Faraday Current Sensor Research Articles

Nonplanar fiber-optic sensing head for the compensation of bending-induced birefringence in Faraday current sensors.

We demonstrate the compensation of bending-induced linear birefringence in single-mode fibers coiled in a nonplanar path by alternating orthogonal bending planes. This effect can be applied for the construction of birefringence-free fiber coils in Faraday sensor heads (e.g., in current sensors) to improve their sensitivity. Validation experiments are presented.

Faraday current sensor using space-variant analyzers

A Faraday current sensor using spatially inhomogeneous analyzers is presented. In the proposed optical architecture, the light waves coming from the sensor head travel in free space reaching a space-variant analyzer, and then they are acquired by a (one-or two-dimensional) photodetector array. This allows a full determination of the Faraday rotation induced by the electrical current, even in the case of a nonideal optical system (e.g., analyzer with poor polarization ratio). The proposed method resembles well-known techniques of phase-shifting interferometry. Validation experiments using linearly variant and azimuthal (radial) analyzers are presented and different detection schemes are compared.

Magnetic Crosstalk Minimization in Optical Current Sensors

We demonstrate that the magnetic crosstalk-induced errors associated with point-type optical (Faraday) current sensors working in three-phase electric systems may be minimized by properly designing the geometry of the system. We propose a novel architecture in which the optical path inside the magnetooptical material is parallel to the direction of the neighboring conductors, except in a small loop around the sensor head. This renders the crosstalk effect at least one order of magnitude smaller than in conventional Faraday sensor architectures.

Evolution of the polarization state in material media with uniform linear birefringence and an applied nonhomogeneous external magnetic field: application to bulk-type Faraday current sensors

In this work we find the exact solution for the evolution of the polarization state of a light wave that propagates in a material medium with uniform linear birefringence and a nonuniform external magnetic field. The obtained results could be used to improve the precision of existing Faraday current sensors.

Faraday current sensor with temperature monitoring

We present a novel optical current sensor based on the Faraday effect that incorporates a temperature monitoring system. The monitoring element is a temperature-dependent birefringent plate placed at the Faraday sensor head. Measurement of the plate retardation permits compensation for the temperature dependence of the Verdet constant. Validation experiments are presented and discussed.

Development of a Portable Fiber-Optic Current Sensor for Power Systems Monitoring

The paper presents a portable fiber-optic current sensor (FOCS), based on the Faraday effect, with a magnetic concentrator. Both the optical sensing head and electronic processing block are illustrated. A detailed experimental study to confirm the performance of the device is also reported. According to the measured values of ac rms current up to 1 kA, a calibration procedure was performed. The paper provides an analysis of the results obtained for various conductor displacements within the concentrator. The well-known temperature dependence of the Faraday current sensor and its influence on the measurement accuracy are tested by means of a special double-layer thermal insulated chamber. The calibrated and characterized FOCS is applied for harmonic analysis of the current. The results clearly illustrate the nature of the sensing process and demonstrate odd-order harmonics presence, as predicted by the mathematical model. The paper indicates that the developed device is suitable for the power systems monitoring.

Faraday plasma current sensor with compensation for reciprocal birefringence induced by mechanical perturbations

A Faraday fiber-optic current sensor was employed to measure the tokamak plasma current. In order to decrease the influence of mechanical perturbations on the sensor sensitivity, a two-pass optical scheme with a variable Faraday mirror at the fiber end is proposed. A decrease, by two orders of magnitude, in the influence of the linear birefringence produced by an external piezoceramic fiber modulator was experimentally observed.

Improved method for Faraday current sensor data processing

There are several practical problems that have made difficult the prompt application of Faraday current sensors in industrial environments. Two of them are the multiplicative noise generated by mechanical vibrations and the normalization of the output signal. In this paper we propose a logarithmical processing and a posterior bandpass filtering of the sensor data in order to solve simultaneously both issues. Experimental results demonstrating the viability of the procedure and a comparison with standard processing methods are presented.

White LED based Faraday current sensor using a quartz wavelength encoder

The use of polychromatic light with chromatic modulation has been shown to provide a different approach to Faraday current sensing. In this contribution the use of purely white LEDs along with a quartz rotary encoder and a multiple reflection Faraday sensing element is described for realizing a practical form of a chromatic modulation Faraday current sensor. It is shown that such a sensor embodiment leads to a system sensitivity, for a chromatic system, which is approximately a factor of three times higher than previously reported (0.65 nm A −1 compared with 0.19 nm A −1).

Elliptical polarization effects in a chromatically addressed Faraday current sensor

Optical polarization processes in a parallel-sided glass element used in a Faraday rotation current sensor have been considered. In such sensors the path length necessary to produce sufficient rotation of the plane of polarization is produced by a multiplicity of reflections within the glass element. It is shown that such reflections induce ellipticity of polarization and that this affects the current-sensing performance of the sensor. Two reflection cases, corresponding to total internal reflections at a glass-air interface and reflections by aluminium-coated surfaces, are considered. The latter are shown to produce higher optical attenuation but a lower degree of elliptical polarization. The implications of the induced elliptical polarization in relation to chromatically modulated polychromatic light are considered. It is shown that the resolution of the Faraday sensing is improved by minimizing the ellipticity of the polarization with the aluminium-coated reflections. However a greater dynamic range and signal strength may be achievable with the total internal reflection element.

