Refraction compensation for use in LiDAR systems

Embodiments discussed herein refer to LiDAR systems that use refraction compensation to improve transmission efficiency of light energy through transmissive mediums such as covers. Refraction compensation can be achieved using a cover or an anti-reflective coating.

FIELD OF THE INVENTION

The present disclosure relates to light detection and ranging (LiDAR), and in particular to refraction compensation for LiDAR systems.

BACKGROUND

Systems exist that enable vehicles to be driven semi-autonomously or fully autonomously. Such systems may use one or more range finding, mapping, or object detection systems to provide sensory input to assist in semi-autonomous or fully autonomous vehicle control. LiDAR systems, for example, can provide the sensory input required by a semi-autonomous or fully autonomous vehicle.

BRIEF SUMMARY

Embodiments discussed herein refer to refraction compensation for LiDAR systems.

In one embodiment, a LiDAR system is provided that can include a laser operative to emit light characterized as having a p-polarization and s-polarization; and a light transmissive cover characterized as having a reflective polarization plane, wherein the laser is aligned with the light transmissive cover such that the p-polarization of the laser is co-planer with the reflective polarization plane of the light transmissive cover.

In one embodiment, a curved cover for use with LiDAR system can be provided that can include a medium comprising a first curve and a second curve, wherein the first and second curves are designed to minimize deformation of exiting and receiving light beams and to prevent formation of ghost images, and wherein a focal length of the first and second curves is infinity.

In one embodiment, a LiDAR system mounted to a windshield of a vehicle is provided that can include a laser system operative to emit light according to a beam field of view towards the windshield; and a windshield cover mounted to the windshield, wherein the windshield cover is operative to adjust the beam field of view to yield an exit beam field of view that compensates for Fresnel properties of the windshield.

In one embodiment, a vehicle system is provided that can include a windshield comprising an anti-reflective (AR) coating that covers a beam transmission portion of the windshield; and a laser system operative to emit light according to a beam field of view towards the beam transmission portion of the windshield, wherein the AR coating is operative to adjust the beam field of view to yield an exit beam field of view that compensates for Fresnel properties of the windshield.

A further understanding of the nature and advantages of the embodiments discussed herein may be realized by reference to the remaining portions of the specification and the drawings.

DETAILED DESCRIPTION

Illustrative embodiments are now described more fully hereinafter with reference to the accompanying drawings, in which representative examples are shown. Indeed, the disclosed LiDAR systems and methods may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Like numbers refer to like elements throughout.

In the following detailed description, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the various embodiments. Those of ordinary skill in the art will realize that these various embodiments are illustrative only and are not intended to be limiting in any way. Other embodiments will readily suggest themselves to such skilled persons having the benefit of this disclosure.

In addition, for clarity purposes, not all of the routine features of the embodiments described herein are shown or described. One of ordinary skill in the art would readily appreciate that in the development of any such actual embodiment, numerous embodiment-specific decisions may be required to achieve specific design objectives. These design objectives will vary from one embodiment to another and from one developer to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming but would nevertheless be a routine engineering undertaking for those of ordinary skill in the art having the benefit of this disclosure.

FIG.1shows an illustrative vehicle100having a LiDAR system110that is attached to and/or incorporated therein according to an embodiment. Vehicle100can be an automobile, a truck, a drone, a plane, a robot, a hand-held device, a stationary device, or any other suitable platform on which LiDAR system110can be used. LiDAR system110can be fully contained within an interior portion of vehicle100, mounted to an exterior portion of vehicle100, or contained partially within the interior portion and partially mounted to the exterior. Interior portions may be portions of vehicle100that are not directly exposed to external environmental factors such as the environment conditions (e.g., water, humidity, sun, ice, wind, etc.) and road conditions (e.g., road debris). The interior portion may be influenced by external environment conditions but to a lesser degree than the exterior portion.

