fiber optics workshop dr feng zhou department of physics indiana university of pennsylvania
TRANSCRIPT
Fiber Optics Workshop
Dr Feng ZhouDepartment of Physics
Indiana University of Pennsylvania
Biography
PhD in Optics in 1989 5 years postdoc in the UK (Strathclyde U.,
Glasgow U., and Imperial College) 5 years lecturer at Nanyang Technological
U in Singapore (teaching & research); 5 years of working experience with
industry in the US (Corning Applied Tech, Nanovision, Phosistor, ESI Newwave Research).
Joined IUP since 2003, the new EO program supported by OP-TEC
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Workshop Outline
This course consists of 2 sessions. This afternoon session is 4 hours (1-5 pm) and tomorrow morning is 1.5 hours (from 8:30-10:00 am).
This lecture is condensed from a 30 hr training course for Certified Fiber Optics Technician.
Sequence ->History -> basic concepts -> optical fibers -> light sources -> detectors -> other components such coupler and optical amplifier
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Objectives
After completing this workshop, participants will be able to: Define the basic technical differences
between optical, wired, and wireless communications systems.
Discuss applications of optical, wired, and wireless communications systems.
Explain some of the factors that can impact optical, wired, and wireless communications systems.
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The Textbook
We will cover all the content except the fiber sensors.
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Fiber Optic Installation Standard
ANSI/NECA/FOA 301 Written for installers Defines installation
in a “neat and workmanlike manner”
Covers premises and outside plant
Free online
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Lennie Lightwave’s Guide
http://www.lennielightwave.com
Session 1:
Basics of Fiber Optics
This session provides • A historical review of Fiber Optics
development• Refractive index, total internal reflection• An understanding of the basics of optic
fiber.
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What is Fiber Optics
Transmitting communications signals using light over hair thin glass or plastic fibers
Concept over a century old
Technology used commercially for 25 years
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History
Interest in the use of light as a carrier for information grew in the 1960's.
As Corning made the first low loss glass fiber in 1970 (<20dB/km), it became feasible to use optical fibers as a practical transmission medium. The high loss was due to impurities.
Charles Kao 2009 Nobel price By 1980 world wide installation of fiber
optic communication systems had been achieved.
By 1990, erbium doped fibers were used as optical amplifiers.
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Future Trends
Fiber is already used in: >90% of all long distance telephony and
>50% of all local telephony; Most CATV networks and LAN backbones; Many video surveillance links.
Fiber is the least expensive, most reliable method for high speed and/or long distance communications
While we already transmit signals at GB/s speeds, we have only started to utilize the potential bandwidth of fiber.
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Advantages of Fiber Optic Comm
What are the advantages?
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Advantage 1
Capacity - Optical fibers carry signals with much less energy loss than copper cable and with a much higher bandwidth. This means that fibers can carry more channels of information over longer distances and with fewer repeaters required. Bandwidth: A measure of the maximum
frequency by which light intensity can be modulated [MHz/km]. The greater the bandwidth, the greater the information carrying capacity .
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Advantage 2 Size and Weight - Optical fiber cables
are much lighter and thinner than copper cables with the same bandwidth. This means that much less space is required in underground cabling ducts. Also they are easier for installation engineers to handle.
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Advantage 3
Security – difficult to tap information; a great
advantage for banks and security installations.
immune to Electromagnetic (EM) interference from radio signals, car ignition systems, lightning etc. They can be routed safely through explosive or flammable atmospheres.
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Advantage 4
Running Costs - The main consideration in choosing fiber when installing domestic cable TV networks is the electric bill. Although copper coaxial cable can handle the bandwidth requirement over the short distances of a housing scheme, a copper system consumes far more electrical power than fiber, simply to carry the signals.
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Application Areas of Fiber Optics
Telecom - telephones and the Internet (capacity)
LANs - local area networks (weight/size)
CATV (cable TV) - for video, voice and Internet connections (low running cost)
Security - CCTV (closed circuit TV) and intrusion sensors (security)
Military - everywhere! (immune to EM & security)
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Basic Fibre Optic System
Electrical
Signal In
Driver
O/pCircuit
Source Detector
Electrical
Signal Out
Transmitter Receiver
FO cable plant
Transmitter: uses laser or LED, etc to covert an electrical signal to an optical signal.
