Distance measurement

Abstract

A laser ranging method comprises separating each of two combs (11, 12) of double-comb radiation into a signal radiation portion (21, 22) and a local oscillator radiation portion (23, 24). After the separation, the signal radiation portions are combined into a combined signal radiation and are guided, in particular by generating a partially reflective reference structure, onto an object (10) of which the distance is to be measured. A signal reflected back from an object is separated into a first signal portion and a second signal portion. The first signal portion (25) is superimposed on the second local oscillator radiation portion (24) to generate a first measurement signal, and the second signal portion (26) is superimposed on the first local oscillator radiation portion (23) to generate a second measurement signal. The distance to the object (10) is determined from the first and second measurement signals and from a reference signal obtained by partial reflection, wherein the vernier effect can be used to extend the ambiguity by comparing the first and second measurement signals.

Description

Technical Field

[0001] The invention belongs to the field of distance measurement based on laser. Background Art

[0002] Among laser-based ranging (Light Detection and Ranging LiDAR) devices, those based on dual optical frequency comb technology (“dual-comb ranging”) offer inherently high accuracy and sampling rates, and do not require any moving parts to scan the distance.

[0003] Dual-comb ranging uses a combination of two optical frequency combs with slightly different repetition rates. One beam (corresponding to one of the combs) is directed towards a target and a reference, both of which at least partially reflect the beam back, and the returned radiation is locally combined, for example in a beam splitter, with another beam (corresponding to the other of the combs; acting as a local oscillator (LO). The resulting interference signal is measured by a photodiode. One of its characteristics is that, instead of high-frequency photodetection, detection of half the repetition rate of the resolving frequency comb is sufficient. However, the measurement suffers from an ambiguity range associated with the pulse repetition rate. This range is typically in the order of centimeters to meters, which is too low for many applications.

[0004] I. Coddington et al., Nature Photonics, 3, 351-356 (2009), have proposed a significant extension of the ambiguity range by switching the roles of the signal and local oscillator (LO) laser beams and by using the Vernier effect: Due to the difference in repetition rate, for targets farther away than the intrinsic ambiguity range, the distance measurement using the first comb as the signal laser beam differs from the distance measurement using the second comb as the signal laser beam. This difference is an integer multiple of the difference in the ambiguity ranges of the two measurement configurations. Therefore, by combining the two measurements, this integer can be determined and thus the value of the absolute distance can be determined.

[0005] However, a disadvantage of this approach is that two measurements of the target must be made in close succession in order to exploit the Vernier effect. The measurements will therefore be relatively slow and require an additional switching mechanism to swap the roles of the signal beam and the local oscillator. This is not robust to drift between consecutive measurements and also makes the system unsuitable for distance measurements of moving targets.

[0006] In DE 102012001754, it is also proposed to use an optical filter that separates one of the frequency combs into two parts so that two measurements can be made simultaneously, with the roles of the two combs interchanged. However, this technique has a significant disadvantage in that the two measurements with the interchanged combs are asymmetric. This can lead to systematic errors and increase the complexity of post-processing of the data. In addition, spectral filtering is complex and expensive, and sacrifices resolution by limiting the bandwidth.

[0007] Ya Liu et al. ("Fast distance measurement with a long ambiguity range using a free-running dual-comb fiber laser"; Frontiers in Optics / Laser Science (2020)) proposed to perform two measurements according to the method mentioned by I. Coddington et al. by switching the roles of the signal and the local oscillator laser beam, followed by another measurement, in which the two measurements are combined into a simultaneous measurement. This method can be used for moving targets, but it also has the other disadvantages mentioned by the method proposed by I. Coddington et al.

[0008] J. Nürnberg et al. (“Absolute laser ranging with sub-µm resolution from a free-running dual-comb Yb:CaF2 laser”, 2021 conference on lasers and electro-optics(CLEO), OSA(2021)) discuss absolute laser ranging using two frequency combs, where a LIDAR setup facilitates two simultaneous measurements of the distance between a reference R and a target T by exchanging the roles of the two frequency combs in the signal and local oscillator arms. The two combs have different polarizations and are combined in a polarization beam splitter. The combined comb radiation is split between a signal branch and a local oscillator branch, the signal branch being directed onto the target (and partially reflected by a wedge-shaped window to produce a reference). The signal reflected from the object (together with the partially reflected reference signal) is superimposed with the local oscillator radiation, and the resulting superimposed signal is separated by a Wollaston prism into two orthogonally polarized measurement signals. A disadvantage of this approach is that one has to rely on polarization to distinguish the two local oscillators belonging to the two frequency combs. Any polarization inaccuracy, e.g. due to unwanted polarization rotations in the local oscillator arms or non-ideal Wollaston prisms / polarization beam splitters, will result in the local oscillator part being located at the wrong photodetector. Any polarization rotation along the signal arm (e.g. caused by a non-cooperative target) will result in spurious ghost pulses in the recorded signal, which will distort the measurement.

[0009] Xu Xinjang et al. (“Underwater dynamic distance measurement using across-sampling dual-comb”, Optics Communications 517, 128319 (2022)) disclose a method for performing underwater dynamic measurement based on cross-sampling of dual combs. A first beam splitter is used to combine two frequency combs into a common signal branch and a common local oscillator branch. The radiation of the signal branch is guided to the target via a circulator, and the reflected signal reaches the second beam splitter through the circulator, where the signal is superimposed with the radiation of the common local oscillator branch to produce an interference signal. This method does not allow for clear separation of the two channels belonging to the two frequency combs. All interference radiation is analyzed by a single common photodiode. This may produce unwanted interference between the two channels. Summary of the invention

[0010] An object of the present invention is to provide a laser distance measurement method and a laser distance measurement device that overcome the disadvantages of the prior art. A particular object of the present invention is to provide a method and a device that are also suitable for measuring longer distances greater than 50 m, greater than 100 m or even greater than 1 km without ambiguity and without the need for calibration measurements before each measurement.

[0011] These and other objects are achieved by the method and the device defined in the claims.

[0012] The laser ranging method according to one aspect of the present invention comprises the following steps:

[0013] Generating a dual-comb radiation comprising a first radiation portion of a first radiation pulse train having a first pulse repetition frequency and a second radiation portion of a second radiation pulse train having a second pulse repetition frequency that is slightly different from the first pulse repetition frequency.

[0014] • Using a first radiation splitter to split the first radiation portion into a first signal radiation portion and a first local oscillator radiation portion.

[0015] • Using a second radiation splitter to split the second radiation portion into a second signal radiation portion and a second local oscillator radiation portion.

[0016] • Combining the first signal radiation portion (having the first pulse repetition frequency) and the second signal radiation portion (having the second pulse repetition frequency) into a combined signal radiation.

[0017] Directing the combined signal radiation to an object whose distance is to be measured. The directing step may include directing the combined signal radiation through a reference object to a target object.

