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

Abstract

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

Description

[0001] The invention relates in particular to a method for optically characterizing a transparent or semi-transparent object and to an optical coherence tomography system.

[0002] In the prior art, for example, optical coherence spectroscopy (OCT) is known for inspecting transparent or semi-transparent objects. 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, an optical coherence tomography system is used to capture a 3D depth profile of the object during a scan.

[0003] In known systems and methods for optical coherence spectroscopy and optical characterization of transparent or semi-transparent objects, there is still room for improvement, in particular with regard to the type, size, structure of the object to be measured and the boundary conditions of the measurement or geometrical characterization of the object.

[0004] Proceeding from this, an improved or novel method for optical coherence spectroscopy, in particular for optical characterization of transparent or semitransparent (translucent) objects, is to be provided. Furthermore, a corresponding optical coherence tomography system is to be provided.

[0005] This problem is solved in particular by the combination of the features of the independent claims. Advantageous embodiments result 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 is generally understood to mean semi-transparent, and therefore may also include, for example, semitransparency / translucency. In a strict sense, the term "optically transparent" in the context of the present invention may mean transparency relative to the infrared and / or visible spectrum. Specifically, infrared transparent objects should be included. Preferably, the method uses an object made of an inorganic material or a non-biological material.

[0007] The proposed optical characterization determines object size, object distance and / or object position based on optical coherence tomography (OCT for short).As already explained at the outset, OCT is a known method for characterizing objects based on coherent optical radiation.

[0008] Optical coherence spectroscopy is known per se. For example, for the optical characterization of objects by means of OCT, reference is made to “Cubic meter volume optical coherence tomography”, Fujimoto et al., Vol. 3, No. 12 / December 2016 / Optical Design, and “Application of along-range swept source optical coherence tomography based scheme for dimensional characterization of multilayer transparent objects”, Eneas N. Morel, Nélida A, Russo, Jorge R. Torga, Ricardo Duchowicz, Opt. Eng. 56(8), 084102 (2017).

[0009] The method proposed herein is characterized by the use of a swept-source laser source, particularly a tunable laser source, preferably a wavelength-tunable laser source, as the light source, which has a coherence length in the decimeter to meter range. Furthermore, the proposed method is characterized by the use of a reference substrate in the beam path of the light source, comprising a partially reflective optical element fixed in the beam path. The reference substrate is used to determine the absolute distance and / or position and / or size of an object or to / from an object.

[0010] The present invention is based on the discovery that the use of a swept-source light 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 can be measured which, for example, move during the measurement and / or emit non-negligible thermal radiation and / or are arranged in an environment that emits non-negligible thermal radiation.

[0011] As mentioned, the proposed combination is suitable for measuring and / or characterizing hot objects, such as partially molten glass or objects made of glass. The method is particularly suitable for optically characterizing glass cylinders or cylindrical glass objects during or during their manufacture, where the objects may be at least partially molten 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 an advantageous axial resolution because, in contrast to known methods, it operates without a Bragg grating. The advantageous axial resolution results, for example, from the fact that, in contrast to methods based on Bragg gratings, the proposed method is essentially unrestricted or unconstrained with respect to the number of reference or measurement points.

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

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

[0015] According to an embodiment, a swept-source laser source is used as the light source, which includes a MEMS VCSEL-based light source (MEMS: Micro-Electromechanical System; VCSEL: Vertical Cavity Surface-Emitting Laser) or a non-moving 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.

[0016] The proposed method with a MEMS VCSEL light source and a reference substrate is advantageously applicable to a relatively large coherence length range and allows particularly accurate coherence / interference measurements over substantially the entire range. For example, a large coherence length can be used when the object is a hot object (such as (partially) molten glass) and a specific distance from the object must be maintained due to the temperature, for example, for large distances between the measuring head or measuring device and the object.

[0017] According to an embodiment, provision may be made for during signal acquisition and / or signal evaluation of optical coherence tomography (OCT) signals:

[0018] a. Use k clock resampling,

[0019] b. Algorithmically determine the minimum, maximum, or zero crossing of the laser source's k clock and use it as the sampling clock, or

[0020] c. Use a k-clock hardware trigger or a k-clock hardware signal.

[0021] The k-clock allows data to be recorded linearly in time, which can then be transformed into linear samples in k-space. This makes it easier to evaluate high-frequency signals. Furthermore, objects can be characterized in near real time, which is advantageous, for example, for geometric characterization of moving transparent or semi-transparent objects during manufacturing processes.

[0022] An exemplary method for algorithmically determining the minimum value, maximum value, or zero-crossing point of the k clock according to b. may be as follows or include the following steps:

[0023] b1) Normalize the k clock by dividing by the envelope;

[0024] b2) performing at least three, preferably directly consecutive, mathematical operations, such as absolute value formation, addition, absolute value formation;

[0025] b3) smoothing, for example by means of a median filter or the like;

[0026] b4) Peak finding or maximum value determination;

[0027] b5) Interpolate the maximum, minimum, and zero-crossing points into linear k-space.

[0028] Due to the absolute value operation b2), for example, the minimum value, the zero crossing point and the maximum value of the k clock cycles can be determined with the aid of the proposed method steps.

[0029] According to an embodiment, according to the method, in optical coherence spectroscopy, a galvanometer scanner, or simply a galvanoscanner, can be used, using a microlens array and / or a bifocal lens, preferably with a high numerical aperture, to scan an object by means of laser light from a light source. In particular, galvanometer scanners, microlens arrays, and bifocal lenses have proven suitable for optical characterization of moving objects, for example during a manufacturing process. Such components are particularly suitable for measuring or characterizing partially molten moving transparent cylinders, for example made of glass.

[0030] Regarding the operating mode of the galvanometer scanner, for example, for moving objects (for example, for objects that move both rotationally and axially), in particular for objects that rotate relatively quickly and move relatively slowly axially, the galvanometer scanner can be programmed in such a way that a point on the object or on the surface of the object (in particular, a point or area aimed at by the galvanometer scanner) is located in the reference frame of the galvanometer scanner. The object can be, for example, a cylinder, an object with a cylindrical geometry, or a cylindrical object. In particular, this is possible in the context of a deflection that can be achieved or obtained using the galvanometer scanner. When the galvanometer scanner reaches the end of its deflection or its deflection range, it is reset in terms of deflection and operates in such a way that it tracks or aims at the next point on the surface.

[0031] For example, 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 by means of an angle sensor. Typically, the movement (translation and / or rotation) of the object can be detected by means of a sensor unit or sensor technology, and the detected movement data can be used for synchronization. Based on such data, for example, the movement path of a point (in particular a target 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 based on the determined movement path. Preferably, the movement or movement path is determined continuously, in particular in real time, and is used for the control and synchronization between the movement on the one hand and the galvanometer scanner and data acquisition on the other hand.

