DETERMINATION OF INDUCED CHANGE OF POLARIZATION STATE OF LIGHT

Abstract: A method of determining induced change of polarization state of light in a polarization element comprising transmitting unpolarized light from a light source at the one end of an optical transmitter waveguide means (21) to polarizer means (31) at the other end; polarizing the unpolarized light by the polarizer means (31); transmitting the polarized light through the polarization element (41, 41A, 41B) using at least one reflective optical element; analyzing the transmitted polarized light from the polarization element by an analyzer means (32); and transmitting the analysed polarized light from the one end of optical receiver waveguide means (22) to a light detector at the other end; wherein the unpolarized light polarized by the polarizer means (31), the polarized light analysed by the analyzer means (32), or both, are non-collimated; and the polarized light in the light path between the polarizer means (31) and the analyzer means (32) is collected and reflected by at least one reflective imaging optical element (51) so that the analyzed light exits the analyzer means (32) from the same side as the unpolarized light enters the polarizer (31). Further, fiber optic sensor devices for determining induced change of polarization state of light in a polarization element, particularly linear birefringence induced by electric voltage, electric field, and mechanical force, and circular birefringence induced by electric current and magnetic field.

Description: DETERMINATION OF INDUCED CHANGE OF POLARIZATION STATE OF LIGHT 1. BACKGROUND OF THE INVENTION The Technical Field The present invention relates to a method of determining induced change of polarization state of light in a polari- zation element, fiber optic sensor devices for determining such induced change of polarization state of light, particularly linear birefringence induced by an electric voltage, an electric field, and a mechanical force, and circular birefringence induced by an electric current and a magnetic field. A magnetic field, an electric field, or a mechanical for¬ ce, or a combination of these, can induce anisotropy in the index of refraction of an optically transparent elec- trooptic, magnetooptic and/or photoelastic material, re¬ spectively, which can be detected by a change of polari¬ zation state of light. Linear birefringence can be induced by an electric field in an electrooptic material by the so-called electrooptic Pockels effect and Kerr effect which can be used in the determination of electric voltage and electric field; circular birefringence can be induced by a magnetic field in a magnetooptic material by the so-called magnetooptic Faraday effect which can be used in the determination of electric current and magnetic field; and linear birefringence can be induced by a mechanical force in a photoelastic material by the so-called photoelastic effect which can be used in the determination of me- chan cal force e.g. acceleration and pressure. Generally, induced change of polarization state of light in a polarization element is used in fiber optic polari- metric optical sensors and the induced change can be determined by: a) transmitting unpolarized light from a light source at the one end of an optical transmitter waveguide means to polarizer means at the other end; b) polarizing the unpolarized light by the polarizer means; c) transmitting the polarized light through a polarization element; d) analyzing the transmitted polarized light from the polarization element by an analyzer means; and e) transmitting the analyzed polarized light from the one end of an optical receiver waveguide means to a light detector at the other end; wherein the unpolarized light is collimated by input coup¬ ling means between the optical transmitter waveguide means and the polarizer means, and the analyzed polarized light is focused by output coupling means onto the optical transmitter waveguide means, said coupling means consis- ting of lenses or graded index rod lenses. Specifically for a fiber optic magnetooptic current sensor, it has been suggested that the transmission of the polarized light through the polarization element is provided by using at least one plane reflective optical surface reflecting a collimated beam. These optical arrangements have a number of disadvantages. First of all, a collimated beam or slightly convergent beam is used which requires auxiliary coupling lenses to obtain an optimal light energy transmission from the op¬ tical transmitter waveguide means to the optical receiver waveguide means. Secondly, the use of conventional lenses as coupling means makes the size of the arrangement inconveniently large be¬ cause the lenses must be surrounded by air in order to obtain a suitably large change of refractive index at the optical surfaces. Also, mechanical holders for the lenses are required which complicates the mechanical construction. Further, in the case of a voltage or electric field sensor using conventional lenses, the air present around the lenses limits the electric field that can be applied to the sensor before disruptive electric discharge occurs. Graded index rod lenses may avoid the problem of air if coupled directly on to the optical waveguide, such as an optical fiber. Such an arrangement can be made compact and the optical interfaces can be filled with an electrically insulating optical cement. However, in order for the graded index rod lenses to collimate the beam from the transmitter, a relative small core diameter (max. 200 um) optical fiber is required which limits the amount of light that can be launched into the optical fiber by the light source, hereby limiting the signal-to-noise ratio and the sensitivity of such a device, unless compensated for by a more powerful light source. Thus, cheap visible light emitting diodes e.g. LED's emit¬ ting light in the range 500 - 700 nm cannot be used because they cannot supply sufficient light through such optical fibers having that small core diameter. Instead powerful and more expensive LED's emitting light in the range 800 - 950 nm must be used. However, LED's and polarizers working at these wavelengths as well as graded index rod lenses are expensive. Therefore, there is a need for a method of determining induced change of polarization state of light in a polarization element which can be implemented in a compact, small and relative cheap fiber optic sensor device. Prior Art Disclosure US Patent No. 3980949 discloses a magneto-optical measuring transducer for measuring both nominal and excessive high currents, said transducer having a first magnetically saturable part and a second paramagnetical part, the outer surfaces of the first and second parts being provided with reflective materials to provide multiple passages of polarized light through the transducer. Nothing is indicated or suggested about providing reflection by means of a reflective imaging optical element and non-collimated light. W.B. Spillman and D.H. McMahon, "Multimode Fiber Optic Sensors Based on the Photoelastic Effect", Proceedings of Fiber Optics and Laser Sensors, Arlington, Virginia, April 5-7, 1983, SPIE, Vol.412, Paper No. 412-17, p.110-114 disclose a multimode fiber optic sensor based on the photoelastic effect in which light from an input fiber is collimated by a graded index rod lens, and the analyzed light is separated into two components and injected into two output optical fibers via graded index rod lenses. UK Patent Application Publication No. 2159944 discloses an optical sensor, in particular an optical pressure sensor, comprising an electrically switchable optical 90° rotator. The sensor uses collimated light and does not comprise a reflective imaging optical element. US Patent No. 4613811 discloses a fiber optic magnetooptic current sensor for measuring current flowing through a conductor comprising two components one of which has a plane reflective surface for reflecting a polarized light beam between the. two components. Nothing is indicated or suggested about reflecting the polarized light by means of a reflective imaging optical element and non-collimated light. Hulshof et al., "Optical Voltage Sensor: Application in Electric Power Systems", SPIE Vol. 798, Fiber Optic Sen- sors II, 1987, disclose a transmission type optical fiber voltage sensor comprising a Pockels cell of bismuth-ger- maniumoxide and conventional lenses or graded index rod lenses as collimating or focusing coupling means. EP Patent Application Publication No. 0284346 discloses an optical interface coupled to a Faraday rotator device disposed in surrounding proximity to an energy transmission line in a magnetooptic current transducer which interface comprises a collimator segment coupled to a spacer lens element focusing a divergent beam of input light from a fiber optic cable into a collimated beam directed to the Faraday rotator device through a po¬ larizing element by a parabolic surface. US Patent No. 4841234 discloses a fiber optic voltage detector comprising an optical probe having an electro- optic material worked into a frusto-conical shape the tip of which is coated with a reflecting mirror. The optical probe comprises a collimator providing collimated light which is reflected by the reflecting mirror. Nothing is indicated or suggested about providing reflection by means of a reflective imaging optical element and non-collimated light. US Patent No. 4948255 discloses a fiber optic optical sensing device comprising an element provided with two total reflection surfaces causing the beam path to invert its direction by 180°. The device uses light collimated by e.g. rod lenses and does not comprise a reflective imaging optical element. 