DE102019002942B4 - Measuring device and method for performing optical coherence tomography with a coherence tomograph - Google Patents

Abstract

Measuring device, in particular for a processing system (12) for processing a workpiece (14) by means of a high-energy processing beam (16), the measuring device (10) having an optical coherence tomograph (18) which comprises: - a beam generating unit (20) for generating a measuring beam (22) and a reference beam (24);- a measuring arm (26) extending from the beam generating unit (20) and in which the measuring beam (22) can be guided optically, so that it can be projected onto a measurement object (28); and- a reference arm (30) extending from the beam generation unit (20), in which the reference beam (24) can be guided optically and which simulates the measuring arm (26) at least in its optical path length, so that the measuring beam (22) and the reference beam (24) can be superimposed after passing through the measuring arm (26) or the reference arm (30) to generate an interference signal; the measuring arm (26) and the reference arm (30) running at least in sections in a common optical fiber (32), wherein the optical coherence tomograph (18) comprises a polarizing beam combiner (34), which is arranged in such a way that the measuring beam (22) and the reference beam (24) are coupled into the common optical fiber (32) with different polarization, so that the measuring beam (22 ) and the reference beam (24) are guided in the common optical fiber (32) with different polarization in such a way that their interaction within the common optical fiber (32) is reduced.

Description

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

Optical coherence tomographs usually include a beam generation unit for generating a measurement beam and a reference beam. Furthermore, optical coherence tomographs of this type have a measuring arm, which extends from the beam generation unit and in which the measuring beam can be optically guided so that it can be projected onto a measurement object, and a reference arm that extends from the beam generation unit, in which the reference beam can be optically guided and 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.

In addition, devices for processing a workpiece by means of a high-energy processing beam are known from the prior art, which are used, for example, in laser material processing processes. A high-energy laser processing beam is used here, for example, to act on one or more workpieces or workpiece parts, for example to weld them together, make laser holes, process surfaces, etc. The device can include a movable processing head that is carried, for example, by an industrial robot and can be movable relative to a workpiece to be machined. Devices of this type 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 measurement 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 using cameras or the like in order to obtain height information during the machining process, which relates, for example, to a surface of the workpiece or a depth of a keyhole produced 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 processing beam optics and the processed workpiece changes. A dispersion adjustment can also be useful in order to take account of dispersion differences between the reference arm and the measuring arm. With regard to such adjustments to the reference arm, reference is made, for example, to the documents DE 10 2013 008 269 A1 and DE 10 2015 015 112 A1 referred.

In order to achieve a high degree of accuracy in the measurements carried out with the optical coherence tomograph, it is basically expedient to match the optical properties of the reference arm and the measuring arm as precisely as possible, since otherwise undesirable transit time differences and/or dispersion differences between the measuring arm and the reference arm can occur. However, such an optical adjustment represents a major 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 length-adjustable reference arm 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 perform temperature compensation. However, the corresponding balancing procedure is complex. In addition, despite the additional temperature measurement, an adjustment can only be made approximately.

A different approach, like him DE 10 2014 216 829 A1 describes, provides for laying 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 effect on both fibers. If the fibers are moved here, for example due to the displacement of a processing head to which the fibers lead, however, it cannot be ensured that the fibers are actually subjected to identical mechanical loads sets are. However, even slight differences between the forces acting on the fibers, which lead to bending, compression, stretching or torsion of the fibers, can have a significant impact on their optical properties. This can cause deviations between the reference arm and measuring arm, even though they are guided in close contact. According to one variant, in DE 10 2014 216 829 A1 also proposed to use a double clad fiber.

DE 103 51 319 A1 discloses an interferometer for optical coherence tomography used in an endoscope and comprising a fiber bundle in which the measuring beam and the reference beam run together. To form a reference plane, a glass plate is placed close to a sample, which provides a reflection and thus defines a reference arm.

Out of U.S. 2008/0123092 A1 discloses an optical coherence tomograph comprising a 2x2 fiber coupler used as a beam splitter and beam combiner. Starting from this fiber coupler, an optical fiber runs to a measuring device, through which both measuring light and reference light are guided to the measuring device.

