System and method for interferometrically measuring a distance

Abstract

The present invention relates to a system (10) for interferometrically measuring a distance between a reference surface (20) and a target surface (22); comprising a tunable radiation source (12) for generating a coherent radiation emission having a wavelength as a function of a modulation parameter; a control module (14) configured to control the modulation parameter such that the wavelength of the generated radiation emission is modulated periodically between a first and a second reference wavelength; an interferometer module (16) configured to generate a first reflection at the reference surface (20) on the basis of a first part of the radiation emission and to generate a second reflection at the target surface (22) on the basis of a second part of the radiation emission, such that an interference signal is generated by way of the first reflection and the second reflection; a detector module (26) configured to capture the interference signal; and an evaluation module (28) configured to perform a phase measurement between the first and the second reference wavelengths on the basis of the captured interference signal and to determine the distance between the reference surface (20) and the target surface (22) on the basis of the phase measurement. The invention also relates to a corresponding method.

Description

[0001] System and method for interferometrically measuring a distance

[0002] The present invention relates to a system and a method for interferometrically measuring a distance.

[0003] Interferometric methods can be used to measure distances and, especially easily, to measure changes in distance relative to a starting value. The progressive development of various measurement setups, their miniaturization, and modern data processing capabilities open up additional fields of application for such techniques, for example, for the positioning and monitoring of components or tools.

[0004] DE 10 2017 113 997 B4 discloses a device and method for interferometric measurement. Interferometry is used to interferometrically measure the rotation of a body around a rotation axis.

[0005] EP 3 418 678 A1 proposes a method and a detector for image formation in correlation with interferometry. In this case, an image of a surface is to be captured in parallel with an interferometric measurement. The method allows for the relative measurement of distances and distance changes.

[0006] EP 2 847 538 A1 discloses a measuring device comprising an interferometer and an absorption medium defining a dense line spectrum.

[0007] EP 2 589 923 B1 describes a frequency scanning interferometer with opposite frequency modulation and multiple laser sources. This interferometric distance measurement is provided by varying the frequencies of multiple light sources. EP 2 149 778 B1 describes a method and a device for measuring the offset using multi-wavelength interferometry. The method provides for simultaneous measurement of interference values ​​for multiple laser beams of different wavelengths.

[0008] EP 3 161 408 B1 proposes a device and method for interferometry with a dual laser source, with frequencies being swept. The method is based on the simultaneous use of two laser sources, with one operating at a variable frequency and the other at a fixed frequency.

[0009] EP 2 877 810 B1 proposes an interferometric distance measuring arrangement, wherein a measuring beam is split into at least two channels in parallel by means of a beam splitter.

[0010] DE 102015 110 362 B4 discloses a method and device for interferometric absolute distance measurement. Interferometry is used to measure absolute distance. The frequency of a coherent light source is cyclically tuned by mechanically adjusting the length of a reference interferometer, and an absorption line at a known wavelength can be used to keep a specific wavelength constant.

[0011] Furthermore, DE 10 2016 103 109 B4 discloses the measurement of a cavity using interference spectroscopy. A coherent light source is tuned across a frequency range. A numerical fit of an interference spectrum is performed to determine a geometric and / or optical parameter of the cavity.

[0012] One of the challenges with known methods and systems is to be able to carry out distance measurements for moving objects, for example during an active movement of the target object or despite a drift, especially when using only a single beam source.

[0013] The object of the present invention is to provide a system and method for interferometric distance measurement that is improved over the prior art. In particular, it should enable the reliable measurement of a variable distance.

[0014] This object is achieved according to the invention by a system or method having the features of the independent claims. Advantageous embodiments are specified in the dependent claims.

[0015] According to this, the object is achieved by a system for interferometrically measuring a distance between a reference surface and a target surface, which system comprises a tunable radiation source for generating a coherent radiation emission with a wavelength dependent on a modulation parameter. The system further comprises a control module configured to control the modulation parameter such that the wavelength of the generated radiation emission is periodically modulated between a first and a second reference wavelength, and an interferometer module configured to generate a first reflection at the reference surface based on a first portion of the radiation emission and to generate a second reflection at the target surface based on a second portion of the radiation emission, such that an interference signal is generated by the first reflection and the second reflection.The system also comprises a detector module configured to detect the interference signal, and an evaluation module configured to perform a phase measurement between the first and second reference wavelengths based on the detected interference signal and to determine the distance between the reference surface and the target surface based on the phase measurement. The invention particularly utilizes the finding that by sweeping a specific wavelength range sufficiently quickly, the interferometric measurement of absolute distances can be performed very accurately. The evaluation of the resulting interferometry signal is particularly simple and fast if a phase counter measures the phase shift when tuning the laser between two defined wavelengths.The high speed of the individual measurements also allows a statistical evaluation of a large number of measured values ​​and thus ensures a high measurement accuracy, even if the distance between the reference surface and the target surface changes relatively quickly.

[0016] A tunable radiation source known per se, in particular a laser, can be used. For tuning, i.e., for variably adjusting the wavelength of the radiation emission, various modulation parameters can be used individually or in combination. For example, the wavelength can depend on a current applied to the radiation source. Furthermore, the wavelength can depend on the temperature of the radiation source. Furthermore, the wavelength can be adjustable by adjusting a cavity of the radiation source, for example, an internal or external cavity.

