Allen Rose

Annealing of linear birefringence in single-mode fiber coils: application to optical fiber current sensors

Annealing procedures that greatly reduce linear birefringence in single-mode fiber coils are described. These procedures have been successfully applied to coils ranging from 5 mm to 10 cm in diameter and up to 200 or more turns. They involve temperature cycles that last 3-4 days and reach maximum temperatures of about 850 degrees C. The residual birefringence and induced loss, are minimized by proper selection of fiber. The primary application of these coils is optical fiber current sensors, where they yield small sensors that are more stable than those achieved by other techniques. A current sensor with a temperature stability of +8.4*10/sup -5//K over the range from -75 to +145 degrees C has been demonstrated. This is approximately 20% greater than the temperature dependence of the Verdet constant. Packaging degrades the stability, but a packaged sensor coil with a temperature stability of about +1.6+10/sup -4//K over the range from -20 to +120 degrees C has also been demonstrated. >

Temperature dependence of the Verdet constant in several diamagnetic glasses

Measured temperature dependences of the Verdet constants of SiO(2), SF-57, and BK-7 are approximately 10(-4)/K within 3-20% of Becquerel formula estimates.

Faraday Effect Sensors: The State Of The Art

The Faraday effect is becoming widely used as an optical method of measuring electric cur-rent or magnetic field. It is particularly advantageous where the measurements must be made at high voltage or in the presence of electromagnetic interference, and where, speed or stability are considerations. In this paper we review the development of the technology over the last twenty years, with an emphasis on the basic principles, design considerations, and performance capabilities of sensors that represent the latest achievements. Faraday effect current sensors are now used routinely in the measurement of large current pulses, and are starting to become available for ac current measurements in the power industry. Recent developments include their extension to the measurement of currents in the milliampere range and substantial reductions in size. Similar devices, in slightly different configurations, can be used for magnetic field measurements. Further improvements, based on new fiber types and new materials, are projected.

Fast, sensitive magnetic-field sensors based on the Faraday effect in YIG

Magnetic-field sensors based on the Faraday effect in ferrimagnetic iron garnets are investigated in terms of their sensitivity, speed, and directionality. Signal-to-noise measurements at 80 Hz on small (typically 5-mm-diameter*3-mm-long) samples of yttrium iron garnet (YIG) yield noise equivalent magnetic fields of 10 nT/ square root Hz. Frequency-response measurements exhibit virtually flat response to approximately 700 MHz. >

Verdet constant dispersion in annealed optical fiber current sensors

The Verdet constant in annealed optical fiber current sensors has been measured at wavelengths from 636 to 1320 nm. The measurements are fitted to two models, one classical and the other an expansion of the classical model that includes a nonlinear term. These measurements and models are compared to previous measurements made in optical fiber and bulk SiO/sub 2/. Our measurements have an average accuracy of /spl plusmn/0.6% and an average measurement uncertainty of /spl plusmn/0.5% over the 636 to 1320 nm range.

Optical, thermo-optic, electro-optic, and photoelastic properties of bismuth germanate (Bi 4 Ge 3 O 12 )

To assess the suitability of bismuth germanate as an electro-optic material for high precision applications, we have confirmed and extended previous data on its refractive index, electro-optic tensor element r(41), and thermal expansion coefficient. In addition, we have measured the thermo-optic coefficient dn/dT, the temperature dependence of the electro-optic coefficient, and the stress-optic tensor elements. From the stress-optic tensor elements and previously published data, we have computed the strain-optic tensor elements. The index of refraction is given, to a good approximation, by the single-term Sellmeier equation, n(2) - 1 = S(0)λ(0)(2)/[1 - (λ(0)/λ)(2)], with S(0) = 95.608 µm(-2) and λ(0) = 0.1807 µm. The thermo-optic coefficient is 3.9 × 10(-5)/°C at 632.8 nm and 3.5 × 10(-5)/°C at 1152.3 nm. The electro-optic tensor element varies between approximately 1.05 and 1.11 pm/V over the spectral range of 550-1000 nm; its normalized effective change with temperature is approximately 1.54 × 10(-4)/°C. The thermal expansion coefficient is 6.3 × 10(-6)/°C over the range 15-125 °C. Values of the stress-optic tensor elements are q(11) - q(12) = -2.995 × 10(-13) m(2)/N and q(44) = -0.1365 × 10(-12) m(2)/N. The strain-optic tensor elements are p(11) - p(12) = -0.0266 and p(44) = -0.0595.

Twisting and annealing optical fiber for current sensors

We demonstrate that twisting a fiber a few turns per meter before it is annealed largely eliminates the residual linear birefringence. This dramatically improves the yield of annealed coils used for current sensing and makes it possible to use fibers that previously had large residual linear birefringence. Twisting the fiber is effective because the residual birefringence, associated with core ellipticity, is reduced to near zero by twisting. A theoretical model of the twisted and annealed fiber current sensor is compared to experimental data. We also show good temperature stability for a sensor made with this new technique.

Fast and accurate low-coherence interferometric measurements of fiber Bragg grating dispersion and reflectance.

We demonstrate fast and accurate measurements of fiber Bragg grating dispersion and spectral reflectance using low-coherence interferometry. Both dispersion and spectral reflectance are obtained in less than 60 seconds, rendering the results immune to errors caused by temperature variations and instrumental drift. To examine the accuracy of the low-coherence technique, we compare the results with independent measurements and demonstrate an agreement better than 1.5 ps for dispersion and 25 pm for spectral reflectance wavelength.

Submicroampere-per-root-hertz current sensor based on the Faraday effect in Ga:YIG.

We demonstrate an optical current sensor that is based on the Faraday effect in gallium-substituted yttrium iron garnet and has a measured sensitivity of ∼3°/A, a noise-equivalent current of ~220nA/Hz, and a −3-dB bandwidth of ∼2.6 MHz. The bandwidth–sensitivity product is a factor of ∼10 greater than that of an all-silica-fiber current sensor with the same diameter.

Magneto-optic sensors based on iron garnets

The use of single crystal bulk and film iron garnets in optical sensors is reviewed. Magneto-optic sensitivity and its stability are important parameters that depend on a variety of factors, including optical design. Polarimetric and diffractive sensor technologies are summarized, and several recent demonstrations of magnetic field, current, and rotation sensing using garnets are described. Garnets also find application as important nonsensing components in sensor systems.