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Is a Laser Data Link 1.5 Million Kilometers Feasible?
An anonymous reader writes "On the Canary Islands last week, a team from Oerlikon Space demonstrated the feasibility of a laser link across a distance of 1.5 million kilometers for the first time ever. In the future, laser links like this one will be able to transmit data across huge distances through the universe far more rapidly and efficiently than is possible using conventional radio links today."
Who would have thought that light could travel such a long distance?
In all seriousness, the problem is not the knowledge a laser can travel that far; its whether you can create precise enough targeting equipment.
A radio signal might be more of a splatter, but at least if you point it "over there" with enough power behind it, it will get there.
As they say their simple hilltop to hilltop test failed because of weather conditions, whats going to happen when they do put 'scopes at the lagrange points?
"Oh sorry, we can't get the data today because its cloudy"
Back onto the radio front, we have Voyager 1 which is 15 billion miles away, proven with radio, that would seem good enough for me.
liqbase
"laser links like this one will be able to transmit data across huge distances through the universe" I think they mean "through the solar system"... laser wouldn't be very efficient "through the universe"... I think we may have other means of communication by the time we need to think about distances that vast.
Do not look into laser with remaining eye.
Hi,
It seems to me that this would be especially useful to reduce the amount of induced radio noise when communicating with L1 (etc) radio telescopes or other instruments potentially sensitive to the normal radio frequencies used for communication, eg keep the comms out-of-band of what is being measured as far as possible.
Rgds
Damon
http://m.earth.org.uk/
Canary Islands and experiments with laser beams? Ahah! There must be sharks there!
Are there sharks there or something?
Because lasers travel at least 42 times as fast as radio waves!
They will all stop at the last mile, rendering the project useless.
Yes they do, since that focus is never perfect. A cheapie laser pointer will show a 1/8" dot at 30 feet and a 1/4" smudge at 60 feet.
Can't you also make a laser out of radio waves? I know they have microwave "lasers" called masers, so do "rasers" exist?
Lasers diffuse over a distance, just like normal light bulbs, albeit a much smaller rate.
So, the farther away you go, the bigger the "dot" the beam casts is. The inverse square law applies. If it didn't, overall power would have been added as the beam travels (the dot would be bigger, but the same brightness). This is a law of physics.
I'd imagine you'd kinda have to aim carefully, but by the time it could 1.5 billion miles the beam would be, at least, hundreds of miles across. Which means you better have a sensitive photo detector, just as you would need sensitive antennae with radio waves.
But having to aim is the point (PUN), really. Concentrating the beam reduces the energy needed to get it there, because the energy is spread out over a smaller area.
If an object 1.5 million kilometres away has a neighbouring quasar, you have bigger worries than communication.
Real Daleks don't climb stairs - they level the building.
What the article doesn't mention is the poor crew that were huddled behind the massive metal crate up by the NOT (Nordic Optical Telescope) on these tiny little white plastic chairs (which had to be weighted down by rocks when they got up). I was up there at the WHT/NOT the other week and happened to pass by their setup, the only potential hint at what they were doing being one of those little yellow hazard signs that simply said 'Laser' on it. Glad they got what they wanted - the weather was pretty terrible for several days, you were basically sitting in cloud.
1.5 million kilometers = 1.6 x 10^-7 light year.
Distance to galactic center = 26,000 light years
Distance to nearest (Andromeda) galaxy = 2.5 million light years
"Faster than radio" probably refers to increased bandwidth, because light-speed is light-speed.
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It seems strange that they didn't aim for the retro-reflector placed by one of the Apollo missions which has been used for 30+ years for laser ranging experiments. It's location is well known. That would give them a real 800,000 km beam path, roughly half of what they claimed.
When the Apollo mission landed on the Moon they left behind a retroreflector that NASA used (still use?) to bounce a laser back and forth to measure the distance from the Earth very accurately. That's 385,000 km. If they were doing that in the late 1960s I don't see any reason why 1.5m km should be that tricky today.
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A house divided against itself cannot stand.
I could be wrong so someone knowing better please correct me.
The inverse square law is applicable only for point sources that are radiating in every direction. The inverse square of distance d arises in the formula that you are interested in the surface of a ball centered at the source with radius d. The surface area is proportional to the square of distance so intensity in some part of the surface relates to the inverse.
Now lasers are not omnidirectional so the inverse square law is not applicable.
It doesn't sound like you know much about mathematics. Please check the relation between the diameter of the laser spot and the power/area ratio, then rethink what the inverse square law actually says.
