DE102018104953A1 - Measuring device and method for determining a physical measurand - Google Patents

Abstract

) aufweisen; einem an einem ersten Kopplereingang anliegenden ersten Lichtwellensignal; einem an einem zweiten Kopplereingang anliegenden zweiten Lichtwellensignal, wobei das erste und das zweite Lichtwellensignal interferierbar sind und eine Messgrößen-bedingte Phasenverschiebung aufweisen, die einer physikalischen Messgröße entsprechend ausgebildet ist; einem an einem ersten Kopplerausgang anliegenden ersten optischen Interferenzsignal, welches eine erste Interferenz des ersten und des zweiten Lichtwellensignals repräsentiert; einem an einem zweiten Koppierausgang anliegenden zweiten optischen Interferenzsignal, welches eine zweite Interferenz des ersten und des zweiten Lichtwellensignals repräsentiert, wobei das zweite optische Interferenzsignal gegenüber dem ersten eine Koppler-bedingte Phasenverschiebung ungleich m·π (mit m ∈

) aufweist; einem ersten optischen Detektor, der eingerichtet ist, an einem ersten Detektoreingang das erste optische Interferenzsignal zu empfangen und an einem ersten Detektorausgang ein erstes elektrisches Ausgangssignal bereitzustellen; einem zweiten optischen Detektor, der eingerichtet ist, an einem zweiten Detektoreingang das zweite optische Interferenzsignal zu empfangen sowie ein zweites elektrisches Ausgangssignal bereitzustellen; und einer Auswerteinrichtung zum Bestimmen der physikalischen Messgröße aus dem ersten und dem zweiten elektrischen Ausgangssignal. Weiterhin ist ein Verfahren zum Bestimmen einer physikalischen Messgröße vorgesehen.

The invention relates to a measuring device for determining a physical measurement quantity with an optical coupler having N (N ≥ 3) coupler inputs for receiving in each case a lightwave input signal and N coupler outputs for outputting a respective lightwave output signal, wherein the lightwave output signals by signal processing in the optical coupler coupler-related phase shifts not equal to m · π (with m ∈

) respectively; a first lightwave signal applied to a first coupler input; a second lightwave signal applied to a second coupler input, wherein the first and second lightwave signals are interferable and have a measurement-related phase shift which is formed corresponding to a physical measurand; a first optical interference signal applied to a first coupler output, which represents a first interference of the first and second lightwave signals; a second optical interference signal applied to a second coupling output, which represents a second interference of the first and the second lightwave signal, wherein the second optical interference signal relative to the first coupler-related phase shift is not equal to m · π (with m ∈

) having; a first optical detector configured to receive the first optical interference signal at a first detector input and to provide a first electrical output signal at a first detector output; a second optical detector configured to receive, at a second detector input, the second optical interference signal and to provide a second electrical output signal; and an evaluation device for determining the physical quantity from the first and the second electrical output signal. Furthermore, a method for determining a physical measured quantity is provided.

Description

The invention relates to a measuring device and a method for determining a physical measured variable.

background

Fiber optic sensors are used in engineering and science to detect mechanical deformations over time. Somewhat more specific is spoken of by sound, vibration or stretching effects. Sound and vibration are periodic phenomena that can be detected, for example, with acoustic sensors (microphones). Here, different principles of action are used, with mechanical vibrations normally being converted into electrical vibrations. For example, in the condenser microphone, the capacity of a movable plate assembly is measured. The piezoacoustic sensor makes use of the fact that a piezoelectric crystal generates an electrical voltage during mechanical excitation. The immersion coil microphone uses the induction effect.

Although elongation effects are in principle similar to those of sound or vibration, the problem here is that the mechanical deformations are often not periodic or at least very low-frequency. Thus, only sensors come into question that have no high-pass characteristic. For example, in the case of a piezoelectric sensor, there is a risk that the electrons released by very slowly acting forces will flow out and thus the voltage no longer corresponds to the respective deformation. A very common approach is the use of a strain gauge, in which an elongation leads to a change in the electrical resistance, which can be easily detected metrologically.

The listed sensors are used in many applications because of their sensitivity and size - not least because they are generally very cost-effective. However, they are difficult to use in harsh environments, for example, in areas where high temperatures prevail or in areas where chemically aggressive substances exist. Fields of application with high field strengths, such as in the vicinity of high-voltage cables or in switchgear, are also problematic. Here, fiber optic measurement principles are showing their strengths. A glass fiber can withstand high temperatures, gets along with many chemical substances and is not conductive. However, the costs for the fiber optic sensor and especially for the measurement technology are higher than with the conventional sensors.

Another strength of fiber optic sensors for the measurement of acoustic effects or elongation effects is that the length changes occurring in the fibers can be measured with high precision. At least, this applies to measuring principles in which the interference of light waves is used as an active principle, such as the Mach-Zehnder interferometer, the Michelson interferometer or the Sagnac interferometer.

In the Mach-Zehnder interferometer, light is first split by a 2x2 coupler. The two light waves then pass through a reference fiber and a sensor fiber. By mechanical excitation, the sensor fiber undergoes an extension or compression, which leads to a slightly changed transit time of the light in the sensor fiber. When the light is made to interfere with the reference fiber by the sensor fiber (again using a 2x2 coupler), the skew differences affect brightness changes in the interference signal. They are recorded and processed by measurement.

The Michelson interferometer works similarly to the Mach-Zehnder interferometer, but does not use two, but only a 2x2 coupler, which is used in both the forward and the reverse direction. As with the Mach-Zehnder interferometer, the 2x2 coupler initially serves as a beam splitter. The light waves are then guided in parallel through the reference and the sensor fiber. At the end of both fibers is a mirror, which sends the light back through the fibers. Finally, if the 2 × 2 coupler is traversed in the opposite direction, interference will occur. The further evaluation is equivalent to the Mach-Zehnder interferometer. Since it is difficult to mirror the fiber ends in terms of manufacturing technology, the Michelson interferometer is hardly used for fiber-optic measurement methods.

Furthermore, the Sagnac interferometer is known. It is an arrangement in which the light is passed through a fiber in which a fiber optic sensor is asymmetrically inserted. The light is simultaneously fed into this fiber from both sides. Due to the asymmetrical arrangement, the two light waves reach the fiber optic sensor with a time offset. Within this short time differential period, mechanical deformation of the fiber optic sensor causes a phase shift between the light beams propagating in the fiber. By interference, the phase shift is finally detected and processed. Similar to the Michelson interferometer, the Sagnac interferometer uses a single coupler for beam splitting and interference. Instead of a 2x2 coupler, however, a 3x3 coupler is used.

In addition to interferometric methods, there are other techniques that can be used to measure the sound, vibration and elongation effects of fiber optics. One example is the fiber Bragg grating, in which a microstructure is embossed into a fiber. This is local, but repeats periodically. Depending on the spaces provided, the fiber on the microstructure reflects a certain wavelength of the incident light. If the distances in the microstructure are changed by stretching or compression, the microstructure reflects other wavelengths, which can finally be measured.

