Research Highlight: Gravitational wave scientists develop new laser mode sensor with unprecedented precision

Figure 1: False colour image of laser eigenmodes that were tested. The colour indicates the phase of the light. Red is 0 degrees, blue is 180 degrees.

​Lasers support certain structures of light called ‘eigenmodes’. An international collaboration of gravitational wave, metasurface and photonics experts have pioneered a new method to measure the amount of these eigenmodes with unprecedented sensitivity.

In gravitational wave detectors, several pairs of mirrors are used to increase the amount of laser light stored along the massive arms of the detector. However, each of these pairs has small distortions that scatters light away from the perfect shape of the laser beam. This scattering can cause excess noise in the detector, limiting sensitivity and taking the detector offline.

From the recently submitted study, Prof Freise (from Vrije Universiteit Amsterdam) says: “Gravitational wave detectors like LIGO, Virgo and KAGRA store enormous amount of optical power – in this work, we wanted to test an idea that would let us zoom in on the laser beam and look for the small wiggles in power that can limit the detectors’ sensitivity.”Lasers support certain structures of light called ‘eigenmodes’. An international collaboration of gravitational wave, metasurface and photonics experts have pioneered a new method to measure the amount of these eigenmodes with unprecedented sensitivity.

Figure 2: A schematic of the apparatus used by the researchers. f is the focal length of the lens.

​A similar problem is encountered in the telecoms industry where scientists want to use multiple eigenmodes to transport more data down optical fibres. OzGrav researcher and lead author Dr Aaron Jones (The University of Western Australia) explains: “Telecoms scientists have developed a way to measure the eigenmodes using a simple apparatus, but it’s not sensitive enough for our purposes. We had the idea to use a metasurface and reached out to collaborators who could help us fabricate one.”

In the study, the proof-of-concept setup the team developed was over 1000x more sensitive than the original way developed by the telecoms scientists. The researchers will now look to translate this work into gravitational wave detectors, where the additional precision will be used to probe the interiors of neutron stars and test fundamental limits of general relativity.

OzGrav Chief Investigator, Prof Zhao (from University of Western Australia) says: “Solving the mode sensing problem in future gravitational wave detectors is essential, if we are to understand the insides of neutron stars.”

Written by Dr Aaron Jones (The University of Western Australia).