Abstract: An optical power limiter and switch, transparent at low light intensity and opaque at high intensity, is comprised of a pair of right triangular prisms separated by a liquid film whose refractive index changes in response to optical energy. A first or input lens focuses the light upon the liquid film at an angle less than the critical angle of total reflection of the prism-liquid interface and a second or output lens refocuses the energy to a detector through an aperture. When the intensity of the optical energy reaches a first predetermined level, the optical energy is defocused and diverted from the detector. When the intensity of the optical energy reaches a second predetermined higher level, the critical angle is achieved whereupon the optical energy is reflected away from the interface rather than through it.

Abstract: A polarmetric fiber sensor comprises a lead fiber (1) and a shorter length of sensing fiber (2). Both fibers comprise higly birefringent optical fiber having orthogonal eigen modes and are spliced together with their eigen modes mutually rotated by 45.degree.. The sensing fiber (2) has a reflective distal end (6). Linearly polarized light from a laser source (3) is launched into one mode of the lead fiber (1) and light reflected from the distal end of the sensing fiber is recovered from the other mode of the lead fiber via a beam splitter (7) and photodiode (8). The resulting electrical signal produced by the photodiode (8) corresponds to the modal birefringence of the sensing fiber (2) and changes in this birefringence are monitored by processing means (9) to detect changes in a physical parameter sensed by the sensing fiber.