Attenuation & Dispersion
Signal Degradation in the Optical Fiber
Signal Attenuation-signal loss
It determines the maximum unamplified or
repeaterless distance between transmitter and
receiver.
Signal Distortion-signal dispersion
•Causes optical pulses broaden.
•Overlapping with neighboring pulses, creating errors
in the receiver output.
•It limits the information carrying capacity of a fiber.
Attenuation
The Basic attenuation mechanisms in a fiber:
1. Absorption:
It is related to the fiber material.
2. Scattering:
It is associated both with the fiber material
and with the structural imperfections in the
optical waveguide.
3. Radiative losses/ Bending losses:
It originates from perturbation (both
microscopic and macroscopic) of the fiber
geometry.
Example: Absorption by Atmospheric
Example: Scattering of light by Atmospheric
The colours of the sky are caused by the scattering of light
Attenuation Units
If P(0) is the optical power in a fiber at the origin
(at z=0), then the power P(z) at a distance z
P(z) =P (0) e -αpz
αp = (1/z) ln [P(0) / P(z)]
Fiber attenuation coefficient
Attenuation coefficient in units of decibels per kilometer,
denoted by dB/ Km, then
α(dB/km) = (10/z) ln [P(0) / P(z)]=4.343 x αp (km-1)
Q: Fiber has an attenuation of 0.4 dB/km at a
wavelength of 1310 nm.
Then after it travels 50 km, what is the optical
power loss in the fiber ?
Q: Optical powers are commonly expressed in
units of dBm, which is the decibel power level
referred to 1 mW. Consider a 30 km long optical
fiber that has an attenuation of 0.4 dB/km at 1310
nm.
Find the optical output power Pout, if Pin is 200 μW
Absorption
Absorption is caused by three different mechanisms:
Absorption is caused by three different mechanisms:
1- Impurities in fiber material: from transition metal ions &
particularly from OH ions with absorption peaks at wavelengths
2700 nm, 400 nm, 950 nm & 725nm.
2- Intrinsic absorption : electronic absorption band (UV region) &
atomic bond vibration band (IR region) in basic SiO2.
3- Radiation defects-damages internal structure of materials
Absorption
1. Absorption by atomic defects
Atomic defects are imperfections in the atomic structure
of the fiber material.
Examples:
•Missing molecules
•High density clusters of atom groups
•Oxygen defects in the glass structure.
•Absorption losses arising from these defects are negligible
compared with intrinsic( energy band) and impurity
absorption.
•Can be significant if the fiber is exposed to ionization
radiations.
1 rad(Si) = 0.01 J/Kg
Absorption
2. Extrinsic absorption by impurity atoms
The dominant absorption factor in silica fibers is the
presence of small quantities of impurities in the fiber
material.
•These impurities include
•OH- (water) ions dissolved in the glass.
•Transition metal ions, such as iron, copper,
chromium and vanadium
Origin :
OH ion impurities in a fiber preform results mainly from the
oxyhydrogen flame used in the hydrolysis reaction of the
SiCl4, GeCl4 and POCl3 starting materials.
Optical fiber attenuation as a function of wavelength yields nominal values of 0.5 dB/km
at 1310 nm and 0.3 dB/km at 1550 nm for standard single mode fiber. Absorption by the
water molecules causes the attenuation peak around 1400nm for standard fiber. The
dashed curve is the attenuation for low water peak fiber.
Absorption
3. Intrinsic absorption by the basic constituent atoms
Intrinsic absorption is associated with the basic fiber
material (e.g pure SiO2).
Intrinsic absorption results from:
1. Electronic absorption bands in the ultraviolet region
2. Atomic vibration bands in the near infrared region
Absorption
1. Electronic absorption (EA) occurs when a photon
interacts with an electron in the valance band and
excites it to a higher energy level. The electronic
absorption is associated with the band gap of the
material. The UV edge of EA follow the empirical
formula
 uv  Ce
E / Eo
Ultraviolet absorption decays exponentially with
increasing wavelength and is small compared with
scattering loss in the near infrared region. UV loss in
dB/km at any  as a function of mole fraction x of GeO2 is
 uv 
154.2 x
 4.63 
10  2 exp 

