DE102024118277A1 - Method and device for measuring distance and thickness - Google Patents

Abstract

In a method for measuring the distance to a transparent object (30) and for measuring the thickness of the transparent object (30), a light source (12) generates measuring light, which is split into reference light and object light, directed into a reference arm (16) and an object arm (18), respectively. A first interference of a reference light component reflected in the reference arm (16) with an object light component reflected from the object (30) in the object arm (18) is detected by a first spectrometer (36; 36a), yielding a first spectrum. A second interference of the reference light component and the object light component is detected by a second spectrometer (36; 36b), which may be identical to the first spectrometer (36), except that the phase of one reference light component is shifted relative to the phase of one object light component by an amount φ. This yields a second spectrum. To measure thickness, the first spectrum is added to the second spectrum, and the sum of the spectra is subjected to a Fourier transform. To measure distance, the first spectrum is subtracted from the second spectrum, and the difference in the spectra is subjected to a Fourier transform.

Description

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention relates to a method and a device for measuring the distance to a transparent object and for measuring the thickness of the transparent object using SD OCT, where SD is the acronym for Spectral Domain and OCT is the acronym for optical coherence tomography.

2. Description of the state of the art

For measuring the surface and thickness profiles of workpieces and other objects, devices based on the principle of SD OCT have been increasingly used for several years. These devices measure the distance to a scattering or at least partially reflective surface of the object at individual measuring points without contact, provided they have a reference arm. Repeating this measurement at a large number of measuring points yields a surface profile of the object.

If the object is sufficiently transparent to the measuring light, the thicknesses between scattering or reflecting interfaces that define the object or are located within the object can also be measured with high accuracy. A measurement at multiple points then yields a corresponding thickness profile. Combining distance and thickness measurements allows, for example, verification that the surface of a coated workpiece has the desired shape and that the coating thickness is constant throughout.

The first devices of this type could only determine the distance for a single measuring point. To scan an area, the measuring head had to be moved relative to the workpiece (or vice versa). From the DE 10 2022 104 416 A1 A device is known in which a scanning unit guides the measuring beam over the workpiece. The applicant markets such devices under the name "Flying Spot Scanner".

SD OCT devices generate raw measurement data in the form of interference spectra, from which the desired distance or thickness values are derived by Fourier transformation. Each distance or thickness value corresponds to a peak that can be displayed on a screen.

For example, if an SD OCT device with a reference arm is used to measure the distances and thicknesses of a glass plate, three peaks will appear: two for the distances to the two surfaces of the plate and one for its thickness. When the glass plate is moved parallel to the direction of the measurement beam, the two distance peaks move, while the thickness peak remains stationary, as the thickness of the glass plate does not change due to the movement. In this way, the displayed peaks can be relatively easily correlated with the quantities being measured. Alternatively, in some devices of this type, the light propagation in the OCT's reference arm can be interrupted. Then, only reflections from the object can interfere. Switching off the reference arm thus causes only the thickness peaks to be displayed, while all distance peaks disappear.

If an object consisting of two layers is to be measured with an SD OCT device containing a reference arm, there are already six peaks that must be assigned to the quantities to be measured: three distance peaks and three thickness peaks (one thickness peak for each layer and one thickness peak for the total thickness of the layer system). With a layer system consisting of three layers, there are four distance peaks and six thickness peaks, and so on.

Especially with multi-layered objects, it can be very tedious to assign the displayed peaks to the desired distances and thicknesses using the measures described above, namely moving the object and/or stopping down the reference arm. In particular, it is not possible to display only the distance peaks and suppress the thickness peaks. The assignment is further complicated when additional peaks appear, caused by double reflections at the object's interfaces.

Furthermore, thickness and distance peaks can overlap completely or partially, so that they may be poorly resolved or not detected at all.

Out of Wen-Chuan Kuo et al., “Balanced detection for spectral domain optical coherence tomography,” Opt. Express 21, 19280-19291 (2013), 10.1364/OE.21.019280 The use of a balanced detection (BD) method in SD OCT is well-known. In this method, two interference spectra shifted by π are acquired by spectrometers and subtracted from each other. This allows background noise and autocorrelation artifacts in biological tissues to be suppressed.

SUMMARY OF THE INVENTION

The object of the invention is to provide a method for measuring the distance to a transparent object and for measuring the thickness of the transparent object using SD OCT, in which, even in the case of a multi-layered object, the measured peaks can be more easily assigned to the distances and thicknesses to be measured.