Other non-interferometric techniques rely on, for example, hollow-core photonic bandgap fibers or use a fiber collimator to detect pressure effects as angular deviation.

Summary

The object of the invention is to specify a measuring device and a method for determining a physical measured variable with which different physical measured variables can be measured with high accuracy in different applications.

As a solution, a measuring device and a method for determining a physical measurand according to independent claims 1 and 15 are provided. Embodiments are the subject of dependent claims.

• - einen optischen Koppler mit N (N ≥ 3) Kopplereingängen, die eingerichtet sind, jeweils ein Lichtwelleneingangssignal zu empfangen, und N Kopplerausgängen, die eingerichtet sind, jeweils ein Lichtwellenausgangssignal auszugeben, wobei die Lichtwellenausgangssignale durch Signalverarbeitung im optischen Koppler jeweils verschiedene Phasen mit einer Koppler-bedingten Phasenverschiebung ungleich m·π (mit m ∈
  
  ) aufweisen;
• - ein erstes Lichtwellensignal, welches an einen ersten der Kopplereingänge angelegt ist;
• - ein zweites Lichtwellensignal, welches an einen zweiten der Kopplereingänge angelegt ist, wobei das erste und das zweite Lichtwellensignal interferierbar sind und eine Messgrößen bedingte Phasenverschiebung aufweisen, die eine physikalische Messgröße entsprechend ausgebildet ist;
• - ein erstes optisches Interferenzsignal, welches eine erste Interferenz des ersten und des zweiten Lichtwellensignals repräsentiert und an einem ersten der Kopplerausgänge anliegt;
• - ein zweites optisches Interferenzsignal, welches eine zweite Interferenz des ersten und des zweiten Lichtwellensignals repräsentiert und an einem zweiten der Kopplerausgänge anliegt, wobei das zweite optische Interferenzsignal gegenüber dem ersten optischen Interferenzsignal eine Koppler-bedingte Phasenverschiebung ungleich m·π (mit m ∈
  
  ) aufweist;
• - einen ersten optischen Detektor, der eingerichtet ist, an einem ersten Detektoreingang das erste optische Interferenzsignal zu empfangen und an einem ersten Detektorausgang ein erstes elektrisches Ausgangssignal bereitzustellen;
• - einen zweiten optischen Detektor, der eingerichtet ist, an einem zweiten Detektoreingang das zweite optische Interferenzsignal zu empfangen und an einem zweiten Detektorausgang ein zweites elektrisches Ausgangssignal bereitzustellen; und
• - eine Auswerteinrichtung, die eingerichtet ist, aus dem ersten und dem zweiten elektrischen Ausgangssignal die physikalische Messgröße zu bestimmen.

In one aspect, a measuring device for determining a physical quantity is provided. The measuring device has the following:

• an optical coupler having N (N ≥ 3) coupler inputs arranged to receive a lightwave input signal, respectively, and N coupler outputs adapted to output a lightwave output signal respectively, the lightwave output signals having different phases with a coupler by signal processing in the optical coupler conditional phase shift unequal to m · π (with m ∈
  
  ) respectively;
• a first lightwave signal applied to a first one of the coupler inputs;
• - A second lightwave signal, which is applied to a second of the coupler inputs, wherein the first and the second lightwave signal are interferable and have a measurement-related phase shift, which is designed according to a physical measurement variable;
• a first optical interference signal, which is a first interference of the represents first and second lightwave signals and is applied to a first one of the coupler outputs;
• a second optical interference signal representing a second interference of the first and second lightwave signals and applied to a second one of the coupler outputs, wherein the second optical interference signal has a coupler-related phase shift not equal to m · π (with m ∈
  
  ) having;
• a first optical detector configured to receive the first optical interference signal at a first detector input and to provide a first electrical output signal at a first detector output;
• a second optical detector arranged to receive the second optical interference signal at a second detector input and to provide a second electrical output signal at a second detector output; and
• - An evaluation device which is adapted to determine from the first and the second electrical output signal, the physical measured variable.

• - Bereitstellen eines optischen Kopplers, welcher N (N ≥ 3) Kopplereingänge, die eingerichtet sind, jeweils ein Lichtwelleneingangssignal zu empfangen, und N Kopplerausgänge aufweist, die eingerichtet sind, jeweils ein Lichtwellenausgangssignal auszugeben, wobei die Lichtwellenausgangssignale aufgrund einer Signalverarbeitung im optischen Koppler jeweils eine verschiedene Phase mit einer Koppler-bedingten Phasenverschiebung ungleich m·π (mit m ∈
  
  ) aufweisen;
• - Anlegen eines ersten Lichtwellensignals an einen ersten der Kopplereingänge;
• - Anlegen eines zweiten Lichtwellensignals an einen zweiten der Kopplereingänge, wobei das erste und das zweite Lichtwellensignal interferierbar sind und eine Messgrößen bedingte Phasenverschiebung aufweisen, die eine physikalische Messgröße entsprechend ausgebildet ist;
• - Bereitstellen eines ersten optischen Interferenzsignals, welches eine erste Interferenz des ersten und des zweiten Lichtwellensignals repräsentiert, an einem ersten der Kopplerausgänge;
• - Bereitstellen eines zweiten optischen Interferenzsignals, welches eine zweite Interferenz des ersten und des zweiten Lichtwellensignals repräsentiert, an einem zweiten der Kopplerausgänge, wobei das zweite optische Interferenzsignal gegenüber dem ersten optischen Interferenzsignal eine Koppler-bedingte Phasenverschiebung ungleich m·π (mit m ∈
  
  ) aufweist;
• - Empfangen des ersten optischen Interferenzsignals an einem ersten Detektoreingang eines ersten optischen Detektors, und Bereitstellen eines ersten elektrischen Ausgangssignals an einem ersten Detektorausgang des ersten optischen Detektors;
• - Empfangen des zweiten optischen Interferenzsignals an einem zweiten Detektoreingang eines zweiten optischen Detektors, und Bereitstellen eines zweiten elektrischen Ausgangssignals an einem zweiten Detektorausgang des zweiten optischen Detektors; und
• - Bestimmen der physikalischen Messgröße mittels einer Auswerteinrichtung aus dem ersten und dem zweiten elektrischen Ausgangssignal.

In another aspect, there is provided a method of determining a physical measure comprising:

• Providing an optical coupler having N (N ≥ 3) coupler inputs arranged to receive a lightwave input signal, respectively, and having N coupler outputs arranged to output a lightwave output signal, the lightwave output signals being respectively one due to signal processing in the optical coupler different phase with a coupler-related phase shift not equal to m · π (with m ∈
  
  ) respectively;
• - applying a first lightwave signal to a first one of the coupler inputs;
• - Applying a second lightwave signal to a second of the coupler inputs, wherein the first and the second lightwave signal are interferable and have a measurement-related phase shift, which is formed according to a physical measurement variable;
• Providing a first optical interference signal representing a first interference of the first and second lightwave signals at a first one of the coupler outputs;
• Providing a second optical interference signal representing a second interference of the first and second lightwave signals at a second of the coupler outputs, wherein the second optical interference signal has a coupler-related phase shift different from m.pi. (With m ∈
  
  ) having;
• - receiving the first optical interference signal at a first detector input of a first optical detector, and providing a first electrical output signal at a first detector output of the first optical detector;
• Receiving the second optical interference signal at a second detector input of a second optical detector, and providing a second electrical output signal at a second detector output of the second optical detector; and
• - Determining the physical quantity by means of an evaluation device from the first and the second electrical output signal.

