Cascaded lidar arrangement and lidar-based methods

Abstract

The invention relates to an FMCW lidar-based arrangement for distance and / or speed measurement and to a lidar-based method for distance and / or speed measurement. The arrangement comprises a cascade of beam splitters associated with a plurality of measurement channels and a light return path for multi-channel LIDAR-based optical measurement of the distance and / or speed of a target object.

Description

[0001] CASCADED LIDAR ARRANGEMENT AND LIDAR-BASED METHODS

[0002] This application claims priority from German application DE 10 2024 102 586 . 7 of January 30, 2024, the disclosure of which is hereby incorporated in its entirety by reference.

[0003] The present invention relates to an arrangement for optical distance and / or speed measurement. Furthermore, the invention relates to the use of a cascaded LIDAR measurement system implemented as a photonic integrated circuit, as well as to a method.

[0004] BACKGROUND

[0005] LIDAR is an optical method for measuring distance and / or speed. Light radiation, primarily from a laser system, is directed at a target object and reflected from there. The reflected light is detected and evaluated. From the properties of the reflected light component, such as the propagation time between the transmitted and received signal, or the frequency properties of the received signal in comparison to the transmitted signal, the distance to the object—and, with suitable modulation, also the relative speed—can be deduced. Various laser systems and measuring methods can be used for this purpose.

[0006] In a frequency-modulated continuous wave (FMCW) LIDAR, a coherent laser beam with a linearly variable frequency is emitted in the direction of a target object. At the same time, a portion of the laser light is coupled out, which corresponds to a local oscillator. The light reflected from the target object is coupled into a monomode waveguide and coherently superimposed with the previously coupled oscillator portion. During the light travel time to the target object and back, a change in the frequency of the light beam occurs, which leads to a beat when the reflected light mixes with the oscillator signal. Such a beat frequency is usually in the range of a few 10s to 100s of MHz up to possibly even a few GHz, and can therefore be precisely digitized and processed electronically.The beat frequency corresponds to the frequency difference between the local oscillator and the signal received from the target object and is proportional to the distance to the target object due to the time-linear frequency shift of the laser.

[0007] The detection of light reflected from the target object and coupled into the waveguide requires that the beam path be designed such that the reflected light is not directed back to the laser, but instead to the differential detector. This prevents instabilities in the laser emission that can arise from such feedback. For this purpose, an optical circulator is used in fiber optics, which typically contains a Faraday rotator and birefringent materials.

[0008] To miniaturize and reduce the cost of FMCW systems, implementation in photonic integrated circuits (PICs) is a desirable solution. However, available technologies do not allow for the manufacture of PIG-based optical circulators. A solution is therefore required to enable the separation of the required laser beam directions without using a circulator.

[0009] In addition, parallel operation of multiple signal channels appears appropriate to achieve the resolution and frame rate required in the automotive sector. Another requirement is to generate laser radiation with highly linear frequency modulation. A simple parallel arrangement of many independent systems in this case involves considerable effort, e.g., characterization and linearity adjustment. The use of a common source for multiple systems is therefore very advantageous.

[0010] There is therefore a need to reduce the space requirements and light losses in FMCW-based distance measurement systems, as well as to achieve increased efficiency of signal detection, lower energy consumption and a reduction in cooling requirements.

[0011] SUMMARY OF THE INVENTION This need is met by the subject matter of the independent patent claims. Further developments and embodiments of the proposed principle are specified in the subclaims.

[0012] In the prior art, a common source is used for several parallel or sequentially controlled channels (e.g. US 11,378,689, US 2021 / 0149056, US 2022 / 0334227). The light from the laser is first split between the channels, and the received light is separated in each individual channel by a separate component, usually a beam splitter or circulator, and guided to a detector. However, in the case of free-space optics, the beam splitter can be used jointly for all channels (US 2021 / 22444). In the prior art, a polarization-dependent beam splitter is used in particular for the single channel (US 11,156,715).

[0013] State-of-the-art solutions include beam splitters that serve either exclusively to split the generated laser light into multiple channels or exclusively to separate emitted and received light. In the latter case, polarization-dependent beam splitters are usually used.

[0014] The inventors now propose an FMCW lidar system with multiple spatially separated and separately analyzable channels, which shares a common laser and requires neither a circulator nor a polarization-dependent beam splitter. This achieves a very small size and reduced costs, since such an FMCW lidar system can be integrated within a photonic integrated circuit.

[0015] Accordingly, the channels are cascaded beam split using beam splitters that have high reflectivity, so that only a small proportion of the laser light is used for one channel. The beam splitters serve simultaneously to split the generated laser light between the channels and to separate the received signal light reflected from the target object, which is used for detection. The received light is decoupled from the detection unit by the beam splitter. The transmitted part of the light coupled out from the beam splitter is used for the channel. The transmittance in the beam splitter is less than 50% (this corresponds to a reflectance greater than 50%), preferably less than 20%, so that several beam splitters can be cascaded per laser light source. The reflected part of the radiation is directed to another beam splitter, where the required radiation for the next channel is coupled out.The remaining laser power after the beam splitter of the last channel in the cascade is divided equally among all channels and fed back as a local oscillator signal. Additional elements of the FMCW lidar system are also described, which serve to suppress any crosstalk artifacts that may occur.