Instead, I imagine the initial linkup might be the limiting step. The system might require an initially higher-power signal (that is broad so that targeting tolerances are lower) to initialize the link, then active feedback could allow the two ends to narrow the beams for lower-energy high-speed data transfer. Maybe the initial phase will use conventional radio signals (or radar) to establish the locations (and relative movement) of the two endpoints of the link. With that information, the two ends can then aim the laser fairly accurately.
I could see it working but the receiver would have to be huge. It's hard enough to hit someone with a gun at a mile using a laser sight (windage which would be comparable to space effect on the laser light). Luckily there is no wind in space, and the motion of objects is measurable and fairly predictable. Obviously over those distances any amount of error or jitter translates to a huge positioning error, but laser-steering systems can also be made quite accurate (not to mention that a laser doesn't have to be perfectly collimated, you can easily tune the aperture so that the beam is 500 m wide at the target... as long as the laser is strong enough, the receiver will still easily be able to measure the signal).
whats going to happen when they do put 'scopes at the lagrange points?
I've been thinking about the Earth/Sun Lagrange points lately. I think they might be an excellent location to test an Earth/Mars transit vehicle. ESL5 is far enough away to be out of Earth's magnetosphere, so it will experience the raw radiation environment. It would be able to remain in position for long periods of time. The only hitch I can see is it may not be easy to get to/from. I can't seem to find any data. If we put a test platform with a "lifeboat" craft there, how quickly could the craft get back here. Is it days away? weeks away? Anybody know?
When our name is on the back of your car, we're behind you all the way!
Correct, they did put corner cubes on the moon (aka retroreflectors, or three mirrored surfaces all at 90 degree angles to one another).
However, the beam size from a collimated laser is a couple miles across at the moon. Typically, receiving a signal back takes a large telescope which counts single-digit photon returns from a Nd:YAG q-switched laser. It's been almost 2 decades since I worked with the stuff (you might search for Satellite Laser Ranging, Goddard Optical Research Facility and MOBLAS or TLRS) and the units that ranged on the moon cubes were at Mt. Haleakala in Hawaii.
It was neat stuff, but I remember one of the PIs saying the spot on the moon was the size of Georgetown (a section of Washington DC), though I can't remember exactly now. The outgoing laser was about 4" in diameter.
Is it just my observation, or are there way too many stupid people in the world?
bounced the signal off the reflector that Neil Armstrong left at the Apollo 11 landing site. Round trip could have come pretty close to 768,800 kilometers... bouncing it back up and down again would have made the link as near as damn it = 1,500,000 kilometers
Donald 'Duck' Dunn: We had a band powerful enough to turn goat piss into gasoline.
I remember this being done with Earth Observation satellites. The EO satellite beams data using an optical link to a satellite that is in geostationary orbit. This satellite then beams the information down through a microwave link. This frees the EO satellite (that producue huge amounts of data) of the need of high-power consuming RF transceivers, reduces the need for ground stations, and is seriously cool. This was done in 2001 between SPOT 4 and Artemis (Press release). Note that SPOT sits in an orbit around 800km, and Artemis is geostationary... They then did the same with an aircraft (see here).
So it is really quite useful. When you consider the amount of data the sensors on board ENVISAT (or even MODIS) produce, this is an important tool.
we have the Gas engine... it works.. lets forget about all this crazy hybird and electric car talk...
While we're at it, Coal Plants do a good job at producing energy and they work too... lets forget about all that fandangled alternate energy source stuff...
While were at it.. smoke signals work too.. no need for complicated technology like telephone and email...
okay.. now that my sarcasm limit has been reached... because something works is not a good reason for ignoring technology that can potentially supercede it...
The retroreflector isn't easy to hit, and they actually get back only one photon every few seconds. This would not yield much bandwidth for communications.
If we start shining huge lasers into space, we're going to end up accidentally blinding aliens. Which might be good (if they're chest-explody types), or bad (if they're hot sex-starved space-babes). Your call.
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The US has a several classes of Signal intelligence and Communication intelligence satellites. I would be shocked if they didn't already use an optical link to send their data to relay satellite for downloading to a ground station. An optical data link would make the satellite "silent" so their data link wouldn't interfere with there intercept receivers. Since both the satellites are in space you wouldn't need to worry about weather an since they are both in geostationary orbit you wouldn't need to worry about aiming. Of course the other benefit is that you could beam the data right from your recon satellite parked over Asia to a relay satellite parked over the US and then right down to a ground station in Virgina. No need to have a ground station in a friendly or not so friendly country.
See my blog http://ilovecookes.blogspot.com/ for light hearted technical information.
Faster Baud rate, not faster event rate. higher frequency signals can carry more information.
