Minimum Detectable Field Research Articles

Optical fibre magnetic field sensors using ceramic magnetostrictive jackets

Optical fibres coated by magnetostrictive ceramic films were tested using a Mach–Zehnder interferometer in open-loop mode. The sensors exhibited excellent linearity and good sensitivity. The response of ceramic-jacketed fibres was not affected by small DC fluctuations due to the linear behaviour of tested ceramic coatings in low magnetic fields. Tested ceramic materials included magnetite, γ-Fe 2O 3, nickel ferrite and cobalt doped nickel ferrite Ni 0.97Co 0.03Fe 2O 3 jackets. The latter one showed the best performance. A minimum detectable field of 3.2×10 −3 A/m for optical fibre jacketed with 2-μm-thick and 1-m-long Ni 0.97Co 0.03Fe 2O 3 material has been achieved. A capability of detecting magnetic fields as low as 2.6×10 −7 A/m using a 10-μm-thick and 1-m-long Ni 0.97Co 0.03Fe 2O 3 fibre jacket is proposed.

Optical response of a D-fiber antenna in a finite-element analysis intended for radio-over-fiber applications

An all-fiber antenna with a piezoelectric polymer-coated circular-core D fiber has been characterized by finite-element analysis. The response of the D-fiber antenna was determined over a wide frequency range from 1 to 500 MHz. The modeling predicts an electric-field-induced phase shift of 2.43 x 10(-5) rad/(V/m)/m at 5 MHz. At frequencies higher than 8 MHz the optical response is dominated by radial resonances of the D-fiber-coating composite. From the simulation results a minimum detectable electric field of 41-muV/m has been achieved with a 1-km length of coated D fiber. In addition, a D-fiber antenna network intended for microcellular communications has been analyzed by shot-noise-limited detection.

Polarized laser-induced fluorescence technique to measure localized electric field induced perpendicularly to magnetic field in the plasma-edge

To develop a direct measurement of the weak electric field induced in a magnetically confined edge plasma, the crossed magnetic- and electric-field effect was investigated on the linear polarization of laser-induced fluorescence. The fluorescence was induced by laser excitation of metastable He atoms 2 1S to 4 1D state due to Stark-induced electric dipole and electric quadrupole moments. In a model-type experiment carried out in a cylindrical hollow cathode plasma column, the radial distribution of the polarization of fluorescence was analyzed to determine the electric field dependence. The measured degree of polarization agreed with estimates from a theoretical model based on the selection rules of the spontaneous radiative transition. The minimum detectable electric field is estimated to be a few tens of V/cm in our scheme. We briefly discuss techniques to increase our measurement sensitivity by the interference between both the forbidden transitions in the laser excitation.

Sensitive Electric Field Measurement by Fluorescence-Dip Spectroscopy of Rydberg States of Atomic Hydrogen

Fluorescence-dip spectroscopy based on an optical double resonance is used to probe the Stark splitting in highly excited Rydberg states of atomic hydrogen. After Doppler-free two-photon excitation to $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3$, fluorescence at Balmer $\ensuremath{\alpha}$ is observed. When the radiation of a second laser is tuned over the resonance between $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3$ and a Rydberg state, the Stark splitting manifests itself as a series of dips in the Balmer- $\ensuremath{\alpha}$ fluorescence intensity. Rydberg states up to $n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}55$ can be identified. The minimum detectable electric field is $5\mathrm{V}/\mathrm{cm}$. This is about an order of magnitude more sensitive than other laser-induced fluorescence techniques.

A microfabricated tip for simultaneous acquisition of sample topography and high-frequency magnetic field

We report a combined μm-level topography and high-frequency magnetic field probe, a 6 μm loop defined on a scanning-force microscope cantilever. Since it functions as a near-zone antenna, it can be used with any suitable detector, and can probe both active and passive samples. We demonstrate its performance by scanning a coplanar waveguide sample at 10 GHz using a microwave network analyzer. With this instrument, the device noise at this frequency is ∼2μΦ0/Hz1/2 at T=300 K, comparable to that found for a superconducting quantum interference device (SQUID) at DC. Unlike the SQUID, however, the thermally limited minimum-detectable field of this system scales with frequency.

