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:
TM1:
- 6 microns in Y
15 microns in Z
Fringe contrast at the four
interferometers is excellent:
TM2:
-16 microns in Y
- 7 microns in Z
(Requirement was ±25 mm)
Reference
Frequency
X1
X12
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:
FIOS1 = 79%
FIOS2 = 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 eLISA
• 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 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
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
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
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
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 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