Receiver: uses a photodiode to convert optical to electric signal.Physical plant: Fiber (SM or MM); Cable (application specific); Connectors; Splices; Panels; and Closures.
FO cable plant
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How to make glass fibers
Three methods:(1) MCVD(2) OVD(3) VAD
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Deposition by MCVD process
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Consolidation When deposition is complete, the tube with
the soot is placed into a consolidation furnace.
During the consolidation process, the tube changes into a solid, dense, transparent glass blank which is called preform.
After the preform cools, it is ready for drawing.
Which part will be the cladding? The core?
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Fiber Drawing Tower
1900-2500oC
Diameter controlbuffer coating
buffer coating
10-20 m/s
Video
How to make a preform How to draw fiber from a preform
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Optics - Reflection of Light
Consider the light strikes a flat mirror at the point P and is then reflected.
How do you describe the reflected beam?
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Law of Reflection
The surface normal is a line perpendicular to the surface at the point the light ray hits
The normal and the incident ray form the plane of incidence.
p
surface normal
reflecting surfaceplane of incidence
Angle of incidence = angle of reflection
The incident ray, normal, and the reflected ray all lie in the incidence plane perpendicular to the reflecting surface.
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Speed of Light
In vacuum: The speed of light c = 3x108 m/s
Light/Matter Interaction Think about the speed of light in air, in
water and in a solid, which speed is fastest? Which speed is slowest? Why?
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Index of refraction, n
Index of refraction, a way to indicate the speed of light in a medium
n = c/v n<1, ~ 1, or > 1? Slow down the speed of light with a huge
n?
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Index of Refraction n
In vacuum, the speed is highest. c = 3x108 m/s.
In traveling through all matter, the velocity is slower, depending on material properties.
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Refractive Indices of Materials
Material nair 1.00027
water 1.33glass 1.5
diamond 2.4
ngas < nliquid < nsolid. Higher refractive index means
more dense of the material.
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Light at a Boundary
“Different refractive indexes” form boundaries.
At a boundary, some light is reflected, some transmitted, and some absorbed depending on material
properties which form the boundary
n1
n2
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Law of refraction
How about the direction of the refracted light?
Amount of bending depends on refractive indexes n1, n2 of the two materials, as given by Snell’s law.
n1sinq1 = n2sinq2
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Two possible cases exist
n1 < n2 n1 > n2
q1
q2
q1
q2
When traveling from less dense to more dense medium, light is bent toward the normal.
When traveling from more dense to less dense medium, light is bent away from the normal.
n1Less denseAir n = 1.0
More densen2glass n = 1.5
n1More denseGlass n = 1.5
Less densen2Air n = 1.0
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Total Internal Reflection (TIR)
From more dense to less dense medium, light is bend away from the normal => So q2 > q1.
What will it happen if we increase q1?
q1
q2
qc
n2 = 1
n1 = 1.5
2n2sin 1n1sin
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Critical angle
At the point when q2 = 90o, q1 has reached a value called the critical angle (q1 = qc ).
n1sinqc = n2sin90o
sinqc = n2/n1qc = arcsin (n2/n1) = sin-1(n2/n1)
Once q1 >= qc, there is no transmitted ray and all the light energy is reflected.
This phenomenon is called Total Internal Reflection (TIR).
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Examples – qc at Interfaces
Ex 1: Given: n1 = 1.49 for glass fiber core and n2=1.47 for the cladding. Find: critical angle qc
Solution
deg60.80)49.1/47.1arcsin( c
Ex 2: For a glass-air interface, n1 = 1.5, n2 = 1.0, and the critical angle is given byqc = sin–1 (1.0/1.5) ≈ 41.8°
In the 1840s, people noticed that the light cold be guided in a jet of water flowing from a tank.
This is now known as total internal reflection (TIR).
