Managing the performance of an optical phased array based on phase shift variations

Abstract

A method comprising: supplying light to a plurality of optical antennas separated by a set of distances in an optical phased array (OPA), wherein a plurality of phase shifters imposes phase shifts on the light supplied to the respective optical antennas of the plurality of optical antennas; and comprising managing a first mode of operation, comprising: for a first subset of optical antennas within a distance from a center of the plurality of optical antennas, determining variations of imposed phase shifts that are less than a phase shift variation range, and for a second subset of optical antennas farther than the distance from the center of the plurality of optical antennas, determining variations of imposed phase shifts that are greater than the phase shift variation range.

Description

REFERENCE TO RELATED APPLICATION(S)

[0001] This application claims priority to and the benefit of U.S. Provisional Application Serial No. 63 / 340,526, entitled "MITIGATION OF SPURIOUS RETURNS IN OPTICAL PHASED ARRAYS," filed May 11, 2022, the entire disclosure of which is hereby incorporated by reference. TECHNICAL FIELD

[0002] This disclosure relates to managing the performance of an optical phased array based on phase shift variations. BACKGROUND

[0003] Optical phased array (OPA) LiDAR (Light Detection and Ranging) systems can generate return intensity maps that include clutter peaks associated with lobes other than an intended lobe of the optical phased array radiation pattern (also referred to as an angular intensity distribution). For example, light associated with a main lobe may be reflected to provide a main signal peak that triggers a detection event, but the radiation pattern typically also includes side lobes on either side of the main lobe (e.g., based on a finite extent of the OPA) and a number of grating lobes at specific angular separations (e.g., based on the finite distances between optical antennas that form the OPA). The grating lobes may be larger than the side lobes, and light from either the grating lobes and / or side lobes may be reflected to provide clutter peaks (e.g.,false or unwanted). If the intensity of these interference signals exceeds the noise floor after digitization of the LiDAR system, they can register as false detection events, which can be problematic for typical LiDAR applications. SUMMARY

[0004] In one aspect, an apparatus generally comprises: an optical phased array (OPA) comprising: a plurality of optical antennas separated by a set of distances, and a plurality of phase shifters configured to impose phase shifts on the light provided to the respective optical antennas of the plurality of optical antennas;a phase shift control module configured to manage the imposed phase shifts, wherein the managing comprises a first mode of operation comprising: for a first subset of optical antennas within a distance from a center of the plurality of optical antennas, determining variations of imposed phase shifts that are less than a phase shift variation range, and for a second subset of optical antennas farther than the distance from the center of the plurality of optical antennas, determining variations of imposed phase shifts that are greater than the phase shift variation range;

[0005] Aspects may include one or more of the following features.

[0006] Determining variations of imposed phase shifts for the first subset of optical antennas and determining variations of imposed phase shifts for the second subset of optical antennas comprises determining variations according to a probability distribution.

[0007] Determining variations according to a probability distribution involves determining variations according to a pseudorandom function.

[0008] The managing further comprises a second mode of operation, comprising: for the first subset of optical antennas and the second subset of optical antennas, determining imposed phase shifts corresponding to a deterministic phase profile.

[0009] The deterministic phase profile comprises a linear phase profile defining a common phase shift to be imposed on each of the plurality of phase shifters.

[0010] The managing further comprises a first scanning mode of operation, wherein a first region is scanned by managing the imposed phase shifts in the first mode of operation for two or more transmit beam angles.

[0011] The managing further comprises a second scanning mode of operation, wherein a second range is scanned by managing the imposed phase shifts in the second mode of operation for two or more transmit beam angles.

[0012] The first area is a subset of the second area.

[0013] The apparatus further includes a control module configured to receive scattered light emitted by the OPA and estimate a range to a target based at least in part on a property of the scattered light.

[0014] The OPA is a first OPA, and the device further comprises a second OPA configured to receive the scattered light emitted by the first OPA and provide the scattered light to the control module.

[0015] The spacings in the set of spacings are each greater than half a wavelength in a spectrum of light provided to the plurality of optical antennas in the OPA.

[0016] The distances in the set of distances are all identical to each other.

[0017] In another aspect, a method generally comprises: providing light to a plurality of optical antennas separated by a set of distances in an optical phased array (OPA), wherein a plurality of phase shifters impose phase shifts on the light provided to the respective optical antennas of the plurality of optical antennas;and managing the imposed phase shifts using a phase shift control module, wherein the managing comprises a first mode of operation comprising: for a first subset of optical antennas within a distance from a center of the plurality of optical antennas, determining variations of imposed phase shifts that are less than a phase shift variation range, and for a second subset of optical antennas farther than the distance from the center of the plurality of optical antennas, determining variations of imposed phase shifts that are greater than the phase shift variation range;

[0018] Determining variations of imposed phase shifts for the first subset of optical antennas and determining variations of imposed phase shifts for the second subset of optical antennas comprises determining variations according to a probability distribution.

[0019] Determining variations according to a probability distribution involves determining variations according to a pseudorandom function.

[0020] The managing further comprises a second mode of operation, comprising: for the first subset of optical antennas and the second subset of optical antennas, determining imposed phase shifts corresponding to a deterministic phase profile.

[0021] The deterministic phase profile comprises a linear phase profile defining a common phase shift to be imposed on each of the plurality of phase shifters.

[0022] The managing further comprises a first scanning mode of operation, wherein a first region is scanned by managing the imposed phase shifts in the first mode of operation for two or more transmit beam angles.

[0023] The managing further comprises a second scanning mode of operation, wherein a second range is scanned by managing the imposed phase shifts in the second mode of operation for two or more transmit beam angles.

[0024] The first area is a subset of the second area.

[0025] The method further includes receiving scattered light emitted by the OPA and estimating a range to a target based at least in part on a property of the scattered light.

[0026] The OPA is a first OPA, and the method further comprises receiving the scattered light emitted by the first OPA into a second OPA and providing the scattered light for estimating the range.

[0027] Aspects may have one or more of the following advantages.