[0016] The use of an FMCW lidar measurement system based on the proposed principle offers several advantages over conventional solutions. Improved speckle statistics and object identification are achievable through the measurability of both polarization directions. For example, by detecting multiple independent input signals (in the sense of being based on a signal intensity that follow the speckle statistics but are independent of each other), it is possible to change the resulting statistics of the intensities of the input signals so that they transition from negative exponential statistics (which leads to a low detection probability) to an Erlang distribution (with a larger k-value), which generally leads to an increased detection probability. This improves the speckle statistics.

[0017] A significant reduction in losses in the signal path is made possible, and thus greater efficiency in signal detection. In particular, when the signal incident from the target object is separated from the emitted light using a beam splitter with a reflectivity of more than 80% or 90%, smaller losses are realized in the receive path compared to the use of a polarization-dependent beam splitter or optical circulators. This means that only a small part of the laser power in the transmit path is transmitted and used in one channel, without the corresponding high power losses that would arise in state-of-the-art systems. According to the proposed principle, the power losses are minimized by using the high reflected power component in the transmit path for the other cascaded channels and for the local oscillator.

[0018] Implementing a measurement system based on the proposed principle on a PIC requires less space and reduces light losses by reducing the number of required components. This results in increased efficiency, lower power consumption, and reduced cooling requirements. Accordingly, system costs are reduced.

[0019] Different aspects of the principle presented here can be combined so that FMCW lidar systems can be optimized for different applications.

[0020] In some aspects, the inventors propose an arrangement for optical distance and / or speed measurement. The arrangement, which is referred to herein as a LIDAR arrangement or LIDAR system, comprises a coherent laser arrangement whose output frequency can be tuned. This can comprise one or more edge-emitting lasers that can be controlled individually. Lasers based on semiconductor material are usually used, but the proposed principle is not restricted to this. In some aspects, the laser arrangement is implemented with a quantum dot laser. This has the advantage that it is robust against light reflected back into the laser arrangement.

[0021] The laser array is designed so that frequency modulation can be applied to the laser light. The frequency modulation can either take place directly within the laser or be performed by a downstream array that, together with the laser, forms the laser array.

[0022] The LIDAR system also includes a plurality of separately evaluable, spatially separated measuring channels that serve to guide the light emitted by the laser array and the light reflected by a target object.

[0023] In the signal path of the laser arrangement in the proposed LIDAR system, a first beam splitter with a first, a second and a third output is arranged. The beam splitter is designed to couple at least a portion of the laser light emitted by the tunable laser arrangement to a first measuring channel coupled to the second output. The portion of light coupled out to the measuring channel, referred to here as the transmitted portion, amounts to less than 50%, preferably less than 20%, and in particular less than 15% or less than 10% of the emitted laser light. The transmitted light portion is guided in the measuring channel through an optics system onto an object to be measured and partially reflected from there. In some aspects, the arrangement comprises an amplifier which is arranged in the measuring channel between the beam splitter and the optics system. The signal amplification depends on application requirements and can, for example, be between 10 dB and 25 dB.When implemented in a PIC, this can be somewhat lower and in particular lie between 15 and 20 dB.

[0024] The third output of the first beam splitter is connected to a first differential detector. In some aspects, the differential detector comprises a light mixer with at least two inputs.

[0025] According to the proposed principle, the LIDAR system comprises at least one further beam splitter, referred to here as the second beam splitter, with a first, a second and a third output. A signal input of the second beam splitter is coupled to the first output of the first beam splitter, for example by one or more reflectors. The light component remaining after the transmitted part has been coupled out at the first beam splitter, referred to here as the reflected light component, is connected to the second beam splitter in a cascade. As with the first beam splitter, the second output of the second beam splitter is connected to a second measuring channel in order to couple out a transmitted part of the light received by the first beam splitter. The third output of the second beam splitter is also connected to a second differential detector.A reflected part of the light received by the first beam splitter can be tapped at the first output.

[0026] An arrangement according to the proposed principle is thus formed with several cascaded beam splitters, each assigned to a corresponding measurement channel to extract a transmitted portion of the laser light. Starting with the second beam splitter in the cascade, an input of each beam splitter is connected to the first output of a beam splitter in the cascade to receive a reflected portion of the laser light from the previous beam splitter. The totality of the transmitted light components from the cascaded beam splitters corresponds to the main beam path of the LIDAR arrangement.

[0027] After the last beam splitter in the cascade, corresponding to the last measurement channel, the reflected light component coupled out to the first output of the last beam splitter is coupled into a light return path. The signal in the light return path is evenly distributed among the measurement channels. For this purpose, the light return path is coupled to the respective differential detectors. The light component reflected from the last beam splitter and coupled to the light return path preferably corresponds to approximately 50% of the original laser radiation.