[0018] · Receive signals reflected from objects irradiated by the combined signal.

[0019] • Splitting the signal into a first signal portion and a second signal portion.

[0020] The first signal portion is superimposed by a first radiation splitter with a second local oscillator radiation portion to generate a first measurement signal.

[0021] The second signal portion is superimposed by a second radiation splitter on the first local oscillator radiation portion to generate a second measurement signal.

[0022] Determining the distance to the object from the first measurement signal and the second measurement signal.

[0023] In this case, the superposition of the corresponding signal portion with the local oscillator portion will cause interference to the components having the same polarization, so that the first and the second measurement signal are interference signals.

[0024] The first and second radiation splitters are separate in that the splitting of the first radiation portion and the splitting of the second radiation portion occur at different locations. This allows directing the respective signal radiation portion to the location where the signal radiation portions are combined (usually to the radiation combiner) and directing the respective local oscillator radiation portion to the respective other radiation splitter.

[0025] After the step of separating the first and second radiating portions, a step of combining the first and second signal radiating portions into a combined signal radiation is performed, ie this step is performed for the first / second signal radiating portions separated from the respective local oscillator portions.

[0026] In other words, combining the first and second signal radiation parts into the combined signal radiation is performed in particular using the first and second signal radiation parts separated by the first / second radiation splitter (ie radiation from one of the output ports of the first and second radiation splitter).

[0027] The corresponding other output ports of the first / second radiation separator are particularly arranged and configured to communicate with each other so that the radiation output from one of the output ports of the first radiation separator is coupled to the output port of the second radiation separator as a first local oscillator part (to be superimposed with the second signal part, wherein the output port serves as an input port of the signal part and the corresponding other local oscillator part), and vice versa.

[0028] The communication between the output ports of the radiation splitters can be achieved by optical fibers (or other waveguides, depending on the implementation) connecting the output ports, or, in particular in a free space configuration, by an appropriate arrangement of components. Such an arrangement may include at least one radiation deflector, for example a reflector, directing radiation from the local oscillator radiation portion output port of one of the radiation splitters to the corresponding port of the other radiation splitter. However, it is also possible to align the two radiation splitters so that such radiation from the local oscillator output port of the first radiation splitter propagates directly (through free space or another substrate, for example glass) to the local oscillator output port of the second radiation splitter, and vice versa, so that no radiation deflector is required.

[0029] By this communication between the output ports it is ensured that the superposition of the first signal portion with the second local oscillator radiation portion and vice versa is done by the correct local oscillator portion only (i.e., e.g., no part of the first local oscillator radiation portion interferes with the first signal portion), independent of any polarization rotation along the local oscillator arms and / or imperfect Wollaston prisms / polarization beam splitters. It is noteworthy that this design feature is independent of the material platform, i.e., applicable to fiber, free space and waveguide settings. This is a great advantage compared to the approaches of, e.g., J. Nürnberg et al. or Xu Xiang et al. in the aforementioned publications.

[0030] The first and second radiation splitters may consist of different dedicated beam splitting devices, for example, fiber beam splitters, in particular when the device is implemented by fiber technology. Alternatively, the first and second radiation splitters may consist of spatially separated, distinguishable parts of a single beam splitting device, for example, when the device is implemented in a free space setup. The way in which the first and second radiation splitters are configured is not a necessary key feature of the method of the present invention, but it may be important that the first and second radiation splitters are separated from each other because the first and second local oscillator parts are different and because the first local oscillator part propagates from the first radiation splitter to the second radiation splitter, while the second local oscillator part propagates from the second radiation splitter to the first radiation splitter. Therefore, the first and second local oscillator parts may have different, opposite propagation directions. It is this concept that makes it possible to distinguish the first and second local oscillator parts without polarization, and therefore makes the measurement more robust compared to the prior art (for example, the technology disclosed by J. Nürnberg et al.).

[0031] The generation of the first / second measurement signal may include a photoelectric conversion, for example, by means of a first / second photodiode.

[0032] The superposition of the first signal portion (which will include at least a portion of the radiation of the first comb) with the portion of the radiation of the second local oscillator (of the second comb) will be an interference signal with a pulse structure, with oscillations within the pulse envelope, and can be used for distance measurement, subject to the well-known ambiguity. The same applies to the superposition of the second signal portion (which will include at least a portion of the radiation of the second comb) with the portion of the radiation of the first local oscillator (of the first comb). The determination of the distance can then include the use of a Vernier effect to extend the ambiguity range by comparing the first and second measurement results, essentially as taught by I. Coddington et al. in Nature Photonics, vol. 3, pp. 351-356 (2009). In contrast to the latter, the method according to the invention includes separating the two combs, allowing the generation of the first and second measurement results simultaneously, so that the method can also be used, for example, to determine the distance and / or speed of moving objects. Furthermore, it works from scratch and does not require any special calibration measurements as is done, for example, by the method of Ya Liu et al. (“Fastdistance measurement with a long ambiguity range using a free-running dual-comb fiber laser”; Frontiers in Optics / Laser Science (2020)).

[0033] Dual-comb radiation can be polarized.

[0034] The combining step may be performed, for example, by a polarization combiner in such a way that a share of the combined signal radiation generated by the first signal radiation portion has a first polarization, and a share of the combined signal radiation generated by the second signal radiation portion has a second polarization different from the first polarization. The step of separating the signals may include separating the signal portion with the first polarization and the signal portion with the second polarization to generate a first signal portion (deriving from the signal portion with the first polarization) and a second signal portion (deriving from the signal portion with the second polarization).

[0035] This has the advantage that a first signal portion having only characteristics of the first frequency comb (i.e. forming the first frequency comb in the frequency domain, i.e. a pulse train corresponding to the first pulse repetition frequency) and being superimposed with a second local oscillator radiation portion having characteristics of the second frequency comb for generating the first measurement signal, and vice versa for a second signal portion having characteristics of the second frequency comb.

[0036] Thus, in these embodiments, only the signal parts intended to be combined for heterodyne signal processing are superimposed, making the process particularly efficient. Essentially all the returned radiation will contribute to the "wanted" measurement signal. Furthermore, because the combined signal radiation parts have orthogonal polarizations, there will not be any "unwanted" interference in the combiner. Such "unwanted" interference would lead to interference signals being detected by the detector and therefore must be filtered out.

[0037] Polarization combiners that generate a combined beam with orthogonal polarization from two polarized beams and that separate the combined beam with orthogonal polarization into two differently polarized beams when operated in opposite directions are known and commercially available.

[0038] As an alternative to combining using a polarization combiner, a polarization maintaining fiber coupler or a single mode fiber coupler or corresponding equivalents for free space or waveguide settings can also be used. In these alternative embodiments, the first signal portion will include contributions from both combs. However, after superposition with the second local oscillator portion, the contribution of the second comb can be cleaned up because the contribution of the second comb will not contribute to the frequency comb generated by heterodyne detection mixed with a local oscillator portion with a slightly offset frequency in a much lower frequency domain (much lower compared to the radiation frequency).

