K.P. Koo

Detection of low-frequency magnetic signals in a magnetostrictive fiber-optic sensor with suppressed residual signal

A correlation is presented between experimental data obtained under various operating conditions and a modified coherent rotation model which predicts the behavior of the residual signal and the sideband noise. The authors identify an operating regime that eliminates the residual signal and gives rise to a reduction in noise levels by a factor of five over previously reported results. Minimum detectable fields of 11+or-2pT/ square root Hz at 1 Hz and 38+or-8 pT/ square root Hz at 0.2 Hz are achieved. >

Easy axis distribution in transversely annealed Metglas 2605 S2

Abstract We have measured the longitudinal engineering magnetostriction over the range 0 to 40 Oe for a series of Metglas 2605 S2sb ribbons annealed at temperatures in the range 250 to 460°C, anneal times in the range 40 to 16000 s and anneal fields in the range 0.1 to 1200 Oe. We analyze the data in terms of a Gaussian distribution of easy axis orientations and find that field annealing can reduce the distribution width from approximately 90° to less than 10° with an attendant improvement in the mangetostrictive response of the ribbon.

Multichannel fiber-optic magnetometer system for undersea measurements

We have designed, fabricated, and operated an undersea array of eight fiber-optic vector magnetometers. Each magnetometer consists of three magnetostrictive transducers aligned on orthogonal axes and incorporated in a single Michelson interferometer. During undersea operation, each interferometer exhibited less than 1 /spl mu/rad//spl radic/ Hz phase noise, and the self-noise of each magnetic transducer was less than 0.2 nT//spl radic/ Hz at 0.1 Hz. We discuss the design and performance of the optical system including noise mechanisms. We present the results of magnetic measurements of the geomagnetic field and the magnetic tracking of ships. >

Preparation of amorphous metallic glass transducers for use in fiber optic magnetic sensors

An important figure of merit for fiber optic magnetometers is the effective C parameter Ceff relating magnetostriction e to applied field H: e=CeffH2. We describe both the annealing and the characterization after annealing of cylindrical metallic glass transducers and present the results of measurements of Ceff for various anneal temperatures, times, and magnetic field strengths for flat Metglas 2605‐S2 samples. Ceff values of (2.0–2.5)×10−6 Oe−2 are observed in the annealed flat strip and cylindrical samples studied here.

Optimization of the fiber/metallic glass bond in fiber-optic magnetic sensors

We present the results of measurements of the response of magnetostriction-based fiber-optic magnetometers to various methods of bonding the optical fiber to the sensing element, in this case a metallic glass ribbon. The measurements were performed using combined Michelson/Mach-Zehnder interferometers which allowed us to obtain separately and simultaneously the strain in the ribbon and the strain in the fiber. We have observed serious degradation in the magnetostriction of metallic glass ribbons bonded to fibers due to loading effects of the bonding adhesive. We present a method for reducing these effects and optimizing the sensor response.

Stability of a fiber-optic magnetometer

The stability of a fiber-optic magnetometer employing metallic glass ribbons was examined over a period of more than 10 h. The magnetometer can be operated either in the open magnetic loop or the closed magnetic loop configuration. The short-term minimum detectable magnetic field for ≈ 10 cm of sensor fiber length was less than 2 gamma in the dc to 1-Hz bandwidth. Long-term stability of approximately 10 gamma was achieved in the closed magnetic loop configuration.

Effect of external perturbations on fiber-optic magnetic sensors

The effects of external nonmagnetic perturbations on a fiber-optic magnetic sensor are analyzed theoretically. It is shown that the deleterious effects of these perturbations can be controlled by maintaining the DC magnetic field at prescribed levels. Experiments have been performed with externally applied stresses and accelerations (vibrations) that validate the theory and yield values for the stress and acceleration responsivities of the sensors studied in this investigation. The analysis and techniques given apply equally well to any interferometric sensor using nonlinear transduction mechanisms. >

Effects of parasitic Fabry–Perot cavities in fiber-optic interferometric sensors

We show theoretical and experimental evidence for increased quadrature point fluctuations and amplitude and phase noise in interferometric fiber sensors owing to the presence of parasitic Fabry-Perot cavities. We demonstrate greater than 2 orders of magnitude reduction of such effects.

Low-frequency Intensity Noise Reduction for Fiber-Optic Sensor Applications

Fiber sensors have recently taken advantage of the 1.3µm diode-pumped laser technology, which offers high output power in a single frequency and orders of magnitude improvement in frequency stability over semiconductor devices [1]. The small frequency jitter [2] of Nd:YAG lasers enables one to attain submicron phase noise in interferometric sensors with as much as 10 meters of optical path difference. Because non-linear fiber sensors, such as magnetic fiber sensors, which make use of a carrier to upconvert low-frequency signals to be detected, are often limited by intensity noise upconverted around the carrier [3], we have measured the intensity stability of those devices . The intensity noise in fiber sensors originates from both the laser instabilities and the drifts in the fiber launch.

Recent developments in fiber optic magnetostrictive sensors

We review developments over the past five years in fiber optic magnetometry based on magnetostriction. Recent work has demonstrated detection of magnetic fields from dc to over 1 MHz with resolution of 10 pT P1Hz at 1 .0 Hz and 0.07 pTpIHz at35 kHz in laboratory devices. We present the basic device operating principles and discuss factors which limit the resolution of fiber optic interferometric magnetic sensors. Results are presented for a number of configurations including a ruggedized, compact, three-axis magnetometer for dc and low-frequency measurements, a single-axis gradiometer, a single-axis heterodyne configuration for narrowband detection of fields in the frequency range 0.01 Hz to 1 MHz, and a magnetostrictive oscillator which exhibits period doubling bifurcations and chaos.