Testbed for prototypes of the LISA point-ahead angle mechanism, Benjamin Sheard, Gerhard Heinzel and Karsten Danzmann Albert-Einstein-Institut Hannover 7 th LISA Symposium Barcelona, 06/16/2008
Point-ahead angle (PAA) Beam divergence with 30 cm 40 cm telescope: 2µrad Rotation axis 60 degrees inclined to the ecliptic plane Nutation of the rotation axis results in significant out-of-plane PAA requiring active compensation [O. Jennrich]
Requirements on the prototypes and on the testbed 7 Requirement Goal 8 Longitudinal pathlength stability: 1.4 pm/ Hz, relaxed towards lower frequencies. amplitude [m/ Hz] 9 11 12 5 4 3 2 1 0 Frequency [Hz] 3 Requirement Goal 4 Angular jitter in active direction: 16 nrad/ Hz, relaxed towards lower frequencies. amplitude [rad/ Hz] 5 6 7 8 5 4 3 2 1 0 Frequency [Hz]
Measurement of longitudinal fluctuations Adapting the existing frequency stabilization setup at AEI for PAAM longitudinal performance tests: 7 Requirement Goal 8 Existing AEI Reference (L=21cm) NPRO QWP HWP HWP identical system 9 pump PZT + Temp current intensity frequency controller controller mixer polariser LO EOM fast PD OPD [m/ Hz] 11 vacuum chamber modematching lens frequency counter FFT 12 13 Spectrum 14 QWP PBS reference cavity radiation shields 15 5 4 3 2 1 0 Freqeuncy [Hz] High gain limit: [Data from M. Troebs] δν ν = δl L The differential cavity length can be recovered from the beatnote fluctuations (differential resonance frequency).
PAAM longitudinal test approach Modify exisiting setup by replacing one of the cavities with a cavity with the PAAM as one of the mirrors. existing system HWP HWP QWP NPRO Thermal shielding fast PD frequency counter FFT Spectrum Vacuum Chamber polariser EOM LO fibre feedthrough modematching lenses mixer pump PZT + Temp current frequency controller intensity controller Aluminium breadboard QPD telescope for DWS Zerodur PAAM baseplate Bonded components on a Zerodur Baseplate. Will use the thermally stable vacuum system being developed. Angular jitter readout via differential wave front sensing. Cavity alignment mirrors
Cavity design 45 degree angle of incidence requires a three mirror cavity. Ring cavity, e.g. isosceles triangle, gives simple separation of reflected beam PAAM mirror is flat, therefore at least one other mirror must be curved (necessarily astigmatic for ring cavity). Compact design desirable to reduce thermal sensivity current cavity geometry: 2a a ρ PAAM a ρ
Experimental overview General setup: laser source: Nd:YAG NPRO lasers. reference cavity: linear ULE cavity in cylindrical vacuum chamber on optical bench. test cavity: triangular ring cavity with 2 bonded mirrors on a Zerodur baseplate. Third mirror to be mounted on PAAM. test setup in a cubical vacuum chamber. Typical operating pressure 5 mbar. thermal stability 5 K/ Hz at mhz. Viton damping between tank and thermal shield layers for mechanical isolation to reduce vibration inside the thermal shield.