[0039] The dual comb radiation may be fiber guided, i.e. at least after the dual comb radiation is generated it may be coupled into an optical fiber, the first and second radiation portions being guided in different optical fibers. Alternatively, a realization by free space components or in a substrate or by a waveguide, e.g. on a chip or other substrate is also possible.

[0040] By means of a free-space component implementation, it is meant that the laser ranging device has a radiation guiding device, which includes a reflector or other reflective surface or other radiation deflector, which directs the radiation to the components described herein via free space or within a substrate (e.g., glass), which components may be discrete components. Thus, it is possible to construct a free-space embodiment of the ranging device such that the radiation propagates within the substrate (1 glass). The radiation can in this case be reflected and guided, for example, via total internal reflection at a glass-air interface.

[0041] In some embodiments, in particular in embodiments implemented by free space components, the device may include a periscope that is arranged in the beam path of one of the signal radiation parts. Such a periscope, in addition to directing the beam to the desired position (e.g., lifting it to the plane where the radiation combiner is arranged), may also have the function of a half-wave plate, i.e., the polarization may be rotated by 90°. Thus, the periscope may ensure that the polarizations between the first and second signal radiation parts (and the shares of the combined signal radiation generated by the first / second signal radiation parts, respectively) are orthogonal for the subsequent separation of the signal reflected back from the target. However, compared to the use of a half-wave plate, the periscope avoids the disadvantage of causing false reflections.

[0042] In principle, the periscope can also be located on the connection path between the local oscillator output port of the first radiation splitter and the local oscillator output port of the second radiation splitter.

[0043] In this case, the input frequency combs need to have mutually orthogonal polarizations, but the operating principle is the same as in the proposed setup.

[0044] Periscopes, in addition to avoiding spurious reflections, have the advantage of allowing more broadband operation than half-wave plates.

[0045] In some embodiments, the laser distance measuring device includes an output coupler - of a type known for lasers with a free space component - arranged in the beam path of the combined signal radiation. The output coupler is configured to extract a reference pulse train from the combined signal radiation, i.e., the coupled-out portion (e.g., substantially less than 50% of the intensity) is used as a reference, while the reflected portion is directed to the target as signal radiation. In some embodiments, the reference is split again by a beam splitter into two different branches, which may have slightly different beam path lengths to account for so-called "dead time" (time when the distance cannot be measured due to the temporal overlap of the target and reference reflections of the signal pulses).

[0046] As an alternative to an output coupler, a wedge-shaped window can be used to couple out a portion of the combined signal radiation to be used as a reference. The wedge-shaped window has two surfaces, each causing a partial reflection, so that two branches are directly generated as references, with the possibility to make the beam path length between the two branches slightly different to handle "down time".

[0047] For determining the distance, in addition to the signal reflected back by the object, a reference can also be taken into account. Such a reference can be generated by reflections from structures (or "reference objects") along the beam path. In particular, partial reflections of the combined signal radiation, for example from the fiber tip and / or from the facets of a dedicated reference object (e.g. a transparent window), can be used to generate a reference pulse train. This partial reflection will generate a reference signal, which undergoes the steps of separation into a first and a second signal part and superposition with the signal reflected back from the object. Since it is possible to distinguish the reference signal from the signal (which, according to the definition used in this article, is reflected back by the object) based on characteristics such as amplitude (which is well known for the reference signal), in post-processing this can be used to determine the distance as the distance between the reflection of the reference signal and the reflection of the structure.

[0048] In addition to or as an alternative to using reflections from structures with well-defined positions (e.g. the mentioned fiber tip or the face of a transparent window; other elements along the beam path), other ways of obtaining a reference may also be used. For example, a reference may be obtained from sum frequency mixing or other non-interference means. If the first and second signal radiation portions do not have orthogonal polarizations, there is another way of obtaining a reference, as is the case in embodiments where the combination is performed using a device other than a polarization combiner. In these embodiments, the combination will generate an interference signal. This interference signal may be used as a reference instead of or in addition to the reference obtained by (partial) reflection at a structure with a well-defined position.

[0049] In a particular embodiment, two partial reflections can be used to generate two reference signals to handle a hypothetical situation in which the distance between the reflection of one of the reference signals and the reflection of the object (or, to be precise, twice the distance) corresponds to an integer multiple of the cavity length of the dual-comb radiation source, so that the reference pulse coincides with the signal pulse, whereby the corresponding other reference signal can be used. As an alternative thereto, even if temporarily overlapping in the measurement, the signal and reference pulses can be distinguished in post-processing, for example, by fitting a model.

[0050] A way to obtain additional partial reflections (besides the partial reflection from the fiber tip) is to place a transparent, e.g. wedge-shaped window in the measurement path. If the thickness of such a window is known, it can be further used to derive the repetition rate and / or calibrate the electronic clock of the device.

[0051] In many embodiments, the dual comb radiation will be in the near infrared or visible range, with a wavelength in the range between 0.3µm and 2µm, for example. The repetition rate of the dual comb can be selected, in particular by selecting an appropriate cavity length according to specific needs. For example, it can be between 50MHz and 10GHz, in particular between 250MHz and 2GHz. In general, increasing the repetition rate - by reducing the cavity length of the dual comb radiation source - allows a larger repetition rate difference to be achieved without aliasing. This, in turn, supports enhanced sensitivity and measurement update rate at the expense of reduced extended ambiguity range.

[0052] The difference in pulse repetition rate between the two combs can be matched to the desired measurement distance to the object, depending on the application. rep The difference can be set, for example, so that the required measurement distance can be resolved using the Vernier technique. The maximum distance without blur is c / (2*Δf rep ).

[0053] For many applications, the ideal situation is c / (2*Δf rep ) value exceeds the desired distance, but not much more for best performance, e.g., no more than 2 or 3 times.

[0054] Therefore, in some embodiments, the following inequality may hold:

[0055] c / (2*Δf rep ) / 3 <d< c / (2*Δf rep );

[0056] where d is the distance to be measured (which is approximately known in many applications).

[0057] Δf rep Another possible design criterion is that if Δf rep It may be advantageous to have Δf be small enough to avoid aliasing. To achieve this, the down-converted frequency comb (usually in the RF domain) generated by the beating of the first and second radiating parts should not be wider than half the repetition frequency. This condition can be expressed as follows: Δf rep <f rep 2 / (2Δv), where Δv is the optical bandwidth of the first and second frequency combs.

[0058] In some specific embodiments, additional dual-comb radiation may be used, the additional dual-comb radiation comprising a third radiation portion of a third radiation pulse train having a third pulse repetition frequency and a fourth radiation portion of a fourth radiation pulse train having a fourth pulse repetition frequency different from the third pulse repetition frequency. The third and fourth repetition frequencies may (but are not necessarily) the same as the first and second repetition frequencies.

