best issue a 'notice to airmen' if you go this route, pointing multiple-tens-of-watt lasers skyward is going to get you into a lot of trouble if you're not careful about it.
Yes, better report to the authorities before you go waving a huge laser at the moon. In my home country, owning a laser this big requires a permit in the first place. You would definitely need to report it to the local aviation authority, who would re-route flights and notify aviators.
A few weeks ago there was a music festival with some big laser decorations and they actually changed the approach path to the largest airport in the country for the weekend.
The lasers used in the lunar ranging experiments are pulsed, in much the same way radar is pulsed, so even though the laser's peak power is very high, the average power could be less than a watt and accomplish the desired objective.
The other thing about this experiment is that it relies on autocorrelation to avoid a need for very much power. Autocorrelation depends on knowing the departure times of the outgoing laser energy and accumulating many samples at the expected return time, which is predictably and reliably related to the transmission time. This integration technique produces a clear return signal that would be lost in noise if only one sample were collected.
In principle, if someone was patient and had good enough equipment, one could detect the reflection of a very small outgoing power level by collecting hundreds or thousands of samples. Autocorrelation is an amazing technique.
