Interferometric dual comb distance measuring device and measuring method

Abstract

An interferometric dual comb distance measuring device and a measuring method are disclosed. The invention relates to an optoelectronic dual comb interferometric distance measuring method and device, wherein the signal comb is chromatically divided into a target signal comb and a non-target signal comb at the emission location, preferably by an optical interleaver in the measuring probe of the device. Only the target signal comb is used as a free optical beam for emission towards the target. The non-target signal comb is used for generating an additional or compensating internal phase difference. The distance to the target is thus based on a first target-dependent phase difference and on a second internal compensating phase difference.

Description

TECHNICAL FIELD

[0001] The present application relates to an optoelectronic distance measuring device and a distance measuring method. BACKGROUND

[0002] Various principles and methods are known in the field of electronic and / or optoelectronic distance measurement. One method is to emit pulsed electromagnetic radiation (e.g. laser light) towards a target to be measured (for which the distance is to be determined) and subsequently to receive a return echo from the target as a backscattering object. In order to make the point on the target object available for the measurement identifiable, visible light can be used in this case. The reflected light radiation is converted into an electrical signal in the device by a light-sensitive element. In this case, optical components (e.g. lenses, wavelength filters, mirrors, etc.) for beam shaping, deflection, filtering, etc. are usually located in the optical emission and / or reception path.

[0003] The distance to the target to be measured can be determined, for example, on the basis of the run time of a pulse or emission pattern and / or the phase of a pulse within a modulation period. Today, such laser distance measuring devices have become widespread as standard solutions in many fields (e.g. geodesy, metrology or industrial measurement), for example in the form of total stations, laser scanners, electronic distance meters (EDM), coordinate measuring machines (CMM) or laser trackers. This applies, for example, to the manufacturing industry, for which non-contact determination and checking of workpiece surfaces is of great importance.

[0004] In order to ascertain the run time of a signal, on the one hand, the so-called time-of-flight (TOF) method is known, which ascertains the time between emission and reception of a light pulse, wherein the time measurement is carried out on the basis of the flanks, peaks or another characteristic of the pulse shape. In this case, the pulse form is understood to be the time-sequentially arranged light intensity curve of the received signal (in particular the received light pulse) taken by the light-sensitive element.

[0005] On the other hand, the so-called phase measurement principle is known, which ascertains the signal run time by comparing the phasing of the electromagnetic radiation of the emitted signal with the received signal. A combination of the TOF scheme and the interferometric method is also known in the art.

[0006] Distance measuring instruments with a stabilized frequency comb laser are known in the field of interferometric distance measurement, for example using a pulsing operation from DE 102009012646 A1. These include tunable resonators for high-precision distance measurement (ppm accuracy) to stabilize the pulse rate or pulse frequency (typically at a pulse length of around 100 fs) so that it is adjusted with ppm accuracy and also CEP stabilization (carrier envelope phase). This stabilization achieves consistency of the optical wavelength and phase.

[0007] With regard to the interferometric measurement scheme, the frequency spacing between continuous wave (CW) lasers used in synthetic wavelength interferometry can be stabilized using an optical frequency comb. Dual-comb schemes are also known, which rely on a heterodyne detection by coherent superposition of a pair of slightly detuned frequency combs, often referred to as "signal comb" and "local comb". For distance measurement, each comb is split, for example by a fiber-based 50:50 coupler. A part of each comb is sent directly to a detector ("reference detector"). The resulting baseband signal contains discrete beat notes or beat frequencies. The other part of the signal comb is routed to the target and back to another photodetector ("measurement detector"), where it is detected together with a second part of the local comb, resulting in another baseband signal containing discrete beat notes. The distance to the target is extracted from the phase (difference) of the baseband beat notes, which is produced by the optical path to the target.

[0008] However, in practice, the phase changes not only by or in dependence on the optical path / distance to the target, but also due to other reasons. For example, the optical path length and with it the phase are influenced by thermal dissipation within the measurement device. Given today's high requirements on distance measurement accuracy, even small temperature changes and with them changes in the optical path length are not acceptable. As a countermeasure, the measurement device can be made temperature-stable. However, this is complex and even impossible, for example for devices with a non- monolithic design (e.g. CMM), in which the measurement radiation is routed along a large optical path to a probe head or measurement head.

