Session Y15: Gravitational Wave Experiment

High Speed Alignment Control of an Optical Resonator

For interferometric gravitational wave detectors, fluctuations in the input laser beam alignment are a critical source of technical noise. In order to maintain optimal sensitivity it is necessary to control the input beam alignment. We introduce a new method for achieving this alignment control using angular actuators based on the electro-optic beam deflection. Compared to piezo-mounted mirror actuators, which have a low bandwidth and intrinsic noise due to moving parts, these actuators promise a much higher bandwidth with no moving parts. This talk presents the experimental demonstration of closed loop alignment control using the electro-optic beam deflectors and report their measured performance.   

Active thermal lensing elements for mode matching optimization in optical systems

In interferometric gravitational wave detectors of the advanced era and beyond, the high laser powers used lead to the generation of thermal lenses in the optics. This can lead to a reduction in the coupling between the various optical cavities comprising the detector, thus reducing its overall sensitivity. We present here an active device which can be used to compensate for such thermal effects, as well as static mismatches between cavities. The device uses a 4 segmented heater to heat a transmissive optic, generating a spherical or astigmatic lens which can be used to compensate other thermal lenses within an optical system. We report on in-vacuum tests of the device, including an interferometric measurement of the wavefront distortions induced by the device, and measurements of the dynamic range and response time. The device was shown to have no observable detrimental effect on wavefront distortion, a focal power dynamic range of 0 to -40\,mD, and a response time of the order 1000\,s.   

Measurement of Thermal Noise in Optical Coatings for Gravitational-Wave Detectors

Interferometric gravitational-wave detectors measure the gravitational-wave-induced strain in the arms of kilometer scale Michelson interferometers. Second-generation detectors, such as Advanced LIGO, are expected to be limited by optical coating thermal noise in the most sensitive region ($30$-$300$ Hz) of the detectors' frequency bands. The direct measurement of coating thermal noise in different optical coatings is essential to both the validation of current thermal noise models as well as the research of future coating material candidates. The THermal noise Optical Resonator (THOR) is a testbed being developed at the University of Florida to directly measure the thermal noise in optical coatings on mirrors in the frequency band around $100$ Hz. This is a presentation on the status of THOR.   

Cryogenic behavior of LEDs for use in third generation LIGO position sensors and actuators

The sensitivity of Advanced LIGO, the second-generation among ground-based, long-baseline interferometric gravitational-wave detectors, is expected to be limited by thermal noise of test-mass optical coatings within the frequency band of interest. To reduce the effects of thermal noise, and thereby increase the sensitivity of LIGO interferometers, the third generation of LIGO detectors will likely be operated with some optical and control components at cryogenic temperatures. In the interest of developing and investigating LIGO subsystems at low temperatures, the University of Florida LIGO Group has constructed a testbed for table-top cryogenic experiments. This presentation focuses on a preliminary investigation into cryogenic Birmingham Optical Sensors and Electro-Magnetic actuators (BOSEMS). BOSEMS are shadow sensors which are used to sense the position of and actuate on the LIGO core optics. Specifically, the low temperature I-V performance and efficiency of LEDs used in BOSEMs will be presented.   

Investigation of Coating Thermal Noise at Cryogenic Temperatures for Third-Generation Interferometric Gravitational-Wave Detectors

Second-generation interferometric gravitational-wave detectors will be limited between 50 and 500 Hz by coating thermal noise (CTN). CTN originates in the motion of the mirror surfaces on the order of $10^{-20}$ m due to thermal excitation and mechanical loss in their coatings. The magnitude of this effect scales with the square root of the available thermal energy, but also depends strongly on coating material parameters. These in turn may also be temperature dependent, making cryogenic mirrors an option to consider for third-generation detectors. The Cryogenic THermal noise Optical Resonator (CryoTHOR) experiment at the University of Florida aims at measuring the CTN of cryogenic mirrors by over-amplifying it using high-finesse cm-scale test cavities; this will make it an invaluable tool to assess the prospect of cryogenic test masses and explore candidate coating materials and techniques in the cryogenic regime. This presentation reports on the development of CryoTHOR.   

Telescope Back-reflection and Space-based Gravitational Wave Observatories

The Laser Interferometer Space Antenna (LISA) represents a class of proposed space-based gravitational wave observatories that will operate in the frequency band between 0.1 mHz and 1 Hz. These missions are characterized by a triangular constellation of three spacecraft (SC), separated by gigameters, in a heliocentric orbit. A reflecting telescope transfers the laser signals between the SC, and laser interferometry is used to measure length changes between proof masses housed on adjacent SC with pm/$\sqrt{\rm Hz}$ sensitivity. One of the proposed telescope designs is an on-axis `quadpod' in which the secondary mirror is axially aligned to the primary mirror. Back-reflected (BR) light from the secondary can introduce phase noise to the measurement signal due to length changes between the telescope structure and the optical bench. We derived a set of requirements for the mode-matched power in the BR field that scale with these length changes. Simulations have demonstrated that the BR power can be sufficiently attenuated by using a specifically patterned anti-reflective region at the center of the secondary mirror. An experimental testbed was built and is currently being used to evaluate the BR light from several secondary prototypes.   

Sensing and actuation system for the University of Florida Torsion Pendulum for LISA

Space-based gravitational wave detectors like LISA are a necessity for understanding the low-frequency portion of the gravitational universe. They use test masses (TMs) which are separated by Gm and are in free fall inside their respective spacecraft. Their relative distance is monitored with laser interferometry at the pm/rtHz level in the LISA band, ranging from 0.1 to 100 mHz. Each TM is enclosed in a housing that provides isolation, capacitive sensing, and electrostatic actuation capabilities. The electronics must both be sensitive at the 1 nm/rtHz level and not induce residual acceleration noise above the requirement for LISA Pathfinder (3*10$^{-15}$ m/sec$^{2}$Hz$^{1/2\, }$at 3 mHz). Testing and developing this technology is one of the roles of the University of Florida Torsion Pendulum, the only US testbed for LISA-like gravitational reference sensor technology. Our implementation of the sensing system functions by biasing our hollow LISA-like TMs with a 100 kHz sine wave and coupling a pair surrounding electrodes as capacitors to a pair of preamps and a differential amplifier; all other processing is done digitally. Here we report on the design of, implementation of, and preliminary results from the UF Torsion Pendulum.   

The UF torsion pendulum and its role in space-based gravitational wave detectors

Space-based gravitational wave observatories like LISA measure picometer changes in the distances between free falling test masses separated by millions of kilometers caused by gravitational waves from sources ranging from super-massive black hole mergers to compact galactic binaries. A test mass and its associated sensing, actuation, charge control and caging subsystems are referred to as a gravitational reference sensor (GRS). LISA has consistently been ranked in the top two of future space missions in the last two Decadal Reviews. With the 2015 launch of LISA Pathfinder (LPF), the expected detection of gravitational waves by aLIGO, and the selection of The Gravitational Universe for the European Space Agency's L3 science theme, LISA is one of the strongest candidates for the next Decadal. Following a successful demonstration of the baseline LISA GRS by LPF, the measurement principle will be carried forward, but improvements in the electronic and optical sensing and control system, the charge control system, and many other components are possible over the next ten years. These improvements will lead to cost savings and potential noise reductions. The UF LISA group has constructed the UF Torsion Pendulum to increase U.S. competency in this critical area and to have a facility where these new technologies can be developed and evaluated. This presentation will introduce this facility and its future role in LISA.