Presentation for Optical Communication: Unit - 1

1. EC8751
OPTICAL
COMMUNICATION

3. Introduction-general optical fiber communication system- basic optical laws
and definitions-optical modes and configurations -mode analysis for optical
propagation through fibers-modes in planar wave guide-modes in cylindrical
optical fiber-transverse electric and transverse magnetic modes- fiber
materials-fiber fabrication techniques-fiber optic cables-classification of optical
fiber-single mode fiber-graded index fiber.
Unit-1 : INTRODUCTION TO OPTICAL
FIBERS

4. INTRODUCTION
Fig. 1.1 Optical fiber waveguide showing the core refractive index n1 surrounded by the
cladding of slightly lower refractive index n2

5. General optical fiber communication system
Fig 1.2 (a) General Communication system (b) Optical Communication system

6. Fiber optical communication Link

7. Basic optical laws and definitions
Ray theory transmission
1. Total internal reflection
2. Acceptance angle
3. Numerical aperture
4. Skew rays

8. Total internal reflection

10. Acceptance angle

11. Numerical Aperture (NA)

16. Skew rays

22. optical modes and configurations

25. mode analysis for optical
propagation through fibers
Electromagnetic Waves
* Comprises of two fields, electric field and magnetic field
* Orthogonal to each other moves with velocity of light.
* Distribution of field is a train of plane of linearly polarized.
Polarisation refers to orientation of the electromagnetic field with respect to some plane.

26.  linearly polarized plane waves
 Elliptically Polarized plane waves
 Circularly polarized waves

27. linearly polarized waves
Any two orthogonal plane waves can be combined
into a linearly Polarized wave. Conversely, any
arbitrary linearly polarized wave can be resolved
into two independent Orthogonal plane waves
that are in phase.
0 0
2 2
0 0
0
1
0
e cos(ω ) e cos(ω )
tan ( )
x x y y
x y
y
x
E E t kz E t kz
E E E E
E
E
 
   
  


28. Elliptically Polarized plane waves
0 0
2
2
2
0 0 0 0
e e E
e cos(ω ) e cos(ω )
2 cos sin
x x y y
x x y y
y y
x x
x y x y
E E
E t kz E t kz
E E
E E
E E E E

 
 
    
   
   
  
   
   
   
   
   

29. Circularly polarized waves
polarized
circularly
left
:
-
polarized,
circularly
right
:
2
&
:
on
polarizati
Circular 0
0
0







E
E
E y
x
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/polclas.html

30. Electromagnetic mode theory for
optical wave propagation

31. Electromagnetic Waves

36. modes in planar wave guide

43. FIBER MATERIALS
 Glass Fibers
 Fluoride Fibers
 Active Glass Fibers
 Chalcogenide Glass Fibers
 Plastic Optical Fiber
 Plastic Clad Silica (PCS) Fiber
Optical fibers are long, thin and flexible strands of optically transparent materials and work as
optical waveguides. The materials - glass or plastic material or a combination of both.

44. Silica glass exhibits the following properties:
 Silica has a good optical transparency in the near infrared (NIR) wavelength region ranging
from 0.85 mm to 1.65 mm. High quality silica glass exhibits lowest attenuation of 0.2
dB/km around 1.5 μm wavelength.
 Long strands of fibers can be drawn from molten silica at reasonably high temperatures.
 Silica-based fibers can be spliced and cleaved without much of practical difficulties.
 A silica fiber has an extremely high mechanical strength against pulling and even bending,
provided that the fiber is not too thick and that the surfaces are well prepared.
 Silica is chemically very stable and does not react with most of the chemicals.

45. Glass Fibers
 Glass is a non-crystalline solid (NCS). Glass in general is a hard substance, usually
brittle and transparent at high temperature.
 Glass is obtained by fusing mixtures of elements, metal oxides, halides, sulfides,
tellurides or selenides.
 Most of the commercially available glasses are prepared by melting and quenching.
 A majority of the commercial glasses, available is materials based on silica (SiO2).
 At room temperature, glass is generally hard. As the temperature is increased beyond
1000°C, silica glass generally softens and further in the temperature around 1400–
1600°C, glass comes into a viscous state. The melting temperature of silica glass can be
reduced by adding soda-lime.
 One of the major advantages of glass is that the properties can be changed by changing
the composition of glass.