2. DISCLOSURE OF THE INVENTION In a primary aspect, it is the object of the present in¬ vention to provide a method and a fiber optic sensor device for determining induced change of polarization state of light in a polarization element which method can be carried out and be implemented in a compact, small, and relatively cheap device which is powered by a low power output light source and easy to mount. It is another object of the present invention to provide such a method and device which can withstand large elec¬ trical fields without being subjected to disruptive electric discharge. Further, it is still another object of the present inven¬ tion to provide such a method and device which has a good temperature stability. Method of Determining Induced Change of Polarization State of Light In one aspect, the present invention provides a method of determining induced change of polarization state of light in a polarization element comprising: a) transmitting unpolarized light from a light source at the one end of an optical waveguide transmitter means to polarizer means at the other end; b) polarizing the unpolarized light by the polarizer means; c) transmitting the polarized light through the polarization element using at least one reflective optical element; d) analyzing the transmitted polarized light from the polarization element by an analyzer means; and e) transmitting the analyzed polarized light from the one end of optical waveguide receiver means to a light de¬ tector at the other end; wherein f) the unpolarized light polarized by the polarizer means, the polarized light analyzed by the analyzer means, or both, are non-collimated; and g) the polarized light in the light path between the po¬ larizer means and the analyzer means is collected and reflected by at least one reflective imaging optical element so that the analyzed light exits the analyzer means from the same side as the unpolarized light en- ters the polarizer. According to the invention, the unpolarized light pola¬ rized by the polarizer means, the polarized light analysed by the analyzer means, or both, are non-collimated whereby auxiliary waveguide coupling means, particularly collimating and focusing coupling lenses, or graded index rod lenses, are avoided. Further, according to the invention in combination with the non-collimated light, the polarized light in the light path between the polarizer means and the analyzer means is collected and reflected by at least one reflective imaging optical element so that the analyzed light exits the analyzer means from the same side as the unpolarized light enters the polarizer, whereby the transmitter and receiver optical waveguide means can be introduced from the same side of the device. In combination the two features provides the effect that simultaneously components can be saved and imaging can be performed over a relatively short length-to-width ratio whereby a compact device having a relatively large proportion of active material compared to the total volume can be obtained. Also, because a reflective surface requires little volume compared to lenses, a relatively small device can be ob¬ tained compared to conventional lens devices because the light path is used more efficiently by applying active ma¬ terial in the space otherwise occupied by graded index rod lenses or conventional lenses and air. Further, because the optical transmitter and receiver wa- veguide means can be introduced from the same side of the polarization element, the device is easy to mount, parti- 9 - cularly in applications where the quantity to be measured only can be approached from the one side. Also, by applying active material in the space otherwise occupied by lenses and particularly by air, it can with¬ stand large electrical fields without being subject to disruptive electric discharge. Further, because e.g. graded index rod lenses can be avoided, relatively cheap large core diameter optical wa¬ veguide means having diameters in the range 200 - 1000 am can be used. Thus, a better transmission efficiency of the optical path through the apparatus from light source to detector compared to sensors using conventional 50 - 100 um optical fibres can be obtained. Further, an increased signal-to-noise ratio and sensitivity is obtained compared to conventional sensors, just as a comparable accuracy is obtained at a lower optical power level. Also, an increase in sensitivity can be obtained by applying more reflective imaging elements which provides multiple transmissions through the polarization element and increases the total optical path in the polarization element. Reflective Imaging Optical Elements According to the invention, the polarized light in the light path between the polarizer means and the analyzer means is collected and reflected by at least one reflec¬ tive imaging optical element so that the polarized light is transmitted one or more times through the polarization element before being analyzed. A simple embodiment involves only one reflective imaging optical element. Therefore, in a preferred embodiment, the polarized light is transmitted through the sensitive polarization element one or more times, preferably once or twice, by one re¬ flective imaging optical element. Generally, to obtain a greater sensitivity, the polarized light can be transmitted through the polarization element a multiple number of times by more reflective optical elements, at least one of which is imaging. Specifically, a polarization element having a reflective plane surface with apertures for the light input and output and a reflective convex surface opposite to the plane surface can provide a total of four transmissions of the polarized light through the polarization element. Therefore, in another preferred embodiment, the polarized light is transmitted through the sensitive polarization element one or more times, preferably one, two or four times, by two reflective elements at least one of which is imaging. According to the invention, reflective imaging optical elements can be provided by methods known in the art. In a preferred embodiment, a reflective imaging optical element consist of a convex surface of the polarization element coated with a reflective coating, the curvature of which can be provided by methods known in the art of lens manufacturing e.g. using ray tracing, provided the aperture of the optical transmitter waveguide means and the polarizer means, the thickness and index of refraction of the polarizer means, the length and index of refraction of the polarization element, the thickness and index of refraction of the analyzer means and the aperture of the analyzer means and optical receiver waveguide means are known . Further, the reflective coating can be selected from re¬ flective coatings known in the art including metallic coatings such as Al, Ag, and Au, and dielectric coatings such as quartz, ZnS, Ti0 2 , Sb^O.-,. In some cases, particularly when expensive or fragile polarization elements are applied, it may be preferred not to provide the convex surface directly on the polarization element, but on e.g. a planoconvex lens. Therefore, in another preferred embodiment, the reflective imaging optical element consists of a planoconvex lens with a convex surface coated with a reflective coating. These lenses can be provided by methods known in the art of lens manufacturing including optical design by ray tra¬ cing. In other cases, it may be preferred to apply a reflective diffractive optical element, particularly to avoid the relatively costly process of manufacturing a convex sur¬ face, which elements can be provided by methods known in the art including methods for manufacturing holographic or computer generated diffractive optical elements (P. Hariharan "Optical Holography", Cambridge University Press, 1984; L. Solymar and D.J. Cooke "Volume Holography and Volume Grating", Academic Press 1981; and R.R.A. Syms "Practical Volume Holography", Claradon Press, Oxford 1990). Therefore, in still another preferred embodiment, the re¬ flective imaging optical element consists of a diffractive optical element. Polarization Element According to the invention, the polarization element con¬ sists of i) a sensitive polarization element comprising a suitable polarization active material in which a magnetic field, an electric field, or a mechanical force can induce anisotropy in the index of refraction, and ii) optionally an auxiliary polarization element providing phase retardation or polarization rotation of po¬ larized light. Polarization active materials some of which have intrinsic birefringence are known in the art and comprise: a) electrooptic materials including crystals such as CdF, GaAs, GaP, 0-ZnS, ZnSe, ZnTe, Bi 4 Ge 3 0 12 , Bi 12 Ge0 2Q , Bi 12 SiO 20 , KH 2 P0 4 , D 2 P0 4 , NH^PO^ NH^PO^ LiNbO.