Detecting a laser before it strikes doesn't make much real-world sense, then again this is a game we are talking about.
But, assuming you had a very, very short window that a laser strike was inbound. Your detectors work on the light speed principle, so the alert traveling to your gunner would be at the same speed. If you deploy sand automatically that shortens the time frame, but if there is a human who makes a decision and presses a button, well, by then the strike should already have either hit or missed.
But assuming it's automatic, another light speed signal has to travel to the launcher, which then ejects (mechanically or otherwise) the sand cannister, which then travels at a speed much less than light speed. The cannister explodes/detonates/spreads the sand somehow and that, too, is moving at much less than light speed. This means you have to detect the incoming strike seconds before it arrives - pretty good for something that has to work in the same physical universe as the ship firing it's lasers and trying to pinpoint where the target is going to be when the lasers arrive.
A better rule would simply be to have the player make a determination at the beginning of the phase whether or not they were going to deploy countermeasures (i.e. sand) and then deploy it.
Then again, it's a game, so a lot can be forgiven.