Temperature has always been a very important requirement in various industrial sectors. Certain industrial applications require a high degree of sensitivity as they are related to critical processes in which accurate, real-time temperature measurement is essential in preventing accidents, failures or breakdowns. In addition, the environments in which these measurements are made tend not to be favourable, so the devices are subjected to high levels of stress that can reduce their useful service life. Examples are the aviation industry where high temperatures inside turbines must be monitored; the petrochemical industry where flammable and explosive substances must be preserved within certain temperature ranges; the food industry where certain products must be stored at sub-zero temperatures, etc.

So there is a demand for highly sensitive, robust thermometers capable of operating over wide thermal ranges. "Today, electronic thermometers are the most widespread temperature detecting devices in industry. Among them, thermocouples are a reliable, cost-effective technology. However, they may not be the best candidates for use in environments with flammable gases etc. In recent decades, various sensors based on optical fibres have emerged as an alternative to overcome this limitation thanks to their passiveness and intrinsic sensitivity to temperature," said Josu Amorebieta, researcher in the UPV/EHU's Applied Photonics Group.

That is why "in this work we developed a thermometer based on a very special optical fibre that is capable of withstanding extremely demanding operating conditions. It is a multi-core optical fibre that we obtain through a partnership agreement we have with the University of Central Florida (UCF).  To draw an analogy: a common fibre, for example, is the fibre that reaches us in our homes; a road with a single lane there and back. Multicore fibre is the equivalent of a motorway in which there are numerous lanes along which light can travel.  So the properties of both data transfer and the quantities of light that can be transmitted along these lanes are multiplied," explained the UPV/EHU researcher.  

"To develop the sensor or thermometer, we conducted a theoretical analysis by exploring in detail the bases relating to the effect of temperature on these fibres in order to predict the most temperature-sensitive geometry. The thermometer designed operates across a wide thermal range from -25 degrees Celsius to 900 degrees Celsius and is robustly packaged to withstand harsh environments," added Josu Amorebieta.

According to the researchers in the UPV/EHU's Applied Photonics Group, "the results suggest that the optical thermometer produced is as accurate as any electronic thermometer with the advantage of being inert, compact and easy to manufacture". Therefore, "we believe that this device is very attractive for industrial applications that call for very sensitive temperature sensing in very demanding environments. This prototype is a step towards commercially viable optical thermometers. Our goal would be to implement it in real environments, in other words, to transfer our lab work to the real world", they added.