EP0489899A1 - Optical fibre parameter measuring device - Google Patents

Abstract

The invention relates to a device for measuring optical fiber of several parameters comprising: a light source (S); at least one optical fiber (1) sensitive to the parameters (P1, P2) measured having stable modes excited by the light source (S), a variation of each parameter (P1, P2) measured introducing phase shifts (deltaPHIi, deltaPHI2) between the different modes called "measurement modes", two by two; at least one coupling means separating the incident light between these measurement modes; at least one means of recombining these measurement modes in order to recreate interference; at least one detector (30) receiving the light flux resulting from the interference of the measurement modes; an electronic processing unit (40) ensuring the extraction of the values of the measured parameters from the signals coming from the detectors (30), characterized in that: the sensitive fibers (1) have two pairs of stable measurement modes, the phase shifts introduced (deltaPHIi, deltaPHI2) by the measurement fiber between the modes of each of the two couples depending on two measured parameters (P1, P2), with linearly independent laws, analysis means producing at least two separate measurements of the phase shifts between the modes of each of the two pairs, the processing unit (40) ensuring the extraction of the independent values of the two parameters measured by the same sensitive fiber, from the two phase shifts.

Description

 MULTI-OPTICAL FIBER MEASURING DEVICE

 SETTINGS

 The present invention relates to a fiber optic measuring device in which the sensitive element is a fiber whose properties vary according to the parameters to be measured.

 Devices of this type have been developed for several years and one can, for example, refer to the work "OPTICAL FIBER SENSORS", edited by B. CULSHAW and J. DAKIN, ARTECH HOUSE (1989).

 More particularly, polarimetric birefringent fiber sensors have been developed. In these known sensors, the measured parameter produces a variation in the birefringence of an optical fiber. A linearly polarized light beam is coupled to this fiber. It is analyzed after transmission in order to measure the phase shift introduced by the fiber between the two orthogonal modes linearly polarized (see the article VARNHAM ET AL. "ELECTRONICS LETTERS" - August 18, 1983 - Vol. 19 n ° 17). The parameter to be measured can be, in particular, the temperature or the longitudinal deformation, for example.

It has also been proposed (FR-A-2 626 367) to produce a multi-point optical fiber temperature sensor. In this device, a light flux of low temporal and polarized coherence is coupled to a birefringence optical fiber comprising a certain number of weak coupling points. The incoming flow is coupled to one of the modes, for example to the fast mode, of the fiber. At each of these coupling points, a small part of the energy of this polarization mode is transmitted to the other polarization mode. We produce the interference of the outgoing waves on each of the modes by placing a polarizer at 45 ° to the neutral axes of the fiber, and we analyzes the flux transmitted with a MICHELSON interferometer and the interferogram thus provided allows access to the phase difference introduced by each of the fiber sections included between two of the coupling points.

It has been shown (article "USE OF HIGHLY ELLIPTICAL CORE FIBER FOR TWO-MODE FIBER DEVICES" by BY KIM ET AL. "OPTICS LETTERS" - September 1987 - vol. 12 n ° 9) that instead of measuring the interference between the two polarizations of a spatially single-mode birefringent fiber, it is possible to have interference with a comparable sensitivity with a non-birefringent fiber having some spatial modes, that is to say a fiber having a weak core diameter used with a source of wavelength less than its cut-off wavelength. In particular, just below this cut-off wavelength, the fiber has three spatial modes: the fundamental LP ₀₁ mode and two LP ₁₁ asymmetric modes with two lobes. In a circular core fiber, these two LP ₁₁ modes are almost degenerate, which gives instabilities. This is why it has been proposed to use a fiber with a very elliptical core. In this case, the LP ₁₁ mode whose two lobes are aligned on the major axis of the elliptical heart (denoted LP _11G Thereafter) is guided just like the fundamental mode LP ₀₁ , while the LP ₁₁ mode whose two lobes are aligned on the minor axis of the elliptical heart (noted LP _11P thereafter) is not guided. Such a fiber therefore has only two spatial modes: LP ₀₁ mode and LP _11G mode, and the analysis of the interference between these two modes makes it possible to detect parameters such as temperature or longitudinal deformation in a manner comparable to the measurement made with a spatially single-mode fiber with two polarization modes. Furthermore, it has been proposed to embed optical measurement fibers in materials, for example composite materials, and to measure the optical properties of these fibers so as to characterize the state of these materials. It has also been suggested (for example in the patent FR-A-2 626 367 already cited) to use the properties of strongly birefringent optical fibers to measure temperatures. It is found, in these applications, that the optical properties of the measurement fibers depend simultaneously on their deformation and on the temperature. The contribution of each of these parameters is difficult to isolate and the methods used so far for this

(for example the use of a reference) lead to the production of relatively complex measuring devices.