I've seen some comments to this post saying that a laser beam dosen't obey the inverse square law and some saying that it does. Actually, everyone is right in a sense. Over "short" distances, laser beams expand at a rate that is slower than inverse square. Over "large" distances, the rate of expansion increases, eventually approaching the inverse square law. The distance that distinguishes "small" from "large" is called the Rayleigh range and it depends on two properties of the beam: the wavelength of the light, and the curvature of the beam's phase fronts at the reference point from which you measure the expansion.
Concrete example:
A 1 micron infrared laser has a 3mm diameter spot with flat phase fronts. The Rayleigh range is 28m
Distance: 1m, 2m, 4m, 8m, 16m, 32m, 64m, 128m
Spot Size: 3.002mm, 3.008mm, 3.030mm, 3.118mm, 3.447mm, 4.531mm, 7.425mm, 13.91mm
The same amount of power is contained within the spot, so the ratio of the intensity (power/area) goes as the inverse square of the ratio of the spot size.
Between 1m and 2m, there is essentially no change in intensity (collimated beam) but between 64m and 128m, the intensity reduces by (1/4)
-Anonymous Physicist
Wrong, they do follow the inverse square law.
See the article you link to, which states that perfect collimation can never be achieved in reality. Thus, like any other source, laser light follows the inverse square law in the far field.
Note that in general, I believe the inverse square law only applies to a point source, or a source which is effectively a point source at the distances involved. For dealing with cases where the source can't be approximated as a point (either because it's really large, or the radiation intensity is being measured very close to the source), RF engineers use the term "near field gain reduction" for the behavior of RF field intensities in close proximity to an antenna, which probably has an equivalent term for optics. As a result, for an optical source with a large aperture in relatively close physical proximity to the aperture, the inverse square law will appear not to apply, but once the "near field" for that source is exited, the inverse square law holds.
retrorocket.o not found, launch anyway?
"Who would have thought that light could travel such a long distance?"
Who would have though the Canary Islands are that big?
-Charlie
Inverse square law applies for isotropic (all directions) as well as directional sources (focused beam). The way the difference is handled is by introducing an antenna gain term, where the gain at a given point in space is defined to be the ratio of the power density due to the directional source to the power density of an isotropic source. In communications applications, you use Friis' Transmission Formula to compute received signal-to-noise ratio which includes a factor Pt*Gt/(4*pi*R^2), which is the power density at a receiving antenna (lense) a distance R from the transmitter, where Pt is the Power Transmitted and Gt is the gain of the transmitting antenna (lense). For a laser it is easy to get a high Gt (very directive) with a small lense because the wavelength is so small, but that still does not get one around the R^2 relationship.
This would work really well in environments that are pretty clear. I only studied a little astronomy but, what if we were to:
- Use radio from the ground to orbit? I think this is pretty common already. Lasers as we know suffer more from weather than radio.
- Use laser from Earth orbit to furthest possible point without a significant signal loss.
- And then, use radio from that point on?
Imagine you're trying to send a signal from a clear area, through a forest, to another clear area. Laser wouldn't work through the forest, but radio would.
I also think that laser would require more power than radio, making it more feasible to have laser power outside of Earth orbit, then using radio for further away.
What do you think?
JPL's been working on it too for a while now... and with similar datarates, and a ground acquisition plan to boot.
http://lasers.jpl.nasa.gov/PAGES/pubs.html#ocd
But, yes, a laser link indeed is desirable. Sure, we can still contact Voyager with radio telescopes, but even from the Mars rovers, notice how it takes so long to get from Mars to grainy B&W picture back on Earth?
Sending back live video feeds and more full colour images sets the data rate bar much, much higher. Getting this much data back quickly is limited by the frequency of the radio waves/light. Laser light has an over 1,000 times shorter wavelength than Ka band radio telescopes can manage (that's what NASA uses now to talk to the Mars probes), which increases the potential amount of data that can be sent in a given timeframe by essentially that amount.
In addition, because laser light is focused so narrowly, it wastes much less energy than a radio antenna which must spray a good portion of space with radio waves in order to hit Earth. Imagine focusing your mag-light in the dark... the narrower the focus, the brighter the beam gets, because more energy is packed into less space. The challenge though, is that you have to aim much more precisely at Earth to compensate for that more focused beam.
Here's a great overview of JPL's long-term vision:
http://lasers.jpl.nasa.gov/PAPERS/REVIEW/overview.pdf
What we really need is a laser that travels a few feet, and makes a swishing noise when you wave it around.
If I recall, one of the requirements for the new Lunar X Prize is the shooting of some high-def video from the lunar surface. (For some *very* pricey stock footage!) I imagine it would be much easier to do that with a high-capacity link, such as what you'd get with a laser. This is the sort of technology that the X Prize (and NASA) should be supporting.