Fibre optic Communication

The communications systems that carry information through guided fibre cable are
called fibre optic system. This comm system uses light as the carrier of information.

The difference between metallic wire system and fibre optic system is that fibre-
optics use light pulses to transmit information down fibre lines instead of using
electronic pulses to transmit information down copper lines.


• SECURE: it is very difficult to tap the fibre cable (military application)

• SAFER: safe and easy to install, require less storage space and easy to
transport. Can be used around volatile liquids and gases.

• SPEED: Fiber optic networks operate at high speeds - up into the gigabits

• LARGE BANDWIDTH: large carrying capacity

• IMMUNE TO CROSS TALK: non conductors of electricity

• IMMUNE TO STATIC INTERFERENCE: non conductors of electricity
• DISTANCE: Signals can be transmitted further without needing to be "refreshed" or

• RESISTANCE: Greater resistance to electromagnetic noise such as radios, motors or
other nearby cables. resistive to environmental extremes.

• Fibre cables are less affected by corrosive liquids and gases. They operate over large
temperature variations.

• MAINTENANCE: Fibre optic cables costs much less to maintain.

•   Electromagnetic spectrum

•   The light frequency spectrum can be divided into 3 bands
•   1] infrared : band of light wavelengths too long to be seen by human eye
•   2] visible light : human eye responds to
•   3] ultraviolet : too short to be seen by human eye
•   With light frequencies, wavelength is stated in microns or nanometers (1 nanometer
    = 0.001 micron or 10 power -9 meter)

• Transmitter, receiver and fibre guide in this system.
• At one end of the system is a transmitter. This is the place of origin for information
coming on to fibre-optic lines.

• It consists of analog or digital interface, voltage to current converter, a light source,
and a source to fibre light coupler.
• the fibre guide is either an ultrapure glass or plastic cable.

• the receiver includes a fibre to light detector coupling device, photo detector, a
current to voltage converter, an amplifier and and A-to-D interface.

• The transmitter accepts coded electronic pulse information coming from copper wire. It
then processes and translates that information into equivalently coded light pulses.
• light source can be modulated by a digital or an analog signal.

• the V-to-C converter serves as an electrical interface between the input circuitry and
the light source.

• light source can be a light-emitting diode (LED) or an injection-laser diode (ILD). The
amount of light emitted by the LED or ILD is proportional to the amount of drive current.

• Thus V-to-C converter converts an input signal voltage to a current i.e used to drive the
light source.
•   The source to fibre coupler (mechanical device) such as a lens, couples the light
    emitted by the source into fibre-optic medium where they travel down the cable.

•   The fibre to light detector coupling device is also mechanical device, couples as
    much light as possible from the fibre cable into the light detector.

•   The light detector is often either a PIN ( p type intrinsic n type ) diode or an APD
    (avalanche photodiode). Both PIN or APD convert the light energy to current.

•    C-to-V convertor transforms the detector current into input signal voltage.
•   The A-to-D interface at the receiver output is also an electrical interface.
•   Analog or digital modulation is used at the end.
    Fibre Types
    Three varieties of optical fibres constructed of glass, plastic or combination of
    glass and plastic are
•   Plastic core and cladding
•   Glass core with plastic cladding (PCS-plastic-clad-silica)
•   Glass core and glass cladding (SCS-silica-clad silica)

•   Plastic fibre have advantages over glass fibres
•   Flexible so More rugged than glass
•   Easy to install, can better withstand stress, less expensive, weigh 60 % less
    than glass
• Disadvantages: high attenuation, they do not propagate light as efficiently as
    glass. Limited to short runs such as within a building.
• Fibres with glass core exhibit low attenuation.
• PCS fibres are better that SCS.
• Less affected by radiation so more used in military.
• SCS adv- best propagation, easier to terminate than PCS
• SCS disadv- least rugged, more susceptible to increases in
attenuation when exposed to radiation
Cable Construction
The cable may include a core, a cladding, a protective tube, buffers, strength
members, protective jackets.

