Pulsating detection work

Written by: Tom Shelley | Published:

A very efficient way of measuring multiple positions in a major project has been unveiled. Tom Shelley reports

Hundreds of position detectors are being read out by a single laser pulse in the Compact Muon Solenoid (CMS), part of the Large Hadron Collider project at Cern in Geneva.
This is achieved by choosing a laser frequency to which silicon photo detectors respond, yet are transparent. So, instead of having to direct the laser pulse to different detectors by lenses and mirrors, it is possible to excite up to 10 detectors behind each other, using low-cost diode lasers – and, by using beam splitters, send the same laser pulse to hundreds of detectors.
While the Cern project is intended to push forward the limits of engineering technology, as well as discover new science, the position measuring system is highly transferable to improving the precision of machine tools and automated manufacturing systems.
Dr Shaukat Hameed Khan, one of the designers of the system – who is now rector of the Ghulam Ishaq Khan Institute of Engineering Sciences and Technology in the North West Frontier Province of Pakistan – has recently been explaining the technology.
The CMS forms part of the Large Hadron Collider at Cern, the largest piece of high-precision engineering on the planet that has been much in the news of late. The superconducting solenoid may be “compact”, he says, but it is still 12m long and 6m in diameter, and is the largest magnet of its type ever constructed, producing a field of 4 Tesla – 100,000 times greater than the earth’s magnetic field. The problem is that positional alignments have to be maintained to within tens of microns in order to produce useful results. However, parts of the site rise and fall by as much as 30mm as a result of changes in water level in Lake Geneva, not to mention the effects of magnetic fields, temperature gradients, and differential thermal expansions of different materials and moisture.
The solution, according to Khan, is to be able to direct things so they stay in alignment, using a Laser Alignment System (LAS). The size of the installation and its many parts makes for an extremely complicated system, with 25,000 silicon strip detectors connected to 75,000 APV (Analogue Pipeline Voltage) chips, with 9.6 million readout channels. And yet it has been a lot less complicated than it would otherwise be, by choosing 50mW diode lasers, producing pulses at around 1075nm wavelength, which is just inside the 400 to 1100nm response curve of silicon photo detectors and just inside the transparency window for silicon, which starts at about 1000nm. This allows up to 10 photo detector arrays to be placed behind each other, with the laser pulses interacting with each detector as they pass through, and beam splitters to direct the laser pulses to multiple cascades. The laser spot position is read on each detector, so that a single laser shot produces many relative positions of many detectors at the same instant.
In order to avoid interference effects resulting from the patterns from the bars interacting with each other as the pulse passes through, it is necessary to use broadband diode lasers with short coherence lengths – in other words, low-cost diode lasers, rather than expensive gas devices. Transmission of each detector device is in the range 13% to 20%, increased by 40% on what it would otherwise by a special anti-reflective coating that reduces reflection to less than 6%.
Just to add to the complexity, everything has to be radiation-hard and able to survive a neutron flux greater than 4 x 10, raised to the power 14 neutrons per square cm and 10 MegaRads of gamma radiation. Optics Labs in Islamabad had to qualify 13 types of glass, three types of optical cement and different dielectric optical coatings for adhesion and abrasion resistance after exposing them to the required radiation fluxes, corresponding to 10 years of CMS life.
The system is still complicated. One mother laser (with a second laser as a backup) produces light that is split into 40 beamlets in the control room. But the system is manageable and, more importantly, it works!
Once the system is stabilised, the centre of gravity of the laser spots can be ascertained with a resolution of 0.2 microns in the X direction and 0.3 microns in the Y direction, provided the detector has a comparable intrinsic resolution.
As well as taking part in the design, many of the optical parts were also manufactured in the Optics Laboratories in Islamabad, including 40 beam splitters and 16 alignment tubes. To make up the assemblies to form the TOB (Tracker Outer Barrel) rods, carbon fibre parts were received by Optics Labs from European suppliers, and then assembled and adhesively bonded together on precision jigs.
These then went to Fermilabs in the USA, where the silicon detectors and front-end electronics were mounted on the frames and sent to CERN. At CERN, the assemblies were tested on electronic jigs, and designed and manufactured by Optics Labs personnel, before being finally integrated and tested as complete rods. The project was completed under the leadership of the RWTH in Aachen, Germany.
While many of the participants seem to have mainly been interested in impressing their fellow researchers, and show little interest in commercialising the technologies they have invented, the position sensing and control system would appear to have many potential applications in improving the precision of integrated production facilities for manufacturing products, ranging from large telescope mirrors to aircraft assemblies and cars.


* Position sensing system uses 1075 nm laser light from diode laser and silicon photo detector bar arrays that are be placed behind each other in series

* The photo detector arrays are sufficiently transparent to allow 10 to be used at a time

* The resulting system is still complex, but vastly less so than it would be otherwise

Email rector@giki.edu.pk

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