Multicolor detector and focal plane array using diffractive lenses

[0002] This invention relates to the use of a microlens array coupled with an associated detector array to effect color separation within each optical pixel of an area array of detection pixel elements (a focal plane array). This invention relates more particularly to a simpler method for simultaneously detecting radiation intensity in each of several wavelength bands when the focal plane array is illuminated with radiation over a broad spectral range. This invention will be particularly useful for multicolor detector arrays operating in the infrared spectral range.

[0003] Traditional sensor systems have relied on filters and beam splitters to direct light in different spectral bands to separate one-color focal plane arrays. Multicolor focal plane arrays therefore provide a large reduction in system complexity in such systems by eliminating complex optics and critical alignments.

[0004] Multicolor focal plane arrays have been proposed and some have been demonstrated using a variety of techniques. Perhaps the most straightforward approach utilizes stacked detectors with differing spectral responses, each of which cover most of the optical pixel area. This requires a detector material technology that allows tuning of the spectral response. For example, stacked detector solutions to two-color focal plane arrays are being developed with Mercury Cadmium Telluride and with Quantum Well Infrared Photo-conductor detectors.

[0005] While versatile, stacked detector technologies are limited either in response wavelength and/or performance and are therefore not suited for many applications, particularly because of the added materials complexity involved in the fabrication of stacked detectors.

[0006] Special patterned filters are often used to make multicolor focal plane arrays. Multicolor charge-coupled device or active pixel sensor focal plane arrays with deposited filters are commonly used in both video and still cameras. However, patterned filters waste photons by rejecting all out-of-band photons and not allowing those to be detected and high performance systems usually need to utilize every available photon for maximum sensitivity. In addition, for long wavelength infrared applications the materials used to fabricate the filter are difficult to deposit and usually require a very thick filter, limiting the transmission and fill factor of the filter as well as the minimum filter size.

[0007] Another approach superimposes a linear diffraction grating onto a refractive microlens for each optical pixel and etches an array of the resulting structures, most often on the backside of the detector substrate. The lens focuses the incident light to a spot much smaller than a pixel while the grating simultaneously diffracts the light off-axis at an angle dependent on the wavelength. Sub-pixel sized detector elements on the front side of the substrate at the focus of the lens and are properly spaced to intercept the diffracted light may then detect the light in two or more spectral bands. For example, the structure resulting from a superposition of a linear diffraction grating onto a refractive microlens is referred as a “dispersive microlens” in the disclosure of Gal, U.S. Pat. No. 5,497,269.

[0008] These approaches to multicolor lens detector systems are often used but have many drawbacks. They are more complicated because they require multiple optical devices to implement a multicolor detector. The patterned filters are inefficient while the other approaches pose fabrication and materials difficulties and are therefore expensive as well. Furthermore, these methods are difficult to apply over wide spectral bands and to extend into the very long wavelength infrared (VLWIR) spectral region; for example it is difficult to make a linear diffraction grating effective over more than an octave in wavelength.

[0009] In the present invention a diffractive microlens coupled to a special multiple detector configuration is used to construct a multicolor optical pixel. An optical pixel or pixel as used herein is defined as an optically active area at the focal plane of an optical system. The invention is a method and apparatus for simultaneous multicolor light detection using a circular, symmetrical diffractive microlens in combination with two concentric detectors, one centrally located and the second concentrically disposed around the first. These two detectors are used to discern two spectral colors or bands out of broadband light illuminating the pixel. Color separation is effected by the inherent strong chromatic aberration of the diffractive microlens. The circular, symmetrical diffractive microlens is designed to focus light in a band Δλ of wavelengths centered about a point λ₀onto the central detector. This approach takes fill advantage of the strong focusing that is possible inside the type of high refractive index material that is typically used in a detector substrate so that light can be focused onto a detector much smaller than the optical pixel, in effect the microlens functions as an immersion lens.

[0010] In an alternative embodiment of the invention, a plurality of such optical pixels may be assembled to form a multicolor focal plane array.

[0011] In yet another embodiment, the present invention is used as a multicolor focal plane array for discrimination of exo-atmospheric targets. The multicolor focal plane array is designed and fabricated for the VLWIR spectral region, where it may be used in the discrimination of exo-atmospheric targets from decoys.

[0012] FIG. 1 is a schematic side view of the preferred embodiment of one optical pixel according to the current invention.

[0013] FIG. 2 is a schematic drawing of the two detectors for one optical pixel.

[0014] FIG. 3 shows a focal plane array of optical pixels configured in a two-dimensional grid suitable for imaging in two infrared colors, simultaneously.

[0015] FIG. 4 is a three-axis plot that shows a three-dimensional representation of one quarter of the calculated distribution of λ=8 μm light intensity on the detectors for the particular case of λ₀=8 μm, optical pixel size=50 μm×50 μm and focal length is 50 μm; one quarter of the detector area is shown.

[0016] FIG. 5 is a three-axis plot that shows a three-dimensional representation of one quarter of the calculated distribution of λ=12 μm light intensity on the detectors for the optical pixel of FIG. 4; one quarter of the detector area is shown.