[0009] DE 102019207192 A1 discloses another countermeasure, proposing to integrate the complete optical measurement assembly into the probe head using PIC technology (PIC, photonic integrated circuit). However, the probe is not passive, but itself dissipates heat, thereby negatively influencing the measurement stability. As a further disadvantage, in contrast to fiber-coupled probes, the focus position or direction of the emitted free beam is fixed. Furthermore, standard assemblies, in particular frequency comb laser modules and assemblies, cannot be reused. SUMMARY

[0010] It is therefore an object of the present application to provide an improved distance measuring device and distance measuring method.

[0011] It is a further object to provide a distance measuring device and distance measuring method which compensates for disturbing influences along the optical measurement path.

[0012] This object is achieved by the features of the independent claims. The features of the present application which are advantageous in the alternative can be inferred from the dependent patent claims.

[0013] The present application relates to a distance measuring method, i.e. an optoelectronic dual comb interferometric distance measuring method for measuring a distance to a target, in particular a distance from an optically reflective surface in a measurement probe to an optically reflective surface of a target object. The method comprises generating a first frequency comb radiation representing a local comb and generating a second frequency comb radiation representing a signal comb. Preferably, the local comb and the signal comb are stabilized and have a free spectral range between 10 GHz and 500 GHz and / or a frequency shift or base offset between 1 GHz and 50 GHz.

[0014] The first beat frequencies representing a reference beat frequency are generated by superimposing the local comb which has travelled along a defined (device internal) optical path and the signal comb. That is, the reference local comb radiation and the reference signal comb radiation are mixed, whereby a first beat frequency as a reference can be determined.

[0015] As is known in principle in the art, these first beat frequencies are used as a reference for determining a respective phase difference or phase shift of a second beat frequency representing a measurement beat frequency, which second beat frequency is obtained by superimposing the local comb which has travelled along a defined optical path and the signal comb which has travelled as a free optical beam to the target and back from the target. Since only the mixed signal comb which is used for the measurement has travelled to the target, the mixed signal comb which is used for the measurement gives a different phase compared to the mixed signal comb which is used for the reference. Thus, a first phase difference is determined, which first phase difference is a result of the distance travelled by the part of the signal comb to the target.

[0016] According to the present application, the signal comb which is used for measuring the target is chromatically split at the point of emission, meaning at or near the point of emission. Then, only a part of this signal comb representing a target signal comb is emitted as said free optical beam and travels to the target. Another part is guided back without being emitted, i.e. it is not guided to the target as a free optical beam.

[0017] This additional signal beam portion (representing a non-target (or internal) signal comb) is used to generate a third beat frequency (representing a compensation beat frequency) by superimposing it with the local comb (which has already travelled along the defined optical path). A second phase difference is then determined between the reference beat frequency and the compensation beat frequency. These second phase differences are independent of the distance to the target, since this portion of the signal comb has not travelled to the target.

[0018] However, both the first phase difference and the second phase difference represent all optical paths from the comb source to the point of emission where the splitting takes place. In reverse, they only differ in the optical path or distance to the target that the measuring signal comb has travelled but the non-target signal comb has not. Thus, by determining the distance to the target on the basis of the first and second values of the phase difference, all phase differences that are not related to the target distance can be compensated or cancelled out. In particular, changes in the optical path length due to temperature changes affect both the non-target signal comb and the target signal comb (correspondingly both the first phase difference and the second phase difference) to the same extent, so that the target distance determination based on the comparison of the first phase difference and the second phase difference remains unchanged.

[0019] For example, the target distance is determined by determining a first distance value from the first phase difference gradient (of the first phase difference) and by determining a second distance value from the second phase difference gradient (of the second phase difference) and subtracting the second distance value from the first distance value.

[0020] Preferably, the signal comb splitting just before emission is a periodic chromatic splitting, for example with a period of 50 GHz. As a further option, the signal comb is split into an odd mode and an even mode by chromatic splitting. Alternatively, the signal comb is split chromatically by bandpass filtering.

[0021] The application also relates to an optoelectronic dual-comb interferometric distance measuring device for measuring a distance to a target, the device comprising a first frequency comb source for providing first or local frequency comb radiation and a second frequency comb source for providing second or signal frequency comb radiation.

