Techniques for compensating for phase impairments in LIDAR systems.

Abstract

[Solution] A system includes transmitting a first optical beam toward a target, receiving a second optical beam from the target to generate a received optical beam, generating a digitally sampled target signal using a local oscillator (LO) signal, a first optical detector, and the received optical beam, generating a digitally sampled reference signal using a reference beam transmitted through a fiber delay device and a second optical detector, estimating one or more phase impairments in the LIDAR system using the digitally sampled reference signal to generate one or more estimated phase impairments, making a first correction for the first phase impairment introduced in the digitally sampled target signal by the local oscillator (LO) signal, and making a second correction for a second phase impairment introduced in the digitally sampled target signal by the received optical beam.

Description

[Technical field]

[0001] This application claims priority under 35 U.S.C. §119(e) to U.S. patent application Ser. No. 16 / 994,325, filed April 19, 2021, and U.S. Provisional Patent Application Ser. No. 63 / 087,432, filed October 5, 2020, the entire contents of which are incorporated herein by reference. [Background technology]

[0002] The present disclosure relates to light detection and ranging (LIDAR) systems. Summary of the Invention [Problem to be solved by the invention]

[0003] Traditional frequency modulated continuous wave (FMCW) LIDAR systems contain phase impairments such as laser phase noise, circuit phase noise, flicker noise from the laser driving electronics, temperature / weather drift, chirp rate offset, etc. These impairments cause reduced detection probability, increased false alarms and range / velocity bias, and further increase the error of estimated target range / velocity.

[0004] In particular, this disclosure describes various embodiments of LIDAR systems and methods that add a reference channel to the system to emulate a target at a predetermined distance, estimate phase impairments based on the reference channel, and correct the signal based on the estimated phase impairments. An embodiment of the present invention includes the ability to estimate phase impairments and the ability to compensate for phase impairments in a return signal. Such impairments include, for example, those due to laser phase noise, circuit phase noise, flicker noise, temperature and weather drift, chirp rate offset, etc., which can lead to reduced detection probability, increased false alarms, and even erroneous range and velocity estimation that can lead to range and velocity bias and increased range and velocity error. [Means for solving the problem]

[0005] In one aspect, the present disclosure relates to a method for compensating for phase impairments in a Light Detection and Ranging (LIDAR) system. The method includes transmitting a first optical beam toward a target, receiving a second optical beam from the target to generate a received optical beam, generating a digitally sampled target signal using a local oscillator (LO) signal, a first optical detector, and the received optical beam, and further generating a digitally sampled reference signal using a reference beam transmitted through a fiber delay device and a second optical detector. The method also includes estimating one or more phase impairments of a LIDAR system using a digitally sampled reference signal to generate one or more estimated phase impairments. The method also includes a first phase impairment caused by the local oscillator (LO) signal in the digitally sampled target signal, and using the one or more estimated phase impairments to perform a first correction for the first phase impairment from the one or more phase impairments in the digitally sampled target signal. The method also includes a second phase impairment caused by the received light beam in the digitally sampled target signal, and using the one or more estimated phase impairments to perform a second correction for the second phase impairment from the one or more phase impairments in the digitally sampled target signal. The method also includes processing the first and second corrections to generate a corrected target signal for transmission to a point cloud. In one embodiment, the method includes estimating the one or more phase impairments by using at least one of a zero-force estimator, an MMSE estimator, a maximum likelihood estimator, and a map estimator. In one embodiment, the method includes performing the first correction by using one or more filters formed based at least in part on an estimate of the first phase impairment. In one embodiment, the method includes performing the second correction by delaying the received light beam through a plurality of digital delays and using a brute-force delay correction filter that selects an optimal delay. In one embodiment, the method includes selecting an optimal delay by performing a Fast Fourier Transform (FFT) of the plurality of digital delays and selecting the delay with the highest peak. In one embodiment, the method includes performing the second correction by using an iterative delay correction filter that iteratively delays the received light beam with different digital delays until the peaks converge. In one embodiment, the method includes performing the second correction by using a deskew compensation correction filter. In one embodiment, the method also includes generating a plurality of digitally sampled reference signals using a reference beam transmitted through a plurality of different fiber delay devices, each fiber delay device being coupled to a respective photodetector.

[0006] According to another aspect, the present disclosure relates to a light detection and ranging (LIDAR) system including: The system includes an optical beam source that irradiates a first optical beam toward a target; a first detector that receives a received optical beam and a local oscillator (LO) signal from the target and generates a digitally sampled target signal; a fiber delay device having a known length and coupled to the optical beam source; a second detector that is coupled to the fiber delay device and generates a digitally sampled reference signal using a reference beam transmitted through the fiber delay device; a phase impairment estimator that estimates one or more phase impairments of the LIDAR system using the digitally sampled reference signal and generates one or more estimated phase impairments; a first phase impairment corrector that corrects a first phase impairment caused by the local oscillator (LO) signal in the digitally sampled target signal; and a second phase impairment corrector that corrects a second phase impairment caused by the received optical beam in the digitally sampled target signal. In one embodiment, the system further comprises at least one of a zero-force estimator, an MMSE estimator, a maximum likelihood estimator, and a map estimator for estimating the one or more phase impairments. In one embodiment, the first phase impairment corrector comprises one or more filters formed based at least in part on an estimate of the first phase impairment. In one embodiment, the second phase impairment corrector comprises a brute-force delay correction filter that delays the received optical beam by a plurality of digital delays and selects an optimal delay. In one embodiment, selecting an optimal delay includes performing a Fast Fourier Transform (FFT) of the plurality of digital delays and selecting a delay with a highest peak. In one embodiment, the second phase impairment corrector comprises an iterative delay correction filter that iteratively delays the received optical beam through different digital delays until peaks converge. In one embodiment, the second phase impairment corrector comprises a deskew compensation correction filter. In one embodiment, a fiber delay device generates a delay proportional to a frequency of the received optical beam such that reflections from distant targets are delayed more and reflections from closer targets are not delayed more. In one embodiment, the system also includes a plurality of different fiber delay elements and photodetectors for generating a plurality of digitally sampled reference signals.

