We describe the fabrication and characterization of a compressive-sampling multispectral imaging (CS-MSI) system that uses single-pixel detectors to capture multiple spectral images concurrently, without mechanical scanning. In particular, we pay special attention to the hardware/software characterization of the CS-MSI system, wherein we discuss the optical realization of the proposed system, measure the effective sensitivity range, and investigate the relationship between the digital micromirror device frame rate and the reconstruction quality. We also compare the imaging performance of the Hadamard-pace variable-density sampling method, which is the CS method implemented with our hardware architecture, with the conventional random sampling method. We propose a compressive-sampling modulation transfer function (CS-MTF) to measure the amplitude response of different spatial frequencies by using different CS methods.
Passive imaging using millimeter waves (mmWs) has many advantages and applications in the defense and security
markets. All terrestrial bodies emit mmW radiation and these wavelengths are able to penetrate smoke, blowing dust or
sand, fog/clouds/marine layers, and even clothing. One primary obstacle to imaging in this spectrum is that longer
wavelengths require larger apertures to achieve the resolutions typically desired in surveillance applications. As a
result, lens-based focal plane systems tend to require large aperture optics, which severely limit the minimum
achievable volume and weight of such systems. To overcome this limitation, a distributed aperture detection scheme is
used in which the effective aperture size can be increased without the associated volumetric increase in imager size.
However, such systems typically require high frequency (~ 30 - 300 GHz) signal routing and down conversion as well
as large correlator banks. Herein, we describe an alternate approach to distributed aperture mmW imaging using optical
upconversion of the mmW signal onto an optical carrier. This conversion serves, in essence, to scale the mmW sparse
aperture array signals onto a complementary optical array. The optical side bands are subsequently stripped from the
optical carrier and optically recombined to provide a real-time snapshot of the mmW signal. In this paper, the design
tradeoffs of resolution, bandwidth, number of elements, and field of view inherent in this type of system will be
discussed. We also will present the performance of a 30 element distributed aperture proof of concept imaging system
operating at 35 GHz.
In this paper, we present novel designs for all optical analog-to-digital converters simulated and realized in
photonic crystal platforms. The designs presented were implemented on both photonic bandgap based
structures as well as self collimation based structures. Numerical simulation results as well as fabrication results
are also included. Characterization results validate the designs presented for a functional all optical two bit
analog to digital converters in photonic crystals. The design presented can be further scaled to higher resolution
conversion as well as to no optical frequencies if necessary.
Compressive imaging (CI) system is a novel electro-optical imaging system design, which uses a single-pixel photo
detector to capture two dimensional (2D) images. Instead of sampling the image directly by the sensor following the
classic Nyquist-Shannon sampling theorem, CI systems insert a measurement layer between the image formation and the
image recording media so that projection measurement matrices used to conduct compressive sampling can be
effectively introduced to the imaging process. The Digital Micromirror Device (DMD) can be used to implement the
projection measurement matrices. The imaging performance of a DMD based CI system relies more than just on the
imaging optics and the pixel size of the sensor. It also depends on the design of the measurement matrices and their
physical representations by the DMD. In the present work, we implemented three compressive sampling methods with
the DMD, namely the random basis under-sampling method, the random sampling method in the Hadamard space and
the variable density sampling method in the Hadamard space. We experimentally demonstrated that the design and
implementation of these methods have a direct impact on the imaging performance of the CI system. We tested the
system with different sampling ratios, DMD mirror configurations and imaging optics. Their influences on the
reconstructed image quality are demonstrated by experimental results. Lastly, we discussed the illumination issue of the
reconstructed image, which is not related to resolution, but is important for our visual perception of the reconstructed
image.
In this paper, a new approach for Confocal Microscopy (CM) based on the framework of compressive sensing is
developed. In the proposed approach, a point illumination and a random set of pinholes are used to eliminate
out-of-focus information at the detector. Furthermore, a Digital Micromirror Device (DMD) is used to efficiently
scan the 2D or 3D specimen but, unlike the conventional CM that uses CCD detectors, the measured data in
the proposed compressive confocal microscopy (CCM) emerge from random sets of pinhole illuminated pixels
in the specimen that are linearly combined (projected) and measured by a single photon detector. Compared
to conventional CM or programmable array microscopy (PAM), the number of measurements needed for nearly
perfect reconstruction in CCM is significantly reduced. Our experimental results are based on a testbed that uses
a Texas Instruments DMD (an array of 1024×768; 13.68×13.68 μm2 mirrors) for computing the linear projections
of illuminated pixels and a single photon detector is used to obtain the compressive sensing measurement. The
position of each element in the DMD is defined by the compressed sensing measurement matrices. Threedimensional
image reconstruction algorithms are developed that exploit the inter-slice spatial image correlation
as well as the correlation between different 2D slices. A comprehensive performance comparison between several
binary projection patterns is shown. Experimental and simulation results are provided to illustrate the features
of the proposed systems.