 Spajer et al (Optics Communications Vol. 60 N ° 5 December 1986, pages 261-264) proposed to avoid the use of a reference fiber and the problem of its temperature by exploiting three spatial modes propagating in a fiber non - birefringent measuring device. The measurement of two parameters, pressure and temperature, can be obtained when they are linked to phase differences between the modes considered by independent linear relations.

 Some results have been obtained, however, a strong dependence on the relationships between phase differences and parameters has been noted. This dependence is probably due to the fact that the phase differences created between three spatial modes, taken two by two, originate from the same physical phenomenon.

The object of the present invention is the production of a fiber optic measurement device for several parameters which makes it possible to obtain high accuracy, ensures the simultaneous measurement of several parameters in one same point and allows to separate the contribution of each of them.

 To this end, the device of the invention comprises: - a light source,

 at least one optical fiber sensitive to the measured parameters having stable modes excited by the light source, a variation of each measured parameter introducing phase shifts between different modes,

 - at least one coupling means separating the incident light between these measurement modes,

 - at least one detector receiving the light flux resulting from the interference of the measurement modes,

 - an electronic processing unit ensuring the extraction of the values of the measured parameters from the signals leaving the detectors.

According to the invention, at least one of the sensitive fibers has at least three stable measurement modes, the phase shift between the modes of each of the pairs depending on the parameters measured with linearly independent laws, analysis means producing at least two separate measurements of the phase shifts between the modes of each of the pairs. Said pairs of stable measurement modes are respectively the linearly polarized modes LP ₁₁ with two lobes, a pair being composed of two modes LP ₁₁ of the same polarization and of orthogonal spatial structure (LP _{11G / G} , LP _{11P / G} for example), the other pair being made up of two LP ₁₁ modes with the same spatial structure and orthogonal polarization (L _{P11G / G} , LP _{11G / P} for example).

The sensitive fiber is advantageously a stress birefringence fiber with a slightly elliptical core, used beyond the cut. Fibers say "Bow-tie" of YORK TECHNOLOGY have these characteristics.

The slight ellipticity of the heart (short axis to long axis ratio of the order of 0.7 to 0.9) ensures two stable LP ₁₁ modes without preventing mode guidance

LP _11P as in the case of a strong ellipticity. Such a fiber therefore has six stable modes of possible measurement: three spatial modes LP ₀₁ , LP _11G and LP _11P each subdivided into two linear polarizations parallel either to the major axis or to the minor axis. These six modes will be noted later

LP _{01 / G} , LP _{01 / P} , LP _{11G / G} , LP _{11G / P} , LP _{11P / G} and LP _{11P / P.}

Preferably, the three stable measurement modes will be an LP ₁₁ mode _, the LP ₁₁ mode with the same spatial distribution and orthogonal polarization and the LP ₁₁ mode with the same polarization and orthogonal spatial distribution, for example LP _{11G / G} , LP _{11G / P} , LP _{11P / G.} The asymmetric modes LP ₁₁ indeed have an interesting similarity with the polarization modes. They also have a spatial orientation and their speed difference is comparable to that of the polarization modes.

 The invention will be described in more detail with reference to the appended figures in which:

 Figure 1 is a block diagram of the device of the invention.

 - Figure 2 is a representation of the spatial modes of the optical measurement fiber concerned by the invention.

 - Figure 3 is a schematic representation of the section of an optical fiber usable as a measurement fiber.

 - Figure 4 is a representation of the mode separator usable at the output of the measurement fiber.

- Figure 5 is a representation of the device of the invention in white light interferometry. - Figure 6 is a representation of the wave trains produced by a broad spectrum source.

 FIG. 7 is a representation of the interferogram produced by interference of the wave trains shown in FIG. 6.

 - Figure 8 is a diagram of the multi-point measurement device according to the invention.

 - Figure 9 is a representation of the operating principle of this multipoint measurement device.

 - Figure 10 is the representation of an interferogram produced by one of the analysis interferometers.

