Testbed for prototypes of the LISA point-ahead angle mechanism · Testbed for prototypes of the...
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Testbed for prototypes of the LISA point-ahead angle mechanism Felipe Guzm´ an Cervantes, Benjamin Sheard, Gerhard Heinzel and Karsten Danzmann Albert-Einstein-Institut Hannover 7 th LISA Symposium Barcelona, 06/16/2008
Transcript of Testbed for prototypes of the LISA point-ahead angle mechanism · Testbed for prototypes of the...
Testbed for prototypes of the LISApoint-ahead angle mechanism
Felipe Guzman Cervantes, Benjamin Sheard, Gerhard Heinzeland Karsten Danzmann
Albert-Einstein-Institut Hannover
7th LISA Symposium
Barcelona, 06/16/2008
LISA PAAM testbed Felipe Guzman Cervantes
Point-ahead angle (PAA)
Rotation axis 60 degrees inclined tothe ecliptic plane
Nutation of the rotation axisresults in significant out-of-planePAA requiring active compensation
Beam divergence with 30 cm – 40 cm telescope: ≈ 2µrad
[O. Jennrich]
LISA PAAM testbed Felipe Guzman Cervantes
Requirements on the prototypes and on the testbed
Longitudinal pathlengthstability: 1.4 pm/
√Hz,
relaxed towards lowerfrequencies.
Angular jitter in activedirection: 16 nrad/
√Hz,
relaxed towards lowerfrequencies.
10−5 10−4 10−3 10−2 10−1 10010−12
10−11
10−10
10−9
10−8
10−7
ampl
itude
[m/√
Hz]
Frequency [Hz]
RequirementGoal
10−5 10−4 10−3 10−2 10−1 10010−8
10−7
10−6
10−5
10−4
10−3
ampl
itude
[rad
/√Hz
]
Frequency [Hz]
RequirementGoal
LISA PAAM testbed Felipe Guzman Cervantes
Measurement of longitudinal fluctuations
Adapting the existing frequency stabilization setup at AEI for PAAM
longitudinal performance tests:
NPRO
vacuum chamber
radiation shieldsreference cavity
QWP PBS
mode-matching
lens
PZT + Temp
frequencycontroller
mixer
LOintensitycontroller
pumpcurrent
QWP HWP HWP
EOM
polariser
frequencycounter
FFT
Spectrum
fastPD
identicalsystem
10−5 10−4 10−3 10−2 10−1 10010−15
10−14
10−13
10−12
10−11
10−10
10−9
10−8
10−7
Freqeuncy [Hz]
OPD
[m/√
Hz]
RequirementGoalExisting AEI Reference (L=21cm)
[Data from M. Troebs]High gain limit:
δν
ν=
δL
L
The differential cavity length can be recovered from the beatnote
fluctuations (differential resonance frequency).
LISA PAAM testbed Felipe Guzman Cervantes
PAAM longitudinal test approach
Modify exisiting setup by replacing one of the cavities with a cavitywith the PAAM as one of the mirrors.
NPRO
mode-matching
lenses
fibrefeedthrough
PZT + Temp
frequencycontroller
mixer
LO
intensitycontroller
pumpcurrent
QWPHWPHWP
EOM
polariser
frequencycounter
FFT
Spectrum
fastPD
existingsystem
PAAM
telescopefor DWS
QPD
VacuumChamber
Thermal shielding
Cavity alignment mirrors
Aluminiumbreadboard
Zerodurbaseplate
Bonded components on aZerodur Baseplate.
Will use the thermally stablevacuum system beingdeveloped.
Angular jitter readout viadifferential wave frontsensing.
LISA PAAM testbed Felipe Guzman Cervantes
Cavity design
45 degree angle of incidence requires a three mirror cavity.
Ring cavity, e.g. isosceles triangle, gives simple separation ofreflected beam
PAAM mirror is flat, therefore at least one other mirror mustbe curved (necessarily astigmatic for ring cavity).
