LOSSES IN FIBER OPTIC SYSTEM CHAPTER 4 LOSSES IN FIBER OPTIC SYSTEM

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.

The Basic Attenuation Mechanisms in a Fiber 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.

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.

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.

Absorption Absorption Atomic Defects Extrinsic (Impurity atoms) Intrinsic Absorption Absorption in Ultraviolet region Absorption in Infrared region

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 Compositional fluctuations Inhomogeneities or defects occurring during fiber manufacture

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.

Rayleigh Scatter Diagram-2: Light scattered during transmission

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.

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)

Microbending loss

Attenuation Attenuation Scattering Losses Absorption Radiative losses/ Bending losses Intrinsic Absorption Extrinsic (Impurity atoms) Atomic Defects Inhomogeneities or defects in fiber Compositional fluctuations in material Microscopic bends Macroscopic bends Absorption in Infrared region Absorption in Ultraviolet region

Signal Distortion in Fibers Signal 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.

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)

Dispersion

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

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

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

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

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

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.

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.

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.

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

Formula PRX  in dBm or dBW PTX  in dBm or dBW Total Losses  in dB 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