OBI – from LTP to NGO and beyond

1. OBI – from LTP to NGO and beyond Alasdair Taylor, Ewan Fitzsimons, Christian Killow, Michael Perreur-Lloyd, David Robertson, Harry Ward University of Glasgow on behalf of the LISA EBB teams at AEI, ASD and TNO

2. Plan of the talk • Retrospective : where we were at the time of the last LISA Symposium • Update on LPF OB • Update on LISA EBB developments • NGO and beyond

3. March 2010 : OB ready for delivery …

4. PFM OB performance highlights Beam alignment to test masses: TM1:- 6 microns in Y 15 microns in Z TM2: -16 microns in Y - 7 microns in Z (Requirement was ±25 mm) Interference Contrast: Fringe contrast at the four interferometers is excellent: Reference = 94% Frequency = 90% X1 = 94% X12 = 92% Optical Throughput: Throughput of the optical chain has been measured from input to the fibre connector on the feed to the OB to the summed output from all ports of the OBI Excellent throughput achieved: FIOS1 = 79% FIOS2 = 80% Generally excellent results !

5. But then …. photodiode problems …

6. LISA 2010 Symposium OB design

7. EBB OB design – main features Fibre injectors Reference interferometer Test mass readout Beam expander Beam dump PAAM metrology Science ifo Acquisition sensors PAAM Beam clip

8. EBB OB CAD design Fibre injectors Reference interferometer Test mass readout Beam expander Beam dump PAAM metrology Science ifo Acquisition sensors PAAM Beam clip

9. LTP Fibre injectors (FIOS) • The LPF fibre injectors had an air-spaced design with separately mounted lens glued into fused silica holder • Holder then catalysis bonded with spacing from fibre end adjusted with ~ 10 micron accuracy using live readout of beam

10. LTP FIOS – evolution to LISA • The LPF fibre couplers met all design requirements • But the design allowed small thermally induced beam pointing drift, due mainly to the CTE of the separately mounted lens • Left: temperature of bench, right: beam position on quadrant photodiode during one thermal cycle, showing a vertical angle variation of the beam from the fibre coupler of 3.4µrad/K • Also, the exit of the fibre was exposed, potentially giving susceptibility to contamination issues if used with high laser power

11. FIOS for eLISA • Essentially monolithic design

12. Monolithic FIOS production

13. Redundancy provided by dual FIOS

14. CQP alignment reference for LTP build • Two quadrant photodiodes mounted on a rigid Invar base • Mounting of beamsplitter and photodiodes by screwed and glued interfaces requiring regular calibration checks to catch any critical component movement • Used as both target and to measure existing beams • Very successfully used for LTP OB build

15. CQP – evolution for eLISA construction • Mark II CQP, featuring improvements derived from experience gained during the LTP OBI build, and designed to take us to the regime where we are dominated by CMM accuracy • Zerodur baseplate • bonded photodiode mounts, beamsplitter and mirrors • larger area photodiodes • longer baseline • optimised shape

16. Beam dumps • In sensitive interferometric measurements small amplitude of stray light can be a significant perturbation • Commercial beam dumps typically use coatings that are not suitable for spaceflight • A spiral beam dump has been designed at Glasgow in which absorption is achieved by multiple reflections and absorption steps • Titanium construction with internal black coating of >98% absorption at 1 micron • Measurement in a heterodyne interferometer of an uncoated test device has shown reduction of return light power of better than 108

17. Beam clip for the transmitted beam • Defines the beam at the entrance pupil of the telescope • 5mm +/-20um aperture with 2 degree tapered profile • Titanium with polished, gold plated 45 degree clipping face • Flexure feet for differential thermal expansion reasons • Isostatic mount to allow for removal from beam path and possible later attachment of an actuated mechanism • Tight positioning accuracy of +/-20um • Integrated beam dump to remove the excess beam power (~81 mW) from the 5mm beam

18. Acquisition sensor • Acquisition sensor • Goodrich KTS320-1.7-RT-OEM : InGaAs CCD, 320 x 256 pixels, pitch 30 μm; power < 4 W • Titanium construction of the mounting frame to complement primarily aluminium architecture of the acquisition sensor and form a ‘pendulum clock’-style thermally stable design • Horizontal stability negligible by analysis over ±10°C • Vertical stability < ± 2 um over ±10°C

19. Photodiode mounting • Thermally compensated ‘pendulum clock’ QPD mount design • Y-Z flexure mechanism with micron precision adjustment over a +/-100um range • Polariser-window fitted to front of QPD Mount, also seals the QPD from contamination • Fully removable FEE and QPD sub-assembly • FEE box with low emissivity internal front surface & high emissivity rear surfaces to encourage radiation away from the QPD chip • Optional low-emissivity thermal shield

20. Photodiode imaging optics • Imaging optics are needed in several places to reduce tilt/piston coupling resulting from beam motions (e.g from test mass jitter) • Some of these lens systems are challenging to design and verify and also expensive to produce • Current planning excludes these from the current EBB build program

21. …. NGO …. • The OB subsystems developed for the EBB are directly used in the compact dual-sided NGO design • …. and will also be directly usable in any “LISA-like” OB

22. Other details …. • See Glasgow posters for greater detail • Precision Measurement of Optical Beams • LISA/NGO Fibre Couplers • Optomechanics for LISA/NGO