-,, LiTaO , I0 , and quartz (Si0 2 ), and electrooptic polymer manufactured by poling of polymers such as epoxy or polymethylmethacrylate containing an electro¬ optic dye such as one or more azo dyes such as Disperse Red 1 (4-(4-nitrophenylazo)-N-ethyl-N-2-hydroxyethyl- aniline), (by poling is meant a process in which the polymer is brought to a phase transition from a mobile state to an immobile state under influence of an electric field, i.e. poling field, such that the alignment to electrooptic dye due to the poling field molecules is preserved after removal of the field. The poling process is described in Allan Gόttsche: "Electrooptic and Magnetooptic Sensors for Advanced Applications in Electric Powersystems", Electric Power Engineering Department, Technical University of Denmark Publication no. 9005 (1990)); b) magnetooptic materials including glasses such as fused silica, and diamagnetic glasses such as BK7, SF6, SF57, SF58, SF59 (Shott), FR-4, FR-5, FR-7 (Hoya), and M-16 (Kigre), and crystals such as quartz (SiO«), EuF~, Tb 3 Al 5 0 12 , LiTbF 4 , ZnSe, CeF 3 , LaF 3 , Bi 4 Ge 3 0 12 , Bi 12 Ge0 20 , CdMnTe, γ 3 F e 5 0 12 (YIG), Tb χ Y χ (IG); and c) photoelastic materials including glasses such as ZKN7, FK5, BK7, SK14, SKI6, F2, LaFN2, SF2, SF4, and SF5 (Schott) . All optical materials possess the magnetooptic Faraday effect. However, the magnitude of the effect varies. Also, some magnetooptic materials simultaneously show electrooptic effect, e.g. materials such as Bi.Ge 3 0- , Bi 12 GeO 20 and Bi 12 Si0 20- The sensitive polarization element can consist of mate¬ rials selected from materials having intrinsic birefrin- gence, and the physical dimensions can be chosen so that the intrinsic birefringence provides for a λ/4 phase re¬ tardation whereby an auxiliary polarization element can be avoided. The auxiliary polarization element can consist of a phase retardation means preferable providing a phase retardation of λ/4 comprising phase retardation materials known in the art including dichroic retardation plates, mica retarda¬ tion plates and quartz retardation plates, e.g. all such plates sandwiched between glass plates. The auxiliary polarization element is preferably used together with electrooptic or photo-elastic materials having little or no intrinsic birefringence. As mentioned below, phase retardation means can be provided for by reflective optical elements which direct the polarized light through .the polarization element whereby an auxiliary polarization element is also avoided. Temperature and Wavelength Stability Enhancement Especially, the phase-retardation means may be incorpora¬ ted so that a high tolerance against wavelength and tem¬ perature instabilities are obtained. Phase retardation based on reflection is rather insensiti¬ ve to temperature changes as a change in physical dimen¬ sions of a reflecting element, unlike birefringent phase retardation elements, does not influence the amount of phase retardation (H. Fabricius "Achromatic Prism Retarder for Use in Polarimetric Sensors", Applied Optics, Vol. 30, No. 4, 1991, pp. 426-429). Therefore, according to the invention, phase-retardation means include phase-retardation means based on reflection. In a preferred embodiment, the reflective imaging optical element consists of an optionally coated convex surface of the polarization element reflecting by total internal reflection. In some cases, particularly where expensive or fragile polarization elements are applied, it may be preferred not to provide the convex surface directly on the polarization element, but instead on an additional optical element. Therefore, in another embodiment, the reflective imaging optical element consists of an optionally coated convex surface of an additional optical element reflecting by total internal reflection. Because phase retardation only depends on total internal reflection based on the indices of refraction of the ele¬ ment itself and the surrounding air, it is pratically in¬ sensitive to variations in temperature. The element con- sists of an optical material with a convex total internal reflecting surface which, if chosen to be parabolic, is able to collimate a divergent beam emitting from the focal point of the parabolic surface, or vice versa, is able to focus a collimated beam in the focal point. If not para- bolic, the convex surface will still be able to provide phase retardation, however, a collimated beam is not obtained. When reflected, optical rays fulfilling the angular requi- rement of total internal reflection will obtain a phase retardation which is a function of the index of refraction of the element and the angle between the incident ray and the reflected ray. Though each ray of the beam obtains a somewhat different phase retardation, it is possible to select the index of refraction of the element, provided that the intensity distribution of the optical beam is known, so that a single reflection of the whole beam in the surface will induce an effect, which when analyzed by the analyzer and integrated on the detector can substitute the effect of a λ/8 phase retardation element. When an optical beam is reflected twice at a single sur¬ face, or reflected at two different surfaces, the total net effect on the optical power analyzed and integrated on the detector can substitute for a λ/4 phase retardation element. Polarizer Means The polarizer means comprises linear polarizers including dichroic sheet polarizers, glass polarizers (e.g. 8612 Polarcor™ from Corning), polarizing beamsplitters, and calcite polarizers. Analyzer Means The analyzer means comprises means to discriminate between the polarization state of light selected by the polarizer means and the phase retardation means, if present, and the polarization state of light leaving the polarization element. The analyzer means comprises the same type of linear pola¬ rizers as mentioned for the polarizer means. Optical Transmitter and Receiver Waveguide Means Optical transmitter and receiver waveguide means comprises optical fibers known in the art such as single and multi¬ mode optical fibers, particularly multimode fibers having large cores are preferred. Light Sources and Detectors Light sources comprises suitable light sources for coup- ling to optical fibers including light emitting diodes (LED). Particularly low cost LED's can be used in connec¬ tion with large core diameter optical fibers. Light detector comprises suitable light detectors for coupling to optical fibers including PIN and avalanche photodiodes and phototransistors which are able to detect the light from the applied light source. The optical power transmitted through the analyzer is proportional to the optical power emitted by the light source, but modulated by the measurement quantity. The modulated optical signal .is detected by the light detector and hereby converted to an electric current, which by electronic amplification means is converted to an electric voltage, the magnitude of which is proportional to the optical power transmitted through the analyzer. The component of the signal proportional to the measurement quantity can be extracted from noise signals and the attenuation due to loss in the optical system can be compensated for by means of electronic signal conditioning means, i.e. signal processing electronics, known in the art of fiber optical sensing and polarimetric optical measurement methods. See e.g. Hulshof et al. ibid; Spillman and McMahon ibid; and EP Patent Application Publications No. 0247842 and 0254396. Flier Optic Sensor Devices The invention relates to sensor devices particularly fiber optic sensor devices for determining induced change of polarization state of light in a polarization element. In another aspect the present invention provides a fiber optic sensor device for determining induced change of polarization state of light in a polarization element, particularly circular birefringence induced by electric current and magnetic field comprising: a) a polarizer means connected to the one end of an opti¬ cal transmitter waveguide means emitting unpolarized light from a light source; b) a polarization element; c) at least one reflective optical element; and d) an analyzer means connected to the one end of an opti¬ cal receiver waveguide means transmitting the analyzed light to detection means; wherein e) said at least one reflective optical element is an imaging optical element reflecting the light in the light path between the polarizer means and the analyzer means so that the analyzed light exits the analyzer means from the same side as the unpolarized light en¬ ters the polarizer. A simple embodiment involves only one reflective imaging element. Therefore, in a preferred embodiment, the polarized light from the polarizer means is transmitted through the sensi¬ tive polarization element one or more times, preferably once or twice, by one reflective imaging optical element. A greater sensitivity is obtained by transmitting the po¬ larized light through the polarization element a multiple number of times by more reflective optical elements, at least one of which is imaging. Therefore, in another preferred embodiment, the polarized light is transmitted through the sensitive polarization element one or more times, preferably one, two or four ti- mes, by two reflective imaging optical elements at least one of which is imaging. The at least one reflective imaging optical element is se¬ lected from a group consisting of: i) a convex surface of the polarization element coated with a reflective coating; ii) a planoconvex lens with a convex surface coated with a reflective coating; and iii) a reflective diffractive optical element. The optical waveguide transmission and receiver means consists of one or more multi-mode optical fibers con¬ nected to one or more polarizer means and analyzer means, respectively. In still another aspect the present invention provides a fiber optic sensor device for determining induced change of polarization state of light in a polarization element, particularly linear birefringence induced by electric voltage, electric field, and mechanical force, comprising: a) a polarizer means connected to the one end of an opti¬ cal transmitter waveguide means emitting unpolarized light from a light source; b) a polarization element comprising phase retardation means; c) at least one reflective optical element; and d) an analyzer means connected to the one end of an opti- cal receiver waveguide means transmitting the analyzed light to detection means; wherein e) said at least one reflective optical element is an imaging optical element reflecting the light in the light