Millimeter-wave imaging is very interesting due to its unique transmission properties through a broad range of atmospheric obscurants such as cloud, dust, fog, sandstorms, and smoke, which thereby enables all-weather passive imaging. Unfortunately, the usefulness of millimeter-wave imagers is often limited by the large aperture sizes required to obtain images of sufficient resolution, as governed by the diffraction limit. To this end, we previously proposed a distributed aperture system for direct non-scan millimeter-wave imaging using an optical upconversion technique. In this proposed approach, an antenna array is employed to sample image signals in the millimeter-wave domain. The sampled millimeter-wave signals are then upconverted to the optical domain using electro-optic modulation techniques. These optical signals are mapped into a similar array on the entrance pupil of the following optical system for direct imaging. Although distributed aperture imaging is not new in both radio astronomy and conventional optical inteferometric imaging, the proposed approach is different in that it physically samples image in the millimeter-wave domain and directly forms the image in the optical domain. Therefore, specific analysis and evaluation techniques are required for the design and optimization of the proposed system. In this paper, we will address these issues, develop techniques to evaluate and enhance the system imaging performance and present methods to optimize the geometric configuration.
Millimeter wave (mmW) imaging is continually being researched for its applicability in all weather imaging. While
previous accounts of our imaging system utilized Q-band frequencies (33-50 GHz), we have implemented a system that
now achieves far-field imaging at W-band frequencies (75-110 GHz). Our mmW imaging approach is unique due to the
fact that optical upconversion is used as the method of detection. Optical modulators are not commercially available at
W-band frequencies; therefore, we have designed our own optical modulator that functions at this frequency range.
Imaging at higher frequencies increases our overall resolution two to three times over what was achieved at Q-band
frequencies with our system. Herein, we present imaging results obtained using this novel detector setup, as well as key
imager metrics that have been experimentally validated.
Millimeter-wave imaging has the unique potential to penetrate through poor weather and atmospheric conditions and
create a high-resolution image. In pursuit of this goal, we have implemented a far-field imaging system that is based on
optical upconversion techniques. Our imaging system is passive, in which all native blackbody radiation that is emitted
from the object being scanned is detected by a Cassegrain antenna on a rotating gimbal mount. The signal received by
the Cassegrain is passed to an optical modulator which transfers the radiation onto sidebands of a near-infrared optical
carrier frequency. The signal is then passed to a low-frequency photodetector that converts remaining sideband energy to
a photocurrent. Even though optical upconversion can produce loss, our system demonstrates low noise equivalent
powers (NEP) due to the low-noise of the photodetection process. Herein, we present our experimental results and
images obtained by using the far-field scanning system, which was assembled with commercially available components.
In addition, we detail efforts to increase the resolution of the image and to compact the imaging system as a whole.
In previous publications, we have described a novel technique for millimeter-wave detection based on optical
upconversion, carrier suppression, and photodetection. Using these techniques, we have been able to achieve NETD's as
low as 1 K /√Hz in both 35 GHz and 95 GHz atmospheric transmission windows. These results were obtained without
the use of millimeter-wave LNA's or cryogenic cooling, which have previously been requirements for reaching these
performance levels.
In this proceeding, we detail efforts to create a scanning single-pixel imager based on this detector technology. The
configuration developed uses a larger 60 cm aperture in a Cassegrain configuration, which is mounted on a gimbal for
far-field imaging. The described system has been used to collect data for perception experiments on the identification of
small watercraft and some of the imagery collected in that experiment is presented herein. In addition, we discuss
phenomenological observations noted during this data collection.
We design and characterize a photonic crystal (PhC) based silicon electro-optic modulator. The device is composed of a
planar photonic crystal with associated input and output dielectric waveguides and a p-i-n diode to inject free carriers for
index modulation. The photonic crystal, which confines light using the self-collimation phenomenon, has two regions of
varying air hole diameters forming a defect area in a host self-collimation lattice. At the interface of the defect with the
host lattice, an impedance mismatch is formed which is modulated using free carrier injection. With sufficient index
modulation the impedance mismatch is large enough to decrease the transmission through the defect region, thus,
modulation the overall transmission of the device. Our analysis shows that with a doping concentration in the range of
1020/cm3, the injected free carrier concentration can exceed 2.5*1019 with a drive voltage of 2.6 V. This free carrier
concentration is sufficient to modulate the refractive index, Δn, greater than .05, which in turn produces a modulation
depth greater than 75%. A fabricated device produces a modulation depth of 80% with a drive current of 4mA.
