Method and optical coherence tomography system for optically characterizing transparent or translucent objects

Abstract

The present invention relates in particular to a method and an optical coherence tomography system for optically characterizing a transparent or translucent object (1), in which object dimensions (wall thickness, inner diameter, outer diameter), object distance, and / or object position are determined based on optical coherence tomography (OCT), a swept-source laser source with a coherence length in the decimeter to meter range is used as the laser source (2), and a reference substrate (13) is used in the beam path (12) of the light source (2) to determine the absolute distance, dimension, and / or position, the reference substrate comprising a partially reflective optical unit fixed in the beam path (12).

Description

[Technical Field]

[0001] In particular, the invention relates to a method for optically characterizing a transparent or translucent object and to an optical coherence tomography system.

[0002] In the prior art, it is known to inspect transparent or semi-transparent objects, for example, by optical coherence spectroscopy (OCT). For example, U.S. Pat. No. 10,890,431 discloses a VCSEL (vertical-cavity surface-emitting laser) OCT system for three-dimensional measurement of transparent objects. In this known method, a 3D depth profile of the object is captured in a scan using an optical coherence tomography system.

[0003] Known systems and methods for optical coherence spectroscopy and optical characterization of transparent or translucent objects still have room for improvement, particularly with respect to the type, size, and structure of the object being measured, and the boundary conditions of the measurement or geometric characterization of the object.

[0004] In light of this, an improved or novel method for optical coherence spectroscopy, in particular for optically characterizing transparent, semitransparent, or translucent objects, is provided, as well as a corresponding optical coherence tomography system.

[0005] This problem is solved in particular by the combination of the features of the independent claims. Advantageous embodiments emerge in particular from the dependent claims and the following description.

[0006] According to an embodiment, a method for optically characterizing a transparent or translucent object is provided. Optically transparent should generally be understood as meaning semi-transparent, and thus may also include, for example, semitransparency or translucency. In the strict sense, the term "optically transparent" in the context of the present invention may mean transparency to the infrared and / or visible spectrum. In particular, infrared-transparent objects should be included. Preferably, the method uses objects made of inorganic or non-biological materials.

[0007] The proposed optical characterization determines object dimensions, object distances, and / or object positions based on optical coherence tomography (OCT for short), which, as already explained in the introduction, is a well-known method for characterizing objects based on coherent optical radiation.

[0008] Optical coherence spectroscopy is known per se, see for example "Cubic meter volume optical coherence tomography", Fujimoto et al., vol. 3, no. 12 / December 2016 / Optica, regarding the optical characterization of objects by OCT, and "Application of a long-range swept source optical coherence tomography based scheme for dimensional characterization of multilayer transparent objects", Eneas N. Morel, Nelida A. Russo, Jorge R. Torga, Ricardo Duchowicz, Opt. Eng. 56(8), 084102 (2017).

[0009] The proposed method is characterized in that a swept-source laser source, in particular a tunable laser source, preferably a wavelength-tunable laser source, with a coherence length in the decimeter to meter range is used as the light source. Furthermore, the proposed method is characterized in that a reference substrate is used in the beam path of the light source, the reference substrate comprising a partially reflecting optical unit fixed in the beam path. The reference substrate is used to determine the absolute distance and / or position and / or dimensions of, to, or from the object.

[0010] The invention is based on the finding that the use of a swept source with a reference substrate is suitable for characterizing at least partially transparent or translucent objects, in particular moving objects, and provides relatively reliable and accurate measurement results, in particular at least partially transparent or translucent objects that, for example, move during the measurement and / or emit non-negligible thermal radiation and / or are placed in an environment that emits non-negligible thermal radiation.

[0011] As mentioned above, the proposed combination is suitable for measuring and / or characterizing high-temperature objects, such as partially molten glass or objects made from glass. The method is particularly suitable for optically characterizing glass cylinders or cylindrical glass objects during or after production, where the objects can be in an at least partially molten state or not yet fully solidified. For example, the proposed method allows for determining the outer and / or inner diameter and / or wall thickness of the object.

[0012] In particular, the proposed method also provides advantageous axial resolution since, in contrast to known methods, the proposed method works without a Bragg grating, which results, for example, from the fact that the method proposed herein is not substantially limited or restricted in terms of the number of reference or measurement points, in contrast to methods based on Bragg gratings.

[0013] Furthermore, the proposed method allows for the measurement of both absolute and relative distances, thus enabling improved optical characterization.

[0014] The proposed method also allows for the characterization of transparent or translucent objects in substantially real time, for example during a production or manufacturing process. In particular, the method is not limited to stationary objects, but rather allows for the characterization of moving objects.

[0015] According to an embodiment, as light source, a swept-source laser source is used, including a MEMS VCSEL-based light source (MEMS: micro-electro-mechanical system; VCSEL: vertical-cavity surface-emitting laser) or an akinetic light source. Such light sources particularly preferably have 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.

[0016] The proposed method using a MEMS VCSEL light source and reference substrate is advantageously suited to a relatively large coherence length range, allowing particularly accurate coherence / interferometry measurements over substantially the entire range. For example, a long coherence length can be used when the distance between the measurement head or measurement device and the object is larger, for example when the object is a hot object such as (partially) molten glass and a certain distance to the object must be maintained due to the temperature.

[0017] According to an embodiment, during signal acquisition and / or signal evaluation of an optical coherence tomography (OCT) signal, ak clock resampling is used, b. The minimum, maximum, or zero crossings of the laser source's k-clock are algorithmically determined and used as the sampling clock; or It may be provided that a ck clock hardware trigger or a k clock hardware signal is used.

[0018] The k-clock allows data to be recorded linearly in time, which can then be converted into linear samples in k-space. In particular, high-frequency signals can be more easily evaluated as a result. Furthermore, objects can be characterized in quasi-real time, which is advantageous for example for the geometric characterization of moving transparent or translucent objects during a manufacturing process.

[0019] The method according to b. for algorithmically determining the minimum, maximum or zero crossing of the k clock can be, for example, as follows or can include the following steps: b1) normalization of the k clocks by division by the envelope; b2) performing at least three, preferably directly consecutive, mathematical operations such as absolute value formation, addition, absolute value formation; b3) smoothing, for example by a median filter; b4) peak finding or maximum value determination; b5) interpolation of maxima, minima and zero crossings into linear k-space.

[0020] By means of the absolute value operation b2), the minimum, zero crossing and maximum values ​​of the k clock can be determined using the proposed method steps as an example.

[0021] According to an embodiment, in optical coherence spectroscopy, an object can be scanned with laser light from a light source using a galvanometer scanner, or galvanometer scanner for short, preferably using a microlens array and / or a bifocal lens with a high numerical aperture. Galvanometer scanners, microlens arrays, and bifocal lenses have proven particularly suitable for optical characterization of moving objects, for example, during manufacturing processes. Such components are particularly suitable for measuring or characterizing moving, partially or partially melted, transparent cylinders, for example, made of glass.

[0022] Regarding the operating mode of the galvanometer scanner, for example, for a moving object, e.g., a rotating, axially moving object, particularly an object that rotates relatively quickly and moves relatively slowly axially, the galvanometer scanner can be programmed so that a point on the object or on the surface of the object, particularly a point or area targeted by the galvanometer scanner, is within the reference frame of the galvanometer scanner. The object can be, for example, a cylinder, an object having a cylindrical shape, or a cylindrical object. This is particularly possible within the context of the deflection achievable or available with the galvanometer scanner. When the galvanometer scanner reaches the end of its deflection or its deflection range, it is reset with respect to the deflection and operated to track or target the next point on the surface.

