Deskew Correction Technology in FMCW-LiDAR System

Abstract

The LiDAR system includes a light source that emits a light beam, a first optical lens that transmits the light beam, an optical window oriented to the first optical lens that reflects a first portion of the light beam to generate an LO signal, a scanner that transmits a second portion of the light beam toward a target, scans the target, and generates a target return signal, a second optical lens that transmits the LO signal and the target return signal to a photodetector (PD), and the photodetector that mixes the target return signal with the LO signal to extract distance and velocity information of the target. The LiDAR system comprises an optical subsystem having an optical axis of the light beam. The LO signal mixed with the target return signal is arranged eccentrically from the optical axis of the light beam in the second optical lens so as to increase an overlapping ratio between the LO signal and the target return signal on a detection surface of the photodetector.

Description

Technical Field

[0001] This application claims the benefit and priority of U.S. Patent Application No. 17 / 842,557, "Descan Correction Technology in FMCW-LiDAR Systems," filed on June 16, 2022.

[0002] The present invention relates to a light detection and ranging (LiDAR) system, and more particularly, to descan correction (correction of the positional deviation of the target return signal) in a frequency-modulated continuous wave (FMCW) LiDAR system.

Background Art

[0003] An FMCW-LiDAR system mixes a local oscillator (LO) signal with a target return signal reflected from a target to extract distance and velocity information. A reflective scanner is used to scan the target plane with laser irradiation light. In order to improve the frame rate, the scan speed of the scanner has been increased. However, increasing the scan speed causes a spatial mismatch (positional deviation) of the target signal with respect to the LO signal in the detector. Such a mismatch reduces the spatial mixing efficiency of the target return signal and the LO signal, and as a result, causes a reduction in the signal-to-noise ratio of the entire LiDAR system.

Summary of the Invention

Means for Solving the Problems

[0004] Hereinafter, aspects of the present invention, that is, aspects of each invention of a LiDAR system equipped with descan correction will be described.

[0005] As an aspect of the present invention, an optical subsystem of a LiDAR system and a method for reducing signal loss due to lag angle descan through vertical (or horizontal) displacement of an imaging lens or tilt of an LO plane are disclosed. Through the vertical displacement of the lens and / or the tilt of the LO plane, both the LO signal and the target return signal can be shifted at the detection plane. The magnitude of these corrections is directly determined by the severity of the lag angle descan. The magnitude and direction of the displacement are adjusted to accommodate different descan conditions. Due to the characteristics of this system, the LO signal has significantly higher sensitivity to these corrections compared to the target return signal. By taking these into account, the overlap between the target return signal and the LO signal increases, improving the coherent mixing efficiency of the LiDAR system (e.g., a high-speed scanning LiDAR system). This improves the signal-to-noise ratio (SNR) of the optical receiver and can enhance the performance of the LiDAR system.

[0006] A LiDAR system according to an aspect of the present invention includes an optical subsystem having an optical axis of the following optical beam. This optical subsystem includes a light source that emits the optical beam, a first optical lens that transmits the optical beam, and an optical window oriented to the first optical lens that reflects a first portion of the optical beam to generate a local oscillator (LO) signal. Behind this optical window, an optical scanner that transmits the remaining portion (second portion) of the optical beam toward the target and scans the target to generate a target return signal is used. The optical subsystem further includes a second optical lens and a photodetector (PD). The second optical lens is configured to transmit the LO signal and the target return signal to the photodetector (PD). The photodetector (PD) is configured to mix the target return signal with the LO signal to extract the distance and velocity information of the target. The LO signal mixed with the target return signal is arranged eccentrically from the optical axis of the light beam in the second optical lens so as to increase the overlapping ratio of the LO signal and the target return signal on the detection surface of the photodetector (PD).

[0007] A method of a LiDAR system according to an aspect of the present invention is a method of optical detection and ranging (LiDAR), comprising the steps of emitting a light beam by a light source, transmitting the light beam to an optical window by a first optical lens, reflecting a first portion of the light beam by the optical window to generate a local oscillator (LO) signal, transmitting a second portion of the light beam to a target, scanning the target to generate a target return signal, transmitting the LO signal and the target return signal to a photodetector (PD) by the second optical lens, the step of arranging the LO signal eccentrically from the optical axis of the light beam in the second optical lens so as to increase the overlapping ratio of the LO signal and the target return signal on the detection surface of the photodetector (PD), and extracting distance and velocity information of the target by mixing the target return signal with the LO signal by the photodetector (PD).

[0008] In some embodiments of the present disclosure, the use of point clouds is shown, but the embodiments of the present invention are not limited thereto, and the use of point sets and the like is also included.

[0009] The above and other aspects of the present invention will become apparent by referring to the detailed description together with the drawings briefly described below. The present invention also includes any combination of two or more of the features or elements described herein, regardless of whether those features or elements are explicitly combined or described in a particular embodiment. The present invention should be construed broadly, and the separable features and elements described in each aspect and embodiment of the present invention are understood to be combinable with each other unless clearly shown to be different in the context of the present disclosure.

[0010] Accordingly, this summary is provided to summarize some embodiments of the present invention and to provide a basic understanding of one aspect of the present invention, and is not intended to limit or narrowly construe the scope or spirit of the present invention in any way. Other embodiments, aspects, and advantages of the invention will become apparent by referring to the following detailed description and the accompanying drawings that illustrate the principles of the described embodiments.

Brief Description of the Drawings

[0011] To clarify the various aspects of the present invention, the drawings referred to in the following detailed description (embodiments) are shown. Note that the same reference numerals in the drawings denote the same elements.

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Best Mode for Carrying Out the Invention

[0027] Hereinafter, various embodiments and aspects of the present invention will be described based on the details described below and the accompanying drawings. The following description and drawings are examples of the present invention and should not be construed as limiting the present invention. Although many details are specifically described so that various embodiments of the present invention can be fully understood, in certain cases, the description of well-known or conventional details may be omitted to simplify the description of the embodiments of the present invention.

[0028] The LiDAR system described in this specification can be implemented in any sensing market such as (but not limited to) transportation, manufacturing, measurement, medical, virtual reality, augmented reality, security systems, etc. According to some embodiments, the LiDAR system described in this specification is implemented as part of the front end of a frequency-modulated continuous wave (FMCW) device that supports spatial recognition of an automated driving assistance system or an automated driving vehicle.

