Method for generating light pulses for a lidar system

Abstract

A method for generating light pulses of a laser radar system is described, the method having the following steps: a) generating a sequence of light pulses by a light source, in particular a laser, the sequence of light pulses comprising at least one first light pulse and at least one second light pulse of different intensity; b) emitting the sequence of light pulses by the laser radar system; c) receiving, by the laser radar system, a reflected portion of the sequence of light pulses by an object; d) analyzing the received portion of the sequence of light pulses for distance measurement and intensity measurement. Furthermore, a corresponding laser radar system, a computer program and a machine-readable storage medium are described.

Description

Technical Field

[0001] The present invention is based on a method for generating light pulses for a lidar system. Background Art

[0002] LiDAR systems or LiDAR sensors are a cornerstone on the path to highly automated driving. They are primarily used to measure distances to objects and generate a map of the environment surrounding the LiDAR system. This allows for both high spatial resolution and a large measurement range, meaning that both nearby and distant objects can be detected. The intensity of the light reflected by the object, or received and evaluated by the LiDAR system, plays a significant role. Excessively high pulse intensities can interfere with meaningful evaluation of the reflected pulses and prevent usable measured values, for example if the detector saturates. Excessively low pulse intensities can prevent the detection of distant objects. Summary of the Invention

[0003] Advantages of the present invention

[0004] A method for generating light pulses for a lidar system is disclosed, the method having the features of the independent patent claims.

[0005] Here, a light pulse sequence is generated, comprising at least one first light pulse and at least one second light pulse of different intensities. This is accomplished using a light source, in particular a laser. The pulse intensity can be defined, for example, by the peak amplitude of the pulses, and the pulse intensities can differ, for example, by a factor of 100 (in particular, 200) to 500. For example, the first and second pulses can have a time interval in the range of 1 ns to 1000 ns, in particular 500 ns.

[0006] The light pulse sequence generated in this way is emitted by means of the lidar system, and the part of the light pulse sequence reflected by the object is received by the lidar system.

[0007] The received part of the light pulse sequence is evaluated and can be used to measure the distance to the object.

[0008] This method is advantageous because it eliminates the need for complex charging circuits for the light source, thus ensuring simple implementation. While a remedial measure could be implemented using a constant current source for the laser source, this presents the problem that, in the event of a malfunction, significantly higher laser power is generated, making eye safety a significant challenge and necessitating complex safety mechanisms. This is not necessary with the disclosed method. Furthermore, the reflected light pulse train can be simply evaluated without the need for complex long filters, keeping the computational overhead within reasonable limits.

[0009] Further advantageous embodiments of the invention are the subject matter of the dependent claims.

[0010] When generating the sequence of light pulses, a first light pulse having a predefined first intensity is advantageously generated. Furthermore, after the first light pulse, a second light pulse having a predefined second intensity is generated. The predefined second intensity is higher than the predefined first intensity. This is advantageous because the lower-intensity first light pulse prevents saturation of the lidar system's detector. By emitting two pulses with differing intensities in the sequence described, the problem of a strong pulse falling into the measurement window of a weak pulse and thus no longer being distinguishable from the weak pulse is avoided, which could distort the evaluation of the distance measurement.

[0011] Advantageously, a predefined first time interval exists between each of the light pulses in a light pulse train. This is advantageous because it allows for separate analysis of the individual light pulses and minimizes computational overhead. For example, long filter banks are not required to analyze the individual pulses of a pulse train. Furthermore, a sufficiently long first time interval enables unique unambiguity in the interpretation of the received signal, since weak light pulses have only a short range. In this case, the weak light pulse arrives at the detector before the stronger light pulse is emitted. Alternatively, a shorter first time interval can be used, in which case the pulses are separated from each other during analysis.

[0012] In accordance with the object, in the analysis of the received portion of the light pulse sequence, i.e. the reflected light, the received first light pulse is analyzed and the received second light pulse is analyzed. Advantageously, this is done separately from one another, in particular sequentially, in the electronic control unit in order to minimize the computing outlay and complexity of the electronic control unit. The analysis of the light pulses is then combined in order to extend the distance measurement range of the laser radar system. This is possible because the measurement range in the near distance is covered in particular by the pulses with lower intensity and the measurement range further away is covered by the pulses with higher intensity. In the case of objects in the near distance, the light is reflected with a higher intensity than in the case of objects at a greater distance, so that saturation of the detector of the laser radar system can occur when the high-intensity pulses are reflected by objects in the near distance. As a result, this makes reliable distance measurement impossible. By emitting light pulses of low intensity, the measurement of the distance can also take place reliably in this case, so that an extended measurement range is achieved by combining the analyses.

