Review of the present status of optical fiber sensors

This paper provides a thorough review of state-of-the-art current sensing techniques. It catalogues the current sensors according to the underlying physical principle in order to point out their strengths and weaknesses.

Current Sensing Techniques: A Review

This paper describes the use of wavelet transforms for analyzing power system fault transients in order to determine the fault location. Traveling wave theory is utilized in capturing the travel time of the transients along the monitored lines between the fault point and the relay. Time resolution for the high frequency components of the fault transients, is provided by the wavelet transform. This information is related to the travel time of the signals which are already decomposed into their modal components. The aerial mode is used for all fault types, whereas the ground mode is used to resolve problems associated with certain special cases. The wavelet transform is found to be an excellent discriminant for identifying the traveling wave reflections from the fault, irrespective of the fault type and impedance. EMTP simulations are used to test and validate the proposed fault location approach for typical power system faults.

Fault location using wavelets

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.

Temperature and vibration insensitive fiber-optic current sensor

The reflection and transmission characteristics of a high-birefringence fiber loop mirror (HiBi-FLM), which is composed of a standard fiber coupler and one-section or multisection high-birefringence fibers (HBFs), are analyzed and discussed in detail. Theoretical reflectivity and transmissivity expressions for HiBi-FLMs with one-, two-, and three-section HBFs were obtained. The procedure for calculating reflectivity and transmissivity for HiBi-FLMs with n-section HBFs is given. Experimental results have verified the theoretical model. The basic characteristics of the one-section HiBi-FLM when strain and high temperature are applied to HBFs were analyzed and investigated theoretically and experimentally. The experimental results are in good agreement with the theoretical analysis. Furthermore, a strain– temperature sensor that makes use of those characteristics, which is new for applications of HiBi-FLMs, has been proposed and demonstrated.

High-birefringence fiber loop mirrors and their applications as sensors.

The authors demonstrate for the first time a near shot noise limited in-line Sagnac interferometer current sensor. It is shown to have a number of advantages over the optical current sensors based on polarimetric Faraday and Faraday/Sagnac loop interferometer topologies, including lower sensitivity to environmental disturbances, less demanding optical components, and easy installation. Its application to power system measurements is also detailed.

In-line Sagnac interferometer current sensor

We analyze scale factor errors associated with integrating imperfect quarter-waveplates into loop and in-line Sagnac interferometer fiber-optic current sensors. We show that relatively large imperfections in the quarter-waveplates can be tolerated in the loop version when the birefringence axes of the two quarter-waveplates are oriented 45/spl deg/ with respect to each other. For the in-line version, we demonstrate an electronic signal processing scheme that desensitizes the scale factor to imperfections in the quarter-waveplate.

Imperfect quarter-waveplate compensation in Sagnac interferometer-type current sensors

This paper presents theoretical and experimental results showing the effects of linear and circular birefringence on the sensing coil of an in-line Sagnac interferometer current sensor. Linear birefringence induces local scale factor variations along the length of the sensing coil which can be suppressed with circular birefringence. We show that by winding the sensing coil with tension in a helix configuration, these variations may be effectively reduced to zero for ordinary single mode fiber. In our opinion, this method of implementation overcomes the need to use annealed fiber in the sensing coil.

Elimination of birefringence induced scale factor errors in the in-line Sagnac interferometer current sensor

The signal-to-noise ratio (SNR) of fiber-optic gyroscopes (FOGs) is directly impacted by the optical power coupled to the interferometric circuit. The optical intensity noise produced by the optical broadband source becomes the major component of the total noise at the output when the optical power reaching the detector is more than a few tens of microwatts. This work shows the implementation of a signal processing scheme capable of promoting reduction of the source intensity noise around the proper frequency of birefringent FOG, where the optimum signal demodulation takes place, yielding a better SNR. The total SNR has been enhanced 6.6 dB by means of a 20.4 dB reduction in the noise component associated only with the source intensity noise.

SNR enhancement of intensity noise-limited FOGs

A fiber optics sensor (10) and method for attaining accurate measurement are provided. A polarization maintaining optic fiber (22) forms a linear optical path. An optical element (40) is used to convert linearly polarized light waves to circularly polarized light waves which propagate along the optical path and pass through a sensing medium (32). Because of external stresses and disturbances, the optical element (40) introduces light of the wrong state of polarization into the optical path. The result is a scale factor error in the measurement and an extra incoherent D.C. light detected at the detector (46). The presence and magnitude of the extra incoherent D.C. light is used to provide a normalizing factor to compensate for the scale factor error.

James N. Blake

Papers

In-line Sagnac interferometer current sensor

Imperfect quarter-waveplate compensation in Sagnac interferometer-type current sensors

Elimination of birefringence induced scale factor errors in the in-line Sagnac interferometer current sensor

SNR enhancement of intensity noise-limited FOGs

Fiber optics apparatus and method for accurate current sensing