FIBER SPLICING

 

-:FIBER SPLICING :-

Ø Fiber splicing:-

Splicing of optical fiber is a technique use to join two optical fibers. This technique is used in optical fiber communication, in order to from long optical links for better as well as long-distance optical signal transmission.

           Splices are permanent joint and are for joining most outside plant cables. There are two types of splices i.e., Fusion and Mechanical.

At the time of splicing two optical fiber, the geometry of the fiber, their proper alignment and mechanical strength must be taken into consideration.

·       Joining two optical cables together.

·       The other, more common, method of joining fiber is called termination   or   connectorization.

·       Splicing is most commonly used in the field but has application in cable assembly houses.

·       In field installation, splicing is a faster and more efficient method and is used to restore fiber optic cables when a buried cable is accidentally severed.

Types of splicing:-

There are two methods of Splicing, Mechanical or Fusion.  Both methods provide much lower insertion loss compared to fiber connector.

Ø Mechanical splicing:-

·       1):--Fiber optic cable mechanical splicing is an alternate splicing technique which does not required a fusion splicer.

   2):-     A mechanical splicer is a junction of two or more optical fiber that is aligned and help in place by an assembly that holds the fiber in alignment using an index matching fluid.

   3):-       Mechanical splicing uses a small, mechanical splice, about 6cm long  and 1cm in diameter that permanently joint the two optical fibers.

   4):-       The snap-type cover, an adhesive cover, or both, are used to permanently fasten the splice.

   5);-      The fiber are not permanently joined, just precisely held together so that light can pass from one to another.(Insertion loss < 0.5dB).

   6):-       Splicing loss is typically 0.3dB. But Fiber mechanical splicing introduces higher reflection than fusion splicing method. 

                7):- Fiber optical cable mechanical splices are small, quite easy to use, and are very handy for either quick repairs or permanent installations.

     8):- Fiber optic cable mechanical splicers are available for single mode or multimode fibers.

 Mechanical splicing doesn’t physical fuse two optical fibers together, rather two fibers are held butt-to-butt inside a sleeve with some mechanical mechanism. Mechanical splicing is mostly used for emergency repairs and fiber testing.

Ø Fusion Splicing :-

      1):-Fusion splicing, is more expensive but has a longer life than mechanical splicing. Fusion method fuses the fiber cores together with less attenuation.(Insertion loss<0.1dB).

       2):- In a fusion splicing process a specialized fusion splicer machine is used to precisely align the two fiber ends the glass end are “Fused” or “welded” together using electric arc or some types of heat.

The fusion splicer performs optical fiber fusion splicing in two steps.

  Precisely align the two fibers.

  Generate a small electric arc to melt the fiber and weld them together.

Fiber splicing, two fibers are literally welded (fused) together by an electric arc. Fusion splicing is the most widely used method of splicing as it provides the most reliable joint between two fibers. Fusions. Fusion splicing is done by an automatic machine called Fusion splicer or fusion splicing machine.

Ø Different between mechanical splicing and fusion splicing:-

 

 

 

 

 

OPTICAL DETECTOR 

  -:Photodiode:-

Introduction

A photodiode is a p-n junction or pin semiconductor device that consumes light energy to generate electric current. It is also sometimes referred as photo-detector, photo-sensor, or light detector.

Photodiodes are specially designed to operate in reverse bias condition. Reverse bias means that the p-side of the photodiode is connected to the negative terminal of the battery and n-side is connected to the positive terminal of the battery.

Photodiode is very sensitive to light so when light or photons falls on the photodiode it easily converts light into electric current. Solar cell is also known as large area photodiode because it converts solar energy or light energy into electric energy. However, solar cell works only at bright light.

The construction and working of photodiode is almost similar to the normal p-n junction diode. PIN (p-type, intrinsic and n-type) structure is mostly used for constructing the photodiode instead of p-n (p-type and n-type) junction structure because PIN structure provide fast response time. PIN photodiodes are mostly used in high-speed applications.

In a normal p-n junction diode, voltage is used as the energy source to generate electric current whereas in photodiodes, both voltage and light are used as energy source to generate electric current.

Photodiode symbol

The symbol of photodiode is similar to the normal p-n junction diode except that it contains arrows striking the diode. The arrows striking the diode represent light or photons. 

A photodiode has two terminals: a cathode and an anode.

