Photonic radar system with adjustment of the polarization state

Abstract

A radar system according to the invention has a central unit (Z), at least one radar transmitting unit (S-1, S-2, S-3, S-n) and at least one radar receiving unit (E-1, E-2, E-3, E-n), which each comprise optical and electrical components and exchange signals via at least one transmission medium (G). In the central unit (Z) and / or at least one radar transmitting unit (S-1, S-2, S-3, S-n) and / or at least one radar receiving unit (E-1, E-2, E-3, E-n), a polarization controller (18, 31, 41) is provided, which is designed to adjust the polarization state of a signal received via the transmission medium (G) on the basis of an electro-optical effect.

Description

[0001] Description

[0002] Photonic radar system with polarization state adjustment

[0003] The present invention relates to a radar system that can be operated, for example, in a vehicle. The present invention further relates to a central unit and a radar receiver unit of such a radar system. The invention also relates to a vehicle equipped with such a radar system.

[0004] For driver assistance systems and safety systems in fully automated driving, the safest possible perception of the surroundings is essential. This is achieved by using sensors such as radar, lidar, and camera sensors integrated into the vehicle to detect the environment.

[0005] For example, an environmental model can be created based on the acquired sensor data using a suitable machine learning model. This can involve perception modules that can recognize learned objects in the environment and forward this information to a planning module. The planning module can then take the recognized objects into account for trajectory planning and safe vehicle control. Of particular importance here is a comprehensive 360° 3D capture, enabling complete 360-degree recording of all static and dynamic objects and the creation of the highest possible resolution 3D models of the environment.

[0006] While cameras provide detailed visual information and allow, for example, the recognition of traffic signs, lane markings, and colors, they deliver poor results in unfavorable lighting conditions, fog, or glare, and provide only inadequate distance readings. Lidar sensor-based systems are capable of ensuring precise distance measurement and can also be used for classification. However, these lidar systems are expensive and complex to set up. Furthermore, lidar systems are susceptible to weather conditions such as rain, fog, or direct sunlight. Radar sensors, on the other hand, deliver reliable and fail-safe data in all weather conditions. Even poor visibility conditions such as rain, fog, snow, dust, and darkness hardly affect their reliability.

[0007] However, the resolution is currently limited. For example, radar sensors currently used in series production in the automotive sector have a resolution of approximately 2°. This is insufficient to meet the requirements for Levels 4 and 5 of automated driving with safe driving functions, as these require radar sensors to deliver three-dimensional images with a high resolution of 0.1° and below, with high insensitivity to interference from their surroundings. This cannot be achieved with conventional radar technology because the resolution of such systems is too low.

[0008] Currently under development are so-called photonic radar systems, in which driver signals in the GHz range can be distributed to a large number of radar sensors using an optical carrier signal in the THz frequency range. This allows for the co-integration of electronic and photonic components on a single semiconductor substrate, enabling extremely compact form factors for the individual radar sensors and, consequently, arrays with a large number of such radar sensors integrated into the vehicle.

[0009] German patent application DE 10 2017221 257 A1 discloses a radar system in which signal transmission between a central unit and a radar transmitter or radar receiver is implemented optically. For this purpose, a radar driver signal is optically generated in the central unit and transmitted via at least one optical fiber to at least one radar receiver and / or at least one radar transmitter. In the radar transmitter, the radar driver signal is then converted into an electrical radar driver signal and used to drive a radar transmitter. A radar echo signal received by a radar receiver is mixed with the electrical radar driver signal in a mixer of the radar receiver. The mixed signal is then modulated onto the optical driver signal by means of a modulation unit, coupled into the optical fiber, and transmitted back to the central unit.The central unit receives the modulated optical signal and evaluates it using an evaluation unit. The result is then provided as radar information.

[0010] Compared to this state of the art, one objective of the invention is to provide an improved radar system.

[0011] This problem is solved by the independent claims. Preferred embodiments of the invention are the subject of the dependent claims.