[0032] Regarding the operation or manipulation of the microlens array, it can be provided that the optical beam under OCT is aligned along a direction of movement or along a path or trajectory of movement of a point on an object (specifically, an object surface), and scanning occurs simultaneously at or for a point on the trajectory or surface. For example, if the movement or trajectory is known, data acquisition and movement can be synchronized, i.e., a point on the object or on the object surface can be tracked with respect to data acquisition.

[0033] When using a two-dimensional (2D) microlens array, in particular the (immediate) vicinity of the track can be scanned, in particular the location of interest on the object or the area around the track. This scanning is particularly suitable for cylindrical objects or cylindrically shaped objects.

[0034] Preferably, the scanning by the microlens array is performed in a fixed manner and further preferably at the repetition rate of the laser light.

[0035] Regarding bifocal lenses or sensor heads with bifocal lenses, it should be noted that these can be used, for example, to optimize the intensity of the signal reflected from a sample or object. Specifically, for example, when the sample or object has (a) periodic fluctuations in inner diameter (e.g., 50 mm within an axial range of 500 mm), for example, when there is axial tilt of the inner surface depending on the position of the object, and / or when the sample or object has (b) a significant step-like change in 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), a sensor head with a high numerical aperture is advantageous, as light reflected from the deflected surface propagates at a considerable angle. In addition to a large aperture, it may be useful to separate the excitation of the sample from the detection of backreflected 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 rear focal points to the position of the receiving fiber end, regardless of the diameter / size of the object. For these reasons, a bifocal design is particularly advantageous for the conditions mentioned.

[0036] A bifocal lens or bifocal aspheric surface can, for example, consist of two semi-convex lenses with focal points fa and fb connected to each other, with the connecting surface extending parallel to the axis of, for example, a cylindrical object. In another embodiment, the bifocal aspheric surface can be formed similar to a zone plate having circular sections with alternating focal lengths fa, fb, fa, fb, etc. This embodiment can be used essentially independently of the characteristics of the object to be examined.

[0037] Another variation of the optical setup with two focal points consists of using a single aspheric surface, replicating the same receiving optical element, such as an optical fiber, in orthogonal arms (downstream of the aspheric surface in the beam path), and deflecting the incident beam into the two 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 elements or fibers are captured by optical multiplexing or with the help of an optical combiner.

[0038] If the size of the object to be measured is (roughly) known in advance or substantially known, the lens distance of the bifocal setup can be automatically adjusted to an optimal bifocal configuration.

[0039] In one embodiment, the detection arm or optical fiber and the receiving arm or receiving optical fiber can be formed from different components or optical fibers. For example, the collimated light beam emitted from the detection fiber can be collinearly coupled to a receiving optical fiber with a high numerical aperture via a microprism or glass plate with a polished end face, such as a glued, angled, partially reflective surface. This prevents light reflected from the sample from being blocked.

[0040] In cases where a deflected surface of an object to be inspected refracts a light beam at a small angle, an area on the opposite surface of the object is "excited" that is not completely opposite relative to the central axis of the object. This can lead to errors in determining distance, size, and / or position. Specifically, in embodiments, the deviation can be compensated by placing an array camera or plenoptic camera behind the sample, which can record or determine the direction or angle of the transmitted light. The direction or angle can be used to correct distance, size, and / or position, for example, by ray tracing.

[0041] Depending on the embodiment, a single-focus setup can also be used. In this case, it may not be possible to record all four reflections from the object surface for all axial positions of, for example, a cylindrical object. In this case, 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 from the cylindrical object can be detected, thereby allowing, for example, the continuous determination of the inner diameter, outer diameter, and wall thickness after an initial 180° rotation.

[0042] According to an embodiment, as already mentioned, the object can 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 data acquisition of the optical coherence tomography is synchronized with the scanning path.

[0043] In other words, the scanning path can be moved at least partially or partially in substantial synchronization with the movement of the object. In particular, one finding of the present invention is 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 if the object is moving. As already mentioned above, in particular, a galvanometer scanner and / or a microlens array can be used to scan a moving object, whereby the synchronization between the galvanometer scanner and / or microlens array and the data acquisition is or will be synchronized with the movement.

[0044] According to the embodiment, it is provided that spatial dithering is superimposed on the scanning path of the galvanometer scanner. In particular, such dithering is not only advantageous for objects of irregular shape, but also supports data acquisition for moving objects. For example, dithering can be generated by appropriately controlling the galvanometer scanner. If the surface of the object is ideally perpendicular to the OCT beam, dithering is not necessarily required. Since real objects (in particular objects that have undergone a manufacturing process) fluctuate in orientation with respect to the surface, the use of dithering can improve the accuracy and / or reliability of the acquired object data. In other words, dithering makes it possible, for example, to obtain back-reflected or reflected light from the object, at least to a certain extent, even if the surface of the object is irregular. For example, dithering can be adjusted by superimposing a deflection or movement orthogonal to the trajectory of a point on the object onto the movement of the galvanometer scanner according to the trajectory of the point.

[0045] According to an embodiment, the method combines several (particularly a plurality of) reflectance curves (or A-scans) obtained along a scan path, in particular to form a combined reflectance curve. A periodogram signal can be determined from the combined reflectance curve thus obtained, and then at least one object size, object distance, and / or object position can be determined from at least one periodogram signal (i.e., from one or more periodogram signals).

[0046] In particular, the distance between boundary surfaces of the object and / or their position or location relative to each other relative to a precisely known reference, which reference is formed, for example, by a fixed partially reflecting optical unit, may be determined.

[0047] Thus, object sizes, distances and positions may be determined relatively accurately and reliably, in particular also when the object is moving and / or when the object structure or geometry varies within certain limits.

[0048] According to an embodiment, for example, those A-scans with a number of backreflections expected based on the geometry of the object to be examined (e.g., flat, cylindrical, etc.) may be filtered out. The expected number of backreflections in OCT may be determined or derived, for example, based on the (coarse) geometry of the object to be examined, the target geometry, and / or the expected geometry. The coarse, target, and / or expected geometry may be provided as a parameter or parameter data set (e.g., as an input parameter or data set, in particular an input parameter data set) to the method (e.g., to a corresponding evaluation unit).

[0049] With regard to the selection of A-scans to be used or available for optical characterization, it is also possible or additionally possible to use (e.g. filter out) those A-scans, e.g. from the group or set of recorded A-scans, which show or have the greatest intensity and / or have the best S / N (signal-to-noise ratio).

[0050] In this case, a periodogram is understood in the general sense to be specifically the distance signature of the reflections caused by the boundary surfaces of the object, also referred to as distance signature for short.

