unit ii transmission characteristics of optical fibers dr.gnanasundari/prof/ece/snsce/ocn/unit ii
TRANSCRIPT
UNIT II TRANSMISSION CHARACTERISTICS OF OPTICAL FIBERS
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Attenuation (fiber loss)
• Power loss along a fiber:
• The parameter is called fiber attenuation coefficient in a units of for example [1/km] or [nepers/km]. A more common unit is [dB/km] that is defined by:
Z=0P(0) mW
Z= llpePlP )0()( mw
zpePzP )0()( [3-1]
p
]km/1[343.4)(
)0(log
10]dB/km[ plP
P
l
[3-2]
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Optical fiber attenuation vs. wavelength
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Types of Attenuation
Absorption Loss:Caused by the fibre itself or by impurities in the fibre,such as water and metals.
Scattering Loss:Intrinsic loss mechanism caused by the interaction of photons with the glass itself.
Bending loss:Loss induced by physical stress on the fibre.
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Absorption
• Absorption is caused by three different mechanisms:
1- Impurities in fiber material: from transition metal & particularly from OH ions with absorption peaks at wavelengths 2700 nm, 400 nm, 950 nm & 725nm.
2- Intrinsic absorption (fundamental lower limit): electronic absorption band (UV region) & atomic bond vibration band (IR region) in basic SiO2.
3- Radiation defects
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Material Absorption Losses• Material absorption is caused by absorption of photons within the fiber.
– When a material is illuminated, photons can make the valence electrons of an atom transition to higher energy levels
– Photon is destroyed, and the radiant energy is transformed into electric potential energy. This energy can then
• Be re-emitted (scattering)
• Frees the electron (photoelectric effects) (not in fibers)
• Dissipated to the rest of the material (transformed into heat)
• In an optical fiber Material Absorption is the optical power that is effectively converted to heat dissipation within the fiber.
• Two types of absorption exist:
– Intrinsic Absorption, caused by interaction with one or more of the components of the glass
– Extrinsic Absorption, caused by impurities within the glassDr.Gnanasundari/Prof/ECE/
SNSCE/OCN/Unit II
Intrinsic Absorption 1
Less significant than extrinsic absorption. For a pure (no impurities) silica fibre a low loss window exists between 800 nm and 1600 nm.
• Graph shows attenuation spectrum for
pure silica glass.
• Intrinsic absorption is
very low compared to
other forms of loss.
• It is for this reason that
fibres are made up of silica
and optical communications
systems work between
about 800 to 1600 nm.
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Intrinsic Absorption 2
• Intrinsic absorption in the ultraviolet region is caused by electronic absorption bands. Basically, absorption occurs when a light particle (photon) interacts with an electron and excites it to a higher energy level.
• The main cause of intrinsic absorption in the infrared region is the characteristic vibration frequency of atomic bonds.
In silica glass, absorption is caused by the vibration of silicon-oxygen (Si-O) bonds.
The interaction between the vibrating bond and the electromagnetic field of the optical signal causes intrinsic absorption. Light energy is transferred from the electromagnetic field to the bond.
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Extrinsic Absorption (metallic ions)• Extrinsic absorption is much more significant than intrinsic
• Caused by impurities introduced into the fiber material during manufacture
– Iron, nickel, and chromium
• Caused by transition of metal ions to a higher energy level
• Modern fabrication techniques can reduce impurity levels below 1 part in 1010.
• For some of the more common metallic impurities in silica fibre the table shows the peak attenuation wavelength and the attenuation caused by an impurity concentration of 1 in 109
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Extrinsic Absorption (OH ions)•Extrinsic absorption caused by dissolved water in the glass, as the hydroxyl or OH ion.
•In this case absorption due to the same fundamental processes (between 2700 nm and 4200 nm) gives rise to so called absorption overtones at 1380, 950 and 720 nm.
•Typically a 1 part per million impurity level causes 1 dB/km of attenuation at 950 nm. Typical levels are a few parts per billion
Absorption Spectrum for OH in
Silica
Narrow windows circa 800, 1300 nm
and 1550 nm exist which are
unaffected by
this type of absorption.Dr.Gnanasundari/Prof/ECE/
SNSCE/OCN/Unit II
Scattering Losses in Fibre
• Scattering is a process whereby all or some of the optical power in a mode is transferred into another mode.
• Frequently causes attenuation, since the transfer is often to a mode which does not propagate well. (also called a leaky or radiation mode).
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Types of Scattering Loss in Fibre
•Two basic types of scattering exist:
Linear scattering: Rayleigh and Mie
Non-linear scattering: Stimulated Brillouin and Stimulated Raman.
•Rayleigh is the dominant loss mechanism in the low loss silica window between 800 nm and 1700 nm.
•Raman scattering is an important issue in Dense WDM systems
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Rayleigh Scattering (I)•Dominant scattering mechanism in silica fibres
•Scattering causes by inhomogeneities in the glass, of a size smaller than the wavelength.
