For two decades, extraordinary optical transmission has amplified exploration into subwavelength systems. Researchers have previously suggested exploiting the spectrally selective electromagnetic field confinement of subwavelength slits for multispectral detectors. Utilizing the finite-difference frequency-domain method, we examine electromagnetic field confinement in both two-dimensional and three-dimensional scenarios from 2.5 to 6 μm, i.e., midwave infrared. We explore the trade space of deep subwavelength slits and its impact on resonant enhancement of the electromagnetic field. This builds a fundamental understanding of the coupling mechanisms, allowing for prediction of resonant spectral behavior based on slit geometry and material properties.
We have developed a multi-spectral SWIR lidar system capable of measuring simultaneous spatial-spectral information for imaging and spectral discrimination through partial obscurations. Our system utilizes a supercontinuum laser source and eight narrowband spectral channels in the 1000 nm to 1600 nm region. The system employs a steering mirror, which enables us to scan the region of interest and collect spectral and spatial data as a point-by-point scan. The system is designed to detect weak signal returns in the few-photon regime. The technique promises more capable classification and target detection of spectrally diverse targets in obscured environments with potential applications for mapping of ground type through forest canopy, pollution monitoring of water ways, and intelligence, surveillance, reconnaissance and target detection (ISRT). Custom targets designed to provide distinct spectral response are employed to ascertain the system’s response. The lidar system is calibrated by measuring the return signal from a highly reflective flat Spectralon target; this enables us to determine the reflectivity of the objects of interest. Spectral response of the targets are analyzed and their estimated reflectivity is reported. The same targets are studied in the presence of two partial obscurants. The objects are easily identified even though the return signal is attenuated by a factor of seven. The general spectral shape of the targets are preserved in the presence of the obscurants. More challenging objects and environments and various methods to recover the spectral response of the objects are currently being pursued.
For two decades, extraordinary optical transmission (EOT) has amplified exploration into subwavelength systems. Researchers have previously suggested exploiting the spectrally selective electromagnetic field confinement of subwavelength slits for multispectral detectors. Utilizing the finite-difference frequency domain (FDFD) method, we examine electromagnetic field confinement in both 2-dimensional and 3-dimensional scenarios from 2.5 to 6 microns, i.e. midwave infrared (MWIR). We explore the trade space of deep subwavelength slits and its impact on resonant enhancement of the electromagnetic field. This builds fundamental understanding of the coupling mechanisms allowing for prediction of resonant spectral behavior based on slit geometry and material properties.
Here we report progress in the fabrication, calibration, and testing of a compact spectral imaging camera. The camera uses a micro-array of Fabry-Perot etalons bonded directly to a broadband focal plane array sensor. The array of etalons adds negligible size and weight to the system compared to a panchromatic imager. Other recent demonstrations of compact spectral imagers in the visible and near infrared (VNIR) have commonly used arrays of etalons in a single order, thereby reducing the system bandwidth and sensitivity to achieve the required spectral resolution. Here, we demonstrate a camera utilizing multiple etalon orders in a spectral multiplexing scheme known as Multiple Order Staircase Etalon Spectrometry (MOSES). An important characteristic of the MOSES system is that there is a relaxed tradeoff between spectral resolution and sensitivity (or etalon throughput). Unlike single-order etalon techniques, MOSES allows for the reconstruction of the spectrum to the bandwidth limit of the detector and reflecting layers. This is important in coastal environmental sensing, where IR spectral features may be desired at the same time as UV light transmitted through shallow water layers. This VNIR system demonstration indicates the feasibility of MOSES devices in other wavebands.
