DE102019002942A1 - Measuring device and method for performing optical coherence tomography with a coherence tomograph - Google Patents

Abstract

The invention relates to a measuring device, in particular for a machining system (12, 12 ') for machining a workpiece (14, 14') by means of a high-energy machining beam (16, 16 '), the measuring device (10, 10') being an optical coherence tomograph ( 18, 18 '), comprising: a beam generating unit (20, 20') for generating a measuring beam (22, 22 ') and a reference beam (24, 24'); a measuring arm (26, 26 ') which extends from the beam generating unit (20, 20') and in which the measuring beam (22, 22 ') can be guided optically so that it can be projected onto a measuring object (28, 28'); and a reference arm (30, 30 ') extending from the beam generating unit (20, 20'), in which the reference beam (24, 24 ') can be guided optically and in which the measuring arm (26, 26') at least in its optical path length simulated so that the measuring beam (22, 22 ') and the reference beam (24, 24') can be superimposed after passing through the measuring arm (26, 26 ') or the reference arm (30, 30') to generate an interference signal. According to the invention, it is provided that the measuring arm (26, 26 ') and the reference arm (30, 30') run at least in sections in a common optical fiber (32, 32 ').

Description

The invention relates to a measuring device, in particular for a machining system for machining a workpiece by means of a high-energy machining beam, the measuring device having an optical coherence tomograph.

Optical coherence tomographs usually include a beam generating unit for generating a measuring beam and a reference beam. Furthermore, such optical coherence tomographs have a measuring arm extending from the beam generating unit, in which the measuring beam can be guided optically so that it can be projected onto a measurement object, and a reference arm extending from the beam generating unit, in which the reference beam can be guided optically and the simulates the measuring arm at least in its optical path length, so that the measuring beam and the reference beam can be superimposed after passing through the measuring arm or the reference arm to generate an interference signal.

In addition, devices for machining a workpiece by means of a high-energy machining beam are known from the prior art, which devices are used, for example, in laser material machining processes. Here, for example, a high-energy laser processing beam is used to act on one or more workpieces or workpiece parts, for example to weld them together, to make laser holes, to process surfaces, etc. The device can include a movable processing head that is carried by an industrial robot, for example can be movable relative to a workpiece to be machined. Such devices also have processing beam optics, by means of which the processing beam can be guided optically onto the workpiece. Alternatively or additionally, the workpiece can also be movable relative to the machining head.

In corresponding processing systems, an optical coherence tomograph can be used, the measuring beam of which can be coupled into the processing beam optics. This allows the machining processes to be monitored three-dimensionally by using the coherence tomograph in addition to conventional two-dimensional monitoring by means of cameras or the like in order to obtain height information during the machining process, for example relating to a surface of the workpiece or a depth of a keyhole generated during laser welding.

In this case it may be necessary to adapt the optical path length of the reference arm to changes in the optical path length of the measuring arm, for example if the optical path length of the measuring arm changes due to the fact that the distance between the machining beam optics and the machined workpiece changes. A dispersion adjustment can also be useful in order to take into account dispersion differences between the reference arm and the measuring arm. With regard to such adjustments to the reference arm, refer to the documents, for example DE 10 2013 008 269 A1 and DE 10 2015 015 112 A1 referenced.

In order to achieve a high degree of accuracy of the measurements carried out with the optical coherence tomograph, it is fundamentally expedient to match the optical properties of the reference arm and the measuring arm as precisely as possible, since otherwise undesirable differences in transit time and / or dispersion differences between the measuring arm and reference arm can occur. Such an optical matching, however, poses a great challenge in practice, since the reference arm and measuring arm can be exposed to different environmental influences. This applies in particular when the reference arm is located in a stationary part of the optical coherence tomograph, but part of the measuring arm is guided to a movable processing head or the like and possibly moves with a robot arm.

In DE 10 2017 001 353 A1 a temperature measuring cable is proposed which is routed parallel to an optical fiber of the measuring arm in order to be able to estimate temperature influences on the measuring arm as precisely as possible. A reference arm adjustable in length is arranged in a stationary part of the corresponding optical coherence tomograph, while the measuring arm is guided to a movable processing head. The temperature readings obtained are then used to carry out temperature compensation. However, the corresponding compensation procedure is time-consuming. In addition, an adaptation can only take place approximately despite the additional temperature measurement.

Another approach like him DE 10 2014 216 829 A1 describes, provides to lay an optical fiber of the reference arm and an optical fiber of the measuring arm in close thermal contact so that environmental influences exert approximately the same on both fibers. If the fibers are moved here, for example due to the displacement of a processing head to which the fibers lead, it cannot be ensured that the fibers are actually exposed to identical mechanical loads. Even slight differences between the forces acting on the fibers that lead to bending, compression, stretching or torsion of the fibers, but can have a significant effect on their optical properties. This can lead to deviations between the reference arm and the measuring arm, even though they are in close contact.

