Browsing by Subject "Waveguides"
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Item Design and Fabrication of Integrated Optical Waveguides and Sidewall Bragg Gratings(2014-11-20) Wang, XinIn this dissertation, a novel design platform with arsenic tri-sulfide (As2S3) on titanium-diffused lithium niobate substrate (Ti:LiNbO3) is introduced to provide physical foundation for integrated optical device applications. LiNbO3 possesses excellent birefringence, electro-optical and acousto-optical effects that enable its high efficiency in nonlinear parametric frequency conversions and flexible tuning capabilities. Secondly, high-quality, low-loss channel waveguide can be made by thin-film metal diffusion or proton exchange with high reproducibility. The mode area size of the channel waveguide is close to single-mode fiber, leading to negligible coupling loss. As2S3 has a large index that provides strong mode confinement and tight bending radii for high integration densities. Both materials exhibit broad transparency: 0.4-5.0 ?m for LiNbO3 and 0.63-11.0 ?m for As2S3, making it possible to extend their applications to mid-infrared (3-20 ?m) regime. On this design platform, a hybrid waveguide structure is optimized for efficient mid-infrared radiation at 4.0-4.9 ?m by phase-matched difference frequency generation (DFG). The hybrid waveguide is designed for single mode operation. A normalized power conversion efficiency of 20.52%W^-1cm^-2 is theoretically predicted on a 1 mm-long waveguide pumped at 50 mW, which is the highest efficiency record for LiNbO3. Using a tunable pump at 1.38-1.47 ?m or signal at 1.95-2.15 ?m, a tuning range at 4.0-4.9 ?m is achieved. Such hybrid optical waveguides are feasible for mid-infrared emission with mW powers and sub-nanometer linewidths. Besides, sidewall Bragg gratings in As2S3-Ti:LiNbO3 waveguides are fabricated by electron beam lithography and metal liftoff process. Spectrum measurements are in good agreement with numerical fittings. The measured rejection bandwidth is at 2.4-6.7 nm. Coupling coefficients ranging from 2.5 mm^-1 to 8.9 mm^-1 are obtained by altering the grating depth. A transmission peak with a 3-dB bandwidth of ~0.25 nm is observed on a 432 ?m -long phase-shifted grating. Such integrated sidewall gratings are useful for various optical devices including optical filters, switches, modulators, lasers, sensors, and wavelength division multiplexing (WDM). In addition, optical refractive index sensors are designed with phase-shifted sidewall gratings in slot waveguide based on silicon-on-insulator (SOI) platform. The designed optical sensors have a minimum detection limit on the order of 10-6, a linear response and a compact device dimension as small as 11.7 ?m offering the capabilities for optical sensor array deployment and lab-on-a-chip integration.Item Green's function methods in 1D nanoscale electron waveguides(2014-12) Corse, William Zachary; Reichl, Linda E.R-matrix theory has been used to analyze a variety of scattering potentials in ballistic electron waveguides. The S-matrix is the principal result of this method. Here we analyze ballistic electron scattering in a 1D waveguide with a step potential at its terminus using Green’s function theory. We calculate the S-matrix for this system, scattering particles’ quasibound states, and the survival probability of a particle initially localized in the step region. We then apply R-matrix theory to the same problem. In doing so, we demonstrate the versatility of the Green’s function approach, but also its relative complexity.Item Investigation of ducts as a “radar pinhole” for detecting objects through a wall(2009-08) Whitelonis, Nicholas John; Ling, Hao; Alu, AndreaThere is a continuing interest in the through-the-wall capabilities of radar. It has been found that walls behave as a low-pass medium, and therefore through-the-wall radar has been restricted to frequencies in the low GHz range. Unfortunately at these lower frequencies the resolution of the radar system is sacrificed. This thesis investigates the possibility of using a duct as a means of detecting objects through a wall. Ducts have been extensively studied in the past; however there has been limited research of ducts with two open ends. In this thesis the difference between an open-ended duct and a duct with two open ends is investigated through measurement and simulation. For simulation an approximate method is used that treats the duct as a waveguide. It is found that a significant amount of power is transmitted through a duct with two open ends. It is then shown that an object can be detected through a wall by using a duct that has been inserted into the wall. Then the two-way insertion loss of a duct with two open ends is determined through measurement and simulation. It is shown that a duct behaves as a high-pass medium and can be used as a propagation channel through a wall. The insertion loss due to the duct and the insertion loss through a concrete wall are comparedItem Investigation of high-frequency propagation channels through pipes and ducts for building interior reconnaissance(2012-05) Whitelonis, Nicholas John, 1984-; Ling, Hao; Neikirk, Dean; Wilson, Preston; Powers, Edward; Alu, AndreaRecently, there is strong interest in the through-wall sensing capabilities of radar for use in law enforcement, search and rescue, and urban military operations. Due to the high attenuation of walls, through-wall radar typically operates in the low GHz frequency region, where resolution is limited. It is worthwhile to explore other means of propagating radar waves into and back out of a building’s interior for sensing applications. One possibility is through duct-like structures that are commonly found in a building, such as metal pipes used for plumbing or air conditioning ducts. The objective of this dissertation is to investigate techniques to acquire radar images of targets through a pipe. First, using the pipe as an electromagnetic propagation channel is studied. A modal approach previously developed for computing the radar cross-section of a circular duct is modified to compute the transmission through a pipe. This modal approach for transmission is validated against measured data. It is also shown that a pipe is a high-pass propagation channel. The modal analysis is then extended to two-way, through-pipe propagation for backscattering analysis. The backscattering from a target is observed through a pipe in simulation and measurement. Next, methods to form two-dimensional radar images from backscattering data collected through a pipe are explored. Four different methods previously developed