DE102014001846B3 - Device for the absolute and spatially resolved measurement of the thermally relevant absorption of a light beam in an optical fiber - Google Patents

Abstract

The invention relates to a device (10) for the absolute and spatially resolved measurement of the thermally relevant absorption of a light beam (80) in an optical fiber (90), comprising a first light source (12) for generating the light beam to be fed into the optical fiber (90). 80), a transparent to a measuring light beam (70) and a temperature-dependent refractive index, the optical fiber (90) in the radial direction over its entire circumference enclosing medium (20), a second light source (14) for generating the measuring light beam (70) and its feeding into the medium (20) in such a way that the measuring light beam (70) passes without intersection with the optical fiber (90), and a detector arrangement (30) for measuring the angle by which the measuring light beam emerging from the medium (20) (72) deviates from the measuring light beam (70) fed into the medium (20).

Description

The present invention relates to a device for the absolute and spatially resolved measurement of the thermally relevant absorption of a light beam in an optical fiber.

Devices for determining the absorption are known from the prior art. For example, describes DE 101 39 906 A1 an optical absorption determination device that experiences a light beam passing through a transparent medium as a result of converting some of its energy into heat. By a reflector arrangement, a laser measuring beam is divided so that the laser beams are guided on opposite sides of the light beam through the medium and / or the laser beam passed at least twice through the medium.

In the manufacture of optical fibers, various thermal and photonic steps are used within the process chain from the raw material to the optical fiber and further to the functional element. These steps can lead to an increase in losses in the fiber, which is also called attenuation. Using spectrophotometers, these losses can be characterized. Unresolved so far, however, for optical fibers, the distribution of losses in the individual components, as z. B. scattering and absorption. However, this subdivision is very important, since the process optimization with respect to the different types of loss can be very different.

Furthermore, there is the interest to quantitatively characterize the effects along the process chain, as this is usually done only on the finished fiber (and not already in the course of production).

In addition, it is not possible to perform a spatially resolved characterization in the millimeter to submillimeter range, since this is the typical sensitivities of spectrophotometers are not sufficient. However, such a characterization option is, for example, for the study of splice sites or photonically modified fiber regions, as z. As fiber Bragg grating represent, of great interest.

Another interesting question for a direct absorption measurement is z. As in the characterization of non-linear absorptions in the optical fibers, which can not be solved by commercial spectrometers or spectral analyzers in principle.

There is currently no known possibility of determining the exclusively absorptive loss fraction of an optical fiber or a thin optical rod, which for the purposes of this description can also be considered as an optical fiber, to be absolute and spatially resolved. Exceptions to this are high absorptions due to defects or impurities, which can be detected by their distinct absorption bands with spectrophotometers. A spatially resolved characterization in the millimeter and submillimeter range, as required for example to test the homogeneity of the fiber, but even for these cases is not possible due to the limited sensitivity of the spectrometer.

Spectral photometry as standard characterization of optical fibers achieves sufficient accuracy for very small losses only by measuring long fibers. It is therefore not suitable for spatially resolved investigations in the millimeter to submillimeter range. In addition, generally only the total loss can be determined without any more precise specification of individual components such as absorption or scattering. In addition, a characterization with respect to non-linear absorptions is generally not possible by means of classical spectrophotometers, since the starting light used for this purpose has a much too low intensity.

The present invention has for its object to provide a new device for measuring the thermally relevant absorption of a light beam in an optical fiber - including thin optical rods include - with the absorption in the fiber with high sensitivity can be determined absolute and spatially resolved ,

This object is achieved with a device according to claim 1. Advantageous developments of the invention are the subject of the dependent claims.

A device according to the invention comprises a first light source for generating a light beam to be fed into the optical fiber, which light beam can also be referred to as an excitation light beam. Typically, the first light source is a laser, such as a fiber laser. Furthermore, the device according to the invention comprises a medium which has a temperature-dependent refractive index and is transparent to a measuring light beam (specified later) and arranged to surround the optical fiber in its radial direction over its entire circumference. It is clear that the medium extends over a certain axial length around the optical fiber. The device according to the invention also comprises a second light source for generating the measuring light beam, which is also typically a laser, such. B. a laser diode in the visible range (for example, at 660 nm). The arrangement is made in such a way that the measuring light beam is fed or irradiated into the medium in such a way that it passes without intersection with the optical fiber or in other words is far enough away radially from the axial center of the optical fiber that the irradiated measuring light beam does not penetrate into the medium the optical fiber propagates. Typically, the measuring light beam is fed orthogonally to the optical fiber in the medium. Finally, the device according to the invention comprises a detector arrangement for measuring the measuring light beam emerging from the medium again. More specifically, the detector arrangement measures by which angle the exiting measurement light beam deviates from the measurement light beam fed into the medium.