Starting from the prior art described above, the present invention is therefore based on the object of coordinating the measuring arm and reference arm of an optical coherence tomograph in a simple and reliable manner with regard to their optical properties and, in particular, of reducing the influence of external conditions on measurement results.

This object is achieved according to the invention by a measuring device with the features of claim 1 and with a method with the features of claim 14. Further developments of the invention can be found in the dependent claims.

The invention is based on a measuring device of the type described at the outset. According to the invention, it is proposed that the measuring arm and the reference arm run at least in sections in a common optical fiber.

• - Erzeugen eines Messstrahls und eines Referenzstrahls mittels einer Lichtquelle des optischen Kohärenztomographen;
• - optisches Führen des Messstrahls in einem Messarm derart, dass der Messstrahl auf ein Messobjekt projiziert wird; und
• - optisches Führen des Referenzstrahls in einem Referenzarm, der den Messarm zumindest in seiner optischen Weglänge nachbildet, sodass der Messstrahl und der

The invention also relates to a method for performing optical coherence tomography with a coherence tomograph, in particular for monitoring the machining of a workpiece by means of a high-energy machining beam. The optical coherence tomograph can be the optical coherence tomograph of the measuring device according to the invention. The method according to the invention comprises the steps:

• - Generating a measuring beam and a reference beam by means of a light source of the optical coherence tomograph;
• - Optical guidance of the measuring beam in a measuring arm in such a way that the measuring beam is projected onto a measuring object; and
• - Optical guidance of the reference beam in a reference arm, which simulates the measuring arm at least in its optical path length, so that the measuring beam and the

Reference beam can be superimposed after passing through the measuring arm or the reference arm to generate an interference signal.

According to the invention, it is proposed that the measuring beam and the reference beam are optically guided at least in sections in a common optical fiber.

The invention makes it possible to reduce the effect of external influences such as, for example, an ambient temperature or a mechanical load on components on the measurement results of an optical coherence tomograph. According to the invention, this is achieved in particular in that external influences affect the reference arm and the measuring arm equally. Furthermore, an optical coherence tomograph can be provided which is provided with a single optical fiber for forming the reference arm and the measuring arm, so that deviations between optical fibers that would otherwise occur can be avoided. In addition, complex methods for compensating for deviations that occur between the optical properties of the measuring arm and the reference arm can at least largely be dispensed with, since their optical properties are essentially identical due to the common optical fiber.

The reference arm can also essentially simulate the measuring arm with regard to its dispersion.

The measuring beam can be deflected via a deflection device. The deflection device can be movable. This can be a deflection device of a machining head of the named machining system.

The beam generating unit can comprise a beam splitter which splits a source beam coming from the light source into the measuring beam and the reference beam. The beam splitter can split the source beam into the reference beam or the measuring beam in a suitable ratio. Due to the often incomplete reflection of the measuring beam on the measuring object, it is useful here that a larger part of the source beam falls on the measuring beam. The light path over which the source beam can be guided to the beam splitter is in this case not to be added to either the reference arm or the measuring arm. Rather, both the reference arm and the measuring arm extend in particular starting from the beam splitter. The light source is preferably arranged in a stationary part of the optical coherence tomograph.

This stationary part can also comprise a spectrometer and / or a circulator. It can thus be provided that the stationary part of the optical coherence tomograph forms an OCT sensor.

The common optical fiber can be a single common optical fiber. In other embodiments of the invention, however, several common optical fibers can also be provided, for example in series in this way are arranged so that they jointly define the optical path of the reference arm and the measuring arm, at least in sections. The at least one common optical fiber can comprise a large part of the total length of the optical path of the reference arm and / or of the measuring arm. According to one embodiment, the common optical fiber can lead from a stationary part of the optical coherence tomograph to the processing head. If the processing head is designed to be movable, its movement can lead to mechanical stress on the optical fiber, for example if it is curved, stretched, compressed and / or twisted due to the movement of the processing head, this movement of the optical fiber according to the invention being applied to the reference arm and affects the measuring arm. The optical properties of the reference arm and the measuring arm are thus changed in the same way when the common optical fiber is exposed to mechanical stress. The same applies analogously to other external influences, such as for example the effect of an ambient temperature on the reference arm and the measuring arm or on the common optical fiber.

A reliable optical structure that allows common optical guidance of the reference beam and the measuring beam in the common optical fiber can be provided in particular if the optical coherence tomograph comprises a polarizing beam combiner which is arranged such that the measuring beam and the reference beam with different Polarization are coupled into the common fiber. The polarizing beam combiner can be arranged at a proximal end of the common optical fiber as viewed from the beam generating unit. In particular, the reference beam and / or the measuring beam leave the beam splitter in a linearly polarized manner. The polarizing beam combiner can be designed in such a way that it changes the polarization direction of the measuring beam and / or the reference beam. Furthermore, the polarizing beam combiner can take over the functions of a polarizing beam splitter when the reference beam and measuring beam run back through the common optical fiber. If the measuring beam and the reference beam are guided in the common optical fiber with different polarization, their interaction within the common optical fiber can advantageously be reduced so that the common optical guidance in the common optical fiber does not lead to incorrect measurement results.

