Reducing impact of the zeroth diffraction order or other low-angle scattered light of a transmission profile in an optical system using metasurfaces

Abstract

An apparatus that includes an optical system including a transmission side and a receiver side The transmission side includes a light source operable to generate light, and a light projecting element arranged to project the light toward a scene. An illumination profile of the light projected toward the scene contains increased intensity near a center of a field-of-view due to 0th-diffraction order rays or other low-angle scattered light. The receiver side includes an image sensor, and receiver optics configured to focus light reflected by the scene toward the optical sensor. The receiver optics includes at least one metasurface configured to provide functionality of an angle- of-incidence filter having a transmission coefficient that reduces, proportionately, an intensity of the illumination profile where the 0th-diffraction order rays or other low-angle scattered light are present.

Description

REDUCING IMPACT OF THE ZEROTH DIFFRACTION ORDER OR OTHER LOW-ANGLE SCATTERED LIGHT OF ATRANSMISSION PROFILE IN AN OPTICAL SYSTEM USING METASURFACESFIELD OF THE DISCLOSURE

[0001] The present disclosure relates to optical systems.BACKGROUND

[0002] In some applications for active illumination, optical diffusers or other light projection elements are used, and the illuminated area is monitored with a receiver module in the form, for example, of a camera, time-of-flight sensor or other sensor. The active illumination is generated at the transmission (Tx) side, and optical signals are sensed and processed at the receiver (Rx) side. In some cases, the Rx sensor is less sensitive toward the edges of the field of view (FoV), which is referred to as relative illumination. A challenge when co-designing the Tx and Rx parts of the system is to tailor the illumination profile of the Tx side to compensate for the relative illumination so that, if the receiver is monitoring a scene with uniform reflectance, the intensity of light on the Rx sensor also will be substantially uniform. In this way, the system can have substantially uniform sensitivity over the full FoV.

[0003] Optical systems may include various types of optical elements. Diffractive optical elements (DOEs), such as meta optical elements (MOEs), employ a flat optic technology and can provide several potential advantages compared to refractive elements. For example, compared to refractive lenses, MOEs can have fewer surfaces and less performance degradation due to tolerances. Further, MOEs can be stacked with flat glass surfaces, can have low thermal impact and / or can be designed easily with high sensitivity (e.g., high numerical aperture) across the field (i.e., telecentric at the image plane).

[0004] A challenge, however, in using MOEs is suppression of unwanted diffraction orders. In a typical application, the 1 or -1 diffraction order is the desired diffraction order for imaging, and all other orders are considered to be unwanted or stray light. Fordiffusers, the higher diffraction orders are of less concern because they have little impact on the illumination. However, the Oth-diffraction order, which also may be referred to as ballistic light, shows up in the central part of the field of illumination. In a diffuser, for example, this light is visible in the field of illumination as an additional dimmed Opdiffraction order contribution from the light source. If the light source is, for example, a collimated beam, the Oth-diffraction order shows up as a bright dot in the center of the FoV; if it is a diverging light source, the Oth-diffraction order shows up as an additional dimmed Oth-order contribution from the light source with a profile similar to the bare light source.

[0005] One problem is that if the illumination profile contains higher intensity near the center of the FoV due to the Oth-diffraction order or other low-angle scattering, that part of the image can be saturated, thereby decreasing dynamic range and sensitivity.SUMMARY

[0006] The present disclosure describes techniques for reducing the impact of the 0th- diffraction order or other low-angle scattered light of a transmission profile in an optical system. For example, the receiver optics can include at least one metasurface with tailored angle-of-incidence transmission to compensate the Oth-diffraction order induced angular brightness profile.

[0007] In one aspect, the present disclosure describes an apparatus that includes an optical system including a transmission side and a receiver side. The transmission side includes a light source operable to generate light, and a light projecting element arranged to project the light toward a scene. An illumination profile of the light projected toward the scene contains increased intensity near a center of a field-of-view due to 0th- diffraction order rays or other low-angle scattered light. The receiver side includes an image sensor, and receiver optics configured to focus light reflected by the scene toward the optical sensor. The receiver optics includes at least one metasurface configured to provide functionality of an angle-of-incidence filter having a transmission coefficient thatreduces, proportionately, an intensity of a light profile, received at the receiver optics, where the Oth-diffraction order rays or other low-angle scattered light are present.