Light propagates through fibers by the principle of TIR. 36
Demonstrations of TIR
More TIR examples:• Light pipes• Right angle prisms
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For TIR to Occur
The refractive index of the 1st medium is greater than the refractive index of the 2nd one. e.g.., glass to air
The incident angle is large. TIR causes 100% reflection (no energy loss).
In no other situation in nature, where light is reflected, does 100% reflection occur. So TIR is unique and very useful.
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Is a dielectric waveguide. Core refractive index is higher than the
cladding refractive index. Light is confined within the optic fiber
by TIR.
Optic Fiber
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Attenuation vs wavelength
v1 O–Si–O bending mode; v13 O–H stretching mode
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Transmission Windows
Fiber Type/ Wavelength
850 nm 1300 nm 1550 nm
Multimode 3 dB/km 1 dB/km NA
Singlemode 1 dB/km 0.4 dB/km 0.25 dB/km
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Structure of an Optics Fiber
Core refractive index is higher than the surrounding material’s refractive index.
Light is confined within the optic fiber by Total Internal Reflection (TIR). Core and cladding are
both made of glasses of different refractive indexes and can not be separated.
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Fiber Technology
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Fiber Core/Cladding Sizes
Here are the three most widely used fiber types:
• 50/125 microns;• 62.5/125 microns; • 8-10/125 microns.
• The buffer has a diameter of 250 microns.
• Human hair has a typical diameter of 75 microns.
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Cross Section of a Bare Fiber
Bare fiber
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Cross Section of a Fiber Cable
Fiber Cable
Metric Units
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Fiber optics uses the metric systems as the standard form of measurement.
Several of the more common terms are: Meter (m): 39.37 inches, or just slightly
larger than a yard. Kilometer (km): 1,000 meters (3,281
feet) or approximately 0.62 of a mile. Micron (m): One millionth of a meter. Nanometer (nm): One billionth of a
meter.
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Numerical Aperture
Numerical Aperture is the “light gathering ability”
The numerical aperture is defined as NA = (n1
2 – n22) = sinqA
It is a measure of the light gathering power of the fiber.
Example: Calculate NA and the acceptance angle of a fiber with a core refractive index of 1.53 and cladding of 1.50.
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About NA
It lies between 0 and 1. A numerical aperture of 0 means that
the fiber gathers no light (corresponding to qA = 0o). A numerical aperture of 1 means that the fiber gathers all the light that falls onto it (corresponding to qA = 90o).
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High NA vs low NA
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The Fiber Loss - Decibel (dB)
A logarithmic unit describing the ratio of two powers.
dB = 10 log10 (Pout/Pin) dB = 10 log10 (power remaining)
CircuitInput power, Pin Output power, Pout
• Used to measure losses (or attenuations).
Example – power ratio and dB
Power remaining dB Ratio 100% 0 1 95% -0.22 0.95 90% -0.46 0.9 80% -0.97 0.8 70% -1.55 0.7 60% -2.22 0.6 50% -3.01 0.5
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Example – power ratio and dB
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Power remaining dB Ratio
40% -3.98 0.4 30% -5.22 0.3 20% -6.99 0.2 10% -10 0.1 5% -13 0.05 2% -17 0.02 1% -20 0.01 0.1% -30 0.001 0.01% -40 0.0001
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dBm
Definition: dBm = 10 log10 (P/1 mW), P –power
Decibels referenced to a milliwatt dBm gives an absolute value dB measures the ratio of two power
Example – dBm and Power in mW
Power in W Power in mW dBm 100W 100,000 50 60W 60,000 47.8 1W 1,000 30 0.5W 500 27 0.1W 100 20 0.02W 20 13 0.01W 10 10 0.001W 1 0 0.0001W 0.1 -10 0.00001W 0.01 -20 54
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Power Budget
System design RX Power = TX Power –aL– Nspliceηsplice –
Nconnectotηconnector - System margin
TX RXOptical fiber
Transmitter Receiver
Express received and transmitter power in dBm, losses in dB.
2 – 10 dB(aging)
Average splicinginsertion lossLoss
Average connectorinsertion loss
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Example
Perform a link budget analysis to determine the margin, and sketch a power budget graph.
