DE19605717C1 - Fibre optic measuring unit with light source transmitting linear polarised light - Google Patents

Abstract

A fibre optic measuring unit with a light source (1), for transmitting linear polarised light, with a fibre sensor (S), which is connected mechanically with a piezoelectric sensor element (6) and optically with the light source. A receiver fibre (r) is connected mechanically with at least one modulator (7,8) and optically with the fibre sensor. Two light detectors (D1,D2) at least are provided, which are optically connected with the receiver fibre, and deliver output signals (U1,U2), which are proportional to the received light intensity. The sensor fibre is a polarimetric light fibre, not a two mode fibre.

Description

TECHNICAL AREA

The invention is based on a fiber optic Measuring device according to the preamble of patent claim 1.

STATE OF THE ART

With the preamble of claim 1, the invention takes to a state of the art, as described by K. Bohnert et al., Coherence-Tuned Interrogation of a Remote Elliptical Core, Dual-mode fiber strain sensor, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 13, No. 1, January 1995, pp. 94-103. There 2 fiber optic measuring devices for measurement electrical AC voltages described, in which the electrical voltage to a cylindrical quartz crystal is created. A resulting, periodic, piezoelectric deformation or change in circumference of the Quartz crystal is on a 2-mode glass fiber as a sensor fiber transfer. The periodic elongation of the Sensor fiber leads to a modulation of the phase difference of the two spatial, optical modes LP₀₁ and LP₁₁ (even), that spread in the sensor fiber. This phase modulation is proportional to the electrical voltage applied. For Measurement is made by a multi-mode laser via the light Sensor fiber, a single mode fiber and 2 modulators with one 2Mode fiber as reception fiber to 2 photodiodes, which detect the interference pattern of the two modes. The differential, optical phase of the modes of the receiving fiber with the help of an electronic control loop and 2s piezoelectric modulators controlled so that the  Phase modulation in the sensor fiber just compensated again becomes. The generated in the control loop and on the piezo modulators The applied control voltage is therefore an image of the one to be measured electrical voltage. Instead of in transmission, the Sensor fiber can also be operated in reflection. On essential feature of the sensor is that interference due to temperature fluctuations and mechanical shocks, which on the connecting fibers between the transmission / Receiver unit and sensor head act, the measurement signal do not interfere. Changes in length of sensor and Receive fiber due to temperature changes also result to optical phase shifts. But these are usually so slow that it can be easily removed from the periodic, electrical induced phase changes can be separated.

In high voltage technology, the electrical to be measured Voltage up to some 100 kV. The Then there are phase shifts in the sensor and receiving fibers relatively large. The required control voltage for compensation the optical phase modulation and / or the length of the on the Modulators of coiled receive fiber take accordingly to. This makes detection more complex. In addition, at higher control signals the hysteresis effects of the piezo modulators Affect signal quality. As a rule, the Control voltage does not exceed a threshold of a few volts exceed.

PRESENTATION OF THE INVENTION

The invention as defined in claim 1 solves the task of a fiber optic measuring device of the beginning mentioned type to develop in such a way that it also for electrical voltages up to a few 100 kV without loss Measurement accuracy and can be used without increasing complexity.

Advantageous embodiments of the invention are in the dependent claims defined.  

An advantage of the invention is that it is relative lower voltages known measuring method without large Changes also apply to high voltages.

BRIEF DESCRIPTION OF THE DRAWING

The invention is illustrated below with reference to embodiments play explained. Show it:

Fig. 1 shows schematically a fiber optical measuring device with a polarimetric sensor fiber in the transmission arrangement,

FIGS. 2a-5a orientations of fiber axes of a feed fiber, the sensor fiber, return fiber and a receiving fiber of the fiber optic measuring device according to FIG. 1,

Fig. 2b-5b excited optical modes in the optical fibers shown in FIGS. 2a-5a,

Fig. 6 shows schematically a fiber optical measuring device with a polarimetric sensor fiber in reflection arrangement,

Fig. 7 is a hollow cylindrical modulator and

FIGS. 8 and 9 arrangements interference when using a polarimetric receiving fiber.

