IT Friends

Early Optical Communications
The French used semaphores to transmit messages in the 1790s
Later systems also sent optical signals through the air
• But clouds, rain, and other atmospheric disturbances can disrupt optical signals sent through the air
• Electric signals through wires avoid that problem
Guiding Light With Water
Light in a stream of water stays inside the water and bends with it
This was first demonstrated in the 1840s
• Image from glenbrook.k12.il.us/gbssci

Refraction (Bending) of Light
Ray A comes from straight up and does not bend much
Ray B comes at a shallow angle and bends a lot more
• Image from seafriends.org.nz

The View From Underwater
Underwater, the light always shines down steeply, even when the Sun is low in the sky
The whole sky appears in a limited round area called “Snells Window”
• Image from seafriends.org.nz

Light Coming Out of Water
Animation on link Ch 1b on my Web page (samsclass.info)
• http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=66
Total Internal Reflection
There is a critical angle at which no light can be refracted at all, so 100% of the light is reflected
• Light is trapped in the water and cannot escape into the air
• This works with any dense medium, such as plastic or glass, the same way it works with water
 Image from glenbrook.k12.il.us

How Light Travels in Fiber
Image from ece.umd.edu/~davis
Bare Fiber
During 1920-1950, thin, flexible rods of glass or plastic were used to guide light
Such “bare” fibers require air outside each fiber
• Image from Wikipedia
Fiber With Cladding
Developed in 1954 by van Heel, Hopkins & Kapany
Cladding is a glass or plastic cover around the core
Protects the total-reflection surface contamination
Reduces cross-talk from fibers in bundles
Medical Imaging
By 1960, glass-clad fibers were available for medical instruments, to look inside the body
The glass was unable to transmit light far enough for communications, because of impurities
• Attenuation (loss of light) was 1 decibel per meter
Decibels
Decibels are a logarithmic scale of power
• Abbreviated dB
A loss of 10 decibels means only 10% of the light gets through
A loss of 20 dB means 1% of the light gets through
• Sunglasses stop 99% of light, so they cause a loss of 20 dB
For communications, loss must be no more than 10 or 20 decibels per kilometer

Optical Fiber in 1966
Charles Kao developed a fiber that could transmit 1 GHz (One billion bits per second)
But attenuation was 1000 dB/km, so it could not transmit light far enough for practical communications
Corning
Corning scientists developed low-attenuation silica glass fibers in 1970
Corning Video: At The Speed of Light
• Link Ch 1c on my Web page (samsclass.info)
Singlemode and Multimode Fiber
Singlemode fiber has a core diameter of 8 to 9 microns
Multimode fiber has a core diameter of 50 or 62.5 microns
Both have a cladding diameter of 125 microns
Optical Fiber in 1977
Telephone signals used infrared light with a wavelength of 850 nm to send data at 6.2 Mbps and 45 Mbps
Loss was 2 dB per km
Repeaters were required every few kilometers
• The repeaters were electro-optical – converting the light to electricity and then back to light
TAT-8
In 1988 AT&T laid the first fiber-optic transatlantic telephony cable
3,148 miles long
Connected North America to France
Repeaters every 40 miles
565 Mbps bandwidth
Used 1300 nm light
Attenuation 0.4 dB/km
• Info from link Ch 1e www.greatachievements.org/?id=3706
Fiber Amplifier
Special fiber with Erbium atoms in it is used to amplify light without changing it to an electrical signal first
Uses stimulated emission, the same principle that makes lasers work
• Image from rp-photonics.com (Link Ch 1g)
Wavelength Division Multiplexing (WDM)
Several signals can be sent through the same fiber simultaneously by using different wavelengths (colors) of light
That means more bandwidth—more data per second
Freeway Analogy
TAT-8 in 1980
• 565 Mbps
• Electro-optical repeaters
TAT-12/13 in 1996
• 2.5 Gbps
• Optical amplifiers
1998
• 20 Gbps
• WDM with 8 wavelengths
Image from www2.rad.com (Link Ch 1j)

Dense Wavelength Division Multiplexing (DWDM)
Uses up to 100 wavelengths through a single fiber
Bandwidth up to 1 Tbps (1000 Gbps)

Lennie Lightwave’s Guide To Fiber Optics: Basics
From jimhayes.com/lennielw
Fiber Optics History
Fiber optics began about 30 years ago in the R&D labs (Corning, Bell Labs, ITT UK, etc.)
First installed in Chicago in 1976
By the early 1980s, fiber networks connected the major cities on each coast.
The 1980s
By the mid-80s, fiber was replacing all the telco copper, microwave and satellite links
In the 90s, CATV started using fiber to enhance the reliability of their networks
• CATV companies also discovered they could offer phone and Internet service on that same fiber and greatly enlarged their markets
Computers and LANs
Started using fiber about the same time as the telcos
Industrial links were among the first as the noise immunity of fiber and its distance capability make it ideal for the factory floor
Mainframe storage networks came next, the predecessors of today’s fiber SANs (storage area networks.)
Other Applications
Aircraft, ship and automobile data buses
CCTV for security
Links for consumer digital stereo
Today fiber optics is either the dominant medium or a logical choice for every communication system
Which Fiber Optics?
“Outside Plant” fiber optics are used in telephone networks or CATV
“Premises” fiber optics are usedin buildings and campuses
Just like “wire” which can mean lots of different things - power, security, HVAC, CCTV, LAN or telephone - fiber optics is not all the same.
Installing Fiber Optics

Fiber is harder to install than 100 Mbps copper Ethernet cable
But fiber is MUCH faster, so the infrastructure won’t need to be upgraded so soon
• And gigabit Ethernet is harder to install
LAN copper cable is delicate. It only has a 25 pound pulling tension limit and kinks will ruin the high speed performance
• Fiber has more strength and greater tolerance to abuse than copper wire
Safety First!
The light in the fiber can burn your retina
NEVER look into a fiber unless you know no light is present - use a power meter to check it
The infrared light is invisible
Fiber Shards
When you cleave fiber, there are small scraps of glass produced.
These scraps are very dangerous!
The cleaved ends are extremely sharp and can easily penetrate your skin
They are even worse in your eyes, mouth, etc.

Safety Rules
Wear glasses or safety glasses
Dispose of all scraps properly: put them on black tape and then into a properly marked trashcan
Work on a black pad which makes the slivers of glass easier to spot
Do not drop scraps on the floor
Do not eat or drink anywhere near the work area
Chemical Safety
Fiber optic splicing and termination use various chemical adhesives and cleaners
Follow the instructions for use carefully
Isopropyl alcohol, used as a cleaner, is flammable
Zero Tolerance for Dirt
Airborne particles are about the size of the core of Single Mode fiber
• They absorb lots of light and may scratch connectors if not removed
• Dirt on connectors is the biggest cause of scratches on polished connectors and high loss measurements
Hygiene Rules
Work in a clean area – avoid dust
Keep dust caps on all connectors
Use lint free pads and isopropyl alcohol to clean connectors

Last modified 1-20-06