Optical system

Abstract

Optical system (1), comprising at least the following components: a light source (2), a first optical device (3) for the directed emission of light from the light source (2) via a transmitting light path (100) and for the directed reception of emitted light via a first receiving light path (101), a second optical device (4) for receiving emitted light in a directed manner via a second receiving light path (101'), wherein either the first or the second optical device (3, 4) is operable for receiving emitted light, a detector (5, 5a, 5b) for detecting the received light and a coupling device (10) for splitting the emitted light between the two optical devices (3, 4) according to a predefinable coupling ratio and for forwarding received light from one of the two optical devices (3, 4) to the detector (5, 5a, 5b).

Description

Technical field

[0001] The invention relates to an optical system.

[0002] The invention further relates to a method for operating an optical system.

[0003] The invention further relates to a LiDAR system.

[0004] The invention further relates to a method for manufacturing an optical system.

[0005] Although the present invention is generally applicable to any optical system, the present invention is described in relation to a LiDAR system. State of the art

[0006] Known LiDAR systems use narrowband laser beams that are deflected in a specific direction. When the laser beam hits an object, its distance can be determined by measuring the reflection of the laser beam from the object at that angle. For this purpose, a linear frequency ramp, based on the principle of FMCW (Frequency Modulated Continuous Wave Radar), is transmitted, and the difference frequency between the transmitted and received ramps is determined through coherent reception. The distance to the object can then be determined from this difference frequency. To detect an object within an area, the area can be illuminated two-dimensionally.

[0007] US patent 9,476,981 B2 discloses an optical phase array comprising a multitude of nanophotonic antenna elements for projecting complex images into the far field. These nanophotonic phase arrays, including the nanophotonic antenna elements and waveguides, can be fabricated on a single silicon chip using complementary metal-oxide-semiconductor (CMOS) processes. Directional couplers couple light from the waveguides to the nanophotonic antenna elements, which emit the light with selected phases and amplitudes, so that the emitted beams in the far field create a desired image.

[0008] Known LiDAR systems can be configured for monoaxial or biaxial operation. In a monoaxial LiDAR system, the same optical components are used for beam deflection in both the transmit and receive paths. The advantage here is that the transmitting laser and the detectors in the receive path always capture the same solid angle, and only one component is needed for deflection. However, the transmitted light must be separated from the received light. This can be achieved using a beam splitter. When using a beam splitter, it's important to note that it always introduces optical losses. While an optical circulator doesn't cause losses, it often leads to crosstalk from the light source directly to the detectors, resulting in false positives in a LiDAR system that require complex electrical filtering to eliminate.Furthermore, no low-loss optical circulators integrated into a silicon chip based on CMOS processes are available.

[0009] Alternatively, a LiDAR system can also be configured for biaxial operation. In a biaxial LiDAR system, two separate optical components are used for the transmit and receive paths. However, it must be ensured that both optical components are aligned to the same solid angle. In particular, the transmit beam of one optical component and the receive cone of the other optical component can only overlap at a specific distance, and at other distances, additional optical losses occur because the optical beams only partially overlap.

[0010] Document DE 10 2017 120 877 A1 discloses an optoelectronic sensor system that can be switched between an autocollimation arrangement and a biaxial arrangement by selectively incorporating a deflecting element. This allows the received light path to be selectively guided through either a first or a second, laterally spaced objective area, in order to adapt the system to different measurement tasks.

[0011] Document WO 2018 / 160240 A2 discloses a method for correcting Doppler effects in optical distance measurement using chirp signals. For this purpose, an upwardly and a downwardly chirped optical signal are used to determine two different distance values ​​from the respective backscattered signals, the combination of which enables the determination of the Doppler effect.

[0012] Document US 9,476,981 B2 discloses an optical phased array integrated on a silicon chip, comprising a multitude of nanophotonic antenna elements. By individually phase-controlling the light emitted by the antenna elements, dynamically complex radiation patterns can be generated in the far field, for example, for LiDAR applications.

