Alexander Abramovici

Use of the Microprecision Interferometer testbed for developing control technology for spaceborne optical interferometer missions

This paper describes the Micro-Precision Interferometer (MPI) testbed and its major achievements to date related to mitigating risk for future spaceborne optical interferometer missions. The MPI testbed is ground-based hardware model of a future spaceborne interferometer. The three primary objectives of the testbed are to: (1) demonstrate the 10 nm positional stability requirement in the ambient lab disturbance environment, (2) predict whether the 10 nm positional stability requirement can be achieved in the anticipated on-orbit disturbance environment, and (3) validate integrated modeling tools that will ultimately tools that will ultimately to be used to design the actual space missions. This paper describes the hardware testbed in its present configuration. The testbed simulation model, as it stands today, will be described elsewhere. The paper presents results concerning closed loop positional stabilities at or below the 10 nm requirement for both the ambient and on-orbit disturbance environments. These encouraging results confirm that the MPI testbed provides an essential link between the extensive ongoing ground-based interferometer technology development activities and the technology needs of future spaceborne optical interferometers.

Optical displacement sensor (ODS): an inertial reference sensor candidate for LISA

We propose here an optical displacement sensor (ODS) as a supplemental or backup sensor for the LISA inertial reference sensor concept. This simple ODS consists of a laser diode and a quad-cell photodiode (both commercially available). The inertial mass reflective surface directs the laser beam onto the quad-cell photodiode. Changes in the inertial mass position and orientation are then extracted from ratios of the differences and sums of the quad-cell photodiode outputs. A simpler proto-type using a 200 microns wide slit has demonstrated a resolution of 10 nm/square root Hz at 1 mHz and 1 nm/square root Hz above 5 mHz. The electronics noise was 1 nm/square root Hz at and above 1 mHz with simple and off the shelf electronics components. Although this ODS current performance does not meet the LISAs system requirement1 of 1 nm/rtHz at 1 mHz, we think that is achievable in the near future.

Overview of the MicroPrecision Interferometer testbed

Gives an overview the Micro-Precision Interferometer (MPI) testbed and its major achievements to date related to mitigating risk for future spaceborne optical interferometer missions. The MPI testbed is a ground-based hardware model of a future spaceborne interferometer. The three primary objectives of the testbed are to: (1) demonstrate the 10 nm positional stability requirement in the ambient lab disturbance environment, (2) predict whether the 10 nm positional stability requirement can be achieved in the anticipated on-orbit disturbance environment, and (3) validate integrated modeling tools that will ultimately be used to design the actual space missions. The paper presents results which represent the latest advancements made on the testbed in the first two areas. Encouraging results from this testbed confirm that MPI provides an essential link between the extensive ongoing ground-based interferometer technology development activities and the technology needs of future spaceborne optical interferometers.

LISA: pointing sensor development stand

We are developing a pointing sensor as part of the technlogy development effort for the Laser Interferometer Space Antenna (LISA) mission. The sensor will measure the angle between two beams, by measuring the phase difference in the heterodyne frequency on different sides of the pupil plane. In LISA, one beam would be from the local laser, while the other beam comes from a different spaceraft. The beam coming from the other space ccraft will have a Doppler shift due to changes in the orbits of the satellites. The phase difference across the aperture will be measured to align the incoming and outgoing beams. We have characterized our pointing noise levels due to electronics over bandwidths of 0.001 to 1 Hz with a heterodyne frequency of 5 MHz. The LISA pointing requirements is on the order of 10 nrad/square root Hz stability on the sky, with a worst case scenario of 1 nrad/square root Hz. We present our first results, in which we have reached 4 micro-radians/square root Hz on the detector. This is equivalent to 70 nrad/square root Hz for LISA.

Bench top interferometric test bed for LISA

The optical paths on the LISA bench must have a length instability of less than 10~pm/square root Hz over time scales of 1s to 1000s. A small rigid interferometer has been constructed to measure the optical path length changes using various bonding techniques. The interferometer was constructed entirely from ultra-low expansion (ULE) glass by optically contacting ULE beamsplitters to a ULE bench. Preliminary results taken with the interferometer operating in air indicate optical path length fluctuations of approximately 100 pm/ sqaure root Hz or less for frequencies between 1 mHz and 1 Hz.

LISA laser noise cancellation test using time-delayed interferometry

The Laser-Interferometer-Space-Antenna (LISA) is a space-based interferometer with arm lengths of 5*10 9 m. Its design goal is to measure gravitational waves with a strain sensitivity of 10-23 at 10 mHz. Unlike in earth-based interferometers the arm lengths can differ by up to 2% or 108 m. For that reason frequency noise in the λ ~ 1 μm laser will not cancel in the direct interference signal. A laser locked to a ULE reference cavity in a 1°μK/square root Hz environment will have about 10 Hz/square root Hz frequency noise. The LISA sensitivity goal requires for the laser noise of less than 10-5 Hz/square root Hz, about a factor 10-6 below what has been achieved (1). Cancellation of laser frequency noise can be achieved by time-delayed-interferometry (TDI) (2,3). We describe a laboratory test of TDI with an unequal arm interferometer. The intent is to ascertain the performance limitations and proof-of-concept for 6 orders of magnitude frequency noise suppression.