Polaro–cryptic mirror of the lookdown as a biological model for open ocean camouflage

With no object to hide behind in 3D space, the open ocean represents a challenging environment for camouflage. Conventional strategies for reflective crypsis (e.g., standard mirror) are effective against axially symmetric radiance fields associated with high solar altitudes, yet ineffective against asymmetric polarized radiance fields associated with low solar inclinations. Here we identify a biological model for polaro–crypsis. We measured the surface-reflectance Mueller matrix of live open ocean fish (lookdown, Selene vomer) and seagrass-dwelling fish (pinfish, Lagodon rhomboides) using polarization-imaging and modeling polarization camouflage for the open ocean. Lookdowns occupy the minimization basin of our polarization-contrast space, while pinfish and standard mirror measurements exhibit higher contrast values than optimal. The lookdown reflective strategy achieves significant gains in polaro–crypsis (up to 80%) in comparison with nonpolarization sensitive strategies, such as a vertical mirror. Lookdowns achieve polaro–crypsis across solar altitudes by varying reflective properties (described by 16 Mueller matrix elements mij) with incident illumination. Lookdowns preserve reflected polarization aligned with principle axes (dorsal–ventral and anterior–posterior, m22 = 0.64), while randomizing incident polarization 45° from principle axes (m33 = –0.05). These reflectance properties allow lookdowns to reflect the uniform degree and angle of polarization associated with high-noon conditions due to alignment of the principle axes and the sun, and reflect a more complex polarization pattern at asymmetrical light fields associated with lower solar elevations. Our results suggest that polaro–cryptic strategies vary by habitat, and require context-specific depolarization and angle alteration for effective concealment in the complex open ocean environment

Polarization Imaging Sensors in Advanced Feature CMOS Technologies

The scaling of CMOS technology, as predicted by Moore\u27s law, has allowed for realization of high resolution imaging sensors and for the emergence of multi-mega-pixel imagers. Designing imaging sensors in advanced feature technologies poses many challenges especially since transistor models do not accurately portray their performance in these technologies. Furthermore, transistors fabricated in advanced feature technologies operate in a non-conventional mode known as velocity saturation. Traditionally, analog designers have been discouraged from designing circuits in this mode of operation due to the low gain properties in single transistor amplifiers. Nevertheless, velocity saturation will become even more prominent mode of operation as transistors continue to shrink and warrants careful design of circuits that can exploit this mode of operation. In this research endeavor, I have utilized velocity saturation mode of operation in order to realize low noise imaging sensors. These imaging sensors incorporate low noise analog circuits at the focal plane in order to improve the signal to noise ratio and are fabricated in 0.18 micron technology. Furthermore, I have explored nanofabrication techniques for realizing metallic nanowires acting as polarization filters. These nanoscopic metallic wires are deposited on the surface of the CMOS imaging sensor in order to add polarization sensitivity to the CMOS imaging sensor. This hybrid sensor will serve as a test bed for exploring the next generation of low noise and highly sensitive polarization imaging sensors

Increasing temporal, structural, and spectral resolution in images using exemplar-based priors

In the past decade, camera manufacturers have offered smaller form factors, smaller pixel sizes (leading to higher resolution images), and faster processing chips to increase the performance of consumer cameras. However, these conventional approaches have failed to capitalize on the spatio-temporal redundancy inherent in images, nor have they adequately provided a solution for finding $3$D point correspondences for cameras sampling different bands of the visible spectrum. In this thesis, we pose the following question---given the repetitious nature of image patches, and appropriate camera architectures, can statistical models be used to increase temporal, structural, or spectral resolution? While many techniques have been suggested to tackle individual aspects of this question, the proposed solutions either require prohibitively expensive hardware modifications and/or require overly simplistic assumptions about the geometry of the scene. We propose a two-stage solution to facilitate image reconstruction; 1) design a linear camera system that optically encodes scene information and 2) recover full scene information using prior models learned from statistics of natural images. By leveraging the tendency of small regions to repeat throughout an image or video, we are able to learn prior models from patches pulled from exemplar images. The quality of this approach will be demonstrated for two application domains, using low-speed video cameras for high-speed video acquisition and multi-spectral fusion using an array of cameras. We also investigate a conventional approach for finding 3D correspondence that enables a generalized assorted array of cameras to operate in multiple modalities, including multi-spectral, high dynamic range, and polarization imaging of dynamic scenes

Polarization Division Multiplexing for Optical Data Communications

Multiple parallel channels are ubiquitous in optical communications, with spatial division multiplexing (separate physical paths) and wavelength division multiplexing (separate optical wavelengths) being the most common forms. In this research work, we investigate the viability of polarization division multiplexing, the separation of distinct parallel optical communication channels through the polarization properties of light. We investigate polarization division multiplexing based optical communication systems in five distinct parts. In the first part of the work, we define a simulation model of two or more linearly polarized optical signals (at different polarization angles) that are transmitted through a common medium (e.g., air), filtered using aluminum nanowire optical filters fabricated on-chip, and received using individual silicon photodetectors (one per channel). The filter model is based upon an input optical signal formed as the sum of the Stokes vectors for each individual channel, transformed by the Mueller matrix that models the filter proper, resulting in an output optical signal that impinges on each photodiode. The simulation results show that two and three channel systems can operate with a fixed-threshold comparator in the receiver circuit, but four channel systems (and larger) will require channel coding of some form. The entire simulation model is designed in Cadence tools and the receiver (including optics) is compatible with standard CMOS fabrication processes. In the second part of the work, we design and manufacture a two channel chip that is used as the light receiver to confirm the simulation results from the first part of the research. Since logistics for the receiver’s chip testing were not favorable we constrained our testing to single channel operation, which we demonstrated functionality using both electrical and optical inputs. In addition, we used data from a pair of optical imagers (one linear and the second with a logarithmic response) to investigate the noise properties of both the optical and electrical signals within the system. In the third part of the work, we provide examples of channel coding that enable the four channel system to operate with positive noise margins. In the fourth part of the work, we define an end-to-end simulation model of two, three or four channel systems that utilize air, fiber, and a pair of mirrors in the optical path from transmitter to receiver. Each of these systems is shown to have positive noise margins (albeit using channel coding on the four channel editions); however, there are many circumstances where the noise margins are quite small. In the final part of the work, we examine the trade-offs between number of channels, signal power, and noise margins, including the use of pulse amplitude modulation within the two channel system