Optical Components and Systems for Simultaneous 3D Hyperspectral Imaging

Abstract

A compact structure for snapshot hyperspectral imaging may include an image slicer, an integral field unit, an integral field spectrograph, and a multiplexed integral field spectrograph. The image slicer includes a first section having a first plurality of mirrors, each mirror of the first plurality having a predetermined longitudinal tilt; a second section having a second plurality of mirrors, each mirror of the second plurality having a predetermined longitudinal tilt; and a ridge extending laterally between the first and second sections, the first section forming a first angle with the ridge and the second section forming a second angle with the ridge opposite the first angle. The integral field unit light source includes a four-mirror design including the image slicer. The integral field spectrograph includes an array of spectrographs and the image slicer.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority to Provisional Application No. 63 / 391,107, filed July 21, 2022, the entire contents of which are incorporated herein by reference.

[0002] Government Rights This invention was made with government support under award 1727095 from the National Science Foundation. The government has certain rights in this invention.

[0003] The present disclosure relates to optical components and systems for simultaneous, real-time three-dimensional (two dimensions in spatial extent [x, y] and one dimension in spectrum [λ]) hyperspectral imaging of a two-dimensional spatial field of view. [Background technology]

[0004] Many fields of business and science use cameras that have spatial resolution as well as spectral resolution that often exceeds the red, green, and blue bands perceptible by the human eye. Spectral high-resolution imaging techniques, also referred to as "hyperspectral imaging," have been developed for these measurements. This hyperspectral imaging allows, for example, the recognition and differentiation of different chemical elements based on their spatially resolved spectra.

[0005] Early hyperspectral imaging systems were based on long-slit grating (or any dispersive element such as a prism) spectrographs and used so-called "push-broom" scanning, in which one dimension was used for spatial determination and the other for spectral determination on a two-dimensional image sensor. New approaches in hyperspectral imaging, and developments in high-resolution sensors and computer hardware, have made full-frame hyperspectral system snapshots possible.

[0006] Conventional hyperspectral imagers, also known as integral field spectrographs (IFS), consist of two parts: 1) an integral field unit (IFU), which reformats the two-dimensional (2D) spatial field formed by an imaging system, such as a telescope or microscope, into a sparsely spaced 2D field of view of elongated slices or light sources, and 2) a conventional diffraction grating spectrograph coupled with a 2D sensor to simultaneously record the spectra of all points in the field of view. Three types of IFUs are commonly used to fabricate IFSs: 1) microlens arrays, 2) coherent fiber optic arrays, and 3) machined or polished glass image slicers, each with its own advantages and limitations. The spectrograph optics of conventional IFSs are typically large due to the need to support the large, sparsely spaced miniature light sources formed by the elongated, long slits or IFUs. Due to the large size of the spectrographs, the intrinsic spectral resolution these spectrographs can achieve (limited by the illumination size of the diffraction grating) typically far exceeds the required resolution. Summary of the Invention [Means for solving the problem]

[0007] One or more embodiments are directed to optical components and systems for snapshot hyperspectral imaging in a compact configuration.

[0008] One or more embodiments are directed to an image slicer for use with a multispectral light source, the image slicer including: a first section having a first plurality of mirrors, each mirror of the first plurality having a predetermined longitudinal tilt; a second section having a second plurality of mirrors, each mirror of the second plurality having a predetermined longitudinal tilt; and a ridge extending laterally between the first and second sections, the first section forming a first angle with the ridge and the second section forming a second angle with the ridge that is opposite the first angle.

[0009] Each of the first and second plurality of mirrors may be a planar mirror.

[0010] One or more embodiments are directed to an integral field unit for use with a multispectral light source, the integral field unit having a four-mirror design and including: an image slicer having a plurality of slicer mirrors that receive light from the multispectral light source and output a plurality of diverging light beams; a collimator mirror that collimates each of the plurality of diverging light beams from the plurality of slicer mirrors into a plurality of collimated light beams; a plurality of reimaging mirrors that output an image of each slicer mirror onto an image sensor; and a plurality of folding mirrors that direct the plurality of collimated light beams from the collimator mirrors onto the plurality of reimaging mirrors.

