Using metasurfaces to reduce impact of zero order diffraction or other low angle scattered light of transmission distributions in optical systems

Abstract

An apparatus includes an optical system including a transmit side and a receive side. The emission side comprises a light source operable to generate light and a light projection element arranged to project light toward a scene. Due to 0-order diffracted light or other low-angle scattered light, the illuminance distribution of light projected to the scene includes an increased intensity near the center of the field of view. The receiving side includes an image sensor and receiving optics configured to focus light reflected by the scene toward the optical sensor. The receiving optics include at least one metasurface configured to provide the function of an incident angle filter having a transmission coefficient that proportionally reduces the intensity of an illuminance distribution in which 0-order diffracted light or other low-angle scattered light is present.

Description

Technical Field

[0001] This disclosure relates to optical systems. Background Technology

[0002] In some applications of active lighting, optical diffusers or other light-projecting elements are used, and receiver modules, such as cameras, time-of-flight sensors, or other sensor types, are employed to monitor the illuminated area. Active illumination is generated on the transmitting (Tx) side, and light signals are sensed and processed on the receiving (Rx) side. In some cases, the Rx sensor has low sensitivity to the edges of the field of view (FoV), a phenomenon known as relative illuminance. A challenge in designing both the Tx and Rx portions of the system is to customize the illuminance distribution on the Tx side to compensate for relative illuminance, ensuring that the light intensity on the Rx sensor is also substantially uniform if the receiver is monitoring a scene with uniform reflectivity. In this way, the system can achieve substantially uniform sensitivity across the entire FoV.

[0003] Optical systems can include various types of optical elements. Diffractive optical elements (DOEs), such as super-optical elements (MOEs), employ planar optics techniques and offer several potential advantages compared to refractive elements. For example, MOEs can have less surface area and less performance degradation due to tolerances compared to refractive lenses. Furthermore, MOEs can be stacked with planar glass surfaces, can have low thermal effects, and / or can be easily designed to have high sensitivity (e.g., high numerical aperture) across the field (i.e., at the telecentricity of the image plane).

[0004] However, a challenge in using MOE is suppressing unwanted diffraction orders. In typical applications, 1st or -1st order diffraction is the desired diffraction order for imaging, and all other orders are considered unwanted or stray light. For diffusers, higher diffraction orders are less of a concern because they have little effect on illumination. However, 0th order diffraction (also known as ballistic light) appears in the central portion of the illumination field. For example, in a diffuser, this light is visible in the illumination field as an additional, weakened 0th order diffraction component from the light source. For instance, if the light source is a collimated beam, the 0th order diffraction appears as a bright spot at the center of the FoV; if the light source is a divergent source, the 0th order diffraction appears as an additional, weakened 0th order component from the light source, and its distribution is similar to that of a bare light source.

[0005] One problem is that if the illuminance distribution near the FoV center contains higher intensities due to 0th-order diffraction or other low-angle scattering, that part of the image can saturate, thereby reducing dynamic range and sensitivity. Summary of the Invention

[0006] This disclosure describes techniques for reducing the effects of 0th-order diffraction or other low-angle scattered light on the transmission distribution in an optical system. For example, a receiving optics may include at least one metasurface having a customized incident angle transmission to compensate for the angular brightness distribution caused by 0th-order diffraction.

[0007] In one aspect, this disclosure describes an apparatus comprising an optical system including a transmitting side and a receiving side. The transmitting side includes a light source and a light projection element, the light source being operable to generate light, and the light projection element being arranged to project light toward a scene. Due to 0th-order diffracted rays or other low-angle scattered light, the illuminance distribution of the light projected toward the scene contains increased intensity near the center of the field of view. The receiving side includes an image sensor and receiving optics configured to focus light reflected by the scene toward the image sensor. The receiving optics includes at least one metasurface configured to function as an angle-of-incident filter with a transmission coefficient that proportionally reduces the intensity of the light distribution received at the receiving optics, in which 0th-order diffracted rays or other low-angle scattered light exist.

[0008] Some implementations include one or more of the following features. For example, in some implementations, at least one metasurface is configured to reduce the intensity of the light distribution received at the receiving optics within a specified angle, the specified angle defining a region around the center of the illuminance distribution where zero-order diffraction or other low-angle scattered light exists. In some implementations, at least one metasurface is configured to compensate for the angular brightness distribution caused by zero-order diffraction, such that, under conditions where light projected onto the scene is uniformly reflected by the scene toward the receiving optics, light incident on the image sensor has a substantially uniform measurement intensity on the photosensitive surface of the sensor.

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

[0010] Other aspects, features, and advantages will become apparent from the following detailed description, drawings, and claims. Attached Figure Description

[0011] Figure 1 An example is shown for reducing the effect of 0th-order diffraction or other low-angle scattered light on the transmission distribution in an optical system. Detailed Implementation

[0012] This disclosure describes a technique for reducing the effect of 0th-order diffraction on the transmission distribution in an optical system.

[0013] like Figure 1As shown, the optical system includes a transmitting side (Tx) and a receiving side (Rx). The transmitting side includes a light source 20, which is operable to emit electromagnetic radiation (e.g., light) 22 of a specific wavelength or wavelength range (e.g., infrared, visible light, or ultraviolet light). For example, the light source 20 can be implemented as a vertical-cavity surface-emitting laser (VCSEL), a light-emitting diode (LED), a laser, or other suitable light-emitting device. The light 22 emitted by the light source 20 passes through a light projection element 24, such as a diffraction diffuser or a diffraction beam splitter. For example, a diffuser spreads the light over a wider viewing area, while a diffraction beam splitter splits the beam into multiple beams propagating in different directions. This can be used to generate a focused beam array using a focusing lens or to generate a collimated beam array using, for example, a collimating lens. After passing through the light projection element 24, the light has an intensity distribution 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-order diffracted rays 28, which increase the intensity distribution at or near the center of the scene.