[0013] Document WO 2018 / 160729 A2 discloses a modular optical 3D sensor system for a vehicle with a central signal processing unit and several decentralized beam guidance and detection units. The modules are interconnected via a fiber optic network to enable flexible integration into the vehicle. Disclosure of the invention

[0014] In one embodiment, the present invention provides an optical system comprising the following components: a light source, a first optical device for directed emission of light from the light source via a transmitting light path and for directed reception of emitted light via a first receiving light path, a second optical device for directed reception of emitted light via a second receiving light path, wherein either the first or the second optical device is operable for receiving emitted light, a detector for detecting the received light, and a coupling device for dividing the emitted light between the two optical devices according to a predefinable coupling ratio and for forwarding received light from one of the two optical devices to the detector.

[0015] In a further embodiment, the present invention provides a method for operating an optical system, comprising the steps - Providing light from a light source, - Splitting the light from the light source between two optical devices according to a predefinable coupling ratio by means of a coupling device, - Emitting the split light for the first optical device via the first optical device, - Receiving emitted light via the first optical device using a first receiving light path or via the second optical device using a second receiving light path and - Forwarding the received light from one of the two optical devices to a detector.

[0016] In a further embodiment, the present invention provides a LiDAR system comprising an optical system according to one of claims 1-10, which can be operated monoaxially or biaxially by means of the optical system.

[0017] In a further embodiment, the present invention provides a method for manufacturing an optical system according to one of claims 1-12 comprising a complementary metal-oxide semiconductor process.

[0018] The embodiments of the present invention allow the advantages of both a monoaxial and a biaxial LiDAR system to be exploited: This makes it possible to select the simpler monoaxial mode for closer targets, where optical losses are less relevant, and the biaxial mode for more distant targets, where optical losses must be minimized. Additionally, the monoaxial mode can be used as a reference for calibrating the biaxial mode at the overlap between the optical beams of the transmit and receive paths.

[0019] The embodiments of the invention increase the flexibility with regard to detection schemes and possible configurations. For example, both Time-of-Flight (ToF) and Frequency Modulated Continuous Wave (FMCW) based detection schemes are possible, as are free-beam optical, fiber-optic, or integrated-optical embodiments.

[0020] Further features, advantages and further embodiments of the invention are described below or become apparent therein.

[0021] According to an advantageous further development, the coupling device is designed to adjust the coupling ratio during the operation of the optical system. This enables a continuous or ongoing adjustment of the respective operating mode of the optical system.

[0022] According to a further advantageous embodiment, the coupling device is designed to adjust the coupling ratio depending on the distance to a target. The advantage of this is that the appropriate operating mode, monoaxial on the one hand and biaxial on the other, can be used depending on the distance to the respective target.

[0023] According to a further advantageous embodiment, a monitoring device is arranged to monitor the set coupling ratio of the coupling device. The advantage of this is that the coupling ratio can be monitored and, if necessary, adjusted accordingly. This increases the reliability of operating the optical system.

[0024] According to a further advantageous embodiment, the monitoring device comprises a coupler, in particular a directional coupler, and a photodiode, wherein the coupler has a coupling ratio of at least 90:10, in particular 99:1. The advantage of this is a simple and cost-effective monitoring device which has a strongly asymmetrical coupling ratio, so that only a small amount of light is required for monitoring and the majority is available for detection by the detector.

[0025] According to a further advantageous embodiment, a mixing device and an oscillation device are arranged such that received light is superimposed with emitted light by the mixing device and then directed to the detector. This makes operation via FMCW (Frequency Modulated Continuous Wave) simple, in which emitted light is superimposed with received light.

[0026] According to a further advantageous embodiment, a second detector is arranged which can be illuminated with light to be detected according to the set coupling ratio of the coupling device. This provides a simple implementation for an optical system to be operated via FMCW.

[0027] According to a further advantageous refinement, the coupling device includes a Mach-Zehnder interferometer. This provides a reliable optical coupling device.

[0028] According to a further advantageous embodiment, the Mach-Zehnder interferometer features two phase shifters, which are thermo-optical, electro-optical, and / or mechanical. One of the advantages achieved is that a 2:2 coupler can be implemented in a simple yet reliable manner. For example, the phase shifters can be implemented by locally increasing the temperature and locally varying the refractive index using the thermo-optical coefficient.

[0029] According to a further advantageous development, at least one, and in particular all, components are integrated on a silicon chip. The advantage of this is a compact physical unit for the optical system.

[0030] Further important features and advantages of the invention will become apparent from the dependent claims, the drawings, and the accompanying description of the figures based on the drawings.

