Optics for open-air visible spectrum communications

I want to communicate 2.7 miles from a peak on a mountain. I will be using a very bright LED and some type of photo-element (photo multiplier tube, avalanche photo diode etc.) to receive the amplitude modulated signal.

One problem I have is: what optics will I use to receive the signal to "zoom in" to the light?

I would like to build a system like this YouTube video.

So in short... Where can I get the optics which the people there ↑↑↑ used. I do not want to buy a telescope because of the expense and then my project won't be considered D.I.Y.

I will be starting out at the night time, and probably not doing any day experiments for a while.

Im sorry if this question is considered off topic, I thought it should fit this site at least because it has to do with communications, and optical is above 275Ghz which is on the chart.

• Why a photo multiplier tube or avalanche photo diode? These devices are designed to be very sensitive, but if you are using visible light and a terrestrial target, sensitivity isn't your problem: discriminating the signal from the roughly $350W/m^2$ of solar power in this spectrum is your problem. – Phil Frost - W8II Dec 18 '13 at 17:18
• @PhilFrost didn't mention this, but I will probably be doing it at night at least start out night. I edited my question – Skyler 440 Dec 18 '13 at 18:52
• Would a simple photo-resistor work? – Skyler 440 Dec 18 '13 at 18:54
• Probably, but a photodiode or phototransistor would probably be cheaper, and better. LEDs (all silicon PN junctions, in fact) also work as photodiodes, you might even be able to use the same optoelectronics for receive and transmit. – Phil Frost - W8II Dec 19 '13 at 13:23
• Ok. awesome, so I will probably get some fresnel lenses on ebay, some reflective foil, a phototransistor or diode and be set? – Skyler 440 Dec 20 '13 at 2:14

What you will want to do is not just concentrate on the receiver, but also the transmitter.

For the receiver you will want a lens to focus the signal from the transmitter down to as tight a point as possible. 2.7 miles means your light will be near enough parallel once it hits the lens*. If you have a 5cm (for example) distance between the LED and the lens, you want a lens with a focal length of 5cm.

• Now, in order to improve your efficiency, and signal strength, build the same setup at the transmitter. Use the same lens at the same distance from the LED, and the output will be a stronger beam; as much of the light that would have escaped out to the sides will be focused into that parallel beam.

Your best bet for getting lenses? There are lens stores online that cater to astronomers, photographers etc. The ones in that video look like Fresnel lenses, which are certainly cheaper than traditional optical lenses.

• Thanks, do you know of any particular store? – Skyler 440 Dec 18 '13 at 2:20
• Would these work? ebay.com/itm/… – Skyler 440 Dec 18 '13 at 2:25
• I would imagine so, as the focal length is 30cm, you'd want to have it as the base of a pyramid, with the transmitter (or receiver) at the peak, and coat the inside with reflective foil to maximise signal. – Rory Alsop Dec 18 '13 at 9:20
• The video or the text on it did mention Fresnel lenses which are significantly cheaper than a traditional lens such as might be used in astronomy. – WPrecht Dec 18 '13 at 14:12