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Unit-1
UNIT – I INTRODUCTION • Introduction, • Ray theory transmission- • Total internal reflection-• Acceptance angle –• Numerical aperture – • Skew rays – • Electromagnetic mode theory of optical propagation –• EM waves – • modes in Planar guide – • phase and group velocity – • cylindrical fibers –• SM fibers.
Fig. 1-2: Digital transmission hierarchy
Fig. 1-3: Operating ranges of components
Fig. 1-5: Major elements of an optical fiber link
Fig. 1-6: Optical fiber cable installations
Fig. 1-7: History of attenuation
Fig. 1-8: Optical multiplexing
Evanescent wave
UNIT 2
TRANSMISSION CHARACTERISTICS OF OPTICAL FIBERS
UNIT 2• Attenuation – • Material absorption losses in silica glass fibers – • Linear and Non linear Scattering losses – • Fiber Bend losses – • Midband and farband infra red transmission –• Intra and inter Modal Dispersion – • all Fiber Dispersion – • Polarization- • non linear Phenomena. • Optical fiber connectors, • Fiber alignment and Joint Losses – • Fiber Splices• – Fiber connectors – Expanded Beam Connectors – • Fiber Couplers.
Absorption in
Infrared region
Absorption
Atomic Defects
Extrinsic (Impurity atoms)
Intrinsic Absorption
Absorption in
Ultraviolet region
Attenuation
Scattering Losses
Compositional fluctuations in material
Inhomogeneities or defects
in fiber
Radiative losses/ Bending
losses
Macroscopic bends
Microscopic bends
Pulse broadening and attenuation
1. Raleigh scattering 1. Mie Non linear scattering : 1. Stimulated Brillouin scattering2. Stimulated Raman scattering
89
2.4.3 Stimulated Roman Scattering (SRS) SRS will deplete short wave power and amplifier long wave.
90
2.4.1 Stimulated Brillouin Scattering (SBS)
The scattering interaction occurs with acoustic phonons over Δf =15 MHz, at 1.55μm, stokes and pump waves propagate in opposite directions.
If spacing > 20 MHz => no effects on different channels
Ps(0) Pp(L)
pumping
SBS
A compressible jacket extruded over a fiber reduces microbending resulting from external forces.
Minimizing microbending losses:
Macrobending due to poor reeling
Minimum safe bend radius —shown full size
Bends are shown full size — and may have caused damage to the fiber
Making use of bending losses
There are many uses of bending losses which are based on either the increase in the attenuation or on making use of the light which escapes from the optic fiber.
Radiative losses / Bending Losses
A fiber optic pressure sensor
Active fiber detector
This makes use of the increased attenuation experienced by the fiber as it bends.
This uses the escaping light.
Pressure causes loss at the bends
Is the fiber in use?
Signal Distortion/
Dispersion
Polarization-modeDispersion
Intramodal Dispersion/
Chromatic DispersionIntermodal Delay/
Modal Delay
Material Dispersion
Waveguide Dispersion
Dispersion
Attenuation only reduces the amplitude of the output waveform which does not alter the shape of the signal.
Dispersion distorts both pulse and analog modulation signals.
The basic need is to match the output waveform to the input waveform as closely as possible.
It is noted that actually no power is lost to dispersion, the spreading effect reduces the peak power.
In a pulse modulated system, this causes the received pulse to be spread out over a longer period.
Dispersion
Dispersion results when some components of the input signal spend more time traversing the fiber than other components.
Dispersion
The difference in width of an input pulse with the width of the same pulse at the output, measured in time, is the dispersion characteristic for that piece of fiber.
Pulse dispersion is usually specified in terms of “Nanoseconds-per-kilometer”.
Dispersion
Dispersion of optical energy within an optical fiber falls into following categories:
Intramodal Dispertion or Chromatic DispersionMaterial DispertionWaveguide Dispertion
Intermodal Delay or Modal Delay)
Dispersion
Polarization –Mode Dispersion
Intermodal delay/ modal delay
Dispersion
Intermodal distortion or modal delay appears only in multimode fibers.
This signal distortion mechanism is a result of each mode having a different value of the group velocity at a single frequency.
Group Velocity: It is the speed at which energy in a particular mode travels along the fiber.
The amount of spreading that occurs in a fiber is a function of the number of modes propagated by the fiber and length of the fiber
Intermodal delay/ modal delay
The maximum pulse broadening arising from the modal delay is
the difference between the travel time Tmax of the longest ray and
the travel time Tmin of the shortest ray .
This broadening is simply obtained from ray tracing for a fiber of length L:
∆T= Tmax – Tmin = n1/c ( L/sinøc –L) = (Ln12/cn2)∆
∆T= Tmax – Tmin = (Ln12/cn2)∆
Intermodal delay/ modal delay
Fiber Capacity:
Fiber capacity is specified in terms of the bit rate-distance product BL.
(Bit rate times the possible transmission distance L)
For neighboring signal pulses to remain distinguishable at the receiver, the pulse spread should be less than 1/B.
Or
Pulse spread should be less than the width of a bit period.
∆T < 1 /B General requirement∆T ≤ 0.1 /B For high performance link
Bit rate distance product BL < n2 c/ n12 ∆
Light rays with steep incident angles have longer path lengths than lower-angle rays.
How to minimize the effect of modal dispersion?
1. Graded index fiber
2. Single mode fiber
Answer is
How to get one mode and solve the problem
V = 2πa / λ x (n12 – n2
2)1/2 = 2πa / λ x (NA)
we could decrease the number of modes by increasing the wavelength of the light.
Changing from the 850 nm window to the 1550 nm window will only reduce the number of modes by a factor of 3 or 4.
Change in the numerical aperture can help but it only makes a marginal improvement.
We are left with the core diameter. The smaller the core, the fewer the modes.