We present a reconfigurable, compact, low loss, optical switch in silicon. The device utilizes the self-collimation
properties of photonic crystal structures and provides a technique for efficiently switching an electromagnetic wave
guided through a pre-engineered dispersion based photonic crystal self-guiding structure. The electromagnetic wave can
be either in the microwave or optical regime based on the constituent materials and dimensions of the photonic crystal
host. We propose that the "loss tangent" of dielectric material in the switching region can be modified by external
"commands" to control the direction of propagation of the self-collimated signal and hence attain switching, thereby redirecting
the light. Based on the geometrical orientation and position of the applied electric field, electromagnetic waves
can be completely redirected (switched), or partially routed towards any arbitrary direction on a Manhattan grid or
network. We have found that the induced loss does not significantly attenuate the waves switched in any direction. The
structure presented can be generalized to an arbitrary N by M interconnected switching network or fabric, where the
switching topology can be dynamically modulated by the application of external fields. To attain switching, the free-carrier
absorption loss of Si is controlled by carrier injection from forward-biased PN junction. The concept device is
designed and analyzed using the FastFDTDTM accelerated hardware based FDTD technology.
We demonstrate methods to enhance electro-optical effect in silicon. In the first method, a tunable PhC device is
proposed to consist of the self-guiding region and the tunable region. The tunable lattice is designed such that it has a
band gap and the self-guiding frequency is located at its bottom band edge of the conduction band. Therefore, the device
output can be tuned by injecting free carriers into the tunable region to slightly reduce its effective index to pull up the
band gap. In the second method we design a self-guiding PhC cavity. Using this cavity, we could switch output light on
and off with an extinction ratio of 17.5 dB by changing only 1e-3 of the effective refractive index of the silicon
background. The third method utilizes a 12-fold symmetric quasi- photonic crytal cavity to enhance electro-optical
effect in silicon. The designed cavity supports whispering gallery modes and one of such modes is found to have Q
value of 2.3e4.
Silicon photonics is an area of active research and commercial interest due in part to its leveraging of the existing mature
fabrication processes and infrastructure of the CMOS integrated circuit industry. Its suitability for use at the telecom
wavelengths, low cost, and compact devices enhance the value of silicon for photonics applications. One critical issue
that continues to be investigated is the efficient coupling of optical signals between the outside world and the photonic
chip, which is hampered by the large optical mode mismatch between the glass fiber and high index contrast silicon
waveguide. We introduce a new device that enables efficient coupling from the fiber to single mode silicon waveguide
called the vertical J-coupler, so named in reference to its parabolic shape. Grayscale lithography is used to fabricate the
three-dimensional topology of the coupler, enabled by the high energy beam sensitive (HEBS) glass grayscale
photomask. The principle of operation is total internal reflection, which is inherently polarization insensitive and
broadband. Electro-magnetic simulations validate the efficient operation of the device while experimental results
demonstrate its successful operation in coupling light into the silicon waveguide.
In this paper, we present novel designs for all optical analog-to-digital converters simulated and realized in
photonic crystal platforms. The designs presented were implemented on both photonic bandgap based
structures as well as self collimation based structures. Numerical simulation results as well as fabrication results
are also included. Characterization results validate the designs presented for a functional all optical two bit
analog to digital converters in photonic crystals. The design presented can be further scaled to higher resolution
conversion as well as to no optical frequencies if necessary.
In this paper, we present novel designs for all optical analog-to-digital converters simulated and realized in photonic crystal platforms. The designs presented were implemented on both photonic bandgap based structures as well as self collimation based structures. Numerical simulation results as well as fabrication results are also included. Characterization results validate the designs presented for a functional all optical two bit analog to digital converters in photonic crystals. The design presented can be further scaled to higher resolution conversion as well as to no optical frequencies if necessary.
Under the DARPA COMP-I (Compressive Optical MONTAGE Photography Initiative) program, the goal of this project is to significantly reduce the volume and form factor of infrared imaging systems without loss of resolution. The approach taken is to use an array of small lenses with extremely short focal lengths rather than the conventional approach of a single aperture lens system with large diameter and focal length. The array of lenses creates multiple copies of the scene on a single focal plane detector array, which are then used to reconstruct an image with resolution comparable to or higher than that of the conventional imaging system. This is achieved by a computational method known as super-resolution reconstruction. Work at the University of Delaware towards this end includes participation in the design and optimization of the optical system along with fabrication of some of the optical elements. Grayscale lithography using a high-energy beam sensitive (HEBS) glass photomask and proportional dry etch pattern transfer are the key techniques enabling the fabrication process. In this paper we will discuss the design of the imaging system while focusing on the fabrication aspects of the project.
