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Abstract
We designed and fabricated a flat multi-level diffractive lens (MDL) that is achromatic in the SWIR band (875 nm to 1675 nm). The MDL had a focal length of 25 mm, aperture diameter of 8.93 mm, and thickness of only 2.6 µm. By pairing the MDL with a SWIR image sensor, we also characterized its imaging performance in terms of the point-spread functions, modulation-transfer functions, and still and video imaging.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The primary driver for the utilization of SWIR cameras has been advancements in the image sensors. However, the conventional refractive lenses used in these systems can often be cumbersome due to their thickness, weight and presence of chromatic aberrations. Combining Diffractive optical elements (DOEs) with refractive lenses have been proposed for correcting chromatic aberrations for many years [1–3], and demonstrated with the advent of semiconductor lithography [4]. It is mistakenly thought that diffractive lenses are unable to correct chromatic aberrations on their own. On the contrary, we have demonstrated achromatic multi-level diffractive lenses (MDLs) in the visible, [5–7] in the LWIR, [8] and in the THz [9–10] bands with excellent imaging performance. Surprisingly, we also showed that MDLs could potentially enable imaging over a practically unlimited operating bandwidth [11].
In this paper, we experimentally demonstrate a single MDL with a diameter of 8.93mm designed to be achromatic in the short-wave infrared regime, λ=875nm to 1675nm with focal length = 25mm and NA = 0.17 (Fig. 1a). The MDL is comprised of concentric rings of equal width (3µm), but varying heights (between 0 and 2.6µm, and up to 100 discrete levels). The distribution of heights is selected by maximizing the wavelength averaged focusing efficiency, which is computed as the ratio of power inside 3 times the full-width at half-maximum (FWHM) in the focal plane to the total incident power [9].
2. Results and discussion
The dispersion of a positive-tone photoresist (S1813, Microchem) was also assumed [5]. The operating bandwidth was 875nm to 1675nm, which was dictated by the quantum efficiency of the InGaAs image sensor [12]. The 320CSX SWIR camera provided by Sensors Unlimited, Inc., a Collins Aerospace company, includes a 320 × 256-pixel InGaAs focal plane array with a 12.5µm pixel pitch, wide dynamic range and linear pixel response. As has been described previously [8], a gradient-assisted direct-binary-search algorithm was applied to perform the optimization of the MDL geometry. The optimized height distribution of the rings is shown in Fig. 1(b). The simulated point-spread functions (PSFs) are shown in Figs. 1(c)–(f) for λ=975nm, 1175nm, 1275nm and 1375nm, respectively. The PSFs were also measured by illuminating the MDL with the collimated beam from a super-continuum source (SuperK Extreme, NKT Photonics) equipped with a tunable filter. The image sensor was placed in the focal plane, while the illumination wavelength was changed. The captured images of the PSFs after dark frame subtraction are shown in Figs. 1(g)–(i). Note that our super-continuum source cuts of for wavelengths above ∼1375nm, so we were not able to characterize the PSFs for wavelengths longer than that.
Fig. 1. (a) Schematic of the SWIR MDL. (b) The optimized height profile with focal length = 25 mm and NA = 0.17. (c-f) simulated and (g-i) measured point-spread functions at the λ = 975 nm, 1175 nm, 1275 nm and 1375 nm, respectively.
The focusing efficiency was simulated as described earlier [7] and computed from the measured PSFs. The results are summarized in Fig. 2(a). The simulated and measured (based on curve fitting) average efficiency for λ=875nm to 1375nm were 91% and 35%, respectively. To investigate this large discrepancy between simulation and experiment, we simulated the impact of ring-width errors (Fig. 2b), ring-height errors (Fig. 2c) and a combination of the two (Fig. 2d). The analyses suggest that focusing efficiency is slightly more sensitive to ring-width error than the ring-height error. In practice, the fabrication likely has a combination of these two errors. And we hypothesize that a combination of errors (for example, 100nm of pixel-height error and 200nm of pixel-width error) can lead to focusing efficiency of 35%. These errors are expected in our current fabrication process. However, commercial semiconductor manufacturing can attain errors far smaller than these can and hence, we believe we can achieve efficiencies very close to the simulated values with a more robust fabrication process. The full-width at half-maxima (FWHM) of the focal spots were computed for each design wavelength (Fig. 2e). Finally, utilizing the simulated wavefront from the MDL, we computed the equivalent lens aberrations in terms of Zernike coefficients (Fig. 2f), confirming relatively low aberrations.
