Lidar system, use and method

Abstract

The invention relates to an arrangement for optical distance and / or speed measurement with a mode multiplexer in the coaxial transmission and reception path, wherein a different mode is used for transmission (i.e. the light emission or the transmitter) than for the reception path. No circulator is required in the transmission path as a result. The mode multiplexer thus takes over the function of branching the reception channel from the transmission channel and can be integrated as a PIC.

Description

[0001] LIDAR SYSTEM, USE AND PROCESS

[0002] This application claims priority from German application DE 10 2023 132 307 . 5 of November 20, 2023, the disclosure of which is hereby incorporated by reference in its entirety. The invention relates to an arrangement for optical distance and / or speed measurement. The invention further relates to the use of a mode multiplexer implemented as a photonic integrated circuit in a LIDAR system, as well as to a method.

[0003] BACKGROUND

[0004] LIDAR systems are systems for optical distance and / or speed measurement. Laser systems are primarily used to illuminate an object and reflect the light. The reflected light is detected and evaluated. The travel time between the transmitted and received signal can be used to determine the distance—and, with appropriate modulation, the relative speed to the object.

[0005] Various laser systems can be used for this purpose. One possible principle is based on a frequency-modulated continuous laser beam, also known as an FMCW system. In an FMCW lidar distance measuring system, a coherent laser beam with a linearly variable frequency is emitted, reflected by a target object and then coupled back into a waveguide. Coherent superposition with a portion of the laser light that was decoupled before emission, which corresponds to mixing with a local oscillator signal, creates a beat due to the frequency changing during the light's travel time to the target object and back. Such a beat frequency is usually in the range of a few 10s to 100s of MHz up to possibly even a few GHz, i.e. in a frequency range that can be precisely digitized and processed by electronics.

[0006] The frequency of this beat is detected accordingly. It corresponds 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.

[0007] To detect the light reflected from the target object and recoupled into the waveguide, while simultaneously preventing this feedback from causing instabilities in the laser emission, the beam path must be designed so that the light is not redirected back to the laser, but instead to the detector. For this purpose, an optical circulator is used in fiber optics, which typically contains a Faraday rotator and birefringent materials.

[0008] In order to miniaturize FMCW systems and reduce costs, such a solution is rather hindering, since the necessary elements cannot be miniaturized without considerable effort.

[0009] Accordingly, there is a need for another solution, in particular without an optical circulator, in which a reflected signal can be received separately from a transmitted signal.

[0010] SUMMARY OF THE INVENTION

[0011] 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] Various solutions have been presented in the prior art. One proposal is the use of a simple beam splitter instead of a circulator. However, this results in half of the received signal being lost, and half of the generated laser radiation being diverted unused. In addition, multiple modes of the received signal can be detected using only a single detector. Instead, the frequency of the local oscillator is shifted differently for each mode so that the signal is detected simultaneously at corresponding frequency intervals. However, this requires free-space optics for coupling. If not all modes contain a detectable signal, the assignment of the signals to the modes, and thus the distance measurement, becomes ambiguous.

[0013] Another solution known to the applicant proposes separate transmitting and receiving apertures, including optics, so that the received light is coupled into a separate waveguide rather than the emitting one. This solution leads to a parallax problem, because the area of ​​the target object illuminated by the transmitter is not identical to the area imaged by the receiver optics, at least not for all distances. Thus, at least for some distances, particularly at close range, there is no overlap between the illuminated and detected object surfaces, meaning that no signal is received and measurement is not possible.

[0014] W02020 / 081188 A1 describes, among other things, the use of a polarization-dependent beam splitter for the functionality of the circulator. Compared to a simple beam splitter, the linear polarization of the laser source is used here to avoid the loss of half of the laser radiation before the emission optics. However, due to strong depolarization when the light is reflected by a scattering target object, it is inevitable that a large portion, usually up to half, of the recoupled radiation remains unused because, due to incorrect polarization, it travels through the beam splitter towards the laser instead of being guided to the detector.

[0015] The inventors now propose a solution for using a mode multiplexer in a LIDAR system to separate the transmit and receive channels in a coaxial FMCW arrangement. This eliminates the need for a circulator and simultaneously achieves a very small size, since the mode multiplexer can be implemented within a photonic integrated circuit (PIC).

[0016] Accordingly, an FMCW lidar system, also implemented in PIC technology, is proposed with a mode division multiplexer in the coaxial transmit and receive channel, whereby a different mode is used for transmitting (i.e., light emission or the transmitter) than for receiving. The mode division multiplexer thus assumes the function of branching the receive channel from the transmit channel. Any portion that may flow back into the laser is quite small due to the mode division multiplexer and can be further reduced through various measures.

[0017] The arrangement allows the use of several modes in the mode multiplexer. These can be detected separately in order to achieve an improvement in the speckle statistics. The term speckle statistics refers to the frequency distribution that results from a signal intensity due to random phase superposition of coherent light scattered by a diffuse object, called speckle. This frequency distribution usually corresponds to an exponentially decreasing frequency and has the consequence that with a comparatively high probability a very small signal is coupled in, which can not be detected or can only be detected with difficulty. The proposed measures make the system more robust than conventional solutions.With additional measures, such as a phase shifter, an active variation of the relative phase position is achieved. This allows for an adjustment of the phase position to one another when multiple modes are optically combined, thus optimizing the signal strength. Overall, the speckle statistics are changed by averaging, which reduces the probability of no signal (which is maximum in negative exponential statistics). The distribution becomes more similar to a Gaussian distribution around a mean intensity value.