The sensitive element of the measurement device is an optical fiber called "sensitive fiber" with birefringence of stress and slightly elliptical core having only a limited number of spatial modes. In practice, as shown in FIG. 2, we consider a symmetrical fundamental mode LP ₀₁ with a lobe and two antisymmetrical secondary modes orthogonal LP _11G and LP _11P with two lobes. Each of these spatial modes is broken down into two linearly polarized, orthogonal modes (respectively LP _{01 / P} , LP _{01 / G} , LP _{11G / P} , LP _{11G / G} , LP _{11P / P} ,

LP _{11P / G} ).

 Two independent phase shift measurements are made, on the one hand between two different polarization modes of the same spatial mode, on the other hand between two different spatial modes and of the same polarization. Thus, a double measurement of the two sought parameters is carried out and the two laws which link the phase shifts to the parameters being linearly independent, one can separate the effect of each of the two parameters and obtain two precise, simultaneous and independent measurements.

Sensitive optical fiber therefore has the property of having stable spatial modes LP _11G and LP _11P . he it is for example a fiber with birefringence of constraints of the "Bow-tie" type. By way of example, a fiber of this type is shown in FIG. 3. It has a slightly elliptical core 33 which ensures the propagation of the two spatial modes LP ₁₁ at slightly different speeds. This difference in speeds avoids coupling between these two spatial modes. Each spatial mode is broken down into two orthogonal polarization modes.

 During their manufacture, the core 33 of these fibers is surrounded by two bars 34 of doped glass which contract strongly upon cooling after the preform has been drawn. They are embedded in the optical sheath 35. The region of the heart is thus subjected to anisotropic stresses, which creates an elasto-optical birefringence.

The measurement device comprises a source S coupled to a measurement fiber 1 so as to favor the coupling of at least three of the LP ₁₁ modes. To this end, a polarizer 2 is placed between the source S and the measurement fiber 1.

Parameters P ₁ and P ₂ applied to this measurement fiber 1 produce a phase difference δΦ ₁ (P ₁ , P ₂ ) between for example the mode LP _{11G / G} and the mode LP _{11G / P} and δΦ ₂ (P ₁ , P ₂ ) between LP _{11G / G} mode and LP _{11P / G} mode.

Experience has shown that δΦ ₁ (P ₁ , P ₂ ) and δΦ ₂ (P ₁ ,

P ₂ ) are linearly independent functions or laws such that, by calibration, the functions P ₁ (δΦ ₁ , δΦ ₂ ) and P ₂ (δΦ ₁ , δΦ ₂ ) are accessible. The measurements of δΦ ₁ and δΦ ₂ thus make it possible to obtain the values of P ₁ and P ₂ sought.

The phase shifts δΦ ₁ (P ₁ , P ₂ ) and δΦ ₂ (P ₁ , P ₂ ) between the LP _{11P / P} mode and respectively LP _{11P / G} and LP _{11G / P} are independent functions allowing the measurement to be carried out . In general, phase shifts obtained respectively between couples of different spatial modes for the same polarization state or between couples of different polarization modes for the same spatial state would provide linearly dependent functions Φ ₁ and Φ ₂ not allowing the extraction of P ₁ and P ₂ .

Linearly independent functions Φ ₁ and Φ ₂ are obtained when these functions represent the phase shifts between a couple of different spatial modes of the same polarization, on the one hand, and a couple of different polarization modes of the same spatial state, on the other hand .

At the output of the measurement fiber 1, the three LP modes ₁₁ are recombined two by two by the analysis means 4 in order to create interference.

To separate or recombine two modes with the same spatial structure and orthogonal polarization, for example LP _{11G / G and} LP _{11G / P} , it is known that a polarizer must be placed at 45 ° from the neutral birefringence axes. The two polarizations project on the same intermediate direction at 45 ° and can therefore interfere. To separate or recombine two modes of the same polarization and of different spatial structure, it is necessary to carry out an equivalent operation of projection. For two LP ₁₁ modes of orthogonal spatial structure, for example LP _11G , _G and LP _{11P / G} , a particularly advantageous solution consists in placing at the output of the fiber, an identical fiber but with a rotation of 45 ° of the axes of the elliptical core . The two spatial modes LP ₁₁ are each distributed over the two spatial modes LP ₁₁ of the fiber at 45 °. We must then separate these modes to operate a function equivalent to the polarizer in the case of polarization modes. The device advantageously comprises a starter fiber and an output fiber, placed on either side of the measurement fiber, and of the same structure as the latter.

