Klaus Bohnert

Temperature and vibration insensitive fiber-optic current sensor

A robust interferometric fiber-optic current sensor with inherent temperature compensation of the Faraday effect is presented. Sensor configurations based on Sagnac and polarization-rotated reflection interferometers are considered. The sensing fiber is residing and thermally annealed in a coiled capillary of fused silica. The capillary is embedded in silicone within a ring-shaped housing. It is theoretically and experimentally shown that the temperature dependence of the birefringent fiber-optic phase retarders of the interferometers can be employed to balance the temperature dependence of the Faraday effect (0.7/spl times/10/sup -4///spl deg/C). Insensitivity of the sensor to temperature within 0.2% is demonstrated between -35/spl deg/C and 85/spl deg/C. The influence of the phase retarders on the linearity of the sensor is also addressed. Furthermore, the sensitivity to vibration of the two configurations at frequencies up to 500 Hz and accelerations up to 10 g is compared. High immunity of the reflective sensor to mechanical perturbations is verified.

Fiber-Optic Current Sensor for Electrowinning of Metals

A highly accurate 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, zinc, magnesium, and others. The sensor offers significant advantages with regard to performance and ease of use compared to state-of-the-art Hall-effect-based current transducers. The sensor makes use of the Faraday effect in an optical fiber loop around the current-carrying bus bars. A novel scheme of a polarization-rotated reflection interferometer and fiber gyroscope technology is used to measure the magneto-optic phase shifts. An appropriate technique has been developed for packaging the sensing fiber in a flexible strip of fiber-reinforced epoxy for loop diameters of up to several meters. Sensor accuracy and repeatability are well within 0.1% over a wide range of currents and temperatures. The sensor calibration is valid, regardless of the given magnetic field distribution, and remains stable under repeated manipulation of the flexible sensing strip.

Highly accurate 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 current measurement for process control and protection in the electro-winning industry, for example at aluminum smelters. The sensor offers significant advantages with regard to performance and ease of installation compared to state-of-the-art Hall effect based current transducers. The sensor exploits the Faraday effect in an optical fiber and measures the path integral of the magnetic field along a closed loop around the current-carrying bus bars. The differential magneto-optic phase shift of left and right circular light waves propagating in the fiber is detected by means of a novel polarization-rotated reflection interferometer. Fiber gyroscope technology is employed for signal detection and processing. The fiber is packaged in a flexible strip of fiber re-enforced epoxy, which can be installed without opening the current-carrying bus bars. Subsequent re-calibration is not necessary. The sensor achieves accuracy within /spl plusmn/ 0.1% over a wide range of currents and temperatures.

Fiber-optic voltage sensor for SF6 gas-insulated high-voltage switchgear.

We present an optical-fiber voltage sensor for 170-kV gas-insulated high-voltage switchgear. The sensor is based on the converse piezoelectric effect of quartz. The full voltage is applied to a cylinder-shaped quartz crystal. The resulting alternating piezoelectric deformation of the crystal is sensed by an elliptical-core dual-mode fiber, which is wound onto the circumferential crystal surface. The fiber is interrogated by low-coherence interferometry. We address the dielectric design of the sensor and verify its dielectric reliability under ac overvoltages and lightning impulse voltages. We then investigate the sensor performance, including accuracy, dynamic range, bandwidth, and temperature dependence.

Fiber optic voltage sensor for 420 kV electric power systems

We present an optical fiber voltage sensor for 420 kV electric power lines. The sensor exploits the converse piezoelectric effect of quartz and measures the voltage by a line integration of the electric field. The alternating voltage is partitioned to a series of four cylinder-shaped quartz crystals, which are embedded in polyurethane resin within a 3.2-m long insulator tube of fiber reenforced epoxy. The alternating piezoelectric deformations of the crystals are sensed by a common elliptical-core dual-mode fiber, which is wound onto the circumferential crystal surfaces. The fiber is interrogated using low coherence interferometry. We determine the dielectric design of the sensor from a numerical analysis of the electric field distribution within and in the vicinity of the sensor. We experimentally verify the dielectric reliability under ac overvoltages up to 520 kV root mean square (rms) and lightning and switching impulse voltages up to 1425 and 1050 kV, respectively. Further, we investigate the sensor performance including accuracy, dynamic range, bandwidth, and temperature dependence.

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.

Polarimetric fiber laser sensor for hydrostatic pressure

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 degrees offset in the core orientation, compensates for temperature-induced phase shifts. The dispersion in the birefringent fiber Bragg grating reflectors is employed to remove the near degeneracy of the polarization mode beat frequencies of a given order and to improve substantially the resolution of the sensor to a few parts in 10(6) of the free spectral range. Further investigations address the effect of the fluid on the integrity of the fiber, the influence of various fiber coatings on the sensor response, and the intrinsic stability of erbium-doped and undoped sensing fibers under fluid pressure.

Mechanical and optical reliability of fiber Bragg grating strain and temperature sensors at high temperature

The focus of this paper are performance and mechanical and optical reliability of fiber Bragg gratings as stress/strain and temperature sensors in high temperature applications for extended periods of time. However, not a particular sensor application will be considered but functionality and reliability of fiber Bragg gratings under this condition will be investigated.

Fiber-optic current sensor with self-compensation of source wavelength changes

We demonstrate a method for self-compensation of scale factor changes of an interferometric fiber-optic current sensor caused by source wavelength shifts, e.g., due to changes in source temperature or drive current. An adequately tailored fiber-optic retarder in the optical circuit introduces wavelength-dependent mixing of the orthogonal polarization modes of the sensor. The resulting change in scale factor balances the variation of the Faraday effect with wavelength. The wavelength dependence of the sensor is suppressed by more than an order of magnitude to <0.2% over wavelength spans of at least 10 nm around 1305 nm. The retarder is designed as an athermal device for operation between -40°C and 80°C.

Dispersion effects in a highly birefringent fiber laser sensor with fiber Bragg grating reflectors

The effects of dispersion in the highly birefringent fiber Bragg grating reflectors of a polarimetric elliptical-core fiber laser sensor are experimentally and theoretically investigated. The laser sensor is designed to measure fluid pressure. It is experimentally and theoretically shown that the wavelength-dependent phase shifts in the gratings can be employed to remove the near degeneracy of the polarization mode beat frequencies of a given order, which substantially improves the resolution of the sensor. A resolution of a few parts in 10(6) of the free spectral range of the laser is demonstrated.