When the core is reduced sufficiently the number of modes can be reduced to just one.
How to get one mode and solve the problem
Step Index Multi-mode
Graded Index Multi-mode
Q: Consider a 1 Km long multimode fiber in which n1= 1.480 and ∆ = 0.10 , so that n2= 1.465.
Then find ∆T= ?
∆T = (Ln12/cn2)∆
Where:
L = 1 Km
n1 = 1.480
n2= 1.465
∆ = 0.10
How to characterize dispersion?• Group delay per unit length can be defined as:
• If the spectral width of the optical source is not too wide, then the delay difference per unit wavelength along the propagation path is approximately For spectral components which are apart, symmetrical around center wavelength, the total delay difference over a distance L is:
d
d
cdk
d
cd
d
Lg
2
1
ω
2
[3-15]
d
d g
2
2
2
22
22
d
dL
V
L
d
d
d
d
d
d
d
d
c
L
d
d
g
g
[3-16]
• is called GVD parameter, and shows how much a light pulse broadens as it travels along an optical fiber. The more common parameter is called Dispersion, and can be defined as the delay difference per unit length per unit wavelength as follows:
• In the case of optical pulse, if the spectral width of the optical source is characterized by its rms value of the Gaussian pulse , the pulse spreading over the length of L, can be well approximated by:
• D has a typical unit of [ps/(nm.km)].
2
2
2
d
d
22
211
c
Vd
d
d
d
LD
g
g
[3-17]
g
DLd
d gg [3-18]
Dispersion
Intramodal Dispersion or Chromatic Dispersion
This takes place within a single mode.
Intramodal dispersion depends on the wavelength, its effect on signal distortion increases with the spectral width of the light source.
Spectral width is approximately 4 to 9 percent of a central wavelength.
Two main causes of intramodal dispersion are as:
1. Material Dispersion
2. Waveguide Dispersion
t
Spread, ²
t0
Spectrum, ²
12o
Intensity Intensity Intensity
Cladding
CoreEmitter
Very shortlight pulse
vg(2)
vg(1)Input
Output
All excitation sources are inherently non-monochromatic and emit within aspectrum, ² , of wavelengths. Waves in the guide with different free spacewavelengths travel at different group velocities due to the wavelength dependenceof n1. The waves arrive at the end of the fiber at different times and hence result ina broadened output pulse.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Material Dispersion
Intramodal Dispersion or Chromatic Dispersion
Material Dispersion:
This refractive index property causes a wavelength dependence of the group velocity of a given mode; that is,
Pulse spreading occurs even when different wavelength follow the same path.
Material dispersion can be reduced:
•Either by choosing sources with narrower spectral output widths OR
•By operating at longer wavelengths.
LASER source will produce far less spectral dispersion or intramodal dispersion than an LED source since it is more nearly monochromatic
Material Dispersion
• The refractive index of the material varies as a function of wavelength,
• Material-induced dispersion for a plane wave propagation in homogeneous medium of refractive index n:
• The pulse spread due to material dispersion is therefore:
)(n
d
dnn
c
L
nd
dL
cd
dL
cd
dLmat
)(2
22ω
22
[3-19]
)(2
2
mat
matg DL
d
nd
c
L
d
d [3-20]
)(matD is material dispersion
Material dispersion as a function of optical wavelength for pure silica and 13.5 percent GeO2/ 86.5 percent SiO2.
Intramodal Dispersion or Chromatic Dispersion
Waveguide Dispersion:
Dispersion arises because the fraction of light power propagating in the cladding travels faster than the light confined to core.
It causes pulse spreading because only part of the optical power propagation along a fiber is confined to core.
Single mode fiber confines only 80 percent of the power in the core for V values around 2.
The amount of waveguide dispersion depends on the fiber design.
Waveguide Dispersion
• Waveguide dispersion is due to the dependency of the group velocity of the fundamental mode as well as other modes on the V number, (see Fig 2-18 of the textbook). In order to calculate waveguide dispersion, we consider that n is not dependent on wavelength. Defining the normalized propagation constant b as:
• solving for propagation constant:
• Using V number:
21
22
22
1
22
22 //
nn
nk
nn
nkb
[3-29]
)1(2 bkn [3-31]
2)( 22/12
22
1 kannnkaV [3-33]
Waveguide Dispersion• Delay time due to waveguide dispersion can then be expressed as:
dV
Vbdnn
c
Lwg
)(22 [3-34]
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Signal Distortion in single mode fibers
• For single mode fibers, waveguide dispersion is in the same order of material dispersion. The pulse spread can be well approximated as:
2
22 )(
)(dV
VbdV
c
LnDL
d
dwg
wgwg
[3-25]
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
)(wgD
Over all Fiber distortion
Over all Fiber distortion
Chromatic & Total Dispersion• Chromatic dispersion includes the material & waveguide dispersions.