[0023] Synchronization between the galvanometer scanner and data acquisition, on the one hand, and the rotation of the object, on the other hand, can be achieved, for example, by an angle sensor. Generally, the movement of the object, whether translational and / or rotational, can be detected by a sensor unit or sensor technology, and the detected movement data can be used for synchronization. Based on such data, for example, a movement path of a point on the object, for example, on the surface of the object, particularly a target point or target area, can be determined. Based on the determined movement path, the galvanometer scanner and data acquisition, on the one hand, and the movement of the object, on the other hand, can be synchronized. Preferably, the movement or movement path is determined continuously, particularly in real time, and is used to control and synchronize the movement, on the one hand, and the galvanometer scanner and data acquisition, on the other hand.

[0024] Regarding the function or operation of the microlens array, it can be provided that the light beam under the OCT is aligned along the direction of movement of an object, in particular a point on the object surface, or along a movement path or trajectory, and scanning is performed simultaneously at or for the trajectory points or surface points. For example, if the movement or trajectory is known, data acquisition and movement can be synchronized, i.e., points on the object or on the object surface can be tracked with respect to data acquisition.

[0025] When using a two-dimensional (2D) microlens array, it is possible to scan the (immediate) vicinity of the trajectory, in particular the area around the trajectory or the location of interest on the object, which is particularly suitable for cylindrical or cylindrically shaped objects.

[0026] The scanning by the microlens array is preferably stationary, and more preferably at the repetition rate of the laser.

[0027] Regarding bifocal lenses or sensor heads with bifocal lenses, it should be noted that this can be used, for example, to optimize the signal strength reflected back from the sample or object. This is particularly true when the sample or object, for example, (a) has a periodic variation in inner diameter (e.g., 50 mm over a 500 mm axial range), e.g., when there is an axial slope of the inner surface depending on the object's position, and / or (b) has a significant stepwise change in outer diameter across the axis, or when measuring objects with a wide range of outer diameters (e.g., 200 mm to 400 mm). For condition (a), a sensor head with a high numerical aperture is advantageously used, since the light reflected back from the sloped surface propagates at a significant angle. In addition to a large aperture, it can be useful to separate the excitation of the sample from the detection of the back-reflected light so that collimated excitation is still possible. For condition (b), a sensor head with a bifocal lens is preferably used, which can map the front and back focal points to the position of the receiving fiber end, regardless of the object's diameter / size. For these reasons, a bifocal design is particularly advantageous for the above conditions.

[0028] A bifocal lens or bifocal asphere can, for example, consist of two semi-convex lenses with foci f and f connected to one another, the connecting surface extending parallel to the axis of, for example, a cylindrical object. In another embodiment, the bifocal asphere can be formed similarly to a zone plate with circular portions with alternating focal lengths f, f, f, f, f, etc., which embodiment can be used substantially independently of the properties of the object to be inspected.

[0029] Another variant of the optical setup with two foci is to use a single aspheric surface, duplicate the same receiving optical element, e.g., a fiber, in orthogonal arms (downstream of the aspheric surface in the beam path), and deflect the incident beam into both arms via a beam splitter. In this case, the focus of one receiving arm is preferably adjusted in front of the object, and the focus of the other receiving arm is adjusted behind the object. The signals received by the element or fiber are captured by optical multiplexing or by an optical combiner.

[0030] If the dimensions of the object to be measured are (roughly) known in advance or are substantially known, the lens distance of the bifocal setup can be automatically adjusted to the optimum bifocal configuration.

[0031] In embodiments, the probe arm or fiber and the receiving arm or fiber may be formed by different elements or fibers. The collimated beam emerging from the probe fiber may be collinearly coupled into a receiving fiber with a high numerical aperture, for example, via a microprism or a glass plate with a polished end face, for example, with a glued, angled, partially reflective surface. This prevents blocking of the light reflected back from the sample.

[0032] When an inclined surface of an inspected object refracts a light beam at a small angle, regions on the opposite surface of the object that are not exactly opposite the object's central axis become "excited." This can lead to errors in determining distance, size, and / or position. In particular, in embodiments, the deviation can be compensated for by placing an array camera or plenoptic camera behind the sample, which can be used to record or determine the direction or angle of the transmitted light. The direction or angle can be used to correct the distance, size, and / or position, for example, through ray tracing.

[0033] Depending on the embodiment, a single focus setup can also be used. In this case, it may happen that not all four reflections of the surface of a cylindrical object can be recorded for all axial positions of the object, e.g., cylindrical. In such cases, the position of the receiving fiber and / or the lens distance of the optical setup can be optimized so that at least three of the four reflections of the cylindrical object can be detected, allowing for the continuous determination of, e.g., the inner diameter, the outer diameter, and the wall thickness after, e.g., the first 180° rotation.

[0034] Depending on the embodiment, as already mentioned, the object may be a moving object. When scanning a moving object, the scanning path used at least partially follows the movement of the object. Advantageously, in this embodiment, the optical coherence tomography data acquisition is synchronized with the scanning path.

[0035] In other words, the scanning path may move at least partially or partially substantially synchronously with the movement of the object. In particular, it is a finding of the underlying invention that scanning a moving object based on the light source, coherence length and reference substrate used allows characterization, in particular measurement, of the object or parts thereof even when the object is moving. As already mentioned above, in particular a galvanometer scanner and / or a microlens array can be used to scan the moving object, whereby the synchronization between the galvanometer scanner and / or the microlens array and the data acquisition is or will be synchronized with the movement.

[0036] According to an embodiment, spatial jitter is superimposed on the scanning path of the galvanometer scanner. In particular, such jitter is advantageous for irregularly shaped objects, and also supports data acquisition for moving objects. Jitter can be generated, for example, by appropriate control of the galvanometer scanner. Jitter is not necessarily required when the object surface is ideally perpendicular to the OCT light beam. Because real objects, especially those undergoing a manufacturing process, vary in surface orientation, the use of jitter can improve the accuracy and / or reliability of the acquired object data. In other words, jitter makes it possible to obtain, at least to some extent, back-reflected or reflected light from an object, even if the object surface is irregular. Jitter can be adjusted, for example, by superimposing a deflection or movement perpendicular to the trajectory of a point on the object onto the movement of the galvanometer scanner following the trajectory of the point.

[0037] According to an embodiment, the method combines several, in particular a plurality of, reflection profiles (or A-scans) acquired along the scan path, in particular to form a combined reflection profile, from which a periodogram signal can be determined and at least one object dimension, object distance and / or object position can then be determined from at least one periodogram signal, i.e. from one or more periodogram signals.

[0038] In particular, the distance between the boundary surfaces of the objects and / or their position or location relative to each other can be determined relative to a precisely known reference formed, for example, by a fixed partially reflecting optical unit.

[0039] It is therefore possible to determine object dimensions, distances and positions relatively accurately and reliably, especially when the object is moving and / or when the structure or shape of the object changes within certain limits.

[0040] According to embodiments, it is possible to filter out, for example, A-scans that have a number of back reflections that is expected from the shape of the object to be inspected, e.g. flat, cylindrical, etc. The expected number of back reflections in OCT can, for example, be determined or derived from the (coarse) shape, the target shape, and / or the expected shape of the object to be inspected. The coarse, target, and / or expected shapes can be provided as parameters or parameter datasets, for example as input parameters or datasets, in particular as input parameter datasets, to the method, e.g. a corresponding evaluation unit.

