Norman E. Fisher

ULTRASONIC HYDROPHONE BASED ON SHORT IN-FIBER BRAGG GRATINGS

We investigate the feasibility of using in-fiber Bragg gratings for measuring acoustic fields in the megahertz range. We found that the acoustic coupling from the ultrasonic field to the grating leads to the formation of standing waves in the fiber. Because of these standing waves, the system response is complex and, as we show, the grating does not act as an effective probe. However, significant improvement in its performance can be gained by use of short gratings coupled with an appropriate desensitization of the fiber. A noise-limited pressure resolution of approximately 4.5 x 10(-3) atm/ radicalHz was found.

In-fibre Bragg gratings for ultrasonic medical applications

We investigate the feasibility of using in-fibre Bragg gratings to measure ultrasonic fields for medical applications. Two signal processing schemes for interrogating the gratings are described. Preliminary results for each scheme (one a homodyne approach, the other a heterodyne one) give noise-limited pressure resolutions of and atm respectively, each within a 1 Hz bandwidth. The second scheme, however, gives a more stable response.

The interrogation of a conventional current transformer using an in-fibre Bragg grating

An alternative approach for measuring large currents at high potentials is presented. The output signal from a conventional current transformer is measured by transducing the voltage developed across the transformer secondary into a shift in the reflected wavelength from an in-fibre Bragg grating using a piezoelectric element. A pseudo-heterodyne detection scheme was used to determine this shift. We demonstrate the schemes performance over a current range of 700 A. The current resolution was about over a frequency range of about 50 to .

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

Summary form only given. The fiber-links used to interrogate optical current sensors utilizing the Faraday effect are prone to vibrational effects, which can corrupt the optically encoded Faraday signal with induced optical noise. This corruption can be further exacerbated if the fiber leads are carried to ground using semi-rigid light-weight insulators. To overcome these problems we have developed a common-mode noise rejection (CMR) scheme, which can compensate for these effects even at very high vibration levels.

Combined ultrasound and temperature sensor using a fibre Bragg grating

We demonstrate the feasibility of using in-fibre Bragg gratings to measure MHz acoustic fields and temperature simultaneously. We achieved a noise-limited pressure resolution of ˜4.5×10-4 Atm/vHz and a temperature resolution of 0.2°C.

A single fibre-optic down-lead Faraday current sensor

Abstract 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.

Faraday current sensors and the significance of subtended angles

Abstract 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.

A novel miniature optical fibre probe for MHz frequency ultrasound

The authors investigate the feasibilty of using in-fibre Bragg gratings to measure ultrasonic fields for medical applications. Two signal processing schemes for interrogating the gratings are described. Preliminary results for each scheme (one a homodyne approach, the other a heterodyne) give a noise limited pressure resolution of 2.2/spl times/10/sup -4/ Atm/Hz/sup 1/2 / and 1.8/spl times/10/sup -2/ Atm/Hz/sup 1/2 /, respectively. The second scheme however, gives a more stable response.

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.

Probe for measuring ultrasonic fields using short in-fiber Bragg gratings

We demonstrate that short in-fiber Bragg gratings coupled with an appropriate desensitization of the fiber, may be successfully used to measure MHz ultrasonic fields.