Optical Comms


September 2010 - First thoughts about Optical Communications..

After some inspiration from some videos of French stations acrive on lightwaves who have done some very interesting tests with Digital television over modulated laser beams I decided to have a go myself!
F9ZG and others have also done FM and AM modulated light testing which also proved to be possible.

Most of the information on the web is about modulating laser beams but there is also now an alternative to lasers with the new High Power LED's that are available.
Laser beams are very dangerous to your eyes even if the light is reflected from another surface when the laser is more than a couple of mW, the long range tests done by amateurs have all used lasers as far as I can tell apart from the group in Australia who have managed >100miles using a high power LED (1Watt) as the light source. After seeing this was possible i have opted for the safer LED option, but i do have some 5mW laser modules for future experiments!

My initial thoughts were to go straight to sending a video signal over light but I thought I had better get an understanding of how the transmitter and receiver worked first..

First tests were sending a 1kHz carrier across the room with a normal low power LED.

One led is biased on with a little less than full brightness, a 1uF capacitor to the joint of resistor and led then the 1KHz audio signal (generated by my pc) is fed into the capacitor - Simples!

A photodiode - any sort will do for first tests, I used a BPW21 from Rapid Electronics, BPW34's are most common but maplin do the SFH2030 which is more modern and lower capacitance.
A low ish noise opamp capable of a couple of MHz, I used a CA3140
The photodiode anode is connected to ground and cathode is connected to - input of opamp, + input of opamp is to ground, 10K feedback from output to input(-), 1uF capacitor on output to an audio amplifier - some amplified pc speakers in my case.

That's it, it worked reasonably well across the room with no lenses at either end to focus the light.
To check it's working if you turn up the speakers you will hear a hum from the mains 50Hz + Harmonics when you point the receiver close to a mains light source!

Video over the new Optical path..

A free running oscillator was built for around 15MHz and a BB212 varicap diode was used to enable quite some frequency modulation for the composite video signal.
At the receiver end I used a low noise BF981 as a preamp for the photodiode and the photodiode was reverse biased to reduce capacitance to allow higher frequency through.
After the pre-amp I used a mar-3 mmic stage and then into a mixer to get enough signal for the 70MHz IF of my satellite receiver, LO was 55MHz
Video tests showed that the 15MHz oscillator needed some attention as it was very poorly modulated! Otherwise a success with fully quieting (poorly) video across the room !

Next was to get some more DX and I decided to go with some simple AM modulation for this and use a more sensitive receiver than the 1st opamp version.. KA7OEI's receiver seems to be about the best going so I built that up and made a huge improvement in sensitivity.
I got a 1W LED from farnell to experiment with which is ~25lumens and some fresnel lenses to build a boxed transceiver with.
The LED is biased up with a BFY51 and modulation applied to the base, seems to be ok for basic tests.
The rx is positioned beheind a 26cm square fresnel lens in a horn shaped wooden box to hold it all together, a sight from my telescope is fitted to the top of the box for alignment.

lens box 1

lens box 2

First DX tests 2km
My first test at any distance was done with the 1W led into a 4" magnifier lens pointing out through a window in the house to a farm track about 2km away. I took the Fresnel lens and KA7OEI receiver to pick up the signal. I drove out to the farm track and did a visual to see if i could make out the red light but nothing! soon after a phone call to the family for assistance i saw the red light very brightly, much brighter than the yard floodlight. I switched on the receiver and aimed the fresnel (which is extremely directional!) and found the 800Hz cw beacon pretty much straight away...wow it was strong but with massive 'hum' from the floodlight in the yard.

Best dx so far.. 11km
The next tests were more ambitious being 11km away on the NY Moors near to Swainby which is visible ffrom the front of the house easily. These tests did not go very well as the first five attempts were failures, mostly due to it being very hard to find your home from a distance when there is so much streetlighting but more related to the aiming of the tx, i was now using the fresnel lens on TX. On the sixth attempt and some very rough aiming on the tx side i could faintly see the red light as it was a very clear night and once again phoned the family for assistance on moving the tx, this resulted in a fairly bright twinkling red spot in the distance, signals were once again very strong. This was definately not optimised as it had flashed up much brighter while the tx was being moved but i was quite happy with the signal strength!


Optical Transverter

After talking to a few of the local optical experts while at the Gateshead Rally in October 2010 I built Stuart, G8CYW's HF to Optical transverter and made some boards for the other locals to also have a go with.


New best DX!
Later in the month a one-way qso was made with Stuart G8CYW at about 60km, he could hear my ssb signal 5/7, I could only just detect Stuarts weak carrier on the receiver despite seeing his light with the naked eye. At this point I think I started to think my receiver was not working properly and have been looking into it ever since!



See Eddie's optical page for some more information



Lots more to come soon!





Updated May 2011