How to suspend a mirror as light as
a one US dollar note?
Jan-Simon Hennig for the Glasgow Sagnac Speed Meter Team
Motivation
§ GW detectors (Michelson interferometer) are limited by quantum noise (QN)
§ QN comprises photon shot noise (SN) and radiation pressure noise (RPN)
§ RPN is dominant at low frequencies and mimics a weak GW signal in
readout of interferometer
§ Continuous readout of position information limited by Heisenberg
§ Solution: Find
quantum-non-demolition
(QND)
variable
- Speed, or momentum of test mass (TM)
- Speed meter configuration reduces RPN
§ Proof of concept experiment requires compact suspensions with excellent
performance to minimise seismic noise and ensure limitation by RPN
§ Input test mass (ITM) as light as a one US dollar note
Figure 1: One US dollar note on a scale showing its weight of 1g and design sensitivity of the Glasgow Sagnac Speed Meter [2]
0.5
M9
M5
LaserStab
M6
input beam
10
0.0
Quantum noise
Suspension thermal noise
Coating Brownian noise
Coating Thermo-optic noise
Substrate Brownian noise
Excess Gas
Total noise
SQL
-17
Displacement [m/√ Hz]
M11
M14
Sagnac Speedmeter: Pin = 1.7 W
-16
M3b
M12 M13
10
-18
10
M2b
M1b
M4
M8
M7
M3a
M10
M1a
M16
PhD3
M15
M2a
PhD4
-19
10
0.5
-20
10
2
3
10
4
10
Frequency [Hz]
5
10
10
0.5
0.0
0.5
1.0
1.5
2.0
(left). Optical layout of the Glasgow Sagnac Speed Meter (right).
1g suspension
General design considerations
§ ITM for triangular high Finesse cavities
§ Light weight ensures limitation by RPN
§ Feedback control realised with electro static drive (ESD)
§ No modifications to the optics
Displacement (m/sqrt(Hz))
10
Mechanical design considerations
§ Quadruple design to implement two stages of vertical isolation
§ Attenuation of seismic motion
§ Assume vertical to horizontal coupling of 1:100
4
5
0
1
10
10
2
3
A
R0
.00
5
0.004
B
B
0.005
4
10
10
Frequency (Hz)
10
5
10
5cm length
10cm length
20cm length
SSM total noise
-10
10
1500
-15
10
0
0
Figure 2: CAD drawing of the 1g input test mass. The optic has small (2 mm x 2 mm) bonding areas
on three flat sides. With this design fibres can be bonded to either sides or top of the optic.
Institute for Gravitational Research
PROJECT:
UNLESS STATED
FINISH:
D
Auxiliary Suspension
1. DEBUR AND BREAK SHARP
EDGES
1
2
30
40
10
20
30
40
50
60
70
80
90
100
50
60
70
80
90
100
Length [mm]
10
-1
10
0
1
10
10
2
3
4
10
10
Frequency (Hz)
10
5
10
40
30
20
10
0
0
Length [mm]
Sagnac Speed Meter
Figure 3: Investigation of suspension thermal noise for
different fibre geometries (top: thin vs thick, bottom: short
vs long).
Figure 5: Profile of a thin fibre pulled at the IGR in
Glasgow. Work carried out by Karl Toland.
SUB-SYSTEM:
ITM suspension
ASSEMBLY:
NAME
DRAWN
CHK'D
APPV'D
20
A4
DO NOT SCALE DRAWING
SURFACE
TEXTURE (μm):
10
50
-20
-25
NOTES (UNLESS OTHERWISE SPECIFIED):
QUANTITY:
DIMENSIONS ARE IN METERS
2
MATERIAL:
Fused Silica
1000
500
10
C
Fibre profile of thin fibre
2000
Indicate ROC side (arrow) and part no.
C
Figure 4: First mechanical
design
considerations.
The current idea is a
quadruple pendulum with
two stages of baldesrings
and a fully monolithic
lowest pendulum stage.
Short vs Long
10
0.00025 X 45°
0.002
-1
10
-5
Total mass: 0.8096g
0.002
-20
10
-25
6
A
-15
10
Diameter [um]
3
-10
10
10
Displacement (m/sqrt(Hz))
2
5um fibre
10um fibre
20um fibre
SSM total noise
Diameter [um]
Test mass and final stage
§ Final suspension stage monolithic
§ 1g mass will be 10 mm diameter mirror with four flat sides
§ Suspension thermal noise will limit sensitivity
§ Fibres with 10 μm diameter and 10 cm length
§ Attachment using ears and hydroxide catalysis bonding
1
Thin vs Thick
-5
DATE
100g suspension
PART NAME:
REV:
DWG NO.
SCALE:5:1
PROJ:
SHEET 1 OF 1
§ 11 suspensions required for central Sagnac and beam steering
§ Auxiliary suspensions with compact footprint 8 cm x 5 cm
§ Large actuation range and sufficient seismic isolation
Triple pendulum design for end test mass (ETM)
Originally developed for AEI SQL experiments [1]
Adaptation of suspended parts
Feedback control with coil magnet actuators and
ESD
§ Modified support structure
§
§
§
§
Suspension point
Figure 7: CAD model of the AEI 10m suspension. Left
shows the original design as for the AEI. Right shows the
modified support structure for the Glasgow SSM.
Eddy current
dampers & actuation
Top mass
Conclusion
§ Quadruple pendulum design is most suitable for 1g mirror
§ Need very thin fused silica fibres due to suspension thermal noise
§ Use hydroxide catalysis bonding to bond ears to the mirror for delicate
welding of fibres
§ The suspension design for other main types of suspensions is completed
Test mass
References
Figure 6: Photographs of an auxiliary suspension. In the centre the test and top mass as well as the Eddy current
dampers are highlighted.
Institute for Gravitational Research, University of Glasgow
[1] C. Graef: Optical Design and Numerical Modelling of the AEI 10m Prototype sub-SQL Interferometer., PhD thesis, 2013,
Leibnitz Universität Hannover
[2] C. Graef et al: Design of a speed meter interferometer proof-of-principle experiment, CQG, vol. 31, 215009, 2014
LIGO-G1501088
Jan-Simon Hennig, [email protected]