1. a) eine Lichtquelle erzeugt Messlicht;
2. b) das Messlicht wird in Referenzlicht und in Objektlicht aufgeteilt;
3. c) das Referenzlicht wird in einen Referenzarm und das Objektlicht in einen Objektarm geleitet;
4. d) es wird eine erste Interferenz von einem Referenzlichtanteil des Referenzlichts, der im Referenzarm von einem feststehenden Reflektor reflektiert wurde, mit einem Objektlichtanteil des Objektlichts, der im Objektarm von dem Objekt reflektiert wurde, von einem ersten Spektrometer erfasst, wodurch ein erstes Spektrum erhalten wird;
5. e) es wird eine zweite Interferenz von dem Referenzlichtanteil und dem Objektlichtanteil von einem zweiten Spektrometer erfasst, das identisch mit dem ersten Spektrometer sein kann, wobei eine Phase des Referenzlichtanteils gegenüber einer Phase des Objektlichtanteils um einen Betrag φ verschoben ist, wodurch ein zweites Spektrum erhalten wird;
6. f) zur Dickenmessung wird das erste Spektrum zu dem zweiten Spektrum addiert und die Summe der Spektren einer Fourier-Transformation unterworfen;
7. g) zur Abstandsmessung wird das erste Spektrum von dem zweiten Spektrum subtrahiert und die Differenz der Spektren einer Fourier-Transformation unterworfen.

This task is solved by a procedure with the following steps:

1. a) a light source produces measuring light;
2. b) the measuring light is split into reference light and object light;
3. c) the reference light is directed into a reference arm and the object light into an object arm;
4. d) a first interference of a reference light component that was reflected in the reference arm by a stationary reflector with an object light component that was reflected in the object arm by the object is detected by a first spectrometer, thereby obtaining a first spectrum;
5. e) A second interference of the reference light component and the object light component is recorded by a second spectrometer, which may be identical to the first spectrometer, wherein a phase of the reference light component is shifted relative to a phase of the object light component by an amount φ, thereby obtaining a second spectrum;
6. f) For thickness measurement, the first spectrum is added to the second spectrum and the sum of the spectra is subjected to a Fourier transformation;
7. g) To measure the distance, the first spectrum is subtracted from the second spectrum and the difference between the spectra is subjected to a Fourier transformation.

The closer the magnitude φ of the phase shift is to π, the more the results obtained in steps f) and g) of the inventive method differ. Ideally, i.e., when φ = π and the two spectra are normalized, only the thickness peaks are output in step f) for the thickness measurement, and only the distance peaks are output in step g) for the distance measurement. However, to distinguish the peaks, complete suppression of the unwanted peaks is often not necessary. It is sufficient if the unwanted peaks are so weak that the desired peaks can be clearly identified. Even if the phase shift φ is only approximately π and lies, for example, between 0.9π and 1.1π, very good suppression of the unwanted peaks is still achieved. With very good spectral normalization, even larger deviations from π can be tolerated, e.g., 0.6·π < φ < 1.4·π. Since setting a phase shift of π is technically easy to achieve, one will generally try to keep the deviation of the phase shift from π small.

The invention can be explained intuitively as follows: the interference patterns involving the reference light component add up to a constant due to the phase shift introduced between the reference light component and the object light component. After the Fourier transform, this leads to the cancellation of the distance peaks, leaving only the thickness peaks. Conversely, when the spectra are subtracted, the interferences to which the reference light component does not contribute cancel each other out, resulting in the suppression of the thickness peaks.

In one embodiment, the first spectrometer is identical to the second spectrometer; that is, only a single spectrometer is used. The two spectra can then be generated sequentially by an intermittently switching the phase by π using a switchable phase shifter located in either the reference or object arm. Alternatively, phase shifters can be arranged in both the reference and object arms, shifting the phases to achieve the desired relative phase shift of φ. The (single) spectrometer thus captures the first and second interference sequentially.

However, such a serial measurement of the two spectra takes time. Furthermore, the phase shifter(s) must be able to switch very quickly at high measurement frequencies, which is technologically demanding. This variant is therefore well-suited for static measurements, but less so for fast scanning measurements.

A preferred embodiment therefore includes a configuration in which the first spectrometer is different from the second spectrometer. In step b), the measuring light is split into reference light and object light by a beam splitter, which optically connects the reference arm and the object arm to the first spectrometer and the second spectrometer, respectively.

