Method for optically characterizing a transparent or semitransparent object, and optical coherence tomography system

Abstract

The underlying invention relates in particular to a method for optically characterizing a transparent or semitransparent object (1) and to an optical coherence tomography system. According to the method, object dimensions (wall thickness, inside diameter, outside diameter), object distances and / or object positions are determined on the basis of optical coherence tomography (OCT), wherein, as a laser source (2), a swept-source laser source having a coherence length in the decimeter range to meter range is used, and in order to determine absolute distances, dimensions and / or positions a reference substrate (13) is used in the beam path (12) of the light source (2), the reference substrate comprising a partially reflecting optical unit which is stationary in the beam path (12).

Description

[0001] Method for the optical characterization of a transparent or semi-transparent object, and optical coherence tomography system >

[0002] Description

[0003] The underlying invention relates in particular to a method for the optical characterization of transparent or semi-transparent objects and an optical coherence tomography system.

[0004] For example, it is known in the prior art to examine transparent or translucent objects using optical coherence spectroscopy (OCT). For example, US Pat. No. 10,890,431 B2 discloses a VCSEL (vertical-cavity surface-emitting laser) OCT system for 3D measurement of transparent objects. In this known method, 3D depth profiles of an object are captured in scans using an optical coherence tomography system.

[0005] There is still room for improvement in the known systems and methods for optical coherence spectroscopy and optical characterization of transparent or translucent objects, particularly with regard to the type, size, structure of the objects to be measured and the boundary conditions of the measurement or geometric characterization of the objects.

[0006] Based on this, an improved or new method for optical coherence spectroscopy, in particular for the optical characterization of a transparent, semitransparent, or translucent object, is to be provided. Furthermore, a corresponding optical coherence tomography system is to be provided.

[0007] This problem is solved in particular by the combinations of features of the independent patent claims. Advantageous embodiments emerge in particular from the dependent claims and the following description.

[0008] According to embodiments, a method for the optical characterization of a transparent or semi-transparent object is provided. Optically transparent should generally be understood in the sense of light-permeable, and can therefore also include, for example, semi-transparency or translucency. In the strict sense, the term "optically transparent" within the scope of the invention can mean transparency with respect to the infrared and / or visible spectrum. In particular, infrared-transparent objects are intended to be included. Preferably, objects made of inorganic or non-biological materials are used in the method.

[0009] The proposed optical characterization method determines object dimensions, distances, and / or positions based on optical coherence tomography (OCT). As already explained above, OCT is a well-known method for characterizing objects based on coherent optical radiation.

[0010] Optical coherence spectroscopy is well known. For examples of the optical characterization of objects using OCT, see "Cubic meter volume optical coherence tomography," Fujimoto et al., Vol. 3, No. 12, December 2016, 1 Optica, and "Application of a long-range swept source optical coherence tomography-based scheme for dimensional characterization of multilayer transparent objects," Eneas N.

[0011] Morel, Nelida A. Russo, Jorge R. Torga, Ricardo Duchowicz, Opt. Eng. 56(8), 084102 (2017).

[0012] The method proposed herein is characterized by using a swept-source laser source as the light source, in particular a tunable laser source, preferably a wavelength-tunable laser source, which has a coherence length in the decimeter to meter range. Furthermore, the proposed method is characterized by using a reference substrate in the beam path of the light source, which includes a partially reflective optic that is stationary in the beam path. The reference substrate is used to determine absolute distances and / or positions and / or dimensions of the object or to / from the object.

[0013] The underlying invention is based on the finding that the use of a swept-source light source with a reference substrate is suitable for characterizing at least partially transparent or translucent objects, especially moving ones, and provides comparatively reliable and accurate measurement results. In particular, at least partially transparent or translucent objects can be measured that, for example, move during the measurement and / or emit a non-negligible amount of thermal radiation and / or are located in an environment that emits a non-negligible amount of thermal radiation.

[0014] As mentioned, the proposed combination is suitable for measuring and / or characterizing hot objects, e.g., partially molten glass or objects made of glass. The method is particularly suitable for the optical characterization of glass cylinders or cylindrical glass objects during or during production, whereby the objects may be in an at least partially molten or not yet fully solidified state. For example, the proposed method makes it possible to determine the outer and / or inner diameters and / or wall thicknesses of objects.

[0015] In particular, the proposed method also provides advantageous axial resolution, since, unlike known methods, the proposed method operates without Bragg gratings. The advantageous axial resolution results, for example, from the fact that, unlike methods based on Bragg gratings, the method proposed here is essentially unlimited or restricted with regard to the number of reference points or measurement points.

[0016] Furthermore, the proposed method enables the measurement of both absolute and relative distances, thus enabling improved optical characterization.

[0017] The proposed method also enables the characterization of transparent or semi-transparent objects essentially in real time, for example, during the production or manufacturing process. In particular, the method is not limited to static objects but allows the characterization of moving objects.

[0018] According to embodiments, a swept-source laser source is used as the light source, which comprises a MEMS VCSEL-based light source (MEMS: micro-electro-mechanical system; VCSEL: vertical-cavity surface-emitting laser) or an akinetic light source. Such a light source particularly preferably has a coherence length in the range of 0.2 m to 100 m or more, or the coherence length of the emitted radiation can have a corresponding coherence length.

[0019] The proposed method, using a MEMS VCSEL light source and a reference substrate, is advantageously suited for comparatively large coherence length ranges, enabling particularly accurate coherence / interference measurements across the entire range. Large coherence lengths can be used, for example, for larger distances between the measuring head or measuring apparatus and the object, for example, when the object is hot, such as (partially) molten glass, and a certain distance from the object must be maintained due to temperature.

[0020] According to embodiments, it can be provided that during signal acquisition and / or signal evaluation of the optical coherence tomography (OCT) signals: a. k-clock resampling is used, b. minima, maxima or zero crossings of the k-clock of the laser source are determined algorithmically and used as a sampling clock, or c. a k-clock hardware trigger or a k-clock hardware signal is used.

[0021] A k-clock enables data to be recorded linearly over time, which can then be transformed into a linear sample in k-space. This makes high-frequency signals, in particular, easier to evaluate. Furthermore, it is possible to characterize objects in quasi-real time, which is advantageous, for example, for the geometric characterization of moving transparent or semi-transparent objects during the manufacturing process.

[0022] An exemplary method according to b. for the algorithmic determination of minima, maxima or zero crossings of the k-clock can be as follows or comprise the following steps: bl) normalization of the k-clock by division by the envelope; b2) execution of at least three, preferably immediately consecutive, mathematical operations, such as absolute value formation, addition, absolute value formation; b3) smoothing, e.g. by means of a median filter or the like; b4) peak finding or determination of the maxima; b5) interpolation of the maxima, minima and zero crossings into the linear k-space.

[0023] Due to the absolute value operation b2), minima, zero crossings and maxima of the k-clock can be determined using the proposed exemplary method steps.

[0024] According to embodiments, the optical coherence spectroscopy method can involve scanning the object with laser light from the light source using a galvanometer scanner, or galvanoscanner for short, using a microlens array and / or a bifocal lens, preferably with a high numerical aperture. Galvanoscanners, microlens arrays, and bifocal lenses in particular have proven suitable for the optical characterization of moving objects, for example, during the manufacturing process. Such components are particularly suitable for measuring or characterizing partially or partly melted, moving transparent cylinders, e.g., made of glass.

[0025] With regard to the mode of operation of the galvanometer scanner, it is possible, for example, with moving objects, e.g. rotating and axially moving objects, in particular with objects that rotate relatively quickly and move axially relatively slowly, that the galvanometer scanner is or is programmed in such a way that a point on the object or on the surface of the object, in particular a point or area targeted by the galvanometer scanner, is at rest in the reference system of the galvanometer scanner. The object can, for example, be a cylinder, an object with cylindrical geometry, or a cylindrical object. This is possible in particular within the scope of the deflection that can be achieved or is available with the galvanometer scanner. If the galvanometer scanner reaches the end of the deflection orits deflection range, the deflection is reset and operated in such a way that it tracks or targets the next point on the surface.

