Optical magnetic-field sensor and method of producing the same

Abstract: An optical magnetic-field sensor includes a base (8) fixed relative to distal ends of optical fibers (2) inserted in the sensor, and optical parts (5,7) for transmitting optical signals between the distal ends of the optical fibers (2) and the optical parts and between an optical element (6) and the optical parts. The optical signals are collimated between the optical parts and the distal ends of the optical fibers by spherical lenses (12) optically aligned with the distal ends of the optical fibers, with synthetic resin members (13) filled in clearances between the distal ends of the optical fibers and the respective spherical lenses. The arrangement facilitates the alignment of optical axes of the optical fibers and the spherical lenses.

Description: This invention relates to an optical magnetic-field sensor which is mainly used in a detection system for finding out fault points at electric power transmission line networks, electric power-distribution line networks, transformer substations and the like, and more particularly to a method of producing such an optical magnetic-field sensor. In order to automatically find out fault points in electric power systems, optical magnetic-field sensors using optical single crystals (for example, BSO or the like) have been recently put to practical use. In this device, the light emitted from a light transmitter is transmitted through a magnetooptical element and received into a light receiver where the light is detected. If electric current values are rapidly changed due to short-circuit or ground-fault, the magnitude of a magnetic field produced around the power-transmission line is changed so that the polarization plane of the light transmitted through the magnetooptical element is changed. The change in polarization plane is detected to judge whether any fault occurs or not. Such an optical magnetic-field sensor includes a magnetooptical element, a polarizer and an analyzer which are built in the sensor, and further optical fibers which are inserted into the sensor and have distal ends fixed to the sensor by means of ferrules. The collimation (forming bundles of parallel beams, and light condensing) of the light from the distal ends of the optical fibers is effected by rod lenses. In practice, cylindrical rod lenses are joined with and fixed to the distal ends of the ferrules, respectively, and other surfaces of the cylindrical rod lenses are opposed to the polarizer and the analyzer, respectively. The light transmitted through one of the optical fibers is fed through the one of the rod lenses to the polarizer, and the light emitted from the analyzer is transmitted through the other rod lens onto the distal end of the other optical fiber. EP-A-345 759 shows a similar sensor, in which distal ends of optical fibers are connected by collimating lenses to an assembly of polarizer, magneto-optical element and analyzer. In the optical magnetic-field sensor described above having rod lenses, however, it is required to adjust the optical axes of the distal ends of the optical fibers and the rod lenses. A rod lens has a refractive index distribution in radial directions formed by adjusting radial distributions of metallic atoms. Therefore, if the center of the optical fiber is not coincident with the center of the refractive index distribution, the light is not collimated with high efficiency. Moreover, as the center of refractive index cannot be found out by judging from an external form of the rod lens, the adjustment or alignment of the optical axes is an operation in two dimensions in every directions, which is a time-consuming and troublesome operation. Moreover, in the event that rod lenses are used, optical axes of the distal end of the optical fiber and the rod lens are frequently not coincident with each other owing to the above reason, so that the collimation of light becomes incomplete to increase losses of quantity of light. Consequently, it has been impossible to enlarge distances between the light transmitter and the light receiver respectively and the fault detecting point to an extent beyond certain distances. It is also mentioned that US-A-4327963 shows a spherical lens having an internal refractive index gradient, used for coupling to or from an optical fiber. In one embodiment the lens is adjacent a fiber having the same diameter as the lens. Between the lens and the fiber there is a transparent coupling medium whereby refraction of the light upon departure from the lens is avoided. It is desired to provide a novel optical magnetic-field sensor, capable of easily carrying out the alignment of optical axes at a collimator portion with higher accuracy in a short time, and also to provide a method of producing such a novel optical magnetic-field sensor. By the invention we provide an optical magnetic-field sensor as set out in claim 1. The invention further provides methods of producing such an optical magnetic-field sensor as set out in claims 6 and 9. Alignment paths may be provided on the base which can locate the spherical lenses and/or means which mount the optical fibers, and facilitate controlled relative movement thereof during construction of the sensor. In particular, axially-extending alignment paths such as uniform grooves are useful. Ferrules used for mounting the optical fibers may be provided with tapering guide surfaces to engage and guide the lenses into alignment. The "optical parts" used herein are intended to include a polarizer and an analyzer. The distal ends of the optical fibers may be fixed to the sensor by means of ferrules. Features and embodiments are now discussed in more detail and with reference to the accompanying drawings in which: Fig. 1 is an enlarged partial sectional view illustrating the region of the distal end of an optical fiber used in an optical magnetic-field sensor embodying the invention; Fig. 2 is an enlarged partial sectional view illustrating the region of the distal end of an optical fiber used in another embodiment of the invention; and Fig. 3 is a schematic view illustrating one example of an optical magnetic-field sensor embodying the invention. First, the general construction of an optical magnetic-field sensor for automatically finding out fault points in electric power systems as one embodiment of the invention will be explained by referring to Fig. 3. The optical magnetic-field sensor shown in Fig. 3 has a U-shaped layout in which components on the input side, a device portion and components on the output side are arranged. However, they may be arranged side by side on a straight line. In more detail, a base 8 on which ferrules 3 are arranged on the input and output sides, respectively, is fixed into a casing 1. In the device portion, a polarizer 5, a magnetooptical element 6 and an analyzer 7 are arranged in that order. The positioning of the magnetooptical element 6 and the respective optical parts is effected by previously providing predetermined design patterns (for example, grooves or protrusion) on the base 8. A collimator portion will then be explained in detail by referring to Fig. 1 illustrating a distal end of the ferrule 3 in section on an enlarged scale. The base 8 is formed with grooves 11 having a constant width and, for example, having a V-shaped cross-section, in which spherical lenses 12 are fixed and supported by means of jigs (not shown). As the components on the input and output sides are substantially identical with each other, the components on the input side will be explained. An optical fiber 2 is fixed in the ferrule 3 arranged in the groove 11 formed in the base 8 by means of a jig (not shown). By positional adjustment of the ferrule 3, the optical axis of the distal end 2a of the optical fiber 2 is brought substantially into coincidence with that of the spherical lens 12. A synthetic resin 13, preferably epoxy resin, is filled in the clearance between the end face 3a of the ferrule 3 and the spherical lens 12 as shown in Fig. 1. Before solidification of the synthetic resin 13, the end face 3a of the ferrule 3 is finely adjusted onto the right and left sides viewed in Fig. 1 by means of a jig (not shown), while light beams are transmitted through the optical fiber 2, so that the distance between the distal end 2a of the optical fiber 2 and the spherical lens 12 is finely adjusted to position the distal end 2a of the optical fiber 2 in a most suitable position for collimating the light beams. After the positioning of the optical fiber 2, the synthetic resin 13 in position is permitted to solidify. As a result of the solidification of the synthetic resin 13, the spherical lens 12, the synthetic resin 13, the end face 3a of the ferrule 3 and the base 8 are maintained as a unitary body whose optical axes in the collimation portion have been adjusted to the optimum condition. It is also possible that, after the ferrule 3 has been fixed, the spherical lens is finely adjusted in the right-left sense to obtain the coincidence or alignment of the optical axes. As an alternative, after the spherical lens 12, the synthetic resin 13 and the end face 3a of the ferrule 3 have been joined as a unitary body but the synthetic resin 13 is not joined to the base 8, the positioning and fixation of the ferrule 3 to the base 8 may be effected mechanically or with an adhesive. As a further alternative, a synthetic resin having an elasticity after solidification may be used as the synthetic resin 13. In this case, after solidification of the synthetic resin filled in the clearance between the spherical lens 12 and the end face 3a of the ferrule 3, the ferrule 3 and/or the spherical lens 12 is finely adjusted onto the right and left so as to adjust the distance therebetween to accomplish the alignment of the optical axes of the optical fiber 2 and the spherical lens 12. Thereafter, the ferrule 3 and the spherical lens 12 are fixed to the base 8. With the optical magnetic-field sensor as described, the light emitted from the end face 2a of the optical fiber 2 on the input side is collimated at the synthetic resin 13 and the spherical lens 12. The collimated light is then passed through the polarizer 5 to become linearly polarized light which is then transmitted through the magnetooptical element 6 during which the light is subjected to Faraday rotation. Thereafter, the light is passed through the analyzer 7 during which its quantity of light is changed dependent upon the Faraday rotation. This quantity of light corresponds to the magnetic field applied to the magnetooptical element 6. The light emitted from the analyzer 7 is then condensed at the spherical lens 12 and the synthetic resin 13 on the output side and is incident onto the distal end 2a of the optical fiber on the output side. In the optical magnetic-field sensor of the embodiment shown, the collimation is performed by the use of the spherical lens 12. The spherical lens utilizes