[0029] Figure 1A shows a LiDAR system 100 according to an embodiment of the present invention. The LiDAR system 100 includes one or more of each of a plurality of components, but may include fewer components or additional components than shown in Figure 1A. In some embodiments, one or more of the components described with respect to the LiDAR system 100 are implemented on a photonics chip. The optical circuit 101 may include a combination of active optical components and passive optical components. The active optical components perform functions such as generating, amplifying, and / or detecting optical signals. In some embodiments, the active optical components include optical beams of different wavelengths and include one or more optical amplifiers, one or more photodetectors, and the like.

[0030] The free-space optical system 115 includes one or more optical waveguides for transmitting optical signals and routing and manipulating the optical signals to appropriate input / output ports of the active optical circuit. The free-space optical system 115 includes one or more optical components such as taps, wavelength division multiplexers (WDMs), splitters / combiners, polarization beam splitters (PBSs), collimators, couplers, and the like. In some embodiments, the free-space optical system 115 includes optical components for converting the polarization state and guiding the received polarized light to a photodetector, for example, using a PBS. Further, the free-space optical system 115 may include a diffractive element that deflects beams of different frequencies at different angles.

[0031] In some embodiments, the LiDAR system 100 includes an optical scanner 102 having one or more scanning mirrors. These scanning mirrors are rotatable along an axis (e.g., the slow axis) that is orthogonal or substantially orthogonal to the fast axis of the diffractive element to direct an optical signal that scans the target environment according to a scanning pattern. Objects within the target environment scatter the incident light to generate a return light beam or a target return signal. The optical scanner 102 can also collect the return light beam or the target return signal and return it to the passive optical circuit components of the optical circuit 101. For example, the return light beam is directed towards the photodetector by a polarization beam splitter. Note that the optical scanner 102 may include, in addition to mirrors and galvanometers, a quarter-wave plate, a lens, an anti-reflection-coated optical window, etc.

[0032] The LiDAR system 100 is provided with a LiDAR control device 110 to control and support the optical circuit 101 and the optical scanner 102. The LiDAR control device 110 includes a processing device necessary for the LiDAR system 100. The processing device according to some embodiments is one or more general-purpose processing devices such as a microprocessor and a central processing unit. Specifically, it is a complex instruction set computing (CISC) microprocessor, a reduced instruction set computing (RISC) microprocessor, a very long instruction word (VLIW) microprocessor, or a processor implementing other instruction sets, or a processor implementing a combination of instruction sets. Further, the above processing device may be one or more special-purpose processing devices such as an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA: Field Programmable Gate Array), a digital signal processor (DSP), and a network processor.

[0033] In some embodiments, the LiDAR control device 110 is provided with a signal processing unit 112 such as a DSP. Thereby, the LiDAR control device 110 outputs a digital control signal for controlling the optical driver 103. The digital control signal is converted into an analog signal via the signal conversion unit 106. For example, the signal conversion unit 106 includes a digital / analog converter. The optical driver 103 supplies drive signals to the active optical components of the optical circuit 101 and drives light sources such as lasers and amplifiers. In some embodiments, a plurality of optical drivers 103 and signal conversion units 106 may be provided to drive a plurality of light sources.

[0034] The LiDAR control device 110 is also configured to output a digital control signal to the optical scanner 102. The motion control device 105 can control the galvanometer of the optical scanner 102 based on the control signal received from the LiDAR control device 110. Specifically, a digital / analog converter can be used to convert the coordinate routing information from the LiDAR control device 110 into a signal processable by the galvanometer of the optical scanner 102. In some embodiments, the motion control device 105 can also send back information regarding the position or operation of the components of the optical scanner 102 to the LiDAR control device 110. Specifically, an analog / digital converter can be used to convert the information regarding the position or angle of the galvanometer into a signal processable by the LiDAR control device 110.

[0035] The LiDAR control device 110 is further configured to analyze the input digital signal. In this regard, the LiDAR system 100 is provided with a photoreceiver 104 for measuring one or more beams received by the optical circuit 101. Specifically, the reference beam receiver as the photoreceiver 104 measures the amplitude of the reference beam from the active optical component and converts the signal from the reference beam receiver into a signal processable by the LiDAR control device 110 by means of an analog / digital converter. Also, the target receiver as the photoreceiver 104 measures the optical signal carrying information regarding the distance and speed of the target in the form of a beat frequency modulated optical signal. In this case, the reflected beam of the optical signal may be mixed with the signal from the local oscillator. The photoreceiver 104 can be provided with a high-speed analog / digital converter for converting the signal from the target receiver into a signal processable by the LiDAR control device 110. In some embodiments, the signal from the photoreceiver 104 can be subject to signal conditioning by the signal conditioning unit 107 before being received by the LiDAR control device 110. For example, the signal from the photoreceiver 104 can be supplied to an operational amplifier of the signal conditioning unit 107 for amplification of the return signal, and the signal amplified by the operational amplifier can be supplied to the LiDAR control device 110.

[0036] In some applications, the LiDAR system 100 can additionally be provided with one or more imaging devices 108 configured to capture an image of the environment, a global positioning system (GPS) 109 configured to provide the geographical location of the system, or other sensor inputs. Also, the LiDAR system 100 can be provided with an image processing device 114. In this case, the image processing device 114 can be configured to receive an image and a geographical location from the imaging device 108 and the global positioning system (GPS) 109 and transmit the image and the location or information related thereto to the LiDAR control device 110 or other systems connected thereto.

[0037] As a process according to some embodiments, the LiDAR system 100 is configured to simultaneously measure distance and velocity in two dimensions using a non-degenerate optical light source. This function enables real-time long-distance measurement of the distance, velocity, azimuth angle, and elevation angle of the surrounding environment.