[0022] The device further comprises an electronic evaluation unit which is designed to determine the distance to the target on the basis of a first phase difference, in particular in the form of a first phase difference gradient, between a first or reference beat frequency, which is generated by superimposing a first (solely) internally travelled local comb and a (solely) internally travelled signal comb, and a second or measuring beat frequency, which is generated by superimposing a second (solely) internally travelled local comb and a signal comb that has travelled to the target and back from the target by emission / transmission as a free optical beam by a measuring probe, which is preferably a fibre-coupled probe.

[0023] According to the application, the apparatus is designed to determine the distance to the target by means of the electronic evaluation unit on the basis of the first phase difference and on the basis of a second phase difference, in particular in the form of a second phase difference gradient, which is the phase difference of the third or compensation beat frequency and the reference beat frequency.

[0024] The measurement probe thus comprises a first optical frequency comb splitting element for chromatically splitting the signal comb into a non-target signal comb and a target signal comb, whereby only the target signal comb serves as the free light beam. The compensation beat frequency is then generated by superimposing the second internally running local comb and the internally running non-target signal comb. The second internally running local comb can thus be split into two (spectral) parts, just like the image signal comb.

[0025] The first optical splitting element is preferably located in the measurement probe such that the internal and target signal combs have a maximum common internal optical path. For example, the optical splitting element is an optical element which substantially terminates the internal optical path, or is the last optical element before emission.

[0026] As a preferred option, the first frequency comb splitting element is embodied as an optical interleaver, in particular a multi-mirror Fabry-Perot interleaver. These optical elements are particularly suitable for splitting a frequency comb. They allow the comb to be cut into subsequent frequency ranges such that the transmitted (measurement) comb and the reflected (compensation) comb still cover the entire frequency range or wavelength spectrum with only a larger free spectral range. For example, a signal comb with an initial free spectral range of 100 GHz results in an internal comb and a target comb each having a free spectral range of 200 GHz. Alternatively, the first frequency comb splitting element is embodied as a dichroic mirror which separates the signal light beam into a first frequency range and a second frequency range without changing the original free spectral range.

[0027] As a further option, the measurement probe is passive and does not comprise any electronics or electrical components. As a yet further option, the probe is structurally separated from the frequency comb source and is optically coupled to the frequency comb source. This design offers the advantage that the probe is not connected to the housing in such a way that it can be positioned "anywhere". Since the advantage offered by the present application is that all influences on the optical path, such as temperature changes of the path to the probe, are compensated, an unrestricted arrangement of the measurement probe is possible without any need to consider thermal insulation, etc. This is particularly advantageous, for example, if the apparatus is embodied as a coordinate measuring machine in which core or central components such as radiation sources and detectors are very far away from the probe head.

[0028] In a preferred embodiment, there is a common routing of the non-target signal comb and the received target signal comb, whereby the device comprises a second optical separation / splitting element having the same optical properties as the first optical separation / splitting element for separating the superimposed non-target signal comb and the superimposed target signal comb prior to detection.

[0029] Preferably, the first optical separation element and, if present, the second optical separation element are fiber-coupled or the measurement device as a whole is implemented as a fiber-optical measurement device.

[0030] Thus, a dual-comb distance measurement device is provided, wherein a part of the laser comb used for measuring the target is split off by a splitting element, preferably implemented as an interleaver, prior to the emission of the measurement comb to the target. This split-off comb constitutes the basis with the same conditions as the actual measurement comb to and from the probe, respectively to the emission point, and carries these "markings" or path characteristics / effective lengths in its phase (phase shift) as well, whereby the measurement comb has an additional phase shift depending on the target distance. Thus, the additional pure target-related phase shift can be extracted or separated from the overall phase shift of the measurement comb, respectively the phase difference in line with the reference beat note, by using the split-off comb which fully represents the propagation path to eliminate the phase shift or "markings" caused by the propagation path to and from the probe. In other words, the phase shift independent of the target distance is "known" or at least implicitly rather large or obtainable and thus can be eliminated by evaluation of the split-off comb, respectively the mutual evaluation of both measurement results of the non-target signal comb and the target signal comb.

[0031] The present invention also relates to a non-transitory computer program product comprising program code stored on a machine-readable medium, in particular of an optoelectronic dual-comb interferometric distance measurement device according to the present invention, and having computer-executable instructions which, when executed, cause a computer to perform the method according to the present invention. BRIEF DESCRIPTION OF DRAWINGS

[0032] In the following, the method according to the present invention and the device according to the present invention are described in more detail by way of example only, based on the specific exemplary embodiments which are schematically illustrated in the drawings, wherein further advantages of the present invention are described.

