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.