Photonic crystals have many potential applications due to their unique abilities to control the propagation of electromagnetic waves. If their bandgap and dispersive properties are modulated by external means, more exciting applications emerge. In this work, we present novel applications and devices created by tuning the bandgap and dispersive properties of periodic photonic crystal structures. We present our designs in both high- and low-refractive index materials. Many tunability alternatives exist including thermal, optical, electrical, microfluidic, and liquid crystals-based. In this paper, we will utilize these methods to implement various photonic crystal-based devices and applications.
Novels diffractive element in the THz waveband offers the potential to realize novel types of devices for communications, sensing, integrated optics, networks, transmission lines, and so on. To this end, diffractive planar elements fabricated on non-flat surfaces make it possible to enrich the "pool of devices" for applications including integrated optics at different waveband, including THz, and to design elements with novel properties and potentials. This can be illustrated most clearly using as an example optical element such as that for optical polychromatic computers. For instance, the diffractive element discussed above can be used as a nonlinear device for polychromatic radiation or multiplexer or a focusing element with selectivity in the multimode regime. Frequency characteristics for such elements are determined by the extent of concavity (convexity) of the surface of the element and by the direction of incidence wave onto it. Therefore, when working on a wavelength λ is not equal to λ0, the position of the focusing area in space (the amount of its displacement) and focusing properties should depend on the direction of incidence of the radiation. Hence, it is possible to distinguish between a signal incident on the "tip" of the element from that falling on its "base," simply by placing radiation receivers at the corresponding points in space. Thus, in this paper we will present detailed simulation results obtained using a parallel FDTD method and the application of the proposed device to focusing and frequency-selective properties of flat conical diffractive elements in THz waveband.
In this paper, we present the design, fabrication, and characterization of large-diameter semiconductor ring lasers with a single out coupling waveguide using AlGaAs/GaAs multiquantum well wafer. We also investigate the influence of the coupling between the ring cavity and the straight waveguide on the threshold current. It was found that the threshold current reduces with the decrease of the coupling between the ring cavity and the waveguide due to the widening of the coupling gap. By optimizing the coupling gap, we achieve a device with the threshold current of as low as 49mA.
We demonstrate nondefect mid-infrared waveguides based on the self-collimation effect in photonic crystals (PhCs). Due to the spatial dispersion properties serving to confine and route light, propagation loss as low as 2.56 dB/mm is obtained in a silicon-on-insulator (SOI)-based waveguide. The efficiency, together with their ability of arbitrary and structureless routing of electromagnetic beams, of these self-collimation devices provide a potential candidate for miniaturizing devices.
In this paper, we demonstrate the design and fabrication of a planar lens based on the dispersion property of a photonic crystal. When a photonic crystal is illuminated with a low frequency within its dispersion diagram it behaves very similar to an isotropic material, whose resultant index is kept a constant, and is determined by the ratio of high index material and low index material. To validate our design, we performed the experiment in millimeter regime, where the photonic crystal lens was fabricated using a CNC micro-milling machine, and a millimeter wave imaging system was built based on a vector network analyzer. For the lens, we have observed its ability to collimate an incident point source both in the amplitude and phase.
Using the special dispersion properties of photonic crystals (PhCs), we present a promising novel coupling device, the terahertz (THz) planar photonic crystal (PhC) lens. Three-dimensional finite-difference time-domain (3D-FDTD) calculations show a 90% power transfer from a 100 mm waveguide to a 10 mm waveguide, and experimental results confirm its high efficiency. Furthermore, the PhC lens couples the wave into a PhC line-defect waveguide is also reported. These achievements manifest the usefulness of the PhC lens as an effective approach to couple the wave into future THz circuits.
In this paper, a procedure for optimizing two-dimensional (2D) photonic crystals (PhC) is presented. In this procedure, the unit cell of a PhC structure is discretized into small grids and converted into a binary sequence. A direct binary search (DBS) method is then used to search through a terrain of possible solutions in order to find a more optimal one. This process is designed for improving the absolute band gap, opening a new one, for a predefined PBG structure. By applying this procedure on a honeycomb array of dielectric rods in air background, the maximum absolute gap-to-midgap ratio (MAGTMR) is increased to more than twice that of the initial structure. To further prove the validity of this procedure, this procedure is also applied to two best-found hexagonal and square lattice structures. The band gap improvements in these two cases indicate that besides structure type, structure symmetry, fill factor, index contrast, and size, shape and orientation of the constituent objects, there are other unknown factors, which affect the absolute band gap of a photonic crystal as well. The convergence property of this procedure is also discussed in this paper. The idea of this procedure can be applied to find the global optimal solution by using a global optimization algorithm, such as simulated annealing (SA), genetic algorithm (GA).
