Charles KaoHong Kong

William BoyleBell Labs

The Nobel Prize in Physics 2009

Kao: "for groundbreaking achievements concerning the transmission of light in fibers for optical communication“

Boyle and Smith: "for the invention of an imaging semiconductor circuit – the CCD sensor"

George SmithBell Labs

• Stone Age

• Bronze Age

• Iron Age

• Ice Age

What is Information Age?

Information Age

The cost of the transmission, storage and processing of data has been decreasing extremely fast

Information is available anytime, any place, and for everyone

Information and knowledge became a capital asset

All of this became possible because of several revolutionary ideas

Laser and semiconductor laser

Transistor

Computer

World-Wide Web

Optical fibers

… Are invented by physicists

Integrated circuits

Telecommunications

Samuel Morse's telegraph key, 1844. Today's information age began with the telegraph. It was the first instrument to transform information into electrical signal and transmit it reliably over long distances.

Alexander Graham Bell’s commercial telephone from 1877.

How it all started …

In 1880 patented a “Photophone” (air-based optical telephone)

Alexander Bell, Helen Keller, and Anne Sullivan, 1894

Speaking into the handset's transmitter or microphone makes its diaphragm vibrate. This varies the electric current, causing the receiver's diaphragm to vibrate. This duplicates the original sound.

• Telephone connection requires a dedicated wire line;• Only one communication is possible at a time

How many channels are possible? How many signals can be transmitted at the same time??

Radio: communication through radio waves

1895

Alexander PopovGuglielmo Marconi

www.nrao.edu

Frequency measured in Hertz1 Hz = 1 cycle/second1 kHz = 1000 cycles/second

Radio stations have to broadcast at different carrier frequencies to avoid cross-talk

Range of frequencies (Bandwidth) needs to be at least 20 kHz for each station

Human ear: 10 Hz-20 kHz

Frequencies of different stations should be at least 20 kHz apart

Even if you transmit only voice, from 0 to 2000 kHz you can squeeze only 2000 kHz/20 kHz = 100 different “talks”.

What if you want to download data?

About 100 bands from 0 to 2000 kHz

Digital transmission: any signal, but transmission speed is still limited by bandwidth!

Binary code is transmitted: “0s” and “1s” – bits of information

Want download speed of 2 Mb/sec? Need bandwidth at least 1 MHz

Mega = million, Giga = billion; 1GHz = 1000 MHz = 109 Hz

Want 100 Mb/sec? need 50 MHz bandwidth just for yourself

Modern cell phones and GPS use gigahertz (GHz) frequenciesBut this is only 1000 MHz/50 MHz = 20 channels at 100 Mb/sec!

Higher carrier frequencies

Wider bandwidth

Higher data rate, more channels

Need more channels? Need higher speed?Use higher frequencies for transmission!

Using light? Optical frequencies ~ 1014 Hz !

How can we send light over long distances?

Air? Only within line of sight; High absorption and scattering, especially when it rains

Are there any “light wires” (optical waveguides)?

Copper wire? High absorption, narrow bandwidth 300 MHz

Glass? Window glass absorbs 90% of light after 1 m.Only 1% transmission after 2 meters.

Sand?!

Transmisson 95.5% of power after 1 km 1% of power after 100 km: need amplifiers and repeaters

Total bandwidth ~ 100,000 GHz!!

Ultra-low absorption in silica glasses

Silica (Silicon dioxide) is sand – the most abundant mineral on Earth

Predicted 1965 (Kao), in first low-loss fiber in 1970

Total internal reflection!

n1 > n2

How to trap light with transparent material??

Light coming from more refractive to less refractive medium experiences total reflection – get trapped there!

Examples of total internal reflection

Water: critical angle ~ 49o

Trapping light in waveguides

Optical fiber!

Main enemy: high attenuationSolution: use near-infrared light around 1.3-1.5 m

Optical fibers

Made by drawing molten glass from a crucible

1965: Kao and Hockham proposed fibers for broadband communication

1970s: commercial methods of producing low-loss fibers by Corning and AT&T.

1990: single-mode fiber, capacity 622 Mbit/s

Now: capacity ~ 1Tbit/s, data rate 10 Gbit/s

Fibers open the flood gate

Bandwidth 100 THz would allow 100 million channels with 2Mbits/sec download speed!

Each person in the U.S. could have his own carrier frequency, e.g., 185,674,991,235,657 Hz.

However, we are using less than 1% of available bandwidth!

And maximum transmission speed is less than 0.00001 of bandwidth

In optical communications, information is transmitted as light signal along optical fibers

However, if we want to modify, add/drop, split, or amplify signal, it needs to be first converted to electric current, and then converted back to photons

This is a slow process (maximum 10 GHz)

Limitations of fiber communications

Futuristic silicon chip with monolithically integrated photonic and electronic circuits

THE DREAM: could we replace electrons with photons, and electric circuits with all-optical circuits?

IBM website

wires waveguides

Charge-coupled deviceMOS capacitor

Photons generate charge which becomes trapped

The charge generated by photons is forced to move one step at a time through the application of voltage pulses on the electrodes.

http://www.ecn.purdue.edu/WBG/Device_Research/CCDs/Index.html

http://www.astro.virginia.edu/class/oconnell/astr121/guide14.html

http://digitalimagingu.com/articles/microscopyimaging.html

The principle behind read-out of a CCD chip. One row at a time is shifted through an A/D converter which makes the output signal digital.

CCDs for astronomyHuge, 100 MPs

SLOAN

CFHT, Hawaii

http://digitalimagingu.com/articles/microscopyimaging.html