Fiber Optics Communications
Lecture 11
Signal Degradation In Optical Fibers
We will look at
• Loss and attenuation mechanism
• Distortion of optical signals
contd
• Signal attenuation (fiber loss) determines
maximum unamplified or repeaters
separation between a transmitter and a
receiver
• Signal distortion mechanisms in a fiber cause
signal pulses to broaden at they travel. Thus
limiting information carrying capacity of fiber
Attenuation
• Attenuation mechanisms
– Absorption
• Related to fiber material
– Scattering
• Associated with fiber material and structural
imperfections in optical waveguide
– Radiative losses of optical energy
• Perturbation(micorscopic and macroscopic) of fiber
geometry
Attenuation
• P(z) at distance z further down the fiber is
 pz
• Where
P( z )  P(0)e
1
z
 p  ln[
P(0)
]
P( z )
is fiber attenuation coefficient (km-1). The units
of 2zαp is nepers.
Attenuation
10
P(0)
 (dB / km)  log[
]  4.343 p (km1 )
z
P( z )
• This fiber loss or fiber attenuation. This
parameter is a function of wavelength
Attenuation
• A 3dB/km loss means the signal power will
decrease by 50% over 1 km and would
decrease by 75% over 2km.
• Example:
• A 30 km optical fiber that has attenuation of
0.8 dB/km at 1300nm. Find optical output
power Pout if 200 μm of optical power is
launched into fiber.
contd
 200 x106W 
 Pin (W ) 
Pin (dBm )  10 log 
  10 log  1x103W   7.0dBm
1
mW




 P (W ) 
 Pin(W ) 
Pout  10 log  out

10
log
 z



 1mW 
 1mW 
 7.0dBm  (0.8dB / km)(30km)  31.0dBm
P(30km)  1031/10 (1mW )  0.79 x103 mW  0.79W
Absorption
• Absorption is caused by
– Absorption by atomic defects in glass
composition
– Extrinsic absorption by impurity atom in glass
material
– Intrinsic absorption by basic constituent atoms of
fiber materials
Absorption
• Atomic defects are imperfections in atomic
structure of fiber material.
• Examples
– Missing molecules
– High density clusters of atom groups
– Oxygen defects in the glass structure
• Atomic defects are small compared to instincts
and extrinsic absorption unless fiber is
exposed to ionizing radiation as nuclear
reactor environment
Absorption
• Absorption is caused by
– Absorption by atomic defects in glass composition
– Extrinsic absorption by impurity atom in glass
material
– Intrinsic absorption by basic constituent atoms of
fiber materials
Absorption
• Impurity absorption results from transition metal ions
such iron, chromium, cobalt, copper and OH (water)
ions.
• Transition metal impurities present in the starting
material used for direct melt fiber range from 1 and 10
ppb causes loss from 1 to 10 dB/km.
• Water impurity concentrations of less than few parts
per billion are required if attenuation is to be less that
20 dB/km
• Early optical fibers had high levels of OH ions which
resulted in large absorption peaks occurring at 1400,
950 and 725 nm.
Contd
• Peaks and valleys in the attenuation curve
resulted in “transmission windows” to optical
fibers.
• Complete elimination of water molecules from
fiber resulted in AllWave fiber by lucent
Absorption
• Absorption is caused by
– Absorption by atomic defects in glass composition
– Extrinsic absorption by impurity atom in glass
material
– Intrinsic absorption by basic constituent atoms of
fiber materials
Intrinsic absorption
• Intrinsic absorption is associated with basic
fiber material (e.g. pure SiO2)
• Intrinsic absorption set fundamental lower
limit on absorption for any particular material
• Results from
– Electronic absorption band in UV region
– Atomic vibration bands in near IR region
• Absorption occurs when a photon interacts
with electron in valence band and excites it to
a higher energy level
UV absorption
• Absorption occurs when a light particle (photon)
interacts with an electron and excites it to a
higher
• energy level. The tail of the ultraviolet absorption
band is shown in figure 2-21.
• UV loss (dB/km) at any wavelength can be
expressed empirically as mole fraction x of GeO2
as
154.2 x
4.63
2
 uv 
46.6 x  60
x10 exp(

)
• UV absorption is stronger for shorter wavelength
Infrared absorption
• In near IR above 1.2μm, the optical waveguide
loss is predominantly determined by presence
of OH ions and inherent infrared absorption
• Interaction between the vibrating bond and
electromagnetic field of optical signal results
in a transfer of energy from field to bond.
• An empirical formula (dB/km) for GeO2-SiO2
is
 48.48
11
 IR  7.81x10 x exp(
)

Scattering Loss
• Scattering losses are caused by the interaction of light
with density fluctuations within a fiber. Density changes
are produced when optical fibers are manufactured.
During manufacturing, regions of higher and lower
molecular density areas, relative to the average density
of the fiber, are created.
• Scattering loss in glass arise from microscopic variations
in material density, compositional fluctuations, and from
structural inhomogeneities or defect occurring during
fiber manufacture
• This gives rise to refractive index variations which occur
within the glass over distances that are small compared
with wavelength
• Index variations cause Rayleigh type scattering of light
Rayleigh Scattering
• Rayleigh scattering loss is inversely
proportional to quadratic wavelength
Loss 
1
4
Bending Losses
• Radiative losses occur when fiber undergoes a bend of finite radius of
curvature
• Two types of bends
– Macroscopic
• Radii that are large compared with fiber diameter (fiber around corners)
– Micorscopic
• Radom microscopic bend of fiber axis that can arise when fibers are incorporated
into fiber
• Macrobending losses
– As radius of curvature decreases, loss increases exponentially until at
certain critical radius the loss becomes observable
• These effects can be explained as follows
– Bound core mode has an evanescent filed tail in cladding which decays
exponentially as a function of distance from the core.
– Part of energy of propagating mode travels in fiber cladding.
– When fiber is bent, field tail on far side must move faster to keep up
with the field in core
Bending Losses
• At a certain critical distance xc from the
center of the fiber, the field tail would have to
move faster than the speed of light to keep up
with core field.
• Since this is not possible, optical energy in
field tail beyond xc radiates away
• Since higher order modes are bound less
tightly to fiber core than lower order modes,
the higher order modes are will radiate out of
the fiber first. Thus, total modes supported by
curved fiber is less than straight fiber.
Bending Loss
• Another radiation loss is caused by random
microbends of optical fiber.
• Microbends are repetitive small scale
fluctuations in radius of curvature of fiber axis.
They are caused by
– Nonuniformities in manufacturing
– Non uniform lateral pressure during cabling (this is
referred to as cabling or packaging losses)