Dynamic polarization modulation of a lidar system in a vehicle

Systems and methods in a vehicle involve transmitting light with an initial polarization from a lidar system, and controlling an external compensator, external to the lidar system, or an internal compensator within the lidar system to change the initial polarization of the light to a new polarization of the light. The method also includes receiving reflected light resulting from reflection of the light from one or more objects, and obtaining information about the one or more objects based on the reflected light.

INTRODUCTION

The subject disclosure relates to dynamic polarization modulation of a lidar system in a vehicle.

Vehicles (e.g., automobiles, trucks, construction equipment, farm equipment, automated factory equipment) increasingly employ sensors to obtain information about the vehicle and its environment. The sensor information facilitates augmentation or automation of vehicle operation. Exemplary sensors include a camera, a radio detection and ranging (radar) system, and a light detection and ranging (lidar) system. Generally, a lidar system projects laser illumination with a polarization state that is linear, circular, or a combination of the two (i.e., elliptical). The lidar system measures reflected light that results from the illumination over the field of view (FOV). That is, the laser illumination is reflected by objects in the FOV, and the intensity of this reflected light is measured to obtain information about the objects. Typically in a vehicle application, the lidar system is located in the passenger cabin to transmit light through the windshield and scans across the FOV. The windshield results in a loss of optical transmission of a particular polarization state. Accordingly, it is desirable to provide dynamic polarization modulation of a lidar system in a vehicle.

SUMMARY

In one exemplary embodiment, a method in a vehicle includes transmitting light with an initial polarization from a lidar system, and controlling, using a controller, an external compensator, external to the lidar system, or an internal compensator within the lidar system to change the initial polarization of the light to a new polarization of the light. The method also includes receiving reflected light resulting from reflection of the light from one or more objects, and obtaining information about the one or more objects based on the reflected light.

In addition to one or more of the features described herein, the lidar system transmits light through a transmissive window of the vehicle, and the controlling the external compensator includes changing the initial polarization of the light to the new polarization prior to the light crossing the transmissive window.

In addition to one or more of the features described herein, the controlling the external compensator includes controlling voltage to a liquid crystal variable retarder (LCVR).

In addition to one or more of the features described herein, the controlling the external compensator is performed to control instantaneous polarization based on angular coordinates of a scanning beam direction provided by the lidar system.

In addition to one or more of the features described herein, the controlling the external compensator is performed between frames of the lidar system based on the detection of the one or more objects in previous frames.

In addition to one or more of the features described herein, the method also includes obtaining a feedback of the new polarization as a function of time and temperature.

In addition to one or more of the features described herein, the obtaining the information about the one or more objects includes obtaining a material, a geometry, and a surface property of each of the one or more objects.

In addition to one or more of the features described herein, the controlling the external compensator includes controlling a magnetically controlled polarization element.

In addition to one or more of the features described herein, the controlling the external compensator includes rotating a phase plate mechanically using a motor.

In addition to one or more of the features described herein, the controlling the external compensator includes rotating a Fresnel prism to an optic axis via a motor stage.

In another exemplary embodiments, a system in a vehicle includes a lidar system to transmit light with an initial polarization, and an external compensator outside the lidar system or an internal compensator within the lidar system. The system also includes a controller to control the external compensator or the internal compensator to change the initial polarization of the light to a new polarization of the light. Reflected light resulting from reflection of the light from one or more objects is received, and information about the one or more objects is obtained based on the reflected light.

In addition to one or more of the features described herein, the lidar system transmits light through a transmissive window of the vehicle, and the controller changes the initial polarization of the light to the new polarization prior to the light crossing the transmissive window.

In addition to one or more of the features described herein, the external compensator is a liquid crystal variable retarder (LCVR) and the controller controls a voltage provided to the LCVR.

In addition to one or more of the features described herein, the controller controls instantaneous polarization of the light from the lidar system based on angular coordinates of a scanning beam direction provided by the lidar system.

In addition to one or more of the features described herein, the controller controls polarization of the light between frames of the lidar system based on the detection of the one or more objects in previous frames.