[0028] In a LiDAR system, an optical wave can be transmitted from an optical source to a target object(s) at a given distance using one OPA, and the light backscattered from the target object(s) can be collected using another OPA. Various techniques, such as modulation and / or time-of-flight, can be used to determine the distance to a target object based on information associated with a detection event. The optical source used in such a system can be a laser or other coherent light source that provides an optical wave having a narrow linewidth and a peak wavelength falling within a specific range (e.g., between about 100 nm to about 1 mm, or a subset thereof), also referred to herein simply as "light."

[0029] In some environments where a LiDAR system may be used (e.g., automotive environments), or other photonic systems that employ transmitter and receiver OPAs, return intensity maps (representing, for example, the intensity of the collected return light) containing clutter peaks associated with sidelobes and / or grating lobes of the optical phased array (OPA) radiation patterns may be exacerbated by the ubiquity of retroreflectors (e.g., traffic signs, pavement markings, bicycle safety reflectors, registration stickers, etc.), which can produce return signals several orders of magnitude higher than those produced by diffusely scattering surfaces.Accordingly, the return signal associated with the mixing of sidelobes or grating lobes at a distant retroreflector can easily exceed the noise floor of a phased-array LiDAR system, generating a false detection event. Mitigating this effect is useful for the commercialization of phased-array LiDAR technology in automotive and other applications.

[0030] In some examples, the photonic systems described herein can be used to probe and measure environments or areas over a wider range of angles than existing implementations. For example, a configuration in which grating lobes of a transmit aperture and a receiver aperture are aligned while the main lobes of the transmit aperture and the receiver aperture are misaligned can enable scanning over angles outside of areas accessible to the main lobes, as described in more detail below.

[0031] Other features and advantages will become apparent from the following description and from the figures and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The disclosure is best understood from the following detailed description when read in conjunction with the accompanying drawings. It is emphasized that, in accordance with common practice, the various features of the drawings are not to scale. On the contrary, the dimensions of the various features have been arbitrarily expanded or reduced for clarity. Fig. Figure 1A is a schematic diagram of an example photonic system. Fig. Figure 1B is a schematic diagram of an example LiDAR system. Fig. Figure 2A is a schematic diagram of exemplary radiation intensity patterns for a transmitter OPA and a receiver OPA. Fig. Figure 2B is a diagram of prophetic exemplary radiation intensity patterns for a transmitter OPA and a receiver OPA as a function of the angle formed with respect to the center of a main lobe. Fig. Figure 2C is a diagram of prophetic exemplary radiation intensity patterns for a transmitter OPA and a receiver OPA as a function of the angle formed with respect to the center of a grating lobe. Fig. Figure 2D is a diagram of a prophetic exemplary radiant intensity pattern product for a transmitter OPA and a receiver OPA as a function of the angle formed with respect to the target direction, with the grating lobes of the transmitter OPA and the receiver OPA aligned. Fig. Figure 2E is a diagram of a prophetic exemplary radiant intensity pattern product for a transmitter OPA and a receiver OPA as a function of the angle formed with respect to the target direction, where the grating lobes of the transmitter OPA and the receiver OPA are misaligned. Fig. Figure 3A is a schematic diagram of exemplary radiation intensity patterns for a transmitter OPA and a receiver OPA. Fig. Figure 3B is a diagram of prophetic exemplary radiation intensity patterns for a transmitter OPA and a receiver OPA as a function of the angle formed with respect to the center of a main lobe. Fig. Figure 3C is a diagram of prophetic exemplary radiation intensity patterns for a transmitter OPA and a receiver OPA as a function of the angle formed with respect to the center of a grating lobe. Fig. Figure 4 is a schematic diagram of an example of noise detection and attenuation. Fig. Figure 5A is a schematic diagram of an example near-field photonic system. Fig. Figure 5B is a diagram of prophetic exemplary radiation intensity patterns for a transmitter OPA and a receiver OPA as a function of the angle formed with respect to the center of a grating lobe. Fig. Figure 5C is a schematic diagram of an example near-field photonic system. Fig. Figure 5D is a diagram of prophetic exemplary radiation intensity patterns for a transmitter OPA and a receiver OPA as a function of the angle formed with respect to the center of a grating lobe. Fig. Figure 6A is a plot of a prophetic example absolute value of the effective amplitude for apodization as a function of antenna number, where the antenna number corresponds to the relative position of each antenna in an optical phased array. Fig. Figure 6B is a diagram of a prophetic example phase for apodization as a function of antenna count, where the antenna count corresponds to the relative position of each antenna in an optical phased array. Fig. Figure 7 is a diagram of a prophetic exemplary radiant intensity pattern product with apodization for a transmitter OPA and a receiver OPA as a function of the angle formed with respect to the center of a lobe. Fig. Figure 8 is a graph of prophetic exemplary parallax errors as a function of the angle of a target with respect to a transmitter OPA for various photonic system configurations. Fig. Figure 9 is a diagram of prophetic exemplary radiation intensity patterns for one transmitter OPA and two receiver OPAs as a function of the angle formed with respect to the center of a lobe. Fig. Figure 10 is a flowchart of an exemplary clutter detection and mitigation scheme. DETAILED DESCRIPTION

[0033] An optical phased array (OPA) can form a steerable optical beam based on the phased array principle. For example, arrays of optical antennas (sometimes called optical emitters) can each emit light with controllable phases (e.g., controlled by phase shifters coupled to the optical antennas), resulting in interference patterns that form one or more optical beams. Typically, each optical antenna can also receive light in addition to emitting light. A transmitting OPA can be characterized by an angular intensity distribution, also called a radiation intensity pattern, which can describe the optical beam formed by the transmitting OPA. Thus, an optical beam formed by the transmitting OPA can include light emitted at one or more angles characterized by the angular intensity distribution.A receiver OPA can receive or collect light, which can be used to characterize a scene or area surrounding the OPAs. The receiver OPA can also be characterized by an angular intensity distribution (radiance intensity pattern), which can also be referred to as a gain pattern. The gain pattern characterizes the angular sensitivity of the receiver OPA to incident light. Both the transmitter angular intensity distribution and the receiver angular intensity distribution can exhibit peaks, also referred to as lobes or array factor peaks, that correspond to local maxima where the respective OPAs have higher transmit power or receive sensitivity. A photonic platform comprising both a receiver OPA and a transmitter OPA can be characterized by a radiance intensity pattern product.In some examples, the radiation intensity pattern product may be determined by multiplying the transmitter radiation intensity pattern and the receiver radiation intensity pattern. In other examples, the radiation intensity pattern product may depend on other factors (e.g., an antenna element factor).

