The Kepler Space Mission has discovered over 1,000 planets. These discoveries may increase the confidence of some people that other civilisations exist. That is an opinion issue, and not what I want to ask about here.
If a hypothetical civilisation far from Earth picked up our human created radio signals, which by now have extended to a sphere around 100 light years across, and decided to use lasers to contact us:
Assuming a 50 light year distance, how much power would it take to send us a laser signal?
Would this laser signal be so spread out, i.e. undetectable, that radio signals would still be a better option for communication?
Normally, I would have assumed that the laser signal would have spread out so much over that distance, that it would be virtually undetectable, (and I would not ask this question), but this Wikipedia article: Interstellar Communication states:
It has also been proposed that higher frequency signals, such as lasers operating at visible light frequencies, may prove to be a fruitful method of interstellar communication; at a given frequency it takes surprisingly small energy output for a laser emitter to outshine its local star from the perspective of its target.
It may well be that sending the signal is not a major issue, but I don't know enough about the divergence of laser light to determine if detecting the signal is the main problem. To hopefully clarify this, would a large, much too large in practice, focusing lens be required?