UOA09-11: Absolute distance measurement
Submitting InstitutionUniversity of Oxford
Unit of AssessmentPhysics
Summary Impact TypeTechnological
Research Subject Area(s)
Physical Sciences: Other Physical Sciences
Technology: Communications Technologies
Summary of the impact
The performance of absolute distance measuring systems has been improved
in terms of accuracy, traceability, reliability and cost through the
introduction of new methodology arising from research at the University of
Oxford. This has brought commercial benefit to a German company making
measurement systems, through the creation of a new product line. New
capabilities for measurement have been delivered to a first customer in
Germany. The research has also resulted in the establishment of new
activity at the National Physical Laboratory, and influenced UK and
European technology roadmaps for future manufacturing.
Particle physicists at the University of Oxford have had a fifteen year
involvement with the development of detectors and accelerators that
require precise alignment and monitoring, using frequency-scanning [laser]
interferometry (FSI) of increasing sophistication. Two key projects laid
the foundations for the impact described here.
ATLAS silicon inner tracking detector (R&D 1991-2002;
To monitor the positions of the many components of this detector for the
Large Hadron Collider, a metrology system was required that was of
sub-micron accuracy, very small, low mass, passive, radiation-hard, able
to monitor hundreds of positions inexpensively and capable of remote
operation. The Oxford ATLAS group proposed , developed, constructed and
installed  a Frequency Scanning Interferometry (FSI) system consisting
of a pair of lasers that drive a `tree' of optical splitters each output
of which feeds an interferometer, yielding an extremely scalable system.
Using the two lasers alternately compensated for an error from drift
during each measurement. Essential advances were a highly efficient
data-acquisition system, a new way of manufacturing reflectors, and
identifying a combination of suitable technologies for fibre splitters. In
addition, given the operating environment, radiation-hard materials and
fibres had to be identified and tested.
Linear Collider Alignment and Survey (LiCAS, 2002-9):
This project developed a robotic survey system (RTRS) for the 35km tunnel
of a future linear collider (ILC). Dr Armin Reichold proposed a short
train of measurement cars that can rapidly and automatically measure a
network of reference position markers. The design objective was an
accuracy of order 200 microns in the tunnel's straightness over 600
metres. A key step was the development by Reichold and his student John
Green of a new data analysis technique based on least-squares spectral
analysis (Lomb Periodograms) of the interferometer intensity data over
time allowing simultaneous observation of multiple reflectors in a single
interferometer. Other major achievements from this research were the
ability to both feed light and read it out through a single fibre for the
first time; the simultaneous presence of light from both lasers in the
interferometers, eliminating the drift error completely; and collimation
of the beam without generation of secondary reflection for long
measurements up to 20 metres.
Having carried out market surveys to find photo diodes, lasers, and
collimators with the required performance, the Oxford team realised that
the system could be extended to longer distances and made significantly
cheaper by adopting existing commercial telecommunications hardware. This
meant switching to wavelengths around 1550nm and using InGaAs detectors.
To adopt this approach, Reichold and his team developed compatible
amplifiers with sufficiently low noise and high bandwidth and determined
what specification of detector would be effective. The project culminated
in a system that amplified a telecoms-class laser to operate 32 FSI
interferometers simultaneously, which was deployed and tested in the RTRS
at the DESY laboratory in Hamburg . This project was cut short after
STFC's withdrawal from the ILC in 2007 but, motivated by the need for
stable magnet positions in future linear colliders, the method was refined
further, ultimately reaching nanometre precision interferometry for
distances up to 10 metres.
Following these two particle and accelerator physics projects, the Advanced
Metrology Using Laser Tracers project (AMULET, 2010-13), led by
Reichold, then developed a new technique called dynamic FSI to give
measurements that were both absolute and traceable [4,5], both essential
attributes for industrial applications. The project was funded by EPSRC
and two partners: the National Physical Laboratory (NPL) and the company
ETALON-AG. A major advance was to use the molecular absorption frequencies
in a gas cell as the primary traceable frequency standard, avoiding the
cumbersome evacuated and thermally stabilised reference interferometers
adopted in conventional approaches.
Other achievements by Reichold's group included
- A method to measure distances at every data point taken during a laser
scan, enabling repeat rates up to the sampling rate of the
analogue-digital converter (up to 2.77MHz for the main project and up to
50MHz for a prototype).
- Creation and evaluation of robust fibre-based reference
interferometers that represent a temporary length standard and remain
stable over the duration of an FSI scan (seconds); these massively
decreased the overall size and cost of the system.
