Sub-SQL beam splitter installed

The core beam splitter for the Sub-SQL Interferometer has been installed into the vacuum system and is now freely hanging in its triple suspension system.

Unlike typical beam splitters used in gravitational wave detectors, the shape of our beam splitter is rectangular rather than round. This is because the beam splitter should be wide enough for secondary internal reflections (ghost beams) to exit the beam splitter through the optical surfaces rather than the barrel of the optic so that those beams can be effectively dumped. A round beam splitter fulfilling this requirement would hit the optical breadboard of the suspension platform interferometer (SPI) located directly underneath. This breadboard is housed in the aluminium box below the beam splitter in the above photos. Additionally, the rectangular shape reduces the mass of the beam splitter by approximately 50% compared to round version. This is critical for reducing the total mass on the optical table, which is limited due to the seismic attenuation system.

The beam splitter side of the optic does not use a uniform coating. Instead it uses a split coating design where only the central part of the surface has a beam splitter coating. The sides are anti-reflection coated to suppress further ghost beams.

Reflectivity scan of the beam splitter

More details about the design of the beam splitter and its suspension system are available in chapter 4 of Philip Koch’s PhD thesis.


Philip Koch defends PhD

Congratulations to Philip Koch for successfully defending his PhD thesis titled “Design, Suspension and Optical Characterization of the Beam Splitter and Output Mode Cleaner for the AEI 10 m Prototype”. Philip’s thesis work covered a range of topics including:

  • the thickness homogeneity of crystalline AlGaAs/GaAs high reflectivity coatings
  • the design, construction and characterisation of the main sub-SQL Interferometer beam splitter and it’s triple suspension
  • building the “scatterometer” – an apparatus to characterise the scattered light produced by optics by measuring the bidirectional reflectivity distribution function (BRDF)
  • the design and construction of an outpupt mode cleaner cavity for the sub-SQL interferometer, with a measured intra-cavity loss of < 0.5%


PhD positions in the AEI 10m Prototype

We have openings for talented and highly motivated students to join the AEI 10m Prototype as PhD students to work on bringing the sub-SQL interferometer online and to its design sensitivity. Topics will include the design and installation of suspended optics, interferometer control, scattered light mitigation, and noise identification and suppression.

The closing date for applications is 19.04.2022. More details and application instructions can be found on the AEI home page:


Luise Kranzhoff completes Masters degree

Congratulations to Luise Kranzhoff for successfully completing her masters studies! Her Masters thesis was titled “A Novel Vertical Inertial Sensor with Homodyne Quadrature Interferometer Readout” where she designed and characterised a new sensor and investigated their potential use as part of the AEI-SAS. Her thesis work was done in collaboration with the University of Birmingham and Vrije Universiteit Amsterdam.

Luise will soon start her PhD studies at the University of Maastricht in the Netherlands.


Robin Kirchhoff defends his PhD

Robin Kirchhoff will receive his PhD after successfully defending his PhD thesis titled “Implementation of an active seismic isolation system for the AEI 10 m Prototype”. Robin’s work was centred around the implementation of active control schemes to seismically isolate the pre-isolation platforms in the 10 m Prototype and how they can be optimised to suppress noise in the Sub-SQL Interferometer.

Congratulations Robin!


Position as Junior Research Group Leader (m/f/d) for Sub-Standard Quantum Limit of Interferometry

As part of the Cluster of Excellence QuantumFrontiers, the QUEST Leibniz Research School in cooperation with the Faculty for Mathematics and Physics invites applications for the position of a Junior Research Group Leader (m/f/d) for Sub-Standard Quantum Limit of Interferometry (Salary Scale 14 TV-L, 100 %) to be filled as of November 1st, 2021. The position is limited to four years.

Information on the position and the application process is available on the AEI homepage.


Damping electro-optic modulator acoustic resonances with epoxy

Sean Leavey and Harald Lück

At the prototype we use a lithum niobate New Focus (now Newport) 4004 broadband electro-optic modulator (EOM) as part of our reference cavity frequency stabilisation servo. During measurements we noticed that the EOM contained high-Q resonances at around 790 kHz and 2 MHz which looked mechanical in origin:

A response measurement of our broadband EOM (change in frequency per volt). The blue trace is the measured response, while the orange is the expected response from a simple model. The resonances around 790 kHz and 2 MHz seem to be acoustic in origin.

These resonances appear on the light as unwanted frequency modulation. The presence of the 790 kHz resonance creates instability in our servo, preventing us from pushing its unity gain frequency higher. A higher unity gain frequency would allow for greater suppression of laser frequency noise in the 100 Hz to 1 kHz band of the reference cavity, which in turn would reduce coupling of frequency noise to the main SQL interferometer where it would otherwise show up as a significant noise source masking the quantum noise that we wish to measure.

In passing, Sean mentioned this to our colleague Rana Adhikari in Caltech, who recalled some old “crystal lore” that mounting the lithium niobate on acoustically impedance matched material can reduce acoustic resonances. This gave Sean and Harald the idea to test this hypothesis by applying damping material to the crystal in a spare New Focus 4004.

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