[0007] In another aspect, the present disclosure relates to a method for compensating for phase impairments in a Light Detection and Ranging (LIDAR) system. The method includes the steps of generating a digitally sampled target signal using a local oscillator (LO) signal, a first photodetector, and a received light beam reflected from a target, generating a digitally sampled reference signal using a reference beam transmitted through a fiber delay device and a second photodetector, estimating a first phase impairment caused in the digitally sampled target signal by the local oscillator (LO) signal, estimating a second phase impairment caused in the digitally sampled target signal by the received light beam, making a first correction to correct for the first phase impairment, and making a second correction to correct for the second phase impairment. In one embodiment, the step of estimating the second phase impairment includes using at least one of a zero-force estimator, an MMSE estimator, a maximum likelihood estimator, and a map estimator. In one embodiment, the step of performing the second correction includes using a brute force delay correction filter that delays the received optical beam by multiple digital delays and selects the optimal delay, using an iterative delay correction filter that iteratively delays the received optical beam through different digital delays until the peak converges, or using a deskew compensation correction filter. [Brief description of the drawings]

[0008] In order to clarify various aspects of the present invention, reference is made to the drawings which are referred to in the following detailed description (embodiments), in which like reference numerals refer to like elements.

[0009] [Figure 1] 1 illustrates an exemplary LIDAR system according to an embodiment of the present disclosure.

[0010] [Diagram 2] FIG. 2 is a time-frequency diagram illustrating how LIDAR waveforms are detected and processed according to an embodiment of the present disclosure.

[0011] [Diagram 3] 4 illustrates various fast variations in phase that can be detected by embodiments of the present disclosure.

[0012] [Figure 4] 4 illustrates various slow variations in phase that can be detected by embodiments of the present disclosure.

[0013] [Diagram 5] 1 illustrates various chirp rate offsets detectable by embodiments of the present disclosure.

[0014] [Figure 6] FIG. 1 is a block diagram of an exemplary LIDAR system with a reference arm in accordance with an embodiment of the present disclosure.

[0015] [Figure 7] 1 illustrates a digital signal processing architecture according to an embodiment of the present disclosure.

[0016] [Figure 8] 1 illustrates an example system for compensating for impairments from a return signal according to some embodiments of the present disclosure.

[0017] [Figure 9] 1 illustrates an example system for correcting phase impairments using multiple delays in accordance with some embodiments of the present disclosure.

[0018] [Figure 10] 1 illustrates an example system for correcting phase impairments using an iterative delay compensation filter in accordance with some embodiments of the present disclosure.

[0019] [Figure 11] 1 illustrates an example system for correcting phase impairments using a deskew compensation correction filter in accordance with some embodiments of the present disclosure.

[0020] [Figure 12]1 illustrates an example system for correcting phase impairments using a joint impairment corrector and a peak detector in accordance with some embodiments of the present disclosure.

[0021] [Figure 13] 1 illustrates an example system for correcting phase impairments using multiple reference channels according to some embodiments of the present disclosure.

[0022] [Figure 14] 1 is a flow diagram illustrating an example method for correcting phase impairments in a LIDAR system according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A LIDAR system and method for compensating for phase impairments according to embodiments of the present invention are described below. The LIDAR systems of embodiments of the present invention can be implemented in any sensing market, including, but not limited to, transportation, manufacturing, metrology, medical, augmented reality (AR), virtual reality, and security systems. In other embodiments, the LIDAR system described herein may be implemented as part of a front-end for a frequency modulated continuous wave (FMCW) device that aids in the spatial awareness of autonomous driving assistance systems and vehicles.

[0024] FIG. 1 illustrates a LIDAR system 100 according to one embodiment. The LIDAR system 100 may include any one or more of a number of components, but may include fewer or additional components than those shown in FIG. 1. According to some embodiments, one or more of the components shown in FIG. 1 may be implemented on a photonics chip. The optical circuit 101 includes a combination of active and passive optical components. In some examples, the active optical components have light beams of different wavelengths and include one or more optical amplifiers, one or more photodetectors, etc.

[0025] The free space optics 115 includes one or more optical waveguides for transmitting optical signals and for routing and manipulating the optical signals to appropriate input / output ports of the active optical circuitry. The free space optics 115 may include one or more optical components such as taps, wavelength division multiplexers (WDMs), splitters / combiners, polarizing beam splitters (PBSs), collimators, couplers, and the like. In one embodiment, the free space optics 115 includes components for converting the polarization state and directing the received polarized light to a photodetector, for example, using a PBS, and may further include a diffractive element that deflects light beams having different frequencies at different angles along an axis (e.g., the fast axis).

[0026] The LIDAR system 100 of this embodiment includes an optical scanner 102 having one or more scanning mirrors. The scanning mirrors are rotatable along an axis orthogonal or substantially orthogonal to the fast axis (e.g., a slow axis) of the diffractive element to direct an optical signal that scans the environment according to a scanning pattern. For example, the scanning mirrors can be rotated by one or more galvanometers. Objects in the target environment can scatter the incident light into a return optical beam or target return signal. The optical scanner 102 may also collect a return light beam or a target return signal, which may be returned to the optical circuit components of the optical circuit 101. For example, the return light beam may be directed to a photodetector by a polarizing beam splitter. Note that the optical scanner 102 may include quarter wave plates, lenses, anti-reflective coated optical windows, etc., in addition to mirrors and galvanometers.