[0028] In some aspects of the proposed principle, the light return path comprises at least one beam splitter with a first and a second output. The first output is connected to an input of a differential detector assigned to the measuring channel. The second output is coupled either to an input of a subsequent beam splitter or to an input of a differential detector assigned to the measuring channel, wherein the beam splitters are arranged in a cascade, and the total number of beam splitters is one less than the total number of measuring channels. With the cascaded beam splitters in the main beam path, a preferably equal portion of the emitted laser radiation for the respective measuring channels is coupled out as transmitted light portion.In some aspects, this even distribution of the signal power is achieved by a variable transmittance of the beam splitters, which increases from the first to the last beam splitter in the cascade. The transmittances of the beam splitters assigned to the measurement channels depend on the total number N of measurement channels. Using half of the laser power in the main beam path as the transmitted part, the transmittance of a beam splitter arranged for measurement channel n with a total of N measurement channels results from: where the numbering n of the measuring channels is carried out in the path of the laser light from the source .

[0029] The reflectance of all beam splitters is significantly higher than 50%. For this reason, the majority of the received light reflected from the target object and the light coupled out at the third output of the beam splitter is guided to the respective differential detector. This system architecture thus exhibits greater efficiency than state-of-the-art solutions. For example, when using conventional polarization-sensitive beam splitters, at most 50% of an unpolarized signal is reflected or guided to the detection unit. Furthermore, optical circulators generally exhibit significantly higher losses than beam splitters.

[0030] In an alternative embodiment, the beam splitters arranged in the main beam path are characterized by identical transmittances, with the transmittance increasing with a higher total number N of measuring channels. With a 50% split between the main beam path and return path, the transmittance of each beam splitter arranged in the main beam path results in this case from 0'5 / y. In order to achieve the same signal strength in all channels with the same object reflectivity, signal compensation is required, for example by variable amplification of the amplifiers assigned to the respective measuring channels.

[0031] In some aspects, measurement channels where lower sensitivity is sufficient have reduced gain. This allows for electrical power savings. The amplifier power can also be varied to compensate for manufacturing tolerances in the beamsplitter transmission or other components.

[0032] In another aspect, the signal extracted from the return path is split using a single element, such as a multimode interference coupler (MMI) or a star coupler. This design further reduces the number of required components, resulting in greater efficiency and a smaller footprint.

[0033] In some systems characterized by a large number of measurement channels, at least one additional amplifier is installed after some channels have been branched off to compensate for the reduced power. In this way, systems with more than 10 to 15 channels are feasible. Likewise, with many measurement channels, at least one additional amplifier can be installed in the feedback path.

[0034] In the above-mentioned embodiments, the signal is guided to the differential detector by the beam splitters regardless of the signal polarization. With essentially specular reflection of the laser light emitted by the target object, i.e. without significant scattering, the polarization of the reflected light is essentially identical to the emitted laser light. In this case, the signal received from the target object is fully detectable because the signal has the same polarization direction as the return path and can therefore be mixed. Normally, however, the light reflected from the target object is scattered, resulting in a large portion of the polarization being lost. In this case, only about half of the received signal is detectable.

[0035] In some aspects of the proposed principle, alternative designs of differential detectors for detecting both polarization directions of the light received from the target object are presented. These aspects make it possible to detect an unpolarized signal without systematic losses. Furthermore, the two polarization directions of the signal are random and statistically independent of each other in their signal strength due to coherent light superposition during light scattering (speckle). Furthermore, the surface properties of a target object can be polarization-dependent. Accordingly, parallel detection of both polarization directions enables two simultaneous, independent measurements, which can significantly increase the probability of detection. Furthermore, the tracking or identification of objects based on the polarization rotation becomes possible.

[0036] In one embodiment, the differential detector associated with the measurement channel comprises a polarization-dependent beam splitter having a first and a second output, configured to couple light signals with mutually orthogonal polarization directions from the two outputs. The input of the polarization-dependent beam splitter is coupled to a third output of a beam splitter associated with a measurement channel, for conducting an unpolarized light signal reflected from a target object.

[0037] The first output of the polarization-dependent beam splitter is configured such that a light signal with an identical polarization direction is coupled to the light return path. For this purpose, the first output is connected to a first light mixer. The second output of the polarization-dependent beam splitter is connected to a polarization rotator, which is configured to rotate the polarization direction of the light signal coupled from the second output of the polarization-dependent beam splitter by 90°, wherein the polarization direction of the rotated signal is identical to the polarization direction of the signal from the light return path of the LIDAR system.

[0038] The output of the polarization rotator is connected to a second light mixer. The differential detector further comprises a second beam splitter with a first and a second output, the input of which is connected to the light return path. The second beam splitter is designed to couple light signals with the same polarization directions from the two outputs. The two outputs are each routed to one of the two light mixers.

