Topics:• Introduction• Definition• Parts of Optical Fibre• Types Step Index Graded Index Fibres• Parameters of Optical Fibres Acceptance Angle Acceptance Cone Numerical Aperture (NA)
Cont……. Topics:• Normalized Frequency• Number of Modes• Attenuation in Optical Fibres• Dispersion1. Intermodal Dispersion2. Intramodal Dispersion• Applications1. Optical fibres in Communication2. Sensors
Introduction:• Optical fibers are long, thin strands of very pure glass usually 125 µm in diameter. They are arranged in bundles called optical cables and used to transmit light signals over long distances.
What is optical fiber• Optical fibers are very fine fibers of glass. They consist of a glass core, roughly fifty micrometres in diameter, surrounded by a glass "optical cladding" giving an outside diameter of about 125 micrometres. They make use of total internal reflection to confine light within the core of the fiber.
Parts of Optical Fiber• Core – thin glass center of the fiber where light travels.• Cladding – outer optical material surrounding the core.• Buffer Coating – plastic coating that protects the fiber.
Transmission of Light Through Optical Fibers• Total Internal Reflection
Reflection & refractionμ2< μ 1 2 μ 2< μ 1 μ 2< μ 1 2 1 1 1= c 1> cμ1 1 μ1 c μ1 Snell’s law Critical angle Total internal reflection 2 sin c 1
Total Internal Reflection • The angle of the light is always greater than the critical angle. • Cladding does not absorb any light from the core. • The extent that the signal degrades depends upon the purity of the glass and the wavelength of the transmitted light.
THE OPTICAL FIBER CLADDING AIR or JACKET (Lower index) Dia ~ 125 µm CORE(Higher index)Dia ~ 5 – 50 µm μclad μcore μclad
PROPAGATION THROUGH AN OPTICAL FIBER fastinput output t fast slow t
Types of optical fibresBased on their transmission properties and thestructure they are of two types1. Single Mode Fibre2. Multimode Fibre (a) Step Index Fibres (b) Graded Index Fibres
Single (Mono) Mode :This is called so because the refractive index of the fibre ‘step’up as we move from the cladding to the core and this type offibre allows single mode to propagate at a time due to verysmall diameter of its core. μ clad μcore μ clad In this fibre, the refractive indices of thecladding and the core remain constant In this fibre, the size of its core (diameter) is typically around 9-10 μm.
Multimode fibre :This is called so because it allow more than one mode topropagate. Over more than 100 modes can propagatethrough multimode fibres at a time. The size of its core istypically around 50 μm or 62.5 μm. Two Types Step Index Fibres Graded Index Fibres
Multimode Step Index Fibres:No of propagating modes α Core diameter/ WavelengthTypically the core diameter is 50 μm to 100 μm and NumericalAperture (NA) varies from 0.20 to 0.29 respectivelyDue to higher value of NA , and larger core size in this case,fibre connections and launching of light is very easyDue to several modes, the effect of dispersion gets increased,i.e. the modes arrive at the fibre end slightly different timesand so spreading of pulses takes place. μclad μ core μ clad
Multimode Graded Index Fibres:In this fibre, the refractive index of the core decreases withincreasing radial distance from the fibre axis.The value of the refractive index is highest at the centre of thecore and decreases to a value at the edge of the core thatequal the refractive index of the cladding. μclad μcore μcladBy this type of fibre design, the dispersion of the modes iscompensated. Also, light wave follow sinusoidal paths alongthe fibre.