[0027] The LIDAR system 100 is provided with a LIDAR controller 110 to control and support the optical circuit 101 and the optical scanner 102. The LIDAR controller 110 contains the processing equipment required for the LIDAR system 100. In one aspect, the processing device is one or more general-purpose processing devices, such as a microprocessor, a central processing device, and in particular 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 combination of instruction sets. Additionally, the processing device may be one or more of special purpose processing devices such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal processor (DSP), a network processor, or the like.

[0028] In one embodiment, the LIDAR controller 110 is provided with a signal processing unit 112, such as a DSP, which outputs a digital control signal for controlling the optical driver 103. The digital control signal is converted to an analog signal via the signal conversion unit 106. For example, the signal conversion unit 106 includes a digital-to-analog converter. The optical driver 103 provides drive signals to the active optical components of the optical circuit 101 to drive light sources such as lasers and amplifiers. In one embodiment, multiple optical drivers 103 and signal conversion units 106 may be provided to drive multiple light sources.

[0029] The LIDAR controller 110 is also configured to output digital control signals to the optical scanner 102. The motion controller 105 can control the galvanometers of the optical scanner 102 based on the control signals received from the LIDAR controller 110. In particular, a digital-to-analog converter can be used to convert the coordinate routing information from the LIDAR controller 110 into signals that can be processed by the galvanometers of the optical scanner 102. In one embodiment, the motion controller 105 can also send information regarding the position or movement of components of the optical scanner 102 back to the LIDAR controller 110. In particular, an analog-to-digital converter can be used to convert the information regarding the galvanometer position into a signal that the LIDAR controller 110 can process.

[0030] The LIDAR controller 110 is further configured to analyze the input digital signal. In this regard, the LIDAR system 100 is provided with an optical receiver 104 for measuring one or more beams received by the optical circuit 101. In particular, the optical receiver 104 as a reference beam receiver measures the amplitude of a reference beam from the active optical component and converts the signal from the reference beam receiver by an analog / digital converter into a signal that can be processed by the LIDAR controller 110. The target receiver, as the optical receiver 104, also measures an optical signal carrying information about the range and velocity of the target in the form of a beat frequency modulated optical signal, where the reflected beam of the optical signal may be mixed with a signal from a local oscillator. The optical receiver 104 may include a high speed analog-to-digital converter to convert the signal from the target receiver into a signal that can be processed by the LIDAR controller 110. In one aspect, the signal from the optical receiver 104 may be subject to signal conditioning by the signal conditioning unit 107 before being received by the LIDAR controller 110. For example, the signal from the optical receiver 104 may be provided to an operational amplifier in the signal conditioning unit 107 for amplification of the return signal, and the amplified signal provided by the operational amplifier to the LIDAR controller 110.

[0031] In some applications, the LIDAR system 100 may additionally be provided with one or more imagers 108 configured to capture images of the environment, a global positioning system (GPS) 109 configured to provide a geographic location of the system, or other sensor inputs. The LIDAR system 100 may also include an image processor 114 that may be configured to receive images and geographic locations from the imager 108 and Global Positioning System (GPS) 109, and to transmit the images and location or related information to the LIDAR controller 110 or other systems connected thereto.

[0032] In operation according to some embodiments, the LIDAR system 100 is configured to simultaneously measure distance and velocity in two dimensions using a non-degenerate optical source. This capability enables real-time long-range measurements of distance, velocity, azimuth, and height (elevation) of the surrounding environment.

[0033] In some examples, the scanning process is initiated by the optical driver 103 and the LIDAR controller 110. The LIDAR controller 110 instructs the optical driver 103 to modulate one or more light beams, respectively, and these modulated signals are transmitted through the passive optical circuitry of the optical circuitry 101 to a collimator of the free-space optics 115. The collimator directs the modulated signals to the optical scanner 102, which scans the environment in a pattern preprogrammed by the motion controller 105. The optical circuit 101 may include a polarizing waveplate (PWP) that converts the polarization state of the light as it exits the optical circuit 101. A portion of the polarized light beam may be reflected back into the optical circuit 101. For example, a lens or collimating system used in the LIDAR system 100 may have natural reflective properties or a reflective coating that causes a portion of the light beam to be reflected back into the optical circuit 101.

[0034] The optical signal reflected from the environment is sent to the receiver (optical receiver 104) through the optical circuit 101. At this time, the polarization state of the light has been transformed, so it is reflected by the polarizing beam splitter along with a portion of the polarized light that was 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, but is reflected to different optical receivers. These signals interfere with each other to generate a combined signal. Each beam signal returning from the target produces a time-shifted waveform, and the time phase difference between these two waveforms produces a beat frequency that is measured by an optical receiver (photodetector). The combined signal can then be reflected back to the optical receiver 104.

[0035] The analog signals received by the optical receiver 104 are converted to digital signals by an ADC (analog-to-digital converter), which are then sent to a LIDAR controller 110, whose signal processing unit 112 receives the digital signals and processes them. In one embodiment, the signal processing unit 112 receives position data from the motion controller 105 and a galvanometer (not shown) and image data from the image processor 114. This enables the signal processing unit 112 to generate a 3D point cloud with information about the distance and velocity of points in the environment as the optical scanner 102 scans additional points. The signal processing unit 112 may also overlay the 3D point cloud with image data to determine the speed and distance of surrounding objects. The system may further process satellite-based navigation position data to provide precise global position information.

