DE10326516B3 - Fiber grating sensor system - Google Patents

Abstract

The invention relates to a fiber grating sensor system (1). In order to provide a structurally particularly simple constructed fiber grating sensor system, which has an improved resolution in the determination of the spectral positions of the Bragg reflections, a fiber grating sensor system (1) with a light source (2), with at least one with the light source ( 2) connected fiber grids (4, 5) of monomode fibers (7, 15) with a plurality of fiber grating sensors (6), with a with the fiber grating (4, 5) connected and a photodetector element (14) having detector (8) for spectral evaluation of the light reflected from the fiber grating sensors (6) and with a mode mixing element (11) for producing depolarized and in its beam form to the requirements of the photodetector element (14) adapted light at the detector input (12) proposed.

Description

The The invention relates to a fiber grating sensor system.

Fiber grating sensor systems are systems where optical fiber gratings, which are Bragg gratings in the Core of optical fibers, as a sensor for measuring temperature, mechanical strains, vibrations or refractive indices become. An overview of the Function of such systems are, for example, Y. J. Rao "In-fiber Bragg grating sensors ", Meas. Sci. Technol. Vol. 8, (1997), pp. 355-375. Multiplex method, which the individual sensors, for example, according to their mean wavelength, after the light transit time or by fiber optic switches, allow the construction of networks that consist of a variety of such Sensors exist.

task of measuring systems based on such optical fiber grating sensors is the registration of the spectral position of the Bragg reflections of these sensors, to draw attention to the size and temporal change the above listed Be able to conclude measured variables. It have to to the measurement task adapted pektrometrische method and Devices are developed which provide a determination of the spectral Position of simultaneously occurring multiple reflection maxima allow. These devices must Compact according to the general technical applications and fault-prone to mechanical and thermal loads of industrial environmental conditions and reasonably priced be. Other requirements include a high optical resolution of the spectral Determination of the maximum, preferably in the picometer range and high dynamics, preferably up to about 1000 measurements per second.

To determine the spectral position of the Bragg reflections of the fiber grating sensors of the network, from which the measured variable, for example, strain or temperature is determined, a variety of methods and devices are known. For example, refer to the following publications: US 5,319,435 ; US 5,426,297 ; US 5,410,404 ; US 5,191,458 ; US 5,748,312 ; US 5,361,130 ; US 5,680,489 ; US 5,513,913 ; US 5,493,390 ; US 5,397,891 ; US 5,591,965 , Disadvantages of these methods and devices include the occurrence of intensity losses, low dynamics and a poor signal-to-noise ratio. In addition, many of these methods and devices are very expensive and expensive.

The disadvantages mentioned are partly avoided by systems which use a spectrometer of diffraction grating and photodetector line to determine the spectral position of the Bragg reflections. Such a system is for example in US 6,233,373 or CG Askins, MA Putnam, EJ Friebele "Instrumentation for Interrogating Fiber Bragg Grating Arrays", Proc, of SPIE Vol. 1444, p. 257-266, 1995. The system has an imaging lens in the beam path of the spectrometer and works with an exact image of the entrance slit. The disadvantage here is that this solution does not take into account measures against polarization dependencies of the measuring system, which generally cause measurement errors (drifts) in bending and pinching of the supply fibers to the fiber grating sensors.

A another solution with spectrometer of diffraction grating and photodetector line is in A. Ezbiri, S.E. Kanellopoulos, V.A. Handerek "High resolution instrumentation system for fiber Bragg grating aerospace sensors ", Optics Communications Vol. 150, pp. 43-48, 1998 described. Again, no measures against polarization dependencies taken into account by the measuring system.

In a publication von Ecke W., Bartelt H., Schwotzer G., Usbeck K., Willsch R. Birkle S., Bosselmann T., Kraemmer P. "Low Cost Optical Temperature and Strain Sensing Networks Using In-Line Fiber Gratings ", Proc. SPIE Europto Series, Vol. 3099, pp. 390-397, 1997 is finally a spectrometer of diffraction grating and photodetector line shown in which a depolarization by a Lyot depolarizer, the acts as a polarization scrambler takes place. This solution is but also very expensive and expensive.

From the DE 101 24 983 A1 It is known to use step index fibers for the purpose of dispersion compensation in order to reverse pulse broadening on long feed lines. From the DE 198 28 154 A1 It is known to minimize mode locking in multimode fibers. From the DE 43 44 076 A is the use of a lens as a ground glass in the imaging system known. Other optical systems are off DE 100 12 291 C1 . EP 1 205 740 A2 , WO 02/095329 A1, DE 197 54 910 A1 known. However, none of these systems provides a satisfactory solution to the above-mentioned problems associated with polarization dependencies.

task The present invention is a structurally particularly simple constructed fiber grating sensor system to provide an improved resolution in the determination of the spectral positions of the Bragg reflections.