the curvature of a spherical surface to form the refractive index gradient, which is different from a rod lens using controlling of the chemical composition. Therefore, the optical center of the spherical lens 12 is coincident with its geometrical center. Consequently, so long as the height of the geometrical center of the spherical lens 12 is previously made coincident with the height of the optical fiber 2 in design by adjusting the depth of the groove 11, their optical axes can be ultimately coincident or aligned with each other with ease only by sliding the ferrule 3 on the base in the axial directions of the optical fiber without requiring the troublesome optical axis adjustment in two dimensions in the rod lens as described above. With the technique described, therefore, the alignment of optical axes can be remarkably easily performed with higher accuracy and less loss of light quantity in comparison with the prior art. In the embodiment shown in the drawing, moreover, as the synthetic resin is filled in the clearance between the spherical lens 12 and the distal end face 3a of the ferrule, there is no need for a spacer having a suitable refractive index and thickness, for example, required to have a surface accuracy for collimation corresponding to the spherical lens 12. The base 8 may be made of, for example, a metal, ceramic material, plastic material or the like. The ferrules 3 may be made of a ceramic material, such as alumina, zirconia or the like. The synthetic resin 13 may be , for example, an epoxy base adhesive, an acrylate base adhesive or the like. The synthetic resin having an elasticity after solidification may be, for example, silicone resin or the like. A spherical lens 12 may be fixed to the end of a ferrule 3 as shown in Fig. 2. In more detail, an optical fiber 2 is movable in the ferrule 3 in the right-left sense as viewed in Fig. 2, and the end of the ferrule 3 is formed with a V-shaped notch or conical recess 14 to which is fixed a spherical lens 12 registering with the inner surfaces of the notch 14. The clearance between the notch 14 and the spherical lens 12 is filled with a synthetic resin as described above. The optical fiber 2 is then finely adjusted onto the right or left to align the optical axes of the optical fiber and the spherical lens. The optical fiber 2 is positioned and the synthetic resin is then solidified. Under this state, the distal end face 23a of the synthetic resin 23 is brought into opposition to a polarizer 5 or a analyzer 7. With this arrangement, collimation can be carried out. The spherical lens 12 with the synthetic resin 23 and the ferrule 3 may be fixed to a base 8. As an alternative, after the spherical lens 12 has been received in the notch 14 of the ferrule 3, the spherical lens 12 is fixed to the ferrule 3 by means of the synthetic resin 23, and the ferrule 3 is then fixed to the base 8 by means of the synthetic resin 23 or another organic adhesive. The base 8 is preferably joined with the ferrules 3 by means of a synthetic resin adhesive. In this case, the adhesive layer between the base 8 and the ferrules 3 may be separate from the synthetic resin 23, and further may be a different material from the synthetic resin 23. Or, the adhesive layer may be merged with the synthetic resin 23, and in this case they are formable simultaneously by casting of the same material. While the outer circumferential surface of the spherical lens 12 is registered with the end face 23a of the synthetic resin 23 in the embodiment shown in Fig. 2, it will be apparent that the spherical lens 12 may partially project from the end face 23a or may be fully received in the notch 14 inwardly away from the end face 23a. The optical fibers 2 are shown fixed in ferrules 3 in the above embodiments. However, the ferrules 3 are not necessarily essential for the optical magnetic-field sensor. The optical fibers 2 may be fixed to the base 8 without using the ferrules. In this case, the techniques usually used in the field of microoptics are applicable. For example, the base 8 is formed in its surface with grooves having a V-shaped cross-section along the extending directions of the optical fibers, and the optical fibers are then arranged in the grooves and fixed to the base 8 by a synthetic resin adhesive. According to the the optical magnetic-field sensor and the method of producing the same, a synthetic resin is filled in the clearance between the spherical lens and the distal end of the optical fiber to position these members so that the distance between the spherical lens and the optical fiber can be determined so as to be best for collimation. Therefore, the optical magnetic-field sensor can be made to be superior in collimation characteristics and can reduce loss of quantity of light. Consequently, it is possible to enlarge the distances between locations for detecting fault points and a light transmitter and/or a light receiver. Moreover, the synthetic resin filled between the distal end of the optical fiber and the spherical lens makes easy the alignment of optical axes of the optical fiber and the spherical lens only by adjusting the distance between the distal end of the optical fiber and the spherical lens in one dimension. Therefore, the alignment of optical axes can be much more easily effected with higher accuracy. As a result, the productivity of high accuracy optical magnetic-field sensors may be significantly improved and their production cost significantly reduced.