[0026] Synchronization between the galvanometer scanner and data acquisition on the one hand, and a rotation of the object on the other, can be achieved, for example, by an angle encoder. In general, movements of the object, be it translation and / or rotation, can be recorded by a sensor unit or sensor technology, and the recorded movement data can be used for synchronization. Based on such data, for example, a movement path of a point, in particular a targeted point or area, on the object, for example on the surface of the object, can be determined, and the galvanometer scanner and data acquisition on the one hand, and the movement of the object on the other hand can be synchronized on the basis of the determined movement path. Preferably, the movement, ora movement path is continuously determined, in particular in real time, and used for control and synchronization between movement on the one hand and galvanometer scanner and data acquisition on the other.

[0027] With regard to the mode of operation of the microlens array, it can be provided that the light beam underlying the OCT is aligned along the direction of movement or along a movement path or trajectory of a point on the object, in particular the object surface, and that the scanning takes place simultaneously at or for points or surface points of the trajectory. For example, if the movement or trajectory is known, the data acquisition and the movement can be synchronized, i.e., points on the object or on the object surface can be tracked for data acquisition.

[0028] When using a two-dimensional (2D) microlens array, it is particularly possible to scan the (immediate) vicinity of the trajectory, in particular the locations of interest on the object or an area located around the trajectory. This scanning is particularly suitable for cylindrical objects or cylindrical objects. Scanning by the microlens array is preferably stationary and, more preferably, at the repetition rate of the laser.

[0029] With regard to a bifocal lens or a sensor head with a bifocal lens, it should be mentioned that this can be used, for example, to optimize the signal strength reflected back from a sample or object. This is particularly true when the sample or object has, for example, (a) periodic fluctuations in the inner diameter (e.g. 50 mm over an axial range of 500 mm), e.g. when there is an axial inclination of the inner surfaces depending on the position of the object, and / or (b) a significant stepwise change in the outer diameter across the axis or when measuring objects with a wide range of outer diameters (e.g. from 200 mm to 400 mm). For condition (a), it is advantageous to use a sensor head with a high numerical aperture, since the light reflected back from an inclined surface propagates at a considerable angle.In addition to the large aperture, it may be useful to separate the excitation of the sample from the detection of the back-reflected light to ensure collimated excitation. For condition (b), a sensor head with a bifocal lens is preferably used, which can map the focus of the front and back surfaces to the position of the receptive fiber end, regardless of the diameter / size of the object. For these reasons, a dual-focus design is particularly advantageous for these conditions.

[0030] A bifocal lens or a double-focus asphere can, for example, consist of two semi-convex lenses with focal points fa and fb that are connected to each other, with the connecting surface running parallel to the axis of the object, e.g., a cylindrical object. In another embodiment, a double-focus asphere can be designed similarly to a zone plate with circular sections with alternating focal lengths fa, fb, fa, fb, etc., whereby this embodiment can be used essentially independently of the properties of the object to be examined.

[0031] Another variation of a dual-focal optical setup involves using a single asphere, replicating identical receptive optical elements, such as fibers, (downstream of the asphere in the beam path) in an orthogonal arm, and deflecting the incoming beam into both arms via a beam splitter. The focus of one receiving arm is preferably tuned to the front of the object, while the other is tuned to the back. The signal received by the elements or fibers is captured either by optical multiplexing or with the aid of an optical combiner.

[0032] If the dimensions of the object to be measured are (roughly) known in advance or essentially known, the lens distances of the dual-focus setup can be automatically adjusted to an optimal dual-focus configuration.

[0033] In some embodiments, the probe arm or fiber and the receptive arm or receptive fiber can be formed by different elements or fibers. A collimated beam emerging from the probe fiber, for example, can be collinearly coupled into the receptive fiber with a high numerical aperture via a microprism or a glass plate with polished end faces, which may have a bonded, angled, partially reflective surface. This can prevent the light reflected back from the sample from being blocked.

[0034] In cases where an inclined surface of the object under investigation refracts the light beam at a small angle, so that an area on the opposite surface of the object is "excited" that is not exactly opposite the object's central axis. This can lead to errors in determining the distance, dimension, and / or position. In particular, it is possible to compensate for the deviation by placing an array camera or a plenoptic camera behind the sample, with which the direction or angle of the transmitted light can be recorded or determined. Using the direction or angle, the distance, dimension, and / or position can be corrected, for example, using ray tracing.

[0035] Depending on the configuration, a single-focus setup can also be used. In this case, it may not be possible to capture all four reflections for the object's surfaces for all axial positions of, for example, a cylindrical object. In such cases, the position of the receptive fiber and / or the lens spacing of the optical setup can be optimized so that at least three out of four reflections of the cylindrical object can be captured, enabling continuous determination of, for example, the inner diameter, outer diameter, and wall thickness, e.g., after an initial 180° rotation.

[0036] According to embodiments, as already mentioned, the object can be a moving object. When scanning the moving object, a scanning path used at least partially follows the movement of the object. Advantageously, in this embodiment, the data acquisition of the optical coherence tomography is synchronized with the scanning path.

[0037] In other words, the scanning path can move, at least partially or in part, substantially synchronously with the movement of the object. In particular, it is a finding of the underlying invention that scanning moving objects based on the light source used, coherence length, and reference substrate enables characterization, in particular measurement, of objects or parts thereof, even when the object is moving. As already mentioned above, a galvanometer scanner and / or a microlens array can be used, in particular, for scanning moving objects, wherein synchronization between the galvanometer scanner and / or microlens array and the data acquisition is or will be synchronized with the movement.

[0038] According to embodiments, a spatial jitter is superimposed on the scanning path of the galvanometer scanner. In particular, such jitter is not only advantageous for irregularly shaped objects, but also supports data acquisition for moving objects. The jitter can be generated, for example, by suitable control of the galvanometer scanner. If the object surface is ideally perpendicular to the OCT light beam, jitter is not absolutely necessary. Since real objects, particularly objects undergoing a manufacturing process, fluctuate with regard to the orientation of their surface(s), the use of jitter can improve the accuracy and / or reliability of the acquired object data. In other words, jitter makes it possible, for example, to obtain a back reflection or reflection rays from the object at least to a certain extent even with an irregular object surface.A jitter can be adjusted, for example, by superimposing a deflection or movement that is orthogonal to the trajectory of the point on the object onto the movement of the galvanometer scanner according to the trajectory of the point.

[0039] According to embodiments, the method combines several, in particular a plurality of, reflection profiles (or A-scans) obtained along the scanning path, in particular into a combined reflection profile. Periodogram signals can be determined from the combined reflection profile thus obtained, and at least one object dimension, an object distance, and / or an object position can then be determined from at least one periodogram signal, i.e., from one or more periodogram signals.

[0040] In particular, distances between boundary surfaces of the object and / or their position or location relative to each other relative to a precisely known reference, formed, for example, by the stationary partially reflective optics, can be determined.

[0041] It is therefore possible to determine object dimensions, distances or positions comparatively accurately and reliably, especially when the object is moving and / or when the object structure or geometry varies within certain limits.

[0042] According to embodiments, for example, those A-scans can be filtered out which exhibit a number of back reflections expected from the geometry of the object to be examined, e.g., flat, cylindrical, etc. The expected number of back reflections in OCT can, for example, be determined or derived from the (rough) geometry, target geometry, and / or expected geometry of the object to be examined. The rough, target, and / or expected geometry can be provided to the method, e.g., to a corresponding evaluation unit, as a parameter or

[0043] Parameter data set, e.g. as input parameter or data set, in particular input parameter data set, can be provided.

[0044] With regard to the selection of the A-scans to be used or usable for the optical characterization, it is also or additionally possible to use, e.g. to filter out, those A-scans which, for example from a group or set of recorded A-scans, show or have a maximum intensity and / or which have the best S / N (signal-to-noise ratio).

[0045] A periodogram is understood in the usual sense, in particular as a distance signature of the reflections caused by the boundary surfaces of the object, in short distance signature.

