Title: Communicating with Light: From Telephony to Cell Phones Revision: February 1, 2006 Authors: Jim Overhiser, Luat Vuong Appropriate Physics, Grades 9-12 Level: Abstract: This series of six station activities introduces the physics of transmitting "voice" information using electromagnetic signals or light. Students explore how light can be modulated to encode voice information using a simple version of Bell's original photophone. They observe the decrease of the intensity of open-air signals by increasing the distance between source and receiver, and learn the advantage of using materials with different indices of refraction to manipulate and guide light signals. Finally, students are introduced to the concept of bandwidth by using two different wavelengths of light to send two signals at the same time.
Special Kit available on loan from CIPT lending library. Equipment: Time Required: Two 80-minute periods NY Standards 4.1b Energy may be converted among mechanical, electromagnetic, Met: nuclear, and thermal forms 4.1j Energy may be stored in electric or magnetic fields. This energy may be transferred through conductors or space and may be converted to other forms of energy. 4.3b Waves carry energy and information without transferring mass. This energy may be carried by pulses or periodic waves. 4.3i When a wave moves from one medium into another, the waves may refract due a change in speed. The angle of refraction depends on the angle of incidence and the property of the medium. 4.3h When a wave strikes a boundary between two media, reflection, transmission, and absorption occur. A transmitted wave may be refracted.
Communicating with Light: From Telephony to Cell Phones Teacher Information Section
Objectives: • To learn about the different components of an optical network, how they function and work together to build a telecommunications system. • To gain familiarity with basic optics concepts including wavelength, amplitude, refraction, reflection and total internal reflection within a technological context. • To understand that light guides (such as optical fibers) prevent signals from dispersing and allow them to be transmitted over large distances.
Historical Background:
Alexander Grahm Bell invented and tested a device called a photophone in 1880. Bell's device consisted of a cylinder with at piece of light-reflecting metal foil cover one end. Bell spoke into the other end of the cylinder and directed sunlight reflected off the foil on to a crude selenium cell (precursor to the modern photocell) that was connected in series to a battery. The battery provided the voltage to power one of Bell’s telephone receivers, and the selenium cell provided the voltage spikes necessary to produce intelligible sound. Changes in the intensity of the light beam were used to transmit the information.
Modern telecommmunication systems that use light change (modulate) the intensity of the beam. However, light was not used to carry voice and other signals until recently. Bell's invention, which he considered to be the most ingenious of all his inventions, never took off. It could transmit voice only 200 meters through free space, which was a critical limitation. It would take the invention of optical fibers and fiber amplifiers, capable of guiding light thousands of kilometers, before light was again used to carry communications signals.
Meanwhile, radio was invented and held the technological upper hand in communications for decades. Reginald Aubrey Fessenden’s (1866-1932) is given credit for the first attempt to transmit voice via a radio signal (telephony). He employed a spark transmitter (invented by Heinrich Hertz) operating at about 10,000 sparks/second. To modulate his transmitter he inserted a carbon microphone in series with the antenna lead.... He experienced great difficulty in achieving intelligible sound.
On the 23 December 1900, Fessenden, after many unsuccessful tries, transmitted words without wires. The first radio transmission was the following: "Hello test, one, two, three, four. If it's snowing where you are, Mr. Thiessen. If it is, telegraph back and let me know." Mr. Thiessen telegraphed back immediately. It was indeed snowing where he was. After all Mr. Thiessen and Professor Fessenden were only 1 mile apart. But not withstanding, the short distance, and the poor quality of the transmission, this date heralded the beginning of radio telephony, which dominated distance communication for several decades.