Objectives and limitations of photodiode

1. Photodiode should be always operated in reverse bias condition.
2. Applied reverse bias voltage should be low.
3. Generate low noise
4. High gain
5. High response speed
6. High sensitivity to light
7. Low sensitivity to temperature
8. Low cost
9. Small size
10. Long lifetime

How photodiode works?

A normal p-n junction diode allows a small amount of electric current under reverse bias condition. To increase the electric current under reverse bias condition, we need to generate more minority carriers. 

The external reverse voltage applied to the p-n junction diode will supply energy to the minority carriers but not increase the population of minority carriers.

However, a small number of minority carriers are generated due to external reverse bias voltage. The minority carriers generated at n-side or p-side will recombine in the same material before they cross the junction. As a result, no electric current flows due to these charge carriers. For example, the minority carriers generated in the p-type material experience a repulsive force from the external voltage and try to move towards n-side. However, before crossing the junction, the free electrons recombine with the holes within the same material. As a result, no electric current flows.

To overcome this problem, we need to apply external energy directly to the depletion region to generate more charge carriers.

A special type of diode called photodiode is designed to generate more number of charge carriers in depletion region. In photodiodes, we use light or photons as the external energy to generate charge carriers in depletion region.

Types of photodiodes

The working operation of all types of photodiodes is same. Different types of photodiodes are developed based on specific application. For example, PIN photodiodes are developed to increase the response speed. PIN photodiodes are used where high response speed is needed.

The different types of photodiodes are

• PN junction photodiode
• PIN photodiode
• Avalanche photodiode

Among all the three photodiodes, PN junction and PIN photodiodes are most widely used.

PN junction photodiode

PN junction photodiodes are the first form of photodiodes. They are the most widely used photodiodes before the development of PIN photodiodes. PN junction photodiode is also simply referred as photodiode. Nowadays, PN junction photodiodes are not widely used.

When external light energy is supplied to the p-n junction photodiode, the valence electrons in the depletion region gains energy.

If the light energy applied to the photodiode is greater the band-gap of semiconductor material, the valence electrons gain enough energy and break bonding with the parent atom. The valence electron which breaks bonding with the parent atom will become free electron. Free electrons moves freely from one place to another place by carrying the electric current.

When the valence electron leave the valence shell an empty space is created in the valence shell at which valence electron left. This empty space in the valence shell is called a hole. Thus, both free electrons and holes are generated as pairs. The mechanism of generating electron-hole pair by using light energy is known as the inner photoelectric effect.

The minority carriers in the depletion region experience force due to the depletion region electric field and the external electric field. For example, free electrons in the depletion region experience repulsive and attractive force from the negative and positive ions present at the edge of depletion region at p-side and n-side. As a result, free electrons move towards the n region. When the free electrons reaches n region, they are attracted towards the positive terminals of the battery. In the similar way, holes move in opposite direction.   

The strong depletion region electric field and the external electric field increase the drift velocity of the free electrons. Because of this high drift velocity, the minority carriers (free electrons and holes) generated in the depletion region will cross the p-n junction before they recombine with atoms. As a result, the minority carrier current increases.

When no light is applied to the reverse bias photodiode, it carries a small reverse current due to external voltage. This small electric current under the absence of light is called dark current. It is denoted by I _λ.

In a photodiode, reverse current is independent of reverse bias voltage. Reverse current is mostly depends on the light intensity.

In photodiodes, most of the electric current is carried by the charge carriers generated in the depletion region because the charge carriers in depletion region has high drift velocity and low recombination rate whereas the charge carriers in n-side or p-side has low drift velocity and high recombination rate. The electric current generated in the photodiode due to the application of light is called photocurrent. 

The total current through the photodiode is the sum of the dark current and the photocurrent. The dark current must be reduced to increase the sensitivity of the device. 

The electric current flowing through a photodiode is directly proportional to the incident number of photons.

PIN photodiode

PIN photodiodes are developed from the PN junction photodiodes. The operation of PIN photodiode is similar to the PN junction photodiode except that the PIN photodiode is manufactured differently to improve its performance.

The PIN photodiode is developed to increase the minority carrier current and response speed.

PIN photodiodes generate more electric current than the PN junction photodiodes with the same amount of light energy.

Layers of PIN photodiode

A PN junction photodiode is made of two layers namely p-type and n-type semiconductor whereas PIN photodiode is made of three layers namely p-type, n-type and intrinsic semiconductor.

In PIN photodiode, an addition layer called intrinsic semiconductor is placed between the p-type and n-type semiconductor to increase the minority carrier current.