We report an approximate simplified calculation of transmission spectra for a telecentric cone of light impinging on a Fabry-Perot interferometer. We model sulfur dioxide sensing and show that the F-number affects the optimum parameters. OCIS codes: (120.2230) Fabry-Perot; (280.1120) Air pollution monitoring
Wedge or staircase micro-optics have become important components for building miniature optical spectrometers. These devices create spectral discrimination through interference between beams resulting from reflections at the surfaces of the optic. The literature has examples of low reflectance wedge spectrometer system where the Fourier transform is used to recover the spectrum (with no inherent bandwidth limit), and high-reflectance, band-limited simplex spectrometers where no data processing is required. Instruments in the first category tend to be for the thermal infrared range, and instruments in the second category are more often encountered in the visible band. This second category includes linear variable filters and discrete etalon staircases. Though in practice, the signal treatment for these two types of spectrometers is radically different, the underlying interference mechanism is identical. It follows, that a single signal processing algorithm must exist which correctly treats the two types of signals. We present a mathematical description of the signal model for such spectrometers. We show that in the case of spectrally uniform reflectance, the signal has a specific relationship to the spectrum’s Fourier transform. We cast the spectral recovery problem as a matrix inversion, and derive formulas for calculating the solution matrix. The solution matrix is shown to yield the exact spectrum when applied to modeled wedge spectrometer signals in both low and high reflectance cases.
At the Naval Research Laboratory Optical Sciences and Remote Sensing Divisions, a compact hyperspectral imaging sensor has been in development using a method of multi-order spectroscopy using Fabry-Perot etalon arrays. This has allowed for the first broadband, ultra-compact spectrometer. A prototype hyperspectral imaging system is now in development for use with an unmanned aerial vehicle. This system will be a “push-broom system” that scans ground data line by line in a row of pixels forming the hyperspectral datacube and will be georeferenced onto a Digital Surface Model of the ground with location (latitude, longitude, altitude) and attitude (azimuth, pitch, roll) GPS-INS 6-degree of freedom parameters. These parameters will be collected through the use of a high-end modern smartphone with its GPS, accelerometer, gyroscope, barometric pressure, and digital compass sensors. In this paper, we discuss the various sensors and systems being utilized with a smartphone for use with the hyperspectral imaging sensor in development.
A 16-band plenoptic camera allows for the rapid exchange of filter sets via a 4x4 filter array on the lens's front aperture. This ability to change out filters allows for an operator to quickly adapt to different locales or threat intelligence. Typically, such a system incorporates a default set of 16 equally spaced at-topped filters. Knowing the operating theater or the likely targets of interest it becomes advantageous to tune the filters. We propose using a modified beta distribution to parameterize the different possible filters and differential evolution (DE) to search over the space of possible filter designs. The modified beta distribution allows us to jointly optimize the width, taper and wavelength center of each single- or multi-pass filter in the set over a number of evolutionary steps. Further, by constraining the function parameters we can develop solutions which are not just theoretical but manufacturable. We examine two independent tasks: general spectral sensing and target detection. In the general spectral sensing task we utilize the theory of compressive sensing (CS) and find filters that generate codings which minimize the CS reconstruction error based on a fixed spectral dictionary of endmembers. For the target detection task and a set of known targets, we train the filters to optimize the separation of the background and target signature. We compare our results to the default 16 at-topped non-overlapping filter set which comes with the plenoptic camera and full hyperspectral resolution data which was previously acquired.
Modern nano-metrology instruments require calibration references with nanometer accuracy in the x, y, and z directions.
A common problem is the accurate calibration in the z direction (height). For example, it is generally not difficult to
obtain accurate x- and y- calibration references for a Scanning Probe Microscope (SPM). It is, however, much more
difficult to obtain accurate z-axis results. It is difficult to control z-axis piezo dynamics because during scanning in the
xy-plane the x-and y-axes move at a constant rate whiles the z axis does not. Furthermore due to the high cost of
producing calibration standards, the microscope is often calibrated at only one height. However, if the relationship
between the measured z height and the actual z height is not linear, then the height measurements will not be correct. In
this paper, we will present a method for the fabrication of calibration references with: (i) sub-10 nm features and (ii)
multiple step heights on one reference, allowing for better calibration of the non-linearity in the z direction.
