Harnessing Spatial Intensity Fluctuations for Optical Imaging and Sensing

Properties of light such as amplitude and phase, temporal and spatial coherence, polarization, etc. are abundantly used for sensing and imaging. Regardless of the passive or active nature of the sensing method, optical intensity fluctuations are always present! While these fluctuations are usually regarded as noise, there are situations where one can harness the intensity fluctuations to enhance certain attributes of the sensing procedure. In this thesis, we developed different sensing methodologies that use statistical properties of optical fluctuations for gauging specific information. We examine this concept in the context of three different aspects of computational optical imaging and sensing. First, we study imposing specific statistical properties to the probing field to image or characterize certain properties of an object through a statistical analysis of the spatially integrated scattered intensity. This offers unique capabilities for imaging and sensing techniques operating in highly perturbed environments and low-light conditions. Next, we examine optical sensing in the presence of strong perturbations that preclude any controllable field modification. We demonstrate that inherent properties of diffused coherent fields and fluctuations of integrated intensity can be used to track objects hidden behind obscurants. Finally, we address situations where, due to coherent noise, image accuracy is severely degraded by intensity fluctuations. By taking advantage of the spatial coherence properties of optical fields, we show that this limitation can be effectively mitigated and that a significant improvement in the signal-to-noise ratio can be achieved even in one single-shot measurement. The findings included in this dissertation illustrate different circumstances where optical fluctuations can affect the efficacy of computational optical imaging and sensing. A broad range of applications, including biomedical imaging and remote sensing, could benefit from the new approaches to suppress, enhance, and exploit optical fluctuations, which are described in this dissertation

Quantum sensing

"Quantum sensing" describes the use of a quantum system, quantum properties or quantum phenomena to perform a measurement of a physical quantity. Historical examples of quantum sensors include magnetometers based on superconducting quantum interference devices and atomic vapors, or atomic clocks. More recently, quantum sensing has become a distinct and rapidly growing branch of research within the area of quantum science and technology, with the most common platforms being spin qubits, trapped ions and flux qubits. The field is expected to provide new opportunities - especially with regard to high sensitivity and precision - in applied physics and other areas of science. In this review, we provide an introduction to the basic principles, methods and concepts of quantum sensing from the viewpoint of the interested experimentalist.Comment: 45 pages, 13 figures. Submitted to Rev. Mod. Phy

Prospects for detecting gravitational waves at 5 Hz with ground-based detectors

We propose an upgrade to Advanced LIGO (aLIGO), named LIGO-LF, that focuses on improving the sensitivity in the 5-30 Hz low-frequency band, and we explore the upgrade's astrophysical applications. We present a comprehensive study of the detector's technical noises and show that with technologies currently under development, such as interferometrically sensed seismometers and balanced-homodyne readout, LIGO-LF can reach the fundamental limits set by quantum and thermal noises down to 5 Hz. These technologies are also directly applicable to the future generation of detectors. We go on to consider this upgrade's implications for the astrophysical output of an aLIGO-like detector. A single LIGO-LF can detect mergers of stellar-mass black holes (BHs) out to a redshift of z~6 and would be sensitive to intermediate-mass black holes up to 2000 M_\odot. The detection rate of merging BHs will increase by a factor of 18 compared to aLIGO. Additionally, for a given source the chirp mass and total mass can be constrained 2 times better than aLIGO and the effective spin 3-5 times better than aLIGO. Furthermore, LIGO-LF enables the localization of coalescing binary neutron stars with an uncertainty solid angle 10 times smaller than that of aLIGO at 30 Hz, and 4 times smaller when the entire signal is used. LIGO-LF also significantly enhances the probability of detecting other astrophysical phenomena including the tidal excitation of neutron star r-modes and the gravitational memory effects.Comment: 5 pages, 6 figures, published in PR

Development of a single beam SERF magnetometer using caesium atoms for medical applications

This thesis describes the design and implementation of a compact zero-field optically pumped magnetometer for human biomagetic measurements. This project aimed to achieve lower operating temperatures and a higher sensor bandwidth than current commercial rubidium-based equivalent sensors. Through careful selection of the sensing alkali, caesium, and all constituent components of the sensor package design, both of these aims are achieved. All of the required systems and components for a single-beam zero-field magnetometer are discussed, including a high efficiency cell heating and monitoring system, multi-axis field control and the optical detection scheme. Through full understanding and development of these systems, miniaturised and microfabricated versions are developed that facilitate the construction of a sensor package with external dimensions of 25 × 25 × 50 mm3. A number of machine learning tools are developed and applied to directly optimise the sensor’s sensitivity through control of the appropriate operational parameters, yielding a factor of five improvement. These techniques also enabled the investigation of the effect of nitrogen buffer gas pressure on the sensor’s measured sensitivity, demonstrating a linear increase in sensitivity with increasing pressure. The prototype sensor demonstrated a significant advancement in terms of bandwidth achieving a linear frequency response up to ' 900 Hz. The external package temperature of the sensor for prolonged timescales (> 1 hour) maintained a skin-safe temperature (< 41 ◦C), with a biomagnetic field level sensitivity, 90 fT/√ Hz, and compact package footprint, less than a square inch. A practical measurement of the magnetic field of a cardiac signal successfully demonstrates the sensor as a suitable biomagnetic measurement tool.This thesis describes the design and implementation of a compact zero-field optically pumped magnetometer for human biomagetic measurements. This project aimed to achieve lower operating temperatures and a higher sensor bandwidth than current commercial rubidium-based equivalent sensors. Through careful selection of the sensing alkali, caesium, and all constituent components of the sensor package design, both of these aims are achieved. All of the required systems and components for a single-beam zero-field magnetometer are discussed, including a high efficiency cell heating and monitoring system, multi-axis field control and the optical detection scheme. Through full understanding and development of these systems, miniaturised and microfabricated versions are developed that facilitate the construction of a sensor package with external dimensions of 25 × 25 × 50 mm3. A number of machine learning tools are developed and applied to directly optimise the sensor’s sensitivity through control of the appropriate operational parameters, yielding a factor of five improvement. These techniques also enabled the investigation of the effect of nitrogen buffer gas pressure on the sensor’s measured sensitivity, demonstrating a linear increase in sensitivity with increasing pressure. The prototype sensor demonstrated a significant advancement in terms of bandwidth achieving a linear frequency response up to ' 900 Hz. The external package temperature of the sensor for prolonged timescales (> 1 hour) maintained a skin-safe temperature (< 41 ◦C), with a biomagnetic field level sensitivity, 90 fT/√ Hz, and compact package footprint, less than a square inch. A practical measurement of the magnetic field of a cardiac signal successfully demonstrates the sensor as a suitable biomagnetic measurement tool