Integrated optical E-field sensors with a balanced detection scheme

Compact and nonperturbing integrated optical E-field sensors are used for electric field measurements. The theoretical sensitivity of an integrated optical sensor system is limited by the shot noise. In conventional sensors, however the minimum detectable electric field strength is limited by the relative intensity noise (RIN) of the laser diode. Achieving the theoretical sensitivity a new sensor system is presented. Using a balanced optical receiver both a signal gain and a reduction of the RIN at the optical receiver is obtained. The principle function and the potential of the balanced detection scheme in comparison with existing concepts as well as first measurements are presented, yielding a sensitivity of less than 300 /spl mu/V/(m/spl radic/Hz) with standard laser diodes and a receiving dipole length of 40 mm.

Highly sensitive Al 0.25Ga 0.75As/In 0.25Ga 0.75As/GaAs quantum-well Hall devices with Si-delta-doped GaAs layer grown by LP-MOCVD

Quantum-well Hall devices based on Si-delta-doped Al 0.25Ga 0.75As/In 0.25Ga 0.75As/GaAs pseudomorphic heterostructure materials grown by the low-pressure metal organic chemical vapour deposition (LP-MOCVD) method have been successfully fabricated. A Si-delta-doped GaAs layer is introduced for the first time in the Hall device to reduce the thermal variation of electron concentrations and to improve its temperature characteristics. A high electron mobility of 8100 cm 2V −1s −1 with a sheet carrier density of 1.5 × 10 12cm −2 has been achieved at room temperature. A temperature coefficient of −0.015% K −1 with a product sensitivity of 420 V A −1T −1 has been obtained. High signal-to-noise (S/N) ratios corresponding to minimum detectable magnetic fields ( B min) of 60 nT at 1 kHz and 110 nT at 100 Hz have been attained due to the reduced low-frequency noise from DX centres and the high mobility. These data belong to one of the best reported results.

Optical fiber magnetic field sensors with ceramic magnetostrictive jackets

Optical fibers coated by magnetostrictive ceramic films were tested with a Mach-Zehnder interferometer in an open-loop mode. The sensors exhibited excellent linearity and good sensitivity. The response of ceramic-jacketed fibers was not affected by small dc fluctuations that are due to the linear behavior of tested ceramic coatings in low magnetic fields. Tested ceramic materials included magnetite, γ-Fe(2) O(3), nickel ferrite, and cobalt-doped nickel ferrite (NCF2) jackets. The latter showed the best performance. A minimum detectable field of 3.2 × 10(-3) A/m for optical fiber jacketed with 2-μm-thick and 1-m-long NCF2 material has been achieved. The capability of detecting magnetic fields as low as 2.6 × 10(-7) A/m with a 10-μm-thick cobalt-doped nickel ferrite jacket is proposed.

Effect of interstrip gap on the sensitivity of high sensitivity magnetoresistive transducers

For high sensitivity magnetoresistive (MR) transducers, the role of interstrip gap is examined as a design parameter. This is modeled using analytic and numerical approximations. As the gap is narrowed the range of linearity of the MR transducer decreases and the sensitivity increases. For gap/width ratio exceeding 1.8 the strips behave essentially as isolated strips. An example using typical design parameters shows that the minimum detectable field is reduced from 6.3 μOe/√Hz to 3.6 μOe/√Hz as gap/width ratio is reduced from ∞ to 0.1.

Coherent detection of freely propagating terahertz radiation by electro-optic sampling

We report the demonstration of an electro-optic sampling technique that allows for the detection of freely propagating terahertz radiation. Coherent sampling is performed in a poled polymer device that is physically separated from the emitter. The poling electrodes in the sampling element are found to have an integrating effect on the incident terahertz field. The shot noise limited minimum detectable field in the polymer is 100 (mV/cm)/√Hz. We discuss methods by which the sensitivity may be significantly enhanced.

Quantum-well Hall devices in Si-delta-doped Al/sub 0.25/Ga/sub 0.75/As/GaAs and pseudomorphic Al/sub 0.25/Ga/sub 0.75/As/In/sub 0.25/Ga/sub 0.75/As/GaAs heterostructures grown by LP-MOCVD: performance comparisons