Briefly summarized, the functioning of the device according to the invention is based on the fact that when a first light beam is introduced into the optical fiber, absorption occurs at defect sites, splice sites or optical "devices" introduced therein, such as a fiber Bragg grating. The absorbed energy leads to a warming in the optical fiber, the heat can penetrate into the medium surrounding it and form a so-called thermal lens there. This thermal lens is due to the fact that the refractive index of the medium changes in accordance with the temperature given at each location. The resulting from the inside outward temperature gradient thus leads to a refractive index gradient, whereby the irradiated measuring light beam is deflected in the medium, which is why it exits at a certain angle with respect to the irradiated measuring light beam again from the medium. The angle change is a measure of the absorption that occurs at the point where the detector array is along the axial extent of the optical fiber. Since the location of the detector arrangement can be detected very accurately, a location-related - ie spatially resolved - measurement is possible. If the optical fiber and the detector arrangement are displaceable relative to one another in the axial direction of the optical fiber, an "absorption profile" along the optical fiber can be created by measurements at several locations. The exact measuring method and the evaluation are described in detail in the aforementioned German patent application DE 10 2011 113 572 B9 whose disclosure content is expressly to be considered as included here, although this document refers to the measurement of bulk materials and surfaces - ie not just optical fibers.

The device according to the invention thus offers the advantage that thermally relevant absorptions of a light beam in an optical fiber can be realized in their absolute values and spatially resolved. In addition, the fact that the medium encloses the optical fiber over its entire circumference provides the advantage that better and more complete heat transfer from the optical fiber to the medium can be achieved, thus increasing measurement accuracy. In addition, the measurement setup is simpler since the detector arrangement has no angular dependence in the circumferential direction of the optical fiber or is not subject to any angle requirements. Moreover, the measurement is more accurate because the temperature field generated in the medium and thus the generated thermal lens is made more homogeneous.

Thus, direct absorption measurements on optical fibers can be carried out for the first time and, as a result, spatially resolved information about the absorption along the investigated fiber can be obtained as well. In this way, the entire process chain from the raw material via the preform to the fiber production can be characterized with regard to the occurring absorption and thereby an optimization of the processes can be achieved. Thus comparatively short optical fibers with a length of, for example, 10 cm can also be characterized or measured, since the more usual summation procedure over a long fiber is no longer required for a meaningful measurement.

By using intensive laser irradiation by means of the first light source, non-linear optical absorption processes can also be very well studied.

With the present device according to the invention it is possible, for example, to measure fibers with a fiber core of a few μm to a few hundred μm, without the device being restricted to these dimensions.

According to an advantageous embodiment of the device according to the invention, the medium is formed as a solid formed from a cuboid, which has a passage opening for performing or receiving the optical fiber. As a material for the cuboid comes in particular an amorphous body such. As quartz glass, or a crystalline body in question, preferably crystals, such as alkali metal and alkaline earth halides, especially fluorides, such as calcium fluoride, magnesium fluoride, potassium fluoride and mixtures thereof, LuAG, quartz and alkali and alkaline earth metal oxide, in particular magnesium oxide and mixtures thereof both among themselves and with other oxides, glass, quartz glass, glass ceramics and translucent plastics, such as polyacrylates (such as Plexiglas etc.). Alternatively, it would also be possible to provide as the medium a gas or liquid contained in a cuvette enclosing the optical fiber over its entire circumference.

According to a further advantageous development of the invention, the cuboid is subdivided into at least two subsections subdivided in the circumferential direction of the optical fiber, so that the optical fiber can be easily inserted into or removed from the through opening by separating the subsections. This simplifies the handling of the measuring device.