These The object is achieved by a fiber grating sensor system according to claim 1 solved. Thereafter, the fiber grating sensor system according to the invention comprises a Light source, at least one with the light source, preferably via a Fiber coupler or optical splitter or the like connected fiber grating of single-mode fibers with a plurality of fiber grating sensors, one preferably via a Fiber coupler or optical splitter or the like with the Fiber grating connected and having a photodetector element Detector for the spectral evaluation of the fiber grating sensors reflected light and an element for the production of depolarized and in its beam form to the requirements of the photodetector element adapted light at the detector input.

at Fiber grating sensor systems, the one for the evaluation of the measuring light Detector, in particular a polychromator, with a photosensitive Element, the quality of evaluation by the predetermined resolution of the photosensitive element, for example a photodetector line, affected. It is therefore a fundamental consideration of the invention, already the incoming light at the detector input the requirements of the photodetector element adapt.

The Wavelength dispersion is in a typical polychromator compact dimensions (for example with a base length less than or equal to 110 mm) 80 pm / CCD pixels, with one pixel being 14 μm wide. The half width of the fiber grating sensor reflections is typically about 100 pm, similar the spectral resolution per pixel in the polychromator.

Should the Bragg wavelength be determined to about 1 pm, is therefore sufficiently high-resolution determination the spectral position of Bragg reflections a sub-pixel approximation algorithm apply. For this it is necessary to know the number of pixels that be illuminated by a Bragg reflex of a sensor and thus For this Algorithm can be used to increase.

This takes place according to the invention an element preceding the detector input, hereinafter is referred to as input element. This input element works for a mode-mixing. For adapting the beam shape to the requirements the photodetector element is on the other hand a reproducible Beam shaping by the input element. This beam shaping is advantageously carried out as beam expansion or beam extension.

The Reflections on a photodetector line are then no longer for example limited to only one pixel, but extend over four, for example to seven pixels of the photodetector line. Resolution and linearity of the sub-pixel approximation to the high-resolution Determination of the Bragg wavelength The sensors are thus significantly improved.

in the In the general case, the light source emits at least partially polarized Light. In addition, you can additional at least weakly polarizing components in the sensor network, For example, branching or connectors occur. Because the picture elements in the polychromator, for example an imaging diffraction grating a polychromator having a polarization dependence, takes place according to the invention the input element next to the spatial Expansion at the same time a depolarization of the incoming light. By the depolarization of the incoming light before or on Input of the detector are the polarization modes of the monomode fiber of the sensor network preferably completely evenly distributed.

The Input element according to the invention thus fulfills a double function. Its use results in both preferably completely illuminated Detector input as well as a preferably complete depolarization of the input light. This is done with structurally simple means an improvement in resolution achieved.

According to the invention is a Combinations of several multimode fibers provided as input element, in particular a combination of a step index fiber (first multimode fiber) with a gradient index fiber (second multimode fiber). Especially is a step index fiber with a fiber output of the monomode fiber connected to the fiber grating while a gradient index fiber connected to the step index fiber as Detector input is used.

Further Advantages, special features and expedient developments of the invention emerge from the dependent claims or their sub-combinations.

According to one embodiment of the invention, the input element has a lens (claim 2). The diffuser may be formed as a matt film, for example in the form of a plastic disk, for example with a thickness of more than one millimeter. But it can also consist of an elastic material, such as silicone. In this transparent matrix are then preferably white scattering bodies with diameters of about 1 to 10 microns, for example, alumina or titanium oxide particles embedded. The thickness of the lens and the particle density are chosen so that the particles are arranged completely overlapping in transmitted light. The Gaps between the lens and the fibers are preferably smaller than the thickness of the lens. The diffuser is advantageously designed such that it performs a lateral or a rotational movement in front of the illuminated fiber, ie vibrates or rotates, in order to perfect the mode mixture in the time average. The modulation periods of the lens are preferably less than or equal to the exposure time for spectral detection in the polychromator. The diffuser can also be designed as a flow cell.

Of the Core diameter of the gradient index fiber is preferably 25 μm to 62.5 μm. Is in the gradient index fiber is the only mode-mixing component, In order to ensure sufficient mixture of modes, it must be enough To be long. As empirical value is of a length of about 100 meters or more to go out.

Become Diffuser and gradient index fiber in the mode-mixing element combined with each other is enough a relatively short one Gradient index fiber, for example with a length of a few centimeters, to ensure adequate beam shaping.