Characterized herein are quantum-well Hall devices in Si-delta-doped Al/sub 0.25/Ga/sub 0.75/As/GaAs and pseudomorphic Al/sub 0.25/Ga/sub 0.75/As/In/sub 0.25/Ga/sub 0.75/As/GaAs heterostructures, grown by low-pressure metal organic chemical vapor deposition method. The Si-delta-doping technique has been applied to quantum-well Hall devices for the first time. As a result high electron mobilities of 8100 cm/sup -2//V/spl middot/s with a sheet electron density of 1.5/spl times/10/sup 12/ cm/sup -2/ in Al/sub 0.25/Ga/sub 0.75/As/In/sub 0.25/Ga/sub 0.75/As/GaAs structure and of 6000 cm/sup -2//V/spl middot/s with the sheet electron density of 1.2/spl times/10/sup 12/ cm/sup -2/ in Al/sub 0.25/Ga/sub 0.75/As/GaAs structure have been achieved at room temperature, respectively. From Hall devices in Al/sub 0.25/Ga/sub 0.75/As/In/sub 0.25/Ga/sub 0.75/As structure, the product sensitivity of 420 V/AT with temperature coefficient of -0.015 %/K has been obtained. This temperature characteristic is one of the best result reported. Additionally, a high signal-to-noise ratio corresponding to the minimum detectable magnetic field of 45 nT at 1 kHz and 75 nT at 100 Hz has been attained. These resolutions are among the best reported results.

Fiber optic magnetostrictive transducers for magnetic field sensing

A number of fiber optic magnetostrictive transducers were fabricated using various metallic glass ribbons made of Co₇₀Fe₁₀B₂₀, Co₇₆Fe₄B₂₀, Co₈₀B₂₀, Ni, Fe₇₉B₁₆Si₅, and Fe₄₀Ni₃₈Mo₄B₁₈ and were characterized by a typical optical fiber Mach-Zehnder interferometer constructed by splicing the fiber transducers to the single-mode-fiber sensing arm. The ac magnetostrictive responses for the metallic glasses measured with the fiber optic sensing system show that the dc-bias field (<i>H_p</i>) for the maximum response with a Co₇₀Fe₁₀B₂₀ ribbon is ~ 0.97 Oe, that with a Ni ribbon is 23 Oe, and with a Fe₇₉B₁₆Si₅ ribbon is 5.7 Oe. A magnetostriction function describing the magnetostrictive response of the transducer was also empirically found. The measured effective magnetostrictive parameters and saturation magnetostrictions are 1.3 x 10^-7/Oe² to 7 x 10^-7/Oe² and -4.6 x 10^-6 to 5.7 x 10^-6, respectively. The optical fiber sensors exhibit minimum detectable fields <i>H</i>_min of the order of between 10^-5 Oe(rms)/√Hz and 10^-7 Oe(rms)/√Hz above 1 kHz at their respective dc-bias fields <i>H_p</i>. The <i>H</i>_min can be much further improved by reducing the level of noises that limit the phase resolution to ~ 10^-4 rad/√Hz in the interferometer.

Field trial of an electrically passive optical-fiber magnetometer

A field trial of an electrically passive optical-fiber magnetometer incorporated into the transient electromagnetic method of geomagnetic surveying is described. The transducer of the magnetometer consisted of metallic glass wire cocoated with a length of optical fiber. The magnetometer was capable of measuring the decay rate of the surface magnetic field produced by the surveying technique. The sensor's minimum detectable field was 2.5 × 10(-8) G/√Hz at 2 kHz.

Low-field magnetic sensors based on the planar Hall effect

Sensitive magnetic field detection devices have been fabricated based on the planar Hall effect. The active material consists of permalloy ultrathin films (6 nm thick) epitaxially grown by molecular beam epitaxy. Uniaxial magnetic anisotropy is induced in the film through ferromagnetic coupling with a Fe/Pd bilayer epitaxially grown on MgO(001). The active layer shows a magnetoresistive ratio ΔR/R=2%. The device gives a sensitivity of 100 V/TA and a minimum detectable field below 10 nT. The detector response is linear over at least four decades. The transverse resistivity is sensitive only to the anisotropic resistivity, and not to the isotropic resistivity term which is highly temperature sensitive. Consequently, the thermal noise at 1 Hz is reduced by four orders of magnitude compared to a similar longitudinal magnetoresistive detector.