[0017] The invention provides in particular that the wavelength of the light emission of a tunable laser source can be adjusted based on an applied current.

[0018] The control module can be configured to control the current for tuning the laser source in such a way that a linear change in frequency occurs. This change in frequency is generally not linearly dependent on the current. In particular, the higher the current, the faster the frequency changes. By appropriate control, a constant rate of change of the frequency of the radiation emission, i.e., a constant df / dt, or a constant rate of change of the wavelength of the radiation emission, i.e., a constant dX / dt, can be achieved.

[0019] For example, an infrared laser can be used. Lasers with other wavelengths can be used in a similar way.

[0020] In particular, the wavelength is modulated in such a way that the first and second reference wavelengths are periodically reached or exceeded.

[0021] In one embodiment, the interferometer module is designed as a Fabry-Perot interferometer, with the reference surface being formed by the end face of a light guide. This advantageously allows for a particularly simple and extremely compact design of the interferometer. Furthermore, the end of the light guide at a measuring head of the system is advantageously used directly as the reference surface, eliminating the need for a separate device, such as a beam splitter for a reference arm of the light emission. Instead, the first part of the radiation emission is reflected at the end of the light guide.

[0022] The second part of the radiation emission, however, is decoupled, for example, from a measuring head, and directed onto the target surface. Additional optical devices may be provided to adjust the direction of the beam or to shape it.

[0023] In a further embodiment, the interferometer module can be designed as a Michelson interferometer, wherein a beam splitter is provided which directs the first part of the radiation emission onto a reference surface formed separately from the light guide, while the second part of the radiation emission is directed onto the target surface.

[0024] Reflections occur of the portions of the radiation emission that strike the reference surface or the target surface. The reflected light is superimposed in a conventional manner, for example, by coupling it into a common light guide and striking a detector, where interference occurs.

[0025] Interferometry requires a measurement cavity, which in the invention can be provided, for example, by a Fabry-Perot interferometer. This results in a reference reflection at the end of the optical fiber, where the radiation emission is extracted and directed toward the target surface. At the interface between the end of the optical fiber and the surrounding medium (such as air, gas, or vacuum), the refractive index changes abruptly, resulting in partial reflection of the incident light. Furthermore, the light reflected from the target surface during the second reflection is coupled into the optical fiber at this interface.

[0026] In the present case, the cavity of the Fabry-Perot interferometer formed between the reference surface and the target surface can be designed with a low finesse (“low finesse”). For example, during the reference reflection, approximately 4% of the incident light is reflected at the reference surface (i.e., at the end of the light guide), while the remaining light is transmitted. The second reflection reflected from the target surface can be provided with a similar intensity in order to achieve a favorable signal-to-noise ratio. If the second reflection occurs at a highly reflective target surface, a spatial filter, for example, can be used to attenuate the intensity of the second reflection. The low finesse of the Fabry-Perot interferometer used allows measurements, in particular, with less highly reflective target surfaces, such as a glass surface.

[0027] When selecting a suitable Fabry-Perot interferometer setup, it is important to consider that for an optimal interference signal, the intensity of the first reflection at the reference surface should be similar to the intensity of the second reflection at the target surface. For a highly reflective target surface, an attenuator can be used to reduce the intensity. In the extreme case of destructive interference, this results in the signal being canceled out, while in the extreme case of positive interference, the signal is doubled.

[0028] The detector module can be configured with a photodiode in a conventional manner. To detect the interference signal, for example, the intensity of the light from the combination of the first and second reflections is measured. Destructive interference results in a lower intensity, while constructive interference results in a higher intensity.

[0029] The course of the intensity measured in this way is evaluated by the evaluation module together with the information on the wavelength of the radiation emission, in particular when the first and / or second reference wavelength is reached.

[0030] In particular, the change in the interferometric phase that occurs when sweeping the range of wavelengths between the first and the second reference wavelength is determined.

[0031] In a further development, the system further comprises a reference module for detecting the wavelength of the radiation emission. The reference module is particularly configured to detect whether the first and second reference wavelengths of the generated radiation emission have been reached based on a further portion of the generated radiation emission.

[0032] In one embodiment, the reference module is configured to detect the reaching of the first and / or second reference wavelength based on an absorption measurement for a reference gas cell.

[0033] In particular, a reference gas cell is used whose absorption spectrum exhibits particularly clearly defined characteristic absorption lines, for example, lines of the rotational-vibrational spectrum. The radiation used for the measurement passes through the reference gas cell, and the transmitted intensity is detected. As the wavelength of the radiation emission changes, the proportion of absorbed radiation also changes, which is detected as a change in the intensity of the transmitted radiation. When the center of an absorption line is reached, the transmitted intensity is minimal, so that even with a poorly defined wavelength setting or its variation, it is recognized that a known wavelength has been reached. For example, a cyanide, acetylene, or carbon monoxide gas cell can be used.

[0034] Furthermore, alternatively or additionally, further methods are conceivable in which, in particular, atomic transitions are used to detect the reaching of a reference wavelength, in particular using a frequency comb.

[0035] In a further embodiment, the control module is configured to control the modulation parameter such that a frequency range of at least 50 GHz, preferably at least 100 GHz, more preferably at least 200 GHz, is covered for the generated radiation emission between the first and the second reference wavelength.