[0041] With regard to the selection of A-scans to be used or used for optical characterization, it is additionally possible to use, e.g., filter out, from a group or set of recorded A-scans, A-scans that show or have the greatest intensity and / or have the best S / N (signal-to-noise ratio).

[0042] In this case, the periodogram is understood in the usual sense as a distance signature, or distance signature for short, of the reflections caused in particular by the boundaries of the object.

[0043] According to an embodiment, an infrared filter, preferably a dual-band mirror, is arranged in the beam path of the light source, providing that thermal radiation in the far-infrared spectral range emitted by the object and / or its surroundings is at least largely filtered out using the infrared filter. The infrared filter is preferably arranged in the beam path upstream of the measurement head used to detect light reflected from the object. In particular, it is possible to prevent radiation in the infrared spectral range of thermal radiation or external radiation from interfering with the measurement or to improve the signal-to-noise ratio of light reflected from or on the object. The use of an infrared filter is advantageous, for example, in the characterization of hot objects or objects that themselves and / or their surroundings emit radiation in the infrared spectral range of thermal radiation, at least to the extent that this leads to a degradation of the measurement of the light reflected from the object by OCT. With regard to infrared radiation, in particular, the spectral range corresponding to a temperature range of 800 K to 2200 K, assuming a blackbody radiator, is considered.

[0044] According to an embodiment, it can be provided that the optical coherence tomography comprises signal recording of the k-clock of the light source with software-based filtering and offset correction, and optionally further comprises subsequent phase extraction, in particular based on the Hilbert transform and / or evaluation of zero crossings and extrema. The filtering can preferably be low-pass filtering.

[0045] Based on the signal recording of the k-clock, linearization in time is possible, which in particular allows for real-time characterization of the object. Subsequent phase interpolation, i.e., phase interpolation of the phase profile of the measurement signal, allows the data or measurement signal, which are recorded linearly in time due to the k-clock used, to be converted into a linear sampling in k-space. This allows for an improvement in the resolution of the method or the system that performs the method.

[0046] Depending on the embodiment, the extraction of the phase profile and the re-interpolation of the data based on this phase profile can be performed for each individual sweep of the swept-source laser, thereby ensuring, for example, that the system, and in particular the signal evaluation and / or processing, is robust to variations in the laser's phase profile.

[0047] According to an embodiment, filtering, particularly low-pass filtering and / or phase interpolation, can be performed after each sweep of the laser. For example, a 10 kHz laser has a frequency of 10 kHz. Particularly advantageous for processing and / or handling measurement signals at such frequencies is an architecture with parallel data processing, such as an implementation on a graphics processing unit (GPU). For example, in an application, the sampling rate of the digitizer can result in a laser repetition rate of 10 kHz to 100 kHz, which, depending on the laser duty cycle, can result in relatively high data rates in the range of 5 to 7 GB / s (gigabytes per second), which cannot be handled satisfactorily or at all by conventional CPUs. Therefore, advantageously, processing and / or handling of the measurement signals is performed on a GPU, which can be implemented, for example, with CUDA (Computer Integrated Device Architecture).

[0048] Examples of filters include band-pass filters, median filters, etc. Examples of interpolators include polynomial interpolation, Chebyshev interpolation, etc.

[0049] According to an embodiment, it is provided that the periodogram signal is determined from the optical coherence tomography signal by an automatic peak picking algorithm from the 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 signal, in particular taking into account the periodogram signal of a reference substrate.

[0050] In this case, the term "peak picking" (or peak finding) is to be understood in the prior art sense as an algorithm capable of determining, for example, local maxima or minima from a signal, in particular a measurement signal and / or a processed measurement signal. Such an algorithm may, for example, involve the determination of zero crossings of the numerical first derivative of the signal, which zero crossings are associated with respective peaks, in particular local maxima or minima.

[0051] In particular, peak picking algorithms can use data on the (rough) shape and / or target and / or expected shape of an object as input parameters or boundary conditions. In particular, when manufacturing an object, such as a glass object such as a glass cylinder, the shape of the object or the desired shape of the object is known or specified. For example, in the case of a cylindrical object to be manufactured, if the target diameter, e.g., the inner and / or outer diameter and / or wall thickness of the object, is known or specified, these data provide at least an approximate location or position of the expected signal, which can be calculated or determined, for example, taking into account the measurement device. For example, when a cylindrical object is measured with light incident approximately radially, reflection signals, i.e., peaks, are expected for reflections on the outer and inner surfaces of the wall of the cylindrical object.

[0052] Based on the measurement signals and specification data relating to the object shape and / or the measurement setup, it is possible to monitor the manufacturing process, i.e. characterize the object during production, in particular with regard to whether the manufactured object meets basic requirements regarding shape and / or dimensions. Quality control and / or control of the manufacturing process are therefore possible. Furthermore, it is possible to provide, for example, a correlation between deviations in the object shape and operating parameters of the production plant and / or a fault analysis of the production plant based on deviations in the object shape.

[0053] The advantages of the proposed method are also, in particular, that it allows real-time characterization, which makes it possible to optimize the production process and / or characterize the quality of the manufactured product already during production: characterization of complex objects after production can be omitted.

[0054] In particular, the fixed partially reflective optical unit proposed in this specification makes it possible not only to determine relative object distances or object dimensions based on the measurement signal, in particular the peaks, but also to determine (absolute) distances, dimensions and / or positions based on the measurement system or optical measurement system used.

[0055] By way of example, according to the specific measurements carried out, the proposed method can be used in particular to characterize cylindrical transparent or translucent objects, such as cylinders, e.g., glass cylinders, with diameters of 50 mm to 700 mm and wall thicknesses of 0.05 mm to 100 mm. The proposed method proves to be suitable for a relatively wide range of applications.

[0056] According to an embodiment, an optical coherence tomography system designed to optically characterize transparent or translucent objects based on optical coherence tomography (OCT) is provided. The coherence tomography system comprises at least one swept-source laser source having a coherence length in the range of 0.2 m to 100 m or more, particularly in the meter range; a reference substrate arranged in the beam path of the at least one light source, e.g., the reference substrate comprising a partially reflective optical unit fixed in the beam path; a control unit having a processing unit, particularly a processing unit having an architecture for parallel data processing, such as a graphics processing unit; and a memory associated with the processing unit, the memory comprising instructions that, when executed by the processing unit, perform a method according to one of the method-related embodiments proposed herein. For purposes of the present invention, a processing unit with an associated memory designed to perform a method according to one of the embodiments proposed herein is generally understood to mean an electronic unit programmed and / or designed to perform a method according to one of the embodiments proposed herein during operation.

[0057] The advantages and benefits of the coherence tomography system and its design arise in particular from the advantages and benefits of the embodiments according to the present method, and in this respect reference is made in particular to the above description.

[0058] 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 accelerating laser light source, and / or the coherence length of the light source is in the range of 0.2 m to 100 m or more.

[0059] According to one embodiment of the optical coherence tomography system, the optical coherence tomography system further comprises at least one galvanometer scanner designed to scan the object and / or at least one microlens array designed to scan the object, in particular these components enable accurate scanning of the object to be characterized, especially when the object is in motion, for example when the object is moving along a movement path during manufacturing.