Also revealed WO 02/084263 A1 an interferometric arrangement for determining a transit time difference. Starting from a beam splitter, the measurement light and reference light run through a fiber to a sample, with a partially transparent mirror being arranged just in front of a focal point of the measurement light, which mirror reflects the reference light.

Proceeding from the prior art described above, the object of the present invention is therefore to coordinate the optical properties of the measuring arm and reference arm of an optical coherence tomograph in a simple and reliable manner and, in particular, to reduce the influence of external conditions on measurement results.

This object is achieved according to the invention by a measuring device having the features of claim 1 and a measuring device having the features of claim 3. Further developments of the invention can be found in the dependent claims.

The invention is based on a measuring device of the type described above. 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 Referenzstrahl nach Durchlaufen des Messarms bzw. des Referenzarms zur Erzeugung eines Interferenzsignals überlagert werden können.

The invention also relates to a method for performing optical coherence tomography with a coherence tomograph, in particular for monitoring processing of a workpiece using a high-energy processing beam. The optical coherence tomograph is 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 guiding of the measuring beam in a measuring arm in such a way that the measuring beam is projected onto a measurement object; and
• - Optical guiding 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 be 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 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 have an equal effect on the reference arm and the measuring arm. Furthermore, an optical coherence tomograph equipped with a single optical fiber for constituting the reference arm and the measurement arm can be provided, 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 measurement 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 processing head of the said processing system.

The beam generation unit can include 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 and the measurement beam in a suitable ratio. Due to the often incomplete reflection of the measurement beam on the measurement object, it is expedient here for a larger part of the source beam to fall on the measurement beam. In this case, the light path, via which the source beam can be guided to the beam splitter, is neither to be added to the reference arm nor to 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 fixed part can also include 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, which are arranged in a row, for example, in such a way 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 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 bent, stretched, compressed and/or twisted as a result of the movement of the processing head, with this movement of the optical fiber affecting 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 subjected to mechanical stress. The same applies analogously to other external influences, such as 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 a joint optical guidance of the reference beam and the measurement beam in the common optical fiber can be provided in particular when the optical coherence tomograph comprises a polarizing beam combiner, which is arranged such that the measurement beam and the reference beam with different Polarization are coupled into the common fiber. The polarizing beam combiner may be located at a proximal end of the common optical fiber as viewed from the beam generating unit. In particular, the reference beam and/or the measurement beam leave the beam splitter linearly polarized. The polarizing beam combiner can be designed in such a way that it changes the direction of polarization of the measurement beam and/or the reference beam. Furthermore, the polarizing beam combiner can perform the functions of a polarizing beam splitter when the reference beam and measurement beam return 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 guiding in the common optical fiber does not lead to erroneous measurement results.

In a 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 are rotated through 90° to one another in the common optical fiber. In particular, the measuring beam or the reference beam can be polarized perpendicularly or parallel or parallel or perpendicularly 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 comprehensible manner.

Unintentional interference between the measuring 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 measuring arm from the beam generating unit to the common fiber by an amount differ, which is larger than the working range of the coherence tomograph. The optical path difference may differ by an amount greater than the coherence length of the reference beam and the measurement beam. The measuring beam and the reference beam then run with a path difference in the common optical fiber, which brings about that the beam components later brought to interference are not already superimposed 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 reflecting element which, after passing through the common optical fiber, sends the reference beam back through the common optical fiber. The reference beam can thus traverse the common optical fiber twice. As a result, the optical properties of the reference arm and measuring arm can be further adjusted 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 reflecting element. The end can be the end of the common fiber that is distal as viewed from the beam generation unit. A first output of the polarizing beam splitter can be set up to decouple 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 set up to decouple the reference beam in the direction of the at least partially reflecting 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 reflecting element and the measurement beam reflected on the measurement object, so that the measurement beam and reference beam run back through the common fiber with different polarization. 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 direction and can therefore be completely separated again.