[0013] In accordance with the object, the first light pulse has a shorter pulse duration than the second light pulse. This is advantageous because, by this, the dazzling of the detector of the laser radar system due to internal reflections of the emitted pulses is reduced or avoided. This is particularly relevant for SPAD detectors. This makes it possible to achieve reliable measurement of short distances. Furthermore, this allows the first pulse and the second pulse to be distinguished on the basis of the pulse duration measured by the detector.

[0014] In accordance with the object, in the generation of the light pulse sequence, a portion of the light required for the generation of the light pulse sequence is coupled out into the optical delay chain, in particular by means of a beam splitter. Furthermore, the coupled-out light which has been delayed by the optical delay chain is coupled into the light path of the portion of the light required for the generation of the light pulse sequence which has not been coupled out. By the coupling out, the delay and the recoupling in, it is possible to generate a light pulse sequence of at least two light pulses with different intensities. This is particularly advantageous when the light is generated by a light source or a laser source with constant pulse energy. This makes it possible to achieve a simple control circuit for the light source or the laser source by means of simply installed optical elements. Furthermore, the pulse intensity of the delayed light can be simply determined by means of the beam splitter. In this way, it is possible to predetermine in a simple manner whether the high-intensity pulse occurs first and then the low-intensity pulse or vice versa. It is also possible to predetermine the time interval between the resulting pulses simply by the configuration of the delay chain without a high outlay on electronic circuits.

[0015] The optical delay chain advantageously includes an optical waveguide and / or free-space propagation of the outcoupled light. The duration required for the outcoupled light to be coupled back in is greater than the duration required for the uncoupled light to be coupled back in. This is advantageous because it allows for simple determination of the time intervals between the pulses.

[0016] The light required to generate the optical pulse train is advantageously introduced into an optical resonator in order to generate at least two light pulses of different intensities. This is advantageous because it can be implemented simply and without complex electronic circuits. Optical resonators are particularly advantageous when multiple pulses are desired in the pulse train.

[0017] The light required to generate the optical pulse train is advantageously introduced into a saturable absorber in order to generate at least two optical pulses of different intensities. Here, a sequence of optical pulses of equal intensity is introduced into the optical absorber. A large portion of the intensity of the first pulse is absorbed by the absorber, so that the first pulse emerging from the absorber has a lower intensity than a second pulse following the first pulse, particularly if the second pulse follows the first pulse within a time period shorter than the relaxation time of the optical absorber. This results in a saturation effect in the absorber, which leads to reduced absorption when the second pulse occurs.

[0018] The saturable absorber expediently comprises a semiconductor material and / or a phosphorescent material. This is advantageous because these materials have saturable absorption properties that are suitable for achieving the aforementioned properties and for corresponding modulation of the pulse intensity of a plurality of pulses at intervals of a few nanoseconds.

[0019] The described method steps are advantageously performed multiple times and / or continuously, with a predefined second time interval between the steps of emitting the respective light pulse trains. The second time interval is greater than the first time interval; that is, the interval between the respective light pulse trains is greater than the interval between the individual pulses within a light pulse train. Multiple and / or continuous execution is advantageous because it allows for the detection of multiple objects and their distances, and for the acquisition of corrected distance data in the presence of motion. Consequently, multiple reflected pulses are available that can be analyzed, resulting in good measurement statistics and preventing or reducing "jumps" in the distance signal. Furthermore, the advantages of this method are particularly evident here, particularly with regard to potential saturation of the detector of the lidar system, as this is avoided or reduced by the described method.

[0020] The reflected light is advantageously received using a SPAD detector, a so-called single-photon avalanche diode detector. This is advantageous because SPAD detectors benefit particularly strongly from the described method due to their dead time and saturation behavior. Since SPAD technology also has problems with dynamic range, as only a small number of SPAD cells are available for measurement, the described method can be advantageously used with this type of detector.

