The Glasgow Sagnac Speed Meter Optical Layout & Suspensions

Jan-Simon Hennig - [email protected] Stanford 26.08.2014 �

The Glasgow Sagnac Speed Meter �Optical Layout & Suspensions�

Jan-Simon Hennig for the Speed Meter Team�


•  It has been shown in theoretical models that Sagnac interferometers can further improve the low frequency sensitivity of advanced gravitational wave detectors compared to Michelson interferometers with same parameters�

•  Experimental verification urgently required! �

Motivation �

[Miao et al, CQG 31, 165010 (2014)] �[Credit: S. Danilishin, UWA] �


•  Proof-of-principle experiment with 1g input test masses and high finesse cavities�

•  Aim of the Glasgow Sagnac Speed Meter (SSM): Show reduction of back action noise in a Sagnac interferometer compared to Michelson interferometer with similar parameters�

•  By design SSM signal is proportional to the time-dependent variation of test mass position (velocity) instead of the position of the test mass as in Michelson interferometer �

•  Velocity not a QND observable, but reduction of quantum radiation pressure noise in our case proposed to be a factor ~2.8 for low frequencies [Graef et al] �

Motivation �


•  Sagnac interferometer with triangular arm-cavities�

•  Balanced homodyne readout�

•  But what about alignment and mode matching? �

Optical Layout�


•  Implemented steering/auxiliary optics�

•  Bright port of interferometer serves as local oscillator �

•  3-4 Different types of suspensions�

Optical Layout�AEI 10m suspension �

Monolithic 1g suspension �

Auxiliary suspensions�


•  For sufficient seismic isolation all optics will be suspended �

•  More than 11 suspensions on one breadboard �

•  A compact design is needed �

•  We aim for a maximal footprint of 6 x 8cm�

•  Mode frequencies for longitudinal, pitch and yaw should be below 5Hz�

•  Vertical mode frequencies below 100Hz�

Challenges�


Auxiliary Suspension Parameters�


•  For actuation we use coil magnet combination �

•  High range wanted �

•  160µm copper wire, 350 turns�

•  N35H magnet (6 x 3mm) �

•  Sweet spot calculation done with Mathematicamodel�

•  1.25cm over 20cm with 10mA �

Actuation �


•  MATLAB/Simulink model ofdouble pendulum suspension �

•  Undamped TFs from supportstructure to optic �

•  Coupled mode frequencies�

Transfer Functions�


•  Copper coil formers introduce Eddy current damping �

•  Damping included in Simulink model�

•  Damping factors similar to MOSES suspensions�

Transfer Functions�


•  Vertical ground motion measured with seismometer �

•  Seismic pre isolation stack in vacuum system �

•  Assuming vertical to horizontal coupling 1:100 �

Seismic Isolation �SSM measurement band �


•  Finalise design for support structure�

•  Get parts and start building �

•  Testing of auxiliary suspension performance�

•  Begin design and calculations for small monolithic suspensions�

Outlook�

LIGO-G1400902-v2 �

Thank you for listening! �Questions? �


•  Analytical calculation of state space matrices for all different modes�

•  Deduced model for double pendulum from GEO triple�

•  Simulink model fed with state space matrices�

•  Linearisation of Simulink model gives transfer functions of the pendulum�

•  BUT: Treatment of non vertical wires not included in model�

Taking on the Challenge�

•  Calculation of pendulum dynamics using Lagrangian formalism�

•  Direct output of transfer functions possible�

•  Treatment of non vertical wires possible�

•  BUT: Defining a new case definition requires time�

MATLAB� Mathematica�


Simulink model (long/pitch) �


•  Usually the clamping of three wires with just one clamp can be realised by having two screws. One wire is clamped in between the screws and the two other wires on each side of the clamp �

•  Adopting this idea results in a quiet large wire separation in x-direction at least at the top mass clamp (7mm) �

•  This wire separation results in pitch frequencies ~10Hz�

•  First idea was to use a different wire separation at test mass�

•  MATLAB model not able to deal with that Mathematica model showed that we get down to pitch frequencies ~5Hz�

•  Non-vertical wires are complicated break-off point�

•  Get pitch frequencies ~5Hz with 4mm wire separation at top and test mass and break off point further away at top mass�

Double clamp �


Double clamp �


•  Initial investigation on wire length (25cm in total) �

•  Started off with 100µm wires, wire separation in x-direction 7mm vertical mode frequency ~200Hz and pitch ~10Hz�

•  Change wire diameter to 50µm vertical mode frequencies ~90Hz and pitch ~10Hz�

•  Change wire separation in x-direction to 7mm at top mass and 4mm at test mass non-vertical wires �

Finding Parameters�

Documents

The Glasgow Sagnac Speed Meter Optical Layout & Suspensions