Download Fiber Optic

Fiber Optic System
TOPIC:
INTRODUCTION TO:
• Basic concept of block diagram
• Characteristics of Optical fibre circuit devices such as LED,ILD,
PIN photodiode and APD
BY :
Mohd Nasir bin Said
Telecommunications Department
ADTEC Kulim
Fiber optic System Block Diagram
Fiber
Coder
Light
Source
Repeater
(long distance)
Light pulses
Fiber
Light
Detector
Decoder
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The three primary building blocks of the link
are the transmitter, the receiver, and the
fiber guide.
The transmitter consists of an analog to
digital converter (coder), and a light source.
The A/D converter is used to convert
continuous analog signals such as voice or
video (TV) signals into a series of digital
pulses.

The digital pulses are then used to flash a
powerful light source off and on very rapidly.
The light source is either a light-emitting
diode (LED) or an injection laser diode
(ILD).The light-beam pulses are then fed into
a fiber-optic cable where they are transmitted
over long distances.
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The optical fiber consists of a glass or plastic fiber core, a
cladding, and a protective jacket ( transmission medium)
Repeaters are used to ensure the signals can be transmitted
efficiently when the two stations are separated far enough from
each other.
The receiver includes a light detector or photocell and a
decoder. The light detector is very often either a PIN (p-typeintrinsic-n-type) diode or an APD (avalanche photodiode). The
light detector, acting as the receiving element, converts the
received light pulses back to pulses of electrical current. The
electrical pulses are amplified and reshaped back into digital
form that is fed to a decoder such as a D/A converter, where the
original voice or video is recovered
Light Sources
Generally, a light source must meet the following requirements:
 It must be able to turn on and off several tens of millions, or even
billions, of
 times per second.
 It must be able to emit a wavelength that is transparent to the fiber.
 It must be able to couple light energy into the fiber.
 The optical power emitted must be sufficient enough to transmit
through optical fibers.
 The performance of the fiber-optic should not be affected by the
temperature variation.
 The manufacturing cost of the light source must be relatively
inexpensive
LED


LEDs are complex semiconductors that
convert an electrical current into light.
The conversion process is fairly efficient in
that it generates little heat compared to
incandescent lights. LEDs are of interest for
fiber optics .

Light-emitting diodes use GaAlAs (gallium
aluminum arsenide) for short-wavelength
devices. Long-wavelength devices generally
incorporate InGaAsP (indium gallium
arsenide phosphide).
LED characteristics:
1. They are small.
2. They possess high radiance (i.e., They
emit lots of light in a small area).
3. The emitting area is small, comparable
to the dimensions of optical fibers.
4. They have a very long life, offering high
reliability.
5. They can be modulated (turned off and
on) at high speeds

LED is an incoherent light source that
emits light in a disorderly way as
compared to ILD, which is a coherent light
source that emits light in a very orderly
way (see Figure1.0).
Incoherent radiation
(a)
Coherent radiation
(b)
Figure 1.0
Radiation patterns for (a) LED ; (b) ILD

LEDs are economical and are common
for short distance,low data rate
applications. They are available for all
three wavelengths but are most common
at 850 and 1310 nm
( 850 nm LEDs are usually the least
expensive )

Light power from an LED covers a broad
spectrum, from 20 to over 80 nm . The
LED is more stable and reliable than a
laser in most environments.
ILD


The word 'LASER' is an acronym standing
for 'Light Amplification by Stimulated
Emission of Radiation'
The diode laser is the most compact
among all commercially available laser
products.


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Injection Laser Diodes are more expensive.
The advantagesof using a laser diode are in the
high modulation bandwidth ( over 2 GHz ), with
high optical output power and narrow spectral
width.
Their application is in long distance, high data
rate requirements. Lasers are common in single
mode optical fiber applications and their light
power covers a very narrow spectrum, usually
less than 3 nm.
This results in a low chromatic dispersion value
and hence high fiber bandwidth.
Their life span is shorter than that of an LED.
Lasers are sensitive to the environment
(especially to temperature variation).

The diode's construction begins with a threeleaded header. Mounted on the header
resides a monitoring photodiode designed to
detect light. Above the photodiode is the laser
diode chip where the laser beam is emitted.
The laser chip has three layers, two clad
layers with an active layer in between.


The light beam is emitted from the active
layer, also known as the Multiple Quantum
Well (MQW).
The glass window on a diode laser not only
transmits the light beam, but also forms part
of the hermetically sealed package. Once the
case is sealed its contents are protected from
hazardous elements, unless it is physically
destroyed. The glass envelope comes in two
different styles, flat window and ball lens
Laser Applications

Biomedical, photodynamic therapy,
entertainment & display, graphic arts,
holography, inspection, instrumentation, laser
pumping, optical data storage, sensing,
pollution monitoring, trace element sensing,
spectroscopy, high resolution spectroscopy,
Raman spectroscopy, fluorescence
spectroscopy, molecular spectroscopy, and
telecommunications.

Here we list the commonly available diode
laser wavelengths and their typical
applications:
635 Pointing, holography, replacement of HeNe laser, DVD, CD
650 Pointing, holography, DVD, CD
660 Pointing, holography, DVD, CD
760 Gas sensing
780 CD ROM
808 Solid state laser pumping
850 Communications
980 Erbium-doped fiber pumping
1060 Replacement of low-power Nd:YAG
1310 Communications
1321 Gas sensing
1460 Raman amplifier
1480 Erbium-doped fiber pumping
1540 Gas sensing
1550 Communications, range finding
1578 Gas sensing
1625 Telecommunications testing
1640 Gas sensing
1790 Gas sensing
Light Detector

Optical detection occurs at the light wave
receiver’s circuitry. The photo detector is the
device that receives the optical fiber signal
and converts it back into an electrical signal.

The most important characteristics of light detectors are :
i) Responsitivity: Responsitivity is a measure of the
conversion efficiency of a photodetector.
ii ) Dark current: Dark current is the leakage current that flows
through a photodiode with no light input.
iii ) Transit time: Transit time is the time it takes a light-induced
carrier to travel across the depletion region.
iv ) Spectral response: Spectral response is the range of
wavelength values that can be used for a given photodiode.
v ) Light sensitivity: Light sensitivity is the minimum optical
power a light detector can receive and still produce a usable
electrical output signal.

The most common types of photo detectors
are the positive intrinsic negative
photodiode ( PIN ) and the avalanche
photodiode (APD ).

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PIN photodiodes are inexpensive, but they
require a higher optical signal power to
generate an electrical signal.
They are more common in short distance
communication applications.
Thank You
Q&A