LiDAR system110may include, among other features, fiber laser120, controller130, and scanning system140. Fiber laser120may be any suitable laser that uses a seed laser and at least one amplifier and also includes an active gain medium that is rare-earth ion doped. In some embodiments, two or more fiber lasers may be used in LiDAR system110. In yet another embodiment, the fiber laser may be replaced with one or more diode lasers. Controller130may be operative to control LiDAR system110. For example, controller130may control operating parameters of fiber laser120. Scanning system140may include the appropriate lenses, mirrors, steering optics, and detectors needed to capture an image of a scene existing within a vicinity of vehicle100. Fiber laser120serves as the source of light pulses that are provided to scanning system140. Scanning system140can control projection of those light pulses in accordance with a field of view of scanning system140. The field of view includes lateral and vertical fields of view in which laser pulses are transmitted to capture an X×Y image every scan cycle. This X×Y image is obtained each scan cycle and any objects detected with the image are detected by returns of the laser pulses. The images are processed by software to determine the location and distance of the objects.

When light passes from one medium (having a first refractive index) to another medium (having a second refractive index), both reflection and refraction of the light may occur. The Fresnel equations describe what fraction of the light is reflected and what fraction is refracted (i.e., transmitted). In LiDAR applications, it is desirable to maximize refraction and minimize reflection. Reflection can cause ghosts, which are undesirable. Normal glass, for example, has a reflection of about 4%. When anti-reflective coatings are applied to the glass, the reflection can be, for example, 1-2% for a few degrees of incidence angles. The angle of incidence is the angle between a ray incident on a surface and the line perpendicular to the surface at the point of incidence, called the normal. The scanning system of a LiDAR system can produce incident angles that exceed 60, 70, 80, 90, 100, 110, 120 degrees or more, thus rendering anti-reflective coatings useless for all but a few degrees of all incident angles produced by the system.

The plane of incidence is the plane which contains the surface normal and the propagation vector of the incoming light. The orientation of the incident light's polarization with respect to the plane of incidence has an important effect on the strength of the reflection. The plane of incidence is the plane made by the incoming propagation direction and the vector perpendicular to the plane of an interface. The component of the electric field parallel to this plane is termed p-like (parallel) and the component perpendicular to this plane is termed s-like. Polarized light with its electric field along the plane of incidence is thus denoted p-polarized, while light whose electric field is normal to the plane of incidence is called s-polarized.

FIG.2shows an illustrative diagram of Fresnel transmittance and reflectance of an air to glass medium transition.FIG.2shows that so long as the Rp (p-polarized light) is minimized, the transmittance (Tp) of the light is maximized. As shown, the transmittance is maintained relatively high from 0-60 degrees of incidence angles.

FIG.3shows an illustrative block diagram of a laser and cover according to an embodiment. Laser310may represent the output portion of a fiber laser or a diode laser system. The light being emitted by laser310may be polarized and has both p-polarization and s-polarization. Steering system320may represent the steering system responsible for controlling the projection of the laser through cover330to the ambient outside world. Cover330represents a cover that at least partially encloses a Lidar system. Cover330may be constructed from one or more light transmissive materials (e.g., plastic, glass, and/or anti-reflective coatings). Cover330may have reflective properties that affect p-polarization (Rp) and s-polarization (Rs) of incident light interfacing with it.

In order to maximize transmission power of incident light through cover330, the light being emitted by laser310is oriented such that its p-polarization is co-planer with the Rp of cover330. That is, during installation of laser310and cover330within a LiDAR system, both laser310and cover330are aligned such that the p-polarization of the laser is co-planer with the Rp of cover330. When the p-polarization of the laser and Rp of cover are aligned, the transmission power can be maximized throughout a range of incidence angles so long as light being emitted by laser310remains co-planer with Rp of cover330.

FIG.4shows that a p-polarized aligned light source can cycle through +60 degrees to −60 degrees and remain in-plane with the Rp of cover430. This illustrates that 120 degrees of incidence angles can be projected on cover430and still maintain a substantially high transmittance power (as shown inFIG.2).

FIG.5shows a cover having a flat surface on both sides of medium510. Collimated beam520interfaces with medium510and reflects a collimated beam520′. The flat surfaces of both sides of medium510ensures that the reflected beam remains collimated, but the reflection introduces a relatively strong ghost. Ghosts are undesirable because they can create false positives.