[0059] In this case, the third and fourth radiation portions may be in a wavelength region that is different from the wavelength region of the first and second radiation portions. For example, the second dual comb may be at double the frequency compared to the first dual comb, and the second dual comb may be generated from the first dual comb using nonlinear optical effects. Alternatively, the second dual comb may be generated by a separate source. In these embodiments, the additional dual comb radiation may be used to perform the same measurement as the dual comb radiation with the first and second radiation portions. The purpose of this is to exploit the frequency dependence of the refractive index to correct for refractive index variations along the measurement path.

[0060] In addition to a method, the present invention also relates to a laser distance measuring device. The device is particularly suitable for carrying out the method. Features of the method described or claimed in this document are also optional features of the device, and vice versa.

[0061] Therefore, the laser distance measuring device for performing the method comprises:

[0062] Dual-comb radiation source;

[0063] A first radiation separator and a second radiation separator;

[0064] ·Radiation combiner;

[0065] Radiative in-coupling and out-coupling structures; and

[0066] • A first and a second detector.

[0067] In this case, one of the output ports of the dual-comb radiation source is connected to the input port of the first radiation splitter, for example via an optical isolator to prevent feedback. Another output port of the dual-comb radiation source is also connected to the input port of the second radiation splitter, for example via an optical isolator. One of the output ports of each radiation splitter is connected to the input port of the radiation combiner, while the other output ports of the radiation splitters are connected to each other. The remaining input port of each beam splitter (corresponding to the port not connected to the input of the dual-comb radiation source) is connected to the first and second detectors, respectively.

[0068] If two ports are described as being "connected", this means in this context that the ports communicate so that radiation coupled out of one port reaches the other port. One possible connection could be a connection via an optical fiber or an on-chip waveguide, but ports via free-space coupling are also considered "connected" in this context.

[0069] A dual-comb radiation source generates two frequency combs with slightly offset frequencies, ie two radiation parts, each radiation part comprising a radiation pulse train, the two radiation pulse trains having slightly different pulse repetition frequencies.

[0070] By connecting to an output port of the radiation source, the first radiation separator separates the first radiation part into a first signal radiation part and a first local oscillator radiation part; and similarly, by connecting to another output port of the radiation source, the second radiation separator separates the second radiation part into a second signal radiation part and a second local oscillator radiation part.

[0071] The radiation combiner is arranged to combine the first and second signal radiation portions into a combined signal radiation by being connected to the output port of each radiation splitter (the output port of the signal radiation portion). The signal radiation is coupled out of the device through the in-coupling and out-coupling structures. The in-coupling and out-coupling structures also receive the signal reflected back from the object and direct it to the output port of the combiner.

[0072] Through the above-mentioned connection between the output ports of the radiation splitters (the output ports for the local oscillator radiation portions), the first radiation splitter superimposes the first signal portion with the second local oscillator radiation portion to generate a first measurement signal, and the second radiation splitter superimposes the second signal portion with the first local oscillator radiation portion to generate a second measurement signal. The first / second measurement signal enters the corresponding input port of the radiation splitter. Although the portion coupled into the input port connected to the frequency dual-comb radiation source can be prevented from returning to the radiation source by an optional optical isolator, the portion reaching the corresponding other input port reaches the first / second detector, so that the first detector is set to receive the first measurement signal by being connected to the remaining input port of the first beam splitter, while the second detector receives the second measurement signal.

[0073] The radiation splitter may be, for example, a polarization-maintaining fiber beam splitter, in particular a 50:50 beam splitter or a beam splitter with different beam splitting ratios.

[0074] The radiation combiner may be a polarization combiner.

[0075] The detector may be a photodiode.

[0076] The device may also include a data acquisition system for processing the data. The data acquisition system is a post-processing unit that is equipped to analyze the first and second measurement signals. The post-processing unit may include at least one analog-to-digital converter and software for determining the distance from the digitized signal in the manner described herein.

[0077] The device may optionally include further units, such as further detectors, such as photodiodes, and may also include a frequency counter to determine the pulse repetition rate of the radiation source. This can be used to scale the distance by the cavity length of the radiation source (which can be determined from the pulse repetition rate). BRIEF DESCRIPTION OF THE DRAWINGS

[0078] In the following, embodiments of the present invention are described with reference to the accompanying drawings. In the accompanying drawings, the same reference numerals represent the same or similar elements. The accompanying drawings are schematic and not drawn to scale. The accompanying drawings show:

[0079] Figure 1 A laser range-finding device is shown;

[0080] Figure 2 Shows Figure 1 A variation of a laser rangefinder;

[0081] Figure 3 An embodiment of a laser distance measuring device is shown, which is equipped to measure distances simultaneously in two different wavelength ranges;

[0082] Figure 4 An embodiment of a laser distance measuring device having a radiation source emitting in two different wavelength ranges is shown;

[0083] Figure 5 shows details of another laser distance measuring device, which is Figure 1 Another variation of the laser ranging device;

[0084] Figure 6 Another embodiment of a laser range finding device is shown, implemented using free space components, the radiation source is not shown; and

[0085] Figure 7 Shows Figure 6 A variation of a part of the arrangement enables an alternative way of generating the reference pulse. DETAILED DESCRIPTION

[0086] Figure 1 The device shown comprises a dual-comb (laser) radiation source 1 emitting two radiation portions 11, 12, each radiation portion being a train of laser pulses with a slight shift in the pulse repetition frequency between the two trains. Such dual-comb radiation sources are known in the literature, for example WO 2016 / 049787 or European patent application 21190032.9. Dual-comb laser sources comprising two lasers synchronized in a suitable manner are also known and can be used for the present invention.

[0087] It is assumed that the two radiation portions 11, 12 are polarized when emitted by the dual-comb laser source 1; in principle, a polarization element can also be added outside the laser source if the emitted dual-comb radiation is not sufficiently polarized. Furthermore, the polarization of the two radiation portions 11, 12 can be the same. In some embodiments, the polarizations can also be different, for example, orthogonal to each other.

[0088] Radiation portions 11, 12 emitted by the dual-comb laser source 1 are coupled into a polarization-maintaining fiber 5. In the described embodiment, the polarization is oriented by a λ / 2 wave plate 3 through which the radiation is guided before being coupled into the fiber 5; the polarization in the fiber is along the slow axis of the fiber. An optical isolator (ISO) 4 prevents feedback from the fiber back to the dual-comb laser source 1.

[0089] Each radiation portion is separated into a signal radiation portion (S) 21, 22 and a local oscillator radiation portion (LO) 23, 24 by a 50:50 polarization-maintaining fiber beam splitter (FBS) 6, 7. The signal radiation portions (S) 21, 22 are then combined together by a combiner (COM) 8, which is a polarization combiner implemented in polarization-maintaining fiber. The first signal radiation portion 21 (produced by separating the first radiation portion 11 together with the first local oscillator radiation portion 23) and the second signal radiation portion 22 (produced by separating the second radiation portion 12 together with the second local oscillator radiation portion 24) have orthogonal polarizations after combination. The combined signal radiation consisting of the first and second signal radiation portions 21, 22 is used to sample the distance d between the end face R of the fiber tip 9 and the object (target T) 10.