[0031] It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified, but also in other combinations or on their own, without leaving the scope of the present invention.

[0032] Preferred embodiments and configurations of the invention are shown in the drawings and are explained in more detail in the following description, wherein identical reference numerals refer to identical or similar or functionally identical components or elements. Brief description of the drawings

[0033] This is shown in schematic form Fig. 1 an optical system according to an embodiment of the present invention; Fig. 2 an optical system according to an embodiment of the present invention; Fig. 3 an optical system according to an embodiment of the present invention; Fig. 4 an optical system according to an embodiment of the present invention; Fig. 5 a coupling device according to an embodiment of the present invention; and Fig. Six steps of a method according to an embodiment of the present invention. Embodiments of the invention

[0034] Fig. Figure 1 shows an optical system according to an embodiment of the present invention.

[0035] In detail, in Fig. Figure 1 shows a combined mono / biaxial LiDAR system 1 for Time-of-Flight (ToF) based LiDAR systems, whose measurement principle is the emission of light pulses, or a time-varying light power, as well as time-resolved detection. Light from a light source 2, preferably from a laser, where the laser can also be integrated on a chip, first strikes an adjustable coupler 10. For monoaxial use, this is set to a coupling ratio of 50:50, meaning that the light is evenly split between two outputs 18, 19. With this setting, half of the light from the light source 2 is directed to a first optical deflection and beam shaping device 3, light path 100, and the other half to a second optical deflection and beam shaping device 4, which is not required for monoaxial operation. This results in losses of, for example, 3 dB.The first optical deflection and beam shaping device 3 sends the light onto an object 20, which partially reflects it back. This reflected light is then received by the first optical deflection and beam shaping device 3 and reflected or directed to the adjustable coupler 10. Based on symmetry conditions and its selected setting, the coupler splits the reflected light in half. One half travels back to the light source 2 and is discarded, while the other half is directed to a detector 5. This split results in a further 3 dB loss along the optical path 101 to the detector 5. The received light is then fed to the detector 5, where it is, for example, converted into an electrical signal.

[0036] To precisely adjust the adjustable coupler 10, another directional coupler 11a with an ideally highly asymmetrical coupling ratio, here 99:1 for example, is arranged at one of its outputs 19 on the side facing away from the light source 2. A small portion of the light power is coupled out at this point and fed to a monitor photodiode 11b. When the coupling ratio of the adjustable coupler 10 is varied, the power at the photodiode 11b varies, with this power being proportional to the coupling ratio. Based on the detected power at the monitor photodiode 11b, the control point at which a balanced coupling ratio is achieved can then be set.

[0037] To guarantee purely monoaxial reception, the second optical deflection and beam shaping device 4 must assume an angular position that differs significantly from that of the first optical deflection and beam shaping device 3, or the light path to the second optical deflection and beam shaping device 4 must be interrupted by an aperture or a damping element. This ensures that no power is received at this point.

[0038] Each of the two optical deflection and beam shaping devices 3, 4 is calibrated; that is, the optimal settings of the optical deflection and beam shaping devices 3, 4 are determined to achieve a deflection direction at the desired angle. This is particularly important when the respective optical deflection and beam shaping device 3, 4 is implemented as an optical phase array. For this purpose, for example, the optical deflection and beam shaping device 3, 4 is illuminated with light from input 28, and the far field of emission from the respective optical deflection and beam shaping device 3, 4 is analyzed. To calibrate both optical deflection and beam shaping devices 3, 4, light power is coupled into each of them sequentially. This is achieved by setting the adjustable coupler 10 first to 100:0% and then to 0:100%.

[0039] Fig. Figure 2 shows an optical system according to an embodiment of the present invention.

[0040] In detail, it shows Fig. 2 an optical system according to Fig. 1, in contrast to Fig. 1 this is now operated in biaxial mode.