[0018] The use of the mode multiplexer according to the proposed principle leads to several advantages over conventional solutions. In addition to improving speckle statistics and avoiding the use of complex optical circulators, the proposed principle is largely independent of the distance to the object being measured. Furthermore, both polarization directions can be measured and evaluated simultaneously. Various aspects of the principle presented here can be combined, allowing FMCW LIDAR systems to be optimized for various applications.

[0019] In some aspects, the inventors propose an arrangement for optical distance and / or speed measurement. The arrangement, referred to for the sake of simplicity as a LIDAR arrangement or LIDAR system, comprises a 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 limited to this. In some aspects, the laser arrangement is implemented with a quantum dot laser. This has the advantage that it is relatively robust against light reflected back into the laser arrangement.

[0020] The laser arrangement is tunable in terms of the output frequency of the laser light, i.e., the arrangement is designed so that a frequency modulation can be imposed on the laser light. This frequency modulation can be effected either in the laser itself or by a downstream arrangement that, together with the laser, forms the laser arrangement.

[0021] In some aspects, the laser arrangement is also designed to amplitude modulate the emitted laser light. This can also be achieved, for example, by modulating the current through the laser or by a downstream optical modulator.

[0022] In the proposed LIDAR system, a first splitter having a first and a second output is arranged in the signal path of the laser arrangement. The splitter is designed to couple at least a portion of the laser light emitted by the tunable laser arrangement to an optical mixer coupled to the second output. The intensity of the portion of the laser light guided to the mixer, i.e. the intensity of the laser light at the second output, is quite low and amounts, for example, to less than 10%, in particular less than 5%, and in particular less than 2% of the laser light emitted by the laser arrangement.

[0023] Furthermore, the LIDAR system according to the proposed principle includes a mode multiplexer implemented as a photonic integrated circuit with a first monomode port and at least one second, predominantly monomode port. Furthermore, the mode multiplexer implemented as a photonic integrated circuit comprises a multimode output waveguide. The first monomode port is connected to the first output of the first splitter, in particular without an interposed circulator.

[0024] The mode multiplexer implemented as a photonic integrated circuit is designed to split a multimode laser light reflected from an object and coupled into the output waveguide into at least two monomode light components, and to provide at least one of the at least two light components at the second, in particular monomode, port which is coupled to the optical mixer.

[0025] The proposed LIDAR system does not require an optical circulator, but achieves that a different mode is used for transmission than for reception. Accordingly, in some aspects, it is proposed to use a mode division multiplexer implemented as a photonic integrated circuit with at least two monomode ports and one multimode port in a LIDAR system, wherein a different mode is used for a transmission channel between one of the at least two monomode ports and the multimode port than for a reception channel for light reflected from an object between the multimode port and the at least one other monomode port.

[0026] In some further aspects, the laser arrangement is designed to emit a single-mode laser light, in particular in its fundamental mode. Alternatively or additionally, the mode multiplexer can be designed to couple a signal applied to the connection, in particular in a fundamental mode, into the multi-mode output waveguide. In such a case, it would be ensured that only one mode of the laser light is radiated onto the object. A change in modes, for example due to a surface condition, is thus caused by the target object, from which further information can be obtained if necessary.

[0027] In some aspects, the first port of the mode multiplexer and the multimode port are bidirectional, i.e., a portion of a reflected light flowing back into the multimode port is emitted in the fundamental mode at the first port. Accordingly, the mode multiplexer implemented as a photonic integrated circuit can be designed to provide another of the at least two monomode light components at the first monomode port, in particular in the fundamental mode. Likewise, in some aspects, the mode multiplexer is designed to provide a light component of the coupled-in multimode laser light that is different from the light component in the fundamental mode at the second port.

[0028] In this context, in some aspects it is provided that the first and at least one second monomode connection are designed with a monomode waveguide, and the multimode connection is designed as a multimode waveguide.

[0029] In some aspects, the proposed arrangement comprises an optical isolator between an output of the laser arrangement and the first single-mode port. This prevents unwanted backflow of reflected light at the first port into the laser arrangement. The optical isolator can be arranged before or after the splitter. In the former case, it also prevents a portion of the light from being reflected back into the laser arrangement by the splitter.

[0030] In a further aspect, the mode multiplexer implemented as a photonic integrated circuit is designed to provide the at least one portion at the second port in a polarization-dependent manner. In other words, the mode multiplexer implemented as a photonic integrated circuit is designed to provide the at least one portion at the second port in a defined linear polarization. In a further aspect, the mode multiplexer is designed to provide, at a third port, in particular monomode back-reflected light with a polarization rotated by 90°.

[0031] A further aspect deals with the processing of different portions of back-reflected signals, be it in different modes or in different polarization directions. In one aspect, the arrangement according to the proposed principle comprises a further splitter which is connected between the second output of the first splitter and the optical mixer. The second splitter is designed to guide at least a portion of the laser light tapped at the second output to a second optical mixer coupled to a second output of the second splitter. In this way, the branched laser light is distributed again and fed to two mixers as a local oscillator signal for the beat.The two splitters can also be realized as a single element in which an incident laser light is divided into three parts, two of which (namely those used as local oscillator signals) should be approximately equal in size.

[0032] In a further aspect, the LIDAR system additionally comprises a polarization-changing element. The element thus rotates the polarization, in particular by 90°. The polarization-changing element is arranged between a first output of the second splitter and the first mixer. The signal for which the polarization is changed is flexible. In this respect, in some aspects, an element that changes the polarization, in particular by 90°, can also be arranged between a second output of the second splitter and the second mixer.

