Lidar system

Abstract

The invention relates to a LIDAR system (10) comprising a tunable laser source (100) suitable for emitting electromagnetic radiation, namely source radiation (15), which is reflected by an object (105). The LIDAR system (10) also comprises a multi-mode waveguide (112) suitable for receiving radiation (16) reflected by the object (105), and a plurality of single-mode waveguides (1151, 1152, …115n) and a coupling device (118, 132) suitable for supplying each of the modes (161, 162) of the reflected radiation (16) that have been guided through the multi-mode waveguide (112) to one of the single-mode waveguides (1151, 1152, …115n). The LIDAR system also comprises a plurality of detection devices (125) each suitable for detecting a heterodyne signal resulting from the reflected radiation (16) that has been guided through an associated single-mode waveguide (1151, 1152, …115n) heterodyning a partial beam of the source radiation (15).

Description

[0001] LIDAR SYSTEM

[0002] DESCRIPTION

[0003] LIDAR (Light Detection and Ranging) systems, particularly FMCW (Frequency-Modulated Continuous Wave) LIDAR systems, are increasingly being used in vehicles, for example for autonomous driving. For example, they are used to measure distances or speeds or to detect objects.

[0004] In general, the measurement principle is based on the coherent superposition of emitted laser light and reflected light that has been reflected by an object. In the following, FMCW LIDAR systems are described in particular. The term "LIDAR system" also refers to coherent LIDAR measurement methods with other types of modulation, such as phase modulation or random modulation. As a rule, the reflecting object is an object with a light-scattering surface. When the light is coupled into a waveguide, the random phase fronts of the individual waves, which were scattered with a random phase by the various surface elements of the surface, are coherently superposed. The result is a random intensity of the coupled-in light, depending on the extent to which the superposition is constructive or destructive.The effect of coherent superposition of random phase fronts is called speckle.

[0005] The resulting intensity distribution after coherent superposition follows a certain statistic, particularly for fully developed speckles, a negative exponential one. In other words, the probability distribution of the resulting intensity is a decreasing exponential function. This means that a very low intensity, approximately zero, has a much higher probability than a high intensity.

[0006] Conversely, to detect an object, a certain minimum signal strength, i.e., a certain minimum intensity, is required. This results in a significant probability that, even with a high mean expected signal strength, the received signal strength will not be sufficient for detection because, during coupling into the single-mode fiber, the phase fronts randomly overlap, predominantly destructively. Therefore, a detection probability close to 100% is practically impossible to achieve with a single measurement.

[0007] Accordingly, concepts are being sought that can reduce or prevent the effects of speckles.

[0008] The present invention is based on the object of providing an improved LIDAR system.

[0009] According to embodiments, the problem is solved by the subject matter of the independent patent claims. Further developments are defined in the dependent patent claims.

[0010] A LIDAR system has a tunable laser source that is suitable for emitting electromagnetic radiation, source radiation, which is reflected by an object. The LIDAR system further comprises a multimode waveguide that is suitable for receiving radiation reflected from the object, as well as a plurality of single-mode waveguides and a coupling device that is suitable for feeding the modes of the reflected radiation guided through the multimode waveguide to one of the single-mode waveguides. The LIDAR system further contains a plurality of detection devices, each of which is suitable for detecting a heterodyne signal of a partial beam of the source radiation with the reflected radiation that has been guided through an associated single-mode waveguide.

[0011] For example, the coupling device may be a mode-specific multiplexer. According to embodiments, the mode-specific multiplexer may be a photonic coupler with an inverse design.

[0012] For example, the single-mode waveguide, the coupling device and the multi-mode waveguide can be realized as a photonic integrated circuit (PIC).

[0013] The LIDAR system may further comprise an evaluation device connected to the detection devices.

[0014] For example, the source radiation can be guided to the object via the multimode waveguide. According to embodiments, the source radiation can be guided to the object via a fundamental mode of the multimode waveguide.

[0015] According to further embodiments, the source radiation can be guided separately outside the multimode waveguide to the object.

[0016] The LIDAR system may further comprise a grating coupler for coupling the radiation reflected from the object into the multimode waveguide.