• Total dispersion is the sum of chromatic , polarization dispersion and other dispersion types and the total rms pulse spreading can be approximately written as:
LD
DDD
chch
wgmatch
)(
)(
[3-28]
LD
DDD
totaltotal
polchtotal
...[3-29]
Polarization Mode dispersion
Core
z
n1 x
// x
n1 y
// y
Ey
Ex
Ex
Ey
E
= Pulse spread
Input light pulse
Output light pulset
t
Intensity
Suppose that the core refractive index has different values along two orthogonaldirections corresponding to electric field oscillation direction (polarizations). We cantake x and y axes along these directions. An input light will travel along the fiber with Ex
and Ey polarizations having different group velocities and hence arrive at the output atdifferent times
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Polarization Mode dispersion
• The effects of fiber-birefringence on the polarization states of an optical are another source of pulse broadening. Polarization mode dispersion (PMD) is due to slightly different velocity for each polarization mode because of the lack of perfectly symmetric & anisotropicity of the fiber. If the group velocities of two orthogonal polarization modes are then the differential time delay between these two polarization over a distance L is
• The rms value of the differential group delay can be approximated as:
gygx vv and pol
gygxpol v
L
v
L [3-26]
LDPMDpol [3-27]
Optical Fiber Connectors
• Some of the principal requirements of a good connector design are as follows:
1- low coupling losses
2- Interchangeability
3- Ease of assembly
4- Low environmental sensitivity
5- Low-cost and reliable construction
6- Ease of connection
Fiber Joints
• Fibers must be joined when– You need more length than you can get on a
single roll– Connecting distribution cable to backbone– Connecting to electronic source and transmitter– Repairing a broken cable
Splices v. Connectors
• A permanent join is a splice• Connectors are used at patch panels, and can
be disconnected
Optical Loss
• Intrinsic Loss– Problems the splicer cannot fix
• Core diameter mismatch• Concentricity of fiber core or
connector ferrules• Core ellipticity• Numerical Aperture mismatch
– Images from LANshack and tpub.com (links Ch 6a & 6c)
Optical Loss
• Extrinsic Loss– Problems the person doing the
splicing can avoid• Misalignment• Bad cleaves• Air gaps• Contamination: Dirt, dust, oil, etc.• Reflectance
Measuring Reflectance
• The reflected light is a fraction of the incoming light– If 10% of the light is reflected, that is a reflectance
of 10 dB– If 1% of the light is reflected, 20 dB– Reflectance is not usually a problem for data
networks, but causes ghosting in analog cable TV transmission
– Angled connectors reduce reflectance
Acceptable Losses
Fiber & Fiber & JointJoint
Loss (max)Loss (max) Reflectance Reflectance (min)(min)
SM spliceSM splice 0.15 dB0.15 dB 50 dB50 dB
SM connectorSM connector 1 dB1 dB 30 dB30 dB
MM spliceMM splice 0.25 dB0.25 dB 50 dB50 dB
MM MM connectorconnector
0.75 dB0.75 dB 25 dB25 dB
Connectors
• There are four types– Rigid Ferrule (most common)– Resilient ferrule– Grooved plate hybrids– Expanded beam
• Top image shows ferrules from swiss-jewel.com (link Ch 6e)
• Lower image shows LC, SC, Biconic, and the obsolete Deutsch 1000
– From thefoa.org (link Ch 6d)
Rigid Ferrule Connectors
• 2.5 mm ferrule»ST
»SC
»FC• Images from thefoa.org (link Ch 6d)
Rigid Ferrule Connectors
• 1.25 mm ferrule• Small Form Factor
»LC
»MU
»LX-5
• Images from thefoa.org (link Ch 6d)
Obsolete Connectors
• Simplex (1-fiber)»SMA
»D4
»Biconic
• Images from thefoa.org (link Ch 6d)
Duplex Connectors
• Old, bulky»FDDI
»ESCON
• Images from thefoa.org (link Ch 6d)
Duplex Connectors
• Newer, smaller• Small Form Factor
»MT-RJ
»Opti-Jack
»Volition
• Images from thefoa.org (link Ch 6d)
Duplex Connectors
• New, popular• Small Form Factor
»Duplex LC
• Images from globalsources.com (link Ch 6f)
Ferrule Polish
• To avoid an air gap• Ferrule is polished flat, or • Rounded (PC—Physical Contact),
or• Angled (APC)
– Reduces reflectance– Cannot be mated with the other
polish types• Image from LANshack (link Ch 6a)
FOCIS
• Fiber Optic Connector Intermateability Standard– A document produced by a connector
manufacturer so others can mate to their connector
– Connectors with the same ferrule size can be mated with adaptors
– But 2.5 mm ferrules can not be mated with 1.25 mm ferrules
Telecommunications• In telecommunications, SC
– and FC
– are being replaced by
– LC• in the USA
– MU• in other countries
Data
• In data communications, SC and ST– are being replaced by
– LC
Connectorizing a Cable
• Epoxy-polish process (Proj. 4)– Strip cable, strip and clean fiber– Inject adhesive, put primer on fiber, insert fiber– Crimp connector, cleave protruding fiber– Air polish, final polish– Clean and inspect by microscope– Test connector loss with power meter
Cable Type and Connectors
• Epoxy-polish process requires a cable jacket and strength member to make the connector durable– It works for simplex, zip, or breakout cables– But loose-tube cables and ribbon cables contain bare fiber,
and cannot be connectorized this way– Distribution cables contain 900 micron buffered fiber – can
be connectorized, but the connectors are not very strong and must be protected by hardware such as a junction box
Breakout Kit
• Provides tubing that protects the bare fiber so it can be terminated– Picture from
fonetworks.com (link Ch 4d)
Mounting Methods for Connectors• Adhesives
– Epoxy (room temperature-cure or oven-cure)– Quick-curing anaerobic adhesives (we used this
method in Proj 4)– Hot-Melt adhesive
• Crimping to hold the fiber– Like the Unicam – see link Ch 6h
• Splicing to preconnectorized pigtails– Image of pigtail from fiberdyne.com (link Ch 6g)
Mounting Methods Comparison• Epoxy-Polish
– Takes longer, but costs less and has lowest loss and reflectance
• Anaerobic adhesive– Faster than epoxy-polish but higher loss because polishing
is difficult
• Crimping– Easier, but more expensive and more loss