[0033] A further possibility in some aspects consists in connecting a first polarization-dependent beam splitter on the input side to the second connection and on the output side to the first and second mixers. The first polarization-dependent beam splitter is designed to split light applied on the input side into two, in particular orthogonal, polarization directions. With the first polarization-dependent beam splitter, the reflected light, which has both polarization directions, can be split by the mode multiplexer into the respective polarization directions and thus fed separately to a mixer each. This may allow further information to be obtained from the object reflecting the light.

[0034] In an alternative embodiment, the arrangement comprises a first polarization-dependent beam splitter between the first output of the first splitter and the first connection. Furthermore, the first polarization-dependent beam splitter is designed to output a light component coming from the first connection in a polarization-dependent manner to a further output connected to the second mixer.

[0035] In some other aspects, the mode multiplexer implemented as a photonic integrated circuit can also be designed such that the at least two monomode light components have a polarization direction that is in particular orthogonal to one another. In these embodiments, a first light component is provided by the mode multiplexer at the second port. A second light component is provided by the mode multiplexer at a third port, which is connected to the second mixer. In this respect, the mode multiplexer therefore comprises further ports, at which light components of the reflected light are provided.

[0036] In some aspects, the mode multiplexer implemented as a photonic integrated circuit is designed to perform a polarization-dependent splitting of the multimode laser light coupled into the output waveguide into at least two monomode light components. The splitting therefore occurs such that the light components have different, in particular orthogonal, polarizations.

[0037] In another aspect, the mode multiplexer implemented as a photonic integrated circuit is configured to provide one of the at least two light components at the second port and to provide a second of the at least two light components at a third, in particular monomode, port, which is optionally coupled to the second mixer. In this context, the different light components can have different modes. This results in a separation and decomposition of the backscattered light components.

[0038] Depending on the design, it is possible to recombine the light components emitted by the mode multiplexer and feed them to a mixer. In some aspects, a coupler can be provided for this purpose, for example in the form of a Y-coupler or a multimode interference coupler. The coupler can be designed as a 2:1 or n:1 coupler, where n is a natural number and represents the number of inputs that are applied to the output. According to these aspects, the coupler is connected to the second and third terminals on the input side and to the first mixer on the output side.

[0039] Another possibility is to place a particularly adjustable phase shifter between one of the second and third terminals and the coupler. This allows for an improvement in the detected signal by selectively adjusting the phase.

[0040] In one embodiment, the arrangement according to the proposed principle comprises a second polarization-dependent beam splitter which is connected on the input side to the second connection. Likewise provided is an element which changes the polarization and which is connected on the input side to a first output of the second polarization-dependent beam splitter. In this context, the element can change the polarization by 90°. Finally, a coupler is provided, in particular a Y-coupler or a multimode interference coupler. The coupler is connected with a first input to a second output of the second polarization-dependent beam splitter and with a second input to the element which changes the polarization. On the output side, the coupler is connected to the first mixer.

[0041] Another aspect relates to a method for detecting a distance and / or speed of an object by means of an optical measurement. In this case, the transmission path for the emitted laser light is separated from the reception path, with different modes being used for the two paths. For this purpose, a mode multiplexer is used which has a connection in particular for coupling in the laser light in the fundamental mode and outputs this to a multi-mode output connection. This also receives the multi-mode laser light reflected from the object and forwards a mode which is different from the fundamental mode to a mono-mode connection. This forwarded signal component is then mixed with a local oscillator signal and a beat frequency is determined from this. The distance and / or speed of the object can be determined from the beat frequency.

[0042] BRIEF DESCRIPTION OF THE DRAWINGS

[0043] 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.

[0044] Figure 1 shows a first embodiment of an arrangement for optical distance and / or speed measurement according to the proposed principle;

[0045] Figures 2A and 2B show two possible signal paths for a mode multiplexer;

[0046] Figures 3A and 3B are illustrations of possible multi-mode light distributions to illustrate some aspects of the proposed principle;

[0047] Figure 4 shows a second embodiment of a LIDAR system with some aspects of the proposed principle;

[0048] Figure 5 shows a third embodiment of a LIDAR system with some aspects of the proposed principle;

[0049] Figure 6 illustrates a fourth embodiment of a LIDAR system incorporating some aspects of the proposed principle; Figure 7 illustrates a fifth embodiment of a LIDAR system incorporating some aspects of the proposed principle;

[0050] Figure 8 is a diagram showing a frequency distribution of signal intensity due to random phase superpositions of coherent light scattered by a diffuse object;

[0051] Figure 9 shows the probability distribution of the amplitude of a signal in the form of a Rayleigh distribution with the frequency distribution shown above;

[0052] Figure 10 shows another embodiment of a LIDAR system incorporating some aspects of the proposed principle;

[0053] Figure 11 is a seventh embodiment of an arrangement for optical distance and / or speed measurement according to the proposed principle;

[0054] Figure 12 shows an eighth embodiment of a LIDAR system incorporating some aspects of the proposed principle;

[0055] Figures 13A and 13B are diagrams of a frequency distribution and of the intensities, respectively, when two signals of random amplitude are superimposed in phase to illustrate some aspects of the proposed principle;

[0056] Figures 14A and 14B show two diagrams of a frequency distribution and of the intensities in case of in-phase superposition of further modes to explain some aspects of the proposed principle;

[0057] Figure 15 is a diagram of the expected total intensity for in-phase mixed single modes with independent amplitudes. DETAILED DESCRIPTION

[0058] 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.

[0059] 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.