[0090] Figure 1 Also shown are collimating optics 19 between the fiber tip 9 and the object 10. If present together with the collimating optics 19, the fiber tip 9 acts as a radiation in-coupling and out-coupling structure for the emitted and received radiation.

[0091] In this case, the end face R of the optical fiber tip 9 is used as a reference plane because a portion of the combined signal radiation is reflected by it (see enlarged panel FC / PC; "FC / PC" indicates that a state of the art optical fiber connector (FC) can optionally be used as the optical fiber tip 9).

[0092] In the figure, the collimating optics 19 are illustrated as a pair of lenses after the fiber tip 9. In principle, other arrangements can do the same job, including a single lens or a combination of three lenses, the first lens collimating the beam directly after the fiber tip 9, and then a pair of lenses expanding the beam to the desired size at a later stage. This optional expansion of the beam reduces diffraction if the beam travels a long distance. This is beneficial if the distance between the fiber end face R and the object 10 is long.

[0093] The combined signal radiation consisting of the first and second radiation portions 21, 22 (with the portion being reflected to produce the derived final signal) is directed onto an object 10, which is illustrated here as a reflector acting as a target (T). The first and second signal radiation portions 21, 22 arrive at the object in orthogonal polarization states. The object struck by the combined signal radiation reflects back a signal which is received via the fiber tip 9 and coupled into the optical fiber. The signal is again separated according to its polarization state by a (polarization) combiner 8, which also again aligns the polarization with the slow axis of the optical fiber, so as to produce a first signal portion 25 (comprising a signal portion having a first polarization) and a second signal portion 26 (comprising a signal portion having a second polarization).

[0094] like Figure 1 As shown, since the grids of the local oscillator radiation portions of the fiber beam splitters 6, 7 are connected to each other, these first and second signal portions 25, 26 are superimposed (combined by interferometry) with the local oscillator radiation portions 24; 23 of the corresponding other frequency combs. The resulting interference signals (first and second measurement signals 27, 28) are then fed to a first photodiode 41 and a second photodiode 42, respectively, for photoelectric conversion and then for signal evaluation for deviation distance using techniques known per se from optical heterodyne detection.

[0095] The evaluation also includes a step of comparing the distance determination results of the two photodiodes to eliminate blur using the Vernier effect, as described by I. Coddington et al., Nature Photonics, vol. 3, pp. 351-356 (2009).

[0096] For the reflected combined signal portion (in the embodiment shown, reflected by the fiber tip 9), a distance derivation after superposition with the corresponding local oscillator radiation portion is also performed, which is used as a reference signal. Since the intensity of the reference signal is well known and different from the intensity of the signal, the signal and the reference signal can be easily distinguished.

[0097] In a first alternative embodiment (second embodiment), the combiner 8 is not a polarization combiner, but a polarization-maintaining fiber coupler. In this alternative embodiment, the two combs (i.e., the two signal radiation portions 21, 22) will be emitted toward the target with the same polarization. This alternative is achieved by Figure 1 The inset in the bottom dashed box indicates.

[0098] In this second embodiment, the light reflected from the target can therefore no longer be separated, since the signal radiation portions 21, 22 share the same polarization. Therefore, both signal combs propagate towards the two photodetectors 41, 42. That is, the signal is separated into two signal portions 25, 26, which are substantially identical. However, the comb with the same repetition rate as the local oscillator (LO) will not be sampled by the local oscillator (because the pulses do not move relative to each other), while the comb with a different repetition rate will be scanned by the local oscillator, which is the same as in the first embodiment. Therefore, after superposition of the first signal portion 25 and the second local oscillator radiation portion 24 in the first polarization-maintaining fiber beam splitter 6, only the signal portion originating from the first radiation portion will be added to the sampling signal at a low (heterodyne) frequency. Similarly, after superposition of the second signal portion 26 with the first local oscillator radiation portion 23 in the second polarization-maintaining fiber beam splitter 7, only the signal portion originating from the second radiation portion will be added to the sampling signal.

[0099] Thus, even without polarization selectivity of the signal separation, there will be similar results compared to the first embodiment, except that additional parasitic signals may appear due to the interferometric combination of the two combs at the combiner 8, which may lead to reduced performance at certain distances. However, for certain distances and setups, this less complex embodiment may be sufficient.

[0100] Figure 2 A further alternative embodiment (third embodiment) is shown. Figure 1 Compared to the embodiment of the present invention, the combiner 8 is replaced by a single-mode fiber coupler. For this configuration, the remaining components do not need to maintain polarization. The working principle of this arrangement is the same as the second embodiment with a polarization-maintaining fiber coupler.

[0101] In the third embodiment, due to the lack of polarization-maintaining fiber, the polarization of the light may be significantly rotated while propagating through the setup. For example, if the polarization state of the LO path is exactly rotated 90 degrees relative to the signal path, this may be a serious problem for the measurement, as there will be no interference. To prevent this problem when using non-polarization-maintaining fiber, additional light pre-processing steps can be included in the setup. An example of this is Figure 2 A birefringent crystal is added between the optical isolator 4 and the optical fiber 5 in the optical fiber. If such a crystal is inserted in the path of one of the two beams, with its polarization axis at 45 degrees relative to the output polarization of the light after the isolator, this beam will be separated into two orthogonally polarized and delayed pulse components. Therefore, for each polarization state in the subsequent optical fiber arrangement, there will be a finite projection on one or the other of these pulse components, enabling interference superposition. Alternatively, sufficient performance can be achieved by placing the optical fiber arrangement on a temperature-controlled substrate.

[0102] For example, J. Owens in Bulletin géodésique 89, 277-291 (1968) has proposed to carry out the same measurement with light of different wavelengths in order to correct for variations in the refractive index along the measurement path. This can also be used in the context of the present invention.

[0103] According to a first possibility of doing this, the first wavelength is given by the wavelength of the laser. To generate light in the second wavelength region, for example, second harmonic generation (SHG) of the laser output can be used, which doubles the frequency of the incident light. The idea in this embodiment is:

[0104] First, light of a second wavelength is generated (eg, using SHG). Figure 1 Embodiment Figure 3 A SHG device 71 is shown at each output port of the radiation source 1 .

[0105] In order to measure distances simultaneously in two wavelength regions, the two wavelengths of light can be coupled into a measurement system suitable for both wavelength regions, such as Figure 3 shown.