The optical coupler may be a 3x3 coupler.

The evaluation device may include: a first amplifier device configured to receive, at a first amplifier input, the first electrical output signal of the optical coupler and to provide a first amplified output signal at a first amplifier output; and second amplifier means arranged to receive at a second amplifier input the second electrical output signal of the optical detector and to provide at a second amplifier output a second amplified output signal.

The amplifier device may be a transimpedance amplifier.

The evaluation device may include: a first filter device configured to receive the first electrical output signal at a first filter input and to provide a first filtered output signal at a first filter output; and a second filter means arranged to receive, at a second filter input, the second electrical output signal and to provide a second filtered output signal at a second filter output. With the aid of the filter devices, in each case an electrical signal smoothing can be carried out.

The first filter means may comprise a first anti-aliasing filter and the second filter means may comprise a second anti-aliasing filter.

The first filter device may comprise a first high-pass filter, and the second filter device may comprise a second high-pass filter. The high pass filter can be used to remove a possible DC component from the signals.

The evaluation device may further comprise: a first comparator configured to receive at a first comparator input the first filtered output signal and to provide a first digital output signal at a first comparator output; and a second comparator configured to receive at a second comparator input the second filtered output signal and to provide a second digital output signal at a second comparator output.

The first and / or the second comparator may have a hysteresis. As a result, a Schmitt trigger function can be provided, so that the one or more comparators are designed as Schmitt triggers.

The evaluation device may include: a first AD converter configured to receive the first filtered output signal at a first AD converter input and to provide the first digital output signal at a first AD converter output; and a second AD converter configured to receive the second filtered output signal at a second AD converter input and to provide the second digital output signal at a second AD converter output. The AD converters may be provided as an alternative to the comparators.

The evaluation device may have signal processing that is configured to process the first and the second digital output signal. It can be provided that the signal processing processes the output signals of the comparators and / or the AD converter.

) aufweist; und die Auswerteinrichtung eingerichtet ist, aus dem ersten, dem zweiten und dem dritten elektrischen Ausgangssignal die physikalische Messgröße zu bestimmen. Die Verarbeitung des dritten optischen Interferenzsignals kann der Verarbeitung des ersten und des zweiten Interferenzsignals entsprechend ausgeführt werden.Furthermore, a third optical detector configured to receive a third optical interference signal from the optical coupler at a third detector input and to provide a third electrical output signal at a third detector output, wherein the third optical interference signal relative to the first and the second optical interference signal a coupler-related phase shift not equal to m · π (with m ∈

) having; and the evaluation device is set up to determine the physical measured variable from the first, the second and the third electrical output signal. The processing of the third optical interference signal may be the processing of the first and the second interference signal are executed accordingly.

) aufweisen; und die Auswerteinrichtung eingerichtet ist, aus allen elektrischen Ausgangssignalen die physikalische Messgröße zu bestimmen. Die Verarbeitung der weiteren optischen Interferenzsignale kann der Verarbeitung des ersten und des zweiten Interferenzsignals entsprechend ausgeführt werden.When using an optical coupler with N coupler inputs and N coupler outputs (N> 3) further optical detectors may be provided which are adapted to receive further optical interference signals from the optical coupler at further detector inputs and to provide further electrical output signals at further detector outputs, all of them optical interference signals in pairs to each other a coupler-related phase shift not equal to m · π (with m ∈

) respectively; and the evaluation device is set up to determine the physical measured variable from all electrical output signals. The processing of the further optical interference signals may be carried out in accordance with the processing of the first and second interference signals.

The optical detectors can be photodiodes.

In connection with the method for determining a physical measured variable, the embodiments described above in connection with the measuring device can be provided accordingly.

The technology can be used, for example, for strain, vibration and sound measurements.

Description of exemplary embodiments

• 1 eine schematische Darstellung zum Prinzip eines Sagnac-Interferometers zur Messung von Längenänderungen in einer faseroptischen Spule nach dem Stand der Technik;
• 2 eine schematische Darstellung zum Prinzip eines Mach-Zehnder-Interferometers nach dem Stand der Technik;
• 3a eine schematische Darstellung zum Verhältnis zweier Lichtwellen bei einer kleinen Längenänderung einer Faserspule bei dem bekannten Mach-Zehnder-Interferometer;
• 3b eine schematische Darstellung von Lichtwellen bei einer Verschiebung um mehr als der Periodenlänge des verwendeten Lichts bei dem bekannten Mach-Zehnder-Interferometer;
• 4 eine schematische grafische Darstellung zum Rückrechnen eines Phasenwinkels auf Grundlage der detektierten Lichtmenge;
• 5a eine schematische grafische Darstellung zum Zusammenhang zwischen Sinuskurve und Zeiger in der komplexen Zahlenebene;
• 5b eine schematische grafische Darstellung zur Abgrenzung von Nulldurchgängen bei positiven Phasenwinkeländerungen und Nulldurchgängen bei negativen Phasenwinkeländerungen in der komplexen Zahlenebene;
• 5c eine weitere schematische grafische Darstellung zur Abgrenzung von Nulldurchgängen bei positiven Phasenwinkeländerungen und Nulldurchgängen bei negativen Phasenwinkeländerungen in der komplexen Zahlenebene;
• 6a eine schematische grafische Darstellung eines Zeigers und eines weiteren Zeigers mit einer konstanten Phasenverschiebung in der komplexen Zahlenebene;
• 6b eine schematische grafische Darstellung zur Bestimmung der Drehrichtung eines Zeigers in der komplexen Zahlenebene mittels Einführung eines weiteren Zeigers;
• 6c eine weitere schematische grafische Darstellung zur Bestimmung der Drehrichtung eines Zeigers in der komplexen Zahlenebene mittels Einführung eines weiteren Zeigers;
• 7a eine schematische grafische Darstellung von zwei Zeigern in der komplexen Zahienebene, die den gleichen Imaginärteil, aber unterschiedliche Phasenwinkel aufweisen;
• 7b eine schematische grafische Darstellung zur Bestimmung der Drehrichtung und des Phasenwinkels eines Zeigers in der komplexen Zahlenebene mittels Einführung eines weiteren Zeigers;
• 8 eine schematische Darstellung eines 3x3-Kopplers zur Erzeugung dreier phasenverschobener Interferenzsignale;
• 9 eine schematische Darstellung eines modifizierten Mach-Zehnder-Interferometers mit einem 3x3-Koppler zur Erzeugung von Interferenzsignalen;
• 10 eine schematische Darstellung einer Auswerteeinrichtung zum modifizierten Mach-Zehnder-Interferometer in zwei verschiedenen Implementierungsvarianten;
• 11a eine schematische grafische Darstellung des Falls eines ersten digitalen Signals mit aufsteigender Flanke und eines negativen zweiten digitalen Signals;
• 11b eine schematische grafische Darstellung des Falls eines ersten digitalen Signals mit absteigender Flanke und eines positiven zweiten digitalen Signals;
• 11c eine schematische grafische Darstellung des Falls eines ersten digitalen Signals mit aufsteigender Flanke und eines positiven zweiten digitalen Signals;
• 11d eine schematische grafische Darstellung des Falls eines ersten digitalen Signals mit absteigender Flanke und eines negativen zweiten digitalen Signals;
• 12a eine schematische grafische Darstellung des Zeitverlaufs zweier ungeglätteter elektrischer Signale;
• 12b eine schematische grafische Darstellung des Zeitverlaufs zweier geglätteter Signale;
• 12c eine schematische grafische Darstellung des Zeitverlaufs der an der Signalauswertungseinheit eingehenden digitalen Signale;
• 13 eine schematische grafische Darstellung der Vibrationsmessung auf Basis des modifizierten Mach-Zehnder-Interferometers;
• 14 eine schematische grafische Darstellung zur Auswertung aller Ausgangssignale eines 3x3-Kopplers;
• 15 eine schematische Darstellung einer Auswerteeinrichtung zum modifizierten Mach-Zehnder-Interferometer in einer weiteren Implementierungsvariante;
• 16a eine schematische grafische Darstellung geglätteter Signale als Eingangssignale für die Berechnung des Phasenwinkels;
• 16b eine schematische grafische Darstellung der Signale aus der Berechnung des Phasenwinkels;
• 16c eine schematische grafische Darstellung der Signale zur Rauschreduktion der berechneten Phasenwinkelwerte und
• 17 eine schematische grafische Darstellung zur genauen Gesamtphasenwinkelbestimmung im Vergleich zur Gesamtphasenwinkelbestimmung mittels Halbwellenzählung.