[0011] The image slicer may include a first section having a first plurality of mirrors each having a predetermined longitudinal tilt, a second section having a second plurality of mirrors each having a predetermined longitudinal tilt, and a ridge extending laterally between the first and second sections, the first section forming a first angle with the ridge and the second section forming a second angle with the ridge that is opposite to the first angle.

[0012] Each of the first and second plurality of mirrors may be a planar mirror.

[0013] One or more embodiments are directed to an integral field spectrograph for use with a multispectral light source, the integral field spectrograph including an image slicer including a plurality of slicer mirrors and an array of spectrographs, each spectrograph associated with a corresponding one of the plurality of slicer mirrors, the array of spectrographs multiplexing the multispectral data onto a two-dimensional image sensor.

[0014] Each spectrograph in the array of spectrographs may include a slicer mirror that acts as an entrance slit for the spectrograph, a collimator mirror that collimates a diverging light beam from the corresponding slicer mirror into a collimated light beam, a micro-diffraction grating that receives the collimated light beam from the collimator mirror and diffracts the light into multiple wavelength bands, and a focusing mirror that directs each of the multiple wavelength bands onto a two-dimensional image sensor.

[0015] Each collimator mirror may be an off-axis parabolic collimator mirror.

[0016] Each row of spectrographs in the array of spectrographs may use an integrated row of off-axis parabolic collimator mirrors.

[0017] Each row of spectrographs in the array of spectrographs may use an integrated row of micro-gratings.

[0018] Each row of spectrographs in the array of spectrographs may use an integrated row of collecting mirrors.

[0019] The integral field spectrograph array may be a four-mirror design that includes a plurality of slicer mirrors in an image slicer that receives light from a multispectral light source and outputs a plurality of diverging light beams, a collimator mirror that collimates each of the plurality of diverging light beams from the plurality of slicer mirrors into a plurality of collimated light beams, a plurality of folding mirrors, and a plurality of collecting mirrors that output an image of each slicer mirror onto an image sensor, and the plurality of folding mirrors direct the plurality of collimated light beams from the collimator mirror onto the plurality of collecting mirrors.

[0020] The image slicer may include a first section having a first plurality of mirrors, each mirror of the first plurality having a predetermined longitudinal tilt; a second section having a second plurality of mirrors, each mirror of the second plurality having a predetermined longitudinal tilt; and a ridge extending laterally between the first and second sections, the first section forming a first angle with the ridge and the second section forming a second angle with the ridge that is opposite to the first angle.

[0021] One or more embodiments are directed to a multiplexed integral field spectrograph that includes a plurality of any of the integral field spectrographs described above.

[0022] The multiple integral field spectrographs may further include a field divider that receives light from the single light source and divides the light from the single light source to be incident on each of the multiple integral field spectrographs.

[0023] Each of the multiple integral field spectrographs receives light from a different light source.

[0024] Each of the plurality of integral field spectrographs may include an array of integral field spectrographs and may further include a field splitter that splits the light from each different light source to be incident on each of the array of integral field spectrographs.