The profile of the refractive index is nearly parabolic thatresults in continual refocusing of the ray in the core, andminimizing the model dispersion.Standard graded index fibres typically have a core diameter of50 μm or 62.5 μm and the cladding diameter of 125 μm. Advantage of Graded Index Fibre over Step Index Fibre Decrease in the modal dispersion
Advantage of Single Mode Fibre over Multi Mode FibreLower signal loss and a higher information capacity or bandwidth than multimode fibres as the signal loss depends on the operational wavelength.These fibres are capable of transferring higher amount of data due to low fibre dispersion. Single mode fibres are known as low loss fibres
PROPAGATION THROUGH AN OPTICAL FIBER By grading the index profile in the core, the pulse broadening was reduced, but it was suggested in 1980 that one could make single-mode fibers which will allow only “one path” through the fiber, thereby removing the pulse broadeninginput output t t μclad μcore μclad
GRADIENT-INDEX OPTICAL FIBER μclad μcore μcladinput output t fast slow tLonger path is now located in lower index region; the larger time takenis compensated by faster travel leading to less pulse broadening
Acceptance angle• acceptance angle: In fiber optics, half the vertex angle of that cone within which optical power may be coupled into bound modes of an optical fiber.• Note 1: The axis of the cone is collinear with the fiber axis, the vertex of the cone is on the fiber end-face, and the base of the cone faces the optical power source.• Note 2: The acceptance angle is measured with respect to the fiber axis.• Note 3: Rays entering an optical fiber at angles greater than the acceptance angle are coupled into unbound modes.
Acceptance angle Cont……… Multimode fiber n0 n0 μ2 0 μ1 c 2 Critical angle: sin c 1
Numerical Aperture Multimode fiber μ0 n0 μ2 0 μ1 c Numerical aperture: 2 2 NA 0 sin 0 sin 0 1 2 for 0 1if 1 2 : 2 2 2 2 1 2 1 2 NA 1 2 2 2 2 2 1 1Here Δ is the relative refractive index difference NA 0.1 0 , max 6Or fractional refractive index
Acceptance Cone LsAcceptance A BCone o θr d θr θ0The coneassociated with Acceptance angle θ0:the angle 2θ0 is 2 2 sin 0 1 2called theacceptance cone 0 sin 1 1 2 2 2
Parameters of Optical fibres 2 2 sin Acceptance angle = 0 1 2 1 2 2 0 sin 1 2 Acceptance Cone = 2θ0 2 2Numerical Aperture (NA) = Sinθ0 = 1 2 2 Skip Distance Ls= d 1 1 0 sin i 1 2 Number of reflections Nr = 1 d 1 0 sin i
Numerical ProblemsActivity 1: The refractive indices for core and cladding for astep index fibre are 1.52 and 1.41 respectivelyCaculate (1) Critical angle (2) Numerical Aperture(3) The maximum incidence angle Hints: Given Here =μ1 = μcore = 1.52, and μ2 = μclad = 1.41 Ans = 68.060 Critical angle θc = sin-1 (μ2/ μ1) 2 2 Ans = 0.568 Numerical Aperture 1 2 2 2 Maximum incidence angle (θ0)= sin 1 1 2 Ans θ0 = 34.60
Numerical ProblemsActivity 2: A light ray enters from air to a fibre. The refractiveIndex of air is 1.0. The fibre has refractive index of core isequal to 1.5 and that of cladding is 1.48. Find the criticalAngle, the fractional refractive index, the acceptance angle andNumerical aperture.Hints: Given Here =μ0 = μair = 1.0; μ1 = μcore = 1.5, and μ2 = μclad= 1.48 Critical angle θc = sin-1 (μ2/ μ1) Ans: θc = 80.630 1 2 Fractional Refractive index = 1.33% of light 1 Ans = 0.568 2 2 Numerical Aperture 1 2 1 2 2Acceptance angle (θ0) = sin 1 2 Ans θ0 = 14.130
Numerical ProblemsActivity 3: Calculate the refractive indices of the core andcladding material of fibre from the following data:NA = 0.22, Δμr = 0.012 and core cladding r coreHints: Fractional Refractive index 1 2 1 Numerical Aperture 2 2 1 2 Ans : μ1= 1.424 = μcore and μ2= 0.988 μ1 = 1.41 = μcladding
Allowed Modes and Normalized Frequency 2Maximum Number of modes that 1 dpropagate successively in the fibre mm NA 2 d mm Hence, number of possible modes will be larger for higher ratio d/λ For multimode fibres mm > 2 OR d 2 For single mode fibres mm < 2 NAAs is evident the parameter mm decides the number of possiblemodes since this parameter depends on core diameter d andthe numerical aperture NA. Therefore, the number of allowedmodes would be different for fibres of different core diameters.