In an alternative embodiment, the at least partially reflecting element can be arranged directly at one end of the common fiber, as a result of which 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 reflecting element is preferably used here, which reflects only up to 5%, up to 10%, up to 15% or up to 20%, based on an intensity, for the reference beam.

The at least partially reflecting element can be produced in a particularly simple manner in that it is formed by grinding the end of the common fiber straight. The optical properties of the at least partially reflecting element can also be adjusted precisely and application-specifically if the at least partially reflecting element is formed alternatively or additionally by coating the end of the common fiber. The coating can include 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 also dielectrics can be used, for example to create Bragg mirrors.

In a development of the invention, the optical coherence tomograph includes a path length adjustment device with an adjustment element that can be displaced between different setting positions, the displacement of which 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 adjustment element comprises a mirror. In addition, the adjustment element can comprise a converging lens, in the focal point of which the mirror is arranged. The mirror and 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 high reliability of 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 bring about a change in path length of twice the magnitude. In addition, it can be provided according to the invention that a beam running towards 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 delay plate, in particular a λ/4 plate. In particular in combination with an adjustment element comprising a mirror, a reliable optical structure of the path length adjustment device can be achieved in this way, 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 adjustment device may comprise a polarizing beam combiner, behind the combining end of which the retardation plate and preferably also the mirror are arranged. As a result, a moving fiber end can be dispensed with, while at the same time the polarization of the beam that passes through the path length adjustment device is advantageously influenced in such a way when passing through the path length adjustment device that the polarization present at the entry into the path length adjustment device corresponds to the polarization at the exit.

The invention also relates to a processing system for processing a workpiece using a high-energy processing beam, which comprises a processing beam source, processing beam optics and a measuring device according to the invention, the measuring beam being optically guidable 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 can be projected.
The processing system may 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 or otherwise displaceable. Forces acting on the common optical fiber generated as a result of a corresponding movement therefore have the same effect on the reference arm and the measuring arm in the same way as all other environmental influences, since these both pass through the common 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 a 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 machining 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
• 4 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 to the joint optical guidance of the reference beam and measuring beam in a joint optical fiber are presented here. In addition, different path length adjustment devices are described. The person skilled in the art will also combine the described embodiments in a different way than shown, if this appears to be appropriate. The exemplary embodiments show measuring devices that are used in laser processing systems. However, the measuring devices shown are also suitable for use in other areas, so that the design as part of a laser processing system is to be understood as an example.

1 12 is a schematic representation of a processing system 12 according to the invention. The processing system 12 is set up for processing a workpiece 14 using a high-energy processing beam 16 . For this purpose, the processing system 12 includes a processing beam source 48. In the case shown, the processing beam source 48 includes a laser for generating a laser beam as processing beam 16, which can be used, for example, for laser welding. Furthermore, the processing system 12 includes a processing beam optics 50 by means of which the processing beam 16 can be directed onto the workpiece 14 . The processing beam optics 50 are arranged in a processing head 54 of the processing system 12, which is indicated only schematically. In the case shown, the machining head 54 is arranged on an industrial robot, by means of which the machining head 54 can be moved relative to the workpiece 14 . A target area of the workpiece 14 can thus be processed by moving the processing head 54 and suitably adjusting the processing beam optics 50 .

The processing system 12 includes a measuring device 10 which has an optical coherence tomograph 18 . The optical coherence tomograph 18 includes a beam generation unit 20 with a light source 56 for generating a measuring beam 22 and a reference beam 24. The light source 56 is arranged in a stationary part 52 of the optical coherence tomograph 18, which can also be referred to as an OCT sensor. The stationary part 52 of the optical coherence tomograph 18 also includes a spectrometer 58 for evaluating measurement signals obtained in a known manner, which are generated by superimposing the measurement beam 22 and the reference beam 24 . Further the stationary part 52 comprises a circulator 60 to which the light source 56 and the spectrometer 58 are connected.