[0039] In an alternative embodiment, the two outputs of the polarization-dependent beam splitter are each connected to a light mixer. The first output of the polarization-dependent beam splitter is designed to couple a light signal with an identical polarization direction to the light return path.

[0040] The differential detector further comprises a second beam splitter with a first and a second output, the input of which is connected to the light return path. The second beam splitter is designed to couple light signals with the same polarization directions from the two outputs. The first output of the second beam splitter is connected to the light mixer, which is also coupled to the first output of the polarization-dependent beam splitter. The second output of the second beam splitter is coupled to the input of a polarization rotator. The output of the polarization rotator is coupled to the light mixer, which is coupled to the second output of the polarization-dependent beam splitter. In this embodiment, a polarization rotation of the signal received from the target object is avoided, whereby signal losses can be reduced.

[0041] In an alternative aspect, the differential detector comprises two polarization-dependent beam splitters, each with a first and a second output. The polarization-dependent beam splitters are designed such that they couple out light signals in polarization directions each at 45° to the polarization direction of the light return path, whereby the light components coupled out from the two outputs of the respective beam splitter have polarization directions that are orthogonal to one another. In addition, the light components coupled out from the first outputs of the beam splitters have identical polarization directions. The same applies to the light signals coupled out from the second outputs of the beam splitters. The input of the first polarization-dependent beam splitter is coupled to a third terminal of a beam splitter leading to a measuring channel in order to receive a signal reflected from a target object.The input of the second polarization-dependent beam splitter is designed to supply a signal split from the light return path. The first outputs of the polarization-dependent beam splitters are each connected to a first light mixer, and the second outputs of the beam splitters are each connected to a second light mixer. In this embodiment, two measurement paths are created from the reflected signal and the light return path, each with identical polarization, without the use of a polarization rotator.

[0042] Another aspect relates to a method for detecting a distance and / or a speed of a target object by means of an optical measurement. In this case, a frequency-modulated laser beam is generated. The laser beam is split into a measuring portion, also called the transmitted portion, and a forwarded portion, also called the reflected portion. The transmitted portion is less than 50% of the emitted laser signal and corresponds to a measuring channel signal. The reflected portion is forwarded to at least one further beam splitter, resulting in a cascaded arrangement comprising a plurality of beam splitters, corresponding to a plurality of spatially separated and separately evaluable measuring channels. At each level in the main beam path, a transmitted and a reflected light portion is coupled out, and the reflected portions are fed to further beam splitters up to the last beam splitter.At the final beam splitter, the reflected portion is fed into a light return path. The transmitted portions, each assigned to a measuring channel, are forwarded to the target object to be measured by an optics system assigned to and connected to the measuring channels.

[0043] The light component in the light return path is divided equally between the measuring channels and fed to light mixers that form part of correspondingly assigned differential detectors. The reflected and returned light component is then mixed with the light component in the light return path to form a local oscillator signal. In the next step, according to the prescribed principle, the light reflected from the target object is received and passed on to the light mixer in the differential detector for each measuring channel. The reflected light and the light passed on from the light return path are mixed to generate a beat frequency. The beat frequency is used to determine a distance and / or a speed.

[0044] A further aspect relates to a method for correcting crosstalk in a LIDAR arrangement according to the proposed principle. In general, crosstalk of the signal from the measuring channels occurs in all previous measuring channels in the cascaded arrangement. This results from the incomplete reflection or transmission of the beam splitters. Accordingly, when a signal reflected from the target object is received in one measuring channel, a large part of the signal is forwarded to the corresponding differential detector due to the higher reflectivity of the beam splitters. However, a remaining smaller part of the signal reflected from the target object is transmitted, whereby this transmitted part is forwarded to the differential detector of a current measuring channel.This means that a small portion of the received signal is assigned to a differential detector corresponding to an incorrect measurement channel, and thus also to an incorrect spatial direction. This can result in erroneous detections.

[0045] For example, with five measurement channels, 16.66% of the signal reflected from the target object is transmitted through the fifth beam splitter in the cascade, while the remaining 83.33% is passed on to the corresponding differential detector. The transmitted signal is fed to the fourth beam splitter, where 14.3% of it is passed on to the differential detector belonging to the fourth measurement channel. This means that around 2.4% of the light received in the fifth measurement channel reaches the differential detector belonging to the fourth measurement channel. Through further transmission, around 1.8% of the signal received in the fifth measurement channel falls on the differential detector belonging to the third measurement channel. With several measurement channels, however, the transmittances are lower, which means that the amount of crosstalk is lower.However, it is possible to characterize the crosstalk to a large extent and calculate it from the measured signal, provided the system is equipped with amplifiers between the beam splitters and the projection optics. In a first step, only some of the amplifiers are energized, for example in every second measuring channel. The signals emitted from the energized measuring channels are then reflected by an object and correspond to the signals to be measured. In the remaining measuring channels, a large part of the signal light is absorbed. In addition, the residual signal received in these measuring channels is passed through the absorbing amplifiers again and attenuated again, so that in these channels no measurable signal, or only a significantly reduced one, reaches the associated differential detectors.This allows the differential detectors to detect the crosstalk signals from the energized measuring channels corresponding to the non-energized measuring channels.