In a further development of the invention, it can be provided that the polarizing beam splitter is set up in such a way that the polarization of the measuring beam and the polarization of the reference beam in the common optical fiber are rotated by 90 ° to one another. In particular, the measuring beam or the reference beam can be polarized perpendicular or parallel or parallel or perpendicular with respect to the beam splitter. As a result, the reference beam and the measuring beam can be guided independently of one another in the common optical fiber in a reliable and easily traceable manner.

Unwanted interference between the measurement beam and the reference beam in the common optical fiber can be avoided or at least reduced in a simple and reliable manner, in particular, if an optical path of the reference arm and an optical path of the measurement arm from the beam generating unit to the common fiber differ by an amount differentiate, which is larger than the working range of the coherence tomograph. The difference in the optical paths can differ by an amount that is greater than the coherence length of the reference beam and the measuring beam. The measuring beam and the reference beam then travel with a path difference in the common optical fiber, which ensures that the beam components that are later brought to interference do not already overlap in the common optical fiber.

In a further development of the invention it can be provided that the reference arm comprises an at least partially reflective element which sends the reference beam back through the common optical fiber after passing through the common optical fiber. The reference beam can thus pass through the common optical fiber twice. As a result, the optical properties of the reference arm and measuring arm can be further matched to one another, since the measuring beam usually also runs back through the common optical fiber.

In addition, according to the invention, a polarizing beam splitter can be arranged between one end of the common fiber and the at least partially reflective element. The end can be the distal end of the common fiber as viewed from the beam generating unit. A first output of the polarizing beam splitter can be set up to couple out the measurement beam in the direction of the measurement object, for example via an OCT scanner. A second output of the polarizing beam splitter can be configured to couple out the reference beam in the direction of the at least partially reflective element. The polarizing beam splitter can also assume the functions of a polarizing beam combiner for the reference beam reflected on the at least partially reflective element and the measurement beam reflected on the measurement object, so that Measuring beam and reference beam with different polarization run back through the common fiber. As a result, the measuring beam and the reference beam can each pass through the optical fiber twice, i.e. they use the fiber together, but remain physically different due to their different polarization directions and are thus completely separable again.

In an alternative embodiment, the at least partially reflective element can be arranged directly at one end of the common fiber, whereby a simple structure can be achieved. This is particularly useful when, as described, the optical paths of the reference arm and measuring arm differ by an amount that is greater than the working range of the coherence tomograph. A partially reflective element is preferably used here, which is reflective for the reference beam for example only up to 5%, up to 10%, up to 15% or up to 20% based on an intensity.

The at least partially reflective element can be produced in a particularly simple manner in that it is formed by a straight cut of the end of the common fiber. The optical properties of the at least partially reflective element can also be adapted precisely and specifically to the application if the at least partially reflective element is alternatively or additionally formed by coating the end of the common fiber. The coating can comprise a coating with a layer thickness in the nanometer range, which is produced for example by vapor deposition or sputtering. Depending on the wavelength range to be reflected, metals such as silver, aluminum, gold etc. or dielectrics can be used, for example to produce Bragg mirrors.

In a further development of the invention, the optical coherence tomograph comprises a path length adjusting device with an adjusting element which can be displaced between different setting positions and whose displacement causes a change in the optical path length of the reference arm and / or the measuring arm. As a result, the optical path lengths of the reference arm and the measuring arm can be matched to one another in a simple and reliable manner.

According to the invention it can be provided that the setting element comprises a mirror. In addition, the setting element can comprise a converging lens, in the focal point of which the mirror is arranged. The mirror and the converging lens can then be moved together between setting positions so that it is not necessary to move a fiber end to adjust the path length. This achieves a high level of reliability in the underlying optical structure. In addition, a large adjustment range and a high adjustment speed can be achieved, since the displacement of the mirror by a certain distance can cause a path length change of twice the circumference. In addition, it can be provided according to the invention that a beam approaching the mirror passes through the converging lens at a different point than a beam running away from the mirror. For example, the rays can be guided on opposite sides of a central axis of the converging lens. Alternatively, the beams can be guided coaxially.