P-type semiconductor

If trivalent impurities are added to the intrinsic semiconductor, a p-type semiconductor is formed.

In p-type semiconductors, the number of free electrons in the conduction band is lesser than the number of holes in the valence band. Therefore, holes are the majority charge carriers and free electrons are the minority charge carriers. In p-type semiconductors, holes carry most of the electric current.

N-type semiconductor

If pentavalent impurities are added to the intrinsic semiconductor, an n-type semiconductor is formed.

In n-type semiconductors, the number of free electrons in the conduction band is greater than the number of holes in the valence band. Therefore, free electrons are the majority charge carriers and holes are the minority charge carriers. In n-type semiconductors, free electrons carry most of the electric current.

Intrinsic semiconductor

Intrinsic semiconductors are the pure form of semiconductors. In intrinsic semiconductor, the number of free electrons in the conduction band is equal to the number of holes in the valence band. Therefore, intrinsic semiconductor has no charge carriers to conduct electric current.

However, at room temperature a small number of charge carriers are generated. These small number of charge carriers will carry electric current.

PIN photodiode operation

A PIN photodiode is made of p region and n region separated by a highly resistive intrinsic layer. The intrinsic layer is placed between the p region and n region to increase the width of depletion region.

The p-type and n-type semiconductors are heavily doped. Therefore, the p region and n region of the PIN photodiode has large number of charge carriers to carry electric current. However, these charge carriers will not carry electric current under reverse bias condition.

On the other hand, intrinsic semiconductor is an undoped semiconductor material. Therefore, the intrinsic region does not have charge carriers to conduct electric current.

Under reverse bias condition, the majority charge carriers in n region and p region moves away from the junction. As a result, the width of depletion region becomes very wide. Therefore, majority carriers will not carry electric current under reverse bias condition.

However, the minority carriers will carry electric current because they experience repulsive force from the external electric field.

In PIN photodiode, the charge carriers generated in the depletion region carry most of the electric current. The charge carriers generated in the p region or n region carry only a small electric current.

When light or photon energy is applied to the PIN diode, most part of the energy is observed by the intrinsic or depletion region because of the wide depletion width. As a result, a large number of electron-hole pairs are generated.

Free electrons generated in the intrinsic region move towards n-side whereas holes generated in the intrinsic region move towards p-side. The free electrons and holes moved from one region to another region carry electric current.

When free electrons and holes reach n region and p region, they are attracted to towards the positive and negative terminals of the battery.

The population of minority carriers in PIN photodiode is very large compared to the PN junction photodiode. Therefore, PIN photodiode carry large minority carrier current than PN junction photodiode.

When forward bias voltage is applied to the PIN photodiode, it behaves like a resistor.

We know that capacitance is directly proportional to the size of electrodes and inversely proportional to the distance between electrodes. In PIN photodiode, the p region and n region acts as electrodes and intrinsic region acts as dielectric.

The separation distance between p region and n region in PIN photodiode is very large because of the wide depletion width. Therefore, PIN photodiode has low capacitance compared to the PN junction photodiode.

In PIN photodiode, most of the electric current is carried by the charge carriers generated in the depletion region. The charge carriers generated in p region or n region carry only a small electric current. Therefore, increasing the width of depletion region increases the minority carrier electric current.

Advantages of PIN photodiode

1. Wide bandwidth
2. High quantum efficiency
3. High response speed

Avalanche photodiode

The operation of avalanche photodiode is similar to the PN junction and PIN photodiode except that a high reverse bias voltage is applied in case of avalanche photodiode to achieve avalanche multiplication.

Applying high reverse bias voltage to the avalanche photodiode will not directly increase the generation of charge carriers. However, it provides energy to the electron-hole pairs generated by the incident light.

When light energy is applied to the avalanche photodiode, electron-hole pairs are generated in the depletion. The generated electron-hole pairs experience a force due to the depletion region electric field and external electric field.

In avalanche photodiode, a very high reverse bias voltage supply large amount of energy to the minority carriers (electron-hole pairs). The minority carriers which gains large amount of energy are accelerated to greater velocities.

When the free electrons moving at high speed collides with the atom, they knock off more free electrons. The newly generated free electrons are again accelerated and collide with other atoms. Because of this continuous collision with atoms, a large number of minority carriers are generated. Thus, avalanche photodiodes generates more number of charge carriers than PN and PIN photodiodes.