[0017] According to embodiments, the LIDAR system further includes an optical element for collimating the source radiation, wherein the optical element is arranged between the multimode waveguide and the object.

[0018] For example, a structure of the multimode waveguide can be chosen such that an extension of the modes in the waveguide is at least as large as a characteristic grain size of the speckle pattern.

[0019] The accompanying drawings provide an understanding of embodiments of the invention. The drawings illustrate embodiments and, together with the description, serve to explain the same. Further embodiments and many of the intended advantages will be apparent from the following detailed description. The elements and structures shown in the drawings are not necessarily to scale. Like reference numerals refer to like or corresponding elements and structures.

[0020] Fig. 1 shows a schematic representation of a LIDAR system according to embodiments.

[0021] Figures 2A and 2B show details of a coupling device.

[0022] Figures 3A and 3B illustrate amplitudes of modes in superimposed radiation .

[0023] Fig. 4A shows an example of a coupling device.

[0024] Fig. 4B shows an example of a two-dimensional array of waveguides.

[0025] Fig. 5 shows a schematic representation of a LIDAR system according to further embodiments. In the following detailed description, reference is made to the accompanying drawings, which form a part of the disclosure, and in which specific embodiments are shown for purposes of illustration. In this context, directional terminology such as "top," "bottom," "front," "back," "over," "on," "in front of," "behind," "fore," "rear," etc., refers to the orientation of the figures just described. Since the components of the embodiments can be positioned in different orientations, the directional terminology is for the purpose of explanation only and is in no way limiting.

[0026] The description of the embodiments is not limiting, as other embodiments exist and structural or logical changes may be made without departing from the scope defined by the claims. In particular, elements of the embodiments described below may be combined with elements of other described embodiments, unless the context indicates otherwise.

[0027] The terms "lateral" and "horizontal," as used in this description, are intended to describe an orientation or alignment that is substantially parallel to a first surface of a substrate or semiconductor body. This can be, for example, the surface of a wafer or a chip (die).

[0028] The horizontal direction can, for example, lie in a plane perpendicular to a growth direction during layer growth. The term "vertical," as used in this description, is intended to describe an orientation that is substantially perpendicular to the first surface of a substrate or semiconductor body. The vertical direction can, for example, correspond to a growth direction during layer growth.

[0029] Fig. 1 shows a schematic representation of a LIDAR system 10 according to embodiments. As shown in Fig. 1, the LIDAR system 10 comprises a tunable laser source 100, which is suitable for emitting electromagnetic radiation or source radiation 15. The source radiation is irradiated onto an object 105 to be detected and reflected therefrom. The LIDAR system further comprises a multimode waveguide 112, which is suitable for receiving radiation 16 reflected from the object 105. The LIDAR system 10 further comprises a plurality of single-mode waveguides 115i, 1152, ...115 nand a coupling device 118. The coupling device 118 is suitable for feeding the reflected radiation 16 guided through the multimode waveguide 112 to one of the single-mode waveguides 1151. The reflected radiation 16 is distributed randomly between the individual modes 161, 162 (and possibly further modes). The modes of the reflected radiation 16 guided through the multimode waveguide 112 are each fed to an associated individual waveguide by the coupling device 118. The coupling device 118 determines the modes that can couple into the individual waveguides 1151. The amplitudes in the waveguides 115i result from the proportion of these modes 161, 162 in the reflected radiation 16.The LIDAR system 10 further comprises a plurality of detection devices 125, each of which is suitable for detecting a superposition signal of a partial beam 17 of the source radiation 15 with the respective reflected modes 161, 162, which has been guided through an associated single-mode waveguide 1151.

[0030] As shown in Fig. 1, the electromagnetic radiation 16 reflected by the object 105 is coupled into a multi-mode, i.e. at least two-mode, waveguide 112. For example, the structure of the waveguide 112 is selected such that the extent of the modes within the waveguide is comparable to the typical grain size or granularity of the speckle pattern (or larger). The grain size or granularity of the speckle pattern refers to a typical extent of a structure of the pattern. The exact size can depend, for example, on the wavelength of the source radiation or the design of the optical element 109. However, at the point of coupling into the waveguide 112 it is typically on the order of magnitude of the light wavelength, which roughly corresponds to the extent of the modes. The grain size can also depend on the distance from the object 105.In general, it may be sufficient in this context if the grain size of the speckle pattern matches the expansion of the modes for a critical distance from the object 105. The optical element 109 may, for example, comprise a lens or a lens arrangement for collimating the electromagnetic radiation.