V number: determines how many modes a fiber supports 2 d 2 dV-parameter V 1 2 2 2 NA Single-mode fiber: V 2.405Normalized d d 2 2Frequency n NA 1 2 Therefore, the number of modes mm in terms of normalized frequency is 2 n 2 a 2 2 2 a mm V n1 n2 NA 2
Numerical ProblemsActivity 4: An optical fibre operating at 1.50 μm has a smallValue of core diameter 5.0 μm and fractional refractive indexdifference of 0.0075. Calculate the normalized frequencyand acceptance angle, given μ2 = 1.4.
Cut-off WavelengthDefinition: the wavelength below which multiplemodes of light can be propagated along a particularfiber, i.e., λ> = λ c, single mode, λ < λc, multi-mode 2 d c NA 2.405
Attenuation in optical Fiber• When light travels along the fibre, there is a loss of optical power, which is called attenuation.Definition:Attenuation: Ratio of optical input power (Pi) to the optical output power (Po)Optical Input power: The power transmitted into the fibre from an optical sourceOptical output power: The power received at the fibre end
Fiber Attenuation Cont……..This relation defines the signal attenuation or absorptioncoefficient in terms of length L of the fibre: 10 P log 10 i L P0 Length L of the fibre is expressed in kilometers Here, the unit of Attenuation is decibels/kilometer i.e. dB/km. The main causes of attenuation in optical fibre are: (a)Absorption (b)Scattering (c)Bending losses Each mechanism of loss is influenced by the properties of fibre Material and fibre structure.
Fiber Attenuation Cont……..Absorption losses over a length L of fiber can be describedby the usual exponential law for light intensity (or irradiance) I L I I 0eWhere I0 is the initial intensity or the irradiance of the light.
The attenuation profile is shown in figure which shows the amount ofAttenuation is also wavelength dependent. In the figure, two absorption peaks at 1.25 μm and 1.4 μm are observed which are respectively due to the peculiarities of the single mode fibre and the traces of water remaining in the fibre as an impurity. For commercially available fibers Loss 0.5 dB/km @ 1310 nm 0.25 dB/km @ 1550 nm
Dispersion• A pulse of light sent into a fibre broaden in time as it propagate through the fibre. This phenomenon is known as pulse dispersion.• Intermodal Dispersion: Different rays take different times to propagate through a given length of the fibre. Or Different modes travel with different speeds• Intarmodal dispersion: Any given source emits over a range of wavelength and because of the intrinsic property of the material of the fibre, different wavelength takes different amount of time to propagate along the same path.
Dispersion in Fiber Optics• Dispersion occurs when photons from the same light pulse take slight different paths along the optical fiber. Because some paths will be longer or shorter than other paths the photons will arrive at different times thus smearing the shape of the pulse.• Over long distances, one pulse may merge with another pulse. When this happens, the receiving device will not be able to distinguish between pulses.The dispersive effects in a single mode fibre are much smaller than amultimode fibre. Due to dispersion, optical pulses in optical fibresspread and hence the signal spread over long distances.
Pulse Dispersion in Optical FibreIn optics, dispersion is the phenomenon in which thephase velocity of a wave depends on its frequency. Such medium is called a dispersive medium.
Intermodal DispersionDifferent rays takedifferent times topropagate through a givenlength of the fibre.In the language of waveoptics, this is known asintermodal dispersionbecause it arises due tothe different modestravelling with differentspeeds.
Intramodal DispersionAny given light source emitsover a range of wavelengthand because of the intrinsicproperty of the material of thefibre, different wavelengthstakes different amounts oftime to propagate along thesame path. This is known asmaterial dispersion orintramodel dispersion
Cont……..There are several factors that cause dispersion inoptical fibres. For Example:(1) In multimode fibres different axial speeds of differenttransverse modes cause intermodal dispersion that limitsthe performance of the fibre.(2) In singe mode fibres, though intermodal dispersionis eliminated, chromatic dispersion occures because ofThe slight variation in the index of the glass with thewavelength of the light.Dispersion limits the bandwidth of the fibre because the spreading optical pulses limit the rate that pulses canfollow one another on the fibre and still remaindistinguishable at the receiver.