path between the polarizer means and the analyzer means so that the analyzed light exits the analyzer means from the same side as the unpolarized light en¬ ters the polarizer (31). A simple embodiment involves only one reflective imaging element. Therefore, in a preferred embodiment, the polarized light from the polarizer means is transmitted through the sen¬ sitive polarization element one or more times, preferably once or twice, by one reflective imaging optical element. A greater sensitivity is obtained by transmitting the po- larized light through the polarization element a multiple number of times by more reflective optical elements, at least one of which is imaging. Therefore, in another preferred embodiment, the polarized light is transmitted through the sensitive polarization element one or more times, preferably one, two or four times, by two reflective imaging optical elements at least one of which is imaging. The at least one reflective imaging optical element is se¬ lected from a group consisting of: i) a convex surface of the polarization element coated with a reflective coating; ii) a planoconvex lens with a convex surface coated with a reflective coating; iii) a reflective diffractive optical element; iv) an optionally coated convex surface of the sen¬ sitive polarization element reflecting by total internal reflection; and v) an optionally coated convex surface of an addi¬ tional optical element reflecting by total in¬ ternal reflection. The optical waveguide transmission and receiver means con- sists of one or more multi mode optical fibers connected to one or more polarizer means and analyzer means, respectively. Specifically, it should be mentioned that simple sensor devices, incorporating only a single optical fiber or a single set of transmitter/receiver optical fibers, when operated according to standard polarimetric practice, can only be used to perform measurements of dynamic signals, that is to say impulse or AC signals, because the DC component of the transmitted optical signal, as obtained after the light has passed the analyzer, is used by the driver circuit of the sensor to compensate for dynamic loss in the optical fibers. Therefore, unless it is otherwise explicitly stated, the optical sensor devices described are used to measure dynamic signals, such as AC or impulse magnetic field, electric field, electric current, electric voltage or dynamic mechanical signals such as vibration, acceleration or dynamic pressure changes. Measurement of static signals (DC signals) can be performed when using more than a single set of transmitter/receiver fibers and polarizers and analyzers. Definition of Expressions Within the present context the expression "fiber optic sensor" is only intended to designate an optical sensor device supplied with fiber optic waveguides for transmission of light to and from the optical sensor device. Therefore, the expression "fiber obtic sensor" does not imply that the fiber optical waveguides themselves are the sensitive polarization elements, wherein a change of polarization state of light occurs. Within the present context the expression "linear bire- fringence" designates the ability of an optical material to split up an incident wave of polarized light into two linearly polarized wave components with the polarization planes coinciding with characteristic directions of the material, which wave components are being transmitted with different propagation velocities, resulting in a phase retardation of the transmitted wave generated by recombination of the linearly polarized wave components after passing of the material, providing change in polarization state of the transmitted optical wave relative to the incident wave. Also, phase retardation can occur by letting a polarized wave undergo a reflection at a non-zero angle, preferably as total internal reflection, or reflection by a reflec- tion coating. Phase-retardation means consists either of one or more media of anisotropy of refractive index, i.e. linear bi- refringent media, or of reflecting surfaces or a combina- tion of the two. A phase retardation of τt/2 radian or λ/4 wavelength i.e. a λ/4 waveplate can convert linearly pola- rized light into circularly polarized light which provides an optimal offset for a working point for the detection of induced linear birefringence, e.g. in electrooptic and photo-elastic sensors. Further, within the present context the expression "cir¬ cular birefringence" designates the phenomenon that a wave of polarized light incident on a medium of anisotropy of refractive index with respect to the direction of rotation of circularly polarized light is separated into two counter rotating circularly polarized polarization com¬ ponents with different propagation velocities resulting in a polarization rotation of the transmitted wave generated by recombination of the circularly polarized polarizations after passing of the material. Circular birefringence causes the plane of polarization of linearly polarized light to rotate. A phase retardation of λ/4 or τr/2 radians causes a change from a linear to a circular polarization state. Also, within the present context, when it is mentioned that the unpolarized light polarized by the polarizer means and the polarized light analyzed by the analyzer means, or both, are non-collimated, the expression "non- collimated" designates that the light emitted by the opti¬ cal waveguide transmitter means propagate as a divergent beam through the polarizer means and polarization element propagating according to the physical laws of optical wave propagation as defined by the numerical aperture and core diameter of the optical transmitter waveguide means and the indices of refraction of the polarizer means and polarization element, and that the light collected and reflected by the at least one reflective imaging optical element is reflected back through the polarization element and analyzer means as a convergent beam propagating according to the physical laws of optical wave propagation as defined by the indices of refraction of the polariza¬ tion element and the analyzer means and the aperture and focal length of the reflective imaging optical element. Further, within the present context it should implicitly be understood that everytime the expressions "polarization element", "auxiliary polarization element", "phase- retardation means", "polarizer means" and "analyzer means" are mentioned, the characteristic optical axes of the devices mentioned are aligned according to the art of polarimetric measurement practice as described in optics litterature. That is to say that for the measurement of induced linear birefringence such as linear electrooptic effect or photo¬ elastic effect, the polarization axes of the polarizer means and analyzer means and the so-called "fast" and "slow" optical axes of the phase-retardation means and the polarization element should all be aligned according to polarimetric measurement practice. Accordingly for the measurement of induced circular birefringence such as the magnetooptic Faraday effect, the polarization axes of the polarizer means and analyzer means and the "fast" and "slow" axes of the auxiliary polarization element, if present, and the "fast" and "slow" axes of intrinsic bire¬ fringence in the polarization element, if such intrinsic birefringence is present, should all be aligned according to polarimetric measurement practice. Polarimetric measurement practice is decribed in the optics litterature, specifically in: 1) Eugene Hecht: "Optics", 2n Edition, Addison-Wesley Publishing Company Inc., World Student Series (1987); 2) A. Yariv and P. Yeh: "Optical waves in crystals", Wiley-Interscience Publication, (1984); 3) Allan Gόttsche: "Electrooptic and Magnetooptic Sensors for Advanced Applications in Electric Powersystems", Electric Power Engineering Department, Technical University of Denmark Publication no. 9005 (1990). 3. BRIEF DESCRIPTION OF THE DRAWINGS Other objects and scopes of the present invention will become apparent from the detailed description and specific examples only illustrating preferred embodiments with reference to the drawings where Fig. IA shows a preferred embodiment of a fiber optic magnetooptic sensor device; Fig. IB shows a cross sectional view along the line Ib-Ib on Fig. IA; Fig. IC shows another embodiment of the device in Fig. IA, where the polarized light is passing the polarization element four times; Fig. 2 shows another embodiment of the device in Fig. 1; Fig. 3 shows a preferred embodiment of a fiber optic magnetooptic sensor device comprising a diffractive opti¬ cal element as the reflective imaging optical element; Fig. 4 shows another embodiment of the device in Fig. 3 comprising one optical fiber transmitter and two optical fiber receivers; Fig. 5A, 5B, 5C and 5D show preferred embodiments of an electrooptic/photoelastic optical fiber sensor device; Fig. 6A shows a preferred embodiment a magnetooptic opti- cal fiber sensor device comprising one optical fiber si¬ multaneously functioning as transmitter and receiver; Fig. 6B shows a preferred embodiment of an electrooptic/- photoelastic sensor device comprising one optical fiber simultaneously functioning as transmitter and receiver; Fig. 7A shows a preferred embodiments of an electrooptic/- photoelastic sensor device comprising a combination of an additional optical element including a reflective imaging optical surface and a plane mirror substituting a λ/4 optical element; Fig. 7B shows another embodiment of the device in Fig. 7A where the sensitive polarization element is positioned between the reflective imaging optical element and the plane mirror; Fig. 7C shows another embodiment of the device in Fig. 7A, where