[0051] According to an embodiment, an infrared filter, preferably a dual-band reflector, may be arranged in the beam path of the light source, and the infrared filter may be used to filter out at least primarily thermal radiation in the far-infrared spectral range emanating from the object and / or its surroundings. The infrared filter is preferably placed in the beam path upstream of a measuring head for detecting the light reflected from the object. In particular, this prevents radiation in the infrared spectral range of thermal radiation or external radiation from interfering with the measured values, or improves the signal-to-noise ratio of light reflected from or at the object. The use of an infrared filter is advantageous, for example, when characterizing thermal 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 would or does lead to impairment of the measured values ​​of the light reflected from the object by OCT. With regard to infrared radiation, a spectral range is specifically considered, which, assuming a blackbody radiator, corresponds to a temperature range of 800 K to 2200 K.

[0052] According to an embodiment, provision may be made for optical coherence tomography to include signal recording of the light source's k-clock with software-based filtering and offset correction, and further optionally including subsequent phase extraction, in particular based on a Hilbert transform and / or evaluation of zero crossings and extrema. The filtering may preferably be a low-pass filtering.

[0053] Signal recording based on k-clocks enables temporal linearization, which in particular allows for real-time characterization of the object. By means of subsequent phase interpolation, i.e., of the phase distribution of the measurement signal, the data or measurement signal recorded linearly in time due to the k-clocks used can be transformed into linear sampling in k-space. This can increase the resolution of the method or the system performing the method.

[0054] Depending on the embodiment, extraction of the phase distribution and re-interpolation of data based on the phase distribution can be performed for each individual sweep of the swept source laser. This can ensure, for example, that the system (particularly signal evaluation and / or processing) is robust to fluctuations in the phase distribution of the laser.

[0055] According to an embodiment, filtering (in particular low-pass filtering and / or phase interpolation) can be performed after each individual frequency sweep of the laser. For example, a 10 kHz laser has a frequency of 10 kHz. Particularly advantageous for processing and / or handling the measurement signals at such frequencies is a specific implementation on an architecture with parallel data processing, for example on a graphics processing unit (GPU). For example, in applications, the sampling rate of the digitizer can result in a laser repetition rate of 10 kHz to 100 kHz, and depending on the laser duty cycle, relatively high data rates in the range of 5 GB / s to 7 GB / s (gigabytes per second) can occur, which cannot be handled satisfactorily or at all with a conventional CPU. Therefore, advantageously, the processing and / or handling of the measurement signals is performed on a GPU, which can be implemented, for example, with CUDA (Compute Unified Device Architecture).

[0056] Examples of filters are bandpass filters, median filters, etc. Examples of interpolation are polynomial interpolation, Chebyshev interpolation, etc.

[0057] According to an embodiment, provision is made for determining a periodogram signal from the optical coherence tomography signal from the periodogram data by means of an automatic peak picking algorithm, and for determining at least one object size, at least one object distance and / or at least one object position from the periodogram signal, in particular taking into account the periodogram signal of a reference substrate.

[0058] In this context, the term "peak picking" (or peak finding) is to be understood in the prior art sense as meaning an algorithm by which local maxima or minima can be determined from a signal (in particular a measurement signal and / or a processed measurement signal). For example, such an algorithm can comprise determining the zero crossings of the numerical first derivative of the signal, which zero crossings are associated with individual peaks (in particular local maxima or minima).

[0059] In the peak picking algorithm, in particular, data relating to the (rough) geometry of the object and / or the target geometry and / or the expected geometry can be used as input parameters or boundary conditions. In particular, when manufacturing objects (e.g. glass objects, such as glass cylinders), the geometry of the object or the expected geometry of the object is known or specified. If, in the case of a cylindrical object to be manufactured, for example the target diameter (e.g. the inner diameter and / or outer diameter and / or the wall thickness of the object) is known or specified, then these data at least approximately provide the location or position of the signal to be expected, which can be calculated or determined, for example, taking into account the measurement arrangement. For example, if a cylindrical object is measured with approximately radial incident light, then the reflection signal (i.e. the peak value) of the reflection on the outer surface and the inner surface of the cylindrical object wall is expected.

[0060] Based on the measurement signals and specification data regarding the object's geometry and / or the measurement setup, the manufacturing process can be monitored, i.e., the object can be characterized during manufacturing, specifically regarding whether the manufactured object meets basic requirements regarding geometry and / or dimensions. This allows for quality control and / or control of the manufacturing process. Furthermore, for example, it is possible to correlate deviations in the object's geometry with operating parameters of the production equipment and / or provide fault analysis for the production equipment based on deviations in the object's geometry.

[0061] The advantage of the proposed method is in particular also that real-time characterization is possible, which allows optimizing the production process and / or characterizing the quality of the manufactured product already during manufacturing. Complex object characterization after manufacturing can be omitted.

[0062] In particular, the fixed partially reflective optical unit proposed herein makes it possible to determine not only relative object distances or object sizes based on the measurement signal (in particular the peak value), but also (absolute) distances, sizes and / or positions based on the measurement system or optical measurement system used.

[0063] For example, and depending on the specific measurements performed, the proposed method can be particularly useful for characterizing cylindrical, transparent or translucent objects (such as cylinders, for example glass cylinders) having a diameter of, for example, 50 mm to 700 mm and a wall thickness of 0.05 mm to 100 mm. As a result, the proposed method is suitable for a relatively wide range of applications.

[0064] According to an embodiment, an optical coherence tomography system is provided, which is designed for optical characterization of transparent or semi-transparent objects based on optical coherence tomography (OCT). The optical coherence tomography system comprises: at least one swept-source laser light source, the at least one swept-source laser light source having 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, the reference substrate comprising, for example, a partially reflective optical unit fixed in the beam path; and a control unit having a processing unit and a memory associated with the processing unit, the processing unit being in particular a processing unit with an architecture for parallel data processing (such as a graphics processing unit), the memory comprising instructions that, when executed by the processing unit, implement a method according to one of the embodiments of the method presented herein. For the purposes of the present invention, a processing unit with an associated memory designed to perform a method according to any embodiment presented herein is generally understood to mean an electronic unit that is programmed and / or designed so that, during operation, it performs a method according to any embodiment presented herein.

[0065] The advantages and advantageous effects of the coherence tomography system and its design result in particular from the advantages and advantageous effects according to the embodiment of the method.In this respect and in particular reference is made to the above statements.

[0066] According to one embodiment of the optical coherence tomography system, the swept-source laser source is or includes a MEMS VCSEL-based light source or a non-moving 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.

[0067] According to one embodiment of the optical coherence tomography system, it 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 allow for precise scanning of the object to be characterized, in particular also when the object is moving, for example when the object is moved along a movement path during manufacturing.

[0068] Typically, this or a corresponding movement path of the object is known or predetermined.The predetermined or known movement path can be used, for example, for controlling a galvanometer scanner and / or for signal evaluation of data acquired via a microlens array.