[0033] In the specific figures:

[0034] Figure 1 An exemplary embodiment of an interferometric dual-comb distance measurement device according to the present invention is shown;

[0035] Figure 2An exemplary spectrum of the local frequency comb and the signal frequency comb is shown;

[0036] Figure 3 An example of splitting the signal comb into a part for measuring the target and a part for compensation purposes is shown;

[0037] Figure 4 The superposition of the local comb with the internal signal comb and the target signal comb is schematically illustrated;

[0038] Figure 5a , Figure 5b A schematic example of the resulting mixed frequency spectrum is illustrated;

[0039] Figure 6 An example of the evaluation of the phase shift between the reference beat frequency and the compensation beat frequency and the measurement beat frequency is schematically shown; and

[0040] Figure 7 Another example of the evaluation of the phase shift between the reference beat frequency and the compensation beat frequency and the measurement beat frequency is schematically shown. DETAILED DESCRIPTION

[0041] Figure 1 A first exemplary embodiment of an interferometric double-comb distance measuring device 10 and / or distance measuring method for measuring a distance to a target 9 according to the present application is shown. The device 10 comprises two frequency comb lasers with stabilized frequencies: a first one, referred to as local oscillator 1, which generates a first frequency comb radiation L (referred to as local comb), and a second one, referred to as signal oscillator 2, which generates a second frequency comb radiation S (referred to as signal comb). The radiation sources 1, 2 are for example micro resonators which are mode-coupled with slightly different free spectral ranges (mode distances), for example 100.00 GHz and 100.01 GHz, in the range of 100 gigahertz, for example. The radiation sources 1, 2 can have a common radiation generator, for example a common base laser or pump diode, the light of which is split into two parts, or can be implemented as two completely separate units, for example two separate comb generators (for example based on micro resonators). It is preferred to use fiber-based lasers, as they provide high mechanical stability, for example in terms of vibrations or shocks, compared to solid-state free-beam lasers, which is particularly advantageous in the case of mobile distance measuring instruments. In any case, the frequency spectrum of the radiation sources 1, 2 has a fundamental frequency shift of several GHz, for example 20 GHz, for example. Figure 2 An example of the spectrum of the comb L and the comb S is given.

[0042] Using the optical fiber 8 and the splitters CI, C2, each comb can be partially routed to the coupler C3 for superimposition (first) local comb L and signal comb S, both of which travel only the internal optical path. The resulting mixed frequency spectrum S&L gives the first beat frequency (referred to as reference beat frequency R), which is detected by the first optical detector DI. Such a detector like the detector DI is for example a PIN diode (photodiode) based on the InGaAs semiconductor material for a wavelength of about 1.5 pm. The detector DI preferably works in an energy-sensitive manner so that the electrons released by the ultra-short laser pulses (which can be provided by the frequency comb-based device in particular) are not lost, but contribute to the electrical detector signal.

[0043] The reference beat frequency R serves as an interferometric reference for determining the distance to the target 9 using the phase shift for the measurement signal and the time length depending on the distance to the target 9, which is known in principle in the art and is further described below. Figure 5a An example of the reference beat frequency R (respectively, the mixed frequency spectrum) is given.

[0044] The radiation S of the signal source 2 is routed by the optical fiber 8 through another optical path to the measurement probe 3, which serves for emitting radiation to the target 9 to be measured and for receiving radiation from the target 9 to be measured. Due to the different lengths of the optical paths, including the distance to the target 9, the received radiation is phase-shifted compared to the internally traveling signal, so that the distance to the target 9 can be determined based on the phase shift or phase difference.

[0045] The exemplary measurement probe 3 is as described fiber-coupled and completely passive. There can be a fiber connector (not shown) between the probe 3 and the circulator 7 or the opto-electrical interface to exchange probes with different beam parameters (beam direction, operating range, etc.). This provides the advantage that the probe 3 can be structurally separated from the laser sources 1, 2 or from the main body or "heart" of the measurement device 10, allowing the probe 3 to be placed essentially freely and unrestrictedly depending on the specific measurement environment, which is particularly required for example in metrology devices such as coordinate measuring machines (CMM). Furthermore, since the probe 3 is passive, no power supply for the probe 3 is required and no electrical discharge heat is generated, which would otherwise cause temperature changes that would adversely affect the measurement stability / accuracy or require additional means for temperature stabilization, which greatly increases the complexity of the design and usually cannot completely eliminate all temperature influences.