the reflective imaging optical surface is a convex surface of the sensitive polarization element; Fig. 7D shows another embodiment of the device in Fig. 7A, where the additional optical element comprising a reflective imaging optical surface is constituted by a plano-convex lens mounted on a plane surface of the sen¬ sitive polarization element; Fig. 7E shows a cross sectional view along the lines VIIe- Vlle on Fig. 7A and Fig. 7D for preferred embodiments using one or two optical fibers, respectively; Fig. 7F shows a cross sectional view along the lines Vllf- VIIf in Fig. 7B and Fig. 7C for preferred embodiments using one or two optical fibers, respectively; Fig. 7G shows another embodiment of an electrooptic/photo- elastic sensor device comprising a combination of a polarization element having a reflective imaging optical surface and an additional optical element including a plane total internal reflective surface having phase retarding properties for the substitution of a λ/4 optical element; Fig. 7H shows a cross sectionsl view along the lines Vllh- Vllh in Fig. 7G for preferred embodiments using one or two optical fibers, respectively; Fig. 8A shows another embodiment of the device in Fig. 7A comprising two phase retardation elements; Fig. 8B shows another embodiment of the device in Fig. 7B comprising two phase retardation elements; Fig. 9 shows an application of a magnetooptic fiber optic sensor device according to the invention placed in a magnetic field integrating core; Fig. 10A shows an application of an electrooptic sensor device according to the invention placed between the elec¬ trodes of a voltage sensor; Fig. 10B shows an application of the temperature and wavelengτh stability enhanced electrooptic sensor device shown in Fig. 7C according to the invention placed between the electrodes of a voltage sensor; Fig. IOC shows an application of the temperature and wavelength stability enhanced electrooptic sensor device shown in Fig. 7D according to the invention placed between the electrodes of a voltage sensor; Fig. 11 shows an application of an electrooptic sensor device along with a magnetooptic sensor device including a magnetic field integrating core, both of which devices according to the invention are placed inside an electri- cally grounded bushing surrounding a single conductor of an electrical cable. Fig. 12 shows an application of a photoelastic sensor device according to the invention placed between a float- ing mass and a support for measuring acceleration; and Fig. 13 shows an application of a photoelastic sensor device according to the invention which is connected via a rod to a membrane for measuring pressure. DETAILED DESCRIPTION According to the invention, the method and apparatus for determining induced change of polarization state of light in a polarization element can be used both for linearly and circularly induced birefringence in the polarization element, depending on whether the polarization active ma¬ terial is magnetooptic, electrooptic, or photoelastic. The only difference being the requirement of phase retardation means in case of linearly induced birefringence in electrooptic and photoelastic materials. Thus, the detailed description of preferred embodiments should be taken to illustrate both types of birefringence, even if not explicitely stated. If not otherwise explicitly stated, the optical axes of all optical elements have been aligned according to polarimetric measurement practice as mentioned earlier. Magnetooptic Optical Fiber Sensor Devices Fig. 1 shows a preferred embodiment of a magnetooptic fiber optic sensor device 10. Unpolarized light is transmitted from a light source (not shown) at the one end of an optical fiber transmitter 21, e.g. 600 μm core multimode plastic-clad-silica fiber TECS™, Technology-Enhanced Clad Silica, FT-600-LMT (3M), to a polarizer means 31 consisting of a linear polarizer, e.g. a 8612 Polarcor™ glass polarizer from Corning, at the other end. The unpolarized diverging light from the optical fiber 21 is polarized linearly by the polarizer 31, and transmitted • through a polarization element 41 consisting of a circularly birefringent magnetooptic material causing polarization rotation when subjected to magnetic fields, e.g. the glass SF6 from Schott. The linearly polarized but polarization rotated light is collected and reflected back through the polarization element 41 by a reflective optical surface 51 obtaining further polarization rotation. The reflective imaging op¬ tical surface 51 is a convex surface of the polarization element 41 which is coated with a reflective coating of aluminum. The reflected light is focused through an analyzer means 32 consisting of a linear polarizer, e.g. a 8612 Polarcor™ glass polarizer from Corning, transmitting the linearly polarised light of a selected polarization on to the one end of an optical fiber receiver 22, e.g. 600 um core multimode plastic-clad-silica fiber TECS™, Technology- Enhanced Clad Silica, FT-600-LMT (3M), transmitting the analyzed light to a light detector (not shown) at the other end of the fiber. Thus, applying a magnetic field, a change of polarization state of light in the polarization element 41 can be de¬ termined from the change of light intensity detected by the light detector. Because of the symmetry of rotation of the polarization element 41, the convex surface 51, and the planoconvex lens 52, if present (see Fig. IB), more sets of polarizers and analyzers and transmitter and receiver fibers can be positioned around the axis of symmetry e.g. for a more thorough polarization analysis for the measurement of static magnetic field or DC current. Fig. IB shows a cross sectional view along the line Ib-Ib on Fig. IA indicating sets of polarizers and analyzers 31,32 and 31A, 32A, and the optical fiber transmitter/re¬ ceivers 22, 22 and 21A, 22A, respectively. Fig. IC shows another embodiment of the device in Fig. IA comprising a second reflective optical element, which optionally may be imaging, but here is shown as a plane surface 54 coated with a reflective coating, except in the two apertures, where the light from the optical transmitter fiber 21 is allowed to be transmitted from the polarizer 3 into the polarization element 41, and the reflected light is transmitted from the polarization element 41 through the analyzer 32 and into the receiver fiber 22. The light from the transmitter fiber 21 is collected by the reflective optical imaging surface 51 and reflected back through the polarization element 41 here shown specifically in a collimated form, reflected forward again, still in a collimated form, by the reflective optical surface 54, and finally collected once again by the reflective optical imaging surface 51 and reflected back through the polarization element 41 again, while being focused on the receiver optical fiber 22. In this way, the polarized light is transmitted through the polarization element 41 a total of four times, hereby yielding a higher sensitivity of the sensor device. It should be noted that the reflections within the polarization element are not restricted to light in a collimated from. Furter, it should be noted that the transmission of the polarized light four times through the polarization element can be accomplished with either one of the reflective optical elements being imaging and the other being non-imaging or with both of the reflective optical elements being imaging. Consequently, it is also possible to let both of the reflective optical surfaces 51, 54 be convex imaging surfaces, or let the reflective optical surface 51 be a plane reflective surface and the reflective optical surface 54 be an imaging convex surface. By designing the two reflective optical elements correctly, it is theoretically possible to select the number of double-passes of the polarized light through the polarization element as any positive integer. Fig. 2 shows another embodiment of the device in Fig. 1 comprising a planoconvex lens 52 having a convex surface coated with a reflective coating of aluminum and coupled to the polarization element 41. Optical cement, optical epoxy, or other materials may optionally be applied between the lens and the polarization element. Fig. 3 shows another preferred embodiment of a magneto¬ optic optical fiber sensor device comprising a diffractive optical element 61 as the reflective imaging optical element. Fig. 4 shows another embodiment of the device in Fig. 3 comprising one optical fiber transmitter 21 and two opti¬ cal fiber receivers 22A, 22B. The number of optical fiber transmitters and receivers can be increased. Thus, one transmitter and four receivers can be used for the mea¬ surement of static signals, e.g. dc current. Electrooptic /Photoelastic Optical Fiber Sensor Devices Fig. 5A show preferred embodiments of an electrooptic/- photoelastic optical fiber sensor device 11. Unpolarized light is transmitted from a light source (not shown) at the one end of an optical fiber transmitter 21, e.g. a 600 um core multimode plastic-clad-silica fiber TECS™, Technology-Enhanced Clad Silica, FT-600-LMT (3M), to a polarizer means 31 consisting of a linear polarizer, e.g. a 8612 Polarcor™ glass polarizer from Corning, at the other end. The unpolarized diverging light from the optical fiber 21 is polarized linearly by the polarizer 31, and retarded by an auxiliary polarization element 41B consisting of a thin disc of linear birefringent material, e.g. a λ/4 quartz retardation plate. The retarded polarized light is then transmitted through a sensitive polarization element 41A and is subjected to induced phase retardation in the element. The element consists of a linear birefringent electrooptic polymer manufactured by poling of epoxy