[0012] The invention is based on the understanding that controlling and adjusting the polarization of the optical carrier signal is necessary because the processing of the optical carrier signal in components of a photonic radar system is polarization-dependent, and the polarization after transmission between the central unit and a radar transmitter or receiver is influenced by the passage through the optical fiber and is subject to fluctuations, for example, due to environmental influences. The invention is further based on the understanding that methods based on the thermo-optic effect for controlling and adjusting the polarization are limited to rise and fall times in the sub-megahertz range, so that higher-frequency changes cannot be compensated and can lead to signal degradation.

[0013] The radar system according to the invention therefore comprises a central unit, at least one radar transmitter unit and at least one radar receiver unit, each comprising optical and electrical components and exchanging signals via at least one transmission medium, wherein a polarization controller is provided in the central unit and / or at least one radar transmitter unit and / or at least one radar receiver unit, which is designed to adjust the polarization state of a signal received via the transmission medium on the basis of an electro-optical effect.

[0014] This method allows for polarization adjustment within the components of a photonic radar system, which can be performed faster than methods based on the thermo-optical effect and thus exhibits greater resilience to environmental influences, such as vibrations. Furthermore, cointegration within an electronically-photonically cointegrated chip (EPIC) can be achieved, reducing the cost of such a system.

[0015] According to one embodiment of the invention, the polarization controller comprises optical phase shifters and optical couplers.

[0016] Preferably, the polarization controller comprises several combinations of paired phase shifters and optical couplers arranged in cascade.

[0017] Advantageously, a control unit is provided which is designed to set the operating point for the optical phase shifters and / or optical couplers by means of a control loop. In particular, the polarization controller can have two optical outputs, one of which is fed to the control unit.

[0018] According to a further embodiment of the invention, the central unit is configured to provide an optical carrier signal, to modulate a radar driver signal onto the optical carrier signal, to send the optical carrier signal modulated with the radar driver signal to the at least one radar transmitter unit and / or the at least one radar receiver unit, and to receive an optical signal from the at least one radar receiver unit to which a radar echo signal is superimposed, wherein the optical signal received from the at least one radar receiver unit is supplied to a polarization controller integrated in the central unit.

[0019] According to a further embodiment of the invention, the radar receiving unit is configured to receive at least one optical signal from the central unit via the transmission medium, to supply the received optical signal to a polarization controller integrated in the radar receiving unit, and to adjust the polarization state of the received optical signal by means of the integrated polarization controller.

[0020] Advantageously, an unmodulated continuous wave signal received from the central unit can be supplied to the polarization controller integrated in the radar receiver unit.

[0021] It is also advantageous to supply a modulated optical driver signal received from the central unit to the polarization controller integrated in the radar receiver unit.

[0022] Finally, the invention also includes a vehicle that has a radar system according to the invention.

[0023] Further features of the present invention will become apparent from the following description and the claims in conjunction with the figures. These show:

[0024] Fig. 1 is a schematic representation of one embodiment of the radar system; Fig. 2 is a schematic representation of one embodiment of a central unit of the radar system with a quickly adjustable optical polarization controller;

[0025] Fig. 3 shows a schematic representation of an embodiment of a radar receiver unit of the radar system with a quickly adjustable optical polarization controller;

[0026] Fig. 4 shows an exemplary implementation of a quickly adjustable optical polarization controller based on electro-optical phase shifters;

[0027] Fig. 5 shows an exemplary implementation of a quickly adjustable optical polarization controller based on electro-optical phase shifters with a control loop for setting the operating point; and

[0028] Fig. 6 shows an exemplary integration of the radar system on the vehicle surface of a passenger car.

[0029] To better understand the principles of the present invention, embodiments of the invention are explained in more detail below with reference to the figures. It is understood that the invention is not limited to these embodiments and that the described features can also be combined or modified without departing from the scope of protection of the invention as defined in the claims. In the following figures, electrical connections are represented by solid lines and optical connections by dashed lines for differentiation.

[0030] Figure 1 shows an exemplary schematic representation of a photonic radar system according to the invention, in which rapidly adjustable optical polarization controllers based on electro-optical phase shifters can be used. For the sake of clarity, the polarization controllers are not shown in this figure, but are explained in detail in connection with the other figures.