[0069] Within the scope of the present invention, embodiments may include or provide for detecting or determining the movement, linear and / or rotational movement, or movement path of an object, and thus at least indirectly detecting or determining the movement path of an object region or point to be measured in each case. For this purpose, for example, a sensor or measuring system configured to detect the movement of an object may be provided.

[0070] According to an embodiment of the optical coherence tomography system, the optical coherence tomography system further comprises at least one infrared filter, preferably a dual-band mirror, arranged in the beam path of the light source, the at least one infrared filter being designed to at least primarily filter out thermal radiation emitted by the object and / or the object's surroundings in the far-infrared spectral range. Such a coherence tomography system is particularly suitable for characterizing thermal objects that emit thermal radiation that is not negligible with respect to optical coherence tomography (OCT) or that impairs the results of the OCT. Such an infrared filter also makes it possible to measure or characterize objects in the surrounding environment that emit thermal radiation that is not negligible with respect to the OCT or that impairs the results of the OCT.

[0071] 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 at least 40 nm.

[0072] 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, in particular for a repetition rate of approximately 10 kHz.

[0073] According to an embodiment, an optical coherence tomography system is provided, which can be specifically designed according to any of the above-mentioned embodiments. The optical coherence tomography system may include a k-clock unit (or k-clock for short), which is used to generate a k-clock signal for signal acquisition and / or signal evaluation of optical coherence tomography, specifically for resampling based on the k-clock. The k-clock unit includes a Fabry-Perot interferometer, that is, an interferometer constructed in the manner of a Fabry-Perot interferometer, which can also be called a k-clock interferometer, or more accurately a k-clock Fabry-Perot interferometer. In this context, in the above-mentioned embodiments according to the present invention, it can be provided to use a k-clock interferometer, which has the described structure and generates a k-clock signal in the manner described here. The embodiments of the k-clock unit described herein are correspondingly applicable to the embodiments according to the method, specifically for generating a k-clock signal.

[0074] A Fabry-Perot interferometer or k-clock interferometer is designed or intended to generate an interferometric signal based on laser radiation from a swept-source laser source, which is fed into the Fabry-Perot interferometer or the Fabry-Perot interferometer is fed with this laser radiation. In this case, the term "fed" is intended to mean that the laser light from the laser source is coupled into the Fabry-Perot interferometer, for example via an optical conductor or light guide.

[0075] The Fabry-Perot interferometer comprises a first mirror, a second mirror and a spacer. The mirrors are attached or connected to the sides of the spacer facing away from each other. The spacer ensures that the mirrors are attached to each other or arranged at a distance from each other, wherein the distance is defined by the length of the spacer. In this case, the mirror surfaces of the mirrors face the spacer and are aligned plane-parallel to each other. Advantageously, the first mirror and the second mirror are wedge-shaped, the mirrors being wedge-shaped in particular relative to a plane perpendicular to the optical axis of the Fabry-Perot interferometer. In this case, the optical axis is the axis extending perpendicular to the plane-parallel mirror surfaces. According to the findings of the present invention, the Fabry-Perot interferometer is particularly well suited for generating a stable, in particular temperature-stable k-clock signal, in particular for the application of characterizing transparent or translucent objects on which the present invention is based.

[0076] Particularly preferably, the mirror has an antireflection layer or coating which prevents the mirror from functioning in the sense of an etalon or etalonization.

[0077] Therefore, the Fabry-Perot interferometer is advantageously designed as a planar surface interferometer.

[0078] In order to fasten the mirror to the opposite side, in particular the flat surface or plane-parallel side, of the spacer, a holding element, in particular a holding ring, may be provided, which presses the mirror onto the spacer without stress.

[0079] The optical path length between the mirrors in a Fabry-Perot interferometer 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 positioned are preferably highly parallel, for example, with a parallelism of less than 0.005 degrees. Such parallelism can be used to achieve a suitable quality or finesse for generating k-clock signals for optical coherence tomography.

[0080] According to an embodiment, the spacer may be made of or consist of a material with an ultra-low or negligible coefficient of thermal expansion or temperature expansion. In particular, the spacer should be made of or manufactured from a material for which temperature-dependent changes in the length of the spacer and the associated changes in the distance between the mirrors are negligible in the case of temperature changes within the operating temperature range of the interferometer relevant for the applications contemplated herein (e.g. ambient temperature, e.g. 15°C to 30°C). Preferably, the spacer should be made of or consist of a material for which temperature-dependent changes in the length of the spacer, in particular perpendicular to the mirror plane, are negligible with respect to the wavelength of the laser light from the swept source laser source within the corresponding operating temperature range. The corresponding material preferably has a relative low thermal expansion coefficient of -2*10 -7 / K to +2*10 -7Thermal expansion coefficients in the range of 1000 Å / K. Examples of materials are glass materials such as glass ceramics having extremely low thermal expansion coefficients (so-called "ultra-low expansion glass materials"), in particular Ultra-low expansion glass (Corning Inc., Corning, New York) or glass ceramics (Schott AG, Mainz, Germany).

[0081] Preferably, the material can be processed, in particular also processed or manufactured into a hollow cylindrical form, in such a way that for the plane-parallel support surfaces of the reflectors on the end faces of the spacer, in the case of the hollow cylindrical form, an angular error of 0.002° to 0.005° or less can be achieved on the corresponding end faces of the hollow cylinder.

[0082] According to an embodiment, the spacer is hollow cylindrical and forms a hollow cylindrical gap or air gap between the mirror surfaces, which is, for example, circumferentially bounded by the outer wall of the spacer. The gap or air gap can be sealed in the region of the contact surface or bearing surface between the reflector and the spacer (i.e., in the region where the reflector is positioned on the spacer or abuts or contacts the spacer) by anodic bonding or anodic bonding, by optical contact bonding, or by gluing. By such measures, the gap or air gap or vacuum gap can be sealed relative to the external atmosphere to avoid interference.

[0083] According to an embodiment, 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 for feeding the k-clock unit, in particular a Fabry-Perot interferometer, with laser light from the light source, wherein the beam splitter is preferably designed to feed a certain proportion 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 (e.g. 99% or the corresponding remainder) can be fed 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).

[0084] According to an embodiment, the optical coherence tomography system (in particular, the k-clock unit) may include a detector (in particular, a high-speed detector) having at least one input optically coupled to an output associated with a second mirror of the k-clock unit (in particular, of the Fabry-Perot interferometer) for detecting an interferometric signal of the Fabry-Perot interferometer. In this case, the second mirror is preferably designed as a transparent mirror or a semi-transparent mirror, so that the interferometric signal can be coupled out at the second mirror and fed to the detector.

[0085] According to an embodiment, the optical coherence tomography system (specifically, the k-clock unit) may further include a data processing unit configured to evaluate the detected interferometry signal, i.e., 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 may be configured to generate the k-clock signal based on the interferometry signal of the k-clock interferometer, the k-clock signal being configured, for example, to perform k-clock-based resampling. This k-clock-based resampling may also be understood as k-space linearization of the OCT signal based on the k-clock signal.