In the following, further embodiments will be explained in more detail with reference to figures of a drawing. Hereby show:

• 1 a schematic representation of the principle of a Sagnac interferometer for measuring changes in length in a fiber optic coil according to the prior art;
• 2 a schematic representation of the principle of a Mach-Zehnder interferometer according to the prior art;
• 3a a schematic representation of the ratio of two light waves at a small change in length of a fiber coil in the known Mach-Zehnder interferometer;
• 3b a schematic representation of light waves with a shift by more than the period length of the light used in the known Mach-Zehnder interferometer;
• 4 a schematic diagram for calculating a phase angle based on the detected amount of light;
• 5a a schematic diagram of the relationship between sinusoid and pointer in the complex number plane;
• 5b a schematic graph showing the delineation of zero crossings at positive phase angle changes and zero crossings at negative phase angle changes in the complex number plane;
• 5c a further schematic diagram for delineation of zero crossings at positive phase angle changes and zero crossings at negative phase angle changes in the complex number plane;
• 6a a schematic graphical representation of a pointer and another pointer with a constant phase shift in the complex number plane;
• 6b a schematic diagram for determining the direction of rotation of a pointer in the complex number plane by introducing another pointer;
• 6c a further schematic diagram for determining the direction of rotation of a pointer in the complex number plane by introducing another pointer;
• 7a a schematic graphical representation of two pointers in the complex Zahienebene having the same imaginary part, but different phase angles;
• 7b a schematic diagram for determining the direction of rotation and the phase angle of a pointer in the complex number plane by introducing another pointer;
• 8th a schematic representation of a 3x3 coupler for generating three phase-shifted interference signals;
• 9 a schematic representation of a modified Mach-Zehnder interferometer with a 3x3 coupler for generating interference signals;
• 10 a schematic representation of an evaluation device for the modified Mach-Zehnder interferometer in two different implementation variants;
• 11a a schematic graphical representation of the case of a first digital signal with rising edge and a negative second digital signal;
• 11b a schematic graphical representation of the case of a first digital signal with a rising edge and a positive second digital signal;
• 11c a schematic graphical representation of the case of a first digital signal with rising edge and a positive second digital signal;
• 11d a schematic graphical representation of the case of a first digital signal with a rising edge and a negative second digital signal;
• 12a a schematic graphical representation of the time course of two unsmoothed electrical signals;
• 12b a schematic graphical representation of the time course of two smoothed signals;
• 12c a schematic graphical representation of the time course of the incoming at the signal processing unit digital signals;
• 13 a schematic diagram of the vibration measurement based on the modified Mach-Zehnder interferometer;
• 14 a schematic diagram for the evaluation of all output signals of a 3x3 coupler;
• 15 a schematic representation of an evaluation device for the modified Mach-Zehnder interferometer in a further implementation variant;
• 16a a schematic graphical representation of smoothed signals as input signals for the calculation of the phase angle;
• 16b a schematic graphical representation of the signals from the calculation of the phase angle;
• 16c a schematic graphical representation of the signals for noise reduction of the calculated phase angle values and
• 17 a schematic diagram for the exact total phase angle determination in comparison to the total phase angle determination by means of half-wave counting.

The measuring method described below can, for example, be used to record a change in length and can be combined with different sensors. In the following, a simple sensor coil will be considered as an example. Of course, it is possible to use other sensor principles, such as a Fabry-Perot configuration in which two fiber ends are coupled across a small gap. Acoustic effects change the distance between the fibers, which can be detected interferometrically. Since the sensors are only examples of the operation, will be omitted description of other sensor configurations.

The structure of a fiber optic sensor significantly affects its frequency response and sensitivity. Thus, for the detection of partial discharges, for example, fiber coils are used, which are manufactured with very thin fibers. Smallest vibrations cause in a fiber coil mechanical deformations, which at the same time affect all windings and are therefore amplified according to the number of turns. As a consequence, such a fiber coil is highly sensitive and, because of the thin fibers, is capable of absorbing very high frequencies.

Even if the fiber coil described is in principle suitable for recording vibration events, there are challenges with regard to the overloading of the evaluation electronics. To measure vibrations, therefore, insensitive sensors are used. In the simplest case, the fiber is glued directly onto a vibrating surface and behaves like a strain gauge.

For the parallel processing of, for example, weak acoustic emissions and powerful vibrations, a plurality of different sensors are to be attached to the object to be monitored. This is not a problem for the vibration sensor, since the weak sound emissions are not detected by the insensitive fiber optic vibration sensors. Conversely, to ensure that the sensitive fiber coil designed for weak acoustic emissions is not affected by vibrations, the fiber coil is evaluated differentially. Instead of a change in length of the fiber as a whole, only the change in length is measured within a very short period of time. Slow mechanical processes, such as vibrations, have little effect and therefore hardly influence the measurement result.

