Sensor device.

p0001The invention relates to a sensor assembly comprising a fiber optic Fabry-Perot interferometer having a single mode excited, mechanically loaded light wave conductor, which in each case comprises a partially reflecting mirror at its two end faces, wherein the first end face is exposed to the polarized light of a light source and from the second end face emergent light feeds a detection device which is connected to a transmitter.

p0002In IEEE Journal of Quantum Electronics, Vol. QE-18, No. 10, October 1982, page 1624, et seq., Fiber optic Fabry-Perot interferometer sensor and its application as well as Sensoranordunungen the above-described genus are discussed. When such sensor arrays are used to detect mechanical or other sizes, converted into mechanical variables physical variables, there are often errors in measurement because constant ambient temperatures allemeinen can not be guaranteed. Temperature variations in fact the optical length changes (product of geometric length and refractive index) of the fiber optic Fabry-Perot interferometer and thus the circulation period.

p0003Starting from this prior art, the invention has the object of a sensor arrangement, in particular to provide a force and pressure sensor which is temperature compensated and therefore at different ambient temperatures provides accurate readings.

p0004This object is according to the invention achieved by the fact that the supply of light over a means for generating a circular polarization, in that the optical waveguide is not or only weakly birefringent, where the mechanical load defines the two principal axes of the induced birefringence, and that the emerging light on a beam splitter arrangement which separates the two main axis directions associated polarized light in accordance with its two components, four photodetectors fed, the outputs of which are connected to four inputs of the evaluation electronics.

p0005The inventive arrangement makes it possible to detect due to the circulation period and the birefringence of two parameters, the detection of the ambient temperature serves for the case of using force and pressure sensor to perform a temperature compensation. But it is also possible that detected in the measurement temperature outputting separately. Other physical parameters as temperatures, forces and pressures, can be captured in pairs also when they affect the circulation period and the induced birefringence different.

p0006In an expedient embodiment of the invention, the beam splitter arrangement of two Wollaston prisms, one of which is aligned with its axes to the principal axes of the optical waveguide and the other with its axes at 45 ° to the principal axes of the optical waveguide.

p0007Expedient refinements and developments of the invention are characterized in the dependent claims.

p0008In the following the invention will be discussed with reference to the Ausführungsfbeispiele shown in the drawing. Show it:<ul><li>Fig. 1 shows a first embodiment of a sensor arrangement according to the invention in a schematic representation,</li><li>FIG. 2 shows a second embodiment of an inventive sensor arrangement in a schmatischen representation,</li><li>FIG. 3 is a longitudinal section through the sensor portion of the embodiment shown in FIG. 2,</li><li>Fig. 4 is a variant of a detection device of the sensor arrangement according to the invention.</li><li>Fig. 5 is a plan view of a sensor arrangement with a double-wound optical waveguide,</li><li>FIG. 6 a side view of a section through the embodiment of FIG. 5 along the line VI-VI,</li><li>Fig. 7 is a plan view of an embodiment of a sensor arrangement with a serpentine laid optical fiber, </li><li>Fig. 8 shows a section through the sensor arrangement of FIG. 7 in a side view and</li><li>FIG. 9 is a side view of the sensor arrangement according to FIG. 7 in the direction of the longitudinal edge of the plunger.</li></ul>

p0009In Fig. 1 shows schematically the construction of a sensor assembly according to the invention, which can be used inter alia as a temperature-compensated force and pressure sensor.

p0010In a housing 1, which can be closed off on all sides, or may be provided with an inlet pipe 2 and an outlet 3, an optical waveguide 4 is arranged, which is not or only weakly birefringent and induced in the by the action of an external force, a birefringence , Such birefringence occurs in particular with an optical waveguide 4 of quartz glass.

p0011As can be recognized in Fig. 1, in the housing 1, a trestle 5 is provided with a support surface 6 on which rests at least a part of the optical waveguide 4. On the bearing surface 6 opposite side of the optical waveguide 4, a plunger 7 is provided which is guided in a guide 8, and a force on the optical waveguide 4 can by be exercised in the direction of an arrow. 9 The stamp 7 is preferably knife-like constructed with a rounded, extending at right angles to the axis of the optical waveguide edge 10, by a local action on the optical fiber 4 is effected. The geometry of the arrangement, two major axis Rich are tions x and y defined, which are illustrated in FIG. 1 by a coordinate system 11. The plunger 7 is in the x direction in the guide 8 in the housing 1 verschierbbar. Its position depends on the acting in the direction of the arrow 9 force is generated by the variable birefringence in the optical waveguide. 4 The acting in the direction of the arrow 9 power can be a directly acting on the force sensor, for example, acting through the weight of a mass force (fiber optic scale), or consisting of a force acting on a predetermined surface pressure derived force (fiber optic pressure sensor).

