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

• Where

is fiber attenuation coefficient (km-1). The units of 2zαp is nepers.

zpePzP )0()(

])(

)0(ln[1

zP

P

zp

Attenuation

• This fiber loss or fiber attenuation. This parameter is a function of wavelength

)(343.4])(

)0(log[

10)/( 1 km

zP

P

zkmdB p

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

dBmWx

Wx

mW

WPdBmP in

in 0.7101

10200log10

1

)(log10)(

3

6

zmW

WPin

mW

WPPout out

1

)(log10

1

)(log10

dBmkmkmdBdBm 0.31)30)(/8.0(0.7

WmWxmWkmP 79.01079.0)1(10)30( 310/31

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

• UV absorption is stronger for shorter wavelength

)63.4

exp(10606.46

2.154 2

x

x

xuv

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

exp(1081.7 11

xxIR

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

4

1

Loss

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)