EP1896813A2

EP1896813A2 - Optical strain gauge strips

Info

Publication number: EP1896813A2
Application number: EP06762219A
Authority: EP
Inventors: Karl-Heinz Haase; Michael Schmidt; Regis Blin
Original assignee: Hottinger Baldwin Messtechnik GmbH
Current assignee: Hottinger Bruel and Kjaer GmbH
Priority date: 2005-06-29
Filing date: 2006-06-27
Publication date: 2008-03-12
Also published as: WO2007000324A3; WO2007000324A2; DE102005030751A1

Abstract

The invention relates to an optical strain gauge strip (D), made from a planar support layer (6) preferably of plastic and a cover layer (19) between which an optical fibre (2, 3, 4) has a frictional fit. The optical fibre (2, 3, 4) comprises at least one section (L) with a Bragg grating (5) for recording the strain on a deformable body or a different component region. The invention is characterised in that the optical fibre (2, 3, 4) is let into at least one guide channel (7, 8, 9) in the support film forming the support layer (6). Said guide channel (7,8, 9) with the optical fibre (2, 3, 4) and the Bragg grating (5) embossed therein is sealed with a covering film as outer layer (19), made from the same material as the support layer (6).

Description

Optical strain gauge

The invention relates to an optical strain gauge according to the preamble of patent claim 1 and its manufacturing method according to the preamble of patent claim 9.

For the metrological evaluation of forces or for monitoring of mechanically loaded components electrical strain gauges are often used, which detect the elongation of kraftbeaufschlagten components.

Such electrical strain gages usually consist of meander-shaped measuring grids produced photolithographically from an electrical resistance material which is applied to a carrier film made of plastic and for mechanical protection, usually with a further one

Plastic protective film is covered. These electrical strain gauges are applied to a deformation-dependent strain to detect a load-dependent strain and convert the strain by a change in resistance of the measuring grid in an electrical signal that is proportional to the strain or the force. However, such electrical strain gauges are sensitive to electromagnetic fields or high-voltage influence and may not be used in potentially explosive atmospheres.

A high voltage insensitive optical strain sensor for measuring the contact force of a current collector for rail vehicles is known from DE 102 49 896 Al. This is between the sanding bar and a Holding frame of the current collector attached to an expansion body whose surface deforms according to the contact force. On this deformation body, a so-called fiber Bragg grating sensor is fixed, which obviously consists of an optical waveguide, in whose application area a so-called Bragg grating is embossed, which changes its reflection wavelength according to the detected strain. For this purpose, the waveguide end with the Bragg grating is simply glued to the deformation body and acts as an electrical strain gauge on a load-dependent surface strain. Since such optical waveguides are always formed in their cross section as a round fiber, a glued fiber has only a narrow contact surface, so that usually only a poor and undefined transmission of the non-positive connection surface is ensured by the low contact to the deformation body. Therefore, a sufficiently reproducible measurement accuracy should not be attainable for an accurate metrological evaluation with such a narrow connection surface.

An embedded optical sensor is known from EP 753 130 Bl, which consists of a linear waveguide with a Bragg grating embedded within multiple layers of reinforcing filaments to form a laminated structure. This sensor is designed to measure both stresses and temperatures in the structure within the laminated structure with a single Bragg grating fiber. However, such a voltage sensor must always be embedded in the measurement object, so that the deformation element must always represent a laminated structure, so that it can not produce thin planar optical strain gauges with comparable dimensions as with electrical strain gauges. From EP 1 129 327 Bl a sensor for measuring mechanical stresses with fiber-optic Bragg gratings is known, which is designed as a planar flat transducer, which can be applied to a deformation body. This voltage sensor is provided as a rosette for measuring a multi-axis voltage, which consists of a waveguide with preferably three successively arranged Bragg gratings, which are aligned at certain angles to each other. To reduce the sensor surface and to avoid larger reflection losses are therefore the curved

Connecting elements between the Bragg gratings strongly tapered executed. In this case, the waveguide with the Bragg gratings and the connecting elements is preferably encapsulated in a rigid epoxy or glued between two parallel rigid plates. The plates can be attached to the surface of deformation bodies and thus transfer the strain applied to the plates to the fiber Bragg gratings whose reflected wavelength changes in proportion to the strain and can be detected. However, it should be difficult to manufacture, the thin

Gluing optical fibers with diameters of about 200 microns and a high flexibility exactly at certain angles to each other between the plates or pour in an epoxy resin, even with small deviations of the predetermined angular position relatively high measurement errors can occur.