[0046] According to embodiments, it can be provided that an infrared filter, preferably a dual-band mirror, is arranged in the beam path of the light source, and thermal radiation emanating from the object and / or the object's surroundings in the range of the far infrared spectrum is at least predominantly filtered out using the infrared filter. The infrared filter is preferably arranged upstream of a measuring head used to detect the light reflected from the object. In particular, it is possible to prevent measured values ​​from being disturbed by radiation or extraneous radiation in the infrared spectral range of the thermal radiation, or the S / N ratio for the light reflected from or at the object can be improved. The use of an infrared filter is advantageous, for example, in the characterization of hot objects.from objects that themselves and / or their surroundings emit radiation in the infrared spectral range of thermal radiation, at least to an extent that would or does impair the measured values ​​of the OCT light reflected from the object. With regard to infrared radiation, a spectral range that corresponds to a temperature range of 800 K to 2200 K, assuming a blackbody radiator, is particularly relevant.

[0047] According to embodiments, the optical coherence tomography may include a signal acquisition of the k-clock of the light source with software-based filtering and offset correction, and further, optionally, a subsequent phase extraction, in particular based on a Hilbert transformation, and / or an evaluation of zero crossings and extrema. The filtering may preferably be a low-pass filter.

[0048] Based on the k-clock signal acquisition, a linearization over time is possible, which enables real-time characterization of objects. Subsequent phase interpolation, i.e., phase interpolation of the phase response of the measurement signals, allows the data or measurement signals recorded linearly over time due to the k-clock used to be transformed into a linear sample in k-space. This can increase the resolution of the method or of a system executing the method.

[0049] According to certain embodiments, the phase response can be extracted and the data reinterpolated based on this phase response for each individual sweep of the swept-source laser. This can, for example, ensure that the system, in particular the signal evaluation and / or processing, is robust with respect to fluctuations in the laser's phase response.

[0050] According to embodiments, filtering, in particular low-pass filtering and / or phase interpolation, can occur after each individual laser sweep. For a 10 kHz laser, for example, with a frequency of 10 kHz. Implementation on an architecture with parallel data processing, for example on graphics processors (GPUs), is particularly advantageous for processing and / or handling the measurement signals at such frequencies. For example, in application cases, the sampling rate of the digitizer, a laser repetition rate of 10 kHz to 100 kHz, and depending on the laser duty cycle, can result in comparatively high data rates in the range of 5 to 7 GB / s (gigabytes per second), which cannot be handled satisfactorily or at all with a conventional CPU. Therefore, the processing and / or handling of the measurement signals is advantageously carried out on a GPU, which can be implemented, for example, in CUDA (Compute Unified Device Architecture).

[0051] Examples of filters are bandpass filters, median filters, and the like. Examples of interpolations are polynomial interpolation, Chebychev interpolation, and the like.

[0052] According to embodiments, it is provided that periodogram signals are determined from signals of the optical coherence tomography by means of an automatic peak-picking algorithm from periodogram data, and that at least one object dimension, at least one object distance and / or at least one object position are determined from the periodogram signals, in particular taking into account periodogram signals of the reference substrate.

[0053] The term "peak picking" (or peak finding) should be understood in the sense of the state of the art as, for example, an algorithm with which local maxima or minima can be determined from signals, in particular measurement signals and / or processed or processed measurement signals. Such an algorithm can, for example, comprise a determination of zero crossings of the numerical first derivative of the signal(s), whereby zero crossings are or are assigned to individual peaks, in particular local maxima or minima.

[0054] In the peak-picking algorithm, data on the (rough) geometry and / or target geometry and / or expected geometry of the object can be used as input parameters or boundary conditions. In particular, when manufacturing objects, for example glass objects such as glass cylinders, the geometry of the object or a desired geometry of the object is known or specified. If, for example, the target diameter of a cylindrical object to be manufactured, for example the inner and / or outer diameter and / or the wall thickness of the object is known or specified, this data can be used to at least approximately determine the location(s) or position(s) of the expected signals, which can be calculated or determined, for example, taking the measuring arrangement into account. If, for example, a cylindrical object is measured with light incident radially, reflection signals, i.e.Peaks are expected for reflections on the outer and inner surfaces of the cylindrical object walls.

[0055] Based on the measurement signals and specified data regarding the geometry of the object and / or the measurement setup, it is possible to monitor a manufacturing process, i.e., to characterize the object during production, in particular with regard to whether the manufactured object meets the underlying requirements regarding geometry and / or dimensions. Thus, quality control and / or monitoring of a manufacturing process is possible. Furthermore, for example, a correlation of deviations in the object geometry with operating parameters of the production facility and / or, based on deviations in the object geometry, a fault analysis for the production facility can also be provided.

[0056] The advantage of the proposed method is that it enables real-time characterization, which allows for the optimization of a production process and / or the characterization of the manufactured products during production. Complex object characterization after production can be eliminated.

[0057] In particular, the stationary, partially reflective optics proposed herein make it possible to determine not only relative object distances or object dimensions based on the measurement signals, in particular peaks, but also (absolute) distances, dimensions and / or positions relative to the measuring system or the optical measuring system used.

[0058] By way of example, and based on actual measurements, the proposed method can be used to characterize cylindrical, transparent, or semi-transparent objects, such as cylinders, for example, glass cylinders, with a diameter of 50 mm to 700 mm and a wall thickness of 0.05 mm to 100 mm. It is shown that the proposed method is suitable for a comparatively broad range of applications.

[0059] According to embodiments, an optical coherence tomography system is provided which is designed for the optical characterization of a transparent or semi-transparent object based on optical coherence tomography (OCT). The coherence tomography system comprises at least one swept-source laser source with a coherence length in the range of 0.2 m to 100 m or more, in particular in the meter range, a reference substrate arranged in the beam path of the at least one light source, which reference substrate comprises, for example, a partially reflective optical system stationary in the beam path, and a control unit with a processor, in particular a processor with an architecture for parallel data processing, such as a graphics processor, and a memory assigned to the processor, which memory comprises instructions which, when executed by the processor, effect a method according to one of the method-related embodiments proposed herein.For the purposes of this invention, a processor with associated memory, configured to carry out a method according to one of the embodiments proposed herein, is generally understood to mean an electronic unit which is programmed and / or configured such that, during operation, it carries out a method according to one of the embodiments proposed herein.

[0060] The advantages and beneficial effects of the coherence tomography system and its design arise in particular from the advantages and beneficial effects of the method-related embodiments. Reference is made in this regard and in particular to the above explanations.

[0061] According to one embodiment of the optical coherence tomography system, the swept-source laser source is or comprises a MEMS VCSEL based light source or an akinetic laser light source, and / or a coherence length of the light source is in the range of 0.2 m to 100 m or more.

[0062] According to one embodiment of the optical coherence tomography system, it further comprises at least one galvanometer scanner configured to scan the object and / or at least one microlens array configured to scan the object. These components, in particular, enable precise scanning of an object to be characterized, especially when the object is moving, for example, when the object moves along a movement path during production.

[0063] Typically, the object's motion path, or a corresponding one, is known or predetermined. The predetermined or known motion path can be used, for example, to control the galvanometer scanner and / or to evaluate the signal data acquired via the microlens array.

[0064] Within the scope of the invention, it is possible or provided in embodiments to detect or determine the movement of the object, a linear movement and / or a rotational movement, or the movement path of the object, and thus at least indirectly the movement path of the respective object area or point to be measured. For this purpose, for example, a sensor or measuring system can be provided that is configured to detect the movement of the object.

[0065] According to embodiments of the optical coherence tomography system, it further comprises at least one infrared filter arranged in the beam path of the light source, preferably a dual-band reflector, which is configured to at least predominantly filter out thermal radiation emanating from the object and / or the object's surroundings in the far-infrared spectrum. Such a coherence tomography system is particularly suitable for characterizing hot objects, which emit thermal radiation that is not negligible with regard to OCT or that impairs the OCT result. Such an infrared filter also makes it possible to measure or characterize objects in environments that emit thermal radiation that is not negligible with regard to OCT or that impairs the OCT result.

