DAVE@ORION.BITNET (09/12/90)
I saw my high school physis teacher put an LED into the earphone jack of a simp le radio shack radio, and about 4 feet away, put a phototransistor (?) into the microphone jack of a small amplifier, the sound was carried by the pulsing LED . I would like to re-create this experiment using a laser from the rooftop of one campus building to another. How would I go about this, is it possible, and what materials would I need? Thanx!
jones@pyrite.cs.uiowa.edu (Douglas W. Jones,201H MLH,3193350740,3193382879) (09/13/90)
From article <153DAVE@ORION>, by DAVE@ORION.BITNET: > I saw my high school physis teacher put an LED into the earphone jack > of a simp > le radio shack radio, and about 4 feet away, put a > phototransistor (?) into the microphone jack of a small amplifier, the > sound was carried by the pulsing LED Many commercially available lasers have a modulation input. All you do is hook the audio (or RF, for that matter) signal you want to transmit to the modulation input, then turn on the laser and aim it at an appropriate photodiode on the opposite rooftop, and you've got a roof-to-root communications system. Problem 1) Impedance matching at the laser modulation input. If it's a BNC connector, it probably wants a 50 ohm impedance line, expecting a fairly low voltage signal. Most audio lines, such as the ones from your tape deck to your amplifier, are about 600 ohm impedance, with correspondingly higher voltages. At audio frequencies, a direct connection between the two may be OK, if you turn the volume way down. At RF frequencies, you may need some kind of matching transformer to get things to work right. Problem 2) Beam spread. Over a long distance, it can pay off to get a telescope for the transmitter. Shine the laser in the eyepiece, and adjust the focus so the beam focuses to a reasonably small spot at the receiver. (You'll have to do this at night). Among other things, this makes the signal far more resistant to raindrops, since one drop will only block a fraction of the beam when you spread it to the size of a telescope aperature. Problem 3) Noise. Ideally, the only light falling on the receiving photodiode should be from the transmitting laser. For short haul signals, a simple chunk of pipe may provide enough of a shield, but for long haul transmission, use another telescope. With the laser off, focus the receiving telescope on the laser's transmitting telescope, then put the photodiode where your eye was and turn on the laser. If your telescopes are reflecting telescopes, you should not that the result is the optical analog of the parabolic reflectors traditionally used in microwave transmitters and receivers. Doug Jones jones@herky.cs.uiowa.edu
pahsnsr@nmt.edu (Paul A. Houle) (09/13/90)
Making an optical link with a laser is very possible, and actually can be very easy if you've got the right kind of laser. It only takes two technological tricks to pull it off - one is that you have to be able to modulate the laser beam, and the other one is to detect the modulations. Number 2 is exactly the same as if you did it with LED's. The same circuits work, and HeNe lasers don't produce enough power to ruin photocells or CdS cells. Some lasers have a modulation jack that you can plug your audio/data (or even video with some models!) source into that will automatically modulate the beam. Most of these lasers use a stable cavity but have a modulator in the beam path -- something that gets dark when you apply electricity through it. There are many physical properties that you can use to get this effect - best known is the "Kerr Cell" which works like a LCD. Controlling power input to the laser is also possible, but tends to add more noise and, for most models, you'll have to considerably modify the power supply. A final trick is to use a bit of mirror that you can vibrate to move the beam on and off the detector. -- ---------------------------------- NEWS FLASH: NICE GUYS DECLARE WAR!!!!!!!!!!!!
wiml@milton.u.washington.edu (William Lewis) (09/14/90)
In article <1990Sep13.020325.10622@nmt.edu> pahsnsr@nmt.edu (Paul A. Houle) writes: > ... Controlling power input to the >laser is also possible, but tends to add more noise and, for most models, >you'll have to considerably modify the power supply. A final trick is to >use a bit of mirror that you can vibrate to move the beam on and off the >detector. I have a HeNe laser that I'd like to use for communication also. The tube itself has no provision for modulation, but the power supply is seperate (built from a kit) so modifying it would be no problem. My question is: how would I go about adding a modulator to the supply? Do I just have to modulate the current? Does anyone have an example schematic? I'm not well versed in high voltage electronics (high voltage to me means "more than TTL levels" =8) ) -- wiml@milton.acs.washington.edu Seattle, Washington | No sig under (William Lewis) | 47 41' 15" N 122 42' 58" W |||||||| construction
berryh@udel.edu (John Berryhill) (09/17/90)
In article <7504@milton.u.washington.edu> wiml@milton.u.washington.edu (William Lewis) writes: > > I have a HeNe laser that I'd like to use for communication also. >My question is: how would I go about adding a modulator to the supply? First off, these questions are better answered in the library than on the net. However, if you want to use your HeNe to transmit a signal, you are going to be much better off modulating the light than its source. In other words, let the HeNe stay on all the time and use something in beam path that can either deflect the beam (an acousto-optic modulator for instance) or vary its opacity (an LCD window with a polarizer crossed wrt the LCD). -- John Berryhill 143 King William, Newark DE 19711
jgk@osc.COM (Joe Keane) (09/18/90)
We had a HeNe with a modulator stuck on it, plus a TTL interface. I don't think these things are too expensive. The bandwidth you can get is pretty impressive. BTW, i don't think modulating the power supply will work so well. The response time is likely to be pretty lousy; i assume the plasma has to heat up and cool down. My stupid laser takes half a minute just to turn on.
dlbres10@pc.usl.edu (Fraering Philip) (09/21/90)
In article <30638@nigel.ee.udel.edu> berryh@udel.edu (John Berryhill) writes:
jb>Followup-To: sci.garbage
(nice try)
jb>In other words, let the HeNe stay on all the time and use something
jb>in beam path that can either deflect the beam (an acousto-optic
jb>modulator for instance) or vary its opacity (an LCD window with a
jb>polarizer crossed wrt the LCD).
Isn't the laser beam polarized already?