Patent Description:
Fixed focus cameras are typically aligned and focused at room temperature. These cameras may experience defocus, also known as a change in back focal length, over operating temperatures associated with various automotive applications due to thermal expansion and contraction of the camera.

<CIT> discloses a camera with compensation for defocusing due to temperature changes.

An camera according to claim <NUM> is provided. It includes a phased metalens positioned between an objective lens and an imager of the camera. The phased metalens is configured to adjust a focus plane of an image in a field of view of the camera in response to changes in an operating temperature of the camera.

In an example having one or more features of the camera of the previous paragraph, the phased metalens is positioned within <NUM> of an imager focal plane.

In an example having one or more features of the camera of any of the previous paragraphs, the phased metalens adjusts the focus plane for multiple frequencies or wavelengths of the light.

In an example having one or more features of the camera of any of the previous paragraphs, the wavelengths range from <NUM> nanometers (<NUM>) to <NUM>.

In an example having one or more features of the camera of any of the previous paragraphs, the phased metalens adjusts the focus plane over a temperature range of about <NUM> degrees Celsius.

In an example having one or more features of the camera of any of the previous paragraphs, the phased metalens adjusts the focus plane over changes in a focal length of about <NUM> microns (<NUM>).

In an example having one or more features of the camera of any of the previous paragraphs, the phased metalens comprises a plurality of sub-wavelength structures positioned at predetermined coordinates across the phased metalens.

In an example having one or more features of the camera of any of the previous paragraphs, the plurality of sub-wavelength structures range from two times to eight times smaller than a wavelength of the light transmitted through the phased metalens.

In an example having one or more features of the camera of any of the previous paragraphs, the unique phase profiles are configured to adjust a phase of the light transmitted through the plurality of arrangements.

In an example having one or more features of the camera of any of the previous paragraphs, the unique phase profiles are based on the arrangement's respective radial distance from a center of the phased metalens.

In an example having one or more features of the camera of any of the previous paragraphs, the unique phase profiles are based on an operating temperature of the camera.

In an example having one or more features of the camera of any of the previous paragraphs, the plurality of arrangements define a plurality of resolution units.

In an example having one or more features of the camera of any of the previous paragraphs, the plurality of resolution units located at a same radius from a center of the phased metalens have identical phase profiles.

In an example having one or more features of the camera of any of the previous paragraphs, the plurality of resolution units located at a different radius from a center of the phased metalens have different phase profiles.

In an example having one or more features of the camera of any of the previous paragraphs, a size of one resolution unit is equal a size of four image pixels.

In an example having one or more features of the camera of any of the previous paragraphs, each image pixel includes about <NUM> to <NUM> sub-wavelength structures.

In an example having one or more features of the camera of any of the previous paragraphs, each resolution unit includes about <NUM> to <NUM> sub-wavelength structures.

In an example having one or more features of the camera of any of the previous paragraphs, as the respective radial distance of the plurality of resolution units increases from a center of the phased metalens, the unique phase profiles increase an amount of phase adjustment for a given wavelength of light.

In an example having one or more features of the camera of any of the previous paragraphs, as a radial distance of the plurality of resolution units increases from a center of the phased metalens, the unique phase profiles increase an amount of phase adjustment for decreasing wavelengths of the light.

In an example having one or more features of the camera of any of the previous paragraphs, as a radial distance of the plurality of resolution units increases from a center of the phased metalens, the unique phase profiles increase an amount of phase adjustment for a given temperature.

In an example having one or more features of the camera of any of the previous paragraphs, as a radial distance of the plurality of resolution units increases from a center of the phased metalens, the unique phase profiles increase an amount of phase adjustment for increasing temperatures.

In an example having one or more features of the camera of any of the previous paragraphs, all light wave-fronts exiting the phased metalens arrive at the imager at a same time.

