Fibre sensor operates at extreme temperatures

Optical fibres can be used in a variety of ways to perform remote sensing operations. Take the case of the Fibre Fabry-Pérot (FFP) interferometer, for example. Working on the principle of light interference produced by two parallel reflecting surfaces either side of a small cavity, they can be constructed in different ways, either with an external cavity, or with the cavity located within the body of the fibre itself. They can be used to measure pressure, temperature or strain, all by detecting changes in the optical path length of the cavity due to environmental influences. They can even be used as chemical sensors because the optical path length in the cavity is related to the refractive index of the medium inside. Researchers at the Missouri University of Science and Technology have come up with a way in which to do just that, in such a way that produces a highly robust device suitable for chemical sensing¹.

In its simplest form, an FFP interferometer is formed by a pair of parallel mirrors separated by a small distance, at the end of an optical fibre. Some of the light rays sent down the fibre toward the interferometer pass through the first mirror, which is only partially reflecting, and are reflected off the second mirror. Some of those rays then pass back through the first mirror and interfere with the light rays that did not enter the cavity between the two mirrors, instead having been reflected off the first mirror. By analysing the interference pattern produced by the reflected rays, it is possible to measure the extra path length traversed by those rays that entered the cavity and reflected off the second mirror. The optical path length of the cavity can be modulated either by movement of the mirrors or by a change in the refractive index of the medium inside.

FFP sensors are attractive alternatives to more conventional electronic sensors, due to their small size and their immunity to electromagnetic interference. They can also operate in environments where it is advisable to use the inert glass material of an optical fibre, such as for use within the body, or where sparks may be hazardous, thereby precluding the use of electrical sensors.

This latest advance in fibre optic sensing used a femtosecond laser to cut a micronotch in a fibre, thereby forming the cavity of the interferometer. Femtosecond lasers are able to remove material by ablation and, due to their extremely short duration pulses, they produce minimal thermal damage. It is even possible to produce features within the bulk of the material by focusing the laser beam or exploiting the phenomena of multi-photon absorption. One advantage of cutting the cavity into the fibre is that there is no need for complicated construction of small parts, such as external mirrored surfaces. Another advantage is that the interferometer is all-glass and can therefore withstand higher temperatures than can sensors constructed with parts that must be fixed in place by a bonding agent.

Fig. 1. Micronotch cut into the side of an optical fibre.

A device was made using this technique and is illustrated in Fig. 1. The micronotch was cut into the side of a single-mode fibre, perpendicular to its length, to a depth of approximately 72 μm, cutting through the core of the fibre through which the light propagates. The cavity between the reflective surfaces was around 30 μm wide. By positioning the cut close to the end of the fibre, the risk of breaking the fibre was minimised, something more likely to occur further down its length when the weakened fibre is exposed to bending forces.

The surfaces of the cavity perpendicular to the light path along the fibre constitute the two mirrors of the interferometer. By carefully controlling the rate of laser ablation, the roughness of these surfaces was kept to a minimum. This was done by continually monitoring the interference signal from the cavity during fabrication, thus ensuring sufficient reflection at the two surfaces to produce optical interference.

The device was tested for its ability to withstand high temperatures. Using a broadband light source and an optical spectrum analyser (OSA), the change in cavity length could be measured as a function of temperature, which ranged from room temperature up to 1100 °C. The device survived repeated tests and the results were reproducible, however the measured coefficient of thermal expansion, based on a linear fit to the data, did not agree with that expected for the glass fibre. The researchers suggest that thermally induced bending may be the cause. Despite this, adequate calibration of the device would enable it to make direct temperature measurements in extreme temperature environments. Also, by creating a notch in the optical fibre, the cavity can remain open and allow the ingress of solutions, which would enable it to operate as a chemical sensor based on measurements of the solution's refractive index.

One can imagine many uses for such devices, from simple pressure and temperature sensors, to the chemical sensors suggested by these researchers. The simple one-step laser machining process is also a favourable manufacturing technique and it may be possible to construct fibres with arrays of such sensors along its length, which could then be addressed individually. This research is yet another demonstration of how optical fibre sensors can be constructed in new ways and applied to solve a variety of novel problems.

1) Wei, T., Han, Y., Tsai, H., Xiao, H. (2008). Miniaturized fiber inline Fabry-Perot interferometer fabricated with a femtosecond laser. Optics Letters, 33(6), 536. DOI: 10.1364/OL.33.000536