[0038] In some embodiments, the scanning process starts from the optical driver 103 and the LiDAR control device 110. The LiDAR control device 110 instructs the optical driver 103 to modulate one or more light beams respectively, and these modulation signals are transmitted through the passive optical circuit of the optical circuit 101 to the collimator of the free-space optical system 115. The collimator guides the modulation signals to the optical scanner 102, and the optical scanner 102 scans the environment in a pattern pre-programmed by the motion control device 105. The optical circuit 101 may be provided with a polarization wavelength plate (PWP) that converts the polarization state of light when the light exits the optical circuit 101. A portion of the polarized light beam can be reflected back to the optical circuit 101. For example, the lens system or collimating system used in the LiDAR system 100 may have natural reflection characteristics or a reflective coating, whereby a portion of the light beam is reflected back to the optical circuit 101.

[0039] The optical signal reflected from the environment is sent through the optical circuit 101 to the receiver (optical receiver 104). At this time, since the polarization state of the light has been converted, it is reflected by a polarization beam splitter together with a portion of the polarized light that has reflected back to the optical circuit 101. As a result, the reflected optical signal does not return to the same optical fiber or waveguide as the light source and is reflected to separate optical receivers respectively. These signals interfere with each other to generate a combined signal. Each beam signal returning from the target generates a time-shifted waveform, and the beat frequency measured by the optical receiver (photodetector) is generated by the time phase difference between these two waveforms. Then, the combined signal can be reflected to the optical receiver 104.

[0040] The analog signal received by the optical receiver 104 is converted into a digital signal by an ADC (analog / digital converter). Then, the digital signal is transmitted to the LiDAR control device 110. The signal processing unit 112 of the device receives and processes these digital signals. In some embodiments, the signal processing unit 112 receives position data from the motion control device 105 and a galvanometer (not shown), and receives image data from the image processing device 114. Thereby, the signal processing unit 112 can generate a 3D point cloud having information on the distances and velocities of points in the environment when the optical scanner 102 scans additional points. The signal processing unit 112 may also overlay the 3D point cloud with the image data to determine the velocities and distances of surrounding objects. This system may further process satellite-based navigation position data to provide accurate global position information.

[0041] FIG. 1B is a time-frequency diagram 200 of a FMCW scanning signal that can be used for a LiDAR system, such as the LiDAR system 100, to scan a target environment in one embodiment. In this example, the scanning signal denoted as f FM (t)201 has a sawtooth waveform (sawtooth "chirp") with a chirp bandwidth Δf C and a chirp period T. The slope of the sawtooth is k = (Δf C / T C ). FIG. 1B also shows a target return signal f FM (t - Δt)202 in one embodiment. The target return signal denoted as f FM (t - Δt)202 is a time-delayed version of the scanning signal f FM (t)201, and Δt is the round-trip time between the target irradiated by the same scanning signal. This round-trip time is given by Δt = 2R / v. Here, R is the distance of the target, and v is the speed of light c, the speed of the light beam. Therefore, the distance R of the same target can be calculated as R = c(Δt / 2). When the target return signal f FM (t - Δt)202 is optically mixed with the scanning signal, a distance-dependent difference frequency ("beat frequency") ΔfR (t) is generated. Beat frequency Δf R (t) has a linear relationship with the time delay Δt depending on the slope k of the sawtooth. That is, Δf R (t) = kΔt. Since the target distance R is proportional to Δt, the target distance R can be calculated as R = (c / 2)(Δf R (t) / k). That is, the distance R has a linear relationship with the beat frequency Δf R (t). The beat frequency Δf R (t) is generated as an analog signal, for example, by the optical receiver 104 of the LiDAR system 100. This beat frequency is digitized, for example, by an analog / digital converter (ADC) in the signal conditioning unit 107 of the LiDAR system 100. The beat frequency signal digitized in this way is digitally processed by a signal processing unit (e.g., signal processing unit 112) in the LiDAR system 100. However, it should be noted that when the target has a relative velocity with respect to the LiDAR system 100, the target return signal f FM (t - Δt)202 generally includes a frequency offset (Doppler shift). Since the Doppler shift is separately detected and used to correct the frequency of the return signal, the Doppler shift is not shown in FIG. 2 for simplicity and ease of explanation. Also, it should be noted that the sampling frequency of the ADC is determined to be the highest beat frequency that can be processed by the system without generating aliasing. Generally, the highest frequency that can be processed is half of the sampling frequency (i.e., the "Nyquist frequency"). For example, without limitation, if the sampling frequency of the ADC is 1 gigahertz, the highest beat frequency (Δf Rmax ) that can be processed without aliasing is 500 megahertz. This limit is determined by R max =(c / 2)(Δf Rmax / k) and can be adjusted by changing the slope k of the sawtooth. In one example, the data samples from the ADC may be continuous, but the subsequent digital processing described below can be divided into "time segments" associated with a predetermined periodicity of the LiDAR system 100. For example, without limitation, the time segment may correspond to the number of chirp periods T or the number of rotations in the azimuth direction by the aforementioned optical scanner.

[0042] FIG. 2A is a diagram showing an example of a detection surface 202 of a LiDAR system without descanning correction. FIG. 2B is a diagram showing an example of an intensity profile on the detection surface 202 shown in FIG. 2A. In recent years, in a LiDAR system, a high-speed scanning mirror may be used to irradiate each scene. For example, one scanning mirror performs high-speed scanning along the X direction (horizontal), and the other scanning mirror performs low-speed scanning along the Y direction (elevation angle). The received light has a changing frequency characteristic, and this is used to extract distance information. As described above, the signal processing unit 112 (see FIG. 1A) can generate a 3D point cloud including information regarding the distance and speed of a target.

[0043] In order to improve the frame rate while maintaining the number of points per frame, usually, the scanning speed of the scanning mirror (e.g., in one direction) is increased. While the light beam (frequency sweep) reaches a distant target and returns to the scanning system, the scanning mirror moves at a high scanning speed (e.g., high rotational speed). When the scanning speed increases, a spatial mismatch (positional deviation) between the target return signal and the LO signal in the detector of the LiDAR system is caused. This mismatch significantly reduces the spatial mixing efficiency between the target return signal and the LO signal, and as a result, the signal-to-noise ratio of the entire LiDAR system may decrease. For example, when the scanning speed of the scanning mirror exceeds 100 Hz and measurements are made at a long distance (> 3000° (angle) / second), the spatial mixing efficiency significantly decreases.