[0046] As mentioned, the difference between the reference light path and the measurement light path (which depends on the distance to the target 9) leads to a phase difference which is used to determine the distance. However, the length of the measurement light path not only changes with the distance to the target 9, but is also susceptible to changes in the environment. In particular, temperature changes affect the effective light path length and can thus lead to errors in the determined distance. As mentioned, it is complex and generally not possible to eliminate such environmental influences, in particular if a flexible or unrestricted mounting of the probe 3 is intended or a compact design with nearby temperature-increasing electrical units is to be achieved.

[0047] The present application thus provides a method and a device for compensating such length changes of the measurement path. The idea is to provide a signal SB which is based on these length changes by splitting the signal comb S before emission, but not on the distance to the target 9. This allows the comb S to separate into a comb signal SB which is independent of the target distance and a comb signal SA which expresses the target distance.

[0048] In this example, the device 10 comprises an optical interleaver as a comb splitting element 5 which is located at the very end of the measurement probe 5, "behind" the lens 4 of the probe 3 in the emission or exit direction. The optical interleaver 5 is, for example, of the Fabry-Perot element type with a plurality of dielectric layers (forming a repetitive bandpass structure which periodically transmits and reflects), allowing a periodic spectral separation of the signal comb S as a periodic signal. The fiber-coupled interleaver 5 of this example has an internally miniaturized free-beam optics.

[0049] The interleaver 5 located at the emission point of the free-beam for measuring the target 9 has the effect that a recurrent part SA of the signal beam is transmitted as a measurement free-beam FB, propagates to the target 9 and is reflected from the target, while another part SB of the signal beam does not leave the probe 3 but is reflected. Figure 4 The periodic separation of the signal comb S is further illustrated.

[0050] Preferably, the separation of the signal beam is at the most distant point of the probe 3, such that the common light path of the non-target signal comb SA and the target signal comb SB is maximized, for which the separated light path of the measurement signal beam SB is essentially only the free-beam channel from the exit point to the target 9 and back. The non-target signal comb SA and the target signal comb SB thus differ in the distance or propagation to the target 9 of the target comb SB; all other influences on the signal comb are common and are accordingly represented in both combs SA, SB.

[0051] Thus, the internal or non-target comb SB after reflection at interleaver 5 and the target comb SA after reflection at target 9 and re-entry through probe 3 travel via fiber 8 and circulator 7 to coupler C4, where they overlap with the (second) local comb L, for which the local comb L is split into spectral parts LA, LB accordingly. This mixed radiation is routed to a second interleaver 6, which works in the same way as the first interleaver 5 and enables separate routing of SA&LA radiation to a second detector D2 and SB&LB radiation to a third detector D3. Figure 5a and Figure 5b The periodic separation of the mixed combs by the second interleaver 6 is further illustrated.

[0052] Thus, a second or measurement beat frequency M of the target signal comb SA and the local comb L is detected by detector D2 and a third or signal beat frequency C of the non-target signal comb SB and the local comb L is detected by detector D3. Referring to Figure 5b Examples of the second measurement beat frequency M and the third measurement beat frequency C are given. As an alternative to two separate detectors D2, D3, a single or common detector can be used to detect both signals SA&LA and SB&LB, e.g. by a detection array.

[0053] As already mentioned above, due to the different optical paths, the beat frequencies M, C of the measurement arms detected by detectors D2, D3 are phase shifted with respect to the reference beat frequency R of the reference arm detected by detector Dl. Due to the fact that not only one measurement optical path exists, but also due to the fact that the probe interleaver 5 splits the two different measurement optical paths, there are different phase shifts of the non-target signal comb SB and the target signal comb SA. This phase shift difference represents the optical path difference (i.e. the distance to the target 9) of the internal comb SB and the measurement signal comb SA, for which the target distance can be determined from the phase shift difference of the internal probe signal comb SB and the external probe signal comb SA. Referring to Figure 6 Distance determination based on the phase shift difference of the target signal SA and the internal signal SB is further illustrated.