Chromatic DispersionFor example, when white light passes through a prism some of thewavelengths of light bend more because their refractive index ishigher, i.e. they travel slower This is what gives us the "Spectrum" ofwhite light. The "red and "orange" light travel slowest and so are bentmost while the "violet" and "blue" travel fastest and so are bent less. Allthe other colours lie in between Figure -The Dispersion of White Light
A TYPICAL OPTICAL COMMUNICATION SYSTEM Electrical Signal Electrical Signal Optical Fiber Optical Source External LED or Laser Modulator Connector/ Optical Splice Optical Detector PIN Diode AmplifierProcessing electronics
Optical Fibre SensorsOptical Fibre sensors are fibre based devices that are used forsensing some typical quantities like temperature ofmechanical strain.These sensors are sometime used for sensing vibrations,pressure, acceleration or concentrations of chemical speciesPrinciple: When a light beam is sent through a optical fibre,then its parameters either in the fibre or several fibreBraggs grating experience subtle change. Then the lightreaches a detector arrangement measure these changes.The light may be changed in five of its optical properties i.e.intensity, phase, polarization, wavelength and spectraldistribution.
Optical Fibre Sensors (A) Intrinsic Sensors (B) Extrinsic Sensors (A) Intrinsic Sensors Fibre Pressure Light A Light Source B Detector Pressure•In this type of fibres, sensing medium is itself fibre•Measure the variation in Intensity of transmitted light signals•Useful in measuring the force being exerted between the two objects•If one apply the pressure then due to micro bending losses the light•intensity at the detector will decrease•If we remove the pressure the intensity will increase.
(B) Extrinsic Sensors•The delivery of light and its collection is done by the fibre•Used to measure vibration, rotation, displacement, velocity, acceleration, torque and twisting.•A major benefit of these sensors is their ability to reachplaces which are otherwise inaccessible .FOR EXAMPLE:Useful to measure the temperature inside aircraft jet engine.Used to measure the internal temperature of electricaltransformer.
(B) Extrinsic Sensors Cont…….. Light LightSource Detector Feed Fibre l Return Fibre The amount of light launched into the return fibre will decrease as the distance between the two fibers is increased. However if length is decreased the light intensity collected by the receiver will decrease. This way these optical fibre sensors are capable of determining small shifts between objects.
Advantages of optical fibres compared withTraditional metal communication lines:1. Fibre optics cables can carry more data as their bandwidth is greater than metal cables2. Fibre optics cables are less susceptible (sensitive) then metal cables to interference.3. Fibre optic cables are much thinner and lighter than metal wires.4. Through fibre optic cables the data can be transmitted digitally rather than analogically.5. Attenuation through fibre cables is very low in transmitting the data over a long distance, so there is no need of repeaters (thing that repeats)
An optical fiber (or fibre) is a glass or plastic fiber that carrieslight along its length. Fiber opticsApplied science EngineeringOptical fibers are widely used as these• permits transmission over longer distances•and at higher bandwidths (data rates) than other forms ofcommunications.•Fibers are used instead of metal wires because signals travelalong them with less loss, and they are also immune toelectromagnetic interference.
Areas of Applications• Carry plain old telephone service (POTS)• For transmission of data• Transmitting broadband signals• In the biomedical industry• Non-Communication Applications (sensors etc…)• Telecommunications• Local Area Networks• Cable TV• Optical Fiber Sensors
The advantages of fiber optic over wire cable• Thinner • Less Expensive• Higher carrying capacity • Digital signals• Less signal degradation • Light weight• Light signal • Non-flammable• Low power• Flexible
DISADVANTAGES OF OPTICAL FIBERS… The terminating equipment is still costly as compared to copper equipment.1. IT has to be handled carefully.2. Optical fiber is more expensive per meter than copper Communication is not totally in optical domain, so repeated electric –optical – electrical conversion is needed.1. Tapping is not possible. Specialized equipment is needed to tap a fiber.1. Optical fiber splicing (joining by interweaving strands) is a specialized technique and needs expertly trained manpower.
DISADVANTAGES OF OPTICAL FIBERS…1. The splicing (joining by interweaving strands) and testing equipments are very expensive as compared to copper equipments.2. Optical fiber can not be join together as easily as copper cable. It requires training and expensive splicing (joining by interweaving strands) and measurement equipment.