Spherical Shell Fibre Optic Magnetic Field Sensors and Magnetometers and Magnetic Gradient Detectors Incorporating Them

This invention relates to magnetic field detection and in particular to fibre optic magnetic field sensors and their use in magnetic gradient detection.

Interferometric fibre optic magnetometers are already known for example from "A fibre-optic dc magnetometer" by K. P. Koo et al. Journal of Lightwave Technology, Vol. LT-1, No. 3, September 1983, P 524-5. They basically comprise an all-fibre Mach-Zehnder interferometer, one arm of which is magnetically sensitised by, for example, being bonded to a strip of magnetostrictive material or coated with a magnetostrictive material. A d.c. magnetic field has the effect of altering the optical path length of the sensitised arm and the interferometer output which is proportional to the differential path length change (.DELTA.L) is thus related to the magnetic field (H), .DELTA.L.alpha.H.sup.2. To overcome inaccuracies due to noise etc, an ac bias field at frequency w is applied to the sensitised arm and then the interferometer output at w is directly proportional to the dc magnetic field.

A fibre-optic magnetic gradient detector is described by K. P. Koo et al in the article on pages 509-513 of the Journal of Lightwave Technology, Vol. LT-1, No. 3, September 1983. In this case both arms of the all-fibre Mach-Zehnder interferometer are magnetically sensitised and ac bias fields at the same frequency are applied to both sensitised portions. The detector described in this article only measures the magnetic gradient in one direction, that is between the two arms.

In our co-pending G.B. Application No. 8504729 published under (Ser. No. 2171514A) (p. Extance-R. E. Jones 17-17) there is described a fibre optic magnetic gradient detector which detects two orthogonal magnetic gradient components simultaneously and employs three sensitised optical fibre portions, two in one arm and one in the other arm of an all-fibre Mach-Zehnder interferometer, and ac bias fields at two different frequencies. The sensitised optical fibre portions are wound in the form of planar circular sensing coils, the single mode optical fibre employed being magnetically sensitised by, for example, being coated with a magnetostrictive material.

According to one aspect of the present invention there is provided a magnetic field sensor comprising a length of magnetically sensitised single mode optical fibre wound in substantially random directions into a spherical shell shape.

According to another aspect of the present invention there is provided a magnetometer comprising an all fibre Mach-Zehnder interferometer in one arm of which is connected a magnetic field sensor comprising a length of magnetically sensitised single mode optical fibre wound in substantially random directions into a spherical shell shape.

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 illustrates schematically an optical fibre magnetic field sensor structure according to the present invention;

FIG. 2 illustrates schematically an all fibre Mach-Zehnder interferometer magnetometer including a sensor of FIG. 1;

FIG. 3 illustrates the relative arrangement of four sensors of FIG. 1 for a magnetic gradient detector, and

FIG. 4 illustrates the arrangement of four sensors of FIG. 3 in a Mach-Zehnder interferometer.

The optical fibre magnetic field sensor structure illustrated in FIG. 1 comprises a length of magnetically sensitised, for example by being coated with a magnetostrictive material, single mode optical fibre 1 formed into a spherical shell shape 2 by being wound in substantially random directions on a support sphere of non-magnetic material. A magnetic field in any direction will thus cause a length change of the optical fibre that can be detected interferometrically, that is an omnidirectional response is obtained. Specific directionality can be obtained by the application of a d.c. bias field in the desired direction. All three perpendicular components of a magnetic field can be measured by applying three perpendicular (mutually orthogonal) a.c. magnetic bias fields, each at a different frequency, to the spherically wound fibre.

If only the magnitude and direction of the magnetic field is required the interferometer may be arranged as in FIG. 2. The interferometer has a sphere of wound fibre 2 disposed in one arm 3 and is controlled by a PZT phase modulator 4 in the other arm 5 in dependence on the outputs of detectors 6 and 7 which are applied to a differential amplifier 8 followed by an integrator 9. Three a.c. bias fields (b.sub.x, b.sub.y, b.sub.z) at right angles to one another and each at respective different frequencies are required to be generated at the spherically wound fibre 2. This may be achieved by solenoids or Helmholtz coils (omitted for clarity). Each component of the magnetic field to be detected is related to a respective ac bias field frequency and is measured as the amplitude of the respective fundamental frequency at the interferometer output.

For a basic magnetic gradient detector two such spherically wound fibres separated by a predetermined baseline spacing, together with respective ac bias field generating means, are required for one component of the gradient and the differential response of the two spherically wound fibres is measured.

A full description of the gradient of a magnetic field requires the knowledge of the nine components of the gradient tensor: ##EQU1## However, from Maxwells' equations div B=0, therefore

so that only two of these gradients need to be measured to determine all three. Hence there is no need to measure

dB.sub.z /dz

say. In addition for this system curl B=0, so that the tensor is symmetrical, that is

Thus only five independent components remain which must be measured in order to determine all nine components, as follows: ##EQU2## Since it is necessary to measure gradients along all three directions according to this reduced tensor, a minimum of four displaced spherically wound sensors are required (FIG. 3). Bias fields are required in the x and y directions only, as is apparent from the reduced tensor. The central spherically wound optical fibre sensor ("golf-ball") 10 is at the origin of the x, y and z axes, and three "golf-balls" 11, 12 and 13 are spaced apart from the origin in the x, y and z directions respectively by baseline spacings dx, dy and dz, respectively. In order to obtain the dB.sub.x /dx term of the reduced tensor, "golf-balls" 10 and 11 each require the application of the same bias field in the x direction, that is bias field b.sub.x.sup.l. For the dB.sub.x /dy and dB.sub.y /dy terms, "golf-balls" 10 and 12 each require the application of the same bias fields in the x and y directions, that is bias fields b.sub.x.sup.ll and b.sub.y.sup.ll. For the dB.sub.x /dz and dB.sub.y /d.sub.z terms, "golf-balls" 10 and 13 each require the application of the same bias field in the x and y directions, that is bias fields B.sub.x.sup.lll and b.sub.y.sup.lll. Thus five bias fields, or rather fields at five different frequencies are required.

The four "golf-balls" 10, 11, 12 and 13 of FIG. 3 are disposed correctly with respect to one another in the x, y and z directions and connected in a two arm all fibre Mach-Zehnder interferometer, the "golf-ball" 10 being disposed in one arm 14 and the "golf-balls" 11, 12 and 13 connected in series, but not necessarily in that order, in the other arm 15. Means for providing the five bias fields at their respective frequencies being required for each arm, as indicated in FIG. 4. Light from a source 16 is input to both arms 14 and 15 by means of a 3 dB coupler 17. The output ends of arms 14 and 15 are coupled by means of a 3 dB coupler 18. Since five frequencies are involved use of a spectrum analyser 19 may be preferable to separate detectors. The output of the spectrum analyser is also employed to control the interferometer via a PZT phase modulator 20. Once the five independent magnetic gradients are determined from output of the interferometer, the remaining four terms of the tensor can be calculated.

Preferably the coating of magnetostrictive material on the fibre of the "golf-balls" is less than 100 microns thick. The coating should be as thin as possible to minimise the effect of the inherent demagnetising field but thick enough to minimise the effect of mass loading on magnetostrictive strain.