Fiberoptic Sensor for Minimally-Perturbing Measurement of Electric Fields in High Power Microwave Environments
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FIBEROPTIC SENSOR FOR MINIMALLY-PERTURBING MEASUREMENT OF ELECTRIC FIELDS IN HIGH POWER MICROWAVE ENVIRONMENTS
M. H. SUN, K. A. WICKERSHEIM, A. KAMAL AND W. R. KOLBECK Luxtron Corporation, 1060 Terra Bella Avenue, Mountain View,
CA 94043
SUMMARY Recently a small, minimally-perturbing, isotropic, and easy-to-use fiberoptic microwave E-field probe has been developed. This probe senses microwave energy by measuring the Joule heating of a susceptor material. It consists of two Luxtron Fluoroptic sensors. One measures the temperature of the susceptor formed as a thin-walled spherical shell, the other measures the temperature of the air around the susceptor so that any ambient temperature change can be subtracted. The differential temperature (6T) obtained between the two sensors is directly related to the microwave E-field strength or the power density corresponding to that field strength at the probe. The packaged probe has a diameter of 3/16", which allows the probe to be easily introduced into ovens and other processing chambers without affecting the leakage of the microwave. The susceptor sphere size used for most of the tests performed in this work is -2.3 mm diameter, although 1.9 mm to 6 mm diameter have also been made for different levels of sensitivity. Using the 2.3 mm sphere with the Luxtron Model 755 thermometry system, the minimum detectable average power is 1-2 watts or Efield of 1.5 kV/m when the probe is placed in a TEl 0 mode waveguide, which 0 corresponds to a 6T of 0.2 C. This probe has a response time of five 0 seconds and is resistant to moisture and to temperatures of up to 250 C. INTRODUCTION The existing commercial stray field probes are large in size and can only be used for low power measurements. To date, a small and minimallyperturbing probe that can be used for high power measurements at points in free space has not been available. The use of Luxtron's Fluoroptic thermometer [1] to measure Joule heating in microwave fields was first applied in the sensing of microwave frequency induced currents in the electroexplosive devices (EEDs) in ordnance systems [2] where the temperature of resistive bridgewires was measured with a very small Fluoroptic sensor and correlated to the current induced. Later to develop a transfer standard for frequencies in the range from 40 to 120 GHz [3] for NIST, custom sensors were made to optimize the sensitivity of the sensor for detection of field strengths in the order of 100 V/M. Solid susceptor spheres made with carbon and silver loaded resins were used. The resin surrounded the fiberoptic temperature sensor. Work by Martin, et al. [4] demonstrated the possible use of a fiberoptic thermal measurement technique in combination with resistive antenna. Recently, Lentz, et al. patented [5] the concept of a susceptor based Efield probe utilizing fiberoptic sensors. The sensor described here utilizes a highly efficient susceptor material formed as a desired thin-walled hollow sphere surrounding a fiberoptic thermal sensor to provide the performance required for on-l
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