Objectives Understand the importance of fiber-optic technologies in the information society Identify the fundamental components of a fiber-optic cable Understand the principles by which light travels within a fiber-optic cable, including refraction and total internal reflection 2

Background on Optical Communications Fiber-optic communication Trapping light inside an optical fiber Can carry any form of information Fiber is an optical medium, which means it is capable of transmitting light Based on total internal reflection (TIR) 3

Tyndall’s Experiment 4

Structure of Fiber-Optic Cables Core Cladding Coating 5

General Structure of Fiber-Optic Cables 6

General Structure of Fiber-Optic Cables (continued) 7

Structure of Fiber-Optic Cables – Cladding Cylindrical material made of glass or specialized plastic Central portion of the fiber Light signal carrying the information travels through the core The diameter of the core can range from a couple of micrometers (µm-one millionth of a meter) to a couple of millimeters (mm-one thousandth of a meter) 8

Structure of Fiber-Optic Cables – Jacket Surrounds the cladding Insulates and protects the fiber from physical damage and environmental effects, such as moisture, that might interfere with the inner workings of the cable Usually made of opaque plastic or another type of material 9

How Light Travels Through Fiber TIR is the basis of fiber-optic communication TIR may be considered to be an extreme case of refraction When a light ray strikes a boundary of two materials with different RIs, it bends, or in other terms, refracts to an extent that depends on the ratio of the RIs of the two materials 10

Fiber-Optic Communication Systems 11 Pulses of electricity corresponding to the bits arrive at the input transducer –A device that converts one form of energy to another Devices similar to light bulbs are used as optical transducers at the input of fiber-optic cables to convert electricity into light –Light emitting diodes (LED) –Laser diodes (LD)

Fiber-Optic Communication Systems (continued) 12 Electrical signals arriving at the input of optoelectronic devices are used to modulate the light source The modulated optical signal is emitted by the source and coupled into the cable Once the light is trapped inside the cable, it travels to the other end, where it is demodulated and an output transducer –Photodiode (PD) or phototransistor

Fiber Optic Data Links

Light Used In Fiber Optics Fiber optic systems transmit using infrared light, invisible to the human eye, because it goes further in the optical fiber at those wavelengths.

Wavelength-Division Multiplexing

Refraction The phenomenon that causes a spoon inside a clear glass of water to appear shorter and bent to an observer looking from the outside Light rays that strike the water/air boundary bend to create an image that is shorter than the actual height of the spoon in the water, because water has a higher RI than air TIR is the phenomenon that makes the side of an aquarium act as a mirror when viewed at an appropriate angle 16

Refractive Index Refractive index of an optical medium = Speed of light in a vacuum (300,000,000 meters per second)/speed of light in the optical medium 17

Refractive Index (continued) 18

Total Internal Reflection 19

Total Internal Reflection (continued) 20

Total Internal Reflection (continued) 21

Types of Fiber-Optic Cables 22 Based on mode of propagation Single-mode fiber Multi-mode fiber Based on refractive indices (physical construction) Step index fiber Graded index fiber

Multi-mode (Step-index), Graded Index, Single Mode Cross sectional views ( should be circles*) Multi-modeGraded IndexSingle Mode 125  m ~10  m ~80  m Accurate alignment less needed for splicing. Higher loss. Major time dispersion of short optical pulses due to different geometric paths. Less used today, but historically important. Accurate alignment less needed for splicing. Higher loss. Reduced dispersion due to lower wave speed in central rays, higher wave speed (lower index) in outer part of core. Used for “last mile” and service drops, with single mode reserved for long runs. Accurate alignment needed for splicing. Best low loss. Most widely used fiber type for long spans. *non-circularity of images is an artifact of computer artwork software.

Step Index Fiber 24 A fiber-optic cable with a uniform refractive index throughout its core is classified as a step index fiber Both single-mode and multi-mode

Graded Index Fiber 25 A cable whose core refractive index is non- uniform and varies gradually is classified as a graded index fiber The value of the RI of a graded index fiber is highest in the center of the core and gradually diminishes towards the cladding

Classification based on Modes of propagation Two types of modes exist in both, the step- and the graded-index fibers: Single-mode: As the name implies, a single-mode fiber sustains only one mode of propagation. Multimode: Multimode fibers contain many hundreds of modes.

Single Mode and Multimode

Single Mode Step-index This fiber has a core of constant refractive index but its diameter is only 8-12 µm. This narrow core limits the propagation of waveguide modes to a single one as the angle required for the higher modes would not be achievable in this configuration. The step in refractive index is small (about 1%) but this is sufficient to produce the total internal reflection required to confine the wave to the core of the fiber.

Cladding Core

Multimode Step-index The fiber has a core of constant refractive index surrounded by a cladding of lower refractive index Step-index fiber is so called because the refractive index of the fiber “steps” up as we move from the cladding to the core of the fiber. The diameter of the core is about µm and this is wide enough to allow several different waveguide modes to propagate down the fiber.

Multimode Step-index Fiber

Multimode Graded-index The core is similar in size to that of multimode step- index fiber However, in this case the refraction index is higher at the centre and decreases outwards in a specified non- linear fashion toward the lower refractive index of the cladding.

Multimode Graded-index This graded refractive index profile causes the modes of higher orders which travel at higher velocities to be reflected in the outer portion of the core and thus travel a longer distance than the lower order modes which are slower but are confined to the inner portion of the core.

Graded-Index Fiber

Multimode Graded-index Fiber

Graded-index Fiber

Overview of the different fibers

Fiber-Optic Technology: Benefits/Drawbacks 38 High transmission rate Immunity to electromagnetic interference Low attenuation High security Small weight and size Low power consumption High installation cost Difficulty in splicing