An integrated photonic Mach-Zehnder interferometer with no electrodes for sensing electric fields

A new configuration for a Mach-Zehnder interferometric electric field sensor device utilizing the electrooptic effect is reported. The lithium niobate device uses no metallic electrodes and operates solely by immersion in an electric field. Reverse poling of one arm of the interferometer provides opposing optical phase changes in the two interferometer arms when placed in an electric field. One fabricated device exhibits a measured minimum detectable field of 0.22 V/m/spl radic/Hz and a frequency response of greater than 1 GHz. Theoretical calculations show that detection of 0.11 V/m/spl radic/Hz is attainable while the upper frequency limit can exceed 6 GHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Magnetic moment fluctuations in samples with near-to-rectangular hysteresis loops

Soft magnetic materials with high permeance are usually used in the development of sensors for low magnetic fields. When the sensor is excited with an alternating magnetic field of frequency ƒ 0, the minimum detectable external magnetic field depends on the magnetic noise level in the sensor. These noises are usually connected both with the Barkhausen effect and with ‘1/ƒ’ noise. The former has a relatively uniform spectral distribution up to frequencies equal to the inverse mean Barkhausen discontinuity duration when the latter has the spectral distribution concentrated around harmonics of the exciting frequency in the sensor response. It is shown in this paper that the spectral distribution of magnetic moment fluctuations can be modulated with the oscillating function depending on frequency with gaps adjacent to even harmonics. These modulations are connected with the magnetic moment change correlation on different exciting field cycles and should become most apparent in samples with near-to-rectangular hysteresis loops. An experiment with a single-axis polycrystalline magnetic Fe-Ni film produced by vacuum deposition of metal on a hot glass layer has confirmed the existence of such a modulation. The number of cycles with correlated magnetic moment fluctuations has been estimated. The correlation increases at higher ƒ 0. It has been shown that the 1/ƒ noise in samples with near-to-rectangular hysteresis loops is generated due to the presence of ‘hard’ domains that do not remagnetize with frequency ƒ 0 and have stochastically wandering bounds that determine the coercive force and top magnetization fluctuations. The potential relief responsible for quasi-stable states of the equilibrium location of domain bounds is due to local internal strains at crystalline defects.

Magneto-optic magnetic field sensor with 1.4 pT/√(Hz) minimum detectable field at 1 kHz

The Letter demonstrates that the sensitivity of magneto-optic magnetic field sensors employing iron garnets can be increased by approximately two orders of magnitude by applying flux concentration. A minimum detectable field of 1.4 pT/√(Hz) was measured when a cylindrical gallium-substituted yttrium iron garnet crystal was combined with two conically tapered nickel–zinc ferrite cylinders.

A high resolution imaging susceptometer

A high-resolution, MicroSQUID (superconducting quantum interference device)-based susceptometer has been constructed for imaging the susceptibility distribution in diamagnetic and paramagnetic objects. The maximum available applied field, produced by the Helmholtz pair, is 300 mu T, which provides an adequate signal-to-noise ratio for common susceptible materials. The calculated variation in the applied field within 20 mm of the center is less than 0.1% of the field at the center. In a 100-Hz bandwidth, the minimum detectable susceptibility-induced field change, (limited by SQUID noise) is 3*10/sup -9/ of the applied field. The sensitivity of the system may be improved by increasing the applied field, for instance by incorporating a superconducting magnet into the magnetometer dewar, and by using lower-noise SQUIDs.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Si MAGFETs optimized for sensitivity and noise properties

New results obtained with multidrain MOSFET magnetic-field sensors (commonly named MAGFETs) are presented. The dependence of the sensitivity and noise characteristics on the device geometries and dimensions and on contact positions has been investigated experimentally. From these results, it appears that the resolution is better when the MAGFET is used as a Hall-MOS sensor rather than as a multidrain magnetotransistor. Although the 1/ f noise in MOS devices has a rather high level, a very good value of 150 nT/√Hz has been obtained for the minimum detectable magnetic field.

Development and analysis of electric field sensor using LiNbO/sub 3/ optical modulator

The sensitivity of an electromagnetic field sensor which uses a LiNbO/sub 3/ electrooptical crystal and an optical-fiber link is improved by using a Mach-Zehnder interferometer, whose half-wave voltage is about 4 V at 1.3- mu m wavelength, and a YAG laser pumped by a laser diode whose output power is 25 mW. The resulting frequency response is about flat from 100 Hz to 300 MHz, and the minimum detectable electric field strengths are 0.22 mV/m at 50 MHz and 0.079 mV/m at 750 MHz. The variation of the sensitivity with the frequency and element length are analyzed using the moment method, and the calculated results agree with the measured results. The measurement of the cross-polarization of the sensor indicates that this property is similar to that of a dipole antenna. The improved sensor can measure an electromagnetic impulse whose peak value is larger than 10 V/m and whose width is wider than 5 ns. >