
a) A linear effect
b) A linear effect varying in time
c) A nonlinear effect
d) An effect depending on the state of polarization of the channels
e) An effect due to nonlinear crosstalk between WDM channels
f) An effect that can be compensated 100% or to a lower extent, depending on the choice ofthe system designer,
i.e. on the system tolerance
g) An effect that can be modeled as a constant-amplitude filter, but with parabolic phase in the frequency domain
h) An effect that can be modeled as a Gaussian amplitude filter in the frequency domain
i) An effect that introduces different group velocity values for different spectral components
j) An effect that must be compensated 100%, always
6) Chromatic dispersion is due to:
a) interference of multiple modes
b) temperature variations
c) dependence on frequency of the propagation constant
d) mechanical stress

7) Cross Phase Modulation
a) Is related to Kerr effect
b) Is related to the interplay of chromatic dispersion and Self Phase Modulation (SPM)
c) Determines impairments that are significant only if the phase matching is reached
d) Can have an increased impact in single mode fibers
8) Cross Phase Modulation between two channels
c) Is strong if the walk off length is shorter than the effective length
d) Is strong if the walk off length is longer than the effective length
9) Dispersion compensation
a) can always be achieved by proper signal chirping (frequency modulation), for any possible accumulated
dispersion
b) Can be accomplished by using dispersion compensating fibers.
c) can be realized by means of dispersion compensating fibers
d) can be realized by means of polarizers
e) Changes in time.
f) Depends on the amplifier spacing.
g) Induces impairments that are lower at higher bit rate.
h) must be dynamically adapted, since the chromatic dispersion effect changes in time
i) must be realized with a higher precision when the bit rate is higher
j) must be realized with a lower precision when the bit rate is lower
k) should not be needed on Return to Zero (RZ) signals
l) should, in some WDM systems, also account for the wavelength-dependence of the dispersion coefficient (i.e.,
dispersion slope)
10) EDFAs are placed along the link with approximately uniform spacing; the spacing is decided based on
a) the link length
b) the bit rate of the channels
c) the fiber dispersion
d) the PMD coefficient
11) Four Wave Mixing
c) Depends on the phase matching
d) Can have an increased impact when using Unequal Channel Spacing
12) Four wave mixing (FWM)
a) Is a single-channel nonlinear effect.
b) In a WDM system transfers power/energy from signals at shorter wavelength to the signalsat longer
wavelengths.
c) Produces a counter-propagating lightwave at lower frequency than the input signal.
d) Given two signals (at ω1 and ω2) produces two lightwaves at ω1 −∆ω and ω2 +∆ω (∆ω=ω2 −ω1).
13) In a optical system, the design requirements for Polarization Mode Dispersion (PMD) ask that
a) The average DGD experienced by the signal must always be greater than 10% of the bittime
b) The average DGD experienced by signal must be lower than 10% (or 15%) of the bit time

c) The average DGD experienced by the signal must always be greater than the bit time
d) The instantaneous DGD experienced by signal must be 0
14) In a WDM system the effect due to SPM (Self Phase Modulation) on the performance of one of the
channels
a) can be reduced by increasing the launch power
b) Can be reduced by proper choice of the in-line DCF’s (dispersion map).
c) can be reduced by proper design of the dispersion map
d) Depends on how many neighboring channels have a walk-off length Lwo higher than the effective length
(Leff).
e) depends on how many neighboring channels have a walk-off length Lwo lower than the effective length (Leff)
f) depends on the value of chromatic dispersion
g) Depends on the value of PMD.
h) depends on the value of the chromatic dispersion
i) increases with the power of the neighboring channels
j) Is higher in SMF than in NDSF or DSF fibers.
k) That the dispersion map can also reduce the impact of the nonlinear effects
l) The average DGD experienced by most of the WDM channels must be lower than the 10%of the bit time
m) The instantaneous DGD experienced by each WDM channel must always be lower than 10% of the bit time
n) The instantaneous DGD experienced by each WDM channel must always be lower than the bit time
o) The instantaneous DGD experienced by each WDM channel must always be lower than 100% of the bit time
p) The instantaneous DGD experienced by each WDM channel must usually be lower than 50% of the bit time
q) The instantaneous DGD is lower than 10% the bit time
r) The residual dispersion experienced by all WDM channel is lower than a given tolerance
s) The residual dispersion experienced by the central WDM channel is lower than a given tolerance
15) In an optical transmission system the effect due to XPM (Cross Phase Modulation) on the performance of
one of the channels
a) Is higher at lower frequency spacing of the channels
b) Can be substantially different if the transmitter modulator has a negative chirp parameter
c) Does depend on chromatic dispersion
d) Depends on phase matching
16) Modulation Instability
a) Is related to Brillouin effect
b) Is related to the interplay of chromatic dispersion and self-phase modulation (SPM)
c) Depends on phase matching
d) Can have a reduced impact by using unequal channel spacing
17) Noise figure of Erbium doped fiber amplifiers (EDFA)
a) is lower than 3 dB
b) is negative
c) is typically around 4-6 dB
d) is higher than 10 dB
18) On a given signal, the optical noise (ASE noise) at the end of the link
a) Depends on the residual chromatic dispersion
b) Can change the PMD coefficient of the link
c) Depends on the amplifier spacing and their noise figure