[0031] The radar system comprises a central unit Z, several radar transmitters S-1, S-2, S-3,..., Sn, and several radar receivers E-1, E-2, E-3,..., En. The central unit is referred to as the backend, and the radar transmitters and receivers as the frontend. Although the radar transmitters and receivers are depicted here as separate units, they can also be implemented using combined units that integrate both a transmitter and a receiver.

[0032] The central unit Z is connected via one or more transmission media G to the radar transmitting units S-1, S-2, S-3,..., Sn and the radar receiving units E-1, E-2, E-3,..., En, wherein the transmission media can in particular be one or more optical fibers. In the case of an integrated unit on which circuits for transmitting and receiving are located, the signal only needs to be sent to this common unit.

[0033] The central processing unit Z generates a frequency-modulated continuous wave (FMCW) signal and processes and evaluates the signals generated by the radar receiver units E-1, E-2, E-3, ..., En. Instead of a frequency-modulated continuous wave signal, a signal with a different waveform can also be generated. This centralized processing and evaluation of the signals allows the individual radar sensors to be designed as small and cost-effectively as possible.

[0034] At the central station, a radar driver signal is generated, which consists of a radar carrier signal with frequency fcarrier and a radar ramp signal with frequency fr. ra m PThe radar driver signal is modulated onto an optical carrier signal. The frequency of the radar driver signal is preferably only a fraction of the carrier frequency required to drive the individual radar transmitters. For example, the signal to be transmitted can be modulated with 1 / 8 of the radar frequency. With a carrier frequency of 77 GHz currently common for road vehicles, this results in a frequency of 9.625 GHz. The optical carrier signal modulated with the radar driver signal is then coupled into the transmission medium G.

[0035] The optical carrier signal modulated by the radar driver signal is coupled out of the transmission medium G by each of the individual radar transmitter units S-1, S-2, S-3,..., Sn by means of a coupling unit (not shown), and the radar driver signal is separated in each case. If this radar driver signal is only a fraction of the radar frequency used, for example, 1 / 8 of the radar frequency, it is first amplified eightfold in each of the individual radar transmitter units S-1, S-2, S-3,..., Sn. The resulting signal then drives the respective radar transmitters in the radar transmitter units S-1, S-2, S-3,..., Sn. The individual radar transmitters include, in particular, radar antennas, each of which then transmits a radar signal.

[0036] An unmodulated continuous wave (CW) signal is transmitted to the individual radar receivers E-1, E-2, E-3, ... , En and also coupled out of the transmission medium G by means of a coupling unit. Radar echo signals are received by the individual radar receivers E-1, E-2, E-3, ... , En and modulated onto the unmodulated continuous wave signal. The resulting signal is coupled into the transmission medium G by means of further coupling units and sent back to the central unit.

[0037] The central unit Z then receives the signals from the radar receivers E-1, E-2, E-3,..., En, evaluates them, and provides derived radar information. This radar information can then be further processed, for example, to create or update an environmental model.

[0038] The radar transmitters and receivers can each be designed as separate electronically and photonically cointegrated chips (so-called "EPIC chips") or implemented on a single electronically and photonically cointegrated chip. Silicon photonics technology can be used for the cointegration of the electronic and photonic components, enabling the monolithic integration of photonic devices, high-frequency electronics, and digital electronics on a single chip. A hybrid implementation using separate electronic (EIC) and photonic (PIC) chips is also possible.

[0039] The integration of optical components into the chip can be achieved, for example, using so-called silicon-on-insulator (SOI) regions, while the integration of electronic components can be accomplished using bulk silicon regions. In SOI regions, a thin silicon layer is separated from the silicon substrate by an insulating layer, such as silicon dioxide. Since silicon is transparent at the near-infrared wavelengths common in optical communication technology, and the refractive indices of silicon and silicon dioxide differ significantly in this wavelength range, various optical components can be implemented using SOI structures. This allows for high signal quality with low parasitic interference, particularly at high data rates.The integration of the RF circuits for the radar antennas, including the frequency multiplier, with the photonic circuit can be implemented in a monolithic EPIC design without additional wire or flip-chip bonding. Furthermore, chips can be optically and electrically tested directly at the wafer level. Additionally, the scalability to large volumes in the highly integrated manufacturing of electronically and photonically integrated circuits enables a significant reduction in assembly costs and a more efficient cost structure.