46. Fluoride Fibers
 Fluoride optical fibers are based on fluoride glasses, e.g., fluoroaluminate or
fluorozirconate glasses.
 Such glasses are usually from heavy metals such as zirconium or lead.
 Fluorozirconate glass (where ZrF4 is the major component) is a typical member of
the halide glass family and is the principal constituents.
 Among the various halide glass ZBLAN glass (ZrF4-BaF2-LaF3-AlF3-NaF) is used
for making the core of the optical fiber.
Drawbacks:
 High brittleness and extremely high cost associated with the fabrication of halide
fibers.
 A major problem associated with pure halide fiber is that fluoride glass has a tendency to
form microcrystallites which increases attenuation.

47. Active Glass Fibers:
 Optical fibers used as channel as passive component - output power available at the
receiver end is always less than the power launched at the input end (transmitter) of
the fiber. However, by incorporating rare-earth elements into a normally passive
fiber, it is possible to induce new optical and magnetic properties in the fiber. These
properties can obtain amplification, phase retardation and other non-linear
behavior of light propagating through such fibers. This kind of fiber is referred to as
active fibers.
 The rare-earth elements include erbium, neodymium, ytterbium.
 The active fibers provide high gain efficiency, resulting from strong optical
confinement in the waveguide structure.

48. Chalcogenide Glass Fibers:
 Chalcogenide glasses contain at least one of the chalcogen elements such as S, Se or Te.
 Chalcogenide glasses exhibit high optical non-linearity, found applications in non-linear
optics ranging from optical amplifiers to all optical switches.
 The most widely investigated material from the Chalcogenide glass family is As2S3
(cladding) is reported to exhibit a loss as low as 1 dB/km.

49. Plastic Optical Fiber:
 Plastic optical fibers are manufactured from a variety of polymers commonly referred to as
plastic materials such as polystyrene, polycarbonates, and polymethylmethacrylate.
 The attenuation of optical signal in such fibers at these wavelengths is very high typically in
the range from 150 dB/km for PMMA to 1,000 dB/km for polystyrene.
 Applications- industrial controls, automobiles, sensors for detecting high-energy particles.
 Low cost of POF makes POF-based optical communication system cheaper than glass fiber.
 Other advantages of POF include lighter weight, operation in the visible region, greater
flexibility, and resiliency to bending, shock and vibration, ease in handling and
connecting needs simple and inexpensive test equipment, splices, and connectors

50. Plastic Clad Silica (PCS) Fiber:
 Plastic clad silica (PCS) is a compromise between high performance silica fiber and less
efficient plastic fibers.
 It consists of a core made of silica glass and cladding made of a compatible polymer of lower
refractive index.
 Commercial Plastic clad silica (PCS) fibers consist of a pure silica core, a soft silicone cladding,
and a protective jacket.
 The advantages of PCS fiber include high light collection efficiency, insensitivity to bending,
excellent transmission.

51. fiber fabrication techniques
Fig: Schematic of fiber drawing
apparatus

52. Four basic methods used for making optical fibers are:
1) Outside Vapor Phase Oxidation (OVPO) or Outside Vapor Deposition (OVD)
2) Vapor Axial Deposition (VAD)
3) Modified Chemical Vapor Deposition (MCVD)
4) Plasma activated Chemical Vapor Deposition (PCVD)

53. Fig: Basic steps in preparing a perform by the OVPO process

54. Fig: Apparatus for VAD

55. Fig: Schematic of MCVD

56. Fig: Schematic of PCVD

57. Fiber Fabrication Method without Involving Preforms
Rod-in-tube method:
 In this method, a solid rod of a glass ( SiO2:GeO2) with higher refractive index is inserted into a
glass tube (say, SiO2) with lower refractive index.
 When the outer tube is heated to a high temperature both the rod and the tube get well connected.
 The combination is then strongly heated so that the combination of the rod and the tube melt and
the bottom of the tube collapses due to surface tension.
 Long fibers can be drawn from the molten material.