In this way, the two interferences, with and without phase shift, can be simultaneously recorded by the spectrometers and output as spectra. This makes the measurement just as fast as a measurement with a [previous method/method - context needed]. conventional SD OCT device. A further advantage is that no switchable phase shifters are required. Instead, beam splitters, which are needed anyway, are used for the phase shift, since beam splitters inherently possess the desired property of generating a phase shift of π. This applies both to beam splitters in the form of beam splitter cubes, as used for free-ray light propagation, and to fiber optic beam splitters such as conventional 3 dB fiber couplers.

To reduce the equipment required, in this embodiment a device for detecting interferences can be used in which the first spectrometer and the second spectrometer are integrated in a common housing and use a common dispersive optical element that spectrally decomposes incident light and directs it onto different rows of light-sensitive cells.

In steps d) and e), the object light can be guided over the object using a scanning device. Preferably, the scanning device deflects the object light not only in one, but in two spatial directions, so that the surfaces of objects can be scanned very quickly.

1. a) eine Lichtquelle, die dazu eingerichtet ist, Messlicht zu erzeugen,
2. b) einen Referenzarm mit einem feststehenden Reflektor, in dem ein erster Teil des Messlichts als Referenzlicht geführt wird,
3. c) einen Objektarm, in dem ein zweiter Teil des Messlichts als Objektlicht geführt wird,
4. d) ein erstes Spektrometer, das dazu eingerichtet ist, eine erste Interferenz von einem Referenzlichtanteil des Referenzlichts, der im Referenzarm von dem Reflektor reflektiert wurde, mit einem Objektlichtanteil des Objektlichts, der im Objektarm von dem Objekt reflektiert wurde, zu erfassen, wodurch ein erstes Spektrum erhalten wird,
5. e) ein zweites Spektrometer, das identisch mit dem ersten Spektrometer sein kann, wobei das zweite Spektrometer dazu eingerichtet ist, eine zweite Interferenz von dem Referenzlichtanteil und dem Objektlichtanteil zu erfassen, wobei eine Phase des Referenzlichtanteils gegenüber einer Phase des Objektlichtanteils um einen Betrag φ verschoben ist, wodurch ein zweites Spektrum erhalten wird, und
6. f) eine Recheneinheit, die dazu eingerichtet ist,
   • - zur Dickenmessung das erste Spektrum zu dem zweiten Spektrum zu addieren und die Summe der Spektren einer Fourier-Transformation zu unterwerfen und
   • - zur Abstandsmessung das erste Spektrum von dem zweiten Spektrum zu subtrahieren und die Differenz der Spektren einer Fourier-Transformation zu unterwerfen.

A further object of the invention is to provide a device suitable for carrying out the method. A device that solves this object comprises:

1. a) a light source designed to produce measuring light,
2. b) a reference arm with a fixed reflector in which a first part of the measuring light is guided as a reference light,
3. c) an object arm in which a second part of the measuring light is guided as object light,
4. d) a first spectrometer designed to detect a first interference of a reference light component reflected in the reference arm from the reflector with an object light component reflected in the object arm from the object, thereby obtaining a first spectrum,
5. e) a second spectrometer, which may be identical to the first spectrometer, wherein the second spectrometer is configured to detect a second interference of the reference light component and the object light component, wherein a phase of the reference light component is shifted relative to a phase of the object light component by an amount φ, thereby obtaining a second spectrum, and
6. f) a computing unit designed to
   • - to measure thickness, add the first spectrum to the second spectrum and subject the sum of the spectra to a Fourier transform and
   • - to measure the distance, subtract the first spectrum from the second spectrum and subject the difference of the spectra to a Fourier transformation.

The advantages and variations described above for the process apply accordingly to the device.

