1. 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.

2. 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

3. 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, (2012)
Conor Mow-Lowry, ET Symposium, March 2017

4. 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

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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

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

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

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

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

16. 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

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

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

19. 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

20. Future inertial sensor work
Improved inertial sensors ( 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

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

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

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

24. 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

25. Conor Mow-Lowry, ET Symposium, March 2017

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