Patent Application
20200073023
The present invention relates to focal plane arrays, and more particularly to a solid-immersion lens focal plane array. Focal plane arrays are light-sensing devices including an array of pixels. Each pixel in the array of pixels includes a photosensor configured to sense light and output a current or voltage indicative of the sensed light. The current output by each photosensor in the array of pixels may be analyzed to determine properties of the sensed light, and analyzed to determine a complete image sensed by the focal plane array.
According to at least one aspect of the present invention, an optical system is provided comprising a solid immersion lens including a first surface and a second surface, and a focal plane array comprising a plurality of pixels, the focal plane array being configured to be in optical contact with the first surface of the solid immersion lens. In one embodiment, the first surface is a planar surface and the second surface is a non-planar surface. In an embodiment, the solid immersion lens is an aspherical lens.
In an embodiment, the solid immersion lens is one of a conic lens and a high-order polynomial lens. In embodiments, the focal plane array includes a planar surface, and the plurality of pixels is arranged on the surface of the focal plane array. In some embodiments, the planar surface of the focal plane array is configured to be in optical contact with the first surface of the solid immersion lens. In an embodiment, each pixel of the plurality of pixels is configured to have a size of less than a vacuum wavelength of electromagnetic radiation incident on the solid immersion lens.
In embodiments, the system includes a lens configured to provide the electromagnetic radiation to the solid immersion lens. In one embodiment, the size of each pixel of the plurality of pixels is equivalent to the vacuum wavelength of the electromagnetic radiation divided by an index of refraction of the lens. In some embodiments, each pixel of the plurality of pixels is configured to minimize a dark current.
According to an embodiment, a method of detecting light with an optical system is provided, the method comprising receiving incident light at an aperture, and directing the incident light to the optical system including a solid immersion lens having a first surface and a second surface, and a focal plane array having a plurality of pixels, the focal plane array being configured to be in optical contact with the first surface of the solid immersion lens.
In an embodiment, the method further includes magnifying, by the solid immersion lens, the focal plane array such that an effective focal length of the optical system is decreased. In one embodiment, the method further includes receiving the incident light at a telescope aperture. In some embodiments, the first surface is a planar surface and the second surface is a non-planar surface. In an embodiment, directing the incident light to the optical system includes receiving, at the second surface of the solid immersion lens, the incident light, and focusing, by the solid immersion lens, the incident light on the focal plane array. In embodiments, the method includes minimizing, by the optical system, a dark current provided by the plurality of pixels.
According to an embodiment, an optical system is provided including a lens including an aperture configured to receive incident light, and direct the incident light to a focal plane array having a plurality of pixels, and means for enabling each pixel of the plurality of pixels to detect the incident light, wherein the incident light has a spot size diameter greater than a physical size of each pixel.
In an embodiment, the system further comprises means for minimizing a dark current of the plurality of pixels. In embodiments, each pixel of the plurality of pixels is configured to have a size of less than a vacuum wavelength of the incident light. In at least one embodiment, the size of each pixel of the plurality of pixels is equivalent to the vacuum wavelength of the incident light divided by an index of refraction of the lens.
Various aspects of at least one embodiment are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. The figures are included to provide an illustration and a further understanding of the various aspects and embodiments, and are incorporated in and constitute a part of this specification, but are not intended as a definition of the limits of any particular embodiment. The drawings, together with the remainder of the specification, serve to explain principles and operations of the described and claimed aspects and embodiments. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every figure. In the figures:
As discussed above, Focal Plane Arrays (FPAs) include an array of pixels having photosensors configured to sense light. A size of each pixel in the array of pixels may be selected according to several design considerations. For example, minimizing a size of each pixel may reduce a physical footprint and cost of the FPA, and may reduce a total dark current. Dark current, which may also be referred to as reverse-bias leakage current, generally refers to a current flowing in the FPA when no illumination is present (for example, in an idle state), and is a parameter to be considered in the sensitivity of the FPA detectors. For example, dark current may be a relevant design consideration because the associated fluctuations and noise that arise from the dark current may limit the sensitivity of the FPA for detecting small signals.
Dark current is proportional to pixel size through the relationship,
i
dark
=A
pixel
·j
dark (1)
where idark is the dark current in units of Amperes (A), Apixel is the pixel area in units of square meters (m2), and jdark is the dark current density of the detector material in units of Amperes per square meter (A/m2). Dark current arises from the thermal generation of electron-hole pairs and diffusion within the detector material. The rate of the generation of electron-hole pairs, and hence the value of jdark, depends on the detector material bandgap energy (which is, in turn, related to the cut-off wavelength of the detector) and operating temperature.
