Rachel J. Cruz

Adaptive beam shaping by controlled thermal lensing in optical elements.

We describe an adaptive optical system for use as a tunable focusing element. The system provides adaptive beam shaping via controlled thermal lensing in the optical elements. The system is agile, remotely controllable, touch free, and vacuum compatible; it offers a wide dynamic range, aberration-free focal length tuning, and can provide both positive and negative lensing effects. Focusing is obtained through dynamic heating of an optical element by an external pump beam. The system is especially suitable for use in interferometric gravitational wave interferometers employing high laser power, allowing for in situ control of the laser modal properties and compensation for thermal lensing of the primary laser. Using CO(2) laser heating of fused-silica substrates, we demonstrate a focal length variable from infinity to 4.0 m, with a slope of 0.082 diopter/W of absorbed heat. For on-axis operation, no higher-order modes are introduced by the adaptive optical element. Theoretical modeling of the induced optical path change and predicted thermal lens agrees well with measurement.

The LISA benchtop simulator at the University of Florida

At the University of Florida, we are developing an experimental Laser Interferometer Space Antenna (LISA) simulator. The foundation for the simulator is a pair of cavity-stabilized lasers that provide realistic, LISA-like phase noise. The light travel time over the five million kilometres between spacecraft is recreated in the lab by use of an electronic phase delay technique. Initial tests will focus on phasemeter implementation, time delay interferometry (TDI) and arm-locking. In this paper we present the frequency stabilization results, results from an electronic arm-locking experiment, software phasemeter performance and results from a first optical experiment to test the TDI concept. In the future, the benchtop simulator will be extended to test several other aspects of LISA interferometry such as clock noise and Doppler shifts of the signals. The eventual long-term use of the LISA simulator will be to provide realistic data streams, including all the noise components, into which model gravitational wave signals can be injected. This will make the simulator a useful testbed for data analysis research groups.

Clock Noise Removal in LISA

The relative motion of the LISA spacecraft will Doppler shift the laser frequencies by around 15 MHz. These Doppler shifts introduce sensitivity to the phase noise of the measurement’s master clock. Using the most stable clocks available this effect would degrade the LISA sensitivity by more than a factor of 100. This clock noise can be removed in post‐processing if the clock phase can be transferred between the spacecraft with a fidelity of a few microcycles/Hz. In the LISA baseline design, the clock phase is phase modulated onto the science lasers exchanged between spacecraft. Interference between the outgoing sidebands and the incoming sidebands contains the information necessary to remove the clock noise in post‐processing. The details of the LISA clock noise removal scheme are described and results of a recent successful demonstration of the clock noise transfer are presented.

Electronic phase delay—a first step towards a bench-top model of LISA

Electronic phase delay (EPD), a new technique for delaying the phase of a signal by an arbitrary amount, is presented as the basis for a model of the Laser Interferometer Space Antenna (LISA). The validity of EPD is demonstrated by constructing a synthetic interferometer (SI) with a single-arm time delay of 1 s. Schemes for studying the phase noise reduction techniques of arm-locking and time delay interferometry using EPD units are presented and discussed, with preliminary results for arm-locking. EPD can also be used as the basis for a bench-top model of LISA which will be used to study LISA interferometry and data analysis methods.

First step toward a benchtop model of the Laser Interferometer Space Antenna

A technique for simulating large optical path lengths by use of digital delay buffers is presented. This technique is used to generate a synthetic interferometer with one arm having an arbitrary length. The response of the interferometer to phase and frequency modulation is measured and found to be in agreement with predictions. This technique could be used to simulate long-baseline interferometric space missions such as the Laser Interferometer Space Antenna.

Dimensional stability of Hexoloy SA silicon carbide and Zerodur glass using hydroxide-catalysis bonding for optical systems in space

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.

Arm‐Locking in a LISA‐like Hardware Model: A Status Report

The Laser Interferometer Space Antenna (LISA), is a proposed mission to detect gravitational waves in the mHz regime. The mission calls for a triangular constellation of three spacecraft, with inter‐spacecraft distances of approximately 5 Gm. Laser interferometry is used to measure fluctuations in the inter‐spacecraft distances to a level of ∼ 10 pm, sufficient for detecting gravitational waves. A major challenge in reaching this precision is addressing laser phase noise, which couples into the distance measurement at a level several orders of magnitude above the gravitational‐wave signal. Arm‐locking is a proposed stabilization method which uses the inter‐spacecraft distance(s) as a frequency reference. In this paper we describe the implementation of arm‐locking in an electro‐optic model of the LISA interferometer. The system includes input phase noise derived from cavity‐stabilized lasers and a single‐arm delay of 1.065 ms. The residual frequency noise of the locked system was measured to be less than 2...

Dimensional stability of hexoloy SA silicon carbide and zerodur materials for the LISA mission

In the LISA mission, incoming gravitational waves will modulate the distance between proof masses while laser beams monitor the optical path length changes with 20 pm/Hz accuracy. Optical path length changes between bench components or the relative motion between the primary and secondary mirrors of the telescope need to be well below this level to result in a successful operation of LISA. The reference cavity for frequency stabilization must have a dimensional stability of a few fm/√Hz. While the effects of temperature fluctuations are well characterized in most materials at the macroscopic level (i.e. coefficients of thermal expansion), microscopic material internal processes and long term processes in the bonds between different components can dominate the dimensional stability at the pm or fm levels. Zerodur and ULE have been well studied, but the ultimate stabilities of other materials like silicon carbide or CFRP are virtually unknown. Chemical bonding techniques, like hydroxide bonding, provide significantly stronger bonds than the standard optical contacts. However, the noise levels of these bonds are also unknown. In this paper we present our latest results on the stability of silicon carbide and hydroxide bonds on Zerodur.In the LISA mission, incoming gravitational waves will modulate the distance between proof masses while laser beams monitor the optical path length changes with 20 pm/Hz accuracy. Optical path length changes between bench components or the relative motion between the primary and secondary mirrors of the telescope need to be well below this level to result in a successful operation of LISA. The reference cavity for frequency stabilization must have a dimensional stability of a few fm/√Hz. While the effects of temperature fluctuations are well characterized in most materials at the macroscopic level (i.e. coefficients of thermal expansion), microscopic material internal processes and long term processes in the bonds between different components can dominate the dimensional stability at the pm or fm levels. Zerodur and ULE have been well studied, but the ultimate stabilities of other materials like silicon carbide or CFRP are virtually unknown. Chemical bonding techniques, like hydroxide bonding, provide sig...

Time Delay Interferometry using the UF LISA Benchtop Simulator

At the University of Florida, we are developing an experimental LISA simulator to test the implementation of various aspects of LISA interferometry including arm‐locking and time delay interferometry (TDI). Realistic light travel times are created in the lab using an electronic phase delay technique and signals are read with a LISA‐like phasemeter. TDI relies on a strong correlation between the LISA signals taken at different times and locations. In this paper, we present results from the first‐generation of the simulator showing that we are able to create LISA‐like optical signals in the lab and measure and recombine them to cancel several orders of magnitude laser phase noise.