[0031] The LIDAR system comprises several, i.e., at least two, individual waveguides 115i, 1152. The coupling device 118 splits the reflected radiation 16 between the various individual waveguides 115i, 1152. The splitting is not random, but mode-specific. In other words, each radiation mode of the multimode waveguide

[0032] 112 is assigned precisely to the single waveguide 1151 whose mode corresponds to the radiation mode. The tunable laser source 100 can, for example, be any laser device, such as a laser diode, that emits electromagnetic radiation in the required wavelength range. An emission wavelength of the laser source 100 can be modulated, for example by changing an impressed operating current. For example, the emission wavelength can be linearly modulated. For example, the emission wavelength can increase linearly and then decrease linearly. Furthermore, the emission wavelength can increase and decrease monotonically. The term "tunable laser source" also includes a combination of a laser device with a modulation device, such as a frequency shifter.

[0033] The electromagnetic radiation 15 or local oscillator radiation emitted by the laser source 100 is split, for example, by beam splitters 121, each splitting off a partial beam of the source radiation. The number of partial beams 17 generated corresponds, for example, to the number n of single-mode waveguides 115i f 1152 , ...115 n .

[0034] In the individual detection devices 125, each of the partial beams 17 is coherently superimposed with an associated reflected beam 16, which has been guided through an associated single-mode waveguide 1151. The partial beams that have been guided through different single-mode waveguides 1151 are separate from one another. They originate from the originally mixed partial excitations of different modes in the multi-mode waveguide 112, which are orthogonal to one another. The resulting superposition or beat signal of the separated partial beams is each detected by the detection devices 125. As shown in Fig. 1, for example, one of the single-mode waveguides 115i, which is provided for supplying the source radiation 15, is also used for returning the reflected beam 16. Accordingly, as shown in Fig.1, a circulator 123 may be provided to separate the object beam 15 from the reflected beam 16. For example, the circulator 123 may comprise a polarization filter that splits the light according to the polarization direction.

[0035] As further illustrated, for example, in order to obtain the smallest possible spot on the object 105, the object beam 15 can be coupled via the coupling device 118 into the fundamental mode of the multimode waveguide 112 and collimated via an optical element 109. The same optical element 109 receives the electromagnetic radiation 16 reflected by the object 105 in the multimode waveguide 112.

[0036] Due to the roughness of the object 105, the imaging on the coupling plane results in a random intensity and phase pattern, called speckle. As a result, the light can be coupled into all modes of the multimode waveguide with a random distribution.

[0037] When coupling into the multimode waveguide, an anticorrelation can occur between the intensities in the fundamental mode and higher modes. The intensity coupled into a mode is calculated from the integral of the product of the electric field of the incident radiation and the mode profile. For example, the coupling efficiency is given by:

[0038] , where Ei is the incident electric field of the signal and E2 is the mode profile, i.e., the electric field of the mode in the waveguide. A is the area of ​​the entrance facet of the multimode waveguide 112 over which two-dimensional integration is performed.

[0039] A very low intensity in the fundamental mode is caused, among other things, by the destructive superposition of the fields incident at different parts of the waveguide surface, as they exhibit opposing phases. In these cases, there may be an excessive probability that the superposition occurs constructively upon coupling into a higher mode.

[0040] This will be explained in more detail below with reference to Figures 3A and 3B.

[0041] Because all modes are coaxially coupled into a common aperture, the expected value of the signal strength in all modes or in all individual waveguides 115i is comparable.