[0040] According to the invention, a solution integrated into an electronically-photonically cointegrated chip is proposed for polarization adjustment, utilizing an electro-optic effect. The electro-optic effect exploits the property of certain materials to change their optical properties, particularly their refractive index, when an electric field is applied or the charge carrier concentrations in the semiconductor are altered. This enables precise and rapid control (in the nanosecond range) of the phase of light passing through the material. This phase can, in turn, be used to adjust the polarization, which can be achieved significantly faster than by utilizing the thermo-optic effect, which limits the possible rise and fall times to frequencies in the sub-megahertz range.

[0041] In particular, the electro-optical effect based on carrier injection, for example via forward-driven PIN diodes, can be used. Alternatively, other electro-optical effects, such as carrier depletion, the Pockels effect, the Kerr effect, liquid crystals, etc., can also be used to vary the refractive index.

[0042] Figure 2 schematically shows an embodiment of a central unit of a photonic radar system according to the invention, in which a quickly adjustable optical polarization controller is integrated.

[0043] The central unit Z generates two optical output signals: a carrier signal modulated with a radar driver signal, which is transmitted to the radar transmitting units and / or the radar receiving units, and an unmodulated continuous wave signal, which is transmitted to the radar receiving units.

[0044] To provide the optical output signals, the central unit Z in the illustrated embodiment comprises two light sources 10 and 11. Each of these can be a laser diode emitting continuous laser light in the near-infrared range. Preferably, the wavelengths of the laser diodes are in a range where optical losses and dispersion are as low as possible, for example, 850 nm, 1310 nm, or 1550 nm. However, so-called ASE light sources (Amplified Spontaneous Emission) can also be used as light sources. These emit light that is generated by spontaneous emission and subsequently optically amplified by stimulated emission.

[0045] The optical carrier signal generated by the light source 10 is fed to an electro-optic modulator 12, which modulates the electrical radar driver signal, consisting of the radar carrier signal and the radar ramp signal, onto the optical carrier signal. The radar ramp signal is generated by a ramp generator 17. For example, the electro-optic modulator 12 can be a Mach-Zehnder modulator, in which the optical carrier signal is first split into two waveguides where phase modulation is performed and then recombined. Depending on their relative phase, the partial signals superimpose to form the modulated output signal. Alternative modulation principles, such as those using directly modulated light sources, are also possible.

[0046] The optical carrier signal modulated by the electro-optical modulator 12 can then first be fed to an optional optical control unit 13, which can, for example, control or regulate the amplitude, phase, or polarization of the modulated optical carrier signal. Subsequently, the modulated optical carrier signal is fed to an optical 1:N switch or beam splitter 14, which ensures that the radar driver signal can be provided as the first output signal on multiple channels for the individual radar transmitter units.

[0047] The second optical output signal generated by the second light source 11 is transmitted to the radar receivers as an unmodulated continuous wave signal. An optional optical control unit 15 can also be provided for monitoring and controlling the amplitude, phase, or polarization. The second optical output signal can also be split into multiple channels for the individual radar receivers using an optical 1:N switch or beam splitter 16.

[0048] To receive and evaluate the signals received by the radar receivers, which are a distance-dependent, time-delayed copy of the signals emitted by the radar transmitters, the central unit Z first comprises a rapidly adjustable optical polarization controller 18, with which the polarization of the received optical signal can be controlled and precisely adjusted. The signal is then fed to an optical receiver and processing unit 19, in which the radar echo signal superimposed on the optical signal is separated by means of an optoelectric conversion. Although only one polarization controller is shown in Figure 2, several polarization controllers can be used if there are multiple optical inputs.