According to one embodiment of the invention, it is proposed that the path length adjustment device comprises a retardation plate, in particular a λ / 4 plate. In particular in combination with an adjusting element comprising a mirror, a reliable optical structure of the path length adjustment device can be achieved, which allows the path length adjustment device to be arranged at different suitable positions in the optical structure of the optical coherence tomograph. The path length setting device can comprise a polarizing beam combiner, behind the combining end of which the retardation plate and preferably also the mirror are arranged. This makes it possible to dispense with a moving fiber end, whereby at the same time the polarization of the beam which passes through the path length adjustment device is advantageously influenced when passing through the path length adjustment device in such a way that the polarization present when entering the path length adjustment device corresponds to the polarization at the exit.

The invention also relates to a processing system for processing a workpiece by means of a high-energy processing beam, which comprises a processing beam source, processing beam optics and a measuring device according to the invention, wherein the measuring beam can be guided optically in the measuring arm in such a way that the measuring beam can be coupled into the processing beam and via the processing beam optics the workpiece is projectable.

The processing system can comprise a processing head, the common optical fiber leading from the stationary part of the measuring device to the processing head. The machining head can be displaceable relative to the workpiece. For example, the processing head can be arranged on a robot arm for this purpose or can be displaced in some other way. Forces acting on the common optical fiber due to a corresponding movement therefore act in the same way as all other environmental influences Point out the reference arm and the measuring arm, as they both run through the same fiber.

It goes without saying that the subject matter of the invention is not limited to the embodiments described above. The embodiments and features described in connection with developments of the invention can be combined as desired by the person skilled in the art without deviating from the subject matter of the invention as defined by the independent claims.

• 1 ein erfindungsgemäßes Bearbeitungssystem mit einer Messvorrichtung in einer schematischen Darstellung;
• 2 eine weitere Ausführungsform eines erfindungsgemäßen Bearbeitungssystem mit einer Messvorrichtung in einer schematischen Darstellung;
• 3 eine schematische Darstellung einer ersten Weglängeneinstellvorrichtung, die erfindungsgemäß verwendet werden kann; und
• 4 eine schematische Darstellung einer zweiten Weglängeneinstellvorrichtung, die erfindungsgemäß verwendet werden kann.

Preferred embodiments of the invention are explained in more detail below with reference to the accompanying schematic drawings. They represent:

• 1 a processing system according to the invention with a measuring device in a schematic representation;
• 2 a further embodiment of a processing system according to the invention with a measuring device in a schematic representation;
• 3 a schematic representation of a first path length adjustment device that can be used according to the invention; and
• 4th a schematic representation of a second path length adjustment device that can be used according to the invention.

Several aspects of the invention are presented below. Different approaches for the common optical guidance of the reference beam and the measuring beam in a common optical fiber are presented. Different path length adjustment devices are also described. The person skilled in the art will also combine the described embodiments differently than shown, provided this appears expedient. In the exemplary embodiments, measuring devices are shown that are used in laser processing systems. The measuring devices shown are, however, also suitable for use in other areas, so that the configuration as part of a laser processing system is to be understood as an example.

1 Figure 3 is a schematic representation of a processing system according to the invention 12 . The editing system 12 is for machining a workpiece 14th by means of a high-energy machining beam 16 furnished. The editing system 12 includes a processing beam source for this purpose 48 . In the illustrated case, the machining beam source comprises 48 a laser for generating a laser beam as a machining beam 16 which can be used for laser welding, for example. The processing system also includes 12 a processing beam optics 50 , by means of which the machining beam 16 on the workpiece 14th can be directed. The processing beam optics 50 is in a machining head 54 of the processing system 12 arranged, which is only indicated schematically. The processing head 54 is arranged in the illustrated case on an industrial robot, by means of which the processing head 54 relative to the workpiece 14th is movable. By moving the processing head 54 and suitable setting of the processing beam optics 50 can thus be a target area of the workpiece 14th to be edited.

The editing system 12 comprises a measuring device 10 who have favourited an optical coherence tomograph 18th having. The optical coherence tomograph 18th comprises a beam generating unit 20th with a light source 56 for generating a measuring beam 22nd and a reference beam 24 . The light source 56 is in a fixed part 52 of the optical coherence tomograph 18th arranged, which can also be referred to as an OCT sensor. The fixed part 52 of the optical coherence tomograph 18th further comprises a spectrometer 58 for evaluating received measurement signals in a known manner, which is achieved by superimposing the measurement beam 22nd and reference beam 24 be generated. The stationary part also includes 52 a circulator 60 to which the light source 56 and the spectrometer 58 are connected.

The beam generation unit 20th further comprises a beam splitter 62 passing through a source optical fiber 64 to the circulator 60 connected and thus with the light source 56 connected is. The light source 56 creates a source beam from which the beam splitter 62 the measuring beam 22nd and the reference beam 24 generated. The source beam fiber also leads 64 the measuring beam 22nd and the reference beam 24 from the beam splitter 62 to the circulator 60 and thus to the spectrometer 58 back.

