A pixel of the CCD (Charged-coupled Device) Array Energy  Thin Insulator Thin Conductor P-Type semiconductor N-Type Excess negative charges Excess positive charges Depletion Region Question: What will happen if apply a positive voltage at A? a.Nothing, no electrons can move b.Electrons in N-type will flow to the right and pile up at insulator until charges balance c.Electrons will flow from P-type to N- type and pile up at insulator until charges balance d.Electrons will flow continuously from P to N to A. A

A pixel of the CCD (Charged-coupled Device) Array Energy  Thin Insulator Thin Conductor P-Type semiconductor N-Type Excess negative charges Excess positive charges Depletion Region Question: What will happen if apply a positive voltage at A? Answer is b. Electrons in N-type will flow to the right and pile up at insulator until charges balance This creates large region of excess positive charges in middle of N- type. +V

A pixel of the CCD (Charged-coupled Device) Array Energy  Thin Insulator Thin Conductor P-Type semiconductor N-Type +V

A pixel of the CCD (Charged-coupled Device) Array Energy  Thin Insulator (TRANSPARENT) Thin Conductor (TRANSPARENT) P-Type semiconductor N-Type +V What will happen to excited electron? a.fall back down to empty energy level b.Move to right c.Move to left d.Stay where it is

A pixel of the CCD (Charged-coupled Device) Array Thin Insulator Thin Conductor P-Type semiconductor N-Type +V e- What will happen to excited electron? Answer is b. Move to right. Attracted to center of N- type with lots of positive charge.

A pixel of the CCD (Charged-coupled Device) Array Thin Insulator Thin Conductor P-Type semiconductor N-Type +V e- These e-’s stuck in upper band. No empty energy levels below. More photons absorbed. More electrons pile up.

Isolating the pixels of the CCD (Charged- coupled Device) Array -V +V -V Electrons pile up… 1 electron per photon absorbed. Electrons confined to region of positive excess charge (pixel). After exposure of CCD. Read out each pixel. Move charges as a unit to readout. little buckets of charge, pass down to end of row and measure electron counter capacitor, measure V created

Columns of pixels move across, counted at right edge, one at a time.

Today- 1. optical transmission of information- optical fiber network. (all long distance telephone and computer communication) 2. Optical storage of information --CD and DVDs. (if time??) See how bunch of physics already covered is put to use in a new way. Light, lasers, reflection and bending of light, focusing light, detection. Main advantages of using light for long distance communication-- already apparent in early discussion. H. W. Longfellow

Paul Revere's Ride Henry Wadsworth Longfellow Listen my children and you shall hear Of the midnight ride of Paul Revere, On the eighteenth of April, in Seventy-five; Hardly a man is now alive Who remembers that famous day and year. He said to his friend, "If the British march By land or sea from the town to-night, Hang a lantern aloft in the belfry arch Of the North Church tower as a signal light,-- One if by land, and two if by sea; And I on the opposite shore will be, Ready to ride and spread the alarm Through every Middlesex village and farm, For the country folk to be up and to arm." next verses talk about moon, night wind, … i. e. noise that could obscure signals

alternatives? (shows benefits of light used today)- 1) “yell really loud at me”- - Does rhyme with “sea”. but sound not carry as far as light. 2) “Row over and tell me” - still rhymes, but slow. Light much faster. Uses another trick of modern optical communication- digital information. Flash a light. Just seeing it tells Paul his friend is there to send signal. All information contained in second pulse of light. If on = by sea, if off, no light= by land. “binary digital signal”- two choices (on/off). One bit of information. Information “digital”. Big Advantage-Very distinct and unambiguous. on or off -- can distinguish even if fog, moonlight, etc. Now days- information coded as series of on and offs that give numbers. Signal is insulated from presence of noise. (same as hiss on tape recorder, etc.)

Why use light ? 1. light- fast. Transmit signals rapidly. 2. Can go long way without being absorbed. (under special conditions!)

Principles behind design of optical communication system Want to send telephone signals from Boulder to Ft. Collins. Put sound information into light pulses. (lots of lantern flashes, a al Paul). 1. What are some problems with sending this way? (Write down 3 per group.) Then be prepared to offer ways to get around the problems. 1. need lots of light flashes in a hurry. Need good way to turn light on and off very fast and detect pulses. 2. Light spreads out all over the place, not much gets to Ft. Collins. Also, clouds and dust block it. 3. Want to send many conversations, no way to separate one light from another. 4. not be able to see light from location. 5. ….

Solutions: 1. confine light to pipe, send it right where want, keeps clouds, dust etc. out, keeps light from spreading out (laser). Send light pulses down thin glass optical fibers. (“light pipes”) Light does not spread out, can be directed to exactly right place, can have multiple fibers to separate conversations, no clouds, etc.

Demo of plastic rod and laser beam- behavior of light inside rod. How to make light stay inside fiber/pipe? a. it doesn’t. It all leaks out but hard to see because so spread out. b. Most reflects off the walls because they are covered with mirror-like reflective surface. c. Only the light that hits wall at too steep an angle reflects. ans. c.