•Inhomogeneities manifested as refractive index variations, present in the glass after manufacture.
•Difficult to eliminate with present manufacturing methods
•Rayleigh loss falls off as a function of the fourth power of wavelength:
in this empirical formula is expressed in microns (μm)
•The Rayleigh scattering coefficient Ar is a constant for a given material.
•For 1550 nm the loss is approximately 0.18 dB per km.
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Rayleigh Scattering (II)
•The Rayleigh scattering coefficient Ar depends:
-The fibre refractive index profile
-The doping used to achieve a given core refractive index
•For a step index germanium doped fibre Ar is given by:
Ar = 0.63 + 2.06.NA dB/km
•For a graded index near-parabolic profile fibre Ar is given by:
Ar = 0.63 + 1.75.NA dB/km
Exercise: Show that for a graded index fibre with a numerical aperture of 0.275 operating at 1330 nm the Rayleigh scattering loss is approximately 0.36 dB/km.
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Scattering Loss
• Small (compared to wavelength) variation in material density, chemical composition, and structural inhomogeneity scatter light in other directions and absorb energy from guided optical wave.
• The essential mechanism is the Rayleigh scattering. Since the black body radiation classically is proportional to (this is true for wavelength typically greater than 5 micrometer), the attenuation coefficient due to Rayleigh scattering is approximately proportional to . This seems to me not precise, where the attenuation of fibers at 1.3 & 1.55 micrometer can be exactly predicted with Planck’s formula & can not be described with Rayleigh-Jeans law. Therefore I believe that the more accurate formula for
scattering loss is
44
1
5 )exp(
Tk
hc
Bscat
eTemperatur : ,JK 103806.1 Js, 10626.6 -12334 Tkh B
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Absorption & scattering losses in fibers
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Typical spectral absorption & scattering attenuations for a single mode-fiber
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Bending Loss (Macrobending & Microbending)
• Macrobending Loss: The curvature of the bend is much larger than fiber diameter. Lightwave suffers sever loss due to radiation of the evanescent field in the cladding region. As the radius of the curvature decreases, the loss increases exponentially until it reaches at a certain critical radius. For any radius a bit smaller than this point, the losses suddenly becomes extremely large. Higher order modes radiate away faster than lower order modes.
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Microbending Loss
• Microbending Loss: microscopic bends of the fiber axis that can arise when the fibers are incorporated into cables. The power is dissipated through the microbended fiber, because of the repetitive coupling of energy between guided modes & the leaky or radiation modes in the fiber.
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Dispersion in Optical Fibers• Dispersion: Any phenomenon in which the velocity of propagation of any
electromagnetic wave is wavelength dependent.
• In communication, dispersion is used to describe any process by which any electromagnetic signal propagating in a physical medium is degraded because the various wave characteristics (i.e., frequencies) of the signal have different propagation velocities within the physical medium.
• There are 3 dispersion types in the optical fibers, in general:
1- Material Dispersion 2- Waveguide Dispersion 3- Polarization-Mode Dispersion Material & waveguide dispersions are main causes of Intramodal
Dispersion.
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Group Velocity• Wave Velocities:• 1- Plane wave velocity: For a plane wave propagating along z-axis in an
unbounded homogeneous region of refractive index , which is represented by , the velocity of constant phase plane is:
• 2- Modal wave phase velocity: For a modal wave propagating along z-axis represented by , the velocity of constant phase plane is:
3- For transmission system operation the most important & useful type of
velocity is the group velocity, . This is the actual velocity which the signal information & energy is traveling down the fiber. It is always less than the speed of light in the medium. The observable delay experiences by the optical signal waveform & energy, when traveling a length of l along the fiber is commonly referred to as group delay.
1n)ωexp( 1zjktj
11 n
c
kv
)ωexp( zjtj
ω
pv
[3-4]
[3-5]
gV
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Group Velocity & Group Delay• The group velocity is given by:
• The group delay is given by:
• It is important to note that all above quantities depend both on frequency & the propagation mode. In order to see the effect of these parameters on group velocity and delay, the following analysis would be helpful.
d
dVg
ω [3-6]
ωd
dl
V
l
gg
[3-7]
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Input/Output signals in Fiber Transmission System
• The optical signal (complex) waveform at the input of fiber of length l is f(t). The propagation constant of a particular modal wave carrying the signal is . Let us find the output signal waveform g(t).)ω(
z-=0 Z=l
c
c
deftf tj)(~
)( [3-8]
c
c
deftg ljtj )()(~
)( [3-9]
bandwidth. signal optical theis
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
...)(2
1)()()(
If
22
2
ccc
c
ccd
d
d
d
)()(
)(~
)(~
)(~
)(
)()(
)(2/
2/
)(
)]()([2/
2/
)(2/
2/
gljlj
d
dltj
lj
ld
djtj
ljtj
tfed
dltfe
defe
defdeftg
c
c
c
c
c
c
c
c
c
cc
c
c
c
gg V
l
d
dl
c
[3-10]
[3-11]
[3-14]
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Intramodal Dispersion• As we have seen from Input/output signal relationship in optical fiber, the
output is proportional to the delayed version of the input signal, and the delay is inversely proportional to the group velocity of the wave. Since the propagation constant, , is frequency dependent over band width sitting at the center frequency , at each frequency, we have one propagation constant resulting in a specific delay time. As the output signal is collectively represented by group velocity & group delay this phenomenon is called intramodal dispersion or Group Velocity Dispersion (GVD). This phenomenon arises due to a finite bandwidth of the optical source, dependency of refractive index on the wavelength and the modal dependency of the group velocity.