[0014] In operation, a portion of the light incident on scene 29 is reflected back towards the receiving side of the optical system, which includes receiving optics 36 to focus (e.g., onto) the incident light toward the photosensitive surface of the optical sensor 30 (e.g., a CMOS, CCD, or SPAD image sensor). The light transmitted through receiving optics 36 is indicated by 38. Assuming proper alignment in the optical system, zero-order diffraction in image space reflects zero-order diffraction in optical space. Therefore, due to the presence of zero-order diffracted rays 28 in the light 26 projected onto scene 29, the illuminance distribution of light 38 can contain higher intensity near its center 39. The presence of higher intensity can adversely affect the dynamic range of the receiver.

[0015] Since 0th-order diffraction projection is an inherent characteristic of diffuser design and illumination source, it will illuminate the field of view (FoV) in a substantially predictable and invariant manner, varying only slightly between devices due to manufacturing tolerances or other tolerances. Therefore, the receiving optics can map each point of the FoV to a predictable and invariant location on the sensor, such that additional illumination from 0th-order diffraction provides a predictable and invariant brightness distribution on the sensor. Furthermore, since the brightness distribution of additional projection from 0th-order diffraction is in a predictable and invariant location in position space, this also applies to angular space, as 0th-order diffraction propagates radially. This means that 0th-order diffraction also has a well-defined distribution in angular space. Therefore, by integrating the angle of incidence (AOI) transmission function into the receiving optics 36, 0th-order diffraction (or other low-angle scattered light) can be compensated for. That is, for example, the function of an AOI filter can be provided by designing the receiving optics to directly have a reduced transmission efficiency corresponding to the angular distribution and intensity of the projected 0th-order diffraction.

[0016] The receiving optics 36 may include, for example, one or more lenses, and preferably one or more metasurfaces. A metasurface is a surface having distributed small structures (e.g., superatoms) arranged to interact with light in a specific manner. For example, a superlens consists of carefully arranged superatoms (e.g., a distributed array of nanostructures) with subwavelength structures. By adjusting the geometry of the superatoms, the phase above the element can be modified in response to a plane wave.

[0017] In some embodiments, one or more metasurfaces in the receiving optics 36 are configured to have a transmission coefficient that proportionally reduces the intensity of the light distribution 38 near the center 39, in which zero-order diffracted rays exist. For example, the transmission coefficient can be customized based on the specifications of the light source 20 and the receiving optics 36. By incorporating the incident angle transmission into the receiving optics 36, the light incident on the image sensor 30 can have a relatively uniform intensity distribution 40 (e.g., assuming that light projected onto the scene is uniformly reflected from the scene to the receiving optics).

[0018] One or more metasurfaces in the receiving optics 36 can be configured to proportionally reduce the intensity of the light distribution within a specified angle, which defines a region around the center of the distribution where zero-order diffraction occurs. That is, the receiving optics 36 is configured to compensate for the angular brightness distribution caused by zero-order diffraction, such that the distribution 46 of light incident on the image sensor 30 has a relatively uniform intensity distribution (e.g., assuming light projected onto the scene is uniformly reflected by the scene to the receiving optics). In some cases, the receiving optics 36 is configured to allow, for example, a variation of up to 5% to 10% in the offset distribution 46 of light incident on the sensor 30 (e.g., a 5% or 10% variation). In some cases, the receiving optics 36 is configured to allow, for example, a variation of up to 50% to 75% in the offset distribution 46 of light incident on the sensor 30 (e.g., a 50% or 75% variation).

[0019] The aforementioned optical systems can be integrated, for example, into compact electronic devices such as smartphones, laptops, televisions, or wearable devices, as well as into larger devices or systems such as motor vehicles.

[0020] While this specification contains numerous details, these should not be construed as limiting the scope of this disclosure or the scope of any claims, but rather as descriptions of specific features of particular implementations. Certain features described in the context of separate embodiments in this specification may also be combined in the same embodiment. Conversely, various features described in the context of a single embodiment may also be implemented separately or in any suitable sub-combination in multiple embodiments. Various modifications may be made to the foregoing examples. Therefore, other embodiments are also within the scope of the claims.

Claims

1. An apparatus comprising: An optical system, comprising a transmitting side and a receiving side, The transmitting side includes: A light source, operable to produce light; and A light projection element is arranged to project light toward a scene, wherein, due to 0th-order diffraction or other low-angle scattered light, the illuminance distribution of the light projected toward the scene includes increased intensity near the center of the field of view, and The receiving side includes: Image sensors; and A receiving optics device configured to focus light reflected by the scene toward an optical sensor, wherein the receiving optics device includes at least one metasurface configured to function as an angle-of-incidence filter with a transmission coefficient that proportionally reduces the intensity of a light distribution received at the receiving optics device, in which the 0th-order diffracted ray or other low-angle scattered light is present.

2. The apparatus according to claim 1, wherein, The at least one metasurface is configured to reduce the intensity of the light distribution received at the receiving optics within a specified angle, the specified angle defining a region around the center of the light distribution, in which the 0th order diffraction or other low-angle scattered light exists.

3. The apparatus according to claim 1, wherein, The at least one metasurface is configured to compensate for the angular brightness distribution caused by 0th-order diffraction, such that, under the condition that light projected onto the scene is uniformly reflected by the scene toward the receiving optics, light incident on the image sensor has a substantially uniform measurement intensity on the photosensitive surface of the sensor.

4. The apparatus according to claim 3, wherein, The receiving optics are configured to allow a shift in the distribution of light incident on the image sensor of no more than 10%.

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

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