Fiber-optic magnetic field sensors

Fiber-optic magnetic field sensors Erin Tate Mississippi State University Bagley College of Engineering, ECE Department Mississippi State, Mississippi 39762 eet44@msstate.edu Abstract The fiber optic magnetic sensor was researched in terms of design, applications, testing, and responses. After several months of general theory lessons on fiber optic cables, lasers, and various optoelectronic devices, a project related to the understanding of a particular fiber optic sensor device was undertaken to apply and test general knowledge to a physical construct. I. INTRODUCTION Fiber optic sensor technology has significantly increased over the past several years. To date, there are over sixty sensor devices developed using fiber optic sensors. These types of sensors have several advantages over other types of sensors used before the development of optical fibers. A few of these include much greater sensitivity, a general foundation for sensing physical disturbances and variances, and geometry flexibility. Fiber optic sensors can also be used for applications involving high voltage, significant electrical noise, corrosive conditions, and high temperatures. The focus of this paper is to analyze developments in the fiber optic magnetic field sensor for small magnetic fields. Advancements have exponentially increased since the theoretical study of these sensors began in the 1980s. Previously, a Superconducting Quantum Interference Device (SQUID) was the only option for magnetic field measurement. SQUIDs are cryogenic (requiring very low temperatures,), enormous in size, expensive, and can only be used under specific laboratory-controlled conditions. Fiber optic magnetic sensors, however, are much simpler and smaller, less expensive, and capable of operation at standard room temperature. These desirable characteristics are making fiber optic magnetic sensors and much more widely used and attractive option than the SQUID. This paper will also discuss design options and various configurations, measurement approaches, sensor responses, and applications of fiber optic magnetic sensors. II. FIBER OPTIC MAGNETIC SENSOR DESIGN The basic design of a fiber optic magnetic sensor is a section of fiber optic cable combined with a magnetostrictive material. Magnetostrictive materials are ferromagnetic materials capable of deformation caused by magnetic fields. All investigated designs incorporate a Mach-Zehnder interferometer. The design of a Mach- Zehnder interferometer is shown in Figure 1[1]. Figure 1: Fiber optic Mach-Zehnder interferometer This interferometer functions by first having a 50/50-ratio fiber optic coupler divide the light from the source evenly between the sensing arm and the reference arm. The reference arm is not connected to magnetostrive material, while the sensing arm is. The sensing arm detects interference associated with the deformation of the magnetostrictive material based on the magnetic field encountered. The light is then recombined and transferred to processing equipment for analysis of the interference pattern. Various patterns correspond with specific magnetic fields. Magnetostrictive materials used to provide the best

possible results compared to the SQUID are found to be Metglas and Nickel [1]. There are several configurations for the magnetostrictive fiber optic cable used for the sensing arm. These configurations are shown in Figure 2 [1] [2]. (a) Ribbon geometry (b) Sandwich geometry (c) Cylinder geometry (d) External sheath coating Figure 2: Fiber cable geometry Metallic glasses, particularly those manufactured by Allied Chemical Corporation, have provided extreme sensitivity in fiber optic magnetic sensors. Epoxy is used to bond the metallic glass to single mode fibers. One of the issues involved with the metallic glass technique is that there is currently no way to successfully bond the metallic glass to the fiber during fiber construction. All tests to date have been conducted under the condition that the fiber was manufactured prior to bonding with the metallic glass. Two specific configurations were used by Koo and Sigel [3] to experiment with the all-fiber interferometer used to measure extremely small magnetic fields. Both configurations are shown in Figure 3 [3]. Figure 3(a) shows the strip geometry, in which two Helmholtz coils producing ac and dc magnetic fields to be measured by a single mode fiber. This fiber is bonded to a metallic glass strip. Figure 3(b) shows the tubular geometry, in which a current carrying copper cylinder is surrounded by a metallic glass strip. The fiber is wrapped around the metallic strip and bonded to it. The magnetic fields are produced by electrical current flowing through the copper cylinder. Figure 3: Fiber optic magnetic sensor (a) strip geometry; (b) tubular geometry. Although design of fiber optic magnetic sensors is generally quite simple, there is a major constraint currently under inspection for a solution. Due to the materials necessary for effective measurement, these sensors can only contain fibers with lengths of approximately one meter. The ability to implement sensors with fiber cables in the kilometer range would be desirable, but is not currently possible with the available technology on attaching the magnetostrictive layer. Another difficulty in the design of these sensors is finding materials with a high magnetostrictive property. Then once new materials are identified, they must be further investigated to find the most suitable bonding and coating techniques.