Fig. 2. (a) Simulated and measured focusing efficiency as a function of wavelength. A systematic study of the fabrication errors with relation to (b) standard deviation of the pixel height error, (c) pixel width error and (d) effect of both pixel width and standard deviation of the pixel height error. The effect of error in pixel width is more prominent among the two with the combined effect bringing down the error tolerance to ∼100 nm for ∼50% efficiency. (e) Full-width at half-maximum (FWHM) as a function of wavelength. (f) Simulated Zernike coefficients of the MDL at λ = 1275 nm.
As has been reported previously [5–7], the MDL was fabricated in a positive-tone photoresist (S1813) atop a glass substrate via grayscale lithography (MicroPG101, Heidelberg Instruments). Photographs and optical micrographs of the fabricated MDL are shown in Figs. 3(a)–(c). In order to characterize the imaging performance of the MDL, we imaged the Air Force resolution chart (Fig. 3d and see Visualization 1) and the Macbeth color chart (Fig. 3e and see Visualization 2). The illumination was via a halogen lamp. The distance between the MDL and the image sensor was ∼26mm; while the distance between the object and the MDL was ∼500mm. Conventional wiener deconvolution was applied to improve the quality of the images. The lines of group 1-4 corresponding to a spatial frequency of 2.83 line-pairs/mm (spatial period of 446µm) were resolved. An image of one of the authors taken using the camera is also shown in Fig. 3(f).
Fig. 3. (a) Photograph of the MDL (inset = optical micrograph). (b) Magnified optical micrograph of the central portion of the MDL. (c) Side view of the MDL. Images captured under ambient indoor lighting of (d) the Air Force resolution chart (see Visualization 1), (e) the Macbeth color chart (see Visualization 2) and (f) a human face. Wiener deconvolution was applied to improve the images.
Recently, metalenses have been touted as promising candidates for flat, lightweight lenses [13–16]. However, we showed that MDLs are far easier to fabricate and offer better imaging performance than metalenses [17]. In order to clarify this point further, we summarize the key characteristics of exemplary metalenses in the IR and compare these to our MDL in Table 1. There are two important points to note. Firstly, the bandwidth of our MDL (800nm) is higher than any metalens demonstrated in the IR so far. Secondly, our MDLs require a minimum feature width of 3µm, which is far easier to achieve compared to the ∼100nm feature widths required by metalenses. Furthermore, the MDL can be replicated inexpensively in polymers using imprint lithography, while metalenses typically require high-refractive-index materials.
Table 1. Summary of the reported work in comparison to previously reported thin flat metalenses.
3. Conclusion
In summary, we demonstrated good imaging performance using a single flat MDL in the wavelength range from 875nm to 1675nm (SWIR band) with diameter = 8.93mm, focal length = 25mm and NA = 0.17. The thickness of the active region of our MDL is only 2.6µm and consequently its weight can be negligible as well. Such flat MDLs can replace multiple lens systems and enable far simpler, lighter and less expensive SWIR cameras.
Funding
Office of Naval Research (N66001-10-1-4065).
Acknowledgements
We thank Brian Baker, Steve Pritchett and Christian Bach for fabrication advice, and Tom Tiwald (Woollam) for measuring dispersion of materials. The support and resources with the computing facilities from the Center for High Performance Computing at the University of Utah and Amazon AWS (051241749381) are gratefully acknowledged. We gratefully acknowledge Sensors Unlimited (Collins Aerospace) for lending us the InGaAs focal plane array.
Disclosures
RM is co-founder of Oblate Optics, Inc., which is commercializing technology discussed in this manuscript. The University of Utah has filed for patent protection for technology discussed in this manuscript.