Compact design desirable to reduce thermal sensivity
current cavity geometry:
a
a
√2a
ρ
ρ
PAAM
LISA PAAM testbed Felipe Guzman Cervantes
Experimental overview
General setup:
laser source: Nd:YAG NPRO lasers.
reference cavity: linear ULE cavity in cylindrical vacuumchamber on optical bench.
test cavity: triangular ring cavity with 2 bonded mirrors on aZerodur baseplate. Third mirror to be mounted on PAAM.
test setup in a cubical vacuum chamber. Typical operatingpressure 10−5 mbar.
thermal stability 10−5 K/√
Hz at mHz.
Viton damping between tank and thermal shield layers formechanical isolation to reduce vibration inside the thermalshield.
LISA PAAM testbed Felipe Guzman Cervantes
Experimental setup
LISA PAAM testbed Felipe Guzman Cervantes
DAQ / Data Analysis
Data Acquisition:beatnote readout: frequency counter Agilent 53131A.temperature readout: 8 channel FPGA based board developedin house.pressure readout: sensors Leybold ITR90 and ThermovacTTR91S.For additional data: DAQ card NI PCI-6221, 10 channels,selectable sample rate.
Data Analysis:LTPDA: data analysis tool developed in house for LISAPathfinder. (talk by Martin Hewitson, Wed. 12:15)developed as a toolbox running on a MATLAB platform.free software that can be downloaded from our website:www.lisa.aei-hannover.de/ltpda
LISA PAAM testbed Felipe Guzman Cervantes
Subsequent requirements: temperature stability
Thermally stable environment is required where pm/√
Hzpath length stability in the mHz band is required.
Thermal expansion of Zerodur is α ≈ 10−7 K−1 therefore therequired temperature stability in the LISA band (for 0.1 mpath) is:
∆T <<∆Lreq.
Lα=
10−12 m/√
Hz0.1 m × 10−7 K−1
= 10−4 K/√
Hz
Temperature stabilisation approaches:Passive:
A sufficient vacuum virtually eliminates convectionThermal shielding can reduce conduction/radiationEasier to implement for “higher” frequencies, e.g. 1 Hz
LISA PAAM testbed Felipe Guzman Cervantes
Thermal shield design
Thermal shield
Lumped capacity tf model(conduction only)
10−6 10−5 10−4 10−3 10−210−7
10−6
10−5
10−4
10−3
10−2
10−1
100
Frequency [Hz]
Mag
nitu
de [−
]
Normalised Transfer Function
Additional passive isolation with styrofoam outside
Cabling “short cuts” thermal shielding / isolation andfluctuations in power dissipation inside thermal shielding (e.g.photodetector electronics) generates local thermaldisturbances
LISA PAAM testbed Felipe Guzman Cervantes
Heat sources inside thermal shield
Heat sources inside thermalshield include:
Photodetector (must bealways on)
CCD (turned off afterlocking)
Temperature sensors
Photodetector most significantheat source.
Temperature readout board
LISA PAAM testbed Felipe Guzman Cervantes
Temperature fluctuations
10−6 10−5 10−4 10−3 10−2 10−1 100 10110−6
10−5
10−4
10−3
10−2
10−1
100
101
ampl
itude
[K/√
Hz]
Frequency [Hz]
lpsd(split(Tank wall))lpsd(split(Photodetector))lpsd(split(Alumnium Breadboard 1))lpsd(split(Alumnium Breadboard 2))Requirement
LISA PAAM testbed Felipe Guzman Cervantes
Temperature fluctuations
10−6 10−5 10−4 10−3 10−2 10−1 100 10110−6
10−5
10−4
10−3
10−2
10−1
100
101
ampl
itude
[K/√
Hz]
Frequency [Hz]
lpsd(split(Zerodur Right))lpsd(split(Zerodur Front))lpsd(split(Zerodur Left))lpsd(split(Zerodur Back))Requirement
LISA PAAM testbed Felipe Guzman Cervantes
Subsequent requirements: pressure stability
Changes in refractive index result in cavity round-trip lengthchanges:
∆L = ∆n Lrt (1)