containing a sufficient concentration of the electrooptic azo dye Disperse Red 1 with the systematic name 4-(4- nitrophenylazo)-N-ethyl-N-2-hydroxyethylaniline. In case of a photoelastic sensor device, the polarization element 41A consists of a linear birefringent photoelastic material, e.g. ZKN7 glass from Schott. The circularly polarized light is collected and reflected back through the sensitive polarization element 41A by a reflective optical surface 51 and hereby being subjected to further phase retardation. The reflective imaging optical surface 51 is a convex surface of the sensitive polarization element 41A which is coated with a reflective coating of aluminum. The reflected light is focused on an analyzer means 32 consisting of a linear polarizer, e.g. a 8612 Polarcor™ glass polarizer from Corning, transmitting a linearly po- larised component of selected polarization on to the one end of an optical fiber receiver 22, e.g. a 600 μm core multimode plastic-clad-silica fiber TECS™, Technology- Enhanced Clad Silica, FT-600-LMT (3M), transmitting the analyzed light to a light detector (not shown) at the other end of the fiber. Thus, applying an electric field or mechanical force, a change of polarization state of light in the sensitive polarization element 41A can be determined by the change of light intensity detected by the light detector. In Fig. 5B, the auxiliary polarization element 41B is po¬ sitioned close before the analyzer means 32, i.e. after the reflective imaging optical surface 51 in the light path, but before the analyzer means 32. In Fig. 5C, the auxiliary polarization element 41B is in¬ tegral with the sensitive polarization element 41A. In Fig. 5D, the auxiliary polarization element 41B includes the reflective imaging optical surface 51. It should be noted that the embodiments of the reflective imaging optical element in Figs. 2,3 and 4 consisting of the planoconvex lens 52 and the diffractive optical ele¬ ment 61 can also be implemented for this electrooptic/- photoelastic sensor device. Likewise more sets of po¬ larizers and analyzers, and auxiliary polarization elements and transmitter/receiver fibers can be applied parallel to what is shown in Fig. IB and Fig. 4. Magnetooptic/ Electrooptic/ Photoelastic Fiber optic sensor Devices Using One Optical Fiber Fig. 6A shows a preferred embodiment of a magnetooptic fiber optic sensor device comprising one optical fiber simultaneously functioning as transmitter and receiver. Unpolarized light is transmitted from a light source (not shown) to a beam splitter or fiber splitter (not shown) and a collimator lens (not shown) at the one end of an optical fiber transmitter/receiver 23, e.g. a 600 um core multimode plastic-clad-silica fiber TECS™, Technology- Enhanced Clad Silica, FT-600-LMT (3M), to a polarizer means 31 consisting of a linear polarizer, e.g. a 8612 Po¬ larcor™ glass polarizer from Corning, at the other end. The unpolarized divergent light from the optical fiber 23 is polarized linearly by the polarizer/analyzer means 31, and phase retarded by an auxiliary polarization element 41B consisting of a thin λ/4 disc of linear birefringent material, e.g. a λ/4 quartz retardation plate, and trans¬ mitted through a sensitive polarization element 41A. The polarization rotated light is collected and reflected back through the sensitive polarization element 41A by a reflective optical surface 51 hereby obtaining further ro¬ tation polarization. The reflective imaging optical ele- ment consists of a convex surface of the sensitive polarization element 41A which is coated with a reflective coating of aluminum. The reflected light is focused and transmited through the λ/4 retarder means providing a total phase retardation of λ/2 resulting in a rotation of the plane of polarization of 7T/4 radians, and through the polarizer/analyzer means 31 consisting of a linear polarizer, e.g. a 8612 Po- larcor™ glass polarizer from Corning, transmitting linearly polarised light of a selected polarization on to the optical fiber transmitter/receiver 23, transmitting the analyzed light via a collimator lens (not shown) and a beam splitter or fiber splitter (not shown) to a light detector (not shown) at the other end of the fiber. Thus, by applying a magnetic field, a change of polari¬ zation state of light in the sensitive polarization element 41A can be determined by the change of light intensity detected by the light detector. Fig. 6B shows a preferred embodiment of an electrooptic/- photoelastic sensor device comprising one optical fiber simultaneously functioning as transmitter and receiver. The device is similar in construction to the device shown in Fig. 6A except for the sensitive polarization element 41A being an electrooptic/photoelastic material and the auxiliary polarization element 41B being a λ/8 plate consisting of a thin disc of linear birefringent material which provides the necessary λ/4 phase retardation when passed twice by the optical beam. Temperature and wavelength stability Enhanced Polarization Sensor Devices Fig. 7A shows a preferred embodiment of an electrooptic photoelastic sensor device comprising an additional optical element 53 including a total internal reflective imaging optical surface 51A providing a temperature and wavelength stable phase retardation and including a reflective optical element optionally imaging, here shown as a plane reflective surface 54. In Fig. 7A the optical path is similar to the one in Fig. 5A and 5B and 5C and Fig. 6A and 6B except for the single reflective imaging optical surface 51 being replaced by a parabolic total internal reflective imaging optical sur¬ face 51A on an additional optical element 53 providing a phase-retardation yielding a net effect on the reflected optical beam substituting for a λ/8 phase retardation. Consequently, two reflections of the optical beam in the surface 51A substitutes for a λ/4 phase retardation, whereby the birefringent auxiliary polarization element 41B can be avoided. The embodiments using two optical fibers are illustrated in Fig. 7E (b) and 7F (b). In Fig. 7E (b) there is shown cross sectional views along the lines Vlle-VIIe in Fig. 7A and 7D. In Fig. 7F (b) there is shown cross sectionsl views along the lines Vllf-VIIf in Fig. 7B and 7C. Tthe embodiments using one optical fiber are illustrated on Fig. 7E (a) and Fig. 7D (a). In the embodiments shown in Figs. 7A-7F, the unpolarized diverging light from the transmitter optical fiber is polarized linearly by the polarizer and transmitted through the sensitive polarization element 41A, entering the additional optical element 53 where it is reflected and collimated by the total internal reflective imaging optical surface 51A and directed on to a plane reflective surface 54 reflecting the light back through its original path. The surface 51A may be optionally coated with a coating of selected refractive index providing optimal amount of phase retardation. As the polarized light is phase retarded twice by the internal reflection, a total effect on the transmitted optical beam is obtained which substitutes for the effect of a λ/4 phase retardation whe the beam is analyzed by the analyzer and the optical powe is integrated on the detector. To get an effect as close as possible to a λ/4 phase retardation, glasses such as BaK4 or PSK3 (Schott) can be used for the manufacture of the optical element comprising the total internal reflective imaging optical surface 51A. (H. Fabricius "Achromatic Prism Retarder for Use in Polarimetric Sensors", Applied Optics, Vol. 30, No. 4, 1991, p. 426 - 429). It should be noted that the phase retarding parabolic surface element providing the effect substituting for a λ/4 phase retarder is not a true λ/4 plate as it provides different retardation angles for parts of the beam emitted at different solid angles. The net effect first appears after the analyzer when the light is integrated on the detector. Fig. 7C shows an embodiment of the device in Fig. 7A, where the additional optical element 53 is avoided by forming the total internal reflective imaging optical surface 51A in the sensitive polarization element 41A itself . Fig. 7D shows an embodiment of the device in fig. 7A, where the additional optical element 53 is constituted by a plano-convex lens 53, mounted on a plane surface of the sensitive polarization element 41A with the convex surface of the lens constituting the total internal reflective imaging optical surface 51A. Optical cement, optical epoxy, or other materials may optionally be applied between the lens and the polarization element. Fig. 7G shows another embodiment of an electrooptic/ photoelastic sensor device comprising a combination of a polarization element 41 having a reflective imaging optical surface 51 and an additional optical element 53 including a plane total internal reflective surface 51A having phase retarding properties for the substitution of a λ/4 optical element. It should be noted that for this embodiment, the variation in phase retardation obtained for individual optical rays emitted at different solid angles from the optical fiber is greater than for the embodiments using an imaging total infernal reflective surface phase retardation means. Therefore the present embodiment provides a poorer substitution for a λ/4 or λ/8 phase retarder. However, the simple geometry using a plane total internal reflective surface simplifies manufacturing of the device. The optical paths which can be used in Fig. 7G are shown in Fig. 7H. The embodiments using two fibers are illustrated in Fig. 7H (b) showing a cross sectional view along the line Vllh-VIIh. The embodiments using one optical fiber are illustrated in Fig. 7H (a). Fig. 8A shows another embodiment of the