The beam generation unit 20 also includes a beam splitter 62 which is connected to the circulator 60 via an optical source beam fiber 64 and is thus connected to the light source 56 . The light source 56 generates a source beam from which the beam splitter 62 generates the measurement beam 22 and the reference beam 24 . Furthermore, the source beam fiber 64 returns the measurement beam 22 and the reference beam 24 from the beam splitter 62 to the circulator 60 and thus to the spectrometer 58 .

Furthermore, the optical coherence tomograph 18 includes a measuring arm 26 in which the measuring beam 22 can be guided optically in such a way that it can be coupled into the processing beam 16 so that the measuring beam 22 can be projected onto the workpiece 14 via the processing beam optics 50 . A movable deflection device 66 is arranged in the measuring arm 26, by means of which the measuring beam 22 can be deflected in such a way that it can be guided within a close range of a point of impingement of the machining beam 16 on the workpiece 14, as a result of which the measuring beam 22, for example, into a keyhole and/or or can be directed directly to the point of impact of the machining beam 16 on the workpiece 14. 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 22 . During operation, the measuring beam 22 is reflected on the workpiece 14 and thus returns via the measuring arm 26 to the stationary part 52 of the optical coherence tomograph 18. In the case shown, the workpiece 14 forms a measurement object 28.

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

In addition, the optical coherence tomograph 18 includes a common optical fiber 32 in which both the measuring arm 26 and the reference arm 30 run in sections. When the optical coherence tomograph 18 is in operation, both the measuring beam 22 and the reference beam 24 are guided optically in the shared optical fiber 32 . The common optical fiber 32 leads from the stationary part 52 to the processing head 54. Thus, both the measuring arm 26 and the reference arm 30 are guided to the processing head 54 via a single fiber, so that the measuring arm 26 and reference arm 30 are at least in the area of the common optical fiber 32, which forms a large part of the measuring arm 26 and the reference arm 30, are exposed to identical environmental influences.

In the following, reference is often made to polarizing beam combiners (PBC) and polarizing beam splitters (PBS). Corresponding terms usually refer to an optical path that starts from the beam generation unit 20 . Those skilled in the art will understand that a component referred to 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 opposite optical direction. Directions of polarization are also in 1 illustrated by small arrows.

The optical coherence tomograph 18 comprises a polarizing beam combiner 34 which is connected to the beam splitter 62 via optical fibers and thus combines the measurement beam 22 and the reference beam 24 . A combining output of polarizing beam combiner 34 is connected to a first end of common optical fiber 32 . In the illustrated case, a source beam generated by the light source 56 is linearly polarized. Therefore, the measuring beam 22 and the reference beam 24 are polarized parallel to one another after passing through the beam splitter 62 . Since the polarizing beam combiner 34 rotates the direction of polarization at an input, the measurement beam 22 and the reference beam 24 are coupled into the common optical fiber with different polarization. In the exemplary embodiment shown, the directions of polarization are rotated by 90° with respect to one another. The measurement beam 22 and the reference beam 24 therefore do not undesirably interfere in the common optical fiber within the optical path from the stationary part 52 to the processing head 54 .

Furthermore, the optical coherence tomograph 18 comprises a polarizing beam splitter 38 which is connected to a second end of the common optical fiber 32 . The second end of the common optical fiber 32 is a distal end when viewed from the beam generating unit 20, whereas the first end is a proximal end. The polarizing beam splitter 38 in turn rotates the polarization of one of the beams 22, 24, in the illustrated case the reference beam 24, so that the measurement beam 22 and the reference beam 24 now exit the polarizing beam splitter 38 polarized parallel to one another but on separate paths.

Starting from a first output of the polarizing beam splitter 38, a section of the reference arm 30 leads to a reflective element 36. This sends the reference beam 24 after traversing the common optical fiber 32 back through the common optical fiber 32 again. In this case, 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 passage directions.