p0012When the housing 1 is closed on all sides, an existing in the interior 12 of the housing 1 medium can act as Homogenisierungsbad which ensures a uniform temperature distribution along the optical waveguide. 4 In a housing 1 with a inlet port 2 and an outlet 3 can in the direction of arrow 13 a a liquid or a gas inflow and outflow in the direction of arrow 14 in order in this way in addition to the first through the Druckstemepl 7 acting parameters (force, pressure) to leave a second parameter (temperature) to act on the optical waveguide 4th As will be described below, can then take place at the same time measuring two parameters in a single sensor. On the other hand it is also possible, with the sensor shown in Fig. 1 to measure the force acting in the direction of arrow 9 force and regarded the ambient temperature of the optical waveguide 4 as interference in determining de mechanical parameter, the detection of the disturbance serves a measurement correction ensure.

p0013The through the housing 1 extending therethrough optical waveguide 4 is provided at its end faces 15 and 16 with partially transparent mirrors, so that it forms a Faster-Fabry-Perot interferometer, apply to the result of the birefringence two main axis directions associated circulation periods, the difference between them as differential phase is referred to. The two main axis directions associated with refractive indices determine, together with the geometric length of the fiber Fabry-Perot interferometer the differential phase along the optical waveguide 4 applied respectively from the temperature and the locally via the plunger 7 acting mechanical stress dependent. In a particularly simple case, the circulation period depends on the x-major axis direction from alone of the first parameter and the differential phase alone by the second parameter. However, in general circulation phase and differential phase are independent linear combinations of the two applied parameters, that the temperature and the force.

p0014The housing 1 of the sensor arrangement is assigned to a left shown by case 1 in Fig. 1 lighting device 17 and a shown in Fig. 1 to the right from the housing 1 the detection device 18. The illumination device 17 has a diode laser 19 whose light is collimated via a collimator lens 20 into a parallel beam. The parallel beam passes through an optical isolator having a polarizer 21 and a λ / 4 plate 22, so that the parallel beam is brought into the state of circular polarization. The circularly polarized parallel beam con stant wavelength is coupled by means of a converging lens 23 in the core of the optical waveguide 4th Here, the optical waveguide per orthogonal polarization direction is excited in a single mode.

p0015Depending on the ambient temperature of the optical waveguide 4 and the displacement of the plunger 7 in the direction of the support frame 5, a Resonatorsignal is obtained at the end surface 16, which is collimated by a converging lens 24th The collimated Resonatorsignallichtbündel acted upon, a first Wollaston prism 25, and via a deflecting mirror 27, a second Wollaston prism 26th

p0016The axes of the first Wollaston prism 25 are orientert at 45 ° to the main axes x and y of the optical waveguide 4 of the fiber Fabry-Perot interferometer. The axes of the second Wollaston prism 26 are adjusted to the principal axes of the fiber Fabry-Perot interferometer. According to divide the polarization directions 25 and 26 the Resonatorsignallichtbündel in pairs on in intensity units I₁, I₂, I₃ and I₄ the Wollaston prisms. The light beam 31 with the intensity I₁ is applied to a first photodetector 35, the light beam 32 to the intensity I₂ a second photodetector 36, the second Wollaston prism 26 exiting light beam 33 with the intensity I₃ of the photodetector 37 and also the second Wollaston prism 26 exiting light beam 34 with the intensity I₄ the fourth photodetector 38th

p0017The outputs of the photodetectors 35, 36, 37 and 38 feed the measuring inputs of a transmitter 39, which assigns the quartet of intensities I₁, I₂, I₃ and I₄ the associated circulation period and differential phase and from the to be determined Meßparameterpaar so on Haputausgant 40 a electrical signal is available, depending only on the on the plunger 7 acting force, while at the auxiliary output 41 whose signal may not be further processed only on the temperature of the optical waveguide 4-dependent signal is available. The signal at the main output 40 is thus a measurement signal temperaturkompeniiertes acting on the optical fiber 4 via the plunger 7 force.

p0018The transmitter 39 either contains an arithmetic unit which calculates the two parameters based on the variation of the circulation period and the variation of the differential phase and experimentally determined coefficients of a matrix, or a read-write memory of as part of a calibration procedure for a variety value pairs for the circulation period and the differential phase contains the associated power / temperature value pairs. After carrying out such verification the evaluation of the evaluation is to determine in each case the associated signal pair for the main exit 40 and 41 for the sites identified by the photodetectors 35, 36, 37 and 38 four intensities.

p0019In the figures 2 and 3, a further embodiment of a sensor arrangement according to the invention is shown. Components that correspond to the arrangement of FIG. 1 are designated by the same numerals. The end face 15 as in the embodiment discussed above, exposed to light of a constant wavelength, which is circularly polarized, which is to be illustrated by the arrow 42nd The end face 16 is coupled to a detection device 18 which corresponds to that described in connection with FIG. 1, the detection device 18. Instead of introducing and extracting on the end faces 15, 16 directly associated convergent lenses 23,24, it is also possible to perform a coupling or decoupling on in the drawing, not shown, polarization-maintaining optical waveguide.