[0086] Depending on the embodiment of the optical coherence tomography system, the detector can be implemented as a balanced detector or a symmetric detector for the purpose of improving the signal-to-noise ratio and suppressing interference signals. The balanced detector can have two inputs: a first detector input that is optically coupled to the output of the Fabry-Perot interferometer or k-clock interferometer associated with the first mirror; and a second detector input that is optically coupled to the output of the Fabry-Perot interferometer associated with the second mirror. In this case, the mirrors are preferably each designed as a transparent mirror or a semi-transparent mirror.

[0087] According to an embodiment, the optical coherence tomography system may further include a circulator having three ports or terminals or terminal interfaces, the laser light source being optically coupled to the first port for feeding laser light from the laser light source (e.g., 0.5-2%, specifically 1% of laser light) to the Fabry-Perot interferometer via the second port of the circulator connected to the input associated with the first reflector. The second detector input of the detector is optically coupled to the third port of the circulator, and the circulator is designed such that the interferometric signal of the Fabry-Perot interferometer, emitted to the second port via the output of the Fabry-Perot interferometer associated with the first reflector, is guided to the first terminal of the detector via the third port of the circulator.

[0088] 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 inline fiber optic attenuator or attenuator is suitable, which is connected between the k-clock unit and the detector and / or is integrated in the optical path, for example in a light-guiding element such as an optical fiber.

[0089] The optical coupling of the aforementioned components (such as the laser light source, beam splitter, circulator, k-clock interferometer, detector, etc.) can be achieved specifically via a light guide, an optical fiber, or a fiber bundle. Preferably, the optical path lengths of the optical connections between the terminals of the detector and the k-clock interferometer are of equal length. This means that the optical connection connecting the output of the first reflector to the first terminal of the detector and the optical connection connecting the output of the second reflector to the second terminal of the detector have the same optical path length.

[0090] The proposed k-clock unit can be used to improve (particularly optimize) the measurement accuracy of the swept-source OCT system (particularly the swept-source OCT sensor system) proposed herein. Therefore, the measurement accuracy (particularly for the intended application) depends on the stability of the k-clock or k-clock unit, which, in the embodiment described above, comprises or forms an interferometric measurement device and measures the wavelength sweep of the swept-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 fluctuations or pressure fluctuations. The FSR is related to the optical path length difference of the k-clock unit (particularly the k-clock interferometer). The design and use of the proposed k-clock unit makes it possible to minimize the corresponding environmental influences and obtain an FSR with which sufficiently accurate measurements can be performed, particularly for the applications envisaged herein.

[0091] Compared to known methods for optically characterizing transparent or translucent objects with the aid of OCT, the proposed method and the OCT system according to the embodiments described herein can achieve a relatively large axial range, i.e. the range parallel to the measurement of the optical radiation, in particular for objects with a diameter of up to 100 mm and larger, for example objects with a diameter of up to 700 mm and larger.

[0092] Furthermore, in an embodiment, the proposed method makes it possible to characterize moving objects, for example during a manufacturing process in which the object is moved, for example performing rotational and / or translational movements. Such an example is the manufacturing of a cylinder or an object made of glass.

[0093] The proposed method and the corresponding optical coherence tomography system embodiments particularly and advantageously allow optical characterization of thermal objects, i.e., objects that emit relatively intense blackbody radiation (i.e., thermal radiation), by means of OCT. Examples include the manufacture of objects or bodies from molten glass, or more generally based on thermoforming. In particular, the object and its geometry can be monitored or characterized during thermoforming during manufacturing, which allows for real-time adaptation and monitoring of manufacturing process conditions and / or reduction of waste. Characterization steps that would normally be required after manufacturing can also be omitted, which simplifies and, in particular, shortens the manufacturing process.

[0094] In embodiments, the proposed method and optical coherence tomography system enable measurement or determination of both absolute and relative distances, particularly in comparison to known methods and devices. Specifically, there is no need 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 problems or characterization of manufacturing processes if an object or product is to be moved according to a predetermined path (e.g., in a straight line) during manufacturing.

[0095] By determining the absolute distance, for example, deviations from the path can be detected and the manufacturing process can be adjusted, or potential sources of errors or malfunctions in the manufacturing process or the production equipment can be identified from the deviation data.

[0096] For example, relative distances can be used to characterize the geometry of the object itself, such as diameter, wall thickness, cross-sectional shape, and the like.

[0097] The proposed method and optical coherence tomography system are particularly flexible and applicable to a relatively wide range of applications. For example, the method is applicable to measurements with relatively large axial depths and simultaneously high resolution (e.g., 10 micrometers) and / or with relatively small axial depths and simultaneously very high resolution (e.g., 1 micrometer).

[0098] Compared to known methods and devices, in embodiments with the proposed method, particularly with the herein proposed algorithm for processing OCT signals, advantageous resolutions can be achieved. In particular, OCT signals or signals derived therefrom with advantageous half-widths can be achieved.

[0099] Embodiments of the present invention are described below with reference to the accompanying drawings, in which:

[0100] Figure 1 schematically illustrates a first device-based OCT implementation for characterizing an object;

[0101] Figure 2 schematically illustrates a second device-based OCT implementation for characterizing an object;

[0102] Figure 3 schematically illustrates a third device-based OCT implementation for characterizing an object;

[0103] Figure 4 The process of the method for determining the periodogram is shown;

[0104] Figure 5 shows exemplary OCT measurement results for characterization of a glass cylinder according to the third embodiment;

[0105] Figure 6 shows exemplary OCT measurement results for characterization of a glass cylinder according to the first or second embodiment;

[0106] Figure 7 is an exemplary graph illustrating the characterization of a glass cylinder having a relatively large axial depth;

[0107] Figure 8 shows exemplary OCT measurement results for the characterization of a moving hot glass cylinder;

[0108] Figure 9 An example structure of a k-clock interferometer is shown;

[0109] Figure 10 For example, the Figure 9 A first specific implementation of a k-clock unit of a k-clock interferometer; and

[0110] Figure 11 For example, the Figure 9 A second specific implementation of the k-clock unit of the k-clock interferometer.

[0111] In the figures, identical or functionally identical elements are denoted by the same reference numerals. The figures merely illustrate exemplary applications or specific implementations, and the invention is not limited thereto or to the advantages or advantageous effects resulting therefrom.

[0112] Figure 1 A first device-based OCT implementation for characterizing an object 1 , which may be, for example, a glass hollow cylinder (also referred to as glass cylinder or cylinder for short), is schematically shown.

