Transcript dep5313-fiber optic communication system topic 2

DEP5313-FIBER OPTIC
COMMUNICATION SYSTEM
TOPIC 2: COMPONENTS IN FIBER OPTIC COMMUNICATION
SYSTEM
TOPIC 2
COMPONENTS IN FIBER OPTIC
COMMUNICATION SYSTEM
(08 : 10)
LEARNING OUTCOME
At the end of learning session, students should
understand
2.1 Optical devices in fiber optic systems
2.2 Types of Fiber optic connections
2.3 Multiplexing and demultiplexing techniques n fiber
optic communication
Block Diagram of Fiber Optic
Communications
Pulses
Information
Input (Voice
or video)
Coder and
Converter
Light Source
Transmitter
Digital data from computer
Pulses
Shaper
Photocell or
light
detector
Decoder
Amplifier
Digital data to computer
Original voice
or video
Block Diagram Function
TRANSMITTER SECTION
(1) CODER/CONVERTER:
• It is a ADC (analog to
digital converter).
• At the input , the Coder
converts analog
signal(analog information
such as voice r video or
computer data) into digital
signals.
• If the input signals are
computer signals, they are
directly connected to light
source transmitter circuit.
(2) LIGHT SOURCE
• Is generally a FOCUS type LED (Light
Emitting Diode) or low intensity laser
beam source (such as Injection Laser
Diode-solid state laser) or in some
cases an infrared beam of light
• The frequency of digital pulses
control the rate, at which light
source turns ON/OFF, in other word,
this is how the digital signals are
converted into equivalent light
pulses and focused at one end of
fiber-optic cable.
Block Diagram Function
(c) Fiber–optic cable
• Fiber optic cable passes the light pulses that are fed to one end of
fiber-optic cable on to the other end.
• The cable has VERY LESS attenuation (loss due to absorption of light
waves) over a long distance.
• Its bandwidth is large; hence, its information carrying capacity is
high.
Block Diagram Function
(1)RECEIVER SECTION - - LIGHT
• DETECTOR or photodetector is
tranducer that detect the light pulses
and then converts it into
proportional electrical signal. signal
into analog signals, such as voice,
video or computer data.
• The electrical signals are then
amplified
• and reshaped into original digital
pulses by the shaper
• If the input signals are computer
signals, the signal can be directly
taken out from the output of the
shaper circuit.
(1) DECODER
• It is a ADC (analog to
digital converter).
• Converts digital signal into
analog signals, such as
voice, video or computer
data.
Pulses
before
shaper
process
Pulses after
shaper
process
5V
0V
Optical Transmitter
•
•
•
The transmitter consists of a lightsource and its drive
circuitry.
The light sources used for fiber optic transmitters need to
meet several criteria:
 It has to be at the correct wavelength
 Be able to be modulated fast enough to transmit data
 Be efficiency coupled into fiber
Two devices commonly used to generate light for fiber
optic communication system:
(a) Light emitting diode (LED)
(b) LASER diode (LD)
(a) Light Emitting Diode (LED)
• An LED is a PN junction diode that is operated with forward bias.
• Combination of electron and holes in depletion region generates
photons of light:
 Photons are allowed to direction normal to the junction, the
diode is called surface-emitting LED.
 If parallel to the junction, the diode is called edge-emitting
LED.
• LEDs can be visible spectrum or infrared.
(a) Light Emitting Diode (LED)
Characteristics
• Light generation - emits light by spontaneous emission.
• Have much lower power outputs than lasers.
• Property of light - is an incoherent light source that
emits light in a disorderly way(No internal order)
• Transmission wavelength within 660-1650 nm.
Typically used at 850nm and 1310nm.
• Diverging light output pattern makes them harder to couple
into fibers, thus limiting them to use with multimode fibers.
• Have moderate bandwidth - Limited to systems operating up
to about 250 MHz or around 10-100 Mbps and shorter
distance multimode systems
• Have a very broad spectral output (40-190nm)
which causes them to suffer chromatic dispersion
in fiber.
LASER Diode (LD)
Laser diodes are sometimes referred to as injection laser diodes or by
the acronyms LD or ILD.
• Laser diode are more complex than LED, although the basic
mechanism is still forward-biased PN junction diode. However, the
simplest diode lasers are structurally similar to LEDs.
• Both generate light from recombination of electron hole pairs at a
forward-biased junction but laser diode operates at higher current
levels.
•
LD Characteristics
• A laser diode emits light through stimulated emission rather than
spontaneous emission, which results in higher output power.
• Laser is a coherent light source that emits light in a very orderly
way.
• Relatively directional light output pattern makes them easily couple
to single mode or multimode fibers.
• A laser diode has a narrower an emission line width (spectral width)
from 0.00001 to 10 nm, compared to common LED
• Transmission wavelength within 780-1650 nm. Primarilly used at
1310nm and 1550nm.
• High bandwidth capability, most being useful to well over 10 GHz
and faster data transmission speed about 10 Gbps. Thus Ideal for
long distance high speed links.
• More expensive- creating the laser cavity inside the device is more
difficult, the chip must be separated from the semiconductor wafer and
each end coated before the laser can even be tested to see if its good.
LED & LD Wavelengths

