A laser fence is a mechanism to detect objects passing the line of sight between the laser

Laser

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

 source and the detector. Stronger lasers can potentially be used to injure someone or something passing the laser beam (see mosquito laser

Mosquito laser

The mosquito laser is a device invented by astrophysicist Lowell Wood to kill large numbers of mosquitoes to reduce the chance of people being infected with malaria. Mosquitoes can carry the blood parasite of the genus plasmodium, which causes malaria...

).

Sometimes laser fences are used in fiction with the ability to stop intruders by blocking or injuring them. It is a concept used frequently in videogames. The concepts of these fictional fences can be compared to other concepts like tractor

Tractor beam

A tractor beam is a device with the ability to attract one object to another from a distance. Since the 1990s, technology and research has labored to make it a reality, mostly at microscopic level. Less commonly, a similar beam that repels is called a pressor beam or repulsor beam...

or repulsor beams.