[0042] Fig. 2A shows the transition from a single-mode waveguide 115i, which carries, for example, a fundamental mode, into the multimode waveguide 112 via the coupling device 118. Incoming radiation 15 is passed on unchanged to the multimode waveguide 112 and vice versa. As shown in Fig. 2B, reflected radiation 16 of the next higher mode is returned via the coupling device 118 into the lower single-mode waveguide 1152. The coupling device is designed such that each mode of the multimode waveguide is coupled to exactly one single-mode waveguide 1151.

[0043] Fig. 3A now shows, as an example, a predominantly destructive superposition of the amplitude of the radiation 15 reflected from the object with the fundamental mode 129 at the transition into the upper waveguide 115i shown in Fig. 2A.

[0044] Fig. 3B shows, as an example, a superposition of the amplitude of the reflected radiation 15 with the next higher or first waveguide mode at the transition into the lower waveguide 1152 shown in Fig. 2B. As can be seen, in this case the superposition with the next higher waveguide mode 130 is predominantly constructive. This results in a lower probability of a signal that is too low and therefore undetectable in both modes simultaneously, compared to completely random, independent couplings. This increases the probability that the signal can be detected in at least one measurement when both signals are measured.

[0045] In particular, if the mode geometry or geometry of the multimode waveguide is selected to match the characteristic grain size of the speckle pattern, the electric field of the waveguide, especially the higher modes, is favored to change on a similar scale to the field of the incident signal. The method would also work with a smaller speckle pattern period.

[0046] As shown in Fig. 1, after coupling into the multimode waveguide 112, the modes are separated by the coupling device 118 and propagated in separate single-mode waveguides 115i. According to embodiments, the light guided in the fundamental-mode waveguide 115i is branched off via the circulator 123.

[0047] The signals detected by the detection devices 125 are each digitized. For example, the beat frequency can be determined using a Fourier transformation. If the amplitude of a frequency is sufficiently far from the background noise in at least one of the measurements, it is assumed that an object has been detected at the distance corresponding to the frequency. For example, an evaluation device 127 can be connected to the detection devices 125 and evaluate the detected signals in the manner described.

[0048] In a LIDAR system, a coherent laser beam with a linearly variable frequency is emitted over time, reflected from a target object, and then coherently superimposed with a portion of the laser light (local oscillator) that was decoupled before emission. This creates a beat due to the frequency changing during the light's travel time to the target object and back. The frequency of this beat is detected. It corresponds, for example, to the frequency difference between the local oscillator and the received signal and, due to the laser's linear frequency shift over time, is proportional to the distance to the target object.

[0049] The evaluation device 127 can, for example, be connected to the tunable laser source 100, so that information about the modulation of the laser wavelength is available in the evaluation device 127. The coupling device 118 couples each mode of the multimode waveguide 112 to exactly one associated single-mode waveguide. For example, the coupling device 118 can be a multiplexer, in particular a multiplexer or photonic coupler with an inverse design (“inverse-designed multi-dimensional photonic transmitter”). Such a device is known, for example, from Yang, KY, Shirpurkar, C., White, AD et al. Multi-dimensional data transmission using inverse-designed silicon photonics and microcombs. Nat Commun 13, 7862 (2022). https: / / doi.org / 10.1038 / s41467-022-35446-4.According to further embodiments, the coupling device 118 can also be a photonic lantern, as is known, for example, from Velazquez-Benitez et al., "Scaling photonic lanterns for space division multiplexing," Scientific Reports, June 11, 2018. For example, the photonic lantern can enable a targeted assignment of individual modes to specific modes in a multimode waveguide embodied as an optical fiber.

[0050] For example, the individual waveguides, the multimode waveguide and the single-mode waveguides 115i, 1152,...115 n and the coupling device 118 can be implemented as a photonic integrated circuit (PIC). The coupling device 118 can, in particular, be a mode division multiplexer. This splits the received signal into orthogonal individual modes, for example.

[0051] According to further embodiments, the individual waveguides can also be implemented as fiber optic elements. For example, in this case, the coupling device 118 can be realized as a photonic lantern. The described design significantly reduces the probability that sufficient signal intensity for detecting the distance of the object 105 will not be achieved due to destructive interference during waveguide coupling caused by speckle.