- Improvement of the vibration and drift tolerance of FSI.
- Optimisation of FSI system costs, in particular for the FSI lasers and
- Exploitation of the new capability of a single FSI interferometer to
observe several distances simultaneously for improved multidimensional
Participating researchers from the University of Oxford included
lecturers Richard Nickerson (1989-present), Armin Reichold (1998-present)
and David Urner (2004-10); PDRAs Steve Gibson (2007-10) and John Dale
(2010-2013), both previously students, Paul Coe (2009-10), Patrick
Brockill (2008-9 and 2011-13), and Grzegorz Grzelak (2003-8); and students
John Green (2004-8), Matthew Warden (2006-12) and Andrew Lancaster
References to the research
(Oxford authors underlined; * denotes best indicators of quality)
1. A.F. Fox-Murphy, D.F. Howell, R.B. Nickerson
and A.R. Weidberg (1996). Frequency Scanned Interferometry (FSI):
The basis of a survey system for ATLAS using fast automated remote
interferometry. NIM A 383 229-237. (7 citations, Scopus)
This was the first paper on FSI for ATLAS.
2. *S.M. Gibson, M. Dehchar, K. Horton, A.
Lewis, Z. Liang, S. Livermore, C. Mattravers
and R.B. Nickerson (2010). A novel method for ATLAS FSI alignment
based on rapid, direct phase monitoring. ATL-INDET-PROC-2010-037,
This conference paper presents the results from operation of the FSI
system at ATLAS, providing continuous measurements every 8 seconds, with
a sensitivity of <50 nanometres over 24 hours. Gibson worked on this
project in Oxford and thereafter moved to CERN.
3. A. Reichold, P. Brockill, S. Cohen, J.
Dale, M. Dawson, T. Handford, M. Jones, G.
Moss, L.A. Rainbow, M. Tacon, C. Uribe-Estrada,
D. Urner, R. Wastie, S. Yang, J. Prenting, M.
Schlösser and G. Grezelak (2008). First data from the Linear Collider
Alignment and Survey Project (LiCAS). 11th European Particle
Accelerator Conference, Genoa, June 2008. http://epaper.kek.jp/e08/papers/tupc118.pdf
This paper presents the design criteria and performance of the tunnel
survey robot (using FSI technology) installed at DESY, Hamburg and
developments in analysis software and understanding of systematic
calibration and errors. Other authors, from DESY and Warsaw, are members
of the LiCAS collaboration.
5. *J. Dale, B. Hughes, A. Lancaster, A. Lewis, A.
Reichold and M. Warden (2013), An absolute distance
measurement system using frequency scanning interferometry and gas
absorption cells. Confidential report submitted to ETALON, 28th
October 2013 (available on request).
Details of the impact
(references e.g. [A] are to sources in section 5)
Laser tracers and laser trackers are widely used for high accuracy,
large-scale dimensional metrology, for example in the calibration of CNC
and CMM machines. In 2010, the best such instruments used differential
interferometry for high accuracy displacement measurements, necessitating
either continuous observation of a single target, or the use of much lower
accuracy absolute distance meters. The AMULET project broke these
limitations, by equipping a prototype laser tracer with dynamic FSI to
perform absolute distance measurements with accuracy comparable to
that of differential interferometry and with other user benefits as
described below. The National Physical Laboratory, as one of the partners
in the project, tested the dynamic FSI technology and confirmed
sub-part-per-million accuracy at up to a 20 metre range [A].
Isis Innovation, the University of Oxford's technology transfer office,
filed two patents  for the FSI system in July 2011, which were licensed
to ETALON-AG in 2012. ETALON identified the FSI technology as, "a
pioneering solution to the precision measurement of absolute length"
New capabilities in absolute distance metrology
The dynamic FSI approach allowed the tracer to be tolerant to beam breaks
and to automatically switch targets. ETALON describe some of the
advantages of dynamic FSI being that it allows traceable measurements with
a "10,000 times faster time resolution and 20 times higher accuracy"
than previous absolute distance systems; that it "recovers gracefully"
from beam breaks and does not require frequent calibration.
