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.
We report a Fourier Transform Spectroscopy scheme for the simultaneous demodulation of a Bragg grating sensor array and the measurement of the intra-grating spectral structure of the individual gratings of the array, based on referencing to a transversely multimode laser. The performance for this reference, relative to that of a single-mode reference, is evaluated in the context of coherence effects, temperature sensing, and high-resolution intra-grating spectral measurement. The results indicate the potential for grating demodulation across the full spectrum of the E, S, C and L optical telecommunications bands.
We demonstrate a Fourier Transform Spectroscopic approach to achieve high-resolution measurement of the structural detail of the reflected spectra from an infrared FBG grating array, with the inherent measurement capability over a wavelength range of 900 nm. Measurements on all gratings in the array are achieved in a single scan of the OPD, from an interferogram captured on a single InGaAs photodiode. We demonstrate the approach for measurements on standard gratings and in the case of a grating subjected to a nonuniform measurand. The OPD is referenced from a visible HeNe beam propagated on the array downlead, and we demonstrate the robustness of the measurement in the presence of the complex transverse modal structure of that reference beam.