High frequency current sensors using the Faraday effect in optical fibers

Abstract

This study investigates the high frequency response of Faraday effect optical fiber current sensors that are bandwidth-limited by the transit time of the light in the fiber. Mathematical models were developed for several configurations of planar (collocated turns) and travelling wave (helical turns) singlemode fiber sensor coils, and experimental measurements verified the model predictions. High frequency operation above 500 MHz, with good sensitivity, was demonstrated for several current sensors; this frequency region was not previously considered accessible by fiber devices. Planar fiber coils in three configurations were investigated: circular cross section with the conductor centered coaxially; circular cross section with the conductor noncentered; and noncircular cross section with arbitrary location of the conductor. Centered, circular coil sensors have a frequency response easily characterized by light transit time. In this category, a 20-turn, 6.4mm diameter annealed fiber sensor was tested up to 1 GHz. It has a 3dB bandwidth of approximately 230 MHz and a sensitivity-bandwidth product of approximately 0.8 MHz-°/A. Both noncentered coils and noncircular coils exhibit a resonant response at the higher frequencies that mimics the response of tapped, recirculating fiber delay lines. Coils with large eccentricity in the cross section feature the same sensitivity in the resonance bands as they do at low frequencies. This frequency response was successfully modeled using a point interaction between the magnetic field and the light in the fiber. The helical travelling wave fiber coils were immersed in the dielectric of a coaxial transmission line to improve velocity phase matching between the field and light. Three liquids (propanol, methanol, and water) and air were used as transmission line dielectrics. Complete models, which must account for liquid dispersion and waveguide dispersion from the multilayer dielectric in the transmission line, were developed to describe the Faraday response of the travelling wave sensors. Large enhancements in current sensor bandwidth are possible without a loss in system sensitivity. A maximum bandwidth of approximately 300 MHz was achieved in a water-dielectric sensor cell containing a 14-turn coil with a 3 cm diameter and a 3 cm helical pitch. This sensor has a sensitivity-bandwidth product of approximately 1.08 MHz-°/A. Other travelling wave current sensors with potentially greater Faraday sensitivity, wider bandwidth and smaller size are investigated using the theoretical models developed for the singlemode fiber coils.