BRIEF DESCRIPTION OF THE DRAWINGS

• 1 eine SD OCT Messvorrichtung gemäß dem Stand der Technik in einer schematischen Darstellung;
• 2 mehrere Graphen zur Illustration wesentlicher Schritte bei der Auswertung der Messdaten;
• 3 mehrere Abstands- und Dickenpeaks, wie sie von der in der 1 gezeigten Messvorrichtung angezeigt werden;
• 4 eine an die 1 angelehnte Darstellung einer erfindungsgemäßen Messvorrichtung gemäß einem ersten Ausführungsbeispiel mit einem Spektrometer;
• 5a das ohne Phasenverschiebung von der in der 4 gezeigten Messvorrichtung gemessene Spektrum;
• 5b die Abstands- und Dickenpeaks, die sich aus dem in der 5a gezeigten ersten Spektrum nach Durchführung einer Fourier-Transformation ergeben;
• 6a das mit Phasenverschiebung von der in der 4 gezeigten Messvorrichtung gemessene Spektrum;
• 6b die Abstands- und Dickenpeaks, die sich aus dem in der 6a gezeigten ersten Spektrum nach Durchführung einer Fourier-Transformation ergeben;
• 7a die Summe der beiden in den 5a und 6a gezeigten Spektren;
• 7b der Dickenpeak, der sich aus dem in der 7a gezeigten Summenspektrum ergibt;
• 8a die Differenz der beiden in den 5a und 6a gezeigten Spektren;
• 8b die Abstandspeaks, die sich aus dem in der 8a gezeigten Differenzspektrum ergeben;
• 9 eine an die 4 angelehnte Darstellung einer erfindungsgemäßen Messvorrichtung gemäß einem zweiten Ausführungsbeispiel mit zwei Spektrometern; und
• 10 eine schematische Darstellung eines Geräts, bei dem zwei Spektrometer mit einem gemeinsamen dispersiven optischen Element in einem Gehäuse vereint.

Exemplary embodiments of the invention are explained in more detail below with reference to the drawings. These show:

• 1 an SD OCT measuring device according to the state of the art in a schematic representation;
• 2 Several graphs to illustrate essential steps in the evaluation of the measurement data;
• 3 several distance and thickness peaks, as seen from the one in the 1 The measuring device shown will be displayed;
• 4 one to 1 Adapted representation of a measuring device according to the invention in a first embodiment with a spectrometer;
• 5a that without phase shift from the one in the 4 Spectrum measured by the measuring device shown;
• 5b the distance and thickness peaks that result from the in the 5a The first spectrum shown is obtained after performing a Fourier transformation;
• 6a that with phase shift from the one in the 4 Spectrum measured by the measuring device shown;
• 6b the distance and thickness peaks that result from the in the 6a The first spectrum shown is obtained after performing a Fourier transformation;
• 7a the sum of the two in the 5a and 6a shown spectra;
• 7b the thickness peak, which results from the in the 7a the sum spectrum shown;
• 8a the difference between the two in the 5a and 6a shown spectra;
• 8b the distance peaks that result from the in the 8a shown difference spectrum;
• 9 one to 4 Adapted representation of a measuring device according to the invention in a second embodiment with two spectrometers; and
• 10 a schematic representation of a device in which two spectrometers with a common dispersive optical element are combined in one housing.

DESCRIPTION OF PREFERRED EXAMPLES

1. Setup of a conventional SD OCT measuring device

The 1 Figure 1 shows a schematic representation of a conventional SD OCT measuring device, designated 10' overall. The measuring device 10' essentially consists of an optical coherence tomograph containing a broadband light source 12', which can, for example, be a superluminescent diode. In the illustrated embodiment, the light used for the measurement is predominantly guided in optical fibers; however, it is equally possible to guide the light exclusively as a free beam, as is known in the prior art.

A first beam splitter 14' divides the measurement light generated by the light source 12' into reference light RL and object light OL. In general, in known SD OCT measuring devices, the measurement light is not divided equally, but asymmetrically. A smaller portion of the measurement light is directed by the first beam splitter 14' as reference light RL into a reference arm 16', and the larger remaining portion as object light OL into an object arm 18'. In fiber-based measuring devices, 2x2 fiber couplers are typically used as the beam splitter 14'.

The reference light RL is reflected as completely as possible in the reference arm 16' and returns to the first beam splitter 14' as the reference light component. Reflection in the reference arm 16' can be achieved using a fiber reflector (e.g., a Bragg reflector). A higher reflectance, independent of wavelength, is achieved with a conventional fixed mirror 20'. For this purpose, the reference light RL exiting the optical fiber can be collimated using a first lens 22' and focused onto the mirror 20' using a second lens 24'.