WAYS OF CARRYING OUT THE INVENTION

In the figures, the same parts have the same reference numerals featured.

Fig. 1 shows schematically a fiber optic measuring device with a quartz cylinder or a piezoelectric sensor element ( 6 ) made of quartz with a polarimetric glass fiber or light fiber or sensor fiber (s) wound on its cylinder circumference in a transmission arrangement. A polarimetric optical fiber has two main optical axes (x, y) orthogonal to each other perpendicular to the fiber direction. At a given wavelength, the two orthogonal polarization states (LP₀₁ (x)) and (LP₀₁ (y)) of the LP₀₁ basic mode are propagable with polarizations parallel to the main optical axes (x, y). The effective refractive indices for the two polarizations are different, ie the optical fiber is birefringent.

From a low-coherent light source ( 1 ), e.g. B. a multimode laser diode, light is transmitted via a highly birefringent and thus polarization-maintaining single-mode glass fiber or single-mode optical fiber or feed fiber ( 2 ) with 2 mutually orthogonal optical main axes (x, y), cf. Fig. 2a, the sensor fiber (s), cf. Fig. 3a, supplied via a glass fiber connection or a splice ( 3 ). In the single-mode optical fiber ( 2 ), the light is polarized parallel to one of the main optical axes (x, y) of an elliptical fiber core ( 14 ), which fiber core ( 14 ) is surrounded by a fiber cladding ( 15 ). The feed fiber ( 2 ) and the sensor fiber (s) are spliced together in the splice ( 3 ) so that their main optical axes (x, y; x ', y') are at an angle of 45 ° to one another, the main optical axes being Sensor fiber (s) are designated with (x ′, y ′). The main optical axes (x, y; x ′, y ′) of these two spliced fibers ( 2 , s) may deviate from this 45 ° angle by a tolerable difference angle (ε) of 30 °, preferably 10 °. In the sensor fiber (s) both orthogonal optical modes or polarization states of the basic mode (LP₀₁ (x ') and (LP₀₁ (y')) are excited and have the same amplitude at a 45 ° angle, cf. Fig. 3b. In the feed fiber ( 2 ), cf. Fig. 2b, z. B. the basic mode (LP₀₁) excited in the y direction. The sensor fiber (s) is with a piezoelectric sensor element ( 6 ), for. B. a quartz cylinder, in operative connection, which acts on an electric field to be measured with respect to its amplitude, which is indicated by a lightning symbol.

After passing through the sensor fiber (s), the light is coupled via a splice ( 4 ) into a further polarization-maintaining single-mode optical fiber or return fiber ( 2 ') and from there via a further splice ( 5 ) into an optical 2 -mode optical fiber or receive fiber (r) .

In a 2-mode optical fiber is at a given Wavelength in addition to the basic mode LP₀₁ also the straight LP₁₁- Spreadable mode. The optical fiber has 2 to each other orthogonal optical main axes (x, y) perpendicular to the Grain direction. Both spatial modes (LP₀₁ and straighter LP₁₁ mode) can be both parallel to the x and parallel to y direction are polarized, they are accordingly as LP₀₁ (x) - and straight LP₁₁ (x) mode or as LP₀₁ (y) - and called straight LP₁₁ (y) mode.

The main optical axes (x ′, y ′; x, y) of sensor fiber (s) and return fiber ( 2 ′) are also at an angle of 45 ° ± ε to each other. The main optical axes (x, y) of the return fiber ( 2 ′) and the receiving fiber (r) are parallel or perpendicular to one another, with a tolerable deviation of ± 10 °; however, they are spliced together so that their optical axes, which point in a z direction orthogonal to the xy plane, have a small lateral offset in the y direction, such that the two spatial modes (basic mode (LP₀₁) and straight LP₁₁- Mode) of the receiving fiber (r) are excited with approximately the same amplitude, cf. Fig. 5b.

Fig. 4a shows the orientation of the fiber axes of the return fiber ( 2 ') and Fig. 4b the optical modes excited therein.