Faraday current sensing employing chromatic modulation

Faraday current sensors using a variety of sensing elements have been investigated extensively for their high sensitivity as well as other advantages [G.L. Lewis et al., Proc. IEE Conf. on The Reliability of Transmission and Distribution Equipment, 1995; Y.N. Ning et al., Optics Lett. 16 (1991); C.M.M. van den Tempel, Appl. Optics 32 (1993)]. Concurrently chromatic modulation techniques have been investigated at the University of Liverpool for use with optical fibre sensors of different types [N.A. Pilling, Ph.D. Thesis, 1992; M.M. Murphy, Ph.D. Thesis, 1991] including Faraday current sensing, for overcoming difficulties with non-referenced intensity modulation systems. In this contribution a brief discussion of the scope of chromatically based Faraday current sensing with particular regard to electric power transmission and distribution industries is given. A novel sensor based upon a Faraday glass block in combination with a BSO crystal in the sensing element is described. The sensor takes advantage of the natural gyrotropy of the BSO which conversely has been previously regarded as a disadvantage. The experimental results obtained indicate that this method offers a novel approach to improving system sensitivity. The extension of the approach from a simple bench top demonstrator to real power systems deployment is also discussed.

Faraday current sensors and the significance of subtended angles

A series of experiments, involving measurements of Faraday rotation for light beams in linear and closed-loop optical current sensors, confirms that the degree of Faraday rotation of the polarization azimuth of a linearly polarized light beam, travelling in the vicinity of a current-carrying wire, is a linear function of the angle subtended at the wire by the beam. The experiments have been used to demonstrate the magnitudes of the errors that occur for different path geometries in a triangular closed-loop bulk-glass current sensor. The results of this work should enable geometrical design criteria for bulk-glass current sensors to be more readily established in future.

Tuning a bulk-glass optical current sensor by controlling conditions external to its reflecting surfaces

In a bulk-glass triangular Faraday current sensor, the stringent conditions required to provide critical-angle reflection at each reflecting surface are difficult to achieve. High tolerances for both the optical geometry of the sensor and the wavelength of the laser source are required. Here we present a method of essentially tuning the sensor for optimum operation by matching the angle of reflection at each surface to the wavelength of the laser source. This is done by controlling the gaseous environment external to each of the reflecting surfaces. The technique is demonstrated using sulphur hexafluoride (SF 6), which has the advantage of a relatively large refractive index.

A single fibre-optic down-lead Faraday current sensor

We demonstrate a single-down-lead common-mode rejection scheme for a bulk optic Faraday current sensor that can eliminate optical noise induced by fibre-link vibration. The new configuration is based upon the creation of two temporally delayed signals at the sensor level. One signal is encoded with the Faraday signal and the optical noise. The other signal contains the noise component only. By using a pulsed laser diode at the emitting point and electric delays at the receiving point, the two signals are combined differentially and time gated to eliminate the optical noise. As the delays at the sensor level cannot be too large due to the finite size of bulk optics configurations, it is neccessary for the laser, photodetector and electronics to work with nanosecond pulses. Methods to alleviate the constraints raised by the needs to process nanosecond pulses are discussed.

Vibration immunity for a triangular faraday current sensor

We demonstrate a common-mode rejection scheme for a bulk-optic triangular Faraday current sensor that can eliminate optical noise induced by fiber-link vibration. The noise floor before applying common rejection was about 30 dB for a 100Å Faraday signal and transceiver vibration levels of approximately 30 g. This was reduced to about 60 dB for the same vibration levels. The sensor's exploitation of Ampere's circuital law is also demonstrated.

A common-mode optical noise-rejection scheme for an extrinsic Faraday current sensor

We demostrate a common-mode rejection scheme for a Faraday current sensor that can eliminate optical noise induced by fibre-link vibration.

Thermodynamic phase noise in fibre interferometers

Phase noise due to thermodynamic fluctuations in the optical path length is evaluated in this paper for basic fibre interferometers. In Mach-Zehnder and Michelson interferometers, where the temperature phase fluctuation (TPF) is that intrinsic to the fibre, this noise has been reported to be comparable to shot noise and a possible limit to sensor sensitivity in practical cases. We show that in Sagnac interferometers, used in fibre gyro and in Faraday current sensors, the TPF noise is decreased with respect to that intrinsic to the fiber because propagation in the same optical path leads to a correlation of the phase fluctuations. In addition, we show that in Fabry-Perot and ring resonators, as multiple reflections increase the effective path length, TPF noise is enhanced and can be dominant over shot noise even for moderate fibre lengths.

Improving the sensitivity of a Faraday current sensor by varying its operating point

By varying the cross polarization angle between the input and output polarizer of a bulk glass rod sensor that utilizes the Faraday effect to measure currents, we show both experimentally and theoretically that the signal-to-optical-noise ratio can potentially be considerably improved. The advantages and limitations are described and possible applications discussed.

A Faraday current sensor using a novel multi-optical-loop sensing element

The design and the performance of an optically based, Faraday current sensing system employing a novel sensing element, using the so-called 'complementary reflections' scheme, is reported. This new design not only increases the overall sensitivity of the device by a factor of three, but also makes it easy to align the current measuring element in the system. With the use of this new optical element, a sensitivity of 6.57*10-5 rad A-1 and a minimum detectable current of 11.3 mA Hz-1/2 have been achieved.

Faraday effect optical current clamp using a bulk-glass sensing element

A new topology for an openable Faraday current sensor is presented with a demonstrated resolution of 7.2 mA/ radicalHz over a measurement range from 1 to 1500 A. The sensitivity of the system is 2.21 x 10(-5) rad/A. Since this sensor is fabricated in an openable form, emulating a conventional current clamp, it can be used either in permanent installations or for short-term diagnostic studies.