OBI from LTP to NGO and beyond Alasdair

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

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

March 2010 : OB ready for delivery …

PFM OB performance highlights Beam alignment to test masses: Interference Contrast: TM 1: - 6 microns in Y 15 microns in Z Fringe contrast at the four interferometers is excellent: TM 2: -16 microns in Y - 7 microns in Z (Requirement was ± 25 mm) Reference Frequency X 12 Optical Throughput: Generally excellent results ! 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: FIOS 1 = 79% FIOS 2 = 80% = = 94% 90% 94% 92%

But then …. photodiode problems …

LISA 2010 Symposium OB design

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

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

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

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

FIOS for e. LISA • Essentially monolithic design

Monolithic FIOS production

Redundancy provided by dual FIOS

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

CQP – evolution for e. LISA 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

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

Beam clip for the transmitted beam • • Defines the beam at the entrance pupil of the telescope 5 mm +/-20 um 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 +/-20 um Integrated beam dump to remove the excess beam power (~81 m. W) from the 5 mm beam

Acquisition sensor • Acquisition sensor – Goodrich KTS 320 -1. 7 -RT-OEM : In. Ga. As 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

Photodiode mounting • Thermally compensated ‘pendulum clock’ QPD mount design – Y-Z flexure mechanism with micron precision adjustment over a +/100 um range – Polariser-window fitted to front of QPD Mount, also seals the QPD from contamination – Fully removable FEE and QPD subassembly – 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

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

…. 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

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