In addition to one or more of the features described herein, the controller obtains a feedback of the new polarization as a function of time and temperature.

In addition to one or more of the features described herein, the information about the one or more objects includes obtaining a material, a geometry, and a surface property of each of the one or more objects.

In addition to one or more of the features described herein, the external compensator is a magnetically controlled polarization element.

In addition to one or more of the features described herein, the external compensator is a phase plate rotated mechanically using a motor.

In addition to one or more of the features described herein, the external compensator is a Fresnel prism rotated to an optic axis via a motor stage.

DETAILED DESCRIPTION

As previously noted, a lidar system is among the exemplary sensors that may be used in a vehicle. The lidar system may be located behind the windshield in the passenger compartment of the vehicle in order to keep it clean. However, there is a loss of optical transmission by the lidar system to outside the vehicle. The loss is directly related to the Fresnel reflection coefficients that determine the amount of light that is reflected from a surface (i.e., the windshield) for a given incident angle, a given polarization state, and given material characteristics. The loss in optical power can be as much as 70 percent in some cases and cannot be entirely alleviated by the application of anti-reflection coatings on the windshield because of the coating design, manufacturing costs, and coating durability.

Embodiments of the systems and methods detailed herein relate to the dynamic polarization modulation of a lidar system in a vehicle. According to an exemplary embodiments, a liquid crystal (LC) phase modulator facilitates tuning the polarization to enhance resolution. Specifically, according to an exemplary embodiment, a liquid crystal variable retarder (LCVR) optical phase plate, which is similar to a quarter wave plate, is used to dynamically modulate the polarization state of the lidar beam to minimize polarization losses. While the location of the lidar system behind the windshield is discussed as an exemplary reason for optical loss, the location of the lidar system and the efficacy of the dynamic polarization modulation technique are not limited by the exemplary case. The windshield or other source of optical loss represents an external constraint that was not designed into the lidar system. Thus, the LCVR or other mechanism, as discussed further, represents an external compensator to address the constraint.

In accordance with an exemplary embodiment,FIG.1is a block diagram of a vehicle100that exhibits dynamic polarization modulation of a lidar system110. The exemplary vehicle100inFIG.1is an automobile101. The lidar system110is shown within the passenger compartment behind a transmissive surface117, which is the windshield115in the case of the vehicle100. The transmissive surface117is transmissive to light from the lidar system110. Cameras130and a radar system140are also shown along with a controller120. The numbers and locations of the sensors and controller120are not intended to be limited by the exemplary illustration. The controller120includes processing circuitry that may include an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality.

FIG.2details an exemplary lidar system110with dynamic polarization modulation according to one or more embodiments. The lidar system110is shown behind a transmissive surface117(e.g., windshield115). The transmissive surface117may have an anti-reflection coating. The lidar system110provides an output with polarization P, as shown. An LCVR210is shown between the lidar system110and the transmissive surface117. As indicated, at block220, information is obtained from the lidar system110to determine control. In the exemplary case, the control is of the voltage to the LCVR210, which affects the polarization P′ of the lidar output downstream of the LCVR210. The information from the lidar system110may indicate the incident beam angle on the transmissive surface117, for example. As part of the processes at block220, which may be performed by the controller120(FIG.1), a look-up table (LUT) may be consulted to associate the information from the lidar system110(e.g., incident beam angle) with a corresponding voltage (i.e., polarization).

While the LCVR210is shown inFIG.2as an exemplary mechanism for controlling the polarization of the signal from the lidar system110, the LCVR210is only one example. Other external compensators may be used instead of the LCVR210according to alternate embodiments. For example, a magnetically controlled polarization element may be used or a phase plate (e.g., quarter wave plate) may be rotated mechanically via a motor. As another example, a phase shift may be achieved by a Fresnel prism rotated to the optic axis via a motor stage. For any of the alternate external compensating mechanisms, a corresponding LUT may be developed and calibrated such that each scan angle (i.e., incident beam angle) is associated with a polarization.