Dynamic FSI systems can also be scaled up to make many hundreds of
measurements for only a small fractional increase in cost compared to the
basic system, simply by using multiple interferometers whose components
are cheap. ETALON have identified that together with being "cheaper to
manufacture than previous technologies", this makes their "Absolute
Multiline™" range of FSI products "relevant and accessible to a much
wider range of purchasers" [B]. ETALON describe their market areas
as "the calibration, monitoring and accuracy enhancement of machine
tools and measuring machines", with customers from the mechanical
engineering, metrology, automotive and aerospace industries, as well as
from research sectors.
New product brought to market
The Absolute Multiline™ system launched in November 2012 [B] became
ETALON's primary FSI-based product. It is a general purpose, absolute
distance interferometer system marketed as having up to 100 measurement
channels and distances between the sensors and system electronics of up to
several kilometres. ETALON's marketing material describes another
advantage of the system as being that, "The sensor signals are not
affected by electromagnetically noisy environments; therefore the
placing of sensors in e.g. energy chains is possible without the
degradation of performance" [C]. The list price of the basic product
([text removed for publication]) with software is in excess of [text
removed for publication].
ETALON's development of the FSI system into a product was accelerated by
research and consultancy contracts undertaken by Reichold and his team at
Oxford, which ETALON identify as "drawing directly on knowledge and
experience from their research" [B]. ETALON-AG secured funding from
the Lower Saxony Development Bank, NBank, to build a prototype Absolute
Multiline™ system for marketing and user training purposes. In total,
ETALON have invested [text removed for publication] in developing and
marketing the Multiline system. This included not only manufacturing the
prototype but also demonstrating it at trade shows in Chester, Stuttgart
and San Diego. They have stated that they wish to "make Multiline one
of the cornerstones of our business".
The first of these systems, with 24 channels, was built at Oxford and
later repackaged by ETALON. This system was sold to the Metrology and
Quality Management group at RWTH Aachen, and installed and commissioned in
June 2013. The group leader at Aachen stated that the system "significantly
enlarged" their capabilities in the field of large volume metrology
and that they were "not currently aware of any other system on the
market that [Aachen] could have purchased to achieve the same
performance". Their most challenging application at July 2013 had
successfully achieved ten simultaneous absolute distance measurements over
distances up to 6m [D]. By July 2013, a second sale had been agreed to the
metrology group at the SLAC National Accelerator Laboratory in the USA.
New budgeted programme and employees at the National Physical
NPL is the UK's national measurement institute, operated commercially
under contract to the National Measurement Office. As a direct result of
their participation in the AMULET project, NPL decided to establish their
own FSI group including a research programme valued at around £700,000
[A]. By July 2013 this included appointing two new, full-time members of
staff, one of whom was Dr Warden, who left the Oxford group to take up
that post. As another benefit, NPL realise licence revenue from any sales
of Absolute Multiline™ and thus receive financial benefit from the systems
sold by ETALON.
Influence on policy via technology roadmaps
One of the themes within NPL's future vision, `Metrology for the 2020s'
(published in March 2012), is `embedded and ubiquitous measurement':
within this theme, laser interferometry systems for accurate and traceable
metrology were identified as having an important contribution to future
processing and production [E]. NPL state that the FSI work will be
instrumental in delivering some of this vision. NPL also identify the
inclusion of FSI on two roadmaps as resulting directly from AMULET [A]:
- The UK National Measurement Office draft Large Volume Metrology
roadmap 2011-19 [F], which forms part of the Engineering and Flow
metrology programme of the governmental Department of Business,
Innovation and Skills.
- The European Association of National Metrology Institutes (EURAMET)
Technical Committee for Length's roadmap for Large Volume and Long Range
Dimensional Metrology, published in June 2012 [G].
Sources to corroborate the impact
A. Performance of dynamic FSI system, attribution of impacts to
Oxford's research and changes to NPL's programmes: Science Area
Leader, Dimensional Metrology and Lead Scientist, Large Volume Metrology,
National Physical Laboratory, letter held on file.
B. Activities, expenditure and benefits to ETALON-AG: CEO of
ETALON, letter held on file.
C. Multiline marketing: http://www.etalon-ag.com/index.php/en/products/multiline
D. Purchase, installation and performance of first system sold:
Chief Engineer, Laboratory for Machine Tools and Production Engineering,
RWTH Aachen University, letter held on file.
E. `Metrology for the 2020s' vision, http://www.npl.co.uk/2020vision/
F. Inclusion of FSI in draft NMO Large Metrology Roadmap 2011-19:
copy held on file.
G. Inclusion of FSI in EURAMET roadmaps, June 2012,
especially Large Volume and Long Range Dimensional Metrology: