signal loss at optical fiber spliced joints
TRANSCRIPT
SIGNAL LOSS AT OPTICAL FIBER SPLICED JOINTS
A PROJECT REPORT SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE AWARD OF THE DEGREE OF
MASTER OF ENGINEERING (M.ENG) IN ELECTRONIC ENGINEERING (COMMUNICATION OPTION)
BY
MOJEKWU, OKWUCHUKWU EMMANUEL PG/M.ENG/07/43630
UNIVERSITY OF NIGERIA NSUKKA FACULTY OF ENGINEERING
DEPARTMENT OF ELECTRONIC ENGINEERING
DECEMBER 2011
2
i
TITLE PAGE
SIGNAL LOSS AT OPTICAL FIBER SPLICED JOINTS
A RESEARCH PROJECT PRESENTED TO THE DEPARTMENT OF ELECTRONIC ENGINEERING UNIVERSITY OF NIGERIA,
NSUKKA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS OF THE AWARD OF MASTER OF ENGINEERING (M.ENGR) IN
ELECTRONIC ENGINEERING
BY
MOJEKWU OKWUCHUKWU EMMANUEL PG/M.ENG/07/43630
ii
APPROVAL/ CERTIFICATION PAGE
This project was submitted to the Department of Electronic Engineering, University
of Nigeria, Nsukka for the award of the degree of Master of Engineering
(Communication Option)
------------------------------------------------------- ---------------------------------- Mojekwu Okwuchukwu Emmanuel Date PG/M.ENG/07/43630 (Student) ------------------------------------------------------- ---------------------------------- Engr. Prof. A. N. Nzeako Date (Supervisor) ------------------------------------------------------ ---------------------------------- Engr. Prof. O. U. Oparaku, Date (Head of Department) ------------------------------------------------------- ---------------------------------- Engr. Prof J.C. Agunwamba Date (Dean Faculty of Engineering) ------------------------------------------------------- ---------------------------------- (External Examiner) Date
iii
DEDICATION
This work is dedicated to my parents late Mr. Mojekwu Ngoka Emmanuel and late
Mrs. Mojekwu Afubene Patricia, for sowing the seed of excellence in my life.
iv
ACKNOWLEDGEMENTS
I sincerely thank God Almighty, my ultimate source of hope and strength for guiding
me throughout the duration of the study.
I express my earnest and sincere thanks to my supervisor, Engr. Prof. A. N. Nzeako
for painstakingly supervising this work and making constructive criticisms.
Engr. Prof. O. U. Oparaku, the Head of the Department, Dr C. I. Ani, Engr. M. A.
Ahaneku, Engr. Fred Eze, Engr. Okonor Obinna, Mr. Uwakwe and other staff of the
department also share in the glory of my academic training.
My appreciation also to Dr L. N. Binh of Monash University; Clayton Australia for
his help in the use of MATLAB© Simulink to model optical network.
My profound appreciation to Engr. C.C. Nzekwe and Engr. Shola Joseph Omoyode of
Astera Engineering Nigeria Ltd; for allowing me to use their facilities.
I thank Mr. Obiora Okoye, MD of Vast Ltd, Engr. Silas Obinwa and John Mathew ;
NAOC AGIP/Eni-Saipem Spa, Nigeria –Sao Tome Joint Development Authority
(JDA –BLOCK 4) and the Petroleum Technology Development Fund (PTDF) for
their financial assistance and help.
I wish to express my appreciation to my classmates especially Val, Dozie, James,
Rhema, Dubem, Ify and Joy for the great moments we shared together.
Outside the classroom, I thank Chief and Mrs. Uche Mojekwu (Ijeneme I), Engr. D.
C. Mojekwu of the Federal College of Education (Technical) Umunze, Bldr (Sir) and
Lady (Dr) J. A. Ezeuchu of Nnamdi Azikiwe University Awka, Dr (Mrs.) Chinyere
Nwabunwanne, Chukwuma Aguata of McCemag Nigeria Ltd; and Ikechukwu D.
Mojekwu (Opokonja), Mrs. Bridget I. Ezimadu, Dame Mercy Onebunne , Dr A. O. C
Mojekwu, Barr C. C. Mojekwu, Air Commodore C. K. S. Mojekwu, Prof Oby
Okonkwo –Mojekwu, Pharm. Ezeuchu Onyiye and Ngozi Ilebo for help rendered.
v
Members of the Catholic Association of Postgraduate Students, St. Peters’
Chaplaincy, University of Nigeria, Nsukka especially 2007/08 executives where I
served as the image maker including Mbachu Vitalis-the President, Okeke Ogo, Izu
Ndukaihe, Uzoma Osualla, Eboh Frank, Ekwem Oge Hannah, Desmond Nnamani,
Nseh Obong Utoh and Chukwuma Azuka for their constant prayers.
I remember the help I got from Bede Ugwu, Oje Obinna, Ufondu Nwankwo, Nwaka,
Joe Ikenyiri, Tony Nkwocha and Anthony Chibuko.
May God reward your efforts.
vi
TABLE OF CONTENTS
Page Cover page i
Approval/Certification ii
Dedication iii
Acknowledgement iv
Table of Contents vi
List of Figures xi
List of Tables xiv
List of Abbreviations xv
Abstract xvii
Chapter One Introduction 1
1.1 Background of the Study 1
1.2 Statement of Problem 2
1.3 Objectives of the Study 3
1.4 Scope and Limitations of the Study 4
1.5 Thesis Outline 4
Chapter Two Literature Review 5
2.1 Splicing Loss Equations for Single Mode Fibers 5
2.1 Splicing Loss Equations for Multi Mode Fibers 6
2.3 Other Derived Applications for Fiber Splicing Loss Equations 7
2.4 Choice of Analytical Models for Splice Loss 10
2.5 Classification of Losses in Optical Fiber 10
2.5.1 Intrinsic Loss 10
2.5.2 Extrinsic Loss 11
vii
2.6 Optical Fiber Transmission Rates 13
2.7 The Optical Link Architecture 15
2.7 The Optical Link Architecture 16
Chapter Three Methodology 16
3.1 Assessment of Losses at Optical Fiber Joints 16
3.2. Splicing Preparation Steps 16
3.3 Measurement Using the OTDR 19
3.4 Bit Error Rate 22
3.5 Development of Matlab Simulation Blocks 23
3.5.1 Optical Carrier Source Model 23
3.5.2 MZIM Modulator Model 24
3.5.3 Linear Fiber Propagation Model 24
3.5.4 Receiver Model 26
3.5.5 Optical Splice Joint Model 27
3.6.6 Assumptions and Simulation Parameters 28
Chapter Four Measurement, Simulation Results and Analysis 29
4.1 Measurement Procedure 29
4.2 Measured Splice Losses 29
4.3 Calculated Lateral and Angular Misalignment Values 30
4.4 Result of Splice Loss Inspection 31
4.5 Result of the Reflectance (Return Loss) Measurement 33
4.6 Determination of the Optical Link Power Budget 36
4.7 Simulation Results 36
4.7.1 Signal Distortions at Scope 36
4.7.2 Bit Error Rate (BER) 38
viii
4.8 Results of One-Way Anova Statistical Analysis 41
Chapter Five Conclusion and Suggestions 42
5.1 Conclusion 42
5.2 Suggestions 42
References 44
Appendix A Optical Fiber Cable Drum Data Sheet 49
Appendix B Matlab Optical Simulation Model Without Traffic 50
Appendix C Matlab Optical Simulation Model With Traffic 51
Appendix D Amplifier Datasheet 52
Appendix E Analysis of Variance (ANOVA) for Fiber Tubes and Colours 53
Appendix F Splice Loss Equations for Single Mode Fibers 54
Appendix G Typical System Initialisation MATLAB m-file 60
ix
LIST OF FIGURES
Figure 2.500 Intrinsic loss in optical fiber waveguide
Figure 2.501 (a) Extrinsic loss from microbending (b) extrinsic loss from
macrobending
Figure 2.502 (a) Insertion loss from misaligned core diameters (b) insertion loss
from angular misalignment (c) insertion loss from air gap (d) insertion
loss from contamination
Figure 2.700 General optically amplified DWDM point-to-point link propagtion in
optical fiber
Figure 3.200 Semantic of fusion splicing of optical fibers
Figure 3.201 Flow chart for fiber fusion splicing
Figure 3.300 Nanjing DVP-730 arc fusion splicer machine (courtesy of Astera
Nigeria Ltd)
Figure 3.301 Nanjing DVP-105 cleaving machine (courtesy of Astera Nigeria Ltd)
Figure 3.302 Nanjing KL-6200 OTDR machine (courtesy of Astera Nigeria Ltd)
Figure 3.303 Fusion splicing of fiber lengths
Figure 3.400 Eye pattern
Figure 3.500 Sinewave source from Simulink block
Figure 3.501 Block diagram of the linear fiber model
Figure 4.400 Bar chart of splice loss measurement
Figure 4.401 Bar chart of calculated lateral misalignment versus angular
misalignment measurement
Figure 4.500 Return loss at point AA
Figure 4.501 Return loss at point AB
x
Figure 4.502 Return loss at point AC
Figure 4.503 Return loss at point AD
Figure 4.700 Signal scope at 0.01dB (a) before and (b) after splice joint
Figure 4.701 Signal scope at 0.019dB (a) before and (b) after splice joint
Figure 4.702 Signal scope at 0.05dB (a) before and (b) after splice joint
Figure 4.720 Eye diagram (BER) before filtration (a) without load (b) with 100%
traffic
Figure 4.721 Eye diagram (BER) after filtration (a) without load (b) with 100%
traffic
Figure 4.722 Eye diagram (BER) at the receiver (a) without load (b) with 100%
traffic
xi
LIST OF TABLES
Table 2.500 Sources of transmission loss in optical fibers (waveguides)
Table 2.600 Optical transmission rates
Table 3.300 Simulation parameters
Table 4.200 Measured splice loss at points AA, AB, AC, AD
Table 4.300 Calculated lateral and angular misalignment values
Table 4.400 Summary of OTDR readings and measured splice loss readings
Table 4.401 Summary of attenuation readings recorded by the OTDR and qouted
factory attenuation values of the fiber
Table 4.402 Summary of splice loss contribution to overall optical links loss
xii
LIST OF ABBREVIATIONS
Amp Amplifier
BER Bit Error Rate
dB Decibel
dBm Power level in dB
DCF Dispersion Compensation Fiber
Demux Demultiplxer
DWDM Dense Wavelength Division Multiplexing
EM Electromagnetic
EMI Electromagnetic Interference
EMP Electromagnetic Pulses
FFT Fast Fourier Transform
Gbps Giga bits per second
GHz Giga Hertz
GSM General System for Mobile Telecommunications
HF Holey Fiber
IEC International Electrotechnical Commission
IFFT Inverse Fast Fourier Transform
ITU International Telecommunications Union
ITU-T ITU – Telecommunication Standardization Sector
Kbps Kilo bits per second
km Kilometer
LAN Local Area Networks
LED Ligth Emiting Diode
MAN Metropolitan Area Networks
Mbps Mega bits per second
MFD Mode Field Diameter
MHz Mega Hertz
Mux Multiplexer
mW Milliwatt
MZDI Mach Zehnder Delay Interferometer
MZIM Mach Zehnder Interferometric Modulator
NA Numerical Aperture
xiii
NEC National Electrical Code
OC-1 Optical Carrier- level 1
Opt Optical
OTDR Optical Time Domain Reflectometer
QoS Quality of Service
Regen Regenerator
RFI Radiofrequency Interference
Rx Receiver
SDH Synchronous Digital Hierarchy
SNR Signal-to-Noise Ratio
SONET Synchronous Optical Network
SSMF Standard Single Mode Fiber
STM-1 Synchronous Transport Module-level 1
STS-1 Synchronous Transport Signal-level 1
TAM Power coefficient for angular misalignment
TLM Power coefficient for lateral misalignment
Tx Transmitter
UV Ultra Violet
WAN Wide Area Networks
WDM Wavelength Division Multiplexing
µm Micrometer
xiv
ABSTRACT
This study analyses the characteristics of signal loss at optical splice joints. This
includes the investigation of splicing loss contribution to the links overall loss.
Splicing losses from four different points spread along an optical link were measured.
The lateral and angular misalignment loss equations for single mode fiber were
employed to determine the lateral separation distances and angular deviations of each
splice joint from the measured losses at the spliced joints. A one way ANOVA test
performed on the splice losses at p-value of <0.05, firstly on the tube and fiber
colours, indicated that the differences in the mean values among the treatment groups
are not great enough to exclude the possibilty that the difference is due to random
sampling variability; there is not a statistically significant difference on the tube.
However there is a significant difference among the colours. The statistical minimum,
maximum and average of the values splicing losses obtained were used to run a
MATLAB simulation developed with simulink blocks for optically modulated
transmission system. The developed simulation model was monitored with several
scopes which were tapped in the link and the effects of splicing losses on the link
were viewed and analysed. The simulation results indicate that a high splicing loss
will increase the level of signal distortion. The attenuation level observed for the fiber
length shows a slight difference from the factory quoted values. Thus at a sufficient
traffic volume, the distortions will have a significant effect on the overall integrity of
the link’s signals.
1
CHAPTER ONE INTRODUCTION
1.1 Background of the Study To handle the ever increasing demand for high-bandwidth services by
telecommunication users in Nigeria, communication providers deploy optical fibers as
the choice of transmission medium. The incredible advancements in communication
engineering in the country have opened an appetite for new services and more
bandwidth requirment that the traditional communication network has ran short of
bandwidth capacity [2]. In addition to high bandwidth demand, many new services
demand high quality of service (QoS) indices, reliability, availability and real-time
deliverability as well as bandwidth elasticity, and bandwidth on demand [39]. Again
the conventional method of signal propagation using microwave has some negative
impacts on the environment. This comes from pollution coming from exhaust of
generators mounted at base stations and radioactive emmissions from the base station
masts.
Optical fibers are dielectric wave guiding devices used to confine and guide light. A
simple optical network includes a laser diode as an optical source and fiber optic as a
medium of transmission and detector as a part of receiver. To achieve this, optical
network cables are joined at strategic intervals to have a closed communication link.
Joints are inevitable in optical fiber links as it is not economical and viable to have a
single continuous long haul of optical cable for signal propagation that will link the
various components of the optical communication system over a considerable
distance. The choice of joint to be made is based on whether a permanent bond or an
easily demountable joint is desired. A permanent bond on an optical fiber cable is
called a splice while an easily demountable joint is called a connector.
2
1.2 Statement of Problem
The introduction of optical fiber transmission system for GSM and other
communication sevices in Nigeria came with varying degrees of problems. These
include absence of regulatory framework, unsatable electricity (power) supply, lack of
qualified and skilled manpower, communication system compatiblity issues,
environmental impact assessment issues, cost implications for the new technology and
issues of returns on investment.
To install the optical cables either for long haul transmission (backbone) or short
spans of few kilometers, the cable must be joined to have complete communication
link. Fiber joining techniques are subject to certain conditions that cause varying
degrees of optical power loss at the joints. These losses depend on parameters such as
the input power distribution to the joint, the length of the fiber between the optical
source and the joint, the geometrical and wave guide characteristics of the two fiber
ends at the joint and the fiber end qualities [1]. The optical power that can be coupled
from one fiber to another one is limited to the number of modes that can propagate in
each fiber. For example, if a fiber which has a greater number of modes is coupled
(connected) to another fiber with less number of modes, a percentage loss in optical
power is inherent from the first fiber to the second assuming all the modes were
excited. The considerations of analytical models developed to calculate the level of
loss at the joints have not been exhausted as the mathematical equations developed
are merely pessimistic or addressed only a part of the multiple of the causes
responsible for the loss at the joints [1]. This is because it is difficult to predict the
exact loss at the joints as the signals propagate down the link owing to the
unpredictable nature of light waves as it moves along a medium especially in multi
mode fibers [1]. Asides these, practical field working conditions where these
3
installations are carried out do not give room for complicated and complex methods
for the calculation and determination of signal losses emanating from fiber joints [3].
1.3 Objectives of the Study
Attenuation and signal degradation as light signals propagate along a fiber wave guide
is an important consideration in the design of optical communication system as it will
help in determining the unaided (without amplifiers and repeaters) permissible
transmission distance between a transmitter and the receiver [5].
Since amplifiers and repeaters are expensive to manufacture, install and maintain, the
degree of attenuation and signal degradation from fiber joints has a large influence on
overall system performance. These distortion mechanisms in a fiber cause the optical
signal pulses to broaden as they travel along a fiber [1]. If these pulses travel
sufficiently far, they will eventually overlap with neighboring pulses, thereby creating
errors in the receiver output thus reducing the integrity of the optical fiber network.
The project aims at determining the signal loss at optical spliced joints. The splicing
loss equations for lateral and angular misalignments proposed by Marcuse D. [4] was
used to determine the lateral distances and angular separations of the spliced fiber
ends with a view of establishing the level of influence of spliced losses on a selected
optical link. The influence of splicing joints on the optical communication link was
viewed and analysed using MATLAB simulation tool.
As Nigeria leading telecommunication providers plan massive deployment of optical
fiber cable, the study therefore will help in policy formulations for optical fiber
deployment in the country, determine the level of technical competence of splicers,
determine the contributions of splicing machines (as a result of machine
malfunctioning) and level of errors introduced by optical cable supplied by vendors to
the overall link’s loss.
4
1.4 Scope and Limitations of the Study
Predictions of joint losses in fiber are very difficult. For instance analytical models
can predict instanteneous result but fail to give accurate results because of the nature
and behaviors of light waves as it propagate along a fiber after a considerable distance
[10]. Again considering the fact that in practical optical fiber transmission system, the
optical components have to interact with a number of digital and electronic circuits
that have a different characteristic behavior with the fiber cable, mathematical
analysis of the joint losses invoke a lot of assumptions that may be too pessimistic and
therefore introduce errors that mar the accuracy of the results [4]. Providing a hands-
on experience and laboratory experimentations can be costly especially in the field of
optical communication. Thus relying only on hardware manipulations to analyze
optical joints is not economically viable [3].
The scope and limitations of this work therefore involve measurement of splicing
losses at optical joints and the applications of lateral and angular misalignment
models [4] to determine the separation lateral distance and angular misalignments.
1.5 Thesis Outline
This work is further organized as follows; in chapter two a review of literature is
carried out. In chapter three, the methodology and development of simulation models
for the analysis of losses of spliced joints are considered. Measurement, simulation
and results analysis are considered in chapter four. Chapter five treats the conclusion
and suggestions made for further studies. Lastly, the work concludes with references
and appendices.
5
CHAPTER TWO
LITERATURE REVIEW
2.1 Splicing Loss Equations for Single Mode Fibers
Splice loss theory for single mode fiber is well established with detailed studies by
various reseachers. Marcuse D. [4] determined the splice loss for step index fiber. The
analysis is based on the principle that the mode field of the single mode fiber is nearly
Gaussian in shape hence the splice losess are related to the corresponding losses of the
Gaussian beams [4]. The gaussian beam shape of the mode field of a single mode
fiber theory was colloborated by Miller C.M. [5] but emphasised that the use of
Gaussian power distribution model for splice analysis is difficult since joints are
located at least 1km or less apart in most trunks as power distribution would exist
after several kilometers of fibers.(each segment of the fiber has a slightly different
steady state power distribution.) He went further to suggest that the best way for
splice loss characterisation should be by measuring a typical source power loss and
with typical lengths of fibers and splices or connectors preceeding it.
The mode field radius of a Gaussian beam is defined as the radial distance at which
the amplitudes are at 1/e of their peak. Given mode radii of w1 and w2 in the
respective fibers, the splice loss for lateral misalignment (offset) for a perfect splice
alignment that is at d = 0 for a single mode fiber is given by [4]:
2.100
For a Gaussian shaped beam, the loss between an identical fibers having lateral
misalignment (offset) loss, that is at is given by [4]:
2.101
6
where LLS = splice loss due to lateral misalignment; at d=0 or ∞, W = spot size =
mode field diamter [6] and d is the lateral misalignment(separation distance).
Equation 2.101 can be compressed as [7] to yield
2.102
For angular misalignment (fiber with tilt), the splice loss is given by [4] to be
2.103
Equation 2.103 can be compressed by [7] to yield
2.104
where LAS is the splice loss due to angular misalignment for single mode fiber, is the
angular misalignment in radians, w is the Gaussian spot size =MFD [6], n2 is the
refractive index of the cladding, λ is the wavelength of the light.
2.2 Splicing Loss Equations for Multi Mode Fibers
Young M, [53] assumed multi fibers to be illuminated uniformly across the core and
within the acceptance half angle θ, with joint index matching fluid. Since the
illumination is uniform, the transmission in the one dimensional case by inspection
simply gave a lateral or radial misalignment of
2.106
where b is the width of the core (the diameter in real fibers), δ is the lateral
misalignment distance and Tδ is the transmission connection in the direction of the
light in the misaligned fiber which can be approximated to be the splice loss.
For a multimode fiber, the lateral misalignment splicing loss is given by [6] as
2.105
7
where LLM is the splice loss due to lateral misalignment for multimode fiber, d is the
lateral misalignment distance, ɑ is the diameter of the fiber.
And for multimode fiber applications,the splice loss between identical fibers due to
angular misalignment can be expressed as [52]
2.106
For the angular misalignment, [53] assumed that the acceptance half angle of the fiber
to be θ. The numerical apperture, NA is equal to nsinθ, where n is the index of the
core. If the second fiber is inclined at angle to the first, then some of the rays
emitted from the first fiber will fall on the second with angle of incidence greater
than θ. Given the assumption of uniform illumination with a full angle of 2θ, this
implies that:
2.107
where is the is the transmission connection in the direction of the light in the
misaligned fiber which can be approximated to be the splice loss.
2.3 Other Derived Applications for Fiber Splicing Loss Equations
Nemota S. and Makinto T. [43] derived the connector (coupling) loss between single
mode fibers that have unequal mode field diameters, lateral, longitudinal and angular
offsets plus reflections. The derived equation is given as
2.108
where ρ = (kW1)2
q = G2+(σ +1 )2
u = (σ +1 )F2 +2σFGsinθ + σ(G2+σ +1 )Sin2θ
F =
8
G =
σ =
k =
n1 = core refractive index of fibers
n3 = refractive idex of medium between fiber
λ = wavelenght of optical source
d = lateral offset
s = longitudinal offset
θ = angular misalignment
W1 = 1/e mode field radius of transmitting fiber
W2 = 1/e mode field radius of receiving fiber
The derived equation for the single mode coupling loss gave a good correlation with
experimental investigations as reported by [44].
Kihara M. et al [45] derived the connector return loss equation for index matching
fiber. The equation is given as
2.109
This approximation can be extended to determine the return loss for a mechanically
spliced fiber joint provided the index matching gel provided to reduce return (
reflectance loss) do not shrink during usage and thermal expansivity of the gel is of
permisible limit [46].
Varoius researchers such as Lin T.Y. [6] have applied the fiber splice loss equation as
derived by [4] in the design and modeling of fiber connectors. This indicates that the
splice loss equations can be used as good approximation for the fiber connector loss
9
equation. The assumption for the splice loss approximation for the splice connector
are that
1. The numerical appertures (NA) of the connecting fibers are the same.
2. The diameters of the connecting fibers are identical.
3. Matching fluids are used during connecting and further testing.
4. The end faces of the fibers contact.
Lizier J. [7] used the splice loss equation in the determination of splicing loss, spot-
size conversion and coupling factor of Holey fiber material. The splice loss formulae
are used to predict Holey fiber (HF) parameters.
Ieda Koji et al [10] applied splice loss equation of [4] in the determination of splicing
and bending loss characterizations of hole assisted fiber. The splice loss of the hole
assisted fiber (HAF) was investigated taking into consideration the mode field
diameter (MFD) mismatch and the refractive index of the matching material of the
fiber material.
Nakajima Kazuhide et al [47] also applied the splice loss in the design of the mode
field diameter (MFD) characteristics of a hole assisted fiber (HAF) which is
insensitive to bending loss. The splice loss equation of [4] was used because the
designed hole assisted fiber transmission characteristics is compatible to conventional
single mode fiber (SMF).
As noted by [6], the practical measurement of connector or splice loss can be divided
into two namely insertion loss and return loss. These two terms are defined as
2.110
2.111
(Signal Power)in
(Signal Power)out = Insertion Loss
(Signal Power)reflected
(Signal Power)in = Return (Reflectance) Loss
10
These two equations are useful in the experimental measurements since these can
quantify input and output power however these can not be used in theoritical analysis
as these represent the physical phenomenon but no design parameters.
2.4 Choice of Analytical Models for Splice Loss
The choice of the splice loss models in equations 2.101 and 2.103 developed by [4]
were adopted based on the type of the optical fiber deployed in the transmission link
being considered. Certain assumptions in the developed model are applicable only to
single mode fibers and may be extended in the analysis of connector type of joints
provided that certain conditions clearly spelt out for instance in [6] are met.
The splice loss model for gap is not considered since the fusion splicing machine will
ensure that the fiber ends contact properly. The splice loss models for angular (offset)
and lateral (longitudinal) separation are thus employed to analyze the separations in
the fiber.
2.5 Classification of Losses in Optical Fibers
Losses in optical fibers are classified into two namely intrinsic and extrinsic.
2.5.1 Intrinsic Loss
Loss due to inherent traits within the fiber; for example, absorption and
scattering.
Absorption loss is light energy being absorbed in the glass, or more
specifically, the removal of light by non-reradiating collisions with the atomic
structure of the optical core.
Scattering loss is the removal of light due to light being "scattered" after
colliding with a variation in the atomic structure of the optical material.
Insertion loss is the total power loss caused by the insertion of a component
such as a splice or connector in an optical fiber system. Intrinsic loss can also be
11
caused by impure molecules from processing issues, pure, but rare molecules, or
impurity intentionally introduced during processing (doping) [34].
2.5.2 Extrinsic Loss
Loss that is induced in an optical transmission system by an external source. If an
external source introduces loss to an optical fiber medium, then the following classes
of loss are obtained.
Microbending in optical fiber, sharp but microscopic curvatures that create
local axial displacements of a few microns and spatial wavelength displacements of a
few millimeters. One frequent cause is longitudinal shrinking of the fiber buffer [9].
But it also can result from poor drawing or cable manufacturing methods and
installation.
Macrobending occurs when the fiber is bent into a visible curvature. A
relatively large-radius bend in an optical fiber may be found in a splice organizer tray
or a fiber-optic cable that has been bent. A macrobend will result in no significant
radiation loss if it is of sufficiently large radius. This depends on the type of fiber.
Single-mode fibers have a low numerical aperture, typically less than 0.15 [9], and are
therefore are more susceptible to bend losses than other types.
Figure 2.500 Intrinsic loss in optical fiber wave guide [20]
12
(c)
(b)
d
θ
d 2a1
2a1
Figure 2.501 (a) Extrinsic loss from Microbending (b) Extrinsic loss from macrobending [9]
Insertion loss from misalignment of mismatched core diameters
Insertion loss from angular misalignment
Insertion loss from air gap (there is no physical contact between the fibers)
Insertion loss from contamination
Dirts,scratches or chips
Fiber core
(a)
(d)
Figure 2.502 (a) – (d) Different optical insertion (splicing) loss mechanisms [4,20]
13
Transmission loss
Causes
1. Absorption
Intrinsic
Impurity
Defects
- UV Electronic transitions - IR molecular vibrations - Transition metals - Rare earths - Interstitials - Matrix impurities - OH vibrations - H2 vibrations - Vacancies - Radiation induced - Thermally induced - H2 induced
2. Scattering Rayleigh Bulk imperfections Waveguide imperfections Brillouin, Raman
- Minute density and concentration fluctuations - Bubbles, inhomogeneities, cracks - Core, clad interfacial irregularities - Spontaneous
3. Waveguide Macrobending Microbending Design Stimulated Raman,
Brillouin
- Curvature induced - Perturbation induced - Radiative - Depends on power density
Table 2.500 Sources of transmission loss in optical fibers (lightguides) [21].
2.6 Optical Fiber Transmission Rates
The advent of high capacity fiber transmission lines necessitated the establishment of
standard signal format for service providers. The signal formats are called
synchronous optical network (SONET) in North America and synchronous digital
hierarchy (SDH) in other parts of the world. These standards define a synchronous
frame structure for sending multiplexed digital traffic over optical fiber trunk lines.
The first level of SONET signal hierarchy is called the synchronous transport signal-
level 1 (STS-1) with bit level of 51.84 Mbps. Higher rate SONET signals are obtained
14
by byte-interleaving N of these STS-1 frames, which are then scrambled and
converted to an optical carrier- level (OC-N) signal. Thus the OC-N signal will have a
line rate exactly N times that of an OC-1 signal. For SDH systems the fundamental
building block is the 155.52-Mbps synchronous transport module—Level 1 (STM-1).
Again, higher-rate information streams are generated by synchronously multiplexing
N different STM-1 signals to form the STM-N signal. The test optical link where the
splicing process was done have 155 Mbps transmission rate.
SONET level Electrical level SDH level Line rate, Mbps Common rate name
OC-1
OC-3
OC-12
OC-48
OC-192
OC-768
STS-1
STS-3
STS-12
STS-48
STS-192
STS-768
-
STM-1
STM-4
STM-16
STM-64
STM-256
51.84
155.52
622.08
2,488.32
9,953.28
39,813.12
-
155 Mbps
622 Mbps
2.5 Gbps
10 Gbps
40 Gbps
Table 2.600 Some commonly used Optical SONET and SDH information
transmission rates [1].
2.7 The Optical Link Architecture
A general optical link may contain all or part of the components mentioned above.
The choice of component depends on the nature of the signals to be handled and
design requirements. The Wavelength Division Multiplexing (WDM) is the optical
technology that couples many wavelength carrier light waves transmitting over the
same single mode fiber. This increases the aggregate bandwidth per single fiber to
integrate the bit-rate of all wavelength channels. The Dense Wavelength Division
Multiplexing (DWDM) technology offers larger (denser) number of wavelengths
15
coupled into a fiber compared to WDM with closer spacing between carries of the
channels [17].
Figure 2.700 shows the DWDM architecture adopted in the implementation of the
MATLAB simulation.
In-line OA wide band for multiwavelength
Opt fiber
mux demux Opt ADM
Tx
Tx
Tx
Tx
Regen
Regen
Regen
Regen
Rx
Rx
Rx
Rx
WDM mux WDM demux
amp
Figure 2.700 General optically amplified DWDM point-to-point link [12,15,17]
16
CHAPTER THREE
METHODOLGY
3.1 Assessment of Losses at Optical Fiber Joints
There exist three possibilities in the assessment of losses at optical fiber joints. These
include:
Measurement surveys using appropriate instrumentation like optical time
domain reflectometer, OTDR. Nanjing Model KL-6200 OTDR was used to
measure the attenuation of the fiber cable against the quoted attenuation
results at the point of manufacture. Also splicing losses at selected points
along the test fiber link were measured and used to calculate the lateral
separation values and angular deviations at the measured spliced joints.
Employing computational methods in order to simulate the propagation
scenarios. Due to the absence of optical spectrometer , the Matlab® Simulink®
event environment was used to model and view graphically, the influence of
splicing loss as the signals propagate down the optical link.
By using simplified mathematical analytic methods. The splicing loss
equations 2.101 and 2.103 developed by Marcuse D [4] were used to obtain
the lateral separation distances and angular deviations of the measured spliced
joints once the measured splice loss values were known.
3.2 Splicing Preparation Steps
Fusion splices are made by thermally bonding prepared fiber ends, where the fiber
ends are first aligned and then butted together. This is done either in a grooved fiber
holder or under a microscope with a micromanipulator [40]. The butt joint then is
heated with an electric arc or a laser pulse so that the fiber ends are melted
momentarily and hence bonded.
17
The following steps are taken when splicing joints at fiber ends:
Step 1: Preparing the fiber - Strip the protective coatings, jackets, tubes, strength
members, etc. leaving only the bare fiber showing. The matching gel is
wiped with spirit or any other appropriate cleaning solvent. The main
concern here is cleanliness of the surface of the fiber lengths.
Step 2: Cleave the fiber - the cleave precision is critical. Fiber tip is also kept free
from contamination to help couple the light from one fiber end to the other.
Step 3: Mechanically join the fibers - Simply position the fiber ends together inside
the splice unit and allow the splicing machine to weld the two fibers
together.
Step 4: Protect the fiber - the completed splice is provided with protection by ferrule
which houses the joint and then heated for the proper bonding. The spliced
length is then kept in a protective encapsulation.
Step 5: The encapsulation is then placed firmly using fasteners at the pole or buried
underground in a trench.
Electric arc or laser fusion welder
Figure 3.200 Semantic of fusion splicing of optical fibers [1]
Micromanipulatable fiber holders
18
Yes
No
No
Yes
Fiber cutting/ removal of protective
coating, jacket
Check if fiber is
damaged?
Fiber cleaning with solvents
and removal of matching index
oil
Fiber tip preparationn
Cleaving
Fusion splicing
Is the splice loss ≤ 0.05dB
Ferrule alignment and setting
Tray arrangement and encaspulation
Final cover up and fiber duct protection
End
Figure 3.201 Flow chart for fiber fusion splicing
Start
19
3.3 Measurement Using the OTDR
The optical time domain reflectometer is used to make single ended measurements in
optical link characteristics and faults tracing. Such characteristics include fiber
attenuation, connector and splice losses reflectance levels from link components and
chromatic dispersion [2]. The OTDR fuctions by injecting a series of optical pulses
into the fiber under test. It also extracts, from the same end of the fiber, light that is
scattered (Rayleigh backscatter) or reflected back from points along the fiber. The
strength of the return pulses is measured and integrated as a function of time, and is
plotted as a function of fiber length.
To get the attenuation level of the optical power in the link,the OTDR was connected
by a patch cord to the joined fiber end, the optical power of the OTDR was lauched.
The process is repeated at the end of fiber length to see if there is any significant
differences in the reading. It may be possible to have different readings due to the fact
that adjacent fibers may have different backscatter coefficients, so the second fiber
reflects more light than the first fiber, with the same amount of light travelling
through it. If the OTDR is placed at the other end of this same fiber pair, it will
measure an abnormally high loss at that joint. However if the two signals are then
combined, the correct loss will be obtained.
For this reason, it is common OTDR practice to measure and combine the loss from
both ends of a link, so that the loss of cable joints, and end to end loss, can be more
accurately measured.
The readings obtained were recorded and taken to the PC-based software to perform
easy data collection and sophisticated data analysis. The screen shots of the values
obtained are shown and tabulated in the next chapter.
20
Figure 3.300 Nanjing DVP-730 arc fusion splicer machine (courtesy of Astera Nigeria Ltd)
Figure 3.301 Nanjing DVP-105 cleaving machine (courtesy of Astera Nigeria Ltd)
21
Figure 3.302 Nanjing Model KL-6200 OTDR machine (courtesy of Astera Nigeria Ltd)
Figure 3.303 Fusion splicing of fiber lengths
22
“0”
“1”
3.4 Bit Error Rate
The bit error rate (BER) is used as a performance index for error rate analysis in
digital systems. The developed simulation model system performance BER is
analysed using Q-factor from the eye diagram which is expressed as [16] contained in
[12]
3.400
where is the magnitude of the eye opening shown in figure 3.30. are
fractions of and . The former are defined for Gaussian pulse shape as [16]
contained in [12] by
Figure 3.400 Eye pattern [17]
3.401
One bit Length
µ1- µ0
Signal with noise
Noise margin
Good sampling
period Jitter
Noise
23
3.5 Development of MATLAB Simulation Blocks
The simulation blocks that will be employed in viewing the optical link will be
limited to the transmitter (source and the modulator), splice joint, fiber cable, and the
receiver.
3.5.1 Optical Carrier Source Model
The ideal laser source model in the Simulink block is achieved by selecting the sine
wave block [28]
Figure 3.500 Simulink sine wave source block [35]
The block models the desired sinosoidal optical carrier obtained from an ideal laser
which takes the mathematical form [12]
3.500
where and are the frequency and phase of the optical carrier respectively,A is
the amplitude, with = 2π x 1.93x1014 rad/s corresponding to the 1550 nm
operating wavelength of the laser source. The amplitude is set for unity for simplicity.
Due to the discrete nature in which MATLAB stores and processes data; one needs to
sample this waveform and other signals to ensure synchronization at certain intervals,
T, in order to process the data through the system correctly [12]. The sample time
fields of some blocks are required, the Nyquist sampling time is inserted here. This
technique maintains the integrity of the signals. According to the Nyquist theorem,
this sampling interval is at least twice the highest frequency in the system. Thus:
3.501
3.502
24
where B is highest sampling frequency.
3.5.2 MZIM Modulator Model
The optical laser source is now injected with a modulated MZIM (Mach Zehnder
Interferometric Modulator) carrier frequency which is modeled according to the
equation [12]
3.503
When multiplied by π gives a value between 0 and π. This value is used to implement
the required phase shifting of the optical carrier. The laser source and MZIM
complete the transmitter section of the optical link.
3.5.3 Linear Fiber Propagation Model
The developed model achieves propagation accuracy by implementing fiber
dispersion effects using time-domain digital signal processing and filtering technique
that has been proven to be efficient in its computational resources. The split step
model has been implemented in [13] and as contained in [12].This fiber propagation
model considers the effects of fiber attenuation and dispersion compensation on the
system performance. The mathematical representation is given by
3.504
where
3.505
L is the fiber length, λ the operating wavelength, v the optical frequency and c the
speed of light. From these equations it can be shown that the equivalent model for the
single mode fiber is expressed by the transfer function as [14,26,27,36]:
3.506
25
Hout (f)
Hin (f)
Since equ 3.506 is in the form of a frequency domain transfer function, it is more
convenient to operate in the frequency domain as apposed to taking the convolution in
the time domain. Determining the output of the fiber (f) given an input
modulated signal (f) (where the ^ symbol refers to the Fast Fourier Transform
(FFT) of xin and xout) is found by:
3.507
Figure 3.501 Block diagram of the linear fiber model [13]
Thus by taking the FFT of the input modulated signal in Simulink then multiplying by
H(f) and finally taking the IFFT (inverse FFT) one can accurately represent fiber
propagation with any additional chromatic dispersion, thus making the model linear.
For the standard single mode fiber (SSMF) fiber model one uses D(λ) SSMF =
+16.744 ps/nm.km at 1550 nm wavelength, L = 90km) with no optical amplifiers and
for the dispersion compensation fiber (DCF) model, smaller length is assume where,
D(λ) DCF = -85 ps/nm.km, L = 17 km. This value of dispersion for the DCF cancels
the dispersion effects of the SSMF.
Given the fiber manufacturers specifications quote of an attenuation level of
0.185dB/km attached in appendix A, this implies a total of 15.17dB attenuation of
optical power after 90km. This results in power attenuation (Simulink gain block) of
0.012 approximately. This value is taken into account in the simulations of the SMF
Simulink model via the Gain block at the top of the model.
H (f)
26
The other fiber models are identically the same with modifications to the parameter
values made as desired for dispersion compensation.
3.5.4 Receiver Model
This model of the receiver attempts to simulate, to within Simulink capabilities, some
of the key principles allowing for the successful receipt of signal. The optical carrier
is implemented by the MZDI, (Mach Zehnder Delay Interferometer). A receiver
model (block: Post Tx tap) is also place before fiber propagation to allow for
comparisons between pre and post fiber effects in eye diagram form. Phase offset and
photodiode/ amplifier noise models are incorporated into the receiver component to
represent the optical to electrical conversion performed by the single-ended
photodiode [13].
The MZDI characteristic is modeled according to the expression
3.508
where
The final term, in equation 3.508 is referred to as phase offset.
The detection of transmitted lightwaves is performed primarily by the photodetector.
In most instances, the received optical signal is quite weak and thus electronic
amplification circuitry is used, following the photodiode, to ensure that an optimized
power signal-to-noise ratio (SNR) is achieved [12]. This power signal to noise ratio is
calculated as follows
3.509
where Isig denote the photocurrent and as the mean squared noise
contributions from the photodetector. Three sources of optical receiver noise include
shot noise ish, the PD dark current noise idk and the thermal (Johnson noise) ith. The
27
total current generated by the photodiode when optical power falls on it is expressed
by [12]
3.510
where
It has been demonstrated that both the shot noise and dark current noise contributions
from the bulk material of the photodiode follow a Poisson process, and is thus random
[12]. The noise sources are expressed mathematically as
3.511
The photodiode amplifier datasheet shown in appendix D as used by [12,15] specify
the values for simulation.
Exact signal recovery is not the major concern of the simulation, hence observed
disparity between the input wave form at the scope of the source and the receiver.
However, inclusion of optical demodulator would have taken care of that.
3.5.5 Optical Splice Joint Model
The model for the splice joint was developed using constant block in Simulink. The
value of the constant block represents distance of the optical link. The gain block
which represents the measured splice loss value was used to multiply the constant
block thus allowing the value of the measured splice loss with respect to optical link
distance value to be represented. For instance, if the link distance is 90km; a constant
block with the value 90 was used and at splice loss value of 0.05dB, a gain block with
value 0.05 was also used to develop the splice joint model.
28
3.3.6 Assumptions and Simulation Parameters
Field Measurement
1. The Numerical Apertures (NA) of the connecting fibers are identical to
allow complete coupling of light.
2. The diameters of the connecting fibers are identical since the fibers ends
were once in the same length.
3. The fibers did not expand in any form as no environmental factor affected
the fiber. This is however too pessimistic as fibers both those buried
underground and those kept on open surface continually under
deformation due to several factors including temperature changes, applied
force on the fiber bundle, etc
Simulations
1. Exact signal recovery was not the major concern of the simulation.
2. There are no transmitter losses.
3. Actual system design configurations in fiber communications are different
from the arrangement of some developed block models in the simulation.
Parameter Magnitude Level of distortion
SSMF (fiber) 90 km (length) +16.744ps/nm.km
DCF (fiber) 17 km (length) -85ps/nm.km
Carrier Frequency 2*pi*1.93*10^14 rad/s -
Wavelength 1550 nm -
Bit Number 256 b -
Speed of Light 3*10^8 m/s -
Sampling Time 2.59*10^-15 sec -
Simulation Time 2.59*10^-12 sec -
Table 3.300 Simulation parameters. CHAPTER FOUR
MEASUREMENT, SIMULATION RESULTS AND ANALYSIS
29
4.1 Measurement Procedure
A Nanjing DVP-105 cleaver was used for the cleaving process. The cleaver machine
has error margin of ± 0.05%. Positioning accuracy and precision were achieved
during cleaving operation by placing well cleaned fiber ends appropriately at the
cutting edge (position) of the cleaver. The fiber ends were inspected for proper
cleaves and bad ones re-cleaved again. The cleaver was wiped intermittently to
remove fiber ends as this may contaminate new fiber lengths being cleaved.
A Nanjing DVP-730 fusion splicer with splice time of 1800mS was used for the
optical fiber fusion splicing. The output of the completed fusion splicing process was
indicated by the LCD panel of the splicing machine. A threshold value of splice loss
of ≤0.05dB was implemented. This was adopted to reduce the splice loss contribution
to the link’s loss. Several values exceeded this value and were discarded. The splicing
process was repeated again using the flow chart shown in figure 3.201 until the
splicing of the fiber lenghts were completed.
The splicing machine was switched off and the splicing programme initialised. This
was done when the machine was recording exceedingly high splicing loss values. The
end power setting of the arc of the splicing machine was steadly maintained as this
ensured that heated fibers melted and bonded homogeneously. This ensured that
splices with un-optimised splicing paramters (errors) such as fuse current too hot, fuse
time being too long, low and high auto fed were avoided. By inspection the lateral and
angular misaligned fiber spliced joints were separated. The splicing loss values
obtained at the cut sections of the fiber cable are shown in table 4.200.
30
4.2 Measured Splice Losses
Tube Number
F.No Fiber Colour
Splice Points AA(dB) AB(dB) AC(dB) AD(dB)
A(Blue) 1 Blue 0.01 0.01 0.02 0.02 2 Orange 0.02 0.02 0.02 0.02 3 Green 0.02 0.02 0.03 0.02 4 Brown 0.04 0.02 0.04 0.02 5 Grey 0.01 0.02 0.01 0.02 6 White 0.02 0.02 0.01 0.02 7 Red 0.03 0.03 0.01 0.02 8 Black 0.03 0.03 0.01 0.02 9 Yellow 0.02 0.03 0.01 0.01 10 Violet 0.01 0.02 0.01 0.03 11 Pink 0.02 0.01 0.01 0.02 12 Turquoise 0.01 0.01 0.01 0.01
B(Orange) 13 Blue 0.02 0.02 0.02 0.02 14 Orange 0.03 0.01 0.02 0.03 15 Green 0.01 0.02 0.03 0.02 16 Brown 0.02 0.03 0.01 0.01 17 Grey 0.02 0.03 0.01 0.01 18 White 0.03 0.02 0.02 0.01 19 Red 0.03 0.02 0.02 0.01 20 Black 0.02 0.03 0.03 0.01 21 Yellow 0.05 0.03 0.03 0.01 22 Violet 0.02 0.03 0.01 0.01 23 Pink 0.03 0.04 0.02 0.01 24 Turquoise 0.01 0.01 0.04 0.01
C(Green) 25 Blue 0.03 0.02 0.03 0.02 26 Orange 0.02 0.01 0.02 0.02 27 Green 0.03 0.01 0.02 0.01 28 Brown 0.01 0.02 0.01 0.01 29 Grey 0.01 0.02 0.01 0.02 30 White 0.01 0.03 0.03 0.03 31 Red 0.01 0.03 0.01 0.03 32 Black 0.01 0.03 0.02 0.02 33 Yellow 0.02 0.04 0.01 0.01 34 Violet 0.03 0.01 0.01 0.02 35 Pink 0.04 0.02 0.02 0.01 36 Turquoise 0.02 0.02 0.01 0.01
D(Brown) 37 Blue 0.02 0.01 0.01 0.01 38 Orange 0.02 0.01 0.03 0.03 39 Green 0.02 0.01 0.02 0.03 40 Brown 0.03 0.01 0.02 0.03 41 Grey 0.03 0.02 0.03 0.02 42 White 0.01 0.03 0.03 0.02 43 Red 0.02 0.03 0.03 0.01 44 Black 0.01 0.02 0.04 0.02 45 Yellow 0.01 0.01 0.01 0.02 46 Violet 0.02 0.02 0.01 0.01 47 Pink 0.02 0.01 0.01 0.02 48 Turquoise 0.02 0.01 0.01 0.02 Max 0.05 0.04 0.04 0.03 Min 0.01 0.01 0.01 0.01 Avg 0.0208 0.0204 0.0187 0.0175
Table 4.200 Measured splice loss at points AA, AB, AC and AD
31
4.3 Calculated Lateral and Angular Misalignment Values Tube Number
Fiber colour
F No
d1 (nm) d2 (nm) d3 (nm) d4 (nm) θe1 (0) θe2 (0) θe3 (0) θe4 (0)
A (Blue)
Blue 1 0.48 0.48 0.68 0.68 - - - - Orange 2 0.49 0.69 0.69 0.69 - - - - Green 3 0.69 0.69 - 0.69 - - 0.03 - Brown 4 - 0.69 - 0.69 0.04 - 0.04 - Grey 5 0.49 0.69 0.49 0.69 - - - - White 6 0.69 0.69 0.49 0.69 - - - - Red 7 - - 0.49 0.69 0.03 0.03 - - Black 8 - - 0.49 0.69 0.03 0.03 - - Yellow 9 0.69 - 0.49 0.49 - 0.03 - - Violet 10 0.49 0.69 0.49 - - - - 0.03 Pink 11 0.69 0.49 0.49 0.69 - - - - Turquoise 12 0.49 0.49 0.49 0.49 - - - -
B (Orange)
Blue 13 0.68 0.68 0.68 0.68 - - - - Orange 14 - 0.48 0.68 - 0.03 - - 0.03 Green 15 0.48 0.69 - 0.69 - - 0.03 - Brown 16 0.69 - 0.48 0.48 - 0.03 - - Grey 17 0.69 - 0.49 0.49 - 0.03 - - White 18 - 0.69 0.69 0.48 0.03 - - - Red 19 - 0.69 0.69 0.49 0.03 - - - Black 20 0.69 - - 0.49 - 0.03 0.03 - Yellow 21 - - - 0.49 0.04 0.03 0.03 - Violet 22 0.70 - 0.49 0.49 - 0.04 - - Pink 23 - - 0.69 0.49 0.03 0.02 - - Turquoise 24 0.50 0.50 - 0.50 - - 0.04 -
C (Green)
Blue 25 - 0.69 - 0.69 0.03 - 0.03 - Orange 26 0.69 0.49 0.69 0.69 - - - - Green 27 - 0.49 0.69 0.49 0.03 - - - Brown 28 0.49 0.69 0.49 0.49 - - - - Grey 29 0.49 0.69 0.49 0.69 - - - - White 30 0.49 - - - - 0.03 0.03 0.03 Red 31 0.49 - 0.49 - - 0.03 - 0.02 Black 32 0.49 - 0.69 0.69 - 0.04 - - Yellow 33 0.69 - 0.49 0.49 - 0.02 - - Violet 34 - 0.49 0.49 0.69 0.03 - - - Pink 35 - 0.69 0.69 0.49 0.04 - - - Turquoise 36 0.69 0.69 0.49 0.49 - - - -
D (Brown)
Blue 37 0.70 0.50 0.50 0.50 - - - - Orange 38 0.70 0.50 - - - - 0.02 0.03 Green 39 0.70 0.50 0.70 - - - - 0.03 Brown 40 - 0.50 0.70 - 0.03 - - 0.03 Grey 41 - 0.71 - 0.71 0.03 - 0.03 - White 42 0.50 - - 0.71 - 0.03 0.03 - Red 43 0.71 - - 0.50 - 0.03 0.03 - Black 44 0.50 0.71 - 0.71 - - 0.03 - Yellow 45 0.50 0.50 0.50 0.71 - - - - Violet 46 0.71 0.71 0.50 0.50 - - - - Pink 47 0.72 0.50 0.50 0.72 - - - - Turquoise 48 0.72 0.50 0.50 0.72 - - - -
Table 4.300 Calculated lateral and angular misalignment values Table 4.300 was generated using excel worksheet. where the values of d and are obtained for the various splice points respectively.
32
Recall that:
from equ 3.281 and 3.282
from equ 3.287 an 3.288
π = 3.1415, n2 = 1.457 (for silica based fiber)
w = MFD [6] (see appendix A)
4.4 Result of Splice Loss Inspection
The results of splice loss measurements (figure 4.400) and calculated lateral and
angular misalignment(figure 4.401) values are shown in accordance with [6,10,11].
The total measurement samples was 240 which corresponds to the total number of
splice joints. The splice loss recorded 0.02dB as the highest occuring value indicating
a good splicing workmanship. However splicing should be geared towards achieving
no loss. The maximum and minimum value of 0.05dB and 0.01dB were recorded
respectively. The calculated average for the splice loss inspection was 0.019dB.
Loss (dB)
0.00 0.01 0.02 0.03 0.04 0.05
Freq
uenc
y (n
umbe
r of o
ccur
ence
)
0
20
40
60
80
nm240 Samplesmax = 0.05dBavg = 0.019dBmin = 0.01dB
Figure 4.400 Bar chart of splice loss measurement
33
Angular misalignment Lateral misalignment
Freq
uenc
y (n
umbe
r of o
ccur
ence
)
0
20
40
60
80
100
120
140
160
Figure 4.401 Bar chart of calculated lateral misalignment versus angular
misalignment measurement.