An optional internal compensator205(e.g., retarder), which is part of the lidar system110, is also shown inFIG.2. According to alternate embodiments, the controller120may control the polarization output by the lidar system110using the internal compensator205rather than changing the polarization output by the lidar system110using an external compensator such as the LCVR210.

As further discussed with reference toFIGS.3and4, the polarization state may be varied within a scan sweep (i.e., within a frame) or from frame-to-frame. The intra frame instantaneous polarization control (according to the embodiment ofFIG.3) may minimize polarization losses. The frame-to-frame control of polarization (according to the embodiment ofFIG.4) may additionally consider objects detected in the FOV of the lidar system to adjust the polarization in a way that enhances detection. Polarization-based lidar detections facilitate the determination of physical characteristics of detected objects such as material, geometry, and surface properties. Generally, light propagation from the lidar system110through the transmissive surface117changes over the FOV, over time, and based on the object being illuminated. The variation of polarization state within a frame, as discussed with reference toFIG.3, addresses the change in light propagation over the FOV. The variation of polarization state from frame-to-frame, as discussed with reference toFIG.4, addresses the change in light propagation based on the object being detected. The feedback discussed with reference toFIGS.3and4addresses the change in light propagation over time.

FIG.3is a process flow of a method300of performing dynamic polarization modulation according to one or more embodiments. The method300facilitates polarization change within a frame of the lidar system110using the LCVR210(FIG.2). The processes at blocks310through340may be performed iteratively for each scanning beam angle of the lidar system110. At block310, obtaining current angular coordinates of the scanning beam direction from the lidar system110refers to the controller120obtaining this information according to an exemplary embodiment. At block320, using a LUT refers to looking up the incident angle on the transmissive surface117that corresponds with the scan angle indicated by the scanning beam direction angular coordinates. The incident angle on the transmissive surface117indicates the optimal polarization state that is pre-calibrated. The voltage (in the case of the LCVR210) to achieve this optimal polarization state is determined as an output from block320.

At block330, the processes include modulating the LCVR210inter frame (i.e., varying the instantaneous polarization) for each scan position across the transmissive surface117. That is, the voltage value determined at block320is applied to the LCVR210. At block340, providing feedback as a function of time and temperature refers to monitoring the actual result of the voltage control (at block330) which may vary over time and temperature. At block350, performing detection over the FOV refers to identifying objects over the FOV of the lidar system110and, as previously noted, may include determining the material, geometry, and surface properties of objects. As previously noted, the mechanism used to change polarization state need not be limited to the LCVR210. Thus, according to alternate embodiments, the value determined at block320and applied to modulate polarization state of the lidar system110at block330may be different according to different mechanisms. That is, whether a different external compensator than the LCVR210is used or an internal compensator205is used instead, blocks320and330may be modified to control the polarization P′ that is ultimately transmitted through the transmissive surface117.

FIG.4is a process flow of a method400of performing dynamic polarization modulation according to one or more embodiments. The method400facilitates polarization change from one frame to the next of the lidar system110. The processes at blocks410through440may be performed iteratively for each frame of the lidar system110. At block410, obtaining the current angular coordinates of the scanning beam direction from the lidar system110refers to the controller120obtaining this information. The lidar system110detects objects from a point cloud of reflections that the lidar system110receives based on transmitting the light at a given polarization state. The particular angular coordinates of objects detected in the point cloud are determined in part from the information received at block410. At block420, using the LUT and detection of objects in previous frames refers to determining alternate polarization states that may provide additional information about the objects.

At block430, modulating the LCVR210refers to applying the voltage determined at block420. As previously noted, the processes at blocks420and430may differ based on an alternate mechanism, internal or external to the lidar system110, for controlling the polarization P′ of light through the transmissive surface117. At block440, providing feedback as a function of time and temperature refers to monitoring the actual result of the voltage control (at block430) which may vary over time and temperature. At block450, performing detection over the FOV refers to identifying objects over the FOV of the lidar system110and, as previously noted, may include determining the material, geometry, and surface properties of objects.