[0021] Furthermore, the present disclosure provides a lidar system comprising at least one device configured to implement the described method. Such a lidar system includes, for example, a light source, an electronic control unit, and a detector for receiving reflected light. Furthermore, deflection mirrors or transmitting / receiving optical components may also be present. Thus, the aforementioned advantages can be achieved.

[0022] Furthermore, the subject matter of the present disclosure is a computer program comprising instructions that enable the disclosed lidar system to carry out all the steps of the described method, thereby achieving the advantages mentioned.

[0023] Furthermore, the subject matter of the present disclosure is a machine-readable storage medium on which the disclosed computer program is stored. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Advantageous embodiments of the invention are shown in the drawings and are explained in more detail in the following description.

[0025] The accompanying drawings show:

[0026] Figure 1 A flow chart showing the disclosed method according to a first embodiment;

[0027] Figure 2 A schematic diagram showing a light pulse obtained by the disclosed method according to the first embodiment;

[0028] Figure 3 shows a flow chart of the disclosed method according to a second embodiment;

[0029] Figure 4 A schematic diagram showing a light pulse obtained by the disclosed method according to a second embodiment;

[0030] Figure 5 shows a flow chart of the disclosed method according to a third embodiment;

[0031] Figure 6 A schematic diagram showing a light pulse obtained by the disclosed method according to a third embodiment;

[0032] Figure 7schematic diagram of an analyzed processed received light pulse sequence for distance measurement is shown in a histogram;

[0033] Figure 8 a schematic diagram of the analyzed processed first and second light pulses is shown;

[0034] Figure 9 a schematic diagram of a lidar system with an optical delay chain is shown; DETAILED DESCRIPTION

[0035] In all figures, same reference signs refer to same device components or same method steps.

[0036] Figure 1 A flow chart of a method for generating light pulses of a lidar system according to the disclosure is shown according to a first embodiment. Herein, in a first step S11 a sequence of light pulses is generated by a light source of the lidar system, wherein the sequence of light pulses comprises at least a first light pulse and a second light pulse. Herein, the light pulses have different intensities.

[0037] In a second step S12, which can overlap with the first step S11, the sequence of light pulses is emitted by the lidar system into its surroundings.

[0038] In a third step S13, a reflected part of the sequence of light pulses is received by the lidar system. Herein, the originally emitted sequence of light pulses can be received again in whole or in parts, possibly in attenuated fashion, depending on what the sequence of light pulses hits on and is reflected back. The reception of the light can be achieved by a respective detector of the lidar system.

[0039] In a fourth step S14, the received part of the sequence of light pulses is analyzed processed in order to derive therefrom a distance of e.g. an object.

[0040] Figure 2 A schematic diagram of the light pulses resulting from the disclosed method according to the first embodiment is shown. Herein, the intensity of the respective pulses is plotted on a vertical axis Tx and the course over time is plotted on a longitudinal axis time. The two pulses of the sequence of light pulses have a predefined first time interval T1 and the respective sequence of light pulses has a predefined second time interval T2. The predefined time interval T2 can be in the range of 1 μβ to 10 μβ, in particular 2 μβ, for example.

[0041] Figure 3A flow chart showing the disclosed method for generating light pulses of a lidar system according to the second embodiment is shown. Here, in a first step S31 a sequence of light pulses is generated by a light source of the lidar system, wherein the sequence of light pulses comprises at least one light pulse. The light source generates light pulses having the same intensity. Thus, the light source can be built more simply and more cost advantageously.

[0042] In a second step S32 a part of the light required for generating the sequence of light pulses is coupled out into an optical delay chain. For this purpose, for example a beam splitter can be used. Thereby the intensity of the resulting pulses can be determined.

[0043] In a third step S33 the coupled-out light which has been delayed by the optical delay chain is coupled in again into the light which has not been coupled out in order to generate a sequence of light pulses of at least two pulses having different intensities.

[0044] In a fourth step S34 which can overlap with the above-mentioned steps, the thus generated sequence of light pulses is emitted by the lidar system into its surroundings.

[0045] In a fifth step S35 a part of the sequence of light pulses which is reflected by an object is received by the lidar system. Here, the originally emitted sequence of light pulses can be received again in whole or in part, possibly in attenuated fashion, depending on what the sequence of light pulses hits and is reflected back. The reception of the light can be effected by a corresponding detector of the lidar system.