The measuring light, directed into the object arm 18' as object light OL, is focused by means of lenses 26', 28', which are part of the object arm 18', and directed onto the object 30 to be measured. In the illustrated embodiment, the object 30 consists of two adjacent glass plates 32, 34, both of which are transparent to the object light OL. In this context, transparent means that a detectable proportion of the object light OL reflected from the interfaces of the lower glass plate 34 can exit the object 30 again. The transmittance of the object 30 should therefore be at least 5% and preferably more than 25%.

The object light OL is partially reflected or scattered at the interfaces of the two glass plates 32, 34. The reflected object light component passes through the lenses 26', 28' back into the optical fiber and interferes in the first beam splitter 14' with the reference light component from the reference arm 16'. The interference signal is coupled out via a second beam splitter 35' and fed to a spectrometer 36', which records the spectral intensity distribution of the interference signal.

From this, an evaluation unit 38' connected to the spectrometer 36' can determine the distances between the interfaces of the glass plates 32, 34 to the measuring device 10', the thicknesses of the individual glass plates 32, 34 and the thickness of the entire object 30.

The 2 This evaluation is illustrated in a schematic representation. On the left, object 30 is shown with the two glass plates 32, 34. At each of the three optical interfaces 40, 42, 44, there is a change in refractive index and thus a partial reflection of the object light OL, indicated by an arrow.

To the right of object 30, three interference spectra 1, 2, and 3 are shown. Interference spectrum 1 would be recorded by spectrometer 36' if only the first interface 40 were in the beam path of the object light OL. The reflection of the object light OL at the first interface 40 leads to an intensity modulation of the spectrum in k-space, which is proportional to the difference in optical path lengths traveled by the reference light RL in reference arm 16' and the object light OL in object arm 18'. The desired distance information is thus encoded in the frequency at which the intensity oscillates in k-space.

Corresponding considerations apply to interference spectra 2 and 3 and the optical interfaces 42 and 44 associated with these interference spectra. The optical path length in the reference arm 16' is defined here such that the modulation of the intensity in k-space becomes higher in frequency the further the relevant optical interface is from the coherence tomograph 40. The increasing modulation frequencies are clearly visible in interference spectra 1, 2, and 3.

Since the object light OL strikes not only the interfaces 40, but also the interfaces 42 and 44, the interference spectra 1, 2 and 3 overlap. The spectrometer 36' therefore only detects the total spectrum 50 shown to the right, which represents an additive superposition of the interference spectra 1, 2 and 3.

A Fourier transform can be used to obtain the spectral components, i.e., the modulation frequencies of spectra 1, 2, and 3, from the total spectrum 50. This is shown on the right in the 2 shown. In the example shown, for each optical interface 40, 42 and 44, a peak is obtained which corresponds to a certain distance value z and is assigned to one of the interference spectra 1, 2 and 3.

The spectrometer 36' also detects interferences that do not result from the interference of the reference light component with the object light component described above, but rather from interference of parts of the object light component caused by reflections at the interfaces 40, 42, 44. These interferences also produce peaks after the Fourier transform is performed, which correspond to the ones shown on the right in the 2 The peaks shown are added and make the classification more difficult.

The 3 Figure 1 shows six exemplary peaks as they are displayed to a user of the measuring device 10' when measuring object 30 with its three interfaces 40, 42, 44.

The graph plots intensity I against distance z. A peak in the middle towers above the other peaks. This is due—though a user cannot definitively verify this—to the fact that two peaks, indicated by dotted lines, largely overlap. It is not immediately clear which of the interfaces 40, 42, 44, or thicknesses any of the peaks belong to. It is not possible to focus solely on the peaks shown in the graph. 2 to display the distance peaks shown or only the thickness peaks.

2. First embodiment - switchable phase shifter

The 4 displays in a message to the 1 The illustration is based on a measuring device 14 according to the invention in a first embodiment. This device has largely the same structure as the one in the 1 The measuring device shown 14' is in accordance with the prior art. Identical or corresponding components are therefore designated with the same reference numerals without an apostrophe.

In contrast to the known measuring device 10', the measuring device 10 according to the invention includes a switchable fiber-based phase shifter 60, which is arranged in the reference arm 16, but could alternatively also be arranged in the object arm 18. The phase shifter 60 is configured to intermittently increase the phase of the transmitted light by π/2, as indicated by a function above the phase shifter. Due to reflection at the mirror 20, the reference light component passes through the phase shifter 60 twice, resulting in a phase shift of π.

The spectrometer 36 alternately records a first interference of the reference light component with the object light component without phase shift (first spectrum) and a second interference of the reference light component with the object light component with a relative phase shift of π (second spectrum).