46.6 x  60



2. The inherent infrared absorption is
associated with the vibration frequency of
chemical bond between the atoms of which the
fiber is composed.
An interaction between the vibrating bond and the
electromagnetic field of the optical signal results in a
transfer of energy from the field to the bond and
thereby giving rise to absorption.
This absorption is quite strong because of many bonds
present in the fiber. Example: GeO2-SiO2.
  48.48 
 IR  7.8110  exp 

  
11
**Optical fiber attenuation characteristics and their limiting mechanisms for a
GeO2 doped low loss water content silica fiber.
A comparison of the infrared absorption induced by various doping
materials in low-loss silica fibers.
Absorption
Atomic Defects
Extrinsic
(Impurity atoms)
Intrinsic
Absorption
Absorption in
Ultraviolet region
Absorption in
Infrared region
Scattering Losses
Scattering losses in glass arise from microscopic variation
in the material density from:
1. Compositional fluctuations-density of components
2. Inhomogeneities or defects occurring during fiber
manufacture-structural defects
These two effects give rise to refractive index variation,
occurring within the glass over distances that are small
compared with the wavelength.
These index variation case Rayleigh-type scattering of
the light and inversely proportional to wavelength.
It decreases dramatically with increasing wavelength
Scattering Losses
Rayleigh scattering in an optical fiber
Combining the infrared, ultraviolet, and scattering losses for
single mode fiber.
Scattering Losses
Compositional fluctuations
Inhomogeneities or defects
in material
in fiber
Radiative losses / Bending Losses
Radiative losses occur whenever an optical fiber undergoes
a bend of finite radius of curvature.
Fiber can be subject to two types of bends:
1. Macroscopic bends
2. Microscopic bends
Macrobending: Light lost from the optical core due to
macroscopic effects such as tight bends being induced in the
fiber itself.
Microbending. Light lost from the optical core due to
microscopic effects resulting from deformation and damage to
the core cladding interface.
Radiative losses / Bending Losses
• For the slight bends losses are unobservable
• By decreasing radius of curvature we will come to the critical value
after which these losses increase drastically, shown as xc in the fig.
• As we know that the electric/magnetic field have a tail in the cladding
region so by bending the cable we come to the extent where cladding
field should move faster to keep up with the core field.
• As this is not possible so the energy in that region radiates away
• Radiation losses depend on the value of xc and radius of curvature R
• As the lower order modes remain close to the core axis and the higher
modes are closer to the cladding so the higher modes will radiate out of
the fiber first.
Radiative losses / Bending Losses
Macrobending losses are normally produced by poor
handling of fiber .
Poor reeling and mishandling during installation can
create severe bending of the fiber resulting in small but
important localized losses
Power loss in a curved fiber
Radiative losses / Bending Losses
Microbending losses:
It is the radiation loss in optical waveguide results from mode
coupling by random microbends.
Fiber curvature causes repetitive coupling of energy between
the guided modes and the leaky or nonguided modes in the
fiber.
Microbending is a much more critical feature and can be a
major cause of cabling attenuation.
These stresses are very difficult to define, however, they can
be caused by:
• nonuniformities in the manufacturing of the fiber
• nonuniform lateral pressures during cabling
• Low temperatures
• High pressures
Microbending losses
Minimizing microbending losses:
A compressible jacket extruded over a fiber reduces microbending resulting from
external forces.
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
Radiative losses / Bending Losses
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.
A fiber optic pressure sensor
This makes use of the increased attenuation experienced by
the fiber as it bends.
Active fiber detector
This uses the escaping light.
Pressure causes loss at the bends
Is the fiber in use?
Radiative losses/
Bending losses
Macroscopic bends
Microscopic bends
Attenuation
Scattering
Losses
Absorption
Intrinsic
Absorption
Extrinsic
(Impurity
atoms)
Absorption
in
Infrared
region
Absorption
in
Ultraviolet
region
Radiative
losses/ Bending
losses
Atomic
Defects
Inhomogeneities
or defects
in fiber
Compositional
fluctuations
in material
Microscopic
bends
Macroscopic
bends
Signal Distortion/Dispersion in Fibers
Optical signal weakens from attenuation
mechanisms and broadens due to distortion effects.
Eventually these two factors will cause
neighboring pulses to overlap.
After a certain amount of overlap occurs,
the receiver can no longer distinguish the
individual adjacent pulses and error arise
when interpreting the received signal.
Pulse broadening and attenuation
Dispersion

The basic need is to match the output waveform to
the input waveform as closely as possible.

