# Range of modern laser beam

Is today there the prospect of communication using lasers a distance of approx. 50-150 light-years? If so, what is the maximum range which can be achieved with modern laser power, eg. 1000W? We assume that the other party will have a very modern detectors (at least such as we have currently or higher).

I must admit that the question is about SETI. Are there projects sending laser pulses to other places in our galaxy?

• If we learn to build large interferometric arrays, yes, which means that the answer, at the moment, is negative, since we have neither funding nor technology at the moment. The range doesn't just depend on the power but also in the bandwidth you want to transmit. If you can make the bandwidth very low, even a little bit of light will go a long way. I do agree, by the way, that optical SETI is the way to go. – CuriousOne Mar 25 '16 at 0:45

## 1 Answer

You can make your own estimate; assume a certain fluence, power per square meter, f, required to activate the detector; start with power P, an initial beam diameter, D, and a beam divergence $\theta$. As the beam travels a given distance z, you can now calculate the divergence, and the increase in the size of the wave front, and the fluence at that distance.

You will now be able to determine the available fluence at every distance, given the assumption that there are no losses, or you can include a loss parameter, such as a certain proportion per meter due to scattering.

The most powerful lasers today can produce brief pulses, under 100 femtoseconds in duration, with perhaps 10 joules per pulse, and an initial beam diameter of one millimeter. Of course you can invent more powerful lasers for your project, but this gives you a start. This system will also require some fantastic degree of pointing stability, for not only will you have to point at a distant star, but you will also have to point at the actual detector in orbit about that star, and actually hit it.

We can actually detect single photons, but for communications you will need to provide a communications protocol which will permit the detector to know that it has received a signal, as opposed to a random photon.

• We can produce very short photon pulses, but I am not aware of a gating mechanism to make the aperture window of the detector so small. Small PMTs can have 100ps response times and very small photodiodes can probably go below the 10ps range, but I am not aware of detectors that can compete with the shortest pulses. Maybe I am just missing something in my understanding of engineering optics. For long distance communication one would make the beam as wide as possible. – CuriousOne Mar 25 '16 at 0:42
• I mentioned pulses as a start on a communications protocol; things are alrady pretty spread out just going to the moon and back for the lunar laser ranging. The silicon avalanche single photon detectors in my current experiments have a 500 micron diameter active area, and the counter clocks at 81 picoseconds, 80% quantum efficiency. You would like to get sufficient signal onto the the detector -- a wider spread makes it easier to find the detector, but wastes power. It's a tradeoff. – Peter Diehr Mar 25 '16 at 1:11
• So I am totally in the ballpark then. One can, of course, get the signal from Alpha Centauri to focus onto a 500um detector... it just needs a really precise mirror... :-) – CuriousOne Mar 25 '16 at 1:18
• Thank you Peter Diehr. Yes. Perhaps for the Alpha Centauri is possible. However, Alpha Centauri is a distance less than 5 ly. For distances of 50 ly by adding a loss and (in theory) it is possible? I have done some calculations but unfortunately for the 50 ly are not "interesting". – Aurelio Mar 25 '16 at 11:25
• The explicit tradeoffs in your model will be the size of the phton collector, the signalling rate, and the initial energy per pulse. By making really large collectors you can detect signals from furthrer away. Review how stellar interferometers work. Ultimately it reduces to signal to noise calculations, and cost per decibel. Have fun! – Peter Diehr Mar 25 '16 at 11:34