[0036] Expressed as a wavelength, a range of approximately 1 nm can be covered. The wavelength range should be selected to be large enough to achieve the required accuracy of the distance determination. On the other hand, it should not be selected too large, since this would increase the speed at which maxima and minima of the interference occur, thus increasing the required data acquisition speed at a constant scan frequency.

[0037] In a further development, the control module is configured to control the modulation parameter such that, for the generated radiation emission, a frequency range between the first and second reference wavelengths is covered with a scanning frequency of at least 10 Hz, preferably at least 100 Hz, more preferably at least 1 kHz, more preferably at least 10 kHz, more preferably at least 20 kHz, more preferably at least 50 kHz. The measurement is thus advantageously performed quickly enough to compensate for typical drift velocities of the target surface, but not too fast with regard to the technical limitations of the detection and processing of the interference signal.

[0038] The scan frequency refers in particular to the number of periods per unit of time within which the frequency range between the first and the second reference frequency is covered.

[0039] In a further embodiment, it can be provided that a target's travel speed, i.e., a speed of the target surface, is detected, and the scan frequency is determined based on the travel speed. In particular, an initial estimate of the travel speed is determined, for example, by detecting a user input. Optionally, a more precise value of the actual travel speed can be determined in a further step.

[0040] In a further development, the control module is configured to control the modulation parameter such that the change in wavelength includes an additional signal, for example, a sawtooth or sawtooth-shaped modulation in the MHz range. In particular, the evaluation module is configured to detect a direction of a distance change based on the additional signal and the detected interference signal. In particular, the amplitude of the modulation is selected such that the most favorable signal-to-noise ratio of the measured interferometric phase is obtained, independent of the interferometric phase.

[0041] This allows phase measurement or phase counting to be used to determine the direction of movement or the change in distance (drift) between the reference surface and the target surface. For example, a bias current from the radiation source is modulated, such as with a sawtooth or sinusoidal modulation. The frequency of the modulated signal can be in the MHz range, for example, if the scan frequency is in the kHz range—in general, the frequency of the modulated signal can be about three orders of magnitude higher than the sweep frequency or scan frequency.

[0042] In one embodiment, the control module is configured to control the modulation parameter after the distance between the reference surface and the target surface has been determined, so that the radiation emission has the first or second reference wavelength. The evaluation module is configured to detect a change in distance between the reference surface and the target surface based on the interference signal.

[0043] The distance change can be recorded as a function of time, i.e., the rate of change in the distance between the reference surface and the target surface is determined. Such a change can occur due to drift, for example, due to temperature-dependent expansion or contraction of a material and a resulting change in distance. Furthermore, the distance change can be actively generated by moving the target surface relative to the reference surface, for example, due to the movement of a tool.

[0044] In such a configuration, the system is specifically configured to first perform an absolute distance measurement by scanning the frequency range between the first and second reference wavelengths at a scanning frequency. The interference signal is detected, and the distance is determined using a phase measurement.

[0045] For this distance determination, the change in the interferometric phase of the interference signal is measured as it sweeps over a known wavelength interval. The wavelength interval results from the difference between the first and second reference wavelengths, between which the change in the interferometric phase is recorded.

[0046] The frequency range is periodically scanned until the confidence of the determined distance lies within a specified value. With a static distance between the reference surface and the target surface, it is assumed that the mean value is more accurate the more measurements are taken. This means that the distance values ​​determined each time the frequency range is scanned are evaluated using statistical methods, and the number of individual measurements taken into account (i.e., the values ​​determined each time the frequency range is scanned) is chosen to be large enough to achieve a specified statistical accuracy of the measurement.

[0047] The invention takes advantage of the possibility of performing the individual measurements – in particular the scans over a well-defined wavelength range – at a high frequency. Even with a certain amount of drift, a sufficient number of measurements can be performed to obtain a good level of confidence. This means that a large number of individual measurements can be averaged, whereby the maximum usable number of measurements or the necessary scan frequency depends on the drift velocity. Determining the drift velocity is also possible in this way. In contrast, with known methods, where a longer time is required to cover the wavelength range, even a relatively low drift velocity can make an accurate measurement or the repeated repetition of the measurement impossible or at least difficult.

[0048] If the absolute distance is not constant, the accuracy can be affected by the relative movement between the reference surface and the target surface. The distribution of the measured individual values ​​can be evaluated directly or implicitly, for example, using a Kalman filter. This can also determine the velocity and / or acceleration between the reference surface and the target surface. Alternatively or additionally, machine learning or artificial intelligence methods can be used to determine the velocity and / or acceleration of a drift.

[0049] In further examples, the phase can be determined when an absorption line is reached. If this value changes over time, the velocity and acceleration can be determined from this.

[0050] Furthermore, it is possible to use a second interferometer to directly measure the movement and, if necessary, take it into account when determining the absolute distance. Alternatively, or in addition, a time-of-fz / g measurement can be used to directly measure the velocity and / or acceleration.

[0051] In a further step, the system can now be configured to control the radiation source so that the generated light emission constantly has a known wavelength. If a change in the interferometric phase occurs, this indicates a change in the distance between the reference surface and the target surface. With a known wavelength, the change in distance can now be determined.