[0034] Some of the examples described herein may include an OPA with a receive aperture (i.e., a receiving OPA, a receiving section, or a receiving subsystem) and a transmit aperture (i.e., a transmitting OPA, a transmitting section, or a transmitting subsystem). Other examples may include separate structures, where the transmit aperture and the receive aperture are not physically connected or are fabricated as standalone devices. Furthermore, the transmit aperture and the receive aperture may be the same.

[0035] In some example implementations, the photonic system described herein may be designed to operate over a specific range of optical wavelengths, for example, the λ = 1500 to 1600 nm band or the λ = 1270 to 1330 nm band, and the base pitch spacing α between the optical antennas may be of a similar size to the optical wavelength. For example, for operation in the 1500 to 1600 nm band, 1000 nm ≤ a ≤ 2000 nm may be typical. Although all grating lobes would be eliminated if α < λ / 2, a practical lower limit on the grating spacings can be set by the refractive indices of available waveguide materials, as well as by the detrimental effects of waveguide-to-waveguide coupling as the spacing is reduced.For example, single-mode silicon waveguides designed for the 1500 to 1600 nm band, if spaced 750 nm apart to completely eliminate grating lobes, would couple with a characteristic coupling length between 50 µm and 200 µm. Thus, phase-dependent light redistribution between waveguides would be unavoidable and would damage the angular intensity distribution. Therefore, in practical implementations, the presence of grating lobes may occur in the transmit (TX) radiation pattern and the receive (RX) gain pattern. However, the grating lobes of the TX OPA and the grating lobes of the RX OPA can be misaligned by applying uniform pitch mismatch (Δa), element factor technique, pitch apodization, random spacing, or other techniques.

[0036] Fig. 1A shows an exemplary optical interface 100 that includes a transmitting OPA 102 and a receiving OPA 104. The transmitting OPA 102 includes an array of transmitting optical antennas 106 with spacing determined by a first spacing α. The receive aperture includes an array of receiving optical antennas 108 with spacing determined by a second spacing a + Δa. The transmitting optical antennas 106 and the receiving optical antennas 108 may be, for example, optical gratings or waveguide facets (referred to as an "end-fire array" configuration). The exemplary optical interface 100 further includes an array of transmitting phase shifters 110 and an array of receiving phase shifters 112, which may be, for example, thermal phase shifters, electro-optic phase shifters, or microelectromechanical phase shifters.In some examples, each of the transmitting phase shifters 110 and each of the receiving phase shifters 112 can be controlled independently. The transmitting phase shifters 110 can modulate the direction and radiation intensity pattern of the transmitted beam emitted by the transmitting OPA 102, and the receiving phase shifters 112 can modulate the receiver radiation intensity pattern (i.e., a gain pattern of the receiver). Transmitting optical power splitters 114 direct an input lightwave 116 into each of the transmitting phase shifters 110, which are optically coupled to respective transmitting optical antennas 106. In this example, the power splitters 114 are connected by waveguides in a binary tree arrangement. A similar or identical binary tree arrangement for receiving optical power combiners 118 combines received light into an output lightwave 120, which can then be further manipulated, transformed, or measured.A similar system (not shown) may also include multiple similar or identical transmitting OPAs 102 or receiving OPAs 104.

[0037] Fig. 1B shows an example of a LiDAR system 130 in which an optical interface using one or more OPAs, such as the one shown in Fig. 1A. The LiDAR system 130 includes a laser system 132, a transmitter module 134 configured to transmit light provided by the laser system 132 (e.g., using an OPA) to a target area, and a receiver module 136 configured to receive light (e.g., using an OPA) and coherently mix the received light with local oscillator (LO) light 138, which may be derived from the laser system 132, in a coherent detector 140. A control module 142 is configured to control various aspects of the transmitter module 134 and the receiver module 136 and to estimate a range to a target associated with a detection event based at least in part on a characteristic of the scattered light received by the receiver module 136.The laser system 132 may provide a continuous wave (CW) light signal having a narrow linewidth and low phase noise, sufficient, for example, to provide a temporal coherence length long enough to perform coherent detection over the timescales of interest. In some implementations, the laser system 132 is a frequency-tunable laser system in which the frequency of the provided light can be swept to perform frequency-modulated continuous wave (FMCW) LiDAR measurements. A particular transmit beam angle 144 from the transmitter module 134 may correspond to a particular receive beam collection angle 146 into the receiver module 136, as described in more detail below.

[0038] Any of a variety of techniques can be used to control a transmit angle or receive angle of a beam of a radiation intensity pattern. Some OPAs have a linear distribution of optical antennas. Steering about a first axis perpendicular to the linear distribution can be provided, for example, by changing the relative phase shifts in phase shifters coupled to each of the optical antennas. Other techniques can be used for steering about a second axis orthogonal to the first axis (e.g., by changing the frequency of the laser system 132).

[0039] Fig. 2A-2E illustrate an exemplary hardware configuration and technique for attenuating grating lobe signals in a case where the phase shifters are programmed to generate a linear phase front in the near field (e.g., by using a common phase shift for each of the phase shifters). Another exemplary configuration or operating mode involving a phase-apodized phase front is described herein. With reference to Fig. 2A-2E, grating lobe misalignment can result in a difference in the main lobe to grating lobe angles of their respective array factors. However, the main lobe direction can still be controlled, for example, using the phase shifters coupled to the optical antennas.

[0040] Fig. Figure 2A shows an exemplary radiation intensity pattern for a transmitter OPA 202 and a receiver OPA 204. A transmitter main lobe 206 and a receiver main lobe 208 overlap, whereas a transmitter grating lobe 210 and a receiver grating lobe 212 do not overlap or have significantly less overlap. Such an arrangement of lobe overlaps may result from tuning phase shifters associated with optical transmitter and receive antennas. In this example, a return signal from an object located near one of the grating lobes is attenuated due to the reduced overlap.