Another arrangement for detecting strains and temperatures containing Bragg gratings optical fibers is known from DE 100 04 384 C2. In this arrangement, the optical waveguide containing the Bragg grating is mounted within a resist layer on a support. For this purpose, the carrier is obviously at the same time a deformation body whose elongation and temperature is to be measured. For attachment of the sensor performing Optical fiber with the Bragg grating, the very thin and flexible fiber is first fixed on an adhesive strip. For this purpose, holes are provided for precise positioning in the adhesive strip, which predetermine the alignment of the optical fiber. Then the

Optical fiber unrolled over the middle of the row of holes and glued onto the adhesive strip carrier. Thereafter, the adhesive strip with the fixed optical fiber is positioned on the support surface and coated with a lacquer layer that penetrates through the rows of holes and the

Optical fiber fixed on the carrier. Subsequently, the adhesive strip is peeled off and the attached optical fiber completely coated with a lacquer layer until it is positively connected to the carrier and completely covered. An extensively manufactured strain sensor of this type is currently hardly usable compared to the strain sensors of electrical strain gauges for reasons of cost and is therefore reserved for special applications only.

The invention is therefore based on the object to provide prefabricated strain gauges of optical waveguides that can be produced in precise uniform manufacturing quality and can be manufactured inexpensively.

This object is solved by the invention specified in claim 1 and 9. Further developments and advantageous embodiments are specified in the dependent claims.

The invention has the advantage that very compact optical strain gauges can be produced by fixing the waveguide with the Bragg gratings in provided guide channels on a carrier foil. The provided guide channels are preferably by a photolithographic etching process or a mechanical processing method made very accurately, so that such optical strain gauges have a high reproducibility and can advantageously be prefabricated as serial parts in large quantities at low cost to be applied in a simple manner on intended deformation bodies or other expansion bodies. Such prefabricated flat and small-area optical strain gauges can also be advantageously fixed in or on fiber composites, which affect the fiber structure only slightly and advantageously also withstand strain changes up to 10% undamaged, as are common in deformation bodies made of fiber composites.

The optical strain gages of the invention have the advantage over electrical strain gauges that they are largely insensitive to electromagnetic fields and high voltage areas. They advantageously have no power supply, so that they are insensitive to power fluctuations on the transmission line and may also be used in hazardous areas. Furthermore, the non-positive connection of the Bragg gratings in the guide channels allows an enclosed connection structure with the flat carrier film, so that a good and defined power transmission is ensured on the Bragg gratings, whereby a high measurement accuracy and in particular a low hysteresis effect can be achieved.

A particular embodiment of the invention, in which the optical waveguides are cast over the entire surface in the guide channels, has the advantage that it can be made very flat easy to manufacture optical strain gauges. Because these optical strain gauges too can be produced from ceramic or glass carrier foils and optical waveguides made of glass materials, these can be advantageously used even at very high temperature loads.

In a particular embodiment of the invention with additional Bragg gratings for temperature compensation is advantageous that thus a temperature-independent strain measurement is possible. Thus, at the same time a separate temperature detection executable, which advantageously also the thermal overload of the optical

Strain gauge and the adjacent remaining components can be detected.

The invention will be explained in more detail with reference to an embodiment which is illustrated in the drawing. Show it:

Fig. 1: the top view of an optical

Strain gauges;

2: a section of a sectional image through the fiber feed region of the optical

Strain gauge, and

3: a schematic representation of an evaluation unit for an optical strain gauge.