[0017] FIG. 6 is a two dimensional graph that shows the calculated relative response curves for the short band and the long band detectors as a function of illumination wavelength for the optical pixel of FIG. 4.

[0018] The following detailed description, and the figures to which it refers, are provided for the purpose of describing example(s) and specific embodiment(s) of the invention only and are not intended to exhaustively describe all possible examples and embodiments of the invention.

[0019] The design and construction of diffractive microlenses is disclosed by Swanson, G. J., in Technical Report854, Binary Optical Technology: The Theory and Design of Multi-level Diffractive Optical Elements, Aug. 14, 1989, Lincoln Laboratory, Massachusetts Institute of Technology, and is expressly incorporated by reference herein.

[0020] As described by Swanson, the effect of the surface profile of a lens on the phase of incident light of wavelength λ can be described by a phase function φ_λ(r). The phase function of a diffractive lens is equal to the phase function of the corresponding refractive lens modulo 2π, in other words, the phase function of a diffractive lens is discontinuous at transition points where the phase shift induced in light of the design wavelength λ₀changes from 2π to 0 or from 0 to 2π. Because the transition points correspond to a 2π phase shift only for light of a single wavelength λ₀, the diffractive lens has strong chromatic aberration, i.e. the focus of light of wavelength λ₀will be sharp but will degrade rapidly as the wavelength deviates from λ₀, except for integer divisions of the light, of the series λ₀/2, λ₀/3, λ₀/4 . . . , which will be well focused as well. Swanson provides a useful method for locating and designing these transition points in diffractive microlenses.

[0021] Diffractive microlenses can be fashioned using the multi-level profile approach known as binary optics as described by Swanson to produce a lens of nearly 100% diffraction efficiency (focusing efficiency) for light of a particular wavelength λ₀. This technique is used to approximate the ideal diffractive microlens profile by substituting instead a set of discrete levels or steps in the diffractive microlens. By properly combining a number of successive etchings, m, with varying etch depths from successive photo-lithographic masks, m masking operations results in 2^mlevels in the multi-level lens. As noted by Swanson, using this technique requires that only four masking operations are needed for a 16-phase level structure that can achieve 99% diffraction efficiency at the design wavelength λ₀.

[0022] A two-color optical pixel is constructed by disposing a diffractive microlens on one side of a thin substrate of suitably high index of refraction and transparent material. Two detectors are disposed on the other side of the substrate, arranged so that a small cental detector acts as a short band detector and is centered on the optical axis of the lens. A detector of much larger sensitive area surrounds the short band detector and serves as the long band detector. Color band separation is achieved because the short band detector will efficiently detect light in a band of width Δλ centered about λ₀, while light of wavelength substantially longer than λ₀will not be focused efficiently onto the small detector. Most of the longer wavelength light will be detected in the long band detector because of its much larger area. The width Δλ of short band optical pixel may be further optimized by adjusting the focal length 𝑓 of the diffractive microlens and adjusting the thickness of the substrate to match.

[0023] Precise spectral response of the detectors also depends on the specific detector size and pixel size used and whether any optically active separation exists between the short band detector and the long band detector. Separate blocking filters may be used to define both the short band cut-on wavelength and the long band cut-off wavelength.

[0024] While the descriptions herein pertain to a two-color optical pixel operating in the VLWIR spectral region, those skilled in the art will appreciate that the invention can be extended to other spectral regions and to multiple colors. Further, those skilled in the art will understand that the diffractive microlens may be fabricated by any of several different techniques. In the preferred embodiment, a close approximation to a diffractive microlens is fabricated by the binary optic technique described by Swanson.

[0025] Those skilled in the art will also understand that a plurality of the multicolor optical pixels of this invention can, and in most practical cases will be, assembled into a multicolor focal plane array. A multicolor focal plane array of these optical pixels comprises a focal plane array and its architecture is comprised of an array of detector elements disposed on one side of a thin, transparent substrate with a high index of refraction and a corresponding array of diffractive microlenses disposed on the other side of the substrate. Each pixel consists of a small, cental short band detector and a concentric, surrounding long band detector receiving light through a diffractive microlens, and each is connected to separate electronic output circuits. Electronic readout of a multicolor focal plane array may be accomplished with appropriate electronic circuitry, well known in the art.

[0026] In the following various figures identical elements and features are given the same reference number, and similar or corresponding elements and features are or may be given the same reference numbers followed by an a, b, c, and so on as appropriate for purposes of describing the various embodiments of the present invention.

[0027] Referring now to FIG. 1 there is shown a two-color optical pixel 1, comprised of a diffractive microlens 3 to focus radiation onto a two-color detector 5. The band of wavelengths defined by λ₀±Δλ/2 detected by the central detector is called the short band (SB), shown at 7. Light of wavelengths longer than λ₀+Δλ/2 will not be focused very effectively onto the central detector by the diffractive microlens and these wavelengths will fall more preferentially onto, and be detected by, the concentric long band (LB) detector 9, which detects the band of wavelengths from λ₀+Δλ/2 to the cut-off wavelength of the detector. Wavelengths of light shorter than λ₀−Δλ/2 should be blocked, either by an external blocking filter or by integrally incorporating a blocking material in the focal plane array design.