[0036] 2 is a time-frequency diagram 200 of an FMCW scanning signal 201 that, in one embodiment, can be used by a LIDAR system, such as LIDAR system 100, to scan a target environment. In this example, the FMCW scanning signal 201, denoted as fFM(t), is a sawtooth waveform (sawtooth "chirp") with a chirp bandwidth ΔfC and a chirp period TC. The slope of the sawtooth is k=(ΔfC / TC). Also shown in Figure 2 is a target return signal 202 in one embodiment. The target return signal 202, denoted as fFM(t-Δt), is a time delayed version of the FMCW scanning signal 201, where Δt is the round trip time to and from the target illuminated by the FMCW scanning signal 201. This round trip time is given by Δt=2R / v, where R is the range of the target and v is the speed of light, c, which is the speed of the light beam. Therefore, the range R of the target can be calculated as R = c(Δt / 2). When the target return signal 202 is optically mixed with the FMCW scanning signal, a range-dependent difference frequency ("beat frequency") ΔfR(t) is generated that is linearly related to the time delay Δt by the sawtooth slope k. In other words, ΔfR(t) = kΔt. Since the target distance R is proportional to Δt, the target distance R can be calculated as R = (c / 2)(ΔfR(t) / k). In other words, the distance R is linearly related to the beat frequency ΔfR(t). The beat frequency ΔfR(t) is generated as an analog signal, for example, in the optical receiver 104 of the LIDAR system 100. This beat frequency is digitized, for example, by an analog-to-digital converter (ADC) in the signal conditioning unit 107 of the LIDAR system 100. The beat frequency signal thus digitized is digitally processed in a signal processing unit (e.g., signal processing unit 112) in the LIDAR system 100. However, it should be noted that the target return signal 202 typically contains a frequency offset (Doppler shift) if the target has a relative velocity with respect to the LIDAR system 100. For simplicity and ease of illustration, the Doppler shift is not shown in FIG. 2 because it is detected separately and used to correct the frequency of the return signal. Also, keep in mind that the sampling frequency of the ADC is determined by the highest beat frequency the system can handle without aliasing, which is generally half the sampling frequency (i.e. the "Nyquist limit"). For example, and not by way of limitation, if the ADC sampling frequency is 1 GHz, then the highest beat frequency (ΔfRmax) that can be processed without aliasing is 500 MHz. This limit is determined by the system's maximum target distance Rmax=(c / 2)(ΔfRmax / k), which can be adjusted by changing the sawtooth slope k. In one example, data samples from the ADC may be continuous, but subsequent digital processing, described below, may be divided into "time segments" that may be associated with a predetermined periodicity of the LIDAR system 100. For example, but not limited to, a time segment may correspond to a number of chirp periods T C or a number of azimuthal rotations by the optical scanner described above.

[0037] FIG. 3 is a graph illustrating various fast variations in phase, or phase disturbances, that can be detected by an embodiment of the present invention. In this embodiment, the variations in the scanning signal Tx are shown at 301 and the variations in the return signal Rx are shown at 303. As shown in Figure 3, variations in laser phase noise, circuit phase noise, and flicker noise, among others, can be detected by a LIDAR system described by an embodiment of the present invention using components such as those shown in Figure 1.

[0038] FIG. 4 illustrates various slow variations in phase that can be detected by an embodiment of the present invention. In this embodiment, the variations in the scanning signal Tx are shown at 401 and the variations in the return signal Rx are shown at 403. As shown in Figure 4, variations in temperature, among others, can be detected by a LIDAR system described according to an embodiment of the present invention using components such as those depicted in Figure 1.

[0039] FIG. 5 illustrates various chirp rate offsets that can be detected by an embodiment of the present invention. In this embodiment, the variation of the scanning signal Tx is shown at 501 and the variation of the return signal Rx is shown at 503. As shown in Figure 5, offsets caused by, among other things, laser calibration and / or component aging can be detected by a LIDAR system described according to an embodiment of the present invention using components such as those shown in Figure 1.

[0040] FIG. 6 is a block diagram of an exemplary LIDAR system having a reference arm 627, in accordance with an embodiment of the present disclosure. As shown in FIG. 6, according to some embodiments, one or more reference arms 627 can be added to the LIDAR system to generate a digitally sampled reference signal 626 that can be used to estimate phase impairments in the transmitted signal as described herein. In this embodiment, the reference arm 627 produces a digitally sampled reference signal 626 corresponding to a target (e.g., target 607) with a known delay having a phase impairment similar to the phase impairment on the received signal from the target. The digitally sampled reference signal 626 is used to estimate the phase impairment for subsequent correction. For example, in one embodiment, the system includes a beam source 601, such as an FMCW laser source. A target arm 605 includes multiple optical components (e.g., lenses or filters) through which a scanning signal 603 passes on its way to a target 607. A return signal 609 is reflected from the target 607 and directed to a photodetector 611. In this embodiment, a local oscillator (LO) signal 613, which is part of the scanning signal 603, is directed to the photodetector 611 before being transmitted to the target 607. From the photodetector 611, a digitally sampled target signal 616 is then passed through the target ADC 615 to the DSP 617.