LISA PAAM testbed Experimental setup Felipe Guzma n Cervantes
DAQ / Data Analysis Data Acquisition: beatnote readout: frequency counter Agilent 53131A. temperature readout: 8 channel FPGA based board developed in house. pressure readout: sensors Leybold ITR90 and Thermovac TTR91S. For additional data: DAQ card NI PCI-6221, channels, selectable sample rate. Data Analysis: LTPDA: data analysis tool developed in house for LISA Pathfinder. (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
Subsequent requirements: temperature stability Thermally stable environment is required where pm/ Hz path length stability in the mhz band is required. Thermal expansion of Zerodur is α 7 K 1 therefore the required temperature stability in the LISA band (for 0.1 m path) is: T << L req. Lα = 12 m/ Hz 0.1 m 7 K 1 = 4 K/ Hz Temperature stabilisation approaches: Passive: A sufficient vacuum virtually eliminates convection Thermal shielding can reduce conduction/radiation Easier to implement for higher frequencies, e.g. 1 Hz
LISA PAAM testbed Felipe Guzma n Cervantes Thermal shield design Lumped capacity tf model (conduction only) Thermal shield Normalised Transfer Function 0 1 2 Magnitude [ ] 3 4 5 6 7 6 5 4 Frequency [Hz] 3 2 Additional passive isolation with styrofoam outside Cabling short cuts thermal shielding / isolation and fluctuations in power dissipation inside thermal shielding (e.g. photodetector electronics) generates local thermal disturbances
Heat sources inside thermal shield Heat sources inside thermal shield include: Photodetector (must be always on) CCD (turned off after locking) Temperature sensors Photodetector most significant heat source. Temperature readout board
Temperature fluctuations 1 0 lpsd(split(tank wall)) lpsd(split(photodetector)) lpsd(split(alumnium Breadboard 1)) lpsd(split(alumnium Breadboard 2)) Requirement 1 amplitude [K/ Hz] 2 3 4 5 6 6 5 4 3 2 1 0 1 Frequency [Hz]
Temperature fluctuations 1 0 lpsd(split(zerodur Right)) lpsd(split(zerodur Front)) lpsd(split(zerodur Left)) lpsd(split(zerodur Back)) Requirement 1 amplitude [K/ Hz] 2 3 4 5 6 6 5 4 3 2 1 0 1 Frequency [Hz]
Subsequent requirements: pressure stability Changes in refractive index result in cavity round-trip length changes: Change of refractive index due to pressure Estimated requirement: L = nl rt (1) n p =2.9 9 Pa 1 (2) 5 mbar p(f ) 2.4 (3) Hz
Pressure fluctuations in vacuum environment 1 lpsd(split(tank)) Requirement 2 3 amplitude [mbar/ Hz] 4 5 6 7 6 5 4 3 2 1 0 Frequency [Hz]
Beat note readout: time series 6.25 x 8 Time origin: 2008 04 25 17:42:32.000 Beatnote 6.24 6.23 6.22 Amplitude [Hz] 6.21 6.2 6.19 6.18 6.17 6.16 0 0.5 1 1.5 2 2.5 Time [s] x 5
Corresponding length spectrum 7 lpsd(split(beatnote)) * 5.141e 016 Requirement 8 amplitude [m/ Hz] 9 11 12 6 5 4 3 2 1 0 Frequency [Hz]
Origin of excess noise: Excess noise shoulder between 1 mhz does not appear to be limited by pressure fluctuations temperature stability inside chamber electronic noise (e.g. photodiode, amplifier) Appears to originate in phase modulation/demodulation (likely candidate is residual amplitude modulation) Possible improvements: temperature stabilisation/isolation of EOM
Phase modulator Fiber coupled EOM high modulation depth. low amplitude modulation. residual amplitude modulation potentially limits performance.
Noise projection with no modulation 7 6 Error signal noise projection Requirement Equivalent Frequency ASD [Hz/ Hz] 5 4 3 2 1 0 4 3 2 1 0 Frequency [Hz]
Noise projection with modulation 7 Error signal noise projection Requirement 6 Equivalent Frequency ASD [Hz/ Hz] 5 4 3 2 4 3 2 1 0 Frequency [Hz]
Comparison 7 Error signal noise projection Requirement 7 lpsd(split(beatnote)) * 5.141e 016 Requirement 6 8 Equivalent Frequency ASD [Hz/ Hz] 5 4 amplitude [m/ Hz] 9 3 11 2 4 3 2 1 0 Frequency [Hz] 12 6 5 4 3 2 1 0 Frequency [Hz]
Beam alignment onto PAAM mirror angular jitter translates as longitudinal fluctuations: manufacture tolerances location of PAAM center of rotation (COR). limited alignment accuracy of beam onto PAAM COR. current requirement assumed to ± 50 µm. 2a a ρ PAAM a ρ
Differential wavefront sensing Telescope design: limited space for beam propagation inside thermal shield: 40 cm. design compromise for sensitivity in both tilt directions: Gouy phase 45. 0 0 optical layout 11 M2 1 QPD1 2 3 4 5 6 7 8 9 PAAM f=50mm f=75mm M1 design of resonant QPD and mixer electronics.!0!0 0 0 200 300
Outlook and Summary Characterization of excess noise source. First observations show a non-linear coupling mechanism of thermal effects into amplitude modulation. Intensity stabilisation appears not to be necessary at present. Implementation of DWS to provide auxiliary angular jitter information. Present performance is considered to be sufficient to place a significant upper limit on the longitudinal noise of the PAAM. Testing of first industrial prototype (TNO) has already started at AEI premises.