Robert Wüest

Influence of residual fiber birefringence and temperature on the high-current performance of an interferometric fiber-optic current sensor

A highly accurate reflective interferometric fiber-optic current sensor for alternating and direct currents up to 500 kA is investigated. The magnetic field of the current introduces a differential phase shift between right and left essentially circularly polarized light waves in a fiber coil wound around the conductor. Technology adopted from fiber gyroscopes is used to measure the current-induced phase shift. The sensor achieves accuracy to within ±0.1% over at least two orders of magnitude of current and for temperatures from -40 to 80°C with inherent temperature compensation by means of a non-90°-retarder. The paper analyzes the influence of key parameters on the sensor accuracy as well as linearity as a function of magneto-optic phase shift. Particularly, we consider residual birefringence in the sensing fiber and its effect on the high-current performance of the sensor as well as optimum parameters for the temperature compensation scheme. Applications of the sensor are in high-voltage substations and in the electrolytic production of metals such as aluminum.

Experimental and theoretical investigations of the high current regime of an interferometric fiber-optic current sensor

The influence of non-perfect circularity of the left and right circular light waves of a reflective interferometric fiber-optic current sensor is theoretically and experimentally investigated for currents up to several 100 kA and temperatures from -40 to 80degC. By control of crucial parameters and appropriate packaging of the sensing fiber accuracy and repeatability within plusmn0.1% is achieved. Applications of the sensor are in high-voltage substations and in the electrolytic production of metals such as aluminum.

Characterization of fiber wave retarders for interferometric fiber-optic current sensors

Fiber-optic wave retarders made from a short section of polarization-maintaining (pm) fiber are crucial components of interferometric fiber-optic current sensors for high-voltage substations [1]. Another potential application of such retarders is in chiral spectroscopy, e. g. for the detection of specific molecules. In a fiber-optic current sensor, as considered here, the magnetic field of the current introduces a differential optical phase shift between left and right circularly polarized light waves during their round trip through a fiber coil that encloses the current conductor (Faraday-effect). Closed-loop fiber gyroscope technology is used to measure the current-induced optical phase shift (Fig. 1). The phase retardation of the retarder which generates the circular waves and its temperature dependence affect the sensor scale factor and are therefore critical to the sensor performance. Using an appropriately designed retarder (Fig. 2) it is possible to compensate for the temperature dependence of the Faraday-effect [1] that is 0.7% per 100°C. The overall retardation can be substantially influenced by splice joints - especially if short beat length fiber is used as fusion splicing modifies the fiber birefringence in the proximity of the joints. Conventional methods to determine the phase and/or group birefringence of pm fibers [2–3] are generally applied to long and homogeneous fiber segments. The alternative method presented here is ideally suited for short fiber sections (of a few millimeters in length) acting as wave retarders and also accounts for the influence of splice joints.

Effects of thermal fiber annealing on the temperature compensation of interferometric fiber-optic current sensors

In this paper, we present an experimental and theoretical study on how thermal fiber annealing influences the temperature dependence of the interferometric fiber-optic current sensor. Such sensors measure the current-induced phase shift (Faraday effect) between left and right circularly polarized light waves in a reflective fiber coil around the current conductor. The fiber-optic phase retarder at the coil entrance that generates the (nearly) circular light waves may also serve to compensate for the temperature dependence of the Faraday effect. Thermal annealing serves to remove disturbing bend-induced fiber birefringence in the sensing coil - but it can also affect the retarder parameters. We investigate, to our knowledge for the first time, how thermal annealing affects the optical phase retardation and its temperature dependence of elliptical-core fiber retarders and show how the retarder parameters must be set prior to annealing in order to achieve a temperature-independent sensor after annealing.

A Study on Different Types of Fiber Coils for Fiber Optic Current Sensors

We consider an interferometric fiber optic current sensor with a fiber coil operated in reflection and compare three different techniques to prepare the coil: thermally annealed coils, stress-free packaging of a bare low birefringent fiber in a fused silica capillary, and coils from highly birefringent spun fiber. In particular we theoretically and experimentally investigate how the fiber retarder that generates the near left and right circular light waves in the sensing fiber must be prepared for temperature compensation of the Faraday effect in the three cases. All three methods can achieve accuracy within ±<0.2% over an extended temperature range but they considerably differ in their practical challenges.

Thermal tuning of fiber quarter-wave retarders for temperature compensation of fiber-optic current sensors

The fiber retarder of an interferometric fiber-optic current sensor is fine-tuned by controlled local heat treatment for temperature compensation of the Faraday effect. The retardation is determined from sensor scale factor measurement.

Reflective Polarimetric Fiber-Optic Thermometer

The design and the realization of a cost effective reflective polarimetric fiber optic thermometer are discussed for several applications. The temperature dependent birefringence of a polarization maintaining fiber is used to deduce the sensor head temperature from measured polarization intensities. Measurements from a fabricated and packaged prototype show that the sensor features a non-ambiguous temperature range of >160°C and an accuracy of ±2°C.