1…2…3…4…5…6
or squares
1…4…9…16…25…36
or primes
1…2…3…5…7…11…13
or even the Fibonacci sequence
0, 1, 1, 2, 3, 5, 8, 13…F(i) = F(i-1) + F(i-2)

Laser Com is more energy efficient since the beam is so much more collimated, but as you point out, that makes it hard- essentially you only will see the signal if it is being intentionally directed at you.

An RF signal from the moon would cover most of the continental United States, the laser in LLCD made a spot a few km in diameter.

Accurately targeting a planet in another star system many light years away is not trivial matter- You have to know where it WILL be years later when your signal gets there… you could cover your bases by blowing up the beam divergence – making it less collimated, but then you are trading away one of the main advantages of laser com.

I can envision a laser being used as a beacon, “Hey look at me! I am obviously the creation of an intelligent species!”

Transmitting data beyond that gets tricky…

A 1 Watt laser at the moon transmitting at the Nd:YAG wavelength of 1064 nm with a divergence of 15 micro-radians would have a beam diameter at the earth of ~6km, for a power flux of ~35nW/m2 or 190 billions of photons per square meter per second- A telescope with 1m2 area is a good fraction of a million dollars.

Thats a lot of photons, but if you are trying to communicate at 1 Gbit per second that’s only 189 photons per bit- and we haven’t looked at the losses from the atmospheric transmission and your receiver and detector…

The moon is very close compared to the nearest star- the earth-moon distance is 1/24-millionth of a light year. The number of photons per square meter at earth will fall as the inverse square of the distance.

At 10 times the distance you will get 100 times less photons per square meter.

At 1 light year you will get just 0.00034 photons per square meter- that means you would need a 30 m diameter telescope just to have a good chance of a single photon from your 1W laser at 1 light year hitting it.
You need a much larger telescope if you want to pick the signal out of the noise…

If you transmit using a diffraction limited beam transmitted from a telescope like the Keck with a divergence of 64 nanoradians you get 18 photons per square meter per second on earth (but the spot diameter is 600 million meters- earth moon distance is ~400 million meters, so you have to point it very accurately at a point where the earth will be 1 year in advance.)

That is 1 Watt at 1 light year using a perfect 10 meter diameter telescope to transmit the beam.

That same transmitter located at Alpha centauri would get you ~1 photon/second per square meter incident at earth…

The flux will scale linearly with the transmitted power, 1/r^2 with the distance- and 1/divergence^2

So it is possible, but it rapidly becomes very expensive and difficult to do- especially if you want the flux to communicate a good number of bits per second.

A megawatt laser transmitted from a 10m telescope at alpha centauri would be a pretty good start…. and the bigger the telescope on earth to receive it the better….

But the main problem is that the ETs would have to target us very precisely… so they would have to know we are here, and know exactly where our planet would be years in the future.

Not impossible, especially if you assume advanced ETI technology…

Of course, the fundamental problem still remains, the transmitting civilization must have reason to believe there will be someone looking at him in the system he is signalling to, at the time he is signalling (making allowances for light-travel time). And because of the super tight beams, it is highly unlikely we might intercept a signal meant for someone else.

You know, the more I think about SETI, the more I’m starting to believe it is a waste of time and effort. If you’re the last man on earth, even if the last girl on earth lives somewhere within a hundred miles of you, its still next to impossible to hook up with her.

Imagine being able to watch 4K ultra-high-definition (UHD) video as humans take their first steps on another planet. Or imagine astronauts picking up a cell phone and video-conferencing their family and friends from 34 million miles away, just the same as they might on Earth. LEMNOS, Laser-Enhanced Mission and Navigation Operational Services, may make these capabilities and more a reality in the near future. The project was named for the island, Lemnos, where the mythical hero Orion regained his sight, according to Greek lore. Similarly, LEMNOS will provide sight for NASA’s next-generation Orion spacecraft.

“Laser communications will revolutionize data return from destinations beyond low-Earth orbit, enhance outreach opportunities from outer space and improve astronauts’ quality of life on long space missions,” said Don Cornwell, director of the Advanced Communication and Navigation division at the Space Communications and Navigation program office at NASA Headquarters. “As we strive to put humans on Mars for the first time, it’s imperative that we develop a communications system to support these activities at the highest level possible.”

Laser communications, also known as optical communications, is the latest space communications technology, able to provide data rates as much as a hundred times higher than current systems. This means, for example, that astronauts could send and receive ultra-high-definition video from the surface of Mars. No mission to Mars has yet had that capability. Something that basic could have wide-reaching applications, allowing the American public to “ride along” as our astronauts explore deep space while also enabling scientific discoveries with much higher resolution images and data.
The Exploration and Space Communications (ESC) projects division at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, has been tapped to build LEMNOS in collaboration with the MIT Lincoln Laboratory in Lexington, Massachusetts. They worked with other NASA centers to determine specific needs the system can fulfill.