Optical fiber sensors are being pursued in developments for sensing a variety of physical parameters including acoustic pressure, temperature, magnetic fields, acceleration and rate of rotation. Optical fiber devices are known in the patented literature for measuring electric current in a conductor by passing light through an optical fiber disposed in the magnetic field surrounding the conductor. Such devices have commonly employed the principle known as the Faraday effect where a beam of plane polarized light propagating longitudinally through an optical fiber in the presence of a magnetic field is caused to rotate (twist) about its longitudinal axis to an extent dependent upon its path length and magnitude of the magnetic field. The extent of rotation is proportional to the current carried by the conductor.
Presently used methods of detecting perturbations in the earth's magnetic field require cryogenic temperatures for increased sensitivity. Obviously, this involves complex supporting apparatus and high operating costs.
The possibility of detecting weak magnetic fields by magnetostrictive perturbations of optical fibers was proposed in Optics Letters, Vol. 5, No. 3, March 1980. An invention according to that idea discussed therein is covered in U.S. Patent application Ser. No. 223,635 filed Jan. 9, 1981 entitled "Magnetostrictive Optical Fiber Cable and Magnetic Field Detector and Method Thereof". That invention relates to providing a magnetostrictive metal in direct physical contact with a light transmitting optical fiber cable for effecting light transmission through the cable in response to a magnetic field. Tests conducted in the proposed detection scheme as well as other arrangements are reported by the authors in Electronics Letters, May 22, 1980, Vol. 16, No. 11, pp. 408-409. The basic principle of that sensor's operation is associated with the measurement of longitudinal strain induced in an optical fiber by a magnetostrictive material. Direct physical contact between a magnetostrictive metal jacket and the optical fiber may interfere with the glass surface and causes light transmission losses.
In order to strain a longitudinal section of the fiber to change its optical path length for increased sensitivity, it is desirable that the surrounding jacket have a high magnetostrictive constant and thick walls which cover a substantial length of the optical fibers. Unfortunately, these desirable characteristics are not possible in one embodiment. Flexibility of the fibers, for example, is greatly reduced. Direct application of the metal to the optical fiber surface if not done properly destroys the surface integrity of the fiber (either core or cladding) resulting in light leakage. Furthermore bending of a metal jacket which has been applied directly to the optical fiber induces optical losses in the fiber known as microbending losses. As the jacket is conformed, surface strains change the refractive index and undulations are induced in the fiber surface. These conditions interfere with light transmission and causes leakage. These and other shortcomings present in the prior art are overcome by the present invention.