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As I'm a Ham and one of my other hobbies is Lasers I built a two-way "lightwave band communications station" some time about 18mo ago and put it on display during a local club meeting. I used a simple AM modulation circuit that varied the current going to a 100mW 658nm laser diode. I set the quescent current at around 80% maximum safe operating level and allowed for about +- 15mA variation based on audio input. It worked out very well and I was able to achieve reliable communication across about 700ft. I'm quite sure I could have made a much longer distance QSO but the main problem encountered was alignment. Even at only a couple hundred feet the precision level needed was incredible. I used a silicon photodiode as the receiver and trying to hit a 1/8"^2 target at 700ft distance is truly a challenge. It took two telescope tripods and a LOT of time to set up. If someone walked nearby it threw off the alignment a bit.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059
Interesting. Did you run into the "speckle" problem associated with lasers? I read somewhere if you randomise the beam by using a spinning frosted glass sheet this goes away giving you a well collimated but incoherent beam immune to speckle.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
That trick scrambles the speckle so you can't see it. We can only see about 16-20 frames per second. I think you would hear any imperfections in the frosted glass as a tone at the frequency you spun it. I think you would also hear noise from the variable transmission of the frosted glass. I'm not sure that helps.
I did not encounter any issues with speckle or aberrations. In fact, the signal was so strong that I had to purposely shift the receiver's photodiode slightly off axis of the beam as there was distortion when head-on. At shorter distances placing a single sheet of white paper over the photodiode to diffuse and filter out most of the light worked equally as well. Beam shape, transverse modes, and artifacts/aberrations were clearly not an issue. Rain drops reflecting the beam elsewhere were, though.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059
Dave_B has a spare phlatlight if anyone wants it, I also have one working and one dead-but-useable-as-sensor unit.
Another option on the Rx end is to use an infrared LED as the sensor, as large area diodes are now available. As all LEDs will tend to be more efficient as solar cells around 60nm more than their emission wavelength this could work very well indeed.
See My LEDs are around 780nm so take 60 from this and the efficiency should peak around 700nm which is only slightly into the infrared so a red phlatlight should still do the trick.
EDIT: 45mV under typical room lighting (7am) into 1Mohm.
Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780
Thanks for this schematic... I have a bunch of 6080 I have gathered for an audio amp, and I'll simulate this one!
Proud Mary wrote ...
This one can be resolved with a projector lens and a parabolic mirror, and maybe a white target so that you don't point the detector bur rather the beam image, increasing the sensitivity
Sigurthr wrote ...
As I'm a Ham and one of my other hobbies is Lasers I built a two-way "lightwave band communications station" some time about 18mo ago and put it on display during a local club meeting. I used a simple AM modulation circuit that varied the current going to a 100mW 658nm laser diode. I set the quescent current at around 80% maximum safe operating level and allowed for about +- 15mA variation based on audio input. It worked out very well and I was able to achieve reliable communication across about 700ft. I'm quite sure I could have made a much longer distance QSO but the main problem encountered was alignment. Even at only a couple hundred feet the precision level needed was incredible. I used a silicon photodiode as the receiver and trying to hit a 1/8"^2 target at 700ft distance is truly a challenge. It took two telescope tripods and a LOT of time to set up. If someone walked nearby it threw off the alignment a bit.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059
Also it is worth noting that many of the new backlight strips from cracked laptop screens can be with care made into a primitive-but-effective optical comms transmitter by winding the flexible Mylar around a toilet roll holder so all the diodes line up.
EDIT: would you believe that this is totally novel, no other reference to this exists anywhere. Sure there are plenty of single diode Tx units but nothing remotely like this, the bandwidth is limited due to phosphor lifetime but thats no biggie for audio use.
Anyone in line-of sight of Guernsey want to try this experiment?
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Optical amateur "radio"