[0060] Typically, the movement path of the object or the corresponding movement path is known or predetermined, and the predetermined or known movement path can be used, for example, to control a galvanometer scanner and / or for signal evaluation of data acquired via the microlens array.

[0061] Within the scope of the present invention, it is possible or provided in embodiments to detect or determine the movement, linear and / or rotational movement of an object or the path of movement of the object and thus, at least indirectly, the path of movement of the object area or point being measured in each case. For this purpose, for example, a sensor or measuring system can be provided which is configured to detect the movement of the object.

[0062] According to an embodiment of the optical coherence tomography system, it further comprises at least one infrared filter, preferably a dual-band mirror, arranged in the beam path of the light source, the infrared filter being designed to filter out at least a large portion of thermal radiation in the far-infrared spectrum range emitted by the object and / or its surroundings. Such a coherence tomography system is particularly suitable for characterizing high-temperature objects that emit non-negligible thermal radiation for OCT or that impair OCT results. Such an infrared filter also makes it possible to measure or characterize surrounding objects that emit non-negligible thermal radiation for OCT or that impair OCT results.

[0063] According to an embodiment of the optical coherence tomography system, the light source has a bandwidth in the range of 20 nm to 100 nm and / or a bandwidth of at least 40 nm.

[0064] According to an embodiment of the optical coherence tomography system, the light source is designed for a repetition rate in the range of 4 kHz to 4 MHz, and in particular for a repetition rate of about 10 kHz.

[0065] According to an embodiment, an optical coherence tomography system is provided, which can be designed in particular according to any of the above-described embodiments. The optical coherence tomography system may include a k-clock unit, or 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 includes an interferometer configured as a Fabry-Perot interferometer, i.e., a k-clock interferometer, which may more precisely be called a k-clock Fabry-Perot interferometer. In this case, the above-described embodiments of the method may use a k-clock interferometer having the above-described structure and generating the k-clock signal in the manner described herein. The embodiments of the k-clock unit described herein also apply correspondingly to embodiments of the method, in particular to embodiments for generating a k-clock signal.

[0066] A Fabry-Perot interferometer or k-clock interferometer is designed or intended to generate an interferometry signal based on laser radiation from a swept-source laser source that is fed into the Fabry-Perot interferometer, or is fed into the Fabry-Perot interferometer, where the term "feeding" is intended to mean that the laser light from the laser source is coupled into the Fabry-Perot interferometer via, for example, a light conductor or light guide.

[0067] The Fabry-Perot interferometer comprises a first mirror, a second mirror, and a spacer. The mirrors are attached or connected to oppositely facing sides of the spacer. The spacer ensures that the mirrors are attached or arranged at a distance from each other, the distance being determined by the length of the spacer. In this case, the mirror surfaces of the mirrors face the spacer and are aligned in a parallel plane. Advantageously, the first and second mirrors are wedge-shaped, particularly with respect to a plane perpendicular to the optical axis of the Fabry-Perot interferometer. In this case, the optical axis is an axis extending perpendicular to the plane-parallel mirror surfaces. According to the findings of the present invention, the Fabry-Perot interferometer is particularly suitable for generating a stable, particularly temperature-stable, k-clock signal, particularly for the application underlying the present invention, which characterizes transparent or semi-transparent objects.

[0068] Particularly preferably, the mirror has an anti-reflection layer or coating that prevents the mirror from acting as an etalon or etalonizing in that sense.

[0069] The Fabry-Perot interferometer is therefore advantageously designed as a planar interferometer.

[0070] To fix the mirror on both sides of the spacer, in particular on the flat or plane-parallel sides, holding elements, in particular holding rings, can be provided which press the mirror against the spacer without stress.

[0071] The optical path length of the Fabry-Perot interferometer between the mirrors is preferably defined by the axial length of the spacer, which extends perpendicular to the plane-parallel mirror surfaces. The end faces of the spacer on which the mirrors are arranged are preferably highly parallel, for example with a parallelism of less than 0.005 degrees. Such parallelism provides a quality or finesse that is particularly suitable for generating k-clock signals for optical coherence tomography.

[0072] According to embodiments, the spacers may be made or constructed from a material with an ultra-low or negligible thermal or temperature expansion coefficient. In particular, the spacers should be made or fabricated from a material such that the temperature-related change in the spacer length and the associated change in the distance between the mirrors is negligible over the operating temperature range of the interferometer relevant to the application contemplated herein, e.g., ambient temperature, e.g., a temperature change of 15°C to 30°C. Preferably, the spacers should be made or constructed from a material such that the temperature-related change in the spacer length, particularly perpendicular to the mirror plane, over the respective operating temperature range is negligible relative to the wavelength of the laser light from the swept-source laser source. The corresponding material preferably has a thermal expansion coefficient within 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).

[0073] Preferably, the material can be processed, and in particular processed or manufactured into a hollow cylindrical shape, so that an angular error of 0.002° to 0.005° or less can be implemented in the case of a hollow cylindrical shape on each end face of the hollow cylinder relative to the plane-parallel support surface of the mirror on the end face of the spacer.

[0074] According to an embodiment, the spacer is hollow cylindrical and forms a hollow cylindrical gap or air gap between the mirror surfaces, which is delimited, for example, in the circumferential direction by the outer wall of the spacer. The gap or air gap can be sealed by anodic bonding, optical contact bonding or cement bonding in the region of the contact or support surface between the mirror and the spacer, i.e., the region where the mirror is located on or abuts or is in contact with the spacer. By such means, the gap or air gap or vacuum gap can be sealed against the external atmosphere to avoid interference.

[0075] According to an embodiment, a beam splitter connected to the light source on the one hand and the k-clock unit on the other hand may be provided for supplying the laser light from the light source to the k-clock unit, in particular a Fabry-Perot interferometer, and the beam splitter is preferably designed to supply a fraction of the laser light from the light source, in the range of 0.5% to 2%, in particular about 1%, to the k-clock unit via an input associated with the first mirror. The remaining laser light, for example 99% or a corresponding fraction, may be supplied to a sensor head or sensor unit or scanning unit of an optical coherence tomography system, for example for recording or performing an A-scan signal (so-called A-scan).

[0076] According to an embodiment, the optical coherence tomography system, in particular the k-clock unit, can comprise a detector, in particular a high-speed detector, having at least one input optically coupled to an output associated with the second mirror of the k-clock unit, in particular the Fabry-Perot interferometer, for detecting the interferometric signal of the Fabry-Perot interferometer, wherein the second mirror is preferably designed as a transparent or semi-transparent mirror, so that the interferometric signal can be coupled in at the second mirror and fed to the detector.

[0077] According to an embodiment, the optical coherence tomography system, in particular the k-clock unit, can further comprise a data processing unit designed to evaluate the detected interferometry signal, i.e., to evaluate the detection signal of the detector, and to generate a k-clock signal based on the evaluation of the detected interferometry signal. Thus, the data processing unit can be designed to generate a k-clock signal based on the interferometry signal of the k-clock interferometer, the k-clock signal being provided, for example, to perform 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.

[0078] According to an embodiment of the optical coherence tomography system, the detector can be implemented as a balanced or symmetric detector for the purpose of improving the signal-to-noise ratio and suppressing interference signals. The balanced detector can have two inputs, where a first detector input is optically coupled to the output of a Fabry-Perot interferometer or k-clock interferometer associated with a first mirror, and a second detector input is optically coupled to the output of a Fabry-Perot interferometer associated with a second mirror. In this case, the mirrors are preferably designed as transparent or semitransparent mirrors, respectively.

