From the March 2021 Scientific American:

Optics. Traveling Photons. A laser-based system could boost deep-space data transfer.

'Laser communication could work with receivers about 20 centimeters across—the size of a personal pizza—and condensed laser beams can carry much more information than radio. But laser signals are transmitted at a lower power level, and processing them once they are received requires a daunting level of amplification.'

Why are laser signals used for long-distance communication transmitted at such a low power level? Such that they need extensive amplification?

Would it be too expensive or energy-intensive to shoot a powerful IR, visible or UV laser beam? Would the equipment be too big to fit on a spacecraft sent to Mars, or wherever?

Would a high-amplitude laser beam destroy its own equipment? ( I thought Donna Strickland won the Nobel for fixing this...)

Or would our atmosphere absorb or scatter the signal? But if so, how does lowering the amplitude get around this?

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    $\begingroup$ I think you mistunderstood the article. They write that because the laser beam would be expanded to 2 km you only detect a tiny fraction of the light and therefore it must be amplified. $\endgroup$ – A. P. Mar 9 at 20:08

Why are laser signals used for long-distance communication transmitted at such a low power level?

Because they can, and at least for now, because they must.

In addition to increased data rate, key reasons for investigating using laser communications for spacecraft are reduced mass, reduced power consumption, and reduced volume compared to radio communications. Using high-power lasers rather than low-power lasers would defeat these purposes.

Using laser communications for spacecraft of any power level comes at a cost compared to using radio communications. A radio communications system between a spacecraft near the Moon and the Earth requires sub-degree pointing accuracy on the part of the spacecraft. That is easily attainable. A laser communications system would require pointing accuracy at the arc second level. That's getting close to spy satellite or Hubble level pointing accuracy. Most spacecraft do not need this kind of pointing accuracy for their instruments.

This increased level of pointing accuracy needed for laser-based communications does not come cheap in dollars, mass, power, or volume. This in turn makes the reduced mass, power, and volume requirements of a laser communications system be offset somewhat by the increased mass, power, and volume requirements needed for pointing accuracy. A laser-based communication system must necessarily operate at significantly lower power levels than a radio-based communication system to be competitive with that radio-based communication system in terms of dollars, mass, power, and volume.

Finally, at least for now, laser-based communication systems have been auxiliary test articles on operational satellites, or primary systems on low-cost experimental satellites. While space agencies do spend monies on experimental vehicles, they don't spend a lot. In both cases, the money, mass, power, and volume needed for a high-power laser-based communication system just is not there. The experimental systems to date have all used relatively low-power lasers, as do the operational systems planned for the future.

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    $\begingroup$ Would you be able to add a few comments on why the optical method needs better pointing accuracy than a radio frequency method? I would have thought you could use a lens to make the optical signal spread in the same way as the radio signal, so the two would then be equivalent on required pointing accuracy and it becomes the power requirement that is the issue. $\endgroup$ – Andrew Steane Mar 10 at 13:16
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    $\begingroup$ @AndrewSteane Broadening the beamwidth of a laser communication system would entirely defeat the purpose of and utterly destroy the advantages of laser communications over radio communications. Laser communication systems are significantly less efficient than are radio communication systems in terms of power consumed versus transmitted power. The tighter beamwidth of a laser, along with its higher frequency, are what make laser communication systems promising. But that tighter beamwidth comes at a cost of increased pointer accuracy. $\endgroup$ – David Hammen Mar 10 at 14:00

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