[0066] According to embodiments of the optical coherence tomography system, the light source has a bandwidth in the range of 20 nm to 100 nm, and / or of at least 40 nm.

[0067] According to embodiments of the optical coherence tomography system, the light source is configured for repetition rates in the range of 4 kHz to 4 MHz, in particular for repetition rates of approximately 10 kHz.

[0068] According to embodiments, an optical coherence tomography system is provided, which can in particular be designed according to one of the embodiments described above. The optical coherence tomography system can comprise a k-clock unit, or a k-clock for short, for generating a k-clock signal for signal acquisition and / or signal evaluation for optical coherence tomography, in particular for k-clock-based resampling. The k-clock unit comprises a Fabry-Perot interferometer, ie an interferometer constructed in the manner of a Fabry-Perot interferometer, which can also be referred to as a k-clock interferometer, or more precisely a k-clock Fabry-Perot interferometer. In this context, in the method-related embodiments described above, it can be provided that a k-clock interferometer is used which has the described structure and generates k-clock signals in the manner described here.The embodiments of the k-clock unit described here apply accordingly to the embodiments according to the method, in particular for generating k-clock signals.

[0069] The Fabry-Perot interferometer or k-clock interferometer is designed or intended to generate interferometry signals based on laser radiation from the swept-source laser source, which is fed to the Fabry-Perot interferometer or with which it is fed. The term "feed" refers to the coupling of laser light from the laser source into the Fabry-Perot interferometer, for example, via optical fibers or light guides.

[0070] The Fabry-Perot interferometer comprises a first mirror, a second mirror, and a spacer. The mirrors are attached to or connected to opposite sides of the spacer. The spacer spacers the mirrors at a distance from each other, with the distance defined by the length of the spacer. The mirror surfaces of the mirrors face the spacer and are aligned plane-parallel to each other. Advantageously, the first and second mirrors are wedge-shaped, with the mirrors being wedge-shaped, in particular, with respect to planes perpendicular to the optical axis of the Fabry-Perot interferometer. The optical axis denotes an axis running perpendicular to the plane-parallel mirror surfaces.According to the findings of the invention, a Fabry-Perot interferometer is particularly well suited for generating stable, in particular temperature-stable k-clock signals, in particular for the application underlying the invention of characterizing transparent or semi-transparent objects.

[0071] Particularly preferably, the mirrors have an anti-reflective layer or coating which prevents the mirrors from acting as such in the sense of an etalon or from etalonizing.

[0072] The Fabry-Perot interferometer is therefore advantageously designed as a plane-surface interferometer. For attaching the mirrors to opposite sides, particularly plane surfaces or plane-parallel sides, of the spacer, retaining elements, particularly retaining rings, can be provided, which press or press the mirrors against the spacer without stress.

[0073] The optical path length of the Fabry-Perot interferometer between the mirrors is preferably defined by the axial length of the spacer, where the axial length is perpendicular to the plane-parallel mirror surfaces. The end faces of the spacer, where the mirrors are positioned, are preferably highly parallel, for example, with a parallelism of less than 0.005 degrees. Such parallelisms can, in particular, achieve a suitable quality or finesse for generating k-clock signals for optical coherence tomography.

[0074] According to embodiments, the spacer can be made of or consist of a material that has an ultra-low or negligibly low thermal expansion coefficient. In particular, the spacer should be made of or consist of a material for which temperature-related length changes of the spacer and associated changes in the distance between the mirrors are negligible during temperature changes within the operating temperature range or operative temperature range of the interferometer relevant for the application envisaged herein, e.g., ambient temperature, for example, 15°C to 30°C.Preferably, the spacer should be made of or consist of a material for which temperature-related length changes of the spacer are negligible in the respective operating temperature range, particularly perpendicular to the mirror surfaces, relative to the wavelengths of the laser light of the swept-source laser. Such materials preferably have a thermal expansion coefficient in the range of -2 * 10'. 7 / K to +2 * 10' 7 / K. Examples of materials are glass materials, such as glass ceramics with extremely low thermal expansion coefficients (so-called "ultra low expansion glass materials"), in particular ULE® ultra low expansion glass (Corning Inc., Corning, NY), or Zerodur® glass-ceramic (Schott AG, Mainz, Germany). The materials can preferably be processed in such a way, in particular also in hollow cylindrical form or manufactured, that for plane-parallel contact surfaces of the mirrors on the end faces of the spacer, or in the case of a hollow cylindrical form on the respective end faces of the hollow cylinder, an angular error of 0.002° to 0.005° or less can be implemented.

[0075] According to embodiments, the spacer is hollow-cylindrical and forms a hollow-cylindrical gap or air gap between the mirror surfaces, which is circumferentially delimited, for example, by the outer wall of the spacer. The gap or air gap can be sealed in the region of the contact or bearing surface between the mirrors and the spacer, i.e., in the region in which the mirrors are positioned on the spacer or abut or contact it, by anodic bonding, by optical contact bonding, or by cementing. Such measures can seal the gap or air gap, or vacuum gap, from the outside atmosphere to prevent interference.

[0076] According to embodiments, a beam splitter connected to the light source on the one hand and to the k-clock unit on the other hand can be provided to supply the k-clock unit, in particular the Fabry-Perot interferometer, with laser light from the light source. The beam splitter is preferably configured to supply the k-clock unit via an input assigned to the first mirror with a proportion of the laser light from the light source in the range of 0.5% to 2%, in particular of approximately 1%. The remaining laser light, e.g., 99% or a corresponding residual proportion, can be fed to the sensor head or the sensor unit or scanning unit of the optical coherence tomography system, for example, for recording or executing the A-scan signals (so-called A-scans).

[0077] According to embodiments, the optical coherence tomography system, in particular the k-clock unit, can comprise a detector, in particular a high-speed detector, with at least one input that is optically coupled to an output associated with the second mirror of the k-clock unit, in particular the Fabry-Perot interferometer, for detecting interferometry signals from the Fabry-Perot interferometer. The second mirror is preferably designed as a transmission mirror or a semitransparent mirror, so that the interferometry signals can be coupled out at the second mirror and fed to the detector.

[0078] According to embodiments, the optical coherence tomography system, in particular the k-clock unit, can further comprise a data processing unit configured to evaluate detected interferometry signals, i.e., to evaluate detection signals from the detector, and to generate a k-clock signal based on the evaluation of the detected interferometry signals. Thus, the data processing unit can be configured to generate the k-clock signals based on the interferometry signals from the k-clock interferometer, wherein the k-clock signals are provided, for example, for performing k-clock-based resampling. This k-clock-based resampling can also be understood as k-space linearization of the OCT signal based on the k-clock signal.

[0079] According to embodiments of the optical coherence tomography system, the detector can be implemented as a balanced detector or symmetrical detector for the purpose of improving the signal-to-noise ratio and suppressing interference signals. The balanced detector can have two inputs, wherein a first detector input is optically coupled to an output of the Fabry-Perot interferometer or k-clock interferometer associated with the first mirror, and a second detector input is optically coupled to an output of the Fabry-Perot interferometer associated with the second mirror. The mirrors are preferably each designed as a transmission mirror or a semitransparent mirror.

[0080] According to embodiments, the optical coherence tomography system can further comprise a circulator with three ports or connections or connection interfaces, wherein the laser light source is optically coupled to a first port for feeding the Fabry-Perot interferometer with laser light from the laser light source (for example, 0.5-2%, in particular 1% of the laser light) via a second port of the circulator connected to an input associated with the first mirror. The second detector input of the detector is optically coupled to a third port of the circulator, and the circulator is configured such that interferometry signals of the Fabry-Perot interferometer emitted to the second port via an output of the Fabry-Perot interferometer associated with the first mirror are directed via the third port of the circulator to the first connection of the detector.

[0081] Particularly preferably, the amplitudes of the signals applied to the first and second terminals of the detector are matched to each other before being applied to the detector. Suitable for this purpose are, for example, inline fiber attenuators or attenuators that are connected and / or integrated in the light-optical paths, for example, in light-guiding elements such as optical fibers, between the k-clock unit and the detector.

