Optical Communication

[Carbon Brush]

I have just heard about this from someone today. It seems that a small number of people in amateur radio are trying out communications links using high power (few watts) red light emitting diodes to transmit a signal, and photodiodes to receive a signal.

I won't go into the electronics complexity of the link. From an optics or astronomy viewpoint there are though some interesting things here.

The optics are A4 size fresnel lenses. Page magnifiers. 99p type! The only 'improvement' is what sounds like a wideband Ha filter on the receiver. Nobody is using even cheap scopes to help.

The path lengths in the UK are tens of kilometres, with over 100KM being known, I believe the record is held in the USA for a link between two accessible mountain tops at almost 300Km.

Line of sight is interesting. Atmospheric refraction is apparently significant and variable. A report of a tower in a photos taken from one site was that it varied from top only visible to almost fully visible.

Another technique under discussion is trying to reflect the light from the underside of a cloud to get beyond the usual horizon.

There has been talk of some sort of forward scatter or ducting(?) of light under certain conditions to get much more than expected range.

How do they set up? After consulting maps, etc, a link is decided on. On each hill a transmitter & receiver are set up. Alignment is confirmed using binoculars to spot the red transmitter.

Anyone else heard about this?

[ronin]

The description matches the description of a fresnel lens used in a light house. Seems first used in 1823. Low power bulb transmitting a signal over large distances.

Another point the visible light we "see" is just another part of the electromagnetic spectrum, so micro waves and radio waves are part of the same thing and have been used for communications for quite some time now.

[Carbon Brush]

Hi Ronin.

As I understand things, in the lighthouse, fresnel lenses were chosen to avoid the problems of a large diameter short FL lens.

In the optical comms, they were chosen for price and simplicity. I asked if a telescope had een tried. The response was that a scope at £100+ was far too expensive.

In this application, recovering a signal from the background noise is amazing. The noise includes the on/off light fixtures radiating not only 100Hz from the mains, but harmoncs up to several KHz.

I suspect though that with more attention to the optics, a viable link could be made using lower emitter power and/or in daylight.

The people I have met who are doing these trials have to be a bit of a Jack of all trades. Optics, electronics design and assembly, and computer work to recover the signals from the noise. They seem to have very little reference material to fall back on and the receivers have to be built from component (not module) level.

Some interesting stuff going on there.