                                                                    Single fiber structure

  The design factors for some types of fiber optic cables are listed below.

  Indoor cables: Fire safety is the number one factor in selecting indoor cables.

  Outdoor cables: Moisture resistance and temperature tolerance are the major
  factors when choosing materials for outdoor environment cables. They also need
  to be ultraviolet (UV) resistant.
  Aerial/Figure 8 Self-Supporting Cables: Aerial cables must endure extreme
  temperature ranges from sunlight heat to freezing snow. They also must survive
  high wind loading.
  Cable Jacket Materials can be Polyethylene (PE) for outdoor application. PE
  has excellent moisture – and weather-resistance properties. It has very stable
  dielectric properties over a wide temperature range.
  Polyurethane (PUR)- for highly flexible cables

  Strength members - For tensile strength the yarns are used like kevlar yarn. It is
  strong and is used to bundle and protect the loose tubes or fibers in the cable. It is
  the strength member to provide tensile strength along the length of the cable during
  and after installation. When a cable is pulled into a duct, the tension is applied to the
  aramid yarn instead of the fibers.

Central Strength Member
Many fiber optic cables has a central strength member, made of steel, fiberglass or
aramid yarn. Central strength members are needed to provide the rigidity to keep the
cable from buckling. Central strength members are common in outdoor cables and
some high fiber counts indoor cables.

Gel Compound
Gel compound fills buffer tubes and cable interiors, making the cable impervious to
water. It needs to be completely cleaned off when the cable end is stripped for
Telephone cable
Different types of fibre optic configurations are

         Multiple strands cable
Refraction and reflection
• the fig show above shows the refraction of light as it passes from one medium to
• the refractive index is the amount of bending or refractions that occurs at the
interface of two materials of two different.

• how a light reacts when it meets the interface of two tranmissive materials that have
different indexes of refraction can be explained with snell’s law. Snells law simply
states that:
  n1 sin θ1 = n2 sin θ2

Where n1 – RI of material 1
      n2 – RI of material 2
      θ1 – angle if incidence
      θ2 – angle of refraction

• when a light ray enters a less dense material, the ray bends away from the normal.
 while when it enters a more dense material, the ray bends towards the normal.

• critical angle is defined as the minimum angle of incidence at which a light ray may
strike the interface of two media and result in an angle of refraction of 90 deg or
greater. (this occurs when light travels from more dense medium to less dense
    Propagation of light through an optical fibre

•   Light can be propagated down an optical fibre cable by either reflection or
    refraction. How the light propagates depends on the mode of propagation and the
    index of profile.

•   Light pulses move easily down the fibre-optic line because of a principle known as
    total internal reflection.
•      "This principle of total internal reflection states that when the angle of incidence
    exceeds a critical value, light cannot get out of the glass; instead, the light bounces
    back in.
•   When this principle is applied to the construction of the fiber-optic strand, it is
    possible to transmit information down fiber lines in the form of light pulses.

•     The core must a very clear and pure material for the light or in most cases near
    infrared light (850nm, 1300nm and 1500nm).

•    The core can be Plastic (used for very short distances) but most are made from
    glass. Glass optical fibers are almost always made from pure silica, but some other
    materials also.
•   Mode means path, there can be only one path for the light called as single
    mode. If there is more than one path from the cable it is called as multimode.

•   Index profile of an optical fibre is the graphical representation of the value of the
    refractive index across the fibre. The RI is plotted in the horizontal axis, while
    the radial distance from the core axis is plotted on the vertical axis.

•   There are two basic types of index profiles : step and graded.

•   The step index fibre has the central core with a uniform RI. The core is
    surrounded by an outside cladding with a uniform RI less than that of the central

•   In step index fibre there is an abrupt change in the RI at the core/cladding

•   the graded index fibre there is no cladding and the RI of the core is non uniform,
    it is highest at the centre and decreases gradually with distance toward the
    outer edge.