Light travels more slowly in material with larger index of refraction n. Causes it to bend more when go from higher to lower n. Light Beam AIR, N=1GLASS, N=1.5 AIR, N=1 Small Reflection

How does amount bent depend on angle of hitting wall? If hits at steeper angle bends a. more, b. same, c. less AIR, N=1GLASS, N=1.5 AIR, N=1 a b c ans. a. Bends more. Like car. Front right tire in air first, goes faster,remember n=c/v, so smaller n, means travels faster. Turns car more toward top. before after (steeper)

What if hit at even steeper angle? AIR, N=1GLASS, N=1.5 AIR, N=1 Bends so much never even leaves glass! “Total internal reflection” Happens at certain angle for every interface. when angle a > sin -1 (n big /n little ), then total reflection. Absolutely everything stays inside, no light leaves! Makes it possible to pipe light beam around like water!! Demos of optical fibers. a

Demo of plastic rod and laser beam- what will happen if dip into mineral oil? Send light through plexiglass rod. If dip rod in mineral oil what will happen to light coming through? a. will get dimmer, b. will stay same, c. will get brighter n = 1.4 n plastic = 1.45 ans. a. will get dimmer. now no change in n from inside to out. no bend and no reflection. key is change in n!

optical fiber about the same diameter as a thick hair. Purity is about 1 wrong atom per billion glass atoms. Light goes about 20 miles, before half light absorbed!! Boost back up about every 50 miles. Copper wire boost electrical signal every ½ mile. Optical fibers under ocean, all over land. Send information at rate of 10 billion bits / seconds, copper wire transmits about 1/100 the speed. laser light high n low n

How to get light into tiny fiber? Lens to focus laser light. why laser light rather than ordinary light? Laser can be focused into much smaller spot. 1. Can get much larger fraction of laser light into tiny fiber. Smaller fiber means cheaper, more flexible.

Want to make absorption of light in fiber as small as possible. How to do it? Write down important physics issues for absorption of light. How to build low absorption fiber. a. make fiber only out of atoms that have excitation energies matching the wavelength of light. b. make fiber only out of atoms that have excitation energies as far as possible from that of the wavelength of light. c. use wide mixture of different kinds of atoms d. use range of different colors of light.

Want to make absorption of light in fiber as small as possible. How to do it? b. make fiber only out of atoms that have excitation energies as far as possible from that of the wavelength of light. Make fiber as pure as possible, pick atoms and wavelengths to have excitation energies as far as possible from photon energy. (near IR, super incredibly pure silicon)

Key things making optical fiber communication possible. 1. Lasers so could get enough light focused into skinny little fiber. 2. fast light detectors- photodiodes like in digital cameras. 3. Fibers with small enough absorption. Physics you already know-- absorption of light by atoms. a. ones with energy levels near energy of light- bad impurities, work incredibly hard at removing them. Ultrapure!! b. ones like glass with energy levels way up in ultraviolet like in air. still some scattering- same as why sky is blue. So less loss in infrared, so light sent down fibers is near infrared where loss is lowest. To send voice telephone call takes 64,000 bits/sec. Range of sound waves from 400 Hz to 3400 Hz. (56 K modem = 56,000 bits/sec) Can send 156,000 phone calls at the same time down one optical fiber! transmission rate of 10 Gbits/s (=10 billion bits/s)

2. Storage of information optically. Store all these digital numbers as tiny little rough spots on CD or DVD. Light reflection-on, no reflect-off. Have to focus the light down into tiny little spot. Determines information density. Really extensive discussion of DVD and CD technology at how things work:

CD and DVD construction: basically the same, but DVDs smaller pits, shorter wavelength light, can have two sides and two reflecting layers, data compression. Information stored as pits in a surface, light reflects off pits. The amount of light that reaches a detector depends on whether a pit is hit or not. Information is stored digitally.

Picture of CD/DVD Player

Digital to Analog conversion

DVD vs CD 1. smaller pits, shorter wavelength. 2. two sided option 3. two layer option.- how to pick out which layer looking at? change focus of laser beam- move lens (little bit). Top layer transmits half. On to off of top or bottom, bigger effect than ½. second pass

Wavelength Division Multiplexing (WDM)

Light travels more slowly in material with larger index of refraction n. Causes it to bend more when go from higher to lower n. Light Beam AIR, N=1GLASS, N=1.5 AIR, N=1 In a rectangular slab, light ray straightens out again when it comes back out! Small Reflection If n is 1.5 for all colors, which travels through the glass most rapidly? a. red, b. blue, c. green, d. they all go at 3 x 10^8 m/s, e. they all go at 2 x 10^8 m/s. e. The speed of light is given by n=c/v, or v= c/n. c = 3 x 10^8 m/s. so if n =1.5 for all colors, v = 3/1.5 x 10^8 m/s. In most materials different colors travel at very slightly different speeds, so they bend differently. That is why a prism works. n = for red, for blue. Difference of 1%. Which faster?

So what about storage of sound? 1) demo of sound on scope with microphone and storage scope. Original phonograph- groove on tin foil went up and down like sound. similar in record player. time air pressure run needle over surface to reproduce sound. Can’t follow perfectly, surface scrapes away, extra hiss from scraping, … “analogue” storage of information surface of record needle

Digital storage. Every 1/44,100 sec, measure pressure, store as binary number. As long as can tell difference between 1 (on) and 0 (off) sound will be perfectly reproduced even if noise! sound on scope. Make signal small, see signal/noise ratio.