[0106] In this embodiment, before detection, it is still necessary to separate the light of the two wavelength regions. To this end, for example, the first and second measurement signals 27, 28 can be separated according to their different wavelength regions, and the obtained separated measurement signals will be detected by separate photodetectors respectively, so that a total of two pairs of photodetectors 41, 42; 43, 44 are used. Figure 3 Wavelength combiners 72 , 73 for separating the measurement signals 27 , 28 are shown.

[0107] In contrast, based on Figure 1 Concept Figure 4 The embodiment relies on using a separate measurement system for each wavelength region. To achieve compensation of the refractive index, Figure 4 The embodiment also uses two different wavelengths λ1 and λ2. Depending on the source used, λ1 and λ2 can be selected independently of each other. For each wavelength, a reference Figure 1 The arrangement described above (the combiner 8 is a polarization combiner POL-COM) is shown in Figure 4 The combined signal radiation of the two wavelengths λ1 and λ2 is combined in a wavelength combiner (WAV-COM) 51 so that the wavelength combined signal radiation is transmitted to the target 10. The wavelength combiner can be, for example, a dichroic mirror (in free space), a wavelength division multiplexer (WDM in optical fiber) or a wavelength combiner implemented with a waveguide (on a chip).

[0108] The wavelength combined signal radiation contains four frequency combs, namely, a dual comb at a first wavelength λ1 and a dual comb at a second wavelength λ2. The reflected signal is first wavelength-separated into two branches in the wavelength combiner 51, and each branch undergoes a reference Figure 1 Describe the steps.

[0109] This signal is then detected on four separate photodiodes (PD1 and PD2 for the first wavelength λ1, and PD3 and PD4 for the second wavelength λ2).

[0110] Figure 5 Also depicted is a method for handling the case where the signal pulse reflected by the object (target 10) and the reference pulse arrive at the photodetector simultaneously, i.e. if the (optical) distance to be measured is an integer multiple of the (optical) laser cavity length (the latter sets the pulse-to-pulse distance). Figure 1 An embodiment of Figure 5 A polarization combiner POL-COM 8 is shown as well as a fiber tip 9 and an object 10 .

[0111] according to Figure 5 , an additional reference pulse is generated by placing a wedge-shaped window 61 in the measurement path. Thus, one reference pulse will originate from the reflection at the tip of the FC / PC fiber ( Figure 5 The reference pulse R1 in the wedge window W 61 ( Figure 5 The reflectivity of the wedge-shaped window 61 may be slightly different from the reflectivity of the FC / PC fiber tip (generating the first reference pulse) so that the two reference pulses are distinguishable. Furthermore, the wedge-shaped window 61 will be placed so that the distance between the reflections R1 and R2 will be different from an integer multiple of the laser cavity length (e.g., the distance may be much smaller than the latter).

[0112] Due to the presence of two reference pulses, it can be ensured that the signal pulse can always be referenced to at least one of the two reference pulses, i.e. at least d1 or d2 can always be determined (see Figure 5 ). Since the distance between the two reference planes (i.e. d1-d1) is known from the measurement with blocked signal paths, we can always find the distance between the first reference plane R1 and the target T. For most target distances, in the absence of overlap between the target pulse and the “primary” reference pulse, the presence of the second signal increases the achievable signal-to-noise ratio. Therefore, both pulse components are useful for the measurement.

[0113] When working with light in two different wavelength regions (e.g., ref. Figure 4 As described above, the wedge window causes slightly different deflections of beams of different wavelengths. A suitable location for the wedge window is directly after the collimating optics 19 (e.g. Figure 5), where the angular offset introduced by the different refractive indices of light of different wavelengths is small compared to the divergence angle. This ensures that after the collimating optics, the beams of different wavelengths propagate more collinearly (as small angular offsets are reduced by the beam size magnification factor).

[0114] As an alternative to a wedged window, a non-wedge window can also be used to create the reference pulse. This becomes particularly useful when working with light in two different wavelength regions, as this allows keeping the beams of different wavelengths more parallel. If a non-wedge window is used, both faces will produce reflections, so that a third reference pulse is generated and can be used. Alternatively, one of the reflections can be ignored. In both cases, an anti-reflection (AR) coating can be used on one side of the non-wedge window to suppress the reflection to be ignored or to adjust the intensity of the reference pulse. In particular, if both reflections are used, the AR coating can reduce the reflectivity from one surface and thus reduce the pulse energy of one of the reference pulses, making it possible to distinguish between the two reference pulses in a post-processing stage.

[0115] The use of a wedge-shaped or non-wedge-shaped transparent window in the measurement path to generate one or two reference pulses is also an option in embodiments where no reference pulses originate from reflections at the fiber tip 9 or where such reflections are not utilized.

[0116] As an alternative to using a transparent window to generate further reflections, it is in principle also possible to distinguish signal and reference pulses in post-processing, even if they temporarily overlap in the measurement. If the signal and reference pulses overlap, the measurement result will be the coherent sum of two corresponding interferograms, one corresponding to the reference pulse and one to the signal pulse. The two signals can then be distinguished by fitting a model of the coherent sum of the two interferograms to the measured signal. With this model, separate interferograms corresponding to the signal pulse and the reference pulse, respectively, can be reconstructed.

[0117] The non-wedged window of the type described above for generating the two reference pulses can also be used as a distance reference. This would require that the non-wedged window be made of a sufficiently stable material, for example ultra-low thermal expansion glass (ULE manufactured by Corning). By using a non-wedged window made of such a material, and measuring its thickness very accurately, two reference planes with a known spacing ∆d can be obtained. These surfaces will generate two reference pulses with a known and well-defined optical delay ∆τ. By measuring this delay with the device according to the invention, it is possible to (a) very accurately calibrate the repetition rate f of the laser rep , or (b) correct for uncertainties in the electronic clock. These calibrations can be performed as follows:

[0118] (a) How to calibrate the laser repetition rate f repThe idea is that the repetition rate difference ∆f between the two frequency combs rep This information, together with the separation between the two reference planes and the delay between the corresponding reference pulses, is sufficient to find f by using the following equation: rep :

[0119]

[0120] : Group velocity of light

[0121] : The delay between two reference pulses measured with the device according to the invention

[0122] : The distance between two reference planes

[0123] : Laser repetition rate

[0124] : The repetition rate difference between the two frequency combs

[0125] (b) Dual-comb data provide ∆f according to the clock used on the data acquisition device rep By using a separate photodiode to measure the laser repetition rate and a data acquisition system that shares the same clock to quantize the signal, a very precise ∆f can be obtained. rep / f rep ratio, independent of the uncertainty of the electronic clock. A LiDAR measurement using a device according to the invention will produce a delay between two pulses in the electronic domain: ∆T = ∆τ * f rep / ∆f rep Since ∆τ is precisely known from previous measurements (e.g., by a separate ultra-high precision laboratory source), and f rep / ∆f rep The ratio of ∆T is also known very accurately (independent of the uncertainty of the electronic clock), so the measured ∆T can be used to correct for the uncertainty of the electronic clock.