Instructions to build your own light radios:
Materials List for Light Emitter Number Catalog Total Item Source Cost per Number Cost Assembly Red LED 92F465 $0.38 $0.38 OR OR OR OR Newark Blue LED 34C1518 $0.97 1 $0.97
OR OR OR OR Yellow LED 34C1534 $0.44 $0.44 Aluminum Newark 95F2580 $5.36 1 $5.36 enclosure box Switch (SPST) Newark 61F1245 $3.59 1 $3.59 $2.99 1/8" Mono plug Radioshack 274-286 1 $1.50 (pkg of 2) Ribbon cable 2 pieces $13.33 for (14 strand, Newark 89F4840 1.5' long $0.06 100' spool 0.05” pitch) 2 strand Pocket radio Radio Shack 12-467 $16.99 1 $16.99 9 V Battery clip Newark 16F433 $0.53 1 $0.53 Resistor (300Ω) Newark 84N1622 $0.12 1 $0.12 Total cost for project: $29.12
Materials List for Light Receiver Number Catalog Total Item Source Cost per Number Cost Assembly DigiKey 425-1004-5-ND $2.27 $2.27 Photodiode OR OR OR 1 OR Hamamatsu S2386-45K $11.64 $11.64 $2.99 1/8" Mono plug Radioshack 274-286 1 $1.50 (pkg of 2) Ribbon cable 1 piece $13.33 for (14 strand, Newark 89F4840 1.5' long $0.03 100' spool 0.05” pitch) 2 strand Amplifier/speaker Radio Shack 277-1008 $12.99 1 $12.99 Total cost for project: $16.97 OR $26.16
Suppliers: • Newark website: www.newark.com • Radio Shack website: www.radioshack.com • DigiKey website: www.digikey.com • Hamamatsu website: www.hamamatsu.com
Tools needed: • Soldering iron, rosin core solder, electrical tape • Drill and drill bits • Pliers, wire stripper
Instructions for Light Emitter
1. Find two of the two-stranded 1.5' pieces of ribbon cable. At each end separate the strands for about 2" and strip the insulation from about 1/4" of each strand. If the strands are the same color, then mark one of them red at each end so you can tell them apart. Do this for both pieces of ribbon cable. 2. Find a mono plug and unscrew the black cap. Take one of the pieces of ribbon cable and solder one end of the strand marked red to the short terminal of the mono plug. Solder the other strand to the long terminal. The plug and wire should look like the picture below. Screw the black cap back on the mono plug.
3. Find a LED and the other piece of ribbon cable (not the one used in step #2). Solder one end of the strand marked red to the long led of the LED. Solder the other strand to the short leg of the LED. The LED and wire should look like the picture below. Wrap the legs of the LED and any bare wires in electrical tape to prevent them from touching and causing a short circuit.
4. Find the 9V battery clip and the 300Ω resistor. Solder one leg the 300Ω resistor to the red wire from the 9V battery clip. Solder the other leg of the resistor to the unmarked strand of the ribbon cable attached to the mono plug. Wrap the 300Ω resistor in electrical tape so that no bare wires are exposed. 5. Find the switch. Solder the black wire from the battery clip to the middle terminal of the switch. Solder the unmarked strand of the ribbon cable attached to the LED to one of the side terminals of the switch. Finally, solder together the strands from each ribbon cable marked red. The circuit will look like the picture below. Wrap the solder joint in electrical tape so that no bare wires are exposed.
to mono plug to LED
300Ω resistor (wrapped in tape)
to 9V battery clip switch
6. Drill a ¼” hole on the top side of the box for the switch. Insert the switch through the hole and tighten the nut on the threaded portion of the switch to lock it in place. Install a battery to the batter clip and tape the battery to the inside of the box to prevent it from rattling around. Your box should look like the picture below.
7. File two dents, each in opposite edges of box, to allow the wire to the mono plug and the wire to the LED to enter the box without getting pinched. Put the box together. 8. Turn on the radio and tune it to a station. Then plug in the mono plug of the light emitter assembly and flip the switch to determine which position is on and which is off. Label the switch positions.
Instructions for Light Receiver
1. Find a two-stranded 1.5' pieces of ribbon cable. At each end separate the strands for about 2" and strip the insulation from about 1/4" of each strand. If the strands are the same color, then mark one of them red at each end so you can tell them apart. 2. Find a mono plug and unscrew the black cap. Take one of the pieces of ribbon cable and solder one end of the strand marked red to the short terminal of the mono plug. Solder the other strand to the long terminal. The plug and wire should look like the picture below. Screw the black cap back on the mono plug.