The optical coherence tomograph also includes 18th a measuring arm 26th in which the measuring beam 22nd can be optically guided in such a way that it enters the machining beam 16 can be coupled in, so that the measuring beam 22nd via the processing beam optics 50 on the workpiece 14th is projectable. In the measuring arm 26th is a movable deflector 66 arranged, by means of which the measuring beam 22nd can be deflected in such a way that it is within a close range of an impact point of the machining beam 16 on the workpiece 14th can be guided, whereby the measuring beam 22nd for example in a keyhole that occurs during processing and / or directly on the point of impact of the processing beam 16 on the workpiece 14th can be directed. Measurements in front of, next to or behind the respective processing area are also possible by shifting the point of impact of the Measuring beam 22nd possible. The measuring beam 22nd is on the workpiece during operation 14th reflects and thus reaches the measuring arm 26th back to the stationary part 52 of the optical coherence tomograph 18th . The workpiece 14th forms a measurement object in the case shown 28 .

The optical coherence tomograph 18th also includes a reference arm 30th in which the reference beam 24 is optically feasible and for monitoring the processing system 12 by means of the measuring beam 22nd the measuring arm 26th at least simulates its optical path length.

It also includes the optical coherence tomograph 18th a common optical fiber 32 , in which both the measuring arm 26th as well as the reference arm 30th run in sections. When operating the optical coherence tomograph 18th become both the measuring beam 22nd as well as the reference beam 24 in the common optical fiber 32 optically guided. The common optical fiber 32 leads from the stationary part 52 to the machining head 54 . Thus, both the measuring arm 26th as well as the reference arm 30th via a single fiber to the processing head 54 guided so that measuring arm 26th and reference arm 30th at least in the area of the common optical fiber 32 covering most of the measuring arm 26th and the reference arm 30th trains, are exposed to identical environmental influences.

In the following, polarizing beam combiners (PBC) and polarizing beam splitters (PBS) are referred to frequently. Corresponding terms usually relate to an optical path from the beam generation unit 20th goes out. The person skilled in the art will understand that a component designated as a beam combiner or beam splitter for a specific optical direction can assume the function of a beam splitter or beam combiner in the reverse optical direction. Polarization directions are also in 1 illustrated by small arrows.

The optical coherence tomograph 18th includes a polarizing beam combiner 34 connected via optical fibers to the beam splitter 62 is connected and thus the measuring beam 22nd and the reference beam 24 combined. A combining output of the polarizing beam combiner 34 is with a first end of the common optical fiber 32 connected. In the case shown, one is from the light source 56 generated source beam linearly polarized. Hence the measuring beam 22nd and the reference beam 24 after passing through the beam splitter 62 polarized parallel to each other. Because the polarizing beam combiner 34 the direction of polarization rotates at an input, the measuring beam 22nd and the reference beam 24 coupled with different polarization in the common optical fiber. In the exemplary embodiment shown, the directions of polarization are rotated by 90 ° to one another. The measuring beam 22nd and the reference beam 24 therefore interfere in the common optical fiber within the optical path from the stationary part 52 to the machining head 54 not in an undesirable way.

The optical coherence tomograph also includes 18th a polarizing beam splitter 38 connected to a second end of the common optical fiber 32 connected. The second end of the common optical fiber 32 is from the beam generating unit 20th viewed from a distal end, whereas the first end is a proximal end. The polarizing beam splitter 38 in turn rotates the polarization of one of the rays 22nd , 24 , in the illustrated case of the reference beam 24 so that the measuring beam 22nd and the reference beam 24 the polarizing beam splitter 38 now polarized parallel to each other but left on separate paths.

Starting from a first output of the polarizing beam splitter 38 guides a section of the reference arm 30th to a reflective element 36 . This sends the reference beam 24 after passing through the common optical fiber 32 again through the common optical fiber 32 back. The polarizing beam splitter 38 in turn rotates the polarization of the returning reference beam so that its polarization in the common optical fiber is identical for both directions.

Starting from a second output of the polarizing beam splitter 38 guides a section of the measuring arm 26th in a known manner to the deflector 66 and about the processing beam optics 50 to the measurement object 28 . From there the measuring beam 22nd reflected and through the polarizing beam splitter 38 , which works in the opposite direction as a polarizing beam combiner, again with the reference beam 24 together in the common optical fiber 32 coupled, with the polarization of the measuring beam as mentioned 22nd and the reference beam 24 also when passing through the common optical fiber 32 are rotated in the opposite direction by 90 ° to each other.

The polarizing beam combiner 34 takes over for the returning rays 22nd , 24 then the function of a polarizing beam splitter so that the measuring beam 22nd and the reference beam 24 the polarizing beam combiner 34 on the way back to the beam splitter 62 and to the circulator 60 polarized again in parallel, but left on separate paths, whereby the relevant beam components only enter the spectrometer 38 can be brought to interference, whereas this cannot already in the common optical fiber 32 interfere in an undesirable manner.