[0079] According to an embodiment, the optical coherence tomography system may further include a circulator having three ports, terminals, or terminal interfaces, wherein the laser light source is optically coupled to the first port of the circulator via a second port connected to the input associated with the first mirror to supply laser light (e.g., 0.5 to 2%, particularly 1% of the laser light) from the laser light source to the Fabry-Perot interferometer. A second detector input of the detector is optically coupled to a third port of the circulator, and the circulator is designed such that an interferometric signal of the Fabry-Perot interferometer emitted to the second port via the output of the Fabry-Perot interferometer associated with the first mirror is directed to the first terminal of the detector via the third port of the circulator.

[0080] Particularly preferably, the amplitudes of the signals supplied to the first and second terminals of the detector are adapted to one another before being supplied to the detector. For this purpose, for example, an in-line fiber attenuator or attenuator connected to and / or integrated in the light-to-light path between the k-clock unit and the detector, for example an optical fiber or other light guide element, is suitable.

[0081] The optical-optical coupling of the aforementioned components, such as the laser source, the beam splitter, the circulator, the k-clock interferometer, the detector, etc., can be achieved, inter alia, via light guides or optical fibers or optical fiber bundles. Preferably, the optical-optical connections between the terminals of the detector and the terminals of the k-clock interferometer have equal optical path lengths. This means that the optical-optical connection connecting the output of the first mirror to the first terminal of the detector and the optical-optical connection connecting the output of the second mirror to the second terminal of the detector have the same optical path length.

[0082] The proposed k-clock unit can be used to improve, and in particular optimize, the measurement accuracy of the swept-source OCT system proposed herein, particularly the swept-source OCT sensor system. Therefore, measurement accuracy, particularly for the intended application, depends on the stability of the k-clock or k-clock unit, which, in the above-described embodiment, comprises or forms an interferometric device and measures the wavelength sweep of a swept-source laser 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 difference of the k-clock unit, particularly the k-clock interferometer. The proposed design and use of the k-clock unit minimizes corresponding environmental influences and allows for an FSR that is sufficient to perform accurate measurements, particularly for the applications envisioned herein.

[0083] Compared to known methods for optically characterizing transparent or translucent objects using OCT, the proposed method and OCT system according to the embodiments described herein can implement a relatively large axial range, i.e., the range measured parallel to the optical radiation, especially for objects having diameters of up to 100 mm or more, e.g., up to 700 mm or more.

[0084] Furthermore, in embodiments, the proposed method allows for characterizing moving objects, for example during a manufacturing process in which the object is moved, for example by rotation and / or translation, an example of this being the manufacturing of cylinders or objects made from glass.

[0085] The proposed method and the corresponding embodiment of the optical coherence tomography system particularly and advantageously enable the optical characterization by OCT of high-temperature objects, i.e., objects that emit relatively strong blackbody radiation, i.e., thermal radiation. Examples include the manufacture 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 their shapes during hot forming during manufacture, which allows for real-time adaptation and monitoring of process conditions for manufacturing and / or reduction of waste. Characterization steps that are usually required after manufacturing can also be omitted, which simplifies and, in particular, shortens the manufacturing process.

[0086] In embodiments, the proposed method and optical coherence tomography system allow for the measurement or determination of absolute and relative distances, in particular in contrast to known methods and devices. In particular, there is no need to provide separate devices for determining relative distances on the one hand and absolute distances on the other hand. Absolute distances may be relevant, for example, for questioning or characterizing a manufacturing process, when, during manufacturing, an object or product moves according to a predetermined path, for example in a straight line.

[0087] By determining absolute distance, for example, deviations from a path can be detected and the manufacturing process can be adjusted, or potential sources of errors or malfunctions in the manufacturing process or production plant can be identified from the deviation data.

[0088] The relative distance can be used to characterize the shape of the object itself, for example, diameter, wall thickness, cross-sectional shape, etc.

[0089] The proposed method and optical coherence tomography system are particularly flexible in use and suitable for a relatively wide range of applications, for example, the method is suitable for measurements with a relatively large axial depth and simultaneously high resolution (e.g., 10 micrometers) and / or measurements with a relatively small axial depth and simultaneously very high resolution (e.g., 1 micrometer).

[0090] Compared to known methods and devices, the proposed method, and in particular the embodiments using the algorithm proposed herein for processing OCT signals, can achieve advantageous resolution, in particular OCT signals or signals derived therefrom with advantageous half-widths. [Brief explanation of the drawings]

[0091] Embodiments of the present invention are described below with reference to the accompanying drawings.

[0092] [Figure 1] 1 illustrates a schematic representation of a first device-based OCT implementation for characterizing an object. [Figure 2] 10A and 10B illustrate schematically a second device-based OCT implementation for characterizing an object. [Figure 3] 10A and 10B illustrate schematically a third device-based OCT implementation for characterizing an object. [Figure 4] 1 shows the progression of a method for determining a periodogram. [Figure 5] 10 shows exemplary OCT measurement results for the characterization of a glass cylinder according to the third implementation. [Figure 6] 10 shows exemplary OCT measurement results for the characterization of a glass cylinder based on the first or second implementation. [Figure 7] 1 is a graph illustrating the properties of a glass cylinder having a relatively large axial depth. [Figure 8] 10 shows exemplary OCT measurements for the characterization of a moving hot glass cylinder. [Figure 9]1 shows an example structure of a k-clock interferometer. [Figure 10] 10 exemplarily shows a first implementation of a k-clock unit having a k-clock interferometer according to FIG. [Figure 11] 10 exemplarily shows a second implementation of a k-clock unit having a k-clock interferometer according to FIG.

[0093] In the drawings, identical or functionally identical elements are designated by the same reference numerals. The drawings merely illustrate exemplary applications or implementations, and the present invention is not limited thereto or to the benefits or advantages resulting therefrom.

[0094] FIG. 1 shows a schematic representation of a first device-based OCT implementation for characterizing an object 1, which may be, for example, a glass hollow cylinder, glass cylinder or cylinder for short.

[0095] The device-based implementation comprises a swept-source laser 2, also referred to as laser 2 for short, followed by a circulator 3. The three terminals of the circulator 3 are connected to the laser 2, a collimator 4, and a beam splitter 5. The beam splitter 5 may have a 90:10 splitting ratio, with a first output 6 associated with a splitting ratio of 90 and a second output 7 associated with a splitting ratio of 10.

[0096] A power monitoring unit 8 is connected downstream of the second output side 7 .

[0097] The first output 6 is followed by a variable optical attenuator 9 (VOA), which is followed by a high speed detector 10 for producing an OCT signal 11 .

[0098] Laser light guides, in particular so-called TEC fibers (TEC: thermally extended core), can be used to connect the components.

[0099] Downstream of the collimator 4, a partially reflecting optical unit 13 or a fixed reference substrate 13 is fixedly connected 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, there is also a galvanometer scanner 14, scanner 14 for short, for beam deflection by deflection angles φ (phi) and θ (theta). The object 1 to be measured or characterized is arranged downstream of the scanner 14 in the beam path.