[0028] The width of the short pass band Δλ is determined by the focal length, aperture, and design wavelength of the diffractive microlens as well as by the size of the central detector. Swanson gives an expression for the maximum wavefront error, maxφ_e, of a diffractive microlens (in air) as $\max {\phi }_{ϵ}=\frac{D}{8\left(F/\#\right)}\left(\frac{\mathrm{\Delta \lambda }}{{\lambda }^2}\right)=\frac{D^2}{8f}\left(\frac{\mathrm{\Delta \lambda }}{{\lambda }^2}\right).$

[0029] Where, D is the lens diameter, F/# is the f-number of the lens, and 𝑓 is the focal length of the lens. If it is assumed that the bandwidth detected by the central detector is constant for a constant wavefront error, then $\mathrm{\Delta \lambda }\propto \frac{{\mathrm{f\lambda }}^2}{D^2}.$

[0030] The method of the current invention comprises constructing an optical pixel of the invention and detecting a plurality of wavelengths of light with the aforesaid appropriate electronic circuitry.

[0031] In the preferred embodiment the two detectors form a 50μm×50 μm square, and the diffractive microlens is a corresponding 50 μm×50 μm square lens. Referring again to FIG. 1 there is schematically shown a ray-trace plot with two ray bundles 11 and 13. Light at the design wavelength λ₀is focused on the center detector as indicated by ray bundle 13 while longer wavelength light principally falls on the surrounding detector as indicated by ray bundles 11, and infrared radiation substantially of a wavelength from 6 μm to 25 μm is split into two bands, the shorter band into one of about 6 μm to 10 μm, the longer band into one of about 10 μm to 25 μm.

[0032] Referring now to FIG. 2 there is shown the detector 5aof the present invention. Detectors made of silicon are preferred over common HgCdTe detectors for this particular embodiment because they display better sensitivity at the longer wavelengths in the VLWIR spectral region. There is shown the central, circular short band detector component 7a, and the concentric, surrounding long band detector 9a, both defined by ion implants into the silicon. The short band detector is centrally located on the optical pixel as a 10 μm diameter circular detector. The long band detector is coextensive with the balance of the 50 μm×50 μm square pixel Indium bumps 15 mounted on metallized areas 17 provide for the connection of each detector to an external electronic readout circuit.

[0033] Referring now to FIG. 3 there is shown a focal plane array 23 comprised of a grid of two-color optical pixels detectors 1athat, in combination with appropriate circuitry 24, well known in the art, form an array that enables a two dimensional image to be formed in the dimensions corresponding to each of the two columns.

[0034] The focal plane array 23 may be incorporated into a multicolor focal plane array sensitive to the VLWIR spectral region, where it may be used in the discrimination of exo-atmospheric targets from decoys. Focal plane arrays used for the discrimination of exo-atmospheric targets from decoys are well known in the art and the optical pixel design of the present invention may be used in these systems instead of the multiple focal plane arrays used in other optical pixel configurations of the prior art, resulting in a consequent improved performance and sensitivity as outlined above.

[0035] Referring now to FIG. 4 there is shown a three-axis graph that shows a three-dimensional representation depicting the calculated light intensity pattern on one quarter of the detector for light of wavelength 8 μm. Referring now to FIG. 5 there is shown a three-axis graph that shows a three-dimensional representation depicting the calculated light intensity pattern on one quarter of the detector for light of wavelength 12 μm. The calculations for both FIG. 4 and FIG. 5 were made for a square 50 μm×50 μm optical pixel which has a diffractive microlens optimized to focus at λ₀=8 μm light at a focal length of 50 μm in silicon (which has a refractive index of n=3.4). The light of 8 μm wavelength is sharply focused in the middle of the pixel (where the central detector is located) with a relative peak intensity of about 50 on the short band detector.

[0036] Referring now to FIG. 5, light of 12 μm wavelength has a relative peak intensity of only about 1 in the region of the central pixel, and most of the 12 μm or longer wavelength light falls onto the surrounding long band detector, which is generally much larger in surface area than the short band detector.

[0037] Calculations of the type shown in FIGS. 4 and 5 were made as a function of wavelength to graph the resulting light intensity shown in FIG. 6. Light intensity was integrated over a 5 μm×5 μm square central detector to generate the short band curve in FIG. 6. The long band curve in FIG. 6 is generated by subtracting the short band integral from 1. Good spectral separation is shown with this calculation.

[0038] It will be appreciated that the invention has been described hereabove with reference to certain examples or preferred embodiments as shown in the drawings. Various additions, deletions, changes and alterations may be made to the above-described embodiments and examples without departing from the intended spirit and scope of this invention. Accordingly, it is intended that all such additions, deletions, changes and alterations be included within the scope of the following claims.