 The axes of the pigtail and the output fiber are oriented at 45 ° from the axes of the measurement fiber; they respectively ensure the separation and the recombination of the different spatial structure modes on the one hand and of the different polarization modes on the other hand.

Separation can also be obtained by prism decoupling (see the article WV SORIN et al. "PHASE VELOCITY MEASUREMENT USING PRISM OUTPUT COUPLING FOR SINGLE AND FEW MODES OPTICAL FIBER" - OPTICS LETTERS, 11, pl06-108, 1986) fiber laterally polished and placed in contact with a medium with a higher index than the fiber core.

 Note that this prism decoupling method can also be used to separate the two polarizations (see article H. C. LEFEVRE et al. "HIGH SELECTIVITY POLARISATION SPLITTING FIBER COUPLERS" S.P.I.E. Vol. 988 (1988), p63-69).

 To make the prism derivator 3 (FIG. 4), the fiber 7 is placed in a support 8 which gives it a curvature and allows it to be polished up to the vicinity of the core. The fiber core is then brought into contact with a medium 9 of index greater than its own index.

Each of the modes (LP _{11G / P} , LP _{11G / G} , LP _{11P / P} , LP _{11p / G} ) has a propagation speed different from that of the others so that it refracts on passing through the medium 9 according to a its own angle. The modes are thus geometrically separated. At least one detector 30 measures the luminous flux produced. It supplies a signal to the processing unit 40 which, after calibration, ensures extracting the values P ₁ and P ₂ of the parameters from Φ _i and Φ ₂ .

If such a measurement fiber can be used in a conventional interferometric assembly, it is particularly advantageous to use the techniques of interferometry in white light (FIG. 5). In this case, the broad spectrum source S can be considered as emitting wave trains 10 of length equal to the coherence length of the source. The wave train is coupled to at least three of the fiber modes, for example LP _{11G / G} , LP _{11G / P} and LP _{11P / G.} At the end of the sensitive fiber element, the initial wave train 10 is broken down into three wave trains 11, 12, 13 separated in time because of the difference in speed of the modes. These three wave trains 11, 12, 13 are recombined by the analysis means 4 but cannot interfere because they have no time overlap. The phase shift information is however spectrally coded and can be restored by passing these three recombined trains through an analysis interferometer 14 which has a variable path difference. The contrast of the fringes then reappears when the path difference of the replay interferometer compensates for the phase shifts of the measurement fiber, the measurement of the path differences restoring a local maximum of contrast makes it possible to detect the phase shifts δΦ _i (P ₁ , P ₂ ), δΦ ₂ (P ₁ , P ₂ ) two by two between the three modes.

 The replay interferometer 14 can be a MICHELSON interferometer for example or an air wedge interferometer where the interferogram is projected onto a strip of photodiodes. These interferometers are known per se.

A fiber optic measuring device has thus far been described making it possible to carry out measurements in one point, more specifically to access the cumulative effects of the parameters observed over the entire length of the measurement fiber. This device is well suited for carrying out multiplexed measurements, that is to say measurements independent of the effect of the parameters studied P ₁ and P ₂ between different successive points A ₁ , A ₂ , A ₃ , etc. of the same optical fiber.

To this end, (FIGS. 8 to 10), at each of these points A ₁ , A ₂ , A ₃ , etc., a slight coupling is created between the different modes passing through the measurement fiber 1. These coupling points are in practice produced by producing a slight twist on the fiber at a given point, bringing this point to a certain temperature and then releasing the twist. These coupling points produce a transfer of part of the energy contained in each of the modes into each of the other modes.

Since the spatial LP ₁₁ modes have an axial orientation similar to the linear polarization modes, a rotation of the axes of symmetry of the measurement fiber creates a coupling between an LP ₁₁ mode and the same spatial orthogonal polarization mode but also a coupling of similar value between the LP ₁₁ mode of departure and the LP ₁₁ mode and spatially orthogonal with the same polarization. The coupling in the spatial structure and orthogonal polarization mode can be considered as second order and therefore negligible.

In this case, the coupling of the source S to the input of the measurement optical fiber 1 is carried out according to one of the modes, for example the LP _{11G / G} mode and the light source S with broad spectrum emits trains d 'wave.