[0100] During operation for measuring or characterizing object 1, laser radiation 12 emitted from collimator 4 first passes through reference substrate 13 and then hits scanner 14, which directs laser radiation 12 onto object 1. Laser radiation 12 reflected from object 1 travels via scanner 14, collimator 4, and circulator 3 to beam splitter 5, which directs the reflected laser radiation 12 or its signal via first output 6 and variable optical attenuator 9 to detector 10. Detector 10 generates an OCT signal from the reflected laser radiation 12 or a corresponding signal for optical characterization of object 1.

[0101] Based on the scanner 14 or by suitable control of the scanner 14, for example, angles φ (phi) and / or θ (theta), the laser radiation 12 can be directed or emitted onto the object 1 such that the impact point x of the laser radiation 12 on the object 1 follows, for example, the movement of the object 1. In the illustrated example, the object 1 rotates counterclockwise, which is indicated by the curved arrow. If the object 1 simultaneously moves, for example, linearly out of the drawing plane of FIG. 1, suitable control of the scanner 14 can also achieve that the impact point x also follows this linear movement. Thus, moving objects can be optically characterized by OCT.

[0102] 2 shows a schematic diagram of a second implementation of device-based OCT for characterizing an object. In contrast to the implementation according to FIG. 1, here 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, e.g., a 2D microlens array, also allows for tracking of a point x on the object 1 or on the object surface, so that the second implementation also allows for characterizing an object 1 that exhibits rotational and / or translational movement (e.g., due to movement out of the drawing plane of FIG. 2). Apart from the 2D microlens array 15, a 1D microlens array 15 can also be used.

[0103] In a further implementation variation, a galvanometer scanner 14 and a microlens array 15 can be used in combination.

[0104] Otherwise, the structure of the implementation according to FIG. 2 corresponds to the structure shown in FIG.

[0105] The implementations according to Figures 1 and 2 correspond to a setup with autocorrelation. Figure 3 shows a schematic representation of a third device-based OCT implementation for characterizing an object 1, which implementation corresponds to a setup with cross-correlation.

[0106] In the setup shown in FIG. 3, similar to FIGS. 1 and 2, a beam splitter 5 is present, which is connected on the one hand to the 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 terminal of the circulator 3 closest to the terminal of the laser 2 in the circulation direction of the circulator 3 is connected to a first terminal 17.1 of a further beam splitter 16. A second terminal 17.2 of the further beam splitter 16 is connected to another variable optical attenuator 18 and a retroreflector 19. The third terminal 17.3 is connected to a collimator 4, which, as in FIGS. 1 and 2, is designed to collimate the laser radiation 12 onto or reflected from the object 1 to be measured or characterized. The fourth terminal 17.4 is connected to a detector 10 via a variable optical attenuator 9. Thus, in the implementation according to FIG. 3 with cross-correlation, the detector 10 is coupled to both beam splitters 5 and 16 to receive the laser signal. The reference substrate 13 and microlens array 15 are not shown in FIG. 3 but may be present, either separately or combined.

[0107] 4 shows an exemplary method sequence for determining the periodogram. The algorithm uses k-clock 20-based resampling 21 for the OCT signal 11. For performance reasons, the algorithm is executed on a processing unit architecture with parallel data processing, such as a GPU.

[0108] For resampling 21, the start times of the k-clock and the OCT signal are adjusted or aligned 22. Prior to resampling, the k-clock is filtered using a zero-phase low-pass filter 23, followed by calculation of the phase unfolding 24 and extraction of the phase profile 25 (so-called "phase unwrapping"). After the k-clock-based resampling of the OCT signal 21, a window function is applied 26, and then a fast Fourier transform (FFT) 27 is applied to the resampling result to determine an OCT periodogram 28. From the OCT periodogram 28, geometric parameters or variables characterizing the object 1, such as diameter, wall thickness, etc., can be determined.

[0109] For example, to measure or characterize a moving object 1 undergoing a relatively fast rotational motion superimposed with a relatively slow translational motion, the mirrors of the galvanometer scanner 14 are synchronized with the data acquisition so that points on the surface of the moving object 1, e.g., a cylinder, are 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 with respect to the incident laser beam.

[0110] Tracking points in space and spatial jitter contributes in particular to reducing the probability of or substantially avoiding signal loss, which leads for example to an incomplete characterization of the shape of the moving object 1.

[0111] All A-scan signals obtained from tracking points in space are combined and used to compute a periodogram signal. By combining the A-signals, robustness to signal loss can be achieved.

[0112] For example, an automatic peak picking algorithm can be used to identify the maximum values ​​in the OCT periodogram 28 .

[0113] 5 exemplarily illustrates OCT measurement results for characterization of a glass cylinder based on a third implementation ("cross-correlation"). In the graph of FIG. 5, the abscissa (x-axis) indicates time t, and the ordinate (y-axis) indicates the wall thickness W, inner diameter ID, and outer diameter AD, each in arbitrary units, for example, mm. The OCT measurement uses a rotating glass cylinder as the measurement object 1. The graph illustrates the progression or determined values ​​of the wall thickness W, inner diameter, and outer diameter AD over time from the OCT measurement associated with the movement of the glass cylinder.

[0114] Figure 6 shows exemplary OCT measurement results for the characterization of a glass cylinder according to Figure 5 based on the first or second implementation ("autocorrelation"). In the graph of Figure 6, similar to Figure 5, the abscissa (x-axis) represents time t, and the ordinate (y-axis) also indicates the wall thickness W, the inner diameter ID, and the outer diameter AD, each in arbitrary units, for example mm.

[0115] FIG. 7 shows an example graph for characterizing a glass cylinder as object 1 used at a relatively large axial depth, in particular at a relatively large distance between object 1 and the measurement device. In the illustrated example, this is approximately 400 mm. In the graph of FIG. 7, the horizontal axis (x-axis) indicates the distance D of object 1 from the measurement device in millimeters (mm), and the vertical axis (y-axis) indicates the OCT signal intensity OS (arbitrary units). As can be seen from the graph, even at relatively large distances, a signal with a low signal-to-noise ratio can be obtained. Furthermore, it can be seen that absolute distances can also be determined, for example, by using a reference substrate 13.

[0116] FIG. 8 shows an example OCT measurement result for the characterization of a moving, hot glass cylinder. An infrared filter designed to filter out thermal radiation in the far-infrared spectrum emitted by the object and / or its surroundings was used. In the graph of FIG. 8, the abscissa (x-axis) indicates time t, while on the ordinate (y-axis) the wall thickness W, inner diameter ID, and outer diameter AD are given, each in arbitrary units, e.g., mm. FIG. 8 particularly shows that the proposed method is also suitable for measuring or characterizing hot, transparent or translucent objects.

[0117] 9 to 11 exemplarily show the structure of a k-clock interferometer of a k-clock unit, and exemplarily show implementation forms of the k-clock unit in an OCT system.

[0118] 9 shows a k-clock interferometer 29 of a k-clock unit for generating k-clock signals for use in, for example, k-clock-based resampling. The k-clock interferometer 29 is configured as a planar interferometer, such as a Fabry-Perot interferometer, and is therefore also referred to herein as a k-clock Fabry-Perot interferometer or simply a Fabry-Perot interferometer.

[0119] The k-clock interferometer 29, hereinafter also referred to as interferometer 29 for short, comprises a spacer 30 having a hollow spacer body 31 with an inner hollow cylindrical gap 32 or air gap 32. Two distal ends 33 of the spacer body 31 are formed with flat surfaces having a parallelism of, for example, less than 0.005 degrees, i.e., a deviation of the parallelism of less than 0.005 degrees. The spacer 30 or spacer body 31 is made of a material with an ultra-low thermal expansion coefficient, for example, a material with a thermal expansion coefficient in the range of + / - 2*10-7 / C.