• Splicing to preconnectorized pigtail– Very easy, but expensive and higher loss
Strip, Clean and Cleave
• Strip – remove 900 micron buffer (if present) and 250 micron coating
• Clean with alcohol and lint-free wipe• Cleave – scribe and snap; goal is a 90 degree
flat break
End-Face Polish
• Polish on a flat glass plate for a flat finish• Polish on a rubber mat for a domed PC finish
(Physical Contact)• Angled PC finish is tilted at 8 degrees to avoid
reflectance (difficult to field-terminate)
Cleaning Connectors
• Keep dust caps on• Use lint-free wipes and reagent-grade
isopropyl alcohol to avoid attacking epoxy• “Canned air” has propellant, so does
compressed air from a hose
Splices
• Splices are a permanent join of two fibers– Lower attenuation and reflectance than
connectors– Stronger and cheaper than connectors– Easier to perform than connectorization– Mass splicing does 12 fibers at a time, for ribbon
cables
Mass Fusion Splicing
• Video from fitel.fiberoptic.com (link Ch 6i)
Fusion Splicing
• Melts the fibers together to form a continuous fiber
• Expensive machine• Strongest and best join for singlemode fiber
– May lower bandwidth of multimode fiber
• Corning videos 1-7 & 12
Mechanical Splicing
• Mechanically aligns fibers• Contains index-matching gel to transmit light• Equipment cost is low• Per-splice cost is high• Quality of splice varies, but better than
connectors• Fiber alignment can be tuned using a Visual
Fault Locator
Connector Return Loss
Coupling Efficiency
s
F
P
P
sourse thefrom emittedpower
fiber theinto coupledpower [5-1]
Source Optical Fiber
sPFP
Examples of possible lensing schemes used to improve optical source-to-fiber coupling efficiency
Fiber-to-Fiber Joint
• Fiber-to-Fiber coupling loss:
• Low loss fiber-fiber joints are either:
1- Splice (permanent bond)
2- Connector (demountable connection)
FFL log10]dB[ [5-8]
Different modal distribution of the optical beam emerging from a fiber lead to different degrees of coupling loss. a) when all modes are equally excited, the output beam fills the entire output NA. b)
for a steady state modal distribution, only the equilibrium NA is filled by the output beam.
Mechanical misalignment losses
Lateral (axial) misalignment loss is a dominant Mechanical loss.
2/12
2step, 21
2arccos
2
a
d
a
d
a
d
a
AcommF
[5-9]
Longitudinal offset effect
Losses due to differences in the geometry and waveguide characteristics of the fibers
ERE
RF
ERE
RF
aL
aaa
aaL
NANAfor )NA
NAlog(20)(
for )log(10)(
[5-10]
E & R subscripts refer to emitting and receiving fibers.
Fiber end face
Fiber end defects
Fiber splicing
Fusion Splicing
V-groove optical fiber splicing
Optical Fiber Connectors
• Some of the principal requirements of a good connector design are as follows:
1- low coupling losses
2- Interchangeability
3- Ease of assembly
4- Low environmental sensitivity
5- Low-cost and reliable construction
6- Ease of connection
Connector Return Loss
198
Connectors
A mechanical or optical device that provides ademountable connection between two fibers or
a fiber and a source or detector.
199
Connectors - contd.
Type: SC, FC, ST, MU, SMA• Favored with single-mode fibre• Multimode fibre (50/125um) and (62.5/125um)• Loss 0.15 - 0.3 dB• Return loss 55 dB (SMF), 25 dB (MMF)
Single fibre connector
200
Connectors - contd.
• Single-mode fiber• Multi-mode fiber (50/125)• Multi-mode fiber (62.5/125)
• Low insertion loss & reflection
MT-RJ Patch Cord MT-RJ Fan-out Cord
201
Coupler• Uses
– Splitter: (50:50)– Taps: (90:10) or (95:05)– Combiners
• An important issue: – two output differ /2 in phase
• Applications: – Optical Switches, – Mach Zehnder Interferometers, – Optical amplifiers, – passive star couplers, ...
202
Coupler Configuration
P1P2
P3
P1P2
P3
1
2
3
1
2
n
1 ……n
203
Coupler - Integrated Waveguide
Directional Coupler
P2 = P0 sin2 kz P1 = P0 - P2 = P0 cos2 kz
k = coupling coefficient = (m + 1)/2
P0
P1
P2
P3
zP4
204
Coupler - Integrated Waveguide Directional Coupler
• A directional coupler
• Different performance couplers can be made by varying the length, size for specific wavelength.
G Keiser
205
Couplers - Fabrication
• Wavelength independent, depends on how light is launched • In the coupling region
– Higher order modes are trapped at the outer surface of the cladding: thus becoming cladding modes
– Lower order modes remain in the original fibre (as the incident angles are still > the critical angle)
• Cladding modes are converted back into core modes at the output ports. • The splitting ratio is determined by the
– length of the taper – thickness of the cladding.
• Multimode Fibres
Cladding modes
Source: Australian Photonics CRC
206
Couplers - Fabrication
• It is wavelength dependent. Resonance occur when the two fibres are close to each other.
• Single Fibres
100% coupling
The coupling length for 1.55 µm > the coupling length for 1.3 µm: – 100 % of light coupling for 1.3 µm to the core of fibre B, and to the core of
fibre A. – 100% of light coupling for 1.55 µm to the core of fibre B
Source: Australian Photonics CRC
207
Couplers - Fabrication• The amount of power transmitted into fibres depend on
the coupling length• The coupling length changes with the wavelength. • The splitting ratio can be tuned choosing the coupling
length.• By choosing carefully the coupler length, it is possible to
combine or separate Two different wavelengths
208
Coupler - Performance Parameters
Coupling ratio or splitting ratio
outT
t
P
PCR
portsalltooutpowerTotal
outputsingleanyfromPower
• Excess LossoutT
ie P
PL
routputpoweTotal
powerInput
21
01010
PP
PLe log
21
21010
PP
PCR logIn dB For 2 x 2 coupler
209
Coupler - Performance Parameters
• Isolation Loss or Crosstalk
• Insertion Loss
i
ti P
PL
inputPower
outputsingleanyfromPower
3
01010
P
PLiso log
portinputotherintobackpowerReflected
portoneatpowerInputisoL
In dB
210
Generic 2X2 Guided-Wave Coupler
Sab
,b
2
1
b
band
a
a,a
2
1
2221
1211
ss
ssSwhere
There are altogether eight possible ways(two ways) for the light to travel.