[0041] In contrast to the monoaxial mode according to Fig. 1. The adjustable coupler 10 is now set to an asymmetric ratio of ideally 100:0%. This means that the light is directed completely onto the first optical deflection and beam shaping device 3 and emitted in the direction of the (measured) object 20. In contrast to monoaxial operation according to Fig. 1. No significant loss occurs at the adjustable coupler 10. The backscattered light from an object 20 is not collected by the same optical deflection and beam shaping device 3, but by the second optical deflection and beam shaping device 4, since the latter is aligned in the solid angle from which back reflection from the object 20 is expected. This is, in particular, the direction in which the first optical deflection and beam shaping device 3 was aligned at the time of emission. With a rapid change in angle, the angles of the two optical deflection and beam shaping devices 3 and 4 therefore differ, since the first optical deflection and beam shaping device 3 is already addressing a different solid angle at the time of reception by the second optical deflection and beam shaping device 4.In this way, the biaxial mode allows for faster scanning of the relevant solid angle range as well as, at fixed scanning speeds, longer light transit times and thus greater object distances.

[0042] Due to the selected coupler setting of 100:0% and symmetry conditions, 100% of the light is now coupled from the second optical deflection and beam shaping device 4 to the detector 5.

[0043] Fig. Figure 3 shows an optical system according to an embodiment of the present invention.

[0044] In the embodiment of the Fig. Figure 3 shows a Frequency Modulated Continuous Wave (FMCW) LiDAR system 1. Here, a portion of the light emitted by the light source 2 (light path 100') is superimposed with the light scattered back from the object 20 (receive path 101). This is similar to the implementation as a ToF system according to the Fig. 1 and Fig. 2. The (FMCW) LiDAR system 1 can be configured differently, e.g., free-beam optical, fiber-optic, or integrated-optical. The following describes monoaxial operation: The light from a light source 2, preferably a laser, first strikes an adjustable coupler 10. This coupler is set to a coupling ratio of 50:50% for monoaxial operation. Thus, half of the light from the light source 2 is directed to the first optical deflection and beam shaping device 3 and emitted onto the object 20 (light path 100), where parts of it are backscattered (light path 101) and received by the first optical deflection and beam shaping device 3. This received light is then directed again, half to a combining adjustable coupler 10.

[0045] A mixer 6a directs this light to a first balance diode 5a. Simultaneously, during the first pass through the adjustable coupler 10, 50% of the light from the light source 2 is decoupled for a local oscillator 12. This is superimposed with the backscattered light in the mixer 6a by means of a diversion device 7, and the resulting beat signal allows both the distance and the speed of the object 20 to be calculated.

[0046] To ensure a precise 50:50 ratio, another diode 11b can be placed in the path of the local oscillator 12. The coupling ratio of the adjustable coupler 10 can then be traversed, and the control point at which the average power is reached can be set. Alternatively, the balance diode 5b can be used; in this case, the diverting device 7 only needs to be calibrated and adjusted so that some of the light reaches the balance diode 5b. The diverting device 7 can also be used to control the amount of light reaching the balance diode 5a as a local oscillator. The purpose of controlling the amount of light is to ensure that the balance diode 5a is neither oversaturated (i.e., supplied with too much light) nor operated too close to the noise limit (i.e., supplied with too little light).

[0047] Fig. Figure 4 shows an optical system according to an embodiment of the present invention.

[0048] Fig. Figure 4 shows an optical system according to Fig. 3, in contrast to Fig. 3 this is now operated in biaxial mode.

[0049] In contrast to the monoaxial mode according to Fig. 3 the coupler 10 will be according to Fig. 4 is now set to a ratio of 90:10 or 99:1. This means that at least 90% of the light is not decoupled and is emitted onto the object 20 via the first optical deflection and beam shaping device 3. The backscattered light from the object 20 is now collected not by the same device, but by the second optical deflection and beam shaping device 4. The superposition from the local oscillator 12 now takes place on a second balancing diode 5b. The local oscillator 12 is driven by light that has been tapped off by the adjustable coupler 10 (light path 100' via output 19 of the adjustable coupler 10).

[0050] Fig. Figure 5 shows a coupling device according to an embodiment of the present invention.

[0051] In detail, in Fig. Figure 5 shows a coupling device 10 in the form of a Mach-Zehnder interferometer, which is implemented using two 2:2 couplers 10', 10". A phase shifter 10a, 10b is arranged in each arm of the interferometer. The phase shifters 10a, 10b can shift the phase in one arm relative to the other and thus adjust the interference at the output coupler 10'. Here, the phase shifters 10a, 10b are implemented thermally, whereby the temperature is locally increased and the refractive index is locally changed by the thermo-optic coefficient. Alternatively or additionally, electro-optical or mechanical phase shifters can also be used.

