Abstract

Recently, a travelling wave Faraday effect fiber current sensor was demonstrated that has increased bandwidth without a loss in detection sensitivity.¹ This device consists of a helically shaped coil of single-mode optical fiber immersed in the dielectric of a coaxial transmission line. Attempts are made to phase match the velocity of the light propagating in the fiber with the relative velocity of the magnetic wave propagating along with the conducted current by increasing the pitch of the fiber helix and increasing the relative permittivity of the dielectric material. Ideally, an infinite bandwidth is possible, but practically, bandwidth is limited by the frequency- dependent properties of the dielectric and the transmission line. Mathematical models of the frequency-dependent Faraday rotation in a travelling wave sensor system have been developed that account for the modal structure of the coaxial line electromagnetic fields, the dispersion of the relative permittivity of the dielectric, and the effects of the optical fiber leads that carry light to and from the fiber coil. Model results are in good agreement with experimental measurements made on several configurations of current sensors. These include systems using high dielectric constant liquids where large absorption occurs at frequencies above a few hundred megahertz. Predictions based on the models indicate that current sensing well beyond 1 GHz is possible by using travelling wave Faraday effect fiber devices.

© 1992 Optical Society of America