Consequently, decreasing the pixel size can decrease jdark relative to a larger pixel size for a given value of jdark, and allow for higher sensitivity at a given operating temperature due to reduced dark current fluctuations and noise. It would also be possible to increase the cutoff wavelength operating temperature of a detector, resulting in a larger value of jdark, while maintaining the same value of jdark relative to a larger pixel FPA of the same material and operating temperature. Accordingly, it may be advantageous to reduce the pixel size of the FPA in order to improve imaging performance of an optical system, or to reduce the size, weight, and power of the optical system to maintain the infrared camera operating temperature.
However, the minimum size of each pixel is subject to certain physical limitations. For example, as will be appreciated by one of ordinary skill in the art, the minimum dimension of image features produced by the lens of the FPA is limited by diffraction such that a spot size of light is physically incapable of being less than a diffraction-limited minimum spot size. If the pixel area is smaller than the minimum spot size, then the incident spot may be spread out over multiple pixels. Requiring multiple pixels to sense the incident spot increases electrical noise without increasing an amount of spatial information collected, thereby lowering the Signal-to-Noise Ratio (SNR) of the system.
In one example, a Solid Immersion Lens (SIL) may be implemented to reduce the spot-size produced at the pixel below the value that can be achieved on the FPA without such a lens. The minimum spot size Full-Width-at-Half-Maximum (FWHM) of a diffraction-limited optical system specified by an f-number of the optical system, F/#, operating at a vacuum wavelength of λ0, and immersed in a medium with refractive index n can be approximated with the Rayleigh criterion as,
FWHM=1.220(F/#)λ0/n (2)
Accordingly, a spot size of a diffraction-limited optical system that includes an immersion lens can be n times smaller than the spot size of an equivalent optical system that does not include an immersion lens. Another consideration in reducing the size in pixels is maintaining a given field of view over the FPA while imaging with a given aperture diameter. When pixel size is decreased, the focal length of the optical system must also decrease if the same field of view is to be maintained.
However, for the purposes of signal collection, it may be advantageous to hold the aperture diameter constant so that the same signal power is collected to be focused onto the pixel elements of the array. Therefore, if the foregoing design considerations are to be observed, decreasing the pixel size while maintaining aperture diameter requires decreasing f-numbers of the optical system.
Due to tolerance sensitivity of the optical assembly, f-numbers are typically limited to the range of f/1. However, the effect of an immersion lens effectively magnifies the FPA and the FPA's individual pixels from the perspective of the incident light without increasing the physical size of the FPA. Consequently, the optical system overall can be built with an f-number below 1, scaled by the refractive index of the SIL, without requiring significantly higher manufacturing tolerances. By addressing the lower limitations on FPA pixel size, the FPA may be smaller and more cost-efficient, and suffer less dark current, while still being capable of sensing a minimum diffraction-limited spot size. Moreover, because the size of each constituent pixel of the FPA may be reduced, the size of the FPA as a whole may be reduced. Embodiments disclosed herein provide systems and methods for achieving an FPA with reduced cost, physical footprint, and dark current while providing decreased focal lengths to achieve the field of view of systems which do not include a SIL.
Examples of the methods and systems discussed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The methods and systems are capable of implementation in other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. In particular, acts, components, elements and features discussed in connection with any one or more examples are not intended to be excluded from a similar role in any other examples.
Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. Any references to examples, embodiments, components, elements or acts of the systems and methods herein referred to in the singular may also embrace embodiments including a plurality, and any references in plural to any embodiment, component, element or act herein may also embrace embodiments including only a singularity. References in the singular or plural form are no intended to limit the presently disclosed systems or methods, their components, acts, or elements. The use herein of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.
References to “or” may be construed as inclusive so that any terms described using “or” may indicate any of a single, more than one, and all of the described terms. In addition, in the event of inconsistent usages of terms between this document and documents incorporated herein by reference, the term usage in the incorporated features is supplementary to that of this document; for irreconcilable differences, the term usage in this document controls.
Each pixel 102 of the plurality of pixels includes at least one photosensor configured to detect incident light. For example, each photosensor may include a p-n junction configured to output a current indicative of an amount of light incident on the p-n junction. The current output by each photosensor may be provided to a computing device configured to interpret the output currents and determine an image indicated by the output currents. For example, the computing device may include a controller, as discussed in greater detail below.
As discussed above, pixel size may be limited by a diffraction limit of incident light. In some examples, it may be disadvantageous to detect incident light with pixels having an area significantly smaller than the smallest spot size of incident light, because an incident image will be oversampled across multiple pixels, thereby reducing the SNR. For example, continuing with the foregoing example, a pixel 102 having a size below the minimum wavelength of light in the image increasingly oversamples the image produced by diffraction-limited optical systems without an immersion lens. Accordingly, embodiments disclosed herein enable the detection of light with pixels 102 having a physical size that is less than the spot size diameter of incident light while maintaining full or near-full detection of the incident light.