The 5a Using the example of an object consisting of a single glass plate, the first spectrum is shown, and the 5b The peaks obtained after Fourier transformation are shown. The intensity I is plotted against the wavenumber k or the distance z. Three peaks are visible, two of which correspond to the distances to the interfaces, and one to the thickness of the glass plate. It is not immediately clear how the three peaks correspond to the quantities being measured.

The 6a and 6b are the 5a and 5b Corresponding diagrams for the second spectrum, generated with the phase shift of π. Here too, the same three peaks are visible after the Fourier transform, which is not surprising, since a phase shift in one of the two arms of the coherence tomograph should not affect the measurement results.

However, in the measuring device 10 according to the invention, the two in the 5b and 6b The diagrams shown are not displayed, but optionally one of the diagrams that are in the 7b or 8b shown.

In the 7a The sum of the two spectra shown in the 5a and 6a are reproduced, and in the 7b The result of the Fourier transform of this sum spectrum. It can be seen that in the 7b Only one peak – namely the thickness peak – appears, while the The other two peaks are almost completely suppressed.

In the 8a The difference between the two spectra is shown, which are in the 5a and 6a are reproduced, and in the 8b The result of the Fourier transform of this difference spectrum. It can be seen that in the 8b Only two peaks appear – namely the two distance peaks – while the thickness peak has been almost completely suppressed.

The measuring device 10 according to the invention thus makes it possible to obtain two phase-shifted spectra by switching the phase shifter 60, which are then added and subtracted. Fourier transformation of the sum spectrum and the difference spectrum yields only the peaks from the thickness measurement and the distance measurement, respectively. Preferably, the measuring device has a rotary control or other input device with which the user can specify whether to display all peaks as described in the 5b or 6b The option to display only the distance peaks or only the thickness peaks is shown. This significantly simplifies the assignment of the peaks to the desired distances and thicknesses.

3. Mathematical description

First, the case of thickness measurement is considered, where only the interference of parts of the object light component is of interest, but interference with the reference light component also occurs. It is assumed that the reference light component coming from reference arm 16 is... $$w_1=A\text{ cos}\left(kx-\omega t\right)$$ and the object light component coming from the object arm 18 through $$w_2=A\text{ cos}\left(kx-\omega t+\varphi \right)$$ is described where ϕ = kL and L is the propagation difference between the reference light component and the object light component. To simplify the mathematical representation, it is assumed that the amplitudes of the two wave components are equal and given by A; in reality, the amplitudes are usually only similar at best.

If the two waves are superimposed in the first beam splitter 14, the following results for the case without phase shift: $$w_1+w_2=2A\text{ cos}\left(\frac{\varphi }{2}\right)\text{cos}\left(kx-\omega t+\frac{\varphi }{2}\right)$$

The interference signal in spectrometer 36 is then given by $$I_a~4A^2{\left(cos\left(\frac{\varphi }{2}\right)\right)}^2=4A^2{\left(cos\left(\frac{kL}{2}\right)\right)}^2$$

If the wave w ₁ is superimposed with the wave w ₂ , which is phase-shifted by π, after activation of the phase shifter 60, the following superposition of the waves results: $$w_1+w_2=2A\text{ cos}\left(\frac{\varphi +\pi }{2}\right)\text{cos}\left(kx-\omega t+\frac{\varphi +\pi }{2}\right)$$

The interference signal in spectrometer 36 is then given by $$I_b~4A^2{\left(cos\left(\frac{\varphi +\pi }{2}\right)\right)}^2=4A^2{\left(sin\left(\frac{\varphi }{2}\right)\right)}^2=4A^2{\left(sin\left(\frac{kL}{2}\right)\right)}^2$$

If one forms the sum of the two interference signals I _a + I _b (cf. 7a) , thus it follows that: $$I_a+I_b~4A^2{\left(cos\left(\frac{kL}{2}\right)\right)}^2+{\left(sin\left(\frac{kL}{2}\right)\right)}^2=4A^2=const.$$ since cos(α) ² + sin(α) ² = 1.

All interference patterns for distance signals, which arise from interference with the reference light component, add up to a constant when summed. After the Fourier transform, only the thickness signals remain. Their amplitude doubles due to the summation, so that the thickness signals stand out even better from the noise.

