[3][4]: 2 [5] The fiber-optic current sensor uses an interferometer to measure the phase change in the light produced by a magnetic field.
[4][5] The optical phase detection circuit, light source and digital signal processor are contained within the sensor electronics; this technology has been proven in highly demanding applications such as navigation systems in the air, on land and at sea.
[7][4]: 5 Interferometric fiber optic current sensors (FOCS) employ circularly polarized light traversing a closed loop path around an electrical conductor's current-generated magnetic flux, which reflects off a mirror and experiences a reciprocal phase shift as the refractive index, and effective path length, is modulated by the presence of a magnetic field which optically induces circular birefringence, and where the interference pattern relative to a reference wave form is a transduced optical intensity value corresponding to the current magnitude.
Such configurations are vulnerable to acoustic perturbations in the fiber optical cables, as changes to the linear birefringence of the fiber cable causes additional phase shifts between the orthogonally polarized modes which must be of equal magnitude to generate circular polarization, as an exact quarter wave displacement between the fast and slow axis modes is required for a circular polarization state, and additional phase shifts in the sensing network cause the circularly polarized measurement photons, which experience a phase shift in the fiber optic current sensing coil in proportion to magnetic field density, to degenerate to a random form of elliptical polarization, which degrades interference measurement abilities as the measurement and reference photon wave forms become non-coherent at the analyzer.
[8][9] In 2013, ABB introduced a 420 kV Disconnecting Circuit Breaker (DCB) that integrates FOCS technology replacing many conventional current transformers, thereby simplifying the engineering and design of the substation.