It is to be appreciated that, although the plurality of pixels includes square pixels 102, the plurality of pixels may be configured according to alternate configurations. For example, each pixel 102 of the plurality of pixels may be configured in a rectangular or circular configuration, or any other configuration. Furthermore, although the pixels 102 of the plurality of pixels are identical, in alternate embodiments, the plurality of pixels may include different types of pixels. The plurality of pixels may further include any number of pixels 102, and is not intended to be limited to the illustrated number of pixels. For example, in some embodiments, the FPA 100 may be configured to have 240 rows of pixels and 320 columns of pixels.
The SIL 204 is generally configured to receive incident light on the top surface 210 of the SIL 204, focus the incident light, and provide the focused light to the bottom surface 212 of the SIL 204. As discussed above, the bottom surface 212 of the SIL 204 may be in optical contact with the array surface 208 of the FPA 202. Accordingly, the SIL 204 may provide the focused light to the pixels 206. From the perspective of the incident light, the FPA 202 and the pixels 206 appear magnified. The implementation of the SIL 204 therefore enables the FPA 202 to be implemented with smaller pixels without changing the apparent pixel size from the perspective of the incident light.
In at least one embodiment, the SIL 204 is an aspherical lens configured to focus light which is incident on the top surface 210, and provide the focused light to the pixels 206 via the bottom surface 212, thereby magnifying the pixels 206. The top surface 210 of the SIL 204 may be a curved surface, and the bottom surface 212 of the SIL 204 may be a planar surface. The SIL 204 may be one of several aspherical shapes. For example, the SIL 204 may be configured in a conic shape, a high-order polynomial shape, or another aspherical shape. In alternate embodiments, the SIL 204 may be implemented with a spherical shape.
As will be appreciated by one of ordinary skill in the art, implementation of an aspherical lens may advantageously correct optical aberrations in off-axis images. As used herein, the term “off-axis images” may refer to images which are focused off of an optical axis of the SIL 204, and “on-axis images” may refer to images which are focused on the optical axis of the SIL 204. For example, a first image 214 may be an on-axis image focused on an optical axis 216 of the SIL 204, and the second image 218 may be an off-axis image focused off of the optical axis 216 of the SIL 204. The SIL 204 may be formed from a compatible lens material. For example, where the SIL 204 is configured to receive infrared light, the SIL 204 may be formed from germanium or silicon. The material forming the SIL 204 may be selected to reduce the diffraction-limited spot size of light incident on the top surface 210 of the SIL 204. For example, if λ0 is the vacuum optical wavelength (outside of the lens), and λeff is the wavelength inside of the SIL medium, the material may be selected to have an index of refraction n which decreases the optical wavelength at the array surface 208 of the FPA 202, where
In at least one example, the SIL 204 is composed of a material having an index of refraction within a range of approximately 1.4-4.6.
The aperture 302 has an aperture diameter 304, hereinafter referred to as Daperture. The FPA 204 has a size measurement 306, hereinafter referred to as the FPA size d′FPA, and is configured to receive incident light having a field of view 308, hereinafter referred to as FOV. Each pixel 206 of the pixels has an individual size measurement 310, hereinafter referred to as a pixel size d′pixel, and is configured to receive incident light having an instantaneous field of view 312, hereinafter referred to as IFOV. In at least one example, the FPA size is approximately equal to the pixel size d′pixel multiplied by the number of adjacent pixels in the dimension in which the size measurement 306 is measured. The aperture 302 is displaced from the FPA 204 by a focal length 314 measured from the center of the aperture 302 to the FPA 204, hereinafter referred to as f′.
The relationships of the foregoing parameters may be represented as follows:
where n is an index of refraction of the SIL 202, F/# is the F-number of the optical system 300, q is a pixel sampling parameter, and d′Airy is a diameter of an Airy disc. As will be appreciated by one of ordinary skill in the art, the diameter of the Airy disc d′Airy is indicative of a minimum spot diameter achievable due to the diffraction limit of light. The pixel sampling parameter q is indicative of a ratio of the diameter of the minimum spot size to the size of the pixel.
As illustrated by
For example, in a first implementation, the SIL 202 has an index of refraction n of 1.5. Each pixel 206 of the pixels has a pixel size d′pixel of 12 μm. The focal length f′ between the aperture 302 and the FPA 204 is approximately 3.5 inches. Applying Equation (4), the instantaneous field of view IFOV, indicative of a field of view of an individual pixel 206 of the pixels, is approximately 0.0116 degrees.
