The document has fiber optics fundamentals (concepts) The document is prepared by Yagnesh Bhadiyadra, Harshil darji, Nisarg sheth, Kairav pithadiya

1. Fiber Optics
Fundamentals
Members:
Yagnesh Bhadiyadra
Nisarg Sheth
Kairav Pithadia
Harshil Darji

2. Motivation
● Fiber optics have very low loss compared to electric wires, the
reason will be explained later in the presentation.
● Fiber optics have very high speed transmission capacity
compared to traditional system.
● For e.g.,Nippon Telegraph and Telephone Corporation
transferred 1 Petabit per second over 50 kilometers over a
single fiber(2012).

3. Introduction
● Fiber optics, or optical fiber, refers to the medium and the technology
associated with the transmission of information as light pulses along a
glass or plastic strand or fiber.
● Optical fiber is divided into three main cross sections i.e. the core,
cladding and jacket.
● The core and cladding are made of glass and the
jacket is made of varying materials (usually PVC).

4. Introduction
● Fiber optics transfer data in the form of photons that pulse through the
optic cable.
● Light passes through the core(SiO2+GeO2), cladding(SiO2) keeps the
light within the core and the jacket prevents from any external
interference.

5. Total Internal Reflection (TIR)
● If θ ≤ θc, the ray will split; some of the ray will reflect off the boundary, and some will
refract as it passes through. This is not total internal reflection.
● If θ > θc, the entire ray reflects from the boundary. None passes through. This is called
total internal reflection.
● Here, n1>n2 and the critical angle is given by θc = sin-1(n2/n1)

6. Total Internal Reflection in
Optical fiber
● The refractive index of Cladding(n2) is less than refractive index of the
core(n1).
● This ensures TIR within the core and due to high contrast between n1 and n2,
this works for considerable range of input beam angles and very minimal data
losses.

7. Types of Fiber Optic cables
● There are three types of Optical fibers used : Single-mode,
Multi-mode and plastic optical fiber.
● Plastic optical fiber is an optical fiber made out of polymer. The
chief advantage of this type over the other glass fibers is its
robustness under bending and stretching.
● Most commonly used fibers are Single-mode and Multi-mode
optical fibers.

8. Single mode
● Single mode transmission is generally used for longer distance
transmission.
● The diameter of core is small compared to multimode transmission.
● The small diameter has less attenuation and hence the distance can travel
longer distances with lesser loss.
● Light source for single mode typically is laser light.

9. Multi mode
● Multimode transmission is used for shorter travel distances e.g. within a
building or campus.
● It has higher core diameter compared to single mode transmission.
● Typical multimode fiber core diameters are 50, 62.5, and 100
micrometers.
● Bigger diameter allows multiple signal transmission and hence more
information transmission.
● Light source used is typically LED light.

10. Optical Fiber Field distribution
● An evanescent wave is a near-field wave with an intensity that exhibits
exponential decay without absorption as a function of the distance from the
boundary at which the wave was formed.
● Evanescence is observed at the edges of an optical fiber where the angle of
reflection is greater than critical angle.
● For the Single Mode propagation, the distribution goes at its peak in the
middle.
MFD-mode field diameter

11. Optical Fiber Field distribution
● The field and intensity distributions for single and multi-mode
propagations are shown.
● For the multi-mode intensity gets zero somewhere near the middle and
again reaches to peak value.

12. Optical Fiber Loss and Attenuation
● The attenuation of an optical fiber measures the amount of light lost
between input and output.
● Optical losses of a fiber are usually expressed in decibels per kilometer
(dB/km). The expression is called the fiber’s attenuation coefficient α
and the expression is
where P(z) is the optical power at a position z from the origin, P(0) is the
power at the origin.

13. Attenuation
Attenuation in an fibre optic can occur due to following reasons.
● Rayleigh Scattering
○ Microscopic variations in the refractive index of the core material
can cause scattering of light which can lead to substantial data loss.
● Absorption
○ Attenuation is caused due to absorption of light due to some
impurities. For e.g 1 parts per million (ppm) of Fe2+ would lead to a
loss of 0.68 dB/km at 1.1um(absorption peak wavelength).
However losses due to metallic ions can be reduced to very low by
refining the glass mixture to an impurity level below 1 ppb.