[0054] Figure 2 Exemplary spectra of the local frequency comb L and the signal frequency comb S are shown. Both spectra show a plurality of lines or modes L0, L1, L2,... and S0, S1, S2,... respectively. In addition to the basic offset of the wavelengths, e.g. S0 is 20 GHz higher than L0, the frequency spacing wl of the local comb L is different (slightly) from the frequency spacing w2 of the signal comb S. Thus, the wavelength shift δw is multiplied with the line number / mode number.

[0055] The beat note or beat frequency is received by superposition of the local comb L and the signal comb S (see Figure 5a). Using a stabilized radiation source, these beat frequencies provide a stable and well-detectable frequency spectrum centered around a center frequency v0 and usually in the MHz range.

[0056] Figure 3 An example is shown of splitting the signal comb S into a part SA for measuring the target and a part SB for compensation purposes. In the upper part of the figure, as in Figure 3 Figure 2 The entire signal comb S is depicted as it arrives at the measurement probe, showing all modes with the pattern S0, S1, S2,....

[0057] The signal comb S enters the interleaver via an input port. The interleaver has a periodic reflection behavior IT-R, as schematically depicted in the second graph of the figure. As a result, only the even modes S0, S2,.... pass through the interleaver and form the target signal comb SA, while the odd modes S1, S3,.... are reflected and form the non-target signal comb SB. Only the target signal comb SA is emitted towards the target, while the non-target signal comb SB is routed back inside the device to be detected independent of any relation to the target distance.

[0058] As an alternative to the depicted alternative splitting of the signal beam S, a bandpass filtering can be used, such that e.g. the lower order modes are split off and sent as target signal SA towards the target, while the higher order modes form the internal / non-target signal SB.

[0059] In any case, the signal comb S (and respectively the two parts SA, SB) share a common optical path up to the splitting point in or at the measurement probe (and respectively the "reunion" point seen in the free beam reception direction) and thus constitute the basis of the same influences or conditions. Therefore, the non-target part SB can be used as a reference or standard for compensating or eliminating these influences, which allows to clearly distinguish or separate them from the influences to be determined for the target distance.

[0060] As mentioned, the reference signal comb SB as well as the measurement signal comb SA received from the target by the measurement probe are routed by optical fibers to be mixed with the local comb for generating beat notes.

[0061] Figure 4 The superposition of the local comb L with the internal signal comb SB and the target signal comb SA is schematically illustrated. Since the target signal comb SA only comprises even modes S0, S2,...., these modes are mixed with the even modes L0, L2,.... of the local comb. The non-target signal comb SB has odd modes S1, S3,.... for which these modes are mixed with the odd modes L1, L3,.... of the local comb.

[0062] ​The upper part of the figure shows the mixed signals of the corresponding modes: the center mode L0 of the local comb and the target signal comb SA0, the first mode L1 of the local comb and the reference signal comb SB1, the second mode L2 of the local comb and the target signal comb SA2, the third mode L3 of the local comb and the reference signal comb SB3, ...

[0063] To separate these modes / beat frequencies for independent detection, a second interleaver with the same optical properties as the measurement probe is used. This second interleaver has, for example, the optical properties of... Figure 4 The rectangle in the upper figure illustrates the repeating reflection behavior of IT-R (see also...). Figure 3 (The second graphic at the top).

[0064] The interleaver outputs signals related to the target signal comb (even mode) L0, SA0, ... through its first output port. Figure 1 SA&LA in the middle), such as Figure 4 The diagram in the middle illustrates this. Through the second output, signals related to the non-target signal combs (odd mode) L1, SB1, ... are provided. Figure 1 (SB&LB in the original text). However, because the interleaver has a fixed frequency-stabilized filter spectrum, the spectra of the local comb and the signal comb are slightly detuned to each other (see SB&LB in the original text). Figure 2 Higher mode orders can no longer be clearly separated above a certain number. However, a few hundred mixing frequencies are still available, which is sufficient for distance determination.

[0065] Figure 5b A schematic example is given of the resulting mixing spectrum BA of the separated target signal and the mixing spectrum BB of the compensation signal. For comparison, Figure 5a The image depicts the mixed spectrum BS of the complete signal comb and the local comb, showing all beat frequencies ν (even beat frequencies and odd beat frequencies). Figure 5a The diagram can be viewed as a schematic example of the beat frequency detected by detector D1, which detects mixed reference radiation.

[0066] In contrast. Figure 5b The spectrum BA related to the target signal shown in the lower part only includes even beat frequencies ν-2, ν0, ν2, ... Figure 5b The spectrum BA shown at the top, which relates to the non-target signal comb, includes only odd beat frequencies ν-1, ν1, ν3, ...