In Fig. 1 of the drawing, an optical strain gauge 1 is shown, which is designed for biaxial strain measurement as a rosette and basically consists of three juxtaposed optical waveguides 2, 3, 4 with embossed Bragg gratings 5 in the incorporated guide channels 7, 8, 9 a carrier film 6 are fixed.

For receiving the optical waveguide 2, 3, 4, a thin elastic carrier film 6 is provided, which is preferably made of plastic, such as. B. polyimide. The carrier film 6 can but also made of other hard elastic plastics, metals, glass or ceramics. In practice, an elastic silica glass film has proven to be very thin and in which the guide channels can be precisely ground. In this case, the carrier film 6 is used to apply the prefabricated optical strain gauges 1 on provided deformation bodies or to integrate in loaded components in a positionally and non-positively. The prefabricated support film 6 is planar, preferably has a rectangular or square base and a thickness of about 0.3 mm. For special designs but also film thicknesses of 0.2 to 0.8 mm are possible. The base depends essentially on the length of the Bragg gratings 5 at the ends of the optical waveguides 2, 3, 4 and the formation of the optical strain gauges 1 for one or two-axis strain detections.

In the uniaxial strain detection only a linearly aligned optical waveguide 3 is provided in a guide channel 2 of the carrier film 6, which has a length of about 8 to 15 mm and a width of about 2 to 5 mm, depending on the required Stör- Nutzsignalabstand.

In the illustrated embodiment of an optical strain gauge 1 for biaxial strain detection by means of three angularly offset disposed Bragg gratings 5, a size of the support film 6 of about 26 x 30 mm is provided. The size is determined not only by the 10 mm long Bragg gratings 5, but essentially also by the

Radii 11 of the bending optical fibers 2, 3, which are relatively large with r = 20 mm in order to keep the measurement errors low. Between the three Bragg gratings 5, an opening angle 18 of 45 ° is provided in each case, through which substantially the width of the optical strain gauges is determined. For shorter Bragg gratings 5 with smaller radii and carrier foils 6 with smaller base areas of about 18 x 20 mm are possible.

In the carrier films 6 are for fixing the

Optical waveguide 2, 3, 4 guide channels 7, 8, 9 or recesses incorporated, whose cross section corresponds at least to the cross section of the optical waveguides 2, 3, 4. For this purpose, optical waveguides 2, 3, 4 are preferably used made of mineral glass fibers with an outer diameter of 0.25 mm, so that the guide channels 7, 8, 9 or depressions at least in the lead-in area 12 to the cross-sectional edge AA each have a depth and width of 0.25 mm have.

The arrangement of the optical waveguides 2, 3, 4 is shown in detail in detail in Fig. 2 of the drawing. From the section of the sectional image A-A it can be seen that four optical waveguides 2, 3, 4, 13 are arranged parallel next to one another in the lead-in region 12. There are three

Optical waveguide 2, 3, 4 provided for strain measurement and an end in the lead-in optical waveguide 13 is used only for temperature compensation. This fourth optical waveguide 13 ends about 2 mm behind the introduction edge and has outside the carrier film 6, a Bragg grating 14 for temperature detection. The other three optical waveguides 2, 3, 4 are continued after the introduction region 12 only with their fiber cladding region.

The optical waveguides 2, 3, 4, 13 are conventional optical fibers made of mineral glass fibers as they are also used for telecommunications as a single-mode fiber with a wavelength of preferably λ = 1550 nm. These glass fibers 2, 3, 4 are preferably made of a fiber core 15, a fiber cladding 16 and a fiber protection layer 17, which may also be omitted. Since the optical light-guiding effect occurs exclusively in the core 15 and cladding region 16 of the glass fiber, the fiber protection layer 17 has been removed as a mechanical protection region behind the introduction region 12. This is also necessary in order to impress the Bragg gratings 5 at the end of the glass fibers 2, 3, 4. Therefore, for these jacket regions after the insertion region 12, three further guiding channels 7, 8, 9 of only 0.125 mm (or less) depth and width are incorporated into the carrier film 6, which corresponds to the diameter of the fiber jacket 16. These guide channels 7, 8, 9 for fixing the glass fibers 2, 3, 4 with its fiber core 15 and the fiber cladding 16 initially run parallel to each other, wherein the two outer glass fibers 2, 4 bend laterally with a radius of 20 mm and after a Abknickungswinkel 18th each of 45 ° straight forward. At least one linear guide channel part is provided at the end of a maximum of L = 10 mm, which ends approximately 2 mm in front of the opposite carrier foil edge.