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.
Dispersion
Dispersion results when some components of
the input signal spend more time traversing the
fiber than other components.

In a pulse modulated system, this causes the
received pulse to be spread out over a longer
period.

It is noted that actually no power is lost to
dispersion, the spreading effect reduces the
peak power.
Dispersion
Dispersion

Pulse dispersion is usually specified in terms of
“Nanoseconds-per-kilometer”.

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.
Dispersion
•
•
•
Dispersion - time domain spreading / broadening of pulses
Types of dispersion
Chromatic dispersion (CD)
Polarization modes dispersion (PMD)
Chromatic Dispersion
 Deterministic
∆λ
 Linear
 Independent of environmental factors
 Can be compensated
Wavelength
• How does a pulse spread?
 Light sources emit a band of spectral width, ∆λ
 Speed of travel depends on the wavelength and fibre design
Chromatic Dispersion
•
•
In a positive dispersion fibre: short wavelength arrive before long ones
Total CD = Material contribution + waveguide contribution
 Material – depend on material and cant be changed
 Waveguide - depend on refractive index profile-can be engineered
•
Broad Spectrum
1
Difference in group velocity
1
1
1
Broadened pulse
1
0
•
Optical
Input at transmitter
Output at receiver
fibreand transmission
Limits
bandwidth
distance esp. as the bit rates increase
Dispersion compensating techniques
•
•
•
•
Electronic chirp implementation on laser diode and modulator chirp (pulse
shaping-reduce width)
In-line compensation
 Dispersion compensating fibre (DCF)/reverse dispersion fibre (RDF)
/Inverse dispersion fibre (IDF)
 fibre Bragg grating (FBG)
 phase conjugation
Post compensation scheme involving electronic equalization at the receiver.
Soliton transmission
Inverse dispersion fibre
 high negative dispersion
LDCF  DDCF   LSMF  DSMF (1)
 negative dispersion slope
Gumaste A and Antony T 2002 DWDM Network Designs and Engineering Solutions (Indianapolis : Cisco Press)
He G S 2002 Optical phase conjugation: principles, techniques, and applications Prog. in Quant. Elect. 26 pp 131–191
Aikawa K, Yoshida J, Saitoh S, Kudoh M and Suzuki K 2011 Dispersion Compensating Fibre Module Fujikura Tech. rev. pp 16-22
IDF
compensation
Compensating fibre - IDF
Transmitting fibre - SSMF
+
Positive dispersion
Negative dispersion
Longer wavelength - slow
Longer wavelength - fast
Shorter wavelength - fast
Shorter wavelength - slow
Negative dispersion
Longer wavelength - fast
Positive dispersion
Longer wavelength - slow
Shorter wavelength - slow
Shorter wavelength - fast
Dispersion
Dispersion of optical energy within an optical fiber
falls into following categories:

Intermodal Delay or Modal Delay)

Intramodal Dispertion or Chromatic Dispersion
Material Dispertion
Waveguide Dispertion

Polarization –Mode Dispersion
Dispersion
Intermodal delay/ modal delay
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.
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
Group Velocity: It is the speed at which energy in a particular
mode travels along 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?
Answer is
1. Graded index fiber
2. Single mode fiber
How to get one mode and solve the problem
V = 2πa / λ x (n12 – n22)1/2 = 2πa / λ x (NA)
How to get one mode and solve the problem
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.
Step Index Multi-mode
Graded Index Multi-mode
Q: Consider a 1 Km long multimode fiber in which
1.480 and ∆ = 0.10 , so that n2= 1.465.
Then find ∆T= ?
Where:
L = 1 Km
n1 = 1.480
n2= 1.465
∆ = 0.10
∆T = (Ln12/cn2)∆
n 1=
How to characterize dispersion?
• Group delay per unit length can be defined as:
g
d
1 d
2 d