58. Double crucible method:
Soft glass fibers are generally fabricated by using double crucible method.

59. Optical fiber cable
 Necessary to incorporate them in some form of cable structure.
 Depend on the type of installation- Aerial, underground, buried underground, submarine &
environmental conditions.
 The environmental conditions can be of two types,
1). Natural external factors 2).Man-made factors.
 The purpose of cabling :
1. provide protection to fibers and ensure proper functioning of the optical fibers.
2. A good cable protects from the external stress and enhances the life of the fibers.
3. Buffer coating layer – protect from lateral force

60. The major components
1. Primary and secondary buffer coating of the fiber
2. Suitable strength material for core of the cable
3. additional strength members in form of steel wires
4. water blocking materials (for under water cables) and
5. sheath materials.

61.  Tight polymer coating with a composite primary layer, an optional buffer layer, and a
polymer secondary coating are used. For Plant application the below diagram is used.
 In loose packaging within a tube or groove the primary coated fibers are loosely
placed in a hard-outer tube reinforced with composite wall.
 The primary coated fibers are laid in V-grooved cylindrical core surrounded by the
outer tube.

62.  For field applications, many primary or secondary coated fibers are generally placed around a
central strength material fastened by a plastic tape surrounded by outer jacket.
 Submarine cables for unburied routes and undersea cables

63. Single Mode Fibers
 Step index: refractive index of core remains constant & cladding with lower refractive
index.
 Intermodal dispersion: broadening of transmitted light pulses
 LP Modes: Difference between core & cladding indices of refraction is small.
 The advantage of the propagation of a single mode within an optical fiber is that the signal
dispersion caused by the delay differences between different modes in a multimode fiber
may be avoided.
 For the transmission of a single mode the fiber must be designed to allow propagation of
only one mode, while all other modes are attenuated by leakage or absorption
 For single mode operation, only the fundamental LP01, mode can exist.
 The cutoff normalized frequency for the single mode occurs at Vc = 2.405.
 Thus single mode propagation of the LP01mode is possible over the range:
 The normalized frequency for the fiber may be adjusted to within the range by reduction of
the core radius and the relative refractive index difference.

64.  Why single mode used in telecommunication?
 MFD (Mode Field Diameter):
The electric field of the first fundamental mode can be written as:
r -distance measured from center of core along the radius
E0 - Field at zero radius
WO - Mode Field Radius
0
2
0
2
0 2
MFD
);
exp(
)
( W
W
r
E
r
E 



65. Birefringence in single-mode fibers
 Because of asymmetries the refractive indices for the two degenerate modes (vertical &
horizontal polarizations) are different. This difference is referred to as birefringence
x
y
f n
n
B 


66. Birefringence

67. Fiber Beat Length
In general, a linearly polarized mode is a combination of both of the degenerate modes. As the
modal wave travels along the fiber, the difference in the refractive indices would change the phase
difference between these two components & thereby the state of the polarization of the mode.
However after certain length referred to as fiber beat length, the modal wave will produce its
original state of polarization. This length is simply given by:

68. Graded Index Fiber

76. Linearly polarized modes
 In a step index fiber the difference between core & cladding indices difference is very
small. This is the weakly guiding fiber approximation. ∆ is less than 0.03% .
 As ∆ in weakly guiding fibers is very small, then HE-EH mode pairs occur which have
almost identical propagation constants. Such modes are said to be degenerate mode.
 The liner polarized modes are derived from basic transverse electric & transverse magnetic
and hybrid modes
(1) each LP0m is derived from HE1m
(2) each LP1m is derived from TE0m TM0m and HE2m
(3) each LPim is derived from HEl+1m and HEl-1m
 i and m are related to an electric field intensity.
 Modes having lowest cut-off frequency is known as lower order LP mode

78. Leaky modes

79. KEY MODAL CONCEPTS

81. Cylindrical Waveguide Equation

93. Fiber-transverse electric and transverse magnetic modes

94. Thanks!
Any questions?