In this paper, a novel effective index method for modeling high index contrast planar photonic crystals is introduced. In this effective index method, the frequency axis is divided into many regions, and the effective index for each region is obtained using the central frequency of this region. Dispersion relationship in a wide frequency region can then be calculated using the effective index for each region to obtain the states in this region with a 2-D simulation and integrating all states in all frequency regions. The validity of this effective index method in approximating an intense three-dimensional (3-D) simulation required by the finite thickness of planar photonic crystals with a less intense two-dimensional simulation is examined. As an example, this method is applied to calculate the band diagram of a planar photonic crystal waveguide. By comparing with the results obtained with the full 3D simulation, we find that this method is valid for all states below the light cone in many bands, and the wider the waveguide, the more the number of bands, which agree with the full 3D results.
In this paper, we presented a new algorithm for the band structure calculation of photonic crystal slabs (PCS) based on the plane wave expansion method (PWEM) with perfectly matched layers (PMLs). The introduction of PMLs is used to truncate the computational region in the vertical direction. In addition, the effective medium tensor is applied to improve the convergence. By using PWM, a general complex eigenvalue problem is then obtained. Two criterions are presented to distinguish the guided modes from the PML modes. The presented scheme can accurately determine the band structure both above and below the light cone. The results obtained using this algorithm have been compared with those by using Finite-difference Time-Domain (FDTD) method and found to agree very well.
In this paper, we review the confinement mechanism of self-collimation in planar photonic crystals. In this mechanism, an approximately flat equi-frequency contour (EFC) below the light cone of the planar photonic crystal can be used to laterally confine the light and total internal reflection (TIR) provides vertical confinement. To this end, self-collimation in both low-index and high-index planar photonic crystals are investigated using the three-dimensional (3D) finite-difference time-domain (FDTD) method and the 3D iterative plane wave method (PWM). It is found that low-loss self-guiding is achievable in both the valence and conduction bands for high-index planar photonic crystals. However, for low-index planar photonic crystals, low-loss self-guiding can be only observed in the valence band. Experimental results show a propagation loss of as low as 1.1 dB/mm for the self-guiding in a high-index planar photonic crystals.
A novel implementation of a variable beam splitter using a photonic crystal (PhC) is proposed. The beam splitter consists of two periodic structures: a non-channel dispersion guiding region and a band gap based splitting structure. The dispersion guiding PhC structure is used to route the optical wave by exploiting the dispersion properties of the lattice. Arbitrary power ratio between output beams can be achieved by varying the parameters of the splitting structure. Within the studied range of splitting structures, high output power was observed and verified experimentally.
In this paper we discuss the design and implementation of integrated planar optical devices realized by exploiting the unique dispersion properties of photonic crystal (PhC) devices. In particular, we demonstrate the ability to focus and spatially route optical beams in the absence of channelized structures. By this we mean that these devices do not contain any form of lateral confinement, in the sense of a physical structure, other than the dispersion properties of the crystal lattice. To this end, lateral control is imposed on the propagating wave by virtue of engineering the band structure of the photonic crystal lattice. Our approach to this effort is based on engineering the dispersion diagram of a given periodic structure outside of its band gap. As such, this allows for the determination of unique propagation characteristics and corresponding devices, as we show in theoretical simulations and experimental results.
A terahertz-scale two-dimensional photonic-crystal waveguide based on a silicon-on-insulator was fabricated, and the optical transmission spectrum was measured. Terahertz beam propagation characteristics were observed using a thermal imaging camera, with incident light in the 10.1-10.7μm range. The measured transmission spectrum was in very good agreement with a three-dimensional finite-difference time-domain calculation.
A procedure for design binary circular subwavelength diffractive optical elements (DOE) is presented. In this procedure, we first generalized the design approach presented by Farn and extend Farn's techniques to include the design of binary circular subwavelength diffractive optical elements, we apply the zeroth-order effective medium theory in every subwavelength area to determine the relationship between index and the fill factor. An example is presented. The body of revolution finite difference time domain method is used to analyze responses of this DOE. The results are compared with the counterparts of its continuous profile.
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