Avalanche photodiodes are used in the applications where high gain is an important factor.

Advantages of avalanche photodiode

1. High sensitivity
2. Larger gain

Disadvantages of avalanche photodiode

Generates high level of noise than a PN photodiode

Photodiode operation modes

A photodiode can be operated in one of the two modes: photovoltaic mode or photoconductive mode.

Operation mode selection of the photodiode is depends upon the speed requirements of the application and the amount of dark current that is tolerable.

Photovoltaic mode

In the photovoltaic mode, the photodiode is unbiased. In other words, no external voltage is applied to the photodiode under photovoltaic mode.

In photovoltaic mode, dark current is very low. Photodiodes operated in photovoltaic mode have low response speed.

The photodiodes operated in photovoltaic mode are generally used for low speed applications or for detecting low light levels.

Photoconductive mode

In photoconductive mode, an external reverse bias voltage is applied to the photodiode.

Applying a reverse bias voltage increases the width of depletion region and reduces the junction capacitance which results in increased response speed. The reverse bias also increases the dark current.

Photodiodes operated in photoconductive mode has high noise current. This is due to the reverse saturation current flowing through the photodiode.

Dark current

Dark current is the leakage current that flows in the photodiode in the absence of light. The dark current in the photodiode increases when temperature increases. The material used to construct the photodiode also affects the dark current.

The different materials used to construct photodiodes are Silicon (Si), Germanium, (Ge), Gallium Phosphide (GaP), Indium Gallium Arsenide (InGaAs), Indium Arsenide Antimonide (InAsSb), Extended Range Indium Gallium Arsenide (InGaAs), Mercury Cadmium Telluride (MCT, HgCdTe).

Germanium, Indium Arsenide Antimonide, Indium Gallium Arsenide and Mercury Cadmium Telluride generates large dark current because they are very sensitive to temperature.

The response speed of Silicon, Gallium Phosphide, Indium Gallium Arsenide and Extended Range Indium Gallium Arsenide is very high.

Performance parameters of a photodiode

Responsivity

Responsivity is the ratio of generated photocurrent to the incident light power.

Quantum efficiency

Quantum efficiency is defined as the ratio of the number of electron-hole pairs (photoelectrons) generated to the incident photons.

Response time or transit time

The response time of a photodiode is defined as the time it takes for light generated charge carriers to cross p-n junction.

Photodiode applications

The various applications of photodiodes are

1. Compact disc players
2. Smoke detectors
3. Space applications
4. Photodiodes are used in medical applications such as computed tomography, instruments to analyze samples, and pulse oximeters.
5. Photodiodes are used for optical communications.
6. Photodiodes are used to measure extremely low light intensities. 

OPTICAL FIBER COMMUNICATION

1. OPTICAL  FIBERS  COMMUNICATION:-

Optical communication is mode of  communiction by which we can transfer the information from one place to another through optical carrier wave.

fiber-optic communication is a method of transmitting information from one place to another by sending light pulse through an optical fiber. The light from an electromagnetic carrier wave that is intensity modulated to carry information.

An optical fiber is a flexible, transparent fiber made by drawing glass, silica or plastic to a diameter slightly thicker than that of human hair.

The filed of optical depends upon the total internal reflection of light rays traveling through tiny optical fibers.

OPTICAL FIBER:-

Optical fiber are used as cable  to transmit light signals from one place to another place without any appreciable loss in the intensity of light. 

The main part of an optical fiber are Center core, Cladding, Coating, Strength Member, Outer Jacket . (CCCSO).

 

                                 (b) Optical Fiber three Layer Structure.

 (1)- Central Core: It is the innermost  core of the optical  fiber made of thin and fine quality  glass or plastic . The diameter of this core is 10 to 100mu m, with refractive index mu1.

(2)-Cladding: It is a layer of glass or plastic surrounding the central core. Its inner and outer diameters are about 100um and 420um respectively.The refractive index of cladding is little less than of the core, but  this difference is very small (~10`3).

(3)-Plastic Jacket: It is a protective plastic jacket , which encloses central core and cladding, to provide safety and strength to optical fiber.