[0052] By modulating an additional signal as described above, the direction of the distance change can be determined.

[0053] Together with the previously recorded absolute distance, an updated absolute distance can be determined based on the now determined relative distance.

[0054] In other cases, the change in the interferometric phase can be caused by a change in the optical properties of the measuring cavity between the reference surface and the target surface. Such a parameter, such as the optical density of the optical medium being traversed, can also be determined from the interference signal. In a further embodiment, the system comprises a device for splitting the radiation emission into two or three partial beams, by means of which separate interferometer modules are supplied with output radiation. For each of the interferometer modules, a first part of the partial beam is reflected by a reference surface and a second part of the partial beam is reflected by a target surface, so that the first and second reflections generate an interference signal. This interference signal can be recorded and evaluated for each of the interferometer modules.This allows the system to measure two or three distances, for example to measure a two- or three-dimensional position of an object.

[0055] In a further embodiment, the system further comprises a calibration mode. The calibration mode can be activated, for example, in the control module and / or another module. The evaluation module, optionally a separate calibration module or another module, is configured to detect a rate of maxima of the interference signal in the calibration mode. The maxima of the interference signal occur during periodic modulation of the wavelength of the radiation emission.

[0056] The rate of maxima can be determined, for example, based on the number of recorded maxima per time interval or based on the time intervals between recorded, consecutive maxima. In particular, it can be determined whether the maxima are recorded at a constant rate.

[0057] In particular, a constant distance between the reference surface and the target surface is assumed. This is achieved in particular by using a reference cavity with a length that is essentially independent of temperature, for example made of a material such as Zerodur®, stainless steel or Invar®, whereby any drift during the measurement period is minimal or not measurable. The distance during the measurement does not have to be known exactly, but it is sufficient that it is constant or that any drift during the measurement is quantitatively known and compensated for during the evaluation. In particular, in calibration mode, no distance between the reference surface and the target surface is determined; instead, this distance is considered known for the duration of the measurement.

[0058] Furthermore, the drift of a reference cavity can be determined using a second laser interferometer. Furthermore, a drift measurement with a single radiation source is possible, although a lower rate is accepted.

[0059] It can further be provided that the control module or the control of the modulation parameter can be calibrated based on the detected rate such that a constant rate of maxima is obtained when modulating the wavelength. In particular, the wavelength of the radiation emission is modulated at a constant rate of change of the frequency of the radiation emission df / dt or at a constant rate of change of the wavelength of the radiation emission, i.e., at a constant dX / dt. When tuning the radiation source, the maxima are then detected at a constant rate.

[0060] In other words, the rate of the maxima during the measurement or during the periodic tuning of the radiation source by calibration can be used to ensure that the modulation of the wavelength occurs with a constant rate of change of the frequency of the radiation emission, i.e. with a constant df / dt, or with a constant rate of change of the wavelength of the radiation emission, i.e. with a constant dX / dt.

[0061] In a further embodiment of the system, a reference cavity can be connected for operation in calibration mode. The reference cavity is designed in particular to comprise a temperature-independent material and to ensure that no or no significant drift of the measured distance occurs.

[0062] The reference cavity can have a length that is essentially independent of temperature, where “essentially” means that no measurable drift occurs within the measurement period or, for example, a drift of less than 10 nm per second.

[0063] The reference cavity can be connected, for example, by connecting it to a measuring head of the system, whereby the reference cavity defines a length between the reference surface and a reference target surface of the reference cavity.

[0064] For example, the cavity can be connected where, outside of the calibration mode, the radiation emission is coupled out and directed onto the target surface or onto a target.

[0065] Furthermore, the interferometer module can be designed, for example with an optical switch, so that the first and second reflections are obtained at the reference cavity.

[0066] In particular, the system may include an integrated reference cavity, wherein the optical switch can be used to switch between a first configuration in which the light emission is coupled out toward the target surface and a second configuration in which the light emission is coupled out toward the reference cavity. For example, two "arms" of optical fibers may be provided for the first and second configurations.

[0067] In other words, a calibration mode can be provided for the system in which the distance to the target is not measured, but rather a fixed distance from an external, possibly internally installed cavity with a length that is essentially independent of temperature. A material that is stable against temperature changes, such as Zerodur®, stainless steel, or Invar®, can be used for this purpose. In particular, it should be ensured that the radiation source or the laser used is modulated in such a way that a constant frequency change df / dt is obtained during "chirping," i.e., the periodic modulation of the frequency. If the maxima are detected at a constant rate during chirping with the cavity, then the frequency change df / dt is constant.

[0068] An external cavity can be connected to the system where the target would normally be located, or an optical switch can be built into the system to create an additional “arm” and switch the light path to the reference cavity.

[0069] In a further development, the system further comprises a second radiation source for generating a second coherent radiation emission with a further wavelength.

[0070] The additional wavelength can, for example, be in the so-called telecom range, i.e. in an infrared range of the electromagnetic spectrum.

[0071] A reference gas cell can be used, for example, with acetylene, cyanide, and / or carbon monoxide. A gas cell containing two gases can also be used, or two lines of the same cell can be used. Furthermore, two reference gas cells can be provided to ensure the correct wavelength of the second radiation emission.