[0113] The specific implementation of the device includes a swept source laser 2 (also referred to as laser 2), followed by a circulator 3. The three terminals of the circulator 3 are connected to the laser 2, the collimator 4, and the beam splitter 5. The beam splitter 5 can have a 90:10 splitting ratio, where the first output 6 is associated with a splitting ratio of 90 and the second output 7 is associated with a splitting ratio of 10.

[0114] A power monitoring unit 8 is connected downstream of the second output 7 .

[0115] The first output 6 is followed by a variable optical attenuator 9 (VOA), which is followed by a high-speed detector 10 for generating an OCT signal 11 .

[0116] Laser light guides, in particular so-called TEC fibers (TEC: Thermally Extended Core) can be used to connect the components.

[0117] In the beam path of the laser radiation 12 from the laser 2, a fixed partially reflecting optical unit 13 or a fixed reference substrate 13 is connected downstream of the collimator 4. Downstream of the reference substrate 13 in the beam path of the laser radiation 12 from the laser 2, there is a portion of the reference substrate 13 for the deflection angle φ The object 1 to be measured or characterized is arranged downstream of the scanner 14 in the beam path.

[0118] During operation for measuring or characterizing an object 1, laser radiation 12 emitted from the collimator 4 first passes through a reference substrate 13 and then strikes a scanner 14, which directs the laser radiation 12 onto the object 1. The laser radiation 12 reflected from the object 1 is passed 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 signal via a first output 6 via a variable optical attenuator 9 to a detector 10. The detector 10 generates an OCT signal from the reflected laser radiation 12 or from a corresponding signal for optical characterization of the object 1.

[0119] Based on the scanner 14, or by appropriate control of the scanner 14, such as the angle (phi) and / or θ (theta), the laser radiation 12 can be directed or irradiated onto the object 1 in such a way 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 example shown, the object 1 performs a counterclockwise rotation, which is indicated by the curved arrow. If the object 1 is simultaneously moved linearly, for example from Figure 1 By suitable control of the scanner 14, it is also possible to achieve that the impact point x further follows this linear movement. Thus, moving objects can be optically characterized by means of OCT.

[0120] Figure 2 A second device-based OCT implementation for characterizing an object is schematically shown. Figure 1 Compared to the embodiment of the present invention, 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 (for example a 2D microlens array) also makes it possible to track a point x on the object 1 or on the surface of the object, so that the second embodiment also makes it possible to characterize a point that exhibits a rotational movement and / or a translational movement (for example, according to the distance from the object 1). Figure 2 In addition to the 2D microlens array 15, a 1D microlens array 15 can also be used.

[0121] In further specific implementation variants, the galvanometer scanner 14 and the microlens array 15 can be used in combination.

[0122] In addition, according to Figure 2 The specific implementation structure corresponds to Figure 1 The structure shown in .

[0123] according to Figure 1 and Figure 2 The specific implementation of corresponds to a setting with autocorrelation. Figure 3 A third device-based OCT implementation for characterizing an object 1 is schematically shown, which corresponds to a setup with cross-correlation.

[0124] exist Figure 3 In the case of the setup shown, something like Figure 1 and Figure 2 , there is a beam splitter 5 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. Figure 1 and Figure 2 In contrast, 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 a further variable optical attenuator 18 and a retroreflector 19. A third terminal 17.3 is connected to a collimator 4, which is connected as shown in FIG. Figure 1 and Figure 2 The fourth terminal 17.4 is connected to the detector 10 via the variable optical attenuator 9. Figure 3 In a specific embodiment with cross-correlation, the detector 10 is therefore coupled to two beam splitters 5, 16 for receiving the laser signal. The reference substrate 13 and the microlens array 15 are Figure 3 Not shown, but may exist alone or in combination.

[0125] Figure 4 The process of an example method for determining a periodogram is shown. The algorithm uses a 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.

[0126] 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 23 using a zero-phase low-pass filter, followed by calculation of the phase evolution 24 and extraction of the phase distribution 25 (so-called "phase unwrapping"). After the k-clock-based resampling 21 of the OCT signal, a fast Fourier transform (FFT) 27 is applied to the resampled result after applying a window function 26 to determine the OCT periodogram 28. From the OCT periodogram 28, geometric parameters or variables characterizing the object 1, such as diameter, wall thickness, etc., can then be determined.

[0127] In order to measure or characterize a moving object 1 that, for example, performs a relatively fast rotational movement superimposed on a relatively 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 (e.g., a cylinder) is tracked in space. If necessary, a specific 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.

[0128] Tracking spatial points and spatial dithering in particular helps to reduce the probability of signal loss or substantially avoid signal loss, which can lead to an incomplete representation of the geometry of the moving object 1, for example.

[0129] All A-scan signals obtained from tracking space points are combined and used to calculate the periodogram signal. By combining the A-scan signals, robustness against signal loss is achieved.

[0130] For example, an automatic peak picking algorithm may be used to identify the maximum values ​​of the OCT periodogram 28 .

[0131] Figure 5 An exemplary OCT measurement result of the characterization of a glass cylinder based on the third embodiment ("cross-correlation") is shown. Figure 5 In the graph, the abscissa (x-axis) represents time t, and on the ordinate (y-axis), the wall thickness W, the inner diameter ID, and the outer diameter AD are each given in arbitrary units, for example, in mm. The OCT measurement uses a rotating glass cylinder as the object to be measured 1. The graph shows the course or the values ​​determined from the OCT measurement of the wall thickness W, the inner diameter, and the outer diameter AD over time, which in turn are correlated with the movement of the glass cylinder.

[0132] Figure 6 The results are shown based on the first or second implementation ("autocorrelation") Figure 5 OCT measurement results for example of the characterization of a glass cylinder. Figure 6 In the curve diagram, similar to Figure 5 , the abscissa (x-axis) represents time t, and the ordinate (y-axis) also shows the wall thickness W, the inner diameter ID and the outer diameter AD, each in any unit, for example in mm.

[0133] Figure 7 The graph is shown for the example of a glass cylinder used as the object 1 at a relatively large axial depth, in particular at a relatively large distance between the object 1 and the measuring device. In the example shown, this distance is approximately 400 mm. Figure 7 In the graph, the abscissa (x-axis) represents the distance D of the object 1 from the measuring device in millimeters (mm), and the ordinate (y-axis) represents the OCT signal intensity OS in arbitrary units. As can be seen from the graph, a signal with a low signal-to-noise ratio can be obtained even at relatively large distances. Furthermore, it can be seen that, for example, by using a reference substrate 13, absolute distances can also be determined.

[0134] Figure 8The results of OCT measurements for the characterization of a moving hot glass cylinder are shown as an example. An infrared filter is used in the measurement, which is designed to filter out thermal radiation emitted from the object and / or its surroundings in the far infrared spectral range. Figure 8 In the graph of , the abscissa (x-axis) represents the time t, and on the ordinate (y-axis), the wall thickness W, the inner diameter ID and the outer diameter AD are each given in arbitrary units, for example mm. Figure 8 It was shown in particular that the proposed method is also suitable for measuring or characterizing hot, transparent or translucent objects.

