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LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4

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Page 1: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

LOSSES IN FIBER OPTIC SYSTEM

CHAPTER 4

Page 2: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Signal Degradation in the Optical Fiber

Signal Attenuation- It determines the maximum unamplified or

repeaterless distance between transmitter and receiver.

Signal Distortion

- Causes optical pulses broaden.

- Overlapping with neighboring pulses, creating errors in the receiver output.

- It limits the information carrying capacity of a fiber.

Page 3: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

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. Radiation losses/ Bending losses:

It originates from perturbation (both microscopic and macroscopic) of the fiber geometry.

Page 4: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Absorption

Light travels best in clear substances. Impurities such as metal particles or moisture in the fiber can block some of the light energy, it absorb the light and dissipate it in the form of heat energy, which caused absorption loss.

The solution is to use ultra-pure glass and dopant chemicals to minimize impurities, and to eliminate loss at the peak wavelength during the process of fiber manufacturing.

Page 5: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Absorption

Absorption is caused by three different mechanisms:

1- Impurities in fiber material - occurs due to electronic transitions between the energy levels and because of charge transitions from one ion to another. A major source of attenuation is from transition of metal impurity ions such as iron, chromium, cobalt, and copper

2- Intrinsic absorption- Intrinsic absorption results from electronic absorption bands in UV region and from atomic vibration bands in the near infrared region. Absorption occurs when a photon interacts with an electronic in the valance band and excites it to a higher energy level

3- Atomic defects -imperfections in the atomic structure of the fiber materials such as missing molecules, high density clusters of atom group.

Page 6: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Absorption

Absorption inInfrared region

Absorption

Atomic DefectsExtrinsic

(Impurity atoms)Intrinsic

Absorption

Absorption inUltraviolet region

Page 7: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Scattering Loss

Small variation in material density, chemical composition, and structural inhomogeneity scatter light in other directions and absorb energy from guided optical wave.

Scattering losses in glass arise from microscopic variation in the material density from

1. Compositional fluctuations

2. Inhomogeneities or defects occurring during fiber manufacture

Page 8: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Rayleigh Scatter

Rayleigh scatter occurs at random when there are small changes in the refractive index of materials in which the light signal travels.

In this case, it is the changes in the refractive index of the core and the cladding of the fiber optic cable.

This loss is caused by the miniscule variation in the composition and density of the optical glass material itself, which is related to the fiber manufacturing process.

Page 9: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Rayleigh Scatter

Diagram-2: Light scattered during transmission

Page 10: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Bending Loss

Bending losses occurs in two forms - macrobending and microbending. When a cable is bent and it disrupts the path of the light signal. The tighter the bends of a cable, the greater it is of the light loss.

Page 11: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Bending Loss

Radiative losses occur whenever an optical fiber undergoes a bend of finite radius of curvature.

Macrobending: Light lost from the optical core due to macroscopic effects such as tight bends being induced in the fiber itself.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

Microbending:Light lost from the optical core due to microscopic effects resulting from deformation and damage to the core cladding interface. Occurs when a fiber is sheathed within a protective cable. The stresses set up during the cabling process cause small axial distortions (microbends)

Page 12: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Microbending loss

Page 13: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Attenuation

Absorption in

Infrared region

Absorption

Atomic Defects

Extrinsic (Impurity atoms)

Intrinsic Absorption

Absorption in

Ultraviolet region

Attenuation

Scattering Losses

Compositional fluctuations in material

Inhomogeneities or defects

in fiber

Radiative losses/ Bending

losses

Macroscopic bends

Microscopic bends

Page 14: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Signal Distortion in FibersSignal Distortion 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.

Page 15: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Dispersion

Dispersion is the spreading out of a light pulse as it travels through the fiber.

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

Three types: Modal Dispersion Chromatic Dispersion Polarization Mode Dispersion (PMD)

Page 16: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Dispersion

Page 17: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Modal Dispersion

Modal Dispersion Spreading of a pulse because different modes (paths)

through the fiber take different times Only happens in multimode fiber Reduced, but not eliminated, with graded-index fiber

Page 18: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Chromatic Dispersion

Different wavelengths travel at different speeds through the fiber

This spreads a pulse in an effect named chromatic dispersion

Chromatic dispersion occurs in both single mode and multimode fiber Larger effect with LEDs than with lasers A far smaller effect than modal dispersion

Page 19: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Polarization Mode Dispersion

Light with different polarization can travel at different speeds, if the fiber is not perfectly symmetric at the atomic level

This could come from imperfect circular geometry or stress on the cable, and there is no easy way to correct it

It can affect both singlemode and multimode fiber

Page 20: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Modal Distribution

In graded-index fiber, the off-axis modes go a longer distance than the axial mode, but they travel faster, compensating for dispersion But because the off-axis modes travel further, they

suffer more attenuation

Page 21: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Insertion Losses

Most important performance indicator of a fiber optic interconnection. This is the loss of light signal, measured in decibels (dB), during the insertion of a fiber optic connector.

Some of the common causes of insertion losses includes:(i) the misalignment of ferrules during connection,(ii) the air gap between two mating ferrules, and(iii) absorption loss from impurities such as scratches and oil contamination

Insertion loss can be minimized by proper selection of interconnect materials, good polishing and termination process of fiber connectors

Page 22: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Types of insertion losses

Coupling Losses- The connector assembly must maintain

stringent alignment tolerances to ensure low mating losses.

- The losses should be around 2 to 5 percent (0.1 to 0.2 dB) and must not change significantly during operation and after numerous connects and disconnects.

Page 23: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Types of insertion losses

Splicing Losses- Optical power loss at the splicing point of two

ends of optical fiber is known as splice loss.- It is important to remember that actual

splice-loss is the measured splice-loss in both directions divided with two.

Page 24: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Types of insertion losses

Connector Loss - Optical loss at connection points in fiber

cables- Connector losses are associated with the

coupling of the output of one fiber with the input of another fiber, or couplings with detectors or other components.

Page 25: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

LOSSES CALCULATION

• Total Loss = Lsplice + Lfiber + Lconn. + Lnon-linear

Lsplice = Splice Loss

Lfiber = Fiber Loss

Lconn. = Connector Loss

Lnon-linear= Non-linear Loss

Gain,G = Gainamp + Gnon-linear

Gainamp = Amplifier Gain

Gnon-linear = Non-linear Gain

Page 26: LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4. Signal Degradation in the Optical Fiber  Signal Attenuation - It determines the maximum unamplified or repeaterless

Formula

COUTION !!! PRX in dBm or dBW PTX in dBm or dBW Total Losses in dB Total Gain in dB PMARGIN in dB

dB = 10 log (Po/Pin) = 10 log (PRX / PTX)

dBm = 10 log (P/1mW) P PRX atau PTX

dBm = 10 log (P/1mW) P PRX atau PTX