[0067] Figure 6 The phase shift between the reference beat frequency and the corresponding beat frequency of the signal arm is schematically illustrated. This will be demonstrated by detectors D1 to D3 (see...). Figure 1The detected signal undergoes a Fourier transform and is separated into single-frequency signals. The phase is determined for each frequency signal νi of the reference signal or the comb index 0, 1, 2, 3, 4, ..., and the corresponding phase is determined for the measured signal.

[0068] Then, the difference between the reference signal detected by detector D1 and the target signal detected by detector D2 (and corresponding D3) is determined, which is indicated by the value (point) of the corresponding comb index in the plotted phase difference diagram: PA (even comb index) indicates the phase difference of the target signal comb, while PB (odd comb index) indicates the phase difference of the non-target signal comb.

[0069] Based on the two distinct signal paths of the internal signal comb and the target signal comb, two sets of phase differences are obtained, each representing the signal travel time and its distance traveled. Each set can be described, for example, by a corresponding linear fit or gradient (the first phase difference gradient GA for the target signal and the second phase difference gradient GB for the compensation signal). Each gradient is proportional to a specific time delay value and thus indicates the distance traveled.

[0070] Therefore, the first distance value L is determined based on the first gradient GA. A And determine the second distance value L based on the second gradient GB. B The distance to the target is ultimately the difference between these two values: L = L A -L B The difference between the two derived values ​​depends only on the target distance and is independent of any change in the optical path length within the device (correspondingly, a change in the optical path from the laser source to the beam exit), because such a change affects the two phase difference gradients (correspondingly the first distance value L) in the same way. A Second distance value L B ).

[0071] Figure 7The evaluation of the phase shift between the reference beat frequencies and the respective beat frequencies of the signal arms for the above mentioned alternative of signal separation by band pass filtering is schematically shown. This band pass filtering does not provide a periodic or alternating mode separation but a separation into two modes or frequency intervals. As schematically shown in the figure, this results in a plurality of phase differences PA of the lower modes of the target signal comb (up to comb index 5 in this example) and a second number of phase differences PB of the higher modes of the non-target signal comb (starting with comb index 6 in this example). Thus, a first phase difference gradient GA of the lower modes and a second phase difference gradient GB of the higher modes can be determined. However, the alternative approach using a periodic filtering, e.g. an interleaver, is superior to the depicted band pass filtering approach, as the periodic filtering allows to use or average both the target path and the inner path over the full range of available modes (full range of even modes and full range of odd modes) (see Figure 6 ). By the repetitive or periodic splitting, there is no cut in the mode or phase difference but only a thinning. In addition, the laser pulse form, although in a similar way for all combs, is not changed by the periodic filtering but by the band pass filtering.

[0072] It is clear that these illustration figures only schematically illustrate possible exemplary embodiments. The various methods can also be combined with each other and with prior art devices or methods, if not mentioned otherwise.

Claims

1. An optoelectronic dual comb interferometric distance measuring method for measuring a distance to a target, the optoelectronic dual comb interferometric distance measuring method comprising the steps of: generating a first frequency comb radiation representing a local comb; and generating a second frequency comb radiation representing a signal comb; generating a first beat frequency representing a reference beat frequency by superimposing the local comb and the signal comb that have travelled along a defined optical path; generating a second beat frequency representing a measurement beat frequency by superimposing the local comb that has travelled along a defined optical path and the signal comb that has travelled as a free beam from a launch position to the target and back from the target; determining a first phase difference between the reference beat frequency and the measurement beat frequency; characterized in that the signal comb is chromatically split into a target signal comb and a non-target signal comb at the launch position, only the target signal comb being used as the free beam launched towards the target; generating a third beat frequency representing a compensation beat frequency by superimposing the local comb that has travelled along a defined optical path and the non-target signal comb; determining a second phase difference between the reference beat frequency and the compensation beat frequency; and determining the distance to the target based on the first phase difference and the second phase difference.

2. The optoelectronic dual comb interferometric distance measuring method according to claim 1, characterized in that the first phase difference has a first phase difference gradient form.

3. The optoelectronic dual comb interferometric distance measuring method according to claim 1, characterized in that the second phase difference has a second phase difference gradient form.

