A new method for the design and manufacture of sensors based on optic fibres

A novel method for the design and manufacture of sensors to measure the temperature and relative humidity of the air, the pH of solutions or the refractive index of liquids based on optic fibre has been devised at the Public University of Navarre.

The sensors are small devices capable of capturing both physical and chemical signals from the surrounding environment and converting them into electrical signals for their subsequent processing. The information thus transformed can be easily quantified, manipulated and processed by electronic and computer systems which keep us constantly up-dated on any change in the environment where the device is located.

In this PhD thesis, the proposed sensors are based on the transformation of the optic fibre by a burnished area thereof. These optic fibres are of standard telecommunications monomode silica and their coating has been partially eliminated over a small area by mechanical burnishing. This perturbation on the burnished zone alters the propagation of the optic signal transmitted in the fibre, giving rise to a controllable correlation between the diminution of the optic potential of the outgoing signal and the mesurando which actuates with the signal over the said transformated zone.

The impact of the perturbation to which the fibre is subjected with the burnishing varies according to the samples. In some, the perturbation is important, in such a way that an amply dynamic range can be achieved for the sensor more than half of its power. This depends on the extent of the burnishing.

Sensors for turbidity and composition

Using this sensor system, the thesis author, Ainhoa Gastón, has studied the optic structure based on this polished fibre, its characteristics, how it changes the transmitted potential, i.e., what the light intensity might be as a function of parameters such as wavelength and external refraction index in contact with the structure, in order to know how to interpret what the sensors’ characterisation curve might be.

This research and the suitable selection of materials have provided the guidelines for the development of the sensors presented and, likewise, constitute a simple procedure for the design of new sensor devices adapted for specific applications.

One of the applications of the new sensors is the study and determination of the refraction index of liquids, in other words, functioning as a refractometer, with the advantage, moreover, of being not only incorporated into the fibre, but also being able to provide the index of non-transparent materials.

Thus, in a process of chemical components manufacture, the sensors allow the exact measurement of what will be, in the end, the refractive index, and in such a way that it will be possible to determine the chemical composition from the data obtained. For example, using a sensor incorporated into a tray on a production line, exactly what is being mixed when different compounds are being mixed can be determined in situ, or the concentration of sugar in a solution can be controlled. This application is useful not only for food processing but also all kinds of chemical processes.

In the same way, the refractive index allows the determination, for example, when the water in a washing machine gets so dirty that it has to be changed. In this sense, although this application is carried out by means of sensors for turbidity, it can equally be effected using the refractive index of the water when it becomes darker and/or dirtier. In this case, the sensor head would have to be incorporated inside the washing machine drum and, via optic fibre, the information on turbidity, etc., would be transmitted directly to the control panel of the washing machine, to the microprocesser which integrates all the functions.

In brief, the sensors designed in this PhD thesis serve to incorporate them into certain networks for controlling conditions such as humidity, temperature, pH, the quality of oil, etc. They can be used in very specific situations, in those cases where other sensors are not viable, in effect, both their operation and resistance characteristics are very good. Moreover, it is important to take into account the current ubiquity of optic fibres, given that the use of these sensors does not require electro-optical transducers in order to obtain the information they offer.

Method of manufacture

The polishing techniques currently used to make the sensors are varied. For example, there are those who polish the fibres incorporated into silicon wafers or glass blocks. The method proposed by Ainhoa Gastón, however, is one where the fibre is polished trapped in a resin, given that it is a much cheaper alternative, much more accessible, even versatile, given that the resin allows the moulding of pellets of varying sizes, with a bigger or smaller curvature of fibres embedded in them. Apart from the resin, this PhD proposes the use of other new materials aluminium powder (aluminium oxide) as an abrasive element.

Likewise, Ainhoa Gastón puts forward an innovative system of moulding of the fibre in multiple stages. One of the peculiarities of this system is that it permits placing different curvatures at different points on the fibre.