The emission wavelength depends on the chemical composition of the diode.
Material
Wavelength range nm
GaAs
GaAIAs
InGaAs
InGaAsP
750-900
800-900
1000-1300
900-1700
Communications LEDs are most commonly made from GaAsP (1300 nm) or GaAs
(810-870 nm)
 Most Laser Diodes emit in the near-infrared spectral region, but others can emit
visible (particularly red or blue) light or mid-infrared light.
 The most common semiconductors used in laser diodes are compounds based on:
 Gallium Arsenide, GaAs - 750 to 900 nm in the infrared
 Indium Gallium Arsenide Phosphide, InGaAsP -1200 to 1700 nm in the
infrared
 Gallium Nitride - near 400 nm in the blue.

Wavelength for Different Colors
Color
Wavelength (nm)
Red
780 - 622
Orange
622 - 597
Yellow
597 - 577
Green
577 - 492
Blue
492 - 455
Violet
455 - 390
Optical receiver (Light Detector)

The main function of the receiver is to convert optical
signal into electrical signal.

An optical receiver consists of:
photo diode semiconductor (photodetector) which
produces current in response to incident light
an amplifier
signal conditioning circuitry
Photo diode
•
Fiber optic receivers use two types of photo diodes:
 positive-intrinsic-negative (PIN) photo diode
 avalanche photo diodes (APD).
•
A junction photodiode is an intrinsic device that behaves
similarly to an ordinary signal diode, but it generates a
photocurrent when light is absorbed in the depleted
region of the junction semiconductor
•
In a photodiode, a reverse bias potential is applied across
the diode, preventing current from flowing in the absence
of light. However, when expose to light, electron-hole
pairs are created, generating a current.
PIN photodiode

consists of a thick doped intrinsic layer sandwiched between
thin p and n regions.

The major feature of a p-i-n PD is that its intrinsic layer is its
depletion layer, where the absorption of photons occurs.
Avalanche Photo Diode (APD)
The APD photodiode structure is relatively similar to PIN photodiode
structure.
• APD internally amplifies the photocurrent by an avalanche process.
• A large reverse-bias voltage (typically over 100 volts) is applied across
the active region that will causes electrons to collide with other
electrons in the semiconductor material. This process is called
avalanche multiplication, and fraction of the electrons part of the
photocurrent.
•
Photodetector Characteristics
•
Since the optical signal generally weakened and distorted when it
emerges from the end of the fiber, the photodetector must meet
strict performance requirements such as:
 A high sensitivity to the emission wavelength range of the
received light signal
 A minimum addition of noise to the signal
 A fast response speed to handle the desired data rate
•
Sensitivity measures the response to an optical input signal as a
function of its intensity. Photodetector’s sensitivity can be measured
in two concepts:
a) quantum efficiency
b) responsivity.
Photodetector Characteristics
a) Quantum efficiency , η
measures the fraction of
incoming photons that
generate electrons at the
detector.