device in Fig. 7A comprising two phase retardation elements. Unpolarized light provided by the optical fiber transmit- ter 21, e.g. a 600 μm core multimode plastic-clad-silica fiber TECS™, Technology-Enhanced Clad Silica, FT-600-LMT (3M), is polarized by the polarizer 31, e.g. a 8612 Polarcor™ glass polarizer from Corning, transmitted through the sensitive polarization element 41A to a parabolic phase retarding surface 51A of an additional optical element 53 collimating and phase retarding the polarized light onto another parabolic phase retarding surface 5IB focusing the polarized light through the sen¬ sitive polarization element 41A, an analyzer 32, e.g. a 8612 Polarcor™ glass polarizer from Corning, and onto an optical fiber receiver 22. Fig. 8B shows another embodiment of the device in Fig. 7B where the sensitive polarization element 41A is placed between two additional optical elements 53A and 53B. 5. EXAMPLES Applications EXAMPLE 1 "Magnetooptic Fiber Optical Sensing" Fig. 9 shows an application of a magnetooptic fiber optic sensor device according to the invention positioned in a magnetic field concentrator core 91 around an AC current carrying conductor 92. The concentrator core served the purpose of concentrating the magnetic field of the current in the conductor in the gap of the core, whereby a measurement of the magnetic field in the total volume of the gap of the concentrator core has proved to be a good approximation to an integra¬ tion of the magnetic field in air along an arbitrary path incirculating the conductor, which according to Ampeeres law yields the current in the conductor. Therefore, when as according to the invention, the gap of the concentrator core is almost fully occupied with the magnetooptic material, the output from the magnetooptic sensor measu- ring the magnetic field in the gap, yields, when correctly calibrated by electronic means, a very reliable measu¬ rement of the current which is only very little sensitive to ambient magnetic fields and field perturbations and only little sensitive to the position of the conductor inside the aperture of the concentrator core. If there are no ambient magnetic fields and no magnetic field perturbing objects present, the magnetic field strength around the conductor is directly proportional to "the current flowing in the conductor and inversely pro¬ portional to the square of the distance to the conductor. Therefore, assuming an unperturbed magnetic field and measuring a small portion of the field by a magnetooptic sensor a result is obtained, which, when properly cali- brated for the distance to the conductor and the sensor characteristics by electronic means, will be a correct measurement of the current. Therefore, for some appli¬ cations, the magnetic integrator core can be avoided and the magnetic field sensor directly applied. Because of the compactness of the magnetooptic sensor it can be used to measure current even where only very little space is available. For example an application of the magnetooptic sensor is to measure current in a 10-20 kV electric power distribution cable, when the sensor with or without integrator core is placed inside a bushing terminating a single conductor of the cable. (Fig. 11). If the application is not critical and only low accuracy is needed, the integrator core can be avoided. EXAMPLE 2 "Electrooptic Fiber Optical Sensing" Fig. 10A shows an application of an electrooptic sensor device according to the invention fixed between the elec¬ trodes 94,95 of a voltage sensor cast in an insulating material. When sensor characteristics are properly calibrated for by electronic means, the output from an electrooptic sensor placed in a homogeneous AC electric field can yield the field strength of the field. When the electrooptic sensor was placed in a fixed geometry between a set of two elec- trodes, the output signal could be calibrated to directly represent the voltage across the electrodes. When the electrooptic material applied was a material with high dielectric strenght such as the electrooptic polymer described in the Ph.D. -thesis by Allan Gottsche: "Magnetooptic and Electrooptic Sensors for Advanced Appli¬ cations in Electric Power Systems", Publication No. 9005, (1990) Electric Power Engineering Department, Technical University of Denmark, it was possible to have high volta- ge directly across electrodes mounted on the electrooptic material. Because, the voltage sensor was cast in an elec¬ trically insulating material, it could be safely operated at voltages, at which a voltage sensor equipped with con¬ ventional lenses would suffer from electric discharge in the air pockets surrounding the lenses. When electrooptic crystals were used as the electrooptic material, only relatively low voltage could be applied directly across the crystal. However, as the crystal is sensitive to electric fields, the electrooptic sensor can be cast into a block of insulating material, the dimension of which is considerably larger than the dimension of the electrooptic crystal. When the block of insulating material is equipped with electrodes and a voltage is applied, the voltage across the crystal is only a fraction of the voltage across the electrodes, as determined by the ratio of the thickness of the crystal to the distance between the electrodes and by the dielectric constants of the crystal and the insulating material. When implemented as described above, the voltage sensor was used to measure voltage even where only very little space was available. For an example an application of the voltage sensor was to measure voltage on a 10-20 kV electric power distribution cable, when the sensor was placed between the conductor and an electrically grounded outer shield of a bushing terminating a single conductor of the cable. Fig. 10B shows an application of an electrooptic sensor device according to the invention fixed between the electrodes 94, 95 of a voltage sensor cast in an electri¬ cal insulating material. The device was similar to the one shown in Fig. 10A, except that phase retardation was provided for by the total internal reflective imaging optical surface of the polarization element as shown more detailed in Fig. 7C. An air space 111 between the top electrode 94 and the polarization element 41 provided the necessary shift in index of refraction to provide for total internal reflection. The device exhibited the same characteristics as described for Fig. 10A, except for being less temperature sensitive. Fig. IOC shows an application of an electrooptic sensor device according to the invention fixed between electrodes 94, 95 of a voltage sensor cast in an electrical insulating material. The device was similar to the one shown in Fig. 10B, except that the total internal reflec¬ tive surface was provided for by the plane surface of the plano-convex lens ( 53 ) made of BaK4 glass (Schott) mounted on a plane surface of the polarization element. The device exhibited the same characteristics as described for Fig. 10B, including good tolerance against temperture shifts. Fig. 11 shows an application of an electrooptic sensor device combined with a magnetooptic sensor device including a magnetic field integrating core, both of which devices according to the invention are placed inside a cable bushing ( 93 ) consisting of an electrically grounded metal tube surrounding a single phase conductor (92) of an electrical 10 kV distribution cable. The magnetooptic sensor device (10) was mounted with the magnetic field integrating core (91) surrounding the single phase conductor (92) and its primary electrical insulation 110. The electrooptic sensor device 11 was mounted between the insulated phase conductor and the surrounding electrically grounded bushing. The electrooptic sensor device was not equipped with electrodes, but simply exposed to the electrical field between the phase conductor and the surrounding bushing. The optical fibers of the sensing devices were led through holes machined in the bushing for that purpose (not shown). The test demonstrated that it was possible to mount and operate the electrooptic and magnetooptic sensor devices within the confined space available between the phase conductor and the outer shielding of the cable bushing of a 10 kV distribution cable. EXAMPLE 3 "Photoelastic Fiber Optical Sensing" Fig. 12 shows an application of a photoelastic sensor device as an accelerometer, according to the invention positioned between a floating mass 97 and a support 98. When the support was accelerated, mechanical tension would cause induced linear birefringence to build up in the material through photoelastic effect because of the iner¬ tia of the mass as stated by Newtons laws. When applied as described above the photoelastic sensor could be used as an accelerometer. A typical example of an application is to monitor vibra¬ tions on machine parts hereby determining the present condition and extrapolated lifetimes of the machine parts. Fig. 13 shows an application of a photoelastic sensor de- vice according to the invention which is connected via a rod (99) to a membrane (100) for measuring dynamic pres¬ sure changes. Differential presssure applied across the membrane resulted in mechanical forces, which via the rod provided mechanical tension in the photoelastic material causing induced linear birefringence to build up through the pho- toeleastic effect. When applied in this way, the photoela¬ stic sensor was used to measure dynamic pressure changes. An application would be to measure pressure fluctuations in circulating fluid due to pump strokes. A typical exam¬ ple of this application would be the measurement of dif¬ ferential pressure between systolic and diastolic blood pressure in the human body.