As in 1 can also be seen, comprises the optical coherence tomograph 18th a path length adjuster 40 with an adjusting element that can be displaced between different setting positions, the displacement of which changes the optical path length of the reference arm 30th causes. Becomes an optical path length of the measuring arm 26th by moving the machining head 54 and / or by shifting the point of impact of the measuring beam 24 on the measurement object 28 changed, can by means of the path length adjustment device 40 the optical path length of the reference arm 30th adjusted accordingly. In the case shown, the path length adjustment device comprises 40 at least two opposing lenses, movable relative to one another, by means of which the reference beam 24 from optical fibers or into these can be coupled out from the beam splitter 62 to the path length adjuster 40 or from the path length adjustment device 40 to the polarizing beam combiner 34 to lead. The adjustment element 42 comprises one of the opposing lenses and a fiber end arranged at the focal point of the lens. The path length can be adjusted by moving the setting element, as the measuring beam 24 is guided collimated between the lenses. The movability of the adjustment element 42 is in 1 indicated by a large double arrow.

2 shows a schematic representation of a further embodiment of a processing system 12 ' with a measuring device 10 ' according to the present invention. The reference symbols of this further embodiment are provided with inverted commas. Reference symbols that already appear in the previous embodiment without apostrophes and with apostrophes in 2 are also present denote the components of the processing system already described in connection with the previous exemplary embodiment 12 ' . With regard to their description, reference is therefore made to the previous exemplary embodiment. In the following, however, it is predominantly the components configured differently that are described.

The measuring device 10 ' includes an optical coherence tomograph 18 ' , which also has a common optical fiber 32 ' disposes of a fixed part 52 ' of the optical coherence tomograph 18 ' to a machining head 54 ' leads. One from a beam generating unit 20 ' generated measuring beam 22 ' and reference beam 24 ' are common optical in the common optical fiber 32 ' guided so that a measuring arm 26 ' and a reference arm 30 ' of the optical coherence tomograph 18 ' in the area of the common optical fiber 32 ' are exposed to the same environmental influences.

About an undesired interaction between beam components of the measuring beam 22 ' and the reference beam 24 ' to avoid going into the spectrometer 58 ' are to be brought to interference, the further embodiment provides that an optical path of the reference arm 30 ' and an optical path of the measuring arm 26 ' from the beam generating unit 20 ' to the common optical fiber 32 ' differ by an amount that is greater than the working range of the coherence tomograph 18 ' is. In addition, according to the further embodiment, polarizing beam splitters or polarizing beam combiners can be dispensed with.

According to the further embodiment, a proximal end is the common optical fiber 32 ' directly to a first output of a beam splitter 62 ' of the optical coherence tomograph 18 ' connected. In the case shown, it is an output of the beam splitter 62 ' , which emits a larger intensity component of a corresponding source beam. At a distal end of the common optical fiber 32 ' is a partially reflective element 36 ' arranged. In the case shown, this is a straight-cut fiber end that is vaporized with a suitable material such as silver in order to adapt the reflection properties in the desired manner. On the partially reflective element 36 ' becomes part of the common optical fiber 32 ' guided beam is reflected and used as a reference beam 24 ' to the beam splitter 62 ' sent back. One through the partially reflective element 36 ' The transmitted portion of the beam is called the measuring beam 22 ' via a deflector 66 ' to a processing beam optics 50 ' and on the target 28 ' guided.

The one from the partially reflective element 36 ' returning reference beam 24 ' is over the proximal end of the common optical fiber 32 ' into the beam splitter 62 ' returned and runs from an opposite exit of the beam splitter 62 ' to a path length adjustment device 40 ' . This includes an adjustment element that can be displaced between different adjustment positions 42 ' that has a mirror next to a lens 46 ' which is at the focal point of the lens. By moving this adjustment element 42 ' can be the optical path length of the reference arm thus formed 30 ' to a variable optical path length of the measuring arm 26 ' be adjusted. The movability of the adjustment element 42 ' is in 2 indicated by a large double arrow. By having the adjustment element 42 ' over a mirror 46 ' has, according to the further embodiment, it is not necessary to move a fiber end when a path length by means of the path length setting device 40 ' set shall be. On the other hand, a fiber end from which the path length adjustment device 40 ' is to be decoupled, installed in a stationary and thus mechanically reliable manner.

From the mirror 46 ' the path length adjustment device 40 ' becomes the reference beam 24 ' to the beam splitter 62 ' returned and emerges from one of the common optical fibers 32 ' adjacent connection of the beam splitter 62 ' from that to another mirror 68 ' leads. From there the reference beam 24 ' again via the beam splitter 62 ' to a circulator 60 ' the beam generation unit 20 ' returned.