[0060] In LIDAR systems, a signal modulated in frequency and / or amplitude is emitted and partially reflected by an object. The reflected part is received. Due to the modulation, this now has a different frequency or amplitude than the laser light that is still emitted. This can be used to obtain information about the distance and direction of movement of the object. Due to a certain surface roughness and generally due to the shape of the object, there is a superposition of different wavefronts reflected from the object. This appears as grainy noise in the reflected portion and is generally referred to as speckle. Speckle is not external noise, but rather an inherent fluctuation in diffuse reflections, since the scatterers are not identical for every cell and the coherent illumination wave is very sensitive to small fluctuations in the phase change.

[0061] Especially in LIDAR applications, useful information can be generated from the distribution of such speckles, for example, where changes in the spatial speckle pattern over time can be used as a measure of surface activity. However, it is advisable to adhere to certain requirements both for the transmission of signals and for the reflected and received signals, so that the existing speckle generation can be used positively and leads to an improvement in signal quality.

[0062] Reception of a reflected signal in a multimode waveguide is described in the prior art only in multimode fibers or in free-space optics. The inventors are not aware of a solution that allows separate detection of different (orthogonal) modes of the multimode waveguide using multiple detection units. Instead, solutions exist that either follow a random distribution on single-mode fibers without speckle-typical statistics (i.e., with a random mix of all modes), or detect differently delayed modes in the local oscillator using a single detector.

[0063] A coaxial arrangement, i.e., the use of the same waveguide as both transmitter and receiver, is known from the state of the art only by using a circulator or a polarization-dependent beam splitter. However, the former is quite large and thus counteracts the desire for downsizing, while the latter results in at least half the radiation being lost.

[0064] As an improvement, the inventors now propose the use of a mode multiplexer for the function of separating the transmit and receive channels in a coaxial FMCW lidar arrangement. This replaces the function of the circulator. Thus, the use of the mode multiplexer solves two problems. Firstly, the use of optical circulators is avoided, and secondly, the speckle statistics are improved, which in turn improves the analysis of the reflected signals.

[0065] As shown in Figure 1, the inventors propose an FMCW lidar system (preferably implemented in PIC) with a mode division multiplexer in the coaxial transmit and receive channels, whereby a different mode is used for transmission (i.e., light emission or the transmitter) than for the receive path. This eliminates the need for a circulator; rather, the mode division multiplexer performs the function of branching the receive channel from the transmit channel.

[0066] Fig. 1 shows the basic structure of a LIDAR system according to the proposed principle. The LIDAR system 1 comprises a laser arrangement 10 as the light source, which in this case is designed as a linear frequency-modulated and narrowband laser. For frequency modulation, the laser can either be used directly, for example by controlling the laser accordingly. However, a further element can also be added which tunes the frequency of the laser light in a defined frequency range. The frequency of the laser is changed up to a few GHz. This is the change in the fundamental frequency of the laser due to the external modulation. The repetition rate with which the modulation takes place is very low in both cases and is in the range of a few kHz or even a few MHz. Ideally, the laser also only emits light in one mode, for example in the fundamental mode.

[0067] The laser arrangement 10 is connected to a beam splitter 11. The beam splitter 11 splits off a very small part, e.g. less than 10% of the laser light or less than 2% or less than 1% of the laser light, and passes this on to an output of a waveguide 13. The other output of the beam splitter 11 is connected to a waveguide 12. The majority of the laser light thus reaches the waveguide 12, the smaller part serves as a local oscillator signal for a mixer and differential detector 16 connected to the waveguide 13. The waveguide 12 continues to a mode division multiplexer 14. The mode division multiplexer has a monomode waveguide connection 142 which is connected to the waveguide 12. Accordingly, the light from the laser becomes monomode, e.g. coupled in the fundamental mode.

[0068] In order to obtain the smallest possible spot on the target object, the signal beam is coupled at connection 142 by the mode multiplexer 14 into the fundamental mode of a multimode output waveguide 141 and then collimated via a downstream optics 15. Further elements, such as MEMS mirrors or other optical assemblies, can be present downstream of the optics to direct the emitted laser light onto an object. The laser light is at least partially reflected by the object and in turn reaches the multimode waveguide 141 via the optics 15 as a reflected portion L.

[0069] The intensity of the received reflected light component L is significantly lower than the emitted component and, moreover, is no longer confined to a single mode. Rather, due to the roughness of the target object, a random intensity and phase pattern, called speckle, occurs during imaging on the coupling plane in every polarization direction. As a result, the light is coupled with a random distribution into all modes of the multimode waveguide 141.

[0070] This aspect is shown in Figures 3A and 3B, which show a section through the coupling plane of the multimode waveguide 141. The figure shows a fundamental mode of the reflected portion as a solid line. Figure 3B, on the other hand, shows a higher mode than the fundamental mode, which results from the phase superposition caused by the roughness of the object. In addition to these two modes, depending on the object and the actual situation, several other modes may overlap. The energy of the reflected laser light is therefore not located in a single mode, but in a multitude of such modes.

[0071] Referring back to Figure 1, the signal reflected from the target object is received in the multimode waveguide with the same optics. Figures 2A and 2B show the splitting of the reflected light into the individual modes by the mode multiplexer 14. The fundamental mode, shown in Figure 2A, is emitted from port 141 to the multimode waveguide 131. At the same time, the fundamental mode of the reflected portion is coupled back to port 142. The light coupled back to the fundamental mode at port 142 is not used in the present embodiment because a circulator would be necessary for this. The light coupled into higher modes is coupled in the mode multiplexer 14 into a second monomode port 143. This is indicated in Figure 2B by a single higher mode. The direction of the arrow indicates the signal flow. Port 143 is also monomode.The single-mode waveguide port is connected to the other input of the mixer 16, see Figure 1.