[0044] As shown in FIGS. 2A and 2B, a LiDAR system (e.g., the LiDAR system 100 in FIG. 1A) may include an optical subsystem having an optical axis 201. For example, the LO signal 203 and the target return signal 205a are designed to overlap at the detection surface 202 of a photoreceiver (e.g., one of the photoreceivers 104 in FIG. 1A). The LO signal 203 and the target return signal 205a are designed to be aligned about the optical axis 201. However, due to the high scan speed of the scanning mirror, the actual target return signal 205b may have a lag angle (delay angle) at the detection surface 202. In this case, as shown in FIG. 2B, the intensity profile of the actual target return signal 205b may be shifted by the lag angle. And due to the high scan speed of the scanning mirror, a spatial misalignment 204 may occur between the actual target return signal 205b and the LO signal 203 at the detection surface 202 of the LiDAR system. This spatial misalignment is a factor that reduces the signal-to-noise ratio (SNR) of the entire LiDAR system. Here, descanning refers to the phenomenon in which the target return signal is angularly shifted at the detection surface, which is caused, for example, by a high scan speed. The FMCW-LiDAR system mixes the target return signal and the LO signal to extract distance and velocity information. Therefore, the performance of the system depends on the ability to properly combine both signals. In a high-speed scanning LiDAR system, descanning may rapidly reduce the signal-to-noise ratio (SNR) of the photoreceiver due to the spatial misalignment between the target return signal and the LO signal at the detection surface. Therefore, it is necessary to maximize the overlap between the target return signal and the LO signal and maximize the coherent mixing efficiency.

[0045] FIG. 3A is a diagram showing an example of the detection surface 302 of a LiDAR system with descanning correction according to an embodiment of the present invention. FIG. 3B is a diagram showing an example of the intensity profile at the detection surface 302 shown in FIG. 3A. As shown in FIGS. 3A and 3B, the optical subsystem of a LiDAR system (e.g., system 100 of FIG. 1A) is configured to reduce signal loss due to lag angle descan through corrections such as vertical (or horizontal) displacement of the imaging lens or tilt of the LO plane. Through such vertical lens displacement and / or tilt of the LO plane, both the LO signal 303 and the target return signal 305 are shifted at the detection surface 302 of a photoreceiver (e.g., one of the photoreceivers 104 of FIG. 1A). Due to the characteristics of the present system, the LO signal 303 is significantly more sensitive to corrections than the target return signal 305. As shown in FIG. 3A, the correction causes the LO signal 303 to have a shift 304, which is much larger than the shift (not shown) of the target return signal 305. For example, the shift 304 of the LO signal 303 may be a value close to the misalignment 204. In such a relationship, in the present system, the misalignment between the LO signal 303 and the target return signal 305 can be corrected.

[0046] The magnitude of the correction is determined by the severity of the lag angle descan. The magnitude and direction of the displacement can be adjusted to accommodate different descan conditions. For example, the magnitude and direction of the displacement are determined based on the scan speed of the scanning mirror. In this way, the overlap between the target return signal 305 and the LO signal 303 can be increased, and as a result, it becomes possible to improve the coherent mixing efficiency of the LiDAR system in a high-speed scanning type LiDAR system or the like. Thereby, the signal-to-noise ratio (SNR) of the photoreceiver is improved, and the performance of the LiDAR system is enhanced.

[0047] FIG. 4A is a diagram showing an example of an optical subsystem 400 of a LiDAR system with descan correction by an offset (first offset) 430 of an imaging lens according to an embodiment of the present invention. FIG. 4B is a diagram showing the descan correction in the optical subsystem 400 shown in FIG. 4A. The LiDAR system of this embodiment may be the LiDAR system 100 shown in FIG. 1A. The optical subsystem 400 may include components such as an optical circuit 101, a free-space optical system 115, an optical scanner 102, and an optical receiver 104. The optical subsystem 400 includes a light source 402 (e.g., disposed within the optical circuit 101). The optical subsystem 400 includes a beam splitter (BS) 404 (or a polarization beam splitter (PBS)), a lens system 411, an optical window 406, a lens system 412, and a lens system 413, which may be components of the free-space optical system 115 or the optical circuit 101. The optical subsystem 400 includes an optical scanner 408, which may include one of the optical scanners 102. Also, the optical subsystem 400 includes a photodetector (PD) 440, which may include one of the optical receivers 104. Additionally, the optical subsystem 400 includes an optical axis 401 for aligning a plurality of optical components.

[0048] As shown in FIGS. 4A and 4B, the light source 402 emits a light beam 420. In the transmission path, the light beam is the transmission signal 421. The light beam 420 passes through a beam splitter (BS) 404 (or a polarization beam splitter (PBS)), and then is focused on an optical window 406, which is, for example, a partially reflective window plate. In the case of a PBS, a polarization wave plate or a Faraday rotator may be used to change the polarization reflected from the optical window 406. The optical window 406 includes a flat or curved reflecting surface that is the LO surface 406a, from which a local oscillator (LO) is generated. The LO surface 406a may be the front surface or the back surface of the optical window 406.

[0049] The optical window 406 can transmit a portion of the optical beam 420 toward the target 409 and reflect a portion of the optical beam 420 by the reflecting surface 406a to generate the LO signal 423. The transmission signal 421 passes through the optical window 406 and is collimated using the lens system 413. Thereafter, the transmission signal 421 propagates to the optical scanner 408 and is transmitted to the target 409 to scan the target.

[0050] The LO signal 423, which is the reflected light from the optical window 406, forms an image on the detection surface of the PD440 by the lens system 412. On the other hand, the target return signal 425 formed by the reflected light from the target 409 is not affected by the optical window 406. The target return signal 425 also forms an image on the PD440 by the lens system 412. A radio frequency (RF) beat signal is generated by the coherent mixing of the LO signal 423 and the target return signal 425.

[0051] The optical beam 420 incident on the lens system 411 is focused on the optical window 406 (e.g., the reflecting surface 406a). This reflecting surface 406a is either the front or back LO signal generation surface and may have the function of retroreflecting light. When the scanner is stationary, no descanning occurs in the optical path of the target return signal, so it is coaxial with the optical path of the LO signal. However, as described above, when the speed of the scanner increases, without descanning correction, the optical paths of the LO signal and the target return signal begin to diverge, resulting in a decrease in the mixing efficiency at the detector.