•   There are three types of optical fibre configurations: single mode step index,
    multimode step index, multimode graded index
has a narrow core (eight microns or less), and the index of refraction between the
core and the cladding changes less than it does for multimode fibers.

Light thus travels parallel to the axis, creating little pulse
Telephone and cable television networks install millions of
 kilometers of this fibre every year.
Simple single-mode step index, outside cladding is the air. RI of the glass core (n1) is 1.5 and RI of the
air cladding (n0) is 1. the large diff in RI results in small critical angle (42deg) at the glass/air interface
which results in wide aperture which couples light easily from source to cable. (cable-weak, limited use)

Practical single mode step index is one that has cladding other than air. The RI of the cladding (n2) is
slightly less than that of the central core (n1) and is uniform throughout the cladding. Such cable is
physically stronger. Critical angle is also higher (77deg),so small acceptance angle, narrow aperture,
difficult to couple light Into fibre from the light source. (9-12)


• contains a core in which the refractive index diminishes gradually from the centre axis
out toward the cladding.

• The higher refractive index at the centre makes the light rays moving down the axis
advance more slowly than those near the cladding. Also, rather than zigzagging off the
cladding, light in the core curves helically because of the graded index, reducing its
travel distance.

• The shortened path and the higher speed allow light
at the periphery to arrive at a receiver at about the
same time as the slow n straight rays in the core axis.

• The result: a digital pulse suffers less dispersion.
•   STEP-INDEX MULTIMODE FIBER has a large core, up to 100 microns in diameter. As a
    result, some of the light rays that make up the digital pulse may travel a direct route,
    whereas others zigzag as they bounce off the cladding.

•   This fibre has large light-to-fibre aperture so allows more light to enter the cable.

•   The light that strike the core/cladding interface at an angle greater than the critical
    angle (ray A) are propagated down the core in zigzag fashion, continuously reflecting
    off the interface boundary.

•   The light rays that strike the core/cladding interface at an angle less than the critical
    angle (ray B) enter the cladding and are lost (also shown by yellow line) in fig below.

•   These alternative pathways cause the different groupings of light rays, referred to as
    modes, to arrive separately at a receiving point. The pulse, an aggregate of different
    modes, begins to spread out, losing its well-defined shape.

•   The need to leave spacing between pulses to prevent overlapping limits bandwidth that
    is, the amount of information that can be sent. Consequently, this type of fiber is best
    suited for transmission over short distances, in an endoscope, for instance.

                    Step index multimode fibre
Two modal distributions
                             Multimode step index fiber

                          Single mode step index fiber
Comparison of fiber structures
single mode has smaller aperture, higher critical angle and small acceptance angle.
While multimode has wider aperture, the acceptance angle less than that of critical
angle, light enters the cable. If acceptance angle greater than critical angle light enters
the cladding and are lost.

When a light ray enter the fibre, they strike the air/glass interface at normal A. the RI of
air is 1, and the RI of glass is 1.5. light entering from less dense to more dense medium.

According to snells law, light rays will refract toward the normal. This causes light rays to
change direction and propagate diagonally down the core at an angle θc, that is different
from the external angle of incidence at the air/glass interface θn.
For the light ray to propagate down the cable, it must strike the internal core/cladding
interface at an angle that is greater than the critical angle θc.

the acceptance angle is the maximum
angle in which external light rays may
strike the air/fiber interface and still
propagate down the fibre with a
response that is no greater than 10 dB
down from the peak value.
Rotating the AA around the fibre axis
describes the acceptance cone of the
fibre input.
1. The terminating equipment is still costly as compared to copper
2. Of is delicate so has to be handled carefully.
3. Communication is not totally in optical domain, so repeated electric –
   optical – electrical conversion is needed.
4. Optical amplifiers, splitters, MUX-DEMUX are still in development stages.
5. Tapping is not possible. Specialized equipment is needed to tap a fiber.
6. Optical fiber splicing is a specialized technique and needs expertly
   trained manpower.
7. The splicing and testing equipments are very expensive as compared to
   copper equipments.


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