[0052] Furthermore, by choosing the right mode geometry, in some situations the probability of non-detection (i.e. a signal that is too low in all channels) can be lower than if the same number of measurements were carried out completely independently. This is because in some cases a signal that is too low is generated due to destructive interference during waveguide coupling. If the next higher waveguide mode has its antinodes at a distance similar to the typical distance between regions of opposite phase in a speckle pattern in the fiber plane, then destructive interference in the fundamental mode means a high probability of constructive interference in the next higher mode that is asymmetric to the fundamental mode.Thus, equally destructive interference in both modes is very unlikely, and the coupling of a signal sufficient for the measurement into at least one of the two modes is very likely.

[0053] Viewed another way, the energy of the incident signal is distributed across all modes. If this energy is almost zero in the fundamental mode, the probability of a higher energy in the higher mode increases. This is particularly the case when, with good imaging, the energy can only be distributed across a few modes.

[0054] Furthermore, comparable expected signal strength values ​​can be achieved in all modes (or in single-mode waveguides after separation) due to the same coupling location. For example, all channels can be coaxially coupled into a common aperture.

[0055] Fig. 4A shows an example of a coupling device 118 suitable for distributing electromagnetic radiation supplied by the multimode waveguide 112 into precisely one of a plurality of associated single-mode waveguides 115i, 1152, 115s, 1154. As explained above, the coupling device 118 can be a photonic coupler 132 with an inverse design. For example, the coupling device 118 can have regions with different refractive indices. The refractive indices can differ discretely or continuously in certain regions.

[0056] Through targeted mode coupling, a signal is generated in each channel that is independent of the other channels, or even anti-correlated. As a result, the probability of not achieving sufficient signal strength for detection in any of the channels is minimized. According to further embodiments, the coupling device 118 can also be implemented in an alternative manner, for example, as a grating coupler or as a photonic lantern.

[0057] Various modifications of the described arrangement are conceivable.

[0058] It goes without saying that the LIDAR system can have more than two single-mode waveguides 115i. The number of branched partial beams 17 or beam splitters 121 increases accordingly. This results in even better speckle pattern statistics.

[0059] According to the described embodiments, only those modes are used that have their antinodes along a line. Accordingly, the individual single-mode waveguides 115i can be arranged one-dimensionally, for example, in a plane.

[0060] According to further embodiments, the single-mode waveguides 115i can be arranged in multiple planes to accommodate a plurality of optical modes. This is illustrated, for example, in Fig. 4B, in which the single-mode waveguides 115i are arranged in two planes. As shown in Fig. 4B, the single-mode waveguides 115i are provided, for example, in two planes arranged one above the other, i.e., along a vertical direction, for example, the z-direction.

[0061] Typically, in an arrangement comprising the multimode waveguide 112, several single-mode waveguides 115i, and the photonic coupler with inverse design 132, and implemented as a photonic circuit, higher modes can only be realized in one arrangement dimension. In this case, for example, coupling of modes of the second dimension into additional waveguides can be realized via suitably designed grating couplers 133. These additional waveguides can each be multimode and can be used as described above.

[0062] If the arrangement comprising the multimode waveguide, a plurality of single-mode waveguides and the coupling device is realized in fiber optics, the coupling device 118 can again be realized as a photonic lantern.

[0063] Instead of evaluating the signals separately, an average signal can be formed from the electrical signals after detection by the detection devices 125. For example, the average signal can be formed in an evaluation device 127 (shown in Fig. 1). The average signal can be formed both before and after the Fourier transformation. Overall, the detected frequencies are searched for in only one signal.

[0064] The electromagnetic radiation can be coupled into or out of the multimode waveguide via the waveguide facet (edge ​​coupling). Alternatively, the electromagnetic radiation can also be coupled into or out of the multimode waveguide via a grating coupler 133. This is shown, for example, in the upper part of Fig. 4B, in which the coupling of the reflected radiation 16 occurs via a grating coupler 133.