[0052] Fig. Figure 6 shows steps of a method according to an embodiment of the present invention.

[0053] In Fig. Section 6 shows the steps of a procedure for operating an optical system. The procedure comprises the following steps:

[0054] In the first step S1, light from a light source is provided.

[0055] In a further step S2, the light from the light source is split between two optical devices according to a predefinable coupling ratio by means of a coupling device.

[0056] In a further step S3, the split light is emitted via the first optical device.

[0057] In a further step S4, emitted light is received via the first optical device using a first receiving light path or via the second optical device using a second receiving light path.

[0058] In a further step S5, the received light is forwarded from one of the two optical devices to a detector.

[0059] In summary, at least one embodiment of the invention has at least one of the following advantages: - Minimizing optical losses. - High flexibility regarding possible distances to targets to be detected. - Simple implementation.

[0060] Although the present invention has been described using preferred embodiments, it is not limited to these, but can be modified in many ways.

Claims

[1] Optical system (1) comprising at least the following components: a light source (2), a first optical device (3) for the directed emission of light from the light source (2) via a transmitting light path (100) and for the directed reception of emitted light via a first receiving light path (101), a second optical device (4) for receiving emitted light in a directed manner via a second receiving light path (101'), wherein either the first or the second optical device (3, 4) is operable for receiving emitted light, a detector (5, 5a, 5b) for detecting the received light and a coupling device (10) for splitting the emitted light between the two optical devices (3, 4) according to a predefinable coupling ratio and for forwarding received light from one of the two optical devices (3, 4) to the detector (5, 5a, 5b). [2] Optical system according to claim 1, wherein, depending on the coupling ratio, either light can be received by means of the first or the second optical device (3, 4). [3] Optical system according to one of claims 1-2, wherein the coupling device (10) is configured to adjust the coupling ratio during the operation of the optical system (1), wherein either light can be received by means of the first or the second optical device. [4] Optical system according to one of claims 1-3, wherein the coupling device (10) is configured to adjust the coupling ratio depending on the distance of a target (20). [5] Optical system according to one of claims 1-4, wherein a monitoring device (11) is arranged for monitoring the set coupling ratio of the coupling device (10). [6] Optical system according to claim 5, wherein the monitoring device (11) comprises a coupler (11a), in particular a directional coupler, and a photodiode (11b), wherein the coupler (11a) has a coupling ratio of at least 90:10, in particular 99:

1. [7] Optical system according to one of claims 1-6, wherein a mixing device (6a, 6b) and an oscillation device (12) are arranged such that received light is superimposed with emitted light of the oscillation direction (12) by means of the mixing device (6a, 6b) and is then directed onto the detector (5, 5a, 5b). [8] Optical system according to one of claims 1-7, wherein a second detector (5b) is arranged which can be supplied with light to be detected according to the set coupling ratio of the coupling device (10). [9] Optical system according to one of claims 1-8, wherein the coupling device (10) comprises a Mach-Zehnder interferometer (10'). [10] Optical system according to claim 9, wherein the Mach-Zehnder interferometer (10') has two phase shifters (10a, 10b) which are thermo-optical, electro-optical and / or mechanical. [11] Optical system according to one of claims 1-10, wherein at least one of the optical devices (3, 4), in particular both, are designed in the form of optical phase arrays. [12] Optical system according to one of claims 1-11, wherein at least one, preferably all components (2, 3, 4, 5, 10, 11) are integrated on a silicon chip. [13] Method for operating an optical system comprising the steps: - Providing (S1) light from a light source (2), - Splitting (S2) the light from the light source (2) onto two optical devices (3, 4) according to a predefinable coupling ratio by means of a coupling device (10), - Emission (S3) of the split light for the first optical device (3) via the first optical device (3), - Receiving (S4) emitted light via the first optical device (3) using a first receiving light path (101) or via the second optical device (4) using a second receiving light path (101'), and - Forwarding (S5) the received light from one of the two optical devices (3,4) to a detector (5). [14] LiDAR system comprising an optical system (1) according to any one of claims 1-11, which can be operated monoaxially or biaxially by means of the integrated optical system (1). [15] Method for producing an optical system according to any one of claims 1-12, comprising a complementary metal-oxide semiconductor process.

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