Advantages:-

1)- Extremenly high  bnadwith.  2)- Longer distance 

3)- Low security risk.    4)- Small size.  5)- Light weight 

Limitations:

1)- Fragility    2)- Difficult to instal   3)- Cost higher than copper cable 

 OPTICAL  FREQUENCY  RANGE:-

s/n	Band	Frequency Range	Wavelength Range	Application ( Typical Services)
1. 2.   3. 4. 5. 6. 7. 8.	Very low frequency  low frequency   Medium frequency High frequency Very high frequency Ultra high frequency Super high frequency Extra high frequency	3kHz to 30kHz 30kHz to 300kHz   300kHz to 3000kHz 3MHz to 30MHz 30MHz to 300MHz 300MHz to 3000MHz 3GHz to 30GHz 30GHz to 300GHz	10km to 100km 1km to 10km   100m to 1km 10m to 100m 1m to 10 m 10cm to 1m 1cm to 10 cm 1mm to 1cm	World Wide telegraphy Long distance point to point service, navigational aids. Broadcasting, Navigation Long distance communication Television ,FM, radar, radio Short distance communication Satellite communication Experimental Amateur, government, optical fiber communication

 

OPTICAL WINDOWS:-

In case of optical transmission, the loss is wavelength dependent. So , there is a specific band of wavelength where the signal attenuation  is minimum, which is known as operating or optical window. The wavelength of operation from the optical window is selected because them offer minimum attenuation.

First optical window: 800-900nm (minimum signal loss 4dB/km).

Second optical window: Centered at 1310nm also called O-band,(offers 0.5dB/km).

Third optical window: Centered at 1550nm (losses of  0.2dB/km (also called C-band).

TRANSMISSION OF LIGHT THROUGH OPTICAL FIBER:-

Total internal reflection (TIR) is the phenomenon that involves the reflection of all the incident light off the boundary.TIR only takes place when both of the following two condition are met:

1- the light is in the more dense medium and approaching the less dense medium.

2- the angle of incidence is greater than the critical angle.

Transmission of light through optical fiber is based on the phenomenon of total internal reflection of light.When a light enters from a denser medium (refractive index μ1), to a rarer medium (refractive index μ2),  μ1>μ2  and the angle of incidence is greater than the critical angle θc, then there in no refracted ray and incident light is totally internally reflected back. Thus, total internal reflection is said to take place.

         Suppose a light ray enters from core (refractive μ1) to  cladding (refractive index μ2) where μ1>μ2 let the angle of incidence at the core cladding boundary be  θi.

       According to snell's law, 

                         μ1 sinθi = μ2 sinθr

                         sinθr= μ1 sinθi / μ2

      

     This  is the condition  which is applicable during the transmission of light through optical signals. In optical fiber, the refractive  index of the core  is greater than that of the cladding.

                    The ray of light travels in the core a guided manner, the optical fiber is also called optical waveguide.    

    ACCEPTANCE ANGLE:-

The maximum  incident angle at which an optical element (lens,fiber) or material will transmit light by total internal reflection .

In optical fiber light depend of total internal reflection then incident ray is greater than the angle of critical angle then fiber propogation work and this fiber propogation is called acceptance anlge.              

                                       Fig- Acceptance angle                                                                                                              

NUMERICAL APERTURE (NA):-

1- Numerical Aperture shows the light collecting ability of the fiber thus its value must be high.

2- As higher the value of NA, better will be the opticaal fiber.

3- The greater value of NA will be achieved only when the difference between the two refractive indices is high and for this either,  μ1 is to be higher or μ2 to be low.

 

Derivation for Numerical  Aperture of Optical Fiber:-

TYPES OF OPTICAL FIBER AND THEIR CHARACTERISTICS:-

1)- Single-Mode optical fiber 

2)-  Multimode optical fiber with stepped index 

3)- Multimode optical fiber with graded-index

SINGLE -MODE OPTICAL FIBER :-

1)- This type of optical fiber transmits only mode of light.

2)- it can carry only one wavelength of light across its length.

3)-single-mode fiber came into existence after multimode fibers. They are more recent than the mutimode cables.

4)-Only laser are used as a light source.

5)- light used in single-mode fibers are not in the visible spectrum.

6)-A distinct disadvantange of single-mode fiber is that they are hard to couple.

7)-This wavelength is usually 1310nm or 1550nm.

LOSSES IN OPTICAL FIBER CABLE:-

ATTENUATION:- 

1)- Attenuation represents the reduction in amplitude of signal.

2)- It is called as the transmission loss and it represent the reduction in the intensity of the light rays propogation through it.

3)- It is measure with respect to the distance travelled by light rays in optical cable.

4)- Attenuation is usualliy expressed indecibel(dB).