These guide channels 7, 8, 9, 10 are incorporated into the carrier film 6, preferably by a photolithographic etching process. But there are also known mechanical or thermal methods by which the fine guide channels 7, 8, 9, 10 can be incorporated by a material removal in the carrier film 6. The middle guide channel 8 for fixing the second optical waveguide 3, however, extends linearly and simultaneously represents the center line of the symmetrical optical strain gauge 1. In this case, the second optical waveguide 3 with the two outer optical waveguides 4, 3 in each case an angle of 45 ° and the two outer to each other an angle of 90 °, so that so that all strains in the two surface axes can be detected. After the removal of material, the optical waveguides 2, 3, 4 are inserted into the three guide channels 7, 8, 9 which are diverging, at the ends of which the three Bragg gratings 5 have already been embossed. Although the three optical fibers 2, 3, 4 are very flexible and with 0.125 mm diameter or less relatively thin, the insertion into the guide channels 7, 8, 9 can be done easily both mechanically and manually, as this by the accurately fitting guide channel dimensions Pressure can be fixed in this. In this case, a firm connection can be achieved both by pressing and by gluing to the carrier film 6, so that not only a form-fitting, but also a firm frictional connection between the optical waveguides 2, 3, 4 and the carrier film 6 takes place.

By virtue of the juxtaposition of the optical waveguides 2, 3, 4, a so-called strain gauge rosette is basically formed, with which all horizontally extending force or expansion components can be detected. As an adhesive for frictional connection of the glass fibers 2, 3, 4 with the carrier film 6, a curable epoxy resin adhesive has been proven in practice, which also has only low hysteresis and excellent transmission of the strain on the optical waveguide 2, 3, 4 guaranteed.

In a particular embodiment of the invention, the optical waveguide 2, 3, 4 is designed as a planar optical waveguide, which is preferably cast in the guide channels 7, 8, 9, 10. For this purpose, for example, an optically conductive polymer substrate or another so-called photoresist is introduced into the guide channel 7, 8, 9, 10 of the plastic carrier film 6, which has a higher refractive index than the carrier film 6. This results in a refractive index jump, through which the polymer substrate as photoconductive plastic acts like an optical fiber. In this case, the polymer substrate is basically the core and the carrier film 6 the sheath with the lower refractive index. In particular with rectangular or square guide channels 7, 8, 9 formed by the introduction of the photoconductive substrate in a simple manner, an optical strip conductor, the light line of certain wavelengths as glass fibers is suitable. In this embodiment, strip-shaped irregularities are imprinted at a distance Λ before introducing the photoconductive layer into the guide channels 7, 8, 9, which then act as Bragg gratings 5. These can represent comb-like elevations or depressions which form a Bragg grating over a length L of 3 to 10 mm, which reflects the light waves fed in at a predetermined wavelength λ _B. Because the

Optical waveguides 2, 3, 4 firmly embedded in the guide channels 7, 8, 9 of the support layer 6 and are firmly connected to these, so that all strains acting on the support layer 6 can be accurately detected. By means of these polymeric optical waveguides 2, 3, 4 as optical strip conductors, very flat designs of optical strain gauges 1 which are thinner than 0.5 mm can be realized.