[0046] In a 5-measurement channel system, where measurement channels 1, 3 and 5 are energized, the crosstalk signal from measurement channel 5 to measurement channel 3 can be subtracted from the detected signal in measurement channel 3, since the ratio of the crosstalk signal strengths in measurement channels 3 and 4 is known according to the calculation above. In addition, no statistical fluctuations resulting from speckle occur, since all signals originate from the same, simultaneously received mode and are therefore in a fixed relationship to one another. Accordingly, the measurement from measurement channel 2, which is also deactivated, can be used to correct the crosstalk from measurement channels 3 and 5 in measurement channel 1. The previously unenergized measurement channels are then energized in a next time interval, while the previously energized measurement channels remain switched off. This allows the crosstalk in the other measurement channels to be characterized.This method can be applied to any number of measurement channels, although correcting crosstalk requires twice the number of measurements. In some aspects of the described principle, the system can be operated in such a way that most of the measurement channels are in operation simultaneously in order to obtain as many measurement points as possible as quickly as possible, or to save measurement effort, thereby reducing power consumption. In the meantime, for every x-th measurement, only every second measurement channel can be operated in order to characterize or eliminate any relevant incorrect measurements due to crosstalk in the event of uncertainties in the interpretation of the measured data. Operating the system in this way enables the detection of a target object in a single step based on a coarse measurement with little spatial resolution.In a further step, by making the measurement more precise, a higher spatial resolution is achieved and, on the other hand, further parameters and properties of the object can be determined.

[0047] BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Further aspects and embodiments according to the proposed principle will become apparent with reference to the various embodiments and examples which will be described in detail in conjunction with the accompanying drawings.

[0049] Figure 1 shows a conventional design of an FMCW lidar measuring system;

[0050] Figure 2 shows a first embodiment of an arrangement for optical distance and / or speed measurement according to the proposed principle;

[0051] Figures 3 and 4 show the signal distribution in the measurement channels and the local oscillators according to some aspects of the proposed principle;

[0052] Figure 5 shows a second embodiment of an FMCW lidar arrangement incorporating some aspects of the proposed principle; Figures 6 and 7 illustrate possible embodiments of a differential detector incorporating some aspects of the proposed principle.

[0053] DETAILED DESCRIPTION

[0054] The following embodiments and examples show various aspects and their combinations according to the proposed principle. The embodiments and examples are not always true to scale. Likewise, various elements can be shown enlarged or reduced in size to emphasize individual aspects. It goes without saying that the individual aspects and features of the embodiments and examples shown in the figures can be combined with one another without thereby impairing the inventive principle. Some aspects have a regular structure or shape. It should be noted that in practice slight deviations from the ideal shape can occur without, however, contradicting the inventive idea.

[0055] Furthermore, the individual figures, features, and aspects are not necessarily shown in the correct size, and the proportions between the individual elements may not always be correct. Some aspects and features are emphasized by being shown enlarged. However, terms such as "top", "above", "below", "below", "larger", "smaller", and the like are correctly represented with reference to the elements in the figures. This makes it possible to infer such relationships between the elements from the illustrations.

[0056] In conventional FMCW lidar distance measuring systems, a coherent laser signal with a linearly variable frequency is emitted at a target object and reflected by the target object. The reflected light is again coupled into a monomode waveguide and superimposed with a portion of the laser light that was coupled out before transmission. The frequency of a resulting beat is passed on to a detector, from which the distance to the target object is calculated. By suitable modulation of the received signal, it is also possible to calculate a speed measurement. Figure 1 shows an FMCW lidar arrangement. A coherent light beam is emitted by a laser arrangement 100. The light signal is guided through the projection optics 104 to a target object 105, with part of the light signal being reflected by the target object. The light signal reflected from the target object is directed to a detector 103.An optical circulator 102 is arranged in the signal path to prevent the light reflected from the target object from being passed on to the laser array and instead to direct this reflected light to the detector.

[0057] The inventors now propose another FMCW lidar arrangement with several parallel measuring channels and a common laser, which requires neither a circulator nor a polarization-dependent beam splitter.

[0058] Figure 2 illustrates one aspect of an arrangement according to the proposed principle. The LIDAR arrangement 1 shown comprises a laser arrangement 100 and five spatially separated and separately analyzable measurement channels 11a to 11e. The number of measurement channels is not limited to 5, but can be larger or smaller. The light emitted by the laser arrangement is guided to an input of a first beam splitter 10a in a cascaded series of five beam splitters 10a to 10e.

