What are Fibre Optics?

Fibre Optics

Fibre Optics? Fibre optics (optical fibres) are the guiding channels through which light energy propagates. These are long, thin strands of very pure glass about the diameter of a human hair designed to guide light waves along their length. The optical fibres are based on the principle of total internal reflection.

What are Fibre Optics? They are arranged in bundles called optical cables and used to transmit light signals over long distances. A single optical fibre consists of three parts: Core - Thin glass center of the fibre where the light travels Cladding - Outer optical material surrounding the core that reflects the light back into the core Jacket - Plastic coating that protects the fibre from damage and moisture

Fibre Optics fibre core glass or plastic cladding plastic jacket

PROPAGATION OF LIGHT IN OPTICAL FIBRE TOTAL INTERNAL REFLECTION

PROPAGATION OF LIGHT IN OPTICAL FIBRE Critical ray Jacket B Cladding µ c i A r c c ɵ c Core µ f Incident ray A ray that is refracted at an angel of 90ᵒ, i.e. travels parallel to core-cladding interface is called critical angel. From figure AC/AB=sin θ c = cos r c But according to Snell s law at core cladding interface sin θ c / sin 90ᵒ = µ c /µ f sin θ c = µ c /µ f Therefore, cos r c = µ c /µ f r c = cos -1 (µ c /µ f ) The angel r c is called critical angel of propagation.

Total Internal Reflection Optical fibres work on the principle of total internal reflection With light, the refractive index is listed The angle of refraction at the interface between two media is governed by Snell s law: n 1sinq 1 = n2sin q 2

Refraction & Total Internal Reflection

Transmission of Light through Optical Fibre

Numerical Aperture and Acceptance Angel The angle of acceptance is the maximum angel that a light ray can make with the axis of the fibre to propagate within the fibre and is given by θ a= sin -1 (µ f 2 - µ c 2 ) 1/2 The numerical aperture of the fibre is closely related to the critical angle and is often used in the specification for optical fibre and the components that work with it The numerical aperture is given by the formula: N.A. = (µ f 2 - µ c2 ) 1/2 = µ f (2 ) 1/2, where = (µ f 2 - µ c2 ) / µ f

Modes of propagation Since optical fibre is a waveguide, light can propagate in a number of modes If a fibre is of large diameter, light entering at different angles will excite different modes while narrow fibre may only excite one mode Multimode propagation will cause dispersion, which results in the spreading of pulses and limits the usable bandwidth Single-mode fibre has much less dispersion but is more expensive to produce. Its small size, together with the fact that its numerical aperture is smaller than that of multimode fibre, makes it more difficult to couple to light sources

Types of fibre Both types of fibre described earlier are known as step-index fibres because the index of refraction changes radically between the core and the cladding Graded-index fibre is a compromise multimode fibre, but the index of refraction gradually decreases away from the center of the core Graded-index fibre has less dispersion than a multimode step-index fibre

Attenuation and Loss in Signals Attenuation in fibre is a measure of the loss of optical power per kilometer when an optical signal propagates through the fibre. Losses in optical fibre result from attenuation in the material itself and from scattering, which causes some light to strike the cladding at less than the critical angle Bending the optical fibre too sharply can also cause losses by causing some of the light to meet the cladding at less than the critical angle Losses vary greatly depending upon the type of fibre Plastic fibre may have losses of several hundred db per kilometer Graded-index multimode glass fibre has a loss of about 2 4 db per kilometer Single-mode fibre has a loss of 0.4 db/km or less

Signal Losses in Fibres The signal loss in the fibre takes place mainly through three mechanism; i) Absorption ii) Rayleigh scattering iii) Geometric effect

Dispersion Dispersion in fibre optics results from the fact that in multimode propagation, the signal travels faster in some modes than it would in others. i) Material dispersion D= {λ ( λ)/c} L d 2 n/dλ 2 ii)waveguide dispersion iii)intermodal dispersion Single-mode fibres are relatively free from dispersion except for intramodal dispersion Graded-index fibres reduce dispersion by taking advantage of higher-order modes One form of intramodal dispersion is called material dispersion because it depends upon the material of the core Another form of dispersion is called waveguide dispersion Dispersion increases with the bandwidth of the light source

Examples of Dispersion

Applications and Uses of Optical Fibre i)medical applications: Widely used for medical diagnostic, endoscopy. ii) Defense services: For privacy purpose the wiring in communications equipments, control mechanism, instruments panel, illumination in military aircraft, ship etc. iii) Sensors: Sensors used for temperature, pressure, rotary, and linear position, liquid vessels, smoke and pollution level use fibre optical technology. iv)entertainment applications: Optical fibre is used in cable television. For the transmission of digital data such as that generated by computers.

The advantages of fibre optic over wire cable Thinner Higher carrying capacity Less signal degradation Light signal Low power Flexible Non-flammable Lightweight

Disadvantage of fibre optic over copper wire cable Optical fibre is more expensive per meter than copper Optical fibre can not be joined together as easily as copper cable. It requires training and expensive splicing and measurement equipment.