Interferometric local sensing for 3G detectors Conor Mow-Lowry for the Birmingham ‘ifolab’ team: Daniel Brown, John Bryant, Sam Cooper, Miguel Dovales, Anna Green, Andreas Freise, David Hoyland, Aaron Jones, Haixing Miao, Daniel Töyrä, and Haoyu Wang.

Which local sensors? BOSEMs/pendulum damping Current damping filters result in sensor noise above the design goal (still well below the current noise floor) CPS/Platform positioning/control A low priority without drastic improvements in other areas Optical levers/SPI Absolute and relative positioning of test-masses is critical Inertial sensors for seismic isolation Reducing input motion for robust operation in a wider variety of conditions. Conor Mow-Lowry, ET Symposium, March 2017

Sensor blending with noisy sensors 3 vertical sensor/actuator pairs 3 horizontal custom made monolithic Accelerometers 4 vertical motorized springs 3 vertical commercial geophones My lessons from the AEI 10m MIMO control is complicated Using multiple sensors with blending is challenging and even more complicated Commercial inertial sensors are not ideal for us (cost, vacuum compatiblity, readout noise) Home-made ones are difficult (mechanical design, wires, magnets) Potential Solution: Interferometric readout, actuation free inertial sensors 3 horizontal sensor/actuator pairs 3 horizontal motorized springs Seismometer A. Wanner et al., Class. Quantum Grav. 29, 245007 (2012) Conor Mow-Lowry, ET Symposium, March 2017

Seismic control block-diagram Initially conservative (damping and DC only), now similar to LIGO’s HAM-ISI control scheme Conor Mow-Lowry, ET Symposium, March 2017

Complementary blending filters Conor Mow-Lowry, ET Symposium, March 2017

Important frequency region: 10 mHz – 100 Hz Tilt-injection, Sensor-blending roll-off Principle suspension modes, ‘Auxilliary’ control signals m-Seismic motion, Principle ground RMS GW sensitivity requirements Conor Mow-Lowry, AEI-Hannover, August 2016

Which local sensors? BOSEMs/pendulum damping Current aLIGO damping filters result in sensor noise above the design goal (still well below the current noise floor) Improved damping reduces RMS and global-control implementation. CPS/Platform positioning/control A low priority without drastic improvements in other areas Optical levers/SPI Absolute and relative positioning of test-masses is critical Inter-platform motion control can further reduce drift and RMS drive forces Inertial sensors for seismic isolation Reducing input motion for robust operation in a wider variety of conditions. Conor Mow-Lowry, ET Symposium, March 2017

Some points to consider… + Drastically reduced readout noise + Non-contact, negligible applied force + Non-magnetic, no wires or cables on proof mass + Vacuum compatible? (Feedthroughs, diodes) + Compact? (cBRS, optical levers) Small dynamic range? (Phasemeters) Complex to design and build (EUCLID) Non-linear (No problem when closed loop) Expensive? (multi-plexing, fibre distribution) No absolute range information? Too large? (BOSEM substitutes) Conor Mow-Lowry, ET Symposium, March 2017

Homodyne Quadrature Interferometer Two independent Michelson interferometers (cross-polarised), Co-propagating after PBS2. Quarter-wave plate adds extra optical pathlength to one ‘arm’ Conor Mow-Lowry, ET Symposium, March 2017

Homodyne Quadrature Interferometer Each photodiode acts like a normal michelson Only linear near the central part of the slope The combination can unwrap the optical phase, for true Cosine and Sine outputs using arctan (or a cordic engine), very fast. Plotted against each other, we find… Conor Mow-Lowry, ET Symposium, March 2017

Homodyne Quadrature Interferometer Sin vs Cos plot creates a circle The centre is shifted to 0,0 Simple arctangent (or a cordic engine) reads out the optical phase, over multiple fringes This repeating pattern is our Lissajous figure. Roughly speaking, deviations from a circle will create non-linearities in the readout. Conor Mow-Lowry, ET Symposium, March 2017

HoQI Experimental Data Approximately 45 degrees between Cos and ‘Sin’ photodiodes (instead of 90) Conor Mow-Lowry, ET Symposium, March 2017

HoQI Experimental Data Conor Mow-Lowry, ET Symposium, March 2017

Interferometer Non-linearities (simulation) Conor Mow-Lowry, ET Symposium, March 2017

Interferometer Non-linearities (experiment) Conor Mow-Lowry, ET Symposium, March 2017

Homodyne Quadrature Interferometer Two independent Michelson interferometers (cross-polarised), Co-propagating after PBS2. Quarter-wave plate adds extra optical pathlength to one ‘arm’ Conor Mow-Lowry, ET Symposium, March 2017

HoQI readout noise Construction and measurements by Sam Cooper Conor Mow-Lowry, ET Symposium, March 2017

The Beam Rotation Sensor (BRS) Ref: LHO aLOG 13250 Source: Krishna Venkateswara

The compact Beam Rotation Sensor (cBRS) Key Ideas Compact design – 30-cm across. Readout sensitivity of ~10-20 picorad/rt(Hz) at 10 Hz. Cross-shape reduces gravity gradient sensitivity. Beam-balance is UHV-compatible, can be mounted directly on the ISI. Source: Krishna Venkateswara

Future inertial sensor work Improved inertial sensors (0.01 - 10 Hz) Install HoQI in commercial geophones (L4Cs and GS-13s, as used in LIGO) Collaborate with Nikhef on Watt’s linkages Work with groups in Brussels and Washington on reviewing readout techniques Conor Mow-Lowry, ET Symposium, March 2017

Inertial Sensors with interferometers Conor Mow-Lowry, ET Symposium, March 2017

Nikhef Watt’s linkage with Michelson Source: Joris van Heijningen

Projected inertial-sensor noise-floor Conor Mow-Lowry, ET Symposium, March 2017

Summary There are numerous uses for interferometric local sensors in GWDs Future detectors will have greater problems with robustness and controls Low-frequency technical noise should drive design considerations Conor Mow-Lowry, ET Symposium, March 2017

Conor Mow-Lowry, ET Symposium, March 2017

Spectra on undamped Multi-SAS bench Source: Joris van Heijningen