[0065] As has been described, the system, in particular the waveguides and the coupling device, can be designed, for example, as a photonic integrated circuit, for example in silicon. According to further embodiments, the material of the few- or single-mode transmit and receive waveguide 115i and of the coupling device 118 can also be silicon nitride. Due to the lower refractive index, the wavelength in silicon nitride is longer, thereby increasing the lateral extension of the modes. As a result, the adaptation of the mode geometry to the speckle geometry can be improved. In this case, the electromagnetic radiation can be coupled, for example, via an adiabatic coupler into a silicon waveguide and the silicon detector.

[0066] Fig. 5 shows a further embodiment of the LIDAR system in which the transmit and receive channels are separated such that the same waveguide is not used for coupling and receiving. More specifically, according to embodiments shown in Fig. 5, the object beam 15 can be radiated into the optical element 109 and onto the object 105 via an optical path that is different from the single-mode waveguides 115i and the multi-mode waveguide 112. For example, the object beam 15 can be supplied via a separate waveguide. The object beam 15 can be radiated onto the object 105 via the same optical element 109 via a different optical arrangement. As a result, a circulator or polarization beam splitter can be omitted.In this case, for example, the coupling device 118 can be used exclusively for coupling the reflected radiation 16 into the single-mode waveguides 115i, 1152.

[0067] Although specific embodiments have been illustrated and described herein, those skilled in the art will recognize that numerous alternative and / or equivalent embodiments may be substituted for the specific embodiments shown and described without departing from the scope of the invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, the invention is limited only by the claims and their equivalents.

[0068] LIST OF REFERENCE SYMBOLS

[0069] LIDAR system

[0070] Source radiation reflected radiation

[0071] Partial beam of the source radiation

[0072] Laser source

[0073] Object optical element multimode waveguide !, 1152 , ...115 n Single-mode waveguide

[0074] Coupling device

[0075] beam splitter

[0076] Circulator

[0077] Detection facility

[0078] Evaluation device

[0079] Fundamental mode next higher mode photonic coupler with inverse design

[0080] Grating coupler first mode of reflected radiation second mode of reflected radiation

Claims

CLAIMS 1. LIDAR system (10) comprising a tunable laser source (100) adapted to emit electromagnetic radiation, source radiation (15), which is reflected by an object (105), a multimode waveguide (112) adapted to receive radiation (16) reflected by the object (105), a plurality of single-mode waveguides (115i, 1152, ...115 n ) and a coupling device (118, 132) which is suitable for coupling the modes (161, 162) of the reflected radiation (16) guided through the multimode waveguide (112) to one of the single-mode waveguides (115i, 1152,...115 n ), a plurality of detection devices (125) each adapted to detect a beat signal of a partial beam of the source radiation (15) with the reflected radiation (16) which is guided through an associated single-mode waveguide (115i, 1152, ...115 n), wherein one of the single-mode waveguides (115i) is arranged to receive the source radiation (15) and to guide it to the object (105) via the coupling device (118, 132) and the multi-mode waveguide (112).

2. LIDAR system (10) according to claim 1, wherein the coupling device (118) is a mode-specific multiplexer (132).

3. LIDAR system (10) according to claim 2, wherein the mode-specific multiplexer (132) is a photonic coupler with an inverse design.

4. LIDAR system (10) according to one of the preceding claims, wherein the single-mode waveguides (115i, 1152, ...115 n ) , the Coupling device (118, 132) and the multi-mode Waveguides (112) are realized as photonic integrated circuits.

5. LIDAR system (10) according to one of the preceding claims, further comprising an evaluation device (127) connected to the detection devices (125).

6. LIDAR system (10) according to one of the preceding claims, wherein the source radiation (105) is guided to the object (105) via a fundamental mode of the multimode waveguide (112).

7. LIDAR system (10) according to one of the preceding claims, further comprising a grating coupler (133) for coupling the radiation (16) reflected by the object (105) into the multi-mode waveguide (112).

8. LIDAR system (10) according to one of the preceding claims, further comprising an optical element (109) for collimating the source radiation (15), wherein the optical element (109) is arranged between the multimode waveguide (112) and the object (105).

9. LIDAR system (10) according to one of the preceding claims, wherein a structure of the multimode waveguide (112) is selected such that an extension of the modes in the multimode waveguide (112) is at least as large as a characteristic grain size of a speckle pattern.

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