From the bar chart in figure 4.401 for the splice loss inspection, it can be seen that
lateral misalignment occurred more frequently than the angular misalignment.
Possible reasons for this are the level of positioning accuracy achieved by the
cleaving machine which is used in cutting the fiber and fusion efficiency of the
splicing machine.
Achieving lower splicing loss may be hard due to time factor and avoidance of
material wastage. The former is of extreme importance especially during emergency
repairs operation in the network to reduce loss of revenue due to service down time.
34
4.5 Result of the Reflectance (Return Loss) Measurement
The return (reflectance) loss measurement test was carried out using the optical time
domain reflectometer (OTDR) machine on tube A of the fiber. The colour of the fiber
is blue. This is only the link currently in use as the others are redundant or awaiting
leasing.
Figure 4.500 Return loss at point AA
It is observed that the splice loss recorded is 0 dB while the measured splice value
was 0.01dB. The discrepancy in the loss is the error introduced by the patchcord. The
slope with high spike indicates splice joint(s) with dirts (contaminations).
Figure 4.501 Return loss at point AB
35
The screen shot at point AB shows that the splice loss is 0.085 dB at reference
kilometer of about 10km.
Figure 4.502 Return loss at point AC
Figure 4.503 Return loss at point AD
As with the observed trend, the measured average at point AB is 0.02 dB. The return
loss is 0.149 dB at a reference distance of about 12km. At 40km, the return loss is
0.077dB. The induced error can be attributed to marginal errors introduced by the
OTDR machine, chromatic dispersion losses from the fiber cable, micro and macro
36
bending losses emanating from the fiber. Table 4.500 shows the difference between
OTDR readings (first reference) and measured splice loss values
S/No Location OTDR reading (dB) Measured splice loss (dB) Difference (%)
1 AA - 0.01 -
2 AB 0.085 0.01 17.64
3 AC 0.106 0.02 0.57
4 AD - 0.02 -
Table 4.500 Summary of OTDR readings and measured splice loss readings
Similarly, table 4.501 shows the the summary of the attenuation readings recorded by
the OTDR and fiber drum quoted values (see appendix A)
S/No Location OTDR attenuation (dB) Quoted attenuation value (dB) Difference (%)
1 AA 0.192 0.186 3.22
2 AB 0.189 0.186 1.61
3 AC 0.187 0.186 0.53
4 AD 0.186 0.186 0.00
Table 4.501 Summary of Attenuation readings recorded by the OTDR and qouted
factory values of the fiber (see appendix A)
S/No Location Links loss (dB) Splice Loss (dB) Contribution (%) 1 AA 1.821 0 -
2 AB 1.605 0.085 5.20
3 AC 6.625 0.106 1.60
4 AD 2.310 0.149 6.45
Table 4.502 Summary of Splice loss contribution to overall optical links loss
37
4.6 Determination of the Optical Link Power Budget
The optical link power buget is determined by establishing the minimum power to fall
on the photodiode in order to ensure a certain bit error rate (BER). The light coupling
efficiency of the transmitter, the loss of the fiber and the sequntial loss contributions
of each element in the link determine the power received at the detector. Power
budget can be expressed by [54] as :
Ptr = Prec + Ploss + Msys 4.600
where Ptr is the average transmitted power, Prec is the average received power, Ploss is
the system lost power and Msys is the system margin or safety factor. Ptr is also
referred to as minimum transmit power and Prec minimum receive sensitivity [55].
The system lost power, Ploss is given by [54] as
Ploss = α f L + αcon + αsplice 4.601
where α f L is the attenuation of the fiber in dB/km (given in appendix A), L is the
fiber length , αcon is the connector losses and αsplice is the splice losses.
Given that Ptr = 32.366dBm and Prec =1.821dBm
Ploss = (4*0.05dB + 0.019*90 + 0.185*90) =18.36dB
Msys = 32.366-18.36-1.821 = 12.185dB
This value will help to determine the transmission power threshold and replacement
policy of fiber cable in the link.
4.7 Simulation Results
4.7.1 Signal Distortions at Scope
The statistical averages obtained in the field measurement were used to model the
magnitude of signals of the splice loss at the splice points in the absence of optical
spectrometer. The mininum, average and maximum values obtained from readings at
38
the different splice points were simulteneously changed in the simulation model.
Signal scopes were attached before and after the splice joints to observe the signal
behaviour as it enter and leaves the joints.
Figure 4.700 Signal scope at 0.01dB before and after spliced joints
Figure 4.601 Signal scope at 0.019dB before and after spliced joints
The magnitude (strength) of the signal at the scope was the major distinguishing
factor for the various splice points. As shown in figures 4.700 (a) and (b) through
4.702 (a) and (b), the signal distortion magnitude increases as the value of splice loss
4.700(a)
4.700(b)
4.701 (a)
4.701 (b)
39
increases. This is because an increased splice loss value will increase the distortion
level of the optical link. When there is traffic in the link, the scopes can be
discriminated and viewed by increasing the signal amplitude in the source lest there
will be no significant differences.
Figure 4.702 Signal scope at 0.05 dB before and after spliced joint.
4.7.2 Bit Error Rate (BER)
Figure 4.720 Eye diagram (BER) before filtration (a) without load and (b) with 100%
Traffic
4.702 (a)
4.702(b)
4.720(a)
4.720 (b)
40
Figure 4.721 Eye diagram (BER) after filtration (a) without load and (b) with 100%
traffic
Figure 4.722 Eye diagram (BER) at the receiver (a) without load and (b) with 100%
traffic
From equations 3.400 and 3.401 in section 3.4 the bit error rate of optical signal in the
link were determined. The BER for figures 4.720 and 4.721 respectively are 10-8 and
10-8. The values were obtained after the increment in the amplitude and gain blocks in
4.721 (a)
4.721 (b)
4.722 (a)
4.722 (b)
41
the Simulink Matlab library. This is also the process through which a proper view of
eye diagram scopes can be seen for better discrimination. The BER for the receiver
can not be readily determined owing to distortions introduced in the link by the fiber
propagation models. A remedy to this is to design a demodulator block capable of
exact signal demodulation.
4.8 Results of One-Way Anova Statistical Anaysis
A one way ANOVA statistical analysis test was performed on the splice losses at p-
value of <0.05, firstly on the tube and fiber colours, using SPSS version 17 as shown
in appendix E. The loss values obtained at cut section AA of the transmission link was
arbitrarly chosen as the control group while the values obtained in the other cut
sections were chosen as treatment groups. The results indicated that the differences in
the mean values among the treatment groups are not great enough to exclude the
possibilty that the difference is due to random sampling variability; there is not a
statistically significant difference on the tube. However there is a significant
difference among the colours. This is understandable due to the large numbers of the
fiber colours which are forty eight in direct contrast of the numbers of the tube which
are four.
This is to say that the errors obtained in the splicing process at the cut section of the
optical link were marginal and could have reduced significantly the overal splicing
loss contributions to the links total loss if the same approach were adopted in splicing
other joints in the link by the field splicers and installers.
However significant improvement can also be obtained in the performance of the
transmission link by transmission engineers during continous monitoring of signal
loss caused by splicing loss as the cable is damaged or cut if optimal replacement
policy for aging splicing machine is implemented.
42
CHAPTER FIVE
CONCLUSION AND SUGGESTIONS
5.1 Conclusion
The design of powerful systems is increasingly becoming a complex problem as long
as the parameters influencing the performance of each component are complex
[26,36,41]. Installed fiber systems have good reliabilty as most fiber failures are due
to complete cable cut. Owing to the emergence of fibers with multi core
configuration, service time restoration to a cut or damaged section of an optical link
can be repaired with minimal downtime rate. Avoidable errors due to improper fiber
splice installation can be reduced in an optical link by proper training and following
established good practices.
Unlike copper cable connectors where electrical isolation is an important design
parameters, the factors affecting the design of fiber splices are mechanical and
environmental. All forces whether axial or radial, acting on an optical fiber cable or
splice will cause the transmission characteristics to deviate [37]. To prevent this, the
design of the fiber strength and splice tray encaspulation must substantially isolate
the fiber spilce joints so that the forces are not converted into serious deformations
[1]. Under a laboratory controlled condition, it is possible to maintain a lossfree
system in a complex system like the optical link. However such environment is not
obtainable in the field of operation, hence network modelling tools in conjuction with
analytical methods are applied to view the performance of such a complicated system
[38,42]. In practice, the mismatch between the two fiber ends having different angular
or lateral misalignment in a single spliced joint is insignifant. Even if alignment is
achieved, the existence of air gaps in the fiber joint result in multiple beam interfernce
that can cause fluctuations in the joint loss of more 1dB [42]. Thus at a sufficient
43
distance the cummulative effects of these individual splice losses can mar the
transmission integrity of an optical link.
