1 NET 535 NEW TECHNOLOGIES DR HANAA ABDALAZIZ

1 NET 535 : NEW TECHNOLOGIES DR. HANAA ABDALAZIZ ABDALLAH

Fiber Optic Technology

Contents: Introduction Construction Principle Working Classifications Application Advantages Disadvantages

Introduction: We hear about fiber-optic cables whenever people talk about the telephone system, the cable TV system or the Internet. We are increasing ability to transmit more information, more quickly and over longer distances. An optical fiber (or fiber) is a glass or plastic fiber that carries light along its length

What are Fiber Optics? Fiber optics (optical fibers) are long, thin of very pure glass about the diameter of a human hair. They are arranged in bundles called optical cables and used to transmit light signals over long distances.

Construction: Optical Fiber consists of 3 regions 1. Core – thin glass center of the fiber where light travels. 2. Cladding – outer optical material surrounding the core 3. Buffer Coating – plastic coating that protects the fiber, and make it strong.

Principle: The principle behind the transmission of the light waves in an Optical Fiber is “Total Internal Reflection” The angle of refraction at the interface between two media is governed by Snell’s law:

Refraction, Critical angle , Total internal reflection Angle of incidence < critical angle Angle of incidence = critical angle Angle of incidence > critical angle

Working: When light traveling in a dense medium hits a boundary at a steep angle (larger than the "critical angle “for the boundary), the light will be completely reflected. This phenomenon is called total internal reflection. Total internal reflection occurs when light enters from higher refractive index to lower refractive index material,

Acceptance Cone qa n 2 cladding n 1 core n 2 cladding -If the angle too large light will be lost in cladding - If the angle is small enough the light reflects into core and propagates

Acceptance Angle Acceptance Cone qa n 2 cladding n 1 core n 2 cladding Acceptance angle, qa , is the maximum angle in which external light rays may strike the air/Fiber interface and still propagate down the Fiber with <10 d. B loss. Note: n 1 belongs to core and n 2 refers to cladding)

Transmission of signal: The light in a fiber-optic cable travels through the core by constantly bouncing from the cladding and the principle called total internal reflection. Hence the cladding does not absorb any light from the core.

Types of Optical Fibers: Material used Glass Fiber Plastic Fiber Mode of transmission Single mode Multi mode Refractive index profile Step Index Graded Index

Classification based on “Material Used”

On the basis of number of modes: on the basis of number of modes of propagation the optical fiber are classified into two types: (i) Single mode fiber (SMF) and (ii) Multi-mode fiber (MMF) Single-mode fibers – in single mode fiber only one mode can propagate through the fiber. This type of fiber has small core diameter(5 um) and high cladding diameter(70 um) and the difference between the refractive index of core and cladding is very small. There is no dispersion i. e. no degradation of signal during travelling through the fiber. The light is passed through the single mode fiber through laser diode.

Multi-mode fiber : Multi mode fiber allows a large number of modes for the light ray travelling through it. The core diameter is (40 um) and that of cladding is(70 um) The relative refractive index difference is also larger than single mode fiber. There is signal degradation due to multimode dispersion. They are not suitable for long distance communication due to large dispersion and attenuation of the signal.

Classification based on “Modes of Transmission”

STEP INDEX FIBER The refractive index of core is constant The refractive index of cladding is also constant

GRADED INDEX FIBER In this type of fiber core has a non uniform refractive index that gradually decrease from the centre towards the core cladding interface. The cladding has a uniform refractive index. The light rays propagate through it in the form of skew rays or helical rays.

Classification based on “Refractive Index profile”

Advantages: Less signal degradation Less Expensive Light weight Digital Signals Non-Flammable Higher carrying Capacity Thinner data security Free From Electromagnetic Interference High Bandwidth Over Long Distances Fiber optics have a large capacity to carry high speed signals over longer distances without repeaters than other types of cables

Disadvantages Higher initial cost in installation Interfacing cost Strength Lower tensile strength More expensive to repair/maintain Tools: Specialized and sophisticated

LOSSES IN OPTICAL FIBER CABLES The predominant losses in optic Fibers are: absorption losses due to impurities in the Fiber material wavelength dispersion because of the use of a non -monochromatic source radiation losses caused by bends and kinks in the Fiber pulse spreading or modal dispersion due to rays taking different paths down the Fiber coupling losses caused by misalignment & imperfect surface finishes o Dispersion is referred to widening the pulse as the light travels through the fiber optics

COUPLING LOSS

DISPERSION LOSS

OPTICAL FIBER ARCHITECTURE TX, RX, and Fiber Link Input Signal Transmitter Coder or Light Converter Source-to-Fiber Interface Fiber-optic Cable Fiber-to-light Interface Light Detector Receiver Amplifier/Shaper Decoder Output

OPTICAL FIBER ARCHITECTURE – COMPONENTS Light source: Input Signal Amount of light emitted is proportional to the drive current Two common types: LED (Light Emitting Diode) o ILD (Injection Laser Diode) Source–to-fiber-coupler (similar to a lens): A mechanical interface to couple the light emitted by the source into the optical fiber Coder or Converter Light Source-to-Fiber Interface Fiber-optic Cable Fiber-to-light Interface Light Detector Amplifier/Shaper Decoder Receiver Light detector: n n n PIN (p-type-intrinsic-n-type) APD (avalanche photo diode) Both convert light energy into current Output

LIGHT SOURCES (MORE DETAILS…) Light-Emitting Diodes (LED) made from material such as Al. Ga. As or Ga. As. P light is emitted when electrons and holes recombine either surface emitting or edge emitting Injection Laser Diodes (ILD) (LASER) Light Amplification by Stimulated Emission of Radiation similar in construction as LED except ends are highly polished to reflect photons back & forth

A LIGHT SOURCES LED (Light emitting diode) ILD (injection laser diode)

DETECTORS • Detector is the receiving end of a fiber optic link. There are two kinds of Detectors 1. PIN (Positive Intrinsic Negative) 2. APD (Avalanche photo diodes) PIN APD

PIN PHOTODETECTOR w The high electric field present in the depletion region causes photo-generated carriers to Separate and be collected across the reverse –biased junction. This give rise to a current Flow in an external circuit, known as photocurrent.

HOMEWORK QUESTION 1: The refractive indices for core and cladding for a step index fibre are 1. 52 and 1. 41 respectively Caculate (1) Critical angle (2) Numerical Aperture (3) The maximum incidence angle

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