p0020The fiber Fabry-Perot interferometer of the embodiment shown in Figures 2 and 3 comprises an optical waveguide 43 which is torsion eye wound on an elastic metal cylinder 44th The metal cylinder 44 shown in Figures 2 and 3 is arranged in a container 45th The optical waveguide 43 extends, as in the embodiment shown in Fig. 1, by two oppositely disposed walls of the container 45 and is on the outside of the container 45 about. Of course, the openings are sealed in the walls of the container filled with a liquid 46 45th The elastic Metallzyliner 44 is provided with a connection piece 49, through which a medium under pressure, in particular a gas or a liquid, into the interior 47 of the metal cylinder 44 can pass. The metal cylinder 44 is mounted on its connecting piece 49 to the left in Fig. 3 side while it is held on the right in Fig. 3 side by means of a bearing plate 48. The liquid provides a heat bath 46 is at a certain temperature. The sensor arrangement according to Figures 2 and 3 is particularly for the monitoring of thermodynamic processes in process measurement, especially in the chemical and pharmaceutical industry, suitable. The sensor arrangement shown in Figures 2 and 3 allows, on the one hand to measure the pressure in the interior 47 and on the other hand the temperature of the liquid 46, wherein changes in the temperature of the liquid 46 does not affect the accuracy of measurement of the pressure in the interior 47th Pressure changes in the interior 47 cause an expansion pressure on the plane formed by the optical waveguide 43 coil in which a function of the mechanical stresses occurring birefringence is induced. In order not to distort the effect generated by the birefringence, it is necessary, as already mentioned, the optical waveguide 43 to wind up torsional stress on the elastic metal cylinder 44th

p0021In FIG. 4 a detection device 50 is shown, which differs from the detection device shown in Figures 1 and 2 18, characterized in that the generation of the four light beams for the photo-detectors 35 to 38 not by means of two Wollaston prisms 25, 26, but occurs mi means of a polarizing beam splitter cube 51 in which 51 is a λ / 2 retardation plate 52 passes through the parallel ray beam generated by the converging lens 24 in part prior to entering the polarizing beam splitter cube, the axis by 22.5 ° to the x axis of the fiber Fabry -Perot interferometer is rotated. The λ / 2 retardation plate 52 divides the parallel beam into two partial beams E and E '. The polarizing beam splitter cube 51 divides the two partial beams E and E 'respectively in the x- and y-Component E_x, e_y, e_x'And E_y'Of which 35 to 38 apply along the principal axes of the Fabry-Perot interferometer propagating wave, again four intensities I₁, I₂, I₃ and I₄ occur photodetectors.

p0022The light incidence plane of the beam splitter cube 51, whose axes are oriented parallel to the principal axes of the fiber Fabry-Perot interferometer is divided with respect to an axis of the fiber Fabry-Perot interferometer are aligned parallel, approximately 1 mm wide masking stripes 53 into two light entry surfaces whereby crosstalk between the two partial beams e and e 'is prevented. The photodetectors 35 to 38 may be realized by two separate into two sectors detector surfaces. Depending on the type of detector used aes may be necessary to focus on the photodetectors 35-36 falling radiation beam additionally with another lens on the detector surfaces of the photodetectors. The evaluation of the output signals of the photodetectors 35 to 38 takes place in the transmitter 39 in the same manner as has been described above in connection with the embodiment of FIG. 1.

p0023While in the Ausführungsbeispeil the plunger shown in FIG. 1 7 the optical waveguide 4 only comes in a single place under tension, are in the expected number in each case several interaction areas between the plunger 7 and the optical waveguide 4 in the figures 5 to 9 embodiments illustrated.

p0024Fig. 5 shows a bearing surface 6 a in a plane spiral and double-wound optical waveguide 4. The cut-like design plunger 7 crosses the double-wound spiral along a diameter, so that a large number of interaction areas is formed by a higher sensitivity is achieved. The ends of the optical waveguide 4 open into fiber optic connectors 54 and are provided at their lying in the fiber optic connectors 54 front sides with partially transparent mirrors. Via a polarization maintaining fiber 55 56 zirular polarized light is coupled in the direction of the arrow. Via a further polarization maintaining fiber 57, a signal for the detection device 18 or 50 is coupled out in the direction of arrow 58th

p0025In Fig. 6 is a side view is shown on a section through the arrangement shown in Fig. 5. Fig. 6 illustrates the flat contact surface 6 with the flat bifilar coil which is formed by the optical waveguide 4. Also evident to be pressed in the direction of the arrow 59 against the optical waveguide 4 plunger 7 with the rounded edge 10. Figures 5 and 6 clearly show the cut-like design of the plunger 7, which has a narrow width and a relative great length. The height of the plunger 7 is determined by the particular application.

p0026In Figures 7 to 9, another embodiment of a sensor arrangement is illustrated in which 7 are also provided several interaction areas between the optical waveguide 4 and the plunger. As seen in Fig. 7, the optical fiber 4 is laid in serpentine fashion, wave-shaped or meander-shaped along the support surface 6, in which at the intersection points with the plunger 7 between it and the optical fiber 4 are each a right angle is formed.

p0027Fig. 8 shows a section parallel to the main extension direction of the plunger 7 and particularly illustrating that the plunger 7 is performed as a result of multiple pad well on the optical waveguide 4.

p0028Finally, FIG. 9 is a left side view of the arrangement shown in Fig. 7, where again the rounded edge 10 of the plunger 7 can be clearly seen.