In this project, the influence of signal loss at optical splice joints of damaged sections
of an optical cable in a link were measured with the loss parameters determined
analytically. The values obtained were used to view propagating signals of the optical
link using the MATLAB simulation software.
5.2 Suggestions
The absence of an optical laboratory, optical measuring instruments and simulation
tools tailored to the analysis of optical communication greatly limited the scope of
this work. Again the data classification protocol at Astera Nigeria Ltd, on the volume
of traffics carried by the link means that the quality of service parameters of the link
can not be determined, analysed and compared with the simulated result. The
challenges faced in this research work were enormous and the following can be
considered for improvement:
Determination of the QoS parameters of an optical link at spliced joints and a
comparism of this with a simulated result.
Design of a demodulator in block in MATLAB© so that the transmitted signals
can be recovered at the receiver of the optical link and hence lost signals can
be determined.
Since the manipulations of optical parameters in MATLAB© can be fastidious
and engaging, the simulation can better be performed using Optisim©,
Optiwaves© and or Labview© simulation packages. These packages are
tailored for the analysis of optical communication hence the performance
effects of the splice loss in a network can also be noted when auxillary
components of an optical link are included in the design.
44
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48
[50] Guanhai Jin. “Advanced Fiber Optic Connectors for Condition Based
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49
APPENDIX A
OPTICAL FIBER CABLE DRUM DATA SHEET [Courtesy of Astera Nigeria Ltd]
50
APPENDIX B
OPTICAL QPSK MODEL WITHOUT TRAFFIC
51
APPENDIX C
OPTICAL QPSK MODEL WITH TRAFFIC
52
APPENDIX D
AMPLIFIER DATASHEET [12,15,17]
53
APPENDIX E ANALYSIS OF VARIANCE (ANOVA) FOR FIBER TUBES
Sum of Squares df Mean Square F Sig.
Splice Point A Between Groups .000 3 .000 .675 .572
Within Groups .004 44 .000
Total .004 47
Splice Point B Between Groups .000 3 .000 2.056 .120
Within Groups .003 44 .000
Total .004 47
Splice Point C Between Groups .000 3 .000 1.126 .349
Within Groups .004 44 .000
Total .004 47
Splice Point D Between Groups .000 3 .000 1.929 .139
Within Groups .002 44 .000
Total .002 47
ANALYSIS OF VARIANCE (ANOVA) FOR FIBER COLOURS
Sum of Squares df Mean Square F Sig.
Splice Point A Between Groups .001 11 .000 .569 .841
Within Groups .004 36 .000
Total .004 47
Splice Point B Between Groups .001 11 .000 2.195 .038
Within Groups .002 36 .000
Total .004 47
Splice Point C Between Groups .001 11 .000 .890 .558
Within Groups .003 36 .000
Total .004 47
Splice Point D Between Groups .000 11 .000 .522 .876
Within Groups .002 36 .000
Total .002 47
54
v
APPENDIX F
Splice Loss Equations for Single Mode Fibers
The incident electric field E at the input end of the fiber can be expressed in terms of
fiber modes as stated by [4]
6.001
where the summation symbol indicates symbollically summation over guided modes
(one for single mode fibers).
Ev is the electric field vectors of the modes (guided and radiation modes) of the fiber
and integration over radiation modes. The symbol v labels the mode (if v = 0 as the
label of the guided mode of the single mode fiber).
Mode orthogonality allows for c0 to be obtained from equ.(6.001)
6.002
H0 is the magnetic field vector of the guided mode, ez is a unit vector in the direction
of the fiber axis , and r and Ø are cylindrical coordinates in the plane at right angles to
the axis of fiber.
The power transmission coefficient finally is obtained from equ. (6.002) by the
relation
T = │c0│2 6.003
The electric field vector of the input field consists of one dorminant transverse
component. If the input field is gaussian; then
6.004
The refractive index n2 equals the cladding index of the fiber, P is the power carried
by the field and is identical to the P parameter in equ. (6.002), w is the width
55
parameter of the gaussian field, β is its propagation constant, μ0 and ϵ0 are the
magnetic susceptibility and the dielectric permitivity of vacuum.
We wish to compare the gaussian field to the mode of the step index fiber,then
6.005
The P parameter is identical to those in equations (6.002) and (6.004), W and U are
related to the important V parameter by
U2 + W2 = V2 = ( )k2ɑ2 6.006
The free space propagation constant of plane waves is k = and ɑ is the core
radius of the fiber. J0 and J1 the Bassel functions and K0 is the modified Hankel
function. The parameter U can be related to the propagation constant βs as follows
U = ( . 6.007
The r- integral in equ.(6.002) must be evaluated numerically. The value of T depends
on the width parameter w of the gaussian beam; T assumes a maximum as a function
of w. At V = 2.4, T = 0.9965, this value of V is close to the largest value where the
fiber supports only one mode. The next larger value ot T comes at V = 2.405, the best
match for fiber mode gaussian field distribution occurs at 2.4.
It can be shown that the optimum value of w divided by the core radius is only a
function of V. This function can be approximated very closely by the empirical
formula in equ 6.008. The accepatable range of values are 1.2 ≤ V ≤ 2.405
6.008
56
For large values of V the emprical approximation w divide by the core radius can be
expressed as
6.009
If a different approach is adopted and the approxiamation of guided mode of a single
mode fiber with refractive index distribution n(r) for r < ɑ and n(r) = n2 for r > ɑ, and
substitute equ (6.004) into wave equation
6.010
and obtain
6.011
for a graded index distribution
6.012 with g = 2, equ. (6.011) can be satisfied.
Equation (6.012) is an infinitely extended parabolic index profile. In this case, we can
find
6.013
And
6.014
We define the V parameter for any value of g by the equation
6.015
This expression is also a good approximation of (6.006), if we use
6.016
57
Equations (6.013) and (6.014) are not correct for actual parabolic index fibers whose
refractive index distributions are given by (6.012) (with g = 2) only for r < ɑ, but
assume the form n(r) = n2 for r > ɑ. Such profiles are referred to as truncated index
distributions.
For the different fiber defects shown in figures 2.501 (a) and 2.501 (b), [4] the
relevant formulae can be obtained by using equations (6.002) and (6.003) with the
field of both fibers represented by gaussian field distributions of the form in (6.004).
For analysis each fiber is represented by the width parameter of the optimum gaussian
field distribution, w1 which belongs to the fiber with radius ɑ1 and w2 belongs to the
fiber with radius ɑ2.
For splice loss with lateral misalignment, the power transmission coefficient can be
expressed as
6.017
The normalised fiber separation distance is defined as
6.018
At d = 0,
6.019
For d → ∞, we obtain asymptotically
6.020
58
For identical fibers, w1 = w2 = W and noting that W = n2kw, then
6.021
6.022
A condensed as expressed by [7] is given as
6.023
where TLM is the power transmission coefficient which reperesent the spilce loss for
lateral misalignment, d is the lateral misalignment and W is the gaussian spot size.
For a fiber with tilt (angular misalignment), the power transmission coefficient is
obtained by
6.024
When the tilt angle becomes large enough to make the exponent of the exponential
function in (6.024) to unity,the transmittted power decreases to 1/e of its maximum
value.
Then this angle is given by the expression
6.025
For identical fibers, w1 = w2 = W and noting that the terms in bracket dimishes faster
than the later, the expression becomes
6.026
The expression for TAM becomes
59
6.027
6.028
A condensed expressed by [7] is given by
6.029
where TAM is the power transmission coefficient which reperesent the spilce loss for
angular misalignment, is the angular misalignment,W is the gaussian spot size, n2
is the refractive index of the cladding and λ is the wavelength of fiber.
60
APPENDIX G
TYPICAL SYSTEM INITIALISATION MATLAB m-file
Initialize opticalmojexsimulation.m
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%
%%%%%%%%%%%%%%%%
% simulation run-time. keep the eye diagram windows on top
% during simulation run-time,
% allow simulations to conclude to view full eye diagram
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%
%%%%%%%%%%%%%%%%
%standard system parameters
bitrate=10*10^9 %bitrate in b/s
bitnum=256 %number of bits in data string,
lambda=1550e-9 %operating wavelength in m
%FIBER PARAMETERS
Dsmf=17e-6 %SMF dispersion factor in s/m^2
Ddcf=-85e-6 %DCF dispersion factor in s/m^2
Lsmf=90000 %SMF fiber length in m
Ldcf=17000 %DCF fiber length in m
speedlight=3*10^8 %speed of light in m/s
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%
%%%%%%%%%%%%%%%%