[0072] It is mixed with the local oscillator signal from splitter 11, and a difference signal is generated as a result of the resulting beat. The frequency of the beat signal results from the difference between the two frequencies of the frequency-modulated oscillator signal and the reflected signal.

[0073] The mode multiplexer 14 thus serves to separate the light paths between the illumination path with the waveguide 12 and the reception path, i.e., it performs the task of directing a significant portion of the received light to the mixer and differential detector 16, rather than back to the laser array. The very small portion of light reflected back into the fundamental mode has, in some embodiments, only a very minor effect on the laser array. Laser arrays implemented with quantum dot lasers, for example, are barely affected by reflected light.

[0074] With a dual-mode multiplexer, as shown here, and a random speckle pattern, it can be expected that, on average, about half of the received light of the detectable polarization direction is guided to the detector. This corresponds to approximately the same efficiency that can be achieved with a polarization-dependent beam splitter, provided the polarization is completely randomly redistributed by the scattering target object. In contrast to the use of a polarization-dependent beam splitter, however, both polarization directions are received in the design shown in Figure 1.

[0075] This can be advantageously exploited with a suitable design of the LIDAR system, allowing the polarization directions to be received and processed separately from one another, yet simultaneously. Figure 4 shows a corresponding possibility for this. Identical components bear the same reference numerals. A further explanation will be omitted unless there are significant changes compared to the previous design.

[0076] Figure 4 shows the LIDAR system based on the proposed principle for detecting both polarization directions. The single-mode waveguide 143 in the receive channel of the mode multiplexer 14 contains both polarization directions of the reflected signal coupled into the multimode waveguide 141.

[0077] An input of a polarization-dependent beam splitter 18 is now connected to the connection of the monomode waveguide 143. A first output leads to the mixer 16 as in the previous exemplary embodiment. The other output of the polarization-dependent beam splitter 18 is connected to a second mixer and differential detector 16'. The local oscillator signal of the intermediate mixer 16' is provided by a second splitter 11', which is connected on the input side to the second output of the splitter 11 and, in turn, with its second output to the first mixer 16.

[0078] The combination of the two mixers 11 and 11' thus splits the signal provided by the laser array into two local oscillator signals. The splitter 11' has a different splitting ratio than the first mixer, for example 50:50, while the mixer 11 has a splitting ratio of 99:1 (with 99 components at the output for the waveguide 12). In another embodiment, the splitters 11 and 11' can also be replaced by a single splitter that splits the input light into three components, with two of the three components (as local oscillator signals) having the same intensity, for example a splitting of 99:0.5:0.5.

[0079] In addition, an element 17 is provided between the second splitter and the first mixer, which rotates the polarization by 90°. In the present embodiment, the element is provided between the second splitter and the first mixer, but it can also be arranged between the second splitter and the second mixer. In other words, the polarization for one of the two oscillator signals is rotated by 90°. In another embodiment, a rotation of the polarization can also be effected in splitter 1 or 11'.

[0080] The polarization-dependent beam splitter 18 splits the signal between the two waveguides, with the two signals having orthogonal polarization to each other. The local oscillator signal from the laser array 10 is also split between two waveguides, with one of the two local oscillator channels still rotated by 90° in polarization, as shown in Figure 4. The rotation occurs in such a way that the two local oscillator signals are superimposed in the mixers 16 and 16' with the correct polarization, i.e., with the same polarization. By detecting both polarization directions, additional information about the target object, in particular about the degree of polarization mixing, can be obtained.

[0081] Furthermore, the signal strengths with different polarization directions are largely independent with respect to speckle statistics. This results in two independent measurements, which significantly reduces the probability of failing to detect a target object due to randomly destructive interference in the speckle pattern.

[0082] The same functionality can be achieved if the polarization rotator 17' is installed in the second signal path, as shown in Figure 5. In addition, in Figure 5, an optical isolator 12' is also installed in the waveguide 12, which prevents the light reflected in the fundamental mode from flowing back into the waveguide 12 and the laser arrangement 10.

[0083] A further embodiment is shown in Figure 6. In this embodiment, the light coupled into the waveguide 12 by the mode multiplexer 14 as the fundamental mode is coupled out of the waveguide 12 by the polarization-dependent beam splitter 18' and guided to the mixer 16'. This also allows the fundamental mode to be detected in this polarization direction.

[0084] Figure 7 shows a further variation of the proposed principle. In this embodiment, the mode multiplexer 14' is designed to separate into three different modes, two of which are used for two independent receive channels. This also enables two independent measurements, since the amplitude of the reflected signal in the two modes can be regarded as independent of one another. This embodiment is also useful when, due to certain parameters on the object, the intensities for the reflected signal are not distributed evenly into the individual modes, but rather some modes are preferred. In addition, the different polarization directions can also be determined separately for individual modes, which provides further information about the object.

[0085] These elements, i.e., the mode multiplexer, the polarization rotator, and the polarization-dependent beam splitter, can be implemented using PIC technology, i.e., as photonic integrated active or passive circuits, allowing for a very small and robust design. The following figures and aspects deal with speckle intensity and a suitable detection of its distribution using an arrangement based on the proposed principle.