<FIG> illustrates a cross sectional view of a camera <NUM> that includes an objective lens <NUM> and an imager <NUM>. While the examples illustrated herein disclose the camera <NUM>, it will be appreciated that the disclosure also applies to other devices or sensors that sense electromagnetic radiation, such as light detection and ranging (LiDAR) sensors. Multiple camera lenses (not shown) of varying geometries may be used in the camera <NUM>, depending on the application requirements. The imager <NUM> may be in electrical communication with a controller circuit (not shown) to process an image <NUM> of an object <NUM> in a field of view <NUM> of the camera <NUM>. In the example illustrated in <FIG>, a focal length of the camera <NUM> is fixed. That is, the camera <NUM> does not include a mechanical or electrical focus adjustment device to refocus the image <NUM> when a focus plane <NUM> of the `camera <NUM> moves away from the imager <NUM> (i.e., a defocus). In an example, thermal expansion and contraction of the camera <NUM>, due to an operating temperature variation, may cause the camera <NUM> to defocus. It will be understood that the focus plane <NUM> may move in a positive direction (i.e., toward the objective lens <NUM>) or a negative direction (i.e., toward the imager <NUM>) along an optical axis <NUM> of the camera <NUM>, due to thermal expansion or thermal contraction of the camera <NUM>.

A typical camera used for advanced driver assistance systems (ADAS) may be required to operate over a temperature range of -<NUM> to +<NUM>. ADAS cameras are typically focused at <NUM>. Depending on the materials used in the camera body (not specifically shown), spacers, and lenses, this temperature range may result in a change in the focal length of the camera <NUM> by as much as <NUM> microns (<NUM>). ADAS cameras, that have fixed focus lenses with relatively large apertures and relatively low f-stops, have a reduced depth of focus compared to more expensive adjustable focus cameras. As a result, the thermal expansion of ADAS cameras over the <NUM> temperature range will cause a significant and measurable (e.g., <NUM>% to <NUM>%) degradation in an image quality, which may negatively affect the ADAS systems. Autonomous vehicle camera requirements are continuing to drive toward smaller camera imager <NUM> pixel sizes (e.g., <NUM>), higher density focal planes (e.g., <NUM> Megapixel arrays), and higher spatial frequency contrast image quality requirements (e.g., greater than <NUM> line pairs/mm). Consequently, the image degradation over temperature for the larger format cameras will be proportionately higher and reduce object detection performance.

For a traditional fixed focus lens system, a change in back focal length of <NUM> - <NUM> would require the same movement by the complete lens system, or could be accomplished by, a) introduction of a lens element index of refractive change (e.g., <NUM>% - <NUM>% or representing a delta change of <NUM> - <NUM>), and/or b) lens element material thickness change (e.g., <NUM> - <NUM>), and/or c) curvature change (e.g., <NUM> radius of curvature), and/or d) smaller contributions by combinations of the above.

To address the defocus issue of the fixed focus camera <NUM>, a phased metalens <NUM> is positioned between the objective lens <NUM> and the imager <NUM> of the camera <NUM>, as illustrated in <FIG>. The phased metalens <NUM> is configured to adjust the focus plane <NUM> of the image <NUM> in the field of view <NUM> of the camera <NUM> in response to changes in the operating temperature of the camera <NUM>. The phased metalens <NUM> accomplishes this by shifting a phase of the incoming light rays via sub-wavelength structures <NUM>, as will be described in more detail below. These sub-wavelength structures <NUM> (also referred to as nanostructures) may be deposited on a relatively thin, generally planar, substrate of optically transparent material (e.g., optical glass), and may be formed of metamaterials with structural features that are capable of manipulating the light waves. In an example, the metamaterials are fabricated using known lithographic processes from compounds such as titanium dioxide, silicon nitride, boron nitride, molybdenum disulfide, or combinations thereof. The metamaterials may be selected based on the wavelengths of the electromagnetic radiation being sensed. In an example, titanium dioxide may be selected for light in the visible and near infrared spectrum. In an example, silicon nitride may be selected for light in the visible spectrum. In an example, boron nitride may be selected for electromagnetic radiation at wavelengths below the visible and near infrared spectrum (e.g., ultraviolet light). In an example, molybdenum disulfide may be selected for electromagnetic radiation at wavelengths in the near infrared spectrum.

