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Natural Science Forum / Physics / Optics / November 2005Large parabolic mirror
Help I'm a Radio amateur beginning to look at optical communications, I see that others are using large Fresnel lenses of about 300mm square for transmission and reception, I wonder if it's possible to make or buy some sort of primary or offset feed parabolic reflector at reasonable cost (i.e. cheap). A reasonable size of dish would be 500-600mm on the long axis. My entirely un-scientific ideas so far are:- 1. A domestic aluminium satellite dish polished to a mirror finish - how difficult/costly? 2. A reflector vacuum formed from acrylic mirror material - would a perforated microwave dish be OK as a former? 3. A segmented parabola made using small mosaic mirror tiles stuck to a metallic dish. 4. A plane parabola formed from sheet mirror material - is there a simple way to gather the light to one point, some sort of lightpipe? I appreciate that none of the above would give a reasonable focus but would they give me light gathering capabilities beyond that I can achieve with easily available Fresnel lenses? I'm guessing that I can correct poor focus by using a supplementary lens to bring the light onto a small area photodiode - am I right? I'll be using Red Lumiled Luxeon IIIs for transmitting but haven't investigated photodiodes yet, modulation will be simple amplitude modulated voice (say 3KHz) or on/off keying - CW. Any other ideas gratefully received Thanks John >Help >I'm a Radio amateur beginning to look at optical communications, I see that [quoted text clipped - 24 lines] >Thanks >John I've worked on motion picture screens under the "Torus" trademark, produced by Stewart Screens. They specialize in making deep curved screens to be used in wraparound flight simulators, etc. They take whatever their screen material is going to be, in your case, thin alum coated mylar, and attach it to an air tight frame. There is an air gap behind the screen with a proximity or simple leaf switch which is hooked up to a vacuum blower. By powering up the blower, they can suck the screen back into different forms such as parabolic, section of a toriod, etc. When the screen reaches the correct shape, the proximity sensor which is placed at the "focal point" senses that the material has been pulled into the correct shape, and then idles the vacuum fan. It works very well, especially where you may have a need to tweak the shape of the refector. And it takes very little vacuum to hold the shape for long periods of time. I built 6 of these things for them over in Korea for flight simulators, and they were typically 40ft wide by 20 ft tall, and it only took 4 standard pie fans to hold the vacuum. Whether or not that would work for what you're planning on doing, who knows, but it is a possibility. Plus they can be made very portable as it can all be made out of simple alum. tubing and fold flat for transport. Again, this method is patented, so I don't know what you would be up against as far as building your own. I would tend to stick with the Fresnel lens material as you can get very large sheets of it (used in almost every large screen tv, video wall cube projectors), and some of them run off axis. Good Luck! Doug -- If you would like to contact me, leave a message in alt.lasers or rec.motorcycles.harley and I will reply with a valid email address. Thank you, The Management Kind of depends upon how much beam spread you want. With an 8" mirror of fairly long focal length, you can get a beam spread so small that at a mile or more, you will not fill a person's head with the beam. The thing here is to figure out what kind of beam spread you want and then figure back how big your source is and that will determine the focal length of the mirror. As to mirrors, the typical glass mirror with an aluminum coating will give you the best results. The mirror is gorun and pollished to a parabolic shape and that is the right shape for the reflection. For larger beam spreads, you dn't need to get the surface as accurate and then thing like mylar on a frame and vacuumed to the desire focal length will work well. -- Why do penguins walk so far to get to their nesting grounds? > I'm a Radio amateur beginning to look at optical communications, I see that > others are using large Fresnel lenses of about 300mm square for > transmission and reception.... At what wavelength are you operating? Can your set up be described as some form of Free Space Optics (FSO)? > > I'm a Radio amateur beginning to look at optical communications, I see > that [quoted text clipped - 4 lines] > > Can your set up be described as some form of Free Space Optics (FSO)? Charles The emitter is a Red Luxeon Lumiled 3 which produces somewhere near 140 lumens at 630nM and yes I think it would be described as a Free Space Optics system, the current distance record is just over 100 miles using simple voice modulation i.e. AM 3KHz bandwidth, on-off (CW) keying is also obviously possible. One option as used for the record is to use Fresnel lens Tx/Rx with spatial diversity using 2 or more transmitters and/or receivers. I'm just looking at all the options before I start building. Thanks John > The emitter is a Red Luxeon Lumiled 3 which produces somewhere near 140 > lumens at 630nM and yes I think it would be described as a Free Space Optics > system, the current distance record is just over 100 miles using simple > voice modulation i.e. AM 3KHz bandwidth, on-off (CW) keying is also > obviously possible. 100 miles? From a very tall building or a samall mountain I suppose. But since I can see quite well the Empire State Building 80 kms distant from a fire tower near where I live (when the weather is good) why not? The hardware used by FSO companies sometimes look like telescopes to me. Maybe you might want to look at the hardware (mirrors, instruments etc.) used by amateur astronomers. Maybe a "used" telescope in decent condition, particularly one made before they were computerized could be found for not much money. Look up the ads in Sky & Telescope. I suppose your chances of establishing a distance record would be better with this kind of equipement (if this is your goal). I hasten to add that I am mostly an optics type and not at all a telecommunications specialist. > One option as used for the record is to use Fresnel lens. Large size Fresnel lenses are messy, imaging wise > Tx/Rx with spatial diversity using 2 or more transmitters and/or receivers. > I'm just looking at all the options before I start building. Your task should be fairly easy without using large optics. For your source use a modulated laser diode. At your receiver use a narrow band filter centered arround the laser filter. This should give you a very good signal / noise. This will be a lot better than the Luxeon diodes. You do need to perform the calculations regarding laser safety. This is very important. > Your task should be fairly easy without using large optics. For your > source use a modulated laser diode. At your receiver use a narrow band [quoted text clipped - 3 lines] > You do need to perform the calculations regarding laser safety. This > is very important. Thanks for the reply but I'm not planning on using a Laser diode, have a look at http://www.bluehaze.com.au/modlight/index.htm Chris Long and Mike Groth have achieved 167Km/104 miles using the earlier 1W Luxeon and relatively simple optics and argue against using Lasers. John ONe little thing to consider is PWM modulation of the sbeam. You can send digital voice signals at 2400BPS of telephone quality sound. There is a chip that does the conversion both ways (TX and RX) at the same time which makes the whole job easy. Beam spreading by the path will tend to make the TX side wanting to do a fairly wide beam angle but the RX side can be a good telescope relative to the TX side. I'll also note that PWM electronics will be quite efficient compared to analog electronics. Sorry that I don't remember the name of the chip but it is from a small company in Boston. I used it in an application and was quite supprised by the quality of the analog while using a 2400baud modem as the transmission medium. That is an interesting article on long distance communications by light! -- Why do penguins walk so far to get to their nesting grounds? > Thanks for the reply but I'm not planning on using a Laser diode, have a > look at http://www.bluehaze.com.au/modlight/index.htm > Chris Long and Mike Groth have achieved 167Km/104 miles using the earlier 1W > Luxeon and relatively simple optics and argue against using Lasers. > John I've had a quick glance at the above reference. However, it is easy to break the coherence of a diode laser if this is a major problem for communictions. Inject the laser into a multimode fiber, roll the fiber on a bobin and collimate the output end. This should give you a suitable source. You will have a nice narrow band source which can be spectrally filtered at the receiver for a high signal / noise. If you arrange your compnents for a similar power and geometry os the lumileds you should have a superior system. |
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