[0082] The optical coupling of the aforementioned components, such as the laser light source, beam splitter, circulator, k-clock interferometer, detector, etc., can be achieved in particular via optical fibers or optical fiber bundles. Preferably, the optical path lengths of the optical connections between the detector terminals and the k-clock interferometer are the same. This means that the optical connections connecting the output of the first mirror to the first detector terminal and the output of the second mirror to the second detector terminal have the same optical path length.

[0083] With the proposed k-clock unit, the measurement accuracy of the swept-source OCT system proposed herein, in particular of the swept-source OCT sensor system, can be improved, in particular optimized. Thus, the measurement accuracy, particularly for the intended application, depends on the stability of the k-clock or the k-clock unit, which in embodiments as described above comprises or forms an interferometric device and measures the wavelength sweep of the sweep-source laser light source or laser. In particular, the proposed k-clock unit can ensure that the free spectral range (FSR) of the k-clock unit is constant and independent of environmental influences such as temperature or pressure fluctuations. The FSR is related to the optical path length differences of the k-clock unit, in particular of the k-clock interferometer.The proposed design and use of the k-clock unit make it possible to minimize corresponding environmental influences and to obtain an FSR with which sufficiently accurate measurements can be carried out, in particular for the application envisaged herein.

[0084] Compared to known methods for the optical characterization of transparent or semi-transparent objects by means of OCT, the proposed method and the OCT system according to the embodiments described herein can implement comparatively large axial ranges, i.e. ranges measured parallel to the light radiation, in particular for objects with diameters of up to 100 mm and more, for example with diameters of up to 700 mm and more.

[0085] Furthermore, the proposed method makes it possible, in certain embodiments, to characterize moving objects, for example, during a manufacturing process in which the object is moved, e.g., performing a rotational and / or translational movement. One example of this is the production of cylindrical bodies or objects made of glass.

[0086] Embodiments of the proposed method and a corresponding optical coherence tomography system particularly and advantageously enable optical characterization by means of OCT of hot objects, i.e. objects that emit comparatively strong blackbody radiation, i.e. thermal radiation. Examples include the production of objects or bodies from molten glass, or more generally based on hot forming. In particular, it is possible to monitor or characterize objects and object geometries during hot forming during production, which on the one hand makes it possible to adapt and monitor process conditions for production in quasi-real time and / or to reduce waste. Furthermore, characterization steps that are otherwise usually carried out downstream of production can be omitted, which simplifies the production process and in particular can be shortened.

[0087] The proposed method and optical coherence tomography system, in particular in contrast to known methods and devices, enable the measurement or determination of absolute and relative distances. In particular, it is not necessary to provide separate devices for determining relative distances on the one hand and absolute distances on the other. Absolute distances can be relevant, for example, for determining or characterizing the manufacturing process if the object or product is to move along a predetermined path, e.g., in a straight line, during production.

[0088] By determining absolute distances, for example, deviations from the path can be detected and the manufacturing process can be adjusted, or potential sources of error or malfunctions in the manufacturing process or production plant can be identified from deviation data.

[0089] Relative distances can, for example, be used to characterize the geometry of the object itself, such as diameter, wall thickness, cross-sectional shape, etc.

[0090] The proposed method and optical coherence tomography system are particularly suitable for a relatively broad range of applications. For example, the method is suitable for measurements with a comparatively large axial depth and simultaneously high resolution (e.g., 10 micrometers) and / or with a comparatively small axial depth and simultaneously very high resolution (e.g., 1 micrometer).

[0091] Compared to known methods and devices, advantageous resolution can be achieved in embodiments using the proposed method, in particular with the algorithms proposed herein for processing OCT signals. In particular, OCT signals or signals derived therefrom can be achieved with an advantageous half-width.

[0092] Embodiments of the invention are described below with reference to the attached figures. They show:

[0093] FIG. 1 schematically shows a first OCT implementation according to the device for characterizing an object; FIG. 2 schematically shows a second OCT implementation according to the device for characterizing an object;

[0094] FIG. 3 schematically shows a third device-based OCT implementation for characterizing an object;

[0095] FIG. 4 shows a process flow for determining periodograms;

[0096] FIG. 5 shows exemplary OCT measurement results for characterizing a glass cylinder based on the third implementation;

[0097] FIG. 6 shows exemplary OCT measurement results for characterizing a glass cylinder based on the first or second implementation;

[0098] FIG. 7 shows an exemplary diagram for characterizing a glass cylinder with a comparatively large axial depth;

[0099] FIG. 8 shows exemplary OCT measurement results for characterizing a moving, hot glass cylinder;

[0100] FIG. 9 shows an exemplary structure of a k-clock interferometer;

[0101] FIG. 10 shows an example of a first implementation of a k-clock unit with a k-clock interferometer according to FIG. 9; and

[0102] FIG. 11 shows an example of a second implementation of a k-clock unit with a k-clock interferometer according to FIG. 9.

[0103] Identical or functionally equivalent elements are designated by the same reference numerals in the figures. The figures merely describe exemplary applications or

[0104] Implementations, without the invention being restricted thereto or to the advantages or advantageous effects resulting therefrom. FIG. 1 schematically shows a first OCT implementation according to the device for characterizing an object 1, which may be, for example, a hollow glass cylinder, also referred to as a glass cylinder or cylinder for short.

[0105] The device implementation comprises a swept-source laser 2, also referred to as laser 2 for short, followed by a circulator 3. The three ports of the circulator 3 are connected to the laser 2, a collimator 4, and a beam splitter 5. The beam splitter 5 can have a splitting ratio of 90:10, with a first output 6 assigned to the splitting ratio 90 and a second output 7 assigned to the splitting ratio 10.

[0106] A power monitoring unit 8 is connected downstream of the second output 7.

[0107] The first output 6 is followed by a variable optical attenuator 9 (VOA: Variable Optical Attenuator), which is followed by a high-speed detector 10 for generating OCT signals 11.

[0108] Laser light guides, especially so-called TEC fibers (TEC: Thermally Extended Core), can be used to connect the components.

[0109] A stationary, partially reflective optic 13 or a stationary reference substrate 13 is arranged downstream of the collimator 4 in the beam path of the laser radiation 12 coming from the laser 2. Downstream of the reference substrate 13 in the beam path of the laser radiation 12 coming from the laser 2 is a galvanometer scanner 14, also referred to as scanner 14, for beam deflection at deflection angles θ (phi) and θ (theta). The object 1 to be measured or characterized is arranged downstream of the scanner 14 in the beam path.

[0110] During operation for measuring or characterizing object 1, the laser radiation 12 emanating from the collimator 4 first passes through the reference substrate 13 and then strikes the scanner 14, which directs the laser radiation 12 onto the object 1. The laser radiation 12 reflected from the object 1 passes via the scanner 14, the collimator 4, and the circulator 3 to the beam splitter 5, which directs the reflected laser radiation 12 or its signals via the first output 6 through the variable optical attenuator 9 to the detector 10. The detector 10 generates OCT signals from the reflected laser radiation 12 or corresponding signals for the optical characterization of object 1.

[0111] Based on the scanner 14, or by appropriate control of the scanner 14, for example of the angle q> (phi) and / or θ (theta), the laser radiation 12 can be directed or irradiated onto the object 1 in such a way that the point of impact x of the laser radiation 12 on the object 1 follows, for example, a movement of the object 1. In the example shown, the object 1 rotates counterclockwise, which is indicated by a curved arrow. If the object 1 moves linearly at the same time, for example out of the plane of FIG. 1, it can also be achieved by appropriate control of the scanner 14 that the point of impact x also follows this linear movement. Thus, moving objects can be optically characterized using OCT.

[0112] FIG. 2 schematically shows a second OCT implementation according to the device for characterizing an object. In contrast to the implementation according to FIG. 1, a microlens array 15 is connected downstream of the collimator 4 and the reference substrate, through which the laser radiation 12 is directed onto the object 1. The microlens array 15, for example a 2D microlens array, also makes it possible to track points x on the object 1 or on the object surface, so that the second implementation also makes it possible to characterize objects 1 that execute a rotational movement and / or a translational movement (e.g., according to a movement out of the plane of FIG. 2). Apart from 2D microlens arrays 15, ID microlens arrays 15 can also be used.