[0041] As shown in this embodiment, the reference arm 627 receives a signal portion 619 of the scanning signal 603, which is provided to a photodetector 621 either directly or after passing through a delay device 623 having a known length and / or delay. In one aspect, the signal portion 619 is received by the photodetector 621 as the scanning signal 603 is simultaneously transmitted through the optical components of the target arm 605. In one aspect, the signal portion 619 is received by the photodetector 621 after the scanning signal 603 is transmitted through the optical components of the target arm 605. In one aspect, the delay device 623 can be a fiber delay device, or the like. In one aspect, the delay device 623 includes a fiber coil having a known length that can create a virtual target (e.g., a fiber target) at a known distance.

[0042] In some scenarios, the distance of the virtual target can be predetermined. The optical signal at the output of the reference delay has the same characteristics as the target return signal 202 depicted in FIG. 2. In one embodiment, in a manner similar to that described in FIG. 2, the virtual target described herein generates a return signal 609 that is a time-delayed version of the scanning signal 603. When the return signal 609 is optically mixed with the scanning signal 603, a reference distance-dependent difference frequency ("reference beat frequency") is generated. In one embodiment, the reference beat frequency is then digitized and conditioned using the procedures described herein, for example in FIG. 1. This digitally sampled reference signal 626 has the same phase impairment characteristics as the received signal from the target. A phase impairment estimator, based at least in part on a digitally sampled reference signal 626 generated by the reference arm 627 and a phase impairment corrector described by an embodiment of the present disclosure (described in more detail below), can compensate for phase impairments in a signal transmitted towards a target (i.e., a “target” signal) and a signal received therefrom (i.e., a “received” signal).

[0043] From the photodetector 621, the reference signal 626 then passes through a reference ADC 625 to a DSP 617. As shown in Figure 6, an embodiment of the present invention generates point cloud data 629 based on the transmission and reception of various signals including a return signal 609, a local oscillator (LO) signal 613, a digitally sampled target signal 616, and a digitally sampled reference signal 626.

[0044] As mentioned above, the phase impairment of the digitally sampled target signal 616 includes two components: an impairment introduced from the local oscillator (LO) signal 613 and an impairment introduced from the return signal 609. Both of these impairments are corrected in various ways by the embodiments described herein, as described in more detail below. For example, FIG. 14 illustrates a technique for correcting both types of phase impairments, FIGS. 7-8 illustrate a technique for correcting a phase impairment arising from a local oscillator (LO) signal 613, and FIGS. 9-12 illustrate a technique for correcting a phase impairment arising from a return signal 609, in accordance with an embodiment of the present disclosure.

[0045] FIG. 7 illustrates a digital signal processing architecture according to an embodiment of the present invention. In one aspect, the signal processing architecture depicted in Figure 7 corresponds to components in the signal processing unit 112 depicted in Figure 1. As depicted in Figure 7, according to some embodiments, the digital signal processing architecture includes one or more reference ADCs 701 and target ADCs 705, as well as a phase impairment estimator 703 and a phase impairment corrector 707. As depicted in Figure 7, the digital signal processing architecture also includes a time domain filter 709, a time to frequency domain transformer 711 (e.g., FFT), a frequency domain filter 713, and a peak detector 715. The phase impairments that are processed by this architecture include various phase impairments, including those previously described. As depicted in Figure 7, according to some embodiments, a digital signal processing architecture generates and processes data such as point cloud data 717. As depicted in Figure 7, in some scenarios, corrections may be performed before one or more digital signal processing steps are performed, such as those performed by the time domain filter 709, the time to frequency domain converter 711, the frequency domain filter 713, and the peak detector 715.

[0046] According to some embodiments, the phase fault estimator 703 can estimate the phase fault using several different approaches. For example, in some embodiments, the phase fault estimator 703 can use a "zero forcing" estimation that passes the reference phase through a filter having a frequency response equal to the inverse frequency response of the reference arm described by the embodiment. In some embodiments, the phase fault estimator 703 can use a minimum mean square estimation (MMSE) that can reduce noise amplification due to the zero forcing estimation near frequencies where the frequency response of the reference arm is typically weak. In one aspect, the phase fault estimator 703 can use a maximum likelihood / MAP estimation that can be based at least in part on statistical data related to the phase fault collected over a period of time.

[0047] According to some embodiments, the phase impairment corrector 707 can correct the phase impairment of the return signal using several different approaches. The phase impairment of the return signal consists of impairments arising from the return signal as well as the local oscillator (LO) signal. For example, in some embodiments, the phase impairment corrector 707 can correct the phase impairment components arising from the local oscillator (LO) signal as well as the return signal path, which will be described in more detail below.

[0048] FIG. 8 illustrates an example system for compensating for impairments resulting from a local oscillator (LO) signal in accordance with some embodiments of the present disclosure. In one aspect, the phase impairment estimator 801 can estimate the phase impairments as disclosed herein, and these impairments can be corrected using a correction filter 807 having filter taps shaped based on the estimated phase impairments. In one aspect, the shape of the filter taps can be calculated using a filter tap calculation module 803 based on the phase impairment estimator 801.

[0049] In one aspect, the filter taps can be shaped to cancel or equalize phase noise impairments and / or mitigate noise amplification due to estimation inaccuracies. In some scenarios, when an instantaneous estimate of the phase impairment is not available, embodiments can use matched filter taps 805 shaped according to the average power spectral density (PSD) of the phase impairment. In some scenarios, when an instantaneous estimate of the phase impairment is not available, embodiments can use a compensating matched filter with different taps and / or shapes at different frequencies to account for variations in phase impairment bandwidth between frequencies.

[0050] In one aspect, the compensation filter 807 can receive the return signal and can also receive inputs provided by the filter tap calculation module 803 calculated by the phase impairment estimator 801 and / or inputs provided by matched filter taps 805 matched to the average PSD. Based on these inputs, the compensation filter 807 can generate a compensated return signal.