[0041] Fig. 2B, Fig. 2C and Fig. 2E quantitatively show an example where the TX-OPA spacing and the RX-OPA spacing differ by 4 nm to misalign the grating lobes when the main lobes are aligned. Fig. Figure 2D quantitatively shows an example where the TX-OPA spacing and the RX-OPA spacing are equal, so that the grating lobes are aligned.

[0042] Fig. Figure 2B shows a graph of a prophetic exemplary transmitter radiation intensity pattern 220 (solid line) and an exemplary receiver radiation intensity pattern 222 (dashed line) as a function of angle with respect to the center of the overlapped main lobes located at 0°.

[0043] Fig. Figure 2C shows a graph of a prophetic example transmitter radiation intensity pattern 230 (solid line) and an example receiver radiation intensity pattern 232 (dashed line) as a function of the angle with respect to the midpoint between the misaligned grating lobes located at 0°.

[0044] Fig. Figures 2B-C show the far-field TX and RX array factor patterns at the main and grating lobes, respectively, representing the relative displacement of the grating lobes when the main lobes are aligned.

[0045] Fig. Figure 2D shows a diagram of a prophetic exemplary radiation intensity pattern product for a transmitter OPA and a receiver OPA as a function of the angle formed with respect to the target direction, with the grating lobes of the transmitter OPA and the receiver OPA aligned. The radiation intensity pattern product can be determined by multiplying a transmitter radiation intensity pattern by a receiver radiation intensity pattern. In some examples, other factors may contribute to the radiation intensity pattern product (e.g., an antenna element factor and parallax). Because the separations of the transmitter OPA and the receiver OPA are equal, the grating lobes of the transmitter OPA and the receiver OPA are aligned, and the main lobes of the transmitter OPA and the receiver OPA are aligned.The aligned grating lobes result in grating intensity peaks 240, which can be of comparable size in radiation intensity to a main intensity peak 242.

[0046] Fig. Figure 2E shows a diagram of a prophetic exemplary radiation intensity pattern product for a transmitter OPA and a receiver OPA as a function of the angle formed with respect to the target direction, where the grating lobes of the transmitter OPA and the receiver OPA are misaligned. Because the pitches of the transmitter OPA and the receiver OPA differ by 4 nm, the grating lobes of the transmitter OPA and the receiver OPA are misaligned when the main lobes of the transmitter OPA and the receiver OPA are aligned. The misaligned grating lobes result in grating intensity peaks 250 that may be of significantly smaller magnitude in radiation intensity compared to a main intensity peak 252.

[0047] Fig. Figures 2D-E show a 24 dB reduction in the return signal from the grating lobe direction for the unmatched case (Δa = 4 nm) compared to the matched case. In general, a larger |Δa| would result in a greater relative reduction in the return signal from the grating lobe.

[0048] Fig. Figures 3A-3C illustrate an example of an alternative operating configuration that can be used in some photonic system implementations. In this example, the antenna array phase shifters are programmed so that one or more grating lobes of the TX OPA overlap with one or more grating lobes of the RX OPA, and the main lobe of the TX OPA is misaligned with the main lobe of the RX OPA. In this operating configuration, the LiDAR can be primarily sensitive to return signals originating from objects within an angular range specified by the grating lobe. The direction of the grating lobe(s) can be controlled, for example, using antenna phase shifters.

[0049] Fig. 3A shows an exemplary radiation intensity pattern for a transmitter OPA 302 and a receiver OPA 304. A transmitter main lobe 306 and a receiver main lobe 308 do not overlap, whereas a transmitter grating lobe 310 and a receiver grating lobe 312 do overlap. Thus, a return signal from any object located at the main lobe is attenuated relative to a return signal from any object located at the grating lobe.

[0050] Fig. 3B shows a graph of a prophetic exemplary transmitter radiation intensity pattern 320 (solid line) and an exemplary receiver radiation intensity pattern 322 (dashed line) as a function of angle with respect to the midpoint between the misaligned main lobes located at 0°.

[0051] Fig. Figure 3C shows a graph of a prophetic example transmitter radiation intensity pattern 330 (solid line) and an example receiver radiation intensity pattern 332 (dashed line) as a function of angle with respect to the center of the overlapped grating lobes located at 0°.

[0052] Fig. Figures 3B-3C show the far-field TX and RX array factor patterns at the main and grating lobes, respectively, representing the relative displacement of the main lobes when the grating lobes are aligned.

[0053] Extending the operational scope to configurations where the grating lobes are aligned (e.g. Fig. 3A-3C), adds several practical capabilities to the photonic system. First, it effectively extends the system's "addressable field of view" beyond the main lobe region, which has a width half of arcsin(λ / 2a), where λ is the wavelength of light and a is the pitch (i.e., the distance between optical antennas) of the OPA. Such configurations and techniques enable the OPA-based LiDAR system to unambiguously address pointing directions at azimuth angles |θ| ≥ arcsin(λ / 2a). This effectively extends the azimuth scanning range of the device, where in the course of scanning across a scene, first the left TX and RX grating lobes overlap while the left grating lobe region is scanned, then the TX and RX main lobes overlap while the main region is scanned, then the right TX and RX grating lobes overlap while the right grating region is scanned.Generally, the user can choose to scan any subset of the full 180° hemisphere in front of the LiDAR unit. Second, the ability to unambiguously target objects in the main and grating lobe regions enables a technique for attenuating a retroreflector grating lobe "image" (or "ghost image" or "clutter image").

[0054] Fig. Figure 4 shows an example of clutter detection and mitigation. During the course of scanning a scene 400 using overlapping TX and RX lobes, when a return signal (in some cases, a large return signal) is measured in one direction, a model of the system radiation intensity pattern product and antenna element factor can be used to predict the intensity and direction of the clutter "ghost" signals that may arise when partially non-overlapping TX and RX lobes coincide with the physical object generating the reflection and resulting return signal (which may still be relatively strong despite the reduction due to the misaligned TX and RX lobes). For example, an object 402A (and 402B) may generate a ghost signal 404A (and 404B).The system can then annotate this direction as affected by grating lobe ghosting and choose to either ignore this data or locally raise the detection threshold to prevent the ghost signal 404A from being registered as a detection event on its own. Such a technique can mitigate the detrimental effects of retroreflectors on application domain performance in some cases. In other examples, the system can "subtract" data corresponding to the ghost signal, obtaining a substantially ghost-free image.