Starting from a second output of the polarizing beam splitter 38, a section of the measuring arm 26 leads in a known manner to the deflection device 66 and via the processing beam optics 50 to the measuring object 28. From there the measuring beam 22 is reflected and through the polarizing beam splitter 38, which travels in the opposite direction works as a polarizing beam combiner, is again coupled together with the reference beam 24 into the common optical fiber 32, with the polarization of the measuring beam 22 and the reference beam 24 also being rotated by 90° to one another in the opposite direction when passing through the common optical fiber 32, as mentioned.

The polarizing beam combiner 34 then assumes the function of a polarizing beam splitter for the returning beams 22, 24, so that the measuring beam 22 and the reference beam 24 again polarize the polarizing beam combiner 34 parallel on the way back to the beam splitter 62 and the circulator 60, but leave on separate paths , As a result of which the relevant beam components can only be brought to interference in the spectrometer 38, whereas they do not already interfere in the common optical fiber 32 in an undesired manner.

As in 1 It can also be seen that the optical coherence tomograph 18 includes a path length adjustment device 40 with an adjustment element 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 . If an optical path length of measuring arm 26 is changed by moving processing head 54 and/or by shifting the point of impingement of measuring beam 24 on measurement object 28, the optical path length of reference arm 30 can be adjusted accordingly using path length adjustment device 40. In the case shown, the path length adjustment device 40 comprises at least two opposing lenses that can be moved relative to one another, by means of which the reference beam 24 can be coupled out of or into optical fibers that are transmitted from the beam splitter 62 to the path length adjustment device 40 or from the path length adjustment device 40 to the polarizing Beam combiner 34 lead. The adjustment element 42 comprises one of the opposite lenses and a fiber end arranged at the focal point of the lens. The path length can be adjusted by moving the adjustment element, since the measuring beam 24 is guided in a collimated manner between the lenses. The displaceability 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 exemplary embodiment without inverted commas and with inverted commas in 2 are also present designate the components of the processing system 12' already described in connection with the previous exemplary embodiment. With regard to their description, reference is therefore made to the previous exemplary embodiment. In the following, on the other hand, the differently designed components are mainly described.

The measuring device 10' includes an optical coherence tomograph 18', which also has a common optical fiber 32', which leads from a stationary part 52' of the optical coherence tomograph 18' to a processing head 54'. A measuring beam 22′ and reference beam 24′ generated by a beam generation unit 20′ are guided optically together in the common optical fiber 32′, so that a measuring arm 26′ and a reference arm 30′ of the optical coherence tomograph 18′ are the same in the area of the common optical fiber 32′ are exposed to environmental influences.

In order to avoid an undesired interaction between beam components of the measuring beam 22' and the reference beam 24', which are only to be brought to interference in the spectrometer 58', the further embodiment provides that an optical path of the reference arm 30' and an optical path of the Measuring arm 26' differ from the beam generating unit 20' to the common optical fiber 32' by an amount which is greater than the working range of the coherence tomograph 18'. 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 of the common optical fiber 32' is connected directly to a first output of a beam splitter 62' of the optical coherence tomograph 18'. In the case shown, this is an output of the beam splitter 62', which outputs a larger intensity component of a corresponding source beam. A partially reflective element 36' is disposed at a distal end of the common optical fiber 32'. In the case shown This is a straightened fiber end that has been vapour-coated with a suitable material such as silver to adjust the reflection properties in the desired way. A portion of the beam guided through the common optical fiber 32' is reflected at the partially reflecting element 36' and sent back to the beam splitter 62' as a reference beam 24'. A part of the beam transmitted through the partially reflecting element 36' is guided as a measuring beam 22' via a deflection device 66' to a processing beam optics 50' and onto the measuring object 28'.

The reference beam 24' returning from the partially reflecting element 36' is fed back into the beam splitter 62' via the proximal end of the common optical fiber 32' and travels from an opposite output of the beam splitter 62' to a path length adjuster 40'. This comprises an adjustment element 42' which can be displaced between different adjustment positions and which, in addition to a lens, comprises a mirror 46' which is located at the focal point of the lens. By moving this adjustment element 42', the optical path length of the reference arm 30' thus formed can be adapted to a variable optical path length of the measuring arm 26'. The displaceability of the adjustment element 42' is in 2 indicated by a large double arrow. Because the adjustment element 42' has a mirror 46', according to the further embodiment it is not necessary to move a fiber end if a path length is to be adjusted by means of the path length adjustment device 40'. On the other hand, a fiber end from which the path length adjustment device 40′ is to be decoupled can be installed in a stationary and thus mechanically reliable manner.