Such embodiments of optically conductive media embedded in the guide channels 7, 8, 9 as optical waveguides 2, 3, 4 can also be carried out with heat-resistant glass or ceramic films as carrier layer 6, in whose guide channels 7, 8, 9 photonic crystals are cast with quartz glass substrates. For this purpose, the Bragg gratings are formed with the help of photonic crystals, with which the strain is detected. The channels can preferably also be realized by a field-assisted ion exchange. The Bragg gratings are then introduced into these channels from outside through a chemical etching process. Such embodiments of optical strain gauges 1 can be used at temperatures up to 900 ⁰ C. .._

For mechanical protection and to prevent the effects of moisture on the optical waveguides 2, 3, 4, the guide channels 7, 8, 9, 10 are additionally coated with the inserted waveguides 2, 3, 4 with a thin cover film 19. The cover 19 is preferably also made of the material of the carrier film 6 such. As polyimide and preferably has a thickness of 0.05 mm and is welded or glued to the support sheet 6, so that the optical strain gauge 1 is hermetically sealed.

Such an optical strain gauge 1 can be applied both to metallic deformation bodies as conventional electrical strain gauges or inserted or glued in fiber composites. With such optical strain gauges 1 are not only strain measurements, but also

Temperature measurements possible, as at the same time the thermal expansion is detectable.

If such an optical strain gauge 1 is applied to a force-loaded deformation body, it can thus be described as follows, the applied force or strain can be detected. Because of the force acting on the expansion body force takes place on the surface of an expansion effect, which is transmitted via the applied thereto carrier sheet 6 on the non-positively fixed therein optical waveguides 2, 3, 4. This also creates a change in length within the Bragg grating region L, since this is formed from a piece of the core 15 of the optical fiber 2, 3, 4, which is surrounded by the shell 16, which has a lower refractive index than the core 15. The optical fiber 2, 3, 4 is is formed above as a single-mode fiber in which the diameter of the 9μm fiber core 15 is sufficiently small so that the light from a preferably infrared light source can propagate along the core 15 in only a single mode. This single mode is essentially guided by the refractive index jump at the core-cladding boundary. The lines 20 of the Bragg grating 5 are a series of preferably regularly spaced perturbations of the effective refractive index n of the core 15. The Bragg grating 5 extends along a length L of the optical fiber 2, 3, 4, where L is normally in the range of 3 up to 20 mm.

For the production of Bragg gratings 5, various methods can be used. In one of these methods, the refractive indices n in the core 15 are generated by masking the optical fibers 2, 3, 4 with a phase mask and irradiating them with strong ultraviolet light. In another method, the index perturbations n are formed by exposing the optical fibers 2, 3, 4 to an interference pattern generated from two intersecting halves of a UV laser beam. The distance Λ between the index perturbations n is determined by the angle at which the two halves of the beam intersect.

In the optical waveguides 2, 3, 4, in which the optically conductive material is poured directly into the guide channels 7, 8, 9, the Bragg gratings 5 are formed by disturbances in the carrier film 6. As a result, a comb-like line pattern is mechanically impressed into the carrier film 6 with the distance Λ, through which the light waves are reflected. In the optical waveguides 7, 8, 9 of photonic crystals with quartz glass substrate, the Bragg grating 5 is preferably formed by irradiation with UV light and a phase mask. The disturbances caused by these methods Nuclear refractive index n is usually on the order of one-thousandth or less.

The optical fibers 2, 3, 4 used for the production of Bragg gratings 5 generally have a protective layer 17 outside of the jacket 16, which preferably consists of a polymer and has no significance for the actual light-guiding function. This protective layer 17 is removed before the optical fiber 2, 3, 4 is exposed to the UV light to form the Bragg grating 5. After irradiation, the stripped portion of the optical fiber 2, 3, 4 may also be recoated to restore its durability. However, since the optical fibers 2, 3, 4 in the present exemplary embodiment run in the guide channels 7, 8, 9 and are protected by both the carrier film 6 and the cover film 19, sufficient mechanical protection of the optical fibers 2, 3 is provided in the optical strain gauge 1 according to the invention , 4 guaranteed.

When the Bragg grating 5 is supplied with a broad spectrum of light as an input signal, most wavelengths penetrate the grating area and form a transmitted output signal. The periodic disturbances of the refractive index n, however, produce a strong Bragg reflection with components of the input signal

Wavelength λ _B , the so-called Bragg wavelength, which results according to the formula λ _B = 2nΛ, where n represents the effective refractive index and Λ the grating period.