[0086] Figure 8 shows the frequency distribution of the signal intensity due to random phase superpositions of coherent light which is diffusely scattered by an object. This type of light is referred to as speckle. It is an exponentially decreasing frequency of intensities. Higher intensities occur with a lower frequency than lower ones, with the difference resulting from a decreasing exponential function. Accordingly, this distribution has the consequence that with a comparatively high probability mainly small signals, possibly lying below the detection threshold, are coupled into a monomode receiving waveguide.

[0087] Since different, mutually orthogonal modes in a multimode waveguide exhibit an independent mode overlap with the speckle pattern, as shown, for example, as a dashed line in Figures 3A and 3B, the independent detection of several such modes enables various independent measurements to be carried out in parallel, thereby significantly reducing the probability of not detecting a sufficient signal in any of the measurements. The principle underlying this application, namely the use of a mode multiplexer to separate the transmitted and received signals, is particularly useful here, as this can provide the individual modes separately.

[0088] The mode multiplexer can combine individual modes in a suitable manner. However, for a parallel measurement, several detectors must be read out in parallel. One possible solution would be to combine several single-mode waveguides into a single-mode waveguide, e.g., using a so-called Y-coupler (see explanations in Figure 10 below) or using a multimode interference coupler. However, if the amplitude and phase of the combined modes are statistically independent of each other, this does not necessarily lead to a significant change in the statistics or the expected value of the intensity. This observation results from the probability distribution of the intensity of a speckle-affected

[0089] Signal with the decaying exponential distribution shown in Figure 6.

[0090] The probability distribution P of the intensity I is: With o as the expected value . The intensity I is proportional to the square of the amplitude E :

[0091] This results in the Rayleigh distribution for the probability distribution of the amplitude of a speckle-dominated signal

[0092] Such a distribution is shown in the diagram in Figure 9. The x-axis shows the amplitude and the y-axis the frequency or probability density.

[0093] If two or more modes with a statistical amplitude distribution similar to Figure 9 and a mutually independent, random phase are coupled together, the frequency distribution reproduces itself except for a statistical but otherwise constant expected value of the amplitude. This results in an intensity distribution which is again similar to the intensity distribution in Figure 9 for a single mode. It follows from this that in general there is no major change in the signal statistics if another optical mode is optically coupled to a received optical mode, whereby the two modes are independent of each other in amplitude and phase. Thus there is no advantage to be achieved by simply combining the modes compared to detecting a single mode. In fact, losses can be expected with realistic coupling.

[0094] Such a simple combination can be seen in Figure 10. In this embodiment, the mode multiplexer is designed with several mono-mode connections, with two or more connections being combined again via a coupler 20.

[0095] The fundamental mode in port 142 is used only for transmitting and emitting the laser light L, while the two higher modes are used for reception at ports 143, 144. Each of these modes is coupled into a single-mode waveguide by the multiplexer 14'. The two modes are combined in the coupler 20 and then fed to the mixer 16. The coupler 20 is a Y-coupler in the present case, but may be another coupler known from the prior art, e.g., a multimode interference coupler (MMI).

[0096] It is also conceivable to optimize a coupler using the inverse design method or to design the mode multiplexer so that the two modes are combined there. In the latter case, the two individual waveguides and the separate coupler in the receive path are omitted. A possible extension of this design would be, for example, the use of a 5-mode waveguide, the distribution of the coupled signal across 4 used receive waveguides, and the combination of these four modes via an Ix4 coupler or a cascade of Ix2 couplers.

[0097] An advantage of this arrangement arises, however, when the two mixed modes are not statistically independent of one another. Such a statistical dependence or correlation can be due to the fact that both modes were excited from the same speckle pattern, i.e. the same optical field. Field distributions are conceivable which can lead to a correlation of the intensities and / or phase positions of the two modes. This gives rise to two general possibilities: that anti-phase excitations become significantly more likely than in-phase excitations, or vice versa. While the first case has a primarily destructive effect, the second case, i.e. that anti-phase excitations are less likely than in-phase excitations, is advantageous. In this way, a higher signal strength is achieved through the mixing (optical combination) of two modes.

[0098] This case occurs when the typical dimensions of the speckle pattern, which correspond to the length scale of the phase variation, are larger than the spatial extent of the multimode waveguide into which the light is coupled. In this case, all modes are essentially excited by a single speckle maximum and thus acquire a similar phase position, as a result of which they interfere constructively during mixing. In this case, when two modes from a three-mode waveguide are mixed, with statistically equal distribution of the amplitudes across all three modes, the theoretical detection efficiency is up to 2 / 3 = 67%, and for a 5-mode waveguide up to 4 / 5 = 80%, since only one of the modes is not used for detection. Such an arrangement can therefore have a higher detection probability than the 50% that could be achieved with a polarization-dependent beam splitter.

[0099] The above requirement for such a case that the typical dimensions of the speckle pattern, which correspond to the length scale of the phase variation, must be larger than the spatial extent of the multimode waveguide, arises especially for distant target objects, since larger speckle patterns often result here.

[0100] A similar advantage can arise if the amplitudes of the received modes are anticorrelated, i.e. if, for a low amplitude in the first waveguide, the probability of a high amplitude in the second waveguide is increased compared to a purely statistical distribution (and vice versa). This would reduce the probability of almost identical amplitudes in both modes to be mixed compared to a statistical distribution. Since this is necessary for almost complete cancellation in antiphase situations, it reduces the probability of very small amplitudes after mixing. Such a situation can often arise with targets at close range, where small speckle patterns can lead to an anticorrelation of the amplitudes between different modes.

[0101] The inventors take advantage of this property to implement various alternatives to the proposed principle, for example, which improve signal statistics. Figure 11 shows a possible implementation of a LIDAR system based on the proposed principle.