[0072] An optical switching unit or an optical switch is provided which can be switched between a first switching state in which the first radiation emission is coupled into the interferometer module and a second switching state in which the second radiation emission is coupled into the interferometer module.

[0073] The optical switching unit is preferably designed such that switching can be carried out within a maximum of 5 ms, preferably in a range between 50 ps and 5 ms.

[0074] In particular, an optical switching unit with the fastest possible switching time is used.

[0075] The optical switching unit or the optical switch can, for example, be designed such that the “switched off” radiation emission is suppressed by at least 50 dB, preferably by at least 60 dB.

[0076] In a further embodiment, the system is configured to measure the distance successively: a) generate a first interference signal by means of the first radiation emission and determine a first distance value; b) generate a second interference signal by means of the second radiation emission and determine a second distance value; and c) determine the distance based on the first and second distance values.

[0077] For example, when determining the first distance value (a), a drift velocity, in particular a temporal change in the distance to be measured, can be determined or estimated. The drift velocity can then be taken into account when determining the distance based on the first and second distance values ​​(c).

[0078] The system can be configured to determine the first distance value (a) such that the confidence determined for the first distance value falls below a specified threshold. For example, the maximum tolerable error after the second measurement can be half the wavelength used.

[0079] The confidence level can be adjustable, for example, between 3 sigma and 6 sigma. The system is specifically designed to use the set confidence level to determine how many individual measurements are necessary to achieve the required accuracy.

[0080] The second distance value (b) can then be determined.

[0081] In order to determine the first distance value with the required confidence, the measurement duration is adjusted, or the number of individual measurements and sweeps carried out is adjusted, i.e. the number of periodic tunings of the radiation source.

[0082] In other words, to improve accuracy with a shorter measurement time, a few measurements can first be performed with the first radiation source, for example, 50 to 100 measurements, to obtain a first distance value for the absolute distance with a specific statistical accuracy of, for example, 5 to 10 pm. Subsequently, the system switches to the second radiation source in a different wavelength range, and a phase measurement is performed. This then yields further distance information, particularly relative distance information, which can be used to improve the accuracy of the first distance value, for example, to obtain an accuracy in the nm range.

[0083] In known systems, similar measurements are only performed with a simultaneous measurement at two wavelengths. The sequential measurement described here is not possible in this case because the distance between the reference surface and the target surface or target can change between the first and second measurements, particularly due to temperature-related drift. With the solution described here, a drift velocity can be estimated based on the data from the first measurement, for example, approximately 500 pm / s, in order to estimate and compensate for the drift during switching using an optical switch.

[0084] This procedure allows for a faster and at the same time very accurate measurement, whereas otherwise a very long measurement with the first wavelength is necessary to obtain the high statistical accuracy.

[0085] In the method for interferometrically measuring a distance between a reference surface and a target surface, a tunable radiation source generates a coherent radiation emission with a wavelength dependent on a modulation parameter. The modulation parameter is controlled such that the wavelength of the generated radiation emission is periodically modulated between a first and a second reference wavelength. A first portion of the radiation emission generates a first reflection at the reference surface, and a second portion of the radiation emission generates a second reflection at the target surface, so that the first reflection and the second reflection generate an interference signal.The interference signal is detected and based on the detected interference signal a phase measurement is carried out between the first and the second reference wavelength and based on the phase measurement the distance between the reference surface and the target surface is determined.

[0086] The method is primarily used to operate the system and therefore offers the same advantages as the system. Furthermore, the method can, in principle, be further developed in the same way as the system.

[0087] Further details and advantages of the invention will now be explained in more detail with reference to an embodiment illustrated in the drawings. They show:

[0088] Fig. 1 is a schematic representation of an embodiment of the system;

[0089] Fig. 2 is a diagram showing central values ​​that are set or measured during an exemplary operation of the system;

[0090] Fig. 3 is a flowchart of an embodiment of the method; and

[0091] Fig. 4 is a schematic representation of another embodiment of the system.

[0092] With reference to Fig. 1, a schematic representation of an embodiment of the system is explained.

[0093] In the exemplary embodiment, the system 10 comprises a tunable radiation source 12.

[0094] In the present embodiment, this is designed as a laser source 12.

[0095] The laser source 12 is coupled to a control module 14. The control module 14 outputs a bias current that determines the wavelength of a radiation emission generated by the laser source 12. The bias current therefore serves as a modulation parameter in the present embodiment.

[0096] In further embodiments, other modulation parameters may be used, such as a temperature or a length of a resonator cavity of the laser source 12.

[0097] The radiation emission generated by the laser source 12 is coupled into a fiber optic cable and reaches an isolator 13, which prevents light from returning to the laser source 12. An interferometer module 16 is optically coupled to the isolator 13 via a fiber optic cable. This interferometer module comprises at least one fiber optic cable 17, at the end of which is a measuring head 20. Light is coupled out as a light beam 24 at an end face of the fiber optic cable 17.

[0098] In the exemplary embodiment, a portion of the generated radiation emission is guided to a reference module 30 and passes through a reference gas cell 32. The light transmitted there is detected by a reference detector 34, wherein in particular the intensity of the transmitted radiation is detected.