Advantageously, the passage opening is designed so that in the situation in which the optical fiber is located in the passage opening, in the radial direction outside the optical fiber, a cavity between the optical fiber and the cuboid remains, and that also at each end of the through hole a seal is provided for sealing the cavity outwardly with optical fiber located in the through hole. In this case, the passage opening can be chosen to be relatively large, so that optical fibers with different diameters are inserted into the passage opening and thus can be measured. The cavity is then filled with a medium which ensures good heat conduction of the heat generated in the optical fiber into the medium enclosing the optical fiber (cuboid). Advantageously, this is air, as has been shown in practice. Alternatively, a good heat-conducting liquid, such as water, may be used. However, other liquids may be used whose thermal conductivity is at least as good as that of water. As a further alternative, gaseous or pulverulent heat transfer media could also be used.

It may be advantageous if the second light source and the detector arrangement are arranged relative to the longitudinal direction of the optical fiber displaceable relative to the medium. In this case, the medium may always be located at the same location relative to the optical fiber, while the second light source and the detector array may be displaced along the optical fiber. For this purpose, of course, a sufficient axial length of the medium is required so that all areas of interest of the optical fiber can be measured. Alternatively, it is of course also possible that the arrangement is such that the optical fiber can be pulled through the passage opening, in which case the medium and the second light source and the detector arrangement can remain stationary relative to each other.

According to an advantageous development of the invention, the device according to the invention comprises an evaluation arrangement for determining the absolute and spatially resolved measurement of the thermally relevant absorptions of the optical fiber and a motor arrangement for the fully automatic displacement of the second light source and the detector arrangement relative to the optical fiber, so that the computer-aided evaluation can be made on the basis of the position data of the second light source and the detector arrangement relative to the optical fiber, which are transmitted to the evaluation arrangement. A human intervention in Messvogang is thereby superfluous, thus improving the handling and the susceptibility to error of the device according to the invention.

Another advantage can be achieved by using as the medium a material which has the greatest possible photothermal sensitivity. The photothermal sensitivity comprises on the one hand the change of the refractive index due to the temperature and on the other hand the change of the refractive index due to the stress induced by the material expansion.

In this way, the effect of the medium can be improved as an "amplifier", since forms in such a medium, a stronger thermal lens at the same incident energy and thus enables a higher accuracy of measurement.

Another object of the present invention is the use of the optical fiber manufacturing apparatus of the present invention, optical elements containing optical fiber, tempered optical elements, lenses, prisms, filters, mirrors, thin film polarizers, laser resonators, and surface-layered optical elements.

Further advantages, features and special features of the present invention will become apparent from the following detailed description of advantageous embodiments of the device with reference to the figures. Show it:

1 a perspective view of a first advantageous embodiment of the device according to the invention and

2 in side view part of a second advantageous embodiment of the device according to the invention.

According to 1 generates a first light source 12 , which is preferably a laser, such as a fiber laser, a light beam 80 - which can also be referred to as excitation light beam - and feeds this light beam 80 in an optical fiber 90 a, which is to be measured and thus is not part of the device according to the invention. The optical fiber 90 is through a passage opening 28 a cuboid made of quartz glass in this embodiment 21 passed. The cuboid 21 is in a lower section 22 and an upper section 23 divided, which only in 2 is shown. The passage opening 28 is designed to be cylindrical with a larger diameter than the diameter of a typical fiber to be measured. This makes it possible to measure optical fibers with different diameters. Between the wall 26 the passage opening 28 and in the through hole 28 located optical fiber 90 is thus a ring-shaped cavity 24 formed, the inner diameter of the maximum possible outer diameter of an optical fiber to be measured 90 is determined.

Laterally from the cuboid 21 is a second light source 14 arranged, which has a measuring light beam 70 generated and so in the cuboid 21 feeds that measuring light beam 70 perpendicular to the light beam 80 or perpendicular to the optical fiber 90 runs, but neither the light beam 80 still the optical fiber 90 crosses or penetrates, so as not to cause any disturbing interference.

