In optics, there are more to it that meets the eye. If safety is your concern, then a collimator (posted by GoForSmoke) is good but you might have to stay away from that somehow. A double collimation might suffice but still way off from real lazers. Also I don't know if there are ways for the particles to be seen but during my experiments, the light can only be seen on the object that might intersect the optical axis. Regarding that, using collimated LEDs defeats your purpose of the tripwire where light particles are seen. Also, you might want to also replace your mirrors with a high reflectance one, probably at around 93% reflectance, tested at 45° angle, passed endurance test at temperature same as your lazer light temperature.