[0099] In the exemplary embodiment, a cyanide gas cell is used as the reference gas cell 32. In further embodiments, other gaseous materials, for example, acetylene or carbon monoxide, or other methods may be used.

[0100] The reference module 30 or the reference detector 34 is coupled to the evaluation module 28, where the recorded values ​​for the intensity of the transmitted light are processed.

[0101] The sudden change in the refractive index at the interface between the light guide 17 and the surrounding medium (such as gas, air, or vacuum) results in a first reflection of a first portion of the light. This surface is therefore a reference surface 20 in the exemplary embodiment.

[0102] This means that the first part of the light is reflected back in the light guide 17, while a second part is coupled out.

[0103] The measuring head 20 is designed such that the second part of the radiation emission, coupled out as light beam 24, strikes a target surface 22 and is reflected there. This second reflection is returned to the measuring head 20 and coupled into the optical fiber 17. At the detector module 26, interference occurs between the light from the first reflection and the light from the second reflection, and an interference signal is measured.

[0104] The system 10 according to the exemplary embodiment thus comprises a Fabry-Perot interferometer. In further exemplary embodiments, other interferometers may be used, in particular a Michelson interferometer.

[0105] The interferometer module 16 is designed such that the resulting interference signal is detected by the detector module 26.

[0106] The detector module 26 is designed in a manner known per se. It comprises, in particular, a photodiode.

[0107] The intensity of the incident interference signal is detected by the detector module 26.

[0108] The evaluation module 28 coupled to the detector module 26 evaluates the detected interference signal and determines the distance d between the reference surface 20 and the target surface 22 based on a phase measurement when passing through a specific wavelength interval.

[0109] In the exemplary embodiment, the intensity of the light transmitted through the reference gas cell 32, measured by the reference detector 34, is used to determine when the light emission is maximally absorbed by the reference gas cell 32. This is determined based on intensity minima. Since the absorption maxima of the reference gas cell 32 occur at clearly defined wavelengths, the wavelength of the radiation emission at this point can be precisely defined. When two absorption maxima are swept across, it is therefore possible to detect when the radiation emission has the corresponding wavelengths. The evaluation module 28 can therefore determine the phase of the signal when sweeping across a precisely defined wavelength interval.

[0110] In a further embodiment, a reference cavity is connected to the system 10. This can be done by providing the target surface 22 from the reference cavity, and by establishing the distance d from the reference surface 20 by an element made of a substantially temperature-independent material. The distance d is then constant, or any drift can be neglected over the measurement period.

[0111] In a variant of this further embodiment, a reference cavity can be integrated into the system 10. An optical switching element can be provided such that the light emission either reaches the reference surface 20 and is coupled out onto the target surface 22, or it can reach another reference surface and the reference cavity via another "arm" of the optics.

[0112] In the further embodiment, a calibration mode can be provided. When the calibration mode is activated, it is assumed that measurements are taken at a fixed distance d. If necessary, an optical switch can be controlled so that measurements are taken via an integrated reference cavity.

[0113] For calibration measurements, the wavelength of the radiation source 12 is periodically modulated, as already explained above, the so-called “chirping”.

[0114] The rate at which the maximums of the interference signal are detected is now checked, and whether the rate is changing or constant. If the rate is constant, this means that the chirping is performed at a constant rate of change of the frequency of the radiation emission, df / dt, or at a constant rate of change of the wavelength of the radiation emission, dA / dt. If this is not the case, calibration can be performed to ensure that this condition is met.

[0115] With reference to Fig. 2, a diagram with key values ​​that are set or measured during exemplary operation of the system is explained. This is based on the above-described embodiment of the system 10.

[0116] The diagram 40 shows a curve of a modulation parameter 48, in this case a bias current 48, which is applied to the radiation source 12 by means of the control module 14.

[0117] In this example, the course of the bias current 48 with linear sections is shown as a triangular course.

[0118] In further embodiments, the bias current 48 may be adjusted non-linearly so that the resulting change in the wavelength of the radiation emission is linear.

[0119] In the exemplary embodiment, it is further provided to superimpose another signal on the modulation of the bias current 48. This then allows for improved evaluation of the resulting signal, in particular to determine the direction of a change in distance between the reference surface 20 and the target surface 22.

[0120] Furthermore, a curve of the measured interference signal 44 is shown. This is essentially sinusoidal.

[0121] In addition, a curve 46 of the signal acquired by the reference detector 34 is shown. Local minima are visible, which indicate the position of the respective absorption maxima of the reference gas cell 32. In the exemplary embodiment, a fit of the acquired curve 46 is performed such that the position of the absorption maxima is determined with the highest possible accuracy; the fit is carried out, in particular, in real time.

[0122] By means of a phase counter, the phase of the interference signal 44 can be determined between the absorption maxima of the reference gas cell 32, i.e. when sweeping over the wavelength interval defined thereby.

[0123] The absolute distance between the reference surface 20 and the target surface 22 can now be determined using the phase.

[0124] The assignment of the absorption maxima to the corresponding bias current 48 also allows the laser source 12 to be adjusted to a precisely known wavelength.

[0125] When operating with a constant, known wavelength of the radiation emission, the interference signal can still be detected. If a change in phase occurs, this is due to a movement between the reference surface 20 and the target surface 22. Such relative movements can therefore be detected during this operation.