[0049] Subsequently, the electrical radar echo signal can be amplified and further processed by optional electrical processing units 20 and 21, for example by phase shifting or filtering. The electrical radar echo signal, possibly further processed, is then fed to analog-to-digital converters 22. Furthermore, optionally, the signals can be processed further in another electrical processing unit 23, for example by an FFT to divide the signal into several frequency components.

[0050] The pre-processed radar echo signal is then fed to the evaluation unit 24, which derives radar information from the signal. This radar information can then be output and further processed, for example, for environmental sensing.

[0051] Additionally, the electrical and optical components of the central unit Z can be controlled via one or more optional control interfaces and units 25, 26. In particular, this includes switching the electrical and optical components on and off, parameterizing them, diagnosing them, etc. Furthermore, communication with the radar transmitters and receivers can also be established via the control interfaces and units 25, 26, and can be carried out electrically, optically, or electrically and optically. For clarity, the control interfaces and units 25, 26 are divided into several blocks in the figure; however, in one implementation, they can also be realized as a single component.

[0052] The generation of the two optical output signals in the central unit Z can be achieved using a single optical source instead of multiple optical sources, as shown in Figure 2. For this purpose, an optical splitter can be connected downstream of the optical source, with one output serving to generate the carrier signal modulated by the radar driver signal and a second output serving to generate the unmodulated continuous wave signal. Figure 3 schematically shows an embodiment of a radar receiver unit E-1 of the photonic radar system according to the invention. The other radar receiver units E-2, E-3, and En are identical in construction. In this embodiment, the radar receiver unit E-1 receives two optical signals from the central unit (backend): a signal modulated by the local oscillator signal (LO signal) and an unmodulated continuous wave signal.

[0053] The modulated optical driver signal received by the central unit Z can first be fed to an optional optical control unit 30 at the input of the radar receiver unit E-1, which can, for example, control or regulation of the amplitude or phase.

[0054] Subsequently, the polarization of the processed optical signal can be controlled and precisely adjusted using a first, quickly adjustable optical polarization controller 31. The signal is then fed to an optical receiving and processing unit 32, where the LO signal superimposed on the optical signal is separated by means of an opto-electrical conversion.

[0055] Subsequently, the LO signal can be amplified in an electrical amplifier 33, for example a transimpedance amplifier, the frequency of the signal can be multiplied to the frequency of the electrical radar driver signal by a multiplier unit 34, and optionally further processed by another electrical processing unit 35, for example by phase shifting or filtering. The LO signal processed in this way is then fed to an electrical mixer 36.

[0056] The radar echo signal received by a radar antenna (not shown) of the radar receiver unit is fed to the electrical mixer 36. This radar echo signal is based on a radar signal originally emitted by an antenna of a radar transmitter unit and is a time-shifted copy of the local oscillator (LO) signal, depending on the distance. The radar echo signal can optionally be amplified beforehand by a low-noise amplifier 37, then optionally amplified again in a further electrical amplifier 38, and subsequently processed by an optional electrical processing unit 39, for example, with respect to amplitude or phase, or by additional filtering. In the mixer 36, the still high-frequency radar echo signal is then mixed with the LO signal and down-converted to a lower-frequency intermediate frequency (IF) range, typically in the sub-gigahertz range.The frequency of the IF signal depends on the time offset between the radar echo signal and the digital local oscillator.

[0057] The unmodulated continuous wave signal received by the central unit Z can first be fed to an optional optical control unit 40 at the input of the radar receiver unit E-1. This unit allows, for example, the control and regulation of the amplitude and phase of the unmodulated continuous wave signal. The processed signal is then fed to a rapidly adjustable optical polarization controller 41, which allows the polarization of the received optical signal to be controlled and precisely adjusted.

[0058] The signal is then fed to a modulation unit 42, which can be configured, for example, as a Mach-Zehnder modulator. Using the modulation unit 42, the down-converted radar echo signal can be modulated onto the unmodulated continuous wave signal, possibly after further processing by an optional electrical processing unit 43. The modulated signal can then first be fed to an optional additional optical control unit 44 and then coupled into a transmission medium (not shown here) and transmitted to the central unit.