a1
a2
b1
b2
Inputfield
strengths
Outputfield
strengths
s22
s21 s12
s11
211
Generic 2X2 Guided-Wave CouplerAssume: Fraction (1- ) of power in the input port 1 appears at output port 1,and the remaining power at the output port 2
1
1
j
jS
If = 0.5, an d input signal defined in terms of filed intensity Ei, then
2
1
2
1
1
1
2
1
,
,
,
,
i
i
o
o
E
E
j
j
E
E
Let Eo,2 = 0, thus in term of optical power
021
12
21
111 PEEEP iooo ,,*
,,
021
12
21
222 PEEEP iooo ,,*
,,
Half the input power appears at
each output
212
Tree and Branch Coupler
Coupling ratio; 1:1 or 1: n, where n is some fraction
Fibre
213
Star Couplers
• Optical couplers with more than four ports.
• Types of star couplers: – transmission star coupler
the light at any of the input port is split equally through all output ports.
– reflection star coupler
214
Fibre Star CouplerCombines power from N inputs and divided them between M outputs
NN
CR 1010 101
10 loglog
Coupling ratio
N
i iout
ine
P
PL
,
log1010Excess loss
1
N
1
N
P1
PN
Power at any one output ).......(, Nio PPPn
P 211
215
Star Coupler - 8 X 8
1
2
3
4
5
6
7
8
1, 2, ... 8
1, 2, ... 8
No of 3 dB coupler NN
N dBc 23 2log
N/2
N2log
Star couplers are optical couplers with more than four ports
216
Star Coupler - 8 X 8 - contd.
• If a fraction of power traversing each 3 dB coupler = Fp, where 0< Fp < 1.
Then, power lost within the coupler = 1- Fp. Excess loss )(log log N
pe FL 21010
NN
CR 1010 101
10 loglog
Coupling ratio
(splitting loss)
Total loss = splitting loss + excess loss
NFLT 10103223110 log)log.( NFLT 10103223110 log)log.(
217
Reflection Star Couplers
The light arriving at port A and is reflected back to all ports.
A directional coupler separates the transmitted and received
signals.
Source: Australian Photonics CRC
218
Y- Couplers
1 X 8 coupler
Y-junctions are 1 x 2 couplers and are a key element in networking.
Ii
I1
I2
219
Coupler - CharacteristicsDesign class No. of CR Le Isolation
port (dB) directivity (-dB)
2 x 2 2 0.1-0.5 0.07-1.0 40 to 55Single mode
2 x 2 2 0.5 1-2 35 to 40Multimode
N x N 3-32 0.33-0.03 0.5-8.0Star
220
Splitters• The simplest couplers are fiber optic splitters. • They possess at least three ports but may have more than 32 for more
complex devices. • Popular splitting ratios include 50%-50%, 90%-10%, 95%-5% and 99%-1%;
however, almost any custom value can be achieved. • Excess loss: assures that the total output is never as high as the input. It
hinders the performance. All couplers and splitters share this parameter. • They are symmetrical. For instance, if the same coupler injected 50 µW
into the 10% output leg, only 5 µW would reach the common port.
OutputOutput
Input
Irfan khan
Launching optical power from source into fiber needs following considerations:
Fiber parameters:
• Numerical aperture
• Core size
• Refractive index profile
• Core cladding index difference
Source parameters:
• Size
• Radiance
• Angular power distribution
Irfan khan
Coupling efficiency:
It is the measure of the amount of optical power emitted from a source that can be coupled into a fiber .
η = PF / PS
PF =Power coupled into the fiber
PS = Power emitted from the light source
Coupling efficiency depends on:
1. Type of fiber that is attached to the source
2. Coupling Process (e.g. lenses or other coupling improvement schemes)
Irfan khan
Flylead / Pigtail:
Short length of optical fiber attached with the source for best power coupling configuration.
Thus
Power launching problem for these pigtailed sources reduces to a simpler coupling optical power from one fiber to another.
Effects to be considered in this case include:
1.Fiber misalignments:
a. Different core sizes
b. Numerical apertures
c. Core refractive index profiles
2.Clean and smooth fiber end faces:
a) perfectly perpendicular to the axis
b) Polished at a slight angle to prevent back reflections
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Optical fiber receptacles:
An alternate arrangement consist of light sources and optical fiber receptacles that are integrated within a transceiver package.
Fiber connector from a cable is simply mated to the built in connector in the transceiver package.
Commercially available configurations are the popular small form factor (SFF) and the SFF pluggable (SFP) devices.
Photodiode, PIN, 1310/1550 nm, LC, SC or FC Receptacle
SFP ,Transceiver, 155 Mb/s STM-1
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Laser diodes with pigtails and Receptacle
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Source to fiber power launchingOptical output of a luminescent source is usually measured by its radiance B at a given diode current.
Radiance: It is the optical power radiated into a unit solid angle per unit emitting surface area and is generally specified in terms of watts per square centimeter per steradian.
Radiance = Power / per unit solid angle x per unit emitting surface area
Solid angle is defined by the projected area of a surface patch onto a unit sphere of a point.