Magnetic Actuators and Suspension for Space Vibration Control

The research on microgravity vibration isolation performed at the University of Virginia is summarized. This research on microgravity vibration isolation was focused in three areas: (1) the development of new actuators for use in microgravity isolation; (2) the design of controllers for multiple-degree-of-freedom active isolation; and (3) the construction of a single-degree-of-freedom test rig with umbilicals. Described are the design and testing of a large stroke linear actuator; the conceptual design and analysis of a redundant coarse-fine six-degree-of-freedom actuator; an investigation of the control issues of active microgravity isolation; a methodology for the design of multiple-degree-of-freedom isolation control systems using modern control theory; and the design and testing of a single-degree-of-freedom test rig with umbilicals

Uncertainty law in ambient modal identification---Part II: Implication and field verification

This paper presents a qualitative analysis of the uncertainty laws for the modal parameters identified in a Bayesian approach using ambient vibration data, based on the theory developed in the companion paper. The uncertainty laws are also appraised using field test data. The paper intends to provide insights for planning ambient vibration tests and managing the uncertainties of the identified modal parameters. Some typical questions that shall be addressed are: to estimate the damping ratio to within 30% of posterior coefficient of variation (c.o.v), what is the minimum data duration? Will deploying an additional accelerometer significantly improve the accuracy in damping (or frequency)? Answers to these questions based on this work can be found in the Conclusions. As the Bayesian approach allows full use of information in the data for given modeling assumptions, the uncertainty laws obtained in this work represent the lower limit of uncertainty (estimation error) that can be achieved by any method (Bayesian or non-Bayesian)

Calibrating and improving the sensitivity of the LIGO detectors

The Laser Interferometer Gravitational wave Observatory (LIGO) is network of three, power recycled Fabry-Perot Michelson interferometers built to detect gravitational waves from astrophysical sources at frequencies between 40 and 6000 Hz. For their fifth science run, from 2005 to 2007, the detectors observed at designed sensitivity, achieving equivalent strain amplitude noise of 3x10^−23 strain/rtHz at 100 Hz. To date, the observatory has not detected gravitational waves. However, even at such sensitivity, the expected detection rate for known astrophysical sources of gravitational waves is likely 0.02 yr^−1. The fundamental noise source of these ground-based detectors limiting the sensitivity below 40 Hz is seismic motion. They use multi-stage passive isolation platforms from which their test masses are suspended from piano wire as single pendula providing isolation from ground motion. The residual test mass motion is controlled by electromagnetic actuators on the suspension system in response to the output of the interferometers, keeping them at their operating point. In the first portion of this thesis, I discuss the absolute calibration of the first generation of LIGO interferometer\u27s gravitational wave readout during their fifth science run, the uncertainty of which is limited by the precision to which we can measure the control system above residual seismic noise. A second generation of detectors, called Advanced LIGO, is currently under construction which will completely replace the first generation. Scheduled to become operational in 2014, they are predicted to improve the sensitivity by ten-fold or more, and will likely improve the detection rate to as much as 40 yr^−1. To achieve this sensitivity at the lower limit of the band, the test masses will be suspended from from multiple cascading pendula. In addition, the multi-stage passive isolation platforms will be replaced with single- and double-stage suspended platforms with built-in active feedback control systems. Prototypes of single-stage active control systems have been in use for two years for a non-invasive upgrade of the LIGO interferometers. In the second portion of this thesis, I present results from these prototypes and demonstrate that their performance can meet the stringent requirement of the second generation of interferometers

Advanced Gravitational Wave Detectors and Detection: Arm Length Stabilization and Directed Searches for Isolated Neutron Stars.

The equations of General Relativity admit wave solutions, known as gravitational waves. Gravitational waves have been indirectly detected through observations of the decay of binary neutron star orbits, but have yet to be observed directly. Advanced LIGO aims to make the first direct detection of gravitational waves using a network of two interferometric gravitational wave detectors. Observations of gravitational waves would not only verify an important prediction of general relativity, but also provide information about some of the most extreme environments in the universe, such as supernovae, black holes, and neutron stars. This thesis covers issues related to both the operation of the Advanced LIGO interferometer and its potential use for neutron star multimessenger astronomy. Principal results include a method for dynamic characterization of long Fabry-Perot optical cavities, the implementation of an auxiliary differential wavefront sensing subsystem for Advanced LIGO arm locking, and the development of a search method for gravitational waves from unassociated gamma-ray emitters in the Fermi 3FGL catalog.PHDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/120826/1/jrsandrs_1.pd