[0113] In further implementation variants, a galvanometer scanner 14 and a microlens array 15 can be used in combination.

[0114] Otherwise, the structure of the implementation shown in FIG. 2 corresponds to that shown in FIG. 1.

[0115] The implementations shown in FIGS. 1 and 2 correspond to a setup with autocorrelation. FIG. 3 schematically shows a third device-based OCT implementation for characterizing an object 1, wherein this implementation corresponds to a setup with cross-correlation.

[0116] In the structure shown in FIG. 3, analogous to FIG. 1 and FIG. 2, a beam splitter 5 is present which is connected on the one hand to a circulator 3 and on the other hand to a high-speed detector 10 or a power monitoring unit 8. In contrast to FIGS. 1 and 2, the connection of the circulator 3 which is closest to the connection of the laser 2 in the circulation direction of the circulator 3 is connected to a first connection 17.1 of a further beam splitter 16. A second connection 17.2 of the further beam splitter 16 is connected to a further variable optical attenuator 18 and a retroreflector 19. A third connection 17.3 is connected to a collimator 4 which, as in FIG. 1 and FIG. 2 is configured to collimate laser radiation 12 onto the object 1 to be measured or characterized, or onto laser radiation 12 reflected from the object 1. A fourth connection 17.4 is connected to the detector 10 via a variable optical attenuator 9. In the implementation according to FIG. 3 with cross-correlation, the detector 10 is thus coupled to both beam splitters 5, 16 for receiving laser signals. Reference substrate 13 and microlens array 15 are not shown in FIG. 3, but can be present individually or in combination.

[0117] FIG. 4 shows an exemplary method for determining periodograms. The algorithm uses k-clock 20-based resampling 21 for the OCT signals 11. For performance reasons, the algorithm is executed on a processor architecture with parallel data processing, such as a GPU.

[0118] For resampling 21, the start times for the k-clock and OCT signals are adjusted or aligned 22. Before resampling, the k-clock is filtered 23 with a zero-phase low-pass filter, followed by a calculation 24 of the phase evolution and an extraction 25 of the phase profile (so-called "phase unwrapping"). After the k-clock-based resampling 21 of the OCT signals, a window function 26 is applied to the resampling result, followed by a fast Fourier transformation (FFT) 27 to determine OCT periodograms 28. From the OCT periodograms 28, geometric parameters or quantities characterizing the object 1, such as diameter, wall thickness, etc., can be determined.

[0119] To measure or characterize moving objects 1, which, for example, perform a comparatively fast rotational movement superimposed on a comparatively slow translational movement, the mirrors of the galvanometer scanner 14 are synchronized with the data acquisition in such a way that a point on the surface of the moving object 1, for example, a cylinder, is tracked in space. If necessary, a certain spatial jitter can be added to the mirrors of the galvanometer scanner 14, in particular to compensate for any non-perpendicular orientation of the surface of the object 1 relative to the incident laser beam.

[0120] In particular, tracking the point in space and spatial jitter contribute to reducing the probability of signal loss, or even essentially eliminating signal loss. Signal loss would, for example, lead to an incomplete characterization of the geometry of the moving object 1.

[0121] All A-scan signals obtained from tracking the point in space are combined and used to calculate periodogram signals. Combining the A-scan signals allows for robustness against signal loss.

[0122] For example, an automatic peak-picking algorithm can be used to identify maxima of the OCT periodograms 28.

[0123] FIG. 5 shows exemplary OCT measurement results for the characterization of a glass cylinder based on the third implementation ("cross-correlation"). In the diagram in FIG. 5, the abscissa (x-axis) denotes time t, and the ordinate (y-axis) shows wall thickness W, inner diameter ID, and outer diameter AD, each in arbitrary units, e.g., in mm. The OCT measurement is based on a rotating glass cylinder as object 1 to be measured. The diagram shows the course or the determined values ​​for wall thickness W, inner diameter, and outer diameter AD over time from the OCT measurement, which in turn is linked to the movement of the glass cylinder.

[0124] FIG. 6 shows exemplary OCT measurement results for characterizing the glass cylinder corresponding to FIG. 5 based on the first or second implementation ("autocorrelation"). In the diagram of FIG. 6, analogous to FIG. 5, the abscissa (x-axis) denotes time t, and the ordinate (y-axis) also shows wall thickness W, inner diameter ID, and outer diameter AD, each in arbitrary units, e.g., in mm.

[0125] FIG. 7 shows an example diagram for characterizing a glass cylinder used as object 1 with a comparatively large axial depth, in particular with a comparatively large distance between object 1 and measuring device. In the example shown, this is approximately 400 mm. In the diagram in FIG. 7, the abscissa (x-axis) denotes the distance D of object 1 from the measuring device in millimeters (mm) and the ordinate (y-axis) denotes the OCT signal strength OS (in arbitrary units). As can be seen from the diagram, signals with a low signal-to-noise ratio can be obtained even at comparatively large distances. Furthermore, it can be seen that, e.g., by using the reference substrate 13, absolute distances can also be determined.

[0126] FIG. 8 shows exemplary OCT measurement results for the characterization of a moving, hot glass cylinder. An infrared filter was used in the measurement, which is designed to filter out thermal radiation emanating from the object and / or the object's surroundings in the far infrared spectrum. In the diagram in FIG. 8, the abscissa (x-axis) denotes time t and the ordinate (y-axis) shows wall thickness W, inner diameter ID and outer diameter AD, each in arbitrary units, e.g. mm. FIG. 8 shows in particular that the proposed method is also suitable for measuring or characterizing hot, transparent or semi-transparent objects. FIG. 9 to FIG. 11 show an exemplary structure of a k-clock interferometer of a k-clock unit as well as exemplary implementations of the k-clock unit in an OCT system.

[0127] FIG. 9 shows a k-clock interferometer 29 of a k-clock unit for generating a k-clock signal, for example, for use in k-clock-based resampling. The k-clock interferometer 29 is constructed as a planar surface interferometer similar to a Fabry-Perot interferometer and is accordingly referred to herein as a k-clock Fabry-Perot interferometer or simply a Fabry-Perot interferometer.

[0128] The k-clock interferometer 29, hereinafter also referred to as interferometer 29, comprises a spacer 30 with a hollow cylindrical spacer body 30 having an inner hollow cylindrical gap 32 or air gap 32. Planar surfaces are formed at the two distal ends 33 of the spacer body 31, which, for example, have a parallelism of less than 0.005 degrees, i.e., a deviation in parallelism of less than 0.005 degrees. The spacer 30 or the spacer body 31 is made of a material with an ultra-low thermal expansion coefficient, for example, a material with thermal expansion coefficients in the range of + / -2*10' 7 / C.

[0129] A first mirror 34 and a second mirror 35 are attached to the distal ends 33 facing away from each other, for example, by means of stress-free retaining rings 36 or retaining bodies that press or press the mirrors 34, 35 against the respective contact surfaces of the distal ends 33. The length of the spacer 30 or the spacer body 31 can be, for example, 95 mm, for example, for an oscillation frequency of the k-clock of or in the range of 1.58 GHz. Suitable materials include, for example, those already mentioned above.

[0130] The mirrors 34, 35 are wedge-shaped and provided with an anti-reflective coating so that the mirrors 34, 35 do not form an etalon as such, but rather the gap or air gap 32, whose axial length is determined by the length of the spacer body 31, or the air or vacuum contained in the air gap 32, forms an etalon defined by the optical path length between the mirrors 34, 35. Mirror surfaces S of the mirrors 34, 35 are plane-parallel and preferably have an angular error of at most 0.002 degrees. The mirror surfaces preferably have an average roughness of 1.5 nm (root mean square, rms) or less. The mirrors 34, 35 are designed as transmission mirrors or semi-transmission mirrors.