[0059] The radar receiver unit shown in Figure 3 also allows for the control of the electrical and optical components of the radar receiver unit via one or more optional control interfaces and units 45, 46.

[0060] Figure 4 shows an embodiment of a rapidly adjustable optical polarization controller based on electro-optical phase shifters. The illustrated implementation is based on electro-optical phase shifters.

[0061] The fiber-side optical input signal is routed to a photonic fiber-to-chip interface 50. The fiber-to-chip interface can be configured, for example, as a spot-size converter (SSC) or a grating coupler (GRC).

[0062] Regardless of the interface design, the incoming optical wave is randomly polarized after passing through the optical fiber and is subject to fluctuations, for example, due to environmental influences. Since integrated photonic waveguides are generally optimized for a specific polarization, typically transverse-electric (TE), using the optimized polarization is preferable to avoid signal degradation.

[0063] When integrating, for example, two-dimensional grating couplers, the design can incorporate a conversion of the incident, random polarization into two output waveguides with the desired polarization. For other structures, such as spot size converters, a polarization rotator and beam splitter (PRBS) can optionally be added, which also features two output waveguides with the desired polarization.

[0064] It should be noted that the distribution of optical power between the two waveguides, regardless of the implemented interface, depends on the incident, random polarization and is therefore susceptible to interference. In the limiting case, the entire optical power is thus concentrated in one of the two output waveguides of the fiber-to-chip interface or the polarization inverter / beam splitter. In a photonic radar system, such a limiting case is generally desirable; however, the entire optical power should be able to be directed in a controlled manner to one of the two output waveguides.

[0065] To control the polarization state, an arrangement downstream of the optional polarization rotator and beam splitter 51, as shown in Figure 4, can be used. In this arrangement, fast optical phase shifters 52 and optical couplers 53 are cascaded. The optical coupler 53 is designed as an optical splitter / combiner, whereby the supplied optical radiation can be transferred, separated, or combined from one optical channel to another by means of interference effects. This can be achieved through various geometries, for example, as a multi-mode interference coupler (MMI) or coupled waveguides.

[0066] The combination of phase shifter pair and optical divider / combiner can be repeated multiple times. This allows for complete control of the polarization state and, furthermore, provides redundancy, enabling continuous, time-independent adjustment. If one optical phase shifter pair cannot be adjusted further due to the risk of thermal / electrical damage or high optical losses, a second pair can be adjusted while the first is gradually detuned. For automatic optimization of the polarization controller's operating point, it is also advantageous to connect the polarization controller to a control loop. For this purpose, one of the two optical outputs can be fed back into the control loop, as illustrated in Figure 5.Here, a control unit 54 can automatically set and optimize the operating point for the optical phase shifters 52 and optical dividers / combiners 53, thus ensuring stable operation in the long term, even under changing conditions. The control loop can be implemented either analogously or digitally, e.g., via a PC, DSP, or integrated circuits.

[0067] Figure 6 schematically shows an exemplary large-area integration of a multitude of antenna chips A, each of which can comprise the described radar transmitting units and / or radar receiving units, onto the vehicle surface of a passenger car. For example, the windshield, rear window, and bumper can be used for integration into the front and rear, and the vehicle floor, roof, and B-pillar for integration along the sides. The central unit of the photonic radar system is not shown in this illustration and can, for example, be implemented as a radar control unit located in the vehicle's interior or engine compartment. Similarly, the figure omits the representation of the transmission media between the central unit and the antenna chips.