[0120] The first mirror 34 and the second mirror 35 are attached to the opposite distal ends 33, for example by a stress-free retaining ring 36 or body pressing the mirrors 34, 35 against respective support surfaces of the distal ends 33. The length of the spacer 30 or spacer body 31 may be, for example, 95 mm, for an oscillation frequency of the k-clock in the range of, for example, 1.58 GHz. Suitable materials are, for example, those mentioned above or below.

[0121] The mirrors 34, 35 are wedge-shaped and provided with an anti-reflection coating, so that the mirrors 34, 35 do not form an etalon by themselves, but rather a gap or air gap 32 whose axial length is given 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.

[0122] The mirror surfaces S of the mirrors 34, 35 are plane-parallel and preferably have an angular error of a maximum of 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 transparent mirrors or semi-transparent mirrors.

[0123] To couple the laser light into or out of the k-clock interferometer 29, or in the region of the mirrors 34, 35, so-called tip-tilt devices or mechanisms can be provided, which, for example, allow tilting about two axes the light guide used for feeding / discharging the laser light into / from the k-clock interferometer 29. The tip-tilt devices may further comprise adjustment mechanisms that allow adjustment in the x- and y-directions, for example perpendicular to the optical axis.

[0124] To avoid environmental influences such as pressure, humidity, etc. from reaching the gap 32 or air gap 32, the mirrors 34, 35 may be hermetically connected to the spacer body 31, for example by anodic bonding, optical contact bonding, or cement bonding.

[0125] 10 and 11 show implementations of a k-clock unit 38 in an OCT system, each showing a laser source or laser 2, specifically a swept-source laser, a beam splitter 39, a k-clock interferometer 29, and an ultrafast detector 40.

[0126] The structure and function of the device shown in FIG. 10 are as follows.

[0127] Laser 2 is photo-optically coupled, for example by a light guide, to an input SE of beam splitter 39. A first output SA1 of beam splitter 39 is photo-optically coupled to k-clock interferometer 29, specifically to an input C1 of k-clock interferometer 29 associated with first mirror 34, for coupling the laser light via first mirror 34. A second output SA2 of beam splitter 39 is photo-optically coupled to the OCT sensor head, as indicated by the arrow.

[0128] The input DE of the detector 40 is photo-optically coupled to the k-clock interferometer 29 at C2, and the output of the k-clock interferometer 29 is associated with the second mirror 35 so that the interferometric signal can be guided to the detector 40 and evaluated thereby.

[0129] In an embodiment, all light-to-optical couplings or connections are preferably implemented by light guides.

[0130] The output DA of the detector 40 is connected to an evaluation unit or signal evaluation, in particular an evaluation unit or data processing unit, for generating the k-clock signal, which is indicated diagrammatically by an arrow.

[0131] In this example, the beam splitter 39 is designed so that 1% of the laser radiation reaches the k-clock interferometer 29 via a first output SA1 of the beam splitter 39, and 99% of the laser radiation reaches the OCT sensor head via a second output SA2.

[0132] The apparatus shown in FIG. 10, as well as the apparatus shown in FIG. 11, which is described in more detail below, allows for efficient and accurate data acquisition based on stable k-clock signals.

[0133] The arrangement or structure according to FIG. 11 differs from that shown in FIG. 10 in that the detector 40 according to FIG. 11 is implemented as a symmetric or balanced detector, or a balanced detector is used.

[0134] 11 has two inputs, a first input DE1 and a second input DE2. The second input DE2 is optically coupled to the output C2 of the k-clock interferometer 29 associated with the second mirror 35, as in FIG. 10. The first input DE1 is also connected to the k-clock interferometer 29, but not to the output C2, but to the output of the k-clock interferometer 29 associated with the first mirror 34, denoted C1*.

[0135] A circulator 41 having three ports P1, P2, and P3 is arranged in the optical path between the beam splitter 39, the k-clock interferometer 29, and the detector 40. In this case, a first output SA1 of the beam splitter is optically coupled or connected to the first port P1, a second port P2 to the input C1 / output C1* of the k-clock interferometer 29, and a third port P3 to the first input DE1 of the detector 40. Laser light incident on port P1 from the beam splitter 39 reaches the input C1 of the k-clock interferometer 29 via port 2. Laser light reaching port P2 from the output C1* of the k-clock interferometer 29 reaches the first input DE1 of the detector 40 via the third port P3.

[0136] The optical path lengths between the output C1* and the first input DE1 and between the output C2 and the second input DE2 are also preferably the same or substantially the same.

[0137] As shown in FIG. 11, by using balanced or symmetric detection it is possible to improve the signal-to-noise ratio and suppress interfering signals.

[0138] For example, the proposed k-clock unit, which may comprise a k-clock interferometer and an associated detector, can be used to obtain a relatively stable k-click signal, which in turn leads to improved measurement results. In particular, it can be achieved that the free spectral range (FSR) of the k-clock unit, and therefore the length scale of the measurement, is substantially constant and independent of environmental influences such as temperature or pressure fluctuations.

[0139] Because the spacer has an extremely low coefficient of thermal expansion, temperature changes have little effect on its length. The remaining sources of FSR change can be attributed to, for example, the effects of temperature, pressure, and humidity on the refractive index of the air inside the spacer. To minimize or eliminate such effects, the mirror can be sealed to the surroundings by anodic bonding, optical contact bonding, or cement bonding, sealing the spacer and thus the gap or air gap.

[0140] The proposed k-clock unit can be relatively easily implemented in the above-mentioned OCT system, as shown in Figures 10 and 11. For this purpose, for example, a small portion (approximately 1%) of the laser light can be sent to an input, e.g., the light guide fiber input of the k-clock. For example, the output signal from the light guide fiber output of the k-clock can be measured with a high-speed photodetector or receiver. As mentioned above, balanced or symmetric detection can be used to increase the signal-to-noise ratio and remove interfering signals. In balanced detection, an optical circulator provides access to both outputs of the k-clock interferometer, which can be measured with a balanced or symmetric detector.

[0141] In balanced or symmetrical detection, the amplitude of each signal, i.e., the signals at the two outputs of the k-clock interferometer, is preferably adjusted before the detector, for example by using a fiber attenuator. The optical path lengths between the detector input and each output of the k-clock are particularly preferably equal.

