Speaker
Description
Suspension Platform Interferometers (SPIs) have proven highly effective at suppressing relative motion between seismic‑isolation platforms, thereby reducing the differential motion of the suspended optics. Because of this success, the interferometric read‑out of inter-platform motion with SPIs is now being considered for implementation in the current generation of gravitational‑wave detectors (GWDs) and also planned for future GWDs, including ET.
The SPI presently operated at the Albert Einstein Institute (AEI) in Hannover uses a heterodyne Mach–Zehnder interferometer. Its performance is limited by differential phase noise in the optical fibers that transport the two heterodyne beams to the vacuum system. Although a reference interferometer can partially cancel this noise during disturbances, a residual high‑frequency fiber-noise remains due to disturbances.
Deep Frequency Modulation Interferometry (DFMI) offers a compelling alternative to make the SPI more robust to disturbances. DFMI eliminates the need for a separate reference interferometer, resulting in a more compact and simpler optical layout. A single, deeply frequency‑modulated laser beam can be generated and distributed to several platforms with ease – a feature that is especially attractive for future detectors such as the Einstein Telescope, where multiple SPIs will be required to control the relative motion of many platform pairs. In addition, DFMI provides absolute ranging, a capability unavailable in heterodyne or homodyne schemes. This work, therefore, investigates DFMI as an alternative to the current heterodyne SPI.
In this presentation, we report on a DFMI-based SPI that has been assembled and tested outside the vacuum system prior to its final installation at the AEI 10 m Prototype. The analytical DFMI read‑out algorithm has been implemented directly in the LIGO‑style Control and Data System (CDS), removing the requirement for dedicated FPGA hardware or specialized software expertise. The DFMI laser’s frequency noise was actively stabilized by offset‑phase‑locking it to an iodine-stabilized reference laser using a FPGA-based GHz phasemeter. A custom digital interface streams the phasemeter data to the CDS in real time, enabling closed‑loop laser frequency control.