b) Responsivity, ρ is the ratio of
current output (photo current)
to light input.
where
λ0 is measured in um (micrometers)
η is the quantum efficiency
It is defined as:
High responsivity equals high
receiver sensitivity.
• Since in fiber optic
communication systems, input
powers are usually in microwatt
level, responsivity is often
expressed as µA/µW.
•
Photodetector Characteristics
Speed of Response
The speed of response and bandwidth of a photodetector depend on
three factors:
 The transit time of the photo-generated carriers through the
depletion region
 The electrical frequency response as determined by the RC time
constant, which depends on the diode’s capacitance
 The slow diffusion of carriers generated outside the depletion
region
Spectral Response
The wavelength that a photo-detector
can respond to depends on its
composition.
• The following graph shows the detector
response curve for different materials.
•
Photodetector Characteristics
Dark Current
• is the current through the photodiode in the absence of light, when it is
operated in photoconductive mode.
•
Is the baseline noise current developed by the random generation of
electrons and holes within the depletion region of a photodiode, without
the addition of an external bias current or light activation
•
The dark current includes photocurrent generated by background
radiation and the saturation current of the semiconductor junction.
•
Dark current sets a floor on the minimum detectable signal, because a
signal must produce more current than the dark current in order to be
detected.
•
Dark current is also a source of noise when a photodiode is used in an
optical communication system.
Typical Performance Characteristics of Detectors
Silicon
Germanium
InGaAs
Parameter
PIN
Wavelength range (nm)
APD
PIN
400 – 1100
APD
PIN
800 – 1800
APD
900 – 1700
Peak (nm)
900
830
1550
1300
1300 (1550)
1300 (1550)
Responsivity
ρ (A/W)
0.6
77-130
0.65-0.7
3-28
0.63-0.8 (0.750.97)
Quantum Efficiency (%)
65 – 90
77
50-55
55-75
60-70
60-70
Gain (M)
1
150-250
1
5-40
1
10-30
Excess Noise Factor (x)
-
0.3-0.5
-
0.95-1
-
0.7
Bias Voltage (-V)
45-100
220
6-10
20-35
5
<30
Dark Current (nA)
1-10
0.1-1.0
50-500
10-500
1-20
1-5
Rise Time (ns)
0.5-1
0.1-2
0.1-0.5
0.5-0.8
0.06-0.5
0.1-0.5
Noise factor

Noise is unwanted components of the signal that tend to disturb
the transmission and processing of the signal in a physical system.

Noise generated by the photodiode is most critical. The three
most predominant types:
1) Thermal Noise- A noise due to the random motion of
electrons or dissipation of heat in the detector resistance.
2) Shot Noise - is a small current produced from the
randomness of the photon-to-electron conversion.
3) dark current Noise - is a very small current present when no
light is incident on the photodetector
SIGNAL-TO-NOISE RATIO SNR

The ratio of the total signal to the total noise shows how much
higher the signal level is than the level of the noise. It is a measure
of signal quality.

The signal-to-noise ratio, SNR (or S/N) at the output of an optical
receiver is defined as the ratio between the signal power and the
noise power and presented as follow:
where:
i2noise = it2 + is2 + id2
RECAP
Tutorial 1
• A Si PIN photodiode is operating at 50 GHz at 300K. The
current is 200 µA, the dark current is 0.5 nA and the load
resistance is 50 M ohm. Find the thermal noise, shot noise,
dark current noise and total noise
Tutorial 2
• The Si PIN photodiode in Exercise 1 has an incident power
of 417 µW and a responsivity of 0.48. Find the SNR.
Tutorial 3
• Suppose we have a system consisting of an LED emitting 10mW at 0.85µm,
a fiber cable with -20 dB of loss, and a PIN photodetector of
responsivity(ρ) 0.5A/W. The detector’s dark current is 2 nA. The load
resistance is 50Ω; the receiver’s bandwidth is 10MHz, and its temperature
is 300K (27oC). The system losses, in addition to the fiber attenuation,
include a -14 db power reduction due to source coupling and a -10dB loss
caused by various splices and connectors.
• Compute the
I. received optic power,
II. the detected signal current and power,
III. the shot noise and thermal noise, and
IV. the signal to noise ratio
Solution
The total system loss is (-20) + (-10) + (-14) = -44dB.
We know loss 10 log10 x = -44dB
So, transmission efficiency of 10-4.4 = 4 x 10-5.
i. The optic power reaching the receiver is then
PR = 4 x 10-5(10) = 4 x 10-4mW = 0.4 µW
ii. Detected signal current / photocurrent
= 0.5 (0.4) = 0.2µA = 200nA
Solution
The dark current only 2nA is small compared to the signal current, so it
can be ignored in this example.
The electrical signal power is
PES = (0.2 x 10-6)2 (50) = 2 x 10-12W
= 2(1.6x10-19) (0.2x10-6)(107)(50)
= 3.2 x 10-17W
Thermal Noise power
PNT = 4 (1.38 x 10-23) (300) (107)
= 1.66 x 10-13W
In this system, the thermal noise is nearly four orders magnitude greater
than the shot noise. The thermal noise limited result applies. We can
compute the SNR from the equation
CONNECTION IN FIBER OPTIC
Fiber optic cable is terminated in two ways :
1) with connectors that can mate two fibers to create a temporary
joint and/or connect the fiber to a piece of network gear
2) with splices which create a permanent joint between the two
fibers.
(1) CONNECTOR

An optical fiber connector terminates the end of an optical fiber,
and enables quicker connection and disconnection than splicing.

The connectors mechanically couple and align the cores of fibers
so that light can pass.