The differential mode of operation can be optically realized with the aid of a Sagnac interferometer. 1 shows a schematic representation of the principle of a Sagnac interferometer for measuring changes in length in a fiber optic coil according to the prior art. A laser 1 generates coherent light through an optical isolator 2 is guided (in particular to avoid unwanted back reflections) and with the aid of a 3x3 optical coupler 3 is divided into three. Two of these three light waves 4a . 4b then go through the delay coil once clockwise and counterclockwise once 5 and the fiber coil 6 , The way through the delay coil 5 is much longer than the fiber coil 6 , Therefore, the light beam over 4b first passes through the fiber coil and slightly delays the light beam 4a , In this short difference time, the fiber coil changes due to the influence of sound or Vibration signals their extent. This leads to a phase shift of the two light waves to each other.

The 3x3 coupler 3 will now go through again in the opposite direction. The two light waves are brought into interference. On the left side of the 3x3 coupler, there are three interference signals 7a . 7b . 7c output. Due to the mode of operation of the coupler, the three signals are identical in terms of the course, but have a phase shift of 2π / 3 (120 °). The interference signals 7b and 7c Now be on a Balanced Detector 8th guided, in which the optical signals are converted into electrical signals and subtracted from each other. The latter is required because the interference signals 7b . 7c are sinusoidal and angle changes of a sine signal will hardly cause changes in the output signal when the phase angle is close to 90 °. The subtraction thus causes an optimization of the operating point.

The Sagnac interferometer has the advantage of being robust against temperature fluctuations. In addition, the frequency behavior can be achieved via the delay coil 5 to adjust. Thus, the structure can be used in principle for the measurement of low-frequency input signals. The problem, however, is that the differential mode of operation has very small amplitudes at the output 9 generated. As a result, a complex analog post-processing and analog-to-digital conversion is required, which causes a high noise sensitivity. Likewise, the approach is not suitable for simultaneously processing high-frequency signals with small amplitudes and low-frequency signals with large amplitudes. If the change in length of a fiber is not differentially but absolutely measured, this is possible.

2 shows a schematic representation of the principle of a Mach-Zehnder interferometer according to the prior art. A 2x2 coupler 20 initially serves as a beam splitter. The split light waves pass through a reference fiber 21a or a sensor fiber 21b , The reference fiber 21a in a further embodiment additionally has a delay coil 22 on. The sensor fiber 21b has a fiber coil 23 on. The reference fiber 21a and the sensor fiber 21b have the Mach-Zehnder interferometer according to 2 have approximately the same length, wherein the difference in length must be below the coherence length of the light source used. A second 2x2 coupler 24 brings the split light waves back to interference. The interference signal 25 Finally, with the help of an optical detector 25a and for example a transimpedance amplifier 26 (transimpedance amplifier, TIA) into an electrical output signal 27 changed. In an alternative embodiment, instead of a transimpedance amplifier 26 a simple resistor can be used to convert the current of the photodiode into a voltage. This arrangement may be extended in a further embodiment by a conventional amplifier. In different versions can be used for the optical detector 25a alternatively or combined photodiodes, phototransistors, photoresistors or solar cells are used.

At rest, the fiber coil 23 The two light waves should extinguish, so that under ideal conditions an output signal 27 is measured from 0V. Acts a sound or vibration signal on the fiber coil 23 on, their extent changes and it comes at the input of the second 2x2 coupler 24 to a phase shift between the split light waves. The interference causes the light waves no longer cancel out completely and it comes to an output signal 27 not equal to 0 V.

Apart from the acoustic properties of the fiber coil, the method is in principle independent of frequency and would enable the handling of signals which have a large dynamic range. The Mach-Zehnder interferometer after 2 However, it has an essential disadvantage: When changing the length of the sensor fiber 21a greater than the wavelength of the light used, there is a phase jump (cf. 3a and 3b) ,

3a shows the ratio of two light waves 30 . 31a with a small change in length of the fiber coil 23 , 3b shows the light waves 30 . 31b at a shift of 31b with respect to the light wave 30 by more than the period length of the light used. Become the light waves 30 . 31a . 31b continuous, so it is not distinguishable by measurement, whether a phase shift by a factor of 0.25 the wavelength (π / 2, 90 °, 3a) or by a factor of 1.25 of the wavelength (5π / 2, 450 °, 3b) is present. This is clear when the gray areas 32a . 32b in the 3a and 3b compared with each other. In fact, the recalculation of the phase angle is based on the detected amount of light (in 4 through the transimpedance amplifier 26 ) is only possible within a phase range from -π / 2 (-90 °) to + π / 2 (+ 90 °), because the sine function is not injective and therefore the arcsine, the inverse function of the sine, only in the interval [-π / 2, π / 2] is defined.

It is assumed that a recalculation of the phase angle φ ₀ between the light wave 30 through the reference fiber and the light waves 31a . 31b through the sensor fiber 21a based on the measured amount of light after interference within a complete period is possible (in the phase interval from 0 to 2π, corresponding to 0 to 360 °) then phase angle φ ₀ greater than 360 ° can be determined by stretching the Sensor fiber, the light-dark transitions in the interference signal 25 are counted and the angle is added within a period. Each change represents a complete period and contributes with 2π (360 °) to the total phase angle φ. The exact total phase angle φ then results according to the following formula, where n is equal to the number of counted periods:

Total phase angle: $$\phi ={\mathrm{<figure-callout\; id="0"\; label="\phi "\; filenames="DE102018104953A1\_0016.png,DE102018104953A1\_0019.png"\; state="\{\{state\}\}">\phi </figure-callout>}}_0+2\mathrm{\pi }n$$

However, the approach outlined here is corresponding with the Mach-Zehnder interferometer 2 not feasible. The reason for this is that when compressing the sensor fiber would have to be exactly the same procedure as when stretching the sensor fiber, wherein the total phase angle is not calculated according to Equation 1 by addition, but according to Equation 2 by subtraction.

Total phase angle: $$\phi ={\phi }_0-2\mathrm{\pi }n$$

Obviously, it is after the Mach-Zehnder interferometer 2 not possible, at the interference signal 25 to detect if the sensor fiber 23 stretched or compressed. 4 illustrates this effect as a graph of measured light intensity / interference over the change in length Δs of the sensor fiber 23 , The fiber is at rest at Δs = 0 (position 40 in 4 ). Positive values for the change in length Δs correspond to an extension, negative values of a compression. For counting periods, it is advisable to detect the zero crossings with positive slope. For these zero crossings, / is negative before the zero crossing and then positive (position 41 ). Exactly such zero crossings are also found for negative Δs (position 42 ).

The solution to the problem is the effect in the complex number plane with real part x and imaginary part y transferred to. The sinus in 4 arises here by a pointer 50 with constant angular velocity and current phase angle φ ₁ to x -Axis in the unit circle rotates and only the imaginary part 53 of the pointer 50 is considered (in 5a is the imaginary part 53 marked by the gray stretch). The pointer rotates 50 at constant speed in a clockwise direction, the results in the 4 shown function course.

