The first space-based gravitational wave observatory, the Laser Interferometer Space Antenna (LISA), will enable us to listen to gravitational waves between 100 μHz and 1 Hz. This ESA-led L-class mission will detect mergers involving ten thousand to ten million solar mass black holes throughout the universe, a million compact galactic binaries involving white dwarfs, neutron stars, and stellar mass black holes and many other gravitational wave signals. LISA will use laser interferometry to measure minuscule changes in the distance between six free falling test masses on board of three spacecrafts which form a 2.5Gm equilateral triangle. I will discuss the science case for LISA, the measurement principle, its technology readiness and the plans towards a launch in 2035.
The Laser Interferometer Space Antenna (LISA) is a large-scale space mission design to directly measure gravitational waves using laser interferometry techniques. The constellation of three spacecraft, each separated by 2:5 Gm, will follow a heliocentric orbit with a constant distance from Earth (~20°). Light exchanges between the spacecraft will be enabled by 300mm telescopes used to simultaneously transmit and receive. Each telescope is part of the interferometer, and each must meet tight requirements on its dimensional stability; below 1pm= pHz in the LISA band, μm-length stability over 10 years of mission duration, and below ppb backscatter of the transmitted light. Here, we present our progress in developing ground support equipment for the LISA telescope ground verification. We also report on recent experimental results of the dimensional stability for the telescope test structure; a key part of the ground support equipment, and simulations of the optical design and internal and external alignment tolerances of the test structure and the telescope within it.
Modern observatories including ultra-stable spectrographs, optical telescopes and gravitational wave observatories rely on ultra-stable structures to meet their science objectives. These structures must exhibit pm to nm level length stability over a few seconds to a few hours and m-level length stability over mission duration of several years in some cases. The use of ultra-low CTE glass substrates provide the required stability while being highly fragile, having limited adaptability while driving turnaround times longer. We characterized structures made using materials that can provide the required stability while improving on the adaptability, turnaround times, structural mass and strength. These include a compound structure made using ALLVAR Alloy, a metal with a negative CTE, a second structure made of HB-Cesic, a full-ULE structure and a metal-ULE hybrid structure. In this work, we present a comparative analysis of the measured length noise and the long-term length stability for these structures.
Many future space missions including Large UV/Optical/Infrared Surveyor and Laser Interferometer Space Antenna require metering structures of exceptional geometric stability in order to meet their science goals. Thermal expansion and contraction of these structures is typically a primary concern. The application of a known high-stability bonding method, hydroxide catalysis bonding, to a new structural material, ALLVAR Alloy 30, with the uniquely useful property among metals of a negative coefficient of thermal expansion is studied. We report on measurements of the shear breaking strength of Allvar mated with fused silica (glass), grade 5 Ti-6Al-4V (Ti64), and aluminum 6061-T6 (aluminum). Measured shear breaking strengths ranged from 3 to 16 MPa with a mean of ∼7.5 MPa. Initial series of tests indicated no significant correlation between bonding strength and surface roughness over the range of surface roughness tested.
A microwave phase-control scheme is proposed and experimentally demonstrated. Two lasers are combined in an optical fiber coupler to generate a beat signal. The beat frequency is tuned by controlling the frequency of one laser. Using the phase shift of the beat waves with different frequencies during the propagation in an optical fiber, the phase of the radio-frequency (RF) signal generated by a photodetector (PD) can be controlled. Using the phase shift during the propagation of beat waves in an optical fiber with different beat frequencies, the phase of the RF signal generated by a PD connected to the fiber can be controlled. A tunable phase shift ranging from 0 deg to 1400 deg is obtained for frequencies from 6 to 10 GHz. This scheme offers the advantages of fast tuning and precise phase control of an RF signal.
Optical systems made for space-based interferometric missions like LISA or SIM must be made of materials that can endure significant accelerations and temperature fluctuations while staying dimensionally stable. Temperature-induced effects can be reduced with thermal shielding techniques and estimated using the thermal expansion coefficient. However, the stability is often limited by virtually unquantified material internal relaxation processes. In this paper we describe the experimental layout and present the status of our experiments to measure the dimensional stability of Zerodur and Hexoloy SA® silicon carbide using hydroxide-bonding and discuss its feasibility for the LISA mission.
The Laser Interferometer Space Antenna (LISA) is a joint NASA/ESA space mission aimed to detect gravitational waves in the 3×10-5 to 1Hz frequency range. Expected sources for LISA include super massive black hole mergers (SMBH), galactic neutron star and white dwarf binaries, and extreme mass ratio inspirals (EMRI). The three LISA spacecraft will travel in a heliocentric orbit trailing or leading earth by about 20°. The distance between the spacecraft will be about 5 million km or 16s light travel time. Laser interferometry will measure the distance with pm/√Hz accuracy. This report focuses on the technology for LISA interferometry.
In the advancing field of gravitational wave interferometry, the desire for greater sensitivity leads to higher laser powers to reduce shot noise. Current detectors[1] such as LIGO and GEO 600 operate with continuous wave lasers at 10-15 W powers, however future versions will operate at 200 W. One of the major challenges of higher power operation is the creation of thermal lenses in optical components, caused by from the absorption of laser light, yielding optical path deformation and concomitant beam aberrations. This effect is especially problematic in transmissive optical components even at very low levels of absorbed power. In environments that restrict the ability to move optical components (such as gravitational wave detectors), this effect can be used for beneficial purposes, specifically for providing adjustable beam-shaping. The method employs an additional laser having a wavelength strongly absorbed by the substrate and can create an aberration-free parabolic lens can be created provided that the heating beam mode is
substantially larger than the transmitted beam mode. The resulting focal length varies inversely with the heating laser power. This idea forms the basis for an adaptive optical telescope. We present experimental and theoretical results on a laser adaptive mode-matching system that uses an argon laser absorbed in a color glass filter. We characterize the dynamic focal range of the lens and measure the resulting aberrations in the transmitted Nd:YAG beam. Our results are in good agreement with a theoretical model incorporating the temperature distribution of the lens and the relevant thermo-optic parameters.
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