References
1. M. P. Hansen and D. S. Malchow, “Overview of SWIR detectors, cameras and applications,” Proc. SPIE 6939, 69390I (2008). [CrossRef]
2. Y. G. Soskind, “Diffractive optics technologies in infrared systems,” In Infrared Technology and Applications XLI (Vol. 9451, p. 94511 T). International Society for Optics and Photonics.
3. G. G. Slusarev, “Optical Systems with Phase Layers,” Soviet Physics-Doclady, Vol. 2, No. 2, pp. 161-163 (1957). Reprinted in: Selected Papers on Holographic and Diffractive Lenses and Mirrors, T. W. Stone and B. J. Thompson eds., SPIE Milestone series, Vol. MS 34, (SPIE, 1991), pp. 412–414.
4. G. J. Swanson, “Binary optics technology: the theory and design of multilevel diffractive optical elements,” Technical Report 854 (MIT Lincoln Laboratory, 1989).
5. P. Wang, N. Mohammad, and R. Menon, “Chromatic-aberration-corrected diffractive lenses for ultra-broadband focusing,” Sci. Rep. 6(1), 21545 (2016). [CrossRef]
6. N. Mohammad, M. Meem, B. Shen, P. Wang, and R. Menon, “Broadband imaging with one planar diffractive lens,” Sci. Rep. 8(1), 2799 (2018). [CrossRef]
7. M. Meem, A. Majumder, and R. Menon, “Full-color video and still imaging using two flat lenses,” Opt. Express 26(21), 26866 (2018). [CrossRef]
8. M. Meem, S. Banerji, A. Majumder, F. G. Vasquez, B. Sensale-Rodriguez, and R. Menon, Broadband lightweight flat lenses for longwave-infrared imaging. arXiv: 1904.09011. (2019).
9. S. Banerji and B. Sensale-Rodriguez, “A Computational Design Framework for Efficient, Fabrication Error-Tolerant, Planar THz Diffractive Optical Elements,” Sci. Rep. 9(1), 5801 (2019). [CrossRef]
10. S. Banerji and B. Sensale-Rodriguez, “3D-printed diffractive terahertz optical elements through computational design,” In Micro-and Nanotechnology Sensors, Systems, and Applications XI (Vol. 10982, p. 109822X). International Society for Optics and Photonics (2019).
11. S. Banerji, M. Meem, A. Majumder, B. Sensale-Rodriguez, and R. Menon, “Imaging over an unlimited bandwidth with a single diffractive surface” arXiv: 1907.06251. (2019).
12. The quantum efficiency is available (labeled STD-SWIR) here: http://www.sensorsinc.com/images/uploads/documents/VIS_NIR_SWIR_FPA_TYPICAL_QE_CHART.pdf
13. S. Shrestha, A. Overvig, M. Lu, A. Stein, and N. Yu, “Broadband achromatic dielectric metalenses,” Light: Sci. Appl. 7(1), 85 (2018). [CrossRef]
14. M. Khorasaninejad, F. Aieta, P. Kanhaiya, M. A. Kats, P. Genevet, D. Rousso, and F. Capasso, “Achromatic metasurface lens at telecommunication wavelengths,” Nano Lett. 15(8), 5358–5362 (2015). [CrossRef]
15. S. Wang, P. C. Wu, V. C. Su, Y. C. Lai, C. H. Chu, J. W. Chen, S. H. Lu, J. Chen, B. Xu, C. H. Kuan, and T. Li, “Broadband achromatic optical metasurface devices,” Nat. Commun. 8(1), 187 (2017). [CrossRef]
16. J. Hu, C. H. Liu, X. Ren, L. J. Lauhon, and T. W. Odom, “Plasmonic lattice lenses for multiwavelength achromatic focusing,” ACS Nano 10(11), 10275–10282 (2016). [CrossRef]
17. S. Banerji, M. Meem, A. Majumder, F. G. Vasquez, B. Sensale-Rodriguez, and R. Menon, “Imaging with flat optics: metalenses or diffractive lenses?” Optica 6(6), 805 (2019). [CrossRef]