[0052] As shown in FIG. 4A, the lens system 411 is arranged to have a vertical offset 430 (or displacement) in a direction perpendicular to the optical axis 401. The lens system 411 is eccentric from the optical axis on a plane perpendicular to the optical axis 401 and can move up and down. The offset 430 (or displacement) in the direction perpendicular to the optical axis 401 may be referred to as a vertical offset. The lens system 411 may have a positive vertical offset 430 (e.g., moving upward from the optical axis 401) or a negative vertical offset 430 (e.g., moving downward from the optical axis 401).

[0053] The optical beam 420 that generates the LO signal 423 is incident on the lens system 411 at a position eccentric from the optical axis 401 before reflection and is also incident on the lens system 411 at an eccentric position after reflection. Therefore, the chief ray 423a of the LO signal 423 in the lens system 412 has a very high sensitivity to the vertical offset 430 of the lens system 411. On the other hand, although eccentricity occurs on the lens system 412 in the optical path of the target return signal, the degree is much smaller compared to the eccentricity that occurs in the optical path of the LO signal. This is because the optical path of the target return signal is a single pass (one-time passage). The chief ray 425a of the target return signal 425 in the lens system 412 does not have such a high sensitivity to the vertical offset 430 of the lens system 411. Therefore, by utilizing the difference in sensitivity to the vertical offset 430 between the optical path of the LO signal and the optical path of the target return signal, adjustment for optimizing the overlap of the beams of the LO signal and the target return signal on the detection surface becomes possible. That is, since the chief ray 425a of the target return signal 425 and the chief ray 423a of the LO signal 423 in the lens system 412 have different sensitivities to the vertical offset 430 of the lens system 411, it becomes possible to increase the overlap of the target return signal 425 and the LO signal 423 on the detection surface by applying the vertical offset 430.

[0054] The LO plane 406a of the optical window 406 is arranged to have a displacement (misalignment) or offset from, for example, the focal plane (not shown) of the lens system 411. If the LO plane is at the focal plane of the lens system 411 and the detection plane of the PD 440 is at the focal plane of the lens system 412, the LO plane becomes an intermediate image of the image formed on the PD. In such a system, it is not sensitive (not affected) to the vertical offset 430 of the lens system 411. However, by introducing a longitudinal displacement or offset along the optical axis 401 between the lens system 411 and the LO plane 406a, it becomes possible to shift and decentre the LO signal at the detection plane of the PD 440. Also, by shifting the position of the optical window 406 in the longitudinal direction along the optical axis 401, a mechanism for adjusting the size of the LO signal 423 is provided, and it becomes possible to further adjust (optimize) the overlap between the target return signal 425 and the LO signal 423.

[0055] As shown in FIGS. 4A and 4B, the LO signal 423 is arranged at a position decentred from the optical axis 401 on the lens system 412. At this time, the chief ray 423a of the LO signal 423 is arranged to have an offset from the optical axis 401 on the lens system 412. Also, the chief ray 423a of the LO signal 423 is arranged at a decentred position in the lens system 412 on a plane perpendicular to the optical axis 401. Therefore, the chief ray 423a of the LO signal 423 can be shifted at the detection plane of the PD 440. As described above, when the optical scanner 408 is operating at a high scan speed, the target return signal 425 has a lag angle at the detection plane of the PD 440. At this time, by arranging the lens system 411 at a position having the vertical offset 430, it becomes possible for the chief ray 423a of the LO signal 423 to shift more greatly than the chief ray 425a of the target return signal 425 at the detection plane of the PD 440. As a result, the overlap between the LO signal 423 and the target return signal 425 at the detection plane of the PD 440 can be increased. For example, the coherent mixing efficiency of the target return signal 425 and the LO signal 423 may be partially proportional to the overlapping ratio (overlap rate) of the LO signal 423 and the target return signal 425 on the detection surface of the PD440.

[0056] The vertical offset 430 can be determined based on the scan speed of the optical scanner 408, the arrangement of the scanner with respect to the detection surface, or the distance to the target. For example, the magnitude of the vertical offset 430 may be determined based on the scan speed. The higher the scan speed, the larger the vertical offset 430. Also, the vertical offset 430 may be set according to the scan speed. In this case, the vertical offset 430 can be determined such that the coherent mixing efficiency of the target return signal 425 and the LO signal 423 is maximized at the scan speed. Also, the severity of the lag angle descanning can be determined based on the scan speed of the optical scanner 408. For this reason, the vertical offset 430 may be determined based on the severity of the lag angle descanning. For example, but not limited to, the vertical offset 430 (or displacement) may be 10 μm, 20 μm, 50 μm, 100 μm, 150 μm, 200 μm, 250 μm, 300 μm, 350 μm, 400 μm from the optical axis 401, or any value therebetween. Depending on the arrangement of the optical subsystem, the vertical offset 430 can have any value. As another example, the direction of the vertical offset 430 may be determined based on the arrangement of the scanner with respect to the detection surface.

[0057] As shown in FIG. 4B, for example, the principal ray 425a of the target return signal 425 may be inclined such that it has an angle 450 with respect to the optical axis 401 on the detection surface of the PD440. The angle 450 between the principal ray 425a of the target return signal 425 and the optical axis 401 in the PD440 may correspond to the scan speed of the optical scanner 408. The higher the scan speed, the larger the angle 450, and accordingly the descanning also becomes larger. The vertical offset 430 is determined based on the angle 450 between the principal ray 425a of the target return signal 425 and the optical axis 401 on the detection surface of PD440. By doing so, the coherent mixing efficiency between the target return signal 425 and the LO signal 423 is maximized, and the descanning can be corrected. As another example, the vertical offset 430 may be determined based on the angle between the principal ray 425a of the target return signal 425 and the optical axis 401 in the lens system 412.

[0058] The magnitude and direction of the vertical offset 430 can be adjusted to correspond to different descanning conditions. In this way, by increasing the overlap between the target return signal 425 and the LO signal 423 and correcting the influence of the lag angle descanning, the performance of the LiDAR system can be improved.

