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.
steve@matt.ksu.ksu.edu (Steve Schallehn) (09/19/90)
In the October issue of QST (a ham radio magazine), there is an article about using lasers for communications. The article includes overviews of different laser communication methods. -Steve Schallehn, KB0AGD Kansas State University
henry@zoo.toronto.edu (Henry Spencer) (09/20/90)
In article <1990Sep19.024913.3455@maverick.ksu.ksu.edu> steve@matt.ksu.ksu.edu (Steve Schallehn) writes: >In the October issue of QST (a ham radio magazine), there is an article >about using lasers for communications. ... Okay, so next question: how does one get a copy of QST without joining the ARRL? It's not available on newsstands, that I know of. -- TCP/IP: handling tomorrow's loads today| Henry Spencer at U of Toronto Zoology OSI: handling yesterday's loads someday| henry@zoo.toronto.edu utzoo!henry
spgreg@earth.lerc.nasa.gov (Greg Macrae) (09/20/90)
In article <3809@osc.COM>, jgk@osc.COM (Joe Keane) writes... >and cool down. My stupid laser takes half a minute just to turn on. The delay before start of your laser is a government mandated safety feature and likely has little to do with the potential performance of the laser itself. ---------------------------------------------------------------------- MacRae | Looking twice through the glass, I see | The bull watching spgreg@csd.lerc.nasa.gov | The torreador. | -MacRae ----------------------------------------------------------------------
wb8foz@mthvax.cs.miami.edu (David Lesher) (09/20/90)
>>In the October issue of QST (a ham radio magazine), there is an article >>about using lasers for communications. ... >Okay, so next question: how does one get a copy of QST without joining >the ARRL? It's not available on newsstands, that I know of. I often see it at newsstands, but in any case, it is generally available at the local public library, at least in the U.S. It's less certain that a University one would get it, however. -- A host is a host from coast to coast.....wb8foz@mthvax.cs.miami.edu & no one will talk to a host that's close............(305) 255-RTFM Unless the host (that isn't close)......................pob 570-335 is busy, hung or dead....................................33257-0335
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?brodie@fps.mcw.edu (09/21/90)
In article <DLBRES10.90Sep20174906@pc.usl.edu>, dlbres10@pc.usl.edu (Fraering Philip) writes: > 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? Only some lasers are polarized. How do they polarize a HeNe laser? simple. Put a small pirce of polarized glass (for lack of a better term..) in the beam path. (very thin, very high quality). Take a look at any Edmund catalog... only about half of the lasers they sell are polarized. Most modulated lasers use something that sticks right inbetween the power supply and the tube, thus modulating the intensity of the beam. (I think..) ------ Kent C. Brodie - Sr. Systems Manager internet: brodie@fps.mcw.edu Faculty Physicians & Surgeons uucpnet: uwm.edu!fps!brodie Medical College of Wisconsin voicenet: +1 414 778 4500 faxnet: +1 414 778 6694 "Researchers have shown that people who drink four or more cups of coffee a day are 40% more prone to heart attacks.... I guess we all now know what happened to *MISTER* Olson...." -Jay Leno
john@frog.UUCP (John Woods) (09/22/90)
In article <1990Sep19.194119.229@eagle.lerc.nasa.gov>, spgreg@earth.lerc.nasa.gov (Greg Macrae) writes: > In article <3809@osc.COM>, jgk@osc.COM (Joe Keane) writes... < >and cool down. My stupid laser takes half a minute just to turn on. > < The delay before start of your laser is a government mandated safety feature Thus giving you enough time to line up your eye with the beam output? ;-) -- John Woods, Charles River Data Systems, Framingham MA, (508) 626-1101 ...!decvax!frog!john, john@frog.UUCP, ...!mit-eddie!jfw, jfw@eddie.mit.edu
rrw@naucse.cse.nau.edu (Robert Wier) (09/22/90)
In article <1990Sep20.001947.29666@mthvax.cs.miami.edu>, wb8foz@mthvax.cs.miami.edu (David Lesher) writes: > >>In the October issue of QST (a ham radio magazine), there is an article > >>about using lasers for communications. ... > > >Okay, so next question: how does one get a copy of QST without joining > >the ARRL? It's not available on newsstands, that I know of. > > I often see it at newsstands, but in any case, it is generally > available at the local public library, at least in the U.S. It's less > certain that a University one would get it, however. > -- > A host is a host from coast to coast.....wb8foz@mthvax.cs.miami.edu > & no one will talk to a host that's close............(305) 255-RTFM > Unless the host (that isn't close)......................pob 570-335 > is busy, hung or dead....................................33257-0335 On the contrary, I've found MOST university libraries get QST, if for no other reason that it's been published so long, or a local ham group gave a subscription. Course, the wonderful state of Arizona cut the periodicals budget back so far that our library no longer gets it ... - Bob Wier -------------- insert favorite standard disclaimers here ---------- College of Engineering Northern Arizona University / Flagstaff, Arizona Internet: rrw@naucse.cse.nau.edu | BITNET: WIER@NAUVAX | WB5KXH or uucp: ...arizona!naucse!rrw
ken@argus.UUCP (Kenneth Ng) (09/30/90)
In article <19149@frog.UUCP>, john@frog.UUCP (John Woods) writes: : In article <1990Sep19.194119.229@eagle.lerc.nasa.gov>, spgreg@earth.lerc.nasa.gov (Greg Macrae) writes: : > In article <3809@osc.COM>, jgk@osc.COM (Joe Keane) writes... : < >and cool down. My stupid laser takes half a minute just to turn on. : < The delay before start of your laser is a government mandated safety feature : Thus giving you enough time to line up your eye with the beam output? ;-) Yup, I can see it now, turn the laser on, hm, no light, check the power cords, yup they are in, hm, maybe if I look down the barrel I can see something blocking the light...... :-) -- Kenneth Ng: Post office: NJIT - CCCC, Newark New Jersey 07102 uucp !andromeda!galaxy!argus!ken *** NOT ken@bellcore.uucp *** bitnet(prefered) ken@orion.bitnet or ken@orion.njit.edu