At each coupling point, the transmitted wave train (LP _{11G / G} ) gives rise to weak wave trains according to the other two modes (L _{11G / P} , L _{11P / G} ). 0n therefore obtains at the output of the fiber, a main train (LP _{11G / G} ) and a series of low intensity trains (LP _{11G / Pi} ), (LP _{11P / Gi} ) where _i represents the coupling point A _i .

At the output of the measurement fiber, the recombination is produced between the light flux contained in the initially excited mode LP _{11G / G} and the outgoing fluxes in the two other modes, LP _{11G / P} and LP _{11P / G.}

The beam thus produced carries a spectral coding representative of the phase shift values δΦ _1i and δΦ _2i linked to the cumulative effect of the parameters P ₁ and P ₂ between the weak coupling points _i and _{i + 1} . These phase shifts are then measured with a read-back interferometer, looking for the path differences which restore maximum contrast.

Rather than recombining the three measurement modes on the same beam and measuring with the read-back interferometer all the phase shifts δΦ _1i and δΦ _2i , it is also possible to recombine in parallel or sequentially the spatial orientation modes orthogonal to each other regardless of polarization and modes of orthogonal polarization regardless of spatial structure. The two recombination beams are then sent to a replay interferometer, in parallel or sequentially, and the two generated interferograms make it possible to measure the values δΦ _1i or else δΦ _2i . This makes it possible to prevent the rise in contrast of phase shifts δΦ _1i which would _occur at the same place as the rise in phase shifts δΦ _2i disturbing the measurement.

Claims

 1. Fiber optic measurement device of several parameters comprising:

 - a light source (S),

- at least one optical fiber (1) sensitive to the parameters (P ₁ , P ₂ ) measured having stable modes excited by the light source (S), a variation of each parameter (P ₁ , P ₂ ) measured introducing phase shifts ( δΦ _i , δΦ ₂ ) between the different modes called "measurement modes", two by two,

 - at least one coupling means separating the incident light between these measurement modes,

 - at least one means of recombining these measurement modes in order to recreate interference,

 - at least one detector (30) receiving the light flux resulting from the interference of the measurement modes,

 an electronic processing unit (40) ensuring the extraction of the values of the measured parameters from the signals leaving the detectors (30),

 characterized in that:

. the sensitive fibers (1) have pairs of stable measurement modes, the phase shifts introduced (δΦ ₁ , δΦ ₂ ) by the measurement fiber between the modes of each of the couples depending on laws which are linearly independent of the parameters measured (P ₁ , P ₂ ),

. said pairs of stable modes of measurement being respectively the linearly polarized modes LP ₁₁ with two lobes, a couple being composed of two modes LP ₁₁ of the same polarization and of orthogonal spatial structure, the other couple being composed of two modes LP ₁₁ of the same spatial structure and orthogonal polarization.

. analysis means producing at least two separate measurements of the phase shifts between the modes of each of the pairs,

. the processing unit (40) ensuring the extraction of the independent values of the two parameters measured by the same sensitive fiber, from the phase shifts.

 2. A device for measuring optical fiber of several parameters according to claim 1, characterized in that the two measured parameters are the longitudinal deformation of the sensitive fiber and its temperature.

 3. Device for measuring optical fiber of several parameters according to any one of claims 1 and 2, characterized in that the sensitive fiber is a birefringent fiber with polarization maintenance produced by internal stresses and having an elliptical core, the orientation is represented by axes.

 4. optical fiber measuring device according to claim 3, characterized in that it comprises a starter fiber and an output fiber placed on either side of the measuring fiber, with the same structure as the measuring fiber, whose axes are oriented at 45 ° from the axes of said measurement fiber, the starter fiber and the output fiber respectively ensuring the separation and the recombination of the different spatial structure modes on the one hand and of the different polarization modes on the other hand go.

 5. Device for measuring optical fiber of several parameters according to any one of claims 1 to 3, characterized in that it comprises a prism derivator ensuring the recombination of the measurement modes at the output of the sensitive fiber by an orientation at 45 ° from the leader fiber of the diverter.

6. A fiber optic remote measurement device according to any one of claims 1 to 4, characterized in that it comprises a broad spectrum source. and a replay interferometer making it possible to measure the phase shifts generated mode to mode in the sensitive fiber.

 7. Device for measuring optical fiber of several parameters according to any one of claims 1 to 6, characterized in that it comprises several couplings allowing the measurement of the two parameters on several segments, the mode couplings being ensured by a rotation local axes of symmetry of the sensitive fiber.