The receiving fiber (r) is wound around 1st and 2nd hollow cylinders made of a piezoelectric ceramic or around piezoelectric modulators ( 7 , 8 ); it is on the output side via a polarizer acting as an analyzer ( 9 ) with 2 optoelectric detectors or photodiodes or light detectors (D1, D2) in optical connection, each of which supplies an output voltage (U1) or (U2) on the output side, for the received light output is proportional. The light detectors (D1, D2) are arranged such that they each detect one of the two antiphase substructures of the interference pattern. The analyzer ( 9 ) at the end of the receiving fiber (r) is aligned parallel to one of the two main optical axes (x, y) of the receiving fiber (r), ie, depending on the position of the analyzer, the x or y polarization of the modes is detected. The analyzer ( 9 ) may deviate from this parallel orientation by a maximum of ± 10 °. For a given change in length of the optical fiber, the relative phase shift of the modes is polarization-dependent, with a difference of typically 20% -30% between the two orthogonal polarizations. With this orientation of the fibers ( 2 , s, 2 ', r) it is achieved that the light of the two polarization modes of the sensor fiber (s) is each divided approximately equally between the two spatial modes of the receiving fiber (r).

The two orthogonal polarization states (LP₀₁ (x ')) and (LP₀₁ (y ′)) of the sensor fiber (s) accumulate one Path difference

ΔL _s = l _S · Δn _{g, s} ,

where l _{s is} the length of the sensor fiber (s) and Δn _{g, s is} the difference in the group refractive indices of the two polarization states.

The two spatial modes of the receiving fiber (r) (depending on the position of the analyzer ( 9 ), these are either the LP₀₁ (x) - and the straight LP₁₁ (x) mode or the LP₀₁ (y) - and the straight LP₁₁ (y ) Mode) accumulate a path difference

.DELTA.L _r = l _r · .DELTA.n _{g, r,}

where l _{r is} the length of the receive fiber (r) and Δn _{g, r is} the difference in the group refractive indices of the two modes. This difference is only insignificantly dependent on the polarization, so that no distinction is made here between the x and y polarization. The fiber lengths l _s and l _r are chosen so that ΔL _s and ΔL _r are the same within the coherence length of the light source ( 1 ).

At the end of the receiving fiber (r) there are light waves with a relative path difference of ΔL _r - ΔL _s ≈ 0, which interfere coherently with one another and light waves with relative path differences of L _s and ΔL _r (with ΔL₅ ≈ ΔL _r ) and ΔL₅ + ΔL _r which interfere incoherently and only provide a constant background to the interference pattern. It is important that ΔL _s and ΔL _{r are} significantly larger than the coherence length of the light source ( 1 ).

In a sensor fiber (s) with a length of the large main axis of the fiber core ( 14 ) of 4 µm, a length of the small main axis of 2 µm, a refractive difference between the fiber core ( 14 ) and the fiber cladding ( 15 ) of 0.03 and a fiber length of An accumulated optical path difference of about 0.2 mm at a wavelength of 780 nm was measured 1 m for the two orthogonal polarizations. The difference between the group refractive indices of the two polarization states of the LP₀₁ basic mode of the sensor fiber (s) is almost independent of the wavelength. In contrast, the group refraction difference between the two spatial modes LP₀₁ and LP₁₁ of the receiving fiber (r) is strongly dependent on the wavelength of the light.

The sensitivity of the fibers, ie the differential phase change for a given change in length of the fiber, is essentially determined by the difference in the effective refractive indices that the interfering waves "see". For fibers with an elliptical fiber core ( 14 ) of the type mentioned above, a change in length of about 100 microns is required for a phase shift of 2π between the spatial modes (LP₀₁) and (LP₁₁). The same phase shift between the orthogonal polarizations of the basic mode (LP₀₁) requires a change in length of about 1.5 mm. If, in the case of an optical voltage sensor, the 2-mode sensor fiber according to the prior art mentioned at the beginning is replaced by a polarimetric fiber (s), 15 times higher voltages can be measured with an unchanged detection system.