[0102] Instead of combining two different modes of the multimode waveguide, it is also possible to combine two differently polarized modes. This is possible, for example, as shown in Figure 11, in that the two polarization directions in the reflected signal from the mode multiplexer at connection 143 are first separated in the beam path by a polarization-dependent beam splitter 21. A polarization rotator 17 is now arranged in one of the signal paths, which rotates signals in one signal path by 90° and thus brings them to identical linear polarization to the other signal path. A coupler is then provided which combines the two signals with the same polarization and feeds them to the mixer 16. This variant can be combined with the ones above, for example to allow two modes of the multimode waveguide to each use both polarization directions, in order to combine a total of four modes.

[0103] When implemented in PIC, combining independent modes with speckle statistics requires no effort for the system's manufacture or operation, but, with good component efficiencies, it provides improved detection probability in certain situations. However, when using both polarization directions, conditions rarely exist in which the two orthogonally polarized modes exhibit a significant correlation in phase and / or amplitude.

[0104] However, the following variant in Figure 12 also produces an advantage for the detection probability in the case of uncorrelated signals in the modes to be mixed. This takes advantage of the fact that a higher expected value of the output amplitude can be achieved if the two combined modes are in phase. In comparison to the embodiment in Figure 10, a phase shifter 21 is additionally installed in one of the two monomode waveguides (coming from the mode multiplexer). The phase shifter is adjustable, i.e. the phase position of the signal and thus of one mode can be varied in a targeted manner by applying a voltage signal.

[0105] In some aspects, the phase position is continuously tuned during the chirp, i.e. a sweep of the frequency modulation of the laser arrangement, e.g. by a total of 2n, in order to achieve an optimally constructive superposition of both modes in a part of the time interval of the measurement process in each case and thus a maximization of the output signal.

[0106] Instead of continuous tuning, it may be sufficient to set a number of discrete phase positions. For example, if the phase shifts 0 and n are set, the phase difference between the two modes at the coupler will be no more than n / 2 for at least one of the two phase positions, which reliably prevents predominantly destructive interference. However, an optimization process can also be carried out which initially maximizes the signal by varying the phase position in order to then perform the most precise measurement possible with optimal signal strength. Such optimization can be achieved, for example, by a time-dependent Fourier transformation during tuning, with subsequent determination of the signal maximum.

[0107] Another approach may be to first perform a measurement without active phase shifting. If no signal can be detected, the measurement can be repeated, for example, with a phase shift of n / 2. If the undetectable signal was caused by an unfavorable phase position, i.e., by an unfavorable speckle pattern, this signal can most likely be measured after changing the relative phase position.

[0108] The phase shifter as in Figure 12 can also be used in combination with the elements of Figure 11. The benefit is shown in the probability distributions of amplitude and intensity for in-phase superposition of two statistically independent modes with speckle statistics in Figures 13A and 13B. For the intensity, the expectation value increases due to the in-phase nature of 1 (for random phase position) to approximately 1.78. In addition, there is a particularly low probability for very small intensities, which in combination causes a significant increase in the detection probability. If four in-phase modes are mixed, the amplitude and intensity probability distributions are shown in Figures 14A and 14B. The probability for very low overall intensities is further reduced, while the expectation value increases to more than three times the expected value of the intensity of an individual mode.

[0109] Theoretically, this can be continued indefinitely, however, with a combination of 10 or more modes, a certain saturation for the expected value of the intensity occurs. This is shown in Figure 15 and is due to the fact that the amplitude of the mixed signal in the case of in-phase superposition is not the sum of the individual amplitudes EJ , but due to the coupling properties of the assumed couplers Af i ~F al which is the theoretical maximum for Y-couplers.

[0110] Real couplers cause additional losses, which leads to a significantly lower maximum achievable expected value than that shown in Figure 15. Thus, with a loss of 5%, this maximum value is already reduced to approximately 3.5.

[0111] In addition, it must be taken into account that the phase position of each channel (except for the first) must be adjusted or optimized separately using a separate phase shifter in order to achieve this signal improvement. This requires additional measurement time for optimization. Tuning all phase positions, in which all essential combinations occur, requires a very fast variation of the phases. Essential here means: For each mixture, a predominantly constructive interference must occur at least once.

[0112] A phase deviation of, for example, n / 4 can be tolerable since this only leads to a slightly lower signal.

[0113] To ensure that at least one superposition with a maximum phase difference of n / 4 occurs for each channel, four different phase positions are required for each channel whose phase is to be varied. With four channels, three channels must be varied, resulting in 43 = 64 combinations. This causes a large number of phase shifts or jumps in the signal, which in the frequency domain after the Fourier transformation leads to a considerable broadening of the signal and thus to a reduction in signal strength, which can overcompensate for the advantage of the mixing.

[0114] A mixture of no more than 4 signals therefore appears to be sensible for randomly scattering measurement objects, whereby the advantage of the concept is already largely exhausted with the mixture of 2 signals.