<FIG> illustrates an example where the wave fronts of the light rays exiting from different regions of the phased metalens <NUM> reach the imager focal plane <NUM> at a same time (i.e., Δt = <NUM>, in phase). In an example, the sub-wavelength structures <NUM> are fabricated on an exit side of the substrate (i.e., the side facing the imager <NUM>). In an example, a cross section of the sub-wavelength structures <NUM> normal to the incident light rays are rectangular. In another example, the cross section of the sub-wavelength structures <NUM> normal to the incident light rays are circular. The phased metalens <NUM> is configured to shift the phase of the incoming light rays such that all light wave-fronts exiting the phased metalens <NUM> arrive at the imager <NUM> at a same time, resulting in good focus for all temperature conditions. That is, the phased metalens <NUM> delays the light wave-fronts by differing amounts, depending on the position of the sub-wavelength structures <NUM> on the phased metalens <NUM>, such that all the light wave-fronts reaching the imager <NUM> are in-phase. The phased metalens <NUM> accomplishes this by achieving near diffraction limited focusing over the incoming light wavelengths using precisely defined nanoscale sub-wavelength resolution structures. In an example, the phase relationship for the phased metalens <NUM> is defined by the design wavelength, a sub-wavelength structure shape, and the phased metalens <NUM> focal length, using the known equation below, <MAT> where λd is the design wavelength, f is the focal length for the converging phased metalens <NUM> and x and y are the coordinates of the sub-wavelength structures <NUM> on the phased metalens <NUM>. To account for the focus variation across the operating temperature range, the phased metalens <NUM> includes the sub-wavelength structures <NUM> arranged in unique phase profiles for the multiple focal lengths within the resolution unit <NUM> that result from the temperature changes of the camera <NUM>. That is, the phased metalens <NUM> includes multiple unique phase profiles designed for multiple offsets of the focal length, so that as the focal length is offset by the temperature change, the light rays exiting the phased metalens <NUM> will remain in phase.

<FIG> illustrates an example of a focus characteristic of the phased metalens <NUM> of <FIG>. The phased metalens <NUM> is configured to adjust the focus plane <NUM> over the temperature range of about <NUM> and over the associated changes in the focal length of about <NUM>. The phased metalens <NUM> adjusts the focus plane <NUM> for multiple frequencies or wavelengths of the light. In an example, the wavelengths range from about <NUM> to about <NUM> (i.e., visible light to near infrared light). In another example, the wavelengths range from about <NUM> to about <NUM> (i.e., visible light only). In another example, the wavelengths range from about <NUM> to about <NUM> (i.e., near infrared light only).

An aspect of the camera <NUM> is that the phased metalens <NUM> is placed in close proximity to the imager <NUM>. In an example, the phased metalens <NUM> is positioned within <NUM> of an imager focal plane <NUM> (i.e., the imaging surface of the imager <NUM>). In an example, a thickness of the metalens <NUM> is less than <NUM>, and preferably less than <NUM>. This relatively thin structure enables the metalens <NUM> to be positioned in the typically narrow space between the fixed focus objective lens <NUM> and the imager focal plane <NUM>. This positioning enables a greater flexibility allowing for the compensation of the thermal driven defocus while otherwise being independent of the existing fixed lens system.

<FIG> illustrates the phased metalens <NUM> viewed along an optical axis <NUM> of the camera <NUM>. The phased metalens <NUM> comprises a plurality of sub-wavelength structures <NUM> (not shown) positioned at predetermined coordinates across the phased metalens <NUM>. In an example, the plurality of sub-wavelength structures <NUM> range from two times to eight times smaller than the wavelength of the light transmitted through the phased metalens <NUM>. In an example, the sub-wavelength structures <NUM> that shift blue light (having wavelengths that range from <NUM> - <NUM>) would have cross sectional dimensions normal to the incident light rays that range from <NUM> to <NUM>. It will be recognized that light with longer wavelengths will require larger sub-wavelength structures <NUM> to cause the phase shift, and that light with shorter wavelengths will require smaller sub-wavelength structures <NUM> to cause the phase shift.

Referring back to <FIG>, the plurality of sub-wavelength structures <NUM> are grouped into a plurality of arrangements having unique phase profiles that define a plurality resolution units <NUM> (RUs <NUM>). That is, the plurality of sub-wavelength structures <NUM> are arranged into RUs <NUM> that have unique phase profiles that delay the light transmitted through the RUs <NUM> by differing amounts of time. These unique phase profiles are configured to adjust the phase of the light transmitted through the plurality of RUs <NUM> based on the operating temperature of the camera <NUM>, and also based on the RU's <NUM> respective radial distance from a center of the phased metalens <NUM>. <FIG> illustrates an example of two separate RUs <NUM> isolated from the plurality of RUs <NUM>, having different phase profiles as denoted by "PHASE PROFILE <NUM>" within the RU <NUM> positioned at "RADIUS <NUM>", and by "PHASE PROFILE <NUM>" within the RU <NUM> positioned at "RADIUS <NUM>". In the example illustrated in <FIG>, the plurality RUs <NUM> located at a same radius (e.g. RADIUS <NUM>) from a center of the phased metalens <NUM> have identical phase profiles, and the plurality RUs <NUM> located at a different radius (e.g. RADIUS <NUM>) from the center of the phased metalens <NUM> have different phase profiles.