• In the case of optical pulse propagation down the fiber, GVD causes pulse broadening, leading to Inter Symbol Interference (ISI).
)ω( ωcω
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Dispersion & ISI
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
A measure of information capacity of an optical fiber for digital transmission is usually specified by the bandwidth distance product in GHz.km.For multi-mode step index fiber this quantity is about 20 MHz.km, for graded index fiber is about 2.5 GHz.km & for single mode fibers are higher than 10 GHz.km.
LBW
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
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]
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
• 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]
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
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
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
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
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Material Dispersion Diagrams
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000Dr.Gnanasundari/Prof/ECE/
SNSCE/OCN/Unit II
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. 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
[3-21]
)1(2 bkn [3-22]
2)( 22/12
22
1 kannnkaV [3-23]
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Waveguide Dispersion• Delay time due to waveguide dispersion can then be expressed as:
dV
Vbdnn
c
Lwg
)(22 [3-24]
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000Dr.Gnanasundari/Prof/ECE/
SNSCE/OCN/Unit II
Waveguide dispersion 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
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
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)Dr.Gnanasundari/Prof/ECE/
SNSCE/OCN/Unit II
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]
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
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]
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Total Dispersion, zero Dispersion
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
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.
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Optimum single mode fiber & distortion/attenuation characteristics
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 by Modifying waveguide dispersion by changing from a simple step-index core profile to more complicated profiles. There are four major categories to do that:
1- 1300 nm optimized single mode step-fibers: matched cladding (mode diameter 9.6 micrometer) and depressed-cladding (mode diameter about 9 micrometer)
2- Dispersion shifted fibers.3- Dispersion-flattened fibers.4- Large-effective area (LEA) fibers (less nonlinearities for fiber optical
amplifier applications, effective cross section areas are typically greater than 100 ).2m
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Single mode fiber dispersion
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Single mode fiber dispersion
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Single mode Cut-off wavelength & Dispersion
• Fundamental mode is with V=2.405 and
• Dispersion:
• For non-dispersion-shifted fibers (1270 nm – 1340 nm)
• For dispersion shifted fibers (1500 nm- 1600 nm)
0111 LPor HE 22
21
2nn
V
ac
[3-30]
LD
DDd
dD wgmat
)(
)()()(
[3-31]
[3-32]
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Dispersion for non-dispersion-shifted fibers (1270 nm – 1340 nm)
• is relative delay minimum at the zero-dispersion wavelength , and is the value of the dispersion slope in .
22
000 )(
8)(
S
0 0 0S.km)ps/(nm2
0
)( 00
d
dDSS
[3-33]
[3-34]
400 )(1
4)(
S
D [3-35]
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Dispersion for dispersion shifted fibers (1500 nm- 1600 nm)
20
00 )(
2)(
S
00 )()( SD
[3-36]
[3-37]
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Example of dispersionPerformance curve for Set of SM-fiber
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Example of BW vs wavelength for various optical sources for SM-fiber.
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
MFD
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000Dr.Gnanasundari/Prof/ECE/
SNSCE/OCN/Unit II
Bending Loss
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000Dr.Gnanasundari/Prof/ECE/
SNSCE/OCN/Unit II
Bending effects on loss vs MFD
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000Dr.Gnanasundari/Prof/ECE/
SNSCE/OCN/Unit II
Bend loss versus bend radius
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
07.0 ;1056.3
m60 ;m6.3
2
233
n
nn
ba
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
K e r r e f f e c t
Innn 20 K e r r n o n l i n e a r i t y i n f i b e r , w h e r e I i s t h e i n t e n s i t y o f O p t i c a l w a v e .
T e m p o r a l c h a n g e s i n a n a r r o w o p t i c a l p u l s e t h a t i s s u b j e c t e d t o K e r r n o n l i n e a r i t y i n A d i s p e r s i v e m e d i u m w i t h p o s i t i v e G V D .
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II
First-order Soliton
Temporal changes in a medium with Kerr nonlinearity and negative GVD. Since dispersion tends to broaden the pulse, Kerr Nonlinearity tends to squeeze the pulse, resulting in a formation of optical soliton.
Dr.Gnanasundari/Prof/ECE/SNSCE/OCN/Unit II