[0059] FIG. 5A is a block diagram showing an example of an optical subsystem 500a of a LiDAR system with descanning correction by an inclined optical window 506 according to an embodiment of the present invention. The optical subsystem 500a is similar to the optical subsystem 400 shown in FIG. 4A, except that the lens system 411 is not offset from the optical axis 401, and the optical window 506 is inclined at an inclination angle (first angle) 530 with respect to the plane perpendicular to the optical axis 401. For example, the optical window 506 is a plate having two parallel surfaces, and the LO surface 506a is the first surface (front surface or back surface) of the plate. The two parallel surfaces of the optical window 506 are arranged at an inclination angle 530 with respect to the plane perpendicular to the optical axis 401.

[0060] Similar to the optical subsystem 400, the LO surface 506a of the optical window 506 can be arranged to have a displacement (shift) or offset from the focal plane (not shown) of the lens system 411.

[0061] As shown in FIG. 5A, the light source 402 emits a light beam 520. In the transmission path, this light beam is the transmission signal 521. The light beam 520 passes through a beam splitter (BS) 404 (or a polarization beam splitter (PBS)), and then is focused on the optical window 506. The optical window 506 transmits a part of the light beam 520 toward the target 409 and reflects a part of the light beam 520 at the LO surface 506a to generate an LO signal 523. The transmission signal 521 that has passed through the optical window 506 is collimated using a lens system 413. Then, the transmission signal 521 propagates to the optical scanner 408 and is transmitted to the target 409 to scan the target. The LO signal 523, which is the reflected light from the optical window 506, forms an image on the detection surface of the PD 440 by the lens system 412. The reflected light from the target 409 forms a target return signal 525, and this target return signal 525 also forms an image on the PD 440 by the lens system 412. A radio frequency (RF) beat signal is generated by the coherent mixing of the LO signal 523 and the target return signal 525.

[0062] As shown in FIG. 5A, since the LO surface 506a is inclined at an inclination angle 530 with respect to the plane perpendicular to the optical axis 401, the LO signal 523 shifts after being reflected by the optical window 506. For this reason, the chief ray 523a of the LO signal 523 is disposed at a position offset from the optical axis 401 on the lens systems 411 and 412. Also, the chief ray 523a of the LO signal 523 is disposed at an eccentric position on the plane perpendicular to the optical axis 401 in the lens system 412. As a result, the chief ray 523a of the LO signal 523 will shift at the detection surface of the PD 440. By tilting the LO plane 506a at the tilt angle 530, the principal ray 523a of the LO signal 523 is shifted more greatly than the principal ray 525a of the target return signal 525 on the detection surface of the PD 440. As a result, the overlap between the LO signal 523 and the target return signal 525 on the detection surface of the PD 440 increases. Since the coherent mixing efficiency of the target return signal 525 and the LO signal 523 is partially proportional to the overlap ratio between the LO signal 523 and the target return signal 525 on the detection surface of the PD 440, it will be improved as a result.

[0063] The tilt angle 530 of the LO plane 506a (or 507a: see FIG. 5B) can be determined based on the scan speed of the optical scanner 408, the arrangement of the scanner with respect to the detection surface, or the distance to the target, similar to the vertical offset 430. For example, the magnitude of the tilt angle 530 may be determined based on the scan speed. In this case, it can be set so that the coherent mixing efficiency of the target return signal 525 and the LO signal 523 is maximized at that scan speed. Also, the tilt angle 530 may be determined based on the angle between the principal ray 525a of the target return signal 525 and the optical axis 401 on the detection surface of the PD 440. In this case, the maximum coherent mixing efficiency between the target return signal 525 and the LO signal 523 can be achieved, and descanning can be corrected. As another example, the tilt angle 530 may be determined based on the angle between the principal ray 525a of the target return signal 525 and the optical axis 401 in the lens system 412. In one embodiment, although not limited, the tilt angle 530 may be 0.1°, 0.5°, 1°, 2°, 3°, 4°, 5°, 20° or any value therebetween. The tilt angle 530 can have any value depending on the arrangement of the optical subsystem. Furthermore, as another embodiment, the direction of the tilt angle 530 may be determined based on the arrangement of the scanner with respect to the detection surface (not limited to a specific direction).

[0064] Figure 5B is a block diagram showing an example of an optical subsystem 500b of a LiDAR system with deskew correction by a wedge-shaped optical window (wedge window) 507 according to an embodiment of the present invention. The optical subsystem 500b is similar to the optical subsystem 500a of Figure 5A, except that a wedge-shaped optical window 507 is used instead of the optical window 506a. For example, the wedge-shaped optical window 507 has an LO plane 507a (front or back: the first plane), and the other plane (the second plane) of the wedge-shaped optical window 507 is arranged perpendicular to the optical axis 401. The LO plane 507a is inclined at an inclination angle 530. The LO signal 523 and the target return signal 525 are transmitted in the same manner as described in Figure 5A.

[0065] Figure 5C is a diagram showing deskew correction in the optical subsystems of Figures 5A and 5B according to an embodiment of the present invention. As shown in Figure 5C, for example, the principal ray 525a of the target return signal 525 is inclined at an angle 550 with respect to the optical axis 401 on the detection surface of the PD 440. The angle 550 between the principal ray 525a of the target return signal 525 and the optical axis 401 at the PD 440 may correspond to the scan speed of the optical scanner 408. The higher the scan speed, the larger the angle 550, and accordingly, the larger the deskew. The inclination angle 530 can be determined based on the angle 550 between the principal ray 525a of the target return signal 525 and the optical axis 401 on the detection surface of the PD 440. Thereby, the coherent mixing efficiency of the target return signal 525 and the LO signal 523 is maximized, and deskew can be corrected. As another example, the inclination angle 530 may be determined based on the angle between the principal ray 525a of the target return signal 525 and the optical axis 401 in the lens system 412. The magnitude and direction of the inclination angle 530 can be adjusted to correspond to different deskew conditions. In this way, the overlap between the target return signal 525 and the LO signal 523 can be increased, and the influence of the lag angle deskew can be corrected, thereby improving the performance of the LiDAR system.