With a spectrometer or a so-called Fabry-Perot filter, the lightwave signals reflected by the Bragg grating 5 can be detected. In this case, the determined wavelength λ, at which a peak occurs in reflection, a value which is dependent on the grating period Λ. When a longitudinal strain acts on the Bragg grating 5, changes the distance Λ, so that the Bragg wavelength λ _B shifts. The Bragg wavelength λ _B behaves approximately proportionally to the strain along the longitudinal axis of the optical waveguides 2, 3, 4. The wavelength change Δλ _B is thus a measure of the introduced into the deformation body

Force. Therefore, such optical strain gauges 1 can be used similarly to electrical strain gauges on provided deformation body preferably in load cells, torque shafts or other force transducers. However, such optical

Strain gauges 1 also used in load tests, for example in aerospace, where the optical strain gauges 1 are then applied directly to the loaded components, in particular the rosettes according to the invention for measuring the unknown force introduction directions are useful. But also for monitoring the operating state of loaded components such optical strain gauges 1 can be used, which can detect a fatigue damage or cracking when exceeding a predetermined limit strain.

To detect the strain-dependent load, however, a special spectral evaluation unit with, for example, a Fabry-Perot filter is provided, which is shown schematically in FIG. 3 of the drawing. Basically, this evaluation unit 21 contains a transmitting and receiving unit 23 for optical waveguides 2, 3, 4, in which the evaluation unit 21 detects the wavelength λ _B reflected by the Bragg gratings 5, 14. It is first in the unloaded state by means of a preferably infrared light source as

Transmitting unit 22 a broadband light signal having a wavelength λ of preferably about 1525 to 1575 nm in the optical waveguides 2, 3, 4 is fed. Due to the relationship λ _B = 2nΛ, a predetermined wavelength λ _{B0 is} reflected by the Bragg grating 5, which is transmitted via a switching unit 24a in a coupler 24b is separated from the radiated light signals. By means of a subsequent known Fabry-Perot filter in the receiving unit 23 or other spectrometer unit then the reflected optical signals can be detected pm with a resolution of 1 and stored as a reference value λ _B o electronically or displayed in a display device 25th

Now occurs when a load-dependent strain on the deformation body on the applied carrier sheet 6 causes a change in length of the optical waveguide 2, 3, 4, so changes over the grating period Λ and the reflected wavelength λ _B i of the Bragg grating 5, which also with help of the Fabry-Perot filter is detected. If the difference between the reference Bragg grating wavelength λ _B o - λ _B i is now formed, a value is obtained which is proportional to the strain or the loading force and which can be displayed in the display device 25 as strain or force. In the present rosette of the three juxtaposed at an angle optical fibers 2, 3, 4 all three strain or force components are evaluated separately and calculated on the known angular position as in electrical strain gage rosettes as individual force components or as a resultant force.

However, such an expansion or force detection is only sufficiently accurate if the ambient temperature is always constantly constant, since such Bragg gratings change their reflected wavelength λ _B also proportional to the ambient temperature. Therefore, you can measure with such optical strain gauges 1 basically without load-dependent strain and the temperature T. Because the Bragg wavelength λ _B shifts as a function of the strain ε and the temperature T according to the relationship: Δλ _B = K _E x ε + K _τ x ΔT Where:

K _E = the sensitivity factor of the elongation; ε = the strain; K _τ = the sensitivity factor of the temperature, and ΔT = the temperature change.

However, since one can not differentiate between the temperature and strain-related Bragg wavelength change Δλ _B according to the above relationship and a constant temperature constancy is not always sustainable, a fourth optical waveguide 13 with Bragg grating 14 located outside the optical strain gauge 1 additionally becomes provided for temperature compensation. By means of this fourth Bragg grating 14, therefore, a further only temperature-dependent wavelength change Δλ _B τ (λ _B τ = K _τ ΔT) is detected with the aid of a receiving unit 26, which in the evaluation unit 21 determines the temperature-dependent wavelength change from the strain and temperature-related wavelength changes Δλ _B Δλ _BT by a computing device 27 for

Temperature compensation is subtracted. This gives a very accurate measure of strain or force which is independent of the temperature of the optical strain gauge 1.

