Q:

Could a civilisation contact us by laser from 50 light years away?

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?  

A:

The divergence of the signal is closely related to the angular resolution of the "antenna" (lens, reflector, ...) you use. This will in turn be a function of the size of the source in relation to the size of the lens - or in the limit, the ratio of dish diameter and wavelength (the familiar $\frac{1.22\lambda}{d}$ relationship)
Typically this means it is easier to get a narrow beam of laser light than radio waves. But the narrower you make it, the better your aim needs to be. How well can you aim your transmission at the point where Earth will be 50 years from now?

A:

When you say "laser", I'm guessing that you mean visible or near-visible light.
The problem is, where are they going to build their laser?  If they build it on their home planet, then it's going to be so close to their sun, that our telescopes won't be able to distinguish the one from the other.  We won't even be able to tell that it's there unless it is a lot brighter (as seen from Earth) than their Sun.
If you're going to transmit a signal from the neighborhood of a star, then it makes a lot more sense to use a wavelength that the star does not emit.