[0059] The beam splitters each have one input and three outputs. The first output of the first beam splitter 10a is coupled to an input of the second beam splitter 10b by a first reflector 12a. The second output of the first beam splitter is connected to the first measuring channel 11a. A first amplifier 101a in the first measuring channel serves to amplify the portion of light coupled out by the laser arrangement, as well as the signal reflected from the target object. The third output of the first beam splitter is led to a first differential detector 13a. The outputs of the other beam splitters, except for the last beam splitter 10e, are also connected in the same way in the system. The signal 111c represents the return path of a signal reflected from the target object via the measuring channel 11c.At the last beam splitter 10e, the third output is connected to a light return path by a reflector 12e, the light return path comprising the beam splitters 14a to 14d.

[0060] Signal amplifiers 101a to 101e are arranged between the beam splitters 10a to 10e and the projection optics 110 in the corresponding measuring channels 11a to 11e. These amplifiers serve to amplify the signal components coupled out of the beam splitters as well as the light signals reflected from the target object. The amplifier elements 101a to 101e can, for example, be semiconductor-based optical amplifiers (SOAs).

[0061] The return path, comprising the beam splitters 14a to 14d, is defined by the reflectors 12e and 15e at the first output of the last beam splitter 10e in the main beam path. Each beam splitter has two outputs. For the beam splitters 14a to 14d in the return path, a first output is each connected to a differential detector 13e to 13b by reflectors 16e to 16b. The respective second outputs for the last three beam splitters in the return path, viewed in the path of the laser light from the source (i.e. 14b to 14d), are each guided by the reflectors 15b to 15e to the inputs of the nearest beam splitters in the return path. However, the second output of the beam splitter 14a is coupled by the reflectors 15a and 16a to a differential detector 13a assigned to the first measuring channel.

[0062] The individual reflectors in the arrangement shown here are merely examples to better illustrate the individual light guides. In a suitable implementation, particularly in PIC, these can also be formed by waveguides or eliminated through suitable optical alignment. According to the proposed principle, the division into several measurement channels is essential.

[0063] According to the proposed principle, each beam splitter coupled to a measuring channel in the LIDAR arrangement serves simultaneously to couple out and transmit a portion of the light to the corresponding measuring channel, to forward a larger portion of the light to another beam splitter arranged in a cascade, and to couple out the light reflected from the target object to a corresponding differential detector.

[0064] The transmittance of the beam splitters increases with each level of the cascade in the main beam path, as seen from the source, in order to provide approximately the same light output for each measurement channel. Therefore, the reflectance of the first beam splitter, located closest to the laser array, is preferably 90% or higher.

[0065] Figure 3 shows an example of the transmittance of the beam splitters. With five channels, 10% of the radiation is coupled into each measurement channel, leaving 50% of the original laser radiation to be passed on to the return path. The respective transmittances of the beam splitters arranged in the return path are also shown in Figure 4. The "Laser" columns indicate the laser light remaining in the main beam path and in the return path. The "Beamsplitter Transmission" columns represent the transmittances of the respective beam splitters. The "Signal" column shows the parts of the laser light coupled out to the measurement channels, and the "RP" column shows the parts of the laser light coupled out from the return path to the differential detectors.

[0066] In other aspects, a uniform signal strength in the measurement channels can be achieved by using identical beam splitters with, for example, 10% transmittance and 90% reflectance by variable gain of the respective amplifiers. With a higher total number of measurement channels, the transmittance is selected to be correspondingly lower. In such an aspect, the gain of the corresponding amplifiers in the measurement channels increases with each level of the cascade in the main beam path, as seen from the source.

[0067] A further aspect of the proposed principle is shown in Figure 5. This shows an implementation of an FMCW lidar system with five measuring channels in a photonic integrated circuit (PIC). The FMCW lidar arrangement comprises a frequency-modulated laser light source 200. A portion of the laser radiation, preferably 50% of the total laser power, is guided through five beam splitters 210a to 210e to the respective measuring channels 211a to 211e. The transmittances of the beam splitters are preferably selected such that equal portions of the laser power are coupled into each measuring channel. The coupled signals are each amplified by an amplifier 201a to 201e arranged between the beam splitter and a projection optics before being forwarded through a downstream optics to a target object. The laser light remaining after the fifth beam splitter is guided to a return path.Here, a single element is used to divide the feedback path signal between the five differential detectors 213a to 213e corresponding to the measurement channels 211a to 211e.

[0068] In some aspects, according to the proposed principle, the splitting element comprises a multimode interference coupler (MMI) with a number of outputs that is identical to the total number of measurement channels. In an alternative embodiment, the splitting element comprises a star coupler. In some aspects, the laser light is coupled out to the measurement channels by MMI-based beam splitter elements. In further aspects, the beam splitters assigned to the measurement channels comprise at least one star coupler.

[0069] Some aspects of the proposed principle enable detection of both polarization directions of a light signal reflected from the target object.

[0070] Figure 6 shows a differential detector according to some such aspects of the FMCW lidar arrangement. A light signal a reflected from the target object, composed of light components with different polarization directions, is fed to an input of a polarization-dependent beam splitter 310. The light signal is split into two mutually orthogonal light components with the respective polarization directions A and B. In addition, a laser light component from the return path RP of the measuring system is fed to an input of a second polarization-independent beam splitter 311. The polarization direction A of the light signal coupled in from the return path is parallel to the polarization direction of the light component coupled out from a first output of the polarization-dependent beam splitter 310, as well as the light components transmitted and reflected from the outputs of the second beam splitter 311.

