We describe experiments aimed at assessing the applicability of fibre Bragg grating sensors to distributive tactile
sensing. Strain signals from flexible surfaces instrumented with Bragg grating sensors are processed using neural
networks so as to obtain the location, shape and orientation of objects placed on the surfaces.
Long period gratings have been inscribed in standard single mode fibre using a fs laser system, a fusion arc and a UV
laser and a comparative study carried out of their thermal behaviour. The fs laser induced gratings can survive
temperatures in excess of 800 °C, however the inscription process can induce considerable birefringence within the
device. Annealing studies have been carried out showing that below 600 °C, all three grating types show a blue shift in
their room temperature resonance wavelengths following cyclic heating, while above 600 °C, the UV and arc induced
LPGs exhibit a red shift, with the fs LPG showing an even stronger blue shift. High temperature annealing is also shown
to considerably reduce the birefringence induced by the fs inscription process.
We report on high power issues related to the reliability of fibre Bragg gratings inscribed with an infrared femtosecond laser using the point-by-point writing method. Conventionally, fibre Bragg gratings have usually been written in fibres using ultraviolet light, either holographically or using a phase mask. Since the coating is highly absorbing in the UV, this process normally requires that the protective polymer coating is stripped prior to inscription, with the fibre then being recoated. This results in a time consuming fabrication process that, unless great care is taken, can lead to fibre strength degradation, due to the presence of surface damage. The recent development of FBG inscription using NIR femtosecond lasers has eliminated the requirement for the stripping of the coating. At the same time the ability to write gratings point-by-point offers the potential for great flexibility in the grating design. There is, however, a requirement for reliability testing of these gratings, particularly for use in telecommunications systems where high powers are increasingly being used in long-haul transmission systems making use of Raman amplification. We report on a study of such gratings which has revealed the presence of broad spectrum power losses. When high powers are used, even at wavelengths far removed from the Bragg condition, these losses produce an increase in the fibre temperature due to absorption in the coating. We have monitored this temperature rise using the wavelength shift in the grating itself. At power levels of a few watts, various temperature increases were experienced ranging from a few degrees up to the point where the buffer completely melts off the fibre at the grating site. Further investigations are currently under way to study the optical loss mechanisms in order to optimise the inscription mechanism and minimise such losses.