Modulated Laser


Concepts Shown:

semiconduction, optical wave-guiding materials


EverReady "Floating Lantern" 6-volt lantern, 3/8" x 2" bolt and nut, 22 guage enameled magnet wire (20 feet), 2 audio cables (Radio Shack #42-2434), Silicon Solar Cell (3V at 100mA) with wired outputs, Amplified speaker with mic input, Mono tape player with headphone output (or stereo tape player with stereo to mono headphone adapter), piece of sandpaper, 2 wire nuts, electrical tape, fiber optic cable (for show & tell only). All materials are available at Radio Shack, except the solar cell which is available from Edmund Scientific, (609) 573-6270, and the bolt which is available from any hardware store. Cost: $35-$60 (depends on the availability of the tape player and amplified speaker)


Setup: Audio Cables: If the cables cannot be found at Radio Shack, they can be built.

1) Cut a headphone cord about 6' long, leaving the jack on one end.
2) Strip the other ends of the cord. If it is a stereo cable, there will be more than two wires, you must use only two. the ground and either the right or left channel. If the cable is mono, only two wires will be present and these are the correct wires.

Transmitter: (schematic shown below)

  1. Put the nut on the end of the bolt and coil the magnetic wire around the bolt about 100 times leaving at least 8" of wire on both ends.
  2. Strip the enamel of the ends of the wire using sandpaper.
  3. Open the flashlights bulb/reflector housing and battery by unscrewing the top.
  4. Cut the wire leading from the switch to the bulb and strip both cut ends.
  5. Poke a small hole in the back of the flashlight, and pass 8" of the stripped ends of one of the audio cables through it. If the wire goes loosely through this hole, tie a knot so the cable will not pull out.
  6. Poke two small holes through the side of the flashlight about 2" apart and pass both ends of the wire-wrapped bolt through them.
  7. Tape the bolt in place.
  8. Using the wire nut and electrical tape (solder if possible) connect one wire from the bolt, one wire fro the auio sender cable, and the wire that goes to the switch.
  9. Connect the remaining three wires.
  10. Reconnect the battery and reclose the whole assembly.
  11. Plug the jack intot he headphone output of a tape player.
  1. Splice each ends of the other auio cable to each output wire from the solar cell.
  2. Plug the jack into the amplified speaker.
Note: You will hardly be able to see the light flicker, but the solar cell will pick up frequencies higher than what the human eye is able to see.
  1. Turn on the flashlight.
  2. Plug the cable from flashlight into the headphone output of the tape player. Remember, if the tape player is stereo, a stereo to mono headphone adapter must be used or it will not work.
  3. Turn on the amplified speaker.
  4. Plug the cable from the solar cell into the input of the amplified speaker.
  5. Dim the lights
  6. Press play on the tape player.
  7. Shine the light into the solar cell at a distance of about 8".
  8. Show fiber optic cabls and discuss science.


Note: Volumes may need to be adjusted all the way up. If a lot of background distortion is heard, either there is too much light striking the cell from other sources or, the flashlight is being jostled around too much.


This experiment shows the basic principles behind fiber optics. Light is simply a form of electromagnetic wave, the same as a radio wave, except with a different wavelength. As with radio waves, signals can be sent on these waves by simply varying the frequency. the problem in the past with using light as the medium for transmitting a signal was that it was extremely difficult to guide it along a path. However, today through the use of transparent materials, engineers have developed cables which transmit images or signals across great distance with extremely little signal loss and requiring low amounts of power. This is due to the critical angle of the fiber optic material and total internal reflection. If the light traveling within a fiber bounces off the wall at an angle less than the critical angle, the signal will remain inside the cable. This is shown by the equation: n1/n2 = sin 90/sin phi(2) where n1 is the index of the material clad around the waveguide, n2 is the index of refraction of the material, phi92) is the critical angle of the wave guiding material. Below this angle the cable has total internal reflection - no signal is allowed to escape if the material is not blemished. Today, fiber optic materials have becoms so refined that if you made a window pane secen miles thick out the glass used in today's telecommunication fibers, you would be able to see right through it. Using these cables, signals can be sent anywhere in the world with relative ease.

In our demonstration, inside the flashlight, the wire wrapped bolt is called an induction coil. Under a DC current (from the battery) the coil acts as a short circuit, as a result, under only DC current, the bulb is on. However, the coil acts as an open circuit for the AC current from the tape player. The AC pulses from the tape player that would normally drive headphone speakers are instead causing the light bult to fluctuate rapidly at the same frequency as the sound recorded on the tape. The reason you can't see the fluctuations is because music ranges between 20 Hz and 20,000 Hz. However even extremely high fidelity cannot achieve this range, more like 80 Hz to 15,000 Hz. To put it into perspective, a TV monitor flashes off and on at 60 Hz and that is hardly visible. The other main principle lies iwthin the semiconducting solar cell. This cell is made up of two layers of silicon, a very thin n-type on top of a thick layer of p-type silicon. when silicon is dped with electrons, to make n-type silicon, the fermi energy level goes up, and vice versa, when doped with holes to make p-type silicon. However, when the two are put together to form a junction, the fermi levels must line up, this causes the conduction band of p-type silicon to be at a much higher level than that of n-type. (see "Rectifier") When a photon strikes the solar cell, it is able to pass through the thin layer of n-type into the p[type. Here, it excites an electron into the conduction band. However, this conduction band represents a state of high energy when compared to that of the n-type, so it wants to be more stable (lower energy) so the electron moves to the n-type silicon. this movement of electrons in one direction is called a DC current. The key to this working in the fiber optic experiments is that this induced current is directly proportional to the intensity of the light striking the solar cell. The light, as explained above varies to the music, thus the current reproduced varies to the music. This current is fed directly into an amplified speaker and the sound is reproduced intact. [eq].



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