philb@orca.UUCP (10/27/86)
I have a small Helium-Neon laser that I would like to attempt to
use for voice communication and desire a little help from those
of you who have done this. Is it possible to modulate the laser
simply by modulating the input power supply? In this case tube is
powered by a 12 volt DC/DC converter. If not then what other
"simple" ways of modulating can be used?
What about detectors? I would like to be able to detect the beam
from a distance of several miles. What kind of detectors work well
in this application? Is it possble with a 4 mWatt laser? Can a lens
help if the intensity is too weak?
Any and all help appreciated, thanks much.
Phil Biehl, N7FWL
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mail: Tektronix Inc., IDG MS 61-028, POB 1000, Wilsonville OR 97070adp@hp-sdd.UUCP (10/28/86)
In article <2200@orca.TEK.COM> philb@orca.TEK.COM (Phil Biehl) writes: > > I have a small Helium-Neon laser that I would like to attempt to > use for voice communication and desire a little help from those > of you who have done this. Is it possible to modulate the laser > simply by modulating the input power supply? No. A HeNe laser tube behaves in much the same way as a flourescent light. That is, the power supply starts it with about 10kv then it runs at about 2 or 3 kv at about 6 milliamps. Trying to modulate the beam by means of the power supply would likely keep turning off the beam. The power supply is usually highly regulated so it would be difficult anyway. As a matter of fact, the beams intensity is highest at the point closest to turning off the beam. That is, if you increase the running voltage, the beam will dim. Not a proper way to modulate it. > In this case tube is > powered by a 12 volt DC/DC converter. If not then what other > "simple" ways of modulating can be used? "simple" this isn't. The idea your looking for is something to put in front of the beam whose opaqueness is controllable. These devices are available, but I dont know the cost involved. > What about detectors? I would like to be able to detect the beam > from a distance of several miles. What kind of detectors work well > in this application? Is it possble with a 4 mWatt laser? Can a lens > help if the intensity is too weak? This can be a toughy. Any general purpose sensor would have to be filtered, which will dim the signal. It helps to have some directional input like a black tube. I think you will find a 4 mWatt HeNe to be too weak for such a purpose, you really want a 10 mWatt to do any real work with:-). You might check into using a laser diode instead of a HeNe. The diodes can be modulated, the beam is IR insead of Red, and they should be pretty inexpensive. I don't think you can use one for a several mile transmission tho (unless you use fiber optics:-) -- **************** Insert 'Standard' Disclaimer here: OOP ACK! ***************** * Tony Parkhurst -- {hplabs|sdcsvax|ncr-sd|hpfcla|ihnp4}!hp-sdd!adp * * OR hp-sdd!adp@nosc.arpa * *******************************************************************************
billr@tekred.UUCP (Bill Randle) (10/30/86)
Actually, you can do some modulation without using a variable opaqueness device in front of the beam. There was an article in Popular Electronics about 16 years ago on how to build a modulator-demodulator for a HeNe laser. I built one up for the laser our high school had, but never did get it to work right. As I recall, it used a "fire-fet" (vacumn tube) to do the modulation, but I don't remember much else. You could probably find the back issue in a library (try the Reader's Guide circa 1969 - 1970) and update it using transistors/ICs. -- -Bill Randle Tektronix, Inc. <many machines>!tektronix!tekgen!tekred!billr
mwg@petrus.UUCP (Mark Garrett) (10/30/86)
++ In article <2200@orca.TEK.COM> philb@orca.TEK.COM (Phil Biehl) writes: > > I have a small Helium-Neon laser that I would like to attempt to > > use for voice communication and desire a little help from those > > of you who have done this. Is it possible to modulate the laser > > simply by modulating the input power supply? > > No. A HeNe laser tube behaves in much the same way as a flourescent > light.... The trouble with this experiment is that optical communications technology is geared towared infrared diode lasers and fiber optics. It's less practical to use red light through air, so there's not much you can buy to do it. However, since you only want a few kHz bandwidth, you can probably do it with even crude aparatus. You need an external modulator. There are electronic devices called Kerr cells that change their opacity with an input voltage but, these are expensive and probably don't work at the right wavelength. Here's an idea that might be fun to try: Modulate the beam mechanically using the vibrations of the sound. First you need something that will change the amount of light let through according to a slight movement, and it must be linear (or at least it helps). Focus the output of the laser through a lens, and beyond the focal point put another lens which collimates the light into a wide beam. Adjust the lenses to give you a slightly narrowing beam so that it focuses on the