14. Attenuation
● Bending
○ Macro-bending
■ Macrobending happens when the fiber is bent into a large
radius of curvature relative to the fiber diameter.
○ Micro-bending
■ Microbendings are the small-scale bends in the core-cladding
interface. Microbending can happen in the fiber
manufacturing process. It is sharp but microscopic curvatures
that create local axial displacement of a few microns (um) and
spatial wavelength displacement of a few millimeters.

15. Reason behind low loss
The graph of loss(dB/km) to wavelength
Loss = 0.1db/km = 2%
source:https://pe2bz.philpem.me.uk/Lights/-%20Laser/Info-999-LaserCourse/C00-M08-Laser-FiberOptic-
CommunicationSystems/module8.htm

16. The v number
● The v number is useful as a parameter deciding the mode of the wave
whether is it single or multi-mode.
dc = diameter of core
= wavelength of the transmitted wave
n1 = refractive index of core
n2 = refractive index of cladding

17. Different modes and the v number
● When the v-number is very small, it’s observed that for single mode
most of the light propagates in the cladding than the core.
● As v-number approaches to around 2.4 (for glass), 80% is transmitted
through the core for the single-mode.
● For multi-mode, as v-number increases from 2.4, the light initially
propagates totally through cladding and then slowly through the core,
reaching the single-mode index after certain increase.

18. Different modes and the v number
source:https://ocw.mit.edu/resources/res-6-005-understanding-lasers-and-fiberoptics-spring-2008/fiberoptics-
fundamentals/

19. Fiber Optic components
● Directional coupler
● Polarizer
● Polarization controller
● Phase modulator
● Frequency shifter

20. Directional coupler
● Coupler allows transfer of energy from one fiber to another.
● This can be made by shaving off the cladding between the fibers.
● The amount of energy transmitted depends on length over which it
takes place and proximity of two cores (length of interaction).
● The evanescent nature is what allows the transmission of light to any
one end.

21. Phase Modulator
● To change the phase of light, phase modulators are used.
● If we stretch the fibre in some way, the change in phase is directly
proportional to change in physical length of fibre.
● It turns out change in phase is also proportional to change in refractive
index of fibre.

22. Phase Modulator
● Phase modulators can be created by wrapping optical fibers around a
piezoelectric cylinder and giving a potential difference across the ends.
● Optical fibers can be bonded to piezoelectric sheets, giving a potential
difference which consequently causes change in length and index.

23. Integrated Optics Waveguide
● Integrated optic waveguides can be prepared out of glass irrespective of
the thickness by adding certain impurities in controlled amounts in
specific areas of the substrate.
● It’s like creating a thin waveguide with a higher refractive index within
the substrate surrounded by a lower refractive index.
● The waveguide can be made appropriately as per needs of single-mode
or multi-mode.

24. Integrated Optic components
● Directional coupler (fixed)
● Directional coupler (variable)
● Polarizer
● Phase modulator
● Frequency shifter
● Intensity modulator

25. Phase modulator
● By applying a potential difference across the slightly electro-optic
substance across the ends, the refractive index change causes a phase
change.
● Frequency modulator can be made using the same method.

26. Fixed and Variable Couplers
● Fixed couplers can be made by combining multiple waveguides together
by bringing them closer to each other. The length between cores stays
constant for fixed couplers.
● For Variable couplers, the length between cores can be changed by
applying potential difference between the two layers.

27. Intensity Modulator
● Combining multiple waveguides can easily be used to construct an
intensity modulator.

28. Applications
● Gigabit ethernet : As network traffic is increasing day by day, the fiber
optic can be used to achieve high data speed as they have higher degree
of flexibility and future bandwidth/speed expansion as opposed to its
copper counterparts
● Networking : With all broadband and MSO applications using a network
structure to deliver its signal, networking applications have a significant
contribution in virtually every area, so fiber optics can be used as
networking cables for the networking.
● Different types of fiber optics can be used as sensors : polarization
maintaining fibers, coated fibers, doped fibers, twin-core fibers, etc.

29. References:
● http://www.timbercon.com/gigabit-ethernet/
● https://ocw.mit.edu/resources/res-6-005-understanding-lasers-and-fiberoptics-spring-
2008/fiberoptics-fundamentals/
● https://pe2bz.philpem.me.uk/Lights/-%20Laser/Info-999-LaserCourse/C00-M08-
Laser-FiberOptic-CommunicationSystems/module8.htm
● https://en.wikipedia.org/wiki/Optical_fiber
● https://www.newport.com/t/fiber-optic-basics