The angle that, seen from the center of a sphere, includes a given area on the surface of that sphere. The value of the solid angle is numerically equal to the size of that area divided by the square of the radius of the sphere
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Radiance (Brightness) of the source
• B= Optical power radiated from a unit area of the source into a unit solid angle [watts/(square centimeter per stradian)]
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Surface emitting LEDs have a Lambertian pattern:
cos),( 0BB [5-2]
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Edge emitting LEDs and laser diodes radiation pattern
LT BBB cos
cos
cos
sin
),(
1
0
2
0
2
For edge emitting LEDs, L=1
[5-3]
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Power Coupled from source to the fiber
rdrdddB
dAdABP
s
r
s
A
sssF
m
f f
max0
0
2
0
2
00
sin),(
),([5-4]
source theof angleemission solid and area : and ssA
fiber of angle acceptance solid
and area : and ffA
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Power coupled from LED to the Fiber
rdrdB
rdrdB
rdrddBP
s
r
s
r
s
r
s
s
s
22
00
0
2
0
max02
0
0
2
0 0
0
0
NA
sin
sincos2max0
210
2220
22stepLED, 2)NA( nBrBrP ss [5-5]
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Power coupling from LED to step-index fiber
• Total optical power from LED:
sincos2
sin),(
2/
0
022
02
2
0
2/
0
BrdBrP
ddBAP
sss
ss
[5-6]
arP
r
a
arP
Pss
s
ss
if )NA(
if )NA(
2
2
2
stepLED, [5-7]
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Power coupling from LED to graded-index fiber
• Power coupled from the LED to the graded indexed fiber is given as
• If the medium between source and fiber is different from the core material with refractive index n, the power coupled into the fiber will be reduced by the factor
a
rnP
rdrnrnBP
ss
r
oginLED
s
2
212
2
21
0
22
22,
2
1
1
nn
nnR
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Power Launching Vs Wavelength
• Optical power only depends on the radiance and not on the wavelength of the mode. For a graded index fiber number of modes is related to the wavelength as
• So twice as many modes propagate for 900 nm as compared to 1300 nm but the radiated power per mode from a source is
• So twice as much power is launched per mode for 1300nm as compared to the 900nm
2
12
2
an
M
2os B
M
P
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Equilibrium Numerical aperture
• For fibers with flylead attachments the connecting fiber should have the same NA. A certain amount of loss occurs at this junction which is almost 0.1 – 1dB. Exact loss depends on the connecting mechanism.
• Excess power loss occurs for few tens of meters of a multimode fiber as the launched modes come to the equilibrium.
• The excess power loss is due to the non propagating modes• The loss is more important for SLED.• Fiber coupled lasers are less prone to this effect as they have very few
non propagating modes.• The optical power in the fiber scales as
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Equilibrium Numerical Aperture
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Lensing Scheme for Coupling Improvement
Several Possible lensing schemes are:
1. Rounded end fiber
2. Nonimaging Microsphere (small glass sphere in contact with both the
fiber and source)
3. Imaging sphere ( a larger spherical lens used to image the source on
the core area of the fiber end)
4. Cylindrical lens (generally formed from a short section of fiber)
5. Spherical surfaced LED and spherical ended fiber
6. Taper ended fiber.
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Examples of possible lensing scheme used to improve optical source to fiber coupling efficiency
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Problem in using lens:
Lensing Scheme for Coupling Improvement
One problem is that the lens size is similar to the source and fiber core dimensions, which introduces fabrication and handling difficulties.
In the case of taper end fiber, the mechanical alignment must be carried out with great precision
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Non Imaging Microsphere• Use for surface emitter is shown• Assumptions: refractive indices shown in
the fig. and emitting area is circular• To collimate the output from the LED,
the emitting surface should be located at the focal point of the lens which can be found as
• Where s and q are object and image distances as measured from the lens surface, n is the refractive index of the lens, n/ is the refractive index of the outside medium and r is the radius of curvature of the lens surface
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continued
The following sign conventions are used1. Light travels from left to right2. Object distances are measured as positive to the left of a vertex and negative to
the right3. Image distances are measured as positive to the right of a vertex and negative
to the left4. All convex surfaces encountered by the light have a positive radius of curvature,
and concave surfaces have a negative radius.
For these conventions, we can find the focal point for the right hand surface of the lens shown in the last fig. We set q = infinity, solve for s yields
s = f = 2RL
So the focal point is at point A. Magnification M of the emitting area is given as
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continued
Using eq. 5.4 one can show that, with the lens, the optical power PL that can be coupled into a full aperture angle 2θ is given by
For the fiber of radius a and numerical aperture NA, the maximum coupling efficiency max is given by
So when the radius of the emitting area is larger than the fiber radius, there’ll be no improvement in the coupling efficiency with the use of lens
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Laser diode to Fiber Coupling
• Edge emitting laser diodes have an emission pattern that nominally has FWHM of
• 30 – 50o in the plane perpendicular to the active area junction• 5 – 10o in the plane parallel to the junction
• As the angular output distribution of the laser is greater than the fiber acceptance angle and since the laser emitting area is much smaller than the fiber core, so that one can use
• spherical lenses• cylindrical lenses• Fiber taper
to improve the coupling efficiency between edge emitting laser diodes and optical fibers
• Same technique is used for vertical cavity surface emitting lasers (VCSELs).
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continued
• Mass produced connections of laser arrays to parallel multimode fiber has efficiencies of 35%
• Direct (lensless) coupling from a single VCSEL source to a multimode fiber results into efficiencies of upto 90%.
• The use of homogeneous glass microsphere lenses has been tested in series of several hundred laser diode assemblies.
• Spherical glass lens of refractive index 1.9 and diameters ranging between 50 and 60μm were epoxied to the ends of 50 μm core diameter graded index fibers having NA of 0.2. The measured FWHM values of the laser output beams were as follows
• b/w 3 and 9μm for the near field parallel to the junction• b/w 30 and 60o for the field perpendicular to the junction• b/w 15 and 55o for the field parallel to the junction Coupling efficiencies in these experiments ranged between 50 and 80%.
