Optical Fiber Telecommunication Systems

This practical assignment focuses on optical fiber telecommunication systems. It involves designing a dispersion compensation fiber (DCF) for full dispersion compensation in each span of an optical fiber communication system. The assignment also explores the optimization of pre- and post-compensation schemes, considering factors like power variations, EDFA gains, and Q-factor calculations. Furthermore, it delves into limiting non-linear effects by determining appropriate channel spacing to ensure that four-wave mixing (FMW) and walk-off length remain within specified limits. The assignment also touches upon the theoretical limits of information capacity in optical fibers due to nonlinearities and reviews the use of random lasing in long telecommunication fibers. The provided solution includes detailed explanations and calculations, referencing relevant research papers.

Running head: OPTICAL FIBER TELECOMMUNICATION SYSTEMS 1
Optical Fiber Telecommunication Systems
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OPTICAL FIBER TELECOMMUNICATION SYSTEMS 2
1) Design a DFC in order to have a full (100%) dispersion compensation for each span.
Note: the DFC length is not a transmission distance-it does not account for the link
length.
The optimization of pre compensation and post compensation schemes is done following the
SMF and DCF scheme. For 100% dispersion compensation, the powers at the SMF input and
DCF input should be varied systematically by employing the variations in the gains of EDFAs
and Q factor calculated for each set of power after a defined number of cascaded spans (Hiranoet
al, 2001).
The average signal input power should take a range of -6 to 10 dBm into SMF and -25 to +25
dBm into DCF for evaluation. Both the core and the cladding are made from a type of glass
known as silica (SiO2) which is almost transparent in the visible and near-IR. In the case that
the refractive index changes in a “step” between the core and the clades the fiber structure is
known as step-index fiber. The higher core refractive index (~ 0.3% higher) is typically achieved
by doping the silica core with germanium dioxide (GeO2. The Q factor contour plot is obtained
for 2, 5, 10, and 15 for 6. Cascaded spans for 100% pre and post dispersion compensation
schemes for RZ and NRZ modulation set-ups. Those are non-intrusive testing (in- service) used
to monitor live traffic for service-affecting faults, and intrusive testing (out-of service) used for
circuit turn-up and troubleshooting faults. An end-to-end QoS test can be run in order to
determine if performance will degrade over time. The RZ modulation format is considered in the
correspondence of the 50% duty cycle(Hirano et al, 2001). The transmission optimum in this
case will take a particular level of SMF and DCF that has contour plots which are obtained from
Q value which is greater than 15.

OPTICAL FIBER TELECOMMUNICATION SYSTEMS 3
Answer to Question 2:
In order to limit non-linear we need to determine the channel spacing so that
a) FMW is less than 10-5 in both fibers (SMF and DCF)
b) Walk-off length is less than 4km in both fibers (SMF and DCF)
a.
The exponential growth in the rate at which information can be communicated through an
optical fiber is a key element in the ‘information revolution’. However, as for all exponential
growth laws, physical limits must be considered. The nonlinear nature of the propagation of light
in optical fiber has made these limits difficult to elucidate. Here we use a key simplification to
investigate the theoretical limits to the information capacity of an optical fiber arising from these
nonlinearities. The success of our approach lies in relating the nonlinear channel to a linear
channel with multiplicative noise, for which we are able to obtain analytical results. In
fundamental distinction to linear channels with additive noise, the capacity of a nonlinear
channel does not grow indefinitely with increasing signal power, but has a maximal value. The
ideas presented here may have broader implications for other nonlinear information channels,
such as those involved in sensory transduction in neurobiology. These have been often examined
using additive noise linear channel models1 but, as we show here, nonlinearities can change the
picture qualitatively.

OPTICAL FIBER TELECOMMUNICATION SYSTEMS 4
b. Single-mode optical fibers have been widely used in optical communications, and effective
fiber lasers have been designed on the basis of active fibers with linear or ring cavities. In fiber
lasers, the distance between the cavity mirrors can reach 270 km (the maximum length is
determined by the linear attenuation and nonlinear dephasing of the waves). In 2009, random
lasing was found in a long telecommunications fiber without any cavity: the positive distributed
feedback required for lasing is due to Rayleigh scattering of light, and distributed amplification
is provided by stimulated Raman scattering. Such a laser can be classified into the group of so-
called random lasers, actively studied recently; the fiber geometry and the weakness of Rayleigh
scattering provide much better output characteristics compared to the other types of random
lasers. The lasing efficiency and beam quality of this laser are comparable to those of fiber lasers
with a conventional cavity. At the same time, it has a number of unique features (unlimited
length, and mode-free spectrum etc.), providing new physical phenomena and new opportunities
for applications in telecommunications and sensor systems. The paper presents a review of recent
results of studies in this area.
The lasing efficiency of this multi-wavelength laser is very low:
2 W pump produced a 2 mW output at a threshold of 1 W. These results were improved in
where 6 mW. Obtained for nine lines at 15 nm, the configuration was reduced to a half-
symmetric version (as with a circular mirror and a Sagnac interferometer of birefringent PCF
fiber and the basic DCF fiber of length 10 km.
The power and spectral characteristics achieved In this case are close to
Zhang et al. [31] modified the half-symmetric configuration pumping was carried out at λ =
1365 nm, and the reflection gratings for the first (1454 nm) and second (1550 nm)
Stokes components were located at the left end. A DCF fiber length with a higher SRS
gain was added to the SMF fiber, making it possible to use a shortest path possible
The concentrate on the most important propagation nonlinearity, namely the intensity
dependence of the refractive index4 n: n ˆ n0 : n2. Here n0 is the linear refractive index and n2 is a

OPTICAL FIBER TELECOMMUNICATION SYSTEMS 5
constant. The nonlinearity is weak, but its effects accumulate over long propagation distances.
Three principal parameters are of interest: the group velocity dispersion b < 10 ps2 km21, the
propagation loss a < 0:2 dB km21 and the strength of the nonlinear refractive index, g < 1 w21<
km21. The propagation loss is compensated by interposing optical amplifiers into the system,
which inject spontaneous emission noise into the system with strength

OPTICAL FIBER TELECOMMUNICATION SYSTEMS 6
References
Chatzi, S. (2013). Optimization of emerging extended FTTH WDM/TDM PONs and financial
overall assessment.
Hirano, M., Tada, A., Kato, T., Onishi, M., Makio, Y., & Nishimura, M. (2001). Dispersion
compensating fiber over 140 nm-bandwidth. In Optical Communication, 2001. ECOC'01.
27th European Conference on (Vol. 4, pp. 494-495). IEEE
Rothnie, D. M., & Midwinter, J. E. (1996). Improved standard fibre performance by positioning
the dispersion compensating fibre. Electronics Letters, 32(20), 1907-1908.
Nuño del Campo, J. (2013). Novel photonic systems and devices exploiting the Raman effect in
optical fiber.