The case of distance measurement is now considered, in which, according to the invention, the two spectra obtained with and without phase shift are subtracted from each other. The relative phase shift between the object light component and the reference light component caused by the phase shifter 60 has no effect on interferences between different parts of the object light component and thus on the thickness measurement. Since these components of the spectrum are identical in both measurements, they cancel each other out when the two spectra are subtracted, with the result that the thickness peaks are suppressed after the Fourier transform, as is the case with the 8b illustrated.

4. Second embodiment - two spectrometers

The 9 displays in a message to the 4 The illustration shows a measuring device 14 according to the invention in a second embodiment. Identical or corresponding components are designated with the same reference numerals.

This one has largely the same structure as the one in the 1 Measuring device 14' shown according to the state of the art.

In this embodiment, two spectrometers 36a and 36b are provided. The first beam splitter is formed by a 2x2 fiber coupler 140, which splits the measurement light coming from the light source 12 into reference light RL and object light OL and optically connects the reference arm 16 and the object arm 18, respectively, to the first spectrometer 36a and the second spectrometer 36b. The signal resulting from the interference of the reference light component with the object light component is thus split between the two spectrometers 36a and 36b, preferably in a 50:50 ratio (so-called 3 dB coupler).

Such fiber couplers (like other beam splitters) have the property that the signals relevant for distance measurement, resulting from the interference of light from the object arm 18 and reference arm 16, are phase-shifted by π. This is ultimately a consequence of energy conservation. Consequently, this embodiment does not require a phase shifter 60.

In comparison to the first embodiment, the one in the 9 The second embodiment shown has the advantage that the two interferences, with and without phase shift, can be recorded simultaneously and output as spectra, rather than sequentially. This makes the measurement just as fast as a measurement using the method described in the 1 The conventional SD OCT measuring device shown is 10'. Another advantage is the elimination of a switchable phase shifter, which, at least in fiber optic versions, is only available with relatively low switching frequencies.

The one in 9 The measuring device shown also includes a scanning device, indicated by 62, which deflects the object light OL variably in two spatial directions. For this purpose, the scanning device 62 has a first scanning mirror 64, which is rotatably mounted about a first axis of rotation 66. A second scanning mirror 68 is rotatably mounted about a second axis of rotation 70, which is oriented perpendicular to the first axis of rotation 66. The scanning mirrors 64 and 68 are driven by galvanometer drives (not shown), which are controlled by a control unit.

The measuring device 10 also includes an f-theta lens 72, which is located in the 9 as indicated by three lenses L1, L2 and L3. In the illustrated embodiment, the f-theta lens 72 focuses the object light deflected by the scanning device 26 such that it always strikes the surface of the object 30 facing the f-theta lens 72 at approximately a perpendicular angle.

To reduce the instrumental effort, the first spectrometer 36a and the second spectrometer 36b can be integrated in a common housing and use a common dispersive optical element that spectrally decomposes incident light and directs it onto different rows of light-sensitive cells.

This is in the 10 This is illustrated. A housing 73 and a dispersive optical element are visible, which here is designed as a transmission grating 74 primarily for the sake of clarity. The transmission grating 74 is arranged in the collimated beam path; a converging lens 76 focuses the diffracted light onto a detector 78. The detector 78 has not one, but two pixel rows 80, 82. Along the first pixel row 80 are arranged pixels 84, which are exclusively for the detection of the first spectrum. Along the second pixel row 82, which runs offset along the x-direction but parallel to the first pixel row 80, are arranged pixels 86, which are exclusively for the detection of the second spectrum. By dividing the spectrum into two pixel rows 80, 82, it is not necessary to provide two complete spectrometers with their own dispersive elements. By spatially offsetting the ends of the two fibers along the x-direction, which are connected to the outputs of the coupler 140, the diffracted light components can each be directed onto one of the two pixel rows 80, 82.

For measurements with two spectrometers 36a, 36b, the values output by the individual pixels 84, 86 must be added or subtracted as part of the summation or subtraction process. This requires that the light incident on the respective pixels 84, 86 has exactly the same wavelength. This can be ensured, for example, by adjusting a calibration table or by interpolating the measured values to equidistant points in k-space (wavenumber instead of wavelength), which is necessary anyway in connection with performing the Fourier transform.