Change of refractive index due to pressure
∆n
∆p= 2.9× 10−9 Pa−1 (2)
Estimated requirement:
∆p(f ) ≤ 2.4× 10−5 mbar√Hz
(3)
LISA PAAM testbed Felipe Guzman Cervantes
Pressure fluctuations in vacuum environment
10−6 10−5 10−4 10−3 10−2 10−1 10010−7
10−6
10−5
10−4
10−3
10−2
10−1
ampl
itude
[mba
r/√Hz
]
Frequency [Hz]
lpsd(split(Tank))Requirement
LISA PAAM testbed Felipe Guzman Cervantes
Beat note readout: time series
0 0.5 1 1.5 2 2.5x 105
6.16
6.17
6.18
6.19
6.2
6.21
6.22
6.23
6.24
6.25x 108 Time origin: 2008−04−25 17:42:32.000
Ampl
itude
[Hz]
Time [s]
Beatnote
LISA PAAM testbed Felipe Guzman Cervantes
Corresponding length spectrum
10−6 10−5 10−4 10−3 10−2 10−1 10010−12
10−11
10−10
10−9
10−8
10−7
ampl
itude
[m/√
Hz]
Frequency [Hz]
lpsd(split(Beatnote)) * 5.141e−016Requirement
LISA PAAM testbed Felipe Guzman Cervantes
Origin of excess noise:
Excess noise shoulder between 1 – 10mHz does not appear to belimited by
pressure fluctuations
temperature stability inside chamber
electronic noise (e.g. photodiode, amplifier)
Appears to originate in phase modulation/demodulation (likelycandidate is residual amplitude modulation) Possibleimprovements:
temperature stabilisation/isolation of EOM
LISA PAAM testbed Felipe Guzman Cervantes
Phase modulator
Fiber coupled EOM
high modulation depth.
low amplitude modulation.
residual amplitude modulation potentially limits performance.
LISA PAAM testbed Felipe Guzman Cervantes
Noise projection with no modulation
10−4 10−3 10−2 10−1 100100
101
102
103
104
105
106
107
Frequency [Hz]
Equi
vale
nt F
requ
ency
ASD
[Hz/√
Hz]
Error signal noise projectionRequirement
LISA PAAM testbed Felipe Guzman Cervantes
Noise projection with modulation
10−4 10−3 10−2 10−1 100102
103
104
105
106
107
Frequency [Hz]
Equi
vale
nt F
requ
ency
ASD
[Hz/√
Hz]
Error signal noise projectionRequirement
LISA PAAM testbed Felipe Guzman Cervantes
Comparison
10−4 10−3 10−2 10−1 100102
103
104
105
106
107
Frequency [Hz]
Equi
vale
nt F
requ
ency
ASD
[Hz/√
Hz]
Error signal noise projectionRequirement
10−6 10−5 10−4 10−3 10−2 10−1 10010−12
10−11
10−10
10−9
10−8
10−7
ampl
itude
[m/√
Hz]
Frequency [Hz]
lpsd(split(Beatnote)) * 5.141e−016Requirement
LISA PAAM testbed Felipe Guzman Cervantes
Beam alignment onto PAAM mirrorangular jitter translates as longitudinal fluctuations:
manufacture tolerances → location of PAAM center ofrotation (COR).limited alignment accuracy of beam onto PAAM COR.current requirement assumed to ± 50 µm.
a
a
√2a
ρ
ρ
PAAM
LISA PAAM testbed Felipe Guzman Cervantes
Differential wavefront sensing
Telescope design:
limited space forbeam propagationinside thermalshield: 40 cm.
design compromisefor sensitivity inboth tilt directions:Gouy phase 45◦.
design of resonantQPD and mixerelectronics.
optical layout
, 12 Jun 2008, cav500_DWS.ps
!100 0 100 200 300
!100
0
100
1
2
3 4 5 6 7 8 9
10
11
M1
M2
PAAM f=50mm f=75mm
QPD1
LISA PAAM testbed Felipe Guzman Cervantes
Outlook and Summary
Characterization of excess noise source.
First observations show a non-linear coupling mechanism ofthermal effects into amplitude modulation.
Intensity stabilisation appears not to be necessary at present.
Implementation of DWS to provide auxiliary angular jitterinformation.
Present performance is considered to be sufficient to place asignificant upper limit on the longitudinal noise of the PAAM.
Testing of first industrial prototype (TNO) has already startedat AEI premises.