[0126] In addition to or as an alternative to using a non-wedge-shaped window in the measurement path, a reference cavity may be used in the measurement path to generate a plurality of reference pulses.

[0127] In the above embodiments, it is assumed that the object whose distance to the reference reflection is to be measured comprises a plane mirror as a target. This is not necessary. Instead, diffuse reflections (e.g., directly from the surface of the object) may also be used. As a further alternative, retroreflectors, e.g., corner retroreflectors, may be used. Using a retroreflector is more practical than a plane mirror because the system is less sensitive to the alignment of the target.

[0128] In the above-described embodiments, it is assumed that the device is implemented by fiber optic components. However, it is also possible to use another platform at least in part. For example, free-space components can be used, which means that the fiber optic beam splitter and / or combiner are replaced by corresponding free-space components and the radiation propagates in free space between the beam splitter and combiner and / or in other parts of the device.

[0129] As a further alternative, the device may also be at least partially implemented by waveguides, for example by using integrated photonics on a chip.

[0130] Figure 6 An embodiment using free space components is shown (the dual comb radiation source generating the two frequency combs 50 is not in Figure 6 ). The fact that the device is implemented using free-space components means that the fiber optic beam splitters and combiners are replaced by corresponding free-space components and the radiation propagates in free space between these components, using beam deflectors 71 (e.g. mirrors) to guide the radiation.

[0131] exist Figure 6 In the example shown, the polarization of the corresponding radiation portion is shown by a small double arrow. In the example shown, the radiation is either linearly polarized (in the horizontal / vertical plane, respectively) or a superposition of two orthogonal linear polarizations (crossed double arrows). Other polarization states can also be used, for example, circular polarization.

[0132] In addition to using free space components to achieve Figure 6 The embodiments have the following special features, which can be implemented in combination (such as Figure 6 ), or in alternative embodiments, may be implemented separately from each other.

[0133] The first and second beam splitters 6, 7 are different spaced apart surface parts of a single beam splitting device 60, which in the embodiment shown is plate-shaped, but may also have a different appearance. This arrangement is advantageous by ensuring that the first and second beam splitters have similar characteristics and by reducing the complexity and the number of devices used. In order to guide the local oscillator parts to the respective other beam splitter, the arrangement has a plurality of reflectors 74 instead of the optical fibers of the previous embodiments.

[0134] The arrangement comprises a periscope 72 which has the effect of a half-wave plate (rotates the polarization by 90°), but avoids spurious reflections. A particular advantage of the invention is that such a periscope 72, which may be advantageous, can be used to rotate the polarization.

[0135] As mentioned above, the periscope 72 may also be located on the connection path between the local oscillator output port of the first radiation separator and the local oscillator output port of the second radiation separator, instead of Figure 6 In this case, with Figure 6 In contrast to the case shown, the input frequency combs 50 will have polarizations that are orthogonal to each other, but otherwise the setup operates in the same manner as Figure 6 The settings are the same.

[0136] The polarization combiner 8 is composed of a Wollaston prism used as a polarization beam splitter (for signal).

[0137] The output coupler 75 is used to separate the combined signal radiation that hits the target from the reference pulse train.

[0138] In the embodiment shown, the reference pulse train is split into two branches by another beam splitter 76 , and the two branches are reflected back by different end mirrors 78 .

[0139] Typically, in embodiments implemented by free-space components, for example, Figure 6 The free-space components in (but not limited to) couple the light into the polarization combiner ( Figure 6 It can be beneficial to route the signal into a single mode fiber after the RF input (#8) in the RF section to allow for mode cleanup and ensure that the two signal portions overlap well when directed to the target.

[0140] In this case, if any light is reflected during the step of coupling the signal portion into the optical fiber, it may be beneficial to block this back-reflected light to prevent ghost pulses in the measurement signal. This can be done, for example, by an aperture in front of the optical fiber tip.

[0141] As an alternative to the arrangement with an output coupler and a further beam splitter, a wedge-shaped window 81 can be placed in the signal beam path to obtain partial reflections of the combined signal radiation beam at the front and rear surfaces of the wedge-shaped window, thereby generating two branches of the reference pulse. Figure 7 shown.

[0142] The wedge in the window avoids etalon effects and results in an angular offset between the two reflected reference pulse portions, which are separately directed back by corresponding end mirrors 78 to generate two pulse copies with corresponding delays.

[0143] In both embodiments, the delay between the two branches of the reference signal path shown may be slightly different.

[0144] Figure 6 Examples and Figure 7The embodiments of are common in that both embodiments create references on different paths to the target so that they both avoid false pulses.

[0145] Furthermore, both embodiments have in common that they provide two reference signals, which, as mentioned above, can be used to handle the hypothetical case where the distance between the reflection of one reference signal and the reflection of the object (or, to be precise, twice that distance) corresponds to an integer multiple of the cavity length of the dual-comb radiation source. In other words, by providing two reference reflections, "down time" (i.e., the time during which the distance cannot be measured due to the temporal overlap of the target and reference reflections of the signal pulse) can be avoided.

[0146] Both embodiments have in common that a second reference reflection is added in a manner that avoids false reflections.

[0147] By further extending one of the proposed methods, additional pulse replicas can be added (which can be used as additional reference pulses).

[0148] In addition, independent of the second reference signal, the reference Figure 6 and Figure 7 Free space designs of the type shown address the problem of avoiding spurious reflections caused by certain optical fibers and free space components (eg, wave plates), for example, by using the periscope 72 described above.

[0149] Furthermore, compared to using, for example, a wave plate or a circulator, Figure 6 and Figure 7 The free-space architecture shown enables broadband operation by increasing the optical bandwidth. For example, the broadband configuration is compatible with multi-wavelength (also called multispectral or hyperspectral) LiDAR, which can provide additional information compared to using a single wavelength.

Claims

1. A laser ranging method, comprising the following steps: o generating a dual-comb radiation comprising a first radiation portion (11) having a first radiation pulse train and a second radiation portion (12) having a second radiation pulse train, the first radiation pulses having a first pulse repetition frequency and the second radiation pulses having a second pulse repetition frequency different from the first pulse repetition frequency; o using a first radiation separator (6) to separate the first radiation portion (11) into a first signal radiation portion (21) and a first local oscillator radiation portion (23); o using a second radiation separator (7) to separate the second radiation portion (12) into a second signal radiation portion (22) and a second local oscillator radiation portion (24); o combining the first signal radiation part (21) and the second signal radiation part (22) into a combined signal radiation; o directing the combined signal radiation onto an object (10); o receiving a signal reflected from the object (10) irradiated by the combined signal; o separating the signal into a first signal portion (25) and a second signal portion (26); o the first radiation separator (6) superimposes the first signal portion (25) and the second local oscillator radiation portion (24) to generate a first measurement signal (27); o the second signal portion (26) is superimposed by the second radiation separator (7) on the first local oscillator radiation portion (23) to generate a second measurement signal (28); as well as o Derivation of the distance to the object (10) based on the first measurement signal and the second measurement signal.