In a second implementation, the SIL 202 has an index of refraction n of 4.3, and the parameters d′pixel and f′ remain unchanged. Applying Equation (4), the instantaneous field of view IFOV is approximately 0.034 degrees. Accordingly, it is to be appreciated that increasing the index of refraction n of the SIL 202 proportionally increases the instantaneous field of view IFOV of each pixel 206 of the pixels, and correspondingly the field of view FOV, thereby magnifying each pixel to incident light without physically increasing the size of each pixel.
Enabling modulation of the instantaneous field of view IFOV by modulating the index of refraction n of the SIL 202 provides increased flexibility in designing the optical system 300. For example, in some conventional systems, increasing the instantaneous field of view IFOV might be achieved by decreasing the F-number F/# of the conventional system. Pursuant to Equation (5), decreasing the F-number F/# may include reducing the focal length f′, which yields an increase in the instantaneous field of view IFOV pursuant to Equation (4). However, reducing the F-number F/# of an optical system may be expensive, whereas providing the SIL 202 and increasing the index of refraction n of the SIL 202 may be less expensive. Accordingly, the implementation and configuration of SIL 202 may enable a more cost-efficient method of increasing the instantaneous field of view IFOV and minimizing a pixel size d′pixel without changing the F-number F/#.
In light of the foregoing, the optical system 300 enables smaller pixels to be implemented in an optical system, without compromising a field of view FOV of the optical system 200. The FPA 204 may be implemented with a same number of pixels as an FPA not implemented in connection with the SIL 202, thereby yielding a more compact FPA. Furthermore, reducing the size of the FPA while maintaining the same number of pixels is more affordable because less material is necessary to construct the smaller FPA. Alternatively, the FPA 204 may be implemented with a larger number of pixels to increase the field of view FOV, reduce the instantaneous field of view IFOV, or accomplish both by adjusting the focal length f′, because more of the smaller pixels may be implemented on a wafer of a given size.
As discussed above, embodiments disclosed herein may enable a dark current to be minimized. For example, pixels of an FPA (for example, the FPA 100 or the FPA 202) may each respectively include a p-n junction configured to generate a current responsive to receiving incident light. The dark current generated within a p-n junction is proportional to the area of the p-n junction. A larger p-n junction is therefore subject to a higher dark current, which causes additional background noise. Accordingly, reducing the area of the pixels of the FPA enables reduction of the p-n junction area, which correspondingly reduces an amount of dark current produced by the p-n junction and lowers background noise.
A reduction in dark current enables a higher SNR by reducing the background noise caused by the dark current. Conventional optical systems may include cooling systems configured to enable a higher SNR by cooling the FPAs. For example, the cooling systems may be configured to cryogenically cool FPAs by providing a flow of cryogenically cooled gas across the FPAs. By mitigating the effects of background noise with the implementation of smaller pixels, less cooling may be necessary to improve the SNR, which may enable the cooling system to be reduced in size and power requirements, or removed entirely. Alternatively, the cooling system may not be altered, thereby maintaining a higher SNR.
Examples disclosed herein are not intended to be limiting. For example, in alternate implementations, the FPAs 100 and 202 may be implemented according to any arrangement of pixels. For example, the FPAs 100 and 202 may be implemented having any number of pixels arranged in any number of rows and columns, wherein the pixels may be individually shaped in any configuration.
As discussed above, the FPA 100 may be coupled to a computing device configured to analyze current generated by the plurality of pixels 102. The FPA 202 may similarly be coupled to a computing device configured to analyze current generated by the pixels 206. For example, the computing device may include a controller. The controller may control certain aspects of the FPA 100. Using data stored in associated memory, the controller may also execute one or more instructions stored on one or more non-transitory computer-readable media that may result in manipulated data. In some examples, the controller may include one or more processors or other types of controllers. In one example, the controller performs a portion of the functions disclosed herein on a processor and performs another portion using an Application-Specific Integrated Circuit (ASIC) tailored to perform particular operations. As illustrated by these examples, examples in accordance with the present invention may perform the operations described herein using many specific combinations of hardware and software and the invention is not limited to any particular combination of hardware and software components.
An optical system including an SIL and an FPA has been described. The optical system described may be particularly effective in connection with infrared FPAs, or optical detectors requiring high sensitivity or very fast (i.e., low F-number) collection optics. The optical system enables spot sizes to be reduced to below the wavelength of light measured outside of the SIL medium, which may be performed continuously across the entire FPA. The optical system allows for smaller pixel pitches, at least because a footprint of an incident image is reduced by the SIL, which reduces the pixel area and, correspondingly, the dark current. The optical system further enables a lens F-number to be reduced by a factor of up to the square of the refractive index of the SIL, thereby improving the optical system's sensitivity.
Having thus described several aspects of at least one embodiment, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the scope of the disclosure. Accordingly, the foregoing description and drawings are by way of example only.