[0051] In one aspect, the systems and components described herein can correct for impairments on a return signal resulting from a received signal from a target. In one scenario, the received signal has a delay equal to the target delay. In one aspect, correcting for phase impairments from the received signal includes compensating for the delay before executing the correction filter. Approaches used by various embodiments can include, but are not limited to, a brute force delay correction filter using multiple delays, an iterative delay correction filter, and a deskew compensation correction filter. In one aspect, the phase impairment estimator 801 and the filter tap calculation module 803 may correspond to components within the signal processing unit 112 described in FIG. 1, and the matched filter taps 805 and the correction filter 807 may correspond to components within the optical circuit 101 in FIG. 1.

[0052] FIG. 9 illustrates an example system for correcting phase impairments using multiple delays in accordance with some embodiments of the present disclosure. In one aspect, the phase impairment corrector can be configured with a brute force delay correction filter that can delay the received optical beam through multiple digital delays and select an optimal delay, which can include, for example, performing an FFT of the digital delays and selecting the delay with the highest peak. As shown in FIG. 9, the technique may use, but is not limited to, a phase impairment estimator 901, computed filter taps 903, one or more correction filters 909-911, a time domain filter 915, a time to frequency domain transformer 917 (e.g., FFT), a frequency domain filter 919, and a peak detector 921.

[0053] In one embodiment, the target arm signal is sent through N different delays 905-907, and a correction filter 909-911 can be applied for each of the possible target delays 905-907. Once multiple delays are applied to the target arm signal, an optimal delay 913 can be selected. In one embodiment, the best match can be defined based on, for example, but not limited to, strongest peak, higher SNR, or other metric used for signal detection. In one embodiment, choosing the optimal delay includes performing a Fast Fourier Transform (FFT) of the multiple delayed light beams and selecting the delay with the highest peak. In one embodiment, the components described in FIG. 9 can correspond to components in the signal processing unit 112 or optical circuit 101 described in FIG. 1.

[0054] FIG. 10 illustrates an exemplary system for correcting phase impairments using an iterative delay correction filter in accordance with one embodiment of the present disclosure. As shown in Figure 10, the technique may use, but is not limited to, a phase impairment estimator 1001, a calculated filter tap 1003, one or more correction filters 1007, a time domain filter 1009, a time to frequency domain converter 1011 (e.g., FFT), a frequency domain filter 1013, and a peak detector 1015. In one aspect, the target arm signal passes through a delay 1005 and then through the correction filter 1007 and other signal processing components. After the peak detector 1015, the signal is returned to the delay 1005 and the process repeats between the optimal delay and peak frequency until convergence. In one embodiment, the initialization for the delay unit 1005 can be based on, for example, adjacent detection points in the current frame and / or detection points from a previous frame. In one embodiment, the components depicted in FIG. 10 correspond to components in the signal processing unit 112 or the optical circuit 101 depicted in FIG.

[0055] FIG. 11 illustrates an exemplary system for correcting phase impairments using a deskew compensation correction filter, according to one embodiment of the present disclosure. 11, the technique may use, but is not limited to, a phase impairment estimator 1101, a calculated filter tap 1103, one or more correction filters 1107, a time domain filter 1109, a time to frequency domain converter 1111 (e.g., FFT), a frequency domain filter 1113, and a peak detector 1115. In one aspect, the target arm signal may be passed through a deskew filter 1105 to apply different delays based on the frequency of the signal. In an embodiment, a linear relationship between the frequency of the peak and the delay may be utilized.

[0056] In one aspect, the deskew filter 1105 can shift the phase impairment estimate based on the peak frequency. In one aspect, the deskew filter 1105 can be used to apply a group delay to the signal that is proportional to the frequency content of the target arm signal. In one aspect, one or more deskew filters can be used to apply the correct group delay when the return signal (Rx) does not have a Doppler shift. In one aspect, the components described in FIG. 11 can correspond to components in the signal processing unit 112 or optical circuit 101 described in FIG. 1.

[0057] FIG. 12 illustrates an exemplary system for correcting phase impairments using a joint impairment corrector and peak detector 1213 according to one embodiment of the disclosure. As shown in FIG. 12, the technique may use, but is not limited to, a phase impairment estimator 1203, a joint impairment corrector and peak detector 1213, a time domain filter 1207, a time-to-frequency domain transformer 1209 (e.g., FFT), a frequency domain filter 1211, along with one or more reference ADCs 1201 and target ADCs 1205. An embodiment of the present invention may generate point cloud data 1215 from these described architectures. In one aspect, rather than correcting for phase impairments up front, a phase impairment corrector may be integrated as part of the peak detector to jointly estimate optimal peaks. In one aspect, the components described in FIG. 12 correspond to components in the signal processing unit 112 or optical circuit 101 described in FIG. 1.

[0058] FIG. 13 illustrates an exemplary system for correcting phase impairments using multiple reference channels according to one embodiment of the disclosure. As shown in Figure 13, the LIDAR system includes a beam source 1301, such as a FMWC laser source. The target arm includes a number of optical components 1305 through which the scanning signal 1303 can pass on its way to the target 1307. The return signal 1309 is reflected from the target 1307 and directed to a photodetector 1311. In this embodiment, a local oscillator (LO) signal 1313 is directed to the photodetector 1311 before being sent to the target 1307. From the photodetector 1311, the target arm signal then passes to a target ADC 1315 and then to a DSP 1317.