remote detector (more or less). Now, in the wide beam put your modulator. One way to make this (I have no idea if it will work!) might be to take two pieces of window screening, paint them black to avoid reflections (always be careful about hitting shinny things with laser light). Fix one piece of screen in the beam, and have the other one attached to a loudspeaker, and also in the beam. lens 2 screens lens 1 |______#_#__________________________ | _/ | # # \_______________ ------ __| _/ | # # laser __|><_ | # # beam converges on detector ------ | \_ | # # ______________ | \ |______#_#___________________________/ | # # .........J # : _____#_____ : (_____X_____) : \ speaker / --- --------- If the vibration of the speaker is equal to the spacing between the metal filaments in the screen, and the space between the filaments is about equal to the width of the filament, you will have linear modulation of the light with an on/off ratio of infinity - that is, when the second screen is fully translated in one direction, the holes in the second screen line up with the filaments of the first and vice-versa, and when it is fully translated in the other direction, the spaces line up with the spaces. If the spaces are too big, paint it again. Probably several coats of very thin paint can give you the right effect. You could possibly construct an electromagnet to replace the speaker if the vibration is too small for the amount of noise you make (you want your receiver to get the information through the light, not because he can hear you blasting your voice all over the neighborhood). One nice thing about voice is that you can understand it even if you only get a piece of the frequency spectrum. So even if your freq response is lousy you can still communicate. As far as a detector goes, you can filter the light through a red camera filter (to get rid of interfering light) and make a long tube to get directionality. You'll have to experiment to see how far away you can get with whatever detector you find. I don't know what's good for red light. Have fun, -Mark mwg@bellcore.arpa allegra!bellcore!mwg
magore@watdcsu.UUCP (M.A.Gore - ICR) (10/30/86)
I would like to mention the Popular Electronics somewhere between 1968 to 1971 did have an article that used a power supply modulated beam. The percentage the beam needs to be modulated need not be great. Today a cheap way would be FSK with a fixed modulation amplitude. It is the brightness of the laser that counts. Anyway you should be able to get a few miles if you use FSK, a filter and some cheap optics.... You present power supply may be a problem if it is filtered. Using a DC/DC converter could mean that you could modulate the chopping frequency and remove the output filter. But if your DC/DC converter uses a voltage multiplier in the final stage you are partly out of luck using what you have (voltage multipliers tend to be filters by design, that is the ones that use diodes and capacitors of course).....Anyway what you *want* is the final chopping frequency to be part of the power supply 'ripple'. Being partly filtered would just mean that your FSK to analog transfer would be non-linear..... Just use a good multi-polled notch filter on your receiver.....Lots of fun.... Good luck.... ps. Many types of lights make ***VERY*** good 60/120Hz transmitters!!! (-: (so watch your receiver design!... another good reason to use FSK/FM) # Mike Gore # Institute for Computer Research. ( watmath!mgvax!root - at home ) # These ideas/concepts do not imply views held by the University of Waterloo.
root@doghouse.gwd.tek.com (Der Kennelmeister) (10/30/86)
In article <2201@orca.TEK.COM> philb@orca.TEK.COM (Phil Biehl) writes: > > I have a small Helium-Neon laser that I would like to attempt to > use for voice communication [...] > I would like to be able to detect the beam > from a distance of several miles. What about right-of-way ? How do you keep people from looking into the beam? (unlikely, assuming you have the beam well above ground, but possible) How do the commercial systems solve this? Snoopy tektronix!doghouse.gwd!snoopy snoopy@doghouse.gwd.tek.com
jtk@mordor.ARPA (Jordan Kare) (10/31/86)
In article <820@tekred.UUCP> billr@tekred.UUCP (Bill Randle) writes: >Actually, you can do some modulation without using a variable opaqueness >device in front of the beam. There was an article in Popular >Electronics about 16 years ago on how to build a modulator-demodulator I built the laser and several modulators from these articles; I believe the article on laser communication was in June 1970 PE. The modulator used a pentode (I used a 6L6) in series with the tube as a variable current source. Most laser tubes can be modulated at least slightly this way, but the percentage of modulation will depend on the laser tube; Metrologic's standard tubes would modulate ~50% before turning off (the arc inside the tube goes out), but Metrologic built special "communications" lasers that would go all the way to zero output before the arc dropped. I doubt that a transistor modulator would work well; the laser supply is ~1500 volts, and the starting spike is ~5 KV. Perhaps if you used a high