[0135] Figures 9 to 11 An example structure of a k-clock interferometer of a k-clock unit and an example implementation of the k-clock unit in an OCT system are shown.

[0136] Figure 9 A k-clock interferometer 29 of a k-clock unit for generating a k-clock signal (e.g., for k-clock-based resampling) is shown. The k-clock interferometer 29 is constructed as a planar surface interferometer in the manner of a Fabry-Perot interferometer and is therefore also referred to herein as a k-clock Fabry-Perot interferometer or simply a Fabry-Perot interferometer.

[0137] The k-clock interferometer 29 (hereinafter also referred to as interferometer 29) includes a spacer 30 having a hollow cylindrical spacer body 31 having an inner hollow cylindrical gap 32 or air gap 32. Flat surfaces are formed at both distal ends 33 of the spacer body 31, which have a parallelism of less than 0.005 degrees, i.e., a deviation of the parallelism is less than 0.005 degrees. The spacer 30 or the spacer body 31 is made of a material having a very low temperature expansion coefficient (e.g., having a coefficient of expansion within + / - 2*10 -7 / ℃ range of thermal expansion coefficient).

[0138] The first and second mirrors 34, 35 are attached to the distal ends 33 facing away from each other, for example, by means of a stress-free retaining ring 36 or a retaining body that presses the mirrors 34, 35 onto corresponding support surfaces of the distal end 33. For example, for an oscillation frequency of a k-clock of 1.58 GHz, the length of the spacer 30 or spacer body 31 can be, for example, 95 mm. Suitable materials are, for example, those mentioned above or below.

[0139] The mirrors 34, 35 are wedge-shaped and provided with an antireflection coating so that the mirrors 34, 35 themselves do not form an etalon, but rather the 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.

[0140] The mirror surfaces S of the reflectors 34 and 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 reflectors 34 and 35 are designed as transparent reflectors or semi-transparent reflectors.

[0141] To couple laser light into and out of the k-clock interferometer 29, or in the region of the mirrors 34 and 35, a so-called tilting device or mechanism can be provided, with which, for example, a light guide for supplying laser light to or releasing laser light from the k-clock interferometer 29 can be tilted or tilted relative to two axes. The tilting device can also include an adjustment mechanism that enables adjustment in the xy direction (e.g., perpendicular to the optical axis).

[0142] In order to avoid influence of the surrounding environment (such as pressure, humidity etc.) on the gap 32 or air gap 32 , the mirrors 34 , 35 may be sealingly connected to the spacer body 31 , for example by anodic bonding, by optical contact bonding or by gluing.

[0143] Figure 10 and Figure 11 By way of example, a specific implementation of the k-clock unit 38 in an OCT system is shown. Figure 10 and Figure 11 A laser light source or laser 2 (particularly a swept source laser), a beam splitter 39, a k-clock interferometer 29 and an ultrafast detector 40 are each shown.

[0144] according to Figure 10 The structure and function of the device are as follows:

[0145] The laser 2 is optically coupled, for example by means of a light guide, to an input SE of a beam splitter 39. A first output SA1 of the beam splitter 39 is optically coupled to the k-clock interferometer 29, in particular to an input C1 of the k-clock interferometer 29 associated with the first mirror 34, for coupling the laser light via the first mirror 34. As indicated by the arrow, a second output SA2 of the beam splitter 39 is optically coupled to the OCT sensor head.

[0146] The input DE of the detector 40 is optically coupled to the k-clock interferometer 29 at C2 (the output of the k-clock interferometer 29 associated with the second mirror 35 ) so that the interferometric signal can be directed to the detector 40 and evaluated thereby.

[0147] In an embodiment, all optical coupling or connection is preferably achieved by light guides.

[0148] 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. This is schematically indicated by an arrow.

[0149] In the present example, the beam splitter 39 is designed such that a 1% portion of the laser radiation reaches the k-clock interferometer 29 via a first output SA1 of the beam splitter 39 and a 99% portion of the laser radiation reaches the OCT sensor head via a second output SA2 .

[0150] Figure 10 The arrangement shown and Figure 11 The arrangement shown, which will be explained in more detail below, allows for efficient and accurate data acquisition based on a stable k-clock signal.

[0151] according to Figure 11 The arrangement or structure of Figure 10 The arrangement or structure shown differs in that, according to Figure 11 The detector 40 is implemented as a symmetrical or balanced detector, or uses balanced detection or a balanced detector.

[0152] Specifically, according to Figure 11 The detector 40 has two inputs, a first input DE1 and a second input DE2. Figure 10 , the second input DE2 is optically coupled to the output C2 of the k-clock interferometer 29 associated with the second mirror 35. 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 (this output is denoted as C1*).

[0153] 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, the first output SA1 of the beam splitter is optically coupled or connected to the first port P1, the second port P2 is optically coupled or connected to the input C1 / output C1* of the k-clock interferometer 29, and the third port P3 is optically coupled or connected to the first input DE1 of the detector 40. Laser light entering from the beam splitter 39 at port P1 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.

[0154] The optical path lengths between the output C1 * and the first input DE1 and also between the output C2 and the second input DE2 are preferably identical or substantially identical.

[0155] By using balance or symmetry detection, e.g. Figure 11As shown, the signal-to-noise ratio can be improved and interference signals can be suppressed.

[0156] Using the proposed k-clock unit (which may include, for example, a k-clock interferometer and an associated detector), a relatively stable k-click signal can be obtained, which in turn leads to improved measurement results. In particular, the free spectral range (FSR) of the k-clock unit, and therefore the length scale of the measurement, can be made essentially constant and independent of environmental influences such as temperature or pressure fluctuations.

[0157] Due to the extremely low thermal expansion coefficient of the spacer, temperature variations have little effect on its length. For example, the effects of temperature, pressure, and humidity on the refractive index of the air inside the spacer can be considered as a residual factor in the FSR variation. To minimize or eliminate such effects, the spacer can also be used to seal the mirror from the surrounding environment and thereby seal the gap or air gap by anodic bonding, optical contact bonding, or gluing.

[0158] The proposed k-clock unit can be Figure 10 and 11 As shown, this can be relatively easily implemented into the OCT system described above. To this end, for example, a small portion (~1%) of the laser light can be delivered to an input, such as a fiber-optic input of a k-clock. The output signal, such as from the fiber-optic output of the k-clock, can be measured with a fast optical detector or receiver. As mentioned, balanced or symmetrical 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 by a balanced or symmetrical detector.

[0159] In balanced or symmetrical detection, the amplitudes of the respective signals (i.e., the signals at the two outputs of the k-clock interferometer) are preferably adjusted before the detector, for example, by means of fiber optic attenuators. It is particularly preferred that the optical path lengths between the detector inputs and the respective outputs of the k-clock are of the same length.