Thus, an offset between corresponding beam components is generated by the fact that the reference beam 24 ' only up to the distal end of the common optical fiber 32 ' runs, whereas the measuring beam 22 ' from the partially reflective element 36 ' further to the measurement object 28 ' and is led back from there. The corresponding to the reference arm 30 ' The optical path section missing at this point is determined by the path length adjustment device 40 ' and a portion of the beam splitter 62 ' to the further mirror 68 ' leads.

In a modification of this embodiment, instead of the partially reflective element 36 ' at the end of the common optical fiber 32 ' a partially transparent mirror in the deflector 66 ' be provided, behind which an additional mirror is arranged so that the reference beam 64 ' reflected at this additional mirror and from there into the common optical fiber 32 ' is returned.

The 3 and 4th show alternative path length adjusting devices 140 , 240 . These can be in the measuring device 10 according to the first embodiment as well as in the measuring device 10 ' according to the second embodiment as an alternative to the path length adjustment devices described there 40 , 40 ' be used. In addition, all of the path length adjustment devices described herein 40 , 40 ' , 140 , 240 be installed either in a reference arm or in a measuring arm. In the latter case, an additional fiber section can be provided in the reference arm which adjusts the lengths, for example, in such a way that the measuring arm and reference arm for a mean setting position of the corresponding setting elements 42 , 42 ' , 142 , 242 are coordinated. Furthermore, both the reference arm and the measuring arm can each be provided with a path length adjustment device.

3 shows a schematic representation of a first alternative path length setting device 140 . This includes a polarizing beam combiner 170 , its inputs with a feeding fiber 172 and a laxative fiber 174 are connected. These can for example be part of a reference arm and the first alternative path length adjustment device 140 include in this.

One from the combining output of the polarizing beam combiner 170 exiting beam is widened and by means of a first lens 176 collimated. The collimated beam then passes through a retardation plate 144 the path length adjustment device 140 through. In the case shown, this is a λ / 4 plate.

Then the collimated beam becomes a second lens 178 that led him to a mirror 146 focused. A distance between the second lens 178 and the mirror 146 is constant. The two optical elements together form an adjustment element 142 from, which is displaceable between different setting positions, whereby a path length setting is made possible. The movability of the adjustment element 142 is in 3 indicated by a large double arrow.

From the setting element, the beam travels back to the polarizing beam combiner 170 and leaves this to the laxative fiber 174 down.

Is the one for the path length adjuster 140 guided beam, for example a reference beam, linearly polarized, this strikes linearly polarized on the retardation plate 144 so that it emerges from it circularly polarized. At the mirror 146 the polarization is then reversed in a known manner so that the reflected light is circularly polarized in the opposite direction. If this light re-emerges through the retardation plate when going back 144 through, linearly polarized light is again generated, the polarization direction of which is perpendicular to the polarization direction of the incoming beam. Since the polarizing beam combiner rotates the polarization direction of the beam once by 90 ° in the described passages and thus rotates through the retardation plate 144 and mirror 146 If the change in polarization caused is compensated, the polarization corresponds to that of the feeding fiber 172 supplied light corresponds to the polarization of the outgoing fiber 174 discharged light, so that the polarization of the light by the path length adjustment device 140 as a whole is not changed. Directions of polarization are in 3 illustrated by small arrows.

The first alternative path length adjuster 140 thus also allows an adjustment element 142 with a mirror 146 to use so that it is not necessary to provide a moving fiber end. In addition, a large adjustment path can be achieved despite the compact design.

4th shows a schematic representation of a second alternative path length adjustment device 240 . This is also on a feeding fiber 272 and a laxative fiber 274 tied up. An incoming beam is made from the incoming fiber 272 coupled out and by means of a first coupling lens 280 collimated. The collimated beam thus generated is passed through a main lens 282 on a mirror 246 the second alternative path length adjustment device 240 focused. From there the beam goes through the main lens 280 guided and collimated hits a second coupling lens 284 , by means of which the in the laxative fiber 274 is coupled.

The beam occurs in each case to a central axis of the main lens 282 offset through this. The main lens 282 can therefore be used as a single lens both to focus the incoming beam onto the mirror 246 as well as to collimate the reflected beam. Similar to the case of the first alternative path length adjuster 140 is a distance between the main lens 282 and the mirror 246 fixed and the two are used together as an adjustment element 242 as with the double arrow in 4th indicated shifted between different adjustment positions in order to achieve a path length adjustment.

In addition to the path length adjustment devices described 40 , 40 ' , 140 , 240 Path length adjustment devices can also be used which have at least one length-adjustable fiber that is, for example, specifically stretched in order to increase an optical path length.