3. Find the photodiode. Solder the long leg of the photodiode to the strand of the ribbon cable marked red. Solder the other strand to the short leg of the photodiode. Wrap the legs of the photodiode and any bare wires in electrical tape to prevent them from touching and causing a short circuit.
4. Find the amplifier/speaker and make sure it has a battery. Plug the mono plug of the photodiode circuit into the input of the amplifier and turn it on. It should make noise in response to varying intensity of light, such as the light from the LED of the light emitter circuit attached to the radio. 5. Enjoy!
If your light emitter/receiver circuits do not work, check for bad connections or short circuits. If it is still not working, contact the CIPT office at [email protected] or (607) 255-9434.
Circuit Diagram for Light Emitter and Light Receiver
Station 1: Talking with a beam of light
Part A: Modulation
Materials: • laser • laser mount • photodiode with amplifier/speaker • various accessories: plastic comb, glass slide, spray bottle, lycopodium powder, barcodes, wire mesh
Instructions: 1. Place the laser under the rubber bands on the laser mount. Use the binder clip to depress the on/off button and keep the laser beam on. 2. Aim the laser beam at the photodiode attached to the amplifier/speaker. Turn the speaker on. 3. Observe the effects of interrupting the laser beam various items. 4. Answer related questions on the Student Data Sheet.
Part B: Alexander’s Photophone
Materials: • latex balloon • Al foil (~1/2" square) Talk • yogurt cup (bottom cut out) • photodiode with amplifier/speaker • Mini Maglight flashlight
Instructions: 1. Stretch a piece of latex over one end of the yogurt cup and secure with a rubber band. Glue the piece of Al foil to the center of the latex. 2. Hold the open end of the yogurt cup device up to your mouth. 3. Have a partner aim the Mini Maglight at the Al foil and focus the beam to a bright spot. 4. Have a second partner hold the photodiode in the reflected beam of light. Make sure the speaker is on. 5. Talk into the yogurt cup and observe what happens 6. Answer related questions on the Student Data Sheet.
Station 2: Good transmission
Materials: • transistor radio • red LED with mono jack • photodiode with amplifier/speaker • magnifying glass with support • 30 cm long 7 mm diameter lucite rod • 2 m fiber optic cable
Instructions: 1. Select a strong station on the transistor radio and plug in the red LED. 2. Turn on the LED circuit and observe—can you see the light flicker? 3. Turn on the amplifier/speaker unit and aim the red LED at the photodiode. Make sure you can receive a good signal. 4. Observe what happens as you increase the distance between the LED transmitter and the photodiode receiver. 5. Place the magnifying glass between the LED transmitter and the photodiode receiver. Using the ability of the magnifier to focus light, can you position it to improve the transmission? 6. Try to improve transmission from LED to photodiode using the lucite rod. Compare transmission with and without the lucite rod. 7. Finally, try to improve transmission with the fiber optic cable. Does bending the fiber affect transmission? 8. Answer related questions on the Student Data Sheet
Station 3: Trapping light
transmitted Materials: beam • small plastic metric ruler • 3 semicircular dish with water • powdered milk • protractor • laser • white poster paper or large index cards reflected • 4 mirror holders beam
Instructions: 1. Full the semicircular dish with water and add a tiny pinch of powdered milk to the water and stir to dissolve. The water should still look clear. 2. Place the semicircular dish on the protractor as pictured with the flat side directly over the 0° — 180° line. 3. Set up two barriers as pictured, each using 2 mirror holders and poster paper. 4. Shine the laser through the water, aiming it from the 90° mark through the origin of the protractor. 5. Move the laser slowly to larger angles, keeping the beam aimed at the origin. 6. Observe the dots where the transmitted beam and the reflected beam fall on the barriers. Compare the brightness of each dot as the angle gets larger. 7. Increase the angle until the transmitted dot disappears. (You may need to adjust a barrier if the beam moves off of it.) Check to make sure your beam is still aimed at the origin point of the protractor and record the critical angle. 8. Prepare two more semicircular dishes as in step #1 and build the set-up pictured below. Move a barrier to check along the flat sides of the dishes and see if any light leaks out of the sides. 9. Answer related questions on the Student Data Sheet.