[0131] For coupling and decoupling laser light into and from the k-clock interferometer 29, or in the area of ​​the mirrors 34, 35, so-called tip-tilt devices or mechanisms can be provided, with which, for example, light guides used to supply / discharge the laser light to or from the k-clock interferometer 29 can be tilted or inclined with respect to two axes. The tip-tilt devices can also have adjustment mechanisms that enable adjustment in the xy direction, e.g., perpendicular to the optical axis.

[0132] To avoid environmental influences, such as pressure, humidity, etc. on the gap 32 or air gap 32, the mirrors 34, 35 can be sealingly connected to the spacer body 31, for example by anodic joining, by optical contact bonding or by cementing.

[0133] FIGS. 10 and 11 show an exemplary implementation of a k-clock unit 38 in an OCT system. Figures 10 and 11 each depict a laser light source or laser 2, specifically a swept-source laser, a beam splitter 39, a k-clock interferometer 29, and an ultrafast detector 40.

[0134] The structure and function of the arrangement shown in FIG. 10 is as follows:

[0135] The laser 2 is optically coupled, e.g., by means of a fiber optic cable, to an input SE of the beam splitter 39. A first output SA1 of the beam splitter 39 is optically coupled to the k-clock interferometer 29, specifically to an input C1 of the k-clock interferometer 29 assigned to the first mirror 34, for coupling laser light via the first mirror 34. A second output SA2 of the beam splitter 39 is optically coupled to the OCT sensor head, as indicated by the arrow. An input DE of the detector 40 is optically coupled to the k-clock interferometer 29 at C2, an output of the k-clock interferometer 29 assigned to the second mirror 35, so that interferometry signals can be passed to the detector 40 and evaluated by it.

[0136] In the embodiments, all light-optical couplings or connections are preferably implemented by optical fibers.

[0137] An output DA of detector 40 is connected to an evaluation unit or signal evaluation unit, in particular an evaluation unit or data processing unit, for generating a k-clock signal. This is indicated schematically by the arrow.

[0138] In the present example, the beam splitter 39 is designed such that a portion of 1% of the laser radiation reaches the k-clock interferometer 29 via the first output SA1 of the beam splitter 39 and a portion of 99% of the laser radiation reaches the OCT sensor head via the second output SA2.

[0139] The arrangement shown in FIG. 10 as well as the arrangement shown in FIG. 11, which is explained in more detail below, enable efficient and accurate data acquisition based on a stable k-clock signal.

[0140] The arrangement or structure according to FIG. 11 differs from that according to FIG. 10 in that the detector 40 according to FIG. 11 is implemented as a symmetrical or balanced detector, or that a balanced detection or a balanced detector is used.

[0141] Specifically, the detector 40 according to FIG. 11 has two inputs, a first input DE1 and a second input DE2. Analogously to FIG. 10, the second input DE2 is optically coupled to an output C2 of the k-clock interferometer 29 assigned to the second mirror 35. The first input DE1 is also connected to the k-clock interferometer 29, but not to the output C2, but rather to an output of the k-clock interferometer 29 assigned to the first mirror 34, which is designated C1*. A circulator 41 having three ports P1, P2, P3 is arranged in the optical path between the beam splitter 39, the k-clock interferometer 29, and the detector 40. The first output SA1 of the beam splitter is optically coupled or connected to the first port P1, the second port P2 to the input C1 / output C1* of the k-clock interferometer 29 and the third port P3 to the first input DE1 of the detector 40.Laser light arriving from the beam splitter 39 at port P1 passes through port 2 to the input C1 of the k-clock interferometer 29. Laser light arriving from the output C1* of the k-clock interferometer 29 at port P2 passes through the third port P3 to the first input DE1 of the detector 40.

[0142] The optical path length between the output C1* and the first input DE1 on the one hand and between the output C2 and the second input DE2 are preferably identical or substantially identical.

[0143] By using balanced or symmetrical detection, as shown in FIG. 11, it is possible to improve the signal-to-noise ratio and suppress interference signals.

[0144] With the proposed k-clock unit, which can include, for example, the k-clock interferometer and the associated detector, comparatively stable k-click signals can be obtained, which in turn leads to improved measurement results. In particular, the free spectral range (FSR) of the k-clock unit, and thus the length scale of the measurement, can be achieved to be essentially constant and independent of environmental influences such as temperature or pressure fluctuations.

[0145] Due to the extremely low thermal expansion coefficient of the spacer, temperature changes have little effect on its length. The influence of temperature, pressure, and humidity on the refractive index of the air inside the spacer, for example, can be considered as a remaining factor for changes in the FSR. To minimize or eliminate such effects, the mirrors and the spacer can be sealed from the environment by anodic bonding, optical contact bonding, or cementing, and thus the gap or air gap. The proposed k-clock unit can be relatively easily integrated into the OCT system described above, as shown in FIGS. 10 and 11. For this purpose, a small portion (~1%) of the laser light can be sent to an input, e.g., a fiber optic input of the k-clock. Output signals, e.g.The k-clock output of an optical fiber can be measured with a fast optical detector or receiver. To increase the signal-to-noise ratio and remove interference, balanced or symmetric detection can be used, as mentioned above. With balanced detection, an optical circulator provides access to both outputs of the k-clock interferometer, which can be measured with the balanced or symmetric detector.

[0146] In balanced or symmetric detection, the amplitudes of the respective signals, i.e., the signals from the two outputs of the k-clock interferometer, are preferably matched upstream of the detector, for example, using fiber attenuators. The optical path lengths between the detector inputs and the respective k-clock outputs are particularly preferably equal.

[0147] From the exemplary embodiments, it becomes particularly clear that the method and the optical coherence tomography system proposed herein enable reliable measurement and geometric characterization of objects and are suitable for determining not only relative object geometries, but also absolute distances with respect to the measuring system and the object. Furthermore, the method is suitable for characterizing hot and / or moving objects. In particular, the method is suitable for characterizing objects, for example, during a manufacturing process, wherein the characterization can be carried out essentially in real time. A. REFERENCE SYMBOL

[0148] 1 object

[0149] 2 swept-source lasers or lasers

[0150] 3 Circulator

[0151] 4 Collimator

[0152] 5 beam splitters

[0153] 6 first exit

[0154] 7 second exit

[0155] 8 Performance monitoring unit

[0156] 9 variable optical attenuator

[0157] 10 High-speed detector

[0158] 11 OCT signals

[0159] 12 Laser radiation

[0160] 13 stationary, partially reflective optics, stationary reference substrate

[0161] 14 galvanometer scanners or scanners

[0162] 15 microlens array

[0163] 16 additional beam splitters

[0164] 17 connections for additional beam splitters

[0165] 18 additional variable optical attenuator

[0166] 19 Retroreflector

[0167] 20 K-Clock

[0168] 21 Resampling

[0169] 22 Synchronization of start times

[0170] 23 zero-phase low-pass filtering

[0171] 24 Calculation of phase development

[0172] 25 Extraction of the phase curve ("phase unwrapping")

[0173] 26 Applying a window function

[0174] 27 Fast Fourier Transformation (FFT)

[0175] 28 OCT periodogram

[0176] 29 k-Clock Interferometer

[0177] 30 spacers

[0178] 31 Spacer 32 Gap I Air gap

[0179] 33 distal end

[0180] 34 first mirror

[0181] 35 second mirror

[0182] 36 retaining ring

[0183] 37 Tip-Ti It device

[0184] 38 kClock unit

[0185] 39 beam splitters

[0186] 40 detector

[0187] 41 Circulator t Time

[0188] W wall thickness

[0189] ID inner diameter

[0190] OD outer diameter

[0191] D Distance

[0192] OS OCT signal I strength

[0193] X point of impact, point

[0194] CI input k-clock interferometer

[0195] C2 Output k-Clock Interferometer

[0196] CI* Output k-Clock Interferometer

[0197] DE Input Detector

[0198] DE1 first input detector

[0199] DE2 second input detector

[0200] DA output detector

[0201] SE input beam splitter

[0202] SA1 first output beam splitter

[0203] SA2 second output beam splitter

[0204] P1-P3 ports, circulator

[0205] S mirror surface

Claims

Claims 1. Method for the optical characterization of a transparent or semi-transparent object (1), in which object dimensions (W, ID, AD), object distances and / or object positions are determined based on optical coherence tomography, characterized in that a swept-source laser source with a coherence length in the decimeter to meter range is used as the light source (2), and a reference substrate (13) is used to determine absolute distances, dimensions and / or positions in the beam path (12) of the light source (2), which reference substrate comprises a partially reflecting optic that is stationary in the beam path (12).