Hyperspectral instruments with physical sizes comparable to that of a bare sensing array are now possible. Compact Fabry-Perot (FP) etalon arrays allow for different spectral sensitivities to be assigned to the different pixels of a sensing array. Such arrays for hyperspectral imaging are commercially available but underutilized due to cost and performance tradeoffs. FP arrays were first made possible by binary or logarithmic fabrication, which reduces the number of lithography steps to log2(k) where k is the number of distinct levels of material required for the integrated optical element. However, significant yield loss results from the several lithography steps required in this process. We introduce a new binary etching technique that allows for the creation of an arbitrary number of distinct levels with a single greyscale lithography technique. Our technique has been used for the fabrication of distinct levels of 1 nm rms flatness with a controlled 10 nm resolution. This technique has been used to fabricate a staircase structure with greater than 100 distinct steps directly on a COTS optical imager. Details of the fabrication technique and characteristics of the optical element will be presented.
Short wave infrared (SWIR) sensors are becoming more common in DoD imaging systems because of their haze penetration capabilities and spectral properties of materials in this waveband. Typical SWIR systems have provided either full motion video (FMV) with framing panchromatic systems or multi-spectral or hyperspectral imagery with line-scanning systems. The system described here bridges these modalities, providing FMV with nine discrete spectral bands. Nine pixel sized SWIR filters are arranged in a repeating 3x3 pattern and mounted on top of a COTS, 2D staring focal plane array (FPA). We characterize the spectral response of the filter and integrated sensor. Spot-scan measurements and data collected with this camera using narrow band sources reveals crosstalk induced nonlinearity in the sensor response. We demonstrate a simple approach to reduce the impact of this nonlinearity on collected imagery.
Short wave infrared (SWIR) spectral imaging systems are vital for Intelligence, Surveillance, and Reconnaissance (ISR)
applications because of their abilities to autonomously detect targets and classify materials. Typically the spectral
imagers are incapable of providing Full Motion Video (FMV) because of their reliance on line scanning. We enable
FMV capability for a SWIR multi-spectral camera by creating a repeating pattern of 3x3 spectral filters on a staring focal
plane array (FPA). In this paper we present the imagery from an FMV SWIR camera with nine discrete bands and
discuss image processing algorithms necessary for its operation. The main task of image processing in this case is
demosaicking of the spectral bands i.e. reconstructing full spectral images with original FPA resolution from spatially
subsampled and incomplete spectral data acquired with the choice of filter array pattern. To the best of author's
knowledge, the demosaicking algorithms for nine or more equally sampled bands have not been reported before.
Moreover all existing algorithms developed for demosaicking visible color filter arrays with less than nine colors assume
either certain relationship between the visible colors, which are not valid for SWIR imaging, or presence of one color
band with higher sampling rate compared to the rest of the bands, which does not conform to our spectral filter pattern.
We will discuss and present results for two novel approaches to demosaicking: interpolation using multi-band edge
information and application of multi-frame super-resolution to a single frame resolution enhancement of multi-spectral
spatially multiplexed images.
Traditional Fabry-Perot (FP) spectroscopy is bandwidth limited to avoid mixing signals from different transmission
orders of the interferometer. Unlike Fourier transformation, the extraction of spectra from multiple-order interferograms
resulting from multiplexed optical signals is in general an ill-posed problem. Using a Fourier transform approach, we
derive a generalized Nyquist limit appropriate to signal recovery from FP interferograms. This result is used to derive a
set of design rules giving the usable wavelength range and spectral resolution of FP interferometers or etalon arrays
given a set of accessible physical parameters. Numerical simulations verify the utility of these design rules for moderate
resolution spectroscopy with bandwidths limited by the detector spectral response. Stable and accurate spectral recovery
over more than one octave is accomplished by simple matrix multiplication of the interferogram. In analogy to recently
developed single-order micro-etalon arrays (Proc. of SPIE v.8266, no. 82660Q), we introduce Multiple-Order Staircase
Etalon Spectroscopy (MOSES), in which micro-arrays of multiple order etalons can be bonded to or co-fabricated with a
sensor array. MOSES enables broader bandwidth multispectral and hyperspectral instruments than single-order etalon
arrays while keeping a physical footprint insignificantly different from that of the detection array.
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