5b shows how the counterclockwise rotating pointer 50 from the position 51 in the position 52 moves while doing the x Axis, from the negative half-plane (y <0) to the positive half-plane (y> 0). The in 5c illustrated counterclockwise rotating pointer 50 passes by position during its movement 53 by position 54 also the x -Axis from the negative to the positive half-plane. Only the respective sine of the pointer 50 on the y Axis (the imaginary parts 55 . 56 ), both cases are in 5b and 5c indistinguishable.

As the direction of rotation of a pointer 50 is in the 6a to 6c outlined. This will be a second pointer 60 needed, to the pointer 50 has a constant phase angle δ, where δ may not correspond to an integer multiple of π. In such a constellation becomes a zero crossing of the first pointer 50 detected, based on the second phase-shifted pointer 60 determine if both hands 50 . 60 move clockwise or counterclockwise. In 6b For example, the first pointer moves 50 from the position 61 counterclockwise to the position 62 , Here is the x -Axis crossed from negative to positive. Simultaneously, the second pointer moves 60 from the position 63 in the position 64 ,

The case corresponds to that in 5b is shown. 6c corresponds to the case in 5c and shows how the pointer is 50 clockwise from position 65 by position 66 moves while doing the x Axis exceeds. At the same time, the phase-shifted second pointer moves 60 from the position 67 in the position 68 , While the two cases in 5b and 5c are indistinguishable, they can be in 6b and 6c delimited solely by the fact that the phase-shifted second pointer 60 in 6b in the upper half-plane lies (y> 0), while in 6c the phase-shifted second pointer 60 in the lower half-plane lies (y <0).

The phase-shifted second pointer 60 allows not only the determination of the direction of rotation, but also the angle determination within the complete unit circle, from -π to + π. 7a illustrates this. In order to determine the phase angle to the measured value, the arc sine function is required, which is uniquely defined only in the interval from -π / 2 to + π / 2, as in 7a is clarified: A sine value 70 can be on the pointers at the positions 71 or. 72 with respective different phase angles from each other φ ₂ or. φ ₃ to be led back.

By additionally considering a phase-shifted pointer, the two positions can be resolved 71 and 72 but separate from each other. So belongs in 7b to the pointer at position 71 the phase-shifted pointer at position 73 and Pointer at position 72 the phase-shifted pointer at position 74 , The phase-shifted hands at the positions 73 and 74 However, they have different values for the arcsine, so that it can be determined by a simple case distinction with respect to the sign of the arcsine of the phase-shifted pointer, whether the measured sine value 70 to the pointer at the position 71 or the position 72 belongs.

As it is necessary, with two phased out hands 50 . 60 To work, it must be clarified how a constant phase-shifted signal can be generated with the help of an interferometer. Here, the operation of the N × N couplers used in fiber optics for interferometry is helpful. NxN couplers cause phase shifts that depend on the number of inputs and outputs. For example, a 3x3 coupler 80 generates from an input signal 81 three in each case by 2π / 3 (corresponding to 360 ° / 3 = 120 °) shifted output signals 82 . 83 . 84 (please refer 8th ). In general, the relation holds that an NxN coupler at the output causes a pairwise phase shift of 2π / N (corresponding to 360 ° / N), where N equals the number of inputs and outputs on the N × N coupler. The relationship applies even if the N × N coupler is driven with two input signals, in which case the phase shift at the output of the NxN coupler refers to the interference signal of the input signals.

The described technology is based in one embodiment on a modified Mach-Zehnder interferometer, wherein an N × N coupler with N ≥ 3 is used for the interference signal. The structure is exemplary with a 3x3 coupler 90 in FIG 9 shown. At the outputs of the 3x3 coupler 90, three phase-shifted interference signals ( 91a . 91b . 91c) generated. At least two of these interference signals 91a . 91b be using an optical detector 91d . 91e (For example, a photodiode, a phototransistor, a photomultiplier or a solar cell) and provided for signal processing transimpedance amplifier 92a . 92b into electrical signals 93a . 93b changed. The further processing of the electrical signals 93a . 93b is implementation-dependent and will be described in detail below. Also implementation-dependent is whether more than two interference signals 91a . 91b be taken into account at the outputs of the N × N coupler 90. That's why the optical detector is 91f and the transimpedance amplifier 92c in 9 shown in gray. It is expressly noted that the use of higher order NxN couplers (N ≥ 4) is possible and may be advantageous as set forth below.

An exemplary embodiment which can be implemented with little implementation effort is explained below.

If it is sufficient for a measurement to work with an accuracy that corresponds to half the wavelength of the light, it is not necessary to change the phase angle φ ₀ within the unit circle. Instead, only the half periods must be counted, which at a stretch or compression of the sensor fiber 21b in the positive or negative direction in the interference signal 91a occur. The input information for the processing therefore does not have to be the time-resolved exact amounts of light from the interference signal 91a but only their zero crossings. This simplifies the post-processing of the output signals 93a . 93b of the modified Mach-Zehnder interferometer substantially as in 10 shown. The components highlighted in gray have no function in implementation variant A.

The transimpedance amplifiers used for signal conditioning 92a . 92b correspond to those in , They are merely listed for the sake of clarity and need not necessarily be provided. For detecting the zero crossings of the interference signal 91a become the interference signals 91a and 91b first on antialiasing filters 100a . 100b guided. They ensure that high frequency noise is reduced at zero crossing.

Subsequently, the smoothed signals 101 . 101b on the comparator units 102 . 102b guided. The comparator units 102 . 102b with comparators 103a . 103b can in one embodiment high-pass filter on the input side 104a . 104b comprising the smoothed signals 101 . 101b Free from DC components that could otherwise interfere with the further processing. At the outputs of the comparator units 102 . 102b are already digital signals 105a . 105b in front. In implementation variant A, more expensive analog-to-digital converters are not needed. Because very broadband comparators are commonly available, through the circuit in 10 even high frequencies are processed.

The further signal evaluation takes place digitally in a signal evaluation unit 106 , If high-frequency signals are also to be processed, the signal evaluation unit can in one embodiment 106 be implemented in the form of a FPGA system (Field Programmable Gate Array). If only low-frequency changes are to be expected at the input, in an alternative embodiment, the signal evaluation unit 106 be realized in the form of an embedded system. The embedded system only needs to pass the zero crossings, mediated by the digital signals 105a . 105b , counting and pass on the result (for example via the interface 107 ). It is expedient, not only whole periods in the interference signals 91a . 91b to count but half periods. In this way, on the one hand, the resolution of the measurement is improved, so that is quantized with half the wavelength of light, and on the other hand ensures that interference on one of the two considered digital signals 105a and 105b not in the form of a drift to large positive or negative values out.

11a to 11d show the four cases that are digitally from the signal processing unit 106 be evaluated. By means of the digital signal 105a become the zero crossings of the interference signal 91a as a flank 110 detected. By means of the digital signal 105b it is decided whether the total phase angle φ should be incremented or decremented. 11a and 11c or. 11b and 11d in each case show whether the first or second half-wave of a period is to be counted. According to the cases in 11a and 11b becomes the total phase angle φ increments, according to the cases in 11c and 11d becomes the total phase angle φ decremented. For a length-accurate evaluation, in this way half the wavelength of light per half-wave is the total phase angle φ or to subtract from the total phase angle φ.