[0055] The use of selectively aligned and misaligned TX and RX grating lobes can enable objects within the grating lobe regions to be unambiguously targeted. For example, the left TX and RX grating lobes can be overlapped to sample the left grating lobe region 406, the TX and RX main lobes can be overlapped to sample the main lobe region 408, and the right TX and RX grating lobes can be overlapped to sample the right grating lobe region 410. Directions in which strong return signals are detected can then be used to annotate directions where interfering "ghosting" images are expected. The data at the points caused by ghosting can then be deleted, or alternatively, the detection threshold at these points can be increased so that the ghosting signal alone does not constitute a detection.In this example, there are two grating lobes, one on each side of a main lobe, corresponding to three respective regions corresponding to different sections of the scanned area. In other examples, there may be any number of grating lobes on one or both sides of a main lobe, resulting in more than three respective regions corresponding to different sections of the scanned area.

[0056] Fig. 1, Fig. 2A, Fig. 3A and Fig. 4 together illustrate various aspects of a first example method for imaging an area. The first example method for imaging an area includes providing light (e.g., input light wave 116) to a plurality of optical antennas (e.g., transmitting optical antennas 106) separated by a first set of distances (e.g., distance α for a case where the distances are all equal) in a first optical phased array (e.g., transmitting OPA 102), wherein phases of light provided to the respective optical antennas are controlled (e.g., by an array of transmitting phase shifters 110) to form a transmitted beam from the first OPA. The transmitted beam is characterized by a first angular intensity distribution comprising a first lobe (e.g., transmitter main lobe 206, transmitter main lobe 306) and a second lobe (e.g., transmitter grating lobe 210, transmitter grating lobe 310).The first example method for imaging a region further includes steering an angle of the beam such that the first lobe of the first angular intensity distribution scans over a first portion of the region (e.g., main lobe region 408) and the second lobe of the first angular intensity distribution scans over a second portion of the region (e.g., left grating lobe region 406, right grating lobe region 410). The first example method for imaging a region further includes receiving light from a plurality of optical antennas (e.g., receiving optical antennas 108) separated by a second set of pitches (e.g., pitch α + Δα) in a second OPA (e.g.,receiving OPA 104), wherein at least one spacing in the second set of spacings differs from at least one spacing in the first set of spacings, and wherein phases of light provided to the respective optical antennas are controlled (e.g., by an array of receiving phase shifters 112) to receive light into the OPA from different directions associated with a second angular intensity distribution including a first lobe (e.g., receiver main lobe 208, receiver main lobe 308) and a second lobe (e.g., receiver grating lobe 212, receiver grating lobe 312). The phases of the first OPA and the phases of the second OPA are configured such that the first lobe of the first angular intensity distribution and the first lobe of the second angular intensity distribution substantially angularly overlap during scanning of the first lobes across the first portion of the range (e.g., as shown in FIG. Fig. 2A) and the second lobe of the first angular intensity distribution and the second lobe of the second angular intensity distribution substantially overlap in angle during scanning of the second lobes over the second portion of the area (e.g., as shown in Fig. 3A).

[0057] Fig. 1, Fig. 2A, Fig. 3A and Fig. 4 together illustrate various aspects of a second example method for imaging an area. The second example method for imaging an area includes providing light (e.g., input light wave 116) to a plurality of optical antennas (e.g., transmitting optical antennas 106) separated by a first set of distances (e.g., distance α) in a first optical phased array (e.g., transmitting OPA 102), wherein phases of light provided to the respective optical antennas are controlled (e.g., by an array of transmitting phase shifters 110) to form a transmitted beam from the first OPA. The transmitted beam is characterized by a first angular intensity distribution comprising a first lobe (e.g., transmitter main lobe 206, transmitter main lobe 306) and a second lobe (e.g., transmitter grating lobe 210, transmitter grating lobe 310).The second example method for imaging a region further comprises steering an angle of the beam such that the first lobe of the first angular intensity distribution scans over a first portion of the region (e.g., main lobe region 408) and the second lobe of the first angular intensity distribution scans over a second portion of the region (e.g., left grating lobe region 406, right grating lobe region 410). The second example method for imaging a region further comprises receiving light from a plurality of optical antennas (e.g., receiving optical antennas 108) separated by a second set of pitches (e.g., pitch α + Δα) in a second OPA (e.g.,receiving OPA 104), wherein at least one distance in the second set of distances differs from at least one distance in the first set of distances, and wherein phases of light provided to the respective optical antennas are controlled (e.g., by an array of receiving phase shifters 112) to receive light into the OPA from different directions associated with a second angular intensity distribution. The second example method for imaging an area further includes characterizing potential detection events (e.g., ghost signal 404A) while scanning the first lobe of the first angular intensity distribution over the first portion of the area based at least in part on whether a detection event (e.g.,Object 402A) was detected or not during scanning of the second lobe of the first angular intensity distribution over the second portion of the area at a beam angle associated with the potential detection event.

[0058] In the Fig. 2A-E and Fig. In the examples shown in Figures 3A-C, the phase shifters addressing each TX and RX antenna are adjusted to produce nearly flat phase fronts in the near field of the TX and RX apertures. In other examples, the phase shifters may be configured to produce other, non-flat phase fronts in the near field.

[0059] Fig. 5A shows a schematic diagram of an exemplary transmitter optical phased array (TX OPA) 500 configured to produce a nearly flat transmitter phase front corresponding to a distribution of phases 502A in the near field of a beam transmitted from a transmitter aperture 506, and an exemplary receiver optical phased array (RX OPA) 501 configured to receive a nearly flat receiver phase front corresponding to a distribution of phases 504A in the near field of light received at a receiver aperture 508. The arrows illustrated in conjunction with the near-field phase front of the transmitted beam represent the relative phase shifts applied by phase shifters to the optical field emitted by the optical antennas of the TX OPA 500.The arrows illustrated in conjunction with the near-field phase front of the received beam represent the relative phase shifts applied by phase shifters to the optical field received at the optical antennas of the RX-OPA 501. In this example, three lobes begin to emerge as the optical field begins to transition from the near field to the far field for each OPA, corresponding to the main lobe and a grating lobe on either side of the main lobe.