Good connectors lose very little light due to reflection or
misalignment of the fibers.
(1) CONNECTOR
Type
CHARACTERISTICS
• available in single mode and
ST
multimode.
Straight Tip • simplex only, twist-on
mechanism.
•simplex only, screw-on
FC
mechanism.
Ferrule
• available in single mode
Connector
and multimode
LC
Lucent/
Local
Connector
•simplex and duplex, push
and latch
•available in single mode and
multimode
Connector
Adapter/Coupler
(1) CONNECTOR
Type
Characteristics
SC
Subscriber •
Connector
•
simplex and duplex, snap-in
mechanism.
available in single mode and
multimode.
FDDI
• 2.5mm ferrules
connectors • duplex multimode
.
connector generally used to
connect to the equipment
from a wall outlet, but the
rest of the network will have
ST or SC
Connector
Adapter/Coupler
(2) SPLICING


Fiber splicing is the process of permanently joining two fibers
together.
There are two types of splices:
a) fusion
b) mechanical
(a) Fusion Splicing



In fusion splicing, two fibers are literally welded (fused) together
by an electric arc.
is done by an automatic machine called fusion splicer, which
mechanically aligns the two fiber ends, then applies a spark across
the fiber tips to fuse them together.
Fusion splicing is the most widely used method of splicing as it
provides lowest insertion loss and virtually no back reflection.
(a) Fusion Splicing
generates spark (high temperature heat)
• Fusion arc in
Splice complete
(b) Mechanical Splicing


Mechanical splicing uses mechanical fixtures to join two fibers
together end to end.
Mechanical splicing join two fiber ends either :
 by clamping them within a structure
 by gluing them together
 is include transparent adhesives and index matching gels.
 Transparent adhesives are epoxy resins that seal mechanical
splices and provide index matching between the connected
fibers.
Types of mechanical splices


The biggest difference between mechanical splices is the way the
fibers are aligned.
Some types of mechanical splices include
• capillary type
• V-groove and rotary devices.
• plastic, glass, metal,
• ceramic tubes
Types of mechanical splices
Capillary type



is the simplest method of making a mechanical splice.
Two fibers are inserted into a thin capillary tube. The tube has a
inner diameter that matches the fiber's cladding diameter.
(The fibers must first have coatings removed and cladding exposed
and cleaned).
These two fiber ends are pushed inwards until they meet. Index
matching gels are often inserted in the center to reduce back
reflections.
Types of mechanical splices
Ribbon V- Groove type




V-groove splices are quite simple and work well for single fiber or
even for fiber ribbons.
For ribbon fibers, capillary type doesn't work anymore. Instead, fiber
ribbon is put in a V-shaped groove array, with each fiber place in its
own v-groove.
Two ribbon fibers are butted together in this V-groove array and then
a cover plate is applied on top.
This method is primarily used for splicing a multi-fiber cable in a
single action.
Types of mechanical splices
Elastomeric type 35



Elastomeric splice is for lab testing or emergency fiber repairs.
Same as V-groove type, it has a single fiber v-groove but the v-groove
is made of flexible plastic.
First an index matching gel is injected into the hole, then one fiber is
inserted until it reaches about halfway. The other fiber is then
inserted from the other end until it meet the first one.
Splices, from left: FUSION SPLICE, ELASTOMERIC, ULTRASPLICE
(capillary splice), camlock, FIBERLOK (V-Groove type), T&T Rotary
Splice
Fusion vs Mechanical splicing
Characteristic
Arc Fusion
Mechanical
Fiber alignment
mechanism
machine is used to
precisely align the two fiber
ends then the glass ends
are "fused" or "welded"
together using some type
of heat or electric arc.
simple alignment
devices to hold the two
fiber ends in an
alignment fixture with a
transparent gel or
optical adhesive.
Loss and back
reflection
lower loss (Typical loss: 0.1 higher loss (Typical loss:
dB )and less back reflection 0.3 dB) and greater
reflectance
Fiber types
are used primarily with
single mode fiber
What else?
work with both single
and multi mode fiber.
REFERENCES
Agrawal, Govind P. (2010). Fiber-Optic Communication Systems. (Fourth Edition).
Wiley Series. (ISBN : 978-0-47050511-3).
Downing , James N. (2005). Fiber-Optic Communications, Thomson Delmar
Learning. (ISBN: 1-4018-6635-2).
George Kennedy, Bernard Davis. (2006). Electronics Communication Systems.(4th).
McGraw Hill.
Jim Hayes, (2010). Fiber Optics. Technician’s Manual, Fourth Edition. Thomson
Delmar Learning.
Joseph C. Palais, (2005) Fiber Optic Communications. Fifth Edition. Pearson /
Prentice Hall. (ISBN 0130085103, 9780130085103).