[0059] In the embodiment shown in FIG. 13, a portion 1319 of the scanning signal 1303 is diverted to multiple reference arms. In one embodiment, each of the reference arms can include a photodetector and a fiber delay having a known length. For example, the reference arm includes a photodetector 1321, a fiber delay 1323, and a reference ADC 1325. Similarly, the Nth reference arm includes its own photodetector 1327, a fiber delay 1329, and an ADC 1331.

[0060] In one embodiment, multiple reference channels can be added with different reference arm delays to improve the estimate of phase impairments. In one embodiment, the system can generate point cloud data 1333 based on transmitting and receiving various signals, including signals from the target environment and local oscillator (LO) signals. In one embodiment, the components described in FIG. 13 can correspond to components in the signal processing unit 112 or optical circuit 101 described in FIG. 1.

[0061] 14 is a flow diagram illustrating an exemplary method for compensating for phase impairments in a LIDAR system, according to an embodiment of the present disclosure. The method begins in operation 1401 by transmitting a light beam (first light beam) to a target. The light beam (first light beam) may include, for example, an FMWC beam.

[0062] In operation 1403, a reflected light beam (a second light beam) is received from a target to generate a received light beam.

[0063] In operation 1405, a digitally sampled target signal is generated using a local oscillator (LO) signal, a photodetector, and a received light beam.

[0064] In operation 1407, a digitally sampled reference signal is generated using a reference beam transmitted through a fiber delay and a photodetector. In one aspect, a number of digitally sampled reference signals are generated using a reference beam transmitted through a number of different fiber delays. Each fiber delay, in one aspect, can be coupled to a respective photodetector. In one aspect, the fiber delays generate a delay proportional to the frequency of the received light beam such that reflections from distant targets are delayed more and reflections from closer targets are delayed less.

[0065] In operation 1409, one or more phase impairments in the LIDAR system are estimated using the digitally sampled reference signal, thereby generating one or more estimated phase impairments. In one aspect, estimating the phase impairments includes using a zero-forcing estimator, an MMSE estimator, a maximum likelihood estimator, or a map estimator.

[0066] In operation 1411, a first correction is performed for a first phase impairment corresponding to a local oscillator (LO) signal, i.e., the first correction corrects for a phase impairment introduced into a target signal digitally sampled from the local oscillator (LO) signal. In one aspect, performing the first correction includes using one or more filters formed based at least in part on the estimate of the first phase impairment, such as matched filter taps as described above.

[0067] In operation 1413, a second correction is performed for a second phase impairment corresponding to the received light beam, i.e., the second correction corrects for a phase impairment introduced into the target signal digitally sampled from the received light beam. In one embodiment, performing the second correction includes using a brute force delay correction filter that delays the received optical beam through a predetermined number of digital delays and selects the optimal delay. In one embodiment, the optimal delay can be selected by performing a Fast Fourier Transform (FFT) of the digital delays and selecting the delay with the highest peak. In an alternative embodiment, the second correction includes using an iterative delay correction filter that iteratively delays the received light beam through different digital delays until the peaks converge. In yet another embodiment, the second correction includes using a deskew compensation correction filter.

[0068] A corrected target signal is generated by processing the first and second corrections in operation 1415. Once the corrected target signal is generated, it can be transmitted to a point cloud.

[0069] Various operations and methods described herein may be performed, in some embodiments, using the signal processing unit 112, the signal conversion unit 106, or the signal conditioning unit 107 described above in Figure 1. The various optical components, fiber delays, and other components may be implemented as the optical circuit 101 described in Figure 1.

[0070] In the above description, a number of specific examples of specific systems, components, methods, etc. are shown to facilitate understanding of the embodiments of the present invention, but those skilled in the art can practice the present invention without the description of these specific examples. In addition, details of known components and methods may be omitted or shown in the form of simple block diagrams, but this is for the purpose of facilitating understanding of the present invention. Therefore, the contents disclosed are merely examples, and even if one example differs from other examples, it is considered to be included within the scope of the present invention.

[0071] When the phrase "one embodiment" or "an embodiment" is used in this specification, it means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase "in one embodiment" or "in an embodiment" in various places in this specification do not necessarily refer to the same embodiment.

[0072] The term "coupled," along with its derivatives, is used to indicate that two or more elements interact with each other. These coupled elements may or may not be in direct physical or electrical contact with each other.

[0073] Although the operations of the methods described herein are shown in a particular order, the order of operations of each method may be changed, certain operations may be performed in reverse order, at least some operations may be performed simultaneously with other operations, and instructions for different operations or auxiliary operations may be performed intermittently or alternately.

[0074] The above-described description of the embodiments of the invention (including the contents described in the Abstract) is not intended to be detailed or exhaustive, nor is it intended to limit the invention to the specific forms disclosed. Specific embodiments and examples of the invention are described herein for illustrative purposes, but various equivalent modifications may be made within the scope of those skilled in the art. As used herein, the word "example" or "exemplary" is used to mean serving as an example, illustration, or explanation. An aspect or design described herein as "example" or "exemplary" should not necessarily be construed as preferred or advantageous over other aspects or designs. Rather, use of the term "example" or "exemplary" is intended to illustrate a concept in a concrete manner. The term "or" as used herein is intended to be interpreted as an inclusive "or" rather than an exclusive "or." That is, unless otherwise specified or clear from the context, the phrase "X includes A or B" refers to any of the natural inclusive permutations. That is, if X includes A, if X includes B, or if X includes both A and B, then in any of the above cases, the condition "X includes A or B" is satisfied. Furthermore, the articles "a" and "an" as used in this specification and the appended claims are to be construed to mean "one or more" unless otherwise specified and unless the singular form is clear from the context. Furthermore, when terms such as "first," "second," "third," and "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