The present invention takes advantage of the fact that in the optical fiber 90 fed light beam 80 at locations where an impurity, an optical device, such as a fiber Bragg grating or the like, undergoes partial absorption. The absorbed energy leads to heating of the optical fiber 90 , This warming spreads into the cuboid 21 or its two subsections 22 and 23 out and calls in the cuboid 21 a change in the refractive index dependent on the local temperature and thus produces a refractive index gradient. This is indicated by two dashed circles in 2 indicated. The refractive index gradient causes the irradiated measuring light beam 70 deflected from its original orbit and emerges as a deflected measuring light beam 72 on the other side of the cuboid 21 at an angle Δ to the original propagation direction of the input measurement light beam 70 out. There he is from a detector array 30 detected and together with the position data of the second light source 14 and the detector assembly 30 to an evaluation device 50 derived from the angle Δ, the change in the refractive index and thus determines the strength of the absorption.

Thus the propagation of the in the optical fiber 90 heat generated by absorption in the cuboid 21 runs as optimally as possible, that is between the cuboid 21 and the optical fiber 90 located cavity 24 with air 25 filled, on the one hand, a very good contact between the optical fiber 90 and the cuboid 21 produces and therefore on the other hand ensures good heat conduction. Of course, liquids or gases can be used, whose heat conduction properties are at least as good as, for example, those of air. To prevent leakage of fluid from the cavity 24 to prevent are at both ends of the passage opening 28 appropriate seals 29 intended.

In order to be able to carry out the measurement described above fully automatically at a plurality of positions along the direction of extension of the optical fiber, a motor arrangement is provided 40 provided, which the second light source 14 and the detector assembly 30 along the longitudinal direction of the optical fiber 90 can move. In this embodiment, therefore, the position of the optical fiber remains 90 in the cuboid 21 always the same. Alternatively, it would also be possible to pull the fiber through the cuboid and the second light source 14 as well as the detector arrangement 30 stationary relative to the cuboid 21 to keep.

The mobility of the motor arrangement 40 for the second light source 14 and the detector assembly 30 is indicated schematically by an arrow A.

The subdivision of the cuboid 21 in a lower section 22 and an upper section 23 offers the advantage that the cuboid 21 can be easily opened and closed again to an optical fiber 90 easily insert or remove, without the optical fiber 90 through the passage opening 28 to thread through. Of course, the subdivision can also take place in a "left" and a "right" subsection or in another manner, wherein the two subsections do not necessarily have to be exactly the same size.

In the 2 which has a view on the front of the cuboid 21 shows, a second embodiment of the device according to the invention is shown. The second light source 14 generates two measuring light beams 70 that are in appropriate optical fibers 15 are fed, whose ends are arranged so that they receive a measuring light beam 70 "Above" and the second measuring light beam 70 "Below" the optical fiber 90 , so on both sides of the optical fiber 90 , in the upper section 23 or the lower section 22 of the cuboid 21 radiate. Alternatively, the arrangement may be made such that the second light source 14 only one measuring light beam 70 generated, which then in a beam splitter in two measuring light beams 70 is divided, or that two second light sources 14 for generating a measuring light beam in each case 70 are provided. Thus, two measuring light beams occur 72 from the cuboid 21 off and are from the detector array 30 recorded and sent to the (in 2 not shown) evaluation 50 forwarded. By using two measuring light beams 70 can be increased due to a difference in the measurement accuracy.

It is clear that the invention does not refer to the use of a (solid) cuboid 21 is limited, but that as a medium 20 in which a refractive index gradient or refractive index gradient can be generated in order to carry out such a photothermal measuring method for determining the thermally relevant absorption, for this purpose also a corresponding gas or a liquid can be used.

In summary, it should be noted that, according to the invention, a photothermal measuring method is used to determine the thermally relevant absorption in a spatially resolved manner, wherein the part of the absorbed light output converted into heat is displayed. This method is known as the LID principle (LID = laser-induced test beam deflection) and was developed at the Institute for Photonic Technologies e. V. developed in Jena. With appropriate motorized adjustment of the second light source 14 and the detector assembly 30 Spatially resolved information with a resolution of about 0.5 mm over the absorption in the optical fiber (or in its core) can be obtained over the length of the cuboid. With the help of a suitable electrical calibration, the absorption values can also be determined absolutely. For this purpose, an electric heater instead of the optical fiber is introduced into the cuboid and induced by a resistance measurement a well-defined heat development to set the resulting angle change Δ in relation to the amount of heat. For example, a heat quantity of 1 mW is generated for this calibration. Further details for the exact execution of the measuring procedure as well as for the electrical calibration can be found in the DE 10 2011 113 572 B9 be removed.