d) Affects the final OSNR, thus can affect the system performance
19) On a given signal, the optical noise (ASE noise) at the end of the link
a) Depends on the residual chromatic dispersion
b) Can change the PMD coefficient of the link
c) Depends on the amplifier spacing and their noise figure
d) Affects the final OSNR, thus can affect the system performance
20) PMD (polarization Mode Dispersion)
a) At a given time is characterized by one differential group delay (DGD) value and by a pairof orthogonally
polarized states of Polarization (SOPs)
b) can be compensated by chirped fiber gratings
c) can be completely removed (i.e., made equal to zero) by fiber design
d) Can be found only in dispersion shifted fibers.
e) Determines a differential group delay (DGD) which is varying both in time and in wavelength.
f) Determines impairments that are not sensitive to the power/presence of other WDM channels
g) Determines the amplifier spacing.
h) Gives a system tolerance which can be reduced by signal chirping.
i) gives a system tolerance which is dependent on the signal bit rate
j) gives a system tolerance which is independent of the signal bit rate
k) induces effects which, on average, scale as √𝐿, being L the length of the link
l) induces effects which, on average, scale as 𝐿2
, being L the length of the link
m) Is a static effect
n) Is acceptable if the instantaneous DGD value is lower than 10% the bit time
o) is due to the EDFA in the link
p) is present only in multi-mode fibers
q) is related to attenuation
r) is typically around 25ps/km1/2
s) requires statistically based design criteria
21) Polarization Mode Dispersion (PMD) impairments
a) can be compensated by using DCF
b) can be reduced by nonlinear effects
c) are not constant in time
d) are accounted for by means of statistical rules
22) SBS effect (SBS: Stimulated Brillouin Scattering)
a) is lower at higher power
b) is produced within the first effective length (around 20km)
c) is related to the interaction of counterpropagating waves
d) is related to the interaction of co-propagating waves
23) Self Phase Modulation (SPM)
a) Can be suppressed by frequency dithering
b) depends on signal power
c) depends on the walk off
d) depends on the walk-off parameter
e) Determines impairments that are sensitive to the power/presence of other WDM channels
f) Determines impairments that depend on the nonlinear coefficient γ

g) has no impact if no dispersion is present
h) induces a phase change, which, coupled with chromatic dispersion, gives intensity variations
i) induces only a phase change
j) Is a time varying effect
k) is lower in SMF than in DSF because of the lower γ
l) is lower when the power is higher
24) Stimulated Brillouin scattering (SBS)
a) produces a counter-propagating lightwave at lower frequency than the input signal
b) produces a co-propagating lightwave at the same frequency of the input signal
c) produces a counter-propagating lightwave at higher frequency than the input signal
d) given two signals (at ω1 and ω2) produces two lightwaves at ω1 −∆ω and ω2 +∆ω (∆ω= ω2 −ω1)
25) Stimulated Raman scattering (SRS)
a) can amplify a co-propagating or a counter-propagating lightwave at lower frequency thanthe input signal
b) can amplify a co-propagating lightwave at the same frequency of the input signal
c) can amplify a counter-propagating lightwave at higher frequency than the input signal
d) given two signals (at ω1 and ω2) produces two lightwaves at ω1−∆ω and ω2+∆ω (∆ω= ω2 −ω1)
26) The OSNR at the end of an amplified link depends on
a) The residual chromatic dispersion
b) The PMD and the length of the link
c) The amplifier spacing and their noise figure
d) The signal launch power at the input of each span
27) The OSNR requirements (i.e., the minimum required OSNR) in a given amplified line depend on
a) The amplifier spacing
b) The bit rate, which determines the electrical filter bandwidth
c) The noise introduced by the photodetector
d) The noise figure of the amplifiers
28) The Q factor value, measured after the receiver placed at the end of an amplified fiber link,depends on
a) The residual chromatic dispersion
b) The PMD coefficient and the length of the link
c) The amplifier spacing and their noise figure
d) Only on the noise introduced by the photodetector
29) XPM (Cross Phase Modulation)
a) has the highest impact on Dispersion Shifted Fibers (DSF)
b) has increased impact when frequency spacing between neighbour WDM channels is larger
c) is a single-channel effect
d) is lower if the walk-off length is lower
30) Zero dispersion wavelength is
a) the wavelength at which PMD = 0
b) the wavelength at which D= 0
c) the wavelength of a WDM signal comb at which the performance is the worst
d) around 1300nm in SMF