A difference generator ( 10 ), to which the output voltages (U1) and (U2) of the light detectors (D1, D2) are fed on the input side, supplies a differential voltage (ΔU) on the output side to a regulator or differential voltage regulator ( 11 ) which detects the differential voltage (ΔU) regulates to 0. On the output side, this differential voltage regulator ( 11 ) supplies a signal (S) which contains both a DC and an AC voltage component. This signal (S) passes through a low-pass filter (TP) to a DC voltage amplifier ( 12 ), which on the output side transmits a compensation voltage (U12) to the modulator ( 8 ). The signal (S) is also fed to a blocking filter ( 13 ) for the resonance frequency of the modulator ( 8 ), which on the output side transmits a compensation voltage (U13) to the modulator ( 7 ). The blocking filter ( 13 ) has the task of preventing the control circuit from oscillating at the resonance frequency of the modulator ( 8 ). The compensation voltage (U13) is proportional to the electrical alternating voltage to be measured and thus at the same time an output signal from the fiber optic measuring device. The optical phase shift caused for a given modulator voltage depends on the geometry and the material of the modulator ( 7 , 8 ) as well as on the type of optical fiber and the length of the optical fiber segment connected to the modulator ( 7 , 8 ). The modulator ( 7 , 8 ) can e.g. B. be designed so that a compensation voltage (U13) of ± 3 V causes a differential optical phase shift of ± 10 ° between the spatial modes LP₀₁ and LP₁₁.

Fig. 6 shows a fiber optic measuring device in which the sensor fiber (s) is operated in reflection. Light from the light source ( 1 ) is transmitted via the feed fiber ( 2 ), a splice ( 17 ), a fiber coupler ( 18 ), a splice ( 19 ), a further feed fiber ( 20 ), which have the same optical properties as the feed fiber ( 2 ) has, and the splice ( 3 ) of the sensor fiber (s) supplied, which is provided at the end with a reflective coating ( 21 ). The light is preferably divided in a ratio of 1: 1 in the fiber coupler ( 18 ). The light is reflected at the end of the sensor fiber (s) in the coating ( 21 ) and passes through the sensor fiber (s) a second time. Via the feed fiber ( 20 ), the splice ( 19 ), the fiber coupler ( 18 ) and the splice ( 5 ), part of the reflected light reaches the receiving fiber (r), at the end of which the interference pattern of the two modes (LP₀₁) and (LP₁₁) behind the analyzer ( 9 ) is detected. To compensate for the path difference accumulated in the sensor fiber (s) between the orthogonal polarization states, 2 hollow cylindrical modulators ( 7 ) and ( 8 ) according to FIG. 1 can be in operative connection with the receiving fiber (r) or only one hollow cylindrical modulator ( 22 ), as in Fig. 6 is shown schematically. In the case of the modulator ( 22 ), the compensation voltage (U13) is applied to the inside or inside wall of the hollow cylinder and the compensation voltage (U12) to the outside or outside wall thereof or vice versa (not shown). The evaluation devices are the same as in FIG. 1.

It should be noted that as a result of the double passage of light through the sensor fiber (s), the accumulated path difference between the orthogonal polarization states and the amplitude of the differential phase modulation double compared to the transmission arrangement according to FIG. 1. This must be taken into account when dimensioning the sensors ( 7 , 8 ; 22 ), the sensor fiber (s) and the receiving fiber (r).

Is the reflection arrangement according to Fig. 6, especially with large distances between a measuring head with the sensor fiber (s) and a transmitting / receiving unit with the light source (1) and the receiving fiber (r) with downstream reception and control devices (9, D1, D2 , 10-13 , TP) because the fiber length required for light transmission between the two units is halved compared to the transmission arrangement according to FIG. 1. Likewise, the number of optical fiber connectors required (not shown) is halved.

Fig. 7 shows a hollow cylindrical modulator ( 22 ') with a grounded electrode coating ( 25 ) on its inner wall and 2 by an electrode-free zone (a) spaced apart and electrically mutually insulated electrode coatings ( 23 , 24 ) on its outer wall. The compensation voltage (U12) is applied to one of the two electrode coatings ( 23 ) and the compensation voltage (U13) to the other ( 24 ). Such a modulator ( 22 ') can be used instead of the modulator ( 22 ) in Fig. 6. It is understood that the inner and outer electrodes can be interchanged. One of the two electrode coatings ( 23 , 24 ) can be attached outside and the other inside on the modulator ( 22 '). It also goes without saying that the modulators ( 22 , 22 ') according to FIGS. 6 and 7 can also be used for the arrangement according to FIG. 1.