Depending on the configuration of the shape of the cuboid or medium, optical fibers of, for example, 10 to 150 μm core diameter, but also thin optical rods with a diameter of 1 to 2 mm or thicker, can be measured with the device according to the invention.

It should be understood that the features of the invention described with reference to the illustrated embodiments, such as the nature, arrangement and configuration of the individual light sources, light beams or arrangements and devices, may be present in other embodiments as well, unless otherwise specified bans itself for technical reasons.

LIST OF REFERENCE NUMBERS

10

contraption

12

first light source

14

second light source

15

optical fibers

20

medium

21

cuboid

22

lower section

23

upper section

24

ring-shaped cavity

25

air

26

wall

28

Through opening

29

seals

30

detector array

40

engine location

50

evaluation

70

fed-in measuring light beam

72

emerging / deflected measuring light beam

80

beam of light

90

optical fiber

A

arrow

Δ

angle

Claims (9)

Contraption ( 10 ) for the absolute and spatially resolved measurement of the thermally relevant absorption of a light beam ( 80 ) in an optical fiber ( 90 ), comprising - a first light source ( 12 ) for generating the optical fiber ( 90 ) to be fed light beam ( 80 ), - one for a measuring light beam ( 70 ) transparent and a temperature-dependent refractive index, the optical fiber ( 90 ) in the radial direction over its entire circumference enclosing medium ( 20 ), - a second light source ( 14 ) for generating the measuring light beam ( 70 ) and its feeding into the medium ( 20 ) in such a way that the fed measuring light beam ( 70 ) without crossing with the optical fiber ( 90 ), and - a detector arrangement ( 30 ) for measuring the angle to which the medium ( 20 ) exiting measurement light beam ( 72 ) of which into the medium ( 20 ) fed measuring light beam ( 70 ) deviates. Contraption ( 10 ) according to claim 1, characterized in that the medium ( 20 ) as solid cuboid ( 21 ) is formed, which has a passage opening ( 28 ) for the optical fiber ( 90 ) having. Apparatus according to claim 2, characterized in that the cuboid ( 21 ) at least two in the circumferential direction of the optical fiber ( 90 ) subdivided subsections ( 22 . 23 ). Apparatus according to claim 2 or 3, characterized in that the passage opening ( 28 ) is designed so that in in the passage opening ( 28 ) located optical fiber ( 90 ) in the radial direction a cavity ( 24 ) between the optical fiber ( 90 ) and the cuboid ( 21 ) remains, and at each end of the passage opening ( 28 ) a seal ( 29 ) for sealing the cavity ( 24 ) to the outside at in the through hole ( 28 ) located optical fiber ( 90 ) is provided. Device according to claim 4, characterized in that the cavity ( 24 ) with air ( 25 ), preferably under atmospheric pressure. Device according to one of the preceding claims, characterized in that the second light source ( 14 ) and the detector arrangement ( 30 ) with respect to the longitudinal direction of the optical fiber ( 90 ) displaceable relative to the medium ( 20 ) are arranged. Apparatus according to claim 6, characterized in that it has an evaluation arrangement ( 50 ) for the determination of the absolute and spatially resolved measurement of the thermally relevant absorption in the optical fiber ( 90 ) and a motor assembly ( 40 ) for the fully automatic displacement of the second light source ( 14 ) and the detector arrangement ( 30 ) relative to the optical fiber ( 90 ), wherein the position data of the second light source ( 14 ) and the detector arrangement ( 30 ) relative to the optical fiber ( 90 ) to the evaluation arrangement ( 50 ) be transmitted. Device according to one of the preceding claims, characterized in that the medium ( 20 ) is formed of a material which has a greater thermal change in the refractive index than the optical fiber ( 90 ). Use of the device according to one of the preceding claims for the production of optical fibers ( 90 ) as well as an optical fiber ( 90 ), tempered optical elements, lenses, prisms, filters, mirrors, thin film polarizers, laser resonators, and surface layered optical elements.

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