The implementation variant A was practically tested for the vibration measurement mentioned above. Both the antialiasing filters 100a . 100b as well as the comparators 103a . 103b were not built in hardware, but simulated in software using the Python programming language. The result based on real measurement data is in 12a to 12c shown. The laser used in the modified Mach-Zehnder interferometer 1 operated at a wavelength of 1550 nm, so that elongations and compressions with an accuracy of ± 775 nm can be detected.

12a shows the measured with the help of an oscilloscope waveform of the electrical signals 93a . 93b at the output of the modified Mach-Zehnder interferometer (cf. 9 ). The antialiasing filters 100a . 100b for the first processing step, 19 points were simulated using a moving average filter. The result, the smoothed signals 101 . 101b accordingly, is in 12b shown. By means of the smoothing by the antialiasing filters 100a . 100b artificial zero crossings are avoided. However, in one embodiment, the use of anti-aliasing filters can be omitted if the interference signals present 91a . 91b are clearly bandlimited. 12c represents the simulated curve of the digital signals 105a . 105b at the output of the comparators 103a . 103b (cf. 10 ).

The calculated graph 130 in 13 is the result of a vibration measurement. It was accordingly 11 the half-waves each counted by a factor of 0.5. In the graph 130 are clearly quantization levels by stepwise incrementing and decrementing according to 11 to recognize. In the present configuration, amplitude values occur 131 from about -25 to about +58 units. Measured from tip to tip this results in a change in length of 83 x 1550 nm = 128.65 μm. As was measured accurately on half-waves, is quantized at this length change in 83 · 2 = 166 stages, which corresponds to an 8-bit conversion. However, it would not be correct to speak of 8-bit accuracy since the maximum amplitude is limited only by the size of the digitally-implemented counter. Resolution and frequency of digital signals 105a . 105b However, are in a reciprocal relationship, since the signal evaluation unit 106 operates with limited clock and therefore is not capable of any fast digital signals 105a . 105b dissolve.

Hereinafter, another embodiment is explained, which is feasible with medium implementation effort.

With the embodiment described above, changes in length can be detected with a resolution corresponding to half the wavelength of the light. The fact that half the wavelength of the light is resolved in this case is due to the fact that two zero crossings occur within one period of the non-phase-shifted input signal, namely in the phase angle position 0 and at the angular position π. Taking into account that the N × N coupler in the modified Mach-Zehnder interferometer generates N output signals which are each phase-shifted by 2π / N, the resolution of the length changes can be improved many times with certain modifications.

In an implementation with a 3x3 coupler this leads to the in 10 sketched structure, where in implementation variant B the in 10 gray deposited components are included. The three output signals 105a . 105b . 105c the comparators 103a . 103b . 103c are in 14 exemplified. In the embodiment described above, only the zero crossings of the digital signal 105a counted. Therefore, a resolution of the change in length of half a wavelength of light is achieved. But all flanks 110 the three output signals 105a . 105b . 105c detected, it is no longer resolved with half the wavelength of the light, but with 1/6 of the wavelength of light.

In general, the resolution when using an NxN coupler depends on whether N is even or odd. If the wavelength of the light is denoted by λ, an odd value for N results in a maximum resolution of λ / 2N per quantization step and a straight-line value for N results in a maximum resolution with λ / N. Thus, a 4x4 coupler has no higher resolution than a 3x3 coupler. Only with a 5 × 5 coupler, an improvement of the resolution would again be achieved (at a wavelength of, for example, 1550 nm results in a resolution of 155 nm per count).

There may be provided a further embodiment with a higher implementation cost.

It was explained above that within one period the phase angle φ ₀ in the interference signal 91a can also be calculated exactly. This allows a significant increase in accuracy compared to the above-described implementation variants B and C but in turn leads to an increase in the implementation effort. Instead of in the implementation variant A used comparators 103a . 103b become two fast analog-to-digital converters 150a . 150b used. Furthermore, the digital signals 151a . 151b to convert into phase angle, which due to the complexity of the required arcsine function with case distinction according to 11 leads to increased complexity of the digital hardware. The structure of the transmitter is in 15 outlined.

The gray deposited components are not needed in one embodiment. They can be used in a further embodiment for noise reduction. The measuring principle was constructed as shown and used for the already mentioned vibration analysis. The antialiasing filters 100a . 100b and the analog-to-digital converters 150a . 150b were realized with the help of a multichannel storage oscilloscope. The scanned data was then processed offline on a PC. The programming language was Python. The wavelength of the laser used for interferometry 1 For example, in one embodiment, it may be 1550 nm.

16a shows the digital signals coming from the modified Mach-Zehnder interferometer 151 a, 151b, which, because of the 3 × 3 coupler 90, are shifted from one another by a phase angle δ = 2π / 3 (corresponding to 120 °) and correspond to light intensity values. The digital signals 151a . 151b are at the maximum value 1 normalized to be compatible with the domain [-1,1] of the arc sine function. 16b shows by means of the arcsine function and the case distinction according to 11 calculated values 160a or. 160b for the phase angle φ ₀ with the digital signal 151a or. 151b as input signal. It is clear on the curves to the phase angle values 160a . 160b disorders 161 to detect the phase angle values π / 2 and 3π / 2. These are due to the fact that the arcsine function is not linear. In areas where the argument is close to +1 or -1 (corresponding to the phase angle values π / 2 and -π / 2 and 3π / 2, respectively), slight changes in the value due to noise will lead to significant phase angle deviations. In these areas, noise thus affects more.

The problem is addressed by the phase angle positions of both digital signals 151a . 151b of the modified Mach-Zehnder interferometer, ie with the addition of the phase-shifted digital signal 151b , be calculated independently. Because of the constant shift by the phase angle δ not both digital signals 151a . 151b at the same time accepting values around ± 1, faults occur 161 for each time always only one of the two curves of the phase angle values 160a . 160b on (cf. 12b) , This phenomenon can be used for noise reduction. Accordingly, the two curves of the phase angle values can be obtained 160a . 160b Assemble phase-dependent in intervals. In an alternative embodiment, noise reduction may be accomplished by calculating the average of the phase angle value curves 160a . 160b be achieved, wherein before the mean value calculation, the shift by the phase angle δ = 2π / 3 is to be considered. This results in the phase angle curve 162 with reduced interference 163 , in the 16c is shown.

In further embodiments, it is possible to use the optical detector 91f with transimpedance amplifier 92c , the antialiasing filter 110c and the analog-to-digital converter 150c the digital signal 151c , and other digital signals for noise reduction, if an N × N coupler with N ≥ 4 instead of the 3x3 coupler 90 and associated transimpedance, antialiasing and analog-to-digital converters are used. This can be done, for example, by means of phase angle-dependent assembly at intervals or by shifting the further digital signals by multiples of the phase angle δ = 2π / N and subsequent averaging analogously to the formation of the phase angle curve 162 , Instead of further analog-to-digital converters, further comparator units can also be used in one embodiment.

