M. N. Deeter

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.

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. >

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.

High frequency magnetic field sensors based on the Faraday effect in garnet thick films

The Faraday effect in the thick epitaxial films of magnetic garnets of the type used in magneto‐optic isolators can be used as the basis for a fiber‐optic magnetic field sensor. These films have uniaxial anisotropy perpendicular to the surface and they contain bismuth to enhance the Faraday rotation. The typical magnetic domain pattern of meandering stripes changes in response to an applied field perpendicular to the film and this changes the polarization of infrared light propagating perpendicular to the film. Theory and experiment show that the speed of operation is limited by relaxation or resonance effects to upper frequencies between 106 and 109 Hz. Maximum sensitivity requires low magnetic moment and large thickness, in conflict with the requirements for high speed.

Fiber-optic Faraday-effect magnetic-field sensor based on flux concentrators.

The principles and performance of a fiber-optic Faraday-effect magnetic-field sensor designed around an yttrium-iron-garnet (YIG) sensing element and two flux concentrators are described. The system design exploits the technique of polarization-rotated reflection in which a single polarization-maintaining optical fiber links the sensor head to the optical source and detection system. In the sensing head, ferrite flux concentrators are magnetically coupled to the YIG sensing element to achieve maximum sensitivity. The system exhibits a noise equivalent field of 6 pT/√Hz and a 3-dB bandwidth of~10 MHz.

Faraday effect current sensor with improved sensitivity–bandwidth product

We report a new design for a Faraday effect current sensor based on yttrium iron garnet that has substantially greater bandwidth than previous designs and is much easier to fabricate. The measured sensitivity is 0.7°/A, with a −3-dB bandwidth of 500 MHz, which gives an improvement in sensitivity–bandwidth product of approximately 45. A noise-equivalent current of 840 nA/Hz1/2 was measured at 1.8 kHz by difference-over-sum processing. The use of turning prisms with phase-preserving coatings greatly simplifies construction, improves electrical isolation, and increases sensitivity through proximity effects.

Novel bulk iron garnets for magneto-optic magnetic field sensing

We report measurements of the magneto-optic response function and frequency response for three bulk iron garnet crystals grown by a flux technique. The samples were the product of an intensive effort to develop iron garnet compositions with properties specifically optimized for magnetic field sensing. Sensitivity enhancement was achieved through both bismuth substitution (for increasing the saturation Faraday rotation) and gallium substitution (for reducing the saturation magnetization). One sample exhibited a value of magneto-optic sensitivity of 25/spl deg//mT for 1.3 /spl mu/m light. Frequency response measurements indicate that bismuth substitution actually improves performance (compared to unsubstituted yttrium iron garnet) in contrast with gallium, which causes substantial degradation. >

Magneto-optic Magnetic Field Sensors Based On Uniaxial Iron Garnet Films In Ofiical Waveguide Geometry

Iron garnet films which exhibit perpendicular uniaxial magnetic anisotropy are promising materials for magnetooptic magnetic field sensing. In an optical waveguide geometry, these materials exhibit large values of saturation Faraday rotation which in turn produce high sensitivity. The domain structure of these films favors magnetization rotation as the primary magnetization process. This process is significantly faster than domain wall motion, which is the primary magnetization process in bulk iron garnet crystals. Data which confirm the high sensitivity and wideband frequency response of these materials are presented. One film exhibits a virtually flat frequency response from DC to at least 1 GHz. Potential problems with waveguide sensors, such as birefringence and optical coupling efficiency appear to be soluble. >

Sensitivity limits to ferrimagnetic Faraday effect magnetic field sensors

In general, the sensitivity of ferrimagnetic Faraday effect magnetic field sensors is a function of both the crystal geometry and composition. The geometrical dependence of the sensitivity in nonellipsoidal crystals, such as cylinders, is complicated by their spatially nonuniform demagnetizing factors. We compare sensitivity data obtained from a variety of cylindrical iron garnet samples with models which predict the effective demagnetizing factor Neff as a function of the length‐to‐diameter ratio. With respect to composition, we present experimental results of sensitivity vs diamagnetic substitution (x) in the iron garnet series Y3Fe5−xGaxO12. As expected, the sensitivity rises sharply as x approaches the compositional compensation point.

Domain effects in Faraday effect sensors based on iron garnets.

Domain-induced diffraction effects produced by two iron garnet thick films and two bulk crystals are compared. The thick films, characterized by a serpentine magnetic domain structure, produced nonlinear response functions; this is in qualitative agreement with a one-dimensional diffraction model. Bulk iron garnet crystals, which exhibited a complex three-dimensional domain structure, produced qualitatively similar effects that diminished with increasing crystal length. Differential signal processing resulted in a linear signal for the thick films and a primarily sinusoidal response for the bulk crystals.