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Fiber-to-Fiber Joints
Interconnecting fibers in a fiber optic system is another very important factor. These interconnects should be low-loss. These interconnects occur at
• Optical source• Photodetector• Within the cable where two fibers are connected• Intermediate point in a link where two cables are connected
The connection can be
• Permanent bond: known as SPLICE
• Easily demountable connection: Known as CONNECTOR
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continued• All joining techniques are subject to different levels of power loss at the
joint. These losses depend on different parameters like
• Input power distribution to the joint• Length of the fiber between the source and the joint• Geometrical and waveguide characteristics of the two ends at the joint• Fiber end face qualities
• The optical power that can be coupled from one fiber to the other is limited by the number of modes that can propagate in each fiber
• A fiber with 500 modes capacity connected with the fiber of 400 modes capacity can only couple 80% of the power
• For a GIN fiber with core radius a, cladding index n2, k=2π/, and n(r) as the variation in the core index profile, the total number of modes can be found from the expression
5.18
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continued
• Eq. 5.18 can be associated with the general local numerical aperture to yield
• As the different fibers can have different values of a, NA(0) and α, so M can be different for different fibers
• The fraction of energy that can be coupled is proportional to the common mode volume Mcomm. The fiber-to-fiber coupling efficiency F is given by
Where ME is the number of modes in the emitting fiber. The fiber-to-fiber coupling loss LF is given in terms of F as
LF = -10 log F
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Case a: All modes equally excited, joint with fiber of the same size having even slight mechanical misalignment can cause power loss
Case b: Propagating modes in the steady state have an equilibrium NA. Joining with an optical fiber of the same core size and same characteristics will face a NA of larger size in the receiving fiber and even a mechanical misalignment cannot cause the power loss.
case b is for longer fibers. Power loss will occur when in the receiving fiber, steady state will be achieved
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Mechanical Misalignment• Mechanical alignment is the major problem when joining two fibers
considering their microscopic size. • A standard multimode GIN fiber core is 50 - 100μm in diameter (thickness
of the human hair)• Single mode fiber has core dia of 9 μm • Radiation losses occur because the acceptance cone of the emitting fiber
is not equal to the acceptance cone of the receiving fiber.• Magnitude of radiation loss depends on the degree of misalignment• Three different types of misalignment can occur
• Longitudinal Separation• Angular misalignment• Axial displacement or lateral displacement
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Axial displacement
• Most common misalignment is the axial displacement.
• It causes the greatest power loss
Illustration:
• Axial offset reduces the overlap area of the two fiber-core end faces
• This in turn reduces the power coupled between two fibers.
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continued
• To illistrate the effect of misalignment, consider two identical step-index fibers of radii a.
• Suppose the axes are offset be a separation d• Assume there is a uniform mdal power distribution in the emitting fiber.• NA is constant for the two fibers so coupled fiber will be proportional to
the common area Acomm of the two fiber cores
• Assignment: show that Acomm has expression
• For step index fiber, the coupling efficiency is simply the ratio of the common core area of the core end face area
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continued
• For Graded Index Fiber the calculations for the power loss between two identical fibers is more complex since n varies across the end face of the core.
• The total power coupled in the common area is restricted by the NA of the transmitting or receiving fiber at the point, depending which one is smaller.
• If the end face of the GIN fiber is uniformly illuminated, the optical power accepted by the core will be that power which falls within the NA of the fiber.
• The optical power density p(r) at a point r on the fiber end is proportional to the square of the local NA(r) at that point
Where NA(r) and NA(0) are defined by eqs. 2.80. p(0) is the power density at the core axis which is related to the total power P in the fiber by
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We can use the parabolic index profile (α=2.0) for which p(r) will be givn as
p(r) = p(0)[1 – {r/a}2 ]P will be calculated as
P = (πa2 / 2) p(0)
The calculations of received power for GIN fiber can be carried out and the result will be
Where P is the total power in the transmitting fiber, d is the distance between two axes and a is the radius of fiber
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continued
The coupling loss for the offsets is given as
For Longitudinal misalignment:For longitudinal misalignment of distance s, the coupling loss is given as
Where s is the misalignment and θc is the critical acceptance angle of the fiber
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Angular misalignment at the joint
When the axes of two fibers are angularly misaligned at the joint, the optical power that leaves the emitting fiber outside the acceptance angle of the receiving fiber will be lost. For two step index fibers with misalignment angle θ, the optical power loss at the joint will be
where
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Fiber Related LossesFiber losses are related to the
• Core diameter• Core area ellipticity, numerical aperture• Refractive index profiles• Core-cladding concentricity
Fiber losses are significant for differences in core radii and NADifferent core radii: Loss is given as
Different NA: Power loss is given as
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Different core index profiles: Coupling loss will be given as
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Fiber End Face Preparation• End face preparation is the first step before splicing or connecting the
fibers through connectors.
• Fiber end must be • Flat• Perpendicular to the fiber axis• Smooth
• Techniques used are• Sawing• Grinding• Polishing
• Grinding and Polishing require a controlled environment like laboratory or factory
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continued
• Controlled fracture techniques are used to cleave the fiber
• Highly smooth and perpendicular end faces can be produced through this method
• Requires a careful control of the curvature and the tension• Improperly controlled tension can cause multiple fracture and can leave a
lip or hackled portion
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Fiber Splicing
Three different types of splicing can be done
• Fusion splicing• V-groove mechanical splicing• Elastic tube splice
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Self-Centering Effect
Influences on Fusion Process
The self-centering effect is the tendency of the fiber to form a homogeneous joint which is consequently free of misalignment as result of the surface tension of the molten glass during the fusion bonding process
Core Eccentricity
The process of aligning the fiber cores is of great importance in splicing. Fibers with high core eccentricity can cause , depending on the position of the relating cores, increased splice losses due to the core offset within the splice
Fiber End Face Quality
The end face quality of fibers to be fused directly influences the splice loss. Thus when cleaving fibers for splicing, the end face of the fiber has to be clean, unchipped, flat and perpendicular to the fiber axis
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Influences on Fusion Process
Fiber Preparation Quality
When preparing the fibers for splicing, it is necessary to ensure that no damage occurs to the fiber cladding
Any damage to the unprotected glass of the fiber can produce micro cracks causing the fiber to break during handling, splicing or storage
Dirt Particles or Coating Residues
Any contamination on the fiber cladding or in the v-grooves can lead to bad fiber positioning.
