Doctor of Philosophy (PhD)
Physics and Astronomy
One hundred years after Albert Einstein predicted the existence of gravitational waves in his general theory of relativity, the Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first direct detection of gravitational waves. Since the first detection of gravitational waves from a binary black hole merger, LIGO has gone on to detect gravitational waves from multiple binary black hole mergers, and more recently from a binary neutron star merger in collaboration with telescopes around the world. The detection of gravitational waves has opened a new window to the universe and has launched the era of gravitational wave astronomy.
With the first detection of gravitational waves now two years behind us, work has already begun on improving the sensitivity of Advanced LIGO and planning for future generations of gravitational wave interferometers. One of the main limiting noise sources for current and future gravitational wave detectors is quantum noise, which includes quantum radiation pressure noise that originates from the quantum nature of the photons that reflect off of the test masses.
Chapter one provides an introduction to gravitational wave sources and detectors. It also describes the noise sources that limit the sensitivity of interferometeric gravitational wave detectors like Advanced LIGO and includes a detailed description of the origin of quantum noise and its effect in interferometers.
Chapter two introduces the concept and properties of optical springs. Much of the experimental work presented in the rest of this thesis utilizes an optical spring.
This thesis investigates quantum radiation pressure noise and techniques to reduce quantum noise in gravitational wave interferometers. The experimental research contained in this thesis uses an optomechanical Fabry-Perot cavity in which one of the cavity mirrors is a microresonator consisting of a micro-mirror suspended by a cantilever structure. Chapter three outlines the design and construction of the optomechanical cavity that is housed in a vacuum chamber and sits on a suspended optical breadboard to provide isolation from seismic motion. Chapter three also includes details on the design of the cantilever micro-mirror used in the optomechanical cavity. The experiments in this thesis can be divided into two main categories: the characterization of optical springs and the measurement of broadband quantum radiation pressure noise.
Chapter four of this thesis focuses on the characterization of optical springs. I present results from an experiment that uses radiation pressure to control an optomechanical cavity and investigates the feedback control needed to keep the system stable. In chapter five, I present results from an experiment in which we create an optical spring using a beamsplitter rather than the canonical example of an optical spring in a detuned Fabry-Perot cavity.
Chapter six of the thesis describes the experiment and results of a broadband measurement of quantum radiation pressure noise. I present a measurement of a noise spectrum in which the effects of quantum radiation pressure noise are observed between 2 kHz and 90 kHz, including a frequency band between 10 kHz and 30 kHz where the quantum radiation pressure noise is visible above all other noise sources.
Chapter seven presents the results from two experiments in which we have successfully reduced the amount of quantum radiation pressure noise. The first experiment is done by detecting the light that is transmitted through the cavity by a photodetector. By detecting the light in transmission of the cavity rather than reflection, we are able to evade the presence of quantum radiation pressure noise in the measurement. The second experiment injects bright squeezed light into the optomechanical cavity in place of the coherent field used in the experiment in chapter six. The injection of squeezed light into the optomechanical cavity successfully reduces the amount of quantum radiation pressure noise.
Finally, having made a measurement of quantum radiation pressure noise and two measurements in which the quantum radiation pressure noise is reduced, I outline a future experiment to measure the ponderomotive squeezing that is produced by the optomechanical cavity and the plans for making a measurement below the Standard Quantum Limit.
Cripe, Jonathan Daniel, "Broadband Measurement and Reduction of Quantum Radiation Pressure Noise in the Audio Band" (2018). LSU Doctoral Dissertations. 4653.