Recent advances in 130 nm CMOS based Schottky barrier diode THz power detectors enable relatively simple, highperformance focal plane arrays. We present a low size, weight and power block camera which uses polymer refractive optics and a 6x6 focal plane array to image the return from an active source operating at 218 GHz. The operating frequency is chosen for multiple reasons: to coincide with atmospheric transmission windows, to image through degraded visual environments, and to leverage recently developed high power sources available at the Naval Research Laboratory. The sensor achieves better than 30 pW/√Hz NEP at video frame rates while lock-in detecting a modulated source. The three and a half pound camera houses a COTs aspheric polymer optic, detector array, signal amplification and lock-in detection, and outputs data over an Ethernet connection. We will present the camera design, performance metrics, and sample imagery
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.
Many components for free-space optical (FSO) communication systems have shrunken in size over the last decade. However, the steering systems have remained large and power hungry. Nonmechanical beam steering offers a path to reducing the size of these systems. Optical phased arrays can allow integrated beam steering elements. One of the most important aspects of an optical phased array technology is its scalability to a large number of elements. Silicon photonics can potentially offer this scalability using CMOS foundry techniques. A phased array that can steer in two dimensions using the thermo-optic effect is demonstrated. No wavelength tuning of the input laser is needed and the design allows a simple control system with only two inputs. A benchtop FSO link with the phased array in both transmit and receive mode is demonstrated.
Many components for free space optical communication systems have shrunken in size over the last decade. However, the steering systems have remained large and power hungry. Non-mechanical beam steering offers a path to reducing the size of these systems. Optical phased arrays can allow integrated beam steering elements. One of the most important aspects of an optical phased array technology is its scalability to a large number of elements. Silicon photonics can potentially offer this scalability using CMOS foundry techniques. In this paper a small-scale silicon photonic optical phased array is demonstrated for both the transmitter and receiver functions in a free space optical link. The device using an array of thermo-optically controlled waveguide phase shifters and demonstrates one-dimensional steering with a single control electrode. Transmission of a digitized video data stream over the link is shown.
Silicon photonics provides the ability to construct complex photonic circuits that act on the amplitude and phase of
multiple optical channels. Many applications of silicon photonics depend on maintenance of optical coherence among the
various waveguides and structures on the chip. Other applications can depend on the modal structures of the waveguides.
All these application require the ability to characterize the amplitude and phase of individual optical channels. Fourier
imaging with high numerical aperture microscope objectives has been used to image the intensity of individual channels
of photonic structures in both real and Fourier space. In other work, holographic imaging of multimode fibers has
allowed modal decomposition. In this work we use interferometric microscopy to image the amplitude and phase of a
variety of silicon photonic structures. These include a multimode interference splitter and a multimode waveguide under
various excitation conditions.
We report new materials that transmit from 0.9 to > 14 μm in wavelength and fill up the glass map for multispectral optics having refractive index from 2.38 to 3.17. They show a large spread in dispersion (Abbe number) and offer some unique solutions for multispectral optics designs. The new IR glasses can be easily molded and also fused together to make bonded doublets. We present the benefits of these new materials through dual-band optics designs and compare to designs using currently available crystalline materials.
Folded path reflection and catadioptric optics are of growing interest, especially in the long wave infrared (LWIR), due to continuing demands for reductions in imaging system size, weight and power (SWAP). We present the optical design and laboratory data for a 50 mm focal length low f/# folded-path compact LWIR imaging system. The optical design uses 4 concentric aspheric mirrors, each of which is described by annular aspheric functions well suited to the folded path design space. The 4 mirrors are diamond turned onto two thin air-spaced aluminum plates which can be manually focused onto the uncooled LWIR microbolometer array detector. Stray light analysis will be presented to show how specialized internal baffling can be used to reduce stray light propagation through the folded path optical train. The system achieves near diffraction limited performance across the FOV with a 15 mm long optical train and a 5 mm back focal distance. The completed system is small enough to reside within a 3 inch diameter ball gimbal.
We report new multispectral materials that transmit from 0.9 to < 12 µm in wavelength. These materials fill up the glass map for multispectral optics and vary in refractive index from 2.38 to 3.17. They show a large spread in dispersion (Abbe number) and offer some unique solutions for multispectral optics designs. One of the glasses developed is a very good candidate to replace Ge, as it has a combination of excellent properties, including high Abbe number in the LWIR, high index of 3.2, 60% lower dn/dT, and better thermal stability at working temperatures. Our results also provide a wider selection of optical materials to enable simpler achromat designs. For example, we have developed other glasses that have relatively high Abbe number in both the MWIR and LWIR regions, while our MILTRAN ceramic has low Abbe number in both regions. This makes for a very good combination of glasses and MILTRAN ceramic (analogous to crown and flint glasses in the visible) for MWIR + LWIR dual band imaging. We have designed preliminary optics for one such imager with f/2.5, 51 mm focal length and 22 degrees FOV using a spaced doublet of NRL's glass and MILTRAN ceramic. NRL's approach reduces the number of elements, weight, complexity and cost compared with the approach using traditional optics. Another important advantage of using NRL glasses in optics design is their negative or very low positive dn/dT, that makes it easier to athermalize the optical system.
This paper reviews recent progress in the design and fabrication of bio-inspired gradient index lenses. Inspired by the
gradient index distributions of the protein layers in biological eyes, we employ nested layers of polymer composites to
create smoothly-varying index distributions within bulk lens substrates. Because the fabrication technique allows for
independent control of the index layers, the index contours, and the final lens surfaces, optical power can be combined
with aberration control in a single element. Gradient-index singlets which correct for spherical aberration and singlets
which correct for chromatic aberration are described as examples of the utility of this class of optics.
The design, fabrication, and properties of one of a new class of gradient-index lenses are reported. The lens is an f/2.25 GRIN singlet based on a nanolayered polymer composite material, designed to correct for spherical aberration. The light
gathering and focusing properties of the polymer lens are compared to a homogeneous BK7 glass singlet with a similar
f-number. The modulation transfer function of the polymer GRIN lens exceeded that of the homogeneous glass lens at
all spatial frequencies and was as much as 3 times better at 5 cyc/mm. The weight of the polymer lens was
approximately an order of magnitude less than the homogeneous glass lens.
Prototype 2-, 3-, and 4- band long wave infrared (LWIR) focal plane arrays (FPA) for missile defense applications have recently been constructed to enhance target discrimination in space-based interceptor seekers. To address issues related to target identification such as algorithm choice and band number, this study created synthesized, optimized (using a genetic algorithm) image cubes (8- 12 mm) of targets and backgrounds compatible with expected mid-course defense scenarios and current multicolor sensors. Each candidate band was weighted using an interacting band edge model for 2-, 3- or 4- band sensors, consistent with a DRS multi-color HgCdTe LWIR FPA. Whitening the binned cubes and assigning red, green, blue colors directly to the whitened data set can prominently display and identify targets. Modified target signatures applied in matched filters searches and spectral angle maps autonomously searched for targets in the synthetic binned image cubes. Target discrimination diminished with decreasing target temperature and/or increasing distance between sensor and targets due to mixing subpixel target spectra with noise background. Spectral angle maps identified target temperatures and materials substantively better than the matched filter in this particular study. Target material and temperature identification improved by increasing number of bands, with greatest improvement for 3 bands relative to 2 bands. Extending detector sensitivity to 6-14 mm failed to improve target identification. This is the first study to systematically examine target identification in synthetic images cubes, consistent with missile defense scenarios and current multi-sensor technology.