[0060] Fig. 5B shows a diagram of a prophetic exemplary transmitter radiation intensity pattern 510A (dashed line) for a transmitter optical phased array and a prophetic exemplary receiver radiation intensity pattern 512A (solid line) for a receiver optical phased array as a function of the angle with respect to the midpoint between the tips of the grating lobes. Referring to Fig. 5A, when optical power is distributed substantially evenly across antennas in the transmitter aperture 506 and collected evenly by the antennas in the receiver aperture 508, for example, by using binary splitter trees, the transmitter optical phased array 500 and the receiver optical phased array 501 can achieve the Fig. 5B. Referring to Fig. 5B, the potentially slow decay of the sinc-squared profile can limit the relative attenuation of ghosts located at the misaligned TX and RX grating lobes relative to the main lobes. This disadvantage may apply to splitter networks other than binary trees, which, for example, produce slow decays in the far-field due to the near-field amplitude profile.

[0061] Fig. 5C-D show an additional mode of operation that can be used in some implementations to further reduce the effect of ghosting.

[0062] Fig. 5C shows a schematic diagram of an example in which phase shifters are configured to generate spatially varying transmitter phases 502C and spatially varying receiver phases 504C in the near field of the transmitter aperture 506 and the receiver aperture 508. The spatially varying phase shifts applied by the phase shifters result in a near-field profile with a smooth, gradual roll-off toward the edges of the aperture.

[0063] Fig. Figure 5D shows a diagram of a prophetic exemplary transmitter radiation intensity pattern 510C (dashed line) for a transmitter optical phased array and a prophetic exemplary receiver radiation intensity pattern 512C (solid line) for a receiver optical phased array as a function of the angle relative to the midpoint between the tips of the grating lobes. The far-field beam profiles correspond to the Fig. 5C and exhibit a sharper roll-off away from the array factor peak. In some examples, the near-field phases can be designed such that, for sufficient grating lobe misalignment, the relative attenuation of a ghost signal with respect to the main signal can be further reduced compared to that shown in Fig. 5B. In some examples, near-field phase misalignment can reduce the peak intensity of the main lobe, thereby imposing a small link penalty on the overall return signal of the system. A representative value of this penalty can range from 0 to 10 dB. However, for some phase-front configurations, the relative attenuation of the ghost signal can greatly exceed the induced link penalty by 10 dB or more in some cases.

[0064] In general, the various phase apodization techniques described herein can be used to improve peak rolloff and reduce ghosting (e.g., the contribution of a ghost signal to the received signal, where the ghost signal is the result of reflection from an object outside the area scanned by the intended lobe). Furthermore, the various phase apodization techniques can be realized in various implementations. Two possible manifestations—example algorithms for determining appropriate phases to be applied to each antenna—are described as follows.

[0065] Fig. Figure 6A is a plot of a prophetic example absolute value of the effective amplitude for apodization as a function of antenna number, where the antenna number corresponds to the relative position of each antenna in an optical phased array.

[0066] Fig. Figure 6B is a diagram of a prophetic example phase for apodization as a function of antenna count, where the antenna count corresponds to the relative position of each antenna in an optical phased array.

[0067] Fig. 6A-B show an example of apodization by applying random phases to each antenna in a transmitter OPA (TX) and a receiver OPA (RX), where each phase is selected from a suitable probability distribution (e.g., a Gaussian probability distribution) with a varying standard deviation across the width of the respective aperture using a suitable function for generating random values ​​(e.g., using a pseudorandom number generation function). The procedure for this example is as follows. First, a desired “effective amplitude apodization” profile, which is Fig. 6A, is selected for the TX and RX apertures. The design of this apodization profile can be similar to the design of a finite impulse response low-pass filter in that the result, namely the far-field profile, can be the Fourier transform of a finite list of values. In some implementations, finite impulse response design techniques can be used for this step. Second, for each antenna, a phase is randomly selected from a Gaussian distribution with zero mean and standard deviation σ = sqrt[-2 In(A)], where A is the value of the effective amplitude apodization profile at that antenna and In is the natural logarithm.

[0068] Fig. Figure 6B shows a diagram of a prophetic example of such an antenna phase profile. Fig. The antenna phase configuration shown in Figure 6B produces a far-field profile whose side lobe intensities are approximately equal to those that would result if the emission sizes of the antennas were Fig. 6A, in contrast to the emission phases shown in Fig. 6B. The Fig. 5D was obtained using this methodology (e.g. the one described in Fig. 6B shown antenna phase configuration).

[0069] Another exemplary methodology for determining the antenna phase configuration involves solving an optimization problem involving a calibrated model of each individual OPA LiDAR system, where the optimization figure of merit may be the minimization of ghost intensity (i.e., the minimization of the TX / RX radiant intensity pattern product in the non-coincident regions) constrained by an acceptable amount of attenuation in the radiant intensity pattern product peak or main lobe. A particularly desirable solution to this optimization problem may result in a deep >10 dB dip in the RX pattern noise floor coinciding with the TX peak in the non-coincident region and a >10 dB dip in the TX pattern noise floor coinciding with the RX peak in the non-coincident region, resulting in a TX / RX radiant intensity pattern product with highly attenuated ghosting.

[0070] The Fig. 6A-B can lead to additional benefits. For example, such phase apodization can lead to the narrowing of the main peak of the TX / RX radiation intensity pattern product, as shown in Fig. 7 shown.