[0071] The first output of the first beam splitter 310 and the first output of the second beam splitter 311 are connected to a first light mixer 315. The signal from the second output of the second beam splitter 311 is coupled by a second reflector 314 to the input of a polarization rotator 312, where the incoming light signal is rotated by 90°. The rotated signal from the polarization rotator and the signal from the second output of the first beam splitter 310 are combined in a second light mixer 316, with a first reflector 314 being arranged between the second output of the first beam splitter and the second light mixer.

[0072] In an alternative embodiment, the signal from the second output of the polarization-dependent beam splitter 310 is passed through a polarization rotator, and the light signal rotated by 90° is coupled into a second light mixer. In such an embodiment, the two outputs of the second beam splitter 311 are each coupled to one of the two light mixers 315 and 316.

[0073] Another aspect of the differential detector is shown in Figure 7. It shows a simpler alternative solution for detecting both polarization directions of the light signal received from the target object.

[0074] In this embodiment, the differential detector 4 comprises two polarization-dependent beam splitters 410 and 411. The beam splitters are designed such that they rotate the polarization direction of incoming light signals by 45° relative to the polarization direction of the light signal guided from the return path RP. This results in two partial beams with mutually orthogonal polarization directions X and Y at the outputs of the respective beam splitters. A portion of the light a reflected from the target object is coupled into the input of the first beam splitter 410. The input of the second beam splitter 411 is connected to the return path RP.The outputs of the two beam splitters are coupled to the two light mixers 415 and 416, with the output signals having a polarization direction parallel to the indicated direction X being connected to the first light mixer 415, and the output signals having a polarization direction parallel to Y being fed to the second light mixer 416. In this way, two measurement paths are created from the measurement signal received from the target object and the light return path, each with identical polarization, without the use of a polarization rotator.

[0075] LIST OF REFERENCE SYMBOLS

[0076] I,2 FMCW Lidar System 3,4,13,103,213 Differential Detector

[0077] 10,14,210,311 beam splitters

[0078] II, 211 Measuring channels 12, 15, 16 Reflectors 100 Laser array 101,201 Amplifier

[0079] 102 Circulator

[0080] 104,110,210 Projection optics

[0081] 214 Multimode Interference Coupler

[0082] 310,410,411 polarization-dependent beam splitter 312 polarization rotator

[0083] 315,316,415,416 light mixer

[0084] A, B,X,Y polarization direction a measurement signal

[0085] RP light return path

Claims

PATENT CLAIMS 1 . Arrangement for optical distance and / or speed measurement, comprising: - a laser array whose output frequency can be tuned; - a large number of separately evaluable and spatially separated measuring channels; - a first beam splitter with a first, a second and a third output, which is connected to the laser arrangement with an input, the second output being connected to a first of the measuring channels and the third output being connected to an input of a differential detector; a portion of the light coupled out from the second output being smaller than a portion of the light coupled out from the first output; and light reflected from an object being guided through the first of the measuring channels to the differential detector; at least one further beam splitter, the at least one further beam splitter being coupled on the input side to the first output of the previous beam splitter, a second output of the at least one further beam splitter being connected to a second of the measuring channels and a third output of the at least one further beam splitter being connected to an input of a further differential detector;and wherein a portion of light coupled out of the second output is smaller than a portion of light coupled out of the first output; and a light reflected from an object is guided through the second of the measuring channels to the further differential detector.; 2 . Arrangement according to claim 1 , further comprising : a light return path coupled to the first output of the last beam splitter viewed in the path of the laser light from the laser arrangement, comprising at least two outputs; each output is coupled to a local oscillator input of a differential detector assigned to the respective measuring channel. 3 . Arrangement according to one of the preceding claims, wherein the light return path comprises at least one of : - a beam splitter having a first and a second output, the first output being coupled to a differential detector corresponding to a measuring channel, and the second output being coupled either to an input of a further beam splitter or to the differential detector corresponding to a measuring channel; - a multimode interference coupler; or - a star coupler . 4 . Arrangement according to one of the preceding claims, wherein the light return path is designed to provide light of substantially the same intensity at each of its outputs. 5 . Arrangement according to one of the preceding claims, in which the light return path comprises at least one beam splitter with two outputs, which is designed to split the beam at the two outputs with an unequal distribution, the output with low light intensity forming one output of the light return path.

6. Arrangement according to one of the preceding claims, wherein the beam splitters have different transmittances, in particular at the first and second outputs; and / or wherein the transmittances are different at at least one of the outputs of the first beam splitter and the at least one further beam splitter.