[0126] As mentioned above, an additional signal can be modulated onto the change in bias current 48. The frequency of such a signal, which is configured, for example, with a sawtooth shape, can be approximately three orders of magnitude higher than the frequency of the wavelength variation between the first and second reference wavelengths.

[0127] This additional modulation is particularly advantageous in this fixed wavelength operation in order to be able to determine the direction of a distance change depending on the time derivative of the interference signal.

[0128] With reference to Fig. 3, a flowchart of an embodiment of the

[0129] The method is explained. Here, too, the above-explained embodiment of Fig. 1 and the measurement embodiment of Fig. 2 are used as a starting point.

[0130] In a first step 50, a radiation emission is generated by the radiation source 12. The wavelength of the radiation emission depends on the bias current 48 as a modulation parameter, which is provided by the control module 14.

[0131] In a further step 52, the bias current 48 is periodically controlled so that the wavelength of the radiation emission periodically sweeps over a predetermined wavelength interval. The absorption of a reference gas cell 32 is measured, and based on the defined absorption maxima, it can be determined when the radiation emission has the corresponding reference wavelengths.

[0132] A first portion of the radiation emission produces a first reflection at the reference surface 20 and a second portion of the radiation emission produces a second reflection at the target surface 22. The reflected rays are superimposed at the detector 26, so that an interference signal is generated, which is detected.

[0133] In a further step 54, the acquired data on the absorption of the light emission and the interference signal are evaluated. The phase change of the interference signal between the first and second reference wavelengths is measured. Based on this phase change, the distance d between the reference surface 20 and the target surface 22 is then determined.

[0134] By repeatedly measuring a value for the distance d, a confidence can be determined using statistical methods. In the exemplary embodiment, the measurement is repeated until a certain confidence is reached. In a further step 56, the laser source 12 is controlled by the control module 14 such that the radiation emission has a defined wavelength. For this purpose, the bias current is adjusted such that a defined absorption maximum is achieved at the reference module 30. This means that the wavelength of the radiation emission is known.

[0135] By operating at a constant wavelength, the interference signal is then used to check whether further phase changes occur. The phase change can then be used to determine a relative movement between the reference surface 20 and the target surface 22.

[0136] In the embodiment, it is particularly provided that the operation with periodic scanning over the wavelength range is carried out alternately with the operation with a fixed wavelength in order to first determine the absolute distance d and then to detect a relative distance change.

[0137] With reference to Fig. 4, a schematic representation of another embodiment of the system is explained. This is based on the system described above with reference to Fig. 1, and only differences are explained in detail.

[0138] Instead of a single radiation source 12, a second radiation source 62 is also provided in the further embodiment, in this case a laser source 62. This is coupled into the system via an isolator 63.

[0139] The second radiation source 62 can also be connected to the control module 14 and controlled by it.

[0140] An optical switch 64 or an optical switch 64 is provided, which on the one hand has two inputs connected to the first 12 and second radiation source 62, and on the other hand has an output optically connected to the interferometer module 16.

[0141] By actuating the optical switch 64, either the first 12 or second radiation source 62 can be connected to the output, so that the interferometric measurement is carried out either with the light emission of the first 12 or the second radiation source 62.

[0142] In a method for operating the second embodiment, an interferometric distance measurement is first performed using the light emission of the first radiation source 12. This is done as explained above. The measurement is performed for a long enough time, i.e., with a sufficient number of individual passes during which the wavelength of the first radiation source 12 is modulated between the first and second reference wavelengths, to obtain a certain predetermined confidence for the resulting first distance value.

[0143] During this first measurement, the drift velocity of the target surface 22 relative to the reference surface 20 is also determined or estimated.

[0144] The optical switch 64 is then controlled to switch to the second radiation source 62, and the measurement is continued with this source. In particular, a phase measurement is now performed and a second distance value is determined.

[0145] In order to compensate for the drift between reference surface 20 and target surface 22, the drift during switching between the first 12 and second radiation source 62 is determined and taken into account based on the determined drift velocity.

[0146] The distance d is then determined based on the first and second distance values. List of reference symbols

[0147] 10 systems

[0148] 12 radiation source; laser source

[0149] 13 Insulator

[0150] 14 Control module

[0151] 16 Interferometer module

[0152] 17 light guides

[0153] 20 Reference surface; measuring head

[0154] 22 Target area

[0155] 24 light beam

[0156] 26 Detector module

[0157] 28 Evaluation module

[0158] 30 Reference module

[0159] 32 Reference gas cell

[0160] 34 Reference detector

[0161] 40 Diagram

[0162] 44 Interference signal

[0163] 46 Absorption

[0164] 48 modulation parameters; bias current

[0165] 50, 52, 54, 56 steps

[0166] 62 Second radiation source; Second laser source

[0167] 63 Insulator

[0168] 64 Optical switch; Switch d distance

Claims

Patent claims 1. A system (10) for interferometrically measuring a distance between a reference surface (20) and a target surface (22); comprising a tunable radiation source (12) for generating a coherent radiation emission with a wavelength dependent on a modulation parameter; a control module (14) configured to control the modulation parameter such that the wavelength of the generated radiation emission is periodically modulated between a first and a second reference wavelength; an interferometer module (16) configured to generate a first reflection at the reference surface (20) based on a first portion of the radiation emission and to generate a second reflection at the target surface (22) based on a second portion of the radiation emission, such that an interference signal is generated by the first reflection and the second reflection; a detector module (26) configured to detect the interference signal;and an evaluation module (28) which is configured to carry out a phase measurement between the first and the second reference wavelength on the basis of the detected interference signal and to determine the distance between the reference surface (20) and the target surface (22) on the basis of the phase measurement; 2. System (10) according to claim 1, characterized in that the interferometer module (16) is designed as a Fabry-Perot interferometer, wherein in particular the reference surface (20) is formed by an end face of a optical fiber (17) or is designed as a Michelson interferometer.