[0071] Fig. Figure 7 shows a plot of a prophetic example of apodized radiant intensity pattern product for a transmitter OPA and a receiver OPA as a function of the angle relative to the peak of the intensity pattern product. Compared to the unpodized, linear phase front case, where the center lobe at 0° can have ~10 sidelobes on each side rising above the noise floor, phase apodization reduces this number to 2 sidelobes on each side for the 2048-element TX and RX arrays in this example. The reduced number of sidelobes, in turn, reduces the azimuthal width of spurious signals associated with the coincidence of these sidelobes on strong reflectors. The practical manifestation of this effect is that the "apparent width" of retroreflectors, which in the unattenuated case can be several times wider than their physical width, can be significantly reduced. Fig. 7, distances between side lobes are expressed in terms of angular offset relative to the center in degrees on the horizontal axis, and relative widths are compared in terms of arbitrary units (denoted as basketball diameters in a range of 200 m).

[0072] Due to the joint penalty associated with phase apodization, the practical application of the attenuation techniques may involve switching between a non-phase-apodized near-field phase profile (e.g., a linear phase profile based on a joint phase shift or another deterministic phase profile, such as a lenticular linear / parabolic phase profile that does not use a probability distribution or pseudorandom function) and phase-apodized near-field phase profiles depending on the circumstances. For example, the OPA LiDAR described herein can be operated in a linear / lenticular linear mode during the initial scan of a scene. In regions of the scene where retroreflectors (e.g.,If either strong return signals (detected by a strong return signal compared to a predetermined threshold) or ghosting (annotated at known angles left and right of strong return signals) are suspected, the system can reconfigure the phase shifters to generate a phase-apodized near-field phase front and resample the affected regions, taking advantage of reduced sidelobe overlap in the former (near retroreflector) case and reduced offset peak overlap in the latter (in case of ghosting) to increase sensitivity to weaker return signals that may otherwise be masked by the interfering return signals. Since the proportion of a scene affected by retroreflectors is expected to be quite low, for example, < 5%, a similarly small amount of temporal overhead is expected in the resampling process.

[0073] Fig. 1 and Fig. 6B together illustrate various aspects of an example device. The example device includes an optical phased array (e.g., transmitting OPA 102) comprising a plurality of optical antennas (e.g., transmitting optical antennas 106) separated by a set of distances (e.g., distance α), and a plurality of phase shifters (e.g., transmitting phase shifters 110) configured to impose phase shifts on the light provided to the respective optical antennas of the plurality of optical antennas. A phase shift control module is configured to manage the imposed phase shifts, wherein the managing comprises a first mode of operation. The phase shift control module may provide analog electrical signals to the plurality of phase shifters. For example, a computer processing unit (CPU) may be disposed proximate to and electrically connected to the plurality of phase shifters.A digital-to-analog converter (DAC) may be used to convert digital signals from example phase shift control modules into analog signals, which are then sent to the plurality of phase shifters. In other examples, an application-specific integrated circuit (ASIC) may be used as a phase control module. The first mode of operation includes, for a first subset of optical antennas within a distance from a center of the plurality of optical antennas (e.g., antennas with antenna numbers between 800 and 1200 in . Fig. 6B), determining variations of imposed phase shifts that are smaller than a phase shift variation range, and for a second subset of optical antennas further than the distance from the center of the plurality of optical antennas (e.g., antennas with antenna numbers less than 800 and greater than 1200 in Fig. 6B), determining variations of imposed phase shifts that are larger than the phase shift variation range.

[0074] In some implementations, OPA LiDAR may use multiple receiver OPAs to collect the light from one or more transmitter OPAs (e.g., multiple transmitters in a multi-static configuration). In this case, additional measures may be taken to identify or suppress retroreflections. For example, a subset of the receive apertures may be configured to overlap with the main lobe of the transmitter beam, while the remainder of the receiver apertures are configured to overlap with the grating lobe. In this case, the receiver apertures configured to overlap in the grating lobes may be monitored for strong return signals and used to reject the ghost return signals that appear in the receiver apertures configured to overlap with the main lobe of the TX beam.

[0075] Multistatic LiDARs can also contain information that can be used to identify and reject retroreflections with some or all receiver apertures configured to overlap with the transmitter's main lobe. Before reaching the far field, the offset between the transmitter and receiver can cause loss at the receiver due to parallax error between the transmitter and receiver. This loss can be geometry-based and can be calculated assuming that the target object is located at the transmitter's main lobe or at the transmitter's grating lobes, as in Fig. 8 shown.

[0076] Fig. Figure 8 shows a plot of predictive example parallax errors as a function of the angle of a target (assumed to be at a distance of 10 m) with respect to a transmitter OPA for various photonic system configurations. Differences between the expected parallax error in the main lobe and the measurement can be used to flag and reject high-intensity retroreflectors coupling into the grating lobe.

[0077] Fig. Figure 9 shows a diagram of prophetic exemplary radiation intensity patterns for one transmitter OPA and two receiver OPAs as a function of angle with respect to a reference angle for the lobes. For multistatic LiDAR configurations (e.g., as in Fig. 9), each RX aperture can use a different spacing than the TX aperture and also different RX apertures. In this case, a different grating lobe coupling is expected for each receiver aperture. Return signals with intensity falling off according to the expected mismatch in grating lobe coupling can be marked as a retroreflector and ignored. In this example, the Rx2 receiver beam grating lobe has predictably less overlap with the transmit beam grating lobe than the Rx1 receiver beam grating lobe.

[0078] Another interference signal that can affect LiDAR systems such as FMCW (frequency modulated continuous wave) LiDAR is frequency domain propagation of a strong reflector into nearby frequency bins, as in Fig. 10. The data generated by a single FMCW LiDAR acquisition at a single polar and azimuth angle consists of a finite time-complex signal. The frequency content of this signal directly reflects the intensity of light received from the system at various distances, with the frequency bin being roughly proportional to the reflector distance. A strong reflector produces a strong sinusoid in the frequency content, which, after a finite-time Fourier transform, produces a peak with a broad frequency spread, potentially masking weak signals beneath the frequency-domain shoulders of this peak.For example, one way to remove the peak and its frequency propagation might be to derive the phase and amplitude of the original sine wave from the Fourier-domain information, subtract this sine wave from the time-domain data, and reevaluate the Fourier transform to reveal previously obscured weak signals. However, other algorithms may also work.