for free-space imaging are applied to the problem of imaging through a pipe: beamforming, matched filter processing, MUSIC, and compressed sensing. In all four methods it is necessary to take into account the propagation through the pipe in order to properly generate a focused radar image. Each method is demonstrated using simulation and validated against measurement data. The beamforming and matched filter methods are found to suffer from poor cross-range resolution. To improve resolution, the MUSIC algorithm is applied and shown to give superior resolution at the expense of more complicated data collection. The final method, compressed sensing, is shown to achieve good cross-range resolution with simpler data collection. A comparison of the tradeoffs between the four methods is summarized and discussed. Two additional extensions are studied. First, a method for computing the transmission through an arbitrary pipe network using the generalized scattering matrix approach is proposed and implemented. Second, a new method for computing joint time-frequency distributions based on compressed sensing is applied to analyze the backscattering phenomenology from a pipe.Item Micromachined Optical and Acoustic Waveguide Systems for Advance Sensing and Imaging Applications(2014-07-08) Chang, Cheng-ChungEvolving from the IC fabrication processes, micromachining technologies allow mass production of 2D or 3D microstructures, which are otherwise difficult to achieve with traditional machining techniques. In this research, novel micromachining processes have been developed to enable new micro optical and acoustic waveguide systems for advanced optical sensing and acoustic imaging applications. The investigated applications include non-invasive cancer detection inside human body, in-field soil characterization, and time-delayed and multiplexed ultrasound and photoacoustic tomography. Micromachining technology enables miniaturized optical waveguide system for efficient light transmission. The small size and light-guiding capabilities are particularly useful for optical sensing at places deep inside the human body or underground. Two micromachined optical waveguide systems were fabricated and tested. The first one was used to conduct oblique incidence diffuse reflectance spectroscopy (OIDRS) for the determination of tumor margins on human pancreas specimens. The second one was used to conduct visible-near-infrared diffuse reflectance spectroscopy (VNIR-DRS) for extracting the compositional information of soil samples. Micromachining technology also makes it possible to utilize single-crystalline silicon as a structural material for acoustic wave propagation. It enables the development of high-performance integrated acoustic circuits and allows direct acoustic signal processing and control. The acoustic properties and propagation inside silicon waveguides were characterized, and the acoustic signal processing using micromachined acoustic waveguide system was investigated. Based on the results, two acoustic waveguide systems were designed and constructed. The first system utilized micromachined acoustic delay lines to passively delay acoustic signal thereby reducing the required transceivers and processing electronics; while the second system employed micromachined acoustic multiplexer to actively control the transmission of acoustic signals. Both techniques are expected to provide new solutions to reduce the complexity and cost of the acoustic receiver systems in ultrasound and photoacoustic imaging.Item A modern representation of the flow of electromagnetic power and energy using the Poynting's vector and a generalized Poynting's theorem(2011-05) Hsu, Hsin I; Driga, Mircea; Grady, Mack W.A comprehensive and rigorous description of instantaneous balance of electromagnetic power defined as the derivative of energy with respect to time is offered by the Poynting's theorem. Such theorem is expressed as the sum of a series of volume integrals representing the volume densities of densities of different components of electromagnetic power and the power flow through the general surface surrounding the entire domain in which the Poynting's vector expresses the instantaneous power leaving the domain (the positive normal is the outward normal to the enclosing surface). The original feature of the present approach is the introduction in the electromagnetic power balance and conservation of the electromechanical energy conversion by the use of the flux derivatives of the fields [D with vector arrow] and [B with vector arrow]. For the moving points (rotors) involved in electromechanical energy conversion, the surface of integration is driven together with them and [permittivity] and [permeatility] remain substantially constant--(a point in movement maintains its properties as [formula]). Then the balance of energy (and power) can be written at each infinitesimal time interval for the electromagnetic energy in which case the elementary mechanical work is produced by mechanical forces of electromagnetic origin. The thermal energy accounts for the Joule (and hysteresis) losses in the system. A treatment of the flow of electromagnetic energy is given for a complete of illustrative relationship in time and frequency domain.Item Towards two dimensional optical beam steering with silicon nanomembrane-based optical phased arrays(2013-08) Kwong, David Nien; Chen, Ray T.Silicon based on-chip optical phased arrays are an enabling technology to achieving agile and compact large angle beam steering. In this work, a single layer array is presented, and approaches to multilayer 3D photonic integration for achieving a 2D array are also discussed. Finally, two dimensional optical beam steering is achieved using both thermo-optic and wavelength tuning. Various structures are considered as an alternative to the conventionally used shallow etched surface gratings to achieve narrow beam widths in the far field along with low switching power. The corrugated waveguide interspersed with 2D photonic crystal for crosstalk suppression is presented as a novel structure for coupling to free space that can provide lithographically defined index contrast in a single fabrication step, along with the smallest beam widths presented to date, at 0.25°. In addition, a polysilicon overlay with an oxide etch stop layer on top of a silicon waveguide is also presented as a grating coupler that achieves narrow far field beam widths. With this structure, two dimensional steering of 20° X 15° is demonstrated with a 16 element optical phased array, with a beam width of 1.2° X 0.4° and maximum power consumption of 20mW per channel.