[0025] The scope of the present disclosure is best understood from the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 1 is a ray tracing perspective view of a machined image slicer integral field unit (MISI) according to one embodiment. [Figure 2] FIG. 1 is a schematic perspective view of a machined image slicer integral field unit (MISI) according to one embodiment. [Figure 3] FIG. 1 is an isometric perspective view of an image slicer according to one embodiment. [Figure 4] FIG. 1 is a schematic perspective view of a machined image slicer compact spectrograph (MICS) according to one embodiment. [Figure 5] FIG. 1 is a perspective view of a MICS according to one embodiment. [Figure 6] FIG. 1 is a ray tracing perspective view of MICS for the chief ray of a single beam. [Figure 7] 10A-10C show different perspectives of the chief ray of MICS for all beams. [Figure 8] 10A-10C show different perspectives of the chief ray of MICS for all beams. [Figure 9] 10A-10C show different perspectives of the chief ray of MICS for all beams. [Figure 10] Schematic side view of MICS for all beams. [Figure 11] Schematic top view of MICS for all beams. [Figure 12] FIG. 1 is a ray tracing perspective view of a configuration with four MICSs sharing a common light feed according to one embodiment. [Figure 13] FIG. 1 is a schematic perspective view of a configuration having multiple MICSs, each with a dedicated light feed, according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0027] An integral field unit is an optical device that divides a 2D spatial field into a 2D array or elongated slice of image elements (pixels) and uses a focusing system to reformat the spatial field into a sparsely spaced point source or long slit field of view, forming the input source, commonly referred to as the "entrance slit" of a diffraction grating spectrograph, for injection into a diffraction spectrograph for use with a multispectral light source.

[0028] 1 and 2, machined image slicer integral field unit (MISI) 100 includes a machined image slicer 110, which includes a plurality of slicer mirrors 115, a collimator mirror 120, a plurality of fold mirrors 130, a plurality of collector mirrors 140, and an array of exit field stops or exit slits, i.e., exit port 150, having corresponding images of the slicer mirrors 115 that are then output at a sensor to a focal plane array 160. Thus, the collection system of MISI 100 is a four-mirror design, i.e., slicer mirror 115, fold mirror 130 between collimator mirror 120 and collector mirror 140.

[0029] A plurality of slicer mirrors 115 in image slicer 110 splits incident beam I into a plurality of diverging beams B' and reflects them to collimator mirrors 120, which in turn collimate these diverging beams B' into collimated beams B that impinge on a corresponding one of folding mirrors 130. Light output from each folding mirror 130 is reflected and focused by a corresponding one of collecting mirrors 140, imaging each slicer mirror 115 at and through a respective exit slit 150.

[0030] In particular, each of the microslicer mirrors 115 is focused at a designated location within the exit port 150 using a collimator mirror 120, e.g., an off-axis parabolic collimator mirror, to collimate the diverging beam emerging from the slicer mirror 115, which is then refocused onto the focal plane array by a corresponding fold mirror 130, e.g., a microflat fold mirror, and collector mirror 140, e.g., a microspherical mirror. In particular, each collector mirror 140 may be approximately one focal length away from the intermediate pupil for each collimated beam B formed by the parabolic collimator mirror 120, thereby making the exit beams effectively telecentric.

[0031] As shown in FIG. 3 , the image slicer 110 is divided into two sections defined by a ridge 112. The slicer mirror 115 includes a first plurality of slicer mirrors 114 and a second plurality of slicer mirrors 116 separated by the ridge 112. The first plurality of slicer mirrors 114 has a common tilt angle for directing the beam upward, and the second plurality of slicer mirrors 116 has a common tilt angle opposite to the common tilt angle of the first plurality of slicer mirrors for directing the beam downward. Dividing the image slicer into two sections reduces the depth of the valleys in each section. The valleys in a conventional image slicer, i.e., an image slicer without a ridge, would be very deep, making the image slicer impractical to manufacture. However, by including the ridge 112, the depth of the valleys can be reduced. The ridges 112 allow the image slicer 110 to direct the image of the slicer mirror 116 onto different fold mirror 130 / collector mirror 140 configurations arranged in an array. The specific design of the image slicer 110 depends on the arrangement of these other components of the MISI 100, and uses the ridges 112 to keep the angle small while also preserving the focal plane size of the sensor.

[0032] In a particular example, image slicer 110 may include 56x2 slicer mirrors, each measuring, for example, 0.036mm x 2.664mm, to divide the field of view into a total of 112 sub-fields (only six of which are shown for clarity). The design of image slicer 110 depends on the downstream configuration and can include additional sections with additional ridges.