[0142] The embodiments reveal, in particular, that the method and optical coherence tomography system proposed herein are suitable for enabling reliable measurement and geometric characterization of objects and determining not only relative object shapes but also absolute distances to the measurement system and 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, and the characterization can be performed substantially in real time. [Explanation of symbols]

[0143] 1 object 2 Swept-source laser or laser 3 Circulator 4 Collimator 5 Beam splitter 6 First Output 7 Second Output 8 Power Monitoring Unit 9 Variable Optical Attenuator 10 High-speed detector 11 OCT signal 12 Laser radiation 13 Fixed partial reflection optical unit, fixed reference substrate 14 Galvanometer scanner or scanner 15 Microlens Array 16 More beam splitters 17 Additional beam splitter terminals 18 Further variable optical attenuators 19 Retroreflector 20K Clock 21 Resampling 22 Adjusting the start time 23 Zero-phase low-pass filtering 24 Calculation of Topological Evolution 25 Extracting Phase Profiles ("Phase Unwrapping") 26 Applying a window function 27 Fast Fourier Transform (FFT) 28 OCT periodogram 29 kClock Interferometer 30 spacer 31 Spacer body 32 Gap / Air Gap 33 Distal end 34 First Mirror 35 Second Mirror 36 Retaining ring 37 Tip tilt device 38k clock units 39 Beam Splitter 40 detectors 41 Circulator t time W wall thickness ID Inner Diameter AD outer diameter D distance OS OCT signal strength X impact point, point C1 k-clock interferometer input C2 k-clock interferometer output C1* k-clock interferometer output DE input detector DE1 Detector No. 1 input DE2 Detector No. 2 input DA detector output SE beam splitter input SA1 Beamsplitter 1st Output SA2 Beamsplitter Second Output P1-P3 ports, circulator S mirror surface

Claims

1. 1. A method for optically characterizing a transparent or translucent object (1), in which object dimensions (W, ID, AD), object distance, and / or object position 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 light source (2), and a reference substrate (13) is used in a beam path (12) of the light source (2) to determine absolute distance, dimension, and / or position, the reference substrate comprising a partially reflecting optical unit fixed in the beam path (12).

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

3. During the coherence tomography signal acquisition and / or signal evaluation, a. k-clock resampling (21) is used; b. The minimum, maximum or zero crossing of the k-clock of the laser source (2) is algorithmically determined and used as a sampling clock; or c. k-clock hardware trigger is used 3. The method according to claim 1 or 2.

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

5. 5. The method according to claim 4, characterized in that the object (1) is moved and the scanning path used in scanning the object (1) at least partially follows the movement of the object (1), and preferably the optical coherence tomography data recording is synchronized with the scanning path.

6. 6. The method of claim 5, wherein a spatial jitter is superimposed on the scan path.

7. 7. The method according to claim 5 or 6, characterized in that a number of, in particular a plurality of, reflection profiles obtained along the scanning path are combined, a periodogram signal is 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 is determined from at least one periodogram signal.

8. 8. The method according to any one of claims 1 to 7, 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 in the range of the far-infrared spectrum emitted by the object (1) and / or its surroundings is at least largely filtered out using said infrared filter.

9. 9. The method according to any one of claims 1 to 8, characterized in that the optical coherence tomography comprises signal recording of the k-clock (20) with software-based filtering (23) and offset correction (24), and optionally further comprises a subsequent phase extraction (25), in particular based on a Hilbert transform and / or evaluation of zero crossings and extrema, the filtering (23) optionally further comprising low-pass filtering.

10. 10. The method according to claim 1, wherein a periodogram signal is determined from the optical coherence tomography signal by an automatic peak picking algorithm from periodogram data (28), and at least one object dimension (W, ID, AD), at least one object distance and / or at least one object position are determined from the periodogram signal, in particular taking into account the periodogram signal of the reference substrate (13).

11. 11. An optical coherence tomography system designed for optically characterizing a transparent or translucent object (1) based on optical coherence tomography, comprising: at least one swept-source laser source (2) having a coherence length in the range of 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), the reference substrate comprising a partially reflecting optical unit fixed in the beam path (12); a control unit having a processing unit, in particular a processing unit having an architecture for parallel data processing; and a memory associated with the processing unit, the memory comprising instructions which, when executed by the processing unit, perform the method according to any one of claims 1 to 10.

12. the swept-source laser source (2) comprises a MEMS VCSEL-based light source or an akinetic laser light source; the coherence length of said 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 at least 40 nm and / or is designed for a repetition rate in the range of 4 kHz to 4 MHz, in particular around 10 kHz, The optical coherence tomography system of claim 11 .

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

14. 14. The optical coherence tomography system according to any one of claims 11 to 13, further comprising at least one infrared filter, preferably a dual-band mirror, arranged in the beam path (12) of the light source (2), the infrared filter being designed to filter out at least a large portion of thermal radiation in the far-infrared spectrum range emitted by the object (1) and / or its surroundings.

15. 15. An optical coherence tomography system, in particular as described in any one of claims 11 to 14, comprising a k-clock unit (38) for generating k-clock signals for signal acquisition and / or signal evaluation of the coherence tomography, the k-clock unit (38) having a Fabry-Perot interferometer (29) comprising a first mirror (34), a second mirror (35), and a spacer (30), the mirrors (34, 35) being attached to opposite sides of the spacer (30) and spaced apart from each other by the spacer (30), the mirror surfaces (S) of the mirrors (34, 35) facing the spacer (30) being aligned in parallel planes, the first mirror (34) and the second mirror (35) preferably being wedge-shaped.

16. 16. The optical coherence tomography system of claim 15, wherein the spacer (30) is made or constructed from a material with a negligibly low coefficient of thermal expansion, such that the temperature-related change in length of the spacer (30), in particular perpendicular to the mirror surface (S), in a relevant operating temperature range is negligible relative to the wavelength of the laser light from the swept-source laser source (2), the material preferably having a coefficient of thermal expansion in the range of -2*10-7 / K to 2*10-7 / K.

17. 17. The optical coherence tomography system of 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), the gap (32) or the air gap (32) being sealed by anodic bonding, optical contact bonding or cement bonding in the region of the contact or support surfaces between the mirrors (34, 35) and the spacer (30).

18. a beam splitter (39) connected on the one hand to the light source (2) and on the other hand to the k-clock unit (38) is provided for supplying the laser light from the light source (2) to the k-clock unit (38), in particular to the Fabry-Perot interferometer (29), the beam splitter (39) preferably being designed to supply a proportion of the laser light from the light source (2) in the range of 0.5% to 2%, in particular about 1%, to the k-clock unit (38) via an input (C1) associated with the first mirror (34); and / or 18. The optical coherence tomography system according to claim 16 or 17, comprising a detector (40) having at least one input (DE) photo-optically coupled to the k-clock unit (38), in particular to an output (C2) associated with the second mirror (35) of the Fabry-Perot interferometer (29), for detecting the interferometric signal of the Fabry-Perot interferometer (29), wherein the optical coherence tomography system further comprises a data processing unit designed to evaluate the detected interferometric signal and to generate a k-clock signal based on said evaluation.

19. In order to improve the signal-to-noise ratio and suppress interference signals, the detector (40) is implemented as a balanced detector (40), preferably having two inputs (DE1, DE2), a first detector input (DE1) being optically coupled to an output (C1*) of the Fabry-Perot interferometer (29) associated with the first mirror (34), and a second detector input (DE2) being optically coupled to an output (C2) of the Fabry-Perot interferometer (29) associated with the second mirror (35), and the optical coherence tomography system further preferably further comprises a circulator (41) having three ports (P1, P2, P3), and the laser light source (2) is connected to the input (C1) associated with the first mirror (34).

19. The optical coherence tomography system of claim 18, wherein the laser light from the laser light source (2) is photo-optically coupled to a first port (P1) for supplying the laser light to the Fabry-Perot interferometer (29) via a second port (P2) of the circulator (41) connected to the first mirror (34), and the second detector input (DE1) is photo-optically coupled to a third port (P3) of the circulator (41), and the circulator (41) is designed such that an interferometric signal of the Fabry-Perot interferometer (29), emitted to the second port (P2) via an output (C1*) of the Fabry-Perot interferometer (29) associated with the first mirror (34), is directed to the first terminal (DE1) of the detector (40) via the third port (P2).

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