L
dω
c dk
2c d
[3-15]
• If the spectral width of the optical source is not too wide, then the delay
d g
difference per unit wavelength along the propagation path is approximately
d
For spectral components which are apart, symmetrical around center
wavelength, the total delay difference  over a distance L is:
d g
2
L 
d
2 d  
 2

 
  

2 
d
2c 
d
d 
d
d

 
d
d
 L

V
 g

 d 2
  L
 d 2






[3-16]
•
d 2
2 
d 2
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:
1 d g
d  1
D

L d
d  V g

   2c  2
2



[3-17]
• 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,  g can be well approximated by:
g 
d g
d
   DL 
• D has a typical unit of [ps/(nm.km)].
[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
Material Dispersion
Cladding
Input
v g ( 1 )
Emitter
v g ( 2 )
Very short
light pulse
Intensity
Intensity
Core
Output
Intensity
Spectrum, ² 
Spread, ² 
1
o
2

0
t

All excitation sources are inherently non-monochromatic and emit within a
spectrum, ² , of wavelengths. Waves in the guide with different free space
wavelengths travel at different group velocities due to the wavelength dependence
of n1. The waves arrive at the end of the fiber at different times and hence result in
a broadened output pulse.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
t
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, n( )
• Material-induced dispersion for a plane wave propagation in homogeneous
medium of refractive index n:
 mat
d
2 d
2
d  2

L

L

L
n
(

)

dω
2c d
2c d  

L
dn 
n  

c
d 
[3-19]
• The pulse spread due to material dispersion is therefore:
d mat
L  d 2 n
g 
 
 2  L  Dmat ( )
d
c
d
Dmat ( ) is material dispersion
[3-20]
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:
It causes pulse spreading because only part of the optical
power propagation along a fiber is confined to core.
Dispersion arises because the fraction of light power
propagating in the cladding travels faster than the light
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:
b
•
 / k  n2
2
2
n1  n2
2
2
2

 / k  n2
n1  n2
[3-29]
solving for propagation constant:
  n2k (1  b)
[3-31]
• Using V number:
V  ka(n1  n2 )1/ 2  kan2 2
2
2
[3-33]
Waveguide Dispersion
• Delay time due to waveguide dispersion can then be expressed as:
 wg
L
d (Vb) 
 n2  n2 
c
dV 
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
[3-34]
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:
 wg
d wg
n2 L  d 2 (Vb)

   L  Dwg ( ) 
V
d
c
dV 2
Dwg ( )
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
[3-25]
Polarization Mode dispersion
Intensity
t
Output light pulse
n1 y // y
n1 x // x
Ey

Ex
Core
Ex
z
Ey
 = Pulse spread
t
E
Input light pulse
Suppose that the core refractive index has different values along two orthogonal
directions corresponding to electric field oscillation direction (polarizations). We can
take 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 at
different times
© 1999 S.O. K asap,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 vgx and vgy then the
differential time delay  pol between these two polarization over a
distance L is
 pol
L
L