Attenuation calculation:-

Attenuation loss αL(dB/km) is calculated by 

       αL(dB/km) = 10/L log Pi/Po

where,

αL(dB/km) = Attenution loss in dB 

Pi = Input Power 

Po = Output power 

L= length of fiber cable 

ATTENUATION FACTOR:-

1)- Material Absorption 

    Intrinsic Absorption 

    Extrinsic Absorption 

2)- Linear Scattering 

     Raylight Scattering

     Mie Scattering

3)- Non-linear Scattering 

      Stimulated Brilliouin Scattering  

      Stimulated Raman Scattering

4)- Fiber Bending 

      Micro Bending

      Macro Bending 

5)-Dispersion 

    Intramodal (Chromatic dispersion)

           1)- Material dispersion 

           2)- Waveguide dispersion

    Intermodal (Modal dispersion) 

Material Absorption:-

Outlines :-

• Basics of Material Absorption 

• Factors of Material Absorption

 • Intrinsic Absorption

 • Extrinsic Absorption

Basics of Material Absorption :- 

 During the fabrication process of fiber optic cable; some of the transmitted light is dissipated as heat. 

 It is called as material Absorption.

Factors of Material Absorption:-

  The major factors responsible for material absorption loss are as follows:

  Intrinsic Absorption due to basic atoms of fiber material .

 Extrinsic Absorption due to impurity atoms. 

 Absorption due to atomic defects in the glass material.

Intrinsic Absorption

              

INTRINSIC APSORPTION:-

 In near infrared region, the intrinsic absorption takes place due to the basic fiber material properties. 

 Usually, pure silica glass shows low intrinsic absorption.

  At the short wavelengths (Ultra violet region); Intrinsic absorption is more dominant. 

 In IR region, the absorption peaks are present around the operating wavelength range 700 nm to 1200 nm. 

 Basically an interaction between vibrating SiO band and electromagnetic field of optical region takes place and it produces intrinsic absorption. 

EXTRINSIC ABSORPTION:-

  Optical fibers are manufactured using melting techniques. During this process, the metallic ions like 𝐶𝑢2+ , 𝐹𝑒2+ , 𝑁𝑖2+etc gets deposited.

  These are metal element impurities, which causes absorption of incoming photons and it is called as extrinsic absorption. 

 Similarly the OH ions form SiOH bond and it has fundamental absorption at 2700 nm. 

 But the harmonics of these fundamental frequencies at 1380 nm, 1250 nm and 950 nm also produces extrinsic absorption.

  This type of absorption can be reduced by reducing amount of impurities and by reducing level of OH ions. 

SCATTERING :-

Outlines :-

Basics of Scattering 

Classification of Scattering 

Linear Scattering 

 Rayleigh Scattering 

 Mie Scattering 

Non Linear Scattering 

 Stimulated Brilliouin Scattering 

 Stimulated Raman Scattering

Basics of Scattering:-

 Due to non uniformities in fiber optic cable; a straight line path of light rays gets deviated. It is referred as scattering. 

 In case of optical cable; some of the optical power from one propagating mode gets transferred to another mode. 

 This transfer of power takes place through the leaky or radiation mode.

  This leaky mode does not continue to propagate with in the fiber core, but it is radiated out from the fiber. It is scattering loss. 

 This loss are mainly caused by interaction of light with density fluctuations within a fiber. 

 Basically the glass is composed of randomly connected network of molecules, which is made up of several oxides and it increases the compositional fluctuations. 

 In case of multimode fibers, there is a higher dopant concentration and greater compositional fluctuations. Thus scattering losses are more. 

CLASSIFICATION OF SCATTERING LOSS :-

     

Linear Scattering:-

  In case of Linear Scattering optical power transferred from one mode to another mode. But there is no change in frequency on the scattering. 

 There are two types of linear scattering .

 Rayleigh Scattering. 

 Mie Scattering.

Rayleigh Scattering:-

  The light from the sun is scattered in atmosphere to give the sky color blue.

  Rayleigh scattering in the glass is having same phenomenon and this scattering takes place in all directions. 

 The Rayleigh scattering produces attenuation in the light rays and this attenuation is proportional to 1 𝜆 4 . Where 𝜆 is optical wavelength.

 Thus if we transmit the data through the fiber optic cable at lower wavelength; the scattering is minimized. 

 The Rayleigh scattering coefficient is denoted by 𝛾𝑅 

                 

Mie Scattering:-

  The scattering caused by hologenetic which are comparable in size with guided wavelength are called as Mie scattering. 