Station 4: Packing the information highway
Materials: • 3 transistor radios • 3 LEDs (red, yellow, blue) with mono jacks • photodiode with amplifier/speaker • various color filters • 30 cm long 7 mm diameter lucite rod
Instructions: 1. Tune each of the radios to a different station. Plug the three LED circuits into the three radios. 2. Send the light from all three LED through the lucite rod and into the photodiode. Listen to the speaker. 3. Next, you will devise a way to separate the signals. Shine only the red LED on the photodiode. Place the color filters one at a time in front of the photodiode to determine which filters stop the red LED signal from being transmitted to the photodiode. Record your results on the Student Data Sheet. 4. Repeat step #3 with the yellow and blue LEDs. Record your results on the Student Data Sheet. 5. Using the information from the previous steps, design a system that will select each individual station when all three are being transmitted. 6. Again, send the light from all three LED through the lucite rod and into the photodiode. Test your system to select each station and record your results. 7. Answer related questions on the Student Data Sheet.
Station 5: Switching lanes
Materials:
• small plastic metric ruler • glass refraction block
• protractor ? • laser • 5” x 8” index card • 2 mirror supports
Instructions: 1. Position the glass block in the center of a piece of paper. Outline the block on the paper. Then remove the block. 2. Position a card barrier with mirror supports and the laser as shown in the diagram. Note where the laser beam falls on the card barrier. 3. Without moving the laser, replace the block in the outline. Note where the laser beam falls on the card barrier. Record your observations. 4. Remove the laser pointer, and using the ruler draw a straight line in the direction of the beam where the laser used to be. Extend the line to the edge of the block. 5. Remove the card barrier and look through the glass block at the line you drew. 6. Take a ruler, sight along one of the edges, and align the ruler with the line you see through the glass block. 7. Draw a line along the edge of the ruler that you sighted. Remove the glass block and connect this line to the edge of the block outline. 8. Using the ruler, draw a line connecting the point where the light entered the block to where the light exited the block. This is the path of the light through the block. 9. At the points where the light entered and exited the block, draw a dotted line perpendicular to the block outline. 10. At the entrance and exit points, measure the angle between the path of the light and the perpendicular line, both inside and outside the block. Record your data. 11. Answer related questions on the Student Data Sheet.
Station 6: Light on a chip
Materials: • 5 plane mirrors • 2 beam splitters (microscope slides) • laser • laser mount • 2 card barriers with circles • 9 mirror stands • grid paper
Instructions: 1. Place the mirrors, beam splitters, and card barriers in the mirror stands. Put the laser on the laser mount. 2. Arrange the laser, card barriers and grid paper as shown below. 3. Keeping the light inside the confines of the grid paper, arrange the five mirrors and two beam splitters so that the laser beam falls on both barriers. 4. Record your arrangement and answer related questions on the Student Data Sheet.
Start here
End here
Student Data Sheet
Communicating with Light: From Telephony to Cell Phones
Name: ______
te·leph·o·ny n. The transmission of sound between distant stations, especially by radio or telephone.
Station 1: Talking with a beam of light
Part A: Modulation
1. Describe what you did to the light to make sound from the speaker.
2. The comb and the wire mesh can create a certain pitch or tone. Describe how you can increase the pitch.
3. In this activity, you "modulated" the light to produce sound. Give your own definition of the word "modulation."
Part B: Alexander’s photophone
4. Describe how your voice affects the light beam and gets encoded on it.
5. Describe all the energy conversions that take place to transmit your voice to the speaker.
Station 2: Good transmission
1. What is the effect of increasing the distance between the LED emitter and photodiode receiver? Give an explanation as to why this happens.
2. The magnifying glass, the lucite rod, and the optical fiber all improved the transmission between the LED emitter and photodiode receiver. How did these objects improve the transmission?