This can cause fiber offset (fiber axis misalignment) and can influence the fusion process extremely like bad cleave angles
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Influences on Fusion Process
Fiber Melting Characteristics
When fibers are brought together for splice some air gaps are present, called gas bubbles
Electric arc should not be too intense or weak.
When electric arc melts the fibers, the glass tends to collapse inwards, filling the gap
Electrode Condition
High quality splices require a reproducible and stable fusion arc.
Fusion arc is influenced by electrode condition.
Electrode cleaning or replacement is necessary from time to time.
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Fusion Splicing
• It is the thermal bonding of two prepared fiber ends
• The chemical changes during melting sometimes produce a weak splice• Produce very low splice losses
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V-groove splicing
• The prepared fiber ends are first butt together in a V-shaped groove• They are bonded with an adhesive• The V-shaped channel is either grooved silicon, plastic ceramic or metal
substrate• Splice loss depends on the fiber size and eccentricity
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Elastic Tube splicing
• It automatically performs lateral, longitudinal and angular alignment• It splices multimode fiber with losses in the range as commercial fusion
splice• Less equipment and skills are needed• It consists of tube of an elastic material• Internal hole is of smaller diameter as compared to the fiber and is
tapered at two ends for easy insertion of the fiber• A wide range of fiber diameters can be spliced• The fibers to be spiced might not be of the same diameter, still its axial
alignment will be maximum
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Optical Fiber Connectors
Principle requirements of a good connector design are as follows:
Coupling loss: The connector assembly must maintain stringent alignment tolerances to ensure low mating losses. The losses should be around 2 to 5 percent (0.1 to 0.2 dB) and must not change significantly during operation and after numerous connects and disconnects.
Interchangeability: Connectors of the same type must be compatible from one manufacturer to another.
Ease of assembly: A service technician should be able to install the connector in a field environment, that is, in a location other than the connector attachment factory.
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Low environmental sensitivity:Conditions such as temperature, dust, and moisture should have a small effect on connector loss variations.
Low cost and reliable construction:The connector must have a precision suitable to the application, but it must be reliable and its cost must not be a major factor in the system.
Ease of connection:Except for certain unique applications, one should be able to mate and disconnect the connector simply and by hand.
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Connector components
Connectors are available in designs that screw on, twist on, or snap in place. The twist-on and snap-on designs are the ones used most commonly.
The basic coupling mechanisms used belong to either butt-joint or the expanded-beam classes. The majority of connectors use a butt-joint coupling mechanism.
The key components are a long, thin stainless steel, glass, ceramic, or plastic cylinder, known as a ferrule, and a precision sleeve into which the ferrule fits.
This sleeve is known variably as an alignment sleeve, an adapter, or a coupling receptacle.
The center of the ferrule has a hole that precisely matches the sizeof the fiber cladding diameter.
Butt-joint connector:
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Connector components
Expanded beam connector :
Employs lenses on the end of the fiber.
These lenses either collimate the light emerging from the transmitting fiber, or focus the expanded beam onto the core of the receiving fiber.
Optical processing elements, such as beam splitters and switches, can easily be inserted into the expanded beam between the fiber ends.
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Connector types
• Connector are available in designs that screw on, twist on, or snap into place
• Most commonly used are twist on, or snap on designs• These include single channel and multi channel assemblies• The basic coupling mechanism is either a Butt joint or an expanded beam
class• Butt joint connectors employ a metal, ceramic or a molded plastic Ferrule
for each fiber
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Expanded Beam Fiber Optic connector
• Expanded beam connector employs lenses on the end of the fibers.• The lenses collimate the light emerging from the transmitting fiber and
focuses the beam on the receiving fiber• The fiber to lens distance is equal to the focal length• As the beam is collimated so even a separation between the fibers will not
make a difference• Connector is less dependent on the lateral alignment• Beam splitters or switches can be inserted between the fibers
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Optical Connector Types
There are numerous connector styles and configurations.
The main ones are ST, SC, FC, LC, MU, MT-RJ, MPO, and variations on MPO.
ST is derived from the words straight tip, which refers to the ferrule configuration.
SC mean subscriber connector or square connector, although now the connectors are not known by those names.
A connector designed specifically for Fibre Channel applications was designated by the letters FC.
Since Lucent developed a specific connector type, they obviously nicknamed it the LC connector.
ST
SC
FC
LC
The letters MU were selected to indicate a miniature unit.MU
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The designation MT-RJ is an acronym for media termination—recommended jack.
Optical Connector Types
MT-RJ
The letters MPO were selected to indicate a multiple-fiber, push-on/pull-off connecting function.
MPO
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SC connectorST connector
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FC connector LC connector
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MU
MT-RJ
MPO
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Summary Coupling efficiency
Flylead / Pigtail
Optical fiber receptacles
Source to fiber power launching
Power coupling calculations
Lensing Scheme for Coupling Improvement
Fiber SplicingSplicing techniquesGood Splice RequirementsSplice PreparationInfluences on Fusion ProcessFusion Splicing Methods
Optical Fiber Connectors
Connector componentsOptical Connector Types
Coupling Losses
Intrinsic lossesExtrinsic losses