v gx v gy
[3-26]
• The rms value of the differential group delay can be approximated as:
 pol  DPMD L
[3-27]
Chromatic & Total Dispersion
• Chromatic dispersion includes the material & waveguide dispersions.
Dch ( )  Dmat  Dwg
 ch  Dch ( ) L 
[3-28]
• Total dispersion is the sum of chromatic , polarization dispersion and other
dispersion types and the total rms pulse spreading can be approximately
written as:
Dtotal  Dch  D pol  ...
 total  DtotalL 
[3-29]
Chromatic & Total Dispersion
• Chromatic dispersion includes the material & waveguide dispersions.
Dch ( )  Dmat  Dwg
 ch  Dch ( ) L 
[3-28]
• Total dispersion is the sum of chromatic , polarization dispersion and other
dispersion types and the total rms pulse spreading can be approximately
written as:
Dtotal  Dch  D pol  ...
 total  DtotalL 
[3-29]
Total Dispersion, zero Dispersion
Fact 1) Minimum distortion at wavelength about 1300 nm for single mode silica fiber.
Fact 2) Minimum attenuation is at 1550 nm for sinlge mode silica fiber.
Strategy: shifting the zero-dispersion to longer wavelength for minimum attenuation and dispersion.
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Variation in the polarization states of an optical pulse as it passes
through a fiber that has varying birefringence along its length.
Signal Distortion/
Dispersion
Intermodal Delay/
Modal Delay
Intramodal Dispersion/
Chromatic Dispersion
Material
Dispersion
Waveguide
Dispersion
Polarization-mode
Dispersion
Signal Degradation
in the Optical Fiber
Signal Distortion/
Dispersion
Attenuation
Scattering
Losses
Absorption
Intermodal
Delay/
Modal Delay
Intramodal
Dispersion/
Chromatic
Dispersion
Radiative
losses
Polarization
-mode
Dispersion
Material
Waveguide
Dispersion
Dispersion
Intrinsic
Absorption
Extrinsic
(Impurity
atoms)
Absorption
in
Infrared
region
Absorption
in
Ultraviolet
region
Atomic
Defects
Inhomogeneities
or defects
in fiber
Compositional
fluctuations
in material
Microscopic
bends
Macroscopic
bends
Characteristics of Single Mode Fibers
These Characteristics include :
1. Index profile configuration
2. Cutoff wavelength
3. Signal dispersion designations and calculations
4. Mode field diameter
5. Signal loss due to fiber bending.
Three dimensional refractive index profiles for (a) matched cladding 1310nm
optimized (b) depressed cladding 1310nm optimized (c) triangular dispersion
shifted and (d) quadruple clad dispersion flattened single mode fibers.
SM-fiber dispersions
Typical waveguide dispersion and the common material dispersion
for three different single mode fiber designs.
SM-fiber dispersions
Resultant total dispersions
Single mode Cut-off wavelength & Dispersion
2a
• Fundamental mode is HE11 or LP01 with V=2.405 and c 
V
• Dispersion:
d
D ( ) 
 Dmat ( )  Dwg ( )
d
  D( ) L 
• For non-dispersion-shifted fibers (1270 nm – 1340 nm)
• For dispersion shifted fibers (1500 nm- 1600 nm)
n1  n2
2
[3-30]
[3-31]
[3-32]
2
Dispersion for non-dispersion-shifted fibers
(1270 nm – 1340 nm)
d
D ( ) 
 Dmat ( )  Dwg ( )
d
  D( ) L 
S0
0 2
 (  )   0  ( 
)
8

2
•
 0 is relative delay minimum at the zero-dispersion wavelength
value of the dispersion slope in
.
ps/(nm 2 .km)
S 0  S (0 ) 
D ( ) 
S 0 
dD
d    0
0 4 
1

(
) 