[0008] Some implementations include one or more of the following features. For example, in some implementations, the at least one metasurface is configured to reduce an intensity of a light profile received at the receiver optics within a specified angle that defines an area about a center of the illumination profile where the Oth-diffraction order or other low-angle scattered light is present. In some implementations, the at least one metasurface is configured to compensate for Oth-diffraction order-induced angular brightness profile so that, under a condition where the light projected onto the scene is uniformly reflected by the scene toward the receiver optics, light incident on the image sensor has a measured intensity that remains substantially uniform across a light sensitive surface of the sensor.

[0009] In some implementations, one or more of the following advantages can be achieved. For example, the impact of the 0th-diffraction order or other low-angle scattered light of a transmission system can be reduced. In some implementations, the dynamic range of the receiver can be improved.

[0010] Other aspects, features and advantages will be readily apparent from the following detailed description, the accompany drawings and the claims.BRIEF DESCRIPTION OF THE DRAWING(S)

[0011] FIG. 1 illustrates an example for reducing the impact of the Oth-diffraction order or other low-angle scattered light of a transmission profile in an optical system.DETAILED DESCRIPTION

[0012] The present disclosure describes a technique for reducing the impact of the Oth- diffraction order of a transmission profile in an optical system.

[0013] As shown in FIG. 1, an optical system includes a transmission side (Tx) and a receiver side (Rx). The transmission side includes a light source 20 that is operable to emit electromagnetic radiation (e.g., light) 22 at a particular wavelength or range of wavelengths (e.g., infra-red, visible, or ultra-violet). The light source 20 can be implemented, for example, as a vertical cavity surface emitting laser (VCSEL), a light emitting diode (LED), a laser, or other appropriate light emitting device. Light 22 emitted by the light source 20 passes through a light projecting element 24 such as a diffractive diffuser or diffractive fan-out element. A diffuser, for example, spreads the light over wider viewing area, whereas a diffractive fan-out element splits a light beam into multiple beams propagating in different directions, which can be used to generate an array of focused spots with a focusing lens or an array of collimated beams using for example a collimator lens. After passing through the light projecting element 24, the light has an intensity profile 26 and is incident on a scene 29, which may include one or more objects. In this example, the light 26 incident on the scene includes 0th-diffraction order rays 28, which increase the intensity profile at or near its center.

[0014] In operation, some of the light incident on the scene 29 is reflected back toward the receiver side of the optical system, which includes receiver optics 36 to focus incoming light toward (e.g., onto) a light sensitive surface of an optical sensor 30 (e.g., a CMOS, CCD or SPAD image sensor). Light transmitted through the receiver optics 36 is indicated by 38. Assuming proper alignment in the optical system, the 0th-diffraction order in the image space mirrors that in the optics space. Thus, the illumination profile of the light 38 may contain higher intensity near its center 39 as a result of the Oth-diffraction order rays 28 present in the light 26 projected onto the scene 29. The presence of the higher intensity can adversely impact the dynamic range of the receiver.

[0015] As the 0th-diffraction order projection is an intrinsic feature of the diffuser design and the illumination source, it will illuminate the field of view (FoV) in substantially the same predictable, non-varying way, varying only slightly between devices as a result of manufacturing or other tolerances. Thus, the receiver optics can map each field point ofthe FoV to a predictable, non-varying position of the sensor such that the extra illumination from the Oth-diffraction order gives a predictable, non-varying brightness profile on the sensor. Further, as the extra projected brightness from the 0th-diffraction order is distributed in a predictable, non-varying position in position space, the same is applicable in angular space because the Oth-diffraction order propagates radially. This means that the Oth-diffraction order also has a well-defined profile in angular space. Accordingly, by integrating angle-of-incidence (AOI) transmission functionality into the receiver optics 36, the Oth-diffraction order (or other low-angle scattered light) can be compensated for. That is, the functionality of an AOI filter can be provided, for example, by designing the receiver optics to directly have reduced transmission efficiency corresponding to the angular distribution and intensity of the projected Oth-diffraction order.

