Fibre Optics

Introduction

1870 Tyndall demonstrated that light can be guided along a curved stream of water

Why?

Total internal reflection

Electronic communication use radio and microwaves to carry information copper wires and co-axial cables

(Limited band width, information carrying capacity is less)

Use of optical fibre in place of wires enhances the number of signals that can be transmitted simultaneously

1960: Light could be guided by a glass fibreHigh Attenuation

1970: Invention of solid state laser, made optical communication practicable

1977: Commercial communication systems based on optical fibres made their appearance

Optical fibres also used in Fibroscopes, useful in medical diagnostics

An optical fibre is a transparent conduit as thin as human hair made of glass or clear plastic, made to guide light waves along its length

Optical Fiber

Practical optical fibre has three co-axial regions

1. Core- Light guiding region

2. Cladding- Co-axial middle region

3. Sheath- Increases mechanical strength of fibre

The refractive index of cladding is always lower than that of the core

Why?

Purpose of cladding to confine the light to core, How?

Critical angle of propagation

All the rays having ray directions less than the critical angle will be trapped in the fibre due to total internal reflection

Only certain ray directions are allowed to propagate

Modes of the fibre: possible number of paths of light in the fibre

Acceptance Angle

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Acceptance angle is the maximum angle that a light ray can have relative to the axis of the fibre and can propagate down the fibre

Fractional Refractive Index Change

Fractional refractive index change should be very less than 1 for effective guide of light; of the order of 0.01

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Numerical Aperture

The light gathering ability of a fibre depends on two factors, Core Size and the Numerical Aperture

The acceptance angle and fractional refractive index change determine the NA of the fibre, NA does not depend on the physical dimensions of the fibre

NA = sin , i.e. The numerical aperture is defined as the sine of the acceptance angle

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Types of Optical Fibres

Single Mode Fibre: Single mode step index fibre

Multimode Fibre: Multimode step index fibre

Multimode Graded index fibre

SMF

Single Mode Step Index Fibre: Refractive index changes abruptly at the core-cladding boundary. Light travels along a single path i.e. along the axis. This fibre has low value of NA and

Intermodal dispersion does not exist (only one mode exist). With careful choice of material, dimension and wavelength dispersion can be made extremely small. Low dispersion makes it suitable for use with high data rates. Fibre is costly

Multimode Step Index Fibre: Its core has larger diameter than SMF. It has higher dispersion, i.e. less efficient transmission. Easy to manufacture and less costly.

Graded Index (GRIN) Fibre: Multimode fibre with a core consisting of concentric layers of different refractive indices. It has higher value at the centre and falls of with increasing radial distance from the axis. Numerical aperture and acceptance angle decreases with radial distance. Number of modes is half than the similar MMF. Less dispersion, manufacturing is more complex

GRIN

Normalized Frequency: V-number

Pulse Dispersion: Pulse-broadening effect by fibres

The pulse that appears at the output of the fibre is wider than the input pulse. Dispersion is measured in units of time, typically nanoseconds and picoseconds

Intermodal Dispersion

It is dispersion between the modes, caused by the difference in propagation time for the different modes. Numerous modes traveling in a fibre travel with different velocities with respect to the fibre axis, leading to a spread of the input pulse.

Intramodal Dispersion: Light in a fibre consists of a group of wavelength. Light of different wavelength travels at different speeds in a medium. A narrow pulse tend to broaden as they travel down the fiber

Waveguide Dispersion: It arises due to guiding property of fibre. The refractive index of any mode changes with wavelength, causes pulse spreading

Large NA- More modes, more dispersion

Attenuation

Signal attenuation is defined as

ratio of the optical output power from a fibre of length L to the input optical power, in case of an ideal fibre the attenuation would be zero

1. Absorption by material

2. Scattering

3. Waveguide and microbend loss

Applications

1. Illumination and Image Transmission: Endoscopes

2. Optical Communications: light signals replace the traditional electric signals. Increased bandwidth is achieved

3. Optical Fibre Sensors: The variation of refractive index of the optical fibre under the influence of external forses is utilized in fabrication of optical fibre sensors

Thermometer: LED, Coil of fibre optic and photo-detectorSmoke and Pollution DetectorLiquid Level Sensor: Useful in filing of petrol tanks

4. Medical Applications: Endoscopes, in Ophthalmology, in Cardiology, treatment of cancer

5. Military Applications: An aircraft, a ship or a tank requires tons of copper wire for communication, that can be reduced by optical fibre. Fibre guided missiles are used in recent wars

Fibre Optics Communication System

Very much similar to a traditional communications system

Transmitter: Converts electrical signal to light signals

Optical fibre: Transmits the signals

Receiver: Captures the signals at the other end of the fibre and converts them to electrical signals

Advantages

1. Cheaper

2. Smaller in size, lighter in weight, flexible yet strong

3. Not hazardous

4. Immune to EMI and RFI

5. No cross talk

6. Wider bandwidth

7. Low loss per unit length