Of the Core diameter of the step index fiber is preferably 50 μm to 200 μm. Preferably The step index fiber in its input area shows a mode mixing Effect. This can be due to large fiber lengths, for example greater than 100 m in dependence be achieved by the fiber properties, or by microbending, which are produced, for example, with wavy or granular plates can. The mode mixture can be optionally modulated, for example by modulation of the pressing force of the bending-generating components. The modulation period is preferably smaller or equal the exposure time for spectral detection in the polychromator. In the exit area contains the step index fiber preferably a mode filter to higher modes slough. The mode filter is preferably as a winding mandrel filter designed with a winding diameter of 10 to 40 mm.

has the first multimode fiber has a larger core diameter and / or a larger numeric Aperture on than the second multi-mode fiber (claim 3), then the first multimode fiber already at partial illumination of her Fashions a larger fashion offer to deliver to the second multimode fiber, as this could lead. The second multimode fiber serves in other words as a mode filter for the broad Fashion range from the first multimode fiber.

to preferably comprehensive but not necessarily complete and uniform illumination of the first multimode fiber from the monomode fiber of the fiber grating is both the length as well as the winding of the first multimode fiber of importance. The length The first multimode fiber is therefore chosen so large that an oversupply. in terms of fashions the absorption capacity the second multi-mode fiber enters (claim 4). Especially advantageous is the use of a step index fiber for mode mixing in lengths from 10 m to 500 m wound to about 40 mm to 100 mm in diameter is to allow for a mode mixing without interference, even within the narrow wavelength ranges, that of the polychromator per photoelement pixel (typically 0.02 nm to 0.1 nm). By the fullest possible illumination of the second multimode fiber has the total intensity there only a very low degree of polarization.

A Magnification of the Detector input gap by selecting a fiber of suitable core diameter as an entrance slit in the polychromator causes an expansion of the figure on the detector. This allows used for evaluation sub-pixel approximation method particularly advantageous become.

Opposite one results in direct illumination of the polychromator by a single monomode fiber through the combination of several multimode fibers a whole series of advantages. In the first place is always a complete and to call stable illumination with very low power losses. This results in addition to a constant, low degree of polarization a stable position of the center of gravity of the light exit bundle. Furthermore results in a good signal-to-noise ratio in the interpolation the exact position of the spectral lines. This allows a fast and simultaneous Determination of the spectral positions of the Bragg reflections.

Instead of The combination of step index fiber / gradient index fiber can also a combination gradient index fiber / step index fiber / gradient index fiber be used. Also the use of a step index fiber as sole element or as the last element at the Polychromatoreingang is possible, if this for has a matched to the figure in the polychromator beam shape. This can, for example, by winding with a winding diameter preferably from 10 mm to 40 mm, to strip off modes away from the fiber core center.

The input element is advantageously used directly as a detector input. In other words, for example, the fiber core of the second multimode fiber serves as the input gap of the polychro mators. However, the input element can also be used at a distance from the detector input. The detector input then serves as an ordinary input gap, which is illuminated by the fiber in the free jet. Such an entrance gap is preferably used together with a fiber of large core diameter, z. B. 200 microns, used to optimize the size of the detector input.

Of the Fiber input at the polychromator is used to suppress reflections between Fiber input and photo elements also obliquely polished or broken. Of the chamfer is included preferably 1 ° to 8 °, where a small bevel angle of 1 °, for example for a polychromator with a base length between fiber and imaging diffraction grating of 100 mm and 200 μm high photoelements is sufficient. The bevel is advantageously positioned vertically such that the repeatedly reflected beam above or below the photoelements incident.

The Invention will be described below with reference to the drawings and the embodiments explained in more detail. It demonstrate:

1 FIG. 2: a schematic representation of a fiber grating sensor system, FIG.

2 : the structure of the polychromator according to the invention and

3 : A schematic representation of a transition from a monomode to a multimode fiber.

In 1 is the schematic structure of a fiber grating sensor system 1 displayed. The fiber grating sensor system 1 consists of a broadband light source 2 , here a superluminescent diode with emission in the wavelength range from 810 to 850 nm. The light source 2 has a fiber optic output and is equipped with a fiber optic splitter 3 connected to the light of the light source 2 via fiber optic connectors (not shown) to two fiber optic sensor arrays 4 . 5 divides. These sensor arrays 4 . 5 contain fiber grating sensors 6 with different average Bragg wavelength. The optical fibers used 7 and components are monomorphic. That of the fiber grating sensors 6 Reflected light is transmitted through the fiber optic splitter 3 in a polychromator 8th directed to the spectral analysis. Subsequently, the sensor measurement data in a data processing device 9 , For example, a PC, further processed. In addition to the acquisition and data evaluation, this includes the visualization and storage of the sensor measurement data and analysis results.