2. Method according to claim 1, characterized in that the swept-source laser source (2) comprises a MEMS VCSEL based light source or an akinetic laser light source and / or that the coherence length is in the range of 0.2 m to 100 m or more.

3. Method according to claim 1 or 2, characterized in that during signal acquisition and / or signal evaluation of the coherence tomography a. a k-clock resampling (21) is used, b. minima, maxima or zero crossings of the k-clock of the laser source (2) are determined algorithmically and used as a sampling clock, or c. a k-clock hardware trigger is used.

4. Method according to one of the preceding claims, characterized in that in the optical coherence spectroscopy, the object (1) is scanned with laser light (12) of the light source (2) using a galvanometer scanner (14), using a microlens array (15) and / or a bifocal lens, preferably with a high numerical aperture.

5. Method according to the preceding claim, characterized in that the object (1) is moved, and a scanning device (1) used scanning path at least partially follows the movement of the object (1) and that, preferably, the data acquisition of the optical coherence tomography is synchronized with the scanning path.

6. Method according to the preceding claim, characterized in that a spatial jitter is superimposed on the scanning path.

7. Method according to claim 5 or 6, characterized in that several, in particular a multiplicity of reflection profiles obtained along the scanning path are combined, periodogram signals are determined from the combined reflection profile thus obtained, and at least one object dimension (W, ID, AD), at least one object distance and / or at least one object position are determined from at least one periodogram signal.

8. Method according to one of the preceding claims, characterized in that an infrared filter, preferably a dual-band mirror, is arranged in the beam path (12) of the light source (2) and thermal radiation emanating from the object (1) and / or the object environment in the range of the far infrared spectrum is at least predominantly filtered out using the infrared filter.

9. Method according to one of the preceding claims, characterized in that the optical coherence tomography comprises a signal recording of the k-clock (20) with a software-based filtering (23) and an offset correction (24), and further, optionally, a subsequent phase extraction (25), in particular based on a Hilbert transformation and / or an evaluation of zero crossings and extrema, wherein, further optionally, the filtering (23) comprises a low-pass filtering.

10. Method according to one of the preceding claims, characterized in that periodogram signals are determined from signals of the optical coherence tomography by means of an automatic peak-picking algorithm from periodogram data (28), and from the Periodogram signals at least one object dimension (W, ID, AD), at least one object distance and / or at least one object position, in particular taking into account periodogram signals of the reference substrate (13), are / is determined.

11. Optical coherence tomography system designed for the optical characterization of a transparent or semi-transparent object (1) based on optical coherence tomography, comprising at least one swept-source laser source (2) with a coherence length in the range from 0.2 m to 100 m or, in particular in the meter range, a reference substrate (13) arranged in the beam path of the at least one light source (2), which reference substrate (13) comprises a partially reflective optical system that is stationary in the beam path (12), and a control unit with a processor, in particular a processor with an architecture for parallel data processing, and a memory assigned to the processor, which memory comprises instructions that, when executed by the processor, bring about a method according to one of claims 1 to 10.

12. Optical coherence tomography system according to claim 11, wherein the swept-source laser source (2) comprises a MEMS VCSEL-based light source or an akinetic laser light source, a coherence length of the light source (2) is in the range of 0.2 m to 100 m or more and / or the light source (2) has a bandwidth in the range of 20 nm to 100 nm, and / or of at least 40 nm, and / or is set up for repetition rates in the range of 4 kHz to 4 MHz, in particular of approximately 10 kHz.

13. Optical coherence tomography system according to claim 11 or 12, further comprising at least one galvanometer scanner (14) configured to scan the object and / or at least one microlens array (15) configured to scan the object (1).

14. Optical coherence tomography system according to one of claims 11 to 13, further comprising at least one infrared filter arranged in the beam path (12) of the light source (2), preferably a dual-band mirror, which is designed to at least predominantly filter out thermal radiation emanating from the object (1) and / or the object environment in the range of the far infrared spectrum.

15. Optical coherence tomography system in particular according to one of claims 11 to 14, comprising a k-clock unit (38) for generating a k-clock signal for the signal acquisition and / or signal evaluation of the coherence tomography, wherein the k-clock unit (38) has a Fabry-Perot interferometer (29) which comprises a first mirror (34), a second mirror (35) and a spacer (30), wherein the mirrors (34, 35) are attached to opposite sides of the spacer (30) and spaced from one another by the spacer (30), and mirror surfaces (S) of the mirrors (34, 35) face the spacer (30) and are aligned plane-parallel to one another, wherein the first and second mirrors (34, 35) are preferably wedge-shaped.

16. Optical coherence tomography system according to claim 15, wherein the spacer (30) is made of or consists of a material which has a negligibly low thermal expansion coefficient, in particular is made of or consists of a material for which temperature-related length changes of the spacer (30) in the respective operative temperature range, in particular perpendicular to the mirror surfaces (S), are negligible in relation to the wavelengths of the laser light of the swept-source laser source (2), wherein the material preferably has a thermal expansion coefficient in the range of - 2 * 10' 7 / K to 2 * 10' 7 / K.

17. Optical coherence tomography system according to claim 16, wherein the spacer (30) is hollow-cylindrical and forms a hollow-cylindrical gap (32) or air gap (32) between the mirror surfaces (S), wherein the gap (32) or air gap (32) in the region of the contact or the contact surface between the mirrors (34, 35) and the spacer (30) is sealed by anodic joining, by optical contact bonding or by cementing.

18. Optical coherence tomography system according to claim 16 or 17, wherein for feeding the k-clock unit (38), in particular the Fabry-Perot interferometer (29), with laser light from the light source (2), a beam splitter (39) connected to the light source (2) on the one hand and to the k-clock unit (38) on the other hand is provided, wherein the beam splitter (39) is preferably configured to feed the k-clock unit (38) via an input (CI) associated with the first mirror (34) with a proportion of the laser light from the light source (2) in the range of 0.5% to 2%, in particular of approximately 1%, and / or comprising a detector (40) with at least one input (DE), which is light-optically coupled to an output (C2) associated with the second mirror (35) of the k-clock unit (38), in particular of the Fabry-Perot interferometer (29), for detecting Interferometry signals from the Fabry-Perot interferometer (29),wherein the optical coherence tomography system further comprises a data processing unit configured to evaluate detected interferometry signals and to generate a k-clock signal based on the evaluation.

19. Optical coherence tomography system according to claim 18, wherein, to improve the signal-to-noise ratio and to suppress interference signals, the detector (40) is implemented as a balanced detector (40), wherein, preferably, the balanced detector (40) has two inputs (DE1, DE2), and a first detector input (DE1) is light-optically coupled to an output (CI*) of the Fabry-Perot interferometer (29) associated with the first mirror (34), and a second detector input (DE2) is light-optically coupled to an output (C2) of the Fabry-Perot interferometer (29) associated with the second mirror (35), wherein the optical coherence tomography system further preferably further comprises a circulator (41) with three ports (PI, P2, P3), wherein the laser light source (2) is light-optically coupled to a first port (PI) for feeding the Fabry-Perot Interferometer (29) with laser light from the laser light source (2) via a first mirror (34) associated input (CI), and the second detector input (DE1) is light-optically coupled to a third port (P3) of the circulator (41), and the circulator (41) is set up such that interferometry signals of the Fabry-Perot interferometer (29) emitted to the second port (P2) via an output (CI*) of the Fabry-Perot interferometer (29) associated with the first mirror (34) are directed via the third port (P2) to the first connection (DE1) of the detector (40).

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