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The Nobel Prize in Physics 2009. William Boyle Bell Labs. George Smith Bell Labs. Charles Kao Hong Kong. Kao: "for groundbreaking achievements concerning the transmission of light in fibers for optical communication“
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The Nobel Prize in Physics 2009 William Boyle Bell Labs George Smith Bell Labs Charles Kao Hong Kong 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"
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
Transistor Laser and semiconductor laser Computer World-Wide Web Optical fibers Integrated circuits … Are invented by physicists
How it all started … 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. In 1880 patented a “Photophone” (air-based optical telephone)
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
Radio: communication through radio waves 1895 www.nrao.edu Frequency measured in Hertz 1 Hz = 1 cycle/second 1 kHz = 1000 cycles/second Alexander Popov Guglielmo Marconi How many channels are possible? How many signals can be transmitted at the same time??
Radio stations have to broadcast at different carrier frequencies to avoid cross-talk Human ear: 10 Hz-20 kHz Range of frequencies (Bandwidth) needs to be at least 20 kHz for each station Frequencies of different stations should be at least 20 kHz apart
About 100 bands from 0 to 2000 kHz 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?
Digital transmission: any signal, but transmission speed is still limited by bandwidth! Binary code is transmitted: “0s” and “1s” – bits of information Mega = million, Giga = billion; 1GHz = 1000 MHz = 109 Hz Want download speed of 2 Mb/sec? Need bandwidth at least 1 MHz Want 100 Mb/sec? need 50 MHz bandwidth just for yourself Modern cell phones and GPS use gigahertz (GHz) frequencies But this is only 1000 MHz/50 MHz = 20 channels at 100 Mb/sec!
Need more channels? Need higher speed? Use higher frequencies for transmission! Higher carrier frequencies Wider bandwidth Higher data rate, more channels 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?!
Ultra-low absorption in silica glasses Transmisson 95.5% of power after 1 km 1% of power after 100 km: need amplifiers and repeaters Total bandwidth ~ 100,000 GHz!! Predicted 1965 (Kao), in first low-loss fiber in 1970 Silica (Silicon dioxide) is sand – the most abundant mineral on Earth
How to trap light with transparent material?? Total internal reflection! n1 > n2 Light coming from more refractive to less refractive medium experiences total reflection – get trapped there!
Water: critical angle ~ 49o Examples of total internal reflection
Trapping light in waveguides Optical fiber!
Main enemy: high attenuation Solution: 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
Limitations of fiber communications 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)
THE DREAM: could we replace electrons with photons, and electric circuits with all-optical circuits? IBM website Futuristic silicon chip with monolithically integrated photonic and electronic circuits wires waveguides
Charge-coupled device MOS 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.htmlhttp://www.astro.virginia.edu/class/oconnell/astr121/guide14.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 astronomy Huge, 100 MPs CFHT, Hawaii SLOAN
