# Long Distance Laser Beams

1. Sep 27, 2015

### Dr Wu

This is probably an impossible question to ask, but I feel bound to give it a try. Assuming a level of technological know-how that has enabled human beings to journey beyond the heliopause - if not to the nearest stars - just how far can it be reasonably supposed for message-bearing laser beams to travel in space before they finally become incoherent? Again, this is like trying to second-guess the future, but is there a fundamental limit to laser technology in this respect? For example: if I posit two or three thousand AU, will everyone fall about laughing? :(

PS. I've tried to get a handle of this via google, but apart from wading through the occasional PhD thesis on laser technology - which are mathematically way above my head - nothing else out there seems to come anywhere near to addressing this query of mine.

2. Sep 27, 2015

### Staff: Mentor

You wouldn't care about coherence, intensity and data transmission rate limit the range.

Neglecting small prefactors, the angular spread of a laser beam is at least (wavelength)/(telescope radius). At a distance of d, the laser beam has a width of d times the angle, so the fraction of light hitting the receiver is $$\left(\frac{(receiver~radius) \cdot (telescope~radius)}{distance \cdot wavelength}\right)^2$$

With 2000 AU, visible light, the 40-meter E-ELT telescope (under construction) and a 3 meter telescope (about the size of Hubble) far away, that fraction is about 4*10-14 in the ideal case.
To convert that to a data rate, we need the laser power. Well, obviously on Earth we'll get a higher laser power than on the distant spacecraft, so uplink will probably exceed downlink significantly. Let's say the spacecraft has a 100 W laser - that is quite a lot. It leads to 2.5*1020 photons per second. Let's say the detector can see nearly all of those reaching the telescope, so we get up to 2.5*20 * 4*10-14 = 107 photons per second. To distinguish between "on" and "off", something like 10 photons are useful for "on", which gives a data rate of about 1 MHz.
The actual data rate will be significantly lower as there are some losses from the atmosphere, the telescopes won't be perfect, there is some background noise, you need some error correction and so on. Adjusting the emitter to hit the receiver with the laser beam is highly non-trivial as well. In addition, the E-ELT will be by far the largest telescope, and you won't be able to use it around the clock (due to funding reasons but also due to practical reasons - Earth is rotating).
To summarize, data transmission is certainly possible, but don't expect live video streams, even if you have very expensive telescopes for data transmission.

Twice per year the Earth will be very close to the sun, which makes transmission in both directions problematic.

Using a telescope to make a laser beam is not easy, but it is done for the lunar laser ranging experiments, with telescopes up to 3.5 meters (APOLLO).

3. Sep 28, 2015

### Dr Wu

Ah, yes, I see now. Thanks for explaining the physics in some detail. . . it all makes sense now. Regarding solar interference: this shouldn't be a problem if the spaceship in question happened to be lying well away from the plane of the solar system, but that's a side issue. Re. your comments about the need to line the emitter up with the receiver: I gather that 'Acquisition, Tracking and Pointing' (ATP) is altogether more taxing with lasers than with radio - not that this should prove to be too much of challenge for tomorrow's technology, of course!

Many thanks :)

4. Sep 28, 2015

### Loren

If the message happens to be some forms of music I know, the distance before it becomes incoherent can be measured on the Planck scale.

5. Sep 28, 2015

### tfr000

Only if the target is near the plane of Earth's orbit... i.e. at the Ecliptic poles, this doesn't happen.

6. Sep 29, 2015

### Staff: Mentor

Sure. I don't see the science case for such a trajectory, however (unless everything else has been explored). Most interesting things happen close to the ecliptic.

7. Oct 2, 2015

### GTOM

How much percent of visible light swallowed by atmosphere if there arent clouds?
What is the hardest part of using a telescope to make a laser beam?

8. Oct 2, 2015

### Staff: Mentor

A small fraction
Getting the phase right everywhere. A high power beam is also problematic for small mirrors or lenses due to thermal considerations.

9. Apr 7, 2016

### Noisy Rhysling

Anybody else thinking about the Motie probe?