[0160] From the embodiments, it is particularly clear that the methods and optical coherence tomography systems presented herein enable reliable measurement and geometric characterization of objects and are suitable not only for determining relative object geometry but also for determining absolute distances relative to the measurement system and the object. Furthermore, the methods are suitable for characterizing hot and / or moving objects. In particular, the methods are suitable for characterizing objects, for example, during a manufacturing process, and can be performed substantially in real time.

[0161] A. Reference numerals

[0162] 1 object

[0163] 2 swept source lasers or lasers

[0164] 3 Circulator

[0165] 4 Collimator

[0166] 5 beam splitter

[0167] 6First output

[0168] 7 Second output

[0169] 8 power monitoring units

[0170] 9Variable optical attenuator

[0171] 10 high-speed detectors

[0172] 11 OCT signal

[0173] 12 Laser radiation

[0174] 13 Fixed partially reflective optical unit, fixed reference substrate

[0175] 14 galvanometer scanner or scanner

[0176] 15 microlens array

[0177] 16 Another beam splitter

[0178] 17 additional beam splitter terminals

[0179] 18 additional variable optical attenuators

[0180] 19 Retroreflector

[0181] 20K clock

[0182] 21 resampling

[0183] 22 Start time adjustment

[0184] 23 Zero-phase low-pass filter

[0185] Calculation of 24 phase evolution

[0186] 25 Extraction of phase distribution ("phase unwrapping")

[0187] 26 Applying Window Functions

[0188] 27 Fast Fourier Transform (FFT)

[0189] 28 OCT cycle diagram

[0190] 29 k clock interferometer

[0191] 30 spacers

[0192] 31 spacer body

[0193] 32 gap / air gap

[0194] 33 distal end

[0195] 34 First Reflector

[0196] 35 Second reflector

[0197] 36 retaining ring

[0198] 37 Tilt Device

[0199] 38 k clock units

[0200] 39 beam splitter

[0201] 40 detectors

[0202] 41 Circulator

[0203] t time

[0204] W wall thickness

[0205] ID inner diameter

[0206] AD outer diameter

[0207] D distance

[0208] OS OCT signal intensity

[0209] X impact point, point

[0210] C1 k clock interferometer input

[0211] C2 k clock interferometer output

[0212] C1*k clock interferometer output

[0213] DE input detector

[0214] DE1 detector first input

[0215] DE2 detector second input

[0216] DA detector output

[0217] SE beam splitter input

[0218] SA1 beam splitter first output

[0219] SA2 beam splitter second output

[0220] P1-P3 ports, circulator

[0221] S mirror

Claims

1. A method for optically characterizing a transparent or semi-transparent object (1), wherein the object size (W, ID, AD), the object distance and / or the object position are determined based on optical coherence tomography, characterized in that A swept-source laser source having a coherence length in the decimeter range to the meter range is used as the light source (2), and for determining 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 fixed in the beam path (12).

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

3. The method according to claim 1 or 2, characterized in that During the signal acquisition and / or signal evaluation of the coherence tomography a. Use k clock resampling (21), b. algorithmically determining the minimum, maximum or zero crossing of the k clock of the laser source (2) and using it as the sampling clock, or c. Use k-clock hardware trigger.

4. The method according to any one of the preceding claims, characterized in that In the optical coherence spectroscopy, the object (1) is scanned by the laser (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. The method according to the preceding claim, characterized in that The object (1) is moved, and a scanning path used when 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. Method according to the preceding claim, characterized in that Spatial dither is superimposed on the scan path.

7. The method according to claim 5 or 6, characterized in that A plurality of reflection curves obtained along the scanning path are combined, a periodogram signal is determined from the combined reflection curve thus obtained, and at least one object size (W, ID, AD), at least one object distance and / or at least one object position is determined from at least one periodogram signal.

8. The method according to any one of the preceding claims, characterized in that An infrared filter, preferably a dual-band reflector, is arranged in the beam path (12) of the light source (2) and is used to filter out at least predominantly thermal radiation in the far infrared spectral range emanating from the object (1) and / or the object's surroundings.

9. The method according to any one of the preceding claims, 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 further optionally comprises subsequent phase extraction (25), in particular based on Hilbert transformation and / or evaluation of zero crossings and extrema, the filtering (23) optionally also comprising low-pass filtering.

10. The method according to any one of the preceding claims, characterized in that A periodogram signal is determined from the optical coherence tomography signal based on the periodogram data (28) by means of an automatic peak picking algorithm, and at least one object size (W, ID, AD), at least one object distance and / or at least one object position is determined based on the periodogram signal, in particular taking into account the periodogram signal of the reference substrate (13).

11. An optical coherence tomography system designed for optical characterization of a transparent or semi-transparent object (1) based on optical coherence tomography, the optical coherence tomography system comprising: At least one swept-source laser light 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); and a control unit having a processing unit and a memory associated with the processing unit, the processing unit being in particular a processing unit having an architecture for parallel data processing, the memory comprising instructions which, when executed by the processing unit, result in the method according to any one of claims 1 to 10.

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

13. The 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. 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 at least one infrared filter being designed to at least mainly filter out thermal radiation in the far-infrared spectral range emitted from the object (1) and / or the environment surrounding the object.

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

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

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

18. An optical coherence tomography system according to claim 16 or 17, wherein 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 arranged to feed the k-clock unit (38) with laser light from the light source (2), in particular the Fabry-Perot interferometer (29), wherein the beam splitter (39) is preferably designed to feed the k-clock unit (38) 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%, via an input (C1) associated with the first mirror (34), and / or The optical coherence tomography system comprises a detector (40) having at least one input (DE) optically coupled to an output (C2) associated with the k-clock unit (38), in particular the second mirror (35) of the Fabry-Perot interferometer (29), for detecting an 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 the evaluation.

19. The optical coherence tomography system according to claim 18, wherein: In order to improve the signal-to-noise ratio and 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 the first detector input (DE1) is optically coupled to the output (C1*) of the Fabry-Perot interferometer (29) associated with the first reflector (34), and the second detector input (DE2) is optically coupled to the output (C2) of the Fabry-Perot interferometer (29) associated with the second reflector (35), wherein the optical coherence tomography system further preferably comprises a circulator (41) having three ports (P1, P2, P3), wherein the laser light source (2) is optically coupled to the first port ( P1) for feeding the laser light from the laser light source (2) to the Fabry-Perot interferometer (29) via a second port (P2) of the circulator (41) connected to an input (C1) associated with the first reflector (34), and the second detector input (DE1) is optically coupled to a third port (P3) of the circulator (41), and the circulator (41) is designed so that the interferometric measurement 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 reflector (34) is guided to the first terminal (DE1) of the detector (40) via the third port (P2).

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