Hubert Braendle

Field test of interferometric optical fiber high-voltage and current sensors

Optical fiber sensors are of considerable interest to the electric power industry. Particularly attractive features as compared to conventional instrument transformers include the inherent galvanic isolation of the sensor head from ground potential, less sensitivity to electromagnetic interference, smaller size, and higher safety.

Fiber-optic dc current sensor for the electro-winning industry

A fiber-optic current sensor for direct currents up to 500 kA is presented. Applications include the control of the electrolysis process for the production of metals such as aluminum, copper, magnesium, etc. The sensor offers significant advantages with regard to performance and ease of installation compared to state-of-the-art Hall effect based current transducers. A novel scheme of a polarization-rotated reflection interferometer and fiber gyroscope technology is used to measure the magneto-optic phase shift. A new technique has been developed for packaging the sensing fiber in a flexible strip of fiber re-enforced epoxy for coil diameters up to several meters. The sensor can be installed without opening the current-carrying bus bars. Subsequent re-calibration is not necessary. Accuracy is within ±0.1% over a wide range of currents and temperatures.

Investigations on the stability of polarimetric fiber laser sensors in harsh environments

The stability of a polarimetric Fabry-Perot fiber laser sensor for fluid pressure up to 100 MPa is investigated. The fluid acts on one of two elliptical-core fiber sections in the laser cavity producing a shift in the differential phase of the two orthogonal polarization modes and thus a variation in the beat frequencies of the corresponding longitudinal laser modes. The second fiber section, with a 90-degree offset in the core orientation, compensates for temperature-induced phase shifts. Dispersion in the birefringent fiber Bragg grating reflectors is employed to enhance the resolution of the sensor to a few parts in 106 of the free spectral range. Investigations on sensor stability address the effect of the fluid on the integrity of the fiber, creep caused by various types fiber coatings, as well as the intrinsic stability of erbium-doped and undoped sensing fibers under pressure and temperature changes. It is found that moisture-free silicone oil does not cause any fiber deterioration even in case of a bare fiber. Silicon nitride and gold coatings (1 mm thick) cause significant signal drift, whereas no drift is observed for carbon coatings. Highly doped elliptical-core sensing fiber exhibited drift in the differential optical phase while un-doped fiber did not.