[0033] A machined image slicer compact spectrograph (MICS) 200 according to one embodiment is illustrated in FIGS. 4 through 11. FIGS. 4 and 6 are rotated from the actual configuration illustrated in FIGS. 7 through 11 to better visualize the array. As can be seen, MICS 200 uses the machined image slicer 110 design and the focusing system of MISI 100, but replaces the common collimator mirror 120 with individual collimator mirrors and the folding mirror 130 with a diffraction grating. Thus, the integral field unit of MISI 100 is transformed into a mini-spectrograph array in MICS 200. By incorporating a diffraction grating directly into the integral field unit, the MICS design eliminates the need for a large common spectrograph after the integral field unit, thereby significantly reducing the size of the integral field spectrograph. Below, the optical design of the MICS is described according to different configurations: a configuration with a single MICS and a configuration with four MICSs, to demonstrate the flexibility and scalability of this design.

[0034] MICS 200 includes an image slicer 110 including a plurality of slicer mirrors 115, a plurality of off-axis parabolic mirrors (OAPs) 220, a plurality of micro-diffraction gratings 230, a plurality of focusing mirrors 140, and a focal plane array 160. As can be seen in Figure 4, the plurality of slicer mirrors 115 in image slicer 110 split an incident beam I into a plurality of diverging beams B' and reflect them to each of the plurality of OAPs 220, which then collimate these beams B' into collimated beams B and direct the collimated beams B to corresponding diffraction gratings in diffraction gratings 230.

[0035] As can be seen in the inset of FIG. 6, the micro-grating 230 divides each of the collimated beams B into a plurality of constituent light beams λ 1 to λ 2 . n , for example, three (red, green, and blue) for white light, and the multiple component light beams are then separately focused as beams Bg onto focal plane array 160. Light output from micro-diffraction grating 230, here a reflective grating, is reflected by corresponding focusing mirrors 140 towards focal plane array 160. Again, the specific design of image slicer 110 will depend on the arrangement of these other components of MICS 200, as well as the focal plane size of the sensor.

[0036] Replacing the fold mirror 130 with a diffraction grating 230 converts each of the four-mirror focusing systems of MISI 100 into a mini-spectrograph. Furthermore, while the focusing system of MISI 100 has a common parabolic collimator mirror 120, resulting in a variable reflection angle between the incoming and outgoing beams at the fold mirror 130, MICS 200 uses individual off-axis parabolic mirrors 220, with the apex of the parent parabola located at the center of the corresponding slicer mirror 115, to collimate the beam reflected by each slicer mirror 115. This design propagates the collimated beam from each slicer mirror 115 in parallel toward each corresponding micro-diffraction grating 230, maintaining a constant reflection angle (or spectrograph angle) for all mini-spectrographs. Thus, the individual MICS 200 shown in Figures 4 through 10 includes an array of spectrographs, i.e., one spectrograph for each slicer mirror 115 of the image slicer 110.

[0037] As can be seen in the specific example shown in Figures 4-11, the image slicer 110 may include 12 x 2 slicer mirrors, each measuring, for example, 20 μm x 0.84 mm, forming a 4 x 6 array of spectrographs, best shown in Figure 4. All mini-spectrographs may have the same grating angle α and spectrograph angle ψ = α - β, where α is the angle of incidence of the beam on the micro-grating 230 relative to the grating normal, and β is the angle of emergence of the diffracted beam relative to the grating normal. All micro-gratings may have the same blaze angle. In this specific example, each micro-grating may be grouped as part of a grating 235, and each grating 235 includes 14 micro-gratings for the 14 mini-spectrographs in that row. Furthermore, as can be seen, the individual collimator mirrors 220, the individual gratings 230, and the individual collector mirrors 140 may each be integrated along one direction of the array, here the row direction.

