CHAPTER-1

OPTICAL FIBER

CHAPTER 1

INTRODUCTION

1.1 OPTICAL FIBER

An optical fiber is a flexible, transparent fiber made of high quality extruded glass (silica) or plastic, slightly thicker than a human hair. It can function as a waveguide to transmit light between the two ends of the fiber. Power over Fiber (PoF) optic cables can also work to deliver an electric current for low power electric devices. Optical fibers are defined as the glass that transmits over long distances. Optical fibers are used to measure the physical parameters such as temperature, strain and pressure etc. optical fibers can be used to make distributed measurements over long distances and also it is not affected by electromagnetic interference.

Optical fibers typically include a transparent core surrounded by a transparent cladding material with a lower index of refraction. Light is kept in the core by total internal reflection. This causes the fiber to act as a waveguide. Fibers that support many propagation paths or transverse modes are called multi-mode fibers (MMF), while those that only support a single mode are called single-mode fibers (SMF). Multi-mode fibers generally have a wider core diameter, and are used for short-distance communication links and for applications where high power must be transmitted. Single-mode fibers are used for most communication links longer than 1,000 meters (3,300ft).

1.1.1 TYPES OF FIBER

There are three types of fiber optic cable commonly used:

1) Single mode

2) Multimode

Fig 1.1 Structure of an optical fiber

1.1.1.1 Single Mode Fiber

Single mode fiber with a relatively narrow diameter, through which only one mode will propagate. It carries higher bandwidth than multimode fiber, but requires a light source with a narrow spectral width. This fiber also gives you a higher transmission rate and up to 50 times more distance than multimode, but it also costs more. Single-mode fiber has a much smaller core than multimode. The small core and single light-wave virtually eliminate any distortion that could result from overlapping light pulses, providing the least signal attenuation and the highest transmission speeds of any fiber cable type.

1.1.1.2 Multi-mode Fiber:

Multi-mode fiber has a little bit bigger diameter, with a common diameters in the 50 to 100 micron range for the light carry component. It gives us high bandwidth at high speeds over medium distances. Light waves are dispersed into numerous paths, or modes, as they travel through the cable's core typically 850 or 1300 nm. Typical multimode fiber core diameters are 50, 62.5, and 100 micrometers. However, in long cable runs, multiple paths of light can cause signal distortion at the receiving end, resulting in an unclear and incomplete data transmission so designers now call for single mode fiber in new applications using Gigabits and beyond.

1.1.2 ADVANTAGES OF OPTICAL FIBER

Fiber optic cables have a much greater bandwidth than metal cables. The amount of information that can be transmitted per unit time of fibre over other transmission media is its most significant advantage. With the high performance single mode cable used by telephone industries for long distance telecommunication. An optical fibre offers low power loss. This allows for longer transmission distances. Fibre optic cables are immune to electromagnetic interference. It can also be run in electrically noisy environments without concern as electrical noise will not affect fibre. In comparison to copper, a fibre optic cable has nearly 4.5 times as much capacity as the wire cable has and a cross sectional area that is 30 times less. Fibre optic cables are much thinner and lighter than metal wires. They also occupy less space with cables of the same information capacity. Lighter weight makes fibre easier to install. An optical fibre has greater tensile strength than copper or steel fibres of the same diameter. It is flexible, bends easily and resists most corrosive elements that attack copper cable.

An optical fibre has greater tensile strength than copper or steel fibres of the same diameter. It is flexible, bends easily and resists most corrosive elements that attack copper cable.

1.2 FIBER OPTIC SENSORS

A fiber-optic sensor system consists of a fiber-optic cable connected to a remote sensor, or amplifier. The sensor emits, receives, and converts the light energy into an electrical signal. Fibers have many uses in remote sensing. Depending on the application, fiber may be used because of its small size, and no electrical power is needed at the remote location. Many sensors can be multiplexed along the length of a fiber by using light wavelength shift for each sensor. By sensing the time delay as light passes along the fiber through each sensor. Time delay can be determined using a device such as an optical time-domain reflectometer and wavelength shift can be calculated using an instrument implementing optical frequency domain reflectometry. Fiber optic sensors are also immune to electromagnetic interference, and do not conduct electricity so they can be used in places where there is high voltage electricity or inflammable material such as jet fuel. Fiber optic sensors can be designed to withstand high temperature.

1.2.1 TYPES OF OPTICAL SENSORS

There are two types of optical sensors are commonly used

1. Intrinsic sensors

2. Extrinsic sensors