[0066] FIG. 6 is a diagram showing an example of the relationship between the mixing efficiency and the scan speed in a system without descanning correction and a system with descanning correction according to an embodiment of the present invention. In a LiDAR system (e.g., systems 100, 400, 500a, 500b), the coherent mixing efficiency is directly proportional to the overlapping integral of the LO signal and the target return signal on the photodetector. The coherent mixing efficiency can be expressed by the following formula. TIFF2025522465000002.tif24136 Here, E S and E LO are the target return signal and the LO signal on the photodetector, respectively, x0, y0 are the displacements of the signal spot due to the lag angle, and det is the detector radius of the circular detector. These parameters can be optimized by relatively changing the LO spot size (LO signal spot size) with respect to the parameters of the lag angle and the target spot size (target return signal spot size). As described above, the LO spot size can be changed by adjusting the position of the optical window with respect to the focal point of the lens system 411 without changing the target spot size. By using the above-described technique, the signal-to-noise ratio (SNR) of the photodetector is improved.

[0067] As shown in FIG. 6, in a LiDAR system without descanning correction, as the scan speed increases, the mixing efficiency 601 decreases due to the influence of descanning. By using the above-described techniques, such as providing an offset in the lens system 411 or tilting the LO surface of the optical window, the overlap between the LO signal and the target return signal on the detection surface increases, and descanning is corrected. Descan correction can be adjusted according to a specific scan speed, thereby improving the mixing efficiency. The mixing efficiency 602 of the corrected LiDAR system is shown in FIG. 6, and in this LiDAR system, the coherent mixing efficiency of the target return signal and the LO signal is designed to be maximized at the operating scan speed (target scan speed).

[0068] FIG. 7A is a block diagram showing an example of an optical subsystem 700a of a LiDAR system with descan correction for a plurality of optical beams according to an embodiment of the present invention. FIG. 7B is a block diagram showing an example of an optical subsystem 700b of a LiDAR system with descan correction for a plurality of optical beams according to an embodiment of the present invention. As shown in FIGS. 7A and 7B, the aforementioned technology can be extended to a multi-beam configuration. In this case, a plurality of beams are transmitted, and a plurality of images are created at a plurality of PDs (photodetectors).

[0069] As shown in FIG. 7A, the optical subsystem 700a includes lens systems 411a and 411b having an offset from the optical axis in a direction perpendicular to the optical axis. The optical subsystem 700a includes lens systems 412a and 412b for transmitting the LO signal and the target return signal to the PDs 440a and 440b. Descan can be corrected by displacing the lens systems 411a and 411b. Although only two optical beams and two sets of lens systems (411a, 411b) are shown in FIG. 7A, the optical subsystem 700a may include a plurality of optical beams and a plurality of sets of lens systems by using the descan correction technique for displacing the aforementioned lens systems.

[0070] The optical subsystem 700b shown in FIG. 7B is similar to the optical subsystem 700a, but is different from the optical subsystem 700a in that the lens systems 411a and 411b have no offset from the optical axis, and the LO surfaces of the optical windows 506 and 506b are inclined at an angle with respect to a plane perpendicular to the optical axis. For example, the optical windows 506, 506b may be plates having two parallel surfaces, or may be wedge-shaped optical windows. Deskew can be corrected by tilting the LO surfaces of the optical windows 506, 506b. Although only two light beams and two sets of lens systems (411a, 411b) are shown in FIG. 7A, the optical subsystem 700b may include a plurality of light beams and a plurality of sets of lens systems by using the deskew correction technique for tilting the aforementioned LO generation surface.

[0071] FIG. 8 is a flowchart showing an example of a method for deskew correction in a LiDAR system according to an embodiment of the present invention. As shown in FIG. 8, in block 802, a light beam is emitted by a light source. In block 804, the light beam is transmitted to the optical window by a first optical lens. In block 806, a first portion of the light beam is reflected by the optical window to generate an LO signal. In block 808, a second portion of the light beam is transmitted to the target to scan the target and generate a target return signal. In block 810, the LO signal and the target return signal are transmitted to a photodetector (PD) by a second optical lens. At this time, the LO signal is disposed at a position eccentric from the optical axis of the optical system in the second optical lens, and the overlapping ratio of the LO signal and the target return signal on the detection surface of the PD is increased. In block 812, the target return signal and the LO signal are mixed by the PD, and the distance and velocity information of the target are extracted.

[0072] In the foregoing description, for the sake of easy understanding of the embodiments of the present invention, a plurality of specific examples of a specific system, component, method, etc. are shown. However, those skilled in the art can implement the present invention even without the description of these specific examples. Also, well-known components and methods may have their details omitted or may be shown in the form of a block diagram, which is for the purpose of facilitating the understanding of the present invention. Therefore, the disclosed content is merely illustrative, and even if one example is different from other examples, it is considered to be within the scope of the present invention.

[0073] When the expressions "one embodiment" or "embodiments" are used in this specification, it means that the specific features, structures, or characteristics described in relation to those embodiments are included in at least one embodiment. Therefore, when the expressions "in one embodiment" or "in embodiments" appear in several places in this specification, they do not necessarily indicate the same embodiment.

[0074] The operations of the methods described herein are shown in a specific order, but the order of the operations of each method may be changed. A specific operation may be performed in the reverse order, or at least some operations may be performed simultaneously with other operations. Instructions for different operations or auxiliary operations can be performed intermittently or alternately.

[0075] The description of the embodiments of the invention described above (including the content described in the summary) is not intended to be detailed and comprehensive, nor is it limited to the disclosed specific forms. Specific embodiments and examples of the present invention are described in this specification for illustrative purposes, but various equivalent changes can be made within the scope recognized by those skilled in the art. The terms "example" or "exemplary" used herein are used to mean serving as an example, instance, or illustration. The aspects or designs described as "example" or "exemplification" in this specification should not necessarily be construed as being more preferable or advantageous than other aspects or designs. Rather, the use of the terms "example" or "exemplification" is intended to represent the concept in a specific form. As used herein, the term "or" is intended to be construed as an inclusive "or" rather than an exclusive "or". That is, unless otherwise specified or clear from the context, the expression "X includes A or B" means any of the natural inclusive permutations. That is, if X includes A, if X includes B, or if X includes both A and B, in any of the foregoing cases, the condition "X includes A or B" will be satisfied. Furthermore, as used in this specification and the appended claims, the articles "a" and "an" are construed to mean "one or more" unless otherwise specified or clear from the context that the singular form is intended. Furthermore, when terms such as "first", "second", "third", "fourth" are used in this specification, these terms are used as identifiers to distinguish different elements and do not necessarily indicate an order according to the numerical designation.