Given: Transmitter output: -14 dBmReceiver sensitivity: -35 dBm
Fiber loss: 2.2 dB/kmCable length: 4 kmConnector/splice: 1 pair with 1.5dB loss/pair
(Do not forget the 2 connector pairs needed for the transmitter and receiver)
Answers
1. Fiber loss _4km x 2.2 dB/km = 8.8 dB_____
2. Connector Loss__1.5x(2) = 3 dB ____3. Fusion splice loss _1.5x1 = 1.5 dB
________
4. Total cable plant loss (1+2+3) _13.3 dB ____
5. Maximum allowable loss (with excess margin factored in)_-14 – (-35) = 21 dB __________
6. Link loss margin_ 21 – 13.3 = 7.7 dB ______
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Power Budget Graph
-35
-25
-15
-5
1km 2km 3km 4km 5km
T R
margin
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Fiber Types
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Fiber Classification
MATERIAL MAKEUP
REFRACTIVE INDEX AND MODE
glass, pcs, plastic, etc
Step-index
Multi-mode
Single-mode
graded-index, etc
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Fiber Classification – glass fiber
Glass fiber – most common made of ultrapure silicon dioxide or fused
quartz. If sea-water were as clear as a fiber, you could
see to the bottom of the deepest trench ~ 11 km-deep Marianas Trench in Pacific (7 miles)
Impurities are added to change the refractive index
Germanium or phosphorus increases the index Boron or fluorine decreases the index
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PCS fiber means plastic-clad silica fiber Glass core and plastic cladding Performance is not as good as glass
fiber No buffer coating needed
Fiber Classification – PCS fiber
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Plastic core and plastic cladding Disadvantage – high loss and low
bandwidth Advantages – low cost and easy to use Applications – loss or bandwidth is not a
concern No buffer coating needed
Fiber Classification – Plastic fiber
Mode
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A path. Singlemode fibers have a single path at a single wavelength. Multimode fibers have multiple light paths at a single wavelength. The number of modes in a fiber will increase as the fiber core increases, or the wavelength decreases.
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Modes in optic fibers
Optical fibers can sustain as few as one mode to greater than 100,000 modes.
n n
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Fiber types
• In defining fiber types, we will not use physical materials for classification.
• Fiber types are classified according to the type of mode structure and light passage paths in the fiber.
• The three fiber types are• step-index (multimode step-index fiber)• graded-index (multimode graded-index
fiber)• single-mode (single-mode step-index
fiber)
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Step-index (SI) Fiber
Step-index fiber was the first fiber developed and the simplest of the three types.
It has many modes depending on the core size and the refractive index differences between the core and cladding.
The different modes travel different distances along the fiber as they bounce down the optical fiber.
It suffers from having the lowest bandwidth.
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Graded-index Fiber
This construction is similar to having many concentric cylinders or tubes of optical material.
The outer layers have a lower refractive index to "speed up" these light rays, compensating for the greater distance traveled.
n To overcome the
lengthening effect, a graded refractive index core was developed.
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Single-mode Fiber
• A fiber only allows a single mode to pass efficiently.
• For a fiber to behave as a single mode, the diameter of the core must be very close to the size of the wavelength of the optical carrier. The core is ~ 8 to 10 µm in diameter.
• A single-mode fiber at 1300 nm may not be single-mode at 820 nm. Most commonly available single-mode fibers are for 1300 and 1500 nm systems.
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Review Advantages of Optic Fiber
1. High Capacity/Large bandwidth 10 Gbps x 160 channels x 108 fibers
2. Low running costo Low loss (attenuation ~ 0.2dB/km)
3. Light weight & Small size 9 times lighter than coaxial cable 10 times smaller than coaxial cable
4. Safety & Security (dielectric, no fire, chemical, etc
hazard) (no radiation leakage) Electromagnetic immunity (no EM
interference)
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Review
The simplest fiber optic cable consists of two concentric layers. The inner portion, the ____, carries the
light. The outer covering is the ______.
The cladding must have a (higher, lower) ______ refractive index than the core.
therefore, the core and cladding material (is, isn’t) _____ exactly the same.
n1 (>, <) _______ n2. n1
n2
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Review