Suspension Platform Interferometer
Suspension Platform Interferometer in a nutshell
The Suspension Platform Interferometer (SPI) measures the relative motion between the seismically isolated optical benches interferometrically. One optical table is installed in each of the three vacuum tanks. The information about the actual table position is used to move the tables back into their initial position by actuators. We aim to achieve a longitudinal displacement of only 100 pm / √Hz at 10 mHz.
How to visualise this displacement?
The following analogy describes quite well what we will do with the Suspension Platform Interferometer and what 100 pm / √Hz at 10 mHz means.
Think of three compact cars: one is located in Hannover, Germany; another one in Chicago, USA and the last one in Nairobi, Kenya. If the car in Chicago and/or Nairobi is driven on by the amount of a human hair's diameter, we will be able to detect this movement in Hannover. We will also be able to tell if the car was moved forward or backwards and by what amount the steering wheel was turned. Furthermore, we will use the information to keep the relative position between all three cars constant, i.e. driving the cars back into their initial positions.
Now let's assume that the cars are optical benches (both are of comparable size and mass) and that the distance between the cities and how much the cars are driven is just one 700,000th of the aforementioned distances. This is the scale on which our experiment takes place.
A more detailed view on the Suspension Platform Interferometer
The optical tables slightly move relative to each other due to the relative seismic noise amplitude, and due to the different frequency response functions caused by mechanical tolerances from manufacturing and assembling processes. Thermal effects might dominate at very low frequencies. This relative motion will be detected by the Suspension Platform Interferometer and controlled by use of voice-coil actuators.
Therefore, the SPI minimizes the arm length fluctuations and reduces the alignment noise for the experiments. In this way much lower power is needed to actuate the suspended mirrors and thus considerably reduce the control noise. Furthermore, all three optical tables can be regarded as one huge optical table. The SPI is also an essential tool for planned geodesy related experiments . Here, two optical tables act as two flying satellites whose longitudinal displacement is a crucial limitation to achieve the mission's goal.
With the SPI we interferometrically measure all three translational degrees of freedom of the table's relative motions, as well as in pitch and yaw. The SPI consists of three heterodyne Mach-Zehnder interferometers (IFO) (see Figure 1 below). One of them is the reference IFO acting as a reference point for the phase measurement, the other two IFOs measure the motion between the central table and the south and west table, which are situated 11.65 m from the central table. The heterodyne frequency has been chosen to be about 20 kHz to realize a control bandwidth of 100 Hz.
By using a hardware phasemeter – a modified version of the phasemeter developed for LISA Pathfinder  – and quadrant photodiodes, we will detect the phase difference between the two beams (a measure for longitudinal table movement) and acquire differential wave-front sensing signals which quantify rotational motions of the table. All this information will be used as error signal to provide feedback signals to the optical benches via coil actuators. In order not to be limited by thermal expansion of the SPI baseplate all in-vacuum components of the SPI will be hydroxide-catalysis bonded on an ultra-low expansion material (CTE=(0.0+/-0.2)*10-7/°C). In this way we will meet the requirement of 10 nrad / √Hz at 10 mHz angular deviation and 100 pm / √Hz at 10 mHz in longitudinal displacement .
A zoom into the optical layout
 M. Dehne, F. Guzmán Cervantes, B. Sheard, G. Heinzel and K. Danzmann: Laser interferometer for spaceborne mapping of the Earth’s gravity field, Journal of Physics: Conference Series 154 (2009) 012023
 G Heinzel, V Wand, A Garciá, O Jennrich, C Braxmaier, D Robertson, K Middleton, D Hoyland, A Rüdiger, R Schilling,U Johann and K Danzmann: The LTP interferometer and phasemeter, Class. Quantum Grav. 21 (2004) S581–S587
If you want to know more about the SPI feel free to contact: Sina.Koehlenbeck'at'aei.mpg.de