Instead of the 2-mode receive fiber (r), a polarimetric optical fiber can also be used. The main optical axes (x, y) of the receiving fiber (r) are then aligned in the splice ( 5 ) at an angle of 45 ° ± ε with respect to the axes of the return fiber ( 2 '). If the receiving fiber (r) is in optical connection via an air gap or directly with the sensor fiber (s), the main optical axes (x, y) of the receiving fiber (r) are either at an angle of 45 ° ± ε or at an angle of 90 ° ± ε with respect to the main optical axes (x ′, y ′) of the sensor fiber (s) arranged. The two orthogonal polarization states (LP₀₁ (x)) and (LP₀₁ (y)) of the receiving fiber (r) accumulate a path difference

ΔL _r - l _r Δn _{g, r} *,

where Δn _{g, r is} the difference in the group refractive indices of the orthogonal polarization states (LP₀₁ (x)) and (LP₀₁ (y)) of the receiving fiber (r). The conditions given above apply again to ΔL _s and ΔL _r . At the end of the receiving fiber (r), the two orthogonal polarization states (LP₀₁ (x)) and (LP₀₁ (y)) are brought to interference with the aid of the optical arrangements indicated in FIG. 8 or 9. The two antiphase interference signals are again detected with the light detectors (D1, D2).

According to FIG. 8, the light bundle emerging from the receiving fiber (r) is focused on a beam splitter (ST) by means of a lens (L) and broken down into two partial beams which are transmitted to the light detectors (D1, D2) via polarizers (P1, P2). reach. The edges of the beam splitter (ST) are at an angle of 45 ° ± 10 ° to the main optical axes (x, y) of the receiving fiber (r). The polarizer (P1) is at an angle of 45 ° ± ε and the polarizer (P2) is at an angle of -45 ° ± ε to one of the two main optical axes (x, y) of the receiving fiber (r).

According to FIG. 9, the two orthogonal polarization states (LP₀₁ (x)) and (LP₀₁ (y)) the receiving fiber (r) are focused by a lens (L) on a Wollaston prism (W) and is brought to interference. The optical axis of the Wollaston prism (W) is at an angle of 45 ° ± ε to the main optical axes (x, y) of the receiving fiber (r). If the optical arrangements according to FIGS. 8 and 9 are sufficiently compact, the lens (L) can also be omitted.

It goes without saying that instead of quartz cylinders and piezoceramics, other piezoelectric components can be used as sensor element ( 6 ) and as modulators ( 7 , 8 , 22 , 22 ').

In principle, the feed fibers ( 2 , 20 ) and the return fibers ( 2 ') can be omitted. The polarized light could e.g. B. transmitted by air or by vacuum and coupled into the light fibers with the aid of lenses.

Instead of the electrical voltage, another can physical quantity can be measured, provided it is a Length change of the sensor fiber (s) causes that clearly can be assigned to this physical quantity.

Reference list

1 light source, multimode laser
2 , 20 single-mode optical fibers, high birefringent single-mode glass fibers, feed fibers
2 ′ single-mode optical fiber, highly birefringent single-mode glass fiber, return fiber
3-5 , 17 , 19 splices, fiber optic connections
6 sensor element, quartz cylinder
7 , 8 , 22 , 22 ' modulators
9 analyzer
10 difference formers
11 regulator, differential voltage regulator
12 DC amplifiers
13 notch filter
14 fiber core
15 fiber cladding
18 fiber couplers
21 reflective coating
23-25 electrode coatings
a electrode-free zone
D1, D2 light detectors, optoelectric detectors, photodiodes
L lens
LP₀₁ optical basic mode
LP₁₁ optical straight mode
P1, P2 polarizers
r 2-mode optical fiber, polarimetric fiber, receive fiber
s polarimetric optical fiber, sensor fiber
S signal at the output of 11
ST beam splitter
U1, U2 output voltages from D1, D2
U12 output signal from 12 , compensation voltage, compensation potential
U13 output signal from 13 , compensation voltage, compensation potential
TP low pass filter
W Wollaston prism
x, y; x ′, y ′ main optical axes
ΔU differential voltage
ε difference angle, tolerance angle.