[0115] LIST OF REFERENCE SYMBOLS

[0116] 1 LIDAR system

[0117] 10 tunable laser arrays

[0118] 11 , 11 ' splinters

[0119] 12 waveguides

[0120] 12 ' optical isolator

[0121] 13 waveguides

[0122] 14-mode multiplexer

[0123] 15 Optics, collimation optics

[0124] 16 , 16 ' mixer with differential detector pair

[0125] 17 elements

[0126] 18 , 18 ' polarization-dependent beam splitter

[0127] 20 couplers, Y-couplers

[0128] 21 phase shifters

[0129] 141 multimode waveguide

[0130] 142 single-mode waveguides

[0131] 143 single-mode waveguide

[0132] L Laser light

Claims

PATENT CLAIMS 1 . Arrangement for optical distance and / or speed measurement, comprising: a laser arrangement whose output frequency can be tuned; a first splitter with a first and a second output, which is arranged in a light path of the tunable laser arrangement and is designed to couple out at least part of the laser light emitted by the tunable laser arrangement to an optical mixer coupled to the second output; a mode multiplexer implemented as a photonic integrated circuit with a first monomode connection, which is connected to the first output of the first splitter, in particular without an intermediate circulator, wherein the mode multiplexer implemented as a photonic integrated circuit comprises a multimode output waveguide;wherein the mode multiplexer implemented as a photonic integrated circuit is designed to split a multimode laser light reflected from an object and coupled into the output waveguide into at least two monomode light components, and to provide at least one of the at least two light components at a second, in particular monomode, port which is coupled to the optical mixer; 2. Arrangement according to claim 1, wherein the laser arrangement is designed to emit a monomode laser light, in particular in its fundamental mode; or the mode multiplexer is designed to couple a signal applied to the first monomode connection, in particular in a fundamental mode, into the multimode output waveguide. 3 . Arrangement according to one of the preceding claims, wherein the mode multiplexer is designed to have a different mode than in the fundamental mode existing light component of the coupled multimode laser light to the second port. 4 . Arrangement according to one of the preceding claims, wherein the at least two monomode light components have different modes.

5. Arrangement according to one of the preceding claims, wherein the mode multiplexer implemented as a photonic integrated circuit is designed to provide the other of the at least two monomode light components at the first monomode connection, in particular in the fundamental mode; 6 . Arrangement according to one of the preceding claims, - further comprising an optical isolator between an output of the laser arrangement and the first single-mode port; and / or - in which the laser arrangement is realized with a quantum dot laser. 7 . Arrangement according to one of the preceding claims, in which the mode multiplexer implemented as a photonic integrated circuit is designed to provide the at least one portion at the second terminal in a polarization-dependent manner. 8 . Arrangement according to one of the preceding claims, further comprising: a further splitter which is connected between the second output of the first splitter and the optical mixer and is designed to couple out at least part of the laser light which can be tapped off at the second output to a second optical mixer which is coupled to a second output of the second splitter; and optionally an element which changes the polarization, in particular by 90 °, between a first output of the second splitter and the first mixer; or optionally an element which changes the polarization, in particular by 90 °, between a second output of the second splitter and the second mixer.

9. Arrangement according to claim 8, further comprising: a first polarization-dependent beam splitter which is connected on the input side to the second connection and on the output side to the first and second mixers, the first polarization-dependent beam splitter being designed to split a light applied on the input side into two, in particular orthogonal, polarization directions.

10. Arrangement according to claim 8, further comprising a first polarization-dependent beam splitter between the first output of the first splitter and the first connection, wherein the first polarization-dependent beam splitter outputs a light component coming from the first connection in a polarization-dependent manner at a further output which is connected to the second mixer.

11. Arrangement according to claim 8, in which the mode multiplexer implemented as a photonic integrated circuit is designed such that the at least two monomode light components have a polarization direction that is in particular orthogonal to one another, a first light component being provided at the second terminal and a second light component being provided at a third terminal which is connected to the second mixer.

12. Arrangement according to one of the preceding claims, in which the mode multiplexer implemented as a photonic integrated circuit is designed for polarization-dependent splitting of the multimode laser light coupled into the output waveguide into at least two monomode light components; and / or in which the mode multiplexer implemented as a photonic integrated circuit is designed to provide one of the at least two light components at the second connection and to provide a second of the at least two light components at a third, in particular monomode, connection which is optionally coupled to the second mixer.

13. Arrangement according to claim 12, further comprising a coupler, in particular a Y-coupler or a multimode interference coupler, which is connected on the input side to the second and third terminals and on the output side to the first mixer; and / or in which a phase shifter, in particular an adjustable one, is arranged between one of the second and third terminals and the coupler. 14 . Arrangement according to one of the preceding claims, further comprising: a second polarization-dependent beam splitter, which is connected on the input side to the second connection; an element which changes the polarization, in particular by 90 °, which is connected on the input side to a first output of the second polarization-dependent beam splitter; a coupler, in particular a Y-coupler or a multimode interference coupler, which is connected with a first input to a second output of the second polarization-dependent beam splitter and with a second input to the element which changes the polarization and is connected on the output side to the first mixer. 15 . Use of a mode multiplexer implemented as a photonic integrated circuit with at least two monomode connections and one multimode connection in a LIDAR system, wherein a different mode is used for a transmission channel between one of the at least two monomode connections and the multimode connection than for a reception channel for a light reflected from an object between the multimode connection and the at least one other monomode connection.

16. Method for detecting a distance and / or speed of an object by means of an optical measurement, comprising the steps: Generating a frequency-modulated narrowband laser beam Splitting the narrowband laser beam into a measurement component and a local oscillator component coupling the measuring component, in particular in the fundamental mode, into a first monomode connection and forwarding the coupled measuring component to a multimode output connection; Receiving and coupling a multimode part of the measurement component reflected at the object into the multimode output port; forwarding the part in a mode different from the fundamental mode to a second single-mode port; Mixing the part with the local oscillator part to generate a beat frequency; Determining a distance and / or speed from the beat frequency; wherein the step of coupling in the measuring part and forwarding the part is carried out with a mode multiplexer implemented as a photonic integrated circuit.

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