[0016] The receiver optics 36 can include, for example, one or more lenses and, preferably includes one or more metasurfaces. A metasurface refers to a surface with distributed small structures (e.g., meta-atoms) arranged to interact with light in a particular manner. Metalenses, for example, are composed of carefully arranged meta- atoms (e.g., a distributed array of nanostructures) with sub-wavelength structures. By adjusting the geometry of the meta-atoms, one can modify the phase above the elements in response to a plane wave.

[0017] In some implementations, one or more metasurfaces in the receiver optics 36 are configured to have a transmission coefficient that reduces, proportionately, the intensity of the light profile 38 near the center 39 where the Oth-diffraction order rays are present. The transmission coefficient can be tailored, for example, based on the specifications for the light source 20 and the receiver optics 36. By incorporating angle-of-incidence transmission into the receiver optics 36, the light incident on the image sensor 30 can have a relatively uniform intensity profile 40 (assuming, e.g., the light projected onto the scene is uniformly reflected by the scene to the receiver optics).

[0018] The metasurface(s) in the receiver optics 36 can be configured to reduce, proportionately, the intensity of the light profile within a specified angle, which defines an area about the center of the profile where the 0th-diffraction order is present. That is, the receiver optics 36 is configured to compensate for the Oth-diffraction order- induced angular brightness profile so that the profile 46 of the light incident on the image sensor 30 has a relatively uniform intensity profile (assuming, e.g., the light projected onto the scene is uniformly reflected by the scene to the receiver optics). In some cases, the receiver optics 36 is configured to allow, for example, up to 5-10% variation (e.g., 5% or 10% variation) in the offset profile 46 of the light incident on the sensor 30. In some cases, the receiver optics 36 is configured to allow, for example, up to 50-75% variation (e.g., 50% or 75% variation) in the offset profile 46 of the light incident on the sensor 30.

[0019] The optical systems described above can be integrated, for example, into compact electronic devices such as smart phones, laptops, televisions, or wearable devices, as well as larger devices or systems such as automotive vehicles.

[0020] While this specification contains many specifics, these should not be construed as limitations on the scope of the disclosure or of what may be claimed, but rather as descriptions of features specific to particular implementations. Certain features described in this specification in the context of separate implementations also may be combined in the same implementation. Conversely, various features described in the context of a single implementation also may be implemented in multiple implementations separately or in any suitable sub-combination. Various modifications can be made to the foregoing examples. Accordingly, other implementations also are within the scope of the claims.

Claims

What is claimed is:

1. An apparatus comprising: an optical system including a transmission side and a receiver side, wherein the transmission side includes: a light source operable to generate light; and a light projecting element arranged to project the light toward a scene, wherein an illumination profile of the light projected toward the scene contains increased intensity near a center of a field-of-view due to Oth-diffraction order rays or other low-angle scattered light, and wherein the receiver side includes: an image sensor; and receiver optics configured to focus light reflected by the scene toward the optical sensor, wherein the receiver optics includes at least one metasurface configured to provide functionality of an angle-of-incidence filter having a transmission coefficient that reduces, proportionately, an intensity of a light profile, received at the receiver optics, where the Oth-diffraction order rays or other low-angle scattered light are present.

2. The apparatus of claim 1 wherein the at least one metasurface is configured to reduce an intensity of the light profile received at the receiver optics within a specified angle that defines an area about a center of the light profile where the Oll'-diffraction order or other low-angle scattered light is present.

3. The apparatus of claim 1 wherein the at least one metasurface is configured to compensate for 0th-diffraction order-induced angular brightness profile so that, under a condition where the light projected onto the scene is uniformly reflected by the scene toward the receiver optics, light incident on the image sensor has a measured intensity that remains substantially uniform across a light sensitive surface of the sensor.

4. The apparatus of claim 3 wherein the receiver optics is configured to allow no more than 10% variation in an offset profile light incident on the image sensor.

5. The apparatus of claim 3 wherein the at least one metasurface is configured to allow no more than 5% variation in an offset profile light incident on the image sensor.

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