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."

Never saw this coming

[LiquidCoooled]

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.

A bit exaggerated?

[Greg01851]

"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.

Re:Question about lasers

[kevmatic]

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.

It's pretty damn cold up there

[Anonymous Coward]

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.

Re:Never saw this coming

[vertinox]

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?

Huh? The logical thing do to would be have the laser communicators in orbit, and the communication from ground to the laser satellites would be via the conventional means. If its cloudy in your town, then the satellite can talk to another town which isn't cloudy and you can use fiber to talk the rest of the way.

Re:Targeting that is going to be a bitch.

[kebes]

Great idea, now try to do the math on all of the floating bodies and the effect of the gravity from neighboring quasars and other space phenomena.

For the mentioned application (communicating inside the solar system to the Lagrange points, for instance), gravitational effects will not be a big deal. The light deflection that the Earth or the moon will cause are negligible. The real challenge in targeting, I would imagine, will be accounting for relative motion between the two ends of the link.

Maybe a single shot of data, but maintaining that connection would be very difficult IMHO.

I expect just the opposite to be true. Once a link has been established, I imagine maintaining it wouldn't be that hard. Why? Probably the optics on both ends will measure the positioning of the incoming laser on their detector. They can then send information to each other about alignment (e.g. "you're drifting to the left...") so that they can actively compensate (the time lag [google.com] between them will be ~5 seconds, or ~10 seconds roundtrip).

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).

Lagrange points

[camperdave]

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?

Laser moon and back feet, more like *miles*

[Overzeetop]

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.

a better test would have

[advocate_one]

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

Probably already in use.

[LWATCDR]

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.

Re:Never saw this coming

[FuzzyDaddy]

Aim is an identical issue with both radio and lasers.

Unlike radio stations, most point to point links (for example, satellite uplinks) use a focus beam. That's what the big dish is for. The tighter the beam, the less area your transmitted power is spread over and the greater your received signal strength. The downside, of course, is that a tighter beam has to be aimed that much more accurately. As a point of reference, most geosynchronous satellites are spaced about 2 degrees apart, which requires a terresterial pointing accuracy of about 1 or 2 degrees. On the other hand, the Arecibo radio telescope has a beam width of a few thousands of a degree.

A laser naturally comes out with a narrow beamwidth, while a radio signal takes a little more work. But the beam width of both can be manipulated to where you need them to be, and the issues of signal strength versus pointing accuracy are identical in both cases.

Friis Transmission Formula

[dunc78]

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.

Radio to Laser to Radio

[teledyne]

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?