[0046] In a sixth step S36 the received part of the sequence of light pulses is analyzed in order to derive therefrom, for example, the distance of the object.

[0047] Figure 4 A schematic diagram of the light pulses generated by the disclosed method according to the second embodiment is shown. Here, the intensity of the respective pulses is plotted on the vertical axis intensity and the course over time on the longitudinal axis time. In the left diagram the light pulse is shown as it is generated by the light source. The light pulse has a predefined intensity. On the right the light pulse generated after having gone through the optical delay chain is shown. Here, the time intervals and the intensities shown are only schematic: the pulse in the left diagram results in the two pulses shown in the right diagram having a predefined first and second intensity.

[0048] Figure 5A flow chart showing the disclosed method for generating light pulses of a lidar system according to the third embodiment is shown. Here, in a first step S51 a sequence of light pulses is generated by a light source of the lidar system, wherein the sequence of light pulses comprises at least a first light pulse and a second light pulse. The light source generates light pulses having the same intensity. The light source can thus be built more simply and more cost advantageously.

[0049] In a second step S52 the sequence of light pulses obtained in the first step S51 is introduced into a saturable absorber. A substantial part of the first light pulse is absorbed by the saturable absorber, so that the transmitted first light pulse has a lower intensity. This also leads to a saturation effect of the absorber or absorber material, which reduces its absorption capacity for a certain time until a relaxation into the unexcited state takes place again. The second light pulse following after the first light pulse within a time period shorter than the mentioned relaxation time of the absorber is thus absorbed in a smaller part, so that the transmitted second light pulse has a higher intensity than the transmitted first light pulse.

[0050] In a third step S53, which can overlap with the first step S51, the thus generated sequence of light pulses is emitted by the lidar system into its surroundings.

[0051] In a fourth step S54 a part of the sequence of light pulses reflected by an object is received by the lidar system. Here, the originally emitted sequence of light pulses can be received again in whole or in part, possibly in attenuated fashion, depending on what the sequence of light pulses hits and is reflected back. The reception of the light can be effected by a corresponding detector of the lidar system.

[0052] In a fifth step S55 the received part of the sequence of light pulses is analyzed in order to derive therefrom, for example, the distance of the object.

[0053] Figure 6 A schematic diagram showing the light pulses generated by the disclosed method according to the second embodiment is shown. Here, the intensity of the respective pulses is plotted on the vertical axis intensity and the course over time on the longitudinal axis time. Two light pulses are shown exemplary in the left diagram as they are generated by the light source. They have a predefined intensity. The right side shows the light pulses generated after experiencing the light absorber. Here, the time intervals and the shown intensities are only schematic: the two pulses in the left diagram lead to the two pulses shown in the right diagram with a predefined first and second intensity.

[0054] Figure 7A histogram shows a schematic diagram of a sequence of received light pulses that has been evaluated for distance measurement. The histogram has different bins on the vertical axis, representing different distances of an object from the lidar system. A limit line thr is also drawn, indicating the intensity limit at which an object is considered present. The vertical axis I represents intensity. Objects represented by intensities 71, 72, and 73 are detected by the lidar system, while objects represented by intensity 74 are not detected because they are outside the maximum range of the weak pulses.

[0055] Figure 8 A schematic diagram of an analyzed sequence of reflected light pulses according to the present disclosure is shown. The respective intensities of the reflected pulses are plotted on the vertical axis I, and a characteristic number for distance is plotted on the longitudinal axis. Up to a certain distance characteristic number d1, the analyzed intensity curve for pulses with high intensity extends parallel to the vertical axis I. A kink then occurs at point 83. This means that up to this distance, reliable distance and intensity information is not available in the close range because the reflected received pulses have too high an intensity. This leads to saturation of the receiver for the reflected pulses, particularly in SPAD detectors. From distance characteristic number d1 or point 83 onward, the intensity curve extends linearly in the schematic diagram, indicated here by 81. The specific appearance of the curve is not important; the main point is that there is a linear or possibly nonlinear, single-shot relationship between distance and intensity. This relationship can then be used to inversely calculate the distance from the received pulse intensity. Therefore, reliable distance information is not available in the close range up to point 83 or distance d1.

