Digitally enhanced heterodyne interferometry (DEHI) is an advanced optical measurement technique that can be used to discriminate multiple interferometric signals at a single photodetector with sub-wavelength sensitivity [1]. DEHI employs a digital pseudo-random noise (PRN) code phase-modulated onto the light source. This then allows various optical signals to be isolated based on their time-of-flight delay, using appropriate demodulation in digital signal processing. Further, this technique shows flexibility, scalability of number of signals, and computational efficiency through the use of field-programmable gate-arrays [2].

Given the sensitivity and advantages, DEHI is a promising technique to be used for simplifying local sensors/sensing for seismic attenuation systems. We have several projects focussed on demonstrating DEHI for local sensors in seismic attenuation systems, including implementations for the TorPeDO multi-stage attenuator system [3, 4], to applications in future gravitational-wave interferometric detector platforms.

Left: Digitally enhanced heterodyne interferometer for monitoring the separation of mirrors M1, M2, and M3. Reflections from the different mirrors are isolated by matching the demodulation delays. EOM, electro-optic modulator; AOM, acousto-optic modulator [1]. Right: TorPeDO multi-stage attenuator system at CGA [4].

[1] D. A. Shaddock, Opt. Let, 32 (22) 2007. {https://www.osapublishing.org/ol/abstract.cfm?uri=ol-32-22-3355}

[2] Lyle Roberts PhD thesis {https://openresearch-repository.anu.edu.au/handle/1885/110523}

[3] David McManus PhD thesis {https://openresearch-repository.anu.edu.au/handle/1885/186119}

[4] TorPeDO Project {https://cga.anu.edu.au/research/projects/low-frequency-gravitational-force-sensing}