4 
 
0
, and
is theS 0
Dispersion for dispersion shifted fibers (1500
nm- 1600 nm)
S0
 ( )   0  (  0 ) 2
2
D( )  (  0 ) S0
[3-36]
[3-37]
Dispersion Calculation
If tmod, tCD, and tPMD are the modal, chromatic, and polarization
mode dispersion times
Then
Then total dispersion tT can be calculated by the relationship.
Note that tmod = 0 for single-mode fibers.
Where
tcd = |DCD | L ∆λ
tPMD = DPMD (fiber length)1/2
Mode-field diameter vs wavelength
Typical mode field diameter variations with wavelength for (a) 1300 nm
optimized (b) dispersion shifted and (c) dispersion flattened single mode fibers
Bending-induced attenuation
Representative increases in single mode fiber attenuation owing to
microbending and macrobending effects
Bending effects on loss vs MFD
Calculated increase in attenuation at 1310 nm from microbending and
macrobending effects as a function of mode field diameter for (a)
depressed cladding single mode fiber and (b) matched cladding single
mode fiber.
Bend loss versus bend radius
International Standards
ITU-T Recommendations for multimode and Single-Mode
Fibers
Chromatic dispersion as a function of wavelength in various
spectral bands for several different optical fiber types
Recommendation G.651
Core diameters :
1) 50 µm
2) 62.5 µm
cladding diameters:( For both fibers)
125-µm
Attenuation :
Range form 2.5 dB/km at 850nm to less that 0.6dB/km at 1310 nm
Light source used:
Vertical cavity surface emitting laser (VSSEL) operating at 850 nm
Ethernet links running at data rates up to 10Gb/s over
distance up to 550 m can use multimode fibers
Recommendation G.652a/b (Standard single mode fiber or 1300nm optimized fiber)
Installed widely in telecommunication networks in the 1990s.
Core diameter:
5 and 8 µm
Cladding diameter:
125µm
Max PMD: 0.2 ps/ √km
Attenuation :
Range form 0.4 dB/km at 1310nm to
less that 0.35dB/km at 1550 nm
This fiber was optimized to have a zerodispersion value at 1310 nm.
With the trend toward operation in the lower-loss 1550-nm spectral
region, the installation of this fiber has decreased dramatically.
If network operators want to use installed G.652 fiber at 1550 nm,
complex dispersion compensation techniques are needed,
Recommendation G.652c/d (low water peak fiber)
It allows operation in the E-band and are used widely for
fiber to the premises (FTTP) installations.
It is created by reducing the water ion concentration in
order to eliminate the attenuation spike in the 1360 to
1460 nm E-band.
It allow operation over the entire wavelength range from
1260 to 1625 nm
Typically a FTTP link transmits three independent
bidirectional channels at 1310, 1490 and 1550nm over the
same fiber.
Recommendation G.653 (Dispersion shifted fiber DSF)
It was developed for the use with 1550 nm lasers.
Zero dispersion point is shifted to 1550 nm where the fiber
attenuation is about half that at 1310nm.
But
It presents dispersion related problems in dense wavelength
division multiplexing (DWDM) applications in the centre of the C
band.
Because
To prevent undesirable nonlinear effects in DWDM systems the
chromatic dispersion values should be positive or negative over the
entire operational band.
Therefore
The use of G.653 fibers for DWDM should be restricted to either the
S band or L band
These fibers are seldom deployed anymore
Recommendation ITU-T G.654 (cutoff wavelength shifted fiber )
Designed for long distance high power transmission.
It has zero dispersion wavelength around 1300 nm
wavelength.
It has very low loss in the 1550nm band, which is
achieved by using pure silica core.
It has a high cutoff wavelength of 1500 nm, restricted to
operation in the 1500 to 1600 nm region.
Typically used only in long distance undersea
application.
Recommendation ITU-T G.655 (Non zero dispersion shifted fiber )
NZDSF was introduced in the mid 1990s for WDM applications.
Principal characteristic:
It has a positive nonzero dispersion value over the entire Cband, which is the spectral operating region for eribium doped
optical fiber amplifiers.
Version G.655b was introduced to extend WDM application
into the S-band.
Version G.655c specifies a lower PMD value of 0.2 ps√km
than the 0.5 ps/√km value of G.655a/b
Recommendation ITU-T G.656
It has a positive chromatic dispersion value ranging from 2
to 14 ps/(nm-km) in the 1460 to 1625 nm wavelength band.
Here dispersion slop is significantly lower than in G.655
fibers
Lower dispersion slope:
It means that the chromatic dispersion changes slower
with the wavelength so that dispersion compensation is
simpler or not needed.
This allows
The use of CWDM without chromatic dispersion compensation
and
Also means that 40 additional DWDM channels can be implemented
in this wavelength band.
Specialty Fibers
Designed to Manipulate or control some characteristic of an
optical fiber.
The light manipulation applications include:
1. Optical signal amplification
2. Optical power coupling
3. Dispersion compensation
4. Wavelength conversions
5. Sensing of physical parameters:
1. Temperature
2. Stress
3. Pressure
4. Vibration
5. Fluid levels
Specialty Fibers
Specialty fibers can be of either a multimode or a single mode
design.
Optical devices that may use such fibers are:
1. Light transmitters
2. Light modulators
3. Optical receivers
4. Wavelength multiplexers
5. Light couplers
6. Splitters
7. Optical amplifiers
8. Optical switches
9. Wavelength add /drop modules
10.Optical attenuators
Example of Specialty Fibers and Their Applications
Generic Parameter Values of an Erbium-Doped Fiber for
Use in the C-Band
Core
Cladding
Core
Cladding
Cross-sectional geometry of four different polarizationmaintaining fibers
End-face patterns of two possible holey fiber structures.