[0056] The evaluation of the low-intensity pulses results in an intensity curve 82, wherein, in this case as well, a linear or possibly nonlinear, single-shot relationship exists between distance and intensity. This applies up to distance d1, so that the distance to the object can be reliably determined up to distance d1. If the intensity curve 81 with the high-intensity pulses is now combined with the intensity curve 82 with the low-intensity pulses, reliable distance and intensity measurements of the object can be achieved over the entire measuring range without the electrical properties of the receiver having a restrictive effect.

[0057] Figure 9A schematic diagram of a lidar system 90 with an optical delay chain 92 is shown. Delay chain 92 includes two beam splitters for coupling in and out portions of a light beam 93, thereby generating two pulses with a predefined first intensity and a predefined second intensity from a single pulse with a predefined intensity. The light beam is generated by a light source 91 and emitted into the surroundings of the lidar system via suitable means (not shown here), such as corresponding optical components.

Claims

1. A method for generating light pulses for a laser radar system (90), the method comprising the following steps: a) generating a light pulse sequence by a light source (91), the light pulse sequence comprising at least one first light pulse and at least one second light pulse of different intensities; b) transmitting the light pulse sequence by the laser radar system (90); c) receiving, by the laser radar system (90), a portion of the light pulse sequence reflected by an object; d) evaluating the received part of the light pulse sequence for distance and intensity measurement, wherein: The step of generating the sequence of light pulses includes: e) generating a first light pulse having a predefined first intensity; f) generating a second light pulse having a predefined second intensity after generating the first light pulse, wherein the predefined second intensity is higher than the predefined first intensity; and m) introducing light having the same intensity required for generating the sequence of light pulses into a saturable absorber to generate at least the first light pulse and the second light pulse, wherein the second light pulse follows the first light pulse within a time period shorter than a relaxation time of the saturable absorber.

2. The method according to claim 1, wherein There is in each case a predefined first time interval between the light pulses of the light pulse train.

3. A method according to any one of the preceding claims, wherein The analysis includes: g) analyzing a first received light pulse; h) analyzing a second received light pulse; i) combining the analysis of the light pulses to extend the distance measurement range of the laser radar system (90).

4. A method according to any one of the preceding claims, wherein The first light pulse has a shorter pulse duration than the second light pulse.

5. A method according to any one of the preceding claims, wherein The step of generating the light pulse sequence comprises: j) coupling out a portion of the light required for generating the light pulse sequence into an optical delay chain (92); k) coupling back the coupled-out light delayed by the optical delay chain (92) into the optical path of the non-coupled-out portion of the light required for generating the light pulse sequence, so as to generate the following light pulse sequence: the light pulse sequence comprises at least two light pulses of different intensities.

6. The method according to claim 5, wherein: The optical delay chain (92) comprises an optical waveguide and / or free-space propagation of the coupled-out light, wherein a duration required for the coupled-out light until the coupling-in is longer than a duration required for the uncoupled-out light until the coupling-in of the coupled-out light.

7. A method according to any one of the preceding claims, wherein The step of generating the optical pulse sequence includes: 1) introducing light required for generating the optical pulse sequence into an optical resonant cavity, so as to generate at least two optical pulses of different intensities.

8. A method according to any one of the preceding claims, wherein The saturable absorber includes a semiconductor material and / or a phosphorescent material.

9. A method according to any one of the preceding claims, wherein The method steps are performed multiple times and / or continuously, wherein a first predefined time interval exists between the light pulses of the light pulse sequence and a second predefined time interval exists between the steps of emitting the light pulse sequence, wherein the second time interval is greater than the first time interval.

10. A method according to any one of the preceding claims, wherein The reflected light is received by means of a SPAD detector.

11. A method according to any one of the preceding claims, wherein The light source (91) is a laser.

12. The method according to claim 5, wherein: A portion of the light required for generating the sequence of light pulses is coupled out into an optical delay chain (92) by means of a beam splitter.

13. A lidar system (90), comprising at least one device configured to carry out the method according to any one of the preceding claims.

14. The laser radar system (90) of claim 13, wherein: The device is an electronic control unit.

15. A computer program product comprising instructions for enabling the laser radar system (90) according to claim 13 or 14 to implement all the steps of the method according to any one of claims 1 to 12. 16 . A machine-readable storage medium having stored thereon the computer program product according to claim 15 .

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