3. System (10) according to one of the preceding claims, characterized by a reference module (30) for detecting the wavelength of the radiation emission; wherein the reference module (30) is particularly configured to detect, based on a further portion of the generated radiation emission, the reaching of the first and second reference wavelengths of the generated radiation emission.

4. System (10) according to claim 3, characterized in that the reference module (30) is configured to detect the reaching of the first and / or second reference wavelength based on an absorption measurement for a reference gas cell (32).

5. System (10) according to one of the preceding claims, characterized in that the control module (14) is configured to control the modulation parameter such that a frequency range of at least 50 GHz, preferably at least 100 GHz, more preferably at least 200 GHz, is covered for the generated radiation emission between the first and the second reference wavelength.

6. System (10) according to one of the preceding claims, characterized in that the control module (14) is designed to control the modulation parameter such that a frequency range between the first and the second reference wavelength with a scanning frequency of at least 10 Hz, preferably at least 100 Hz, more preferably at least 1 kHz, more preferably at least 10 kHz, more preferably at least 20 kHz, more preferably at least 50 kHz.

7. System (10) according to one of the preceding claims, characterized in that the control module (14) is configured to control the modulation parameter such that the change in wavelength includes an additional signal; for example, a sawtooth-shaped modulation in the MHz range; wherein, in particular, the evaluation module is configured to detect a direction of a distance change based on the additional signal and the detected interference signal.

8. System (10) according to one of the preceding claims, characterized in that the control module (14) is configured, after the distance between the reference surface (20) and the target surface (22) has been determined, to control the modulation parameter such that the radiation emission has the first or second reference wavelength; wherein the evaluation module (28) is configured to detect a change in distance between the reference surface (20) and the target surface (22) based on the interference signal.

9. System (10) according to one of the preceding claims, characterized in that the system further comprises a calibration mode; wherein the evaluation module (28) is configured to Calibration mode to detect a rate of maxima of the interference signal; where the maxima of the interference signal occur during periodic modulation of the wavelength of the radiation emission; wherein a constant distance between the reference surface (20) and the target surface (22) is preferably assumed; wherein it is preferably detected whether the maxima are detected at a constant rate; wherein preferably the control module (14) is calibrated such that a constant rate of maxima is obtained, wherein in particular the modulation of the wavelength of the radiation emission takes place at a constant rate of change of the frequency of the radiation emission df / dt.

10. System (10) according to claim 9, characterized in that a reference cavity can be connected for operation in calibration mode; wherein the reference cavity preferably has a substantially temperature-independent length; wherein the reference cavity is preferably connected by connecting it to a measuring head of the system (10), the reference cavity defining a length between the reference surface (20) and a reference target surface of the reference cavity; wherein the interferometer module (16) is preferably designed such that the first and second reflections are obtained at the reference cavity.

11. System (10) according to one of the preceding claims, characterized in that the system further comprises a second radiation source (62) for generating a second coherent radiation emission having a further wavelength; wherein an optical switching unit (64) is provided which can be switched between a first switching state, in which the first radiation emission is coupled into the interferometer module (16), and a second switching state, in which the second radiation emission is coupled into the interferometer module (16); wherein the optical switching unit (64) is preferably designed such that switching can be carried out within a maximum of 5 ms, preferably in a range between 50 ps and 5 ms.

12. System (10) according to claim 11, characterized in that the system (10) is configured to measure the distance successively: a) generate a first interference signal by means of the first radiation emission and determine a first distance value; b) generate a second interference signal by means of the second radiation emission and determine a second distance value; and c) determine the distance based on the first and second distance values; wherein preferably when determining the first distance value (a) a drift velocity is determined, and the drift velocity is taken into account when determining the distance based on the first and second distance values ​​(c); wherein preferably the system (10) is configured to determine the first distance value (a) such that a confidence determined for the first distance value falls below a predetermined threshold value.

13. A method for interferometrically measuring a distance between a reference surface (20) and a target surface (22); wherein by means of a tunable radiation source (12), a coherent radiation emission having a wavelength dependent on a modulation parameter is generated; wherein the modulation parameter is controlled such that the wavelength of the generated radiation emission is periodically modulated between a first and a second reference wavelength; based on a first part of the radiation emission, a first reflection is generated at the reference surface (20) and based on a second part of the radiation emission, a second reflection is generated at the target surface (22), so that an interference signal is generated by the first reflection and the second reflection; the interference signal is detected; and based on the detected interference signal, a phase measurement is carried out between the first and the second reference wavelength, and based on the phase measurement, the distance between the reference surface (20) and the target surface (22).

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