Claims

Claim 1 A light detection and ranging (LiDAR) system, comprising: A light source that emits a light beam; A first optical lens that transmits the light beam; An optical window oriented towards the first optical lens, which reflects a first portion of the light beam to generate a local oscillator (LO) signal; A light scanner that transmits a second portion of the light beam towards a target, scans the target, and generates a target return signal; A second optical lens that transmits the LO signal and the target return signal to a photodetector (PD); The photodetector (PD) that mixes the target return signal with the LO signal to extract distance and velocity information of the target; An optical subsystem having an optical axis of the light beam, comprising: The LiDAR system, wherein the LO signal mixed with the target return signal is arranged eccentrically from the optical axis of the light beam in the second optical lens so as to increase the overlapping ratio of the LO signal and the target return signal on the detection surface of the photodetector (PD). Claim 2 The LiDAR system according to claim 1, wherein: The principal ray of the LO signal is arranged eccentrically from the optical axis of the light beam in the second optical lens, and the coherent mixing efficiency of the target return signal and the LO signal is proportional to the overlapping ratio of the LO signal and the target return signal on the detection surface of the photodetector (PD). Claim 3 The LiDAR system according to claim 1, wherein: The optical window is arranged at a position displaced from the focal plane of the first optical lens. Claim 4 The LiDAR system according to claim 1, wherein: The first optical lens is arranged at a position having a first offset in a direction perpendicular to the optical axis of the light beam. Claim 5 The LiDAR system according to claim 4, wherein: The first offset corresponds to the overlapping ratio of the LO signal and the target return signal on the detection surface of the photodetector (PD). Claim 6 The LiDAR system according to claim 4, wherein: The first offset is determined based on the scan speed of the light scanner, and the maximum coherent mixing efficiency of the target return signal and the LO signal occurs at the determined scan speed. Claim 7 The LiDAR system according to claim 6, wherein the first offset is determined based on an angle between an optical axis of the light beam and a principal ray of the target return signal on a detection surface of the photodetector (PD), the LiDAR system. **Claim 8** The LiDAR system according to claim 1, wherein a reflective surface of the optical window is inclined at a first angle with respect to a plane perpendicular to the optical axis of the light beam, the LiDAR system. **Claim 9** The LiDAR system according to claim 8, wherein the first angle corresponds to an overlapping ratio of the LO signal and the target return signal on a detection surface of the photodetector (PD), the LiDAR system. **Claim 10** The LiDAR system according to claim 8, wherein the optical window is composed of a wedge-shaped window, the reflective surface of the optical window includes a first surface of the wedge-shaped window, and a second surface of the wedge-shaped window is arranged perpendicular to the optical axis of the light beam, the LiDAR system. **Claim 11** The LiDAR system according to claim 8, wherein the optical window is composed of a plate having two parallel surfaces, the reflective surface of the optical window includes a first surface of the plate, and these two parallel surfaces are arranged at the first angle with respect to a plane perpendicular to the optical axis of the light beam, the LiDAR system. **Claim 12** The LiDAR system according to claim 8, wherein the first angle is determined based on a scan speed of the light scanner, and a maximum coherent mixing efficiency of the target return signal and the LO signal occurs at the determined scan speed, the LiDAR system. **Claim 13** The LiDAR system according to claim 12, wherein the first angle is determined based on an angle between an optical axis of the light beam and a principal ray of the target return signal on a detection surface of the photodetector (PD), the LiDAR system. **Claim 14** The LiDAR system according to claim 1, wherein the optical subsystem includes a second light source that emits a second light beam, a third optical lens that transmits the second light beam, a second optical window that is oriented to the third optical lens and reflects a first portion of the second light beam to generate a second LO signal, and a light scanner that transmits a second portion of the second light beam toward the target, scans the target, and generates a second target return signal. A fourth optical lens that transmits the second LO signal and the second target return signal to a second photodetector (PD); A second photodetector (PD) that mixes the second target return signal with the second LO signal to extract distance and velocity information of the target; and The second LO signal mixed with the second target return signal is arranged eccentrically from the optical axis of the second light beam in the fourth optical lens so as to increase the overlapping ratio of the second LO signal and the second target return signal on the detection surface of the second photodetector (PD). A LiDAR system. **Claim 15** A method for optical detection and ranging (LiDAR), comprising: Emitting a light beam by a light source; Transmitting the light beam to an optical window by a first optical lens; Reflecting a first portion of the light beam by the optical window to generate a local oscillator (LO) signal; Transmitting a second portion of the light beam to a target, scanning the target to generate a target return signal; Transmitting the LO signal and the target return signal to a photodetector (PD) by a second optical lens, wherein the LO signal is arranged eccentrically from the optical axis of the light beam in the second optical lens so as to increase the overlapping ratio of the LO signal and the target return signal on the detection surface of the photodetector (PD); Extracting distance and velocity information of the target by mixing the target return signal with the LO signal by the photodetector (PD). **Claim 16** The method according to claim 15, wherein Transmitting the light beam to the optical window by the first optical lens Includes transmitting the light beam to the optical window by a first optical lens arranged at a position having a first offset in a direction perpendicular to the optical axis of the light beam. **Claim 17** The method according to claim 16, wherein The first offset is determined based on the scan speed of the light scanner, and the maximum coherent mixing efficiency of the target return signal and the LO signal occurs at the determined scan speed. **Claim 18** The method according to claim 16, wherein The method wherein the first offset is determined based on an angle between an optical axis of the light beam and a principal ray of the target return signal on a light detection surface of the photodetector (PD).

19. The method according to claim 15, wherein reflecting the first portion of the light beam by the optical window includes reflecting the first portion of the light beam by the optical window inclined at a first angle with respect to a plane perpendicular to the optical axis of the light beam.

20. The method according to claim 19, wherein the first angle is determined based on a scan speed of the optical scanner, and a maximum coherent mixing efficiency of the target return signal and the LO signal occurs at the determined scan speed.

Images