Claims: P E N T C L A I M S 1. A method of determining induced change of polarization state of light in a polarization element comprising: a) transmitting unpolarized light from a light source at the one end of an optical transmitter waveguide means (21) to polarizer means (31) at the other end; b) polarizing the unpolarized light by the polarizer means (31); c) transmitting the polarized light through the polariza- tion element (41, 41A, 41B) using at least one reflec¬ tive optical element; d) analyzing the transmitted polarized light from the polarization element by an analyzer means (32); and e) transmitting the analyzed polarized light from the one end of optical receiver waveguide means (22) to a light detector at the other end; c h a r a c t e r i z e d in that f) the unpolarized light polarized by the polarizer means (31), the polarized light analyzed by the analyzer means (32), or both, are non-collimated; and g) the polarized light in the light path between the po¬ larizer means (31) and the analyzer means (32) is col¬ lected and reflected by at least one reflective imaging optical element so that the analyzed light exits the analyzer means (32) from the same side as the unpolarized light enters the polarizer (31). 2. A method according to claim 1, c h a r a c t e r ¬ i z e d in that the polarized light from the polarizer means (31) is transmitted through the sensitive polariza¬ tion element (41, 41A) one or more times, preferably once or twice, by one reflective imaging optical element. 3. A method according to claim 1, c h a r a c t e r ¬ i z e d in that the polarized light is transmitted through the sensitive polarization element (41, 41A) one or more times, preferably one, two or four times, by two reflective optical elements at least one of which is imaging. 4. A method according to claims 1 - 3, c h a r a c - t e r i z e d in that the at least one reflective imaging optical element is selected from a group consisting of: i) a convex surface (51) of the polarization element (41, 41A, 41B) coated with a reflective coating; ii ) a planoconvex lens ( 52) with a convex surface ( 51 ) coated with a reflective coating; iϋ ) a reflective diffractive optical element (61); iv) an optionally coated convex surface (51A) of the sensitive polarization element (41A) reflecting by total internal reflection; and v) an optionally coated convex surface (51A, 51B) of an additional optical element ( 53) reflecting by total internal reflection. 5. A method according to claims 1 - 4, c h a r a c ¬ t e r i z e d in that the optical transmission and receiver waveguide means consists of one or more multi mode optical fibers connected to one or more polarizer means and analyzer means, respectively. 6. A fiber optic sensor device for determining induced change of polarization state of light in a polarization element, particularly circular birefringence induced by electric current and magnetic field comprising: a) a polarizer means (31) connected to the one end of an optical transmitter waveguide means (21) emitting unpo¬ larized light from a light source; b) a polarization element (41); c) at least one reflective optical element; and d) an analyzer means (32) connected to the one end of an optical receiver waveguide means (22) transmitting the analyzed light to detection means; c h a r a c t e r i z e d in that e) said at least one reflective optical element is an imaging optical element reflecting the light in the light path between the polarizer means (31) and the analyzer means (32) so that the analyzed light exits the analyzer means (32) from the same side as the unpo¬ larized light enters the polarizer (31). 7. A device according to claim 6, c h a r a c t e r ¬ i z e d in that the polarized light from the polarizer means (31) is transmitted through the sensitive polariza¬ tion element (41, 41A) one or more times, preferably once or twice, by one reflective imaging optical element. - 49 - 8. A device according to claim 6, c h a r a c t e r ¬ i z e d in that the polarized light is transmitted through the sensitive polarization element (41 ) one or more times, preferably one, two or four times, by two reflective imaging optical elements at least one of which is imaging. 9. A device according to claims 6 - 8, c h a r a c ¬ t e r i z e d in that the at least one reflective imaging optical element is selected from a group consisting of: i) a convex surface (51) of the polarization element (41, 41A) coated with a reflective coat¬ ing; ii) a planoconvex lens (52) with a convex surface (51) coated with a reflective coating; and iii) a reflective diffractive optical element (61). 10. A device according to claims 6 - 9, c h a r a c ¬ t e r i z e d in that the optical transmission and receiver waveguide means consists of one or more multi mode optical fibers connected to one or more polarizer means and analyzer means, respectively. 11. A fiber optic sensor device for determining induced change of polarization state of light in a polarization element, particularly linear birefringence induced by electric voltage, electric field, and mechanical force, comprising: a) a polarizer means (31) connected to the one end of an optical transmitter waveguide means (21) emitting unpo- larized light from a light source; b) a polarization element (4lA, 41B) comprising phase retardation means; c) at least one reflective optical element; and d) an analyzer means (32) connected to the one end of an optical receiver waveguide means (22) transmitting the analyzed light to detection means; c h a r a c t e r i z e d in that e) said at least one reflective optical element is an imaging optical element reflecting the light in the light path between the polarizer means (31) and the analyzer means (32) so that the analyzed light exits the analyzer means (32) from the same side as the unpolarized light en¬ ters the polarizer (31). 12. A device according to claim 11, c h a r a c t e r - i z e d in that the polarized light from the polarizer means (31) is transmitted through the sensitive polariza¬ tion element (41A) one or more times, preferably once or twice, by one reflective imaging optical element. 13. A device according to claim 11, c h a r a c t e r ¬ i z e d in that the polarized light is transmitted through the sensitive polarization element (41A) one or more times, preferably one, two or four times, by two reflective imaging optical elements at least one of which is imaging. 14. A device according to claims 11 - 13, c h a r a c ¬ t e r i z e d in that the at least one reflective imaging optical element is selected from a group consisting of: i) a convex surface (51) of the polarization element (41A, 41B) coated .with a reflective coating; ii ) a planoconvex lens ( 52) with a convex surface (51) coated with a reflective coating; iii ) a reflective diffractive optical element (61); iv) an optionally coated convex surface (51A) of the sensitive polarization element (41A) reflecting by total internal reflection; and v) an optionally coated convex surface (51A, 51B) of an additional optical element (53) reflecting by total internal reflection. 15. A device according to claims 11 - 14, c h a r a c ¬ t e r i z e d in that the optical transmission and receiver waveguide means consists of one or more multi mode optical fibers connected to one or more polarizer means and analyzer means, respectively.

Inventors: GOETTSCHE ALLAN, GOETTSCHE, ALLAN
Applicants: GOETTSCHE ALLAN, GOETTSCHE, ALLAN
Application year: 1992
Main classification: G01N21
IPC codes: G01D5, G01J4, G01L1, G01L9, G01R15, G01R33, G02B6