Hubert Brändle

Distributed feedback fiber laser sensor for hydrostatic pressure

A polarimetric distributed feedback fiber laser sensor for the measurement of fluid pressure up to 50 MPa is investigated between room temperature and 250/spl deg/C. The isotropic pressure acts directly on the fiber, which eliminates the need of a mechanical transducer. A pressure resolution of about 20 kPa (0.2 bar) is demonstrated. Furthermore, the protection of the silica fiber against deterioration by the fluid and the thermomechanical effects of various fiber coatings on the sensor response are studied.

Interrogation of a Birefringent Fiber Sensor by Nonreciprocal Phase Modulation

The technique of nonreciprocal phase modulation, known from Sagnac-type interferometers, is adopted to interrogate the differential phase shift of the orthogonal light waves in a birefringent fiber sensor. The sensing section of the novel interferometer consists of a series of two birefringent fibers with balanced differential group delay. A key element is a 45°-Faraday rotator which swaps the polarization states of the orthogonal waves after their round trip through the two fiber sections. An integrated-optic birefringent phase modulator introduces the nonreciprocal phase modulation at the detector end of the sensor. The interferometer is applied to voltage sensing.

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.

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.

Nonlinearities in the high-current response of interferometric fiber-optic current sensors

The nonlinearities in the response of an interferometric fiber-optic current sensor associated with deviations of the light waves from perfect circular polarization are theoretically and experimentally investigated. The consequences for inherent temperature compensation of the Faraday effect by means of a non-90°-retarder are investigated for currents up to several 100 kA and temperatures between -40°C and 80°C. The results are of particular interest to sensors for high direct currents in the electro-winning industry where measurement accuracy to within ±0.1% is required up to 500 kA.

Towards commercial use of optical fiber current sensors

Summary form only given. Optical fiber current sensors for high-voltage electric power substations have been under investigation in the 1980s and 1990s. So far, however, a number of technical shortcomings such as unacceptable sensitivity to temperature and vibration as well as concerns about their long-term reliability have contributed to prevent extended commercial use. We report a sensor which, we believe, meets all important performance requirements. Two configurations of the sensor have been investigated. In the Sagnac configuration the magnetic field of the current induces a differential optical phase shift, proportional to the current. In the reflection configuration left and right circular polarizations are co-propagating in the coil and are reflected at the coil end.

Interrogation of birefingent fiber sensors using non-reciprocal phase modulation

A novel reflective fiber interferometer with non-reciprocal phase modulation adapted from Sagnac-type fiber interferometers is used to measure the differential optical phase shift in polarimetric fiber sensors. The sensitive part consists of two birefringent fiber sections with balanced differential group delay. A key element is a 45°-Faraday-rotator that swaps the polarization states of the orthogonal light waves after their round trip through the two fiber sections. An integrated-optic birefringent phase modulator at the detector end of the sensor introduces the non-reciprocal phase modulation. The application of the interferometer to voltage sensing is experimentally demonstrated.

Nonlinear Phenomena in the Response of Interferometric Fiber-Optic Current Sensors

The nonlinearities in the response of an interferometric fiber-optic current sensor associated with inherent temperature compensation of the Faraday effect are investigated at currents up to several 100 kA and temperatures between -40 and 80degC.

Theoretical and experimental investigations on the calibration stability of fiber-optic current sensors

A fiber-optic current sensor with flexibly packaged sensing fiber for direct currents up to 500kA is presented. Accuracy and calibration stability are well within 0.1%. Applications are in the electro-winning industry such as aluminum smelters.

Towards crystalline electro-optic fibers for high-voltage sensing

Single crystal Bi₄Ge₃O₁₂ fibers for high-voltage sensing are grown using the micro-pulling down technique. Optimum growth conditions are determined and the fibers characterized as to their optical properties and performance under voltage.