 This is a linear scattering which is always in forward direction. 

 Factors responsible for Mie scattering are as follows.

        Cylindrical structure of cable is not perfect. 

        Imperfection of core and cladding interface. 

        Core and cladding refractive index is not uniform through out of fiber. 

        There are fluctuation in core diameter.  

        Due to Bubble or strain in fiber. 

 Mie scattering results significant attenuation depending upon fiber material, size, design and manufacturing process. It can be reduce by following steps. 

       Removing imperfections during glass manufacturing process. 

        Controlling the coating of fiber.

       Increase refractive index difference Engineering Funda YouTube Channel                   between core and cladding.

Non Linear Scattering:-

 When the optical power is transferred from one mode to other mode or same mode         with different frequency; Non Linear scattering happens. 

 This scattering takes place either in forward or backward direction. 

 It produces optical gain but there is a shift in frequency.

 This shift in frequency results loss of signal and creates attenuation. 

 There are two types of Non linear scattering:-

     Stimulated Brilliouin Scattering 

     Stimulated Raman Scattering
Stimulated Brilliouin Scattering:-

  When the laser light beam is travelling in optical cable; there are variations in an electric field of this beam. 

 These variations in electric field produces acostic vibrations in the optical cable.

  That means incident photon of acostic frequency as well as it produces a scattered photon. 

 This type of scattering is called as stimulated Brillouin scattering and this scattering is usually in opposite direction to that of incoming beam. 

 The scattered light looks like upper and lower sidebands, which are separated from the incident light by the modulation frequency. 

 During this scattering, a frequency shift is produced which varies with the scattering angle. This frequency shift is maximum in the backward direction.

Stimulated Raman Scattering SRS:-

  Raman scattering basically represents inelastic scattering of photons. 

 When a laser light is travelling through optical cable; the spontaneous scattering takes place. 

 In this process, some of the photons are transferred to the near frequencies. 

 When the scattered photons lose their energy then it is called as stokes shift and when the scattered photons gain energy then it is called as antistokes shift. 

 But if the photons of other frequencies are already present then the scattering of such photons takes place and in this case the two photons are generated. It is called as stimulated Raman Scattering. 

 This scattering is similar to Simulated Brilliouin Scattering but in SRS instead of acaustic photon; a high frequency optical phonon is created. 

 SRS can occur in both forward and reverse direction.

Fiber Bending Loss:-

Outlines:-

  Basics of Fiber Bending Loss

  Types of Fiber Bending Loss 

     Macroscopic Bending Loss 

     Micro bending Losses or Mode Coupling Losses

Basics of Fiber Bending Loss:-

 If there is abrupt change in the radius of curvature of fiber; then the radiation loss takes place from fiber.

  If there is sharp bend of the fiber then there is a probability of mechanical failure of optical cable. 

 Usually the higher order modes are not tightly bound to the core layer; so due to the sharp bends, the radiation losses of such modes take first. 

Types of Fiber Bending Loss:-

  There are two types of fiber bending losses

   1. Macroscopic bending losses 

   2. Microbending Losses or Mode coupling Losses
Macroscopic Bending Loss:-

Macroscopic Bending Loss:-

 There is a radiation loss, when the radius of curvature of bend is greater than the diameter of fiber. Such losses are also referred as large radius losses. 

 As the radius of curvature of bend decreases, such losses increase exponentially. 

 There is a certain critical value of radius of curvature upto which such losses can be observed. 

 In optical cable; the wavefornt perpendicular to the direction of propagation must be maintained to achieve this the part of mode, which is on the outside of bend has to travel faster. 

 It indicates that, the light rays travelling through cladding; should travel faster. 

 It is not possible, so the energy associated with that part is lost through radiation. 

Microbending Losses or Mode Coupling Losses:-

Microbending Losses or Mode Coupling Losses:-

 These are the losses due to small bending or small distortion.

 If there are small fluctuations in the radius of curvature of fiber axis, then microbends are created and light rays radiate out from these microbends. 

 The Microbends are formed due to two main reasons: 

         Non uniformities in the core radius, while manufacturing the cable. 

         During the cabling of fibers, non uniform lateral pressure can be created. 

 To minimize the losses due to microbends we should takes following steps: 

      While manufacturing the cable; a precise control of core diameter is maintained.        A compressible jacket is fitted over the fiber, so that when the external pressure is applied then the deformation of jacket place and there will not be creation of microbends in the core layer of fiber.