[0068] The radar system, the central unit, and the radar receiver according to the invention can be used, in particular, in any road vehicles, such as passenger cars, commercial vehicles, trucks, or buses, where radar-based environmental sensing is employed, for example, for driver assistance systems or for automatic or autonomous driving functions, but is not limited to this. Application in radar-based environmental sensing in other technical fields is also possible. [List of reference numerals]

[0069] Central processing unit

[0070] S-1, S-2, S-3, Sn Radar transmitting unit E-1, E-2, E-3, En Radar receiving unit G Transmission medium 10, 11 Light source 12 Electro-optical modulator

[0071] 13, 15 optical control unit

[0072] 14, 16 1 : N-switch or divider 17 Ramp generator

[0073] 18 Polarization unit 19 Optical receiving and processing unit 20, 21, 23 Electrical processing unit 22 Analog-to-digital converter 24 Evaluation unit 25, 26 Electrical control interface and unit

[0074] 30, 40, 44 optical control unit

[0075] 31, 41 Polarization converter 32 Optical receiving and processing unit 33, 38 Electrical amplifier 34 Frequency multiplier

[0076] 35, 39, 43 Electrical processing unit 36 ​​Electrical mixer 37 Low-noise amplifier 42 Electro-optical modulator 45, 46 Electrical control interface and unit 50 Fiber-to-chip interface 51 Polarization rotator and beam splitter 52 Fast optical phase shifter 53 Optical splitter / combiner 54 Control unit A Antenna chip

Claims

Patent claims 1. Radar system comprising a central unit (Z), at least one radar transmitter (S-1 , S-2, S-3, Sn) and at least one radar receiver (E-1, E-2, E-3, En), each comprising optical and electrical components and exchanging signals via at least one transmission medium (G), characterized in that a polarization controller (18, 31, 41) is provided in the central unit (Z) and / or at least one radar transmitter (S-1 , S-2, S-3, Sn), and / or at least one radar receiver, which is designed to adjust the polarization state of a signal received via the transmission medium (G) on the basis of an electro-optical effect.

2. Radar system according to claim 1, wherein the polarization controller (18, 31, 41) comprises optical phase shifters (52) and optical couplers (53).

3. Radar system according to claim 2, wherein the polarization controller (18, 31, 41) comprises several combinations of paired phase shifters (52) and optical couplers (53) arranged in cascade.

4. Radar system according to claim 2 or 3, wherein a control unit (54) is provided which is configured to set the operating point for the optical phase shifters (52) and / or optical couplers (53) by means of a control loop.

5. Radar system according to claim 4, wherein the polarization controller (18, 31, 41) has two optical outputs and one of the two optical outputs is supplied to the control unit (54).

6. Central unit (Z) for use in a radar system according to any one of claims 1 to 5, wherein the central unit (Z) is configured to provide an optical carrier signal, to modulate a radar driver signal onto the optical carrier signal, to transmit the optical carrier signal modulated with the radar driver signal to the at least one radar transmitting unit (S-1, S-2, S-3, Sn) and / or the at least one radar receiving unit (E-1, E-2, E-3, En) and from which the at least one The radar receiving unit (E-1 , E-2, E-3, En) receives an optical signal to which a radar echo signal is superimposed, wherein the optical signal received by the at least one radar receiving unit (E-1 , E-2, E-3, En) is fed to a polarization controller (18) integrated in the central unit (Z).

7. Radar receiving unit (E-1 , E-2, E-3, En) for use in a radar system according to any one of claims 1 to 5, wherein the radar receiving unit (E-1, E-2, E-3, E-n) is configured to receive at least one optical signal from the central unit (Z) via the transmission medium (G), to supply the received optical signal to a polarization controller (31, 41) integrated in the radar receiving unit (E-1 , E-2, E-3, En) and to adjust the polarization state of the received optical signal by means of the integrated polarization controller (31 , 41 ).

8. Radar receiving unit (E-1 , E-2, E-3, En) according to claim 7, wherein an unmodulated continuous wave signal received from the central unit (Z) is supplied to the polarization controller (41) integrated in the radar receiving unit (E-1 , E-2, E-3, En).

9. Radar receiving unit (E-1 , E-2, E-3, En) according to claim 7, wherein a modulated optical driver signal received from the central unit (Z) is supplied to the polarization controller (31) integrated in the radar receiving unit (E-1 , E-2, E-3, En).

10. Vehicle comprising a radar system according to any one of claims 1 to 5.

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