[0038] The MICS200 is designed to simultaneously acquire high-quality spectral information over a 2D field of view within a compact space by utilizing modern large-format focal plane arrays (FPAs) with wide multiplexing capabilities. Given an FPA with a specific physical size and pixel format, the instantaneous spatial and spectral sampling size and hyperspectral field of view (nx, ny, nλ) can be adjusted depending on the measurement requirements. For example, larger-sized optics can be used to achieve higher spectral resolution. However, this reduces the number of mini-spectrographs that can be accommodated on the sensor and reduces the coverage of the instantaneous spatial field of view of the IFS. Nevertheless, the compact size of the MICS allows multiple MICS to be used within a single instrument, easily doubling or quadrupling the field of view, as illustrated in Figure 12.

[0039] FIG. 12 shows an exemplary system 300 including four MICSs 200a-200d fed by a common source, e.g., telescope 10. A 2x2 field splitter 320 divides the telescope's focal plane into four subfields, one for each MICS. The inset in FIG. 12 illustrates details of the field splitter 320. Four two-lens folded relay systems direct the four subfields to respective ones of the four MICSs 200a-200d. In this particular example, the telescope's field of view is split twice: first by the field splitter 320 into four subfields, and then each subfield is further split by the image slicer of each MICS 200a-200d. Additional field splitters can be cascaded, and additional MICSs can be included, as needed. The MICS image slicer 110 is placed at the focal point of the telescope 10, either directly or through an optical relay.

[0040] Another method of increasing the hyperspectral field of view through multiplexing is illustrated in Figure 13, where a system 400 includes an array of smaller telescopes 20, a corresponding array of MICSs 200 for each telescope 20, and a corresponding array of image sensors 30. Each MICS 200 covers a mosaic field of view, and the mosaic fields of all the MICSs are combined together to form an image spanning the entire field of view. Again, a MICS image slicer 110 is placed at the focal point of each telescope 20, either directly or through an optical relay.

[0041] Alternatively, each MICS 200 in the array may include multiple MICS 200a-200d along with a field splitter 320 for each telescope 20, ie, using system 300 of FIG.

[0042] The present disclosure is not limited to the above-described embodiments, which are merely exemplary. Those skilled in the art will understand that the disclosed system and / or method may be embodied in other specific forms without departing from the spirit or essential characteristics of the disclosure. Accordingly, the presently disclosed embodiments are considered to be illustrative and not restrictive. The present disclosure is not exhaustive and should not be construed as limiting the claimed invention to the particular disclosed embodiments. In light of the present disclosure, those skilled in the art will appreciate that modifications and variations are possible in light of the above teachings or may be acquired from practice of the present disclosure.

[0043] The scope of the invention is indicated by the appended claims rather than by the foregoing description. [Explanation of symbols]

[0044] I incident beam B Collimated beam B' diverging beam 10 telescope 20 Telescope 30 Image Sensor 100 Machined Image Slicer Integral Field Unit (MISI) 110 Machined Image Slicer 112 Ridge 114 first plurality of slicer mirrors 115 Slicer Mirror 116 second plurality of slicer mirrors 120 Collimator mirror 130 Folding Mirror 140 Condenser mirror 150 Exit port 160 focal plane array 200 Machined Image Slicer Compact Spectrograph (MICS) 200a~200d MICS 220 Off-Axis Parabolic Mirror (OAP) 230 Micro diffraction grating 235 Diffraction Grating 300 System 320 field divider 400 System

Claims

1. 1. An image slicer for use with a multispectral light source, comprising: a first section having a first plurality of mirrors, each mirror of the first plurality of mirrors having a predetermined longitudinal tilt; a second section having a second plurality of mirrors, each mirror of the second plurality of mirrors having a predetermined tilt in the longitudinal direction; a ridge extending laterally between the first section and the second section, the first section forming a first angle with the ridge and the second section forming a second angle with the ridge opposite the first angle; Image slicer with.