4. The optoelectronic dual comb interferometric distance measuring method according to claim 1, characterized in that the distance is determined by determining a first distance value from the first phase difference gradient and a second distance value from the second phase difference gradient and subtracting the second distance value from the first distance value.

5. The optoelectronic dual comb interferometric distance measuring method according to claim 1 or 2, characterized in that the signal comb is chromatically split periodically.

6. The optoelectronic dual comb interferometric distance measuring method according to claim 3, characterized in that the signal comb is split into an odd mode and an even mode by the chromatic splitting.

7. The optoelectronic dual comb interferometric distance measuring method according to claim 1 or 2, characterized in that the signal comb is chromatically split by bandpass filtering.

8. The optoelectronic dual comb interferometric distance measuring method according to claim 1, characterized in that the local comb and the signal comb are frequency stabilized and have: a free spectral range between 10 GHz and 500 GHz; and / or a frequency shift between 1 GHz and 50 GHz.

9. An optoelectronic dual comb interferometric distance measuring apparatus for measuring a distance to a target, the optoelectronic dual comb interferometric distance measuring apparatus comprising: a first frequency comb source for providing a first frequency comb radiation representing a local comb; and a second frequency comb source for providing a second frequency comb radiation representing a signal comb; and an electronic evaluation unit designed to determine the distance to the target on the basis of a first phase difference, which is the phase difference between: a first beat frequency representing a reference beat frequency, which is generated by superimposing a first internally running local comb and an internally running signal comb; and a second beat frequency representing a measurement beat frequency, which is generated by superimposing a second internally running local comb and a signal comb running to the target and back from the target as a free optical beam by means of a measurement probe, characterized in that the device is configured to: determine the distance to the target on the basis of the first phase difference and on the basis of a second phase difference, which is the phase difference between a third beat frequency representing a compensation beat frequency and the reference beat frequency, and for this purpose, the measurement probe comprises a first frequency comb separation element for chromatically dividing the signal comb before emission into a non-target signal comb and a target signal comb, only the target signal comb being used as the free optical beam, and the compensation beat frequency is generated by superimposing a second internally running local comb and the internally running non-target signal comb.

10. The optoelectronic double-comb interferometric distance measuring device according to claim 9, characterized in that the first phase difference has a first phase difference gradient form.

11. The optoelectronic double-comb interferometric distance measuring device according to claim 9, characterized in that the second phase difference has a second phase difference gradient form.

12. The optoelectronic double-comb interferometric distance measuring device according to claim 9, characterized in that a first optical separation element is located in the measurement probe, such that the non-target signal comb and the target signal comb have a maximum common internal optical path.

13. The optoelectronic double-comb interferometric distance measuring device according to claim 12, characterized in that the first optical separation element is an optical element that terminates the internal optical path.

14. The optoelectronic double-comb interferometric distance measuring device according to any one of claims 9 to 13, characterized in that the first frequency comb separation element is embodied as an optical interleaver.

15. The optoelectronic double-comb interferometric distance measuring device according to any one of claims 9 to 13, characterized in that the first frequency comb separation element is embodied as a multi-mirror Fabry-Perot interleaver.

16. The optoelectronic double-comb interferometric distance measuring device according to any one of claims 9 to 13, characterized in that the first frequency comb separation element is embodied as a chromatic frequency comb beam splitter.

17. The optoelectronic double-comb interferometric distance measuring device according to any one of claims 9 to 13, characterized in that the measurement probe is: passive; and / or structurally separate from the frequency comb source and optically coupled to the frequency comb source.

18. The optoelectronic double-comb interferometric distance measuring device according to any one of claims 12 to 13, characterized in that a second optical separation element having the same optical properties as the first optical separation element for separating the superimposed non-target signal comb and the superimposed target signal comb prior to detection.

19. The optoelectronic dual comb interferometry distance measuring device according to claim 18, characterized in that the first optical separation element and, if applicable, the second optical separation element are fiber-coupled.

20. The optoelectronic dual comb interferometry distance measuring device according to any one of claims 9 to 13, characterized in that the optoelectronic dual comb interferometry distance measuring device is embodied as a coordinate measuring machine.

21. A non-transitory computer program product comprising program code stored on a machine-readable medium having the optoelectronic dual comb interferometry distance measuring device according to claim 9 and having computer-executable instructions which, when executed, cause a computer to perform the optoelectronic dual comb interferometry distance measuring method according to claim 1.

Images