Caltech shrinks optical accelerometers
A light alternative to tracking mobes
The practical limit to tracking a smartphone-owner’s movements in real time – beneath the resolution of GPS and associated location-tracking technologies – is the sensitivity of the motion sensor. Researchers at Caltech say they’ve taken a step towards very fine motion sensors, using nano-scale waveguides.
Today’s smartphones use electrical accelerometers. While these are already small enough for the application, sensitivity is limited, as researcher Oskar Painter explains.
“Most sensors are completely limited by thermal noise, or mechanical vibrations—they jiggle around at room temperature, and applied accelerations get lost in that noise,” he explains in Caltech’s announcement.
The green proof masses are suspended across
cavities - not visible here - which use lasers
to sense their movement. Scanning electron
microscope image: Martin Winger & Caltech
Instead of an electrical accelerometer (a displacement circuit measuring the movement of a “proof mass”), Painter’s group focussed on miniaturising optical accelerometers, which use reflected light beams to measure the proof mass displacement. While already used in high-sensitivity science applications – like the Laser Interferometry Gravitational-Wave Observatory – such detectors are hardly miniature (the LIGO mirrors are separated by kilometres).
Down at micro-scale, Painter’s group created a proof mass etched into a chip. These are suspended across optical cavities that are 20 microns long and one micron wide, and a few tenths of a micron thick. Two waveguides (“nanobeams”) in the cavity, one of which is attached to the proof mass, direct a laser and detect how its reflections change with movement of the proof mass.
The sensitivity of the detector reaches down to a few femtometers, the researchers say.
Even cooler – forgive the pun – because everything in the system is so small, the lasers damp unwanted movement in the proof mass.
“In our device, the light applies a force that tends to reduce the thermal motion, cooling the system,” Painter said – down to a temperature of three Kelvins (about –270°C) in the current devices.
That gives the detector the flexibility to sense both very large and very small accelerations, making it suitable for oil and gas exploration, bio-medical sensors, and aircraft applications.
Oh, and yes: if commercialised, a nano-scale optical accelerometer would be sensitive enough to map a shopper’s movements within a supermarket, allowing them to be bombarded with aisle-specific advertisements.
Thanks a lot. ®
Re: Gives a new meaning...
No, I'm just rummaging for loose change.
US export rules
Expect the US government to get jumpy about such things. You need good accelerometers in precision inertial navigation systems, you know, the sort you put in cruise missiles and nuclear missiles so that they can be accurately targeted without having to worry about GPS jamming (or whatever). If all of a sudden you start getting inertial nav that good in a mobile phone then that starts to get worrying; anyone would be able to do it. Let's hope they don't make the gyros better too, though presumably the same trick could be applied to them.
Built in car sat nav sometimes has an inertial side to it to help when GPS drop out (tunnels, etc). This sort of thing could make a portable sat nav like a TomTom as good as a built in. It would also make the EU's Galileo GPS lookalike less worthwhile. Mobiles / sat navs could use accurate inertial navigation plus the occasional GPS fix and cope much better with an obscured sky view. Galileo's improved performance in the Urban Canyon environment wouldn't be needed as much.
It would be distressing indeed if the primary use of this tech ends up being the hideous pettiness of serving up geoloc advertising. What's the ****ing point of advertising washing powder on someone's ****ing mobile when they're already stood in that aisle in the supermarket right next to the ****ing washing powder? They're probably about to ****ing buy some washing powder anyway. All in all it's just another way for the advertising brokers like Google to hold companies to ransom - "If you don't advertising your product right here then someone else will". How does that benefit anyone at all, other than Google/Apple/etc???????? It's bad enough getting spam email as it is, but getting spammed every few yards in the supermarket is going to result in flat batteries, overloaded mobile networks and ****ing cross punters.
3K in one direction only
I will bet that "3K" temperature isn't what most people think of as a temperature, but rather only applies to the motion of the proof mass in the direction of measurement - the thermal vibrations in that axis ONLY are reduced to the level you'd see if things were at 3K. However, I'll bet the over-all vibrations, in all 3 axis, are normal temperatures.