Abstract: An intruder alarm for protecting the perimeter of an area utilizes a multimode optic fiber as the deformable sensing element, wherein a length of multimode optic fiber is buried in the ground of an area or perimeter to be protected. As coherent light from a laser is directed through a length of optic fiber, the output light pattern therefrom is speckled. When a deformation of the fiber occurs, even a small amount, the speckle pattern changes and is detected electronically indicating that a disturbance has taken place.

Abstract: This invention is directed to a laser-fiber optic interferometric strain gauge. This is a strain gauge that can be read out by means of fiber optics, which has no power other than optical at the strain gauge. Laser light is directed into two single mode fibers, which fibers are attached to a supporting member the strain of which is to be measured. The ends of the fibers are brought into close proximity and the light coming out of the two fibers interferes. When the supporting member is strained the fringes move giving a measure of the strain.