Signal Degradation in Optical Fibers

Signal Attenuation & Distortion in Optical Fibers

• Signal attenuation (fiber loss) largely determines the maximum repeaterless separation between optical transmitter & receiver.

• Signal distortion cause that optical pulses to broaden as they travel along a fiber, the overlap between neighboring pulses, creating errors in the receiver output, resulting in the limitation of information-carrying capacity of a fiber.

Signal degradation in optical fibers

Attenuation– Signal attenuation is one of the most important properties of an

optical fiber – The degree of attenuation in a fiber has a large influence on the

system cost.– Signal Attenuation

– Therefore, the unit of attenuation is decibels/kilometer (dB/km).

Attenuation - Standard Fibre

4

600 800 1000 1200 1400 1600 1800

0.2

0.5

1.0

2.0

5

10

0.1

Wavelength (nm)

Att

enua

tion

(dB

/km

)

1 st

win

do

w-

1 97 5 2 n

d w

ind

ow

1 9

8 0

3 rd

win

do

w1 9

8 5

1300 nm

MM-fibre, GaAs-laser or LED

InGaAsP FP-laser or LED

80nm 180 nm

SM-fiber, InGaAsP DFB-laser, ~ 1990 Optical amplifiers

Fourth Generation, 1996, 1.55 mWDM-systems

1550 nm

Types of Attenuation

Absorption Loss:Caused by the fibre itself or by impurities in the fiber, such as water and metals.

Scattering Loss:Arise from microscopic variations in the material density, from compositional fluctuations and from structural defects occurring during fiber manufacture.

Bending loss:Loss induced by physical stress on the fibre.

Material Absorption Losses• Material absorption is caused by absorption of photons within the fibre.

– 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)

• Dissipated to the rest of the material (transformed into heat)

• In an optical fibre Material Absorption is the optical power that is effectively converted to heat dissipation within the fibre.

• 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 glass

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

fibers are made up of silica

and optical communications

systems work between

about 800 to 1600 nm.

Intrinsic Absorption(minimized by suitable choice of core and clad comp)

• Intrinsic absorption in the ultraviolet region is caused 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 due to the interaction of photos with molecular vibrations within the glass.

•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.

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

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.

Scattering Losses in Fibre

• Linear Scattering: cause the transfer of some or all of the optical power contained within one propagating mode to be linearly 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).

Scattering Losses in Fibre

Nonlinear Scattering:

•Optical waveguides do not always behave as completely linear channels whose increase in output power is directly proportional to the input optical power.

•Several nonlinear effects occur, which in the case of scattering cause disproportionate attenuation, usually at high power levels.

•Causes the optical power form one mode to be transferred in either the forward or backward direction to the same, or other modes at a different frequency.

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

Rayleigh Scattering Transmission loss factor

Dominant scattering mechanism in silica fibers.

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:

K is the Boltzmann’s constantn is the refractive index of the mediumλ in this empirical formula is expressed in microns (μm)

Mie Scattering(inhomogeneities comparable to wavelength)

• Irregularities at core clad interface• Δ variations along the length of the fiber• Diameter fluctuations ,strains and bubbles

Can be reduced by Careful extrusion and coating of the fiber. Removing imperfections due to manufacturing

process Increase Δ

Stimulated Brillouin Scattering(backward process)

• Modulation of light through thermal molecular vibrations within the fiber.

• USB and LSB separated from the incident light by the modulation frequency .

• Produces acoustic phonon and a scattered photon in a single mode fiber.

• This produces an optical frequency shift which varies the scattering angle.

• Brillouin Scattering is only significant above a threshold power density. The threshold power is given by

• Where

Stimulated Raman scattering(forward scattering)

• Optical power threshold-3 times than brillouin scattering threshold.

• Produces optical phonon and scattered photon in single mode fiber. The threshold power is given by

Fiber Bend Loss

• Optical fibers suffer radiation losses at bends or curves on their paths.

• This is due to the energy in the evanescent field at the bend exceeding the velocity of light in the cladding.

• Causes light energy to be radiated from the fiber.

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

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