Referring again to <FIG>, in an example, a size of one RU <NUM> is equal the size of four image pixels <NUM> of the imager <NUM>. The maximum useful image resolution is limited to the Nyquist frequency, i.e., the resolution in pixel size scaled to the camera <NUM> imager focal plane <NUM> pixel size. In this example, this is equivalent to the size of four image pixels <NUM>. In an example, for the camera <NUM> with image pixels <NUM> measuring <NUM> x <NUM> in size, the limiting resolution is an area of <NUM> x <NUM>. Within this area, image information is sub-resolved or is not able to be reproduced or imaged, as such the area of <NUM> x <NUM> is the limiting dimension of the RU <NUM>. Table <NUM> below illustrates an example of a scale of various characteristics of a <NUM> x <NUM> phased metalens <NUM>.

Referring to Table <NUM>, in an example, each RU <NUM> includes about <NUM> to <NUM> sub-wavelength structures <NUM>. In this example, a focus characteristic encompassing a range of <NUM>-<NUM> discrete wavelengths with <NUM>-<NUM> discrete temperature offsets may be included within a single RU <NUM>.

<FIG> are plots of radial distances of the sub-wavelength structures <NUM> from the center of the phased metalens <NUM> versus a phase adjustment of the light. In this example, three colors (i.e. wavelengths) of visible light (blue, green, and red), at three operating temperatures (-<NUM>, <NUM>, and +<NUM>) are used to illustrate how the phased metalens <NUM> adjusts the phase of the exiting light rays. In this example, as the respective radial distance of the plurality of RUs <NUM> increases from the center of the phased metalens <NUM> towards the periphery of the phased metalens <NUM>, the unique phase profiles increase an amount of phase adjustment for a given wavelength of light. Referring to <FIG> (blue wavelength), the center of the phased metalens <NUM> is indicated at (<NUM>, <NUM>) where the phase adjustment for the three temperatures are nearly zero. As the sub-wavelength structures <NUM> are moved away from the center of the metalens <NUM>, the phase adjustment increases for the three temperatures indicated.

In the examples illustrated in <FIG>, as the radial distance of the plurality of RUs <NUM> increases from the center of the phased metalens <NUM>, the unique phase profiles increase an amount of phase adjustment for decreasing wavelengths of the light. Comparing <FIG>, with <FIG>, and with <FIG>, the phase adjustment for the blue light in <FIG> is greater than that for the green light of <FIG>, which is in turn greater than that for the red light of <FIG>. It will be understood that the wavelength of light increases from blue light to green light to red light.

In the examples illustrated in <FIG>, as the radial distance of the plurality of RUs <NUM> increases from the center of the phased metalens <NUM>, the unique phase profiles increase an amount of phase adjustment for a given operating temperature of the camera <NUM>. Referring to <FIG>, the plots of constant temperature show increasing phase adjustment as the radial distance of the sub-wavelength structures <NUM> increases from the center of the phased metalens <NUM>.

In the examples illustrated in <FIG>, as the radial distance of the plurality of RUs <NUM> increases from the center of the phased metalens <NUM>, the unique phase profiles increase an amount of phase adjustment for increasing operating temperatures of the camera <NUM>. Referring again to <FIG>, as the temperature increases from -<NUM> to <NUM>, the amount of phase adjustment also increases.

Claim 1:
A camera (<NUM>) configured to adjust a focus plane (<NUM>) of an image in a field of view (<NUM>) of the camera (<NUM>) in response to changes in an operating temperature of the camera,
characterized by:
a phased metalens (<NUM>) positioned between an objective lens (<NUM>) and an imager (<NUM>) of the camera (<NUM>), wherein:
the phased metalens (<NUM>) is configured to adjust a focus plane (<NUM>) of an image in a field of view (<NUM>) of the camera (<NUM>) in response to changes in the operating temperature of the camera; and
the phased metalens (<NUM>) comprises a plurality of sub-wavelength structures (<NUM>) grouped into arrangements having respective phase profiles that vary based on the operating temperature of the camera (<NUM>).