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Interferometer gets set for space

20 Oct 2004

Scientists build a monolithic glass interferometer that boasts picometer stability.

By exploiting a new bonding technique, a team of UK researchers has built a monolithic glass interferometer that is ultrastable and robust enough for space-flight. The prototype instrument was fabricated by scientists from the Rutherford Appleton Laboratory (RAL) and the University of Glasgow.

If all goes well a final version will be launched into space in 2008 as an integral part of a European Space Agency (ESA) mission called LISA Pathfinder [Laser Interferometer Space Antenna] to demonstrate technology for the detection of gravitational waves in space. For LISA to have any chance of success it is important that the interferometer's parts are stable to the picometer level and robust enough to survive the vibrations of the launch.

RAL's project manager Martin Caldwell says that these requirements have now been met: "The interferometer is at the heart of an experiment that recently demonstrated picometre stability, and what ESA calls 'technology readiness'. We are soon to start work on the actual flight instrument."

The 20x20x7 cm-sized glass "bench" actually consists of four separate Mach-Zehnder interferometers made from a total of 20 glass mirrors and beamsplitters. A technology called silicate bonding, pioneered by Stanford University and further developed by the Glasgow team, is used to permanently "lock" the parts to a glass base plate, without the use of any non-glass material. This achieves the required long-term positional stability of better than 10 picometers over 20 minutes.

"Essentially it works like chemical welding," explained Caldwell. "You put a little drop of silicate hydroxide between the surfaces to be joined and it cross-links the bonds of the glass to create a very permanent and stable bond."

To build the instrument, the team use a template to define the position of the parts on the base plate and then apply the pieces one by one. The team has less than 1 minute to fine tune the alignment of a part before the bond becomes permanent.

In order to prevent small changes in temperature from destroying the stability of the interferometer, the glass base plate and the parts to be mounted are made from low thermal expansion glass. The result is effectively a monolithic structure that is immune to thermal effects.

"We can't use normal optical fabrication methods because the use of metallic or epoxy materials, even in small amounts, would produce thermal expansion effects that would prevent the picometer level of performance being reached," said Caldwell.

The instrument development was funded by ESA and led by EADS Astrium, Germany. The stability tests took place at TNO, Netherlands, in a high thermal-isolation chamber. Tesat and Innolight (Germany), Contraves (Switzerland) and University of Hannover (Germany) provided the high-stability laser, modulator and readout electronics that were also needed.

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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