2 shows the schematic structure of the polychromator according to the invention 8th , The beam path inside the polychromator 8th is represented symbolically. The monomodal input light guide 10 becomes an input element 11 guided. For this purpose, a transition to a multi-mode fiber, the output end side as input gap 12 of the polychromator 8th serves. The polychromator 8th has as the only imaging element an immovable diffraction grating 13 on. The exit gap of the polychromator 8th forms a CCD (Charge Coupled Device) line 14 in silicon technology with 2048 pixels. The pixels have a height of 200 microns and a width of 14 microns. Instead of the CCD line 14 however, any other suitable detector may be used.

Advantageously, the polychromator 8th housed together with the fiber optic, optoelectronic and electrical components in a common housing (not shown). An air-permeable light trap in the housing wall compensates for air pressure changes. A particularly high mechanical stability is achieved by using a full-surface stress-free, vibration-damped base plate for the optical structure.

3 Finally, in a highly simplified, schematic manner shows the transition from an incoming single-mode fiber 15 on a gradient index fiber 16 with an intermediately coupled step index fiber 17 in the upstream input element 11 ,

The mono-mode fiber 15 has a fiber core diameter of about 5 μm and a numerical aperture of NA = 0.12 at an operating wavelength of 800 nm.

The transition from this monomode fiber 15 on the gradient index fiber 16 with a fiber core diameter of about 50 microns and a numerical aperture of NA = 0.2 takes place via an intermediate step index fiber 17 with a fiber core diameter of about 200 μm and a numerical aperture of NA = 0.3. The length of the gradient index fiber 16 is irrelevant to this arrangement. It is for example 0.1 to 1 m. The length of the step index fiber 17 is about 100 m, with these in the initial area, near the monomode fiber 15 , has irregular curvatures with period lengths of 0.5 mm to 5 mm. The gradient index fiber 16 can instead of the 50 micron core diameter. for example, have a core diameter of 62.5 microns. Preferably, the diameter of the gradient index fiber 16 at the optimum for a sub-pixel approximation image spot diameter of about 2 to 5 pixel widths of the CCD line 14 of the polychromator 8th customized.

In other words, the single-mode optical fiber output of the fiber grating sensor network illuminates a step index fiber 17 , which in turn is a gradient index fiber 16 illuminated, which eventually serves as the entrance of the polychromator.

to further improvement of the transition from the monomode fiber on the multimode fiber is the input face of the Roughened multimode fiber. The fiber ends are in common Way, in the case without roughening preferably by melting interconnected, with roughened multimode fiber input with 10 μm to 1000 μm spacing opposite each other positioned.

All in the description, the following claims and the drawing Features can both individually and in any combination with each other invention essential be.

1

Fiber grating sensor system

2

light source

3

fiber optic splitter

4

first sensor array

5

second sensor array

6

Fiber grating sensor

7

optical fiber

8th

polychromator

9

Computing device

10

Input guide

11

input element

12

entrance slit

13

diffraction grating

14

CCD line

15

Single-mode fiber

16

graded

17

Step index fiber

Claims (4)

Fiber grating sensor system ( 1 ), with a light source ( 2 ), with at least one with the light source ( 2 ) connected fiber grids ( 4 . 5 ) of monomode fibers ( 7 . 15 ) with a plurality of fiber grating sensors ( 6 ), with one with the fiber grating ( 4 . 5 ) and a photodetector element ( 14 ) detector ( 8th ) for the spectral evaluation of the fiber grating sensors ( 6 ) reflected light, with an input element ( 11 ) for producing depolarized and in its beam form to the requirements of the photodetector element ( 14 ) adapted light at the detector input ( 12 ), wherein the input element ( 11 ) a step index fiber ( 17 ) and a gradient index fiber ( 16 ) having. Fiber grating sensor system ( 1 ) according to claim 1, wherein the input element ( 11 ) has a lens. Fiber grating sensor system ( 1 ) according to claim 1 or 2, wherein the step index fiber ( 17 ) has a larger core diameter and / or a larger numerical aperture than the gradient index fiber ( 16 ) having. Fiber grating sensor system ( 1 ) according to one of claims 1 to 3, wherein the step index fiber ( 17 ) has a large length and / or elements for generating microbends in such a way that a mode oversupply arises, which increases the receptivity of the gradient index fiber ( 16 ) exceeds.