[0079] Fig.Figure 10 shows a flowchart of an exemplary clutter detection and attenuation scheme 1000 in an exemplary FMCW LiDAR system. A retroreflector signal 1002, a target signal 1004, and a noise signal 1006 are summed at a summer 1008. A Fourier transform 1010 is performed on the output of the summer 1008. A Fourier transform signal 1012 shows that the detection of the target signal 1004 is masked by the retroreflector signal 1002. An attenuation algorithm 1014 attempts to reduce the contribution of the retroreflector signal to the Fourier transform signal 1012. An exemplary attenuation algorithm 1014 finds a peak in the zero-padded Fourier transform signal 1012, synthesizes a sine wave with an appropriate complex amplitude, and subtracts the synthesized sine wave from the original output of the summer 1008.Finally, the Fourier transform 1010 is re-evaluated, resulting in an attenuated retroreflector signal 1016 that includes a target signal peak 1018 that was previously obscured.

[0080] In some aspects, the mitigation techniques described herein generally involve an optical phased array system in which the TX and RX phased optical arrays have mismatched grating lobes. This enables a linear or lens-linear mode of operation with maximum link budget, as well as a reduction in ghosting (i.e., spurious returns) associated with the multiple grating orders emitted by the TX and RX antenna arrays. It also enables a phase-apodized mode with even further reduced grating lobe ghosting, as well as reduced sidelobe ghosting (reduced spurious lateral extension of strong reflectors) in exchange for a small link penalty.These techniques may involve using these modes separately or in tandem, where a scene is first sampled using the former mode of operation and then regions of the scene suspected of containing noise are resampled using the latter mode of operation.

[0081] Although the disclosure has been described in connection with particular embodiments, it is to be understood that the disclosure is not intended to be limited to the disclosed embodiments, but on the contrary is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims, which scope is to be given the broadest interpretation so as to encompass all such modifications and equivalent structures as permitted by law. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 63 / 340,526

[0001]

Claims

[1] A device comprising: an optical phased array (OPA) comprising: a plurality of optical antennas separated by a set of distances, and a plurality of phase shifters configured to impose phase shifts on the light provided to the respective optical antennas of the plurality of optical antennas; a phase shift control module configured to manage the imposed phase shifts, wherein the managing comprises a first mode of operation comprising: for a first subset of optical antennas within a distance from a center of the plurality of optical antennas, determining variations of imposed phase shifts that are smaller than a phase shift variation range, and for a second subset of optical antennas farther than the distance from the center of the plurality of optical antennas, determining variations of imposed phase shifts that are greater than the phase shift variation range. [2] The apparatus of claim 1, wherein determining variations of imposed phase shifts for the first subset of optical antennas and determining variations of imposed phase shifts for the second subset of optical antennas comprises determining variations according to a probability distribution. [3] The apparatus of claim 2, wherein determining variations according to a probability distribution comprises determining variations according to a pseudorandom function. [4] The apparatus of claim 1, wherein said managing further comprises a second mode of operation comprising: for the first subset of optical antennas and the second subset of optical antennas, determining imposed phase shifts corresponding to a deterministic phase profile. [5] The apparatus of claim 4, wherein the deterministic phase profile comprises a linear phase profile defining a common phase shift to be imposed on each of the plurality of phase shifters. [6] The apparatus of claim 4, wherein said managing further comprises a first scanning mode of operation, wherein a first region is scanned by managing the imposed phase shifts in the first mode of operation for two or more transmit beam angles. [7] The apparatus of claim 6, wherein said managing further comprises a second scanning mode of operation, wherein a second region is scanned by managing the imposed phase shifts in the second mode of operation for two or more transmit beam angles. [8] The apparatus of claim 7, wherein the first region is a subset of the second region. [9] The apparatus of claim 1, further comprising a control module configured to receive scattered light emitted by the OPA and estimate a range to a target based at least in part on a property of the scattered light. [10] The apparatus of claim 9, wherein the OPA is a first OPA, and the apparatus further comprises a second OPA configured to receive the scattered light emitted by the first OPA and provide the scattered light to the control module. [11] The apparatus of claim 1, wherein the spacings in the set of spacings are each greater than half a wavelength in a spectrum of light provided to the plurality of optical antennas in the OPA. [12] The apparatus of claim 1, wherein the distances in the set of distances are all identical to each other. [13] A method comprising: Providing light to a plurality of optical antennas separated by a set of distances in an optical phased array (OPA), wherein a plurality of phase shifters impose phase shifts on the light provided to the respective optical antennas of the plurality of optical antennas; and Managing the imposed phase shifts using a phase shift control module, wherein the managing comprises a first mode of operation comprising: for a first subset of optical antennas within a distance from a center of the plurality of optical antennas, determining variations of imposed phase shifts that are smaller than a phase shift variation range, and for a second subset of optical antennas farther than the distance from the center of the plurality of optical antennas, determining variations of imposed phase shifts that are greater than the phase shift variation range. [14] The method of claim 13, wherein determining variations of imposed phase shifts for the first subset of optical antennas and determining variations of imposed phase shifts for the second subset of optical antennas comprises determining variations according to a probability distribution. [15] The method of claim 14, wherein determining variations according to a probability distribution comprises determining variations according to a pseudorandom function. [16] The method of claim 13, wherein the managing further comprises a second mode of operation comprising: for the first subset of optical antennas and the second subset of optical antennas, determining imposed phase shifts corresponding to a deterministic phase profile. [17] The method of claim 16, wherein the deterministic phase profile comprises a linear phase profile defining a common phase shift to be imposed on each of the plurality of phase shifters. [18] The method of claim 16, wherein said managing further comprises a first scanning mode of operation, wherein a first region is scanned by managing the imposed phase shifts in the first mode of operation for two or more transmit beam angles. [19] The method of claim 18, wherein said managing further comprises a second scanning mode of operation, wherein a second range is scanned by managing the imposed phase shifts in the second mode of operation for two or more transmit beam angles. [20] The method of claim 19, wherein the first region is a subset of the second region. [21] The method of claim 13, further comprising receiving scattered light emitted by the OPA and estimating a range to a target based at least in part on a property of the scattered light. [22] The method of claim 21, wherein the OPA is a first OPA, and the method further comprises receiving the scattered light emitted by the first OPA into a second OPA and providing the scattered light for estimating the range.

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