FIG.6shows a curved cover600according to an embodiment. Curved cover600can include medium610that has first curve611and second curve612. Using curved cover600can reduce high Fresnel reflection because the incident angle is small. Another benefit is that reflection at the surfaces is not collimated and can reduce the formation of ghost images. However, the curved cover has the potential to deform the exiting beam and receiving beam. First and second curves611and612are designed and shaped to minimize deformation of the exiting beam and receiving beam. For example, curves611and612can be designed to the following equation:
R1*n*(n−1)−(n−1)*n*R2+d*(n−1){circumflex over ( )}2=0,
where R1 is the radius of curve611. R2 is the radius of curve612, n is the refractive index of medium610, and d is the thickness of medium610. The focal length of curved cover600can be designed to be infinity. If desired, an anti-reflective material can be applied to one or both curves B11and B12. Because the incidence angle is relatively small, the anti-reflective material would be effective throughout the Lidar scanning sweep. In addition, there is no need to account for the p-polarization of laser beam with curved cover600.

FIG.7shows a windshield cover designed to be used in combination with windshield, according to an embodiment. Windshield710and cover720are shown. Windshield710is representative of one many different windshields that may exist on a vehicle. As such, the characteristics of the windshield may vary from vehicle to vehicle such as the angle of the windshield (from hood to roof) and any curvature(s) that may exist within the windshield. Windshield710is illustrated to be a substantially flat structure that exists at a fixed angle. Cover720can be coupled directly to windshield710. It may be desirable for the interface between windshield710and cover720to be flush so that there are no variations in the interface, thereby providing a consistent interface to interface transition.

Laser steering system730may direct light signals through cover720and windshield710. Steering system730may control both the vertical and horizontal field of view of the light signals being projected through windshield710. The vertical field of view may range from a beam steered maximum angle732to a beam steered minimum angle734. Cover720is designed so that it selectively adjusts exiting angles of the light originating from steering system730. As shown, cover720has a triangular cross-section that increases in thickness as it spans from top of windshield710to bottom of windshield710. This variation in thickness influences light transmission such that exit angles out of windshield710are changed relative to their respective originating beam steering angle. Maximum exit beam736and minimum exit beam738are shown. In some embodiments, cover720can be a prism.

For example, if windshield710has an angle of 22 degrees relative to horizontal axis740and that cover720has a 3 degree angle and a refractive index of 1.5. The incident angle of beam ranges from −4 degrees (for734) to about 17 degrees (for732), and the exiting beam ranges from minimum exit angle738of about −15 degrees to maximum exit angle736of about 14 degrees. The existence of prism720increases the range of the vertical field of view by a factor of about 1.4.

It should be appreciated that cover720can take any suitable shape to achieve desired exit angles. For example, cover720may be custom made for each windshield of each make and type of vehicle to compensate for idiosyncrasies of each windshield to yield desired vertical fields of view. This way, the same Lidar system can be used with any vehicle without a need for modifications to adapt to the windshield. Instead, cover720is customized for each windshield to allow an unmodified Lidar system to be used.

FIGS.8A and8Billustrate side and front views of vehicle800having a windshield mounted LiDAR system (WMLS)850, according to an embodiment. Vehicle800is a generic representation of any vehicle that can transport persons or cargo from one location to another. Vehicle800has windshield810, which has an exterior surface811that is exposed to the elements and an interior surface812that interfaces with the interior cabin of the vehicle. WMLS850can be mounted to the interior surface812of windshield810. A thin layer of flexible material having refractive index similar to the windshield can be attached to both sides of the windshield with no air gap. The surface of the film exposed to air is AR (anti-reflective) coated so that when the windshield is sandwiched between two flexible films, the optical transmission loss through the film-windshield stack is minimized. The AR-coated thin material860may cover a portion of windshield810. As illustrated, AR coating860may cover a portion of windshield810that is larger, the same, or smaller than a cross-sectional area of WMLS850that interfaces with windshield810. AR-coated film860may be applied to exterior surface811, interior surface812, or both. As illustrated inFIGS.8A and8B, WMLS850is center mounted on windshield810along center axis820and near roof815, such that it is positioned near the location of rear-view mirror817. It should be understood that the position of WMLS850is merely illustrative and that WMLS850can be positioned anywhere on windshield810. If desired, more than one WMLS850can be mounted to windshield810. In addition, one or more LiDAR systems according to embodiments discussed herein can be mounted anywhere on vehicle800.

