Patent ID: 8461822

Claim:
A fiber optic current sensor or magnetic field sensor comprising: a sensing fiber exposed to a magnetic field of a current I, wherein said sensing fiber is looped in a sensor coil plane, and wherein said sensing fiber has a Verdet constant V, at least one retarder arranged between a polarization-maintaining fiber and said sensing fiber for converting light between linear and elliptical polarization, wherein a principal axis of said polarization-maintaining fiber is rotated with respect to a normal of said sensor coil plane by an angle β and wherein said retarder introduces a retardation ρ=π/2+ε between light waves polarized along its principal axes, with ε being an additional, non-zero phase shift; and a control unit generating a signal proportional to a phase shift Δφ, which signal is proportional to 4 S(ε, δ, β, φ F )·V·N·I=4 S(ε, δ, β, φ F )·φ F , wherein N is a number of loops of said sensing fiber, δ is a linear birefringence induced phase shift of said sensing fiber, φ F is a Faraday phase shift and S is a scale factor, wherein a normalized derivative of S with respect to temperature T is Z ⁡ ( ɛ ⁡ ( T ) , δ ⁡ ( T ) , β , φ F ) = 1 S ⁢ ∂ S ⁡ ( ɛ , δ , β , φ F ) ∂ T wherein β, ε and Q = 1 ρ ⁢ ∂ ρ ∂ T are such that  Z + 1 V ⁢ ∂ V ∂ T  < t 1 is fulfilled over a range of values of φ F and for a non-zero value of the linear-birefringence-induced phase shift δ and its given temperature dependence P = 1 δ ⁢ ∂ δ ∂ T , at least at a temperature T 0 , in particular in a center of an operating temperature range of the sensor, with t 1 being a first threshold value smaller than 7*10 −5 1/K, in particular smaller than 2*10 −5 1/K.