Fiber Bragg Grating Method (FBG)
Fibre Bragg gratings (FBG’s) offer a method for labelling short sections of optical fibres in fibre optic sensors (FOS’s) to allow detection of changes in the local environment around the fibre such as strain, pressure and temperature. An attractive feature of FBG sensors is the ability to fabricate arrays of sensors at multiple locations along a single fibre, as illustrated in Figure 1. The FBG’s are uniquely identifiable using optical techniques such as wavelength division multiplexing, where, in their quiescent state, the gratings are arranged to reflect different wavelengths back along the optical fibre. The measured quantity such as strain or static pressure can then be determined from the wavelength reflected from each grating.
The FBG’s are typically etched on the FOS fibre by UV laser illumination using a phase mask or an interferometer. FBG’s have sensing gauge lengths of around 0.1 – 10 mm, and act as a wavelength selective mirror in the core of the fibre (see Figure 2a). FBG sensors in this form are then primarily temperature and strain dependent. These variables generate changes in the grating period and/or the effective refractive index of the propagating wavelength mode. The concomitant changes in the reflected wavelength may subsequently be detected using a spectrometer by interrogating the fibre output using suitable sensors (see Figure 2b). From the spectral output, strain and static pressure are obtained from the fibre as recently proven in a project with Airbus UK.
Advanced FBG systems ensure the measured quantities are wavelength encoded. This allows demodulation schemes to be used that are insensitive to source power fluctuations and to connector and bend losses. Furthermore, in these advanced FOS’s, the FBG’s are intrinsic to the optical fibre, and an array of FBG’s may be readily multiplexed into a single optical fibre to provide multiple measurement points along the fibre with a spatial resolution as high as 0.1 mm and data rates in the order of kilo Hertz (kHz). Finally long fibre lengths can be encoded at multiple points and interrogated without any significant loss of signal.
Fibre optic sensors (FOS’s) have been extensively studied for nearly 3 decades, and offer a number of significant benefits when compared with conventional pressure and strain gauge sensing systems. For example they offer small dimensions (typically, 80 – 125 µm in diameter), low weight, a large operating temperature range and have highly flexibility structures – a 0.2 mm diameter fibre can have a bend radius as low as 2 mm. FOS’s also can be used in harsh environments, being chemically inert, resistant to high radiation environments, and are immune to electro-magnetic interference. In addition, FOS’s may be readily incorporated into composite parts or in pressure vessels with minimal effect upon the mechanical integrity of the structure. Furthermore, with the multipoint and multiplexing methods available when using FBG’s, long fibre lengths can be routed through large aircraft structures for example with the acquisition of many thousands of data points at kHz data rates.
From recent static pressure measurements with FBG FOS systems in a wind tunnel, the major problems encountered with the FBG’s were the dependency of the output signal on the fibre mounting method and the sensitivity of the fibre interrogation optics to vibration. It was found that although the signal output gave the correct aerodynamic characteristic, the magnitude of the output was highly dependent on how the fibre was mounted onto the surface of interest. Further work needs to be completed on the optics stability and the fibre mounting method and calibration before FBG systems will be ready for flight test. Within AIM² these further development steps are done to enable in-flight testing with these new pressure sensors. As the fibers can be realised as thin lightweight foil FBG will enable surface pressure measurements with less impact to the flow and the structure.