1. 1. A method for correcting phase impairments in a Light Detection and Ranging (LIDAR) system, the method comprising: transmitting a first light beam towards a target; receiving a second light beam from the target to generate a received light beam; generating a digitally sampled target signal using a local oscillator (LO) signal, a first photodetector, and the received light beam; generating a digitally sampled reference signal using a reference beam transmitted through a fiber delay and a second photodetector; estimating one or more phase impairments of the LIDAR system using the digitally sampled reference signal to generate one or more estimated phase impairments; a first phase impairment caused by the local oscillator (LO) signal in the digitally sampled target signal, and performing a first correction for the first phase impairment from the one or more phase impairments in the digitally sampled target signal using the one or more estimated phase impairments; a second phase impairment caused by the received light beam in the digitally sampled target signal, and performing a second correction for the second phase impairment from one or more phase impairments in the digitally sampled target signal using the one or more estimated phase impairments; processing the first and second corrections to generate a corrected target signal for transmission to a point cloud; Including, A method of performing the second correction using a brute-force delay correction filter that delays the received light beam through a plurality of digital delays and selects an optimal delay.

2. 2. The method of claim 1 , The method of estimating the one or more phase impairments using at least one of a zero-force estimator, an MMSE estimator, a maximum likelihood estimator, and a map estimator.

3. 2. The method of claim 1 , The method of performing the first correction using one or more filters formed based at least in part on an estimate of the first phase impairment.

4. 2. The method of claim 1 , A method of selecting the optimal delay by performing a Fast Fourier Transform (FFT) of the plurality of digital delays and selecting the delay with the highest peak.

5. 2. The method of claim 1 , A method of performing the second correction by using a deskew compensation correction filter.

6. 2. The method of claim 1 , The method further includes generating a plurality of digitally sampled reference signals using a reference beam transmitted through a plurality of different fiber delays, each fiber delay being coupled to a respective photodetector.

7. A method for correcting phase impairments in a light detection and ranging (LIDAR) system, the method comprising: transmitting a first light beam towards a target; receiving a second light beam from the target to generate a received light beam; generating a digitally sampled target signal using a local oscillator (LO) signal, a first photodetector, and the received light beam; generating a digitally sampled reference signal using a reference beam transmitted through a fiber delay and a second photodetector; estimating one or more phase impairments of the LIDAR system using the digitally sampled reference signal to generate one or more estimated phase impairments; a first phase impairment caused by the local oscillator (LO) signal in the digitally sampled target signal, and performing a first correction for the first phase impairment from the one or more phase impairments in the digitally sampled target signal using the one or more estimated phase impairments; a second phase impairment caused by the received light beam in the digitally sampled target signal, and performing a second correction for the second phase impairment from one or more phase impairments in the digitally sampled target signal using the one or more estimated phase impairments; processing the first and second corrections to generate a corrected target signal for transmission to a point cloud; Including, The second correction is performed by using an iterative delay correction filter that iteratively delays the received light beam with different digital delays until the peaks converge.

8. A light detection and ranging (LIDAR) system comprising: a light beam source for irradiating a first light beam onto a target; a first detector that receives a received optical beam and a local oscillator (LO) signal from the target and generates a digitally sampled target signal; a fiber delay device having a known length coupled to said optical beam source; a second detector coupled to the fiber delay device and configured to generate a digitally sampled reference signal using a reference beam transmitted through the fiber delay device; a phase impairment estimator that estimates one or more phase impairments of the LIDAR system using the digitally sampled reference signal to generate one or more estimated phase impairments; a first phase impairment corrector for correcting a first phase impairment caused by the local oscillator (LO) signal in a digitally sampled target signal; a second phase impairment corrector for correcting a second phase impairment caused by the received light beam in the digitally sampled target signal; The second phase impairment corrector delays the received optical beam by a plurality of digital delays and includes a brute force delay correction filter that selects an optimal delay.

9. The system of claim 8, The system further comprising at least one of a zero-force estimator, an MMSE estimator, a maximum likelihood estimator, and a map estimator for estimating the one or more phase impairments.

10. 9. The system of claim 8, The system, wherein the first phase impairment corrector comprises one or more filters formed based at least in part on an estimate of the first phase impairment.

11. 9. The system of claim 8, selecting the optimal delay by performing a Fast Fourier Transform (FFT) of the plurality of digital delays and selecting the delay with the highest peak.

12. 9. The system of claim 8, The fiber delay device creates a delay proportional to the frequency of the target signal such that reflections from distant targets are delayed more and reflections from close targets are delayed less.

13. 9. The system of claim 8, The system further comprises a plurality of different fiber delay elements and photodetectors for generating the plurality of digitally sampled reference signals.

14. A light detection and ranging (LIDAR) system comprising: a light beam source for irradiating a first light beam onto a target; a first detector that receives a received optical beam and a local oscillator (LO) signal from the target and generates a digitally sampled target signal; a fiber delay device having a known length coupled to said optical beam source; a second detector coupled to the fiber delay device and configured to generate a digitally sampled reference signal using a reference beam transmitted through the fiber delay device; a phase impairment estimator that estimates one or more phase impairments of the LIDAR system using the digitally sampled reference signal to generate one or more estimated phase impairments; a first phase impairment corrector for correcting a first phase impairment caused by the local oscillator (LO) signal in a digitally sampled target signal; a second phase impairment corrector for correcting a second phase impairment caused by the received light beam in the digitally sampled target signal; The second phase impairment corrector comprises an iterative delay correction filter that iteratively delays the received optical beam through different digital delays until a peak converges.

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