Questions
1) A link is made of 120km SMF (D= 16ps/nm/km, α = 0.25dB/km, PMD = 0.5ps/ km). First, a single-
channle 10Gb/s system has to be designed and operated, then the system has to be upgraded to 8×10Gb/s.
The sensitivity at BER = 10−9
is S=−30dBm. At the receiver end, a AWG is used to demultiplex the
channels (insertion loss: −3dB). No optical pre-amplifier can be used. As dispersion compensating fiber
(DCF) you can use N DCF modules (L= 3km, α = 0.5dB/km, D=−160ps/nm/km).
a) Determine how many DCF modules are needed in both cases (10Gb/s and 8×10Gb/s).
b) Assuming the DCF is placed before a booster, sketch the scheme of the system and com-pute the minimum
output power (in mW) of the booster, in order to reach the sensitivity in both cases (10Gb/s and 8×10Gb/s).
c) Comment on the role of cross phase modulation and four wave mixing: would you expectany of them to be
important and why?
2) A system of overall capacity of 120Gbit/s has to be installed on a 4√ ×80 km-long fiber link (D=
16ps/nm/km, PMD = 0.2ps/ km). Please discuss how and why you would choose between a 3 × 40Gbit/s
and a 12 × 10Gbit/s system and which system solutions should be implemented in your chosen option.
3) A system with overall capacity 20Gb/s has to be designed on a link made of 1600km of NZDF (D=
1.5ps/nm/km, PMD = 0.4ps/√km, α= 0.2dB/km) with in-line amplifiers. Considering either 8×2.5Gb/s or
2×10Gb/s, specifically indicate for both cases:
a) What can be the dispersion compensation issues (if any).
b) What is the PMD tolerance of the systems.
c) What could be the impact of Four-Wave Mixing and Cross Phase Modulation effects and,if relevant, how you
could cope with them.
Are the two systems equivalent? Would you choose one and why?
[NOTE: Assume that power budget and OSNR issues are already taken into account]
4) A system with overall capacity of 10Gbit/s has to be installed on a 100km-long fiber link (D =
20ps/nm/km, PMD = 1.2ps/√km).
a) Discuss which one you would choose between a 4×2.5Gbit/s and a 10Gbit/s system, explaining your choice.
b) Discuss which system solutions should be implemented in your chosen option.
5) A WDM system having overall capacity of 200 Gbit/s has to be installed on a 10 × 100 km-long fiber link
(D = 0.1 ps/nm/km, dispersion slope: negligible; PMD = 0.03 ps/√km). Please discuss the design issues of
two options, i.e., a 5×40 Gbit/s and a 20×10 Gbit/s system. Assume that both systems should be operating
within a spectral region of 30 nm, and that, according to ITU-T frequency grids, in that region the
channels might be spaced by 200 GHz, 100 GHz, or 50 GHz.
6) A WDM system having overall capacity of 320 Gbit/s has to be installed on a 9 × 100 km-long fiber link (D
= 2 ps/nm/km, PMD = 0.05 ps/√km); please discuss the design issues of two options, i.e. a 8 × 40 Gbit/s and
a 32 × 10 Gbit/s system. Assume that both systems should be operating within a spectral region of 30 nm,
and that, according to ITU-T frequency grids, in that region the channels might be spaced by 200 GHz,
100 GHz, or 50 GHz.
7) A WDM system having overall capacity of 640√Gbit/s has to be installed on a 9 × 100 km-long fiber link (D
= 16ps/nm/km, PMD = 0.15ps/ km). Please discuss the design issues of two options, i.e., a 16×40Gbit/s and a
64×10Gbit/s system. Assume that both systems the channels can be spaced by 100GHz, or 50GHz, if both
options are possible.
8) A WDM system is made of 60 × 10Gbs channels in the C band (35nm), and should be installed over
2×100km of NZDF (D= 1ps/nm/km, α= 0.2dB/km, PMD: negligible). Please discuss:
a) which type of impairment will be the most critical in your opinion, and why;
b) with respect to the above principal impairment, what type of design solutions are necessaryto make the system
work on this link;
c) what are the issues related to the above solutions, i.e., what effects have to be accountedfor in order to
overcome the above principal impairment.

9) Consider an optical communication system having an overall capacity of 120Gbs. The link is made of
dispersion shifted fiber (D= 0.2ps/nm/km, PMD = 0.15ps/km1/2
, α= 0.2dB/km) and is L= 150km long
without in-line amplifiers. With the following assumptions:
output power sensitivity values demux loss
PTX = 0dBm/channel S={
−30dBm @10Gbs
−18dBm@40Gbs
` = 4dB
discuss, when realizing the link either as a 12×10Gbs or a 3×40Gbs system,
a) what kind of amplifiers you would use (if any), having at your disposal both a booster(Pout =13dBm) and a pre-
amplifier (minimum required power at 40Gbs: −25dBm/channel), in order to satisfy the sensitivity
requirements;
b) whether dispersion compensation is required;
c) PMD tolerance and impact of nonlinear effects.
Finally, based on the above choices, tell whether the two systems are equivalent or one of them is preferable.
10) Consider an unusual fiber link having the following parameters:
a) length L= 50km
b) chromatic dispersion coefficient D= 160ps/nm/km
c) polarization mode dispersion coefficient PMD = 30ps/km1/2
Based on the above parameters and considering only linear impairments, please check which of the following single-
channel systems can be successfully installed, and why:
d) 1Gbit/s
e) 2.5Gbit/s
f) 10Gbit/s
g) 40Gbit/s

Exam design of optical communication systems

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