Wireless temperature sensors can be interrogated remotely

The sensors themselves are essentially just modified crystals that split light to give temperature readings in the order of millikelvins over a broad range from around -120°C to +680°C.

‘The point about this is that the crystals can be anywhere in the area where we’re measuring the temperature,’ said project collaborator Prof Mike Glazer of Oxford University.

‘We can pump the light in from anywhere and we can interrogate from anywhere, with no wires and no cable, and we can work at huge distances away if we want.’

The phenomenon of birefringence, where a non-uniform ‘anisotropic’ crystal splits light into two separate rays, is complex but well characterised.

Indeed, previous research has already shown that the size of the effect will increase or decrease in proportion to the temperature of the crystal. Therefore, in theory, you could calibrate such crystals to be highly accurate temperature gauges.

However, the use of birefringence in this way has significant problems in practice. This temperature-measuring ability of highly birefringent crystals is badly compromised by changes in the thickness and orientation of the crystal. This adds expense to the manufacture and calibration of such crystals and makes them almost unusable in situations where, for example, vibration could alter the orientation of the crystal.

Counterintuitive

Prof Glazer, along with colleagues at Warwick University, came to the somewhat counterintuitive solution of using a novel crystal based on lithium tantalate that actually has zero birefringence in its resting state — so it’s optically isotropic, much like ordinary glass.

However, temperature changes actually induce a rapid increase in birefringence in these materials that is virtually independent of the crystal’s thickness and position, making it resistant to vibration and cheaper to manufacture.

‘We’ve already demonstrated with a very simple rig that we can measure temperature changes in the millikelvin, and I’m quite sure with more effort we can get into microkelvin, which raises all sorts of interesting questions.’

Applications

Thanks to the accuracy and complete flexibility of the system, Glazer believes it could have a range of applications, especially in places where electrical or magnetic interference is an issue.

There is particular interest in using the sensors for magnetic resonance imaging (MRI), which has subtle temperature effects on the human body that are not yet fully understood.

Glazer also added that he had been approached to investigate the potential of his temperature sensing at the bottom of oil wells, which currently use great lengths of optical fibres.

‘So long as you have a line of sight, we could envisage dropping one of these things down an oil well, without the need for any cables.’