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• DE 102013008269 A1 [0005]
• DE 102015015112 A1 [0005]
• DE 102017001353 A1 [0007]
• DE 102014216829 A1 [0008]

Claims (14)

Measuring device, in particular for a machining system (12, 12 ') for machining a workpiece (14, 14') by means of a high-energy machining beam (16, 16 '), the measuring device (10, 10') being an optical coherence tomograph (18, 18 ') ), which includes: - A beam generating unit (20, 20 ') for generating a measuring beam (22, 22') and a reference beam (24, 24 '); - A measuring arm (26, 26 ') extending from the beam generating unit (20, 20') and in which the measuring beam (22, 22 ') can be guided optically so that it can be projected onto a measuring object (28, 28'); and - A reference arm (30, 30 ') extending from the beam generating unit (20, 20'), in which the reference beam (24, 24 ') can be guided optically and in which the measuring arm (26, 26') at least in its optical path length simulates so that the measuring beam (22, 22 ') and the reference beam (24, 24') can be superimposed after passing through the measuring arm (26, 26 ') or the reference arm (30, 30') to generate an interference signal; wherein the measuring arm (26, 26 ') and the reference arm (30, 30') run at least in sections in a common optical fiber (32, 32 '). Measuring device according to Claim 1 , wherein the optical coherence tomograph (18) comprises a polarizing beam combiner (34) which is arranged such that the measuring beam (22) and the reference beam (24) are coupled into the common fiber (32) with different polarization. Measuring device according to Claim 2 , wherein the polarizing beam combiner (24) is set up such that the polarization of the measuring beam (22) and the polarization of the reference beam (24) in the common optical fiber (32) are rotated by 90 ° to one another. Measuring device according to one of the preceding claims, wherein an optical path of the reference arm (30 ') and an optical path of the measuring arm (26') from the beam generating unit (20 ') to the common optical fiber (32') differ by an amount that is larger than the working range of the coherence tomograph (18 '). Measuring device according to one of the preceding claims, wherein the reference arm (30, 30 ') comprises an at least partially reflective element (36, 36') which the reference beam (24, 24 ') after passing through the common optical fiber (32, 32') again back through the common optical fiber (32,32 '). Measuring device according to Claim 5 wherein a polarizing beam splitter (38) is arranged between one end (36) of the common fiber (32) and the at least partially reflective element (36). Measuring device according to Claim 5 wherein the at least partially reflective element (36 ') is arranged directly at one end (36') of the common fiber (32 '). Measuring device according to Claim 7 wherein the at least partially reflective element (36 ') is formed by a straight cut of the end (36') of the common fiber (32 ') and / or by a coating of the end (36') of the common fiber (32 '). Measuring device according to one of the preceding claims, wherein the optical coherence tomograph (18, 18 ') comprises a path length adjustment device (40, 40', 140, 240) with an adjustment element (42, 42 ', 142, 242) that can be displaced between different setting positions, the displacement of which causes a change in the optical path length of the reference arm (30, 30 ') and / or of the measuring arm (26, 26'). Measuring device according to Claim 9 , wherein the path length setting device (140) comprises a retardation plate (144), in particular a λ / 4 plate. Measuring device according to Claim 9 or 10 wherein the adjustment element (40 ', 140, 240) comprises a mirror (46', 146, 246). Processing system for processing a workpiece (14, 14 ') by means of a high-energy processing beam (16, 16'), comprising a processing beam source (48, 48 '), processing beam optics (50, 50') and a measuring device (10, 10 ') One of the preceding claims, wherein the measuring beam (22, 22 ') can be optically guided in the measuring arm (26, 26') in such a way that the measuring beam (22, 22 ') can be coupled into the processing beam (16, 16') and via the Processing beam optics (50, 50 ') can be projected onto the workpiece (14, 14'). Processing system according to Claim 12 comprising a processing head (52, 52 '), wherein the common optical fiber (32, 32') leads from a stationary part (54, 54 ') of the measuring device (10, 10') to the processing head (50, 50 ') . Method for performing optical coherence tomography with a coherence tomograph (18, 18 '), in particular for monitoring the processing of a workpiece (14, 14') by means of a high-energy processing beam (16, 16 '), comprising the steps: Generating a measuring beam (22, 22 ') and a reference beam (24, 24') by means of a beam generating unit (20, 20 ') of the optical coherence tomograph (18, 18'); - Optical guidance of the measuring beam (22, 22 ') in a measuring arm (26, 26') such that the measuring beam (22, 22 ') is projected onto a measuring object (28, 28'); and - optically guiding the reference beam (24, 24 ') in a reference arm (30, 30') which simulates the measuring arm (26, 26 ') at least in its optical path length, so that the measuring beam (22, 22') and the reference beam (24, 24 ') can be superimposed after passing through the measuring arm (26, 26') or the reference arm (24, 24 ') to generate an interference signal; wherein the measuring beam (22, 22 ') and the reference beam (24, 24') are optically guided at least in sections in a common optical fiber (32, 32 ').

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