The reference beam 24' is returned from the mirror 46' of the path length adjustment device 40' to the beam splitter 62' and emerges from a connection of the beam splitter 62' adjacent to the common optical fiber 32', which leads to a further mirror 68'. From there, the reference beam 24' is in turn fed back via the beam splitter 62' to a circulator 60' of the beam generation unit 20'.

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

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

The 3 and 4 show alternative path length setting devices 140, 240. These can be used both in the measuring device 10 according to the first embodiment and in the measuring device 10' according to the second embodiment as an alternative to the path length setting devices 40, 40' described there. In addition, all path length adjustment devices 40, 40', 140, 240 described herein can 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 are matched to one another for a middle setting position of the corresponding setting elements 42, 42', 142, 242. Furthermore, both the reference arm and the measuring arm can each be provided with a path length adjustment device.

3 14 shows a schematic representation of a first alternative path length adjustment device 140. This comprises a polarizing beam combiner 170, the inputs of which are connected to an input fiber 172 and an output fiber 174. FIG. These can be part of a reference arm, for example, and integrate the first alternative path length adjustment device 140 into it.

A beam emerging from the combining output of the polarizing beam combiner 170 is expanded and collimated by a first lens 176 . The collimated beam then passes through a retardation plate 144 of path length adjuster 140 . In the case shown, this is a λ/4 plate.

The collimated beam is then directed to a second lens 178 which focuses it onto a mirror 146 . A distance between the second lens 178 and the mirror 146 is constant. The two optical elements together form an adjustment element 142 which can be displaced between different adjustment positions, as a result of which path length adjustment is made possible. The displaceability of the adjustment element 142 is in 3 indicated by a large double arrow.

From the adjustment element, the beam travels back to the polarizing beam combiner 170 and exits to the outgoing fiber 174 .

If the beam guided to the path length adjustment device 140, for example a reference beam, is linearly polarized, it strikes the retardation plate 144 in a linearly polarized manner, so that it exits the latter in a circularly polarized manner. The polarization is then reversed in a known manner at the mirror 146, so that the reflected light is circularly polarized in the reverse direction. If this light passes through the retardation plate 144 again as it travels back, linearly polarized light is again generated, the direction of polarization of which is perpendicular to the direction of polarization of the incoming beam. Since the polarizing beam combiner rotates the direction of polarization of the beam by a total of 90° in the described passes and thus compensates for the polarization change caused by retardation plate 144 and mirror 146, the polarization of the light fed in through the incoming fiber 172 corresponds to the polarization of the light through the outgoing fiber 174 extracted light so that the polarization of the light is not changed by the path length adjuster 140 as a whole. Directions of polarization are in 3 illustrated by small arrows.

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

4 FIG. 12 shows a schematic representation of a second alternative path length adjustment device 240. This is also connected to an infeed fiber 272 and an outfeed fiber 274. FIG. An input beam is coupled out of the input fiber 272 and collimated by a first coupling lens 280 . The collimated beam thereby produced is focused by a main lens 282 onto a mirror 246 of the second alternative path length adjustment device 240 . From there, the beam is guided through the main lens 280 and is collimated when it hits a second coupling lens 284, by means of which it is coupled into the outgoing fiber 274.

In this case, the beam passes through the main lens 282 offset in each case to a central axis thereof. The main lens 282 can therefore be used as a single lens both to focus the incoming beam onto the mirror 246 and to collimate the reflected beam. Similar to the case of the first alternative path length adjuster 140, a distance between the main lens 282 and the mirror 246 is fixed and the two are collectively referred to as the adjuster 242 as indicated by the double-headed arrow in FIG 4 indicated shifted between different adjustment positions to achieve a path length adjustment.