In some embodiments, AR-coated film860can serve the same function as cover330ofFIG.3or cover720ofFIG.7without requiring a cover. For example, WMLS850may exist without a cover and is flush mounted directly to interior surface812. This way, no cover is present to potentially interfere with the laser beams and AR coating860is provided to adjust p-polarization (Rp) and s-polarization (Rs) of incident light interfacing with windshield810. AR coating860may have a variable thickness, similar to that described above in connection with cover720. In other embodiments, multiple AR coatings may be used. For example, assume the AR coating occupies a fixed area. A first portion of the area (e.g., top half) may have an AR coating with first characteristics, and a second portion of the area (e.g., bottom half) may have an AR coating with second characteristics. In one example, the AR coating comprises an area the size of the beam transmission portion. A first portion of the area comprises a first refraction characteristic, and a second portion of the area comprises a second refraction characteristic. The first and second refraction characteristics are different.

AR-coated film860may be constructed from a thin and flexible material that is bonded to windshield810. A first side of AR-coated film860may have a refractive index that is substantially similar to a refractive index of the windshield. In addition, an adhesive binding the coating to the windshield may also have a refractive index that substantially matches the refractive index of the first side and the windshield. A second side of AR-coating film860can adjust p-polarization (Rp) and s-polarization (Rs) of incident light interfacing with the windshield. In some embodiments, the second side's anti-reflective coating is selected based on the wavelength of the light source.

It is believed that the disclosure set forth herein encompasses multiple distinct inventions with independent utility. While each of these inventions has been disclosed in its preferred form, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense as numerous variations are possible. Each example defines an embodiment disclosed in the foregoing disclosure, but any one example does not necessarily encompass all features or combinations that may be eventually claimed. Where the description recites “a” or “a first” element or the equivalent thereof, such description includes one or more such elements, neither requiring nor excluding two or more such elements. Further, ordinal indicators, such as first, second or third, for identified elements are used to distinguish between the elements, and do not indicate a required or limited number of such elements, and do not indicate a particular position or order of such elements unless otherwise specifically stated.

Moreover, any processes described with respect toFIGS.1-8, as well as any other aspects of the invention, may each be implemented by software, but may also be implemented in hardware, finnware, or any combination of software, hardware, and firmware. They each may also be embodied as machine- or computer-readable code recorded on a machine- or computer-readable medium. The computer-readable medium may be any data storage device that can store data or instructions which can thereafter be read by a computer system. Examples of the computer-readable medium may include, but are not limited to, read-only memory, random-access memory, flash memory, CD-ROMs, DVDs, magnetic tape, and optical data storage devices. The computer-readable medium can also be distributed over network-coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. For example, the computer-readable medium may be communicated from one electronic subsystem or device to another electronic subsystem or device using any suitable communications protocol. The computer-readable medium may embody computer-readable code, instructions, data structures, program modules, or other data in a modulated data signal, such as a carrier wave or other transport mechanism, and may include any information delivery media. A modulated data signal may be a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal.

It is to be understood that any or each module or state machine discussed herein may be provided as a software construct, firmware construct, one or more hardware components, or a combination thereof. For example, any one or more of the state machines or modules may be described in the general context of computer-executable instructions, such as program modules, that may be executed by one or more computers or other devices. Generally, a program module may include one or more routines, programs, objects, components, and/or data structures that may perform one or more particular tasks or that may implement one or more particular abstract data types. It is also to be understood that the number, configuration, functionality, and interconnection of the modules or state machines are merely illustrative, and that the number, configuration, functionality, and interconnection of existing modules may be modified or omitted, additional modules may be added, and the interconnection of certain modules may be altered.

Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that the particular embodiments shown and described by way of illustration are in no way intended to be considered limiting. Therefore, reference to the details of the preferred embodiments is not intended to limit their scope.