Claims: An optical magnetic-field sensor having optical fibers (2) inserted into the sensor, at least one optical element (6) for optically sensing a magnetic field and optical paths for transmitting optical signals between distal ends (2a) of said optical fibers and said element or elements, said optical paths including lenses (12) optically aligned with the respective distal ends (2a) of the optical fibers (2) and acting to collimate light from the optical fiber on the light input side and to converge the light passing through the optical element (6) to the optical fiber on the light output side, characterised in that (i) a base (8) is provided relative to which said distal ends (2a) of said optical fibers (2) are fixed, (ii) said lenses (12) are spherical lenses, and (iii) synthetic resin (13,23) is filled in the gaps between said lenses (12) and said distal ends (2a), whereby on the light input side a diverging light beam from the distal end of the fiber is collimated into a parallel light beam by the refractive index difference as a result of the curvature of the spherical surface of the adjacent said lens and at the light output side a parallel light beam is converged to the distal end of the adjacent fiber by the refractive index difference as a result of the curvature of the spherical surface of the other said lens. A sensor according to claim 1, wherein said base (8) has in its surface grooves (11) respectively permitting adjustment of the position of at least one of said spherical lenses (12) and said distal end (2a) of the optical fiber when producing the sensor. A sensor according to claim 1 or claim 2 wherein said sensor includes a ferrule (3) receiving the optical fiber movably therein and formed with a recess (14) in which the spherical lens (12) is received so as to be inscribed on the inner surface of the recess (14). A sensor according to any one of the preceding claims, wherein said synthetic resin is made of one of epoxy based adhesive, acrylate base adhesive and silicone resin. A sensor according to any one of the preceding claims wherein the lens (12) is of larger diameter than the distal end (2a) of the fiber. A method of producing an optical magnetic-field sensor according to any one of claims 1 to 5 including the steps of filling the synthetic resin (13,23) between the distal ends (2a) of the optical fibers (2) and the spherical lenses (12), adjusting distances between the distal ends (2a) of the optical fibers and the spherical lenses (12) before solidification of the synthetic resin to align the optical axes of the distal ends and the lenses with each other, and then solidifying the synthetic resin. A method according to claim 6, wherein said distances between the distal ends of the optical fibers and the lenses are adjusted along alignment paths (11) formed in a surface of the base (8). A method according to claim 6 or claim 7, wherein said synthetic resin is filled between the distal ends (2a) and the spherical lenses (12) which are received in recesses formed in ferrules (3) provided for receiving the optical fibers movably therein, respectively. A method of producing an optical magnetic-field sensor according to any one of claims 1 to 5 comprising the steps of filling the synthetic resin (13,23) between the distal ends (2a) of the optical fibers and the spherical lenses (12) and adjusting distances between the distal ends (2a) and the spherical lenses (12) after solidification of the synthetic resin to align the optical axes of the distal ends and the optical lenses with each other. A method according to claim 9, wherein said synthetic resin is a resin having elasticity after solidification. A method according to claim 9 or claim 10, wherein said synthetic resin is silicone resin.

Inventors: ASAI YUJI, OKAJIMA HISAKAZU, ASAI, YUJI, OKAJIMA, HISAKAZU
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1991
Main classification: G01R33
IPC codes: G02B6, G01R33, G01R31, G01R15