Dispersion Loss in Optical Fiber:-

Outlines:- 

 Basics of Dispersion Loss 

 Types of Dispersion Loss 

 Intramodal Dispersion

  Types of Intramodal Dispersion 

 Material Dispersion 

 Wave Guide Dispersion 

 Intermodal Dispersion 

Basics of Dispersion Losses :-

 Dispersion is basically one of the limiting factors which decides, how much data can ne transmitted through optical cable.

  Due to dispersion, broadening of the output pulse takes place as well as there can be Inter Symbol Interference ISI. 

 All these factors, limit the information carrying capacity of optical cable. 

 The two major sources of dispersion are material dispersion and waveguide dispersion. 

 Material dispersion arises due to frequency dependent response of a material used to manufacture the cable. 

 When the speed of wave in a waveguide depends on its frequency then waveguide dispersion takes place.

Types of Dispersion Losses:-

  There are two types of dispersion:-

 1. Intramodal Dispersion 

 2. Intermodal Dispersion

Intramodal Dispersion Losses:-

  The light source is used at input side. This converts an electrical signal into optical signal. 

 But this light source does not emits single wavelength. 

 In actual practice, this light sources emits band of wavelength. If the LED is used as lights source then this problem is more savior. 

 So the different spectral components will reach at the output at different times. 

 This gives the spreading of output pulse. This is called as Intramodal dispersion. 

Types of Intramodal Dispersion Losses:-

 There are two types of Intramodal dispersion 

   1. Material Dispersion

   2. Waveguide Dispersion

Material Dispersion Losses:-

 The material dispersion depends on the refractive index of material used to manufacture the fiber cable. 

 The group velocity is the function of wavelength of light and the group velocity is also the function of refractive index of the material.

  Now depending on the light source, each spectral component of input source will be having different wavelength. 

 Thus each component is traveling with different speed through optical fiber. 

 This gives the spreading of the output pulse. 

 This is called as the material dispersion. It is denoted by 𝐷m.

Material Dispersion Losses:-

 It is given as 𝐷𝑚 = 𝜎𝑚/𝐿𝜎𝜆

Where,

 𝜎𝑚 = width of pulse spread because of material dispersion 

𝜎𝜆 = Spectral width of source 

L = length of fiber cable 

  In terms of wavelength 𝐷𝑚 = 𝜆𝑆0/4 [1 − (𝜆0/𝜆)] 4 

Where,

 𝑆0 = Zero Dispersion slope

 𝜆0 = Zero dispersion wavelength

 It is also given by 𝐷𝑚 = 𝜆/𝑐 |𝑑 2𝑛/𝑑𝜆 2 |

 Where, 𝑛 = Refractive Index

Wave Guide Dispersion Losses:- 

 Whenever the optical signals are passing through the fiber optic cable, then the optical cable is acting as wave guide.

  Now there is a variation in the wavelength of each spectral component emitted from the source. 

 As well as the angle made by each light ray with respect to the axis of optical cable will be different. 

 Because this angle is the function of wavelength of light. 

 Since there is variation in the angles, all the light rays are not reaching to the output at the same time. 

 This gives dispersion at the output. This is called as waveguide dispersion. 

 In case of multimode fibers almost all the light rays are travelling away from cut off axis. 

 So in this case the waveguide dispersion is negligible.

Waveguide Dispersion Losses ;-

 It is given as 𝐷𝑤 = 𝜎𝑤 /𝐿𝜎𝜆 

Where, 

𝜎𝑤 = width of pulse spread because of waveguide dispersion 

𝜎𝜆 = Spectral width of source 

L = length of fiber cable

Intermodal Dispersion Losses:-

  This type of dispersion is also called as ‘Modal Dispersion’

  This dispersion takes place in case of multimode fiber optic cables. 

 Here the different mode are travelling with different group velocities inside an optical fiber. 

 Some modes are travelling with maximum speed, while some are travelling with minimum speed. 

 Thus there is difference between the transit time of these modes. 

 So all the modes are not coming to the output at the same time. 

 This gives spreading of output pulse. 

 This type of dispersion is called as intermodal dispersion. 

 In case of multimode step index fiber, this dispersion is highest. 

 It can be reduced by choosing an optimum refractive index profile.

  In case of graded index fiber it is less by factor of 100 times. 

 For single mode fiber, it is almost zero.