3. Of the magnifying glass, the lucite rod, and the optical fiber, which was most effective at improving transmission? Give an explanation why.
4. Why is optical fiber used to guide light signals around the world, not something larger in diameter like the lucite rod?
5. Cell phone reception can be a problem. Why do you think the traditional "landline" telephones have clearer, non-interrupted signals?
Station 3: Trapping Light
1. What happened to the comparative brightness of the transmitted and reflected beams as you moved the laser beam more parallel to the flat side of the dish?
2. Just before the transmitted beam disappeared, what was its angle relative to the flat side of the dish?
3. Record the angle at which the transmitted dot disappeared: ______°. This is called the "critical angle."
4. For what range of angles was all the light reflected? ______° to ______°. This is called total internal reflection.
5. Would you expect the critical angle to change if a fluid other than water were placed inside the dish?
6. Explain how an optical fiber guides light without letting any leak out.
7. What could happen to the light if the fiber gets bent too sharply (assuming it does not break)?
Station 4: Packing the information highway
1. What is the advantage of sending several wavelengths of light through one fiber?
2. "Bandwidth" refers to the range of wavelengths that can be transmitted through a fiber. Explain why greater bandwidth is better.
3. Complete the following table for testing filters: Color of LED Filter colors that allow Filter colors that prevent transmitter transmission transmission RED
BLUE
YELLOW
4. Complete the following guide for selecting only one color of light if the red, yellow and blue LED are all being transmitted. To transmit only... I need the following filters... RED BLUE YELLOW
5. If you had two additional photodiodes with amplifier/speakers, diagram how you would set them up so that each speaker plays a different station at the same time.
6. The process of separating signals carried by different wavelengths of light is called "demultiplexing." Why is demultiplexing important?
Station 5: Switching lanes
1. What happened to the path of the laser beam when you inserted the glass block?
2. Describe how this could be used as an optical switch to route a light signal from one fiber into a different one.
3. In the space below, diagram the block and the path of the laser light through the block. Write in the angles that you measured.
4. Did the light bend towards the normal or away from the normal when entering a material with higher index of refraction? When entering a material with lower index of refraction?
5. A similar switch design called a "bubble switch" redirects light by the presence of a bubble in a fluid. Below, sketch the path a laser beam would take if it hit the side of the bubble at an angle. Sketch a dashed line to show the path of the beam if the bubble were not present.
bubble
laser
Station 6: Light on a chip
1. On the diagram below, draw an accurate representation of your model of a photonic chip. Show the locations of all mirrors and beam splitters.
Start here
End here 2. What are some possible advantages of chip that uses light to process information rather than electrons?
Post-Lab Analysis: Optical networks
Optical communications networks are used to transmit information all around the world at literally the speed of light. Without them, the internet would not be possible.
Optical communications networks have the following components: • Modulator—encodes an electrical signal (e.g. from a telephone) on light by varying its intensity • Fiber—transmits the signal and guides it • Wavelength multiplexer—combines many signals, each encoded on its own wavelength of light, to travel simultaneously through one fiber • Fiber amplifier—increases the amplitude of the signal after it diminishes from traveling several kilometers through the fiber • Optical switch—routes a signal to a different fiber to reach its final destination • Wavelength demultiplexer—separates light from a fiber into its different wavelength components in order to separate the many signals that travel simultaneously down one fiber • Detector—senses the variations in light intensity and converts this to an electrical signal (e.g. to hook up to a speaker)
detectors λ 1 optical wavelength λ1 wavelength fiber switch demultiplexer multiplexer amplifier λ2 λ2 fiber
λn-1 λn-1
λn λn
modulator pulses detector
laser electrical signal electrical (e.g., voice) signal (e.g., voice)
In the table below, describe all the activities you performed in this lab to demonstrate the functioning of the basic components of an optical network:
Function Activity you did to demonstrate this function
Modulation
Guiding light
(with a fiber)
Wavelength
Amplification
Switching
Wavelength
demultiplexing
Detection