Be smart and get that laser out of there. If that beam reflects into an eye, you will end up in court on serious charges.
A phototransistor at the back of a tube restricts the view into a very narrow cone. If the view reflects or sources light into the detector and the some one or thing blocks it, there you have a tripwire.
I know that lasers are TV/movie fascinating. So is fire but I don't play with that, have people walk through it either.
You want to make a safe laser effect, look into interference patterns or onto the wall displays you can make reasonably safe.
Look up collimation, you can fake a laser beam using a led and optics that will be far brighter for the power used but not as full-power-retina-etch-focused as the laser.