The present invention relates generally to magnetometers and more particularly to optical fiber interferometric magnetometers having temperature compensation.
It is well known that optical fiber sensors can be utilized in the detection and measurement of magnetic fields. In the typical device, light is directed into the sensor and is split equally into two fiber arms. One arm is merely an optical fiber and, thus, acts as a reference. The other arm, a sensor arm which can be an optical fiber either clad with a magnetostrictive material or wrapped around a magnetostrictive core, is perturbed by the magnetic field of interest.
When the light from these two arms is recombined onto the surface of a detector, the resulting current depends on the light intensity, which in turn depends on the relative phase of the light wave in each arm. As the local magnetic field changes in value, the dimensions and index of refraction of the magnetostrictive arm also change resulting in a phase change which further causes a change in the intensity of the light.
A major problem with these sensors, however, is that small temperature changes in either arm can cause significant changes in the phase of light independent of any magnetic field change. This is because temperature changes cause the length of the arms and their indices of refraction to change, and as a result, temperature fluctuations cannot be readily discerned from magnetic field fluctuations.
For example, for nickel clad silica fibers, it has been shown that the relationship of magnetic field change to temperature change is approximated by the equation: .DELTA.T(.degree.C.)=0.25H(Oe). This means that a one oersted change in the magnetic field yields the same phase change as a 0.25.degree. C. change in temperature (see Heaton, "Thermal Straining in a Magnetostrictive Optical Fiber Interferometer", Applied Optics, Vol. 19, No. 22, Nov. 15, 1980, pp. 3719-3720). Therefore, when measuring very small changes in a magnetic field, correspondingly smaller temperature changes become increasingly significant.