The fourth optical waveguide 13 could also be integrated into the carrier foil 6 in the case of a modified optical strain gauge 1. Then, however, the associated guide channel 10 would have to be dimensioned such that the Bragg grating 14 rests loosely to exclude the effects of stretching. Such an additional optical fiber with a Bragg grating 14 for temperature compensation is also provided for linear optical strain gauges 1 with only a straight optical fiber 3. It can be the additional Optical fiber are also used simultaneously to a pure temperature measurement. Furthermore, optical strain gauges 1 can be formed, in which a plurality of rosettes or a plurality of linear optical fibers are arranged on a larger support film surface, which allow a flat strain detection, to determine an analysis of the voltage curve, for example, on complicated components.

Claims

Optical strain gauge patent claims

1. Optical strain gauge (1) containing a planar support layer (6) on which at least one optical waveguide (2, 3, 4) is arranged non-positively, having at least one portion with a Bragg grating (5) for detecting the strain , characterized in that the carrier layer as a thin carrier film (6) and in which the optical waveguide (2, 3, 4) within a predetermined guide channel (7, 8, 9) is embedded.

2. Optical strain gauge according to claim 1, characterized in that the carrier film (6) is elastic and consists of metal, glass, ceramic or a hard plastic, in which at least one linear guide channel (8) is recessed, the cross-section round, square or V-shaped and has at least one diameter corresponding to the diameter of the waveguide section (L) with the Bragg grating (5).

3. Optical strain gauge according to claim 1 or 2, characterized in that in the carrier film (6) three guide channels (2, 3, 4) are provided which start parallel in a common insertion region (12), wherein the central guide channel (8). is then linearly continued, and bend the two adjacent guide channels (7, 9) at an angle of 45 ° arcuate and terminate in a linear region of at least the length L.

4. Optical strain gauge according to one of the preceding claims, characterized in that the Optical waveguides (2, 3, 4) are formed as optically conductive glass fibers and for strain detection with the Bragg gratings (5) in the guide channels (7, 8, 9) pressed force fit or fixed with a curable adhesive.

5. Optical strain gauge according to one of claims 1 to 3, characterized in that the optical waveguides

(2, 3, 4) are formed as optically conductive plastic or optically conductive glass whose refractive index is higher than the refractive index of the carrier film (6) and which are embedded in the one or more guide channels (7, 8, 9) and in their End regions have a linearly aligned Bragg grating (5).

6. Optical strain gauge according to one of the preceding claims, characterized in that the carrier film (6) by a cover layer (19) which consists of the same material as the carrier layer (6) is covered.

7. Optical strain gauge according to one of the preceding claims, characterized in that the carrier film (6) is square or rectangular and firmly connected to a cover as a cover layer (19), the at least the guide channels (7, 8, 9) with the optical fibers (2, 3, 4) and the Bragg grating sections (L) covers.

8. Optical strain gauge according to one of the preceding claims, characterized in that in addition to the optical waveguide (2, 3, 4) for strain detection additionally provided an optical waveguide (13) with a Bragg grating (14) for temperature compensation or temperature detection is, whose Bragg grating section is arranged independently of strain on or in the carrier film (6).

9. A method for producing an optical strain gauge according to one of the preceding

Claims 1 to 8, characterized in that at least one guide channel (7, 8, 9) is incorporated by an etching or mechanical material-removing manufacturing process in the cut to a designated surface carrier sheet (6) in which at least one

Optical waveguide (2, 3, 4) with impressed Bragg grating (5) positively and positively secured or cast.

10. The method according to claim 9, characterized in that the carrier film (6) or at least the guide channels (7, 8, 9) are covered with a cover layer (19).