2. The image slicer of claim 1 , wherein each of the first and second plurality of mirrors is a plane mirror.

3. 1. An integral field unit for use with a multispectral light source, comprising: A four-mirror design, an image slicer having a plurality of slicer mirrors that receive light from the multispectral light source and output a plurality of diverging light beams; a collimator mirror that collimates each of the plurality of diverging light beams from the plurality of slicer mirrors into a plurality of collimated light beams; a plurality of focusing mirrors that output the image of each slicer mirror onto an image sensor; a plurality of folding mirrors that direct the plurality of collimated light beams from the collimator mirror onto the plurality of collecting mirrors; Includes a four-mirror design, an integral field of view unit comprising:

4. The image slicer includes: a first section having a first plurality of mirrors, each mirror having a predetermined longitudinal tilt; a second section having a second plurality of mirrors each having a predetermined tilt in the longitudinal direction; a ridge extending laterally between the first section and the second section, the first section forming a first angle with the ridge and the second section forming a second angle with the ridge opposite the first angle; and 4. The integral field unit of claim 3, comprising:

5. The image slicer of claim 4 , wherein each of the first and second plurality of mirrors is a plane mirror.

6. 1. An integral field spectrograph for use with a multispectral light source, comprising: an image slicer including a plurality of slicer mirrors; an array of spectrographs, each spectrograph associated with a corresponding one of the plurality of slicer mirrors, the array of spectrographs multiplexing multispectral data onto a two-dimensional image sensor; 1. An integral field spectrograph comprising:

7. Each spectrograph in said array of spectrographs comprises: a slicer mirror that serves as an entrance slit for the spectrograph; a collimator mirror for collimating the diverging light beam from the corresponding slicer mirror into a collimated light beam; a micro-diffraction grating that receives the collimated light beam from the collimator mirror and diffracts the light into a plurality of wavelength bands; a focusing mirror that guides each of the plurality of wavelength ranges onto the two-dimensional image sensor; 7. The integral field spectrograph of claim 6, comprising:

8. 8. The integral field spectrograph of claim 7 wherein each collimating mirror is an off-axis parabolic collimating mirror.

9. 9. The integral field spectrograph of claim 8 wherein each row of spectrographs in the array of spectrographs uses an integrated row of off-axis parabolic collimator mirrors.

10. 10. An integral field spectrograph according to any one of claims 7 to 9, wherein each row of spectrographs in the array of spectrographs uses an integrated row of micro-gratings.

11. 10. An integral field spectrograph according to any one of claims 7 to 9, wherein each row of spectrographs in the array of spectrographs uses an integrated row of collecting mirrors.

12. The array of integral field spectrographs is a four mirror design, the four mirror design comprising: a plurality of slicer mirrors in the image slicer that receive light from the multispectral light source and output a plurality of diverging light beams; a collimator mirror that collimates each of the plurality of diverging light beams from the plurality of slicer mirrors into a plurality of collimated light beams; A plurality of folding mirrors; a plurality of focusing mirrors that output an image of each slicer mirror onto an image sensor, the plurality of folding mirrors directing the plurality of collimated light beams from the collimator mirror onto the plurality of focusing mirrors; 7. The integral field spectrograph of claim 6, comprising:

13. The image slicer includes: a first section having a first plurality of mirrors, each mirror of the first plurality of mirrors having a predetermined longitudinal tilt; a second section having a second plurality of mirrors, each mirror of the second plurality of mirrors having a predetermined tilt in the longitudinal direction; a ridge extending laterally between the first section and the second section, the first section forming a first angle with the ridge and the second section forming a second angle with the ridge opposite the first angle; 7. The integral field spectrograph of claim 6, comprising:

14. A multiplexed integral field spectrograph comprising a plurality of integral field spectrographs according to any one of claims 6 to 9.

15. 15. The multiplexed integral field spectrograph of claim 14, wherein the multiple integral field spectrographs further include a field splitter that receives light from a single light source and splits the light from the single light source for incidence on each of the multiple integral field spectrographs.

16. 15. The multiplexed integral field spectrograph of claim 14, wherein each of the plurality of integral field spectrographs receives light from a different light source.

17. 17. The multiplexed integral field spectrograph of claim 16, wherein each of the plurality of integral field spectrographs comprises an array of integral field spectrographs and further comprises a field splitter that splits the light from each different light source to be incident on each of the array of integral field spectrographs.

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