Claims (10)

1. Fiber optic measuring device

• a) with a light source ( 1 ) for emitting linearly polarized light,
• b) with a sensor fiber (s) which is mechanically connected to a piezoelectric sensor element ( 6 ) and optically to the light source ( 1 ),
• c) with a receiving fiber (r) which is mechanically connected to at least one modulator ( 7 , 8 ; 22 , 22 ') and optically connected to the sensor fiber (s), and
• d) with at least 2 light detectors (D1, D2) which are in optical connection with the receiving fiber (r) and on the output side supply signals (U1, U2) which are proportional to the received light intensity, characterized in that
• e) that the sensor fiber (s) is a polarimetric light fiber, not a two-mode fiber.

2. Fiber optic measuring device according to claim 1, characterized in

• a) that the main optical axes (x ′, y ′) of the sensor fiber
• (s) at an angle of 45 ° ± ε to the direction of polarization of a light beam incident from the light source ( 1 ) and
• b) also at an angle of 45 ° ± ε to mutually orthogonal optical main axes (x, y) of the receiving fiber (r), where ε is a predeterminable tolerance angle.

3. Fiber optic measuring device according to claim 2, characterized in

• a) that the tolerance angle ε 30 °,
• b) in particular that the tolerance angle ε is 10 °.

4. Fiber optic measuring device according to one of the preceding claims, characterized in that the receiving fiber (r) via an analyzer ( 9 ) with the light detectors (D1, D2) is in optical connection. 5. Fiber optic measuring device according to claim 4, characterized in that the analyzer ( 9 ) at the end of the receiving fiber (r) parallel to one of the two optical. Main axes (x, y) of the receiving fiber (r) is aligned with a tolerance angle of ± 10 °. 6. Fiber optic measuring device according to one of the preceding Claims, characterized in that the receiving fiber (r) is a 2-mode optical fiber. 7. Fiber optic measuring device according to one of the preceding claims, characterized in that

• a) that the light source ( 1 ) emits low-coherent light and
• b) that for the sensor fiber (s) and for the receiving fiber (r) at least approximately the condition: l _s · Δn _{g, s} = l _r · Δn _{g, r} applies, with l _s = length of the sensor fiber (s), Δn _{g, s} = difference between the group refractive indices of the two orthogonal polarization states (LP₀₁ (x ′)) and (LP₀₁ (y ′)) of the basic mode (LP₀₁) of the sensor fiber (s) with the two mutually orthogonal optical main axes (x ′, y ′) , l _r = length of the receive fiber (r) and Δn _{g, r} = difference of the group refractive indices of the two spatial modes (LP₀₁) and (LP₁₁) of the receive fiber (r),
• c) in particular that the light source ( 1 ) has a coherence length which is less than l _s · Δn _{g, s} .

8. Fiber optic measuring device according to claim 1, characterized in

• a) that the receiving fiber (r) is a polarimetric optical fiber and
• b) that the main optical axes (x, y) of the receiving fiber (r) are at an angle of 45 ° ± ε to the direction of polarization of a light beam transmitted by the sensor fiber (s), where ε is a predeterminable tolerance angle,
• c) in particular that the receiving fiber (r) via a beam splitter (ST) and 2 downstream polarizers (P1, P2) or
• d) is in optical connection with the light detectors (D1, D2) via a Wollaston prism (W).

9. Fiber optic measuring device according to one of the preceding claims, characterized in that

• a) that the at least one modulator ( 22 ) is hollow cylindrical,
• b) that on the inner wall a low-frequency compensation potential (U13) and
• c) that a higher-frequency compensation potential (U12) is present on the outer wall
• d) or vice versa.

10. Fiber optic measuring device according to one of claims 1 to 8, characterized in

• a) that the at least one modulator ( 22 ') is hollow cylindrical and
• b) has on its inner or outer wall 2 electrically mutually insulated electrode coatings ( 23 , 24 ),
• c) one of which has a low-frequency compensation potential (U13) and
• d) the other is in electrical connection with a higher-frequency compensation potential (U12).