III. MEASUREMENT TECHNIQUES AND RESPONSES The use of magnetostrictive materials in fiber optic magnetic sensors is by far the most popular method used, but another method known as the Faraday rotation approach also exists. While much less commonly used, the Faraday rotation approach is another option. This approach is very complex in design and expensive to implement because rare earth elements are necessary to develop a sufficient Faraday Effect to result in adequate sensitivity levels. These rare elements are also difficult to make soluble in the glass of fibers. The operation involves applying a magnetic field longitudinally to the fiber. This field will produce rotation in the direction of linear polarization of the fiber. The remainder of research focused on magnetostrictive material implementation on optical fibers. A magnetostrictive coefficient represents the change in the dimension of the material over length of the fiber the material coats. Cubic crystals are often used in fiber optic magnetic sensors. Magnetostrictive coefficients of some commonly used cubic crystals are shown in Table I [2]. The general measurement and analysis process begins with alterations of the optical pathlength caused by either expansion or contraction of the magnetostrictive materials surrounding the optical fibers. The signal arm s pathlengtth was adjusted by having it located within the magnetic field. This adjustment of pathlength phase shift can be detected in amounts as small as micro radians. These detections are made possible by phase tracking techniques that allow the interferometer to remain at maximum sensitivity. These techniques cancel low frequencies cause by changes in air current and temperatures. To create the most accurate and effective testing and measurement scenarios, several parameters must be carefully measured and taken into account when analyzing measurement data. Parameters capable of affecting measurements include the linearity of response to applied magnetic field, effects of dc magnetic-bias field on device output, impact of magnetic field noise on sensitivity, sensitivity as a function of the frequency of the applied field, and the orientation response [3]. The following figures show examples of the interferometer compared with these characteristics. Figure 4: Linearity of interferometer responses versus magnetic fields [3] Making use of these values and comparing them to the magnetic field strength curves of different metals and alloys provides identification of regions where the slightest changes in magnetic field result in very significant changes in dimension.

Figure 5: Frequency responses of three magnetic sensors, each using a different type of metallic glass [3] Figure 6: Interferometer responses versus magnetic biasfields [3] All fiber samples bonded with metallic glass strips or coatings have shown outstanding response linearity. The response peaking at optimum bias conditions leads to identification of the maximum magnetostriction in the metallic glass coupled with the fiber. The frequency response will vary depending on the geometry of the sensor, as shown in Figure 5. Strip geometries have generally been found to have sharp resonances under 15 cm long. Stressing and loading of the fibers can also change the optical signal s phase and amplitude. Another crucial parameter to explore is the sensitivity of the metallic glasses used in the sensor. This sensitivity determines the smallest magnetic field the fiber optic sensors is capable of detecting. The smaller sensitivities correspond to smaller detectable magnetic fields. These fields are generally between the orders of Oe/m of fiber. Nonmagnetic perturbations can cause skewed data and measurements. Such disturbances to magnetic field measurement include stress and acceleration on the sensor [4]. Research and experimental testing has led to the ability to control and practically eliminate these nonmagnetic effects on fiber optic magnetic sensors. External perturbations can affect the elements of a fiber interferometer as well as the transuding element of the system [4]. For the fiber optic magnetic sensor, this would be the magnetostrictive material. One method of controlling the influences of external forces is called the magnetic dither approach [4]. This approach is applied to low-frequency fiber optic magnetometers and is naturally protected from nonmagnetic disturbances involved with the interferometer. Effects on the magnetostrictive material are less easily controlled, and they produce outputs that do not correspond with true magnetic field signals. A possible solution to this problem is the use of dual detectors to reduce the effects of intensity fluctuations [4]. The magnetic element may also function as an open-loop element at a frequency depending on the sensor in use, known as a dither frequency. The most effective way of reducing the effects of external nonmagnetic perturbations on a fiber optic magnetic sensor is to keep the dc magnetic field at desirable levels for proper measurement [4]. These levels are determined by the materials used in the design of the sensors as well as conditions of operation. IV. APPLICATIONS OF FIBER OPTIC MAGNETIC SENSORS Fiber optic magnetic sensors allow for much more practical and common measurements of magnetic fields than SQUID technology allows. With their ability to take accurate measurements at room temperatures rather than cryogenic temperatures, a controlled laboratory is not necessary to measure small magnetic fields. One use of this type of sensor is as a gradient field magnetometer. These are useful when needing to determine the spatial magnetic field gradient of a dipole of object from a distance. This device consists of a dual

arm interferometer configuration in which both arms are composed of a fiber surrounded by a metal jacket. This configuration allows for the canceling of the earth s field when taking measurements. Another application of fiber optic magnetic sensors is use in magnetic antennas used to detect electromagnetic signals. This same type of sensor can also be used in the design of magnetooptic compass, which provides magnetic heading. References [1] Picon, L.L.; Bright, V.M.; Kolesar, E.S.;, "Detecting low-intensity magnetic fields with a magnetostrictive fiber optic sensor," Aerospace and Electronics Conference, 1994. NAECON 1994., Proceedings of the IEEE 1994 National, vol., no., pp.1034-1039 vol.2, 23-27 May 1994 [2] Giallorenzi, T.; Bucaro, J.; Dandridge, A.; Sigel, G.; Cole, J.; Rashleigh, S.; Priest, R.;, "Optical fiber sensor technology," Quantum Electronics, IEEE Journal of, vol.18, no.4, pp. 626-665, Apr 1982 [3] K. P. Koo and G. H. Sigel, Jr., "Characteristics of fiber-optic magnetic-field sensors employing metallic glasses," Opt. Lett. 7, 334-336 (1982) [4] Bucholtz, F.; Koo, K.P.; Dandridge, A.;, "Effect of external perturbations on fiber-optic magnetic sensors," Lightwave Technology, Journal of, vol.6, no.4, pp.507-512, Apr 198