In the 9 In the illustrated embodiment, the scanning device 62 and the f-theta lens 72 can be arranged in a separate housing, which defines a measuring head. This is connected via the [unclear text] in the 9 The optical fiber is connected to the other parts of the measuring device 14, which are located in a separate housing. A signal line for controlling the scanning device 62 runs along the optical fiber in this case. With such a setup, the Particularly sensitive parts of the measuring device 10 can be arranged at a greater distance from the measuring head. The measuring head, especially if it does not include a scanning device 62, can be very compact and therefore used even in confined spaces in production facilities.

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• DE 10 2022 104 416 A1 [0004]

Cited non-patent literature

• Wen-Chuan Kuo et al., “Balanced detection for spectral domain optical coherence tomography,” Opt. Express 21, 19280-19291 (2013), 10.1364/OE.21.019280 [0010]

Claims (10)

Method for measuring the distance to a transparent object (30) and for measuring the thickness of the transparent object (30) comprising the following steps: a) a light source (12) generates measuring light; b) the measuring light is split into reference light and object light; c) the reference light is directed into a reference arm (16) and the object light into an object arm (18); d) a first interference of a reference light component reflected in the reference arm (16) by a stationary reflector (20) with an object light component reflected in the object arm (18) by the object (30) is detected by a first spectrometer (36; 36a), thereby obtaining a first spectrum; e) A second interference of the reference light component and the object light component is recorded by a second spectrometer (36; 36b), which may be identical to the first spectrometer (36), wherein a phase of the reference light component is shifted relative to a phase of the object light component by an amount φ, thereby obtaining a second spectrum; f) For thickness measurement, the first spectrum is added to the second spectrum and the sum of the spectra is subjected to a Fourier transform; g) For distance measurement, the first spectrum is subtracted from the second spectrum and the difference of the spectra is subjected to a Fourier transform. Procedure according to Claim 1 , where the magnitude φ lies between 0.9·π and 1.1·π. Procedure according to Claim 1 or 2 , in which the first spectrometer (36a) is different from the second spectrometer (36b), and in which the measuring light in step b) is split into reference light and object light by a beam splitter (140), which optically connects the reference arm (16) and the object arm (18) on the one hand with the first spectrometer (36a) and the second spectrometer (36b) on the other hand. Procedure according to Claim 3 , in which the first spectrometer (36a) and the second spectrometer (36b) have a common housing (73) and a common dispersive optical element (74) that spectrally decomposes incident light and directs it onto different rows (80, 82) of light-sensitive cells (84, 86). Method according to one of the preceding claims, wherein in steps d) and e) the object light is guided over the object (30) by means of a scanning device (62). Device (10) for measuring the distance to a transparent object (30) and for measuring the thickness of the transparent object (30), comprising: a) a light source (12) configured to generate measuring light, b) a reference arm (16) with a stationary reflector (20) in which a first part of the measuring light is guided as reference light, c) an object arm (18) in which a second part of the measuring light is guided as object light, d) a first spectrometer (36; 36a) configured to detect a first interference of a reference light component reflected in the reference arm (16) by the reflector (20) with an object light component reflected in the object arm (20) by the object (30), thereby obtaining a first spectrum, e) a second spectrometer (36; 36b), which may be identical to the first spectrometer (36), wherein the second spectrometer (36, 36b) is configured to detect a second interference of the reference light component and the object light component, wherein a phase of the reference light component is shifted relative to a phase of the object light component by an amount φ, thereby obtaining a second spectrum, and with f) a computing unit (38) configured to: - add the first spectrum to the second spectrum for thickness measurement and subject the sum of the spectra to a Fourier transform, and - subtract the first spectrum from the second spectrum for distance measurement and subject the difference of the spectra to a Fourier transform. Device according to Claim 6 , where the magnitude φ lies between 0.9·π and 1.1·π. Device according to Claim 6 or 7 , in which the first spectrometer (36a) is different from the second spectrometer (36b), and with a beam splitter (140) that optically connects the reference arm (16) and the object arm (818) on the one hand with the first spectrometer (36a) and the second spectrometer (36b) on the other hand and is designed to split the measuring light into reference light and object light. Device according to 8, wherein the first spectrometer (36a) and the second spectrometer (36b) have a common housing (73) and a common dispersive optical element (74) which spectrally decomposes incident light and directs it onto different rows (80, 82) of light-sensitive cells (84, 86). device according to one of the Claims 6 until 9 , with a scanning device (62) which is used for this purpose is directed to guide the object light over the object (30).