2. The method according to claim 1, wherein: After the combining step, the share of the combined signal radiation generated by the first signal radiation portion (11) has a first polarization, and the share of the combined signal radiation generated by the second signal radiation portion (12) has a second polarization different from the first polarization; and wherein the step of separating the signals comprises separating a signal portion having the first polarization and a signal portion having the second polarization to generate the first signal portion (25) and the second signal portion (26).

3. The method according to claim 2, wherein: For the combining step and the separating step, a polarization combiner is used.

4. The method according to claim 1, wherein: For the combining step and the separating step, a polarization-maintaining fiber coupler or a single-mode fiber coupler is used.

5. The method according to any of the preceding claims, further comprising generating a reference pulse train using a partial reflection of the combined signal radiation, and also for the reference pulse train, performing the separation step and the step of superimposing with the second local oscillator radiation portion (24) and the first local oscillator radiation portion (23), respectively, to generate a first reference signal and a second reference signal, and using the first reference signal and the second reference signal in the step of deriving a distance to the object (10), the distance being the distance between the partial reflection and the object.

6. The method according to any one of the preceding claims, further comprising the step of generating additional dual-comb radiation, the additional dual-comb radiation comprising a third radiation part having a third radiation pulse train and a fourth radiation part having a fourth radiation pulse train, the third radiation pulse having a third pulse repetition frequency, the fourth radiation pulse having a fourth pulse repetition frequency different from the third pulse repetition frequency, the wavelength region in which the third radiation part and the fourth radiation part are located is different from the wavelength region of the first radiation part and the second radiation part, and the additional dual-comb radiation is used to perform the same measurement as the dual-comb radiation having the first radiation part and the second radiation part so as to correct the refractive index change along the measurement path.

7. A method according to any one of the preceding claims, wherein: The step of deriving the distance comprises comparing the first measurement signal and the second measurement signal and using a Vernier effect to reduce blurring.

8. A method according to any one of the preceding claims, wherein: The difference between the first repetition rate and the second repetition rate is Δf rep , and wherein, for the distance d to the object (10), the following inequality holds: c / (2*Δf rep ) / 3 <d< c / (2*Δf rep ).

9. A method according to any one of the preceding claims, wherein: An output port of the first radiation separator (6) for the first local oscillator radiation portion (23) communicates with an output port of the second radiation separator (7) for the second local oscillator radiation portion (24) to guide the first local oscillator radiation portion (23) to the second radiation separator for superposition with the second signal portion, and to guide the second local oscillator radiation portion (24) to the first radiation separator (6) for superposition with the first signal portion.

10. The method according to claim 9, wherein: A first local oscillator portion propagates from the first radiating splitter to the second radiating splitter in a first propagation direction, and a second local oscillator portion propagates from the second radiating splitter to the first radiating splitter in a second propagation direction opposite to the first propagation direction.

11. The method according to claim 9 or 10, wherein: The output port of the first radiation splitter (6) for the first local oscillator radiating portion (23) communicates with the output port of the second radiation splitter (7) via at least one of: an optical fiber, an arrangement including at least one radiation deflector (74), a waveguide on a substrate, or wherein the first radiation splitter and the second radiation splitter are aligned such that radiation from the local oscillator output port of the first radiation splitter propagates directly to the local oscillator output port of the second radiation splitter, and vice versa.

12. A laser distance measuring device, comprising: o a dual-comb radiation source (1) generating a first radiation portion (11) having a train of first radiation pulses and a second radiation portion (12) having a train of second radiation pulses, the first radiation pulses having a first pulse repetition frequency and the second radiation pulses having a second pulse repetition frequency different from the first pulse repetition frequency; o a first radiation separator (6) configured to separate the first radiation portion (11) into a first signal radiation portion (21) and a first local oscillator radiation portion (23); o a second radiation separator (7) configured to separate the second radiation portion (12) into a second signal radiation portion (22) and a second local oscillator radiation portion (24); o a radiation combiner (8) arranged to combine the first signal radiation portion (21) and the second signal radiation portion (22) into a combined signal radiation; o a radiation in-coupling and out-coupling structure arranged to radiationally couple the combined signal out of the device and to receive a signal reflected from an object (10) radiated by the combined signal and direct it to the combiner (8); o wherein an output port of the first radiation splitter (6) for the first local oscillator radiating portion (23) communicates with an output port of the second radiation splitter (7) for the second local oscillator radiating portion (24); Thereby, the first radiation separator (6) superimposes the first signal portion (25) with the second local oscillator radiation portion (24) to generate a first measurement signal; and thereby, the second radiation separator (7) superimposes the second signal portion (26) with the first local oscillator radiation portion (23) to generate a second measurement signal; o a first detector (41) arranged to receive the first measurement signal; and a second detector (42) arranged to receive the second measurement signal; o wherein the device is equipped to obtain a distance to the object (10) based on the first measurement signal and the second measurement signal.

13. A laser distance measuring device according to claim 12, comprising an optical fibre arrangement, wherein: The first and second radiation splitters (6, 7) and the combiner (8) are optical fiber components, and wherein the optical fiber arrangement comprises a connection between the first and second radiation splitters and the combiner and a connection between the first and second radiation splitters (6, 7).

14. The laser distance measuring device according to claim 12, which is realized by a free space component.

15. The laser distance measuring device according to claim 14, wherein: The first radiation splitter and the second radiation splitter are spatially separated parts of a single beam splitting device (60).

16. The laser distance measuring device according to claim 14 or 15, comprising a periscope (72), wherein: One of the first signal radiating portion (21) and the second signal radiating portion (22) passes through the periscope, or wherein: The periscope is located on a connection path between a local oscillator output port of the first radiation splitter and a local oscillator output port of the second radiation splitter.

17. The laser ranging device according to any one of claims 14 to 16, comprising an output coupler (75) arranged in the beam path of the combined signal radiation and configured to extract a reference pulse train from the combined signal radiation.

18. The laser ranging device according to claim 17, further comprising another beam splitter (76) arranged to split the reference pulse train into two branches so as to generate two reference signals.

19. The laser ranging device according to any one of claims 14 to 16, further comprising a wedge-shaped window (81) placed in the beam path of the combined signal radiation so as to cause two partial reflections, each partial reflection generating a reference signal.

20. The laser ranging device according to claim 12, which is implemented by a waveguide.

21. A laser ranging device according to any one of claims 12 to 20, comprising a partially reflective outcoupling facet which reflects a portion of the combined signal radiation back towards the combiner.

22. The laser distance measuring device according to any one of claims 12 to 21, further comprising a transparent window, such as a wedge-shaped window, placed in the measurement path.

23. The laser distance measuring device according to any one of claims 12 to 22, wherein: The first detector and the second detector (41, 42) each include a photodiode.

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