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

Claims (13)

Measuring device, in particular for a processing system (12) for processing a workpiece (14) by means of a high-energy processing beam (16), the measuring device (10) having an optical coherence tomograph (18) which comprises: - A beam generating unit (20) for generating a measuring beam (22) and a reference beam (24); - A starting from the beam generating unit (20) extending measuring arm (26) in which the measuring beam (22) can be guided optically, so that it can be projected onto a measurement object (28); and - A reference arm (30) extending from the beam generating unit (20), in which the reference beam (24) can be guided optically and which simulates the measuring arm (26) at least in its optical path length, so that the measuring beam (22) and the reference beam ( 24) after passing through the measuring arm (26) or the reference arm (30) can be superimposed to generate an interference signal; the measuring arm (26) and the reference arm (30) running at least in sections in a common optical fiber (32), 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 optical fiber (32) with different polarization, so that the measuring beam ( 22) and the reference beam (24) are guided in the common optical fiber (32) with different polarization in such a way that their interaction within the common optical fiber (32) is reduced. measuring device claim 1 , wherein the polarizing beam combiner (24) is set up such that the polarization of the measurement beam (22) and the polarization of the reference beam (24) in the common optical fiber (32) are rotated by 90 ° to each other. Measuring device, in particular for a processing system (12') for processing a workpiece (14') by means of a high-energy processing beam (16'), the measuring device (10') having an optical coherence tomograph (18'), which comprises: - a beam generating unit ( 20') for generating a measurement beam (22') and a reference beam (24'); - a measuring arm (26') which extends from the beam generating unit (20') and in which the measuring beam (22') can be guided optically, so that it can be projected onto a measurement object (28'); and - a reference arm (30') extending from the beam generation unit (20'), in which the reference beam (24') can be guided optically and which simulates the measuring arm (26') at least in its optical path length, so that the measuring beam (22 ') and the reference beam (24') can be superimposed after passing through the measuring arm (26') or the reference arm (30') to generate an interference signal; the measuring arm (26') and the reference arm (30') running at least in sections in a common optical fiber (32'), with an optical path of the reference arm (30') and an optical path of the measuring arm (26') diverging from the Beam generation unit (20') differ from the common optical fiber (32') by an amount that is greater than the working range of the coherence tomograph (18'), so that the measuring beam (22') and the reference beam (24') with a path difference in of the common optical fibre, which ensures that beam components brought to interference later do not already interfere in an undesired manner in the common optical fibre. Measuring device according to one of the preceding claims, wherein the reference arm (30, 30') comprises an at least partially reflecting element (36, 36') which the reference beam (24, 24') after passing through the common optical fiber (32, 32') returns again through the common optical fiber (32, 32'). measuring device claim 4 , A polarizing beam splitter (38) being arranged between one end (36) of the common fiber (32) and the at least partially reflecting element (36). measuring device claim 4 , wherein the at least partially reflecting element (36') is arranged directly at one end (36') of the common fiber (32'). measuring device claim 6 , wherein the at least partially reflecting 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 adjustment positions, the displacement of which causes a change in the optical path length of the reference arm (30, 30') and/or the measuring arm (26, 26'). measuring device claim 8 , wherein the path length adjustment device (140) comprises a retardation plate (144), in particular a λ/4 plate. measuring device claim 8 or 9 , wherein the adjusting element (40', 140, 240) comprises a mirror (46', 146, 246). Processing system for processing a workpiece (14, 14') using 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 guided optically 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 claim 11 comprising a processing head (52, 52'), the common optical fiber (32, 32') leading 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') of a measuring device according to one of Claims 1 until 10 , In particular for monitoring a 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 generation unit (20, 20') of the optical coherence tomograph (18, 18'); - optically guiding the measuring beam (22, 22') in a measuring arm (26, 26') in such a way 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 follows 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; the measuring beam (22, 22') and the reference beam (24, 24') being optically guided at least in sections in a common optical fiber (32, 32').

Images