Becomes the calculated phase angle φ ₀ combined with the value of the total phase angle φ, the total phase angle curve is obtained 170 represented in 17 , For comparison, the total phase angle curve 171 which results from the method of implementation variant B with the sole counting of half periods. At one wavelength of the laser 1 of 1550 nm corresponds to each stage 172 a change in length of 775 nm. The average value method for noise reduction according to the phase angle curve 162 used, the errors are transmitted 163 also on the total phase angle curve 170 , This is visible on the basis of further disturbances 173 , The errors 173 can be avoided by piecewise calculation from the digital signals 151a . 151b ,

The proposed technology has several advantages over the prior art.

Strain, vibration and sound measurements can only be made with a conventional Mach-Zehnder interferometer in special situations, since the measuring range is limited to the wavelength of the light used. For this reason, Mach-Zehnder interferometers are generally not used for such applications. The described technology is not affected by such restrictions. Compared to the Sagnac interferometer, the method and the measuring device can be realized with a very simple evaluation electronics. This increases the robustness and lowers the costs. Improved strain, vibration and sound measurements can be made.

Compared to the Sagnac interferometer the frequency response down (to low frequencies) is not limited, and it is not a long compensation coil, but only a short fiber optic reference strand needed. This reduces the costs.

The method and the measuring device make it possible to measure any desired extensions with micrometer precision. A limiting factor here can be the fiber, which can break when excessively stretched. Furthermore, the method and the measuring device allow to record analog measurement data with a minimum of analog circuit technology. In particular, in an embodiment, for example, can be dispensed with analog-to-digital converter, so that can be spoken by a fully digital measuring sensor.

The features disclosed in the above description, the claims and the drawings may be important both individually and in any combination for the realization of the various embodiments.

Claims (15)

A physical quantity determining apparatus comprising: - an optical coupler having N (N ≥ 3) coupler inputs arranged to receive a lightwave input signal, respectively, and having N coupler outputs configured to output a lightwave output signal, respectively, the lightwave output signals by signal processing in the optical coupler in each case a different phase with a coupler-related phase shift not equal to m · π (with m ∈

) respectively; a first lightwave signal applied to a first one of the coupler inputs; a second lightwave signal applied to a second one of the coupler inputs, the first and second lightwave signals being interferable and having a measurement-related phase shift corresponding to a physical measurand; a first optical interference signal representing a first interference of the first and second lightwave signals and applied to a first one of the coupler outputs; a second optical interference signal representing a second interference of the first and second lightwave signals and applied to a second one of the coupler outputs, wherein the second optical interference signal has a coupler-related phase shift not equal to m · π (with m ∈

) having; a first optical detector configured to receive the first optical interference signal at a first detector input and to provide a first electrical output signal at a first detector output; a second optical detector configured to receive the second optical interference signal at a second detector input and to provide a second electrical output signal at a second detector output; and - an evaluation device, which is configured to determine the physical measured variable from the first and the second electrical output signal. Measuring device after Claim 1 , characterized in that the optical coupler is a 3x3 coupler. Measuring device after Claim 1 or 2 , characterized in that the evaluation device comprises: a first amplifier device which is set up at a first amplifier input receive first electrical output signal and provide a first output signal at a first amplifier output; and a second amplifier configured to receive, at a second amplifier input, the second electrical output signal and to provide a second output signal at a second amplifier output. Measuring device after Claim 3 characterized in that the first amplifier means comprises a transimpedance amplifier and the second amplifier means comprises a transimpedance amplifier. Measuring device according to at least one of the preceding claims, characterized in that the evaluation device comprises: - a first filter device which is adapted to receive the first electrical output signal of the optical detector at a first filter input and to provide a first filtered output signal at a first filter output; and a second filter means arranged to receive the second electrical output signal of the optical detector at a second filter input and to provide a second filtered output signal at a second filter output. Measuring device after Claim 5 , characterized in that the first filter means comprise a first anti-aliasing filter and the second filter means comprise a second anti-aliasing filter. Measuring device after Claim 5 , characterized in that the first filter device comprises a first high-pass filter and the second filter device comprises a second high-pass filter. Measuring device according to at least one of the preceding claims, characterized in that the evaluation device comprises: - a first comparator which is adapted to receive the first filtered output signal at a first comparator input and to provide a first digital output signal at a first comparator output; and a second comparator configured to receive at a second comparator input the second filtered output signal and to provide a second digital output signal at a second comparator output. Measuring device after Claim 8 , characterized in that the first and / or the second comparator have a hysteresis. Measuring device according to at least one of the preceding claims, characterized in that the evaluation device comprises: - a first AD converter which is adapted to receive the first filtered output signal at a first AD converter input and the first digital output at a first AD converter output Provide output signal; and a second AD converter configured to receive the second filtered output signal at a second AD converter input and to provide the second digital output signal at a second AD converter output. Measuring device according to one of Claims 8 to 10 , characterized in that the evaluation device has a signal processing which is adapted to process the first and the second digital output signal. Measuring device according to at least one of the preceding claims, characterized in that it further comprises: - a third optical detector which is adapted to receive at a third detector input a third optical interference signal from the optical coupler and at a third detector output a third electrical output signal to be provided, wherein the third optical interference signal with respect to the first and the second optical interference signal, a coupler-related phase shift not equal to m · π (with m ∈

) having; and - the evaluation device which is set up to determine the physical measured variable from the first, the second and the third electrical output signal. Measuring device after Claim 12 , characterized in that the following further is provided: - further optical detectors which are adapted to receive at further detector inputs further optical interference signals from the N coupler outputs (N> 3) and to provide further electrical output signals at further detector outputs, the further optical interference signals in pairs a coupler-related phase shift not equal to m · π (with m ∈

) respectively; and - the evaluation device which is set up to determine the physical measured variable from the further electrical output signals of the optical detectors. Measuring device according to at least one of the preceding claims, characterized in that the optical detectors are photodiodes. A method of determining a physical measurand, comprising: providing an optical coupler having N (N ≥ 3) coupler inputs arranged to receive a lightwave input signal, respectively, and having N coupler outputs arranged each having a lightwave output signal output, wherein the lightwave output signals by signal processing in the optical coupler each have a different phase with a coupler-related phase shift not equal to m · π (with m ∈

) respectively; - applying a first lightwave signal to a first one of the coupler inputs; - Applying a second lightwave signal to a second of the coupler inputs, wherein the first and the second lightwave signal are interferable and have a measurement-related phase shift, which is formed according to a physical measurement variable; Providing a first optical interference signal representing a first interference of the first and second lightwave signals at a first one of the coupler outputs; Providing a second optical interference signal representing a second interference of the first and second lightwave signals at a second of the coupler outputs, wherein the second optical interference signal has a coupler-related phase shift different from m.pi. (With m ∈

) having; Receiving the first optical interference signal at a first optical detector, and providing a first electrical output signal at a first detector output; Receiving the second optical interference signal at a second optical detector, and providing a second electrical output signal at a second detector output; and - determining the physical measured variable by means of an evaluation device from the first and the second electrical output signal.

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