7. Arrangement according to one of the preceding claims, wherein at least one of the measuring channels comprises an amplifier arranged between the beam splitter and the projection optics. 8 . Arrangement according to one of the preceding claims, wherein the outputs of the first and of the at least one second beam splitter have approximately the same transmittance. 9 . Arrangement according to one of the preceding claims, comprising at least one amplifier which: - is arranged between the first output of a beam splitter and the input of a subsequent beam splitter; and / or - is arranged between the last beam splitter before the return path and / or - which is arranged in the return path. 10 . Arrangement according to one of the preceding claims, wherein at least one of the differential detectors assigned to the measuring channels comprises: - at least one polarization-dependent beam splitter connected to the third output of a beam splitter associated with a measuring channel, the polarization-dependent beam splitter having a first and a second output, and ■ wherein the polarization-dependent beam splitter is designed to couple light signals with mutually orthogonal polarization from the first and the second output, and ■ wherein the light signal from the first output has a polarization that is identical to the polarization of the light emitted from the light return path, and ■ wherein the first output of the polarization-dependent beam splitter is coupled to a first light mixer, and ■ wherein the second output of the polarization-dependent beam splitter is coupled to an input of a polarization rotator, and ■ wherein the polarization rotator is configured to rotate a light signal by 90° and to supply it to a second light mixer; - a second beam splitter coupled to the light return path, comprising a first and a second Output, with the outputs of the second beam splitter being connected to the first and second light mixers respectively. 11 . Arrangement according to one of the preceding claims, wherein at least one of the differential detectors assigned to the measuring channels comprises: - at least one polarization-dependent beam splitter connected to the third output of a beam splitter associated with a measuring channel, the polarization-dependent beam splitter having a first and a second output, and ■ wherein the polarization-dependent beam splitter is designed to couple light signals with mutually orthogonal polarization from the first and the second output, and ■ wherein the light signal from the first output has a polarization which is identical to a polarization of the light emitted from the light return path , and ■ wherein the outputs of the polarization-dependent beam splitter are each coupled to a first and a second light mixer; - a beam splitter coupled to the light return path, comprising a first and a second output, wherein ■ the first output is connected to the light mixer which is coupled to the first output of the polarization dependent beam splitter ■ the second output is coupled to a polarization rotator, and ■ the output of the polarization rotator is coupled to the second light mixer. 12 . Arrangement according to one of the preceding claims, wherein at least one of the differential detectors assigned to the measuring channels comprises: - at least one first and one second polarization-dependent beam splitter, each having a first and a second output with mutually orthogonal polarization, and ■ where the polarization directions of the polarization-dependent beam splitters are rotated by 45° against the polarization of the light return path, and ■ wherein the input of the first polarization-dependent beam splitter is coupled to the third output of the beam splitter belonging to the measuring channel, and ■ wherein the outputs of the first polarization-dependent beam splitter are each coupled to a first and second light mixer, and ■ wherein the input of the second polarization-dependent beam splitter is coupled to a signal split from the light return path, and ■ wherein the outputs of the second polarization-dependent beam splitter are respectively coupled to the first and second light mixers, and ■ where the polarization of the light signals coupled to the first light mixer from the light return path and from the measuring channel are identical, and ■ where the polarization of the light signals coupled to the second light mixer from the light return path and from the measuring channel are identical.

13. Use of a cascade implemented as a photonic integrated circuit comprising at least two beam splitters, each having at least three outputs, and at least one light return path in a LIDAR system, wherein the beam splitters simultaneously serve to divide the generated laser light into several channels and to separate emitted and received light. 14 . Method for detecting a distance and / or speed of an object by means of an optical measurement, comprising the steps: - Generating a frequency-modulated laser beam; - splitting the laser beam into a measuring part and a transmitted part, the measuring part being smaller than the transmitted part; - Forwarding the measuring component to an object to be measured through an optic connected to the measuring channels; - coupling the forwarded part to at least one further cascaded beam splitter corresponding to a separately evaluable measuring channel; - Splitting the transmitted laser beam into a measuring portion and a transmitted laser beam at each cascaded beam splitter; - coupling the forwarded light component from the last cascaded beam splitter viewed from the laser source to at least one local oscillator; - Distribution of the light provided by the local oscillator to light mixers according to the measurement channels; - Receiving and forwarding a light reflected from the object to be measured through a corresponding measuring channel to the respective light mixer; - Mixing the reflected light and the light component generated by the local oscillator to generate a beat frequency; - Determining a distance and / or speed from the beat frequency; wherein the step of dividing the generated laser light into several channels and the step of separating emitted and received light in the same beam splitter are carried out for each measuring channel.

15. Method according to claim 14, wherein the measurement component is amplified before being transmitted to an object to be measured.

16. A method according to any one of the preceding claims, further comprising: activating certain amplifiers coupled to selected measurement channels; Reading the output signals from all measuring channels; - Calculating the crosstalk from the activated measuring channels from the detected signals from the non-activated measuring channels arranged alongside in the cascade; - Correction of the measurement signals detected from the activated measurement channels.

17. Method according to one of the preceding claims, wherein some of the measuring channels are deactivated at least temporarily during operation.

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