voltage transistor (750 V) with an overvoltage bypass (e.g. a string of neon tubes or high voltage Zeners)... Probably the most common "external" modulators are Kerr, Pockels, and Faraday cells, using an optically active material (mononitrobenzene, KDP) in, repectively, a transverse electric, longitudinal electric, and longitudinal magnetic field. For details, see physics/quantum optics texts, particularly 60's vintage. None are trivial to make. For quick and dirty modulation (with poor linearity) consider a shutter blade attached to a loudspeaker voice coil... For long range communications, optics are necessary at both ends. At the transmitter, a small telescope of good optical quality will reduce the beam divergence from ~1 mrad (several meters at 4 mi. range) to a tenth or less of that, PROVIDED you have a good way to focus and align it (an observer at the far end with a CB?? but then why do you need the laser to communicate? :-)) Receiving optics can be lower quality, but should be fairly large (foot-square fresnel lenses are about $10). For daylight operation, a narrow band interference filter (centered at 633 nanometers) will block everything but the laser light; Edmund used to sell these fairly cheaply. (Note that such filters will not work with converging light; you need additional optics to make a parallel beam again). Ordinary photodiodes (even "solar cells") will work for audio communications, but are not especially sensitive. Use them as current sources driving a current-to-voltage converting op-amp. PIN diodes and photomultipliers are better, but fairly expensive. Back when I did this stuff, I used to get on the ham bands and tell people that I'd been working "real VHF -- the 633 nanometer band" :-) Jordin Kare jtk@mordor.UUCP
dje@datacube.UUCP (11/10/86)
I did most of this in 1972 for the Mass state High school science
fair. Beyond audio modulation, I used two carriers and amplitude
modulation to send two audio signals across a He-Ne laser.
For a modulator, use current modulation. The current should be
maintained between the working current of the laser (say 10mA) and
0.1 Times that. If your current drops to 0, the laser extinguishes.
Since most convenient input signals are voltages, and a current is
needed, a transconductance amp (Voltage to current) is best. The
voltage is around 600V, and the power is around 10mA * 600V or 6W. I
used 600V worth of NE-2 neon lamps to prevent the plate voltage from
exceeding 600V (especially during laser firing). At the time,
Hi-Voltage transistors were hard to get, so I used a pentode vacuum
tube. Other than the need for a 200V screen grid supply, it was
perfect. Grid bias was provided with a resistor in the cathode. The
circuit was from a late sixties or early seventies Popular
Electronics.
600V of Neon lamps
+1600VDC Supply <-----| [_______________()-()-()-()------> Gnd
He-Ne Laser |
----- Plate
0.1uF ----------------> +200V Screen grid
Modulation In <----| |--------------- Control Grid
| ___ Cathode
\ | |
1MOhm / /
\ \ 5K Ohm Pot
| /
--------------> GND
Nowadays, 600V 10W power transistors are widely available, but
they're much less forgiving to overvoltage, etc than tubes.
For a reciever, I used a photo-transistor. Biasing to correct for
ambient light is important. You only want the AC component.
I Didn't go for distance, so I didn't use any optics. Just a dark
shield around the detector.
Dave Erickson
------------------------
Datacube Inc. 4 Dearborn Rd. Peabody, Ma 01960 617-535-6644
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[ihnp4 | mirror]!datacube!djeboreas@bucsb.bu.edu.UUCP (The Mad Tickle Monster) (11/15/86)
[In taste testing, line eaters preferred Pepsi to Coke by more than 2 to 1!] Strictly off the top of my head, why not use a variable beamsplitter hooked to some sort of electromechanical gadget? Edmund Scientific sells them, among other places. They may be a bit expensive, though, for tinkering around with. The circular ones are more expensive, from what I remember, and the flat ones might even be easier to use for this -- rig something to pull them in and out of the path a little. Say, a wire hooked to a lever, hooked in turn to a speaker cone (this would generate enough motion to be significant, but would probably be too crude to use). You could probably even send both beams out, and use the relative strengths to generate the receiver signal; this could compensate for dust and fog and such. . . . Have fun, and tell me how it goes! (Next thing you know, all the radio stations will move up there, just like they did in the early days). Michael Justice KA9QEU -- +--------------------------------------------------------------+ | Michael A. Justice | | BITNet: cscj0ac@bostonu CSNET: boreas%bucsb@bu-cs | | UUCP: ....!harvard!bu-cs!bucsb!boreas (boreas@bucsb.UUCP) | | ARPA: boreas@bucsb.bu.edu | | AT&T: (home:) 787-4189 (work1:) 353-2784 (work2:) 353-9063 | | "Space: The Final Front" -- Ronald Reagan | | (well, he COULD have said it. . . .) | | Disclaimers: "I didn't write it! It was my AI project!" | | "Perhaps it was a result of anxiety." -- _Mad_Max_ | +--------------------------------------------------------------+